EGU General Assembly 2023  

The progressive localization of deformation has long been recognized as a fundamental phenomenon of the macroscopic failure of rocks. Our recent analyses using X-ray tomography during triaxial compression indicate that fractures and higher magnitudes of shear and dilative strain spatially localize as rocks are driven closer to macroscopic failure. Similarly, geophysical observations of low magnitude seismicity in southern and Baja California show that deformation localizes toward the future rupture plane of M>7 earthquakes. These sets of observations indicate that deformation can increase in localization toward failure, and that deformation can temporarily decrease in localization (delocalize) during this overall increase. These observations indicate that the spatial organization of deformation may be used to recognize the acceleration of the precursory phase leading to large earthquakes and the macroscopic, system-scale failure of heterogeneous materials. However, such efforts will require identifying the conditions that promote phases of delocalization, and how these perturbations in the overall trend of increasing localization influence the timing of macroscopic failure. In this presentation, I will describe these analyses, and new work that aims to identify which characteristics of the fracture networks determine the localization at a particular level of stress, and the change in localization from one stress step to the next in triaxial compression experiments at the confining stress conditions of the upper crust.

How to cite: McBeck, J., Renard, F., and Ben-Zion, Y.: Episodic delocalization in the upper crust: Implications for earthquake forecasting, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1798, https://doi.org/10.5194/egusphere-egu23-1798, 2023.

Hotspots, sites of mid-plate or excessive volcanism, overlie plumes of hot rock that rise in the solid state from Earth’s deep mantle. Estimated rates of lateral hotspot motion since Late Cretaceous time have been as low as ≈3 mm/yr to as high as ≈80 mm/yr. We focus on geologically current (i.e., neotectonic) motions because the precision and accuracy of relative plate motions in the MORVEL set of relative plate angular velocities (DeMets et al., 2010) are an order of magnitude greater than plate motion estimated for earlier time intervals.  Prior efforts to estimate neotectonic relative motion between hotspots from trends of hotspot tracks found no significant difference from zero motion (i.e., they are consistent with fixed hotspots) and no useful upper bound on the rate of motion (e.g., Minster et al., 1974; Gripp & Gordon, 1990, 2002).

Our recent analysis builds on methods to objectively estimate the uncertainty of hotspot trends. We use the uncertainties estimated by Gripp & Gordon (2002) and by Wang et al. (2019a) for the hotspot trend data set of Morgan & Morgan (2007).  The objectively estimated uncertainties tend to be larger than those assigned by Morgan & Morgan (2007), especially for slow moving plates.  In a global inversion of the observed trends, a chi‐square test indicates that the trends and adopted uncertainties are consistent with fixed hotspots (p=0.08; p < 0.05 would indicate significance). When we conduct a two‐tier analysis, however, the motion between groups of hot spots is significant. The group‐means of trend‐perpendicular component of velocity range in nominal magnitude from 0 to 6 mm/a with a median of ≈3 mm/a.  Wang et al. (2019b) applied these data and uncertainties to investigate how well the hotspot motion predicted by Doubrovine et al (2012) in their Global Moving Hotspot Reference (GMHRF) fit the observations.  Surprisingly the GMHRF fits the observed trends much worse than they are fit assuming fixed hotspots.

The key to our newest analysis, which attains far higher resolution than before, is the novel use of Monte Carlo inversion to find directions and rates of hotspot motion that misfit the observed trends by angles consistent with the uncertainties in the trends. We obtained one million pseudorandom realizations of the direction of motion of each of 53 hotspots and inverted for the rate of hotspot motion that best fits the observed hotspot trend data.  We examined speeds ranging from 0 to 15 mm/yr in increments of 1 mm/yr.  For 60% of the realizations the best-fitting hotspot speed is 0 mm/yr, i.e., no motion between hotspots. Forty per cent of the realizations are fit better with some motion between hotspots with merely 2% of the realizations being fit acceptably close to the misfit expected given the size of the uncertainties.  No realizations gave an acceptable fit with motion less than 2 mm/yr or with motion exceeding 8 mm/yr; the 95% confidence interval is 2–4 mm/yr, significantly different from zero, but low enough to strongly support the use of the fixed hotspot approximation.

How to cite: Gordon, R., Gaastra, K., Mifflin, G., and Wang, C.: Neotectonic Rates of Motion Between Hotspots, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4551, https://doi.org/10.5194/egusphere-egu23-4551, 2023.

Natural faults host various types of migrating slow earthquake phenomena, with migration speeds much lower than seismic wave speeds and different moment-duration scaling from regular earthquakes. To advance the obtained quantitative understanding of the migration process and long duration of slow earthquakes,  I study a chain reaction model in a population of brittle asperities based on a rate- and state-dependent friction on a 3-D subduction plate boundary. Simulation results show that the migration speed is quantitatively related to frictional properties by an analytical relation derived here. By assuming that local pore water in front of the migration drives rapid tremor reversal and is so local as to hold a constant stress drop, the application of the analytical solution to observational results suggests that (i) the temporal changes of observed migration speeds for the rapid tremor reversal could be explained by about 70% reduction of the effective normal stress; (ii) effective normal stress for the deeper extension of seismogenic segment in the western part of Shikoku is about 1.5 times greater than that in the central part. Applying rupture time delay between slow earthquake asperities for a duration longer than the regular earthquake, I also conclude that (iii) the characteristic slip distance of rate-and-state friction for low-frequency earthquakes is roughly between 30 μm and 30 mm; (iv) the stress and strength drops of very low-frequency earthquakes is much smaller than 1 MPa.

References:

Ariyoshi, K. (2022). Extension of aseismic slip propagation theory to slow earthquake migration. Journal of Geophysical Research: Solid Earth, 127, e2021JB023800. https://doi.org/10.1029/2021JB023800

How to cite: Ariyoshi, K.: Physical interpretation of slow earthquake migration process based on a friction law, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-204, https://doi.org/10.5194/egusphere-egu23-204, 2023.

With the growing need for CO2 storage, risk management is essential to secure the storage sites; these risks include fault reactivation, ground surface deformation/sea-bottom (uplift), well and caprock integrity, and CO2 leakage; managing these risks could be achieved by understanding the hydromechanical behaviors of rock induced by the reservoir pressure build-up caused by CO2 injection. However, this remains a crucial challenge because the rock's mechanical and hydraulic properties are poorly constrained. Moreover, the conventional monitoring methods usually consider CO2 plume migration only, which is not enough to understand the induced pressure front that occurs far beyond the real pressure plume. Although several techniques could image the geomechanical deformation and investigate the surface deformation well, these monitoring methods do not provide a complete image regarding the deformation migration from the subsurface to the surface due to the limited measurement points in addition to the cost issue.

 In this paper, we will introduce Rayleigh scattering-based Distributed Optical Fiber Strain Sensing (DFOSS) as an effective tool for subsurface and surface geomechanical monitoring to track the dynamic responses at each spatial location along the cable due to the deformation caused by injection; this technology could overcome other conventional methods' limitations including continual spatiotemporal measurements, cost-effective installation: vertically along the wellbore and horizontally into the ground surface, covered area and sea bottom. We will review several laboratory and field experiments from our previous studies. First, we will show the laboratory results from the first laboratory test to track the movement of the CO2 plume as it enters the clay-rich critical regions in the reservoir–caprock system using DFOSS and monitoring of the injected water in a sandstone sample using DFOSS in the second test. Both results demonstrated that DFOSS could provide high-resolution information on deformation and fluid activity. Next, we will show our subsurface monitoring field results, where we conducted several water injection tests in a shallow well. We monitored the injection process by installing DFOSS in a monitoring well. Our outcome confirmed that DFOSS could provide critical information for rocks' properties and fluid migrations by geomechanical monitoring, and it could be a real-time and permanent monitoring tool for wellbore, caprock integrity, and CO2 leakage. Finally, we will show the surface deformation monitoring results, where we installed the fiber cable into the surface horizontally in a shallow trench; the airbag inflation and deflation tests were conducted under the fiber cable to simulate uplift and subsidence caused by the fluid injection and production in the subsurface. The results suggested that DFOSS could locate any anomaly along the cable.

 Our results demonstrate that installation of DFOSS in fiber cables horizontally into the surface around the injection site and vertically in a well to incorporate well-based strain sensing with surface monitoring, allowing geomechanical monitoring (horizontally into the surface and vertically in the subsurface) in three dimensions via a cost-effective, real-time and permanent monitoring system.

How to cite: Amer, R., Xue, Z., and Hashimoto, T.: Geomechanical Monitoring at CO2 storage sites with Distributed Fiber Optic Strain Sensing: Insights from laboratory and Field experiments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1507, https://doi.org/10.5194/egusphere-egu23-1507, 2023.

Earth tides exert small gravitational variations in the subsurface which lead to pore pressure changes and water level fluctuations in groundwater monitoring wells. This groundwater response to Earth tides has been used to estimate subsurface hydraulic and geomechanical properties. However, existing approaches are based on simplifying assumptions and their reliability has not been tested for realistic conditions. To simulate how Earth tides affect the subsurface, we developed and verified a numerical model that couples hydraulic and geomechanical theories. We modelled the response of a semi-confined aquifer which exchange water with an observation well for the dominant M₂ Earth tide component. We reveal that undrained (i.e., groundwater does not flow in response to stress) and confined (i.e., groundwater is under pressure) conditions are necessary for the analytical solution to be valid. For the M₂ frequency we assess that this occurs at depths ≤ 50 m and requires specific storage at constant strain s_ε ≥ 10^-6 m^-1, hydraulic conductivity of the aquitard k_l ≤ 5 • 10^-5 ms^-1 and aquifer k_l ≥ 1 • 10^-4 ms^-1, respectively. Further, we illustrate that established analytical solutions are valid in unconsolidated systems, whereas consolidated systems require additional consideration of the compressibility ratio between the porous medium and the porous skeleton (i.e., inclusion of the Biot coefficient). Overall, we find that a priori knowledge of the subsurface system increases the reliability of the groundwater response interpretation. Our results improve understanding of the effect of Earth tides on groundwater systems and provide a framework for evaluating subsurface properties.

How to cite: Bastias, J., Rau, G., and Blum, P.: Numerical simulation provides conditions for interpreting the groundwater response to Earth tides, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1637, https://doi.org/10.5194/egusphere-egu23-1637, 2023.

According to well-established hypotheses based on field observations of natural faults, viscous deformation may localize following pre-existing brittle fractures. The weak behaviour can be explained by brittle grain size reduction and phase mixing, which may activate grain size sensitive processes in the viscous field. To prove this hypothesis, it is necessary to perform experiments to observe the strain and stress evolution in faulted and non-faulted rocks. Pec et al. (2012) performed experiments on granitic rocks by shearing manually crushed granitic powder between coarse solid granitic forcing blocks. However, in their study, there are unavoidable boundary conditions between the forcing blocks and the gouge, and a comparison to an intact rock without fracture is difficult.

In our study, we reduce the boundary conditions to a minimum and can directly compare the stresses and microstructural evolution during deformation of intact and fractured granitic ultramylonites at 650°C, confining pressure of 1.2GPa, and a constant displacement rate of 10^-8m/s. We perform these experiments on initially solid cylindrical samples in two experimental sets: In set A, we slowly apply the load and confining pressure, to ensure an intact rock sample is deformed. In set B, we create fractures before the experiment starts but already in the closed system of the experimental setup. Once experimental P/T conditions are reached, both experimental sets are deformed to different finite strains to investigate the associated microstructural evolution. The deformation is disseminated in the set A experiments, but localizes strongly along the fracture in experimental set B. The strain is accommodated by viscous granular flow incorporating an impressive grain size reduction of up to 1000x and dissolution/precipitation processes. In addition, the stress records show that in experiments A, initially a 30% higher yield stress has to be overcome before steady state flow, while in set B steady state flow is reached directly without a strain softening increment. In both sets, steady state stresses range around 300MPa, i.e. far below the confining pressure.

Applying microstructural observations and mechanical data of our experiments to deformation of granitoid crust in nature reveals that fractures serve to reach mechanical steady state earlier compared to non-fractured crust. As a matter of strain, however, both settings may yield at the same mechanical strengths of resulting shear zones. It is important to note that polymineralic fine-grained ultramylonites are up to four times weaker than monomineralic quartz, presenting an important behaviour of efficient strain localization and rheological properties substantially below those of the end member minerals.

Pec, M., Stünitz, H. and Heilbronner, R., 2012. Semi-brittle deformation of granitoid gouges in shear experiments at elevated pressures and temperatures. Journal of Structural Geology, vol. 38, pp. 200-221. https://doi.org/10.1016/j.jsg.2011.09.001

How to cite: Nevskaya, N., Zhan, W., Stünitz, H., Berger, A., and Herwegh, M.: Experimental evidence for viscous deformation and strain localization in fractured granitoid rocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2823, https://doi.org/10.5194/egusphere-egu23-2823, 2023.

The present mean convergence rate of Himalaya is ~ 15 mm/year. In comparison to the convergence and stress accumulation, only few stress release events represented by greater than M5+magnitude earthquakes in the Himalayan region have been observed. Understanding the constraints leading to the disparity in stress accumulation and stress release, is crucial to understand the stress accommodation mechanism and seismicity in the Himalayas. The current active subduction boundary is marked by Main Frontal Thrust separating the sub-Himalayas and the Gangetic alluvial plains. The rock types within the Main Frontal Thrust sheet show two primary types of sandstone protoliths, and gouges exhibiting cataclastic to foliated microstructural features. In this study, we have performed rotary-shear velocity step experiments on the powdered samples of the sandstone within the Main Frontal Thrust to determine their frictional properties at slow (creep) to fast (seismic) velocity under 10 MPa effective normal stress condition.  We discuss these results and their implications on seismic nucleation in Himalayan Main Frontal Thrust.

How to cite: Sarkar, D. P. and Hirose, T.: Frictional properties of sandstone gouges within Himalayan Main Frontal thrust: constraints on seismicity of shallow crustal deformation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3062, https://doi.org/10.5194/egusphere-egu23-3062, 2023.

Stress in active subduction forearcs is controlled primarily by friction along the megathrust and the gravitational force. The competing deviatoric compressive and tensional stresses generated by megathrust friction and gravity, respectively, are of the same order of magnitude and result in very low deviatoric stress in the forearc. The near neutral stress state in subduction forearcs is supported by observations of stress reversal, that is a change from deviatoric compression to deviatoric tension, caused by very small megathrust shear stress drops (<10MPa) after recent large megathrust earthquakes. However, studies that quantify and compare the stress state in subduction forearcs at various stages of the seismic cycle are still limited. Here, we use two-dimensional finite-element force-balance models to quantify and constrain forearc stresses at different locations along the Chilean and Japanese subduction margins that are at different stages of the seismic cycle. The models consider forearc topography, slab geometry, crustal thickness, and water load to quantify the elastic stress in the forearc due to gravity and friction along the megathrust. The models indicate low deviatoric stress values (10s of MPa) in the subduction forearcs, which imply a weak forearc crust in areas of active seismic deformation. We validate the model results by estimating seismic stress drop values of forearc earthquakes from high-quality seismic waveform recordings. We estimate spectral corner frequency using both single-spectrum and spectral-ratio estimates and depth-dependent shear-wave velocity models. Spectral-ratio estimates provide more robust corner-frequency estimates that we employ to validate and interpret the increasing stress drop trend down to depths of ~50-60 km. The slight depth dependency of seismic stress drop values is consistent with depth dependency of deviatoric stress obtained from the finite-element models. Moreover, we find that average seismic stress drop values are systematically lower or similar to maximum deviatoric stress obtained from our models, which is consistent with a partial stress release during earthquakes in the forearc. Our results suggest a relation between seismic stress drop values and ambient stress in subduction forearcs.

How to cite: Bocchini, G. M., Dielforder, A., Kemna, K. B., Harrington, R. M., Hampel, A., and Oncken, O.: Ambient Stress in Subduction Forearcs Constrained by Numerical Models and Earthquake Static Stress Drop Values, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3401, https://doi.org/10.5194/egusphere-egu23-3401, 2023.

Borehole breakouts, rock compressive failure at the wellbore wall, are one of the most widely utilized stress indicators, providing useful site-scale in situ stress states for a variety of geo-engineering projects. A 1 km deep vertical borehole drilled to study earthquakes in southeast Korea showed borehole breakouts rotated in azimuth at several depths by as much as 35° from the average azimuth, enlarging uncertainty in representative stress orientation. These breakouts developed in a highly fractured tuffaceous rock at a depth range from 840 m to 1000 m and breakout rotation always occurred adjacent to fractures and faults. While breakout rotation adjacent to fractures/faults has often been observed previously, there are several issues that have to be addressed regarding such a rotation, that is, would it be a local perturbation associated with drilling that can be ignored when assessing representative in situ stress states?; what aspect of fracture perturbs the stress indicator? To address these questions, we carried out a series of 3D finite element modeling, in which the rock mass consists of a single competent rock type (metamorphosed tuff) with a thin and soft planar fracture crossing the model. A borehole penetrates the center of the model vertically. The fracture orientation was varied from model to model for a given far-field boundary stress condition. The model results show that the rotation of breakouts increases generally (but with wide scattering) with an increase in slip tendency of the fracture. A more detailed analysis shows that the azimuthal rotation of breakouts tends to increase in a clearer manner with an increase in the horizontal shear displacement (or shear strain) component along fracture having relatively high slip tendency. For the reasonable values of mechanical properties assumed in the model, the breakout rotation can be as high as ~34° from the boundary stress orientation imposed in the model. Such stress rotation occurs throughout the extent of the fracture and is reflected in breakout rotation. The model results are quite comparable to the breakout rotations observed in the borehole. Our study suggests that breakout rotation is not just a local feature around the borehole but reflects a site-scale stress rotation associated with the presence of fractures having specific orientations and slip direction.

How to cite: Kang, M. and Chang, C.: Rotation of borehole breakouts by the effect of fractures/faults: observation and numerical model study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4865, https://doi.org/10.5194/egusphere-egu23-4865, 2023.

How to cite: Linsalata, F. and Spada, G.: Strength of the lithosphere derived by geological and geophysics data: the Graham Land (Antarctic Peninsula) case study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5060, https://doi.org/10.5194/egusphere-egu23-5060, 2023.

The stress field of Earth's upper crust is crucial for geodynamic processes and of key importance in planning and managing the utilization of the subsurface, such as geothermal energy extraction, stimulation of enhanced geothermal systems, or safety assessment of deep geological repositories. The contemporary 3-D stress state also provides the basis to assess the impact of induced stress changes which can lead to the reactivation of pre-existing faults, the generation of new fractures, or subsidence due to long-term depletion.

However, information on the stress state of Earth's crust is sparse and often not available for the areas of interest. So far, the stress tensor orientations and stress regimes have been systematically compiled and provided by the World Stress Map (WSM) project in a public-domain database. Yet, the acquisition of stress tensor orientations is not necessarily accompanied by the determination of the stress magnitudes, which, however, are required when investigating questions related to stability and hazard mitigation strategies of georeservoirs. To estimate the 3-D stress state, geomechanical-numerical modelling is applied. For the calibration of such models, stress magnitude data are essential. A major challenge is to bridge the scale gap between the widely scattered data that is required for model calibration and the high-resolution small-scale geological model in the target area. Ziegler et al. (2016) presented a multistage approach to resolve this challenge. For this, two successively calibrated models are created – one large-scale model with coarse resolution but available stress magnitude data for calibration, and one local model with fine resolution, e.g., based on a 3-D seismic survey of the target area, but without any stress data. Synthetic data obtained through the large-scale model is used to calibrate the small-scale local model.

First, we validated the multistage approach by means of generic models to rigorously quantify the associated introduced uncertainties. For this purpose, we implemented a highly simplified model lithology with only vertical stratification and no lateral changes. In a second step, we applied the multistage approach in a real-world setting and demonstrated the applicability on a local model of Unterhaching, south of Munich/Germany, where a geothermal district heating plant is located. Here, a local high-resolution model based on a 3-D seismic survey (Budach et al., 2018) has been successfully calibrated on a regional-scale stress model of the Bavarian Molasse Basin. The results of the local-scale model agree with the large-scale model. At the same time, stress change due to rock property variability, only resolved in the local-scale model, is shown.

How to cite: Morawietz, S., Ziegler, M., Reiter, K., Heidbach, O., Moeck, I., Budach, I., von Hartmann, H., and Ziesch, J.: A stress field model for the Unterhaching geothermal plant: Challenges and solutions in local model calibration, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5935, https://doi.org/10.5194/egusphere-egu23-5935, 2023.

We aim to quantify the likely ranges of magnitudes and directions of forces that may explain present-day natural stresses within the Eurasian plate. We first focus on one of the driving forces, horizontal gravitational stresses (HGSs) resulting from lateral variations in gravitational potential energy, which are particularly relevant in the context of the Eurasian plate because there are no major slabs attached to it (i.e., no slab pull force). We show that different published models of lithospheric density including lateral variations in the lithosphere-asthenosphere boundary result in significantly different HGSs. Other driving forces are mantle convective tractions including dynamic topography, and plate interaction tractions with bounding plates. Second, we include observed major faults into a 2D spherical cap elastic model of the Eurasian plate. We show results of forward FEM calculations based on the best model parameters of Warners et al. (2013) and compare them with observed stress directions. We propose different objective functions that quantify the (relative contributions to the) misfit of the modelled and observed stresses, fault slip directions, and magnitudes, the deviation of the net torque on the plate from zero, and the model representation error. Our study represents a stepping stone towards a Bayesian inference workflow to constrain the dynamics of the Eurasian plate of which we show preliminary results.

Warners-Ruckstuhl, K. N., R. Govers, and R. Wortel, 2013, Tethyan collision forces and the stress field of the Eurasian Plate: Geophys. J. Int., v. 195, no. 1, p. 1–15, doi:10.1093/gji/ggt219.

How to cite: Gutierrez Escobar, R., Garcia Sancho, C., and Govers, R.: New steps toward estimating the driving and resistive forces on the Eurasian plate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6297, https://doi.org/10.5194/egusphere-egu23-6297, 2023.

Fault gouges of granitoid composition represent the principal non-cohesive tectonites within fault zones in the continental crust. The spatial distribution and strength of granitoid fault gouges is therefore crucial for understanding how weak the upper continental crust could be due to the formation of fault zones. Although several laboratory investigations reported the mechanical weakening of granitoid gouges in shear experiments, the deformation mechanism responsible for such behavior remains not well understood.

To address this issue, we conducted two series of shear experiments on granitoid gouges by using a ring shear apparatus. The starting gouge powders were derived from crushed granitoid mylonite with a median grain size of 45 μm. In a first set of experiments, gouges were sheared at a sliding velocity of 100 μm/s for a displacement of 15 mm. Temperatures explored ranged from 20°C to 650°C in order to determine the temperature dependence of gouge strength. The second set of experiments is identical to the first ones, except that the applied sliding velocity was set at 1 μm/s to study how fault slip rate influences the strength of gouges.

We observe that differences in gouges strengths as a function of sliding velocity and temperature: At a sliding velocity of 100 μm/s, the steady-state shear stress (τ) remains relatively constant at τ=76-82 MPa over the entire temperature range. Contrastingly, at a sliding velocity of 1 μm/s the steady-state shear stress remains temperature-insensitive with τ≈75 MPa up to tempertures of 450°C, but decreases then to τ≈50 MPa at 650°C (Fig.1 a). Furthermore, the amount of decrease of shear stress is strain dependent (Fig.1 b). At even slower sliding velocity of 0.1 µm/s, the shear stresses decrease further to τ≈38 MPa.

Microstructurally, all gouges deformed at T≦450°C show typical cataclastic features, where angular clasts with grain size of ~10 μm are surrounded by a fine-grained matrix. Intergranular fracture arrays in Riedel- and Y-shears are well developed over the entire cross section, indicating homogeneous bulk deformation. In contrast, gouges sheared at 650°C with τ≈50 MPa show strain localization in a principal slip zone. It is shear plane parallel with widths up to ~50 µm. Inside the principal slip zone, all grains are dramaticly reduced to nm-size and tightly packed. No intergranular fracture arrays are observed. Outside the principal slip zone, rounded grains with size of ~5 μm are loosely packed, with meniscus cement growing in between. The aforementioned strain localization is enhanced at temperature above 450°C and slip rate below 1μm/s, suggesting that viscous creep mechanisms (e.g. pressure solution) control the deformation process at slow sliding velocities, which is not the case in fast rate experiments. Our results show that the activation of viscous creep mechanisms leads to significant fault zone weakening, while contrasts in grain size keep deformation localized.

Figure 1 Shear stress plotted as a function of temperature. Shear stress data collected at (a) 15mm displacement in steady-state, and at (b) 5mm displacement.

How to cite: Zhan, W., Nevskaya, N., Niemeijer, A., Berger, A., Spiers, C., and Herwegh, M.: Weakening of granitoid gouge in hydrothermal ring shear experiments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6615, https://doi.org/10.5194/egusphere-egu23-6615, 2023.

The North Alpine Thrust Front (NATF) is an example of the classical onshore fold-and-thrust belt and foreland system [1]. There are ongoing heat production projects in this area. However, complex compaction and stress fields require detailed investigation for safe and economical drilling activities. Previous field investigation of the wedge and foredeep shed light on the possible driving mechanisms for overpressure generation in the wedge, foredeep and footwall in the SE of Germany. To do this, 20 deep wellbores are selected in this area and their geophysical and drilling data are investigated [2]. This study is a complementary work to find possible explanations for observations through numerical modeling. Examining the mechanics behind these complex deformations is beyond the capabilities of the critical taper theory. However, a large strain geomechanical numerical simulator coupled with critical state soil mechanics constitutive model can provide useful insights in this regard. Geomechanical forward modeling requires boundary conditions at far distances. Also, except some basic geometrical features, other deformations are not predefined and they are developing during the simulation. Therefore, it is not only insightful regarding the final shape of the system, but also progressive development of the deformations is trackable [3].  A plane-strain framework is implemented to simulate the interested processes through the Elfen software [4]. A quasistatic criterion is assumed throughout the simulation to decrease the possible boundary effects of the loading. Adaptive-remeshing helps to capture the large-strain behavior of the system in a reasonable computational time. Data from different sources of the drilling, geophysical tools and field observation is used to tune the model and test the capability of the model to estimate the required properties. Numerical simulations result in a similar geometry which is observed in the field works. Obtained stress values and pore pressure are comparable to the field data.  The differences between the simulation results and field observations can be attributed to the assumptions which were made during the simulation. For example, thermal impacts and possible diagenetic processes are neglected during the simulation. Also, a homogeneous material is assumed for the different layers, while in the real case, there are heterogeneities inside the layers.

1. Pfiffner, O, A. (1986) “Evolution of the North alpine Foreland Basinn in the central Alps”, Foreland Basins, 219-228.

2. Drews, M., Duschl, F. (2022) “Overpessure, vertical stress, compaction and horizontal loading along the North Alpine Thrust Front, SE Germany”, Marine and Petroleum Geology, 143, 105806.

3. Albertz, M., Sanz, P, F. (2012) “Critical state finite element models of contractional fault-related folding: Part 2. Mechanical analysis”, Tectonophysics, 150-170

4. Rockfield (2017) “Elfen explicit manual (Version 4.10)”, Swansea, UK, Rockfield Software.

How to cite: Mahmoodpour, S., Drews, M., and Duschl, F.: Geomechanical forward modeling of the stress field, pore pressure and compaction in the North Alpine Thrust Front, SE Germany, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7038, https://doi.org/10.5194/egusphere-egu23-7038, 2023.

Natural faults are rough at all scales and can be described with fractal geometry. This deviation from planarity results in geometric asperities and a heterogeneous stress field. Analytical and numerical studies have shown that roughness introduces additional shear resistance on the fault and promotes failure in the medium surrounding the fault because of the elevated stresses. These studies generally assume a small slip on the fault, i.e., much smaller than the minimum roughness wavelength, λ_min. It is important to examine the effects of roughness on shear resistance and near-fault stresses at large sliding, as well as the assumptions incorporated in the derivation of the analytical solutions.

In this study, we examine the effects of fault geometry on the shear resistance and near-fault stresses at large sliding numerically, using a method that is based on the mortar finite element formulation, in which non-matching meshes are allowed across the fault, and the contacts are continuously updated. This enables modeling slip larger than λ_min and the overriding of asperity contacts. We begin with simulations of an elastic medium and show that, indeed, the roughness results in large and heterogeneous stresses on and off the faults, which increase with the roughness level. However, except for small slip values, the increase of shear resistance with slip is much smaller than the linear increase predicted by the analytical models, which assume small and uniform slip. For self-similar geometry, with Hurst exponent of H = 1, the average shear resistance increases with slip at a decreasing rate. For self-affine geometry, with H < 1, it initially increases with slip, then decreases at a slip larger than λ_min /2. Although overriding of asperities is allowed in the simulations, as slip increases, significant stress concentrations are developed on the fault, which may not be realistic for natural rock surfaces. To account for that, we implement wear laws into the method and model the evolution of stresses during a quasistatic slip and cycles of dynamic earthquakes. The wear process redistributes and bounds the stresses on the fault and allows a more realistic characterization of stress distribution near the fault.

How to cite: Tal, Y. and Wise, L.: Shear resistance and near-field stresses on rough faults, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7497, https://doi.org/10.5194/egusphere-egu23-7497, 2023.

After many large earthquakes aftershocks activity can reach depths greater than the base of the seismogenic zone that is defined by background seismic activity. This observation is generally explained by strain rate induced embrittlement associated with the increase of post-mainshock strain rate, which favors a transition from ductile to brittle behavior. However, the underlying physical processes are not well understood. Here we integrate geological and geophysical data for the 2016–2017 Central Italy seismic sequence with laboratory experiments to provide a geological and physical interpretation for the post-mainshock transient deepening of the base of the seismogenic zone.

The base of the seismogenic zone in the central-northern Apennines is set typically at 9-10 kilometers and corresponds to the top of the phyllitic basement. Structural studies on exhumed basement rocks highlight a heterogeneous basement fabric consisting of competent, 10 to 200 m wide, quartz-rich lenses surrounded by an interconnected and frictionally weak phyllosilicate-rich matrix. The matrix controls the overall rheology of the basement due to its interconnectivity, and promotes aseismic deformation because its rate-strengthening behavior.

Following each major (Mw > 5.5) event of the 2016–2017 sequence, a dramatic and abrupt increase in seismic rate is observed below 10 km, hence within the basement. Here we document the presence of seismicity clusters made of more than 4 earthquakes and characterized by small magnitude (Mw < 2.5), small dimensions (< 500 meters), small temporal duration (< 14 days) and a swarm-like behavior. Furthermore, these clusters are often represented by multiple or repeating earthquakes with a cross correlation coefficient higher than 0.7 for all the three components. These observations suggest that the increase of shear stressing rate within the basement is responsible for deepening of seismicity. To further explore this idea, we performed laboratory experiments on rocks from exhumed outcrops of basement rocks. We found that shear stressing rate promotes accelerated creep on the phyllosilicate-rich matrix and dynamic instabilities on the quartz-rich gouge belonging to the lenses.     

Our integrated analysis suggests that the mainshocks of the 2016-2017 seismic sequence promoted an increase of shear stressing rate within the basement allowing the phyllosilicate-rich matrix to creep faster hence favoring the loading and the repeated failures of locked seismogenic patches represented by the quartz-rich lenses.

How to cite: Volpe, G., Locchi, M. E., Pozzi, G., Tinti, E., Scuderi, M., Marone, C., and Collettini, C.: Structural and frictional control on the transient deepening of the seismogenic zone following major earthquakes in Central Italy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8377, https://doi.org/10.5194/egusphere-egu23-8377, 2023.

We study the present-day thermo-mechanical state of the Alpine Himalayan Collision Zone to understand the physics controlling the observed crustal differentiation and the underlying continental-wide geodynamics. We found that the stability of the lithosphere is regulated by a thermodynamically controlled critical crustal thickness (C_r), which is close to the average thickness of the continental crust. Regions with thickness higher than C_r, representing orogenic lithosphere, and higher than average potential energy undergo weakening and dissipate the acquired internal energy, compared to their foreland lithospheres that have crustal thickness close to C_r. A weaker orogenic lithosphere deforming in a dissipative mode to release the acquired potential energy manifests as zones of diffused rather than focused deformation. We additionally find that the energy dissipation path that the orogenic lithosphere takes to either attain C_r (cratonization) or to undergo runaway instability (rifting) is modulated by the feedback between the thermal and mechanical relaxation of the lithosphere. The internal energy stored in the crust from heat-producing elements, fastens the dissipation of the acquired potential energy.

How to cite: Kumar, A., Cacace, M., and Scheck-Wenderoth, M.: Continental lithosphere deformation is a response to its thermodynamically controlled critical crustal thickness, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8980, https://doi.org/10.5194/egusphere-egu23-8980, 2023.

For the safe usage of the subsurface the stress state is of great importance, e.g., for borehole stability, mitigation of induced seismicity or the search and long-term safety of a high-level nuclear waste deposit. However, the state of knowledge concerning the stress state in Germany is limited as only unevenly distributed stress measurements are available which frequently provide only one component of the stress tensor. The SpannEnD (Spannungsmodell Endlagerung Deutschland) project aims to improve this knowledge with the help of a 3D geomechanical-numerical model. The model is calibrated on available stress magnitudes and enables a continuum-mechanics based prediction of the stress state and its local variability for Germany.

The 3D geomechanical-numerical model comprises the upper lithosphere and contains 22 lithological units parametrized with individual mechanical properties (Young’s modulus and Poisson’s ratio) and densities. Linear elasticity is assumed and the finite element method (FEM) is used to solve the equilibrium of forces. Overall, the model contains about 11 million hexahedral elements resulting in a lateral resolution of 2.5 x 2.5 km² and a vertical resolution of up to 250 m. The model is calibrated by adaptation of displacement boundary conditions with magnitudes of the minimum (S_hmin) and maximum horizontal stresses (S_Hmax). The model results show an overall good fit with these stress magnitudes used for calibration indicated by a mean of the absolute stress differences of 4.6 MPa for S_hmin and 6.4 MPa for S_Hmax. Furthermore, the results agree well with additional data sets excluded from calibration but used for validation, e.g., with a mean of the absolute stress differences of 1.1 MPa for vertical stress magnitudes and an absolute mean deviation of the orientation of S_Hmax with regard to World Stress Map data of 11.9°.

How to cite: Ahlers, S., Reiter, K., Hergert, T., Henk, A., Röckel, L., Morawietz, S., Heidbach, O., Ziegler, M., and Müller, B.: SpannEnD – Prediction of the recent crustal stress state of Germany using a 3D geomechnical-numerical model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10150, https://doi.org/10.5194/egusphere-egu23-10150, 2023.

An essential feature of plate tectonics is that lithospheric deformation is localized at plate boundaries with substantially larger magnitude than that in plate interiors, suggesting that lithospheric rheology is weaker at plate boundaries than in plate interiors. Numerous mantle convection modeling studies that approximate this empirically derived lithospheric rheology using different formulations or proxies (e.g., pre-existing weak zones, faults, reduced coefficient of friction or yield stress, …) have largely reproduced the observed features of lithospheric deformation. While the rheological formulations in theoretical modeling studies have become increasingly more sophisticated often with an expressed goal to understand the cause of plate tectonics and initiation of subduction, it is important to place constraints on lithospheric rheology using in-situ observations including flexural (i.e., vertical motion) and seismic response to different forcings. Laboratory studies indicate that lithospheric deformation is controlled by frictional sliding, low-temperature plasticity (LTP) and high-temperature creep with increasing temperature. Observations of lithospheric flexure and seismicity at Hawaiian Islands (i.e., plate interior setting) in response to volcanic construction suggest that internal frictional coefficient µ_f is 0.25, while LTP is significantly weaker than that derived from laboratory studies [e.g., Mei et al., 2010], based on modeling studies of loading response of Hawaiian lithosphere with realistic elasto-frictional-plastic-viscous rheology [Zhong and Watts, 2013]. Further studies [Bellas and Zhong, 2021; Bellas et al., 2020; 2022] showed that µ_f is 0.3 and activation energy of LTP needs to be reduced from laboratory derived value of 320 KJ/mol to 190 KJ/mol to fit the flexural and seismic deformation at Hawaii, and that the same rheological parameters reproduce the observed elastic thickness at other oceanic islands and seamounts on lithosphere of different ages. The Japan subduction zone shows characteristic features of subducting lithosphere with its outer rise and trench topography and transition from shallow normal/extensional faulting to deep reverse/compressional faulting seismic deformation (i.e., neutral plane) [e.g., Craig et al., 2014]. Dynamic deformation models of subduction have been formulated, using realistic slab buoyancy force and elasto-frictional-plastic-viscous rheology, to interpret the observations of trench-outer rise topography and neutral planes [Han et al., 2022]. The modeling indicated that the observed neutral plane in the Japan subduction zone is consistent with the rheology for subducting lithosphere with LTP activation energy of ~220 KJ/mol and µ_f~0.3, which are similar to that inferred for the plate interior at Hawaii. The modeling also found that µ_f<0.1 that is required to generate mobile-lid or plate tectonic convection in mantle convection models [e.g., Moresi and Solomatov, 1998] would not generate the extensional to compressional stress transition (i.e., neutral plane) in the Japan subducting lithosphere, further suggesting the importance of in-situ observational constraint on lithospheric rheology and dynamics of plate tectonics. 

How to cite: Zhong, S., Han, S., and Bellas, A.: Constraints of Flexural and Seismic Observations on Lithospheric Rheology at Plate Interior and Plate Boundary Settings, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11786, https://doi.org/10.5194/egusphere-egu23-11786, 2023.

For many underground operations such as geothermal energy exploitation, mining, oil and gas production or the storage of high-level radioactive waste, active tectonic or induced seismicity is of concern. Seismicity usually occurs on pre-existing faults that are reactivated under adequate stress conditions. Thus, an assessment of the reactivation potential of faults can aid in the identification of areas particularly prone to the occurrence of seismic events or such areas where adequate geotechnical measures have to be taken to avoid anthropogenic fault reactivation. A tool for the assessment of the fault reactivation potential is the so called slip tendency, which is the ratio between the maximum resolved shear stress on the fault plane and the normal stress. Such an analysis requires information about the stress field acting on the fault plane and information about the fault geometry, fault orientation and frictional properties. Information about these parameters can be very limited, since 3D fault geometries are often only extrapolated from geological surface data. Furthermore, stress data is usually sparse, only available pointwise and unevenly spatially distributed. Geomechanical-numerical modelling can be used to derive a spatially comprehensive description of all six independent components of the stress tensor from the available stress data.   

For Germany, an estimate of the stress tensor is provided by the geomechanical-numerical model by Ahlers et al. (2022). Furthermore, fault geometries as part of geological models of the German federal states are available for large parts of Germany. We use both the stress data derived from the geomechanical-numerical model and the fault geometry data from the federal state models to calculate slip tendencies for more than 10.000 faults and fault segments. The resulting slip tendency is generally the highest in the northern Upper Rhine Graben area where it routinely reaches values of 0.7 and more. In the Alpine and Alpine Foreland region the slip tendency is generally the lowest with values only very rarely exceeding 0.3. In North Germany slip tendency values range mainly between 0.3 and 0.6 but with both higher and lower values being fairly common. In general, faults striking in NNE-SSW direction and NW-SE direction display the overall highest slip tendencies whereas faults striking in ENE-WSW direction show very low slip tendencies. With increasing depth slip tendencies generally decrease strongly. However, there are still major areas in Germany where either no fault geometries or only insufficient fault geometries are available. Furthermore, pore pressure has a major influence on the slip tendency. For our calculations, we assume hydrostatic pore pressure. While overpressured pore fluid is documented for example for the Molasse Basin in South Germany, no spatially comprehensive pore pressure data set is currently available for the whole of Germany.

How to cite: Röckel, L., Ahlers, S., Morawietz, S., Müller, B., Reiter, K., Heidbach, O., Hergert, T., Ziegler, M., Henk, A., and Schilling, F.: Slip tendency analysis of 3D faults in Germany, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12847, https://doi.org/10.5194/egusphere-egu23-12847, 2023.

The upper Earth crust is increasingly used by mankind, to extract, transport, store or dispose materials, energy, or waste. Regardless of the objective, long term safety and stability is essential and thus, the contemporary stress state of the upper crust is one of the key variables. To estimate a continuous description of the 3‑D stress tensor, geomechanical numerical models are used. The most important parameters to set up such models are the knowledge of the underground structures, the distribution of rock properties as well as the stress data, on which the models are calibrated. In the model, the vertical stress results from the gravitational volume forces due to the density distribution and the horizontal stresses from the Poisson effect as well as appropriate lateral displacement boundary conditions. The latter are determined by finding a best-fit with respect to given stress magnitude data of the maximum and minimum horizontal stress S_Hmax and S_hmin, respectively.

A unique dataset of stress magnitude data has been recently acquired within the exploration phase for deep geological repository of radioactive waste in Switzerland. Nine cored boreholes in three potential siting areas have been drilled and besides a wide range of logging runs, and laboratory tests of rock properties, more than 120 Mini-Hydraulic Fracturing (MHF) and Sleeve Re-Opening (SR) tests were successfully performed in different stratigraphic units to estimate the magnitudes of S_hmin and S_Hmax

Here, we present a 3‑D geomechanical-numerical model that shows both, the best-fit to the measured stress magnitudes as well as the range of stress magnitude variability in the volume of the different stratigraphic units. This variability results from MHF/SR measurements uncertainties and from the variation of rock properties within the lithologies. Furthermore, one has to assess how representative each MHF/SR measurement is for a larger rock volume. To represent the stress variability within the lithologies, many model simulations that cover the distribution of possible rock parameters were performed. The distribution is given by the cumulative density function (CDF) for the Youngs modulus and the Poisson number for each stratigraphic unit. Based on the range of model simulations we visualize the variation of the stress components along virtual well paths in analogy to the statistical variation. Such plots allow to quantify and visualize the potential variation of the present-day stress state within the stratigraphic column because of the petro-physical variability within the stratigraphic units. Furthermore, using the CDF, we can assign to each model simulation a probability that allows us also to estimate a probability distribution of the stress variability in the different units.

How to cite: Reiter, K., Heidbach, O., Ziegler, M., Giger, S., Garrard, R., and Desroches, J.: If you get the stress data, you've always asked for, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13718, https://doi.org/10.5194/egusphere-egu23-13718, 2023.

One main goal of the Continental Deep Drilling Program (KTB) of the Federal Republic of Germany was to establish a continuous stress profile from the surface to the final drilling depth of 9.1 km. To characterize stresses with depth, several independent methods were applied: analyses of borehole failure such as borehole breakouts/drilling-induced tensile fractures; hydraulic fracturing mini-tests at several intervals ≤3 km depth as well as two modified hydraulic tests at 6 and 9 km depth; and core disking and strain retardation of core samples. Focal mechanisms of induced seismic events from fluid injection experiments were inverted for stress estimates at different depths. Since then, the KTB is known as a world-class site with regard to crustal stress data. In particular, stress magnitude estimates are still among the deepest and fewest high-quality estimates derived at crustal depth.

The GEOREAL fluid injection experiment aims to characterize the geothermal potential at the KTB site at 4 km depth and to refine the adaptive reservoir stimulation concept employing near-real-time microseismic monitoring with direct feedback on hydraulic parameters. Additionally, a goal of GEOREAL is to investigate spatial and temporal stress variations at this depth. We noticed new borehole breakouts in the open hole section of the pilot well KTB-VB, likely due to the massive fluid production and injection experiments between 2002 and 2005. Together with new logging and seismic data from GEOREAL, these stress estimates will be used to further characterize the stress field from the borehole to the reservoir scale.

The GEOREAL hydraulic stimulation will include a series of hydraulic tests at ≥3.9 km to investigate the effect of pressure build-up and release, the role of continuous and periodically varying flow rates, the effect of relaxation phases and maximum injection pressure on the spatio-temporal propagation of induced seismicity. Induced events will be monitored with high precision using a 12-level geophone chain in the KTB main hole at only ~300 m distance to the stimulation interval. This will be used to determine stress estimates from focal mechanism inversion of induced events on a 100-m source scale.

To better understand the role of the local stress field we use a 3-D geomechanical-numerical model (10 x 10 x 10 km³) of the KTB. This offers a unique opportunity to utilize the detailed knowledge of the subsurface at the KTB site, in particular due to the existing 3-D structural model, high-quality rock property estimates from laboratory work, high-quality stress magnitude data, and new information from GEOREAL. The model provides a continuous description of the 3-D stress field including its changes due to the variability of rock properties to assess the in-situ stability of the intact rock mass and faults. This allows for further detailed studies that require the undisturbed in-situ stress state as one key observable and input parameter to characterize deep geothermal reservoirs and associated processes such as induced seismicity.

How to cite: Boese, C., Bohnhoff, M., Heidbach, O., and Dresen, G.: Integrated stress determination at the KTB deep crustal laboratory, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13795, https://doi.org/10.5194/egusphere-egu23-13795, 2023.

The Nordland area in NW Norway is one of the tectonically most active areas in Fennoscandia. It exhibits patterns of extension, which are in contradiction to the first-order regional stress pattern that reflects compression from ridge-push. The regional stress field stems from the interaction of ridge push and GIA (glacial isostatic adjustment); the local stress field mainly results from gravitational stresses, as well as the flexural effects of sediment erosion and re-deposition.

We develop 3D finite element numerical models of crustal scale, using existing geometric constraints from previous geophysical studies. Internal body forces, induced by variations in density, topography or Moho depth, already yield significant deviatoric stresses, which are often omitted in stress models. We show that these can strongly influence the near-surface stress regime, in particular for the continental-margin setting we are considering. Similarly, existing weakness zones (such as faults) control the local stress field.

We apply the far-field stress fields (GIA, ridge-push, sediment redistribution) as effective force boundary conditions to the sides or base of the model. This way, we can account for all stress sources at once, but can also vary them separately in order to examine their relative contributions to the observed stress and strain rate fields.

We compare our models to the stress and strain observations derived from different recent seismological and geodetic data sets. These point to a correlation of seismicity with major changes in the crustal geometry.

How to cite: Gradmann, S., Keiding, M., Olesen, O., and Maystrenko, Y.: The 3D stress field of Nordland, northern Norway - insights from numerical modelling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13955, https://doi.org/10.5194/egusphere-egu23-13955, 2023.

The Canary Islands are an archipelago of eight islands and several islets in the Atlantic Ocean that have been built up by intraplate magmatism. The more recent subaerial eruption took place in La Palma Island during the last four months of 2021 (September 19^th to December 13^th). This volcanic activity formed the Tajogaite volcanic vent and several minor vents following an eruptive fissure roughly trending N310ºE. The eruption was preceded by intense shallow (<12 km depth) volcanotectonic activity that continued during the whole eruptive process, reaching more than 11,000 earthquakes located. After the first shallow pre-eruptive seismic swarm, the seismicity was mainly located at two different depth levels with hypocenters located at 10-15 and 30-40 km depth.

Seismicity record in the island for previous historical eruptions is very scarce and we have used this seismic episode to explore the lithospheric strength in this intraplate geodynamic setting corresponding to an old (~156 Ma) oceanic lithosphere. Geotherms and brittle and ductile rheological laws with different thermo-mechanical properties have been used to calculate strength envelopes. We have combined the study of the lithospheric strength and the vertical distribution of the seismicity from that period to estimate the extension of the brittle mechanical layer that conditioned the hypocentral locations.

How to cite: Martín-Velázquez, S., Gomez-Ortiz, D., Martín-Crespo, T., De Ignacio, C., Arnoso, J., and Montesinos, F. G.: Lithospheric rheology and strength in La Palma Island (Canary archipelago), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14316, https://doi.org/10.5194/egusphere-egu23-14316, 2023.

The active Taiwan mountain belt is located in a complex geodynamic setting that involves two subduction processes. To the northeast, the Philippine Sea Plate subducts northward beneath the Eurasian Plate at the Ryukyu Trench, while in the southwestern part, the Eurasian Plate subducts eastward under the Philippine Sea Plate, where it obliquely collides with the Luzon Volcanic Arc. The Taiwan thrust-and-fold belt is created as a result of this ongoing arc-continent collision. Regardless of the predominance of compression in the overall structure of the island, several studies have also reported normal faulting. This study aims to estimate the local and regional stress field using earthquake focal mechanism data to contribute to a better understanding of crustal deformation in the complex tectonic setting of Taiwan.

Manually clustered earthquake focal mechanisms are inverted to obtain an estimate of the principal stress (σ₁, σ₂, σ₃) orientations and the stress ratio (σ₁-σ₂)/(σ₁-σ₃), from which the direction of the maximum horizontal stress (S_H) is calculated. The initial data set contains 11,587 earthquake focal mechanisms compiled from several sources dating between 1990 and 2020. All deep earthquakes in the Ryukyu subduction zone were removed from the data set. The Chi-Chi 1999 and other major earthquakes and aftershocks were also removed as they may reflect a distorted stress field. After preprocessing, a database consisting of 8,510 events with focal depths between 1-144 km and magnitudes M_L=0.7-5.9 was used in the inversion. Depth division was performed in a regular 7 km grid up to 28 km depth, all events deeper than 28 km being considered in the same layer.

Preliminary results show that, to the southwest, the notable clockwise rotation of S_H from SW-NE to a W-E direction and a change in the fault type from strike-slip to reverse to the east coincides with the interaction between the ENE-striking reactivated inherited structures of the Eurasian continental margin and the NNE-striking thrust faults of the foreland thrust-and-fold belt. To the centre-east, results show normal faulting in the upper crust, which changes to reverse faulting with depth, suggesting that there is a stress transition at approximately 14 km. Beneath that depth, there is a general state of compression. Ongoing research aims at integrating these results with those of numerical modelling and with field data in an effort to understand the locus of deformation and the occurrence of extensional tectonics in compressional settings, here and in other mountains belts worldwide.

This research is part of project PGC2018-094227-B-I00 funded by the Spanish Research Agency of the Ministry of Science and Innovation of Spain. Olivia Lozano acknowledges funding from the same agency through contract PRE2019-091431. Funding from SERA European Union H2020 INFRAIA-2016-2017 Agreement, 170522 is also acknowledged.

How to cite: Lozano Blanco, O., Lund, B., Ayarza, P., Álvarez-Marrón, J., Brown, D., and Wu, Y.-M.: Evidence of Synorogenic Extension in the Upper-Middle Crust in Central Taiwan , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14361, https://doi.org/10.5194/egusphere-egu23-14361, 2023.

The Ruhr region is characterized by centuries of coal mining at depths reaching more than 1000 meters. After the closure of the last mines, their controlled flooding started. The Floodrisk project investigates ground uplift, stress changes due to pore pressure changes and the reactivation potential of faults to explain induced seismicity. We focused on monitoring the eastern Ruhr area and are investigating in detail the relationship between mine water rise, tectonic stress and induced seismicity in the Haus Aden drainage area.

In the region of the former "Bergwerk Ost", which had the highest seismicity in the Ruhr area during active mining, the RUB has installed a network of up to 30 short-period seismic stations. Continuous monitoring of seismicity and mine water levels is available for this region from the active mining phase through the post-mining phase to flooding. The temporal evolution of the mine water level after the pumps were shut down in mid-2019 shows a strong correlation with the temporal evolution of the observed microseismicity. Over 2200 induced events have been located since the beginning of flooding, showing spatial clustering. A comparison of the mine galleries, which today serve as the main underground waterways, with the localizations of the events shows that most of the events occur about 300 m below the main pillars located between the longwall panels.

This study provides a compilation of the regional stress state in the eastern Ruhr area based on the mine measurements, which were re-evaluated to derive the regional stress component and compared with stress orientations from independent sources (information on stresses in deep boreholes and earthquake focal mechanisms). The spatial distribution of stress orientations in the Ruhr region shows a rather homogeneous stress pattern with only very few locations where stress orientations differ significantly from the average.

Based on the geometry of the pillars, shafts and longwall panels, a generic numerical FE-model was developed using the compiled stress data for model calibration. The results indicate increased vertical stresses within and below the pillars as a result of stress arching. The horizontal stress changes are minor, thus differential stress increases in the vicinity of the event localizations.

How to cite: Niederhuber, T., Rische, M., Röckel, T., Müller, B., and Schilling, F.: Flooding Induced Seismicity in the Ruhr Area – a geomechanics numerical modelling approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15990, https://doi.org/10.5194/egusphere-egu23-15990, 2023.

Bavali Shear Zone (BSZ), the western extension of the Moyar Shear Zone (MSZ), located between the Coorg Block in the north and Nilgiri Block in the south is a 100 km long, steep-dipping and WNW-striking dextral shear zone, which is a less studied part of the MSZ. The majority of the rocks in the BSZ are schistose, including hornblende-biotite±epidote schist, garnet-biotite-sillimanite±muscovite±chlorite schist and talc-tremolite-actinolite-chlorite schist. The gneissic varieties comprise of amphibolite gneiss, granulite gneiss, quartz-feldspar gneiss, hornblende-biotite gneiss, and garnet-biotite gneiss. Other rock types include high-grade metamorphic rocks such as pyroxene granulite and charnockite, banded magnetite quartzite, micaceous quartzite with/without sillimanite, metapyroxenite, amphibolite and mylonite. Several felsic intrusive like granite, diorite, syenite, quartz-feldspar leucosomes and mafic/ultramafic intrusive such as gabbro and anorthosite are found in some places.

The dominant structural trend of the BSZ is WNW with strikes varying between 110⁰N and 130⁰N. The steep-dipping and variably oriented pre-shear zone fabrics are preserved in low-strain domains. The BSZ is steep dipping and characterized by steeply-plunging stretching lineation with a persistent dextral sense of shear. The shear zone shows N-down kinematics in vertical sections perpendicular to the shear zone fabrics. The β-axis of poles to the shear zone fabric and the orientations of the hinges of the folds related to shearing share low-angle obliquities with the stretching lineations. It indicates that the shear-related folds have a reclined to steeply-inclined geometry, and the fold hinges are broadly collinear with the stretching direction. The last deformation in the BSZ with down-dip stretching lineation clearly shows the features of the transpressional shear zone with a dextral sense of movement and top-to-the-north kinematics.

U-Th-(total) Pb monazite chemical dating was performed on structurally constrained monazites from the BSZ. Monazites from one garnet-biotite-sillimanite-chlorite schist, one garnet-biotite gneiss and one mylonite close to the BSZ were dated. The monazite hosted in the garnet-biotite-sillimanite-chlorite schists provide prominent ages of 751±40 Ma, the garnet-biotite gneiss yielded a peak at 742±15 Ma and the single mylonite sample collected close to the BSZ yield a distinct age peak at 745±67 Ma. All the metamorphic monazites from the BSZ show a prominent mid-Neoproterozoic age, lacking in the adjoining Coorg Block, Nilgiri Block and Western Dharwar Craton. We, therefore, assign this mid-Neoproterozoic metamorphic chemical ages retrieved from monazites to the ductile deformation in the BSZ that reoriented all the pre-shearing fabrics and speculate that the BSZ collision orogeny preceded the eventual integration of the Greater India landmass with the Gondwanaland during the early-Palaeozoic.

How to cite: Nanda, S. and Rekha, S.: Structure and monazite geochronology along the Bavali Shear Zone, the western extension of the Moyar Shear Zone, southern India, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-281, https://doi.org/10.5194/egusphere-egu23-281, 2023.

Partial melting often occurs alongside sites of rapid deformation in the Earth’s mantle and crust. Microstructural molten and crystalline components align in response to deformation, leading to anisotropies in mechanical, transport, and seismic properties detectable by remote sensing. Here, we investigate the co-evolution of melt, shape, and crystallographic preferred orientations (MPOs, SPOs & CPOs) at early stages of experimental shear deformation, constraining their contribution to observable signatures. We characterized the microstructures of partially molten (2-4 wt% melt) olivine-basalt aggregates deformed in general shear at a temperature of 1250°C under a confining pressure of 300 MPa, at shear stresses of τ = 0-175 MPa and shear strains of γ = 0-2.3. We then used the Gassman poroelastic differential effective medium method to calculate resultant seismic anisotropy.

The grain-scale network of melt pockets developed a strong preferred orientation parallel to the maximum principal stress at γ < 0.4. At higher strains, the orientation of the grain-scale melt pockets remained parallel to the maximum principal stress, but incipient, sample-scale melt bands formed at ~25° antithetic to the direction of shear.  While the orientation of individual melt pockets evolved quickly, grain SPOs and CPOs required larger strains (γ > 2) to strengthen and change. A weak SPO and CPO were induced during sample preparation, with grain long axes oriented perpendicular to the direction of maximum principal stress and a- and c-axes girdled perpendicular to the long axis of the sample. At the highest explored shear strain, a strong SPO was established and the girdled a-axes of the CPO rotated to align nearly parallel to the shear plane, developing clusters parallel to the shearing direction.  

These results yield two key conclusions about the orientation of melt networks in deforming partially molten rocks. First, the grain-scale and sample-scale alignments of melt pockets are distinct. At the grain scale, melt pockets align approximately parallel to the maximum principal stress, but the en echelon arrangement of melt pockets yields a sample-scale MPO at ~25^o to the maximum principal stress (20^o to the shear plane). Second, the relative timescales of melt and solid microstructural evolution are different, and are directly reflected in changes to seismic anisotropy. The grain-scale MPO reacts to a change in the orientation of the maximum principal stress after only a small amount of strain; in contrast, CPOs and SPOs require much higher strains before responding to a change in stress conditions. Seismic anisotropy is greatest when olivine a-axes and the grain-scale orientation of melt pockets are in relatively close alignment, so anisotropy will quickly decrease with any change in the orientation of the stress field that results in a rotation of the MPO away from the orientation of olivine a-axes. Perturbations to a local stress field can thus be observed almost immediately due to a rapidly reorienting melt network, making MPOs a more valuable predictor of instantaneous change than CPO-driven anisotropies.

How to cite: Seltzer, C., Peč, M., Zimmerman, M., and Kohlstedt, D.: Co-evolution of melt and crystal phases in experimentally sheared partially molten rocks and generation of seismic anisotropy during rapid deformation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-539, https://doi.org/10.5194/egusphere-egu23-539, 2023.

Recognition of seismically induced microstructures is important to unravel the different deformation processes during seismic cycles, especially at the base of the upper crust where many earthquakes nucleate. Deformed quartz veins related to a strike-slip shear zone within the Schobergruppe (Austroalpine Crystalline Complex, Eastern Alps) contain intense kinking in elongated quartz grains. The kink band boundaries are inclined into the general dextral sense of shear. Cathodoluminescence (CL) images reveal that the entire thin section contains a very high density of intragranular, sub-planar microstructures developed as thin dark CL lamellae accompanied with nanometre-scale fluid inclusions. Based on the oscillating orientation variation across low angle boundaries (misorientation angle 1-9°) these lamellar microstructures are referred as short-wavelength undulatory extinction microstructures - SWUE (Trepmann and Stöckhert, 2013, Solid Earth, 4). Only grains with SWUE, orientated parallel to the foliation, are kinked. In general, kinked microstructures mainly develop in strongly anisotropic material or within lamellar minerals, i.e. micas. Deformation at high stresses (e.g. at greenschist conditions or during coseismic loading) can produce a strong anisotropic microstructure in quartz by the development of deformation lamellae. Trepmann and Stöckhert (2013) showed in deformation experiments of quartz that SWUE preserve evidence of an earlier coseismic stress peak, even when overprinted during subsequent crystal plastic creep deformation at lower stress. The SWUE in the deformed Schober quartz veins are interpreted in a similar way. These microstructures were primary deformation lamellae developed during coseismic loading. Pseudotachylyte veins within the analysed shear zone next to the sample give evident for a seismic event. Subsequent overprint by ongoing creep at lower stresses is recorded by the vein quartz mylonites. The densely spaced sub-planar microstructures cause a high anisotropy of the quartz grains, which finally were kinked. Electron Backscatter diffraction data give evidence of different slip system active in the formation of the deformation lamellae/SWUE and the subsequent kinking. The opposite direction of the Burges vectors (based on Weighted Burges Vector analysis, Wheeler et al., 2009, Journal of Microscopy, 233) in the kink band domain is consistent with sinistral shearing along the anisotropic deformation lamellae/SWUE in the dextral sheared kink band. Kinked micas (muscovite and biotite) in the mica-rich host rock, next to the kinked quartz vein sample, point to seismic induced kinking. However, the question arise if the kinking of micas and quartz was caused instantaneously during the same seismic event or if the quartz kinking is related to post-seismic creep at transient high stresses during ongoing deformation at lower greenschist facies conditions under dextral sense of shear (Bestmann et al., JGPR – Solid Earth, 126).

How to cite: Bestmann, M., Pennacchioni, G., Grasemann, B., Kilian, R., Morales, L. F. G., Wheeler, J., and Bezold, A.: Seismic induced anisotropy and kinking in quartz, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-878, https://doi.org/10.5194/egusphere-egu23-878, 2023.

The study addresses how the deformation mechanisms influence the microstructural properties of deformation bands developed in high-porosity sandstones of the Kachchh (Kutch) Rift Basin (KRB), India. The KRB located at the western continental margin of the Indian plate is neotectonically active and is affected by the periodic reactivation of multiple ~W-striking dip-slip/strike-slip faults. We investigated the porous sandstones of Bhuj and Jhuran Formation, which contain cataclastic shear bands, disaggregation bands, and cementation bands. High-resolution optical and scanning electron microscopy images were used to analyze the microstructures, grain size and shape, and porosity for both the host rock and deformation bands. The cataclastic bands display preferential alignment of elongated grains parallel to the boundaries of deformation bands. The mechanisms of deformation are dominated by varying degrees of cataclasis and pressure solution that have caused grain reorganization, rolling, flaking of grain edges, and intragranular and transgranular fracturing. This resulted in grain size and porosity reduction within the deformation bands. The porosity loss in deformation bands is of three to five orders of magnitude compared to the host rock. The grain size distribution results are consistent with a power-law model that have low exponent (D) values between 0.89 and 1.00 for deformation bands. Our findings suggest that cataclasis within deformation bands appears to be dominant in the extensional stress state, however shear-related disaggregation of grains is observed in both extensional and compressional stress regimes.

How to cite: Shaikh, M., Mukherjee, S., Dasgupta, S., Rajkhowa, D., Maurya, D., and Chamyal, L.: Insights into microstructural and petrophysical properties of deformation bands in porous sandstone, Kachchh (Kutch) Rift Basin, India, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1582, https://doi.org/10.5194/egusphere-egu23-1582, 2023.

Raman spectroscopy on carbonaceous material (RSCM) is a method widely used to infer the temperature of metamorphism in metasediments. Furthermore, anomalies in the Raman signature of carbonaceous particles contained in fault zones have been interpreted as reflecting short-lived heating events due to friction during earthquakes. These applications of RSCM rely all on the assumption that temperature is the main factor controlling the reorganization of carbonaceous material (CM) and its expression by Raman spectroscopy. Nonetheless, few examples of natural fault zones and shear zones recently raised questions about the possible role of strain in such reorganization.

In this work, we studied the influence of deformation on the Raman spectra of CM. We carried out experiments of deformation on low-grade shales in the Paterson and Griggs-type rigs, corresponding to low and high pressure conditions, for variable conditions of temperature and strain rates, simulating deformation at the brittle-ductile transition. In parallel, we examined in these experiments the evolution of CM using the intensity ratio (IR), defined as the intensity ratio of the Defect band over the Graphite band of the Raman spectrum.

Deformation proceeded as a combination of slip on discrete planes, corresponding to macroscopic stick-slip events, and of ductile shear in few hundreds of µm’s-thick zones. The effect of stick-slip events on RSCM signal is unclear, principally because the deformed layers are too thin to contain CM to be analyzed. In contrast, in zones of distributed strain we observed a systematic and significant increase in IR compared to undeformed domains, reflecting an enhancement in the structural organization of CM. On the basis of additional experiments of static heating we carried out in parallel, we show that the IR increase cannot be connected to local and transient increase in temperature during the experiment, and has therefore to be connected to strain itself.

The zones of concentrated ductile strain are characterized by comminution of the clasts of quartz and feldspar initially present, accompanied by the development of a porosity at the submicron-scale. Chemical analysis revealed that along with intense grain-size reduction, the chemical composition of the feldspar was modified. On the basis of these microstructural and chemical evidence, it appears that the elementary processes behind ductile deformation include intense mechanical fracturing and mass transfer along the grain boundaries.

As a conclusion, it appears that strain has an undisputable effect to increase IR of . When applied to nature, the relevant temperature and mechanical realm of these experiments is the brittle-ductile transition, for temperature of the order or below 300°C. These conditions are also the ones corresponding to seismic deformation. Therefore we show here that Raman spectra in fault zones reflect not only the temperature history (including coseismic flash-heating), but also, and to a large extent, the strain history of the rock.

How to cite: Raimbourg, H., Benjamin, M.-M., Romain, A., Abdeltif, L., and Rémi, C.: The record of ductile strain by Raman spectroscopy of carbonaceous material – deformation experiments and applications to the brittle-ductile régime, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2302, https://doi.org/10.5194/egusphere-egu23-2302, 2023.

To understand the deformation microstructures and seismic properties at the top of a subducting slab in warm subduction zones, deformation experiments of epidote blueschist were conducted in simple shear by using a modified Griggs apparatus. Deformation experiments were performed under high pressure (0.9–1.5 GPa), temperature (400–500 °C), shear strain (γ) in the range of 0.4–4.5, and shear strain rate of 1.5×10^-5–1.8×10^-4 s^-1. After experiments, crystallographic preferred orientations (CPOs) of minerals were determined by electron backscattered diffraction (EBSD) technique, and microstructures of deformed minerals were observed by transmission electron microscopy (TEM). Seismic velocity and anisotropy of constituent minerals and whole rocks were calculated using the CPOs and elastic constants of each mineral. The CPO of glaucophane showed the [001] axes aligned subparallel to shear direction and the (010) poles aligned subnormal to the shear plane at low shear strain (γ ≤1), while the [100] axes aligned subnormal to the shear plane at high shear strain (γ >2). The CPO of epidote showed non-systematic fabric at a shear strain of γ <2, but it showed the (010) poles aligned subparallel to shear direction and the [100] axes aligned subnormal to the shear plane at a shear strain between 2< γ <4. At a high shear strain of γ =4.5, the alignment of the (010) epidote poles had altered from subparallel to subnormal to the shear plane, while the [001] axes were aligned subparallel to shear direction. TEM observations and EBSD mapping revealed that the CPO of glaucophane was developed by dislocation creep, somewhat affected by the cataclastic flow at high shear strain. On the other hand, the CPO development of epidote was considered to have been affected by dislocation creep under a shear strain of 2< γ <4, but the CPO was highly affected by cataclastic flow with rigid body rotation under a high shear strain (γ >4). The average seismic velocity of P-wave (Vp_aver) and S-wave (Vs_aver) of whole rocks (epidote blueschists) were in the range of 7.19–7.63 km/s and 4.22–4.47 km/s, respectively, and the AVp (seismic anisotropy of P-wave) and Max.AVs (maximum seismic anisotropy of S-wave) were in the range of 4.5–11.0% and 3.91–6.40%, respectively. The Vp_aver and Vs_aver of experimentally deformed epidote blueschist were reduced about 8–13% and 6–11%, respectively, compared to the seismic velocities of lithospheric mantle surrounding the slab. The delay time of S-wave calculated from subducting oceanic crust composed of epidote blueschist was generally increased with increasing the subducting angle of the slab and volume proportion of glaucophane. Our experimental results indicate that the magnitude of shear strain and rheological contrast between component minerals plays an important role on the formation of CPOs of glaucophane and epidote. In addition, our calculational results of seismic properties suggest that volume proportion of constituent minerals, CPO types of glaucophane and epidote, and the subducting angle of the slab are important factors to control seismic velocity and anisotropy observed in warm subduction zones.

How to cite: Park, Y. and Jung, H.: Deformation microstructures and seismic properties of experimentally deformed epidote blueschist and implications for seismic velocity and anisotropy in warm subduction zones, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3196, https://doi.org/10.5194/egusphere-egu23-3196, 2023.

At nucleation depths of earthquakes in the continental crust (7-15 km), cataclastic processes and fluids interact in a complex way, affecting the mechanical properties, deformation mechanisms and fabric of fault rocks. In this study, we analyzed the effects of cumulative displacement, fault orientation and slip localization on the fabric of low-displacement cataclasite-pseudotachylyte-bearing faults in granodiorite and discuss the feedbacks between deformation mechanisms potentially controlling the transition to unstable slip.

The samples were stem from a well-exposed outcrop of the Gole Larghe Fault Zone (Southern Alps, Italy), which was active 30 Ma ago as a dextral transpressive fault at depths of earthquake nucleation (9-11 km, 250-280°C). Faults and shear fractures were digitized from an orthorectified photomosaic over an area of about 65 m² to quantify their spatial arrangement. Samples were stem from faults and shear fractures which accommodated increasing cumulative displacements from 0 to 4.8 m, with strikes ranging from N074 to N125. Samples were characterized by means of microstructural (field emission scanning electron microscope, optical cathodoluminescence), mineralogical (X-Ray powder diffraction), geochemical (Energy Dispersive X-Ray Spectroscopy, EMPA) and image analysis (clast size distribution and shape parameters) investigations.

Although fractures are uniformly distributed in the analyzed outcrop, 69% of the total displacement is accommodated along two main pseudotachylyte-bearing fault strands. Cataclasites consist of fragments of the wall rock (quartz, plagioclase and K-feldspar), in a matrix of K-feldspar, chlorite and epidote. With increasing displacement, the average grain size of quartz and plagioclase clasts decreases, the fractal dimension of the clast size distribution increases (from 1.6 to 2.8 in two dimensions) and the faults develop multiple domains of foliated cataclasites and non-foliated, highly comminuted ultracataclasites. If ultracataclasites or pseudotachylytes are present in the fault rocks, an increase of the displacement/thickness ratio suggests strain localization. The boundaries of quartz and plagioclase clasts in cataclasites are generally jagged, and clasts with equivalent diameters of less than 5 μm are rare, suggesting partial corrosion of the clast’s boundaries and dissolution of the smallest fragments. Elongated clasts are often oriented at an acute angle with fault boundaries, forming foliated cataclasite domains. Their iso-orientation is more intense in faults having a higher resolved normal stress (assuming a constant far-field stress tensor), i.e., the P-shears. Foliation is associated with an incipient mineral segregation of the matrix minerals, with epidote and titanite aligned along the foliation surfaces and K-feldspar and chlorite in low-strain sites.

In agreement with experimental results, once slip localizes along highly comminuted horizons, slip appears to be further localized along it, suggesting slip weakening behavior associated with cataclastic flow. Diffusive mass transfer processes enhanced by comminution and fluid ingression allow a residual part of the displacement to be accommodated by frictional-viscous mechanisms (creep), especially at high driving stresses. 

How to cite: Mittempergher, S., Di Toro, G., Aretusini, S., Gratier, J.-P., Bistacchi, A., and Giovanardi, T.: Slip localization by cataclasis and fluid-rock interaction in seismogenic crustal faults (Gole Larghe Fault, Italy), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3794, https://doi.org/10.5194/egusphere-egu23-3794, 2023.

Oceanic Transform Faults (OTFs) are major plate boundaries that regularly offset the axis of Mid-Oceanic Ridges and are able to generate Mw 7 earthquakes. Yet, very little is known on the evolution of fault slip mechanics at depth, mainly due to rare exposures of deep sections at the seafloor. The Atobá Ridge is one of the rare structures where the roots of an active transform fault are exposed and accessible. This transpressive ridge is part of the northern transform fault of the St. Paul transform system in the Equatorial Atlantic (Maia et al., 2016). There, ultramafic mylonites are tectonically exhumed along the inner thrust faults of a positive flower structure.

Here we study the deformation mechanisms in ultramafic mylonites and ultramylonites sampled at the Atobá ridge. We show that all samples experienced ductile deformation at 750-900°C in the spinel stability field, resulting in a pervasive grain size reduction. We propose fluid-assisted dissolution-precipitation creep as the main deformation mechanism, leading to dissolution of orthopyroxene and formation of lens-shaped olivine and interstitial minor phases’ neoblasts (pyroxenes, spinel, and amphibole). Orthopyroxene neoblasts formed by this mechanism mimic the Crystallographic Preferred Orientation of the olivine neoblasts. Fluid-assisted dissolution-precipitation creep allows deformation of stiff minerals at significant lower stresses and temperatures than dislocation creep, possibly leading to an intense strain localization. This mechanism, previously reported in ophiolites and orogenic contexts (Hidas et al., 2016; Prigent et al., 2018), is described for the first time in the oceanic transform environment. Similar microstructures have been observed in mylonites from other OTFs, suggesting that this mechanism could be more widespread and could even represent one of the main deformation law in the lower oceanic lithosphere, with important implications on the mechanics and structures of (oceanic) transform faults and long-lived detachments.

This work is supported by PRIN2017KY5ZX8.

REFERENCES

Maia, M., Sichel, S., Briais, A., Brunelli, D., Ligi, M., Ferreira, N., Campos, T., Mougel, B., Brehme, I., Hémond, C. and Motoki, A., 2016. Extreme mantle uplift and exhumation along a transpressive transform fault. Nature Geoscience, 9(8), pp.619-623.

Hidas, K., Tommasi, A., Garrido, C. J., Padrón-Navarta, J. A., Mainprice, D., Vauchez, A., Barou, F., & Marchesi, C. (2016). Fluid-assisted strain localization in the shallow subcontinental lithospheric mantle. Lithos, 262(October), 636–650. https://doi.org/10.1016/j.lithos.2016.07.038

Prigent, C., Guillot, S., Agard, P., & Ildefonse, B. (2018). Fluid-assisted deformation and strain localization in the cooling mantle wedge of a young subduction zone (Semail ophiolite). Journal of Geophysical Research: Solid Earth, 123. https://doi.org/10.1029/ 2018JB015492

How to cite: Bickert, M., Kaczmarek, M.-A., Maia, M., and Brunelli, D.: Fluid-assisted deformation processes at the roots of oceanic transform faults., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5111, https://doi.org/10.5194/egusphere-egu23-5111, 2023.

In the dry lower crust, earthquake-induced fracturing can increase permeability for fluids to infiltrate and flow, thus facilitating fluid-rock interactions, and potentially altering the strength and rheology of fault systems. Understanding the mechanisms that create and reduce porosity requires a detailed microstructural analysis. Here, we analyze microstructures that have recorded primary and secondary porosity generated by the dynamic rupture propagation of a lower crustal earthquake, and that were subsequently reworked during post- and interseismic viscous creep.

An exhumed lower crustal section comprised largely of anhydrous anorthosites cross-cut by a coeval network of pseudotachylytes (solidified melts produced during seismic slip) and mylonitized pseudotachylytes (overprinted during the post- and interseismic viscous creep), is found at Nusfjord, Lofoten, Norway. We study the microstructures using synchrotron X-ray microtomography (SμCT), focused ion beam scanning electron microscopy (FIB-SEM) nanotomography, electron backscatter diffraction (EBSD) analysis, and SEM imaging.

SμCT data reveals that porosity is dispersed and poorly interconnected within a pseudotachylyte vein (0.16 vol% porosity overall), and noticeably increased along the grain boundaries of garnet grains (1.07 – 1.87 vol%). The increased porosity around garnet is formed due to a net negative volume change (-DV) during garnet growth, as there is a localized increase in density of ~1.00 g/cm³ when a recrystallizing garnet overgrows a pseudotachylyte matrix (plagioclase + amphibole). Efficient healing of the earthquake damage zone (0.03 vol% porosity) resulted in the preservation of only a few but relatively large interconnected primary pores along fractures in the anorthosite. Fractures were healed by the growth of plagioclase neoblasts nucleated from extremely comminuted fragments of the host anorthosite, and by the precipitation of barium-enriched K-feldspar filling intragranular pores. Fluid-rock interaction was so efficient at sealing the porosity that a FIB-SEM transect along one of these microfractures revealed a myrmekite intergrowth replacing K-feldspar.

Porosity is dramatically decreased in the mylonitized pseudotachylyte (0.03 vol% overall), and focused mainly within monomineralic domains of plagioclase (0.07 – 0.11 vol%). These are interpreted as recrystallized and sheared survivor clasts of wall-rock fragments, while the polymineralic domains are primarily derived from the overprint of the original pseudotachylyte veins. The plagioclase grains in both domains are more-or-less equant, very fine grained (< 25 μm), lack a crystallographic preferred orientation, grain boundaries occasionally aligned to form quadruple junctions, and are well-mixed amongst the hydrous phases (polymineralic domain), suggesting that both domains deformed primarily by grain-size sensitive diffusion creep and viscous grain boundary sliding. The polymineralic domain has the least porosity (~0.01 vol%), which reflects the efficient precipitation of phases (amphibole, biotite, and feldspars) into transient pores during creep cavitation.

A porosity reduction on the order of 90% from a pristine to a mylonitized pseudotachylyte may eventually result in shear zone hardening, and development of new pseudotachylytes overprinting the mylonites. Therefore, earthquake-induced rheological weakening of the lower crust is intermittent, occurs when a fluid can infiltrate a transiently permeable shear zone, and may stop when the porosity becomes clogged.

How to cite: Michalchuk, S. P., Zertani, S., Renard, F., Plümper, O., Chogani, A., and Menegon, L.: Dynamic evolution of porosity in lower crustal faults during the earthquake cycle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5451, https://doi.org/10.5194/egusphere-egu23-5451, 2023.

Many metamorphic rocks have a fabric. What is often not clear is how much deformational or metamorphic processes contributed to the formation of these fabrics. Are foliations always the result of strain? When does intrinsic crystallographic anisotropy alone lead to the formation of structural elements? Understanding the relative contributions of deformation and metamorphism in rock fabrics is fundamentally important because it is foundational to understanding the role of stress in reacting and deforming rocks.

To this end, we make a major advance in our understanding of fabric development in reacting rocks by showing in time-resolved (4D) synchrotron microtomography (µCT) experiments that when a gypsum dehydration reaction occurs in a differentially stressed sample the reaction products develop orthogonally to the largest principal stress. This is an important finding because we can show with our µCT data that this preferred orientation forms early in the reaction and at very small strains (<1%). Using a simple kinematic model we can demonstrate that it cannot have formed because of reorientation during mechanical compaction. It remains to be established if it is nucleation or growth of bassanite that is being affected by the stress or both. Our experiments suggest that metamorphic transformations may be inherently anisotropic when reacting under the influence of a non-hydrostatic stress state. 

The consequences of this are many. For example, there will be cases in natural rocks where the interpretation of a lineation, foliation or crystallographic preferred orientation as formed by strain may be incorrect. Moreover, the physical properties (e.g. hydraulic and mechanics) of metamorphic rocks could also be significantly anisotropic from early in a transformation. Mass transport pathways might initialise as channelled or partitioned conduits which would have an impact during subduction and in thin-skinned tectonics. Our data reveal a critical new finding related to the very common geological occurrence of reacting rocks experiencing a differential stress.

How to cite: Gilgannon, J., Freitas, D., Rizzo, R., Wheeler, J., Butler, I., Seth, S., Marone, F., Schlepütz, C., McGill, G., Watt, I., Plümper, O., Eberhard, L., Amiri, H., Chogani, A., and Fusseis, F.: A non-hydrostatic stress state forms fabrics during metamorphic reactions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6483, https://doi.org/10.5194/egusphere-egu23-6483, 2023.

Mechanisms driving the long-term dynamics of plate interfaces remain poorly-constrained. To date, the rheology of the crust is considered to be controlled by solid-state diffusion processes such as crystal plastic deformation (dislocation creep). Yet, most minerals formed at high-pressure conditions are mechanically very strong (garnet, omphacite, glaucophane, zoisite, kyanite) and can only be deformed plastically at unrealistically high stresses or temperatures. A growing number of studies point to the crucial role of fluid-rock interactions and mineral transformations in the development of crustal shear zones of low viscosity. The rock weakening is interpreted as being induced by dissolution and precipitation processes at grains boundaries in chemical disequilibrium. Here, we tackle the eclogite rheology conundrum by performing the first deformation experiments at high-pressure conditions (> 2 GPa) on a two-phase aggregate representative of the lower crust.

Shear experiments were performed in a new generation of Griggs-type apparatus (Univ. Orléans) at 850°C, 2.1 GPa and a shear strain rate of 10⁻⁶ s⁻¹. The starting material consists of mixed powders of plagioclase and clinopyroxene separated from an undeformed gabbro (Kågen, Norway) and hot-pressed with a grain size lower than 100 µm. Experiments have been conducted with 0.2% added water.

Mechanical data indicate that the samples are first very strong with a peak differential stress between 1.0 and 1.4 GPa. Then, a significant weakening is observed with a stress decrease of 0.5 GPa. The high-strain samples are characterized by a strain gradient and a reaction gradient, both increasing toward the center of the shear zone. The nucleation of new phases leads to a drastic grain size reduction and phase mixing. The intensities of both are positively correlated with the strain intensity. The nature, distribution and fabric of the reaction products vary also progressively with strain intensity. At the peak stress, the reaction products are restricted to grain boundaries where they form corona structures, while in the high-strain samples, they occur throughout the sample replacing most of the starting material. The primary plagioclase and clinopyroxene grains show incipient dynamic recrystallization, whereas reaction products never do. The nano-porosity reported in the samples attests to the presence of free-fluid phase along the reactive grain boundaries, despite the high-pressure conditions. This nano-porosity requires grain boundary sliding (GBS) processes to form, as indicated by the spatially associated quadrupole junctions.

Our results show that strain at eclogite-facies conditions is preferentially localized by GBS-accommodated dissolution and precipitation creep in reactive zones. We suggest that this dominant deformation process take place in rock at chemical disequilibrium in the presence of a free-fluid phase. Therefore, deformation along deep plate interfaces should be initiated and governed by transient and local transformation weakening, allowing long-term deformation at far lower stresses than dislocation creep.

How to cite: Soret, M., Stünitz, H., Précigout, J., Osselin, F., Lee, A., and Raimbourg, H.: Deep crustal dynamics driven by local and transient transformation weakening, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6569, https://doi.org/10.5194/egusphere-egu23-6569, 2023.

It has been recently proposed that high-pressure genesis of abiotic hydrocarbon can lead to strain localization in subducted carbonate rocks¹. However, the mechanical effects of the migration of these hydrocarbon-bearing fluids on the infiltrated rocks still need to be constrained.

In this study, we investigate omphacitite (i.e. omphacite-rich rock) adjacent to a high-pressure methane-rich fluid source from the Western Italian Alps (Italy) using a multiscale and analytical approach including petrographic, microstructural, X-ray compositional mapping and electron backscatter diffraction analyses (EBSD). In the field, omphacitite bands are 1-5 metres thick and tens of metres long and are adjacent to carbonate rocks affected by high-pressure reduction and methane-rich fluid production.

Hand specimens and thin sections display a brecciated structure, with omphacitite fragments ranging in size from a few microns to several centimetres, surrounded by a matrix of jadeite, omphacite, grossular, titanite, and graphite. X-ray compositional maps and cathodoluminescence images highlight oscillatory zoning and skeletal textures in jadeite, omphacite and garnet in the matrix, suggesting a fast matrix precipitation under plausible disequilibrium conditions. CH₄ and H₂ are found in fluid inclusions in the jadeite grains. This feature suggests a potential link between the genesis of CH₄ in the adjacent carbonate rocks and the brecciation event.

EBSD analysis was performed on omphacitite clasts close to their borders, where omphacite grain size varies between a few microns and a maximum of 100 microns. Those omphacite grains display no crystallographic preferred orientation, abundant low angle boundaries and low (< 5°) internal lattice distortion. We interpret these textures as formed by pervasive and diffuse micro-fracturing related to the brecciation occurring at high pore fluid pressure, reaching sub-lithostatic values. This study suggests that at high-pressure conditions in subduction zones, the genesis and migration of hydrocarbon-bearing fluids can trigger fracturing in adjacent lithotypes.

1 Giuntoli, F., Vitale Brovarone, A., Menegon, L., 2020. Feedback between high-pressure genesis of abiotic methane and strain localization in subducted carbonate rocks. Sci. Rep. 10, 9848. https://doi.org/10.1038/s41598-020-66640-3

This work is part of project that has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 864045).

How to cite: Giuntoli, F., Menegon, L., Siron, G., Cognigni, F., Leroux, H., Compagnoni, R., Rossi, M., and Vitale Brovarone, A.: Hydrocarbon-bearing fluid migration produces brecciation at high pressure condition in subduction, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7017, https://doi.org/10.5194/egusphere-egu23-7017, 2023.

X-ray tomographic imaging has become a very valuable tool for the analysis of (rock) materials, both for visualising complex 3D microstructures and for imaging internal features such as damage, mineral reaction, and fluid/rock interactions quantitatively. The validity of the results derived from X-ray tomography, however, hinge on the  accuracy of the image segmentation. There are many methods for image segmentation (from simple manual thresholding to machine learning and deep learning approaches), which can produce a high range of variability in the segmentation results. Accuracy of segmentation results is seldom checked and thus calling the reproducibility of the results into question. In this contribution we show how metamorphic reactions themselves can be used to constrain accuracy and highlight the benefits of deep learning methods to extend this over many large datasets efficiently.

Here, we demonstrate a methodology that uses deep learning to achieve reliable segmentation of time-series volumetric images of gypsum dehydration reaction, on which standard segmentation approaches fail due to insufficient contrast. We implement 2D U-net architecture for segmentation, and, to overcome the limitations of training data obtained experimentally through imaging, we show how labelled data obtained via machine learning (i.e., Random Forest Classification) can be used as input data and enhance the neural network performances. The developed deep learning algorithm proves to be incredibly robust, as it is able to consistently segment volume phases within the whole suite of experiments. In addition, the trained neural network exhibits short run times (<7 minutes for ~250 MB of image volumes) on a local workstation equipped with a GPU card.  

To confirm the precision achieved by our workflow, we consider the theoretical and measured molar evolution of gypsum (CaSO4.2H2O) to bassanite (CaSO4.½H2O) during the dehydration. Within all time-series experiments, errors between the predicted theoretical and the segmented volumes fall within the 5% confidence intervals of the theoretical curves. Thus, the segmented CT images are very well suited for extracting quantitative information, such as mineral growth rate and pore size variations during the reaction. To our knowledge, this is the first time an internal standard is used to unequivocally measure the accuracy of a segmentation model.  Being able to accurately and unambiguously measure the volumetric evolution during a reaction enables high-level modelling and verification of the physical (hydraulic and mechanical) properties of rock materials involved in tectono-metamorphic processes.

How to cite: Rizzo, R. E., Freitas, D., Gilgannon, J., Seth, S., Butler, I. B., Wheeler, J., Marone, F., Schlepuetz, C., McGill, G., Plümper, O., and Fusseis, F.: Deep learning and chemical constraints allow accurate segmentation of µCT data from metamorphic rocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7120, https://doi.org/10.5194/egusphere-egu23-7120, 2023.

Stylolites are common microstructures in rocks, where mineral dissolution in a fluid localises in more or less discrete seams. Stylolite formation strongly affects rock porosity, pore connectivity and thus fluid flows. However, details of porosity evolution associated with stylolite nucleation, propagation and growth remain unclear, leading to the debate whether stylolites are conduits or barriers for fluids.

In this contribution, we use an exceptionally large high-resolution SEM-BSE mosaic (102,600 × 18,239 pixels, 0.17µm/pixel) to investigate the detailed microstructures of stylolites from the Eilean Dubh limestone of the NW Highlands in Scotland. Advanced image analyses indicate that porosity self-organises systematically around stylolites suggesting a four-stage growth process for stylolites: In stage 1, primary pyrites in the rock matrix concentrate stress and dissolution, leading to the formation of porosity. In stage 2, the dissolution porosity self-organises, triggering a chemical-hydraulic-mechanical feedback loop that facilitates further dissolution. In stage 3, the porous zones enlarge and grow along the direction of the smallest principal stress (process zone), concentrating the synkinematic precipitation of pyrites from an external fluid in the core (core zone). In stage 4, the isolated domains connect forming a stylolite with a core zone in the center (highest porosity) surrounded on both sides by a process zones (higher-than-matrix porosity), suggesting a conduit phase during stylolite formation. Since the stylolite-hosting Eilean-Dubh limestones were episodically imbricated below the Ullapool thrust and the stylolites formed sub-parallel to the internal thrust planes, we speculate that synkinematic fluids during the stylolite formation were pumped episodically into the rock during activity of the Ullapool thrust. The detailed stylolite microstructures and the proposed process for stylolite growth suggest that stylolite in carbonate may act as a conduit for a fluid flow.

How to cite: Hou, Z., Fusseis, F., Schöpfer, M., and Grasemann, B.: Syn-kinematic porosity evolution during nucleation and growth of stylolites (Eilean-Dubh limestone, NW Scotland), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8015, https://doi.org/10.5194/egusphere-egu23-8015, 2023.

The VPFFT-ELLE numerical scheme is a micro-dynamic approach able to simulate the microstructure evolution of polycrystalline and polyphase aggregates during deformation and recrystallisation (www.elle.ws). The approach has been widely used to simulate the dynamic recrystallisation of ice, halite and olivine aggregates, among others. A limitation of this numerical scheme is that is limited to viscoplastic deformation by dislocation glide. However, at homologous high temperatures and low stresses, point-defect mediated mechanisms such as dislocation climb and diffusion creep are important in order to accommodate plastic deformation in polycrystalline aggregates.

We present an update of the approach by incorporating a new scheme able to simulate dislocation climb and diffusion creep in an elasto-viscoplastic regime. The approach is based on a previously small-strain version of a formulation based on the fast Fourier transforms (FFT) for the prediction of micromechanical fields in polycrystals (Lebensohn et al., 2012). The new approach integrates a rate-sensitivity, crystallographically-based constitutive model of a single crystal deforming by climb and glide, combined with an isotropic, linear model for the parametric simulation of active diffusion within grains and at grain boundaries.  

An overview of the theoretical basis of the approach is presented with some benchmarks and applications to the prediction of the effective behaviour and evolution of crystallographic-preferred orientation of highly anisotropic minerals. As expected, the incorporation of both new mechanisms produces a relaxation of differential stresses and the integration of additional scale lengths in the models. However, challenging features are remaining, such as an efficient integration with the recrystallisation processes (grain boundary migration and nucleation of new grains), or appropriate incorporation of transient creep processes.

Lebensohn et al (2012) An elasto-viscoplastic formulation based on fast Fourier transforms for the prediction of micromechanical fields in polycrystalline materials, International Journal of Plasticity, 32–33, 59–69. 

How to cite: Griera, A. and Lebensohn, R.: A full-field approach to simulate microstructure evolution during high-temperature deformation of polycrystalline aggregates including dislocation (glide and climb) and diffusion creep mechanisms.  , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8486, https://doi.org/10.5194/egusphere-egu23-8486, 2023.

The Muddy Mt. thrust, Nevada, U.S.A, is an iconic contractual fault juxtaposing Paleozoic carbonates onto the autochthonous Mesozoic Aztec sandstone.  The minimum thickness of the thrust sheet is estimated at 4-5 km.  In contrast to many thrusts that exhibit shale-carbonate interfaces, the Muddy Mt. thrust has a mineralogically simple bimaterial interface defined by the carbonate-silicate interface.  The deformation associated with the thrust exists as a variable cataclastic zone on the order of 10-100m. Previous work has documented the extensive evidence of brittle deformation at a range of scales.  Questions at hand during such studies have included the systematic variation in fracture intensity; the contribution of fluid pressure, if any, to the thrust mechanics; the nature of deformation and permeability changes in the footwall porous sandstone and measurement of temperature transients along discrete slip surfaces.

Structures within the deformation zone, and along the thrust interface provide evidence of both seismic and interseismic displacements.  The latter comprise simple fracture, brecciation, shattered dolomite, extensive and cyclic cataclasis and fragment reduction producing tectonic foliation with evidence of cannibalized fragment, gouge injection, discrete slip surfaces and, notably, intracrystalline (plastic) deformation of carbonate aggregates.  Given that macroscopic structures, such as those listed, ultimately depend on atom-to-grain scale processes, this contribution examines the fine-scale textural attributes of aggregates proximal to the thrust interface that in many cases have grain/particle sizes sizes less than a micrometre.  The objective is to extract micromechanical behaviour that can be relevant to the displacement record of crustal-scale faults.  The microstructures and microfabrics of the interface rocks have been examined by analytical STEM, EBSD of samples prepared by focussed ion-beam techniques, with longer range elemental variations determined by mXRF.

How to cite: White, J. C., Merrithew, S. L., and Phillips, N. J.: Micro- to nanostructural analysis of deformation along the Muddy Mt. thrust, Nevada, U.S.A., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9254, https://doi.org/10.5194/egusphere-egu23-9254, 2023.

The subduction interface is characterized by downdip changes in seismic and aseismic behavior that is controlled by metamorphic reactions, deformation conditions, rheological properties, and fluid sources and pathways. Recent seismological evidence shows that a mixture of brittle-ductile deformation at the downdip limit of the seismogenic zone is responsible for producing slow slip and very low-frequency earthquakes. One way to address the possible mechanics and causes behind this dual deformation behavior is by observing exhumed rocks that provide us with the opportunity to make direct geological observations for microstructure and deformation mechanism analysis. Our project aims to understand the deformation mechanism hosted in the dominant mineral phases of metacherts and blueschist from an exhumed paleomegathrust in the Franciscan Subduction Complex. 

The Mesozoic Franciscan Subduction Complex consists mostly of underplated clastic sediment-rich terranes that are metamorphosed from prehnite-pumpellyite to blueschist facies. Angel Island, in the San Francisco Bay, is composed of blueschist facies metasedimentary and metabasic rocks containing potential paleo-megathrust faults that correlate to the source depths of slow slip earthquakes. Subduction-related faults and shear zones crop out in sea cliffs around the perimeter of the island and enable us to observe deformation features in characteristic subduction zone lithologies. We focus on a proposed subduction-related shear zone in blueschist-facies metachert juxtaposed against mafic/ultramafic rocks. Centimetric-scale field maps show coeval brittle-ductile features such as mutually cross-cutting extension fractures, kink folds, veins, and mylonitic foliations hosted in a wide variety of rheologically distinct lithologies. These lithologies contain synkinematic sodic amphibole and stilpnomelane demonstrating that this fault was active at blueschist facies. The main deforming mineral phases are sodic amphiboles, phyllosilicates, and quartz. The structural fabric comprises of shear fabrics, kink folding, dismembered veins, flattened grains, solution seams, and compositional layering. Amphiboles and phyllosilicates define the foliation of these rocks whereas quartz exhibits evidence of dislocation creep. Using EBSD-generated phase maps, we try to determine the deformation mechanisms hosted within the metacherts and blueschists along the paleomegathrust. Paleostress during megathrust creep will be constrained using EBSD-generated grain size statistics. Our study highlights the complexity of deformation within these lithologies while presenting evidence for possible deformation mechanisms witnessed at slow slip depths.

How to cite: Das, M., Rowe, C., and Mookerjee, M.: Microstructural investigations along a blueschist facies paleomegathrust: Implications for deformation mechanisms for deep slow slip behavior, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10817, https://doi.org/10.5194/egusphere-egu23-10817, 2023.

Deformation microstructures can be used to decipher multiple stages of deformation in ancient terranes through the assessment of classical high-temperature minerals, fabrics, and textures (e.g., alteration minerals, reduction of grain size) as well as low-temperature cross-cutting features (e.g., brittle fracture).  Microstructural studies are typically necessary, in conjunction with hand-sample and/or outcrop work, to fully characterize the spectrum of deformation events and mechanisms in a system and to understand the impact that future deformation events may have on similar rock masses.  

In this study, we focus on samples from SE Nebraska, where crystalline basement, chiefly the Central Plains orogeny, dates from the Proterozoic assembly of the central craton of North America. This basement was deformed by the evolution of two major features: (1) the 1.1 Ga Mid-Continent Rift System (MCRS) and (2) the >300 Ma Nemaha Uplift (NU). Due to the uplift on the flanks of the MCRS and the NU, ~60 m of these basement rocks were recovered in a fragmented small-diameter core. This so-called “Capitol/Capital Beach core”, one of exceedingly few basement cores in SE Nebraska, was drilled in 1887 as part of an unsuccessful search for rock salt or brine. Pieces of it were distributed as souvenirs, then painstakingly reclaimed and rearticulated by the state geologist. We present a first-ever textural and microstructural analysis of the basement rock from this historic core.

Preliminary thin section petrography indicates that the groundmass of the basement rock is primarily quartz, twinned feldspar, mica and some opaques (zircon?) with remnant intergrowths and textures consistent with cooling from a melt; therefore, we consider the basement in this area to be granitic. Under crossed polars, we observe undulose extinction in the quartz grains, implying high strain. Quartz grains also show subgrain development at grain boundaries. Thin sections and hand samples also reveal the development of phyllosilicate shear zones, altered from the precursor micas that are folded and kinked themselves. Furthermore, we note at least two generations of brittle fracture in quartz and feldspar grains, some of which propagate through and displace the phyllosilicate shear zones and are filled with alteration minerals, including possible epidote.

Our microstructural data implies that the granitic basement has undergone no less than four discrete phases of deformation since accretion. We associate two of these phases with the evolution of the MRCS and NU evolution, but other two phases of deformation occurred earlier, probably during the Cambrian to Mississippian, an interval about which little in terms of regional tectonism and deformation is known.  Our work highlights the importance of cratonic uplift events in the fracture of rock masses, even on the microscopic scale, during basement evolution.  It also portends important insights from continued investigation.

How to cite: Burberry, C., Joeckel, R. M., and Waszgis, M.: Microscale deformation of a Proterozoic Granite near the nexus of the Mid-Continent Rift and Nemaha Uplift, SE Nebraska, USA, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11375, https://doi.org/10.5194/egusphere-egu23-11375, 2023.

Subducted metabasic rocks play an important role in defining the rheology of the subduction zone interface and are believed to be the primary source of volatiles for arc magmatism and fluid-induced seismicity. The rheology of metabasic rocks along the subduction zone interface are typically modeled based on the strength of omphacite, however metabasic rocks are made up of a variety of different minerals. In this study we present a detailed microstructural analysis of zoisite from an eclogite collected on Kini beach in Syros, Greece and make the argument that

 zoisite may be important in defining the rheology of metabasic rocks along the subduction zone interface.The studied sample is a prograde blueschist to eclogite facies rock primarily containing glaucophane, omphacite, zoisite, and garnet with minor amounts of chlorite retrogression at the rims of large omphacite grains. A hand sample (~12 cm in diameter) was polished and several thin sections were made for analyses. The hand sample shows strain is primarily localized into zoisite-rich layers suggesting zoisite was the weakest mineral when the sample was deforming. Backscatter electron images of the zoisite layers show a homogeneous mixture of zoisite and clinozoisite, where clinozoisite makes up ~10% of the zoisite layers. Zoisite grains are on average twice as large as clinozoisite with an average grain size of 61 μm, whereas clinozoisite has an average grain size of 36 μm. Both phases have strong shape and crystallographic preferred orientations. Both phases show poles to (100) maxima roughly orthogonal to the reference foliation, while poles to (010) develop a girdle sub-parallel to the foliation plane, and poles to (001) are uniformly distributed. EBSD analysis shows that a few zoisite and clinozoisite grains develop subgrain boundaries, suggesting dislocation related mechanisms played a minor role. Based on our analysis, zoisite is interpreted to have deformed primarily by a combination of rigid body rotation and dissolution-precipitation, which lead to the precipitation of clinozoisite. Further analyses will be conducted on zoisite as well as other common minerals within the sample.

How to cite: Morales, L. F. G., Tokle, L., and Salvadori, L.: Microstructural analysis of zoisite from a naturally deformed eclogite (Syros, Greece): Implications on the rheology of metabasic rocks along the subduction zone interface, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11713, https://doi.org/10.5194/egusphere-egu23-11713, 2023.

Amphibole is a hydrous mineral form under metamorphic conditions of the lower crust and in subduction zones. Despite its abundance and ubiquitous texture, the deformation mechanisms of amphiboles are not well understood. We characterize the microstructure of three amphibolites with a high modal fraction of hornblende or actinolite (≥70%) that were deformed under different conditions of pressures and temperatures. We investigated three natural amphibole samples from different localities in India: amphibolite from the Chitradurga shear zone (CSZ) with P-T conditions of ~5 kb and ~600 ºC, hornblendite from the Mayodiya Ophiolite Complex (MOC) from NE Himalaya, with P-T conditions of 7.8-8.2 kb, and 770-820 ºC, and hornblendite from the Koraput Alkaline Complex (KAC) with P-T conditions of 7.6-8.4 kb, and 860-883 °C. To depict the deformation and recrystallization mechanism/s we used electron backscatter diffraction (EBSD) to analyze the grains’ shape, orientation, and the small, intragrain misorientations. The CSZ exhibits elongated actinolite grains with sharp boundaries, tabular shape, and areas of localized deformation illustrated by high-amplitude ‘V-shaped’ kinking of the actinolite grains. The MOC hornblendite exhibits wedge-like elongated grains with crosscutting relations and twinning in the hornblende grains. The KAC hornblendite exhibits very large grains (several mm) with smaller grains at their periphery and highly lobate grain boundaries. MOC and KAC hornblendites show large porphyroblast grains with high intragrain misorientations surrounded by smaller matrix grains and low intragrain misorientations, consistent with a recrystallization fabric. In addition, in the MOC sample, the orientation of grains away from the porphyroblasts shows the continuous spread of their orientation compared with the porphyroblast. Relying on the microstructural observations, we interpret that the CSZ sample was deformed under brittle kinking and the MOC and KAC samples (with elevated P-T conditions) were deformed and recrystallized under temperature-dependent mechanism (e.g., dislocation creep, diffusional processes). Interestingly, although the apparent difference in deformation mechanism, all samples show the same [001] alignment of their intragrain misorientation axis, which does not fit with the common (100)[001] slip system for amphiboles. The different deformation mechanisms will be discussed in light of the microstructural observations and the ability to use the intragrain misorientation axis as a proxy for assessing the deformation mechanism and/or slip system.

How to cite: Meher, B. and Boneh, Y.: Delineating microstructural features of deformation and recrystallization of Ca-rich amphibole from naturally deformed amphibolites , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11779, https://doi.org/10.5194/egusphere-egu23-11779, 2023.

A key characteristic of subduction zones is that lithologies in slabs undergo prograde devolatilization while simultaneously being subject to deformation. A growing number of studies suggest that dissolution-precipitation creep may be the dominant deformation mechanism when fluid is present [1] and that the related fabric development and weakening are most pronounced during metamorphic reaction [2]. Here we investigate the microstructural and textural evolution of meta-ophicarbonate lenses hosted in Atg-serpentinite and Chl-harzburgite of the Milagrosa and Almirez ultramafic massifs in the Nevado-Filábride Complex (Betic Cordillera, S. Spain), which record high-pressure alpine subduction metamorphism. Similar rocks typically occur along tectonized lithological contacts between peridotite and mafic or sedimentary units, thus they can be ideal archives of focused deformation, prograde devolatilization and large scale fluxing by fluid from high-P serpentinite dehydration in subduction zones. In Milagrosa, serpentinite-hosted ophicarbonates —formed after variable mixtures of Ca-carbonate and serpentine— underwent prograde metamorphism to foliated antigorite-diopside-dolomite rocks and Ti-clinohumite-bearing diopside marbles (550 – 600 °C, 1.0 – 1.4 GPa). In Almirez, ophicarbonates were transformed to high-grade assemblages of Ti-clinohumite, olivine, diopside, chlorite, aragonite and dolomite (650 – 680 °C, 1.7 – 1.9 GPa) [3]. We combine microstructural and textural data obtained from electron backscatter diffraction (EBSD) and optical cathodoluminescence (CL) imaging to investigate the relationship between deformation and metamorphic devolatilization during prograde metamorphism. In Milagrosa, Atg-serpentinites show a fabric with a crystallographic preferred orientation (CPO) of antigorite typical of high-P Atg-serpentinites in subduction zones. Antigorite in antigorite-diopside-dolomite rocks displays the same fabric, with a c-axes maximum coinciding with the pole to the foliation, and diopside c-axes parallel to lineation. Dolomite and calcite in the Milagrosa meta-ophicarbonate lenses have a consistent CPO with their c-axes orientated subparallel to that of antigorite, which correlates with the poles to (100) of diopside. In foliated meta-ophicarbonates at Almirez, diopside, dolomite and calcite show a very similar distribution of their crystallographic axes with respect to foliation. CL imaging reveals concentric core-rim zoning of diopside and dolomite in both localities. Clusters of aragonite inclusions indicate that all zoning generations in diopside formed at high-P by growth and/or dissolution-reprecipitation.

These zoning patterns and low intra-grain misorientations point to diffusion creep by dissolution-precipitation as the dominant deformation mechanism. The strong correlation of diopside, carbonate and antigorite CPOs may have been caused by oriented crystal growth under differential stress and anisotropic fluid flow in the reacting ophicarbonate. We infer that antigorite dehydration in meta-ophicarbonate at 580 – 600 °C is related to transient ductile deformation enhancing fluid drainage, followed by compaction and an increased bulk rock strength once all antigorite devolatilizes. This causes deformation and fluid flow during dehydration of the host serpentinite at 650 °C to focus around the meta-ophicarbonate lenses, shielding carbonate from dissolution at subarc conditions.

[1] Malvoisin & Baumgartner (2021) G³; https://doi.org/10.1029/2021GC009633

[2] Stünitz et al. (2020) JSG; https://doi.org/10.1016/j.jsg.2020.104129

[3] Menzel et al. (2019) JMG; https://doi.org/10.1111/jmg.12481

M.D.M acknowledges funding of Junta de Andalucía (Postdoc_21_00791)

How to cite: Menzel, M. D., Hidas, K., and Padrón-Navarta, J. A.: The interplay of deformation and devolatilization – texture evolution during subduction of meta-ophicarbonates, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12331, https://doi.org/10.5194/egusphere-egu23-12331, 2023.

How to cite: Guggisberg, A., Lebihain, M., Moore, J., and Violay, M.: Effect of stress biaxiality on fracture energy and microstructures of tensile cracks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12383, https://doi.org/10.5194/egusphere-egu23-12383, 2023.

Semi-brittle deformation arises as the manifestation of combined microphysical mechanisms dominated by pressure (including dilation, fracturing, frictional sliding) and temperature (including intragranular diffusion of matters, dislocation motion and annihilation, twinning, and grain-boundary sliding). Although each of the deformation mechanisms is individually relatively well understood, the combination and interaction between the two sets of processes remain poorly known.

In order to have a better understanding of the semi-brittle behavior in the lithosphere, we conducted a series of uniaxial compression experiments on samples, cored from Carrara marble – polycrystalline calcite with low porosity and isotropic texture, at confining pressure of 400 MPa in gas pressure medium, temperature of 200 ºC and constant strain rate of 1 × 10^-5 /s. Ultrasonic waves were transmitted through the specimen by a pair of piezoelectrical crystals to monitor the in-situ microstructural states of the sample. The focus of this work is to investigate the contribution of each mechanism as a function of strain accumulation. Accordingly, deformation is ceased at a different stage in each experiment, including the onset of inelastic behavior (at ~0.3% strain) and yield point (at ~1.1% strain) and stages after the yield point (at 1.8%, 3.8% and 7.5% strains).

The mechanical testing results from the 5 runs are essentially reproducible before yielding, and after yielding the difference among the experiments is at most ~20 MPa in stress at the same strain. Strain hardening occurred in the later three specimens deformed to relatively large extent with stress increasing, for example, from 140 to 300 MPa in the sample deformed by 7.5%. The wave velocity, occasionally increasing at the start of deformation in some experiments, generally decreases with increasing strain. Preliminary observations by scanning electron microscopy and EBSD on deformed samples show pervasive distribution of fractures, twins and high misorientation within individual grains, confirming that all the aforementioned mechanisms in semi-brittle regime have potentially contributed to the deformation at such experimental conditions. Further analysis quantifying the evolution of fracture density, twin density and intracrystalline plastic strain with increasing deformation is currently being undertaken.

How to cite: Qu, T., Brantut, N., Wallis, D., and Harbord, C.: Semi-brittle deformation of Carrara marble: snapshots during strain accumulation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14135, https://doi.org/10.5194/egusphere-egu23-14135, 2023.

The Alpine collisional deformation of the upper Iberian margin in the western Axial Zone of the Pyrenees occurred under conditions of moderate paleotemperature (Caldera et al., 2021). Carbonates from the upper Cretaceous of the Eaux-Chaudes fold nappe (ECFN) preserve evidence of ductile deformation at both large- and microscale under greenschist facies. The ECFN provides evidence of high ductile strain achieved under considerable burial conditions (ca. 10 km), contrasting with the standard view of near-surface Alpine deformation of the upper part of the Iberian collided margin. The mylonites observed in the km-scale overturned limb of the ECFN feature strong shear deformation characterized by intrafolial folds, S-C fabrics, mineral lineation and boudinaged, asymmetric dolomite bodies.

Microscale observations by EBSD indicate that grain-shape and crystallographic preferred orientations (CPO) are well-developed in calcite aggregates. Deformation is mild in the normal fold limb, with spaced pressure solution seams. CPO and inverse pole figure (IPF) results from high-strain zones advocate for dislocation creep as the main deformation mechanism by basal-slip along the a- and m- axis. Dynamic recrystallization is also observed as well as local four-node.

In general, calcite-rich aggregates are characterised by fine recrystallized matrix, ranging in grain size between 12-30 µm, and therefore indicating relatively low-stress conditions (20-60 MPa) using the more common piezometers. The spatial distribution of the secondary phase content, such as dolomite and quartz, conditioned the strain partitioning in the polymineralic mylonites. On one hand, grain size reduction of the calcite phase was favoured in areas between small, spaced dolomite grains. On the other hand, calcite grain growth was favoured in shadow zones of large dolomite porphyroclasts. Dolomite phase shows dominant ductile-brittle behaviour expressed by low internal crystal plasticity developing weak CPO and featuring extensional fractures along grain boundaries. Quartz is the less common mineral phase and also affected by ductile-brittle deformation with occasional undulose extinction.

Complementing the microstructural results, we delve deeper into alpine paleotemperatures along the ECFN by Raman spectroscopy of carbonaceous material (RSCM) and microprobe analysis using Powell et al. (1984) and Covey-Crump (1989) methods. Results from all methods are consistent with the ductility observed. The higher values are obtained in the mylonites of the autochthon and overturned limb ranging 340-360ºC. On the other hand, the normal limb features paleotemperatures in a lower range of 300-330ºC. Those temperatures are in accordance with the deformation mechanisms observed.

The microfabrics and paleotemperature results here documented from the Eaux-Chaudes Massif show for the first-time ductile mechanisms of deformation in Alpine mylonites derived from post-Variscan sedimentary rocks during the Alpine orogeny in the Pyrenees.

Caldera, N., Teixell, A., Griera, A., Labaume, P. & Lahfid, A. (2021). https://doi.org/10.1111/ter.12517

Covey-Crump, S.J. & Rutter, E.H. (1989). https://doi.org/10.1007/BF00387202

Powell, R., Condliffe, D. M. & Condliffe, E. (1984). https://doi.org/10.1111/j.1525-1314.1984.tb00283.x

How to cite: Caldera, N., Griera, A., López-Sánchez, M. A., Guardia, M., Labaume, P., Lahfid, A., and Teixell, A.: Microstructure and paleotemperature record of the upper Cretaceous rocks from the Alpine collision in the western Pyrenean Axial Zone (Eaux-Chaudes fold nappe)., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14287, https://doi.org/10.5194/egusphere-egu23-14287, 2023.

While mineral plasticity is often seen as the most important process to deform rocks in the deep, viscous realm, but an increasing number of studies is highlighting the contribution of mineral reactions and chemical processes to bulk strain. This issue is especially acute at the brittle-viscous transition, where plasticity is hampered by the low temperature conditions. In this work, we studied the deformation processes operating in mica+quartz assemblages, representative of greenschist to amphibolite-facies mylonites, with a special focus on the respective contributions of plasticity and dissolution-precipitation processes.

Assemblages made of phlogopite (Phl) and quartz (Qz) were experimentally deformed in simple shear (Griggs-type apparatus) at 800°C, 10kbar, shear strain rate ~10^-5 s^-1 and 0.1 wt. % added H₂O.  For well-constrained grain sizes (63μm < Phl < 125μm, 10μm < Qz < 20μm), we varied the phase proportions of phlogopite (10, 20, 30, 50, 70 and 100% vol. Phl).

Mechanical results indicate at first order a decrease in strength as the proportion of mica is increased. Samples for 10% vol. Phl deform at differential stresses of ∼1200 and at 20% Phl at ~900MPa, while 100% vol. Phl sample deform for differential stresses as low as ~300MPa. However, the weakest behavior is observed for 30% vol. of mica. The strength of the latter sample lies well out of the iso-strain/iso-stress curves, pointing to the fact that the assemblage does not behave as a simple mechanical mixture of the two end-member mineral phases.

In all samples, an important grain size reduction is noticeable for mica flakes. Grain size reduction is also present for quartz grains, especially with decreasing proportion of mica in the assembly. Most of the strain is accommodated by quartz as an interconnected network in Phl-poor samples, while in Phl-rich samples, phlogopite grains are interconnected. In quartz, a weak intragrain deformation is observed for most samples with no evidence for dynamic recrystallization Quartz is largely reworked as attested by trace element variations, with the reworked quartz proportion decreasing with increasing abundance of phlogopite Only the 10% vol. Phl sample is characterized by incipient polygonal subgrains between parent quartz grains (Qz1). Quartz is reworked mainly by dissolution-precipitation, with newly formed product (Qz2) surrounding original, inherited quartz grains. Qz2 is characterized by a large microporosity, and is enriched in aluminum compared to Qz1. The contribution of dissolution-precipitation seems to be maximal in the weakest sample (30% vol. Phl). Therefore, our study shows that dissolution precipitation processes are instrumental to weaken the two-phase material and that bulk strength strongly deviates from composite flow laws in case of a purely mechanical mixing.

How to cite: Alaoui, L. K., Airaghi, L., Stünitz, H., Raimbourg, H., and Précigout, J.: Effect of phlogopite on the strength of mica-quartz assemblage and underlying chemical processes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14586, https://doi.org/10.5194/egusphere-egu23-14586, 2023.

The microstructural evolution of clay-rich sedimentary rocks starts with the settlement and alignment of particles. With the accordance between shape preferred orientation and crystallographic preferred orientation (CPO) in the case of disk-like clay particles, the parallel alignment can be quantified measuring the clay CPO.

In order to quantify the influence of the sedimentation conditions on the CPO of primary layering we performed sedimentation experiments combined with in-situ synchrotron diffraction measurements. The experimental procedure involved a sediment suspension drip inserted at the top of a 30 cm water column at regular intervals. The fluid in the column was either deionized water or seawater, and the sediment suspension contained either kaolinite with of disk-like particle shapes or a mixture of kaolinite and polycrystalline illite, the latter with more compact particle shapes. Time-resolved CPO development in the experiments was measured at ESRF, beamline ID22.

The formation of a CPO is readily observed at the water-sediment interface with an initially higher CPO strength in deionized water experiments than in seawater. The resulting sediment shows a pronounced layering in both fluids when using pure kaolinite. In the layered sediments in deionized water the CPO strength varies strongly between different layers. At some measurement positions, a high initial CPO strength drops fast in the first 150 minutes, interpreted to result from dewatering-related reorganisation of the microstructure. A stable CPO strength can be observed after ~200 min. In the seawater experiments the CPO strength does not vary in different layers and increases slowly but constantly with time and overburden indicating a successive rotation of particles. CPO in kaolinite experiments is higher than in kaolinite + illite experiments as compact particles locally inhibit the alignment.

The evolution of clay particle orientation and therefore microstructure can be quantified in space and time. It is suggested that the initial microstructure is crucial for the progressive development of the rock during diagenesis and hence e.g. resulting physical properties. 

How to cite: Kühn, R., Kilian, R., Lang, D., Morales, L., Grendal, O., and Stipp, M.: Particle alignment during sedimentation – a 4D approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14690, https://doi.org/10.5194/egusphere-egu23-14690, 2023.

The formation of crystallographic preferred orientations (CPO/texture) in sediments is often attributed to rigid grain rotation of minerals and aggregates, plastic-brittle deformation and dissolution-precipitation processes. Due to their platy habit, clay minerals have a large shape anisotropy and are expected to develop a CPO most readily under favorable conditions. Here, we present an experimental approach in order to quantitatively explore the influence of particle settling and subsequent compaction in an undisturbed, ideal environment.

For the experiments, a powder of idiomorphic kaolinite grains (texture-forming components) was mixed with a fine-ground, illite aggregates (texture-inhibiting components) in mass proportions of 0, 30, 50, 70 and 100 %. The illite aggregates have compact shapes and are built up of submicron-sized crystallites. The modal particle size of both components is about 5 µm. Particles were mixed with artificial seawater and the resulting sludge was left to settle in 80 cm high acrylic tubes. For each composition three samples were produced: Sedimentation-only and two drained compaction experiments. The latter were carried out in a in a mechanical press, progressively applying a uniaxial load up to 0.4 MPa and 4 - 8 MPa, respectively. Compaction resulted in a volume decrease of 30 and 60 vol-%. The CPO of clay minerals was measured using high energy X-ray diffraction at beamline P07b at Deutsches Elektronen-Synchrotron (DESY) in Hamburg and pole figure data was directly extracted using single peak evaluation.

The results show that sedimentation alone, can yield a strong texture of the clay minerals. In the compaction experiments, texture strength is logarithmically related to the applied load, i.e. increase of texture strength is decreases at higher loads. Texture strength is linearly related to shortening and porosity reduction. In all cases, kaolinite texture strength is inversely correlated with illite aggregate content.

The results indicate that only a fraction of clay minerals is deposited flat on the sediment surface. Further, alignment during settling is hampered in the presence of particles with compact shapes. It is interpreted that rigid body rotation is limited to the very initial stages of settling and compaction. Additional texture strengthening is hampered by the lack of particles prone to rotate. Sediment surface processes as well as rigid body rotation during initial loading in the first few centimeters of burial are thus the most important processes in the formation of a CPO in clay rich sediments. Subsidence history beyond principal porosity elimination does not have a strong impact on the CPO. Therefore, the texture strength of a fine-grained sedimentary rock can be indicative of settling and early compaction conditions. In return, texture-related properties of artificial clays can be regulated by controlling those conditions.

How to cite: Lang, D., Kuehn, R., and Kilian, R.: Clay mineral alignment during early stages of burial, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14897, https://doi.org/10.5194/egusphere-egu23-14897, 2023.

Microstructures in quartz are widely used to draw inferences on conditions and history of deformation. In particular quartz recrystallization microstructures are often assumed to provide an interdependent information on e.g., deformation temperatures, strain rates or grain boundary mobility. Piezometric approaches attempt to relate a given recrystallized grain size or low angle boundary (LAB) structure to differential stress during deformation.

Here, a decimeter-scale, sheared quartz vein from the Central Pyrenees hosted in phyllites of the Hospitalet Massif was studied by means of EBSD. The sheared part of the vein consists of a mm-scale, planar, parallel layering representing an estimated shear strain > 5. The layering is defined by domains of different crystal orientations and a pronounced contrast in recrystallization behavior. Domains with the c-axis about normal to the kinematic section consist of almost single grains which stretch to aspect ratios > 40, show a very homogeneous LAB pattern, little dynamic recrystallization (< 20 vol%) and recrystallized grain sizes have a volume weighted mode of ~8 µm. Based on boundary misorientations and microstructure, recrystallization is inferred to proceed predominantly by a combination of subgrain rotation, and geometric recrystallization. The second type of domains consists of non-recrystallized remnants of old grains with low aspect ratios, a very heterogeneous LAB structure and c-axes within the kinematic plane, for example normal to the foliation. Recrystallization at overall > 50 vol% proceeds in localized, often conjugated bands. Bands and c-axes of recrystallized grains within those bands are inclined in mutually opposite directions, unrelated to the host crystal orientation. The grain size varies systematically for different band types, e.g. bands in a C' orientation contain the largest recrystallized grain size (~12 µm). Recrystallization is inferred to proceed, at least partially, by nucleation and local grain boundary migration. The third type of domain shows mixed behavior, whereat more highly sheared parts correlate with a smaller recrystallized grain size and c-axis directions changing towards the normal of the kinematic section.

Neither the dependence of grain size on recrystallization processes, nor the orientation-dependent recrystallized grain sizes and LAB structure are easily compatible with a single controlling factor, i.e. flow stress. The data suggest that especially the orientation of host grains plays a non-negligible role in controlling recrystallization mechanisms, LAB structure and dynamically recrystallized grain sizes. This relationship needs to be considered when investigating highly deformed rocks which usually exhibit a strong degreee of preferred orientation.

How to cite: Kilian, R. and Stipp, M.: Orientation-dependent recrystallization and extreme ductility in a sheared quartz vein, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15284, https://doi.org/10.5194/egusphere-egu23-15284, 2023.

Olivine is the most abundant mineral in Earth’s mantle, and its rheology is likely to control upper-mantle convection. While the rheology of olivine is widely studied, little is known about the rheology of olivine grain boundaries and their effect on deformation in the mantle. Forsterite bicrystals, synthesized by direct bonding of highly polished single-crystal plates, were tested in this study to investigate sliding along the single grain boundary at high temperature (1300^°C). Prior to deformation, the bicrystals were polished and scratch markers were scribed perpendicular to the grain boundary to track grain-boundary sliding. Bicrystals were deformed in shear loading between two alumina pistons in a uniaxial creep apparatus at 1 atm with applied axial stress ranging from 1 to 30 MPa. The specimen deformation was measured in real time using a high-resolution (~1 μm) linear variable differential transducer. Each test was carried out until attainment of a quasi-steady state deformation rate to determine the creep parameters. Post-deformation microstructural analysis was conducted using a scanning electron microscope (SEM) and electron backscattered diffraction. Our study established that the creep-rate law for bicrystals is different than single and polycrystalline forsterite. Bicrystals are weaker and shows up to 1 order of magnitude higher deformation rates. SEM microstructures reveal the sliding of scratch markers, which is direct evidence of grain-boundary sliding in forsterite. However, the strain geometry is complex, and further experiments are necessary to determine the overall strain distribution in the sample. Here we present the rationale of our research, and we compare our results on grain-boundary sliding in forsterite with the earlier literature.

How to cite: Singh, S. P., Thom, C., Hansen, L., Marquardt, K., Wheeler, J., Mariani, E., and Mecklenburgh, J.: Direct Observation of Grain Boundary Sliding in Forsterite Bicrystals, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15498, https://doi.org/10.5194/egusphere-egu23-15498, 2023.

In the southern Apennines, Mesozoic carbonates are characterized by the superimposition of different brittle deformation structures formed during different phases of rifting, orogenic shortening and post-orogenic extension. In the western Matese area, fault zones developed in the Triassic dolostones commonly exhibit the complex juxtaposition of different fault rocks, wide damage zones and pulverized dolomite rocks. Significantly, pulverization causes a drastic change in the petrophysical behavior of deformed rock masses through dynamic shattering. For these reasons, a better description of the processes that steer the “dolomite flour” spatial distribution through fault zones represents a fundamental aspect of the mechanical stratigraphy assessment related to subsurface infrastructure building and of the fault zone hydraulic behavior.

In this work, we present a multi-scalar and multi-methodological approach to provide a possible structural evolutionary scenario for the Matese Triassic dolostones. We analyzed fault zones outcropping along the eastern side of the Telese plain in the Ailano and Alife area. The fault zones consist of mature damage zones where pulverized rocks occur in patches of variable size (10s 100s of meters) immediately adjacent to the fault core. The fault zones show complex arrays of slip surfaces, including a major N-S normal fault crosscut and reactivated by NE-SW and NW-SE strike-slip faults. Extensive powder bodies are heterogeneously distributed within the damage zone and adjacent to the fault cores of both systems of faults. Fault zone architecture is associated with different brittle structural facies: i) cataclastic to ultra-cataclastic facies; ii) wide volumes of pulverized “dolomite flour”; iii) foliated cataclasite; iv) cataclastic shear bends; v) hydrothermalized dolomite; vi) unstrained lithons.

Fault analysis suggests that the N-S normal fault systems accommodated extension during the Mesozoic passive margin extensional phases. The NE-SW and NW-SE strike-slip fault systems developed in a successive phase of orogenic shortening. The shortening-related strike-slip faults propagation overprint the inherited Mesozoic fault zone through extensive pulverization processes of dolostones with the formation of a wide volume of “dolomite flour”.

How to cite: Diamanti, R., Camanni, G., and Vitale, S.: Structural evolution of long-lasting fault zones developed in dolostones in the south-western Matese area (southern Apennines, Italy)., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15512, https://doi.org/10.5194/egusphere-egu23-15512, 2023.

Earthquakes are triggered by the sudden release of strain energy accumulated in the Earth’s crust and mantle. Around 55 earthquakes are located every day around the world, and 16 large earthquakes of magnitude greater than 7 are expected in any given year. These events are responsible for many deaths and for major natural disasters. The periodicity of rupture events is controlled by complex variables such as fault surface roughness, fault geometry, fluid-rock interactions, fluid pressure oscillations and the mechanics of the fault rock. Seismology, rock mechanics experiments and modelling have provided vital insights into the behaviour and frictional properties of faults, but the brittle-viscous response of rock to earthquake rupture and the passage of shock waves is currently not understood.

Natural olivine exposed at the Earth surface, but derived from Earth’s upper mantle, display microstructures where brittle, frictional and viscous deformation coexist. Here we study the microstructures and textures locked in the geological record of the Premosello peridotite to understand the transient brittle-viscous deformation mechanisms triggered during large earthquakes, and their contribution to energy dissipation and build-up during the earthquake cycle.

In this study we use electron backscatter diffraction (EBSD) in the SEM, as well as TEM analyses, to detail the micron to nanoscale structures of olivine in samples from the shallow upper mantle, collected near thick pseudotachylytes in proximity of the Mohorovičić discontinuity, which juxtaposes these peridotites with the lower crustal granulites of the Ivrea-Verbano Zone.

How to cite: Mariani, E., Bagshaw, H., Bilton, M., and Gardner, J.: The effect of earthquake rupture on the brittle-viscous flow of olivine, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15541, https://doi.org/10.5194/egusphere-egu23-15541, 2023.

Grain size is a critical parameter for viscous deformation processes and controls the rheological behaviour (weakening and strain localizations in mylonites) of polycrystalline aggregates. Recently,  Ghosh et al., (2022) used Tana quartzite (~200 μm) to develop a new grain-size-insensitive flow law (stress exponent, n = 2; activation energy, Q = 110 kj/mol). The n = 2 is interpreted to indicate the serial operation of grain interior (dislocation glide) and grain boundary (dissolution-precipitation, DPC + Grain boundary sliding, GBS) accommodation processes. It is shown that all the previous results obtained from coarse-grained (∼100 - 200 μm) quartz aggregates (high total strain, significant recrystallization) plot within a factor of ∼5 times the strain rate predicted by this flow law. Moreover, a number of earlier studies with fine-grained (3.6 to 12 μm) quartz reported an n-value range of 1.7 to 2.5 (Kronenberg and Tullis 1984; Fukuda et al., 2018), similar to the values obtained from coarse-grained samples. A deviation from the grain-size-insensitive creep-dominated process is expected at those grain-size ranges and poses the question: how much weakening is possible by grain-size-reduction within the continental crust?

Using a Griggs-type apparatus, we deformed fine-grained (~ 3-4 μm) as-is and 0.1 wt.% H₂O-added novaculites, representing a fully recrystallized shear zone material, at the same condition as coarse-grained Tana quartzite (~200 μm). The as-is novaculite is ~3.5 times stronger than the H₂O-added novaculite. A similar strength difference (~4 times) was also observed between the as-is and H₂O-added Tana quartzite. The H₂O-added novaculite is ~4 times weaker than the H₂O-added Tana. The as-is novaculite is ~4.5 times weaker than the as-is Tana quartzite. Thus, the addition of 0.1 wt.% H₂O causes a similar order of weakening as the smaller grain size; the as-is novaculite has a similar strength as the data predicted from the H₂O-added Tana flow law. The maximum strength difference (more than an order of magnitude) arises between the end-member samples i.e., the as-is Tana and H₂O-added novaculite, which indicate the combined effect of grain size and H₂O. The n (1.91 to 2.19) and Q (118 kj/mol) values measured from novaculites are similar to the Tana and the Tana flow law can express all the novaculite data within the error associated with the Griggs-type apparatus, only by manipulating the A-value of the flow law. Therefore, the n and Q values are largely grain-size-insensitive, while the strength differences due to grain size and added-H₂O can be expressed by varying the A-value. Further, the similarity in the n and Q values indicates that the same deformation mechanisms (i.e., dislocation creep accommodated by grain boundary processes) operate in both materials. Microstructural analysis of the novaculite reveals the development of a core-shell structure of individual grains due to grain boundary migration (GBM). A combination of deformation mechanisms (dislocation glide, GBM, GBS) are mutually dependent and interact to achieve the bulk sample strain. The variations in A-value may reflect the efficiency of the GBS mechanism depending on grain size and H₂O content. Finally, the implications of these results on crustal strength will be discussed.

How to cite: Ghosh, S., Stünitz, H., Raimbourg, H., and Précigout, J.: Rheology and Deformation Processes of Fine-grained Quartz Aggregate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15832, https://doi.org/10.5194/egusphere-egu23-15832, 2023.

The relationship between Raman peak intensity and crystal orientation via knowledge of the: Raman tensor of a given vibrational mode, incident laser light and Raman scattered light polarisation vectors is well established. Thanks to Loudon’s work in 1964 the Raman tensor structure is known for all 32 crystal classes[1]. Many researchers have exploited this to determine Raman tensor coefficients to study the nature of semiconductor and covalently bonded materials e.g.[2,3] using polarised Raman microscopy with the aim to determine crystal orientation locally.

In 2019 Ilchenko et al. demonstrated the feasibility of quantitatively mapping crystallographic orientation of polycrystalline materials in 2D, and in 3D exploiting the confocal nature of a Raman microscope[4]. The novelty of this work overcomes the need to serially collect Raman spectra at each map pixel for multiple combinations incident and scattered polarization needed to compute the local crystal orientation. A new generation of this technology, quantitative Raman imaging of crystallographic orientation (qRICO), rapidly collects Raman spectra of up to 20 combinations of incident and scattered polarization in a simultaneous manner.

Development work using ideal semiconductor materials has demonstrated that qRICO delivers the ability to produce crystallographic images of sample microstructures with sample stage step / pixel sizes down to 0.5 µm, contiguous scanning areas on the order of 10 x 10 cm and crystal orientation accuracy better than 2°. Thus, qRICO provides access to a very wide range of the microstructure length scales seen in geological materials and is amenable to typical geological specimen dimensions and shapes. Fundamentally qRICO is not limited to polished planar sample surfaces and is not restricted to surface studies for transparent materials.

In this work, in addition to non-natural polycrystalline materials, we will present high resolution as well as large area map examples of orientation mapping results on natural diamonds containing defect structures, polycrystalline quartz particles and multiphase petrographic thin slices. These examples will be used to illustrate the potential of qRICO for understanding geological materials in terms of grain boundaries, phase boundaries, orientation gradients, and crystallographic orientations and texture in relation to the conventional information contained in the underlying Raman spectra such as chemical gradients and internal stress.

[1]  R. Loudon, The Raman effect in crystals, Adv. Phys. 13 (1964) 423–482. https://doi.org/10.1080/00018736400101051.
[2]  C. Kranert, C. Sturm, R. Schmidt-Grund, M. Grundmann, Raman tensor elements of β-Ga2O3, Sci. Rep. 6 (2016) 35964. https://doi.org/10.1038/srep35964.
[3]  X. Zhong, A. Loges, V. Roddatis, T. John, Measurement of crystallographic orientation of quartz crystal using Raman spectroscopy: application to entrapped inclusions, Contrib. Mineral. Petrol. 176 (2021) 89. https://doi.org/10.1007/s00410-021-01845-x.
[4] O. Ilchenko, Y. Pilgun, A. Kutsyk, F. Bachmann, R. Slipets, M. Todeschini, P.O. Okeyo, H.F. Poulsen, A. Boisen, Fast and quantitative 2D and 3D orientation mapping using Raman microscopy, Nat. Commun. 10 (2019) 5555. https://doi.org/10.1038/s41467-019-13504-8.

How to cite: Bowen, J., Heinig, M., Reischig, P., Bachmann, F., Ilchenko, O., Pilgun, Y., and Barbee, O.: Raman based crystallographic orientation mapping with qRICO: first applications for geological materials characterisation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16744, https://doi.org/10.5194/egusphere-egu23-16744, 2023.

The grain size of polycrystalline ice affects key parameters related to planetary evolution such as the rheological and dielectric properties of Earth's glaciers and ice sheets as well as the ice shells of ice satellites. Although past experiments have studied the grain growth of pure water ice as well as polycrystalline ice doped with air bubbles and insoluble particles, the effect of soluble ions, such as Cl^- and SO₄^2-, which are commonly found in glaciers, on the growth of polycrystalline ice is not clear. To investigate the effect of soluble impurities on the grain growth kinetics of polycrystalline ice, we conducted annealing experiments on polycrystalline ice samples doped with different concentrations of KCl (10^-2, 10^-3, 10^-4, 10^-5 mol/L) and MgSO₄ (10^-2, 10^-5 mol/L), respectively. Ice powders were obtained by spraying a fine mist of solution (ultra-pure water + KCl or MgSO₄) into liquid nitrogen. The powders were then dried and uniaxially pressed into a cylinder at 30 MPa and -30°C and then hydrostatically pressed into a cylindrical ice sample at 100 MPa and -30°C for 15 min. The samples were annealed for a maximum of 320 h at a hydrostatic pressure of 20 MPa (corresponding to about 2 km glacier depth) and different constant temperatures (-5, -10, -15, -20, -25°C). After each experiment, we took microscopic images of the polished sample surface using an optical microscope equipped with a cold stage. Machine learning methods combined with human quality check were utilized on the images to distinguish the grain boundaries. Then a grain size was measured as the average equivalent grain diameter with a geometry factor applied. For KCl, at -5°C (the eutectic point of KCl solution is -10.7°C), lowest-concentration-doped ice (10^-5 mol/L) grew slightly faster than higher-concentration-doped ice (10^-2, 10^-3, 10^-4 mol/L), and both are faster than pure ice; at -10°C and -15°C, there was no significant difference between the growth rates of both doped ice with different concentrations and pure ice; while at -20°C and -25°C (well below the eutectic point of ice and KCl), the growth rates of higher-concentration-doped ice (10^-2, 10^-3 mol/L) were slower than those of pure ice and lower-concentration-doped ice (10^-4, 10^-5 mol/L), which are similar to each other. For MgSO₄, at -5°C (the eutectic point of MgSO₄ solution is -3.6°C), the growth rates of doped ice with different concentrations are not significantly different from that of pure ice; while at -10°C and lower temperatures, the growth rates of doped ice are slower than that of pure ice. We propose that at temperatures well above the eutectic point, grain growth may be largely influenced by partial melts and temperatures well below the eutectic point, soluble impurities impede grain growth. These results help quantifying the contribution of different creep mechanisms, grain-size sensitive and insensitive, under natural conditions and will contribute to future estimation of the rheological strength of glaciers and ice shells.

How to cite: Wang, Q., Fan, S., and Qi, C.: Grain growth of polycrystalline ice doped with soluble impurities, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17008, https://doi.org/10.5194/egusphere-egu23-17008, 2023.

Junggar Basin, located in the northern Xinjiang, is one of the most important oil and gas bearing sedimentary basins in China. Zhong-4 Block, located in Fukang Sag at the southern edge of the basin, is rich in oil and gas resources but low in exploration. Strike slip faults in the study area play an important role in controlling the distribution of oil and gas. Based on seismic and logging data, the static characteristics of strike slip faults are determined, the formation mechanism of strike slip faults is clarified, and the relationship between strike slip faults and oil and gas distribution is clarified. The results show that: (1) North east trending R-type shear fault, northwest trending P-type shear fault and north-south trending T-type extension fracture fault are developed in Zhong-4 block. The cross section of the strike slip fault is steep, with flower like structure, and obvious dolphin effect and ribbon effect can be seen; On the plane, it is characterized by segmented development, consisting of oblique overlapping segment, linear segment and braided structure segment; (2) The faults in Zhong4 Block are generally strike slip faults formed under the single shear mechanism consisting of NWW, NE and NS directions. According to the comprehensive geometric and kinematic characteristics, the fault zone in the work area mainly follows the rightward Riedel single shear mode. The included angle between R shear fault, P shear fault and main fault is mostly 15 °~25 °, which is widely developed in the work area. There are two stages of strike slip activities in the study area. The first stage is left strike slip in the west of Dong 6 well area in the early Jurassic, and the second stage is right strike slip in the whole study area in the late Jurassic; (3) The fault block traps developed in the strike slip fault oblique overlap zone are the main places for oil and gas enrichment in the study area, while the vertical sealing of the fault controls the oil and gas enrichment horizon. When the normal stress of the section is large and the vertical sealing of the fault is strong, oil and gas are enriched in the deep Sangonghe Formation; When the vertical sealing of the fault is poor, oil and gas are distributed in deep Sangonghe Formation and shallow Toutunhe Formation.

How to cite: xiulin, L. and youlu, J.: Characteristics of Jurassic Strike-slip Faults in Block 4 in Junggar Basin and Their Relationship with Hydrocarbon Distribution, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1023, https://doi.org/10.5194/egusphere-egu23-1023, 2023.

Structural analysis coupled with geochemical study of syn-tectonic mineralizations unraveled the role of fluids during the polyphase tectonic evolution of the Val D’Agri, a seismically-active intermontane basin located in the Southern Apennines fold and thrust belt, hosting the largest onshore oil field in western Europe. In this basin, the structural control on present-day fluid circulation is still not well constrained. For this reason, the aim of this work is to reconstruct the Val d’Agri fault system (VAF) architecture and paleo-fluid circulation during the basin tectonic evolution. The VAF evolution was caused by non-coaxial polyphase stress regimes that can be summarized in: 1) Upper Miocene-Lower Pliocene compressional regime; 2) Upper Pliocene-Lower Pleistocene late orogenic strike-slip regime 3) Early Pleistocene-Present post-orogenic extensional regime. Based on new field work, mapping, and structural analysis, we recognized that the VAF is organized in different oriented faults sets. The main sets are N-S-, NE-SW-, and ESE-WNW-striking faults, where the last one has the higher degree of maturity. U-Pb dating of calcite slickenfibres highlighted the long-term tectonic history of these fault sets, with episodes of activation and reactivation of inherited faults. In particular, we recognize: N-S-striking normal faults that reactivated inherited Upper Miocene-Lower Pliocene thrusts; Upper Pleistocene NE-SW-striking normal-lateral faults; one Miocene ESE-WNW-striking normal-lateral fault, with also evidence of reactivation in more recent time. Clumped isotopes analysis together with optical and cathodoluminescence observations of about 50 syn-tectonic calcite mineralizations allowed us to link specific fluid pathways to the different stress regimes. Indeed, bed-parallel veins and mineralizations sampled along transpressive, transtensive, and normal fault sets show that: 1) during the Upper Miocene-Lower Pliocene compressive tectonic phase, fluid circulation occurred in a closed system, characterized by host-rock buffered fluids; 2) during Upper Pliocene-Lower Pleistocene late orogenic phase, fluid circulation occurred in an open system, characterized by meteoric water with low to moderate residence time; 3) during the Lower Pleistocene-Present extensional phase, fluid circulation occurred in an open system, characterized by the mixing of meteoric water and uprising deep high temperature fluids. We highlight that the reconstruction of paleo to present-day fluid circulation is a valid tool for assessing natural and induced seismic hazard in seismically active areas where hydrocarbons are exploited and fluids are injected into the crust, such as in the Val d'Agri basin.

How to cite: Schirripa Spagnolo, G., Agosta, F., Aldega, L., Billi, A., Bernasconi, S., Prosser, G., Smeraglia, L., and Carminati, E.: Upper Miocene to Present fault architecture and fluid pathways in the Val d’Agri basin (Southern Italy), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1686, https://doi.org/10.5194/egusphere-egu23-1686, 2023.

The progressive localization of deformation has long been recognized as a fundamental phenomenon of the macroscopic failure of rocks. Our recent analyses using X-ray tomography during triaxial compression indicate that fractures and higher magnitudes of shear and dilative strain spatially localize as rocks are driven closer to macroscopic failure. Similarly, geophysical observations of low magnitude seismicity in southern and Baja California show that deformation localizes toward the future rupture plane of M>7 earthquakes. These sets of observations indicate that deformation can increase in localization toward failure, and that deformation can temporarily decrease in localization (delocalize) during this overall increase. These observations indicate that the spatial organization of deformation may be used to recognize the acceleration of the precursory phase leading to large earthquakes and the macroscopic, system-scale failure of heterogeneous materials. However, such efforts will require identifying the conditions that promote phases of delocalization, and how these perturbations in the overall trend of increasing localization influence the timing of macroscopic failure. In this presentation, I will describe these analyses, and new work that aims to identify which characteristics of the fracture networks determine the localization at a particular level of stress, and the change in localization from one stress step to the next in triaxial compression experiments at the confining stress conditions of the upper crust.

How to cite: McBeck, J., Renard, F., and Ben-Zion, Y.: Episodic delocalization in the upper crust: Implications for earthquake forecasting, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1798, https://doi.org/10.5194/egusphere-egu23-1798, 2023.

Modelling Discrete Fracture Networks (DFN) in naturally fractured reservoir (NFR) implies identifying and understanding the fracture spatial distribution and relationships to stratigraphic interfaces (crosscutting or abutment) in 3D. However, capturing fracture geometric parameters in the subsurface has always been a challenging task. To palliate the lack of data, and to better constrain modelling inputs, outcrop analogs are often used. While Simonson (1978) showed that mechanical contrasts at bed interfaces are essential to control fracture abutment, Cooke (2006) showed that bed/inter-bed thickness ratio is also an important parameter to account for.

Our goal is to predict fracture network geometry in stratified sedimentary rocks. To this purpose, we present a new original python toolbox. We performed an integrated approach that quantifies and automatically computes the bed interface’s compliance to let fractures go through (or not). Accounting for abutment, cross-cutting relations and bed thickness data, a compliance value is calculated for each interface using 1D scanline or 2D outcrop photograph. The process comprises (1) a field survey data or a digitized image (stratigraphy and fracture pattern) and (2) the processing of the data. First, we applied the method to existing classification and quantification from other authors as case study to check the feasibility of the code. Second, we applied the method to naturally fractured and stratified carbonates located in SE France and Centre Albania.

How to cite: Namongo Soro, P. J. F., Lamarche, J., Viseur, S., Messaadi, F., and Richard, P.: Python toolbox for fracture stratigraphy quantification and mechanical interface characterization, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4374, https://doi.org/10.5194/egusphere-egu23-4374, 2023.

Modern concepts of fault strength account for compositional and hence rheological heterogeneity of natural fault zones. An important implication of heterogeneity is strain partitioning between stronger and weaker phases and a gradual transition from brittle to ductile deformation. While a broader brittle-ductile transition zone is consistent with deformation fabrics in fault zones, it remains uncertain how broad the transition zone is and whether deformation fabrics record truly simultaneous brittle-ductile deformation or successions of brittle and ductile deformation driven by temporal changes in temperature, strain rate, and/or pore fluid pressure. Here we present data from a major fault zone of the Permo-Triassic Khao-Khwang fold-and-thrust belt, Thailand. The fault zone is located in marine calcareous shales with Permian limestones in the foot- and hanging wall. Thrusting occurred at peak temperatures of ~200°C. Deformation fabrics document frictional sliding, cataclasis, mineral veining, and pressure solution in the main fault zone and local mylonitization at the base of the hanging wall limestone. Thus, the fault zone records competing brittle and ductile deformation and incipient strain partitioning between the shales and limestones, consistent with a broad brittle-ductile transition zones due to rheological heterogeneity. Radiogenic strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) of syntectonic vein carbonates and calc-mylonites, tell, however, a slightly different story. Vein carbonates from the fault zone all have Permian seawater-like ⁸⁷Sr/⁸⁶Sr ratios documenting a rock-buffered pore fluid during brittle deformation. By comparison, the calc-mylonites record less radiogenic ⁸⁷Sr/⁸⁶Sr ratios hinting at a mantle source. The Sr ratios are similar to those of syntectonic mafic dykes that intruded Khao-Khwang fold-and-thrust belt. We interpret the distinct Sr ratios of the calc-mylonite to document that mylonitization was triggered by migration of hot magmatic fluids. In this case, strain partitioning between the shales and limestone were likely insignificant without the impact of the hot fluids, although the limestones were already close to the brittle-ductile transition at ambient temperatures of ~200°C. Taken together, our findings suggest that the transition from frictional deformation and pressure solution in shales to ductile shearing in limestones might be rather abrupt than gradual.

How to cite: von Hagke, C., Dielforder, A., Glodny, J., and Morley, C. K.: Is mixed brittle-ductile deformation simultaneous or successive? Insights from the strontium isotope systematics of deformation structures in a heterogenous fault zone, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4874, https://doi.org/10.5194/egusphere-egu23-4874, 2023.

The characterisation of the fracture network is a fundamental step for modelling the circulation of different types of geofluids at multiple scales, including at the reservoir scale. Due to the difficulty of sampling the fracture network with a spatial continuity, fracture networks are often derived from a stochastic approach applied to ground truth parametrized datasets. Classical field methods for collecting and analysing fracture data sets (i.e., scan-lines or scan-areas) have limited sample area and the results might be affected by local factors (e.g., facies variations and faults). In poorly- to non-vegetated regions, field-derived data can be integrated with satellite and aerial images providing a “big-picture” of the study area. The availability of open-source, high-resolution aerial and satellite images (e.g., Google Maps, Bing Maps) coupled with the development of specific software and tools (e.g., NetworkGT; FracPaQ) allow to digitize and analyse very large and spatially continuous data sets of fractures at multiple scales for a single case study.

In this work, we test the use of Bing Maps imagery for the analysis of the fracture network affecting the Kuh-e-Asmari anticline, in the Zagros fold-and-thrust belt. The Kuh-e-Asmari anticline is considered as an outcropping analogue of fractured reservoirs and is in a scarcely vegetated area, covered by high resolution open-source aerial images (~2.6 m/pixel). We obtained three different image-derived fracture data sets by manually digitizing the fracture network at three different scales (1:50000, 1:5000, and 1:500) in QGIS. Each data set has been analysed using the NetworkGT plugin within QGIS. In detail, we analysed the orientation, length distribution, abundance, and topology of each fracture network data set, and we have compared the results with structural data from scan-lines performed in the field on the same anticline.

Results proved to be scale-dependent, with each scale having its pros and cons, as follows. The 1:50000 data set is the only spatially continuous data set, allows to rapidly map the main tectonic features (e.g., major faults and fracture corridors) but it is not accurate enough in terms of the definition of orientation sets and length analyses. The 1:5000 data set is potentially spatially continuous and allows to analyse fracture and connectivity distributions with high detail, by highlighting strongly fractured/connected elongated zones, possibly representing damage zones of known, previously mapped, fault strands. The 1:500 scale cannot guarantee the spatial continuity of the data set but, if applied to a small area, provide reliable results.

Results obtained in this work suggest that the manual interpretation of open-source aerial images is a viable way for the characterization of fracture networks in poorly vegetated areas only if the right scale is chosen. For the case study presented here we suggest interpreting the fracture network at 1:5000 or similar scales. A successful testing of semi-automatic or automatic algorithms for lineament detection is required to perform a spatially continuous interpretation and analysis at the highest possible resolution, and/or to analyse the fracture dataset at multiple scales (more than the 3 investigated here).

How to cite: Mercuri, M., Tavani, S., Aldega, L., Trippetta, F., Bigi, S., and Carminati, E.: The usage of open-source aerial images for the characterisation of a fracture network. Insights from a multi-scale approach in the Zagros Mts., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4909, https://doi.org/10.5194/egusphere-egu23-4909, 2023.

Studies of the shallow part (Tmax< 150°C) of ancient and active prisms documents that ductile (e.g., folds), brittle and localized (e.g., faults) deformation, and diffuse deformation (e.g. scaly fabric), likely contemporaneous at the scale of geological time, may occur in the same outcrop. Moreover, the source areas of precisely located shallow slow earthquakes (downdip locating the transition from brittle to ductile deformation) span a range of depths from <1 to ~15 km below the seafloor, overlapping the temperature range of megathrust earthquake rupture in some convergent margins (e.g. Nankai and Japan Trench), and propagating up to the seafloor in other ones (e.g. Costa Rica). This implies that the shallow part of the accretionary prisms cannot be uniquely defined as characterized by ductile/brittle behavior or as prone to localization or de-localization of deformation.

To better understand how deformation affects the frontal part of accretionary prisms, the general architecture and internal structure of the frontal part of accretionary prisms need to be considered.  Field evidence of the deformational structures occurring in the frontal part of active and ancient accretionary prisms from the mesoscale to the regional scale suggests that recumbent and isoclinal folding are frequent in the shallow part of accretionary prisms, developing in pre-lithification to poorly metamorphosed rocks. In this framework, diffuse scaly fabric and pervasive boudinage, traditionally considered as evidence of shear delocalization, could be alternatively seen as the result of the progressive deformation envisioned in literature for the formation of recumbent folds i.e.: buckling with the development of an overturned fold limb and subsequent kinematic amplification by coaxial strain components with vertical maximum shortening. However, the plethora of brittle structures accommodating the change in shape on the hinge and limbs of the folds, have a different distribution in space and accommodate a different strain than brittle fracture associated with faults and/or thick shear zone. Therefore, this interpretation has implications in terms of the distribution in space of brittle structures and in the distribution of shear strain in the frontal part of subduction zones.

How to cite: Remitti, F., Festa, A., Nirta, G., Barbero, E., and Mittempergher, S.: Internal architecture of the frontal part of subduction accretionary prism: the role of folding in brittle diffuse deformation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5184, https://doi.org/10.5194/egusphere-egu23-5184, 2023.

The NW-SE striking Pag anticline, in the External Dinarides fold and thrust belt, provides an appropriate field site for studying fold- and fault-related deformation patterns at different scales in a transpressional setting due to outstanding exposures. We performed a multiscale structural analysis together with petrographic and isotopic characterization of syntectonic calcite cements. Results indicate that the Pag anticline, is a box fold developed mainly by detachment folding in response to NE-SW oriented compression. Depth-to-detachment calculation indicates that the basal thrust of the anticline is located in the Upper Jurassic evaporitic complex, at a depth of about 2.5 km. The geometry of the fold is strongly controlled by the interaction and overstepping of a major thrust-backthrust fault pair. In the northern sector, backthrust activity produced a northeastern facing, steeply dipping to near vertical backlimb. Moving southward, the forelimb gradually becomes vertical to overturned and fold asymmetry switches to a southwestward facing. Late- to post-folding tightening resulted in non-cylindrical and compartmentalized deformation by near vertical N-S right-lateral, and E-W, left-lateral, strike-slip faults trending oblique to the fold axis. These fault sets make a wider angle than expected for transversal conjugate strike-slip faults commonly associated with folding, possibly due to lateral propagation from inherited, folded soft-sediment extensional faults. Paleostress analysis indicates that the evolution of the Pag anticline occurred in a stress field that switched from contractional to transpressional configuration, maintaining a N40-50° oriented major stress axis. Petrographic and isotopic data support infiltration of meteoric fluids into exposed carbonates in the pre-folding stage, followed by mixing with marine fluids during folding at shallow burial conditions and, eventually, meteoric fluid circulation along strike-slip faults in the late to post-folding stage. Stable isotope ratios suggest that fluid flow evolved from a bedset confined system to an open one in the late to post-folding stage. As such, a major role to control paleofluid flow is played by the transversal sets of low-displacement near orthogonal strike-slip faults. This suggests that, in reservoir structural characterization, particular attention should be paid to the presence of low-displacement strike-slip faults because of their role to enhance fluid flow mixing and channeling. The presence and, particularly, the abundance of such deformation features are difficult to constrain in buried fractured reservoirs by seismic reflection imaging because of low displacement values. We, consequently, stress the importance of studying field analogues by multidisciplinary approaches for better understanding the relationships between folding, faulting, the associated incremental deformation patterns, and the impact on fluid flow.

How to cite: Lucca, A., Mittempergher, S., Succo, A., Bistacchi, A., Meda, M., and Storti, F.: Deformation pattern, paleostress, and paleofluid evolution in the Pag anticline, External Dinarides of Croatia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5306, https://doi.org/10.5194/egusphere-egu23-5306, 2023.

The study carbonates are exposed in the axial zone of the southern Apennines ftb, Italy. These rocks form a natural laboratory to investigate the time-dependent variation of primary heterogeneities and both dimensional properties and distribution of high-angle joints, veins, and shear fractures. We integrate field-based stratigraphic and structural analyses of the platform carbonates with both petrographic and microstructural investigations of selected specimens to unravel their depositional setting and diagenetic evolution. Furthermore, we focused on the modalities of pressure solution-assisted deformation of bed interfaces and single beds to investigate their possible control on the geometry, distribution, and relative timing of the high-angle fractures. The platform carbonate succession includes a Pliensbachian in age, well-layered, lagoonal carbonate unit including both mud-and grain-supported limestone beds, and a Toarcian in age oolithic carbonate unit. Our results showed that pressure solution mainly localized within bed-parallel interfaces and within the grain-supported limestone beds. During early-to-burial diagenesis, precipitation of calcite rims and blocky calcite cements predominantly occurred in the latter beds, in which ghosts of meniscus cements associated to the earliest diagenetic stages are documented. The fracture intensity (P10) computed for the earliest high-angle fracture set shows the highest values in correspondence of the grain-supported beds. During the Oligo-Miocene Apennine orogeny, the mechanical layering of the carbonates evolved due to flexural slip of the layered carbonates, and formation of intra-carbonates thrust faults with flat-ramp-flat geometries were documented. Specifically, there formed s-c-c’ tectonites along the primary interfaces juxtaposing the single bed packages, and oblique-to-bedding reverse faults offsetting the single beds developed due to pressure-solution assisted deformation.The Plio-Quaternary transtension dissected the platform carbonates, and caused both their uplift and exhumation from shallow crustal depths. By computing the fracture density (P20) and intensity (P21) associated to transtension, the highest values were calculated within the coarser grained carbonate beds. Independently from their thickness, these carbonate beds are the most fractured ones in the study succession. These results hence showed that the platform carbonates acquired the mechanical properties during early-to-burial diagenesis, that later permitted strain clustering within the stiffer, grains-supported beds. We invoke that chemical/physical compaction and diffuse cementation of these carbonate beds were responsible for the Plio-Quaternary fracture distribution throughout the platform carbonates. Moreover, we also documented similar P20 and P21 variations, which are consistent with the profound control exerted by primary mechanical interfaces on vertical growth of transtensional fractures. Indeed, we interpreted the similar P20 and P21 variations as a result of shear stress localization along per-existing, stratabound fractures, which ruptured these interfaces forming small-scale process zones (high P20 values) at their extensional quadrants (high P21 values). Ongoing petrophysical analysis will shed lights on the dimension, distribution, and overall connectivity of the pore system aiming at assessing the overall control exerted by bed-parallel stylolites and pressure solution seams on the efficiency of the mechanical interfaces abutting the stratabound transional fractures.

How to cite: Manniello, C., Todaro, S., Abdallah, I., Prosser, G., and Agosta, F.: The role of pressure solution in the evolving fracture stratigraphy properties of Mesozoic platform carbonates, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5630, https://doi.org/10.5194/egusphere-egu23-5630, 2023.

Geofluids play an important role to seismic faulting both at hypocentral depth during earthquake nucleation and at shallower crustal levels during seismic rupture propagation. Pre- to co-seismic anomalies of crustal fluid circulation have thus far been identified by hydrogeochemical and seismological (Vp/Vs) monitoring and are generally interpreted as potential precursors, triggers, and/or facilitators of large magnitude earthquakes. Structural and geochemical studies on syn-tectonic mineralizations have highlighted different patterns of fluid ingress, circulation and fluid-rock interaction during the seismic cycle of thrusts and normal faults. Understanding fault rock-fluid relationships in exhumed faults is useful for revealing the role of fluids also in still ongoing seismic cycles from different tectonic settings. We present a review of published studies and original data on the chemical-physical characteristics of syn-tectonic mineralizations formed by fluids that assisted fault-related deformation in the Apennines (Italy). We use our data to build a general model of fluid circulation during thrusting and normal faulting during the seismic cycle, and define a multi-technique workflow to identify tectonic-related physical/chemical equilibria/disequilibria in fluid-rock systems. The proposed workflow relies upon multiscale structural analysis, stable C, O, and clumped isotope analysis, radiometric dating and burial-thermal modeling. The chosen study area is the Central Apennines fold-and-thrust belt, where post-orogenic extensional deformation, characterized by numerous Mw ≥ 6.5 earthquakes, currently overprints Neogene-Quaternary folds and thrusts. We show that thrusting mostly develops in closed fluid systems where fluid and host rock are close to chemical equilibrium. Subsequent normal faulting is characterized by a dominant upward and/or downward open fluid circulation system with an overall range of δ¹⁸O of paleofluids from -10‰ to +13.5‰ (V-SMOW). Isotopic and thermal fluid-rock disequilibria are particularly evident during pre-/ and co-seismic extensional deformation with mineralizations that are up to 30° C warmer and 16° C colder than the host rock and are associated with the ingress of exotic (meteoric or deep crustal) fluids.

How to cite: Curzi, M., Aldega, L., Billi, A., Boschi, C., Carminati, E., Vignaroli, G., Viola, G., and Bernasconi, S.: Fluid-rock chemical-physical equilibrium/disequilibrium inferred from tectonic carbonates in the Apennines (Italy): an advanced approach to track seismic cycles and earthquakes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6476, https://doi.org/10.5194/egusphere-egu23-6476, 2023.

The study of faults in the upper crust has generated interests in modelling their impact on fluid flow and the mechanical behavior of the earth's crust. Fault damage zones are important structures with multiple implications for resource management and earthquake studies. This work aims to characterize the distribution and growth of damage around faults, and to study its impact on the Displacement - Damage thickness (D-T) scaling law. Two complementary approaches of field analyses and analog modelling of normal faults are developed to answer this question. We presents new results of fault damage mapping, D-T scaling in carbonate rocks and the first analog modelling experiments of fault damage zones inspected in-plane. The results show a heterogeneous and asymmetric distribution of damage around faults, mainly influenced by fault interactions during their growth (segmentation, conjugate faults). A D-T law specific to wall damage is established and shows a normal correlation between D and T for less than c. 100 m of fault displacement, and also confirms the existence of a damage thickness threshold after c. 100 m of displacement. To explain this law, we propose a damage zone growth model controlled by the interaction and coalescence of fault segments. Analogue modelling shows a failure mode transition during fault growth, from a segmented dilatational-shear mode to a localized compactional-shear mode. They also demonstrate that initiation of segmentation, segment activity selection, interaction and coalescence processes control the development of fault damage zones and the D-T law. Furthermore, the thickness of the faulted brittle layer is a main controlling parameter of segmentation, strain localization and the fault damage thickness threshold observed.

How to cite: Soliva, R., Mayolle, S., Dominguez, S., and Wibberley, C.: The growth of fault damage zones; field data and analogue modelling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6717, https://doi.org/10.5194/egusphere-egu23-6717, 2023.

A better understanding of stress perturbation around faults, strain propagation, and fluid flow in the upper crust require characterization of the evolution of fault damage zones. Numerous studies provide significant amounts of data and description from a broad variety of faults, however, fault damage evolution is not clearly understood.

In this study, we investigate experimentally damage zones dynamic evolution during normal faults population growth. With this aim, we used a sandbox type device and multilayered analog model, monitored by high-resolution cameras. Several types of damage zones are evidenced and we performed an accurate description of their growth from initiation to mature damage. New damage types including conjugate link damage and graben damage are described for the first time and show similarities with observations in nature. We highlight the new concept of “fault system damage” as the increase of deformation and secondary fault density by the interaction of major faults. We also show that fault damage zones grow by segment linkage into corridors of fault segments formed in the first stages of model deformation. Based on these observations, we propose and discuss the new concept of “segment selection in corridors” as the process of the onset of fault maturation and their damage zone development.

How to cite: Mayolle, S., Soliva, R., Dominguez, S., and Wibberley, C.: Fault damage zone growth in analog models: typology and segment selection process into fault corridors, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6753, https://doi.org/10.5194/egusphere-egu23-6753, 2023.

Hydrothermal veins can form when pre-existing anisotropy gets dilated by increase in fluid pressure (P_f). In addition fluctuation in P_f can result in the deposition of economically important deposits, such as gold in veins. Therefore, statistical analysis of dilational quartz veins can help to identify the mineralization potential of an area. This present study is focused on this aspect. The metavolcanic rocks of Dharwar Craton (Southern India) are replete with quartz veins, but mineralization is restricted to certain domains in the vicinity of Gadag, Hutti, and Kolar. To study the topological importance in mineral exploration, an experiment is carried out in different parts (e.g., Gadag, Hutti, Raichur, Gadwal) of Dharwar Craton. Orientation of dilational quartz veins, thickness, and spacing data are collected from different transects of the above-mentioned areas. Variations in driving pressure ratio (ΔRʹ), fractal dimension (Dc), Weibull modulus (α), vein intensity (Vi), and coefficient of vein spacing (C_v(S)) are calculated from vein data of each transect. These parameters are integrated to plot a three-axes “mineralization potential plot”, also known as P-D-F plot (Lahiri et al., 2020). The distance between the origin (0,0,0) and the point representing the transect in the P-D-F plot gives the “mineralization potential parameter” (Md). It is established that a transect in mineralized zone is represented by a point that lies closer to origin than non-mineralized zone. However, it is our observation that the points representing mineralized zone show more clustering on the P-D-F plot, whereas the points representing non-mineralized zone are dispersed.  We infer that in order to identify a zone as having high mineralization potential, the Md value should not only be lower but also the clustering should be higher.

How to cite: Biswas, S. and Mamtani, M. A.: Mineralization potential from vein statistics at regional scale – an example from the Dharwar Craton (Southern India), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7033, https://doi.org/10.5194/egusphere-egu23-7033, 2023.

The Permian–Mesozoic Asturian (NW Spain) hosts a series of world-class fluorspar deposits - a critical raw material for the EU - with more than 15 Mt. extracted to date. Asturian fluorspar deposits are hosted in silicified Paleozoic and Triassic carbonate rocks, spatially associated with the fault-bound margins of the basin. The Asturian fluorspar deposits show an important structural and lithological control, being hosted in fault-fill veins, jogs and breccias associated with steeply-dipping extensional faults and related folds, and strata-bound bodies replacing carbonate rocks. The general paragenesis comprises fluorite, calcite, quartz, (±) barite, and minor sulfides including pyrite, sphalerite, chalcopyrite, (±) galena. Formation of these fluorspar deposits has been assigned to the Permian volcanic activity during the extensional event following the Variscan orogeny, or the Mesozoic opening of the Atlantic realm.

For this study, we have sampled and measured the orientation of the main ore body at La Collada underground works. The ore body is hosted in a fault-fill vein moderately dipping to the SSW (mean plane is 214/47; n=41), ranging in thickness from a few meters to 15m. Observed sub-vertical veins of calcite are kinematically compatible with the main SSW-dipping fault-fill vein, and suggest concomitant ore deposition and extensional faulting. Laminated textures and localized high-dilation breccias in the main fault-fill vein suggests multi episodic fault activity associated with fluid overpressures (fault-valving?). S/C structures and fault slickensides (n=6) indicate oblique right lateral reactivation. Three representative samples from the main fault-fill vein were taken for U–Pb dating by LA-ICP-MS. Calcite spot analysis yielded two age populations (155Ma, 131Ma.) thus suggesting multiepisodic precipitation of calcite, and a potential open system. Obtained radiometric ages are consistent with rift initiation in the Asturian basin, and opening of the Bay of Biscay since Upper Jurassic times. Coupled with structural geology and textural analysis, LA-ICP-MS U-Pb dating of hydrothermal calcite aids in constraining the timing of fluid-flow events and fault activity responsible for ore deposit formation and re-mobilization, and refines the regional context of the Asturian fluorspar deposits.

How to cite: Granado, P., Ruh, J. B., Guillong, M., and Rodríguez-Terente, L.: New constraints from structural data and U-Pb calcite geochronology on La Collada fluorspar ore body (Asturian basin, NW Spain), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7298, https://doi.org/10.5194/egusphere-egu23-7298, 2023.

Under diagenetic conditions between ca. 50 ℃ to 250 ℃ the systematics of cement precipitation and differential infill makes network porosity, and thus permeability and strength, scale and thermal history dependent. Using examples of regional opening-mode fractures in sandstones from the Cambrian Flathead Formation, Wyoming, a low-enthalpy geothermal outcrop analog, we show that quartz deposits preferentially fill fractures up to ca. 0.05 mm wide with a transition from mostly sealed to mostly open fractures over a narrow size range of opening displacements from 0.05 to 0.1 mm. Scale- and diagenesis- dependent connectivity can be described using use rule-based node descriptions to rapidly measure diagenesis sensitive connections within the context of current field practice.  In our example, although networks have trace connectivity, effective connectivity for fluid flow is greatly reduced by quartz cement. Near some faults, trace connectivity increases as initially wide porous fractures preferentially shear and wing cracks form, increasing fracture intersections (Y-nodes). However, pore space is lost due to the development of quartz-cemented microbreccia. Macro-scale trace connectivity increases, but porous connectivity diminishes and thus potential for fluid flow is markedly lower. We illustrate how diagenesis-sensitive contingent nodes can be used to extrapolate permeability estimates to locations having different thermal histories.

How to cite: Laubach, S. and Forstner, S.: Scale-dependent fracture patterns and flow in low-enthalpy geothermal targets: the role of diagenesis and contingent nodes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8776, https://doi.org/10.5194/egusphere-egu23-8776, 2023.

Stylolite, fracture and fault networks are important fluid pathways, especially in low permeable rocks such as limestone and therefore important for subsurface applications including geothermal energy production. These systems grow in both time and space and have a given correlation length. Below the correlation length the system becomes saturated and shows a given scaling, for example in roughness for stylolites. Whereas above the correlation length the roughness or width of the growing system becomes constant. The position of this length varies with time and space whilst also being influenced by the system size. This becomes important when the systems connect, for example fractures that grow and merge together such that they have a given size. In this contribution we show with numerical simulations and natural examples how stylolite, fracture and fault networks scale in time and space, how their correlation length is evolving and how they become connected. We discuss the implications for scaling of larger networks as well as implications for deformation and fluid flow.

How to cite: Koehn, D., Hafermaas, D., Köhler, S., Lang, J., Mathur, B., Prabhakaran, R., and Smith, R.: Scaling, connectivity and correlation length of stylolite, fracture and fault networks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8940, https://doi.org/10.5194/egusphere-egu23-8940, 2023.

Fragmentation in deformation bands and faults is relatively complex with multiple fracture sets breaking single grains into anisotropic splinters. An understanding of the development of deformation bands and faults and the associated mechanical and permeability evolution is important for many applications including treatment of faults in reservoir models, flow properties of faults in geothermal systems and CCS (Carbon Capture and Storage), and seismic hazards. Various mathematical models have been developed to capture rock fragmentation process at single and multi-grain scale as well as to study the hydraulics in large scale geological discontinuities. However a multi-scale approach is needed to understand the implications of the changes in mechanical properties and permeability due to small scale rock fragmentation on the large scale bands, faults and fractures. This study aims to employ an extended Discrete Element Method (DEM) approach with multi-scale aggregates to model the evolution of deformation bands in porous sandstones.

Grain failure in rocks can be caused by different loading conditions, such as compressive loading, shear displacement, thermal, hydraulic or chemical effects. In this study, simulation of the comminution  process of the polyhedral shaped grains is achieved under compressive and shear loading. Single grain fragmentation is realised with the Mohr-Coulomb approach, which is a classical failure criterion for brittle particle systems. The failure criterion is combined with the Weibull statistical distribution that captures the grain size effect. A multiple grain model is simulated with DEM combined with the Mohr-Coulomb-Weibull concept. The computations are carried out with an open-source discrete numerical modelling framework YADE. The study sheds light on the influence of initial rock properties (porosity, grain size and shape) and deformation mechanism (compaction, shearing and combinations) on the development of deformation bands and faults.

How to cite: Mathur, B., Koehn, D., and Smith, R.: Modelling of Fragmentation Process in Deformation Bands and Faults, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8991, https://doi.org/10.5194/egusphere-egu23-8991, 2023.

Reservoir rocks, such as carbonates, are rapidly becoming key elements for the energy transition. The damage of these reservoir rocks when placed under a stress field must be characterized, to better predict storage capacity distribution. In the shallow subsurface, carbonate rocks accommodate the stress by developing structures at the mesoscale, such as fractures, deformation bands or stylolites, depending on porosity or fluid content. Those are localized, showing a finite damaged area, outside which the relative host rock can accommodate the applied stress in a different way, usually overlooked in low temperature and pressure conditions.

In this study, we highlight a new marker of accommodation of shortening, characterized by heterogeneous quartz grain rotation in non-porous carbonate matrix. The studied rock is an upper Cretaceous bioclastic calcarenite from the Cotiella Massif (Spain). This rock is composed of 85% carbonate (micrite and recrystallized microsparite), 10% quartz, and <5% of nanometric porosity. It hosts a fracture pattern including fractures, stylolites and deformation bands that correspond to different tectonic stages. However, we focus on investigating the quartz grain orientation in the grains outside the deformation bands, in both the far-field and near-field host rock. We investigated the fabric (typology, distribution and orientation) of thousands of quartz grains using X-ray microtomography on cylindrical cores of 8-26 mm diameter. Each segmented quartz grain is approximated with a best-fit ellipsoid whose major axis (L1) and minor axis (L3) give us information about the average orientation of the quartz grain. We show that the typology of the quartz grains, namely the size and average shape is similar in all our samples.

The average orientation of all quartz grains at the core scale reveals subtle preferences, without clear correlation to the orientation of neither the stylolites nor the deformation bands. We observe that in half of the samples studied, the quartz grain fabric is not controlled by the bedding. Instead, there are two distinct patterns of grain orientation, with the quartz grain fabric either reflecting the early burial stage or revealing a later reorientation perpendicular to one of the major shortening directions. These directions are either striking parallel to the local shortening flow (NE-SW) or to the regional orogenic flow (N-S), that is attributed to the Pyrenean orogeny. Evidence of dissolution-recrystallization are observed in quartz, but the diagenetic conditions constrainthis mechanism from being a robust hypothesis to explain the change of quartz fabric, but rather favour the rigid rotation of quartz in a micritic matrix. The examination of both the quartz grains and the carbonate matrix with EBSD suggests a local strain accumulation within the carbonates in the vicinity of quartz grains. Although the mechanisms causing this rotation need to be better understood, measuring the grain typology and orientation on a considerable number of grains with the aid of X-ray microtomography could result in a new method of deformation quantification in carbonate rocks.

How to cite: Taxopoulou, M. E., Lartigau, M., Aubourg, C., Beaudoin, N., Charalampidou, E.-M., and Callot, J.-P.: Rigid rotation of quartz grains in a low-porosity calcarenite: an indicator of early deformation? , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9301, https://doi.org/10.5194/egusphere-egu23-9301, 2023.

Although the origin of outer-arc extension fractures in folded sequences is well-understood and documented in many natural examples, geometric and geomechanical factors controlling their spacing are hitherto unexplored. This study investigates the formation of bending-induced tensile fractures during constant-curvature forced folding using two-dimensional Distinct Element Method (DEM) numerical modelling. The DEM model comprises a central brittle layer embedded within elastic layers; the layer interfaces are cohesionless. Folding of this three-layer system is enforced by a velocity boundary condition at the model base, while a constant overburden pressure is maintained at the model top.

The models illustrate several key stages of fracture array development: (i) Prior to the onset of fracture, the neutral surface is located midway between the layer boundaries, consistent with pure bending; (ii) Once the outer-fibre stress equals the tensile strength of the layer, fractures nucleate and propagate through the brittle layer; (iii) The rate of fracture formation as a function of curvature decreases nonlinearly, with new fractures developing approximately midway between two existing fractures; (iv) Eventually no new fractures form, irrespective of any further increase in fold curvature, a state referred to as fracture saturation.

On the basis of these numerical model results, an approximate analytical solution for fracture spacing based on classic beam theory is developed. The predicted range of fracture spacing as a function of fold curvature is in good agreement with the numerical model results. Importantly, the analytical solution reveals which geometric and geomechanical factors control fracture spacing, namely layer thickness, radius of curvature, Young’s modulus, tensile strength and confining pressure. The fracture spacing to layer thickness ratio at saturation however depends only (nonlinearly) on the ratio of tensile strength to overburden pressure.

The numerical model results are qualitatively compared with field observations at outcrops located in the Montpellier Fold region. The folded lithologies are of Jurassic age and comprise (brittle) limestones, with (ductile) marl intercalations. Fracture-bound limestone blocks located within the fold hinges and observed on the fold-profile plane are laterally bound by V-shaped veins that thin towards the fold core. In the inner-arc of each vein-bound limestone block, the marl interbed thickens towards the veins, whereas in the outer-arc it thins towards the veins. Clearly, the thickness distribution of the marl interbeds reflects non-uniform loading, which is consistent with the loading conditions hypothesised on the basis of the theoretical models.

How to cite: Schöpfer, M., Grasemann, B., and Hinsch, R.: Numerical and analytical modelling of bending-induced tensile fractures, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9680, https://doi.org/10.5194/egusphere-egu23-9680, 2023.

Deformation of intraplate sedimentary basins essentially results from tectonic stresses that propagate from plate boundaries. Yet, the detailed characteristics of this far field propagation are often poorly constrained, especially in age. Recent developments of U/Pb dating of calcite precipitations in fault planes and veins hopefully greatly help to frame the schedule of deformation. This is the case for the Meso-Cenozoic Paris basin, liable to have recorded several Cenozoic events due to its location in the western Eurasian plate, like Pyrenean and Alpine collisions and Oligocene extension (ECRIS).

Our study focusses on the southern Paris basin area, north of the Morvan massif, where Alpine deformations are expected. Field data reveal almost exclusively strike-slip deformations with variable σ₁ direction. These results are about to be confirmed at the microscopic scale using Anisotropy of Magnetic Susceptibility and of P-Wave Velocity methods. Associated U-Pb dating of synkinematic calcites from Jurassic strata indicates distributed Eocene deformation and an absence of Oligocene or Miocene ages. Such ages suggest the record of Pyrenean and possibly ECRIS tectonic events. Strikingly, Alpine deformation and especially stress propagation leading to the growth of the Jura fold-and-thrust belt during Miocene does not imprint the studied area although the Alpine orogen is located closer than the Pyrenean belt.

How to cite: Brown, S., Missenard, Y., Robion, P., Beccaletto, L., Allanic, C., and Haurine, F.: Structural investigations and U/Pb dating in the southern Paris basin do not record any Alpine deformation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9789, https://doi.org/10.5194/egusphere-egu23-9789, 2023.

Long-lived, mature faults can be architecturally complex. The in-depth unravelling of their complexity, including understanding the dynamic and multiscalar evolution of their mechanical properties, requires detailed multidisciplinary studies. Indeed, an integrated approach is the only viable solution to the deconvolution of the at times very complex internal architecture of brittle fault zones, which represents a phenomenal archive of faulting history and conditions through time and in space. Fault zone architectures are commonly characterized by the spatial juxtaposition of “brittle structural facies” (BSFs), which progressively form and continuously evolve during faulting. The mutual spatial and temporal relationships of BSFs impact directly on the bulk static and dynamic permeability structure of fault zones. The permeability structure, in turn, plays a significant role on the distribution of georesources and on seismogenesis in the brittle upper crust, where both natural and induced seismicity are often associated with fluid migration and overpressure. Detailed models of the complexity of fault zones and of their static and dynamic permeability structure are thus necessary to refine our understanding of fluid pathways and of the mechanisms leading to fluid compartmentalization and possible overpressure in the crust. We present a multidisciplinary workflow to decipher exhumed complex fault zones by integrating detailed structural analyses with geochronological dating of the deformation events recorded by the constituent BSFs and systematic in-situ permeability measurements to connect deformation structures to deformation age and hydraulic properties. The absolute dating of BSFs constrains how fault hydraulic properties change not only through space but also in time (during seismic or orogenic cycles, for example) as the fault architecture (and permeability structure) progressively develop. As a case study, we apply this workflow to the Zuccale Fault in the northern Apennines (Italy), a major low-angle fault that formed and was repeatedly reactivated during a time interval spanning at least the last 22 Myr. A stark spatial heterogeneity of its present-day permeability (up to four orders of magnitude) emerges as a key structural and hydraulic feature, even for tightly juxtaposed BSFs. Results show how complex fault architectures steer the 3D hydraulic structure of the brittle upper crust with direct implications on styles and modes of fluid ingress and flow.

How to cite: Viola, G., Curzi, M., Giuntoli, F., and Vignaroli, G.: Structural-, in-situ permeability- and K-Ar geochronological constraints from complex and heterogeneous fault zones: new perspectives on fluid circulation and seismogenesis in the upper crust, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12282, https://doi.org/10.5194/egusphere-egu23-12282, 2023.

The nucleation and subsequent vertical and lateral growth of normal faults have been the focus of many studies in extensional settings as these are important conduits for heat or fluid transfer. However, little attention has been attributed to normal fault growth in flexural foreland basins. The flexural downbending of the foreland plate generates extensional stresses, leading to the formation of normal faults in flexural foreland basins. Therefore, besides having an important role for subsurface fluid flow, normal faults in foreland basins are a fingerprint of the tectonic events associated with the downbending of the lower plate during collisional tectonics. In this study, we quantify the Oligocene to Early Miocene spatial and temporal evolution of the normal faults and their growth styles in the Northern Alpine Foreland Basin (NAFB). For this approach we use two 3D seismic volumes in the depth domain, located in the German part of the NAFB, to interpret both normal faults and prominent seismic horizons. This allows us to construct throw-depth and throw-length profiles to quantify changes in fault-parallel and down-dip throw patterns. This analysis allows us to reconstruct how the normal faults in our study area grew in 3D over time. Throw-depth profiles of the faults generally record throw maxima for either the foreland unconformity reflector or Oligocene aged reflectors, subsequently decreasing both up-and downward. This implies that following nucleation, the faults grew both upward into the syn-flexural fill of the basin and downward into the pre-flexural basement. Furthermore, comparing throw-depth and throw-length profiles of the faults in both seismic volumes shows that lateral fault growth is coupled with the fault tip moving towards younger stratigraphy. This is highlighted in the eastern German NAFB, where the vertical linkage between a lower segment that nucleated in the Eocene-Rupelian and an upper segment that nucleated in the Late Chattian is observed. To summarize, the observations imply that normal faults in the German NAFB were newly formed during flexural downbending of the European plate, preferably nucleating at the top of the plate before growing downward. This is different from fault growth observed in purely extensional settings, where faults typically grow upward instead of downward. Finally, the variable normal fault activity in the different seismic volumes in the German NAFB point out basin-parallel variations in flexural subsidence, possibly driven by spatiotemporal variations in Alpine frontal thrusting and/or slab pull.

How to cite: Eskens, L., Andrić Tomašević, N., Müller, M., and Herrman, R.: Growth of normal faults in flexural foreland basins: a case study of the Northern Alpine Foreland Basin, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14635, https://doi.org/10.5194/egusphere-egu23-14635, 2023.

Folded sedimentary layers usually exhibit brittle mesostructures such as faults, joints, veins, and stylolites which accommodate the internal strain of strata during folding but also before strata started to be tilted, i.e. during Layer Parallel Shortening (LPS), and after tilting, i.e. during late stage fold tightening (LSFT) when shortening can no longer be accommodated by fold growth. We have established a fracture sequence in the Mirabeau anticline (SE France) using orientation data and relative chronology with respect to bedding attitude, which comprises the early-, syn- and late-folding fractures related to the folding event (1). This field-based approach is compared to 12 ages out of 32 samples analyzed by LA-ICP-MS U-Pb geochronology of selected syn-kinematic calcite, in order to provide an absolute time frame for the fracture development, along with a potential validation of the sequence of deformation. Along a section across the fold strike axis, the deformation related to the contraction lasted at least 12 Ma (or 26 Ma with uncertainties), bracketed between 52 Ma (± 8 Ma) and 40 Ma (± 6 Ma), indicating that the fold developed in response to the so-called Pyrenean-Provençal phase. In depth however, the sequence of deformation as inferred from the structural study of the fracture network is challenged by the absolute age of the syn-kinematic calcite: syn-folding flexural-slip surfaces are dated from 52 Ma (± 8 Ma), i.e. older than early-folding reverse faults associated to LPS (46 ± 4 Ma).  

In order to better constrain the onset of Layer-Parallel Shortening (LPS), we further analyzed the bedding parallel sedimentary stylolites through the inversion of their roughness to better define the burial depth range during which the Jurassic and Upper Cretaceous formations of interest were submitted to a vertical stress σ1. Once projected on the burial-time model of the investigated strata, the depth range, going up to 3 km, constrains the time at which σ1 presumably switched from vertical to horizontal in response to the onset of tectonic loading. Altogether, this work questions the limits of absolute dating in understanding fold-fracture relationships, and forewarn of potentially misleading interpretation of absolute ages when associated with deformation features. The example of the Mirabeau Anticline enables a discussion about the compatibility and complementarity of different approaches to date mesoscale and macroscale deformation features in fold-and-thrust belts where the sedimentary record is not well preserved.

Keywords : Absolute dating, fracturation, fold, carbonates

(1) Lacombe O., Beaudoin N., Hoareau G., Labeur A., Pecheyran C. & Callot J.P., 2021. Dating folding beyond folding, from layer-parallel shortening to fold tightening, using mesostructures : Lessons from the Apennines, Pyrenees and Rocky Mountains.  Solid Earth, 12, 10, 2145-2157

How to cite: Zeboudj, A., Lacombe, O., Beaudoin, N., Callot, J.-P., Lamarche, J., and Guihou, A.: To what extent does U-Pb geochronology of calcite cements help to constrain the sequence of fracture development in folded strata? The Mirabeau Anticline (SE France) as a case study., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14859, https://doi.org/10.5194/egusphere-egu23-14859, 2023.

Structural analyses of vein and faults related with the growth of the South Pyrenean fold and thrust belt are coupled with the U-Pb ages of fracture-filling calcites compiled in Cruset et al. (2020) and Muñoz-López et al. (2022). Fractures include conjugated vein systems and reverse, strike-slip and normal faults cemented by calcite. The results reveal the orientation of tectonic stresses during the Pyrenean compression and their spatial and temporal evolution since the Late Cretaceous.

 Late Cretaceous to late Palaeocene U-Pb dates between 70.6 and 55.3 Ma registered compressional deformation in the inverted Mesozoic salt-related extensional basins corresponding to the Bóixols and Upper Pedraforca thrust sheets. Calculated paleostresses show a predominant N-S direction of tectonic transport, with more pronounced NW-SE directions in the western termination of the Bóixols and in the northern sector of the Upper Pedraforca.

Fracture data and U-Pb ages between 47.9 and 42.3 Ma measured in the Lower Pedraforca thrust sheet reveal N-S and NW-SE directions of tectonic transport during the middle Eocene, similar to those in the Bóixols and Upper Pedraforca thrust units. Contrarily, younger late Eocene to Oligocene U-Pb ages between 36.2 and 28.4 Ma measured in the lowermost Cadí thrust sheet reveal an homogeneous N-S trend during the emplacement of this unit.

Fractures cutting the Bóixols and Upper Pedraforca thrust sheets, and filled with cements yielding U-Pb ages from 48.8 to 25.68 Ma, show the same directions of tectonic transport than those measured for the Late Cretaceous-Paleocene. These Eocene and Oligocene ages also register the post-emplacement deformation of upper tectonic units on top of the Lower Pedraforca, Cadí, Montsec and Serres Marginals lower thrust sheets.

This research was funded by the project ALORBE (PIE-CSIC-202030E310), DGICYT Spanish Projects PID2021-122467NB-C22 and PGC2018-093903-B-C22 Ministerio de Ciencia, Innovación y Universidades/Agencia Estatal de Investigación/Fondo Europeo de Desarrollo Regional, Unión Europea. David Cruset acknowledges the Spanish Ministry of Science and Innovation for the "Juan de la Cierva Formación” contract FJC2020-043488-I AEI/10.13039/501100011033.

References

Cruset, D. et al., 2020. Quantifying deformation processes in the SE Pyrenees using U-Pb dating of fracture-filling calcites. Journal of the Geological Society, 177: 1186-1196.

Muñoz-López, D., et al. 2022. Spatio-temporal variation of fluid flow behavior along a fold: The Bóixols-Sant Corneli anticline (Southern Pyrenees) from U–Pb dating and structural, petrographic and geochemical constraints. Marine and Petroleum Geology, 143: 105788.

How to cite: Cruset, D., Muñoz-López, D., Vergés, J., and Travé, A.: Paleostress evolution in the South Pyrenean fold and thrust belt based on the structural analysis of fractures and U-Pb dating of carbonates, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15471, https://doi.org/10.5194/egusphere-egu23-15471, 2023.

The evolution of deformation structures and associated past fluid flow is a key to better appraise both the reservoir properties of a rock and the regional evolution of an area. Indeed, the meso-scale fracture network often develops over a long period of time, granting access to a long term evolution of a past fluid system (i.e. the temperature, origin and migration pathways of the fluids) on the one hand. On the other hand, major thrusts are believed to enable episodic fluid migrations on larger spatial scales, in relation to their activity calendar, whether this activity is over or ongoing. It is seldom however to reconstruct both the past fluid flow and the present-day fluid flow on the same structure, yet it can be very enlightening about the evolution of the thrust connectivity at depth since before a fold developed over it.

In this study we reconstructed the deformation pattern and associated past fluid system related to the development of the Sierra de Orba Anticline (Spain), and we compare it to the current-day fluids resurging in the vicinity of the fault. The Sierra de Orba anticline is part of a fault-propagation fold system that developed in the northern part of the Jaca Basin, Southern Pyrenean foreland, Spain. This fold affects the sedimentary succession including the Triassic decollement level, the Upper Cretaceous to Paleocene carbonate strata and the Middle Eocene marls. The N110 striking major thrust, where the Upper Cretaceous Marboré Fm. lies unconformably onto the Eocene marls, was sampled, along with the fracture network in the hangingwall and the footwall of the thrust. The fracture network includes a sequence of prefolding sets of joints and veins, striking E-W and N060 and flexural-slip related reverse faults. Then, the network encompasses the orogeny history since likely the forebulge development until the strata tilting during folding. Syn-kinematic calcite cements were characterized petrographically in both joints and faults, then studied by means of oxygen and carbon isotopic measurements coupled with fluid inclusion microthermometry. Results highlight that the fluid system recorded an alternation between meteoric fluids (δ¹⁸O signature of the fluid: -5‰ SMOW) heated up at 70-80°C, and evolved seawater (δ¹⁸O signature of the fluid: +5 to +10‰SMOW) heated up at 70-100°C. That past fluid system around the main thrust did not record any deep-sourced fluids, unlike similar structures in the southern Pyrenean foreland. The major gas content of current day fluids was analysed from a resurgence in the footwall of the thrust. This gas is especially nitrogen-rich (>86%), and is characterized by an heavy δ¹³C signature of the methane content (4‰PDB). Both these features could relate to deep processes, such as an oxidation of a potential abiotic carbon that can be related to a serpentinization process. This hypothesis needs to be further evaluated by means of a study of noble gas content. Beyond regional implications, this case study illustrate how a combination of geochemical proxies can help to unravel the evolution of a fluid system at a fold-thrust scale, from its onset to today.

How to cite: Beaudoin, N., Battani, A., Aubert, I., Léon, A., Aubourg, C., Emmanuel, L., and Stuart, F.: Depicting the fluid system evolution in a major thrust and associated fracture network, from layer parallel shortening to today: the Sierra de Orba anticline, Jaca basin, Spain., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16265, https://doi.org/10.5194/egusphere-egu23-16265, 2023.

Mid-to-lower crustal rock exhumation is common in orogenic belts, but the deformation process exposing these rocks remains debated. Distributed deformation in low viscous crust extruding mid-to-lower crustal rocks as channel flow and localized deformation along shear zones imbricating rigid blocks are two end-members that account for crustal thickening and unroofing. At the northwest of the Early Paleozoic orogenic belt in the South China Block, the Jiuling Massif includes orogenic root rocks exhumed from deep crustal level. Their structural pattern and exhumation history can improve our understanding on how continental mid-to-lower crust is deformed, thickened, and finally transported to the surface. Structural analysis reveals that two major mid-crustal ductile shear zones and their splays are developed at temperatures of ∼350°C–550°C. Anisotropy of magnetic susceptibility (AMS) shows that the Southern Jiuling Batholith has a modified AMS pattern by syn-orogenic compression, suggesting a gradually deformed rigid block. Combining surface geological evidence and deep structures by gravity modeling, we find shear zones rooted in basal décollement incrementally stacked the rigid granitic blocks. Along strike, the major shear zones evolved differently with more splays at their eastern portions. Thus, tectonic imbrication gradually evolve to pervasive flow-like deformation as shear zones continue to splay and form an anastomosed shear zone system. The complexed structures by splayed shear zones segmenting and imbricating small rigid blocks may correspond to the geophysically low-velocity zone in the crust, so shear zone splaying is a linking mechanism between tectonic imbrication and viscous flow deformation of the crust, and reconcile these two end-members.

How to cite: Chu, Y., Feng, Z., Lin, W., Meng, L., and Xin, G.: Basal Décollement Splaying characterizes Mid-Crustal Deformation and Exhumation of orogenic cores in an intracontinental orogen, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16523, https://doi.org/10.5194/egusphere-egu23-16523, 2023.

Helium-rich gases (up to 6%) upwell along a fault that bounds the western edge of the granitic Morvan Massif (northeastern part of the French Massif Central) and the sedimentary rocks of the southeastern edge of the Paris Basin. The sampled gas is mainly composed of nitrogen (90 %.). The radiogenic ³He/⁴He (0.02 Ra, where Ra is the atmospheric ratio) and nucleogenic ²¹Ne/²²Ne (0.031) imply that the gas is basement-derived, in good agreement with the presence of granite in the area. The high radiogenic He concentration can be explained by a rock/ water system isolated over geological time scales and might be linked with the presence of a close-by fluorite ore deposit, dated 130 Ma, located above the fault and at the basement/sediment unconformity in Pierre-Perthuis, formed by leaching of granite (Gigoux et al., 2015).

The possible link between the fluorite ore and the He-rich gases should imply that groundwater in which He released from U and Th in granite was trapped for several millions of years in the granite or in the permeable basement/sediment unconformity reservoir, below the sedimentary cover probably reaching 1.5 km in the area at the end of the Cretaceous. Deep groundwaters trapped in the reservoir accumulated helium before tectonic exhumation of the Morvan Massif and remobilization of fluids to the surface through the Bazois fault, whose onset began 40 My ago (Lenoir et al., 2021). Mixing between old deep crystalline-water with superficial water is evidenced by the presence of high amount of atmosphere-derived nitrogen.

The high chloride content of the waters may originate from the marine water trapped in the Early Jurassic clays, and released by faulting, or by water-rock interaction and hydrolysis of minerals, reinforcing the link between chlorine, fluorine, and water from granite.

This geological example shows that in quiescent area, inactive fault can act as a fluid pathway over long time scale.

How to cite: Battani, A., Gyore, D., Brigaud, B., Bernard, A., Sarda, P., and Stuart, F.: Long time degassing of crustal fluids along inactive fault in an intracratonic basin (Morvan, France), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16602, https://doi.org/10.5194/egusphere-egu23-16602, 2023.

Mineral dendrites are an example of a pattern which forms in rocks when they are infiltrated by the hydrothermal, manganese-rich fluids. As these fluids mix with other oxygenated fluids within the fractured rock, manganese oxide is formed. The oxide then precipitates, forming intricate, branched patterns. Several models of this process have been proposed, which vary in complexity. One model assumes crystallization of manganese oxides directly on the surface of the growing dendrite, causing it to elongate. Another model involves an initial growth of small nanoparticles of manganese oxide, which then aggregate into larger structures. The evolution of the system in both models is described by the system of reaction-diffusion equations.

We study this process using lattice-Boltzmann method to track the evolving concentrations of the species involved in reaction. Next, we analyze the dependence of the morphology of the resulting patterns on the physical parameters characterizing the reaction and growth, such as initial concentrations of manganese ions and oxygen molecules, reaction rates, nucleation thresholds or surface energy of the dendrites. Our study has been focused on planar structures, growing along fractures or bedding planes. We have investigated the impact of multiple infiltrations of manganese-bearing fluid on the morphology of the dendrites. We compare the numerical results to the morphologies of the real systems with the aim of reconstructing the hydrochemical conditions prevailing during their growth

How to cite: Woś, D. and Szymczak, P.: Computer modeling of mineral dendrite growth, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-236, https://doi.org/10.5194/egusphere-egu23-236, 2023.

Dissolution of carbonate rocks is a complex process in which the interplay of flow, transport, reaction, and geometric evolution plays an important role. The nonlinear couplings between these processes may lead to the formation of intricate patterns, including spontaneously formed channels (wormholes) [1].  It has been long established that the shapes of the dissolution patterns depend on fluid flow and mineral dissolution rates [2]. Recently, it also has become increasingly clear that pore-scale processes can impact large-scale morphologies [3,4]. However, the effects of pore-scale mixing on large-scale patterns remain unclear.

In this work, we investigate the effect of pore-scale mixing processes on the evolution of dissolution channels. Pore space is represented by a network of cylindrical tubes with the diameter of each segment increasing in proportion to the local reactant consumption. The inlet concentration of each pore is controlled by local mixing rules. Two different mixing protocols are considered: full mixing, in which the incoming reactant fluxes are assumed to be completely mixed at the intersection, and streamline routing, where the tracer follows the streamlines into the outgoing pores. We found that streamline routing enhances the flow focusing particularly strongly in moderate Damköhler number regimes where relatively wide dissolution channels appear spontaneously in the system. With the same initial conditions as the full mixing case, the winning channels obtained with streamline routing not only propagate faster but also could grow at a different location in the system. The enhanced flow focusing caused by streamline routing produces thinner wormholes and leads to shorter breakthrough times. Lastly, the evolution of velocity distribution is also found to be distinctive depending on the mixing rule.

[1] Hoefner, M. L. and Fogler,  H. S. AIChE J. 34: 45–54, 1988

[2] Golfier, F., et al.  J. Fluid. Mech. 457: 213-254, 2002

[3] Li, L., Peters, C. A., & Celia, M. A. Adv. Water Res., 29: 1351–1370, 2006

[4] R. Roded, P. Szymczak, R. Holtzman, Geophys. Res. Lett. 48:e2021GL093659, 2021

How to cite: Sharma, R. P., Kang, P. K., and Szymczak, P.: Effects of mixing at pore intersections on large-scale dissolution patterns, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-275, https://doi.org/10.5194/egusphere-egu23-275, 2023.

Water-rock interaction determines how the geochemical cycles evolve from Earth surface to the deep interior, related to the fluxes, time, and reactivity between fluid phases and solids. Thus, quantifying mass balances from the global scale to the local one, for understanding planetary geodynamics as well as optimizing geothermal doublets, require understanding how the driving force is controlled between solids and dissolved phases [1, 2]. As a consequence, water-rock budget has a major role to drive the porosity accessible to flow. The standard approach is to consider the chemical potentials of the bulk phases assuming the interface to be infinitely thin and therefore thermodynamically negligible, except with highly divided materials and/or super-confined solutions. Our work is based on previous investigations evidencing the formation of an interphase layer/domain, up to one micron thick, having distinct thermodynamic features with respect to the bulk phase properties [3].  

Herein, diffraction limited FTIR micro-spectroscopy in transmission mode, based on confocal microscope coupled to broadband supercontinuum laser or synchrotron beam, was employed as an energetic probe to monitor the thermodynamic characteristics of liquid water as a function of beam location in a synthetic fluid inclusion (one pore micro-scale closed cavity). FTIR hyperspectral data was recorded to illustrate distance-dependent vibrational energy (absorption signatures) at room and homogenization temperatures. The vibrational energy was transformed to Gibbs free energy using a partition function [4].

The results showed that Gibbs free energy changes by 600 to 1000 J/mol up to 1μm far from the water-quartz interface. This variation indicates a significant change in the chemical reactivity of liquid water over a thick domain, rather defining an “interphase” instead of an “interface.” We observed that the thermodynamic property of this interphase domain has a thermal dependency, and by increasing the temperature the chemical potential has a higher value. The Gibbs free energy variation with T can be interpreted by either an enthalpic or an entropic contribution, or a combination of both. This surprising discovery calls for a shift in the paradigm of the bulk phases dominance in water-rock interaction.

References

1. Putnis, A., Fluid–Mineral Interactions: Controlling Coupled Mechanisms of Reaction, Mass Transfer and Deformation. Journal of Petrology, 2021. 62(12): p. egab092.

2. Putnis, A., J. Moore, and H. Austrheim, Fluid-rock reaction mechanisms and the inevitable consequences for mass transport and texture formation. 2022, Copernicus Meetings.

3. Bergonzi, I., et al., Oversolubility in the microvicinity of solid–solution interfaces. Physical Chemistry Chemical Physics, 2016. 18(22): p. 14874-14885.

4. Bergonzi, I., et al., Gibbs free energy of liquid water derived from infrared measurements. Physical Chemistry Chemical Physics, 2014. 16(45): p. 24830-24840.

How to cite: Mozhdehei, A., Mercury, L., and Slodczyk, A.: Experimental Measurements of Micron-thick Interphase Thermodynamics along a Water-Rock Interface, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-463, https://doi.org/10.5194/egusphere-egu23-463, 2023.

Basaltic rocks are considered excellent candidates for CO2 storage by in situ mineral trapping, due to their large presence on Earth’ surface and their higher reactivity with CO2 to form calcium-rich minerals. Often carrying a high-volume fraction of vesicles, basaltic rocks can be an important reservoir horizon in petroleum systems. When the vesicle network has been filled by earlier mineralization the basalts can act as impermeable seals and traps. Characterizing the spatial and temporal evolution of the porosity and permeability is critical to understand the CO2 storage potential of basalts. We exploited X-ray computed tomography (XCT) to investigate the precipitation history of an amygdaloidal basalt containing a pore-connecting micro-fracture network now partially filled by calcite as an analogue for CO2 mineral trapping in a vesicular basalt. The fracture network likely represents a preferential pathway for CO2-rich fluids during mineralisation. We quantified the evolution of basalt porosity and permeability during pore-filling calcite precipitation by applying novel numerical erosion techniques to “back-strip” the calcite from the amygdales and fracture networks. We found that permeability evolution is dependent on the precipitation mechanism and rates, as well as on the presence of micro-fracture networks, and that once the precipitation is sufficient to close off all pores, permeability reaches values that are controlled by the micro-fracture network. These results prompt further studies to determine CO2 mineral trapping mechanisms in amygdaloidal basalts as analogues for CO2 injections in basalt formations.

How to cite: Macente, A., Dobson, K. J., MacDonald, J., Wadsworth, F. B., and Vasseur, J.: The Evolution of Paleo-Porosity in Basalts: Reversing Pore-Filling Mechanisms Using X-Ray Computed Tomography, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-478, https://doi.org/10.5194/egusphere-egu23-478, 2023.

Since the impoundment of the Shivajisagar Reservoir behind the Koyna Dam in 1962, numerous earthquakes have been felt in the Koyna-Warna Seismogenic Region of Western India. The mesoscopic and microscopic observations on the basement granitoid core samples, recovered under the Continental Deep Drilling Program of the Ministry of Earth sciences, reveal the precipitation of calcite and the formation of clay minerals (illite and chlorite) along the fractures and faults. The presence of these secondary minerals alongside the primary minerals like quartz and feldspar is further supported by X-ray Diffraction, which also points to the fracture scale chemical alteration as a result of fluid-rock interactions. It's interesting to note that the precipitation of these hydrophilic clay minerals along faults and fractures might promote slip by raising fluid pressure and lowering the shear strength of the faults. Thus, secondary mineralization due to fluid-rock interaction may have a contribution to the release of strain in form of seismic tremors. On the other hand, the neoformation of these hydrophilic clay minerals along fault/fracture surfaces may also cause rheological incongruity, which could lower the density as well as P and S wave velocities. Besides, hydrogen atoms in clay-bound water may influence neutron capture, leading to over-optimistic estimations of neutron porosity. Additionally, our study supports past geophysical anomalies found in the KFD1 borehole and infers that the geophysical anomalies correlating to the growing fracture density and fault system of the basement rocks are caused by chemical alteration due to fluid-rock interaction and subsequent secondary mineralization. So, this research offers important new understandings of geochemical activity in the context of geophysics and serves as a bridge between geochemistry and geophysics.

How to cite: Halder, P., Sharma, A., Shukla, M. K., and Kumar, K.: Is secondary mineralization playing a pivotal role in recurring seismicity at Koyna-Warna Seismogenic Region of India: a geochemical perspective?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-626, https://doi.org/10.5194/egusphere-egu23-626, 2023.

Crustal rocks involved in orogenic processes frequently bear evidence for widespread fluid circulation. The hydration history of the granitic continental crust in inverted passive continental margins is of particular interest, as granitoids experience rheological weakening by fluid-rock interaction processes. Regrettably, it is often unclear if hydration occurs during rifting or during tectonic inversion. Hence, it is difficult to appreciate the interplay of pre- and syn-orogenic fluids inside continental crustal segments of rifted margins. The geochemical fingerprint of ancient hydration events is stored in hydrous minerals that crystallized directly from circulating paleo-fluids. Thus, such minerals can shed light on the nature of these ancient fluids, as well as provide temporal constraints if they can be dated. Hence, advances in geochronological methods applied to hydrous minerals may prove pivotal in untangling the history of fluid circulation in the granitic continental crust in orogens.

We applied U–Pb geochronology of epidote [i.e., Ca₂Al₂(Al,Fe³⁺)Si₃O₁₂(OH)] in hydrothermal veins hosted by a late Carboniferous/early Permian calc-alkaline granodiorite in the inverted Adriatic passive continental margin (hereafter “Err nappe”), both located in the eastern Swiss Alps. During Jurassic rifting leading to the break-up of Pangea, the continental crust in the Err nappe was hydrated, as seawater-derived fluids percolated along syn-rift faults. However, geochronological data of epidote reveal that the hydration of the granitic continental crust in the Err nappe occurred also later during inversion. Epidote U–Pb geochronology returned two age clusters: (1) 85.2 ± 9.7 Ma, related to Late Cretaceous compression; and (2) 59.9 ± 2.7 Ma, related to subsequent Paleocene extension. These age clusters unveil two distict events of fluid circulation, which are consistent with the timing of tectonic inversion and deformation proposed in the literature. As confirmed by Pb–Sr–O–H isotope geochemistry of epidote, Late Cretaceous fluid circulation was likely mediated by fluids released by underlying units undergoing metamorphism during Eo-Alpine compression. Notably, the Paleocene fluids circulating during extension were most likely surficial in origin (i.e., meteoric water and/or modified/connate seawater), and they percolated into the granitic continental crust by exploiting extensional faults.

In the context of existing data, our results show that the hydration of the granitic continental crust of the Adriatic passive continental margin was mediated by a repeated series of fluid circulation events. Our work advocates that the use of a multi-methodological approach, combining new geochemical and geochronological, tools provides unprecedented insight into complex processes of fluid circulation in the continental crust, and beyond.

How to cite: Peverelli, V., Berger, A., Wille, M., Mulch, A., Putlitz, B., Lanari, P., Pettke, T., and Herwegh, M.: Complementing the hydration history of an inverted passive continental margin using epidote U–Pb geochronology and isotope geochemistry, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-732, https://doi.org/10.5194/egusphere-egu23-732, 2023.

The mobility of arsenic in aquatic environments is controlled by oxidation states of arsenic, stability of solid phases, and chemical composition of water (Meng et al., 2002). Binding of arsenic in the environment may occur through precipitation of low-solubility salts (Magalhães, 2002), like mimetite Pb₅(AsO₄)₃Cl. The aim of this study is to experimentally investigate reactions between cerussite (PbCO₃) and solutions containing AsO₄^3- at various conditions favouring mimetite formation. These observations may provide a new recognition for As immobilization, which might be relevant in remediation of contaminated natural waters.

The mechanism of cerussite reaction with arsenate solutions (50 mg As/L) was studied at pH 2 – 8 using synthetic cerussite powder and fragments of natural cerussite crystals (Mibladen, Morocco). The reaction was carried out by direct contact of 500 ml of As-containing solution with PbCO₃, in presence of Cl^- ions.Cerussite was reacted for up to 4 weeks at in situ and ex situ setups. X–Ray Diffraction (XRD), Scanning Electron Microscopy with Energy Dispersive Spectrometry (SEM-EDS) and Electron Microprobe Analysis (EMPA) were used for analysis of the solid products of the experiments. The solutions were tested for Pb with Atomic Absorption Spectroscopy (AAS) and for AsO₄^3-  using colorimetry.

Precipitation of mimetite on cerussite powder crystals is observed already after 1 day of the reaction with arsenate solutions, at the whole range of pH. Mimetite forms hexagonal rods or needles less than 1 µm in size precipitating in the form of incrustations on PbCO₃ crystals. Their size depends on the pH: a fine-grained precipitate forms at higher pH. Observations of natural crystals show replacement of cerussite by polycrystalline mimetite crust. The crust made of columnar and needle crystals is porous allowing for solution penetration and progress of the reaction. The replacement features indicate similarity to pseudomorphic reactions, and the mechanism elucidated as interface coupled dissolution - precipitation. Overall, cerussite replacement by mimetite reduces AsO₄^3- concentration from 50 ppm to below 1 ppm. It also depends on the pH.

This research was funded by AGH University of Science and Technology project No 16.16.140.315.

Magalhães, M. C. F. (2002). Arsenic. An environmental problem limited by solubility. Pure and Applied Chemistry, 74(10), 1843–1850.

Meng, X., Jing, C., & Korfiatis, G. P. (2002). A Review of Redox Transformation of Arsenic in Aquatic Environments. Biogeochemistry of Environmentally Important Trace Elements, 70–83.

How to cite: Stępień, E. and Manecki, M.: Experimental model of cerussite PbCO3 replacement by mimetite Pb5(AsO4)3Cl at pH 2 – 8, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1817, https://doi.org/10.5194/egusphere-egu23-1817, 2023.

The 3^rd and 4^th generation of synchrotron light sources with their high brilliance, fluxes and beam energies allow the development of innovative X-ray translucent rock deformation apparatus that maximise these capabilities. Following on from the development of the Mjolnir triaxial deformation rig (Butler et al., 2020), we present an upscaled design: Heitt Mjolnir, covering a wider temperature range and larger sample volume while operating at similar pressure, enabling a wide range of time-resolved investigations. This device is designed to characterise coupled hydraulic, chemical and mechanical processes, occurring at various temperatures, from the µm to the centimetre scale in cylindrical samples of 10 mm diameter and 20 mm length. Heitt Mjolnir can simultaneously reach confining pressures of ≤30 MPa (hydraulic), 500 MPa of axial stress while the sample’s pore fluid pressure is controlled in a dedicated fluid channel and can reach 30 MPa. This apparatus has an internal heating system and is able to reach temperatures of 573 K in the sample with a minimal vertical thermal gradient of <0.5 K/mm. This portable and modular device has been successfully deployed in operando studies at TOMCAT (SLS) and I12 JEEP (DLS) beamlines for 4D X-ray microtomography with scan intervals of a few minutes. Heitt Mjolnir allows the 4D characterisation of low-grade metamorphism, fluid-rock interaction and deformation processes. It enables spatially and temporally resolved fluid-rock interaction studies at a wide range of conditions and, by covering most geological reservoirs, will be particularly valuable for geothermal, carbonation or subsurface gas storage research.

How to cite: Freitas, D., Butler, I., Elphick, S., Gilgannon, J., Rizzo, R., Pluemper, O., Wheeler, J., Schlepuetz, C., Marone, F., and Fusseis, F.: Heitt Mjolnir: an internally heated triaxial rock deformation apparatus for operando experiments at up to 573 K at Synchrotron imaging beamlines, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3424, https://doi.org/10.5194/egusphere-egu23-3424, 2023.

The equilibration of minerals in the presence of an aqueous fluid phase, with which it is out of equilibrium, has been described in terms of a number of potential mechanisms, specifically mainly either by solid state exchange of elements within the solid phase and the aqueous solution, or by interface-coupled dissolution-precipitation where equilibration is approached by the incremental dissolution of the parent solid and the coupled precipitation of a new product solid (Ruiz-Agudo et al., 2014). The conditions determining the equilibration mechanism can be defined by the specific chemical potential differences at the mineral interface, the kinetics of potential reactions, the solubility of the solid phase in the specific fluid and physical properties such as the mineral: fluid ratio as well as the surface area: fluid ratio, temperature and pressure. We focus on the mechanism of ion exchange in a range of minerals and in most cases ion-exchange in the presence of an aqueous solution occurs by interface-coupled dissolution-precipitation (Putnis and Putnis, 2022).

References

Putnis C.V. and Putnis A. 2022. A mechanism of ion exchange by interface-coupled dissolution-precipitation in the presence of an aqueous fluid. J. Crystal Growth, 600, 126840

Ruiz-Agudo E., Putnis C.V., Putnis A. 2014. Coupled dissolution and precipitation at mineral-fluid interfaces. Chemical Geology 383, 132-146.

How to cite: Putnis, C. V. and Putnis, A.: A mechanism of ion exchange by interface-coupled dissolution-precipitation in the presence of an aqueous fluid, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3792, https://doi.org/10.5194/egusphere-egu23-3792, 2023.

The A-type granites of Jharsuguda at the boundary between the Singhbhum Craton and Rengali Province have rare earth potential. Two populations of zircon are present. One of them represents the unaltered zircons of magmatic origin, as evidenced by the presence of oscillatory zoning and Th/U ratios > 0.7. The other population consists of the altered equivalents of these primary zircons. These zircons have Th/U ratios < 0.1, suggesting their metamorphic origin. Occasionally, single grains of zircon containing both altered and unaltered domains are also encountered. The unaltered zircons comprise a very small proportion. The altered zircons appear dark in CL images and are characterized by low EPMA totals with non-formulae elements like Al, P, Ca, Fe, Y, and REEs. Numerous mineral inclusions, including those of U- Th-bearing ones, are typical of these altered zircons. Xenotime inclusions are typical of altered zircons with xenotime overgrowths in the granites. Rare Nb-rich inclusions are also present in these altered zircons.   In both the granites and pegmatite, fluid-mediated alteration resulted in fergusonite-(Y) and other Nb-Ta-REE oxides in the cracks and fractures. Pseudomorphs of fergusonite-(Y), Nb-Ta-REE oxides, and allanite are also formed by replacing earlier zircon grains. The size of these fergusonite grains ranges from a few micrometers in granite to up to 500 micrometers in pegmatites. The LA-ICPMS U-Pb dating zircons yield a primary age of 2.95 Ga and metamorphism ages of 2.80 Ga and 2.45 Ga. Selected fergusonite-(Y) grains from the pegmatite suggest a 2.1 Ga U-Th-Pb EPMA age for these fergusonites. Rims of some zircon grains in both granites and pegmatite also yield an age of 2.1–2.2 Ga. The age data suggest that the Jharsuguda granites and pegmatite underwent at least three metamorphic/tectonothermal events, during the last of which fergusonites are formed by the remobilization of REEs and HFSEs, probably in the presence of complexing ligands like F^– and PO4^–, as confirmed by EPMA X-ray element maps of altered zircons.

How to cite: panda, S., Roy, A., and Pruseth, K.: Fluid-mediated alteration of zircons to fergusonite-(Y) in A-type granites and pegmatite from the Jharsuguda district, India, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3812, https://doi.org/10.5194/egusphere-egu23-3812, 2023.

Moonstones are a gem-quality feldspar with a special exsolution structure and are well-known for the unique moonshine effect of themselves. However, formation of such “moonshine” is still a mystery. One possibility to reveal it, is by understanding, the formation mechanisms of moonstone exsolution fashion, which is constituted by the lamellae inclusions and the associated zoning patterns (banding structures). Here, by combining the mineralogical- and geochemistry techniques, we investigated the chemistry and textures of the chemical patterns in moonstones in detail. Two different color moonstones (orangish and grayish) are the object of study. Although Raman and EPMA analyses indicate that, both moonstones are orthoclase (Or_73.65~90.38), the orange moonstone is colored by hematite inclusions while the gray one is by magnetite inclusions. The orange moonstone has two lamellae types, which are An-containing albite phase (An_6.53~18.93) and K-high albite phase (≈An_6.23). The An-containing albite lamellae demonstrated a µm-size zone with a decrease of An content (18.93 to 6.53) from the zone center to edge. In contrast, the gray moonstone does not show any zoning structure. Those allow for further analyses focusing on the zone structures, in combination of XRD diffraction structure analysis, La-ICP-MS whole-rock principal element analysis and phase diagram simulation, and by which, we proposed a two-stage-growth process for exsolution structure that is formed in the   orange moonstone. The first stage of exsolution results in oligoclase lamellae, and the second stage results in K-high albite lamellae, in which part of Ab-rich phase became to individual K-high albite lamellae, while part of Ab-rich phase continues to dissolve around the oligoclase, forming the zoning structure. We formulated that the gray moonstone has only one formation stage which corresponds to the second stage of the orange moonstone. Our detail descriptions of moonstone might be a valuable contribution to further the study of moonshine effect.

How to cite: Cao, K., Zhao, F.-F., Hou, Z.-L., and Zheng, L.-S.: Formation of the zoning parttern in moonstones, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5330, https://doi.org/10.5194/egusphere-egu23-5330, 2023.

The Bergen Arcs, in Norway, consist of several arcuate nappes formed during the Caledonian orogeny 440-420 Ma ago (Bingen et al., 2001; Glodny et al., 2008) when the western margin of Baltica was subducted below Laurentia. This Caledonian orogeny overprinted many of the anorthosites that formed the 930 Ma old (Bingen et al., 2001) granulitic basement. This overprint resulted in both amphibolites and eclogites and have been observed in shear zones within the rocks of the well-studied island of Holsnøy, located on the western margin of the Lindås Nappe. On the adjacent island of Radøy, the Caledonian overprint is associated with amphibolite facies shear zones (Mukai et al., 2014; Moore et al., 2020).

In the northern margin of the Bergen Arcs, near the Bergen Arcs Shear Zone, the much less-studied island of Krossøy also exposes the anorthosites from the old granulitic basement and here the Caledonian overprint also resulted only in amphibolite facies metamorphism. The anorthosites in Krossøy are intruded by a series of subparallel mafic granulitic dykes forming the Krossøy dyke swarm, that has never previously been described elsewhere in the Bergen Arcs. The style of deformation in the granulites and the textural evolution in the amphibolite facies overprint are also markedly different from the rocks on Holsnøy and Radøy. The development of ductile Caledonian shear zones may have been facilitated by initial brittle failure of the basement accompanied by fluid infiltration (Jamtveit et al., 2018). Here we investigate the influence of this deformation and fluid infiltration on different features observed on these rocks such as: the occurrence of plagioclase coronas around the garnets on the dykes; the presence of different types of symplectites; the variability of size, deformation and composition observed on the anorthositic feldspars; or the local changes of fluid composition along cm- long fractures. We will show our first analytical results on some of these key features and discuss their relevance in the context of the previous studies of the Bergen Arcs.

Bingen, B., David, W. J., & Austrheim, H. (2001). Zircon U-Pb geochronology in the Bergen Arc eclogites and their Protereyoic protoliths, and implications for the pre-Scandian evolution of the Caledonides in western Norway. In GSA Bulletin (Issue 5). https://doi.org/10.1130/0016-7606(2001)113<0640:ZUPGIT>2.0.CO;2

Glodny, J., Kühn, A., & Austrheim, H. (2008). Geochronology of fluid-induced eclogite and amphibolite facies metamorphic reactions in a subduction-collision system, Bergen Arcs, Norway. Contributions to Mineralogy and Petrology, 156(1), 27–48. https://doi.org/10.1007/s00410-007-0272-y

Jamtveit, B., Moulas, E., Andersen, T. B., Austrheim, H., Corfu, F., Petley-Ragan, A., & Schmalholz, S. M. (2018). High Pressure Metamorphism Caused by Fluid Induced Weakening of Deep Continental Crust. Scientific Reports, 8(1). https://doi.org/10.1038/s41598-018-35200-1

Moore J., Beinlich A., Piazolo S., Austrheim H. & Putnis A.  (2020). Metamorphic differentiation via enhanced dissolution along high permeability zones. Journal of Petrology 61, 10. https://doi.org/10.1093/petrology/egaa096

Mukai H., Austrheim H., Putnis CV. & Putnis A. (2014). Textural evolution of plagioclase feldspar across a shear zone: implications for deformation mechanism and rock strength. Journal of Petrology. 55, 1457-1477. https://doi.org/10.1093/petrology/egu030

How to cite: Filiberto, L. H., Austrheim, H., and Putnis, A.: Deformation and fluid-infiltration influence in the evolution of the Krossøy dyke-swarm in the northern part of the Bergen Arcs, Norway, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5460, https://doi.org/10.5194/egusphere-egu23-5460, 2023.

Retrogression and hydration commonly affect large portions of the crust, causing variable degrees of chloritization, sericitization and/or serpentinization depending on the protolith and the conditions of fluid ingression. Retrograde overprint involving hydration is a strongly exothermic process, and leads to a thermal perturbation around the pressure–temperature conditions of hydration, which in the case of chloritization of felsic rocks typically occurs at <500°C. These conditions of retrogression overlap with the closure temperatures of some isotopic systems commonly used for geochronology, for example ⁴⁰Ar/³⁹Ar in micas and feldspars. The exothermicity of hydration therefore disturbs the recorded apparent ages and cooling histories of reworked terranes. Using an average metapelite composition as case study, we estimate that hydration and retrogression of a high-grade amphibolite facies assemblage to a low-grade greenschist paragenesis involves approximately a twofold increase of the mineral-bound water content and releases about 50 kJ.kg^-1 latent heat. Using a simple 1-dimensional numerical model, we solve the heat equation for a steady-state continental geotherm that is advected towards the surface and track the cooling rates for markers that exhume from different depths. Assuming enthalpy production at 380°C to simulate exothermic hydration, the cooling rate is significantly reduced until the markers are exhumed to the temperature/depth of hydration and reaction. The calculated cooling paths feed into KADMOS (Moulas & Brandon, 2022), a set of MATLAB routines designed to calculate apparent ⁴⁰Ar/³⁹Ar ages as function of customized thermal histories. KADMOS solves the equation of ⁴⁰Ar production from ⁴⁰K decay and thermally-activated diffusive loss of ⁴⁰Ar for time (Fig. 1). Our results reveal that for intermediate exhumation rates, spherical muscovite grains with <100 µm in diameter are affected by a ~10% age error when latent heat is considered (Fig. 1b). Such muscovites in rocks exhuming with a velocity of, for example, 4 mm/year would record an apparent ⁴⁰Ar/³⁹Ar age of c. 10 Ma (Fig. 1a) and be affected by an absolute age error of ~1 Ma from thermal buffering by hydration, yielding an apparent age of 10 ± 1 Ma (Fig. 1b). Our calculations indicate that latent heat released from exothermic hydration may significantly disturb low-temperature isotopic systems, thereby complicating the cooling histories and obscuring the temporal constraints deduced from state-of-the-art geochronological systems.

Figure 1 – Exhumation velocity vs. grainsize contoured for apparent ⁴⁰Ar/³⁹Ar age in muscovite (a) and relative error when latent heat is considered (b)

REFERENCES

Evangelos Moulas, & Mark T Brandon. (2022). KADMOS: a Finite Element code for the calculation of apparent K-Ar ages in minerals (Version 1). Zenodo. https://doi.org/10.5281/zenodo.7358138

How to cite: Schorn, S., Moulas, E., and Stüwe, K.: Hot when wet: the consequences of exothermic hydration on geochronology, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5769, https://doi.org/10.5194/egusphere-egu23-5769, 2023.

Quartz veins in metamorphic basement rocks document periods of hydrous fluid mobility. Here, we present a study of vein formation in the Agly Massif, eastern French Pyrenees, which was subjected to metamorphism during the Hercynian and Alpine orogenies and during Mesozoic extension between the two (Siron et al., 2020).

In this study, fourteen quartz samples have been selected for ⁴⁰Ar/³⁹Ar dating of the fluids inside fluid inclusions (FIs) by stepwise crushing. The results are characteristic for this method: all samples show anomalously high ages in the first part of the experiments decreasing to essentially flat plateaus in the final steps. The plateau ages are interpreted as the time of quartz vein formation, ranging from 117 to 62 Ma, i.e., mid-Cretaceous to early Paleocene. The initial values indicate the presence of another trapped argon component, with  ⁴⁰Ar/³⁶Ar intercepts >6000. An additional nine K-feldspar samples from the same veins are dated by incremental heating.

The quartz veins show considerable variation in mineral content, including feldspars, biotite, muscovite, chlorite, and minor amounts of epidote, almandine, apatite, ilmenite, titanite, and scapolite. Mineral assemblages including quartz, chlorites, epidote, muscovite point to crystallization in the greenschist facies around ca 300°C (Palin, 2020).

Preliminary FI data are collected from primary FIs occurring in vein quartz  from the Souanyes and Bélesta areas. FIs from Souanyes are two-phase (liquid + vapor with constant ratio) aqueous inclusions with high salinity (26.0 NaCl_eq). FIs from Bélesta are aqueo-carbonic multi-phase inclusions (liquid water + gaseous phase, usually supercritical at room temperature ± a cubic salt ± a carbonate, measured using Raman spectroscopy). During microthermometric measurements, these FIs show metastable behavior (e.g., lacking salt re-nucleation after melting) or experienced post-trapping modifications (salt precipitation after cooling) that prevent to obtain an accurate salinity. However, a salinity of ca. 26.3 wt% NaCl_eq can be deduced. The lack of freezing of the gaseous phase during cooling reveals the presence of contaminant gas (N₂, measured using Raman spectroscopy) within CO₂.

In summary, most quartz veins in the Agly massif formed during the Cretaceous, which is consistent with recent thermochronology (Odlum & Stockli 2019). The vein mineralogy points to emplacement in the greenschist facies of high-salinity aqueous fluids, locally with CO₂ and N₂.

• Odlum, M., Stockli, D.F. (2019) Thermotectonic evolution of the north Pyrenean Agly Massif during early Cretaceous hyperextension using multi-mineral U-Pb thermochronometry. 38: pp. 1509-1531. John Wiley & Sons Ltd.
• Palin, R.M. (2020) Metamorphism of pelitic (Al-Rich) rocks. In module Earth Systems and Environmental Sciences, 2: pp. 1-12. Elsevier Inc. 
• Siron G, Goncalves P, Marquer D, Pierre T, Paquette J-L, Vanardois J. (2020) Contribution of magmatism, partial melting buffering and localized crustal thinning on the late Variscan thermal structure of the Agly massif (French Pyrenees). Metamorph Geol. 38: pp. 799-829. John Wiley & Sons Ltd.

How to cite: Chalid, I., Kuiper, K., Kriegsman, L., Ferrando, S., Brouwer, F., and Wijbrans, J.: Quartz vein formation in the Agly massif, French Pyrenees: 40Ar/39Ar dating, mineral chemistry, and fluid inclusion study., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5808, https://doi.org/10.5194/egusphere-egu23-5808, 2023.

Water contamination caused by the overuse of fertilizers has become a concern in many areas throughout the last decades. The intensive use of phosphate fertilizers has led to high concentrations of phosphates in ground waters and effluents, but also to high levels of other toxic elements, especially cadmium. Cadmium can be found in high concentrations in phosphate rocks which are used to synthesize fertilizers, resulting in high concentrations of cadmium in some fertilizers that are then used on fields. Various materials have been studied for cadmium capture in solution and both calcium carbonate and apatite have shown good uptake capacities toward this element. Furthermore, calcium carbonate minerals can be replaced by apatite through a pseudomorphic dissolution-precipitation mechanism when immersed in a solution containing phosphate (Jonas et al., 2014; Klasa et al., 2013; Pedrosa et al., 2016; Wang et al., 2012). Here, we report on the capture of cadmium from solution during the replacement reaction of Carrara marble by hydroxyapatite (Wang et al., 2019). Cubes of Carrara marble have been reacted in sealed hydrothermal reactors at 200°C in solutions containing various concentrations of phosphate and cadmium for times between 4 and 60 days. The samples were then sectioned and analysed by Scanning Electron Microscopy (SEM), BackScattered Electron (BSE) imaging, Electron Dispersive X-ray Spectroscopy (EDS) and Raman Spectroscopy. The nanoscale reaction on the sample surface has been observed with in-situ Atomic Force Microscopy (AFM) in fluid flow and static solutions. The coupled dissolution-precipitation reaction observed and the capture of cadmium by the newly formed phase will be presented.

References:

Jonas, L., John, T., King, H.E., Geisler, T., Putnis, A., 2014. The role of grain boundaries and transient porosity in rocks as fluid pathways for reaction front propagation. Earth and Planetary Science Letters 386, 64–74. https://doi.org/10.1016/j.epsl.2013.10.050

Klasa, J., Ruiz-Agudo, E., Wang, L.J., Putnis, C.V., Valsami-Jones, E., Menneken, M., Putnis, A., 2013. An atomic force microscopy study of the dissolution of calcite in the presence of phosphate ions. Geochimica et Cosmochimica Acta 117, 115–128. https://doi.org/10.1016/j.gca.2013.03.025

Pedrosa, E.T., Putnis, C.V., Putnis, A., 2016. The pseudomorphic replacement of marble by apatite: The role of fluid composition. Chemical Geology 425, 1–11. https://doi.org/10.1016/j.chemgeo.2016.01.022

Wang, L., Ruiz-Agudo, E., Putnis, C.V., Menneken, M., Putnis, A., 2012. Kinetics of Calcium Phosphate Nucleation and Growth on Calcite: Implications for Predicting the Fate of Dissolved Phosphate Species in Alkaline Soils. Environ. Sci. Technol. 46, 834–842. https://doi.org/10.1021/es202924f

Wang, M., Wu, S., Guo, J., Zhang, X., Yang, Y., Chen, F., Zhu, R., 2019. Immobilization of cadmium by hydroxyapatite converted from microbial precipitated calcite. Journal of Hazardous Materials 366, 684–693. https://doi.org/10.1016/j.jhazmat.2018.12.049

How to cite: Julia, M. and Putnis, C. V.: Removal of cadmium from solution during the replacement of calcium carbonate by hydroxyapatite in the presence of phosphate., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6221, https://doi.org/10.5194/egusphere-egu23-6221, 2023.

Fluid-rock interaction relies on the fluid’s ability to migrate through rocks, utilising permeable pore space. While we understand permeability in rocks that interact with fluids to evolve dynamically, e.g. in dehydration or carbonation reactions, we have very little quantitative information on these dynamics, as direct measurements of permeability in reacting rocks are inherently difficult.

Here, we present a series of permeability measurements that capture the evolving fluid transport properties of dehydrating gypsum samples. To derive these measurements, we used an X-ray transparent deformation rig to document gypsum dehydration in 4-dimensional µCT datasets and then modelled the permeability evolution for a segmented sub-volume numerically. In doing so, we were able to characterise the grain-scale porosity and permeability evolution of a dehydration reaction for the first time. We present analyses from two experimental time-series run at a fixed confining pressure, temperature and pore fluid pressure (Pc = 20 MPa; T = ~125 C; Pf = 5 MPa) but contrasting stress states: one with the largest principal stress (Δσ = 16.1 MPa) parallel to the sample cylinder axis and another the largest principal stress (Δσ = 11.3 MPa) being radial. In both cases, as pore space formed due to the negative change in the solid molar volume during the reaction, permeability evolved and increased congruently with porosity in time until ultimately reaching average values of 3.14E-13 m² and 4.55E-13 m², respectively. A clear spatial heterogeneity of fluid flow develops at the grain-scale along with the fabrics in the samples. Importantly,  the calculated permeability tensors are anisotropic from the onset, but  develop over different spatiotemporal trajectories and have different preferred orientations in the two experimental geometries: If the anisotropy is expressed as 1-(min_eigenvalue/max_eigenvalue) of the permeability tensor (where isotropy = 0), then the experiment with the largest principal stress applied radially has a final anisotropy of 0.45, with fluid flow efficiently focussed into a vertical lineation. In the case with an axial largest principal stress, the final anisotropy of permeability is 0.30 with fluid flow being channelled along a foliation that developed orthogonally to σ1.

Our results suggest that the spatial and temporal developments of permeability during a dehydration reaction are controlled by the orientation and relative magnitudes of the principal stresses of a tectonic environment, and that these two parameters exert a strong control on the efficiency of drainage and thus reaction progress. This has consequences for our understanding of fluid movements in thrust tectonics and subduction zones, but also in applications such as the in-situ carbonation of ultramafic rocks.

How to cite: Fusseis, F., Gilgannon, J., Jacob, A., Freitas, D., Rizzo, R., and Butler, I.: Emergent permeability in dehydrating rocks is controlled by the stress state and orientation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7041, https://doi.org/10.5194/egusphere-egu23-7041, 2023.

Abstract:Trapiche describes a gem texture that is characterized by the symmetric six “arms” radiating outward from the cores in gemstone minerals. These is a unique growth pattern, which however is still poorly understood. Here, we document the Trapiche in Trapiche rubies, and discuss formation mechanism of the patterns. The six arms of the Trapiche ruby radiate from a hexagonal core, which separate a single crystal into six growth sectors with internal bandings. Microscopic observations indicate a dendritic growth of the arms. Main branches are dominantly formed by the tube-shaped inclusions, and a part of which exhibit solid minerals, which including graphites, sulfides, calcites. The tube inclusions spatially have a 30° angle to the radiating direction of the arms, and pointing to the direction perpendicular to the hexagonal prism cylindrical {10Ī0}. Our Raman and EPMA analyses suggest that the origin of the ruby may be related to marble. Original source of the inclusions is aluminum-rich fluid with a high amount of CO_2, which originated and evolved from magma. Our detail textural and chemistry on Trapiche ruby may suggest that during the ruby formation, a high crystallization driving force is necessary for arms to be a dendritic pattern which can overcome the growth interface of the bandings.   

How to cite: Cui, X., Zhao, F., and Hou, Z.: Texture formation in Trapiche rubies, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7042, https://doi.org/10.5194/egusphere-egu23-7042, 2023.

Cycling of fluids and their carried metals in the continental crust results in the local enrichment of certain elements, such as Pb, Zn, Cu, at upper crustal levels. Upper crustal ore deposits play a significant role for the advance of our core technologies facilitating communication and transportation. Determining where and how metals are cycled at crustal scale is crucial to infer potential ore deposits.

This study focuses on base metal (Pb, Zn, Cu) quartz vein mineralization in the Moldanubian Domain in central Czech Republic. During the waning stages of the Variscan orogeny, the Moldanubian was affected by MP-HT metamorphism due to underplating of the Saxothuringian Domain and the Brunia microplate [1, 2]. The continuous compressional stress regime led to the collapse and subsequent uplift of the central Moldanubian Domain. This rapid uplift triggered decompressional melting, leading to the formation of a batholith known as the Moldanubian pluton [2]. Collapse occurred along two large scale fault systems perpendicular to the prevailing stress regime [2, 3]. We suggest tectonic movement led to fluid infiltration of the migmatized upper crust preserved as quartz veins with Pb-Zn-Cu mineralization. Once the Moldanubian Domain reached upper crustal levels, rehydration of the rocks and passive enrichment of metals in the fluid occurred.

Petrographic observations show that the composition of fluids changed over time. The first generation of fluids generated translucent quartz with comparable few and small (5 - 30 µm) inclusions whereas the second generation of fluids produced more and bigger (10 - 70 µm) fluid inclusions that incorporate solid phases in 10 % of observed inclusions. The last phases to precipitate in cavities are the base metal sulfides which appear as pyrite, galena, and sphalerite.

Microthermometry data supports a gradual change of fluid composition as first-generation fluid inclusions show NaCl_equiv values lower than 1 wt.-%, while second-generation fluid inclusions are significantly higher in salinity with 3 - 7 wt.-% NaCl_equiv. Raman spectroscopy of fluid inclusions of second-generation quartz show enrichment of CH₄ and N₂ in the gaseous phase, representing a reducing environment. LA-ICPMS data of single fluid inclusions will be used to generate a geochemical fingerprint of the fluids responsible for ore generation.

[1] Schulmann, K., et al., An Andean type Palaeozoic convergence in the Bohemian Massif. Comptes Rendus Geoscience, 2009. 341(2-3): p. 266-286.

[2] Verner, K., et al., Formation of elongated granite–migmatite domes as isostatic accommodation structures in collisional orogens. Journal of Geodynamics, 2014. 73: p. 100-117.

[3] Žák, J., et al., A plate-kinematic model for the assembly of the Bohemian Massif constrained by structural relationships around granitoid plutons. Geological Society, London, Special Publications, 2014. 405(1): p. 169-196.

How to cite: Alt, I., Fusswinkel, T., Wagner, T., Vroon, P. Z., and Brouwer, F. M.: Base metal ore mineralization in the upper crust of the Moldanubian Domain, Bohemian Massif, CZ: generation and source, a question of fluid flow, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7599, https://doi.org/10.5194/egusphere-egu23-7599, 2023.

The oxidation state of sulfur in slab fluids is controversial, with both dominantly oxidized and reduced species proposed. Here we use in situ X-ray absorption spectroscopy analysis of sulfur-in-apatite to monitor changes in the oxidation state of sulfur during high-P metasomatism by slab fluids in the subduction channel. Our samples include a 73 cm continuous transect of reaction zones between a metagabbroic eclogite block and serpentinite matrix from a mélange zone on the island of Syros, Greece. The block core consists of garnet, omphacite, phengite, paragonite, epidote-clinozoisite, and rutile. In this region, apatite is only observed as elongate inclusions in omphacite cores. From the core outwards micas are increasingly replaced by epidote-clinozoisite, garnets are smaller and more frequent, pyrite + bornite is observed as inclusions in recrystallized omphacite, and apatite is increasingly abundant in the matrix and inclusions in garnet. A major transition at 48 cm separates an assemblage of Ca-Na amphibole, omphacite, chlorite, pyrite, and apatite from the inner garnet-bearing eclogite assemblages. Omphacite disappears from the assemblage at ~56 cm and amphibole compositions sharply transition to tremolite at 59 cm. Finally, the assemblage tremolite + talc + pyrite is observed after ~70 cm.

Apatites in the eclogite assemblages exclusively display S⁶⁺ peaks in their absorption spectra. This includes apatite inclusions in omphacite in the least altered lithology, as well as matrix apatite and isolated apatite inclusions in garnet in the outermost metasomatized eclogite zone. In the intermediate pyrite-rich (~1–5 vol %) amphibole + omphacite + chlorite zone, apatite displays a strong S^1- absorption peak in most grains, with rare analyses showing mixed S^1- and S⁶⁺. Finally, apatite in the outermost tremolite-bearing assemblages only displays a S⁶⁺ peak. The pyrite-rich zone at 48 cm occurs at the initial interface between the serpentinite matrix and eclogite block, characterized by a dramatic decrease in Na content and Mg#. Our data suggest that reduction of S⁶⁺ in infiltrating fluids to S^1- in pyrite became focused as Fe diffused across the steep Mg# gradient, resulting in pyrite precipitation. In contrast, S reduction in the Mg-rich tremolite-dominant portions of the transect was limited by a lack of Fe, resulting in low modes of pyrite and fluid buffered S⁶⁺ in apatite. Finally, S⁶⁺-bearing apatite is also observed in reaction zone lithologies from elsewhere on Syros, suggesting our observations are not isolated.

Two important conclusions are drawn from these data and observations: (1) In the case of Syros, slab fluids at eclogite-facies conditions carried oxidized S⁶⁺, and (2) The interaction of these fluids with eclogites composed of ferrous-Fe silicates resulted in extensive sulfide precipitation.

How to cite: Walters, J., Marschall, H., Grützner-Handke, T., Klimm, K., Konecke, B., and Simon, A.: Apatite as a monitor for sulfur redox reactions during fluid-rock interaction in the subduction channel, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7876, https://doi.org/10.5194/egusphere-egu23-7876, 2023.

Dendritic crystallisation is a key geological pattern-forming mechanism, typical of, and recording information about, rapid crystallization events. In this contribution we report on clinopyroxene (Cpx) dendrites in a basaltic rock fulgurite, which formed due to electrical discharge impacting a basaltic rock. Unusually, these dendrites exhibit a curved morphology. The curved, tapering main dendrite branches are up to 50 µm long, range from 3 µm to 100 nm thickness, and are surrounded by several higher orders of branches, which are also curved. The morphological curvature corresponds to lattice rotation, so branches have consistent elongation directions in crystal coordinates. Total rotation exceeds 180° for some branches, with the highest curvature found being 7° per µm. Such “bent” Cpx dendrites have been observed in experiments (e.g. Hammer et al. 2010), but the mechanism of bending was not previously understood.

By combining microstructural observations with crystallographic orientation maps from electron backscatter diffraction analyses of multiple Cpx dendrites, their three-dimensional morphological and crystallographic configuration was reconstructed. Dendrites feature a planar latticework of branches parallel to the Cpx (010) plane. Branches in this plane are elongated either parallel to {001}* (i.e. normal to the (001) plane) or <10-1>, and exhibit strong and weak lattice curvature, respectively. Sprouting out of this plane are branches parallel to {021}* (originating from {001}* branches) and <12-1> (originating from <10-1> branches), both types being weakly curved. Regardless of the crystallographic direction parallel to elongation, all branches exhibit a crystallographic rotation axis parallel to [010] of Cpx. Furthermore, the rotation sense is consistent regardless of elongation direction in crystal or sample coordinates.

The crystallographic control on the sense of bending and on the rotation axis indicates that bending is not caused by sample-scale compositional, thermal, or mechanical gradients. Instead, asymmetric compositional and thermal fields around branch tips are responsible for bending, supported by the fact that compositional gradients exist in the glass surrounding dendritic crystals. The specific cause of bending is inferred to be asymmetric distribution of melt supersaturation at branch tips, resulting from unequal growth rates of different facets. Branch-tip morphology alone poorly explains the constant sense of rotation of all branches, as the sense of morphological asymmetry is unlikely to be consistent for all branch types. The [010] rotation axis implies that lattice rotation is accomplished by incorporation of a single sign of [001](100) edge dislocations, with a maximum inferred density of 2*10¹⁴ m^-2.

This work provides new insights into fundamental processes occurring during rapid crystallization of Cpx and other minerals. Furthermore, microstructural observations suggest that higher degree of undercooling correlates with greater lattice curvature. Bent dendrites may thus encode information about spatial variations in the cooling rate and/or undercooling of samples. Finally, the consistent [010] rotation axis is expected to be preserved during recrystallization, offering a potential way to identify curved dendritic growth stages even after recrystallization.

References:

Hammer et al. (2010), Geology 38:367-370. https://doi.org/10.1130/G30601.1

Griffiths et al. (2022), J Petrol, egac125. https://doi.org/10.1093/petrology/egac125

This contribution was funded by the Austrian Science Fund (FWF): P 33227-N

How to cite: Griffiths, T., Habler, G., Ageeva, O., Sutter, C., Ferrière, L., and Abart, R.: What is the cause of lattice rotation in clinopyroxene dendrites?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9162, https://doi.org/10.5194/egusphere-egu23-9162, 2023.

We recently showed that the dehydration of alabaster, natural gypsum rock with randomly oriented grains, can be accelerated by a factor of two through the application of an elastic differential pre-stress of ~ 5 MPa applied via a uniaxial constant-displacement boundary condition (https://doi.org/10.1038/s43246-021-00156-9). Here, we present a novel series of gypsum dehydration experiments using a new in-situ experimental cell monitored with fast synchrotron transmission small- and wide-angle X-ray scattering (SAXS/WAXS) to investigate if an acceleration of the kinetics also occurs at constant uniaxial stress. Prior to stressing and heating, the loaded sample chamber was flushed with nitrogen to remove atmospheric moisture and finally locked, filled with the nitrogen atmosphere pressurised to 1 bar. Six increasing uniaxial stresses in the interval [0;10] MPa were studied at a dehydration temperature of 142˚C. A strongly nonlinear acceleration of dehydration rate is observed over the studied stress interval. At 10 MPa, the reductions of induction and characteristic time amount to ~60% and ~50%, respectively. 2D SAXS patterns generally evolve from isotropic to highly anisotropic shapes, indicating preferential growth of nano-scatterers. Post-mortem scanning-electron imaging reveals that the phase transformation occurs via pseudomorph replacement. These results are largely consistent with our previous experiments and support the notion that tectonic stresses affect mineral transformation kinetics.

How to cite: Schrank, C., Blach, T., Ji, Y., Vu, P., Wang, X., Jones, M., Kirby, N., Seibt, S., and Regenauer-Lieb, K.: Stress sensitivity of gypsum dehydration kinetics at constant uniaxial stress under dry conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9856, https://doi.org/10.5194/egusphere-egu23-9856, 2023.

We consider a porous medium infiltrated by a reactive fluid which triggers coupled dissolution/precipitation reactions at pore surfaces. To study these processes, we model the porous medium as a system of interconnected pipes with the diameter of each segment increasing in proportion to the local reactant consumption. Moreover, the topology of the network is allowed to change dynamically during the simulation: as the diameters of the eroding pores become comparable with the interpore distances, the pores are joined together thus changing the interconnections within the network. With this model, we investigate different growth regimes in an evolving porous medium, allowing for both erosion and precipitation of the dissolved material.

The interplay of flow, transport and reaction in such a system can give rise to a variety of patterns: from spontaneous channeling to nearly homogeneous transformation of the entire rock matrix into the product phase. Interestingly, even if the product phase has a larger molar volume than the parent phase, clogging in such a system can be avoided, due to the interplay of dissolution and precipitation resulting in the continuous creation of new flow paths. These results can be relevant for the analysis of carbonation reactions, in which an important goal is to avoid clogging of the pore space that can lead to permeability reduction and the overall slowdown of the process.

How to cite: Szymczak, P. and Budek, A.: Channeling, clogging and permeability oscillations: different macroscopic regimes in mineral replacement   , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10432, https://doi.org/10.5194/egusphere-egu23-10432, 2023.

Strontium (Sr) is a common trace element in calcite which is incorporated during calcite precipitation through either inorganic or organic pathways. The ratio of Sr to calcium (Ca) in carbonate rocks and minerals has been widely used in studies of paleoceanography, marine sediment diagenesis, and hydrothermal alteration of oceanic crust. The partitioning coefficient of Sr (K_Sr) describes fundamentally the partitioning of Sr between calcite and aqueous solutions, but is a complicated function of environmental conditions and water composition. It has long been recognized that K_Sr is strongly dependent on the precipitation rate of calcite (R_p), which has been formulated in a surface kinetic model [1] and a subsequent ion-by-ion model [2]. We re-evaluate available experimental data of Sr partitioning in calcite and find apparent dependence of K_Sr on calcite oversaturation and solution pH [3]. An ion-by-ion model is developed that successfully reproduces the observed K_Sr values at given solution chemistry [4]. Our model also reproduces observed K_Sr-R_p relationships at various temperatures of 5, 25 and 40 ^oC. This model provides an opportunity to evaluate effects of past seawater composition on Sr partitioning and their possible roles in reconstructing seawater Sr/Ca ratio in the geological history [5], in using pore fluid Sr concentration to extract sediment-fluid exchange rates in deep sea carbonate sediments [5], and in understanding Sr partitioning in biogenic calcite such as foraminifera.

Reference

[1] DePaolo, D.J., Surface kinetic model for isotopic and trace element fractionation during precipitation of calcite from aqueous solutions. Geochimica et Cosmochimica Acta, 2011. 75(4): p. 1039-1056.

[2] Nielsen, L.C., J.J. De Yoreo, and D.J. DePaolo, General model for calcite growth kinetics in the presence of impurity ions. Geochimica et Cosmochimica Acta, 2013. 115: p. 100-114.

[3] Zhang, S. and D.J. DePaolo, Equilibrium calcite-fluid Sr/Ca partition coefficient from marine sediment and pore fluids. Geochimica Et Cosmochimica Acta, 2020. 289: p. 33-46.

[4] Jia, Q., et al., A model for pH dependent strontium partitioning during calcite precipitation from aqueous solutions. Chemical Geology, 2022. 608: p. 121042.

[5] Zhang, S., R.J. Zhou, and D.J. DePaolo, The seawater Sr/Ca ratio in the past 50 Myr from bulk carbonate sediments corrected for diagenesis. Earth and Planetary Science Letters, 2020. 530: p. 115949.

How to cite: Zhang, S., DePaolo, D., and Jia, Q.: Strontium partitioning in calcite and its controling factors, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10955, https://doi.org/10.5194/egusphere-egu23-10955, 2023.

Clay-rich rocks occur in a wide range of tectonic settings. They are of great interest, for example, for the mechanical properties of shallow subduction zone interfaces, but also for natural barriers in nuclear waste deposits or as subsurface caprocks for CO₂ storage. In contact with a polar fluid (e.g., water), the interaction between clay minerals and fluid can lead to swelling or, under confined conditions, build-up of swelling stress. Many studies have focused on the closure of cracks in clay-rich sedimentary rocks by swelling (also referred to as ’self-sealing’). However, less is known about how water-clay interactions affect the stress state of clay-rich rocks and whether they may induce slip along pre-existing faults. We try to address this knowledge gap in the present study by conducting triaxial shear experiments.

The experiments are performed using oblique saw-cut cylindrical samples, where the top half consists of a clay-rich rock (Opalinus claystone) and the bottom half of a permeable sandstone (Berea sandstone). To estimate the frictional properties of the sandstone-claystone interface, dry experiments are performed at 4 to 25 MPa confining pressure and constant axial displacement of 0.1 mm/min. Fluid injection experiments, where fluids are injected through the permeable footwall sandstone, are performed at 10 and 25 MPa confining pressure, constant piston position (no axial displacement), and an initial differential stress of about 70 % of the expected yield stress. The effect of water-clay interactions on the stress state is estimated by comparing the fluid pressures required to initiate slip when a non-polar fluid is injected (no water-clay interactions are expected) and when a polar fluid is injected (water-clay interactions will occur). In some experiments, the sample assemblage is equipped with fiber optics strain sensors glued to the surface of the sample to distinguish between (poro)elastic deformation of the matrix, deformation due to water-clay interaction, and elastic relaxation due to slip along the saw-cut.

For fluid injection experiments with a non-polar fluid (decane), the mechanical data indicate that slip along the saw-cut occurs at fluid pressures close to what is expected based on the friction slip envelope determined for the dry state. For fluid injection experiments with a polar fluid (deionized water), a differential stress drop already occurs when the water initially reaches the sandstone-claystone interface at ambient fluid pressure (0.1 MPa), which is not expected based on the dry friction slip envelope. The fiber optics strain sensor data indicate that swelling of the claystone is followed by a microstructural collapse before slip along the saw-cut likely occurs. In summary, our data suggest that water-clay interactions may initiate slip due to (1) the alteration of the friction slip envelope, (2) build-up of swelling stress, and (3) collapse of the claystone microstructure. However, to what extent these three mechanisms contribute to the according differential stress drop requires further research.

How to cite: Rast, M., Madonna, C., Selvadurai, P. A., Wenning, Q., Ruh, J. B., and Salazar Vásquez, A.: Fault slip in clay-rich rocks due to water-clay interactions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11542, https://doi.org/10.5194/egusphere-egu23-11542, 2023.

The Yuli belt in eastern Taiwan attests to tectonic evolution involving subduction metamorphism because two metamélange units contain high-pressure (HP) blocks or layers of metaigneous rocks and serpentinites enclosed in metasedimentary schists. Metasomatic reaction zones occur locally along the contact between serpentinite (SP) and pelitic schist (PS). In the Tsunkuanshan area, we recognized five metasomatic zones. From PS to SP, the dominant minerals in each zone are: (I) albite, chlorite, phengite; (II) albite, amphibole, biotite, stilpnomelane; (III) chlorite, phengite, albite, epidote; (IV) epidote, chlorite, albite; and (V) talc, chlorite. Minor garnet and glaucophane are present in zone I and II, respectively. Field and petrographic observations combined with whole-rock major elements data suggest that this rock association was formed by diffusive exchanges between the PS and SP. Zones I - IV and PS samples show identical rare earth element (REE) patterns, indicating that these zones are of PS protolith. Hence, the original boundary between the PS and SP is likely in the zone IV and V. The isocon method was applied to quantify the mass balance among the metasomatic zones. The result shows mass changes in zones: I: +12%; II: -4%; III: +50%; IV: +56%. Enrichment of Ca is present in the entire reaction zones, especially the zone IV (up to 91%), whereas Na is only enriched within the zone I (~80%) and II (~89%). These enrichments are likely due to the involvement of external fluids. The formation of these metasomatic rocks was not only controlled by diffusive exchanges between PS and SP, but also by the Na and Ca rich fluid infiltrations. The existence of glaucophane within the zone II indicates that the metasomatism occurred under HP metamorphic conditions in a paleo-subduction interface.

Keywords: Fluid-rock interactions, isocon method, Na and Ca rich fluids, high-pressure, Yuli belt.

How to cite: Dewangga, D. D., Tsai, C.-H., Lee, H.-Y., Lo, W.-H., Iizuka, Y., and Lee, C.-Y.: Mass transfer between serpentinites and metapelites in a paleo-subduction interface: a case study from the Yuli belt, eastern Taiwan, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11811, https://doi.org/10.5194/egusphere-egu23-11811, 2023.

Pervasive carbonation of serpentinized peridotite to carbonate-quartz rock (listvenite) due to infiltration of CO2-bearing fluid is a remarkable process because it can be geologically fast and it increases the rock’s carbon content from initially zero to > 30 wt% CO2. This pervasive conversion is related to an overall solid volume increase while at the same time requiring high time-integrated fluid rock ratios with permeability and diffusivity on all scales. Thus, porosity has to be created dynamically during reaction progress as otherwise fluid pathways become clogged by the reaction products carbonate and quartz, which is one of the major obstacles for artificial carbon storage by peridotite carbonation. Processes that can renew porosity and permeability during carbonation are fracturing and veining – in response to tectonic stress [1] or induced by reaction and crystal growth [2], or a combination of both –, reaction-enhanced ductile deformation [3,4], and spatial decoupling of dissolution and precipitation with solute transfer at different length scales.

Using SEM, EBSD, TEM and FIB nano-tomography, we investigated the microstructural record of local solute transfer and its role for porosity renewal in natural carbonate-bearing serpentinites, transitional serpentine-carbonate-quartz assemblages and listvenites from the Samail Ophiolite, Oman. The clearest indicators of pervasive replacement accommodated by local solute transfer are pseudomorphic replacement structures where carbonate and quartz occur in distinct microstructures corresponding to different inherited peridotite and serpentinite textures. A common pseudomorphic replacement structure in the Samail carbonated peridotite are quartz-fuchsite intergrowths replacing bastite (pseudomorphs after orthopyroxene) in listvenite with harzburgite protoliths. A local strong crystallographic alignment of quartz in each of the bastite/pyroxene pseudomorphs suggests that the anisotropic porosity structure of bastite serpentine favored oriented, epitaxial growth of quartz. Transitional serpentine-carbonate-quartz assemblages show that the first quartz and carbonate generations precipitate coeval but spatially separated, with distinct crystal habits. FIB nano-tomography, STEM analysis and high resolution SEM on ion-polished samples of a transitional serpentine-dolomite-quartz rock from the carbonation reaction front show nano-porous fluid channels in fibrous serpentine at high angle to a highly serrated carbonate-serpentine replacement contact. These nano-scale fluid channels facilitated bidirectional mass exchange of Ca, C, Mg and Si bearing solutes between sites of preferential replacement, such as the cores of serpentine mesh textures, and larger-scale permeability networks along veins and fractures. These observations imply that massive and pervasive solute transfer through the reacting serpentine matrix is possible on a small scale, without clogging of porosity by immediate co-precipitation of quartz or Mg-silicates. Spatial decoupling of dissolution and precipitation was likely caused by the dynamically evolving composition of the reacting fluid and/or due to the influence of differential stress and volumetric strain – a mechanism that can compensate on a local scale for the volume expansion expected of isochemical carbonation reactions.

[1] Menzel et al., Solid Earth, 2022; https://doi.org/10.5194/se-2021-152

[2] Kelemen & Hirth, EPSL, 2012; https://doi.org/10.1016/j.epsl.2012.06.018

[3] Menzel et al., Nature Communications, 2022; https://doi.org/10.1038/s41467-022-31049-1

[4] Kelemen et al., JGR, 2022; https://doi.org/10.1029/2021JB022352

Funding: Junta de Andalucía (Postdoc_21_00791); DFG grants UR 64/20-1, UR 64/17-1; and EU Horizon 2020 Transnational Access EXCITE _C1_2022_34.

How to cite: Menzel, M. D., Urai, J. L., Plümper, O., Ohl, M., and Schwedt, A.: Multiscale solute transfer and porosity evolution during pervasive replacement of serpentine by carbonate and quartz - insights from the Oman ophiolite, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11886, https://doi.org/10.5194/egusphere-egu23-11886, 2023.

To evaluate fluid-rock interaction in subducted sediments buried down to seismogenic depths (250-330℃), we describe mineral reactions along the prograde path and their influence on the fluid budget, fluid-mobile element concentrations, and δ⁷Li of fluids. We focused in particular on metapelitic rocks from two paleo-accretionary complexes, the Kodiak accretionary complex, USA, and the Shimanto belt, Japan.

In metapelites from Kodiak and Shimanto, illite-to-chlorite transformation is the main mineral reaction in the temperature range from 250 to 330℃. Such reaction requires additional H₂O and the plausible explanation is the consumption of pore water, contributing to the increase in the salinity of the pore fluid from 250 to 330°C, trapped as inclusions in quartz veins. Textural evidences, mineral reactions, and mass-balance calculations suggest that the system behaved as closed in both studied sites in terms of major elements. However, trace elements provide a slightly different picture. Indeed, fluid-mobile elements (FME; Li, B, Rb, Sr, Cs, Ba) indicate opposite trends between the two sites: In Kodiak, FME whole-rock concentrations are preserved from 250 to 330℃, in agreement with FME concentrations in illite and chlorite that suggest redistribution between rock-forming minerals. In contrast, samples from Shimanto show significant loss of all studied FME from 250 to 330℃, reflecting a decrease in the FME content of individual mineral phases.

Further insight into the fluid-rock interactions were provided by the analysis of δ⁷Li both in quartz and its fluid inclusions (FI) by applying crush-leach technique. In Kodiak, the fluid is characterized by relatively higher δ⁷Li than Shimanto (+8.1 to +17.07‰ in comparison to +2.53 to +10.39‰, respectively). Such variations can be accounted for by mineral reactions and lithium concentrations in individual phases. Chlorite is the main host of lithium. In the Kodiak complex, lithium concentrations in chlorite remains statistically identical between 250℃ and 330℃ (⁓240 ppm), whereas in the Shimanto belt significant decrease of lithium is observed in chlorite (from ⁓320 ppm in chlorite at 250℃ down to ⁓120 ppm at 330℃). Hence, the higher δ⁷Li of fluids in Kodiak is explained by the chlorite crystallization as it preferentially consumes ⁶Li and the fluid remains enriched in ⁷Li. Conversely, fluids from Shimanto are isotopically lighter than from Kodiak, consistent with lithium loss in chlorite as temperature increases. Therefore, δ⁷Li of fluids in both Kodiak and Shimanto examples can be accounted for by local redistribution of lithium between reacting phyllosilicates and their isotopic fractionation.

Overall, major elements, FME, and δ⁷Li of fluids point to a local redistribution of elements in the Kodiak complex, suggesting that the system behave as closed, as the studied units are underplated as a part of thick turbiditic sequence far from any large-scale fault zones. In the Shimanto belt, the loss of FME suggest rather the open system opposite to major elements, as they are more sensitive indicators of transfers between rock and fluid. Such opposite trend between Kodiak and Shimanto is largely controlled by (i) the amount of internal strain within the different units and (ii) the proximity to large-scale fault zones.

How to cite: Rajič, K., Raimbourg, H., Richard, A., and Lerouge, C.: Fluid-rock interaction in sediments subducted to the seismogenic zone: Implication from mineral reactions, fluid-mobile elements and δ7Li isotopes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11946, https://doi.org/10.5194/egusphere-egu23-11946, 2023.

Solubility of quartz in aqueous fluids over a wide range of temperatures and pressures is crucial for our understanding of water–rock interaction. Experimental data cover a temperature range of 20 to 1130 °C and pressures from 1 to 20 kbar, and this dataset provides a useful basis for critical comparison and development of thermodynamic models for mineral solubility and aqueous solutes. Thermodynamic models for quartz solubility in pure water (1982-2021) are based on one of the following approaches: (1) successive hydration of solute, (2) correlation with solvent density, (3) virial equation of state for solute-solvent mixtures, or (4) Helgeson-Kirkham-Flowers (HKF) electrostatic equation of state. Predictions from these models generally converge at hydrothermal and supercritical temperatures in the fluid density range of 0.6-1.0 g cm^-3. At high temperatures and fluid densities, the models based on successive hydration or virial expansion loose their physical meaning; the electrostatic approaches tend to overestimate the quartz solubility, although the large number of parameters in the HKF equation of state offers remarkably large calibration flexibility. At low pressures and low fluid densities, the individual approaches diverge: the density and virial models correctly approach the limiting case of ideal gas, but the virial equations of state tend to predict consistently higher solubilities. In this study we develop a more physically rigorous density-based model for quartz and test its performance in low-pressure aqueous fluids. Thermodynamic properties of aqueous solute are formulated as a function of hydration number that is typically variable at very low pressures (< 400 bar). At partial hydration, the thermodynamic properties of species are mainly controlled by enthalpy of stepwise hydration reactions and long-range solute-solvent interactions are minimal. At complete inner-sphere hydration the thermodynamic properties of aqueous species become a combined contribution of unhydrated species properties, mechanic interaction in the hydration sphere and standard-state conversion terms. When complete hydration of a species is achieved, Gibbs energy of species becomes linear with log water density at constant temperature, thus mimicking the linear log K – log density relationship for the mineral solubility equilibrium.

How to cite: Salomone, F. and Dolejš, D.: Quartz solubility in low-density aqueous fluids: evaluation and development of thermodynamic models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12318, https://doi.org/10.5194/egusphere-egu23-12318, 2023.

Geothermal heat is a crucial source of renewable energy. Its present and future exploitation can be enhanced by the understanding of the in–situ structural and mineralogical processes and of how these processes may change the reservoir productivity.  Fossil hydrothermal system may thus be used as analogues to study in-situ fluid-rock interaction processes in active geothermal systems.

We present the results of a structural-mineralogical study carried out in the lithocap of the Allumiere high-sulphidation epithermal system (Tolfa Mountains district, northern Latium, Italy). We integrated measurements of attitude of faults and fractures from field analysis with a virtual outcrop model constructed from drone imagery to model the distribution of major faults at the scale of the entire quarry. We then characterized the textures and mineralogical compositions of the alteration facies using optical petrography, Scanning Electron Microscope (SEM), X-ray diffraction analysis and field-based short-wave infrared (SWIR) spectrometer and we mapped their distribution in relation to major faults orientation.

We interpreted that initial argillization was promoted by circulation of highly reactive fluid(s) along a major fluid conduit, probably in the form of a network of fault and fractures. Fluid circulation promotes hydrolytic alteration of the Plio-Quaternary pyroclastic rocks, forming a highly silicified carapace at the immediate vicinity of conduits. Enrichment in alunite and kaolinite increases towards distal areas, which in turn fades out into illite-smectite-bearing zone, where the country rock appears less altered.  Latest fracturing and fluid circulation occurs along two major sets of faults, oriented NE-SW and NW-SE that sharply put in contact different mineralogical facies.

We propose that initial alteration induces a mineralogical-mechanical zonation that control latest reactivations of the system. Strain localization promotes a massive mineralization of alunite and kaolinite by continuous dissolution and precipitation along major faults.

How to cite: Marchesini, B., Tavani, S., Mercuri, M., Aldega, L., Mondillo, N., Pizzati, M., Balsamo, F., and Carminati, E.: Structurally controlled kaolinite-alunite mineralization in the lithocap of a fossil geothermal system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15099, https://doi.org/10.5194/egusphere-egu23-15099, 2023.

Faults that cut carbonate rocks are the most important seismogenic sources worldwide, while the fault weakening and recovery mechanism in carbonate fault rock still remains controversial. In this study, the structures of an exposed normal fault zone hosted in dolostones with chert bands from the Yuguang basin southern marginal fault (YBSMF), northeast of the Shanxi graben system, North China, was studied by field-based structural analysis, microstructural and fabric investigations. The microstructural observations show that the fault slip surface exhibits a range of slip-related structures including slickenlines, truncated clasts and nano-scale amorphous materials/fragments. On the fault slip surface, the carbonate fault rock contains a large number of nanoparticles. These nanoparticles were shaped into two forms, single spherulitic nanoparticle and agglomerated nanoparticles. The slip zone, under the slip surface, is characterized by cataclasite, with various foliatization in red injection band. EBSD analysis results show weak CPOs, with the (0001) planes of the dolomite fragments nearly parallel to the slip surface. Our microstructural investigations in the dolomite fault rocks, combined with previous publications, suggest the single spherulitic nanoparticles can be the result of thermal decomposition of dolomite along the major slip surface of the normal fault. Nano powder lubrication caused by the rolling of single spherulitic nanoparticle plays a key role during carbonate fault slipping. The thermal pressurization of pore fluid leads to laminar grain flow along the fracture and finally forms foliations in the red injection layers. The transformation from single spherulitic nanoparticles to agglomerated nanoparticles by superplastic diffusive mass transfer results in the recovery of friction strength at the fault plane. After the coseismic slip (or during afterslip), a relatively thick portion in the principal slip zone suffers a temperature increase, leading to the plastic deformation and formation of CPOs of dolomite in the principal slip zone. We inferred that nanoparticles can be produced by thermal decomposition transformation, which facilitate and inhibit earthquake behavior on fault surfaces. The postseismic strength recovery can be generated partly by agglomerated nanoparticles. We consider that nanoparticles produced by thermal decomposition of dolomite play a key role in carbonate fault instability during coseismic slip.

How to cite: Huang, J., Zhang, B., Zou, J., and He, H.: Fault instability and slipping in thermally unstable rock, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15834, https://doi.org/10.5194/egusphere-egu23-15834, 2023.

Melt production and migration are important phenomena in the lower and middle crust. The presence of melt in a volume of rock has a significant impact on its rheology and on the structure of the Earth’s crust in general. Once partial melting starts, the molten portion can either flow towards shallower crustal levels or stay in the area where it originated. A partially molten rock can eventually solidify and be brought to the Earth’s surface, where the distribution of the former melt can provide insight into the conditions in which the rock formed.

Here we present a set of numerical experiments utilising an innovative DEM-continuum model to simulate partial melting and the initial formation of porosity channels. This hybrid model incorporates porous flow to solve for pervasive percolation of melt and a network of springs on a second grid to represent the linear elastic behaviour of the host rock. The latter also includes phenomena such as fracture formation and propagation.

We show under which conditions melt-filled fractures can emerge from local areas of melt production and evolve into larger melt channels. Importantly, our models show that leucosome patterns seen in outcrops can be used to constrain the relative rates of melt diffusion along grain boundaries, syn-migmatisation deformation, and local melt production. Based on the relative rates of these processes, we define regimes with specific melt pattern distributions. The definition of these regimes can be a useful tool for the interpretation of the history of a migmatite.

How to cite: Fedrizzi, G., Piazolo, S., Koehn, D., and Pegler, S.: The Use of Leucosome Patterns in Migmatites to Decipher Rates of Melt Production, Melt Percolation and External Deformation: Insights from Numerical Modelling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16256, https://doi.org/10.5194/egusphere-egu23-16256, 2023.

The Śnieżnik Massif forms the eastern part of the Orlica-Śnieżnik Dome (OSD), located in the north-eastern part of the European Variscan Belt. The OSD, which exposes the root zone of the Variscan Orogen, comprises mostly orthogneisses containing small bodies of ultra-high pressure (UHP) eclogites. Previous studies on the metamorphic conditions recorded by these eclogites yielded inconsistent results. Some authors suggest that they were metamorphosed in conditions of ~1.9-2.2 GPa and ~700-750 °C ¹. Others, however, argue that the eclogites experienced nearly-UHP peak metamorphic conditions of ~2.6-3.0 GPa and 800-930 °C.²

This study provides the first evidence of UHP metamorphic episode recorded in eclogites from the OSD, as coesite inclusions were discovered in garnet and omphacite grains. This finding is consistent with our results obtained using Grt-Cpx-Ky-Ph-Coe/Qtz geothermobarometry and phase equilibria modelling, which both indicated conditions of peak metamorphism of ~2.8 – 3.2 GPa and ~830-870 °C, partially overlapping the coesite stability field.

We also applied quartz-in-garnet elastic barometry to provide additional constraints on the pressure conditions of metamorphism. About 60 inclusions of quartz were identified using Raman spectroscopy. The residual pressure calculated from the spectral shifts of 464 cm^-1 characteristic quartz Raman band reaches a maximum of ~0.73 GPa. This corresponds to the entrapment pressure of ~2.1 GPa, calculated based on the elastic solution for an isotropic spherical inclusion. This estimation contradicts the results coming from methods based on equilibrium thermodynamics. Moreover, such low peak pressure would not explain the presence of the observed coesite inclusions. We hypothesize that the discrepancy might be related to viscous relaxation of garnet host grains under such high peak metamorphic temperatures.

References

[1]     Štípská, P. et al. The juxtaposition of eclogite and mid-crustal rocks in the Orlica-Śnieżnik Dome, Bohemian Massif. J. Metamorph. Geol. 30, 213–234 (2012).

[2]     Majka, J. et al. Integrating X-ray mapping and microtomography of garnet with thermobarometry to define the P-T evolution of the (near) UHP Międzygórze eclogite, Sudetes, SW Poland. J. Metamorph. Geol. 37, 97–112 (2019).

How to cite: Nowak, M., Szczepanski, J., and Dabrowski, M.: Discrepancy between equilibrium thermodynamics-based P-T calculations and quartz-in-garnet elastic barometry in coesite-bearing eclogite (Śnieżnik Massif, NE Bohemian Massif), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-785, https://doi.org/10.5194/egusphere-egu23-785, 2023.

Deformation, chemical reactions, fluid flow in geological formations, and many engineering materials, such as cement, are coupled processes. Most existing models of chemical reactions coupled with fluid transport assume the dissolution-precipitation process or mineral growth in rocks. However, dissolution-precipitation models predict a very limited extent of reaction hampered by pore clogging and blocking reactive surfaces, which will stop reaction progress due to limited fluid supply to reactive surfaces. Yet, field observations report that natural rocks can undergo 100% hydration/carbonation. Mineral growth models, on the other hand, preserve solid volume but do not consider its feedback on porosity evolution. In addition, they predict an unrealistically high force of crystallization on the order of several GPa that must be developed in minerals during the reaction. Yet, experiments designed to measure the force of crystallization consistently report values on the order of hundreds of MPa, which is close to the failure limits for most rock types. Recent experimental and observational studies suggest that mineral replacement is a coupled dissolution-precipitation process that preserves porosity and is associated with the change in the solid volume. Volume change associated with chemical reactions has multiple practical implications. It might be hazardous, causing damage to building materials or deterioration of caprock permeability and leakage of waste fluids, at least along the injection wellbore. Or it might be useful. For example, reaction-driven mineral expansion associated with the hydration of some solid additives may be utilized in plugging and abandonment of old petroleum wells to prevent leakage between plug and caprock or between plug and casing. In a geological context, mineral expansion plays an important role in pseudomorphic replacement and vein formation. Here, we propose a new model for reaction-driven mineral expansion, which preserves porosity and limits unrealistically high build-up of the force of crystallization by allowing inelastic failure processes at the pore scale. First, we look at fluid-rock interaction at the pore scale and derive effective rheology of a reacting porous media. We use a two-phase continuum medium approach to investigate the coupling between reaction, deformation, and fluid flow on a larger scale. Our micromechanical model based on observations assumes that rock or cement consists of an assembly of solid reactive grains, initially composed of a single, pure phase. The reaction occurs at the fluid-solid contact and progresses into the solid grain material. We approximate the pores and surrounding solid material as an idealized cylindrical shell to simplify the problem and obtain tractable results. We derive macroscopic poroviscoelastic stress-strain constitute laws that account for chemical alteration and viscoelastoplastic deformation of porous rocks. Our model explains many experimental observations on natural and engineering geomaterials, such as the possibility of achieving a complete reaction, preservation of porosity during chemical reactions, moderate values of the force of crystallization, and dependence of mechanical rock properties on fluid chemistry.

How to cite: Yarushina, V., Podladchikov, Y., and Wang, H.: Effective rheology of (de)compacting reactive porous media, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3323, https://doi.org/10.5194/egusphere-egu23-3323, 2023.

The interpretation of phase equilibria and reactions in geological materials is underpinned by standard thermodynamics that assumes that the stress in the systems is hydrostatic and homogeneous (i.e., the same for all the phases involved). However, stress gradients and non-hydrostatic stresses are common in rocks, and can be developed even in porous systems with coexisting solid minerals and fluids. In rocks with interconnected porosity, a fluid will always experience a hydrostatic stress gradient. On the contrary, the solid grains will experience different levels of stress due to the changes in the contact area between the grains. Therefore, rocks that are porous or have a granular structure will always experience stress gradients at the small scale, even if their macroscopic stress state is “lithostatic”.

The presence of a heterogeneous-stress distribution at the grain scale casts doubts on the predictive power and accuracy of existing multiphase thermodynamic models. However, currently there is still not an accepted theory which extends thermodynamics to include the effect of non-hydrostatic stress on reactions, and the use of several thermodynamic potentials in stressed geological system is still debated (e.g. [1-3]). Even experiments have not been conclusive, because the direct effect of the applied non-hydrostatic stress on the thermodynamics of the reactions cannot be separated from the indirect effect caused by local stress concentrations [4].

We have investigated the problem of the direct effect of a homogeneous non-hydrostatic stress on the solid-fluid equilibrium with molecular dynamics simulations. With such simulations the energy of the system, the pressure of the fluid and the stress of the solid can be monitored until the stressed system reaches the equilibrium conditions. Our results show that for simple systems at the stress range expected in the lithosphere, the shift of the pressure and temperature of the fluid-solid equilibrium is small for geological applications, consistent with theoretical predictions [5,6]. On the contrary, the mean stress of the solid is largely affected by the applied non-hydrostatic stress and can deviate substantially from the pressure of the fluid. These results suggest that hydro-mechanical-chemical models should not use the pressure of the fluid as a proxy of the mean stress of the solid, and therefore should not equate the thermodynamic pressure of the reaction to the mean stress of the solid. However, our analysis does not take into account the indirect effect of stress heterogeneities at the sample scale. Spatial variations of stress can reach GPa level and can therefore indirectly affect phase equilibria.

MLM is supported by an Alexander von Humboldt research fellowship.

References

[1] Wheeler, J. Geology 42, 647–650 (2014);

[2] Hobbs, B. et al. Geology 43, e372 (2015);

[3] Tajčmanová, L. et al. Lithos 216–217, 338–351 (2015)

[4] Cionoiu, S., et al. J. Geophys. Res. Solid Earth 127, e2022JB024814 (2022)

[5] Sekerka, R. et al. Acta Mater., 52(6), 1663–1668 (2004)

[6] Frolov, T. et al. Phys. Rev. B Condens. Matter Mater. Phys. 82, 1–14 (2010)

How to cite: Mazzucchelli, M. L., Moulas, E., Kaus, B., and Speck, T.: Fluid-mineral equilibrium under stress: insight from molecular dynamics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3681, https://doi.org/10.5194/egusphere-egu23-3681, 2023.

Minerals and multiphase rocks in general may have non-trivial material models (constitutive relations) with respect to their volume change as a response of changing pressure and temperature (P-T) conditions. However, natural minerals within rocks do not freely expand/contract. When mineral phases are enclosed by phases that have different thermoelastic properties, a difference in volumetric strain develops upon the loading/unloading of the host-inclusion system. The difference of the volumetric strain between the two phases can lead to the significant stress build up in the vicinity of the host-inclusion interface. This behavior is in fact expected in geological scenarios where mineral reactions and phase transitions are responsible for significant volumetric changes. One of the most classical problems in elasticity theory is the Lame problem of an internally and externally pressurized thick cylinder. When adapted for spherical symmetry, this problem has been extensively used in geological applications in order to evaluate the stress distribution around a pressurized rock or mineral. Using linear elasticity theory and standard mineral properties it can be shown that the level of stresses that can develop around pressurized inclusions may be in the order of ~ 1 GPa. Such stress predictions are well beyond typical values of the yield stress of rocks which leads to large plastic deformations. Therefore, the incorporation of plasticity and finite strains is crucial in such models.

Here we present new analytical and numerical solutions for the classic host-inclusion problem assuming hyperelastic-plastic materials that follow a Drucker-Prager (non-associative) plasticity model under finite strains. Our analytical solution is based on the recently published solution of Levin and others (2021) that reduces to the Murnaghan model for purely hydrostatic loading. Our solutions have been developed to consider the effects of physical and geometrical non-linearities in deforming geomaterials. For stiff mineral hosts that can support GPa-level differential stresses, non-linear formulations provide accurate stress predictions even if the effects of geometrical non-linearities are ignored. For systems that reach the plastic yield, a plastic zone develops that can lead to the reduction of the pressure difference between the host and the inclusion phase. Nevertheless, the development of a plastic zone is occurring simultaneously to the development of pressure variations at the mineral hosts. Therefore, the development of pressure gradients in host-inclusion systems from the mineral to the outcrop scale are to be expected when the host material reaches the yield conditions.

Acknowledgments:

E.M. would like to acknowledge the Johannes Gutenberg University of Mainz for financial support. Y.P., K.Z., A.V. and V.L. were financially supported by Russian Science Foundation (project No. 19-77-10062) in the part related to the geomechanical problem statement and its analysis, and by Ministry of Education and Science of Russian Federation (grant №075-15-2019-1890) in the part related to the development of analytical and numerical algorithms for problem solving.

How to cite: Moulas, E., Vershinin, A., Zingerman, K., Levin, V., and Podladchikov, Y.: Numerical and analytical solutions for the large-strain elastoplastic Lame problem and its geological applications, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3909, https://doi.org/10.5194/egusphere-egu23-3909, 2023.

Despite the occurrence of high-grade metamorphic rocks next to and along crustal-scale shear zones, the temporal character of their formation and evolution is difficult to extract. We utilize the major-element diffusion in the compositional re-adjustment of garnet from metapelites in two crustal-scale shear zones as a complementary method to extract cooling rates from deforming/reacting rocks. The two thrust zones, the Nestos Thrust Zone (NTZ) in Rhodope, Greece, and the Main Central Thrust (MCT) in Sikkim, Himalaya, exhibit inverted metamorphic zonation. We applied phase equilibria modelling and geothermometry to constrain the peak- and the post-peak-temperature conditions relevant for the cooling-rate estimates. Results are 50–80 ◦C/Myr in the footwalls of both thrust zones, in consistency with published estimates using geochronology methods for MCT. However, results are much less (~0.5–5◦C/Myr) for the base of the MCT hanging wall. The estimated cooling rates are between 300 and 2500 ◦C/Myr for the NTZ hanging wall. The exceedingly fast cooling rates indicate the operation of transient and proximal thermo-mechanical processes consistent with the contribution of thrust related viscous heating during metamorphism. The very slow cooling rate of the MCT hanging wall may reflect a complex thermal history or other overlooked processes.

References:

Burg, J.-P., Moulas, E., 2022. Cooling-rate constraints from metapelites across two inverted metamorphic sequences of the Alpine-Himalayan belt; evidence for viscous heating. Journal of Structural Geology 156, 104536. https://doi.org/10.1016/j.jsg.2022.104536

How to cite: Burg, J.-P. and Moulas, E.: Cooling-rate constraints from metapelites across two inverted metamorphic sequences of the Alpine-Himalayan belt; evidence for viscous heating, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4185, https://doi.org/10.5194/egusphere-egu23-4185, 2023.

Quartz is a common constituent of most rocks in the Earth continental crust and it undergoes the α-β transition at depths controlled by the geotherm. Despite the α-β quartz transition representing one of the most well-known and largely studied phase transitions in geological sciences, only few works report the behaviour of this transformation at high pressure (i.e. in the relevant conditions of the deep crust). Hence, it is important to investigate this transformation through an experimental approach at lower-crust pressure and temperature (P-T) conditions.

In this study, we performed deformation experiments at high P-T conditions on novaculite (quartzite) samples using a Griggs apparatus equipped with acoustics and a multi-anvil press at the European Synchrotron Radiation Facility (ESRF, beamline ID06). Experiments were performed at 1-3 GPa and up to 1000°C.

Measurements in the Griggs apparatus indicate that the expected P-wave velocity increase in the β-field is not observed at high pressure. Diffraction data from ESRF show that the transition becomes smoother at high pressure and results in a smaller crystal lattice change than it does at low pressure, consistently with the P-wave velocity measurements in the Griggs apparatus.

In addition, on the temperature-up path we are able to observe the expected negative thermal expansion of β-quartz but, interestingly, this behaviour is not visible on the cooling path. As a possible explanation, we suggest a competing effect of stress and temperature on the crystal lattice parameters. Moreover, at the transition, in a short temperature range, the intensity of quartz diffraction peaks decreases significantly. Acoustic measurements seem to indicate that this could be also related to a transient increase in attenuation. Further experiments will be performed at the ESRF coupling X-ray diffraction and acoustic measurements to assess the relationship between crystal structure and Vp changes.

Our results question the interpretation of seismic contrasts in the deep crust as due to the α-β quartz transition. However the existence of a high attenuation region might reflect the presence of this transformation.

How to cite: Mingardi, G., Gasc, J., Moarefvand, A., Crichton, W. A., and Schubnel, A.: Alpha-Beta quartz transition in the lower continental crust: perspective from diffraction and acoustic data at high P-T conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6362, https://doi.org/10.5194/egusphere-egu23-6362, 2023.

We propose a 3D model for pipe and chimney formations in tight rocks in sedimentary basins. It is an adaption of a model for hydraulic fracturing in an anisotropic stress field by fluid injection (fracking). The trigger for chimney formation is high overpressure in permeable units, such as reservoirs or aquifers. The permeable units serve as a source of high-pressure fluid that drives the chimney formation. The numerical model is based on cells that “fracture” when the fluid pressure exceeds the least compressive stress and random rock strength. The locally highest points in the reservoir rock become the most likely places for chimney formation. Fracturing implies that cells have their permeability changed from their initial value to a value that represents an average permeability of an open fracture network. Chimney growth appears as chains of cells (branches) emanating from the base of the cap rock. These chains of cells grow towards the surface. The branches have an enhanced permeability during ascension because the fluid pressure in the fracture network is greater than the least compressive stress. The fluid pressure keeps the fracture network open. When the branches reach the hydrostatic surface, the fluid pressure drops below the least compressive stress and the fracture network closes. The model produces pipe structures and chimneys as accumulations of branches that reach the surface. The degree of random rock strength controls how pipe-like the chimneys become. The chimney, which is formed by branches of the fractured cells, drains the reservoir for overpressured fluid. Chimney formation stops when the overpressure in the reservoir is reduced below the least compressive stress at the base of the caprock. The fracture permeability of the chimney branches controls how many branches are produced, and thereby how wide the chimney becomes. A “low” permeability produces wide chimneys with many branches, and a “high” permeability gives narrow chimneys made of just a few branches. The model is demonstrated in a setting similar to the chimneys observed in the cap rock over the Utsira aquifer in the North Sea. By using the proposed model, the permeability of such chimneys is estimated to be of the order of 10 micro-Darcy.

How to cite: Wangen, M., Wang, H., and Yarushina, V.: A 3D numerical model for chimney formation in sedimentary basins, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7206, https://doi.org/10.5194/egusphere-egu23-7206, 2023.

Transient creep of the lower crustal minerals such as feldspar is important to explain postseismic deformation following a large continental earthquake. However, transient creep of feldspar is poorly understood and the flow law parameters are unknown so far. Therefore, we performed constant strain rate deformation experiments on synthetic fine-grained anorthite aggregates under wet conditions using a Paterson-type gas deformation apparatus. We conducted tests at temperatures from 1000 ºC to 1200 ºC and confining pressure of 400 MPa. Typical strain rates in our experiments were 1x10^-4 s^-1, 2.5x10^-4 s^-1, 5x10^-4 s^-1, and 7.5x10^-4 s^-1, including some strain rate stepping experiments. In general, the transient creep accounted for 6-8% of the total strain (~10-15%), which is high compared to 2-3 % transient deformation observed in previous experiments on anorthite, quartz, and olivine aggregates. Inspection of the microstructures of deformed samples using transmission electron microscopy reveal dislocation activity and antiphase domain boundaries. Analysis of steady-state creep data indicates that the samples were deformed at the boundary between diffusion and dislocation creep with a power law stress exponent of ~1.4 and an activation energy of 272 kJ/mol. Because a constitutive equation for transient creep of feldspar is not well established, we estimated transient creep flow law parameters using inter-granular and intra-granular models. In the intergranular model for a polycrystalline aggregate, where grains are randomly oriented,  it is assumed that low strain (i.e., transient creep) is accommodated by individual grains with soft/easy slip orientation and high strain (steady-state creep) is accommodated by grains with hard/strong slip orientation. In contrast, in the intra-granular model, both transient creep and steady-state deformation are dominated by intragranular processes, such as long-range elastic interactions of dislocations. In the intragranular approach, we find that the full stress vs. strain curve (i.e., including transient and steady-state creep) can be modelled using a stress exponent of ~1.5 and an activation energy of ~200 kJ/mol. Applying the intergranular model, we get a stress exponent of ~3 and an activation energy of ~130 kJ/mol for transient creep of anorthite aggregates. Extrapolated to natural strain rates, these two approaches will have different implications in modelling postseismic deformation.

How to cite: Masuti, S. and Rybacki, E.: Transient rheology of feldspar, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8545, https://doi.org/10.5194/egusphere-egu23-8545, 2023.

Elastic geo-thermobarometry has become an important technique in determining the pressure-temperature (P-T) conditions of entrapment during metamorphism. A prerequisite is that the inclusion’s over- or under-pressure is not reset during exhumation. This would be the case if the host-inclusion pair interacts elastically only, which is an oversimplification. It is thus not yet been fully understood how fast the inclusion pressure may become reset. In this study, we performed heating experiment on an almandine-rich (from an eclogite) and a spessartine-rich garnet (gem-stone) under 1) graphite (dry), 2) forming gas (5% H₂ and 95% N₂) and 3) water vapour (wet) buffered conditions at high T and room P. Raman spectroscopy is used to measure the same quartz and zircon inclusions at room T before and after different heating times. In wet and forming gas conditions, the Raman band wavenumber changes are dependent on time, decreasing for quartz and increasing for zircon inclusions. Under dry condition, the Raman band wavenumber exhibits a small amount of shift and becomes stable shortly. Raman mappings reveal that the stress heterogeneity of the garnet host develops stronger at the early stage of the wet heating experiments and fade away afterward, potentially indicating a diffusion-like behaviour of the dislocation density. A visco-elastic model is performed to fit the measured data. The calculated flow law parameters of garnet around quartz inclusions is comparable to the flow law extracted from deformation experiments, while zircon shows substantially faster relaxation rate. This study highlights that fluid can be an important trigger for fast viscous relaxation together with temperature, time and inclusion mineralogy. The study may have implications for elastic thermobarometry, garnet rheology, and the preservation of coesite inclusions.

How to cite: Zhong, X., Wallis, D., Kingsbery, P., and John, T.: On the sensitivity of inclusion pressures after entrapment: the drastic effect of aqueous fluid on garnet viscous relaxation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9047, https://doi.org/10.5194/egusphere-egu23-9047, 2023.

Understanding mechanisms leading to volcanic eruptions are of fundamental importance in geology and volcanology. A prerequisite to a volcanic eruption is the generation of sufficient overpressure in a magma reservoir, enough to exceed the strength of the rock, potentially triggering the volcanic eruption. In geological models, the pressure buildup in magma reservoir is often linked to magma recharge and volatile exsolution. Another mechanism, that is often overlooked in conventional geological models, is related to the isochoric rise of gas bubbles in almost incompressible magma saturated with volatiles. Predicting volcanic eruptions using numerical models is complicated by the need to solve coupled physical processes spanning multiple temporal and spatial scales.

We present a coupled thermo-chemo-hydromechanical mathematical model for predicting the pressurization of a magmatic reservoir. The model predicts porous and free convection of partially crystallized magma due to thermal and compositional heterogeneities, and compaction of crystals due to density difference between solid and liquid phases. We describe thermodynamic equilibrium and thermo-mechanical properties of phases using the nonlinear equation of state obtained through direct Gibbs energy minimization. We resolve the multi-scale processes within the magma reservoir using high-resolution numerical modeling based on supercomputing.

We demonstrate through numerical experiments that the two mechanisms, volatile exsolution due to retrograde boiling, and rising of gas bubbles in a  nearly isochoric system, could lead to pressure buildup in a magma reservoir, sufficient to exceed rock strength. We study systematically the relative importance of these mechanisms in a simplified problem setup.

How to cite: Podladchikov, Y., Utkin, I., and Khakimova, L.: Mechanisms of pressure buildup in magma reservoir: insights from numerical experiments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9717, https://doi.org/10.5194/egusphere-egu23-9717, 2023.

Widely accepted model of Fickian linear diffusion of inert or trace-like elements is restricted to ideal solution models of components with equal molar mass. Simultaneous diffusion of multiple concentrations without mechanical stresses is well-described by the classical Maxwell-Stefan model, which is limited to the use of concentration gradients. Quantitative predictions of concentrations evolution in real mixtures should be treated instead by modified Maxwell-Stefan closure relations, which result in a correct equilibrium limit due to the use of the chemical potential gradients instead of concentration gradients. There is no linearity and tensorial homogeneity assumptions on flux-force relationships of classical irreversible thermodynamics. Coupling the multicomponent diffusion to mechanics results in pressure gradients that contribute to the Gibbs-Duhem relationship. Note, it was demonstrated that current models used for describing chemical diffusion in presence of stress gradient don’t remain invariance with respect to the choice of units, such as mole and mass, and the thermodynamic admissibility is doubted [1].

We develop a new thermodynamically admissible model for multicomponent diffusion in viscously deformable rocks. Thermodynamical admissibility of this model ensures non-negative entropy production, while maintaining invariance with respect to the choice of units and reference frame. We demonstrate the correct Fickian limit and equilibrium limit with zero gradients of chemical potentials of individual components instead of concentration gradients in classical Maxwell-Stefan model. The model satisfies conventional Gibbs-Duhem and Maxwell relationships under pressure gradients and represents the natural coupling to the viscous multi-phase models featuring spontaneous flow localization.

For numerical purposes, we develop the optimal pseudo-transient scheme for diffusion fluxes coupled to viscoelastic bulk deformation. This new effective damping techniques are compared to analytical solutions. The developed model is applied for radial garnet growth with multicomponent diffusion under pressure gradient, hydration porosity waves and melt transport in the Earth’s crust.

1. Tajčmanová, L., Podladchikov, Y., Moulas, E., & Khakimova, L. (2021). The choice of a thermodynamic formulation dramatically affects modelled chemical zoning in minerals. Scientific reports, 11(1), 1-9.

How to cite: Khakimova, L., Moulas, E., Utkin, I., and Podladchikov, Y.: Nonlinear Multi-Component Maxwell-Stefan Diffusion Model In Deforming Rocks: Chemo-Mechanical Coupling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12140, https://doi.org/10.5194/egusphere-egu23-12140, 2023.

In the plagioclase-rich lower continental crust, hydrous epidote-group minerals will, among other phases, replace plagioclase in the presence of minor amounts of fluids. It has previously been shown that this reaction has a significant impact on the strength of plagioclase aggregates, with reacting aggregates being much weaker than their unreacted counterparts (Stünitz and Tullis, 2001). Hence, reactions taking place in the lower continental crust may have a strong influence on its deformation behaviour and thus on its strength. Yet, it still remains unclear if the observed weakening is due to the nucleation and growth of inherently weaker product phases, e.g., epidote-group minerals, or due to inhibited grain growth in a polyphase aggregate as a result of Zener pinning. We experimentally investigated the relative strength of pure epidote and pure plagioclase aggregates at a confining pressure of 1 GPa, two different temperatures (550 and 650 °C) and two different strain rates (5·10^-5 and 5·10^-6 s^-1) using a solid-medium Griggs-deformation apparatus. Furthermore, we also investigated potential strength differences due to differences in grain size by deforming aggregates with a grain-size range of either 90-135 μm or <25 μm. After deformation under 650 °C, the epidote aggregates reveal the nucleation and growth of new phases indicating that epidote was no longer stable. The amount of product phases found in the epidote aggregates scales with the duration of deformation. At the explored experimental conditions, the compressive strength of plagioclase and epidote aggregates depends on temperature and strain rate with a decrease in strength with an increase in temperature or a decrease in strain rate. At identical conditions, the epidote aggregates are either significantly stronger or show a similar strength as the plagioclase aggregates. Microstructural analyses of the recovered samples reveal that deformation in both aggregates was almost exclusively accommodated by grain fracturing and occasionally slip along cleavage planes, and remained non-localized except for the epidote aggregate deformed at 650 °C with a strain rate of 5·10^-6 s^-1, exhibiting kinetically-controlled faulting due to reaction.

Stünitz, H. and Tullis, J. (2001). Weakening and strain localization produced by syn-deformational reaction of
plagioclase. International Journal of Earth Sciences, 90(1):136{148.

How to cite: Incel, S., Mohrbach, K., and Renner, J.: Deformation of epidote and plagioclase in the semi-brittle regime, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12371, https://doi.org/10.5194/egusphere-egu23-12371, 2023.

Fluid-rock interaction is one of the most important factors regarding the evolution of the Earth’s crust, as it is strongly affecting its petrophysical properties and enabling chemical transport. Therefore, its impact on the Earth’s crustal chemical reservoirs and geodynamic processes can be significant. Fluid-mediated mineral reactions are dependent on the availability of fluids and their capability to percolate through the rock and interact with the minerals, often through pre-existing fluid pathways.

The Kråkenes Gabbro is a mafic enclave, embedded in the felsic gneisses of the Western Gneiss Region in Norway. Although the whole region reached (ultra-)high pressure metamorphic conditions, the gabbro remained in a metastable state and preserved its igneous textures and magmatic minerals. The dry and low permeability gabbro is cut by a N-S-trending fracture network of mode-I cracks, which opened during exhumation. These fractures served as fluid pathways for an aqueous fluid to infiltrate the rock and trigger mineral reactions. Along these fractures the dry gabbro is “hydrated” under amphibolite-facies conditions. The resulting amphibolite reaction front is sharp on outcrop scale and propagates on dm-scale into the gabbro. A complete profile of rock spanning 32 cm in length was taken perpendicular to the vein, including sample material from the vein, the alteration zone, and the mostly pristine gabbroic wall rock.

The gabbro-amphibolite-transition is displayed by the development of a hydrous mineral assemblage, accompanied with a densification and therefore porosity formation. The main cause of this is a drop in the abundance of plagioclase during the amphibolitization. Thermodynamic analysis using Thermolab were done to predict the amphibolite mineral assemblage from the original bulk rock composition of the gabbro. The calculations reveal that mainly H₂O is added to the system and minor further element transport is needed. Furthermore, we observe that even the most reacted amphibolite still contains unaffected gabbroic mineral relicts and the main chemical reactions during amphibolitization are limited to a few minerals. The incoming fluid is consumed as soon as the hydrous phases of the amphibolite are formed. As amphibolitization favors porosity formation, a free fluid phase remains in the pore space as soon as the gabbro at the reactive surface of the affected minerals is completely transformed. The fluid progresses through the newly formed pore space and advances as a sharp the amphibolitization front.

In order to test our hypothesis, we formulate a reactive flow model based on local equilibrium thermodynamics, mass balance and Darcy flow, that simulates the hydration of the dry gabbro to amphibolite including the porosity and fluid pressure evolution. Results confirm the formation of a sharp reaction front and the decrease in porosity during the hydration as a potential physical explanation for the observations without the further need for kinetically delayed reactions. We conclude that the metastability of gabbro is mostly controlled by the availability of fluid to the rock.

How to cite: Bläsing, S., John, T., and Vrijmoed, J. C.: How to hydrate almost non-permeable, dry and mafic crust – A mechanistic view on the Kråkenes Gabbro (Western Gneiss Region, Norway), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12498, https://doi.org/10.5194/egusphere-egu23-12498, 2023.

We study the systematics of reaction fronts in multi-component systems using Thermolab. The methodology is based on a finite difference approach for solving the transport problem in combination with lookup tables generated from precomputed thermodynamic equilibria covering the compositional space. The lookup tables generated from Gibbs minimization using linear programming combined with a discrete compound approach are validated against full analytical solutions of the Gibbs minimization problem. We focus on ternary ideal fluid or melt solutions in equilibrium with pure phases as exact solutions are feasible. We show that linear programming techniques yield similar results as a complete analytical solution and that both can be used in stable reactive transport codes.

How to cite: Vrijmoed, J. C. and Podladchikov, Y. Y.: Reaction fronts in multi-component fluid-rock interaction using analytical solutions of the Gibbs minimization problem from Thermolab, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13988, https://doi.org/10.5194/egusphere-egu23-13988, 2023.

Hydration and dehydration reactions as well as the associated fluid flow are important features of geodynamic processes. For example, hydration of rocks can significantly decrease rock strength and generate shear localization or fluids liberated by dehydration reactions in subducting rocks can flow into the mantle wedge and cause melting and magmatism. However, several aspects of (de)hydration related fluid flow and the propagation of (de)hydration reaction fronts remain unclear.

Here, we study hydration and dehydration reactions with hydro-chemical numerical models based on continuum mechanics and local equilibrium thermodynamics. For simplicity, we mainly consider 1D isothermal models. We focus on the propagation velocity and direction of the (de)hydration reaction front. We define hydration as an increase of chemically, or lattice, bound water in the solid phase. Therefore, hydration requires fluid flow towards the hydration reaction front. Contrary, dehydration is a decrease of chemically bound water in the solid phase. Hence, dehydration requires fluid escape from the dehydration reaction front.

Our models show that hydration requires a negative sign of the solid volume change with pressure increase across the reaction boundary, whereas dehydration requires a positive sign of solid volume change with pressure increase across the reaction boundary. The reason for this difference in sign is due to the fluid flow associated with the (de)hydration reaction which is driven by the fluid pressure gradient following Darcy’s law. Thus, for hydration to happen it must occur on the lower fluid pressure side of the reaction front compared to the side with more porous fluid. Porosity is directly related to the solid density change, so it is larger on the high solid density side of the reaction front. Therefore, the hydration reaction requires that the rock that should be hydrated is on the lower fluid pressure side of the front. Opposite can be reasoned for the dehydration front. We also include in our models the case of zero porosity, and hence zero permeability, on one side of the (de)hydration reaction front. This zero-permeability limit involves a singularity at the reaction front due to the multiplication of zero permeability with an infinite pressure gradient. We resolve this singularity in our numerical algorithm by applying a fully conservative form of the governing equations. Resolving this zero-permeability limit is in agreement with the well-established theory of non-linear degenerate parabolic equations. We apply our model to two natural settings: First, eclogite shear zones in the Bergen Arcs, Norway, where hydration of dry granulite formed eclogite. Second, olivine veins in the Erro-Tobbio unit, Ligurian Alps of Italy, where dehydration of serpentinite during subduction formed olivine.

How to cite: Schmalholz, S. M. and Podladchikov, Y.: Hydration versus dehydration reactions: increase versus decrease of solid density with pressure rise, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14012, https://doi.org/10.5194/egusphere-egu23-14012, 2023.

The strength of the lithosphere strongly influences the plate tectonics and mantle convection. The flow behavior of the lithospheric mantle is largely controlled by low-temperature plasticity of olivine, the dominant mineral in the upper mantle. Many experimental studies have explored the low-temperature rheological behaviors of olivine but result in strengths that are highly variable when extrapolated to geological conditions. Kumamoto et al. (2017) performed nanoindentation experiments using Berkovich and spherical indenters on olivine at room temperature and proposed that the strength of olivine depends on the length scale of deformation, with experiments on smaller volumes of material exhibiting larger yield stress, that is, the indentation size effect (ISE). However, their nanoindentation tests were done at room temperature, while traditional creep tests were often done at elevated temperatures of ⩾400°C, the temperature dependence in the ISE must be considered in synthesizing experimental results from different studies. Here, we conducted nanoindentation experiments on a single crystal of Fe-free olivine, eliminating the influence from grain size, using a diamond Berkovich indenter at temperatures of 28, 100, 200, 400 and 600°C. In all tests, the hardness decreases with increasing contact depth that is characteristic of the ISE. Taking our data into the classic hardness-depth relationship of H = H₀(1+h^*/h_c)^1/2, where H is hardness, H₀ is the so-called “infinite hardness”, corresponding to the hardness at the infinite indentation depth, h_c is contact depth, and h^∗ is the material length scale parameter. We found h^∗decreases with increasing temperature, which can be attributed to an increase of the storage volume of geometrically necessary dislocations during nanoindentation test. The decrease of h^∗means that the ISE weakens with increasing temperature, suggesting that at lithospheric temperatures the size effect is not strong enough to explain the disagreements between different experiments and between experiments and geophysical observations. Other aspects, such as grain size effect (Hall-Petch effect) and strain-weakening mechanisms may contribute significantly and need to be revisited.

How to cite: Qi, C. and Wang, Q.: Temperature dependence of indentation size effect in olivine and its implications to low-temperature plasticity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17012, https://doi.org/10.5194/egusphere-egu23-17012, 2023.

Eclogitization constitutes one of the most emblematic transformations in continental subduction zones, where conversion of initially dry lower crustal rocks into eclogite facies rocks correlates with the occurrence of seismogenic events. This reaction is generally considered to occur at high pressure conditions during hydration of dry granulite. Several models using « ad hoc » diffusion equation exist to model this hydration process and the consequences of reaction-induced changes in terms of rheology and density. However, to our knowledge, there is no quantitative model allowing to physically explain how fluids propagate inside a dry rock (i.e. with no porosity at all) and how reaction-induced alteration front widens over time. In this study, we therefore propose a new fully coupled hydro-chemical model wherein a two-phase flow model is coupled with the eclogitization reaction. We use a mass conservative approach, solving total mass and solid mass equations, in a closed isothermal system. Fluid and solid densities are calculated with lookup tables from equilibrium thermodynamics. Our model shows that a fluid pressure pulse generates a pressure gradient that can be associated with the densification reaction when the pressure required for the eclogitization is reached. This reaction generates a large increase in porosity (0 to ~16%) and subsequent porous fluid flow inside the initially dry granulite. This process is then sustained as long as the fluid pulse is maintained, and ends shortly after the fluid pressure pulse stops. However, high pressure within the reacted area can persist for a long period of time. A parametric study allowing to constrain both the duration and the widening of the reaction area is proposed as well as an application to the emblematic case study of the eclogitized granulites of Holsnoy (Bergen Arcs, Norway).

How to cite: Bras, E., Yamato, P., Duretz, T., Schmalholz, S., and Podladchikov, Y.: Fluid-pressure induced eclogitisation of a dry granulite: Insights from Hydro-Chemical model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17232, https://doi.org/10.5194/egusphere-egu23-17232, 2023.

The hydration of initially dry, lower crustal metamorphic rocks during orogenesis is a commonly observed phenomenon and hydration/reaction interfaces are also often preserved, providing a unique insight into the evolution of the lithosphere. Often the interfaces between unreacted and reacted rock are very sharp, even on a thin section scale, and various explanations have been proposed to account for the abrupt changes in mineral assemblage on such a small spatial scale. Common to a wide range of specific examples is the role of an infiltrating aqueous fluid that is generally assumed to be required for the reaction to take place, although other features of such reaction fronts can differ widely in terms of density changes and the apparent difference in metamorphic grade across a sharp interface. 

The examples discussed here all involve the hydration of basement granulite rocks formed during the Caledonian Orogeny and now exposed in the Bergen Arcs in Norway. All stages of hydration can be observed from totally unreacted dry granulites with a wide range of composition to either eclogite facies or amphibolite facies overprints. In these cases the density changes across the interface can either be positive (in the case of eclogite formation from anorthosite granulites), can be negative (in the amphibolitisation of basic rocks) or virtually zero (during the amphibolitisation of garnet bearing anorthosites). The preservation of volume across such interfaces has led to investigations of the coupling between the consequent stress generation and mass transfer, which in turn focusses on the evolution of porosity/permeability in the parent dry rock. The extent of hydration in the Bergen Arcs as a whole ( 90% hydration of ~10⁵ km³ of granulite) suggests a plentiful supply of aqueous solution introduced seismically by fracturing and the consequent generation of shear zones from which hydration fronts spread. The hydration to either eclogite or amphibolite, often observed at the same structural level (i.e. depth in the crust) continues to be an enigma.

Although the details of the reactions and density changes are different, a common feature is the need for an infiltrating aqueous solution and hence the question of what drives the fluid and the hydration reaction and finally why the reaction stops at the sharp interfaces observed in the field. Terminated reactions can be studied by the extent of alteration around fracture planes by modelling the likely fluid pressure gradients that drive Darcy flow from the fluid source towards the reaction interface. Fracture planes represent zones of localised high permeability that facilitate the infiltration of fluid. The difference in fluid saturation between the fracture plane and the alteration halo is thought to be responsible for both the degree of reaction and the difference in assemblage. Additionally, the width of the initial fracture plane is thought to be proportional to the extent of the alteration halo. Examples will be given of hydration fronts associated with amphibolite facies shear zones as well as the observation of eclogite fingers within a partly hydrated granulite host. Combined reaction-fluid flow models attempt to explain these phenomena.

How to cite: Putnis, A., Moore, J., and Podladchikov, Y.: What controls the preservation of hydration interfaces in high grade metamorphic rocks?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17238, https://doi.org/10.5194/egusphere-egu23-17238, 2023.

We discuss a model for temperature-induced rock dehydration that features fluid liberation through mineral reactions, diffusion of chemically released species, and flow through porous media. This model can be derived either by considering standard conservation laws and flux definitions (Pl¨umper et al.[2017], Beinlich et al. [2020]) or, alternatively, using the variational framework of GENERIC (General Equations for Non-Equilibrium Reversible Irreversible Coupling)(Zafferi et al. [2021]) introduced by M. Grmela and H.C. ¨ Ottinger. The latter approach is based on the abstract definition of thermodynamical driving potential and operators characterizing the reversible and dissipative contributions of the processes. By doing so we can show that local equilibrium assumptions are recovered as fast limit of irreversible processes. Ultimately, we rigorously prove that the PDE model so derived admits solutions using a discretization strategy that imitates the numerical implementations.

References

Andreas Beinlich, Timm John, Johannes C Vrijmoed, Masako Tominaga, Tomas Magna, and Yuri Y Podladchikov. Instantaneous rock transformations in the deep crust driven by reactive fluid flow. Nature Geoscience, 13(4):307–311, 2020.

Oliver Plümper, Timm John, Yuri Y Podladchikov, Johannes C Vrijmoed, and Marco Scambelluri. Fluid escape from subduction zones controlled by channel-forming reactive porosity. Nature Geoscience, 10(2):150–156, 2017.

Andrea Zafferi, Dirk Peschka, and Marita Thomas. Generic framework for reactive fluid flows. ZAMM-Journal of Applied Mathematics and Mechanics/Zeitschrift für Angewandte Mathematik und Mechanik, page e202100254, 2021.

How to cite: Zafferi, A., Huber, K., Peschka, D., Vrijmoed, J., John, T., and Thomas, M.: A porous-media model for reactive fluid-rock interaction in a dehydrating rock, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17245, https://doi.org/10.5194/egusphere-egu23-17245, 2023.

Oscillatory zoning occurs in a multitude of minerals growing in both magmatic systems (e.g. zircon, plagioclase, clinopyroxene) and in solid rock (e.g. garnet). Despite the ubiquity of oscillatory growth zoning in minerals, the processes responsible for such compositional zoning remain enigmatic. It has been argued that such zones may form in response to fluctuations in intensive properties, such as temperature, pressure, and magma/fluid chemistry, and/or extensive properties such as surface reaction rates and the creation of a compositional boundary layer during diffusion. However, numerical models that simulate the evolution of a growing crystal remain relatively rare. Here we aim to provide insight to the conditions that attribute to oscillatory mineral zoning of major elements during crystal growth by presenting forward models of diffusion-controlled crystal growth, incorporating multicomponent diffusion and local equilibrium thermodynamics. Two methods are presented, one each in chemical potential and concentration space. These models further constrain the conditions that allow for oscillatory growth zoning. Allowing better insight into the processes occurring during crystal growth in the crust.

How to cite: Moore, J., Khakimova, L., Podladchikov, Y., and Baumgartner, L.: Oscillatory zoning during the growth of single crystals; a comparison of chemical potential and concentration gradient driven numerical models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17278, https://doi.org/10.5194/egusphere-egu23-17278, 2023.

Radial microcracks surrounding retrogressed SiO₂ inclusions in UHP garnets are key microstructural observations allowing to constrain the mechanisms of exhumation of ultra-high-pressure (UHP) rocks. The major challenge lies in identifying whether the microstructures formed during their ascent from mantle depths, or as a consequence of transient variations in the tectonic regime. By combining petrographic observations, petrochronological data and numerical thermo-mechanical modelling, we show that radial cracks around SiO₂ inclusions in ultrahigh-pressure garnets from Dora Maira are caused by ultrafast decompression during the early stage of exhumation (< 0.5 Ma). Decompression rates higher than 10^-14 s^-1 are, for the first time, inferred from natural microstructures independently of current petrochronological estimates¹. We demonstrate that the SiO₂ phase transition generates shear stresses sufficiently large to trigger plastic yielding, resulting in the generation and propagation of radial and bent shear bands, mimicking the fractures observed in UHP garnet. Our results question the traditional interpretation of the exhumation from great depth of ultrahigh-pressure tectonic. Instead, we propose that such ultrafast decompression rates are related to transient changes in the stress state of the buried continental lithosphere, favoring an exhumation mechanism involving nappe stacking.

1 Rubatto, D. & Hermann, J. Exhumation as fast as subduction? Geology 29, 3–6 (2001).

How to cite: Duretz, T., Luisier, C., Tajčmanová, L., and Yamato, P.: Garnet microstructures suggest ultra-fast decompression of ultrahigh-pressure rocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17302, https://doi.org/10.5194/egusphere-egu23-17302, 2023.

Thanks to plate tectonics, the Earth lithosphere is composed of very different lithologies, most of which consisting of peridotites, usually covered by either oceanic or continental crust. Depending on several parameters including composition, pressure, temperature, and strain rate, lithospheric materials can deform smoothly and silently or generate seismic ruptures. Collision belts and subduction systems, including subducted materials being heated and sheared in the mantle transition zone, are characterized by intense seismicity; in contrast, the bottom of lithospheric plates, known as lithosphere-asthenosphere boundary (LAB), is not associated with any seismicity, giving the impression that oceanic plates have the intrinsic ability to maintain their basal stress at relatively low values. Comparing results from experimental geophysics, field geology, geodynamics modelling and seismology, I discuss the representativity of experimental findings and potential consequences on our understanding of the rheology of the lithosphere.

The idea that lithospheric materials at intermediate depths or deeper cannot support high deviatoric stresses is still supported by many studies in geosciences or physics. Plenty of authors start by recalling that brittle failure cannot occur at high pressure, and thus conclude that deep earthquakes and their shallow counterparts should consist of totally different events relying on totally different physical processes. Yet, deep seismicity is characterized by double-couple mechanisms and thus is an actual proof of seismic ruptures at great depths. Here I recall achievements from experiments under synchrotron radiation, suggesting that differential stresses can reach several gigapascals within subducting slabs at intermediate depths (30-300 km). In either peridotites or lawsonite blueschists, high-energy X-rays reveal differential stresses above 2 GPa for confining pressures of 1-1.5 GPa, and reaching ≈ 3 GPa for confining pressures of 2.5-3.5 GPa. This is further supported by both field geology studies and numerical modelling.

While mean stresses in seismogenic zones exhibit severe deviations from lithostatic pressure, the base of lithospheric plates deforms in a way that never triggers seismicity. The coupling between lithospheric plates and the underlying asthenosphere is still a matter of debate. According to global dynamics modelling, a basal shear stress as low as only 10-100 MPa would suffice to allow decoupling at the LAB. While partial melting has recently been favoured as an explanation for plate motion, experimental results on an analogue (germanium peridotite) suggest a solid-state lubrication process, involving grain-boundary disordering, and would confirm that mechanical stresses do not exceed 200 MPa at the LAB (60-120 km).

How to cite: Ferrand, T. P.: How high can mechanical stresses be within lithospheric materials?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17411, https://doi.org/10.5194/egusphere-egu23-17411, 2023.

Systematic reorientation of diamagnetic fabrics of Taunus quartzite due to experimental impact cratering

Sonal Tiwari¹, Amar Agarwal¹, Thomas Kenkmann², Michael H. Poelchau²

¹Department of Earth Sciences, Indian Institute of Technology-Kanpur, Kanpur – 208016, India.

²Geology, University of Freiburg, 79104 Freiburg, Germany

Corresponding author´s email: sonaljp20@iitk.ac.in

Abstract

Para- and ferromagnetic fabrics are known to provide essential clues for understanding impact cratering processes. However, research on the effects of shock waves on diamagnetic fabrics is lacking. We, therefore, conducted a hypervelocity impact experiment on a block of diamagnetic Taunus quartzite and studied the changes in diamagnetic fabrics. Taunus quartzite was formed by a low-grade Variscan metamorphism that overprinted a 405 Ma old sandstone. It consists of c. 91 vol % quartz and a fine-grained, greenish phyllosilicate-bearing matrix (c. 8 vol %), along with small amounts of rutile, chromite, zircon, monazite, and iron oxides.

The experiment was carried out on a 20 cm Taunus quartzite cube with a two-stage light-gas gun of the Fraunhofer Ernst-Mach Institute for High-Speed Dynamics (EMI) in Freiburg (EMI), Germany. The gun has a 8.5 mm calibre launch tube. The 0.3690 g basalt sphere projectile was accelerated to 5.457 kms^-1, with a target chamber pressure of 1.2 mbar. The projectile diameter (d_p) was 6.18 mm. Later, 14 mm-diameter nonmagnetic diamond bits were used to drill oriented cylindrical cores from unshocked and shocked Taunus quartzite blocks.  The AMS of the unshocked and shocked specimens was determined at room temperature in KLY-4S Kappabridge (AGICO). Following the AMS measurements, the cylindrical specimens were cut to make thin sections, which were studied under a Leica DM4 scanning optical microscope.

Hypidiomorphic grain texture, serrated grain boundary, grain boundary migration, ataxial veins, Boehm lamellae, and recrystallized quartz represent the natural microscopic features. Impact-induced microstructures include trans- and intragranular microfractures. Our AMS results demonstrate that in the crater subsurface, the reorientation of the diamagnetic fabrics is concentrated in a zone of ~4 projectile diameters (25 mm) width directly below the point of impact. Higher reorientation in this zone indicates the concentration of damage. The damage is concentrated directly below the point of impact. Another important observation is that weak shock waves have caused an increase in the bulk susceptibility. These results, thus, show that the changes in diamagnetic fabrics can be used as a proxy for plastic deformation caused by shock waves at low peak pressures.

Figure. The images show the specimens' position (black dots), the point source (brown dot), the impact crater (brown arc), and the variation in the orientation of k3.

How to cite: Tiwari, S., Agarwal, A., Kenkmann, T., and Poelchau, M. H.: Systematic reorientation of diamagnetic fabrics of Taunus quartzite due to experimental impact cratering, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-608, https://doi.org/10.5194/egusphere-egu23-608, 2023.

Detailed rock magnetic, facies and textural analyses were carried out across the San Bartolo lava flow (Stromboli) to understand the flow dynamics of lava channel-fed 'a'a entering the water. Having been emplaced 3 ka, San Bartolo is the most recent lava flow field to have been emplaced beyond the Sciara del Fuoco, and underlies the inhabited area on the north-eastern side of the island. One of the remarkable features of the San Bartolo lava flow is the formation of several lobes due to the interaction with seawater. Field analysis shows three facies: 1. Stalling of flow fronts at the coastal interaction to form a littoral barrier to further flow, 2. Ramping of subsequently emplaced units behind this barrier, and creation of a degassed ponded volume, 3. Creation of tubes through the barrier to feed a seaward bench of pahoehoe. Around 12 lobes were identified. All the lobes show similar facies, but each lobe provides a case-type example of the emplacement history and the associated structures. For example, lobe 1 exhibits tube formation associated through the flow front barrier; while lobe 12 shows the formation of inflated pahoehoe lava. Preliminary AMS results show well-confined flow fabrics with a one-to-one relationship to field structures. The samples collected from ramped flow have vertical flow fabrics, while those from tube structures and inflated pahoehoe have horizontal fabrics. Preliminary palaeomagnetic data have characteristic remanent magnetisation (ChRM) directions for all the sampled lobes, with their a₉₅ overlapping, suggesting rapid emplacement. In addition, the average ChRM direction falls in the geocentric axial dipole (GAD) field for Stromboli.

How to cite: Shajahan, R., Zanella, E., Harris, A. J. L., Drovanti, L., Robustelli Test, C., Calvari, S., Gurioli, L., Mana, S., and van Wyk de Vries, B.: Lava-water interaction and formation of associated facies: a multidisciplinary study of the San Bartolo lava flow of Stromboli, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1000, https://doi.org/10.5194/egusphere-egu23-1000, 2023.

To better constrain the drift history of the South Qiangtang block and the closure of the Paleo-Tethys Ocean, we report a paleomagnetic result isolated from 25 sites (199 samples) of Late Triassic volcanic rocks from the Xiaoqiebao Formation. The directions of the characteristic remanent magnetization isolated from these rocks pass both fold- and reversal tests, and are likely primary magnetizations. On base of these data, we estimate that the South Qiangtang block occupied a paleolatitude of 30.1°N±4.6 at 222Ma. When combined with existing paleomagnetic constraints, these new results indicate that the South Qiangtang block moved rapidly northward between the middle Permian and Late Triassic, at an average south‐north speed of ~13.4 cm/yr during middle Permian to Late Triassic. Our new data further suggest that the Paleo-Tethys likely closed by ~222Ma, and the north-south width of the Neo- Tethys Ocean was reached ca. 7000 km at this time.

How to cite: Wei, B., Cheng, X., Domeier, M., and Wu, H.: Paleomagnetism of Late Triassic Volcanic Rocks From the Southern Qiangtang Block, Tibet: Constrains on the Closure of the Paleo-Tethys Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1701, https://doi.org/10.5194/egusphere-egu23-1701, 2023.

The origin and drift history of the Lhasa block in South Tibet is crucial towards unraveling the evolution of the Neo-Tethys Ocean, which has not yet been well constrained by the paucity of paleomagnetic data, especially for the late Paleozoic. Hence, a systematic paleomagnetic investigation of 50 sandstone samples (6 sites), 166 volcanic samples (21 sites) and 76 limestone samples (9 sites) from the middle Permian (267.8 ± 5.0 Ma) Luobadui Formation was conducted in the Lhunzhub area. The results reveal an Eocene re-magnetization component in the sandstone samples, but stable high temperature (field) components obtained from most volcanic and limestone samples can successfully pass the fold, reversal and paleosecular variation tests, which likely represents primary magnetization. On this basis, the middle Permian paleomagnetic pole position (P_lat= 40.9°N, P_long=324.5°E, N=27 sites (dp/dm=3.3/6)) and paleolatitude (~15.9°S) of the Lhasa block are presented. Combined with published paleomagnetic data from other Tethyan continental blocks, this new constraint reveals that the Lhasa block was located in the interior of the Neo-Tethys Ocean at about 268 Ma. In further considering the geological records of the Lhasa block, we propose that the block rifted from the northwestern margin of the Gondwana-Australian plate prior to the middle Permian, and the Neo-Tethys Ocean represented by the Bangong Co-Nujiang and Yarlung-Zangbo Suture Zones coexisted during the middle Permian.

How to cite: Xing, L., Cheng, X., and Wu, H.: Locating the Lhasa Block within the Neo-Tethys Ocean at ~268 Ma: Paleomagnetism and Its Paleogeographic Implications, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1736, https://doi.org/10.5194/egusphere-egu23-1736, 2023.

Paleomagnetic data from the Middle Devonian are typically difficult to interpret. Directions and paleointensities often do not fit with dipolar field behavior or expected paleogeography. The reason why the geomagnetic field cannot be reconstructed with traditional methods has been topic of debate, but no consensus has been reached. We would like to understand what happened to the geomagnetic field during the Middle Devonian and why the configuration of the field was potentially unusual.

We aim to expand the existing paleomagnetic record for the Middle Devonian by sampling a site in Braunfels, Germany. This site consists of relatively unaltered pillow lavas. Petrographic and rockmagnetic analyses indicate the presence of magnetite and minor maghemite in the samples. We obtained paleomagnetic directions using alternating field (AF) and thermal demagnetization experiments. The directions are scattered and do not cluster around paleomagnetic directions that are expected for Germany in the Devonian.

Paleointensity data were acquired using the ZIIZP-Thellier method, resulting in a field intensity of approximately 6 µT, which equals a VADM of 8-15 ZAm². The latter is in line with very low field intensities generally reported for the Devonian.

Various mechanisms have been suggested to explain the typically scattered and ambiguous Devonian paleomagnetic data, such as significant overprinting, tectonic rotations and a non-dipolar field configuration. Our results confirm an extremely weak magnetic field, but this alone does not explain the scattered directions. To exclude the possibility that the scattered directions are related to (partial) overprinting we use Quantum Diamond Microscope imaging to assess the magnetizations of individual magnetic grains instead of the bulk magnetic signal of the sample. With this method, a distinction can be made between e.g., different generations of magnetizations, revealing information on the Middle Devonian geomagnetic field that was previously inaccessible by considering the magnetic moments of bulk samples alone.

How to cite: de Boer, R., van der Boon, A., Königshof, P., and de Groot, L.: Paleomagnetic and micromagnetic measurements of Middle Devonian pillow lavas from Germany, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1885, https://doi.org/10.5194/egusphere-egu23-1885, 2023.

Magnetic susceptibility (both in-phase, ipMS, and out-of-phase, opMS, components) and its anisotropy were investigated in artificial specimens with pure synthetic magnetite and in rocks with titanomagnetite with variable Ti-content reaching X_Usp = 0.6 of ulvospinel end member. The opMS of pure magnetite is in its absolute values three orders lower than the ipMS. The phase angle is then lower than 0.2° in absolute values, which is mostly lower than the sensitivity of phase angle determination. Consequently, the opMS can be considered effective zero. In low-Ti titanomagnetite (X_Usp~0.2), the intensity of the ipMS variation with field is very low, hardly reaching 1% of the initial value. In high-Ti titanomagnetite (X_Usp~0.6), the intensity of ipMS variation is relatively strong reaching 50% of the initial value and that of opMS variation is even much stronger reaching multiples of the initial value.

In artificial specimens with magnetite, the opMS is isotropic from the statistical point of view as indicated by the values of the F-statistics and by confidence angles. In titanomagnetite-bearing rocks, the opAMS ellipsoids resemble the ipAMS ellipsoids, the degree of opAMS being significantly higher than that of ipAMS. The principal directions of ipAMS and opAMS are related closely in specimens with high-Ti titanomagnetites and only poorly in specimens with low-Ti titanomagnetites. In specimens with high-Ti titanomagnetites, there is a linear and very strong (R² = 0.9433) correlation between the degree of ipAMS and root squared degree of opAMS.

The results of the present research are mainly applicable to the rocks whose magnetism is dominantly carried by titanomagnetite and pure magnetite. The opAMS indicates solely the titanomagnetite fabric unaffected by magnetite whose fabric may often originate in different conditions or in different time.

How to cite: Hrouda, F., Jezek, J., and Chadima, M.: Anisotropy of out-of-phase magnetic susceptibility in titanomagnetite-bearing rocks due to weak field hysteresis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2234, https://doi.org/10.5194/egusphere-egu23-2234, 2023.

Classically, paleomagnetic data is obtained from bulk samples, where the signal is obtained from the sum of all magnetic moments within the sample volume. This typically includes stable and unstable remanence carriers. With the advance of microscope imaging techniques (e.g., SQUID microscopes, Quantum Diamond Microscopes) it is now possible to spatially isolate the contribution of single grains or clusters of grains. Yet, despite recent advances in the inversion techniques to derive full vector information from the magnetic microscopy data, there is still no inversion protocol capable of obtaining the magnetization directions of each magnetic particle without using a priori information or physically separating the spatial position of each anomaly. Here we present an algorithm capable of automatically identifying the position of each magnetic anomaly source by combining signal processing and tridimensional Euler deconvolution. Subsequently, we recover both the direction and the intensity of the magnetic moment of each magnetic source with no need for any kind of additional information. The method presented here is a potential technique that may help increase the statistical accuracy of data obtained in paleomagnetic studies from magnetic microscopy data, especially in the case of complex characteristic remanent magnetization.

How to cite: Ferreira de Souza Junior, G., Ivan Ferreira da Trindade, R., Uieda, L., Fu, R., and Araujo do Carmo, J.: Full vector inversion of magnetic microscopy data using Euler deconvolution as a priori information, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4479, https://doi.org/10.5194/egusphere-egu23-4479, 2023.

Micromagnetic tomography (MMT) is a new promising paleomagnetic technique that obtains magnetic moments for individual iron-oxides. These magnetic moments are inferred from surface magnetometry data obtained with quantum diamond microscopy (QDM), and iron-oxide positions determined with micro X-Ray computed tomography (MicroCT). Different to classical techniques, MMT does not depend on bulk measurements of samples. This makes it possible to only select the most reliable magnetic recorders. To make this improvement possible, MMT first has to deal with the presence of undetected magnetic carriers in basaltic rock samples used in previous MMT studies. Although particles smaller than 1 µm are good recorders of the magnetic field and may be visible in surface magnetometry, they are not detected by MicroCT. This violates one of the foundations of MMT and may disturb magnetic moments of other detected grains. However, it is currently unknown how many of these small disturbing particles are present in Hawaiian basaltic samples. We know that the smallest disturbing grains have a diameter of around 40 nm, since particles smaller than this threshold become superparamagnetic and cannot store magnetic signals. For this reason we want to obtain a grain-size distribution for iron-oxides from 20 nm to 10 µm to cover the complete range of grains that are capable of storing Earth’s magnetic field. This requires a combination of FIBSEM slice-and-view and MicroCT techniques; FIBSEM detects single and pseudo-single domain grains with sizes between 20 nm and 1 µm and MicroCT detects multi-domain grains with sizes larger than 1 µm. Subsequently, FIBSEM and MicroCT data are combined to obtain the full spectrum of grain sizes. Unfortunately, grains are not uniformly distributed in the samples, so a scaling by volume would not produce a realistic spectrum. Therefore, based on observations that iron-oxides grains cluster on the interfaces of other minerals, we calculated how many times FIBSEM mineral interfaces from FIBSEM data fit the mineral interfaces from MicroCT data. Then, this factor is used to scale the FIBSEM iron-oxides to MicroCT iron-oxides and to obtain a complete distribution of all grain sizes. Interestingly, this distribution shows a clear peak in grain size at 70-80 nm. Furthermore, the smallest grain fraction is fitted a lognormal trend, but the fraction larger than 0.18 µm are fitted an exponential decay trend. With these trendlines in place we have finally acquired a realistic set of boundary conditions for the distribution of iron-oxide particles in basaltic rocks. This enables us to populate models with a realistic distribution of particles, which ultimately may shed light on the disturbing presence of small iron-oxides in MMT results. If we know the effect of these disturbances, we will understand which grains MMT can solve with highest certainty, ultimately leading to paleomagnetic interpretations on grain scale.

How to cite: Out, F., de Boer, R., Walmsley, J., and de Groot, L.: Building Physical Models of the Distribution of Iron-oxides in Basalts: ‘How MMT deals with MicroCT resolution limitations’, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5045, https://doi.org/10.5194/egusphere-egu23-5045, 2023.

Magnetic fabrics analysis is widely applied in reconstructions of the tectonic evolution of orogenic belts. The main advantage of this method is the ability to trace even weak deformation in rocks. The main goal of this work is to shed new light on the Cretaceous-Neogene tectogenesis of the Tatra Mts by investigating the para- and ferromagnetic fabrics of nappe units and post-thrusting sequences. It is worth mentioning, that this study provides the first magnetic fabric data from the thrust nappe system of the High Tatra Mts (the most elevated part of the Tatra Mts). We focused on Cretaceous marly limestones, the youngest part of the Mesozoic nappe system in the Tatra Mts, and Oligocene post-thrusting shales/siltstones. Petromagnetic methods combined with paleotemperature estimations enabled us to identify the magnetic mineralogy and its origin. The subsequent analysis of para- and ferromagnetic fabrics supported by the previously obtained mineralogical results let us recognize and characterize different stages of the tectonic evolution of the Tatra Mts. Even though the paleotemperatures recorded for Oligocene and Cretaceous rocks are higher in the High Tatra Mts than in the Western Tatra Mts, petromagnetic features of rocks sampled in both areas remain similar. Anisotropy of Magnetic Susceptibility (AMS) fabric of post-thrusting cover is governed by phyllosilicates. A consistent, approximately NE-SW-oriented magnetic lineation of tectonic origin is present in most analyzed sites and documents weak deformation presumably related to the Miocene uplift of Tatra Mts. Mean ferromagnetic mineral in Oligocene clastic sediments is magnetite. Origin of the AARM lineation in this unit is presumably related to the crystallization of the secondary magnetite on a preexisting phyllosilicate matrix and/or the stress field present during magnetite formation. The magnetic fabric of the Cretaceous marly limestones is controlled mainly by the orientation of phyllosilicates and differs significantly among the High and Western Tatra Mts. AMS results from The Western Tatra Mts’ sites document consistently the NW direction of alpine nappe thrusting. On the contrary, the AMS fabric in the High Tatra Mts shows no clear evidence of tectonic deformation. Ferromagnetic mineralogy of the Cretaceous nappe unit is complex and combines magnetite, hematite and maghemite in various proportions, with the usual dominance of magnetite. In the High Tatra Mts, the AARM fabric carried by magnetite is characterized by sub-vertical magnetic lineation, which we interpret as the impact of local transpression-related deformation associated with the Miocene uplift.

How to cite: Staneczek, D., Szaniawski, R., and Marynowski, L.: Multi-stage tectonic evolution of the Tatra Mts recorded in the para- and ferromagnetic fabrics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6126, https://doi.org/10.5194/egusphere-egu23-6126, 2023.

The Gamaye pluton (Kedougou-Kenieba Inlier, West African Craton) is elongated along a N-S direction, over about 20 km, to the east of the sinistral, transcurrent Senegal-Mali Shear Zone (SMSZ). It is assumed to have been emplaced at around 2045±27 Ma (Rb-Sr whole-rock and feldspar age; Bassot & Caen-Vachette, 1984) and, as other granitoids from the Kedougou-Kenieba Inlier, it displays a spatio-temporal relationship with world-class gold mineralization (Lawrence et al., 2013a, b). The pluton is made of a leucocratic, coarse-grained, locally porphyritic granite, associated with a subordinate, mesocratic, fine-grained facies, mostly found in a small (1.5 x 4 km) body along the western border of the pluton. Preliminary apatite LA-ICP-MS U-Pb dating yield ages with quite large uncertainties, which highlight, however, magmatic pulses with distinct emplacement ages: 2294.6±68.3 Ma (western mesocratic body), 2160±34.8Ma (main leucocratic facies) and 1922.7±53.1 Ma (a tiny mesocratic body in the east of the pluton). The western part, particularly the democratic body, and the southern part of the pluton, close to the SMSZ, are mylonitized, displaying a S-C fabric, whereas the northern, central and eastern parts are almost isotropic. However, a study of the microstructures shows that these parts of the Gamaye pluton have also undergone solid-state deformation and dynamic recrystallization. Measurements of the anisotropy of magnetic susceptibility, conducted on about 50 samples, reveal paramagnetic signatures, with bulk susceptibilities lower than 0.5×10^-3 SI. The shape of the magnetic fabric ranges from oblate to prolate, and the degree of anisotropy increases towards the western limit of the pluton and the SMSZ, together with the rock strain intensity. There is also a deflection of the magnetic foliation and lineation in that direction: in particular, when approaching the western border, the magnetic fabric is dominated by NNW-SSE-trending, steeply-dipping foliations (parallel to the C plane of the S-C fabrics) and gently-plunging lineations (concordant with the strike-slip movement of the SMSZ). It is concluded that the Gamaye pluton has been emplaced along the SMSZ and/or has been deformed by this transcurrent regional discontinuity.

Reference

Bassot, J.P., Caen-Vachette, M., 1984. Données géochronologiques et géochimiques nouvelles sur les granitoïdes de l’Est du Sénégal: implications sur l’histoire géologique du Birimien de cette région. In: Klerkx, J., Michot, J. (Eds.), African Geology, Belgium, Tervuren, pp. 196–209.

Lawrence, D.M., Treloar, P.J., Rankin, A.H., Harbidge, P., Holliday, J., 2013a. The geology and mineralogy of the Loulo mining District, Mali, West Africa: evidence for two distinct styles of orogenic gold mineralization. Econ. Geol. 108, 199–227.

Lawrence, D.M., Treloar, P.J., Rankin, A.H., Boyce, A., Harbidge, P., 2013b. A fluid inclusion and stable isotope study at the Loulo mining District, Mali, West Africa: implications for multifluid sources in the generation of orogenic gold deposits. Econ. Geol. 108, 229–257.

How to cite: Dembele, A., Bolle, O., Poujol, M., Dabo, M., and Bouaré, M. L.: Structural analysis by anisotropy of magnetic susceptibility and U-Pb geochronology of the Gamaye pluton (Kédougou-Kéniéba Inlier, West Africa), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6247, https://doi.org/10.5194/egusphere-egu23-6247, 2023.

The magnetic information stored in volcanic rocks is a valuable archive of the history of the behavior of the Earth’s magnetic field. Micromagnetic Tomography (MMT) allows to determine magnetic moments of individual iron-oxide grains in rocks. Theoretically this enables us to separate contributions from non-ideal recorders and ideal recorders, overcoming the difficulties arising from bulk measurements. Here we present results from two sister specimens from the 1907-flow from Hawaii’s Kilauea volcano to which MMT was applied. One specimen was imaged both by the Quantum Diamond Microscope in Harvard and by the MicroCT scanner Nanoscope–S in Delft, producing magnetic moments of 1,646 individual grains. The sister sample underwent stepwise AF-demagnetization: a step toward classic paleomagnetic analysis, from which we present (preliminary) results. In MMT, individual grains are allocated a magnetization through a least-squares inversion. For the first sample, we produced more than one magnetization for each grain, because each grain was present in multiple unique inversion ‘tiles’ (smaller sub-areas  due to computational constraints). This enabled a statistical analysis of the (robustness of) results, presented here. For the second sample (preliminary) demagnetization results per grain are presented. We also present results of an investigation into a parameter for selecting grains that can be reliably resolved from the statistical analysis. For both samples only relatively large iron-oxide grains (diameter > 1.5 -  2 µm) were resolved, as the resolution of the MicroCT was limited. However, any analysis of magnetism at grain level is a step in understanding how magnetizations are stored in individual grains, and is of importance for those specimens that only contain large iron-oxides.

How to cite: Kosters, M., de Boer, R., Out, F., Cortes Ortuno, D., Volk, M., and de Groot, L. V.: Magnetic analysis of individual iron oxide grains; application of Micromagnetic Tomography to a natural sample., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6373, https://doi.org/10.5194/egusphere-egu23-6373, 2023.

Our understanding of paleogeography through Earth history relies heavily on apparent polar wander paths (APWPs), which represent the time-dependent position of Earth’s spin axis relative to a given tectonic plate. However, there are a number of limitations associated with conventional approaches to APWP construction. First, the paleomagnetic record contains significant uncertainty in individual pole positions that is not propagated into APWPs. This traditional approach makes it difficult to incorporate age and positional uncertainty into synthesized paths and assigns equal weight to paleomagnetic poles with vastly different numbers of underlying sites. Second, the effective propagation of site-level uncertainties into the APWP requires a transformation that renders traditional parametric assumptions (i.e., Fisher statistics) on the pole level ineffective. Here, we overcome these limitations with a bottom-up Monte Carlo uncertainty propagation scheme that operates on site-level paleomagnetic data. To demonstrate our methodology, we present a large compilation of site-level Cenozoic paleomagnetic data from North America, which we use to generate a high-resolution APWP. We show that even in the presence of significant noise, polar wandering can be assessed with unprecedented temporal and spatial resolution.

How to cite: Gallo, L., Domeier, M., Sapienza, F., Swanson-Hysell, N., Vaes, B., Zang, Y., Arnould, M., Robert, B., Rolf, T., and van der Boon, A.: Toward finer resolution APWPs, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10000, https://doi.org/10.5194/egusphere-egu23-10000, 2023.

Stalagmites are potential candidates for high-resolution reconstruction of the Earth’s magnetic field and paleoclimatic variations. Here we provided a pristine record of a geomagnetic event recorded in two stalagmites from the Bat Cave, Central Region of Portugal. SQUID microscopy reveals a high concentration of magnetic particles. FORC diagrams and hysteresis parameters point to non-interacting single-domain magnetite as the main magnetic carrier. Carbon and oxygen isotope compositions are interpreted as primary and provide the record of a Greenland interstadial. Paleomagnetic data show a gradual variation of the magnetic declination and inclination from the normal component to an antipodal reverse component. Return to the normal component is abrupt and is not apparently associated with visible hiatus in the precipitation rate of the stalagmite. Preliminary U-Th dating point to an age of ~50-55 Kyr for the geomagnetic event recorded in both stalagmites. However, owing to the speleothem’s high detrital content and low U concentration, alternative techniques are being explored to improve the uncertainty in radiometric dating. This geomagnetic event can possibly correspond to the Laschamp geomagnetic excursion dated at ~41 Kyr or to a newly discovered ~50 kyr short-lived geomagnetic reversal.

Acknowledgments: This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020- IDL, MIT-EXPL/ACC/0023/2021, and PTDC/CTA-GEO/0125/2021).

How to cite: Font, E., M. Sánchez-Moreno, E., Lima, E., Brás, A. R., Spangenberg, J. E., Edwards, L., Trindade, R., Dimuccio, L., Hasözbek, A., Parés, J., Jiménez Barredo, F., Araujo de Carmo, J., and Feinberg, J.: A (new?) ~50 Kyr geomagnetic event recorded in a stalagmite from the Bat Cave, Portugal, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10068, https://doi.org/10.5194/egusphere-egu23-10068, 2023.

Earth and planetary rocks record magnetic variations and are crucial recorders to further our understanding of the formation and evolution of the Earth’s magnetic field. The acquisition and stability of remanent magnetization in rocks are controlled by the magnetization state of magnetic particles. Where the thermoremanent magnetization (TRM) of pseudo-single domain (PSD) magnetite particles probably dominates the natural remanence of many igneous rocks. These PSD particles are therefore important carriers of paleomagnetic information. Nevertheless, the extension of Néel's (1955) elegant analytical theory of single domain (SD) particles to larger PSD particles meets with substantial technical challenges. Understanding the unblocking of energy barriers between energetically favorable domain states is important when predicting the TRM behavior of natural particles. While categorizing different magnetization states is manageable for simple SD particles, it will become increasingly difficult with increasing particle size. Mapping the full energy landscape for PSD particles can be challenging and time-consuming. 

Here we present an automated process that can categorize all possible local energy minima (LEM) from micromagnetic modeling results. The automated process determines if magnetic states with approximately equal total energy are in the same reversible region. We incorporate calculations of energy barriers between LEMs for an assemblage of natural magnetite particles obtained by focused-ion-beam (FIB) nano-tomography. The technique was tested for a suite of PSD particles from single-vortex to multi-vortex states, where we systematically studied the energy landscape and relaxation time as a function of temperature. To map the energy landscape, LEM and energy barriers between all possible LEM were calculated for temperatures between room temperature and Curie temperature. Combining these results in a statistical model allows for predicting the TRM acquisition of individual particles and isotropic ensembles of equal particles. The results are discussed in terms of the TRM behavior of natural PSD magnetite, magnetic stability, and the implications for paleomagnetic research.

How to cite: Beguin, A., Nikolaisen, E., and Fabian, K.: Micromagnetic Modeling of Thermoremanent Magnetization in Small Natural Pseudo-Single Domain Magnetite Particles, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10086, https://doi.org/10.5194/egusphere-egu23-10086, 2023.

The apparent polar wander (APW) path of the Pacific plate during Cenozoic and Late Cretaceous time has important paleogeographic, global tectonic, geodynamic, and paleoclimatic implications. Here we present recent progress on our assessment of Pacific plate APW with a focus on new poles determined from analysis of the skewness of marine magnetic anomalies due to seafloor spreading, which produces high precision paleomagnetic poles.

Our recent and new results include paleomagnetic poles for chron C24r (57–54 Ma; Woodworth et al. 2023), chron 21n (48−46 Ma; Woodworth et al. in preparation), chron 27r (63−62 Ma; Gaastra et al., in preparation), and chron 30n−31n (69−66 Ma; Ritchey et al., in preparation).  The new chron 24r pole extends the northward progression of the 69–58 Ma track in the previously determined Pacific plate apparent polar wander path beyond the southern end of the 46–10 Ma track, which is now extended by our chron 21n pole.  That the two tracks overshoot one another implies either a brief interval during which the Pacific plate moved substantially and rapidly southward relative to a mantle reference frame or to an episode of true polar wander (TPW, the re-orientation of the solid Earth relative to the spin axis).  The reconstructed plate geometry and relative plate motions are inconsistent with ancient plate driving forces having been capable of driving the plate southward.  Considering our results together with drill-core paleomagnetic paleolatitudes, we find that the most likely explanation is a ≈7° episode of TPW between ≈54 Ma and ≈50 Ma.  

The new high precision paleomagnetic poles for the Pacific plate, when compared with continental paleomagnetic results from Torsvik et al. (2012) reconstructed to a common frame of reference, allow an updated test of two plate motion circuits relating the Pacific plate to the circum-Pacific continents. Analysis of the plate-motion circuit through Antarctica (or through Australia) continues to indicate a significant paleomagnetic misfit that increases with age during Cenozoic time up to a maximum of ≈7−8° at ≈50 Ma. Analysis of the fixed hotspot circuit indicates a smaller insignificant misfit of up to ≈5° that also increases with age.

If the plate circuit through Antarctica is flawed, as indicated by the paleomagnetic results, any or all of the following may have contributed to the flaw: (1) intraplate deformation (cf., Kreemer & Gordon, 2014), (2) unrecognized diffuse oceanic plate boundaries (e.g., Gordon & Stein, 1992), (3) motion between East and West Antarctica not localized across mid-ocean ridge segments, and (4) horizontal extension across large expanses of submerged continental area in the south Pacific (e.g., Sutherland et al., 2020).

How to cite: Gordon, R., Woodworth, D., and Gaastra, K.: Cenozoic and Late Cretaceous Apparent Polar Wander of the Pacific Plate: Implications for True Polar Wander and Global Plate Motion Circuits, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10221, https://doi.org/10.5194/egusphere-egu23-10221, 2023.

One of the key aspects of understanding the rapid exhumation of Himalayan core, is to understand the variation of deformation intensity occurring along the major tectonic unit i.e., the Main Central Thrust (MCT). Quantification of magnetic fabrics, derived from analysis of anisotropy of magnetic susceptibility (AMS), defines the strain path in the form of an ellipsoid and provides a rich source of information on the structural evolution of rocks throughout the MCT. The approach of this study is to integrate field and AMS parameters, where field foliation from 73 planes has orientation trending NW-SE with a moderate dip of about 60⁰ (best fit great circle) The magnetic foliation is found to be parallel to the field foliation. Variation in orientation direction of magnetic lineation implies the dominance of superposed folding in the study area. The degree of anisotropy (P_j), which quantifies the intensity of preferred orientation of magnetic minerals show values ranging from 1.1 to 1.8. The AMS study reveals that the shape parameter(T) of susceptibility ellipsoid, in most of the samples are positive representing an oblate ellipsoid. To be more precise, this study reflects the inter-relationship between field structures, superposed folding, magnetic fabric parameters to understand the exhumation of Himalayan core.

How to cite: Borgohain, A., Gairola, P., Bhatt, S., Rana, V., and Banerjee, S.: Deciphering the variation of magnetic fabric intensity across the Main Central Thrust in Garhwal Himalayas, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12219, https://doi.org/10.5194/egusphere-egu23-12219, 2023.

Ancient detrital zircon grains containing magnetite inclusions have the potential to record the Earth’s magnetic field as far back as the Hadean. However, this requires magnetite to be either a primary inclusion or to be a secondary inclusion that forms shortly after zircon crystallization. Microstructural and TEM analysis of Jack Hills zircon show that magnetite, the magnetic recorder in these zircon crystals, is secondary in nature and is associated with the mobility of Fe. However, the timing of Fe mobility within Jack Hills zircon is poorly constrained. Here we undertake nanoscale characterization of highly magnetic zones of zircon, identified by quantum diamond microscopy (QDM), using atom probe tomography (APT). The APT data show the presence of Pb-bearing nanoclusters and these record isotopic compositions consistent with formation during two discrete thermal events at 3.4 Ga and < 2 Ga. The older population of clusters contain no detectable Fe. However, the younger population of clusters are Fe-bearing. This result shows that the Fe required to form secondary magnetite was not present in the zircon prior to 3.4 Ga and that remobilization of Pb and Fe, the latter associated with magnetite formation, took place after 2 Ga, during an annealing event that took place more than one billion years after deposition of the Jack Hills sediment at 3 Ga. This use of APT to date Fe mobility provides a novel approach to temporally constrain the formation of intragranular secondary magnetite inclusions in highly magnetic areas of zircon grains.

How to cite: Reddy, S., Taylor, R., Harrison, R., Saxey, D., Rickard, W., Tang, F., Borlina, C., Fu, R., Weiss, B., Bagot, P., and Williams, H.: Temporal constraints on Fe mobility in Jack Hills zircon, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14336, https://doi.org/10.5194/egusphere-egu23-14336, 2023.

Introduction. This study investigates the Cheka block (pluton) of alkaline granitoids (Southern Urals, Chelyabinsk Oblast). The objective of this study was to further investigate its existing deformation model after previous studies though the methods of fracture analysis, petromagnetic studies and geochemical analysis.

The Cheka pluton is composed of the Cheka Mountain and has a meridional strike and dimensions of 6.5 km long and 1-2 km wide. The pluton is composed of alkaline rocks of three intrusion phases: first – monzodiorites, second – alkaline syenites, third – alkaline granites and granosyenites. The pluton is Triassic and intrudes Carboniferous volcanics. The western contact of the Cheka pluton is limited by a dextral fault. The pluton is situated in the Magnitogorsk zone.

During the formation of the pluton, extension changed to compression. This led to formation of a right-lateral transpression setting with a system of meridional strike-slip and near-slip extension zones. These changes were followed by low-grade metamorphism.

This study can be split into two sections: structural analysis and geochemical/isotopes description. The first part was partially conducted previously and presented in 2022.

Materials and methods. To reconstruct structural evolution of the pluton a combination of petromagnetic studies, magnetic mineralogy and fracture analysis were used as well as supporting aerial and satellite imagery. 62 core samples and over 180 fracture measurements from 7 locations were used for each method respectively. Petromagnetic data was collected by drilling procedures, processed using MFK-1 kappabridge at room temperature and after heating to 470 °C and analyzed in Anisoft 5.1.08 software. Magnetic mineralogy lab analyses were performed with interpretation using Max UnMix software. Fracture analysis was conducted in Stereonet v.11.3.0.

As the second part of the study geochemical analyses were conducted – silicate geochemistry and ICP-MS at 6 locations.

Results and discussion. Petromagetic studies showed the magma flow to have an orientation of 036°. Analysis of tectonic fractures points to the Riedel fracture model with main fracture zone orientation (compression) of 039°. Since the magma flow and compression orientation match a deformation model can be constructed. Also based on the magma flow orientation, types of protectonic fractures were identified (S, Q, L).

Geochemical analyses showed that the elemental signature of the pluton matches the upper crust the best and shows signs of subduction. Silicate geochemistry shows a clear trend in Na₂O concentration, while K₂O concentrations do not. This pattern is interpreted as a sign of low-grade metamorphism (prehnite-pumpellyite facies).

A full deformation model was created based on two methods with additional supporting data providing strong evidence for the Riedel based deformation model, which corresponds to previous structural and geochemical findings. The model suggests that the Cheka pluton was formed in a general right-lateral transpression setting with following tectonic developments and related low-grade metamorphism.

Financial support. The reported study was funded by RFBR and Czech Science Foundation according to the research project № 19-55-26009. Centre of collective usage ‘Geoportal’, Lomonosov Moscow State University (MSU), provided access to remote sensing data.

How to cite: Shestakov, P., Tevelev, A., Kazansky, A., Pravikova, N., Koptev, E., Volodina, E., and Borisenko, A.: Deformation model of the Cheka pluton of alkaline granitoids: petromagnetic and geochemical data (Southern Urals), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14498, https://doi.org/10.5194/egusphere-egu23-14498, 2023.

The Arbus igneous complex (SW Sardinia, Italy) represents a good example of a short time lived post-collisional composite pluton emplaced at shallow crustal level in the external zone of the Variscan chain. The pluton almost consists of granodiorite and leucogranite rock-suites emplaced at 304 ± 1 Ma within a main NW trending thrust separating the metamorphic wedge from the fold and thrust belt foreland. The pluton emplaced into a dilatational step over connecting two NW–SE dextral shear zones which belongs to a regional network of post-collisional strike-slip structures marking the transition from col- lision to post-collisional extension. The microstructure observed for quartz and K-feldspar confirms the lack of significant post-emplacement deformation, indicating only limited high-temperature sub-solidus recrystallization. Anisotropy of magnetic susceptivity data and field-structural analysis have been carried out to reconstruct the geometry of the pluton and the trajectories of magmatic flow in relation to regional deformation structures. Overall, the magmatic and the magnetic fabrics are broadly discordant with the metamorphic foliation of the country rocks, defining an EW trending elliptical asymmetric sill rooted in the SW quadrant. The reconstructed architecture combined to petrologic observation indicates that accretion of the pluton involved injection of multiple dykes through a sub-vertical feeder zone, combined to lateral flow of the roof controlled by inherited collisional structure. The duration of magmatic activity and the cooling history of the contact metamorphic aureole have been evaluated through a suite of 2D thermal models. All these observations, together with the available geochronological constraints are suggestive of very rapid construction of the pluton. The proposed emplacement model is fully consistent with the regional phase of strike-slip tectonics and widespread magmatism accommodating the large rotation of the Corsica-Sardinia block during the Carboniferous-Permian transition.

How to cite: Cifelli, F., Secchi, F., Casini, L., Naitza, S., Carta, E., and Oggiano, G.: An integrated structural and AMS study to define the emplacement of the Arbus pluton (SW Sardinia, Italy), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15137, https://doi.org/10.5194/egusphere-egu23-15137, 2023.

The interpretation of magnetic fabric (studied mainly by means of anisotropy of magnetic susceptibility) has become one of the well-established, fast, and reliable rock fabric proxies used in many branches of the Earth science. In the usual case, the magnetic fabric ellipsoid reflects the crystallographic or shape preferred orientation of the grains of the main rock-constituent mineral. If two (or more) sets of mineral grains are present, their combined contribution towards the whole-rock magnetic fabric is proportional to their relative content, their value of bulk susceptibility and their degree of anisotropy. This can be the case of, for example, combined contribution of differently oriented paramagnetic and ferromagnetic sub-fabrics, normal and inverse magnetic fabric, or anisotropic fabric and isotropic matrix. In order to overcome some misconception on how various magnetic fabrics can contribute to the overall rock fabric, we present a simple toolbox for visualizing a combined contribution of two pre-defined end-member magnetic sub-fabrics. These end-member fabrics are defined by their bulk susceptibility, degree of anisotropy, shape of anisotropy ellipsoid and the orientations of its principal directions. The toolbox is part of Anisoft software which enables the instant visualization how magnetic fabric changes as a function of the relative content of the end-member sub-fabrics.

How to cite: Chadima, M.: A simple toolbox for evaluating the combined contributions to the whole-rock magnetic fabric, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17170, https://doi.org/10.5194/egusphere-egu23-17170, 2023.

The Reykjanes Ridge is located in the North Atlantic Ocean, southwest of Iceland. Here, the oceanic crust is characterized by a series of V-shaped ridges (VSRs) and V-shaped troughs (VSTs), the formation of which has been linked to three alternative hypotheses: i) thermal pulsing, ii) propagating rifts, and iii) buoyant mantle upwelling. During International Ocean Discovery Program Expeditions 384 and 395C, a transect of five sites were drilled eastwards of the modern Mid-Atlantic Ridge (between 20-30°W) at a latitude of ~60°N, to investigate VSTs/VSRs formation.

In this preliminary study, we analyze basalt samples from four sites, two from VSRs and two from VSTs, using rock magnetic and anisotropy of magnetic susceptibility (AMS) techniques, to investigate the differences between VSRs and VSTs. We analyzed the samples at the CIMaN-ALP (Peveragno) and INGV (Rome) Laboratories of paleomagnetism through bulk susceptibility, AMS, stepwise demagnetization of natural remanent magnetization through alternating field and temperature, hysteresis loops and FORC diagrams, and susceptibility vs temperature curves. Rock magnetism was used to determine the rock magnetic properties of each sample and investigate its correlation with the degree of alteration observed in the basalts. The AMS was measured to determine the magnetic fabric as a proxy of the magmatic fabric, where, for instance, lava flow-like fabric would be typical of an unaltered basalt.

Preliminary results suggest that basalts from VSTs are generally characterized by higher susceptibility values, while the AMS shows a mixed behavior (well defined or dispersed) independently from the structural position. Further rock magnetic data, integrated with petrological, structural and geochemical data will be correlated to the pervasiveness of alteration in each site, the age of basalts and their distance from the Mid-Atlantic Ridge to test the three hypotheses.

How to cite: Satolli, S., Di Chiara, A., and Friedman, S.: Preliminary rock magnetic and anisotropy of magnetic susceptibility results from the Reykjanes Ridge basalts, Atlantic Ocean (IODP Expeditions 384 and 395C), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17243, https://doi.org/10.5194/egusphere-egu23-17243, 2023.

The deformation pattern in the interior of cratonic blocks has critical implications for regional structural evolution but still remains an unresolved question. The southwestern Ordos Basin is at the convergence of multiple blocks, leading to intense and complex tectonic stress at this location. However, how the interior of the Ordos block responds to the complex stress field around it is still elusive. This study analyzes the characteristics and evolutionary history of faults in the southwestern part of the inner Ordos Basin using 3D seismic data. Three dominant sets of faults trending NW, NEE, and N–S have developed in the study area. All three sets of faults have subvertical dip angles and straight fault traces and structures that are common in such fault zones, such as flower structures and en-echelon and horsetail arrangements (all indicating strike-slip movement) are present. Structural deformation varies from the center to the periphery of the Ordos block. All three sets of fault systems exhibit small displacements. The polarity of the strike-slip changed in different periods, reflecting the transformation of the stress field in the southwestern Ordos Basin. This illustrates that the formation of cratonic faults is controlled by the regional stress field, while the related strain pattern in the interior of the cratonic block is very different from the deformation around its periphery. These characteristics also demonstrate the special nature of cratonic fault deformation.

How to cite: Wang, Z. and Huang, L.: Characteristics and evolution of strike-slip faults in a stable cratonic block, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1208, https://doi.org/10.5194/egusphere-egu23-1208, 2023.

The Calabria-Lucania border region represents the transitional area between the Southern Apennines and the Northern sector of the Calabrian arc. Roughly the whole Apennine chain is struck by more or less intense earthquakes. While the northern and central parts of the chain are characterized by foreland contraction and hinterland extension, the Southern Apennine is characterized by a strike-slip kinematics in the eastern sector and by an extensional regime in the western sector. Strike-slip earthquakes have been observed also in the axial part of the Campania-Molise Apennines, rightly beneath the active extensional sector.

The Calabria-Lucania border region is considered a seismic gap in the Apennine chain; few paleo-earthquakes, with magnitude ranging from 5 to 7, have been recorded in the area. During 2010-2014 the region was affected by a low-moderate instrumental seismicity (known as Pollino seismic sequence): thousands of earthquakes occurred. Analysis of that seismicity revealed a shallow hypocentral distribution located into the first few km below the surface, and focal mechanisms of the strongest events of the sequence are consistent with upper crustal extensional deformation. While the shallow seismicity of Calabria-Lucania border region has been deeply studied after the 2010-2013 sequence, the sporadic deep seismicity needs a more detailed analysis.

As highlighted by previous studies, instrumental seismicity recorded from 2013 to 2015 reveals the presence of a sporadic deep (from 9 to more than 20 km) seismicity. The events located between 9 and 17 km deep have transcurrent to transpressional kinematics with NE-SW trending P axes; while deeper events show a strike-slip kinematics with NW-SE trending P axes.

We analyzed deep (> 10 km) seismicity recorded in the area from 2013 to nowadays. Starting from the picking of seismograms of more than 40 events (with M between 2.4 and 3.8), we analyzed the focal mechanisms of events computed using at least six good first motion observations. According to Ferranti et al., 2017, our results highlight the presence of a strike-slip/oblique kinematics at depths of more than 20 km. Between 10 and 20 km depth both dip-slip and strike-slip kinematics are present, with a predominance of the last ones, confirming the presence of a transition zone.  

Finally, we inverted the focal mechanisms dataset to infer about the stress field active in the region.

How to cite: De Matteo, A., Capuano, P., and Falanga, M.: Insights on the seismotectonic of Calabria-Lucania border region (Southern Italy): different tectonic styles at different depths, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2686, https://doi.org/10.5194/egusphere-egu23-2686, 2023.

The most common method of detecting subsurface structures on continental and marine surfaces is seismic imaging. Although technological advancements have been made, seismic analysis of carbonates remains challenging due to their strong petrophysical heterogeneity, which becomes more challenging when faults are incorporated. This work aims to produce unmigrated forward-seismic models to grasp the deformation behavior of carbonate-bearing fault systems and the related seismic response changes. The porous and faulted carbonates outcropping at the Majella Massif (central Italy) are here used as case study as an analogue of carbonate ramp reservoirs exploited worldwide. Field and laboratory petrophysical data of fault rocks collected at increasing distances from the fault planes show a damage zone/fault core architecture characterized by a decreasing porosity and an increase in shear modulus moving from host rocks towards fault planes. Starting from these observations, unmigrated stacked seismic models have been built simulating fault zones with both increased and decreased porosities with respect to the host rocks. Fault zones with lower porosity than the host rock show slight diffraction hyperbolas, while the diffractive component is pronounced in seismic images of fault rocks with higher porosity than the host rock. Such hyperbolas can be enhanced or weakened by modifying the dip angle of the fault plane or the width of the damage zone but a key role seems to be played by the decreased porosity. The existence of diffraction hyperbolas in unmigrated seismic models is then interpreted as evidence of a damage zone characterized by larger porosity compared to the host rock. Migrated stacked sections would not provide any evidence of increased porosity in the damage zone due to loss of information about the diffractive component resulting from the processing. Consequently, the absence of diffraction hyperbolas in actual unmigrated seismic images is suggested to be related to a decreased porosity in the fault zone. This can be related to cataclasis and solution/cementation of the damage zone rocks as observed in the study area, and related to confining stress acting at depth or fracture filling that counteracts the fracture-related increase in secondary porosity. On the other hand, diffraction hyperbolas in unmigrated seismic images can represent a clue of the presence of large-porosity fault zones.

How to cite: Tomassi, A., Trippetta, F., and de Franco, R.: Seismic signature of carbonate fault rocks changes with changing petrophysical properties: insights from unmigrated seismic forward modelling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3880, https://doi.org/10.5194/egusphere-egu23-3880, 2023.

The Philippine Sea Plate (PSP) has subducted northwestward beneath the Eurasian Plate (EP) in the northeast Taiwan and has obliquely collided with the EP creating the mountain building. The offshore area of northeastern Taiwan is subject to post-collisional collapse and under extensional regime, forming a series of normal faults. The NE-SW trending Ilan Fault is situated between the Ilan Plain and the Hseushan Range. The Ilan Fault is also call North Ilan Structure (NIS). According to the Taiwan Earthquake Model published by TEC team in 2020, the probability for a Mw 6.9 earthquake happened in NIS in future fifty years is estimated about 13%. However, if the NIS extends to the offshore area, the risk will become greater. This study aims to understand the possible offshore extension of the Ilan Fault and analyze the structural activity. For that, we have collected 24-channel sparker reflection seismic profiles and sub-bottom profilers across the possible fault trace. Based on sparker seismic profiles and sub-bottom profiles, four major normal faults, Fa, Fb, Fc and Fd, trending NE are observed. Faults Fa and Fb are active and have outcropped to the seafloor. A negative flower structure is observed along Fault Fa, which indicates that the regional stress is under transtension. Furthermore, slope failures, slumps and sliding surfaces are found. Overlying on the Last Glacial Maximum (LGM) unconformity, sedimentary layers are tilted in the hanging wall of Fault Fb, indicating Fault Fb is a growth fault. On the other hand, Faults Fc and Fd dip to the NW and SE, respectively. They bound a graben trending NE and only ruptured to the LGM unconformity. It implies that Faults Fc and Fd are no more active.

How to cite: Chang, C.-C., Hsu, S.-K., Lin, L.-K., Tsai, C.-H., and Lin, H.-S.: Offshore extension of the Ilan Fault in northeast Taiwan, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4803, https://doi.org/10.5194/egusphere-egu23-4803, 2023.

The offshore area of northern Taiwan is subject to post-collisional collapse and under an extensional regime. Because of the change from a compressional environment to an extensional environment, a series of normal fault structures has occurred. The reverse faults in the Gongliao area of Taiwan, include the Longdong Fault, Wentzukeng Fault, Aodi Fault, and Fangjiao Faul, are distributed from inland to the coastline. However, their prolongations to the offshore area are unknown. In order to understand the possible fault extension to the offshore area and the possible fault activity, we conducted Sparker reflection seismic surveys in the offshore area of Gongliao. The Sparker seismic system is suitable for shallow water surveys and can provide high-resolution shallow structures. To understand the geological structures in our study area, sequence stratigraphy, seismic facies and faults identification are used to analyze our seismic profiles. According to the sequence stratigraphy, the transgressive surface, the last glacial maximum (LGM) unconformity, and last maximum flooding surface can be identified. In addition, the sand wave base surface, syn-rift unconformity and the acoustic basement are defined. We have identified 5 normal faults (i.e. Fa, Fb, Fc, Fd and Fe) in the prolongation of the onshore faults. All the identified faults cut through the basement and caused large offsets, forming half-graben basins. For the activity of these faults, the strata in the half-graben basin A formed by Fault a, is tilted, which was probably caused by the continuous growth of the fault. The strata in the half-graben basin B formed by Faults b and c, are inclined below the LGM unconformity, but the strata above the LGM unconformity are relatively flat. In contrast, only the sediments in the upper part of the half-graben basin B show the characteristics of sequence stratigraphy, which means the sediments deposit controlled by sea level change. In summary, we infer that the faults b and c were active before the LGM and relatively stable recently. Faults d and are covered by thick sediment layers, indicating early structures not active at all.

How to cite: Lin, Y.-W., Hsu, S.-K., Tsai, C.-H., Cho, Y.-Y., and Lin, L.-K.: Post-collisional Geological Structures off Gongliao District in Northern Taiwan, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4924, https://doi.org/10.5194/egusphere-egu23-4924, 2023.

Although located far from any active plate boundaries, Central Australia features significant seismic activity, with a total of four M>6 events in the last four decades. The most recent such event was the May 25^th, 2016 Petermann Ranges earthquake (Mw = 6.1), which occurred close to the border triangle between the Northern Territory, South Australia and Western Australia. The last tectonic reactivation of the region that hosted the earthquake occurred during the intraplate Petermann Orogeny that terminated about 540 Ma ago. It is commonly assumed that although recently inactive, this region still constitutes a lithospheric-scale zone of weakness, so that stresses imposed on the rigid Australian plate at its edges can localize and lead to seismicity here. Previous studies have shown that the Petermann earthquake occurred on a splay fault in the direct vicinity of the Woodroffe Thrust, one of the principal shear zones of the Petermann Orogeny. It occurred at a very shallow depth (<= 5 km) and had a thrust mechanism with a NW-SE oriented rupture plane. Due to its shallow depth, it created a surface rupture that was mapped over a length of about 20 km.

In the present study, we utilized a temporary deployment of 11 seismic stations that was installed in the aftermath of the Petermann earthquake to characterize its aftershock sequence. Since only a single permanent station was operating within a radius of 400 km around the rupture area before the deployments, we do not have much information about the earliest part (first two weeks) of the aftershock series.

We combine two different event catalogs, a handpicked one comprising 1231 events for the first 3.5 months of the aftershock sequence, and a semi-automatically derived one that contains a total of 4918 events. We derived an optimal 1D velocity model and station corrections from the handpicked catalog, and relocate all events with this model. In a second step, we apply a double-difference relocation to the entire dataset. Most of the relocated events occurred at depths between 2 and 4 km, and outline a tight NW-SE striking an NE-dipping plane that aligns well with the mapped trace of the surface rupture. As already indicated in previous studies, we also find scattered aftershock activity within the footwall of the fault.

We further applied a template-matching algorithm in order to further decrease the completeness magnitude of the catalog and to get the best possible estimate for event rate decay over time. Moreover, we present fault plane solutions for a few of the largest aftershocks.

How to cite: Sippl, C., Brenn, G., Shamsalsadati, S., and Tkalčić, H.: The aftershock series of the 2016 Petermann Ranges earthquake in Central Australia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5141, https://doi.org/10.5194/egusphere-egu23-5141, 2023.

The northern Tyrrhenian Sea separates the northern Apennines of the Tuscany coast (Italy) from the Corsica island (France). 

A comprehensive revision of a vast dataset of vintage public seismic reflection profiles was conducted, re-elaborating via dedicated vectorization codes to improve their resolution and interpretability. 

The bathymetry of this sector shows a very regular and almost flat geometry of the seafloor. Despite this, a close look at seismic profiles reveals an articulated topography of the pre-Neogenic deformed acoustic basement. This is organized in thrust-related structural highs and narrow, N-S and NNW-SSE trending basins, filled by sedimentary successions separated by unconformities.

Nowadays, the sedimentary sequences associated with the most recent evolution of the Tyrrhenian Sea completely sutured the previous morphology.

To date, we found strong evidence regarding the role of structural inheritance in shaping the current architecture of the shallow crust. We identified an intimate relationship between the thrust-related structural highs and the basins' position at the antiforms' forelimb and backlimb. Indeed, the Tuscan Shelf neogenic basins started to develop as intermontane fault-controlled basins along the flanks of the inherited antiforms.

We performed a structural analysis of the faults bounding the basins and a detailed seismic-stratigraphic analysis of the Neogenic succession deposited into such basins to reconstruct the Tyrrhenian Sea extension's initial phases and embed it into the broader evolution of the Mediterranean region.

The evolution of the sedimentary basins from the middle Miocene to the Pleistocene provides a more comprehensive and robust picture of the Tyrrhenian Sea. We were able to track the progressive activation and deactivation of high-angle normal faults controlling the basins deposition and the eastward migration of the extensional front. Such a setting influenced the asymmetrical or symmetrical evolution of the basins. Intriguingly, and partially in contrast with previous works, no evidence of low-angle normal fault was observed.

We also present the first reconstruction of a 3D geological model of the southern Tuscany offshore between Elba Island and Monte Argentario promontory (Italy). 

Such a model poses new questions on the crustal-scale mechanisms responsible for the extensional process, also establishing a unique starting point for fully unraveling the Tuscan Shelf and the Tyrrhenian Sea early stages of evolution.

How to cite: Buttinelli, M., Mazzarini, F., Musumeci, G., Maffucci, R., and Cavirani, I.: Tectonic-Sedimentary evolution of the Tuscan shelf (Italy): seismic-stratigraphic analysis of the Neogenic succession in the northern Tyrrhenian Sea between Elba Island and Monte Argentario promontory, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5350, https://doi.org/10.5194/egusphere-egu23-5350, 2023.

Unveiling the geometry and kinematics of subsurface seismic faults is important for earthquake hazard assessment. Earthquake focal mechanisms can provide such fundamental aspects of faulting; however, they often require additional information to distinguish faults and auxiliary planes. We attempt to identify faults using a stress inversion technique, in which fault planes in individual focal mechanisms are selected based on the fault instability parameter. This stress inversion algorithm developed by Vavryčuk (2014) selects a higher instability nodal plane as the fault and finds 70-80% faults correctly to derive reliable stress results. Our tests using synthetic and simulated focal mechanism data with faults known beforehand show that faults are correctly identified especially when the instability of the selected fault plane is significantly higher than that of the auxiliary plane, which can be quantitatively expressed by the instability ratio of the fault plane to that of the auxiliary plane being higher than ~1.3. This constraint can improve further the ability to identify subsurface seismic faults. We apply this technique to the case of geothermal-induced earthquakes that occurred in Pohang, South Korea during 2016-2017. A total of 53 well-constrained and well-located focal mechanism data are inverted to derive a stress condition, during which faults are identified as higher instability nodal planes. These earthquakes occurred in spatially distinct portions of the region associated with water injection through two respective boreholes (PX-1 and PX-2). For the PX-2-related earthquakes, 70% of identified faults are well aligned in their locations and orientations with a large-scale fault, indicating that these earthquakes occurred on the patches of this fault. This fault is responsible for the 2017 Mw 5.5 main earthquake. Fault planes whose instability ratio is higher than ~1.3 are all consistent with this plane. There is more variation in identified fault orientations in PX-1 earthquakes. However, a few fault planes with high instability ratios are generally subparallel to one another. The locations and orientations of these high instability ratio planes are well aligned with a large-scale fault, which is different from, but subparallel to the PX-2 fault. This study demonstrates the possibility of identifying and imaging subsurface seismic faults only using faulting mechanics without other additional information.

How to cite: Chang, C.: Identifying seismic fault geometry from focal mechanisms based on fault instability ratio during a stress inversion, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5622, https://doi.org/10.5194/egusphere-egu23-5622, 2023.

The accurate characterization of microearthquake sequences allows seismologists to shed light on the physical processes involved in earthquake nucleation, the deformation processes underlying rupture activation and propagation, and to image faults geometry at depth. The current methodologies used for this purpose first need the event detection and the phase-picking - usually manual-based - and earthquake locations, which require plenty of work even by expert analysts particularly in the case of microearthquake signals, commonly noise contaminated. Thus, improving standard procedures through semi-automatic or fully-automatic workflows would be an essential step forward towards the more efficient analysis of seismic sequences.

Here we show the results of a semi-automated template matching and machine-learning based workflow applied for the characterization of the foreshock-mainshock-aftershock microearthquake sequence occurred close to Castelsaraceno village (High Agri Valley, Southern Apennines, Italy) in August 2020. The analyses were performed on seismic data mainly recorded by a local seismic network belonging to the High Agri Valley geophysical Observatory (HAVO) deployed in the study area and located at a maximum epicentral distance of ~20 km from the seismicity cluster.

The application of the semi-automated single-station template matching technique to the continuous data-streams of the two nearest stations of the HAVO network (from 28th July to 12th October 2020) allowed us to detect more than twice the number of microearthquakes previously identified by standard manual detections. The phase-picking was automatically performed through a deep-learning algorithm (Phasenet) on the 202 ultimate detected microearthquakes. Finally, an automatic multi-step absolute and relative earthquake location procedure was carried out.

A total of 76 events were identified as belonging to the Castelsaraceno sequence, which occurred in a short time span (7-12 August) and in a limited range of depths (10 -12 km). Both the Ml 2.1 foreshock doublet and the Ml 2.9 mainshock occurred on 7 August ruptured the same seismogenic patch, thus suggesting the presence of a persistent asperity. The integrated analysis of the aftershocks distribution, the focal mechanism of the mainshock, and the geological framework of the study area, allowed revealing the seismogenic fault, not currently mapped in literature: a NE-SW striking (225°), high-angle (55°) fault with a left-lateral transtensional (rake -30°) kinematic. We also hypothesize that the seismic sequence occurred at depth in a brittle layer of the crystalline basement confined between two regions with more ductile rheology; futhermore, the estimated b-value (0.73±0.04) indicates the occurrence of the sequence in a relatively low-heterogeneity material and suggests the unimportant effect of pore-fluid pressure in driving its evolution. 

How to cite: Panebianco, S., Serlenga, V., Satriano, C., Cavalcante, F., and Stabile, T. A.: The analysis of the Castelsaraceno microearthquake sequence (southern Italy) through a semi-automated template matching and machine-learning approach reveals an anti-apenninic fault, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5677, https://doi.org/10.5194/egusphere-egu23-5677, 2023.

Subsurface characterisation of geothermal fields is important for the expansion of geothermal energy as a low-carbon resource. Faults and fractures provide secondary permeability, thus, their characteristics are crucial parameters in deep geothermal fields. Analysis of ambient seismic noise provides a relatively cheap and widely accessible method for constraining faults and fractures in geothermal settings.

Three-component (3C) beamforming is an array-based method that extracts the polarizations, azimuths, and phase velocities of coherent waves as a function of frequency from ambient seismic noise, offering a comprehensive understanding of the seismic wavefield. 3C beamforming can be used to determine surface wave velocities as a function of depth and the direction of propagation of waves. It is assumed that anisotropic velocities relate to the presence of faults, giving an indication of the maximum depth of the permeability essential for fluid circulation and heat flow throughout a geothermal field. Previous results suggests that some structures have a stronger effect on surface wave velocities than others. Numerical models are essential to study these relationships in more detail.

Here we present a numerical simulation of wave propagation through a model of the subsurface, with anisotropy depicted as faults. This is employed by a rotated staggered grid (RSG) finite-difference (FD) scheme. We model a homogeneous half-space with a fault-like structure (40 m width), changing fault parameters, such as depth, width, velocities and internal conditions of the fault (“fill”). We generate surface waves from a single source as well as multiple sources emulating an ambient noise wavefield.

We then use 3C beamforming on the synthetic data to characterise the modelled wavefield and observe the types of waves present. The polarisation and beam power of the synthetic data denote the composition of the synthetic wavefield and what percentage are retro- and prograde Rayleigh waves and Love waves. To investigate the strength of anisotropy introduced by a single fault we propagate surface waves across the fault in different directions, estimating velocities from array recordings using the beamformer. We are further able to assess the sensitivity of Rayleigh waves towards anisotropy at depth by considering Rayleigh waves at different frequencies sampling different depths.

How to cite: Kennedy, H., Löer, K., Gilligan, A., and Finger, C.: Characterising faults in geothermal fields using surface waves: a numerical study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5756, https://doi.org/10.5194/egusphere-egu23-5756, 2023.

The Pollino area, one of the largest seismic gaps in Italy, has been struck between 2010 and 2014 by a long-lasting seismic sequence. More than 10,000 small-to-medium earthquakes followed a temporal evolution typical of a seismic swarm and, to a lesser extent, of  aftershocks following the two strongest events: a M_L 4.3 on 28 May 2012 and a M_L 5.0 on 25 October 2012. A delay of almost 4 months separated the two main events, with the first event occurring two years after the beginning of the swarm. A slow slip event began about three months before the strongest earthquake. High V_P and high V_P/V_S values have been found in the swarm area, where clusters of events of similar waveforms have been identified in recent works. The distribution of seismicity has been driven by pore fluid pressure diffusion with relatively low diffusivity value.

The present work aims to provide the first 3D images of scattering and absorption of the Pollino area at different frequency bands, measured through peak delay mapping and coda-attenuation tomography, respectively. We collected 870 earthquakes from the 2010 - 2014 seismic sequence and surrounding area, characterized by M_L > 1.7, already applied in a recent tomographic work. We used the manual P-wave pickings of the waveforms to compute the peak delay as the lag between the P-wave onset and the maximum of the envelope. Instead, the coda window has been fixed for the entire dataset at 30 seconds after the origin time of the earthquakes, lasting for 15 seconds. This late lapse time allows us to interpret Q_c^-1 as a marker of the absorption.

The preliminary results show a high scattering anomaly characterizing the seismogenic volume of the sequence and the newly identified faults surrounding the focal area. A strong scattering contrast has been identified south of the M_L 5.0 plane. This contrast is likely related to the presence of a segment of the Pollino Fault that acts as a barrier for the Southern propagation of the sequence. High attenuation anomalies in areas already marked by high V_P and high V_P/V_S confirm the role that fluids played in this complex sequence. These results, together with the recent outcomes, could give more insights about the seismic hazard of this complex area.

This work was supported by the PRIN-2017 MATISSE project (no. 20177EPPN2), funded by the Ministry of Education and Research.

How to cite: Napolitano, F., Amoroso, O., De Siena, L., Gabrielli, S., and Capuano, P.: 3D scattering and absorption imaging of the Pollino seismic gap (Italy), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7651, https://doi.org/10.5194/egusphere-egu23-7651, 2023.

The scientific project TESIRA (TEst Site IRpinia fAult), funded in 2021 by the University of Naples “Federico II”, aims at acquiring multidisciplinary geophysical data above an active fault in an intramontane basin of the Southern Apennines and to achieve, through the integration of this multivariate dataset, an accurate 3D geophysical imaging of the shallow structure of the fault zone in order to understand the link between shallow faulting and petrophysical changes, which affect rock permeability and surface degassing. The target structure is the southern branch of the Irpinia Fault, one of the structures with highest seismogenic potential in the Mediterranean region, causing the 4^th Italian earthquake of last century (1980, Ms=6.9, Pantosti & Valensise, 1990) and generating a modest surface throw at Pantano San Gregorio Magno (Salerno).

A microgravimetric survey and a 3D and 2D Electrical Resistivity measurements survey were acquired between September 2021 and January 2022. 3D seismic data were acquired in July 2022, using two overlapping arrays with a dense geophone distribution covering an area of about four hectares, with a detail of 2.5x2.5m. Moreover, four 2D seismic profiles intersect the 3D volume. An aeromagnetic survey, an extension of the gravimetric survey and a sampling of the CO₂ surface degassing will be completed within this year. We will show the preliminary results of the individual surveys. Later, the different geophysical and geochemical measurements will be integrated using cooperative inversion and machine learning techniques.  We hope that this multidisciplinary approach will provide a more comprehensive understanding of the interaction between surface faulting and basin development in this key area of the Southern Apennines.

References

Pantosti, D.; Valensise, G.; [1990] Faulting Mechanism and Complexity of the November 23, 1980, Campania-Lucania Earthquake, Inferred From Surface Observations, JGR, 95, 319-341

How to cite: Bruno, P. P. G., Ferrara, G., Improta, L., Maraio, S., Di Fiore, V., Iacopini, D., Fusco, M., Punzo, M., Paoletti, V., Cavuoto, G., and De Martini, P. M.: High-resolution 3-D geophysical imaging across a seismogenic fault: the TEst Site IRpinia fAult (TESIRA) project, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11176, https://doi.org/10.5194/egusphere-egu23-11176, 2023.

Reykjanes Peninsula (SW Iceland) is an extraordinary place from the geophysical perspective. Lying on the on-shore part of the Mid-Atlantic Ridge, interlaced by volcanic systems and hosting several high temperature geothermal areas, the seismic activity on the Peninsula is generally persistent on a microseismic level, but occasionally reaching up to M_L~5-6. Throughout the years, many temporary seismic stations or small to medium size local seismic networks have been deployed there for various purposes, from geothermal prospection monitoring to short time passive seismic experiments. We analyzed 10 years of natural seismicity recorded by the semi-permanent local seismic network REYKJANET (in operation since 2013) together with several permanent stations of the SIL regional seismic network present in the area of interest, in total number of 22 stations covering an area of about 1200 km². The timespan, 2013-2022, contains times of relative quiescence, several small tectonic earthquake swarms as well as very active periods of volcano-tectonic origin, with dyke intrusions and larger earthquakes of magnitudes up to M_L- 5.4. We study the distribution of epicenters of the background seismicity as well as for the several seismically active periods with a consistent set of stations.

We compare several different automatically derived earthquake catalogues using different detection and location algorithms, and their common features such as the upper and lower limit of the epicenter occurrence, seismogenic faults, aseismic zones and void areas. The dataset contains dyke intrusions related events during the the 2021-2022 Fagradalsfjall volcano-tectonic event.

Depending on the exact quality criteria and method used, we deal with over 100,000 events with high quality stable locations imaging the subsurface beneath the Reykjanes Peninsula.

How to cite: Doubravová, J., Magnússon, R. L., Konrádová, D., Horálek, J., Fischer, T., Águstsdóttir, T., and Gudnasson, E. Á.: 10 years of seismicity on the Reykjanes Peninsula, SW Iceland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11539, https://doi.org/10.5194/egusphere-egu23-11539, 2023.

In this study, we explore the potential to image the seismic velocity structure of moderate-sized, upper-crustal seismogenic fault zones by means of local earthquake tomography (LET) methods. The study region, located in the transition zone between the Central and Western Alps, represents one of the most seismically active and hazardous areas within the Alpine Arc. Over the past 500 years, several damaging earthquakes with Mw up to 6.2 are documented in historical earthquake catalogs in the vicinity of the Rhone-Simplon Fault (RSF), the dominant tectonic feature of this region. In particular, two major seismogenic structures are imaged by instrumental seismicity on either side of the RSF. To the north, seismicity occurs in the approximately NE-SW striking, 30–40 km long Rawil Fault Zone (RFZ). To the south, diffuse seismicity occurs within the hanging wall of the Pennine Basal Thrust, forming the Penninic Fault Zone (PFZ).

Owing to the dense instrumentation and above-average seismic activity in the study region, the Pg and Sg travel-time data recorded since the year 2000 by the Swiss Seismological Service is of exceptionally high quality and allows for high-quality 3D LET images of the uppermost crust, potentially imaging the damage zones of seismogenic faults. Relative double-difference locations of a recent earthquake sequence within the RFZ, on the other hand, indicate that the width of this fault zone is only on the order of 1 km. Imaging such narrow fault zones with standard LET methods therefore requires model parametrizations with grid spacing of few kilometers and less. Such dense grid spacing, however, poses several challenges in terms of model resolution and the reliability of LET inversion results, especially in less well constrained parts of the model.

To minimize such effects, we therefore tested and applied two different LET inversion strategies to derive 3D Vp and Vs models. The first strategy follows the common approach to compute a minimum 1D model as initial model for the 3D LET. The second strategy uses a coarser 3D regional LET model as initial model for the high-resolution 3D inversion. Synthetic tests suggest that a minimum image resolution of 5x5x3 km can be achieved with the current data, covering a region of about 125x125 km between 0 and 10-15 km depth. The 3D Vp and Vs models derived with the two initial-model strategies are remarkably similar within well resolved parts of the model. This similarity indicates that the anomalies in these parts are well constrained by the data and the solution is stable with respect to differences in the two initial models. On the other hand, the results suggest that results derived with the 3D initial model are more reliable in regions of reduced resolution. Additional synthetic tests were performed to document the potential resolution for hypothetical damage-zone scenarios and to support the interpretation of the derived models presented in this study.

How to cite: Diehl, T., Lee, T., Kissling, E., and Wiemer, S.: High-resolution local earthquake tomography of seismogenic structures along the Rhone-Simplon fault zone (Swiss Alps), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13042, https://doi.org/10.5194/egusphere-egu23-13042, 2023.

The demand for reliable high-resolution reflection seismic exploration campaign in hard-rock environments increases continuously. The target of such surveys varies and covers e.g. mineral exploration, geothermal reservoir characterization or the exploration of potential nuclear waste deposit sites. Although the exploration targets are very different, the expectations on the seismic images and the challenges for data acquisition and processing are similar. The expected structures are often steeply dipping with varying strike directions and conflicting dip situations. Furthermore, the reflection seismic data often has to be acquired on land, possibly in populated areas, or in areas with severe accessibility restrictions. These limitations lead to irregular or sparse datasets in combination with sometimes low signal-to-noise ratio for the target reflections in the recorded data.

Therefore, robust imaging methods are needed to generate high resolution reflection seismic images for such kind of data. Focusing prestack depth migration methods have proven to deliver improved image quality compared to standard time- or depth migration approaches. We show the results of our focusing pre-stack depth migration techniques applied to a vintage 3D seismic data set (ISO89) acquired in 1989 around the German deep continental drill site (KTB). We show a comparison to other previously obtained seismic images for the same data set and how the image quality evolved over time.

How to cite: Hlousek, F. and Buske, S.: New insights into vintage 3D reflection seismic data in hard-rock environment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13872, https://doi.org/10.5194/egusphere-egu23-13872, 2023.

The Istituto Nazionale di Geofisica e Vulcanologia (INGV) monitors the Italian peninsula seismicity by using the data recorded by the Italian National Seismic Network (RSN) together with the ones gathered by other permanent regional networks (PRN). Earthquakes are real-time located by the INGV surveillance system and manually revised by the Italian Seismic Bulletin (BSI) group analysts.

Starting from a catalog composed by homogeneous absolute locations (CLASS; Latorre et al., 2023), obtained by using a 3D regional-scale velocity model, we generated a catalog of relative seismic locations (CARS) of about 310,000 events occurred in Italy during 1981-2018.

We inverted absolute P- and S- waves arrival times derived from data collected by RSN plus PRN for the period 1981-2008 and only by RSN for 2009-2018 to apply the double-difference relocation algorithm (Waldhauser and Ellsworth, 2000).

For the second period, we combined the absolute travel times with relative ones obtained by waveforms cross-correlations analysis performed on pairs of similar events. The time domain cross‐correlation method proposed by Schaff et al., 2004 and Schaff & Waldhauser, 2005 was applied to seismograms of all pairs of events separated by 10 km or less and recorded at common stations. Seismograms were filtered in the 1–15 Hz frequency range using a four pole, zero phase band‐pass Butterworth filter. The correlation measurements were performed on 1.0 s long window for P- and S-waves. We collected a total of ~17 million P- and ~23 million S-wave delay times, retaining all measurements with correlation coefficients greater than 0.7.

1D velocity models characterising 18 different (geologically, seismically and tectonically homogeneous) Italian macroareas (after Pastori et al. B2-2019-2021, Wp1-task4) were used in the location procedure.

To cope with the memory limits (15,000 events with less than 200 readings) of the HypoDD code so to use it in a steady operational mode, we first subdivided the study region in the 18 macroareas related to the velocity models, then we additional discretize each in 100x100km2 cells, overlapped by the 80% in longitude and latitude. We repeatedly produced hypocentral locations of the same events that we merged by computing a final weighted mean location.

We present the double-difference catalog of Italian seismicity, allowing to depict alignments clearer with respect to the starting catalog (CLASS), to be related to seismogenic faults and/or to regional structures along the whole Italian peninsula.

How to cite: Michele, M., Di Stefano, R., Chiaraluce, L., Latorre, D., and Castello, B.: CARS - Catalog of Relative Seismic Locations of 1981-2018 Italian Seismicity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14033, https://doi.org/10.5194/egusphere-egu23-14033, 2023.

The Fucino basin (central Italy) is the largest Plio-Quaternary tectonic depression of the Apennines extensional belt. The basin is bounded to the north and east by two main normal fault systems striking WSW-ENE and NW-SE, respectively. These fault systems controlled the syntectonic depositions of lacustrine and coarse clastic sequences that reach a total thickness of 1.5 km. The NW-SE fault system is the source of the Mw 7.0, 1915 central Italy earthquake and of previous M6-7 earthquakes recognized through paleoseismic trenching. On the other hand, current activity and seismogenic potential of the WSW-ENE structures are uncertain. The shallow architecture of both fault systems (< 2 km depth) is well defined by surface data and seismic reflection profiles, but the fault’s deep geometry is poorly known. Large uncertainties also regard the crustal structure underneath the basin at seismogenic depths (i.e.; 5-15 km depth) despite a close deep seismic reflection profile (i.e., CROP11 line). The instrumental seismicity occurring beneath the Fucino basin is scarce. On the contrary, an intense activity concentrated to the north (2009, Mw 6.3, L’Aquila sequence) and 25-30 km to the south, where both low-to-moderate sequences and diffuse swarm-like seismicity were recorded in recent years. In 2008-2009, a dense passive seismic survey was carried out in the Fucino area to investigate the basin seismic response and local site effects. The temporary network included 18 stations, with an average spacing of 2-3 km, operating in continuous mode with a sampling rate of 125 Hz and equipped with 5 second seismometers. In this study, we re-processed the data recorded by the Fucino temporary network, integrated by the permanent stations of the Italian seismic network and Abruzzo regional network installed on the surrounding ridges, to construct a new earthquake catalog and perform a local-scale passive tomographic survey. We used a standard (STA/LTA) algorithm to detect very local weak events in addition to those used in the previous site-effects study. P- and S-wave arrival times of the detected seismic events were hand picked and weighted according to a standard scheme. Seismograms for stations deployed in the Fucino basin show strong complexities especially for P-waves onsets that are often masked by background noise. We used the final dataset in terms of P- and S-waves arrival times as input for a Local Earthquake Tomography targeting the upper crustal velocity structure and active faults underneath the Fucino basin and surrounding ridges. The tomographic model, presented in terms of Vp and Vp/Vs, aimed at recovering the crustal heterogeneities with a spatial resolution finer with respect to previous tomographic surveys of central Apennines. The 3D distribution of Vp and Vp/Vs and of relocated events helped us to identify the velocity contrasts related to the main faults and to improve our knowledge on their geometry at depth.

How to cite: De Gori, P., Improta, L., Vassallo, M., Cara, F., De Luca, G., Frepoli, A., Bagh, S., and Valoroso, L.: A local earthquake tomography model of the Fucino fault-controlled basin (central Apennines, Italy) obtained through a very dense temporary network, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14206, https://doi.org/10.5194/egusphere-egu23-14206, 2023.

Precise earthquake locations are a prerequisite for determining real fault structures. To improve the precision of the event location, a few relative locations methods are commonly used to refine event locations. Relative relocation methods reduce effects of an imperfect velocity model and errors due to arrival time measurement. We performed comparative tests of tree different relocation methods: HypoDD (HD), GrowClust (GC) and Master event (ME). We tested the efficiency and differences in the event locations using these three methods on dataset from REYKJANET seismic network operating in Iceland on Reykjanes Peninsula. All these methods provide substantially focused shapes of clusters compared to the absolute event locations but the locations of individual events differ evidently depending on the method used.

We also aimed at an effect of the control parameters of HD, GC and ME on final location results and their optimization as well as computational and memory demands.

How to cite: Konrádová, D., Horálek, J., and Doubravová, J.: Comparison of relative locations methods and their accuracy for determining fault structures, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14260, https://doi.org/10.5194/egusphere-egu23-14260, 2023.

Machine learning is rapidly becoming ubiquitous in the Earth Sciences promising to provide scalable algorithms for data-mining, interpretation, and model building. Initially heralded for its ability to exclude complicated physics from data analysis, recent innovations seek to merge machine learning  solutions with conventional physics-based methods in order to enhance their capability

We present a novel eikonal tomography approach for Rayleigh wave phase velocity and azimuthal anisotropy based on the elliptical-anisotropic eikonal equation, by formulating the tomography problem as the training of a physics informed neural network (PINN). The PINN eikonal tomography (pinnET) neural network utilizes deep neural networks as universal function approximators and extracts traveltimes and medium properties during the optimization process. Whereas classical eikonal tomography uses a generic non-physics-based interpolation and regularization step to reconstruct traveltime surfaces, optimizing the network parameters in pinnET means solving a physics constrained traveltime surface reconstruction inversion, tackling measurement noise and resolving the underlying velocities that govern the physics. The fast and slow velocity and the anisotropic direction information can be directly evaluated from the trained medium property networks. Checkerboard tests indicate that the input velocity model can be well recovered by using this approach and synthetic data.

We demonstrate this approach by applying it to multi-frequency surface wave data from ChinArray phase II sampling the north-eastern Tibetan plateau. We are able to use much less data to achieve similar subsurface images because of the benefit of including the physics constraint while reconstructing the traveltime surfaces. We are able to obtain excellent results using only 10 sources.  Comparing results from pinnET with conventional eikonal tomography, we find good agreement with distinct low velocity structures beneath the Songpang-Ganzi block, Qilian and Western Qinling Orogen. Large phase velocity uncertainties occur in a small part of the southeastern Ordos Block, the western Songpan-Ganzi Block and the eastern Sichuan basin, which correspond to the reduced data coverage dependent on the selection of the 10 sources. We also verify the accuracy and reliability of the pinnET by choosing only one station as virtual source, the retrieved velocities show relatively good resolution which is much better than in conventional eikonal tomography using similar sized datasets. The method is memory efficient because compressing the traveltimes as outputs to a NN is a concept akin to compressed sensing and offers advantages over traditional anisotropic eikonal tomography or neural network approaches.

How to cite: de Ridder, S., Chen, Y., Rost, S., Guo, Z., and Chen, Y.: Rayleigh wave tomography in the north-eastern margin of the Tibetan Plateau by way of training physics-informed neural networks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15556, https://doi.org/10.5194/egusphere-egu23-15556, 2023.

We re-located 70 earthquakes belonging to the seismic sequence started on 2021 March 27^th, with a mainshock of Mw 5.2 at 13:47 UTC, in the Central Adriatic region (Italy off-shore) by using the on-purpose designed code by Zhao et al. (2007, 2011), modeling the sP converted phases.

The mainshock of the 2021 seismic sequence occurred about 20 km north of the Palagruza island, 80 km from the Gargano promontory and about 40 km from the Croatian island of Lastovo. It was felt in many central-southern Italian regions, from Ancona to Foggia, and in Central Dalmatia. All the epicenters of this seismic sequence lie in the open sea, about 100km to the SE and about 50 km to the NW of the 2003 Jabuka seismic sequence, and the 1988 Palagruza seismic sequence, respectively.

Though the seismicity in the central Adriatic Sea has been recorded by improving seismic networks, especially in recent decades, the precise location of the Adriatic offshore earthquakes was hampered mainly by the large distance of the closest stations, and by the large gap in the distribution of seismic stations.

The possibility to model the sP depth phases enables us to estimate the epicentral parameters and focal depths of these offshore earthquakes more accurately, thanks to the peculiar ray-path that mimics the presence of a receiver approximately on top of the hypocenter. The refined earthquake locations allow us to make inferences on the structure responsible for the seismicity of the 2021 seismic sequence, a thrust fault NW-SE striking and ~35° NE-dipping, and on its seismotectonic context.

The use of depth-phase arrival times to constrain the off-network events' locations is of particular interest to Italy due to both the peculiar shape of the peninsula and the extreme scarcity of seafloor stations, whose cost and management are very expensive and complex.

We present the first attempt to apply this off-network location technique to the Italian offshore seismicity with the aim of improving the hazard estimation of these hard-to-monitor regions.

How to cite: Di Stefano, R., Ciaccio, M. G., Baccheschi, P., and Zhao, D.: Accurate Earthquake Locations of the Adriatic Thrust Fault of the 2021 Seismic Sequence with sP Depth Phases., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15881, https://doi.org/10.5194/egusphere-egu23-15881, 2023.

A new approach is proposed for measuring the local dispersion curves of surface waves in weakly anisotropic media using a single, multi-component station, which consists of translation and rotation or strain. We directly extract the local azimuth-dependent phase velocity of the Rayleigh wave from the 6C amplitude ratio using seismic arrays deployed in Southern California. The extracted dispersion curves match well with the theoretical 2φ azimuthal anisotropy term. And the estimated fast wave direction is also consistent well with results calculated from SKS and beamforming methods which demonstrates the feasibility of studying local seismic anisotropy directly from 6C amplitude observations.

How to cite: Tang, L., Igel, H., and Montagner, J.-P.: Seismic Anisotropy from 6C Observations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1069, https://doi.org/10.5194/egusphere-egu23-1069, 2023.

Distributed Acoustic Sensing (DAS) in geothermal wells is a particularly attractive technology to implement as part of routine seismic monitoring of geothermal plant operations. It brings a large network of sensors close to the monitoring target – the operated reservoir – increasing the sensitivity towards low magnitude events and allows the application of processing procedures inspired by large network or array processing. However, the technical management of the large flow of produced data and the suitability of the strain-rate acquisitions to monitor locally induced seismicity was yet to be fully assessed.

We present the results of a continuous 6-month monitoring period that aimed at testing an integrated system designed to manage the acquisition, the processing and the saving of DAS data collected from behind casing at the Schäftlarnstraße (SLS) geothermal project (Munich, Germany). The data management system links the existing on-site infrastructure to a cloud Internet-of-Things (IoT) platform integrated into the company’s IT infrastructure. The cloud platform has been designed to deliver both a secure storage environment for the DAS records and optimized computing resources for their continuous processing.

With a special focus on seismic risk mitigation, we investigate the potential of the monitoring concept to provide sensitive detection capabilities, despite operational conditions, while ensuring efficient data processing in order to strive for real-time monitoring. Further analysis of the records confirm additional logging capabilities of borehole DAS. We also evaluate the ability of DAS to provide reliable seismic source description, in particular in terms of location, moment magnitude, and stress drop.

Using two detected local seismic events, we demonstrate the relevance of the system for monitoring the SLS-site in an urban environment, while complementing advantageously the surface seismometer-based monitoring network.

How to cite: Azzola, J. and Gaucher, E.: Continuous seismic monitoring of a geothermal project using Distributed Acoustic Sensing (DAS): a case study in the German Molasse Basin., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1292, https://doi.org/10.5194/egusphere-egu23-1292, 2023.

We have used two seabed fibre optic cables connecting Ny Ålesund and Longyearbyen at Svalbard, North of Norway, to track several whales for several weeks. Exploiting that we have access to two fibres we demonstrate that it is possible to track several whales in a fairly large region. It is possible to create sound records of whales that can be used for identification and discrimination between various species. The localization method has also been tested by using a small air gun to confirm the localization method used for whales. Examples of earthquake recordings, ship traffic monitoring and distant storms will be shown.

Based on the rapid and promising developments within DAS technology, there is a growing interest for using fibre optic cables at the moon. Some challenges and possibilities related to Lunar DAS applications will be discussed.

How to cite: Landrø, M.: Using DAS-fibres at ocean floor and lunar surface, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2091, https://doi.org/10.5194/egusphere-egu23-2091, 2023.

We present a novel seismic acquisition and processing technique to efficiently evaluate the local dispersion curves of Rayleigh waves for subsequent inversion of shear velocities and near-surface characterization.

The proposed approach consists of computing the ratio between the (time derivated) horizontal spectra H(f)=(∂_tV_x(f)²+∂_tV_y(f)²)^1/2  and the pseudo-divergence spectra D(f), with D being the sum of the horizontal gradients of the horizontal components (i.e. D=∂_xV_x+∂_yV_y).

The processing method itself is comparable to the commonly used H/V approach, except that the H/D spectral ratio provides a direct estimate of the frequency-dependent phase velocities c_R(f)  instead of the site frequency amplification(s). This is demonstrated using synthetic data.

We describe how the D component can be obtained in practice, i.e. by finite-differencing closely spaced horizontal phones or potentially using Distributed-Acoustic-Sensing (DAS) and fibre-optic deployed at the surface. Some limitations about wavelength dependency and impact of Love waves are discussed, as well as potential mitigation measures.

A field test on several hours of ambient noise data collected in Germany with multi-component geophones results in realistic values of Rayleigh wave velocities ranging from ~770 m/s at 10 Hz to ~500 m/ at 30 Hz. Thanks to the local and omni-directional nature of the estimation, the minimal number of required channels and the applicability to ambient noise, we believe that the proposed H/D method can be an attractive alternative to expensive array-based techniques.

How to cite: Edme, P., Sollberger, D., Kiers, T., Schmelzbach, C., Bernauer, F., and Robertsson, J.: Divergence-based estimation of Rayleigh wave dispersion curves, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2814, https://doi.org/10.5194/egusphere-egu23-2814, 2023.

A work describes a deeply etched, long active length, high sensitivity short Fabry-Perot cavity nano-strain resolution sensor. The presented sensors exhibit high spectral sensitivity, low intrinsic temperature sensitivity which is for about 40 times lower than in case of FBG, small size and mounting comparable to conventional Fiber Bragg gratings. The sensor high potential is not only high sensitivity and low temperature intrinsic sensitivity, but also in short cavity length and its tunability, which can be simply accomplished in one production step. This brings versatility in interrogation with different general purpose and cost-efficient VIS-NIR widely available linear detector array-based spectrometers, while still providing strain sensing resolution within the range of few 10 nε. A strain resolution of 20 to 70 nε was demonstrated when using a cost-efficient VIS spectrometer. Furthermore, the sensor structure can be combined with multimode telecom lead-in fibers and low-cost broadband LEDs intended for automotive/lightning applications, which allow production of cost efficient solutions.

How to cite: Pevec, S. and Donlagic, D.: Nano-strain resolution fiber-optic Fabry-Perot sensors based measuring systems, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3061, https://doi.org/10.5194/egusphere-egu23-3061, 2023.

Ocean water temperature measurements are fundamental to atmospheric and ocean sciences. Obtaining them, however, often comes along with major experimental and logistic challenges. Except for the uppermost ocean surface temperature, which can be measured from satellites, temperature data of the ocean is often poorly sampled or nonexistent, especially in deep-water regions.

Although Distributed Acoustic Sensing (DAS) technology has become popular because its high sensitivity to strains and mechanical vibrations, our work focuses on its usage on tens-of-kilometer-long underwater fibre-optic (FO) telecommunication cables to measure temperature anomalies at the seafloor at millikelvin (mK) sensitivity. This is possible because of the lack of dominant strain signals at frequencies less than about ∼1 mHz, as well as the poor coupling of the fibre with these signals while remaining highly sensitive to slow ambient temperature variations that locally affect its optical path length. DAS allows us to observe significant temperature anomalies at the continental shelf and slope of the Mediterranean sea, South of Toulon, France over periods of several days, with variability remaining relatively low at the deep ocean. By means of this approach, oceanic processes such as near-inertial internal waves and upwelling can be monitored at unprecedented detail.

Our observations are validated with oceanographic in-situ sensors and alternative Distributed Fibre Optic Sensing (DFOS) technologies established for temperature sensing. We outline key advantages of DAS thermometry over the aforementioned sensors in terms of spatial coverage, sensitivity, versatility and highest attainable frequency. At the current state of the art, DAS can only measure temperature anomalies as opposed to absolute temperature, a drawback that could be compensated via single temperature calibration measurements.

How to cite: Pelaez Quiñones, J., Sladen, A., Ponte, A., Lior, I., Ampuero, J.-P., Rivet, D., Meulé, S., Bouchette, F., Pairaud, I., and Coyle, P.: Monitoring temperature at the ocean seafloor with fibre optic cables and DAS, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3437, https://doi.org/10.5194/egusphere-egu23-3437, 2023.

Volcano seismology is essential for understanding, monitoring, and forecasting eruptive events. The use of distributed acoustic sensing (DAS) technology can be particularly useful for this purpose because of its high temporal and spatial resolution, which may help to overcome the challenges of deploying and maintaining seismic arrays on volcanoes.

Between 2020 and 2022, we installed 4 km of optical fibre on Stromboli volcano, Italy, whose persistent activity is well-suited for investigating the related dynamic strain rate. The cable was buried at a depth of 30 cm and the layout geometry was designed to provide wide coverage while being constrained by natural obstacles and topographical features. Seismometers were also installed along the fibre. DAS data were collected using a Febus A1-R interrogator, and the acquisition period increased from one week in 2020 to over four months in 2022. We recorded volcanic tremor, ordinary explosions (several per hour), two major explosions in 2021 and 2022, and the entire sequence of a pyroclastic flow in 2022. 

DAS and seismic data show good agreement in both time and frequency domains after converting strain rate to velocity and vice versa using different methodologies. Beamforming of DAS data shows a dominant signal in the 3-5 Hz frequency band coming from the active craters. We will also present preliminary results of major explosions and pyroclastic flow. This experiment demonstrates that DAS can be used for monitoring volcanic activity.

How to cite: Biagioli, F., Métaxian, J.-P., Stutzmann, E., Ripepe, M., Trabattoni, A., Bernard, P., Longo, R., Diana, G., Innocenti, L., Capdeville, Y., Bouin, M.-P., and Lacanna, G.: Using Distributed Acoustic Sensing to Monitor and Investigate Eruptive Events at Stromboli Volcano, Italy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3955, https://doi.org/10.5194/egusphere-egu23-3955, 2023.

Water resource management is a crucial socio-economic issue that requires developing high-resolution monitoring techniques, including non-invasive geophysical methods. Among them, passive seismic interferometry takes advantage of natural ambient seismic noise to recover the slight variations of the seismic wave velocity induced by changes in the groundwater level. In this study, we present the time and space monitoring of groundwater changes artificially generated by infiltration ponds at the exploitation field of Crépieux-Charmy (Lyon, France).  We deployed 3km of optical fibre and a dense array of fifty 3C geophones around infiltration basins. We recorded several cycles of filling-emptying with a DAS using a 2m spatial sampling (i.e., 1500 fibre sensors). The recorded signals are mainly associated with local anthropogenic noise (highways, trains, pumping, etc.). We could track seismic velocity variations with high temporal and spatial resolutions using ambient noise interferometry techniques. These variations are associated with the interaction between the water diffused from the basins and water table variations. This dynamic information helps understand and model water exchanges on the ground. The study confirms the possibility of groundwater monitoring using DAS records of ambient noise for seismic interferometry in a highly urbanized zone.

How to cite: Nziengui Bâ, D., Coutant, O., and Jestin, C.: Groundwater monitoring using fibre-optics and DAS: Application to the Lyon water catchment area., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4256, https://doi.org/10.5194/egusphere-egu23-4256, 2023.

In recent years, fiber-optic distributed acoustic sensing (DAS) has been gradually applied to seismology because of its long-distance and dense observation capability. It is a great challenge to effectively process the massive seismic data recorded by DAS. At present, the seismic data processing methods based on deep learning have achieved great success, especially in the tasks of seismic detection and arrival-time picking. However, due to the differences between DAS and geophone, such as sensing principles, spatial and temporal sampling rates, and noise intensity. The seismic arrival time picking model based on deep learning, which is trained by geophone seismic data with low spatial and temporal sampling rates and low noise intensity, severely degrades in performance on DAS seismic data with high spatial and temporal sampling rates and high noise intensity. In addition, a new seismic arrival time picking model is trained by fully supervised learning, which usually requires a large number of seismic data with accurate labels. However, the huge cost of manual picking and the lack of effective automatic picking models make it very difficult to build large-scale DAS seismic data sets with accurate labels. Therefore, it is very difficult to build an arrival time picking model based on fully supervised learning for DAS seismic data.

In this study, we propose a DAS seismic arrival time picking method based on fractional lower order statistics. Based on the difference of probability density function between noise and seismic signal, the proposed method uses alpha-stable distribution modeling noise (generally follow a Gaussian distribution) and seismic signal (generally follow a non-Gaussian distribution), and uses fractional lower order statistics under the assumption of alpha-stable distribution as the characteristic function to pick the arrival time.

Synthetic and actual DAS data tests show that the proposed method has better performance and robustness to random noise than other methods based on characteristic functions, such as STA/LTA, AR-AIC and kurtosis. Since the actual DAS seismic data has no ground truth of arrival time, we have further the performance of the proposed method on the geophone seismic data set. The proposed method provides better results on geophone seismic data and the data after up-sampling them to the typical time sampling rate of DAS.

How to cite: Wang, X., Wang, H., Wang, Y., Zeng, S., and Wang, Y.: Arrival Picking for Distributed Acoustic Sensing seismic based on fractional lower order statistics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4769, https://doi.org/10.5194/egusphere-egu23-4769, 2023.

Underground laboratories have become indispensable in the understanding of physical processes during e.g., hydraulic stimulation and seismic monitoring of deep geothermal reservoirs or CO₂ storage target reservoirs. They provide a test bench and constitute the bridge between small-scale laboratory studies and full-scale pilot sites. Here, we present results from multiple active source seismic campaigns in one of the Swiss underground laboratories: the Mont Terri Rock Laboratory. Here, DAS fibres are cemented behind the casing of multiple monitoring boreholes and active shots are taken with a P-wave sparker. This dense array of active seismic measurements enables us to obtain a baseline characterisation of the P-wave velocity of the rock before any activity (e.g., injection) takes place. During stimulations, dynamic measurements with an active sparker source are recorded, followed by a time-lapse monitoring approach where seismic measurements are collected through active seismic campaigns in set time intervals in the months after stimulations. In this way we can create high-resolution, four-dimensional monitoring and characterisations of the rock body and potential earthquakes during the full monitoring period. We show different configurations and measurements settings with their effect on the DAS recordings of active signals.

How to cite: Tuinstra, K., Rinaldi, A. P., Lanza, F., Zappone, A., Fichtner, A., and Wiemer, S.: Active-source seismic experiments with DAS for monitoring reservoir rock in underground laboratories, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5455, https://doi.org/10.5194/egusphere-egu23-5455, 2023.

Rotational motions can be recorded directly or derived from translational motion recordings. Fairly new rotational sensors allow seismologists to directly record and investigate rotational motions. In order to further investigate and compare recently developed rotational sensors an experiment was made in Fürstenfeldbruck. Within this scope, a vibroseis truck was operated starting from 20 November 2019, 11:00 UTC until 21 November 2019, 14:00 UTC. We recorded 480 Sweep signals at 160 different locations. The truck was operating at 30%, 50%, and 70% relative to a peak force output of 276 kN exciting the ground vertically and each sweep lasted 15 seconds starting with 7 Hz increased up to 120 Hz. We derived back azimuths of each sweep from 6 rotational sensors and calculated root mean squares of each component. We observed that within the first day, the North component of all sensors recorded the largest ground motion energy SV type of energy is dominant. The sweep sources were distributed over two North–South profiles and two East–West profiles.  While the truck moved to the east and its location moved from west to south of the rotational sensors, the signals dominate more and more on the East component.. From our preliminary results, we state that although having different signal to noise ratios all rotational sensor calculated the direction of each sweep. Thus, we can follow the movements of vibroseis truck using all rotational sensors.

How to cite: Izgi, G., Eibl, E., Krüger, F., and Bernauer, F.: Investigating Vibroseis Sweeps using 6 Rotational Sensors in Fürstenfeldbruck, Germany, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5701, https://doi.org/10.5194/egusphere-egu23-5701, 2023.

Seismic monitoring requires high temporal-spatial resolution and low deployment cost. Distributed acoustic sensing (DAS), as an emerging sensing technology for recording seismic data in recent years, can leverage communication cables for seismic monitoring, providing strong support for more intensive and real-time observation of geological activity. However, the traditional DAS phase unwrapping algorithms (PUAs) derived from Itoh requires the phase difference of adjacent pixels to be less than π, and thus make mistakes in the case of severe noise or large disturbance. In this paper, to the best of our knowledge, two-dimensional (2D) PUA is used to obtain seismograms in DAS for the first time. Satisfactory phase unwrapping is achieved by the 2D PUA method based on the transport of intensity equation (TIE), due to its robustness and noise immunity. Dynamic strain measurements in 80 m straight fiber-optic cable using homemade high-performance DAS, combined with TIE-based 2D PUA produce high-quality seismograms. Time Difference of Arrival (TDOA) Algorithm is applied based on the sensing signal of reliable channels in the seismograms, realizing the high-precision localization of the source. 2D PUAs apply to all phase-demodulation-based sensing techniques and are suitable for recovering spatially correlated objects such as seismic waves, thus having great potential in the field of seismic monitoring.

How to cite: Sun, J., Wang, Y., Zhang, J., Wan, A., Zhang, S., Ye, Z., Liu, F., Zhang, G., and Wang, Z.: Two-dimensional phase unwrapping algorithm aided high-precision source positioning with DAS, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5955, https://doi.org/10.5194/egusphere-egu23-5955, 2023.

Distributed Acoustic Sensing (DAS) is an acoustic sensor instrument that turns a single optical fiber into a dense array of thousands of equally spaced seismometers. Geoscientists and companies have an interest in investing in DAS technologies for better understanding the Earth by observing natural and anthropogenic seismic events or assisting in large infrastructure monitoring with low installation and maintenance costs. However, this type of instrument generates a significantly larger amount of data than conventional seismometers, data that can be complex to store, exploit and interpret.

Several strategies for classifying seismic events from fiber-optics DAS data exist in the scientific literature. Conventional approaches rely on the use of features that describe the waveforms and frequency content of signals recorded individually at virtual stations along the fiber; they do not integrate the spatial density of information permitted by DAS. Several studies on dense seismological arrays have introduced similarity measures between the different time series data such as cross-correlations, dynamic time warping (DTW) or compression-based dissimilarity.

This study aims to quantify the contribution of spatial features for DAS data streams classification. We have chosen to explore spatial features related to both standard statistical measures (e.g., spatial mean, median, skewness, kurtosis), and advanced signal processing measures (e.g., auto-correlations, cross-correlations, DTW). This set of measures allows enriching a list of already used time series features which includes waveform, spectrum and spectrogram. A Random Forest (RF) classifier is then trained, and a Random Markov Field (RMF) algorithm is used after classification to account for redundant spatial and temporal information.

The evaluation of the spatial feature contribution is based on the output of the RF-RMF processing chain. Anthropogenically-triggered seismic data were acquired at the FEBUS Optics test bench. We consider five seismic sources: footsteps, impacts, excavators, compactor and fluid leaks. A class of noise is added as the RF-RMF algorithm is developed for processing DAS streams inherently affected by  noise.  Accurate  classification results can be obtained using only time features, and ongoing tests show a 2% increase in the correct classification rate with the use of both time and spatial features. The improvement allowed by the addition of spatial features is tangible but limited on our test dataset, but we think it should have a much greater impact on natural sources and we will discuss this perspective.

How to cite: Huynh, C., Hibert, C., Jestin, C., Malet, J.-P., and Lanticq, V.: Contribution of spatial features for classifying seismic events from Distributed Acoustic Sensing (DAS) data streams, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6189, https://doi.org/10.5194/egusphere-egu23-6189, 2023.

In synthetic tests, rotational ground motion recordings proved to be beneficial for the wavefrom inversion for seismic moment tensors. In a next step, we want to verify these findings using real measurements. To do so, we installed two broadband rotational collocated to translational ground motion sensors in the West Bohemia / Vogtland area in summer 2022.

The area is characterised by regular seismic swarm activity, the last one occurring in December 2021. The seismic swarms are known to be connected with crustal flow of mantle fluids. However, the detailed mechanism of this connection is not well understood yet. Full seismic moment tensors, especially their non-double-couple part, will contribute to investigate the connection between swarm activity and fluid flow. So far, a lacking number of moment tensors and difficulties in the reliability of the non-double-couple part hampered the analysis in the study area. Including rotational ground motion recordings to waveform inversion will help to overcome these difficulties.

In seven months, we have recorded 120 events with magnitudes larger than M ≥ 0 in a distance of up to 35 km, thereof 35 around Nový Kostel, the center of the swarm activity. Considering that rotational sensors are about 2-4 times less sensitive than translational sensors (depending on the local phase velocity of the location) this is already a great success. Here, we show details of the sensor installations, first data analysis, and an estimate on the magnitude of completeness from rotational measurements.

How to cite: Donner, S., Lehr, J., Hoffmann, M., Krüger, F., Heimann, S., Abreu, R., and Durand, S.: Rotational ground motion recordings in the West Bohemia / Vogtland region for waveform inversion for seismic moment tensors, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6379, https://doi.org/10.5194/egusphere-egu23-6379, 2023.

In recent years, the use of rotational sensors and DAS has become a topic of increasing interest within the seismological community because of their increasing sensitivity and affordability. We analyze the sensitivity of wavefield gradients, in the form of normal strain and rotation, to localized shallow velocity changes in a homogeneous medium.
We performed several numerical simulations, using a suitably modified 3D-SEM code, to observe, in addition to wavefield itself, the normal strain and rotation as a direct output.
We analyzed two case studies in which a velocity anomaly is placed in a homogeneous medium. In the first case the velocity change between the anomaly and the surrounding medium is 10%, in the second case 70%. We analyzed the sensitivity of these new observables in terms of phase shift and amplitude change.
We observe a very local effect of the wavefield gradients, which show larger amplitude near the boundary between the medium and the anomaly, while away from it they behave like the displacement wavefield itself.

How to cite: Bracale, M., Brossier, R., Pedersen, H., and Campillo, M.: Effect of shallow heterogeneities on wavefield gradients measurements, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6422, https://doi.org/10.5194/egusphere-egu23-6422, 2023.

DAS technology is particularly suitable for microseismic monitoring application in geothermal environments. This instrumentation can resist to high temperatures (up to about 100°C or more) higher than the operational temperature of standard acquisition instruments (e.g., geophones), allowing the fiber to be located very close to the reservoir. For this reason, DAS is particularly useful for induced seismicity monitoring of Enhanced Geothermal System (EGS). Being of recent development, this acquisition technology still lacks appropriate modeling and analysis tools able to handle such a large amount of data without losing efficiency. Furthermore, open-access DAS datasets are still a rarity, if compared to other geophysical datasets (e.g., seismological data). Therefore, we aim to generate an open-access synthetic (but realistic) DAS dataset that may help the geophysical community to develop “ad hoc” data analysis methods suitable for this kind of data. In the presented work we make use of the spectral element modeling software 'Salvus', developed by Mondaic, which also allows the simulation of DAS data. In particular, it outputs a strain measurement between all points defined as receivers in the simulation. Using the repositories of DAS data collected at the geothermal test site Frontier Observatory for Research in Geothermal Energy (FORGE) located in Utah (USA), we tried to simulate realistic DAS acquisition conditions of seismic events related to low-magnitude natural seismic activity from the nearby Mineral Mountains and microseismic events related to hydraulic stimulation operations for the generation of an EGS.

In order to obtain realistic synthetic data, we first analyze the spectral properties of real noise waveforms by using the Power Spectral Density (PSD) Analysis. Starting from observed PSDs we model the synthetic noise waveforms using a stochastic approach. Then we add it to the synthetic event traces and compare them with the observed ones. We finally test a semblance-based event detector on a 1-hour continuous waveforms of synthetic data to evaluate the performance of the detector in different operational conditions (e.g., different noise levels and inter-event times).

How to cite: Pecci, D., Porras, J., De Solda, M., Grigoli, F., Stucchi, E., and Iannelli, R.: Modeling and analysis of Distributed Acoustic Sensing (DAS) data in Geothermal environments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6915, https://doi.org/10.5194/egusphere-egu23-6915, 2023.

Optical networks for data transmission have become a pervasive infrastructure in the last years in order to cope with the increasing bandwidth request, thus there is a huge potential to be employed as a wide fiber optic sensing network. In the terrestrial scenario such networks are usually arranged on meshed topologies densely covering large areas of hundreths or thousands of kilometers. On the network's nodes, dedicated hardware is used to routed the data traffic between the connections' endpoints. Such nodes are interconnected by optical fiber links of hundreds of kilometers long, repeated every tenths of kilometers using optical amplifiers.

To fulfill the modern traffic requirements, optical networks are evolving towards multi-service autonomous, flexible, software defined entities based on a centralized intelligence orchestrating the networking functions and communicating with the network elements using standardized interfaces. This trend opens the perspective of using the optical network for evironmental sensing, such as earthquake detection or anthropic activities monitoring. 

Indeed, distributed acoustic sensing (DAS) systems based on Rayleigh scattering have demonstrated that optical fibers are excellent sensors of mechanical stress. However, such systems are expensive and pose some limitations on the maximum reach, so they cannot be deployed extensively. In this context, re-using the already deployed optical data infrastructure to support and integrate dedicated system sensing may be highly beneficial. In this work, we propose an optical data network architecture exposing sensing functionalities with minimum or no additional hardware simply by exploiting the pervasiveness of the telecommunication infrastructure and getting data from the physical quantities already monitored for data transmission purposes. Such architecture on a typical terrestrial optical data network is outlined in figure.

Modern coherent transceivers based on digital signal processing already track the evolution of the transmitted optical signal phase and polarization to recover the transmitted data at the receiver side. As those quantities are strongly affected by external strain, they already contain environmental information. Furthermore, some polarization-based processing can be implemented on cheaper non-coherent transceivers available at each amplifier site as data-service channel, providing several sensing sources.

In addition, further optical devices such as add-drop multiplexer or optical amplifiers typically have several other sensors already embedded (power monitors, temperature sensors) or they can be equipped with some others which can provide environmental data from other physical quantities.

The set of all such environmental data streams produced by the network elements constitutes the streaming telemetry fed to a network controller. A post-process agent may be implemented by exploiting the computational power available in typical network elements to perform local data analysis and reduce the amount of data sent to the sensing controller. By cross-processing the data coming from the network elements, a sensing controller is able to detect and localize events making the network act as a smart grid by continuously monitoring large areas and providing early warning signals.

To support our proposal, in this work we show the results of an experimental activity aimed at detecting and localizing anthropic activities in the city of Turin using a deployed fiber ring.

How to cite: Virgillito, E., Straullu, S., Bratovich, R., M. Rodriguez, F., Awad, H., Castoldi, A., Proietti, R., D'Amico, A., Aquilino, F., Pastorelli, R., and Curri, V.: Exploiting Terrestrial Meshed Optical Data Networks as Environmental Sensing Smart Grids, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6998, https://doi.org/10.5194/egusphere-egu23-6998, 2023.

The Distributed Acoustic Sensing (DAS) method re-purposes fiber optic cables into a very-dense array of strain/strain-rate sensors, capable of detecting different types of seismic events. However, DAS data are characterized by lower SNRs compared with standard seismic sensors, mainly because of a) strong directivity effects, 2) ground coupling inhomogeneities, and 3) site effects. Hence, beyond the array geometry, specific noise sources may reduce the potential of DAS for seismic monitoring. Previous research has already shown successful case-studies for event detection/location. Nevertheless, a coherent test on the performances of various arrays of different sizes and geometries is still lacking.

In this study, an extensive DAS database is organized for such a goal, including 15 DAS arrays that recorded at least one seismic event (located at a range of distances from the arrays). P wave arrival times are exploited to estimate the epicentral parameters with a Markov Chain Monte Carlo method. Then, to analyze the effects of cable geometry and potential sources of noise/ambiguity on the location uncertainties, a series of synthetic tests are performed, where synthetic traveltimes are modified as follows: a) adding noise with equal variance to all the DAS channels (SYNTH-01), b) adding noise characterized by an increasing variance with the distance from the event (SYNTH-02), c) simulating the mis-pick between P and S phases (SYNTH-03) and d) adding noise with a variance influenced by cable coupling inhomogeneities (SYNTH-04). Results show that the epicentral locations with automatic P wave arrival times have different degrees of uncertainty, given the geometrical relation between the event and the DAS arrays. This behavior is confirmed by the SYNTH-01 test, indicating that specific geometries provide a lower constraint on event location. Moreover, SYNTH-04 shows that simulating cable coupling inhomogeneities primarily reproduces the observed location uncertainties. Finally, some cases are not explained by any of the synthetic tests, stressing the possible presence of more complex noise sources contaminating the signals.

How to cite: Bozzi, E., Piana Agostinetti, N., F. Baird, A., Becerril, C., Biondi, B., Fichtner, A., Klaasen, S., Lindsey, N., Nishimura, T., Paitz, P., Shen, J., Ugalde, A., Walter, F., Yuan, S., Zhu, T., and Saccorotti, G.: Effects of cable geometry and specific noise sources on DAS monitoring potential, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7309, https://doi.org/10.5194/egusphere-egu23-7309, 2023.

In the past decade, significant progress has been made in the acquisition and processing of seismic wavefield gradient data (e.g., recordings of ground strain and rotation). When combined with conventional multicomponent seismic data, wavefield gradients enable the estimation of local wavefield properties (e.g., the local wave speed, the propagation direction, and the wave type) and the reconstruction of spatially under-sampled seismic wavefields. However, the seismological community has yet to embrace wavefield gradient data as a new observable.

We present TwistPy (Toolbox for Wavefield Inertial Sensing Techniques), an open-source software package for seismic data processing written in Python. It includes routines for single-station polarization analysis and filtering, as well as array processing tools. A special focus lies on innovative techniques to process spatial wavefield gradient data and, in particular, rotational seismic data obtained from dedicated rotational seismometers or small-aperture arrays of three-component sensors. Routines currently included in the package comprise polarization analysis and filtering in both the time domain and the time-frequency domain (for three-component and six-component data), dynamic tilt corrections, and beamforming (Bartlett, Capon, and MUSIC beamformers).  

With TwistPy, we attempt to lower the barrier of entry for the seismological community to use state-of-the art multicomponent and wavefield gradient analysis techniques by providing a user-friendly software interface.

Extensive documentation of the software and examples in the form of Jupyter notebooks can be found at https://twistpy.org.

How to cite: Sollberger, D., Heimann, S., Bernauer, F., Eibl, E. P. S., Donner, S., Hadziioannou, C., Igel, H., Yuan, S., and Wassermann, J.: TwistPy: An open-source Python toolbox for wavefield inertial sensing techniques, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7563, https://doi.org/10.5194/egusphere-egu23-7563, 2023.

Volcano monitoring has been experiencing significant improvements in recent years, yet eruption forecasting and scenarios have still lack of understanding, due to the poor observations in low amplitude events and hindered by surface external noise of similar amplitudes. Volcanic events have been shown to be accurately recorded with fiber optic techniques at the surface. In this study, we present preliminary results of fibre optic cable deployed in a new 200 m deep borehole on the southern flank of Etna at about 6 km away from the summit crater. This borehole has been designed primarily for the future deployment of a new strain sensor type. We benefited from the drilling of this new borehole to deploy a single-mode fibre optic cable. We connected an interrogator and recorded dynamic strain rate during several periods: first, in 2020 for several days during the completion of the borehole and the final stage of the drilling; second, in 2021 for several weeks during an active volcanic period; and in December 2022 during a quiet activity period of several months. We present a selection of records of noise while drilling, local volcano-tectonic earthquakes and volcanic events and tremor that occurred during those periods. These examples show the benefit of deploying a fibre in a borehole far from the active area and demonstrate the great variety of signals fibre optic can record is such configuration.

How to cite: Jousset, P., Currenti, G., Napoli, R., Pulvirenti, M., Pelligrino, D., Cunow, C., Larocca, G., Bonaccorso, A., Leto, G., and Krawczyk, C.: Fibre-optic dynamic strain borehole sensing at Etna volcano, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8327, https://doi.org/10.5194/egusphere-egu23-8327, 2023.

Submarine telecom cables criss-cross the oceans, connecting islands to continents and providing internet, financial and media services to consumers all around the world. Laser reflectometry as well as other optical techniques can potentially transform the optical fibers in these cables into sensors which can detect vibrations and ground motion from earthquakes, ocean waves, currents as well as permanent deformation of the seafloor. The goal of the ERC (European Research Council) funded project - FOCUS is to apply laser reflectometry on submarine fiber optic cables to detect deformation at the seafloor using BOTDR (Brillouin Optical Time Domain Reflectometry). This technique is commonly used monitoring large-scale engineering infrastructures (e.g. - bridges, dams, pipelines, etc.) and can measure very small strains (<< 1 mm/m) at very large distances (10 - 200 km), but until now has never been used to study movements at the seafloor.

Within the framework of the FOCUS project, and in collaboration with the “Conseil Regional” of Guadeloupe, in 2022 we began long-term monitoring of a network of submarine telecom cables that link the islands of the Guadeloupe archipelago. These cables connect the larger island of Basse Terre and Grande Terre to the smaller southern islands of Les Saintes, Marie Galante and La Desirade, with segment lengths ranging from 30 to 70 km. This network was deployed recently (in 2019) and is the property of the Conseil Regional of Guadeloupe, operated with the assistance of Orange. All cables contain twelve fiber pairs, of which three pairs are in use by mobile phone operators and thus unused fibers were available for this scientific monitoring project. In June 2022, we established BOTDR baselines on 8 optical fiber segments, in several cases in both directions. In December 2022, we repeated the measurements of the same fiber segments, allowing us to detect any strain along the cable over this period.

Here, we report that using the BOTDR technique, we detect significant strain signals  (50 micro-strain and more) in several locations along the cable network. These signals, which can be positive (elongation) or negative (shortening) occur typically in areas of steep seafloor slopes or in submarine valleys/canyons. Our tentative interpretation is that stretching and shortening of the cable (representing about 1 cm over a few hundred meters) is occurring, most likely due to sea-bottom currents. These currents may be related to the late summer/early autumn hurricane season, with the passage of tropical storm Fiona in Sept. 2022 dropping heavy rains, causing torrential floods and debris flows in some of the larger rivers with possible impacts further offshore. A longer time-series and more detailed analysis are necessary to test this preliminary hypothesis.

How to cite: Gutscher, M.-A., Quetel, L., Cappelli, G., Quillin, J.-G., Nativelle, C., Lebrun, J.-F., and Philippon, M.: Monitoring a commercially operating submarine telecom cable network in the Guadeloupe archipelago (Lesser Antilles) using Brillouin Optical Time Domain Reflectometry (BOTDR), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8569, https://doi.org/10.5194/egusphere-egu23-8569, 2023.

The increasing evidence of rotational motions due to earthquakes is now motivating civil engineers to investigate the effects of rotational ground motions on structures. With the advancement in instrumentation techniques, rotational sensors have been developed in the past few years, which can measure three components of rotational waves in addition to the translational waves. Conventionally, buildings are designed to withstand horizontal and vertical translational ground motions to minimize the damage to human life and financial losses during an earthquake. Damage to the structure is identified at two levels; (i) structural and (ii) material. The structural damage in reinforced concrete buildings is visible in the form of cracks and spalling concrete, which reduces the overall load-carrying capacity of the building. The damage at the material level is not visible to the human eye. This damage can be identified using coda wave interferometry techniques. In this method, a high cross-correlation between the coda of two waves passing a point on different days of experiment indicates a negligible change in the shear wave velocity of the material. In comparison, a lower cross-correlation signifies considerable change in the material properties.    

In order to understand how rotational motions affect reinforced concrete structures and how these can be simulated, the present work makes a novel attempt to use the newly developed rotation measuring sensors, BlueSeis 3A and IMU50, to understand the damage in a model concrete bridge structure (BLEIB). We employ advanced sensors in addition to conventional broadband and ultrasonic sensors on the 24m long two-span continuous reinforced concrete bridge equipped with various non-destructive sensing techniques and subjected to a variable pre-tension force of up to 450kN and various static loads. As an initial analysis, we first identify the bridge's first three fundamental frequencies and mode shapes from both recorded translational and rotational data. The analysis shows that the same fundamental frequencies are obtained from the recorded translational and rotational data. However, we expect to see a difference in the mode shapes. Theoretically, rotations are maximum at the bridge support and minimum at the centre of the bridge span. This behaviour is the reverse of what we observe from translational motions, where maximum translations are observed at the centre of the span while minimum at the supports. As the study plans to simulate rotational motions for reinforced concrete structures, a finite element model of the prototype bridge is also developed, and the fundamental frequencies and mode shapes of the model are validated with those obtained from the recorded data. This work shall be extended to applying coda wave interferometry to the rotational data recorded on the bridge to understanding the change observed in material properties when the bridge is subjected to active and passive forces.

How to cite: Dhabu, A., Bernauer, F., Liao, C.-M., Hadziioannou, C., Igel, H., and Niederleithinger, E.: Using Rotational Motions to understand material damage in Civil Engineering structure, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8851, https://doi.org/10.5194/egusphere-egu23-8851, 2023.