Unravelling the time-space distribution of diagenetic events modifying the pore network of reservoir rocks is a classical task of hydrocarbon research. Nevertheless, it is not always easy to reach that picture, as it needs to constraint a number of variables driving that distribution along the geological history of sedimentary basins.

Here we present the results of an integrated study performed on 15 samples of Palaeozoic reservoir sandstones coming from 3 hydrocarbon wells in the Illizi Basin. In particular, the topic of debate that promoted this study is the possible thermal effect on the Illizi Basin reservoirs rocks of the Cenozoic magmatic activity occurred in Hoggar dome, south of the studied region, and in other sector of the basin as pointed out by several magmatic intrusions.

The study was performed by combining: i) the reconstruction of the relative diagenetic timing obtained by petrographic observations; ii) microthermometric analyses of fluid inclusions trapped in diagenetic minerals; iii) low-T thermochronology (both Fission tracks and U-Th/H analyses) on clastic apatite grains; iv) vitrinite reflectance analysis; v) geohistory analysis of sampled wells. These data were used to constrain different thermal models, focussing in particular to the possible evidence of a Tertiary thermal overprint.

The results of the study can be summarized as follows:

• Several diagenetic minerals precipitated in the pore network of the studied rocks; among these, precipitation conditions for quartz, calcite, ankerite and to a minor degree feldspars were constrained through fluid inclusion microthermometric analyses;
• All these phases precipitated in a relatively narrow range of temperatures nicely correlated with burial depth of samples, from fluids with quite homogenous salinity (78-113 °C and 9.2-14.5 NaCl eq. %) suggesting a relatively limited time in which most of cements precipitated;
• The data on the thermal maturity of organic matter (vitrinite reflectance along the Mesozoic sequence, and vitrinite reflectance equivalent, mainly from chitinozoans, for the Silurian-Devonian sequence) seem to suggest a heating higher than the one currently observed. This may be compatible both with an episode of magmatic activity or with a late Cretaceous-Tertiary burial now eroded (in the wells studied, no more than 500-700m);
• Thermochronology shows a continuous burial until temperatures compatible with those observed by vitrinite reflectance although a minor thermal episode (i.e. with a temperature variation of the order of 10°C) is allowed.

Based on this integrated data set, different thermal scenarios have been tested, excluding or including a Cenozoic additional heating, in order to estimate the effect of the adopted thermal model on the age of cement precipitation in the pore system of studied reservoir rocks.

How to cite: Di Giulio, A., Nicola, C., Grigo, D., Scotti, P., Zattin, M., Amadori, C., Tamburelli, S., Consonni, A., and Ortenzi, A.: DIAGENETIC HISTORY VS. THERMAL EVOLUTION OF PALAEZOIC RESERVOIR ROCKS IN THE ILLIZI BASIN (Algeria), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2386, https://doi.org/10.5194/egusphere-egu2020-2386, 2020.

Evaporitic salt is prevailed in marine sedimentary basins, and the discovered hydrocarbon reservoirs are generally associated with salt structures in the world; accordingly salt structures have attracted much attention from academic and industry during the past decade. Tarim Basin that locates in northwest China, is the largest marine sedimentary basin in China with great hydrocarbon resources potential. Previous studies of salt structures in this basin mainly focus on its strong sealing capacity and structural traps created by salt structures. However, besides its extreme impermeability and low viscosity, rock salt has another unique thermal properties, featured by a large thermal conductivity as high as 5~6 W/(m.K), usually 2~3 times greater than that of other common sedimentary rocks, but a relatively low radiogenic heat production. This strong contrast in thermal properties could change the evolving thermal regime and associated thermal history of the source rocks around salt bodies, but has not been understood well. Herein based on the theoretical models and interpreted salt bearing seismic profiles from the Kuqa Foreland Basin, northern Tarim Basin, we use the 2D finite element numerical experiments to investigate the impacts of salt structures on basin geothermal regime and associated hydrocarbon thermal evolution. Our results show that, owing to its high efficiency in heat conduction, the salt rocks would result in obviously positive temperature anomalies (3~13%) above the salt body and negative temperature anomalies (11~35%) in the subsalt, enhancing and restraining the thermal maturation of source rocks above and below the salt body, respectively. The amplitude and extent of geothermal effects of salt structures depend on the thermal conductivity, geometry, thickness and burial depth of the salt bodies. The thermally affected area around the salt body can be 2 time of salt radius laterally and 2~3 times of salt thickness vertically. Salt structures in the Kuqa Foreland Basin can prominently cool the subsalt formation temperature and accordingly reduce the thermal maturity (Ro) of Jurassic source rocks as much as 18%, enabling the source rocks to be still of gas generation other than over-mature stage as expected previously, which is favor for deep hydrocarbon preservation below salt. In particular, salt structures in the west and east Kuqa Foreland Basin show strong differences in their thickness, geometric pattern, burial depth and composition, the thermal effects of salt structures on thermal maturation of subsalt source rocks should differ accordingly, which is supported by the observed tempo-spatial variation of Ro for Jurassic source rocks in this basin. Finally, we propose that the geothermal effects of salt structures will be of great importance in the deep hydrocarbon resources potential assessment and exploration in marine sedimentary basins in China.

How to cite: Liu, S. and Wang, L.: Quantifying the Effects of Salt Structures on Source Rocks Thermal Evolution of the Marine Sedimentary Basins, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4584, https://doi.org/10.5194/egusphere-egu2020-4584, 2020.

The Tertiary Piedmont Basin (TPB) in NW Italy represents an episutural basin developed since the Late Eocene in the retrobelt of the Western Alps and in the foreland of the Northern Apennine. During Oligo-Miocene time, up to 3 km-thick clastic deposits filled the basin recording the tectonics associated with the shift from the Alpine collisional thickening and the progressive NE-migration of the Apennine. The continental thickening was also accompanied by the opening of the Liguro-Provençal Basin and the drift of the Corsica-Sardinia block. Because of this key-position, the tectono-sedimentary and thermochronological history of the TPB has been the object of extensive investigations (Maino et al., 2013 and reference therein). However, several questions regarding its burial-exhumation history are still open. In order to define the thermal history of the source-sink system, we combined literature data with new detrital apatite fission-track analyses and zircon U-Pb dating from Upper Priabonian to Lower Miocene syn-tectonic deposits. Results from AFT analysis show: i) a single reset population at 24.8 ± 1.2 Ma (Late Chattian) in the lowermost Late Priabonian sample; ii) partially annealed apatite grains from Early Rupelian sample; iii) unannealed Late Rupelian-Miocene samples with AFT age populations spanning from Late Cretaceous to Late Oligocene in time. Data from the Ligurian Alps crystalline massifs report similar AFT cooling ages between 22.9 ± 5.3 - 24.0 ± 1.4 Ma. This, combined with our data, shows that the bottom of the TPB sequence experienced ~110 °C heating and subsequent cooling together with its nearby margin. The heating experienced by the basin combined with reconstructed sedimentary thickness (before the exhumation/cooling event) of ca. < 3 km, implies an elevated geothermal gradient of about 60 °C/Km, which is anomalous for a thickened orogenic crust. Furthermore, one sample from Upper Oligocene sedimentary rocks contains an AFT detrital population age (33.6 ± 2 Ma) consistent with a youngest U-Pb age peak of 33.6 Ma from co-magmatic zircon grains, which likely reflects the age of volcanites today buried under the Po Plain (Di Giulio et al., 2001). Detrital zircon U-Pb ages show two main populations at ca. 290 and ca. 460 Ma, which are expected products of a Variscan source now exposed in the Ligurian Alps and Southern Alps. Our new geo-thermochronological data overall suggests a distributed Oligocene thermal signal, the origin of which is discussed. Possible explanations are: 1) a > 3 km of focused erosion associated with tectonic deformation occurred in the TPB and nearby margin and/or 2) an anomalously high heat flow event driven by asthenospheric rise as a consequence of the Liguro-Provençal rifting.

Maino M., Decarlis A., Felletti F., Seno S. (2013) Tectono-sedimentary evolution of the Tertiary Piedmont Basin (NW Italy) within the Oligo–Miocene central Mediterranean geodynamics. Tectonics, 32, 593–619.

Di Giulio, A., Carrapa B., Fantoni R., Gorla L., Valdisturlo L. (2001) Middle Eocene to Early Miocene sedimentary evolution of the western segment of the South Alpine foredeep (Italy). Int. J. Earth Sci., 90, 534-548.

How to cite: Amadori, C., Langone, A., Marini, M., Simone, R., Carrapa, B., Maino, M., and Di Giulio, A.: Unexpected thermal history of a syn-collisional basin revealed by geo- and thermochronology: the case of the Tertiary Piedmont Basin (Western Alps, Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5815, https://doi.org/10.5194/egusphere-egu2020-5815, 2020.

A realistic reconstruction of the time-temperature history of sedimentary basins is critical to understand basin evolution and to predict oil maturation as well to assess reservoir quality. Carbonate rocks undergo diagenetic processes that modify their mineralogical and petrophysical properties. Understanding the temperature at which those processes occur and determining the geochemistry of the driving fluids is critical to constrain their occurrence and evolution in space and time.

Here, we put to the test the joint application of two independent techniques: the traditional fluid inclusion microthermometry (FIM) and the more recent clumped isotopes thermometer (∆₄₇). We compare thermal information acquired by Δ₄₇ thermometer and FIM on diagenetic carbonates having precipitated at temperatures between 60°C and 130°C in Upper Triassic reservoirs (depths of 1820-2450 m) from the well-known Paris Basin, and having suffered 120°C during maximum burial for about 20 Ma. A conventional diagenesis study (petrography, O-C isotope geochemistry) has been accomplished in samples from three different cores drilled in carbonate-cemented siliciclastic reservoir units of Norian age (Grés de Chaunoy Formation) and located in the northern part of the basin depocenter. A complete cement paragenesis was reconstructed highlighting three different burial cements: two non-ferroan blocky calcite phases (Cal1 and Cal2) and one non-ferroan dolomite phase of saddle type (Dol1). The progressively more negative δ¹⁸O_carb suggests a possible increase in temperature, going from Cal1 to Dol1, whereas the consistently negative δ¹³C could indicate the involvement of continental fluids.

FIM indicates homogenization temperatures (Th) spanning from 60°C to 95°C (mode 67.5°C) for Cal1, 70°C to 110°C (mode 84°C) for Cal2, and 100°C to 130°C (mode 115°C) for Dol1. Δ₄₇ measurements overall reveals lower temperatures for calcite cements, indicating probable thermal re-equilibration of the fluid inclusions, and a fairly similar temperature for the saddle dolomite cement. Uncertainties in the temperatures obtained through FIM and ∆₄₇ thermometry and in the successively calculated δ¹⁸O_fluid, may lead to an erroneous assessment of the time of precipitation of the different diagenetic phases and to an erroneous thermal history and fluid-flow reconstruction. 

This work emphasizes the necessity of better understanding the limitations and applicability fields of these thermometric tools, especially when applied to burial diagenetic phases precipitated at temperatures above 100°C and/or in reservoirs having experienced temperatures in the gas window.

How to cite: Vergara, N. A., Gasparrini, M., Corrado, S., and Bernasconi, S.: Joint application of fluid inclusion and clumped isotope (Δ47) thermometry to burial carbonate cements from Upper Triassic reservoirs of the Paris Basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20718, https://doi.org/10.5194/egusphere-egu2020-20718, 2020.

The upper Triassic Streppenosa and Noto Formations are considered the main source rocks of the Hyblean Plateau in south-eastern Sicily, that represents the present-day deformed foreland of the Sicilian fold-and-thrust belt. This work focusses on the Upper Triassic Streppenosa and Noto Formations, penetrated by the Eureka 1 onshore well (south-eastern Sicily, Italy) in order to constrain the burial-thermal history of this basin of the western Tethys. According to previous paleogeographic reconstructions, starting from Norian, the palaeogeographic scenario consisted, moving from north to south, of a wide carbonate platform (Sciacca Fm.), adjacent to two different domains: the euxinic lagoon/basin of the Noto Formation, and, to the south, the basin of the Streppenosa Formation. Eureka 1 well is located in the inner portion of the platform-basin system and its Triassic succession consists of alternation of black shales and micritic, microbial dolomitic laminated limestones. A detailed description of the sedimentological facies from cores samples has been performed together with detailed organic petrography/Raman spectroscopy and clay mineralogy on fine grained sediments to assess thermal maturity of the Streppenosa and Noto Fms. The main facies consist of light-grey limestones (wackestone-mudstone) with scattered sub-angular intraclast, light grey finely laminated limestones, dark grey-black laminated mudstones, brownish undulated algal laminae saturated with bitumen. The cores are often bitumen saturated and interrupted by different sets of open microfractures, veins filled with calcite, and stylolites (parallel and vertical with respect to lamination) that may enhance and/or inhibit at places the fluid flow. Concerning thermal maturity, the studied interval falls in the lower-mid portion of the oil window, with robust agreement among the geothermometers derived from the three adopted techniques.

How to cite: Maniscalco, R., Corrado, S., Balestra, M., Schito, A., Casciano, C. I., Forzese, M., Montalbano, S., Pellegrino, A., Bachtiar, A., Palmeri, G., and Di Stefano, A.: Sedimentological features and thermal maturity signature of the upper Triassic Streppenosa and Noto Formations, source rocks in the Hyblean Plateau (SE Sicily, Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21703, https://doi.org/10.5194/egusphere-egu2020-21703, 2020.

Many regions of the Earth’s mantle deform in grain size-sensitive creep regimes. The grain size right below the transition zone is believed to be very small, and the grain size should in subsequent depths be mainly controlled by normal grain growth. The grain size evolution is commonly predicted using either existing grain growth laws in combination with grain boundary diffusion coefficients or by extrapolating empirically determined grain growth laws. Effects of Zenner pinning and different ratios of second phases have been studied, while the role of anisotropic grain boundary properties is currently neglected¹.  The grain boundary energy varies with the orientation of the grain boundary plane, as expressed through the typical crystal habitae (Wulff-shapes). Individual crystals in a polycrystalline material maintain a grain boundary energy anisotropy during grain growth. Here we study how grain boundary anisotropy impacts grain boundary migration and normal grain growth rates by three-dimensional phase-field simulations². We imply grain boundary energy minimization by faceting/varying the grain boundary plane to minimize the grain boundary energy. The ideal grain boundary energy anisotropy for the solid-solid interface is taken from experimentally investigated grain boundary plane distributions and grain boundary energy distributions on periclase (MgO). We compare the grain size evolution in simulations with isotropic and anisotropic grain boundary energy of cubic crystal symmetry. We found that the grain boundary energy anisotropy has a significant influence on grain boundary migration and grain growth kinetics².

<r>²- <r₀>²= kt

The change in grain size is given as variation between the initial and final average grain radius, r. The time is t, and a material-specific parameter that accounts for the grain boundary energy anisotropy is extracted from the simulations as

k = A·µ_gb·σ_gb

Where, the grain boundary energy, σ_gb varies with orientation, and the grain boundary mobility, µ_gb assumed to be isotropic. We found that the rate of grain growth for periclase, A is a factor of 3 smaller compared to an isotropic material.

These results are of three-fold importance:

A better prediction of grain size evolution will need to develop an anisotropic theory for grain growth, that address our lake in knowledge regarding pressure effects on both grain boundary energy anisotropy and diffusion.

1. Rohrer, G. S. Annu. Rev. Mater. Res. 2005

2. Salama et al. Acta Materialia 2020

How to cite: Salama, H., Marquardt, K., Kundin, J., Shchyglo, O., and Steinbach, I.: The role of grain boundary energy anisotropy on the grain size evolution during normal grain growth., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21694, https://doi.org/10.5194/egusphere-egu2020-21694, 2020.

A relocation of mica grains in marbles can be observed as trace of newly precipitated calcite material in cathodoluminescence microscopy analysis. Mica grains relocate by rotation and/or translation from foliation parallel to new irregular orientations. The mica grains can be either located at calcite grain boundaries or within large calcite grains.

The process erases deformed, inclusion-rich calcite material and creates undeformed and mostly inclusion-free grains and can therefore be regarded as postdeformational. Not every mica present relocates and the choice of whether a specific mica relocates cannot be related to a specific primary orientation. Furthermore, no significant difference in composition between relocated and non-relocated mica grains can be observed. Newly precipitated calcite has less Mg than the dissolved grain material.

The precipitation of new calcite material at the calcite-mica interface is supposed to be the initial trigger leading to dissolution of inclusion-rich, deformed calcite material at the opposite side of the mica grain. The newly precipitated calcite material inherits the already existing calcite grain‘s crystallographic orientation.

Assuming this process occurs to a larger extent in a material, it might modify a deformation-related microfabric. Therefore, an interpretation in terms of deformation conditions should be done carefully, considering postdeformational dissolution-precipitation processes.

How to cite: Kühn, R., Duschl, F., Leiss, B., and Schulze, T.: Relocated micas in marble – indicators for postdeformational microfabric modification, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13710, https://doi.org/10.5194/egusphere-egu2020-13710, 2020.

Diffusion creep and the wet low temperature version, pressure solution, are major deformation mechanisms in the Earth. Pressure solution operates in many metamorphosing systems in the crust and may contribute to slow creep on fault surfaces. Diffusion creep prevails in areas of the upper mantle deforming slowly, and possibly in most of the lower mantle. Both mechanisms contribute to localisation since small grain sizes can deform faster.

However, there has been limited attention paid to the evolution of microstructure during diffusion creep. In some experiments grains coarsen; in some but not all experiments grains remain rather equant. We have developed a grain-scale numerical model for diffusion creep, which indicates that those processes are very important in influencing evolving strength. Our models illustrate three behaviours.

1. Strain localises along slip surfaces formed by aligned grain boundaries on all scales. This affects overall strength.
2. Diffusion creep is predicted to produce elongate grains and then the overall aggregate has intense mechanical anisotropy. Thus strength during diffusion creep, and localisation on weak zones, is influenced not just by grain size but by other aspects of microstructure.
3. Grain coarsening increases grain size and strength. Our most recent work shows how it interacts with ongoing deformation. In particular grain growth can lead to particular grain shapes which are directly related to strain rate, and influence strength. Consequently, understanding localisation during diffusion creep must encompass the effects of diffusion itself, grain boundary sliding and grain coarsening.

How to cite: Wheeler, J., Evans, L., Gardner, R., and Piazolo, S.: Strain localisation during diffusion creep: influence of grain coarsening and grain boundary sliding, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9953, https://doi.org/10.5194/egusphere-egu2020-9953, 2020.

About 50 years ago, John Ramsay and colleagues established the thorough foundation for the field-scale observational and mathematical description of the structures, deformation, and kinematics in ductile shear zones. Since then, these probably most important instabilities of the ductile lithosphere enjoyed an almost explosive growth in scientific attention. It is perhaps fair to say that this tremendous research effort featured four main themes:

[1] The historic scientific nucleus – quantification of shear-zone geometry, strain and associated kinematic history from field observations

[2] Qualitative and quantitative analysis of microphysical deformation mechanisms in the field and the laboratory

[3] Shear-zone rheology

[4] The development of physically consistent mathematical models for shear zones, mainly using continuum mechanics.

In concert, these four cornerstones of shear-zone research enabled tremendous progress in our understanding of why and how ductile shear zones form. So, what are some of the outstanding problems?

A truly comprehensive model for ductile shear zones must account for the vast range of length and time scales involved, each easily covering ten orders of magnitude, as well as the associated intimate coupling between thermal, hydraulic, mechanical, and chemical processes. The multi-scale and multi-physics nature of ductile shear zones generates scientific challenges for all four research themes named above. This presentation is dedicated to highlighting exciting challenges in themes 2, and 3 and 4.

In the microanalytical arena [2], the nano-scale is an exciting new frontier, especially when it comes to the interplay between metamorphism and ductile deformation. The nano-frontier can be tackled with new synchrotron methods. I showcase some applications to fossil shear-zone samples and discuss opportunities for in-situ experiments. In the domain of rheology [3], I present some simple experiments with strain-softening materials and field observations that support the notion: transient rheological behaviour is very important for shear localisation. In the modelling domain [4], some recent examples for the intriguing physical consequences predicted by new multi-physics and cross-scale coupling terms in ductile localisation problems are illustrated.

How to cite: Schrank, C.: Ductile shear zones – future perspectives, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4273, https://doi.org/10.5194/egusphere-egu2020-4273, 2020.

We revisit large shear strain deformation experiments on Carrara marble and observe that anisotropic porous domains develop spontaneously during shearing. Specifically, as samples are deformed periodic porous sheets are documented to emerge and are found to transfer mass. These results imply that viscous shear zones may naturally partition fluids into highly anisotropic bands. As this hydro-mechanical anisotropy is produced by creep, each porous sheet is interpreted to represent a transient dynamic pathway for fluid transport. It is unclear how long each porous domain is uniquely sustained but it is clear that sheets are persistently present with increasing strain. Our results forward the idea that viscous shear zones have dynamic transport properties that are not related to fracturing or chemical reaction. We believe these new results provide experimental foundation for changing the paradigm of viscosity in rocks to include dynamic permeability. In our view making this change in perspective could alter many classical interpretations in natural banded mylonite zones, for example shear zone parallel syn-kinematic veining may be the result of pore sheet instability and ductile fracturing.

How to cite: Gilgannon, J., Waldvogel, M., Poulet, T., Fusseis, F., Berger, A., Barnhoorn, A., and Herwegh, M.: Evidence that viscous shear zones spontaneously establish hydro-mechanical anisotropy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5250, https://doi.org/10.5194/egusphere-egu2020-5250, 2020.

Present exposure of the ductile Caledonian retrowedge in northwestern Scotland records the evolution of a shear zone that was exhuming while actively deforming, providing a natural laboratory to study strain localization in a progressively cooling system. Examination of rocks from two detailed transects across this region consistently show a transition from microstructures that are dominated by interconnected phyllosilicate networks in a quartz-rich matrix with feldspar porphyroclasts, to interconnected fine-grained regions of mixed quartz + phyllosilicate + feldspar. These polyphase regions are demonstrably weaker than surrounding quartz layers and likely deform by grain-size sensitive mechanisms including diffusion-accommodated grain boundary sliding.

Microstructures characterized by a quartz-rich matrix and interconnected phyllosilicates undergo quartz recrystallization by high temperature grain boundary migration and are dominated by prism a slip. In contrast, fine-grained polyphase microstructures record quartz recrystallization dominated by subgrain rotation and activation of rhomb a and basal a slip systems. We propose transient hardening occurs in quartz-dominated regions as quartz with a strong Y-axis maximum undergoes the switch from prism a easy slip to basal a easy slip during cooling, and thus partitions strain into interconnected phyllosilicate layers. In response, interconnected phyllosilicate layers undergo mechanical comminution, becoming increasingly mixed by grain-size sensitive creep processes to form polyphase layers as they accommodate an increased proportion of strain. This transition from quartz-rich matrix with phyllosilicate interconnected weak layers to fine-grained, polyphase weak layers could be of first-order importance in strain localization within polyphase mylonitic and ultramylonitic rocks.

How to cite: Lusk, A. and Platt, J.: Development of interconnected fine-grained polyphase networks during progressive exhumation of a shear zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12715, https://doi.org/10.5194/egusphere-egu2020-12715, 2020.

Deformation localisation in rocks can lead to a variety of structures, such as shear zones and shear bands that can range from grain to crustal scale, from discrete and isolated zones to anastomosing networks. The heterogeneous strain field can furthermore result in a wide range of highly diverse fold geometries.

We present a series of numerical simulations of the simple-shear deformation of an intrinsically anisotropic non-linear viscous material with a single maximum crystal preferred orientation (CPO) in dextral simple shear. We use the Viscoplastic Full-Field Transform (VPFFT) crystal plasticity code (e.g. Lebensohn & Rollett, 2020) coupled with the modelling platform ELLE (http://elle.ws) to achieve very high strains. The VPFFT-approach simulates viscoplastic deformation by dislocation glide, taking into account the different available slip systems and their critical resolved shear stresses. The approach is well suited for strongly non-linear anisotropic materials (de Riese et al., 2019). We vary the anisotropic behaviour of the material from isotropic to highly anisotropic (according to the relative critical resolved shear stress required to activate the different slip systems), as well as the orientation of the initial single maximum orientation, which we vary from parallel to perpendicular to the shear plane. To visualize deformation structures, we use passive markers, for which we also systematically vary the initial orientation.

At relatively low strains the amount of strain rate localisation and resulting deformation structures highly depend on the initial single maximum orientation in the material in all anisotropic models. Three regimes can be recognised: distributed shear localisation, synthetic shear bands and antithetic shear bands. However, at very high strains localisation behaviour always tends to converge to a similar state, independent of the initial orientation of the anisotropy.

In rocks, shear localisation is often detected by the deflection and/or folding of layers, which may be parallel to the anisotropy (e.g. cleavage formed by aligned mica), or by deflection/deformation of passive layering, such as original sedimentary layers. The resulting fold patterns vary strongly, depending on the original orientation of layering relative to the deformation field. This can even result in misleading structures that seem to indicate the opposite sense of shear. Most distinct deformation structures tend to form when the layering is originally parallel to the shear plane.

de Riese, T., Evans, L., Gomez-Rivas, E., Griera, A., Lebensohn, R.A., Llorens, M.-G., Ran, H., Sachau, T., Weikusat, I., Bons, P.D. 2019. Shear localisation in anisotropic, non-linear viscous materials that develop a CPO: A numerical study. J. Struct. Geol. 124, 81-90.

Lebensohn, R.A., Rollett, A.D. 2020. Spectral methods for full-field micromechanical modelling of polycrystalline materials. Computational Mat. Sci. 173, 109336.

How to cite: de Riese, T., Bons, P. D., Gomez-Rivas, E., Griera, A., Llorens, M.-G., and Weikusat, I.: Influence of initial preferred orientations on strain localisation and fold patterns in non-linear viscous anisotropic materials , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13160, https://doi.org/10.5194/egusphere-egu2020-13160, 2020.

Hydrocarbon exploration is extending from the shallowly buried to deeply buried strata with increasing demands for fossil fuels. The variable storage and percolation capacities that intrinsically depend on the pore geometry restrict the hydrocarbon recovery and displacement efficiency and trigger studies on the micro-scale pore structure, fluid flow capacity, and their controlling factors. Minerals within sandstone are the results of the coupling control of depositional factors and diagenetic alternations, which determine the microscopic pore geometry and subsequently affect the fluid flow capacity. In order to investigate the impacts of mineralogy on the pore structure and fluid flow capacity, integrated analyses including porosity and permeability measurements, casting thin section (CTS), scanning electron microscopy (SEM), pressure-controlled mercury porosimetry (PCP), rate-controlled mercury porosimetry (RCP), nuclear magnetic resonance (NMR), and X-ray diffraction (XRD) are conducted on the deeply buried sandstone samples in the Jurassic Sangonghe Formation of the Junggar Basin. Microscopic pore structure is characterized by the combination of SEM, CTS, PCP, and RCP and fractal theory. Fluid flow capacity is evaluated by the innovative application of film bound water model in NMR and mineralogy is quantitatively measured by XRD. The results indicate that the deeply buried sandstone is rich in quartz (54.2%), feldspar (25.1%), and clay (14.2%), with dominant kaolinite (5.04%) and chlorite (5.38%) cementation. The reservoir has a wide pore-throat diameter distribution with three peaks in the ranges 0.01–1, 10–80, and 200–1000 μm. Pores are tri-fractal and can be divided into micropores, mesopores, and macropores, with average porosity contributions of 50.11, 21.83, and 28.04%, respectively. The movable porosity of deeply buried sandstone ranges from 1.75 to 8.24%, primarily contributed by intergranular (avg. 2.34%) and intragranular pores (avg. 2.56%). Most of the fluids are movable in intergranular pores but are irreducible in intragranular pores. Correlation analyses between mineralogy and pore structure suggest that quartz provides preservation to intergranular porosity, which increases pore size and macropores porosity and reduces heterogeneity of the pore system. The influence of feldspar reverses and becomes poor owing to the simultaneous clay precipitation and complex roles of feldspar dissolution in microporosity. Chlorite, kaolinite, and illite, all act as destructions to intergranular porosity. They enhance the mesopores and micropores porosities, reduce the pore size, and increase the microscopic heterogeneities of the macropores, micropores, and whole pore system. The relationships between mineralogy and fluid flow capacity indicate that quartz is favorable for the fluid flow capacity, but feldspar and clay play negative roles. The reversed impacts of quartz and feldspar lay in their opposite controls on pore size. However, both pore size and hydrophilia should be taken into account when considering the effects of clay minerals. These negative effects are associated with types, contents, and hydrophilic degrees of clay minerals, in which I/S and illite exhibit the strongest negative impacts. The fluid flow in the intergranular and intragranular pores is generally enhanced by higher quartz content, but reduced by higher clay content. Irreducible fluids in the intergranular and intragranular pores are determined by chlorite and kaolinite contents, respectively.

How to cite: Qiao, J., Zeng, J., and Feng, X.: Impacts of mineralogy on micro-scale pore structure and fluid flow capacity of deeply buried sandstone reservoirs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1516, https://doi.org/10.5194/egusphere-egu2020-1516, 2020.

The coupling between fluid transport, chemical reactions, and deformation constitutes one of the frontiers of geoscientific research. From an analytical perspective, a fundamental challenge is posed by the fact that sub-nanometre- to micrometre-scale structures play a vital role in the macroscopic (centimetre- to metre-scale) response of deforming, reacting, fluid-bearing rocks. Sample analysis with conventional laboratory techniques quickly becomes prohibitively expensive and laborious when more than four orders of magnitude in length scales need to be resolved. This issue is particularly challenging in very fine-grained rocks such as mylonites and shales.

Here, we investigate calcite-vein-bearing shales to illustrate how synchrotron X-ray fluorescence microscopy, ptychography, and small- and wide-angle transmission scattering can be used for the quantitative multi-scale analysis of micro- and nano-textures in rock. These analytical techniques are applied to thin sections or thin rock slabs on the centimetre-scale and provide information on length scales from hundreds of micrometres down to angstroms. Therefore, the considered array of synchrotron techniques covers up to eight orders of magnitude in length scale in terms of chemical and structural information. In our case study, we demonstrate how this suite of analytical techniques can be employed to reveal, for example, the relative timing of mineralisation events, trace-element chemistry, texture, and the structural width of fluid pathways such as grain boundaries.

How to cite: Schrank, C. E., Jones, M. W. M., Kewish, C. M., and van Riessen, G. A.: Synchrotron multi-modal, multi-scale chemical and structural imaging of vein-bearing shales, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6968, https://doi.org/10.5194/egusphere-egu2020-6968, 2020.

Graphitic carbon exhibits a large range of structures and chemical compositions, from amorphous-like compounds to crystalline graphite. The graphitic carbon-bearing rocks are widely occurred in low- to high- grade metamorphic massif and fault zone. The carbonaceous material in the rock will gradually transform from an amorphous into an ordered crystalline structure by thermal metamorphism, which is called graphitization. The degree of graphitization is believed to be a reliable indicator of peak temperature conditions in the metamorphic rock. In many low-grade metamorphic rocks, graphitic carbon (e.g., soot, low-grade coal) is often associated with brittle fault gouge whereas in high-grade metamorphic rocks, graphitic carbon (crystalline granite) are most commonly seen in marble, schist or gneiss. In recent years, graphitic carbon-bearing rocks have been reported from natural fault zones (reviwers paper see Cao and Neubauer 2019 and references therein). The graphitic carbon grains in our samples tend to enrich in slip-surface or micro-shear zone with strain localization in fault, performed as dislocation glide of deformation. The graphite LPO shows slip system in the direction of basil <a> combined basil <a> slip and weak prism <a> slip systems, suggesting a low-temperature to a medium to high temperature deformation conditions, which is in consistent with the results of Raman Spectra of Carbonaceous material (RSCM) thermometry. We also proposed that the graphitic carbon formed in the rocks can significantly affect the mechanical properties of the fault during the process of faulting. This process can effectively cause reaction weakening and strain localization, which is thought to play an important role as solid lubrication in fault weakening.

How to cite: Cao, S., Neubauer, F., and Lv, M.: Graphitic material in fault zones: Implication for strain localization and rheological weakening, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9086, https://doi.org/10.5194/egusphere-egu2020-9086, 2020.

Development of brittle and brittle-ductile shear zones involve partitioning of large shear strains in bands, called C-shear bands (C-SB) nearly parallel to the shear zone boundaries. Our present work aims to provide a comprehensive understanding of the rheological factors in controlling such SB growth in meter scale natural brittle- ductile shear zones observed in in Singbhum and Chotonagpur mobile belts.  The shear zones show C- SB at an angle of 0°- 5° with the shear zone boundary. We used analogue models, based on Coulomb and Viscoplastic rheology to reproduce them in experimental conditions.

These models produce dominantly Riedel (R) shear bands. We show a transition from R-shearing in conjugate to single sets at angles of ~15^o by changing model materials. However, none of the analogue models produced C-SB, as observed in the field. To reconcile the experimental and field findings, numeral models have been used to better constrain the geometrical and rheological parameters. We simulate model shear zones replicating those observed in the field, which display two distinct zones: drag zone where the viscous strains dominate  and the core zone, where both viscous and plastic strains come into play.  Numerical model results suggest the formation of  C- SB for a specific rheological condition. We also show varying shear band patterns as a function of the thickness ratio between drag and core zones.

How to cite: Roy, A., Roy, N., Saha, P., and Mandal, N.: Development of C- shear bands in brittle-ductile shear zones: Insights from analogue and numerical models., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-960, https://doi.org/10.5194/egusphere-egu2020-960, 2020.

Deformation of lithospheric rocks regularly localizes into high-strain shear zones that include fine-grained ultramylonites. Occurring as quasi-straight layers of intimately mixed phases that often describe sharp transitions with the host rock, these structures may channelize fluid flow^[1,2] and could serve as precursors for deep earthquakes^[3]. However, although intensively documented, ultramylonites originate from still unknown processes. Here I focus on a mylonitic complex that includes numerous mantle ultramylonites in the Ronda peridotite (Spain). Among them, I was able to highlight one of their precursors that I better describe as a long and straight grain boundary, along which four-grain junctions are observed with randomly oriented grains of olivine and pyroxenes. This precursor starts from a pyroxene porphyroclast and extends to an incipient, weakly undulated ultramylonite, where intimate phase mixing arises with asymmetrical grain size distribution. While the finer grain size locates on one side, describing a sharp – but continuous – transition with the host rock, the grain size gradually increases towards the other side, giving rise to a smooth transition. All phases have a very weak lattice preferred orientation (LPO) in the ultramylonite, which strongly differs from the host rock where olivine is highly deformed with evidence of high dislocation densities and a strong LPO. Altogether, these features shed light on the origin of mantle ultramylonites that I attribute to a migrating grain boundary, the sliding of which continuously produces new grains by phase nucleation, probably at the favor of transient four-grain junctions. Nucleated grains then grow and progressively detach from the precursor as it keeps on migrating depending on the dislocation densities in the host rock. Although such an unusual grain boundary remains to be understood in terms of source mechanism, these findings provide new constraints on the appearing and development of ultramylonites.

[1] Fusseis, F., Regenauer-Lieb, K., Liu, J., Hough, R. M. & De Carlo, F. Creep cavitation can establish a dynamic granular fluid pump in ductile shear zones. Nature 459: 974–977 (2009)

[2] Précigout, J., Prigent, C., Palasse, L. & Pochon, A. Water pumping in mantle shear zones. Nat. commun. 8: 15736, https://doi.org/10.1038/ncomms15736 (2017)

[3] White, J. C. Paradoxical pseudotachylyte – Fault melt outside the seismogenic zone. J. Struct. Geol. 38: 11-20 (2012)

How to cite: Précigout, J.: On the Origin of Ultramylonites, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3938, https://doi.org/10.5194/egusphere-egu2020-3938, 2020.

Quartz veins in poly-metamorphic settings often accommodate the latest deformation state and therefore can provide important information. Identification of microfabric (microstructure and crystallographic preferred orientation, CPO) evolution of quartz during mylonitization, and especially of the grain-scale interplay between brittle and crystal-plastic processes, has different relevant implications: e.g., on understanding the efficiency of fluid mobility through deforming quartz that can dramatically influence the rheology and the degree of chemical exchange. However, in order to interpret the microstructure and the related deformation processes it is necessary to relate these especially to the deformation temperature. Particularly the CPO and the Ti-in-qtz geothermometry is used to constrain the deformation temperature. However, both methods have to be applied with great caution because even when many times used some fundamental processes are not fully understood yet.

Here we present results from deformed quartz veins from the Prijakt Nappe (Autroalpine Unit, Schober Mountains, Central Eastern Alps). These veins localized ductile shear and eventually seismic faulting (recorded by the occurrence of pseudotachylytes) within Eo-Alpine eclogite-facies shists. The veins formed shortly after the eclogitic peak, but the temperature of their deformation remains unconstrained. CL imaging reveals critical details for understanding the role of microfracturing and fluid-rock interaction during initial stages of shear localization, the onset of dynamic recrystallization and the resetting of the Ti-in-quartz geochemistry. Even when optical-light-microscopy and EBSD analysis indicate crystal plastic deformation by subgrain rotation CL and orientation contrast (OC) imaging gives evidence of brittle stage of deformation at least for some of the deformation microstructure. Microshear zones show a bulk dark-CL, but still bright tones in cores of new recrystallized grains similar to the CL signature of the host coarse quartz crystals. CL dark tones also match with the pattern of subgrain boundaries. This reflects fluid permeability pathways along subgrain and grain boundaries (identified by widespread fluid inclusions) and the associated partial resetting of Ti concentrations. The CPO of the new grains within the micro-shear zones rotate with the sense of shear around the kinematic Y-axis and cannot be related to the activity of specific slip systems. In contrast the partial single girdle of c-axis within the ultramylonite with its elongated substructured grains and its characteristic layered microstructure can be related to the activity of several slip systems. Misorientation axis analysis indicates that prism

How to cite: Bestmann, M., Huet, B., Grasemann, B., and Pennacchioni, G.: Microstructural evolution of amphibolite-eclogite facies quartz veins under low greenschist facies deformation condition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7480, https://doi.org/10.5194/egusphere-egu2020-7480, 2020.

Stress and strain are different physical entities. Do the stress and strain determined from e-twins in a sample of polycrystalline calcite have similar principal orientations and similar shape ratios? Köpping et al. (2019) tackled this question by applying Turner’s (1953) classical method of paleostress analysis to natural data. However, despite the assumption of the method, the orientations of P- and T-axes of an e-twin lamella do not have a one-to-one correspondence with the principal orientations of the stress that formed the lamella. And, the method cannot determine a shape ratio. Another difficulty arises when one tackles the question: Natural calcite has usually been subjected to polyphase tectonics with different stress conditions. One has to separate stresses and to evaluate corresponding strains from a sample. Once lamellae are grouped according to the stresses, the strain achieved by the formation of a group of twin lamellae is easily evaluated by the method of Conel (1962) if the total strain represented by a group is small.

The present authors tackled the question by combining Conel’s strain analysis method with a novel method of paleostress analysis of mechanical twins, which clusters the directional data of e-twins by means of a statistical mixture model and determines stresses for each group of data. And, the appropriate number of stresses is determined by means of Bayesian information criterion. The method also determines the probabilities of each lamella to be formed by the stresses, which are called the memberships of the lamella. The strain achieved under a stress condition can be computed using the memberships. We applied this integrated stress-strain analysis method to Data Sets I and II from two calcite veins in a Miocene forearc basin deposit in central Japan. Since the sampling area was close to a triple-trench junction, the young formation has experienced polyphase tectonics.

As a result, we obtained the consistent stress and strains from both of the data sets. Three stresses were obtained from Data Set I, and the corresponding strains were 0.17, 0.25 and 0.13%. Two stresses were obtained from Data Set II, and the strains were 0.39 and 0.42%. The stress and strain determined from the data sets for each deformation phase were consistent with each other. That is, the principal axes had difference as small as < 20 degrees, and the shape ratios of stress and strain had also similar values. It is not straightforward to generalize this result, but both the stress and strain analyses seem to give appropriate results, providing that polyphase deformations are coped with.

How to cite: Wakamori, K. and Yamaji, A.: The integrated stress-strain analysis of calcite twins: Consistent stress and strain determined from natural data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12819, https://doi.org/10.5194/egusphere-egu2020-12819, 2020.

Strain localisation and fabric development in the lower crust is controlled by the active deformation mechanisms. Understanding the driving forces of such deformation aids in quantifying the stresses and rates of the deformation processes. Here we show that diffusion creep plays a major role in deformation of gabbro lenses at upper amphibolite facies conditions. The Kågen gabbro in the North Norwegian Caledonides intruded the Vaddas Nappe at 439 Ma at pressures of 7-9 kbar, temperatures of 650-900°C (depths of ∼26-34 km). The Kågen gabbro on south Arnøya is made up of undeformed gabbro lenses with sheared margins wrapping around them. This contribution analyses the evolution of the microstructures and fabric of the low strain gabbro to high strain margins. Microstructural and textural data indicate that preferential crystal growth of amphibole grains in the extension direction has produced the deformation microstructure and the CPO. Dissolution precipitation creep is inferred to be the dominant deformation mechanism, where dissolution of the gabbro took place in reacting phases of clinopyroxene and plagioclase, and precipitation took place in the form of new minerals: amphibole, garnet and zoisite. Synchronous deformation and mineral reactions of clinopyroxene suggests mafic rocks can become mechanically weak during the general transformation weakening process, i.e. the interaction of mineral reaction and deformation by diffusion creep. Deformation and metamorphic reaction were both important transformation processes during diffusion creep deformation of the margins of the gabbro lenses. The weakening is directly connected to a transformation process that facilitates diffusion creep deformation of strong minerals (pyroxene, garnet, zoisite) at far lower stresses than dislocation creep. Initially strong lithologies can become weak, provided that reactions can proceed during deformation, the transformation process itself is an important weakening mechanism in mafic (and other) rocks, facilitating deformation at low differential stresses.

How to cite: Lee, A., Stunitz, H., Battisti, M. A., and Konopasek, J.: Dissolution precipitation creep as a process for the strain localisation in gabbro, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13523, https://doi.org/10.5194/egusphere-egu2020-13523, 2020.

Microinclusions of Fe-Ti-oxides in rock-forming plagioclase are protected from subsea-floor alterations by the silicate matrix and are stable carriers of the paleomagnetic record of oceanic gabbros. We studied plagioclase-hosted microinclusions in oceanic gabbro (“gabbro 1241” from the Vema Lithospheric section, Mid-Atlantic ridge, Pertsev et al., 2015) with a complex petrogenetic history. Important events in the gabbro evolution caused consecutive transformations of the micro-inclusions and presumably affected their paleomagnetic records.

The earliest generation of the microinclusions was present as ulvospinel and presumably titanomagnetite which were probably formed by sub-solidus oxidation exsolution of early-magmatic plagioclase (An#42-45). An increase of the anorthite component around the early generation of microinclusions to typical values of late-magmatic plagioclase (An#53) suggest involvement of the late-magmatic fluid accompanying residual melt at 800-900°С (Pertsev et al., 2015). Subsequently, the gabbro was locally affected by hydrothermal alteration at about 600°С as a result of interaction of the gabbro with reduced brine containing 20-21% NaCl. The reducing conditions of this process ensured a “non-oxidative” character and primarily cooling driven exsolution of the microinclusions and transformation of the titanomagnetite microinclusions into ulvospinel-magnetite (“Usp-Mt₁”) intergrowths at about 500°С, which is close or higher than the Curie temperature (Tc) of the exsolving titanomagnetite but lower than the Tc of the newly forming Mt_1, and the acquired magnetisation may be referred to chemical remanence. The further evolution of the micro-inclusions correlates with low-temperature hydrothermal alteration induced by inflow of seawater-derived fluids during tectonic unroofing of the lithospheric section (Pertsev et al., 2015). The homogeneous ulvospinel inclusions and ulvospinel of the “Usp-Mt₁”-inclusions were replaced by “Ilm-Mt₂” -aggregates under more oxidizing conditions: 3 Fe₂TiO₄(Ulv) + 0.5 O₂= 3 FeTiO₃(Ilm) + Fe₃O₄(Mt₂). The Mt₁ was more stable to increase of fO₂ that resulted in simultaneous presence of the “Ilm-Mt₂”-, “Mt₁/Ilm-Mt₂“- and “Usp-Mt₁” -inclusions with two generations of magnetic phases (magnetite) within a single plagioclase grains.

Thus, despite of protection by silicate matrix the microinclusions of Fe-Ti-oxides in rock-forming plagioclases evolve under the influence of petrogenetic processes. It is important to note that bulk-rock AF demagnetization of 14 specimens (extracted from “gabbro-1241“) in the interval 100-250 Oersted (Oe) and 300-700 Oe revealed both moderately-grouped (k=27.4) and variable (deviated with angle 40-104°) directional components of magnetisation, which may have resulted from the presence of different generations of magnetite. Further magnetic investigation of separates of plagioclase single grains will allow to evaluate capacities of plagioclase-hosted Fe-Ti-micro-inclusions to save initial and “newfound” paleomagnetic information and to serve as stable sources of paleomagnetic record in regions of mid-oceanic ridges.

Funding by RFBR project 18-55-14003 and FWF project I 3998-N29 is acknowledged.

Pertsev, A. N., Aranovich, L. Y., Prokofiev, V. Y., Bortnikov, N. S., Cipriani, A., Simakin, S. S., & Borisovskiy, S. E. (2015). Signatures of residual melts, magmatic and seawater-derived fluids in oceanic lower-crust gabbro from the Vema lithospheric section, Central Atlantic. Journal of Petrology, 56(6), 1069-1088.

How to cite: Ageeva, O., Pilipenko, O., Pertsev, A., and Abart, R.: Microstructure evolution of plagioclase-hosted Fe-Ti-oxide microinclusions in oceanic gabbro, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8851, https://doi.org/10.5194/egusphere-egu2020-8851, 2020.

Gabbros are the main component of the oceanic crust and represent ~2/3 of the total magmatic crustal thickness. At the interface between magmatic, tectonic and hydrothermal processes, gabbros from slow spreading ridges may have a complex mineralogy and microstructural evolution. This includes structures that vary from purely magmatic fabrics, with layering and magmatic alignment of minerals, to rocks deformed from subsolidus temperatures to the lower-T brittle-ductile conditions. Such a variation is normally accompanied with changes in mineralogy, microstructures and crystallographic preferred orientations (CPO) of the main phases of these rocks, which in turn affect their seismic properties. Here we present a database of the CPO-derived seismic properties of 70 samples collected during the IODP Expedition 360 (site U1473). Initial results show that the dominant phases are plagioclase and clinopyroxene[MOU1] , and different samples may have different contents of olivine, enstatite, magnetite, ilmenite, chlorite and amphibole. Maximum velocities can be either parallel to the strongest concentration of (010) poles of plagioclase or olivine/clinopyroxene [001], depending on the proportions between these phases. Anisotropy of P waves vary from ~5% in the more isotropic gabbros with weak magmatic fabric to a maximum of ~10% in more mylonitic terms. A similar effect is observed for the S-waves. Destructive interference between plagioclase CPO vs. clinopyroxene/olivine reducing anisotropy is possibly observed. This is because the maximum Vp in a foliated gabbro is parallel to the maximum concentration of poles to (010), and perpendicular to olivine and clinopyroxene. As the lineation in our gabbros is generally marked by olivine and clinopyroxene [001] (instead of the fast direction [100]), this possibly cause anisotropy reduction. When present in the more mylonitized gabbros, amphibole has strong CPOs and help to increase the general anisotropy of P and S waves. The elastic constants calculated from these aggregates will be used as input for more physically robust calculations using differential effective medium approaches to better understand the effect of melt inclusions in these rocks by the time of their deformation in the lower crust.

How to cite: Morales, L. F. G., Allard, M., and Ildefonse, B.: A database for the seismic properties of slow spreading mid-ocean ridges gabbros, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5297, https://doi.org/10.5194/egusphere-egu2020-5297, 2020.

In the Ulten Zone (Tonale nappe, Eastern Alps, N Italy), numerous peridotite bodies occur within high-grade crustal rocks. Peridotites show a transition from coarse protogranular spinel-lherzolites to fine-grained mylonitic garnet-amphibole peridotites (Obata and Morten, 1987). Pyroxenites veins and dikes, transposed along the peridotite foliation, show a similar evolution from coarse garnet-free websterites to fine-grained garnet + amphibole clinopyroxenites (Morten and Obata, 1983). This evolution has been interpreted to reflect cooling and pressure increase of pyroxenites and host peridotites from spinel- (1200 °C, 1.3-1.6 GPa) to garnet-facies conditions (850 °C and 2.8 GPa) within the mantle corner flow (Nimis and Morten, 2000).

The newly formed garnet occurs as exsolution within porphyroclastic, high-T pyroxenes, and crystallises along the pyroxenite and peridotite foliation.

Textural evidence and crystallographic orientation data indicate that the transition from spinel- to garnet-facies conditions was assisted by intense shearing and deformation. Pyroxene porphyroclasts in garnet clinopyroxenites show well-developed crystallographic preferred orientation (CPO), high frequency of low-angle misorientations, and non-random distribution of the low-angle misorientation axes. These features indicates that pyroxene porphyroclasts primarily deform by dislocation creep on the (100) [010] slip system. Dislocation creep is accompanied by subgrain rotation recrystallisation, which promotes the formation of new, smaller and equant pyroxene grains around porphyroclasts. The grain size reduction promotes a switch in the deformation mechanism from grain-size insensitive creep (i.e. dislocation creep) in the porphyroclasts to grain-size sensitive (GSS) creep in the small recrystallised grains. The switch from dislocation to GSS creep is accompanied not only by grain size reduction of pyroxenes, but also by the formation of garnet exsolutions in pyroxenes and garnet crystallisation along foliation. We suggest that garnet crystallisation triggers the pinning of the recrystallised matrix, stabilising the fine-grained microtexture for GSS creep process, and finally contributes to the rheological weakening of pyroxenites.

Pyroxenites and peridotites of Ulten Zone thus offer a unique opportunity to investigate the effects of mantle deformation and weakening on the processes that control the material exchange between crust and mantle at subduction zones.

Morten, L., & Obata, M. (1983). Bulletin de Minéralogie, 106(6), 775-780.

Nimis, P. & Morten, L. (2000). Journal of Geodynamics, 30(1-2), 93-115

Obata, M., & Morten, L. (1987). Journal of Petrology, 28(3), 599-623.

How to cite: Zanchetta, S., Menegon, L., Pellegrino, L., Tumiati, S., and Malaspina, N.: Reaction-induced strain localisation in garnet pyroxenites during mantle corner flow: an example from the Ulten Zone (Eastern Alps, N Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19034, https://doi.org/10.5194/egusphere-egu2020-19034, 2020.

The convective motion of Earth’s upper mantle is controlled by two main deformation mechanisms: grain size-insensitive dislocation creep and grain size sensitive diffusion creep. Grain size thus plays a key role in upper mantle deformation, as it has a significant impact on the viscosity of the upper mantle. Moreover, grain size also affects seismic velocities as well as seismic attenuation.

Despite the importance of grain size and its evolution during deformation, there is still a lack of experimental data on grain growth of olivine at upper mantle pressures. For this reason, we here investigate olivine grain growth at pressures ranging from 1 GPa to 12 GPa and temperatures from 1200 to 1400ºC. The experiments were done using piston cylinder and multi-anvil apparatuses. We used as a starting material olivine aggregates with small amounts of pyroxene (<10%) produced via sol-gel method.

Our results indicate that grain growth is reduced at increasing pressures.  This suggests that the enhanced grain growth due to the temperature increase with depth may be offset, thus facilitating a change from dislocation to diffusion creep in the deep upper mantle. This might have an important impact on the dynamics of the upper mantle.

How to cite: Ferreira, F., Thielmann, M., and Marquardt, K.: Pressure dependence of olivine grain growth at upper mantle conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18346, https://doi.org/10.5194/egusphere-egu2020-18346, 2020.

The ability of water to enhance plastic deformation of a quartz aggregate has been experimentally demonstrated during the sixties (e.g. Griggs and Blacic 1965), however the processes involved are still questioned. Notably the processes combining the effect of water and pressure during the deformation are still not completely understood. Pressure strongly influences the strength of fine-grained (3.6 - 4.9 µm) wet quartz aggregates (Kronenberg and Tullis 1984), where diffusion creep operates (Fukuda et al. 2018) but its effect on coarser-grained material expected to deform only by dislocation creep is not well constrained. To re-assess the effect of pressure on quartz crystal plastic deformation, natural wet quartzite samples from the Tana quarry in northern Norway (grain size ≈ 150 µm) have been deformed using a Griggs-type apparatus at varying confining pressures (from 0.6 to 2.0 GPa). All the samples with 0.1 wt. % H₂O added were shortened coaxially up to 30% strain at constant strain rate (≈10^-6 s^-1) and temperature (900°C).

All mechanical records show that quartzite flow stresses decrease systematically with increasing pressure. These results allow to determine the strength of quartzite as a function of water fugacity, such as introduced in the flow law by Kohlstedt et al. (1995) to account for both pressure and water effects. In our case, the fugacity coefficient is m≈1 when using a stress exponent of n=2.

Microstructure and image analyses of samples reveal that the bulk strain results mainly from crystal plastic deformation of original grains whereas the recrystallization processes are limited volumetrically (less than 5%) and restricted to the boundaries of original grains. Deformation is not strongly partitioned into recrystallized domains compared to flattened original grains. Optical and SEM-cathodoluminescence images revealed the presence of cracks in conjunction with recrystallization (even for high-pressure samples) and associated chemical/fluid interaction, but the cracks do not contribute significantly to the bulk strain of the samples.

In order to determine the amount of water used for the deformation and the redistribution of H₂O during deformation, the H₂O content of the quartzite has been calculated from FTIR (Fourier Transform InfraRed spectroscopy) measurements for both, grain interiors and grain boundaries. The H₂O concentrations decrease inside grains with the onset of deformation with respect to the starting material. H₂O is primarily stored in the grain boundary region. There is no systematic correlation with pressure. Thus, pressure dependence of H₂O weakening is not restricted to fine-grained materials at high pressure and temperature. Deformation redistributes water from the grain interiors to their grain boundaries.

References:
Fukuda, J., Holyoke III, C.W., and Kronenberg, A.K. (2018). J. Geophysical Res.: Solid Earth, 123(6), 4676-4696.
Griggs, D. T., and Blacic J. D. (1965). Science, 147(3655), 292‑295.
Kohlstedt, D. L., Evans B., and Mackwell S. J. (1995). J. Geophysical Res.: Solid Earth, 100(B9), 17587-17602.
Kronenberg, A. K., and Tullis J. (1984). J. Geophysical Res.: Solid Earth, 89(B6), 4281‑4297.

How to cite: Nègre, L., Stünitz, H., Raimbourg, H., Précigout, J., Jeřábek, P., and Pongrac, P.: Effect of confining pressure on the strength of wet quartzite revisited, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5595, https://doi.org/10.5194/egusphere-egu2020-5595, 2020.

To investigate the role of strong and weak secondary phases on the recrystallized grain size of quartz, we performed grain size analyses on quenched samples from general shear experiments on quartz-garnet and quartz-muscovite mixtures. Six general shear experiments were conducted in the Griggs apparatus; three with mixtures of quartz-garnet (vol.% garnet 5, 15, 30) and three with mixtures of quartz-muscovite (vol.% muscovite 5, 10, 25). The starting powders for both set of experiments were synthetic mixtures of quartz-muscovite or quartz-garnet with 0.1 wt.% water added. The quartz-garnet experiments were conducted at 900°C, a pressure of 1.2 GPa, and a shear strain rate of ~10^-5 s^-1, while the quartz-muscovite experiments were conducted at 800°C, a pressure of 1.5 GPa, and a shear strain rate of ~10^-5 s^-1. At these deformation conditions quartz is stronger than muscovite and weaker than garnet. We observed that the bulk strength of the aggregate decreases with a greater volume percent of muscovite and increases with a greater volume percent of garnet. Garnet at these conditions does not deform plastically. The presence of secondary phases within the deforming aggregate causes stress concentrations and partitioning of strain rate between the different phases relative to the measured bulk stress and strain rate. The degree of partitioning is primarily related to the rheology and volume percent of the phases. Due to the piezometric relationship between recrystallized grain size and stress, we can use the quartz recrystallized grain size to determine the local stress of quartz in the experiments and compare it to the measured bulk stress. The results from these analyses will provide new insight into the effect of strain partitioning in general and of strong and weak secondary phases on quartz rheology.

How to cite: Tokle, L., Hirth, G., Morales, L., and Stunitz, H.: The effect of garnet and muscovite on the recrystallized grain size of quartz from general shear experiments , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8179, https://doi.org/10.5194/egusphere-egu2020-8179, 2020.

Forming in oceanic and continental subduction zones during high to ultra-high-pressure metamorphism, eclogites play an important role in convergent settings. To improve our understanding of eclogite deformation behaviour with respect to its phase abundance of omphacite and garnet, synthetic eclogite samples containing 25, 50 and 75% volume fraction of garnet have been deformed in a modified Griggs-type solid-medium apparatus. Deformation conditions were set to a confining pressure of 2.5 GPa, 900 °C and 3*10^-6 s^-1. Detailed microstructure analysis via optical and electron microscope imaging (SE, BSE and EBSD) served for identification of the dominant active deformation mechanism.
All eclogite samples show a foliation which is defined by a shape preferred orientation (SPO) of omphacite and intercalated foliation sub-parallel garnet aggregates. Another common feature is a weak crystallographic preferred orientation (CPO) of omphacite which is present throughout all samples. In accordance to uniaxial shortening the CPO resembles an s-type texture with a point maximum of [001] axis parallel to the foliation and a maximum of [010] axis perpendicular to the foliation. The stereographic projection of garnet crystallographic orientation is almost distributed randomly. Nevertheless, both phases show an intracrystalline misorientation, indicating activation of crystal plastic processes. With increasing garnet content the grain average misorientation is increasing in both omphacite and garnet crystals. On the other hand, deformation twinning in omphacite is decreasing with increasing garnet content. Further, all samples show indication of brittle deformation of both garnet and omphacite, increasing with increasing garnet content. In samples with a 25% volume fraction of garnet micro-cracks are primarily orientated perpendicular to the foliation, getting more randomly distributed in samples with a 50 and 75% volume fraction of garnet.
In conclusion, all samples show similar microstructures and textures indicating that similar mechanism are active during deformation. However, the overall dominant deformation behaviour is switching from crystal plastic to brittle with increasing garnet content.

How to cite: Klackl, S., Kraus, K., Renner, J., Grasemann, B., and Rogowitz, A.: HP/HT deformation experiments of garnet-omphacite aggregates – influence of volume fractions on deformation mechanisms., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8629, https://doi.org/10.5194/egusphere-egu2020-8629, 2020.

The Naab area is situated on the western border of the Bohemian Massif, 60 km south of the KTB (Kontinentalen Tiefbohrung). The main super-deep borehole of the KTB reached a depth of 9,101 meters in the Earth's continental crust. The fission-track data for the KTB and the Naab area present contrasting signatures. The apatite fission-track ages in the upper section of the KTB borehole and surrounding area are in the range 50-70 Ma (Wagner et al., 1994; Wauschkuhn et al., 2015). The apatite fission-track ages of the Naab basement are older than those of the KTB area, and span a broader range: 120-200 Ma (Vercoutere, 1994). The distributions of the confined-track lengths range from unimodal over skewed and mixed to bimodal, with mean lengths in the range 11-13 µm. In broad terms, this can be interpreted as that the Naab samples contain both an older and younger (in particular pre- and post-late Cretaceous) fission-track population. The aim of our research is to investigate the applicability of lab-based models to geological data, using improved measurement techniques.

We studied eighteen samples dated by Vercoutere (1994) from the Palaeozoic basement and seven large rock samples from the Rotliegend strata north of the Luhe fault.  We intend to extend the confined-track length measurements of Vercoutere (1994), aiming to achieve higher resolution through methodological innovations made possible by computer-controlled motorized microscopes. Improved statistics increase the resolution of the modelled thermal histories, which permits to better distinguish systematic from statistical differences between the modelled palaeotemperatures and geological estimates. Experiments have shown that the rate of length increase permits to distinguish older from younger tracks (Jonckheere et al., 2017). This allows us to distinguish between tracks formed before and after the Late Cre­taceous to Palaeocene exhumation. The etch rate of a confined track is also an indicator of its individual thermal history, supplementing the information gleaned from its etchable length under fixed conditions. We compiled a comprehensive, high-resolution confined-track-length dataset. The Naab thermal histories were determined using modern modelling algorithms, implementing the most recent empirical equations.

References

Jonckheere R., Tamer M., Wauschkuhn F., Wauschkuhn B., Ratschbacher L., 2017. Single-track length measurements of step-etched fission tracks in Durango apatite: Vorsprung durch Technik.American Mineralogist 102, 987-996.

Vercoutere C., 1994. The thermotectonic history of the Brabant Massif (Belgium) and the Naab Basement (Germany):   an apatite fission track analysis. Ph. D. thesis, Universiteit Gent, pp. 191.

Wagner G.A., Hejl E., Van Den Haute P., 1994. The KTB fission-track project: Methodical aspects and geological implications. Radiation Measurements 23, 95-101.

Wauschkuhn B., Jonckheere R., Ratschbacher L., 2015. The KTB apatite fission-track profiles: building on a firm foundation? Geochimica et Cosmochimica Acta 167, 27-62.

How to cite: Novakova, L., Jonckheere, R., Wauschkuhn, B., and Ratchbacher, L.: Thermal history modelling of the western margin of the Bohemian Massif using high-resolution apatite fission-track thermochronology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15152, https://doi.org/10.5194/egusphere-egu2020-15152, 2020.

Clinoptilolite, a micro-porous natural zeolite comprising tetrahedra of silica and alumina that commonly occurs in volcanic tuffs through devitrification of natural glasses, has numerous uses in the manufacturing, agriculture and building industries; it also has applications in veterinary and human medicines. Field observations and microstructural investigations in the natural clinoptilolite-tuff from Nižný Hrabovec (Slovak Republic) – one of the world’s economically most important high-quality clinoptilolite deposits – show evidence of strain localization. Brittle faults formed along pre-existing joints with plumose structures that had acted as a pathway for local infiltration of iron-, manganese- and potassium-rich fluids. Fault displacement formed structures that are indicative of both velocity hardening, with dissolution precipitation creep (SC/SCC’ foliation), and velocity weakening, with several phases of ultra-cataclasites forming along principal slip surfaces. Rock-fluid interaction is characterized by a high-mobility of K, with K-feldspar decorating SC/SCC’ foliations, infiltrating fractures in fault damage zones and precipitating as idiomorphic crystals in open cavities and along fault surfaces. Microstructures such as polished slickensides, injection of fluidized cataclasites, clast cortex grains in cataclasites and truncated grains along principal slip surfaces suggest that seismic slip probably occurred along some of the faults.

How to cite: Hou, Z., Rice, A. H. N., Tschegg, C., Berger, T., and Grasemann, B.: Strain localization associated with brittle faulting in a natural clinoptilolite-tuff (open-pit mine Nižný Hrabovec, Slovak Republic), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5167, https://doi.org/10.5194/egusphere-egu2020-5167, 2020.

Marmara granitoid (47 Ma) is a representative example of the Eocene post-collisional magmatism which produced several granitic plutons in NW Anatolia, Turkey. It is a W-E trending sill-like magmatic body which was concordantly emplaced into the metamorphic basement rocks of Erdek Complex and Saraylar Marble. The granitoid is represented by deformed granodiorite which displays well-developed lineation and foliation in meso-scale defined by the elongation of mica and feldspar crystals and recrystallization of quartz however, in some places, magmatic textures are preserved. Deformed granodiorite is broadly cut by aplitic and pegmatitic dikes and contains mafic enclaves which display the same deformation indicators with the main granitoid.

Microstructural analysis shows that the solid-state deformation of the Marmara granitoid is classified as ductile deformation with high temperatures and ductile-to-brittle deformation with relatively lower temperatures. Evidence for the ductile deformation of the granitoid is represented by chessboard extinction of quartz, grain boundary migration (GBM) and subgrain rotation recrystallisation (SGR) which exhibits that the deformation temperature changed from 600 ^oC to 400^oC. Bulging recrystallization (BLG), grain size reduction of amphibole, biotite and plagioclases and microcracks on plagioclases were considered as overlying ductile-to-brittle deformation signatures which develop between 300-<250 ^oC temperatures.

All of these field and micro-structural data collectively suggest that the shear sense indicators such as micafish structures and δ type mantled porphyroclasts displayed stair-steppings pointing out to a right lateral movement, indicating that the structural evolution and deformation history of Marmara granitoid was controlled by a dextral shear zone.

How to cite: Bayrak, S. B., Güraslan, I. N., Ünal, A., Kamacı, Ö., Altunkaynak, Ş., and Yiğitbaş, E.: Deformation history of the Marmara Granitoid and implications for a dextral shear zone in NW Anatolia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9231, https://doi.org/10.5194/egusphere-egu2020-9231, 2020.

Shear zones in the high-grade terranes represent the tectonic- fossils of strain history. One such shear zones, namely Balaram-Jogdadi shear zones defining the terrane boundary of the Ambaji granulites of the South Delhi terrane Aravalli –Delhi Mobile belt, NW India, provide evidence for strain variation during exhumation of lower-middle crustal rocks. Compilation of field and microscopic analysis of various samples of mylonite from shear zones suggest that the part of shear zone contains high-grade mineral assemblages such as cordierite, sillimanite, spinel, garnet in quartzo-feldspathic mylonite rock and exhibit signature of thrusting in which garnet behaved as brittle phase and quartz and feldspar grain show ductile deformation. 2D and 3D strain analysis estimate a plane to flattening type of strain pattern. Principal strain planes are used to calculate the strain ratios for estimation of variation of strain along the shear zone. This study indicates high-grade mylonite accommodates high strain. The flow of rigid porphyroclasts estimates mean kinematic vorticity number varies from 0.47 to 0.68, which indicates the dominance of pure shear during shearing. Vorticity by the Rs/θ method in quartz grain estimates ranges from 0.7 to 0.95, suggesting a non-steady strain towards the end of deformation. High-grade mylonites were overprinted by low-temperature mylonitisation marked by minerals like quartz, feldspar, biotite in which feldspar porphyroclast shows brittle deformation and quartz, biotite show ductile deformation. Several shear kinematics indicate top-to-NW sinistral strike-slip shearing. Thus it has been interpreted that the shear zone had undergone non-steady strain. The initial thrusting phase was dominated by more pure shear component. The strike-slip shearing part was dominated by more simple shear component. Monazite geochronology sets the age of shearing at 834-778 Ma suggesting the exhumation was a transition event between Grenville to Pan-African orogeny.

Keywords: Shear zone, Deformation, Vorticity, 3D strain analysis, Monazite dating

How to cite: Saraswati, R. and Biswal, T. K.: Non-steady strain in the terrane boundary shear zone of the Ambaji Granulite, NW India: Implications for understanding towards the dynamics of emplacement of the lower-middle crustal rocks., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2897, https://doi.org/10.5194/egusphere-egu2020-2897, 2020.

Small seismological events recorded in southern Norway in the period of 2017-2018 were used to calculate the sudden co-seismic temperature increase using a simple stress-drop model. In order to estimate the net production of thermal energy, both, industry explosions and natural events were included.  The range of resultant average temperature rise, with a maximum of ~ 143 ° C for a M_{w =}3.5, is proposed as an additional constituent that explain the weakness areas related to the high amount of intracrustal seismicity, mainly regarding the anisotropic thermal expansion of rocks and the flash heating thermal process produced by historical earthquakes with magnitudes over . The temperature values were subsequently used to estimate the thermal energy in 2D and 3D cumulative patterns in the area, as well as the total amount of energy that is available regarding seismic activity. The results were correlated with existing geological information, considering lineaments and heat flux data. Areas with high values of thermal energy seems to be spatially linked with both high heat flux zones and high density of lineaments, mainly to the south of the Trøndelag Platform as well as to the south of the Møre basin.

How to cite: Estay, R.: Thermal energy pattern related with the temperature increasing due to intraplate and induced seismicity, Southern Norway, 2017-2018, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10759, https://doi.org/10.5194/egusphere-egu2020-10759, 2020.

Frictional energy generating during an earthquake has been well studied in the last decades and quite a few laboratory experiments have been carried out recently with the objective to quantify and describe this type of energy in a better way.  In our research we modelled the temperature rise during a simulated seismic event and the consequent equivalent heat released using the ANSYS® Mechanical software. Our approach is using the Finite Element Method to model a symmetrical fault plane where several parameters such as density, pressure, structural and thermal material characteristics are set according the conditions of a compressional tri-axial test. Natural and forced models were explored applying the Mohr-Coulomb failure criteria. Using a temporal window similar to a realistic situation, we are capable to observe the differences that occur during the stick-slip behavior in the co-seismic rupture process. On the other hand, the time lapse allows us to observe model and infer how the heat is generated and transferred around the fault plane. As a preliminary result, a variation of approximately 1.5°C was obtained simulating the conditions for a laboratory induced micro-seismic event modelled as a tri-axial test under 10 MPa of confining pressure and 20 MPa as vertical pressure, with velocities in the order of 1.5 mm/s.

How to cite: Uslar, A.: Simulation of frictional heat generation due to underground motion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10795, https://doi.org/10.5194/egusphere-egu2020-10795, 2020.

A long standing debate in seismology revolves around the nonexistent heat flow anomaly across the San Andreas fault. Given the fault’s average slip rate and age, a strong San Andreas fault, i.e. characterized by a relatively high static friction coefficient of µ>=0.6, should produce a significant local heat flow anomaly across the fault [1]. Since the work of Lachenbruch and Sass [1], this anomaly has not been observed and although many possible causes for the lack of a heat flow anomaly have been explored, the static or dynamic weakness of the San Andreas fault remains a favorable explanation [2,3].

Recently, we have introduced the ENergy COnserving Seismicity (ENCOS) framework that relates elastic deformation energy loading rates to the long-term average energy release of the seismic process. Within the presented implementation of ENCOS for Southern California with an elastic loading rate between 300 MW and 1.9 GW, the two most significant parameters are the static friction coefficient and the average efficiency. In particular, they are the most significant sources of uncertainty in harnessing the GPS-derived strain rates and the stress data within the ENCOS framework.

Here, we show how ENCOS can be leveraged in combination with the constraints from heat flow measurements and observed seismicity to restrict the parameter space of the average efficiency and the static friction coefficient. This can help to reduce the uncertainty of the ENCOS model parameters, such as the elastic deformation energy loading rate, and opens a new viewpoint on the heat flow paradox.

[1] Lachenbruch, A. H., and Sass, J. H. (1980), Heat flow and energetics of the San Andreas Fault Zone, J. Geophys. Res., 85(B11), 6185–6222.

[2] Scholz, C. H. (2013). The Strength of the San Andreas Fault: A Critical Analysis. In Earthquakes: Radiated Energy and the Physics of Faulting (eds R. Abercrombie, A. McGarr, G. Di Toro and H. Kanamori).

[3] E. E. Brodsky et al. (2020), The State of Stress on the Fault Before, During, and After a Major Earthquake, Annu. Rev. Earth Planet. Sci. 48:2.1–2.26.

How to cite: Ziebarth, M. J., Anderson, J. G., von Specht, S., Heidbach, O., and Cotton, F.: From Elastic Deformation Loading Rates to Heat Flow Anomalies: Constraints on Seismic Efficiency and Friction Coefficient, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18763, https://doi.org/10.5194/egusphere-egu2020-18763, 2020.

In the brittle part of the crust, deformation is usually perceived to be the result of displacement along fault planes, whose behaviors are controlled by their frictional properties. However, fault zones not only consist of a narrow fault core where slip occurs, they are also surrounded by a complex structure which is of key importance in the mechanics of faulting, hence in determining the overall energy budget. Indeed, as pointed out by the numerous field, geophysical, mechanical and laboratory observations, if the behavior of fault zones is intrinsically linked to the properties of the main sliding plane, it also depends on those of the surrounding medium.  In parallel, fault displacements may induce a substantial change in the physical properties of the surrounding medium. As a consequence, to improve our understanding of active fault zones, fault slip and the evolving physical properties must be studied as a unique system of stress accommodation and no longer as two distinct entities. To tackle this problem, we have developed a micromechanics-based constitutive model, thermodynamically argued, that can determine the inelastic behavior at macroscopic scale that arises from structural rearrangements at microscale. It is therefore the compulsory tool to emulate the strong coupling between the bulk and the fault that prevails during earthquakes. With this code, we can reproduce the strain rate sensitive, non-linear stress-strain relationship that leads to off-fault damage as a seismic event is propagating. We explore different scenarii and we show that there is a unique off-fault damage pattern associated with supershear transition of an earthquake rupture, that is also observed in the field.  We define, in return, the impact of damage on the propagation of the earthquake in itself and the generated waves. We conclude by assessing the kinetic energy, the dissipated energy and the radiated energy to define how energy is consumed within crustal systems during seismic events.

How to cite: Thomas, M. Y. and Bhat, H. S.: Impact of coseismic off-fault damage on the overall energy budget., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8030, https://doi.org/10.5194/egusphere-egu2020-8030, 2020.

Episodic tremor and slow slip (ETS) is observed in several subduction zones down-dip of the locked megathrust, and may provide clues for preparatory processes before megathrust rupture. Exhumed rocks provide a unique opportunity to evaluate the sources of rheological heterogeneity on the subduction interface and their potential role in generating ETS-like behavior. We present data from two subduction interface shear zones representative of the down-dip extent of the megathrust: the Condrey Mountain Schist (CMS) in northern CA (greenschist to blueschist facies conditions) and the Cycladic Blueschist Unit (CBU) on Syros Island, Greece (blueschist to eclogite facies). Both complexes highlight the propensity for fluid-mediated metamorphic reactions to produce strong rheological heterogeneities:

In the CMW, hydration reactions led to progressive serpentinization of peridotite bodies that were entrained from the overriding plate and underplated along with oceanic-affinity sediments. The margins of each peridotite-serpentinite lens show extreme strain localization accommodated by dislocation glide and minor pressure solution in antigorite, whereas lens interiors show evidence for more distributed, alternating, frictional-viscous deformation, with abundant crack-seal veins occupied by antigorite, brucite and oxides that are in some places also ductilely sheared. Deformation in the surrounding metasedimentary matrix was purely viscous.

In the CBU on Syros Island, dehydration reactions in MORB-affinity basalts, subducted and underplated with oceanic and continental-affinity sediments, led to progressive development of strong eclogitic lenses within a weaker blueschist and metasedimentary matrix. The eclogite lenses are commonly coarse-grained and massive and show brittle deformation in the form of dilational and shear fractures/veins filled with quartz, white mica, glaucophane and/or chlorite. Brittle deformation in the eclogites is coeval with ductile deformation in the surrounding blueschist and metasedimentary matrix, indicating concurrent frictional-viscous flow.

Although we cannot easily distinguish transient deformation processes in exhumed rocks, we can use the following three approaches to assess whether these heterogeneities could have generated deformation behaviors similar to deep ETS: 1) We measure displacements within, and dimensions of the heterogeneities in outcrop/map-scale to estimate the maximum possible seismic moment that would be released when the frictional heterogeneities slip;  2) We compare deformation mechanisms inferred from field and microstructural observations to their expected mechanical behavior from rock deformation experiments; and 3) We use seismo-thermo-mechanical modeling to examine expected slip velocities and moment-duration ratios for frictional-viscous shear zones that are scaled to observations from nature and the lab.  

All three approaches suggest that frictional-viscous heterogeneities of the types and length-scales we observe in the exhumed rock record are compatible with ETS as documented in modern subduction zones.

How to cite: Behr, W., Tewksbury-Christle, C., Kotowski, A., Cannizzaro, C., Blass, R., and Gerya, T.: Hydration- and dehydration-induced rheological heterogeneities on the deep subduction interface, and possible relationships to episodic tremor and slow slip, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3373, https://doi.org/10.5194/egusphere-egu2020-3373, 2020.

The low velocity layer (LVL) in modern subduction zones is a 3-5 km thick region that parallels the top of the downgoing slab and is characterized by anomalously high V_p/V_s ratios (1.8-2.5) consistent with 2.5-4% fracture porosity at near-lithostatic pore fluid pressures. The LVL has been previously interpreted as partially hydrated, relatively undeformed oceanic crust at the top of the downgoing slab, but collocation of the LVL with episodic tremor and slow slip events (ETS) in modern subduction zones suggests that the LVL may alternatively represent the seismic signature of a subduction interface shear zone. 

To test this hypothesis, we use field & structural observations, geochronology, and seismic velocity calculations to compare and contrast the bulk seismic properties of a fossil subduction interface shear zone (Condrey Mountain Schist, CMS, northern CA) to properties of modern LVLs. Specifically, we 1) determined thicknesses of underplated packages (interpreted to represent the maximum thickness of the actively deforming interface) using depositional age discontinuities and high resolution structural mapping, 2) averaged the bulk rock seismic velocities weighted by mapped lithologic proportions and corrected for pressure-temperature effects, and 3) used field evidence of modifying factors (e.g., microcracks, fluid-filled veins, mineral anisotropy) to further refine the possible range of seismic velocities and effects on V_p/V_s ratio.

The CMS greenschist- to blueschist-facies units were subducted to ~25-35 km (450°C, 0.8-1.0 GPa) with limited retrogression or exhumational overprint. These rocks were underplated episodically at depth in three packages individually up to 4.5 km thick from 155-135 Ma, based on detrital zircon data. Each package is dominantly composed of metasedimentary rocks with m- to km-scale metamafic and serpentinized ultramafic lenses. Strain localization to ~1 km thick ductile shear zones between underplating episodes is collocated with km-scale serpentinized ultramafic lenses at the base of each package. Deformation was distributed and ductile with rare macro- or micro-scale prograde brittle failure in the metasedimentary or metamafic units. In the serpentinized ultramafics, ductile shear zones wrap massive blocks with prograde brittle fracture. Maximum fracture porosity estimated from relict veins is ~10%. Average V_p/V_s for the CMS is ~1.6 (lithology alone) but up to 3.0 (accounting for maximum fracture porosity).

The fossil subduction interface shear zone preserved in the CMS is consistent in both thickness and seismic signature with the LVL in modern subduction zones. Estimated V_p/V_s is higher than the LVL but assumes that all fractures are simultaneously open. The total thickness of the CMS (10+ km) is greater than the LVL, however, so previously underplated material must lose its anomalous seismic signature during underplating (e.g., due to fluid loss and transport up the slab during or after underplating). Our results support the hypothesis that LVLs in modern subduction zones represent the seismic signature of the subduction interface shear zone.

How to cite: Tewksbury-Christle, C., Behr, W., and Helper, M.: Rock record constraints on the seismic signature of subduction interface shear zones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3285, https://doi.org/10.5194/egusphere-egu2020-3285, 2020.

Veins that form contemporaneously with deformation are the best recorders of the fluids circulating in the depths of orogenic and subduction zones. We have analyzed syn-kinematic quartz veins from accretionary prisms (Shimanto Belt in Japan, Kodiak accretionary prism in Alaska) and tectonic nappes in collisional orogens (Flysch à Helminthoïdes in the Alps, the southern domain of the variscan Montagne Noire), which formed at temperature conditions between 250 and 350°C, i.e. spanning the downdip limit of large subduction earthquakes and the generation of slow slip events. In all geological domains, veins hosted in rocks with the lower temperature conditions (~250-300°C) show quartz grains with crystallographic facets and growth rims. Cathodoluminescence (CL) imaging of these growth rims shows two different colors, a short-lived blue color and a brown one, attesting to cyclic variations in precipitation conditions. In contrast, veins hosted in rocks with the higher temperature conditions (~350°C), show a homogeneous, CL-brown colored quartz, except for some very restricted domains of crack-seal structures of CL-blue quartz found in Japan, Kodiak and Montagne Noire. Based on laser ablation and electron microprobe mapping, the variations in CL colors appear correlated with the trace element content of quartz, the short-lived CL-blue being associated with the substitution of Si⁴⁺ by Al³⁺+Li⁺/H⁺.

Due to their ubiquitous presence in various settings, the variations in CL colors in the lower T range reflect a common, general process. We interpret these cyclic growth structures as a reflection of deformation/fracturing events, which triggered transient changes in (1) the fluid pressure through fluid flow and (2) the chemistry of the fluid due to enhanced reactivity of the fractured material. The CL-blue growth rims delineate zones where quartz growth was rapid and crystals incorporated a large proportion of Al and Li. Crystal growth continued at a lower pace after fluid pressure and composition evolved to equilibrium conditions, leading to the formation of CL-brown quartz with few substitutions of tetrahedral Si. The variations in fluid pressure fluctuated at values close to lithostatic conditions, as indicated by growth in cavities that remained open.

The crack-seal microstructures have been interpreted as the result of slow-slip events near the base of the seismogenic zone (Fisher and Brantley, 2014; Ujiie et al., 2018). Our observations on quartz composition suggest that the quartz in crack-seal microstructures records episodic variation in fluid pressure and composition, similar to vein quartz at T<~300 °C. In contrast to the cooler and shallower domain, the variations are significantly smaller, as recorded by the very limited extent of the CL-blue domains, and most if not all of the quartz growth occurred under constant physico-chemical conditions, including a near lithostatic fluid pressure. 

We conclude that quartz trace element content is a useful tool to track variations in fluid conditions. In particular, at seismogenic depths (i.e. near 250°C), fluid pressure varies significantly around a lithostatic value. In contrast, deeper, near the base of the seismogenic zone where slow slip events occur (i.e. near 350°C), the variations in fluid pressure conditions are smaller.

How to cite: Raimbourg, H., Famin, V., Rajic, K., Erdmann, S., Moris-Muttoni, B., and Fisher, D.: Record by quartz veins of earthquakes and slow slip events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5472, https://doi.org/10.5194/egusphere-egu2020-5472, 2020.

In plate-convergent settings, fluid-saturated sediments dehydrate during subduction. The sediments are subsequently accreted to the upper plate. Along their dehydration-deformation path, the initial unconsolidated soft marine sediments become thick, foliated, impermeable meta-sedimentary sequences. Fluid flow through such ‘non’-porous low-permeability rocks is concentrated in fracture networks, ranging from the mm- to the km-scale. We study the interplay between ductile and brittle deformation processes and fluid flow by investigating calcite veins in slates from the exhumed European Alpine accretionary wedge across scales (µm to km). These slates experienced peak metamorphic temperatures between 200°C and 330°C and represent the transition between the upper aseismic and seismic zone. With the use of Synchrotron X-ray Fluorescence Microscopy (SXFM), we investigate the slates by visualizing trace-element distributions. This technique shows that alternating cycles of slow pressure-dissolution processes and brittle fracturing persist over long time scales. At the micron-scale, pressure solution of the initial carbonate-rich slates is indicated by an enrichment of newly recrystallized phyllosilicates on cleavage planes and in pressure shadows. These ductile deformation features are mutually overprinted by calcite veins (aperture 10 µm), which are nicely visualized with Sr-SXFM maps. Increasing compaction and recrystallization in the slate-rich matrix leads to progressed dehydration resulting in an increased pore fluid pressure and subsequent hydrofracturing. The micron-sized fractures are immediately filled in with minerals, which are oversaturated at that time in the fluid, resulting in the formation of (i) micron-veinlets. Micron-veinlets collect (ii) into mm-cm sized veins, which themselves form (iii) vein arrays and (iv) mega-arrays, respectively at the 50-100 m and 300-400 m scale. This upscaling of fluid pathways indicates a localised fluid transport through the accretionary wedge, which has important implications for the understanding of the mechanical stability of the accretionary wedge and related seismic activity.

How to cite: Akker, I. V., Schrank, C. E., Jones, M. W. M., Kewish, C. M., Berger, A., and Herwegh, M.: The coupling of dehydration and deformation results in localised fluid flow in the accretionary wedge – a novel study of calcite veins, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6845, https://doi.org/10.5194/egusphere-egu2020-6845, 2020.

Slow slip events (SSEs) are part of a spectrum of aseismic processes that relieve tectonic stress on faults. Their occurrence in subduction zones have been suggested to trigger megathrust earthquakes due to perturbations in fluid pressure. However, examples to date have been poorly recorded and physical observations of temporal fluid pressure fluctuations through slow slip cycles remain elusive. Here, we use a newly developed two-phase flow numerical model — which couples solid rock deformation and pervasive fluid flow — to show how crustal stresses and fluid pressures within subducting megathrust evolve before and during slow slip and regular events. This unified 2D numerical framework couples inertial mechanical deformation and fluid flow by using finite difference methods, marker-in-cell technique, and poro-visco-elasto-plastic rheologies. Furthermore, an adaptive time stepping allows the correct resolution of both long- and short-time scales, ranging from years to milliseconds during the dynamic propagation of earthquake rupture.

Here we show how permeability and its spatial distribution control the degree of locking along the megathrust interface and the interplay between seismic and aseismic slip. While a constant permeability leads to more regular seismic cycles, a depth dependent permeability contributes substantially to the development of two distinct megathrust zones: a shallow, locked seismogenic zone and a deep, narrow aseismic segment characterized by SSEs. Furthermore, we show that without requiring any specific friction law, our model shows that permeability, episodic stress transfer and fluid pressure cycling control the predominant slip mode along the subduction megathrust. Specifically, we find that the up-dip propagation of episodic SSEs systematically decreases the fault strength due to a continuous accumulation and release of fluid pressure within overpressured subducting interface, thus affecting the timing of large megathrust earthquakes. These results contribute to improve our understanding of the physical driving forces underlying the interplay between seismic and aseismic slip, and demonstrate that slow slip events may prove useful for short-term earthquake forecasts.

How to cite: Petrini, C., Dal Zilio, L., and Gerya, T.: Episodic fluid pressure cycling controls the interplay between Slow Slip Events and Large Megathrust Earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17557, https://doi.org/10.5194/egusphere-egu2020-17557, 2020.

In subduction zones, megathrust seismicity depends on the hydrogeological, thermal, physical and mechanical properties of the sediments before they enter the subduction zone and how these properties evolve through the subduction process. In particular, fluids are progressively released by compaction and/or mineral dehydration reactions as burial increases, resulting in the build-up of pore fluid pressure in low-permeability sediments that strongly affects fault behavior through its control on effective normal stress.

We use porosity, logging and chemical data to characterize compaction state and bound water content of sediments at Site C0024. This site was drilled in March 2019 during International Ocean Discovery Program Expedition 358 in the anticline overlying the frontal thrust of the Nankai margin, a few kilometers landward of Sites C0006 and C0007 that were previously drilled during the NanTroSEIZE project. Sites C0024, C0006 and C0007 transect the décollement and overlying accreted Upper Shikoku Basin and wedge slope deposits. At Site C0024, the main frontal thrust at ~820 mbsf, the top of its footwall (up to ~870 mbsf) and its hanging-wall were logged. Two intervals were cored in the hanging-wall (~0-320 mbsf and ~510-652 mbsf). Four sedimentary facies were identified : (1) the slope apron (~0-4 mbsf) composed of silty clay to clayey silt hemipelagites, (2) accreted trench wedge sediments (~4–519 mbsf) composed of hemipelagites with volcanic ash layers, silt and sand turbidites, (3) outer trench-wedge sediments (~519-555 mbsf) mainly composed of hemipelagites with rare silt turbidites and volcanic ash layers and (4) accreted Upper Shikoku Basin (>555mbsf) composed of hemipelagites and volcanic ash layers. The Pliocene to Miocene accreted Upper Shikoku Basin deposits at the frontal thrust are correlated with undeformed Upper Shikoku Basin deposits at reference Sites C0011 and C0012 seaward in the incoming Shikoku Basin.

Following previous studies, we use Cation Exchange Capacity (CEC) to determine bound water content and interstitial porosity in the cored interval. Unlike total porosity commonly measured onboard, interstitial porosity is representative of the compaction state of sediments. We use interstitial porosity data to calibrate resistivity-derived porosity through the hanging-wall, the décollement and below. Resistivity-derived porosity is obtained with a resistivity model accounting for the high surface conductivity of clays based on CEC, exchangeable cation composition, LWD resistivity and gamma ray logs. We also document the evolution of the structure of micro- to macropores with depth using low pressure nitrogen adsorption/desorption, mercury injection capillary pressure and nuclear magnetic resonance.  Finally, we compare the porosity dataset at Site C0024 with that of Sites C0006 and C0007 in the frontal thrust and reference Sites C0011 and C0012 in the entering Shikoku Basin to characterize the evolution of the compaction state of sediments during accretion.

How to cite: Dutilleul, J., Conin, M., Bourlange, S., and Géraud, Y. and the Expedition 358 Science Party: Characterization of porosity from Cation Exchange Capacity and resistivity data at IODP Expedition 358 Site C0024: insights on compaction state, stress and deformation history at the frontal thrust of the Nankai margin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11685, https://doi.org/10.5194/egusphere-egu2020-11685, 2020.

Long term crustal flow of the Makran subduction zone is computed with a kinematic finite element model based on iterated weighted least squares fits to data. Data include 91 fault traces, 56 fault offset rates, 76 geodetic velocities, 1962 principal stress azimuths, and velocity boundary conditions. Model provides long-term fault slip rates, velocity, and distributed permanent strain rates between faults in the Makran region from all available kinematic data. Due to low seismicity of western Makran compared to its eastern part we defined two models to evaluate the possibility of creep in the Iranian Makran subduction. One model assumes that geodetic velocities measured adjacent to the Makran subduction zone reflect a temporarily locked subduction zone will be referred to as the “seismic deformation model”. In contrast, another model called the “half creeping deformation model” assumes that the western part of Makran may creep smoothly without any locking. In order to verify the models, the estimates of fault slip rates are compared to slip rates from merely analysing geodetic benchmark velocities or paleoseismological studies or published geological rates which have not been used in the model. Our estimated rates are all in the range of geodetic rates and are even more consistent with geological rates than previous GPS-based estimates. Another verification for the model is comparison of the computed interseismic velocities at GPS benchmarks to GPS measurements. While neither model accurately predicts these interseismic velocities at benchmarks, the half creeping deformation model is more accurate for Chabahar station than the seismic deformation model. These results have important earthquake and tsunami hazard implications. For example, Fault slip rates are the main component of time-dependent seismic hazard studies and can be used to estimate activity rates for more sophisticated earthquake models.

How to cite: Ghadimi Moghaddam, H., Khodaverdian, A., and Zafarani, H.: An investigation on possibilty of creep on the Makran subduction zone based on deformation data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-864, https://doi.org/10.5194/egusphere-egu2020-864, 2020.

Geophysical data indicate that the Hikurangi subduction margin on New Zealand’s East Coast contains a large gas hydrate province. Gas hydrates are widespread in shallow sediments across the margin, and locally intense fluid seepage associated with methane hydrate is observed in several areas. Glendhu and Honeycomb ridges lie at the toe of the Hikurangi deformation wedge at depths ranging from 2100 to 2800 m. These two parallel four-way closure systems host concentrated methane hydrate deposits. The control on hydrate formation at these ridges is governed by steeply dipping permeable strata and fractures, which allow methane to flow upwards into the gas hydrate stability zone. Hydrate recycling at the base of the hydrate stability zone may contribute to the accumulation of highly concentrated hydrate in porous layers.
To improve the characterisation of the hydrate systems at Glendhu and Honeycomb ridges, we estimate hydrate saturation and porosity of the concentrated hydrate deposits. We first estimate elastic properties (density, compressional and shear-wave velocities) of the gas hydrate stability zone through full-waveform inversion and iterative geostatistical seismic amplitude versus angle (AVA) inversion. We then perform a petrophysical inversion based on a rock physics model to predict gas hydrate saturation and porosity of the hydrate bearing sediments along the two ridges.
Our results indicate that the high seismic amplitudes correspond to the top interface of highly concentrated hydrate deposit, with peak saturations around 35%. Because of the resolution of the seismic data we assume that the estimated properties are averaged over layers of 10 to 20 meters thickness. These saturation values are in agreement with studies conducted in other areas of concentrated hydrate accumulations in similar geologic settings.

How to cite: Turco, F., Gorman, A., Crutchley, G., Azevedo, L., Grana, D., and Pecher, I.: Methane hydrate saturations at the Southern Hikurangi margin (New Zealand) estimated from seismic and rock physics inversion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11486, https://doi.org/10.5194/egusphere-egu2020-11486, 2020.

Fluids can circulate in all levels of the crust, as veins, ore deposits and chemical alterations and isotopic shifts indicate. It is furthermore generally accepted that faults and fractures play a central role as preferred fluid conduits. Fluid flow is, however, not only passively reacting to the presence of faults and fractures, but actively play a role in their creation, (re-) activation and sealing by mineral precipitates. This means that the interaction between fluid flow and fracturing is a two-way process, which is further controlled by tectonic activity (stress field), fluid sources and fluxes, as well as the availability of alternative fluid conduits, such as matrix porosity. Here we explore the interaction between matrix permeability and dynamic fracturing on the spatial and temporal distribution of fluid flow for upward fluid fluxes. Envisaged fluid sources can be dehydration reactions, release of igneous fluids, or release of fluids due to decompression or heating.

Our 2D numerical cellular automaton-type simulations span the whole range from steady matrix-flow to highly dynamical flow through hydrofractures. Hydrofractures are initiated when matrix flow is insufficient to maintain fluid pressures below the failure threshold. When required fluid fluxes are high and/or matrix porosity low, flow is dominated by hydrofractures and the system exhibits self-organised critical phenomena. The size of fractures achieves a power-law distribution, as failure events may sometimes trigger avalanche-like amalgamation of hydrofractures. By far most hydrofracture events only lead to local fluid flow pulses within the source area. Conductive fracture networks do not develop if hydrofractures seal relatively quickly, which can be expected in deeper crustal levels. Only the larger events span the whole system and actually drain fluid from the system. We present the 10 square km hydrothermal Hidden Valley Mega-Breccia on the Paralana Fault System in South Australia as a possible example of large-scale fluid expulsion events. Although field evidence suggests that the breccia formed over a period of at least 150 Myrs, actual cumulative fluid duration may rather have been in the order of days only. This example illustrates the extreme dynamics that crustal-scale fluid flow in hydrofractures can achieve.

How to cite: Bons, P. D., de Riese, T., Gomez-Rivas, E., Naaman, I., and Sachau, T.: Hydrofractures and crustal-scale fluid flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18248, https://doi.org/10.5194/egusphere-egu2020-18248, 2020.

In marine rift basins, rift-climax deep-water clastics in the hanging wall of rift- or basin-bounding fault systems are commonly juxtaposed against crystalline basement rocks in the footwall. Displacing highly permeable, unconsolidated sediments against low-permeable rock distinguishes these faults significantly from others displacing hard rock. Due to limited surface exposure of such fault zones, studies elucidating their structure and evolution are rare. Consequently, their impact on fluid circulation and in-fault, near-fault, and hanging wall sediment diagenesis are also poorly understood. Motivated by this, we here investigate a well-exposed strand of a major basin-bounding fault system in the East Greenland rift system, namely the Dombjerg Fault which bounds the Wollaston Forland Basin, NE Greenland. Here, Upper Jurassic and Lower Cretaceous syn-rift deep-water clastics are juxtaposed against Caledonian metamorphic basement.

Previously, a ~1 km-wide zone of increased calcite cementation of the hanging wall sediments along the Dombjerg fault core was identified (Kristensen et al., 2016). Now, based on U/Pb calcite dating, we are able to show that cementation and formation of this zone started during the rift climax in Berrisian/Valanginian times. Using clumped isotope analysis, we determined cement formation temperatures of ~30-70˚C. Temperatures likely do not relate to the normal geothermal gradient, but to elevated fluid temperatures of upward directed circulation along the fault.

Vein formation within the cementation zone clusters between ~125-100 Ma in the post-rift stage, indicating that fracturing in the hanging wall is not directly related to the main phase of activity of the adjacent Dombjerg Fault. Vein formation temperatures range between ~30-80˚C, signifying a shallow burial depth of the hanging wall deposits. Further, similar minor element concentrations of veins and adjacent cements argue for diffusional mass transfer, which in turn infers a subdued fluid circulation and low permeability of the fracture network. These results imply that the chemical alteration zone formed an impermeable barrier quickly after sediment deposition and maintained this state even after fracture formation.

We argue that the existence of such a cementation zone should be considered in any assessments that target basin-bounding fault systems for, e.g., hydrocarbon, groundwater, geothermal energy, and carbon storage exploration. Our study highlights that the understanding fluid flow properties as well as fault-controlled diagenesis affecting the fault itself and/or adjacent basinal clastics is of great fundamental and economic importance.

References:

Kristensen, T. B., Rotevatn, A., Peacock, D. C. P., Henstra, G. A., Midtkandal, I., and Grundvag, S. A. (2016). Structure and flow properties of syn-rift border faults: The interplay between fault damage and fault-related chemical alteration (Dombjerg Fault, Wollaston Forland, NE Greenland), J. Struct. Geol., 92, 99-115, doi:10.1016/j.jsg.2016.09.012.

How to cite: Salomon, E., Rotevatn, A., Kristensen, T. B., Grundvåg, S.-A., Henstra, G. A., Meckler, A. N., Gerdes, A., and Roper, R. A.: Fault-controlled diagenesis and fluid circulation along a major syn-rift border fault system – insights from the Dombjerg Fault, Wollaston Forland Basin, NE Greenland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14590, https://doi.org/10.5194/egusphere-egu2020-14590, 2020.

Comb-veins are mineral-filled fractures oriented perpendicular to fault surfaces, with their intersection with the fault surface generating lineations that are perpendicular to the downdip slip direction. Despite the large occurrence along normal faults within seismogenic extensional tectonic settings (i.e. Greece, Turkey, Italy), their origin, geochemical signature, and kinematics are still poorly constrained. Here we present the first multidisciplinary study, combining field to microscale observations (optical microscope and cathodoluminescence) with geochemical-geochronological analyses (U-Th dating, stable-clumped isotopes, Strontium isotopes, whole-rock geochemistry, and fluid inclusions), on calcite-filled comb-veins cutting through the principal surface of the seismogenic Val Roveto Fault in the central Apennines, Italy. We show that comb-veins precipitated in Late Pleistocene time (between 300 ky and 140 ky) below the present-day outcrop level at a maximum depth of ∼350 m and temperatures between 32 and 64°C from deep-seated fluids modified by reactions with crustal rocks and with a mantle contribution (up to ∼39%). The observed geochemical signature and temperatures are not compatible which those of cold meteoric water and/or shallow groundwater (maximum temperature of 12 °C) circulating within shallow aquifers (≤ 500 m depth) in the study region. Therefore, we propose that deep-seated crust/mantle-derived warm fluids were squeezed upward during earthquakes and were hence responsible for calcite precipitation at shallow depths in co-seismic comb fractures. As comb-veins are rather common, particularly along seismogenic normal faults, we suggest that further studies are necessary to test whether these veins are often of co-seismic origin. If so, they may become a unique and irreplaceable tool to unravel the seismic history and crustal-scale fluid circulation of active faults.

How to cite: Smeraglia, L., Bernasconi, S. M., Berra, F., Billi, A., Boschi, C., Caracausi, A., Carminati, E., Castorina, F., Doglioni, C., Italiano, F., Rizzo, A. L., Uysal, I. T., and Zhao, J.: Comb-veins as a marker for crustal-scale fluid circulation: insight from geochronological (U-Th dating), geochemical, and field to microstructural analyses along the seismogenic Val Roveto Fault (central Apennines, Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-660, https://doi.org/10.5194/egusphere-egu2020-660, 2020.

Zannone is a very important island, located in the Neogene-Quaternary extensional domain of the Tyrrhenian back-arc basin, as it is the unique spot where the Paleozoic (?) crystalline basement is hypotesized to be exposed in central Apennines. The exposure of such hypothetical basement in the Zannone Island is very problematic as it implies very large normal displacements (> 3 km) along surrounding faults. No such displacements are known along faults close to Zannone Island.

In this work, we study the hypothetical Paleozoic crystalline basement exposed in the Zannone Island with the main aim of understanding its geological nature and relationships with the surrounding rocks. We use a multidisciplinary approach including 1) interpretation of seismic reflection profile; 2) field survey; 3) petro-textural observations; 4) microthermometry on fluid inclusions; 5) geochemical analyses of stable and clumped isotopes; 6) Illite crystallinity and mineralogical analyses of clays and host rocks; 7) analyses of minor gaseous species (He, Ne, and Ar concentrations and isotope ratios) in fluid inclusions; 8) U-Pb geochronology of syn-tectonic calcite, and 9) K-Ar dating of syn-kinematic clay minerals.

Our results show that the hypothetical Paleozoic (?) crystalline basement exposed on the Zannone Island is, instead, represented by siliciclastic rocks of very low metamorphic grade. This is testified by the quartzarenites nature of the rocks, the presence of chloritoid and by the observed incipient foliation marked by fine-grained white micas and disposed parallel to the bedding. The contact between such siliciclastic rocks and the overlapping Triassic Dolostone is represented by a low-angle thrust cut by sets of high-angle normal faults with associated calcite mineralizations. K-Ar dating on clay minerals in fault gouge reveals that at least one event of authigenesis (i.e. fluid-assisted tectonic activity) occurred in Zannone Island <22 Ma ago. U-Pb dating on sin-tectonic calcite mineralizations allowed to constrain the compressional deformation and subsequent normal faulting in the study area at around 7 Ma. This result is consistent with the 1) described emplacement of imbricate thrust sheets onshore close to Zannone Island and 2) syn-tectonic sediments-filling basins observed by seismic reflection studies. Microthermometry on fluid inclusions and stable isotopes analyses on syn-tectonic mineralizations highlighted the involvement of two different fluids during tectonic processes. One characterized by low salinity (as NaCl equivalent; i.e. meteoric-derived fluids) and one by high salinity (as NaCl equivalent; i.e. deep crustal-derived fluids). Microthermometry on fluid inclusions allowed to constrain a wide range of P-T entrapment conditions. For this reason, we highlighted a transition from lithostatic toward hydrostatic pressure during precipitation of syn-tectonic mineralizations.

How to cite: Curzi, M., Billi, A., Carminati, E., Albert, R., Aldega, L., Bernasconi, S., Boschi, C., Caracausi, A., Cardello, L., Conti, A., Drivenes, K., Franchini, S., Gerdes, A., Rizzo, A. L., Rossetti, F., Smeraglia, L., Sørensen, B. E., Van der Lelij, R., Vignaroli, G., and Viola, G.: Deciphering orogenic and post-orogenic fluid-assisted deformations by coupling structural, mineralogical, geochemical, and geochronological investigation methods. An example from Zannone Island, Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-970, https://doi.org/10.5194/egusphere-egu2020-970, 2020.

Active faults are characterized by creation/destruction of secondary (tectonic) permeability in response to a continuous interplay between deformation and fluid pressure fluctuations during the seismic cycle. The study of the paleofluid circulation in fault rocks can thus provide insights into the hydraulic and mechanical behavior of the seismogenic crust.

This work integrates data from field geology with geochemical and geochronological constraints to understand the spatio-temporal evolution of the paleofluid circulation in the Mount Morrone Fault System (MMFS), a ~25 km-long tectonic structure activated during the extensional Quaternary phase of the central Apennines (Italy). The MMFS cuts through a Mesozoic-Cenozoic multilayer carbonate succession for a cumulative stratigraphic offset of about 2 km. Fluvio-lacustrine and slope deposits (Middle-Late Pleistocene) occur at its hanging wall and are variably involved by faulting. The MMFS is currently classified as a silent seismic fault, with an estimated Mw= 6.5-7.0 potential magnitude and recurrence time at 2.4 ka for an expected earthquake.

The structural survey focused on the western strand of the MMFS cutting through a succession of Sinemurian dolomitized limestones. A composite network of NW-SE-striking, SW-dipping fault surfaces defines the structural architecture of the MMFS in the study outcrops, with high angle (dip > 55°) faults that systematically cut and displace medium-to-low angle (dip in the order of 30°-50°) faults. Both fault systems are characterized by dominant dip-slip movement and normal kinematics. Lenses of cm-thick cataclasites often occur along the slip surfaces. Cataclasites are made by sub-angular to sub-rounded carbonate clasts (up to 1 cm-wide) dispersed in a very fine-grained matrix. Layers of cm-thick carbonate concretions occur associated with the cataclasites, testifying for pulses of fluid discharge along the fault surface during the tectonic activity of the MMFS. Microstructural investigations document that: (i) carbonate concretions show an internal texture of fibrous vein having fiber growth direction roughly perpendicular to the vein wall, and (ii) the basal portions of the carbonate concretions are fractured and incorporated within the underlying cataclasites through the deposition of a new calcite cement. The geochemical (δ¹³C and δ¹⁸O stable isotope) analyses on selected samples attest for a progressive chemical shift of the mineralizing fluid from marine (in host rock and in cataclasites) to meteoric waters (in carbonate concretions). The U-Th dating of carbonate concretions and calcite slickenfibers constrains the fault-controlled fluid circulation to the Middle Pleistocene, with ages spanning from 270 to 180 ka. Significantly, the dating of carbonate concretions documents a 12-kyr cyclicity of the fluid infiltration in the fault zone.

The development of the secondary permeability in the MMFS thus corresponds to a combination of faulting and tensile fracturing, in response to a cyclic increasing of the shear stress and the pore pressure during the seismic cycle. The polyphasic deformation system of the MMFS constitutes a record of fault activation and reactivation episodes that could contribute to define the recurrence model of seismic events on regional-scale faults.

How to cite: Vignaroli, G., Argante, V., Rossetti, F., Petracchini, L., Soligo, M., Brilli, M., Yu, T.-L., and Shen, C.-C.: Cyclicity of paleofluid infiltration in the active Mount Morrone Fault System (central Apennines, Italy) constrained by carbonate concretions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2866, https://doi.org/10.5194/egusphere-egu2020-2866, 2020.

Vein-hosted fluid inclusions may represent remnants of subsurface paleo-fluids and therefore provide a valuable record of fracture-controlled fluid flow. Isotope data (δ²H and δ¹⁸O) of fluid inclusions are particularly useful for studying the provenance and type of paleo-fluids circulating in the subsurface. Although isotopic analysis of sub-microliter amounts of fluid inclusion water is not straightforward, major steps forward have been made over the past decade through the development of continuous-flow set-ups. These techniques make use of mechanical crushing at a relatively low-temperature (110˚C) and allow for on-line analysis of both δ²H and δ¹⁸O ratios of bulk fluid inclusion water. However, continuous-flow techniques have mostly been used in speleothem research, and have not yet found a widespread application on vein systems for hydrogeological reconstructions.

We used isotope data of fluid inclusions hosted in calcite vein cements to reconstruct regional fluid migration pathways in the Albanian foreland fold-and-thrust system. Tectonic forces during thrust emplacement typically instigate distinct phases of fracturing accompanied by complex fluid flow patterns. The studied calcite veins developed in a sequence of naturally fractured Cretaceous to Eocene carbonate rocks as a result of several fracturing events from the early stages of burial onward. Fluid inclusion isotope data of the veins reveal that fluids circulating in the carbonates were derived from an underlying reservoir, which consisted of a mixture of meteoric water and evolved marine fluids, probably derived from deep-seated evaporites. The meteoric fluids infiltrated in the hinterland before being driven outward into the foreland basin. The fluid inclusion isotope data furthermore show that meteoric water becomes increasingly dominant in the system through time as migration pathways shortened and marine formation fluids were progressively flushed out.

The diagenetic stability of fluid inclusions is of key interest in the study of their isotope ratios. Recrystallization, secondary fluid infiltration and isotope exchange processes could potentially drive alterations of fluid inclusion isotope signatures after entrapment. In this case, however, isotope signatures of fluid inclusions seem to have remained largely unaltered, despite the Cretaceous to Tertiary age of the vein system. Oxygen isotope exchange processes between the fluid inclusion water and host mineral could have been inhibited at the relatively low temperatures of vein formation (i.e. <80˚C). Although more research into the diagenetic stability of fluid inclusion isotope ratios is required, the fluid inclusion isotope record has potential as a powerful tool for fluid provenancing in subsurface fluid flow systems.

How to cite: de Graaf, S., Nooitgedacht, C., Vonhof, H., van der Lubbe, J., and Reijmer, J.: Isotope analysis of vein-hosted fluid inclusions: A case study on fracture-controlled fluid flow in the Albanian foreland fold-and-thrust belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20149, https://doi.org/10.5194/egusphere-egu2020-20149, 2020.

Micro-cracks in fault damage zones can heal through diffusive mass transfer driven by differences in chemical potential, with rates controlled by temperature and pressure. The diffusion of pore fluid pressure in fault damage zones accelerates mass diffusion and assists healing processes. In this work, we use fluid flow model coupled with heat transfer and crack healing to investigate, through different scenarios, the role of subsurface warm fluid migration, along damage zones, in enhancing healing and re-shaping the fault permeability structure. Our results show that if the flow communication exists between the bed and only one side of the damage zone and not the other side, it leads to an asymmetric permeability structure caused by healing in the side circulated by fluids (ex: Rapolano geothermal area, Italy). Another scenario is when the damage zone adjacent to the fault core is not the interval with the highest permeability, as conventionally expected, which is the case of the Alpine Fault, New Zealand. As shown by our simulations, this can be due to healing by diffusive mass transfer, favored by the localized high geothermal gradients and the upward fluid migration through the fault relay structure.

How to cite: Yehya, A. and Rice, J. R.: Influence of fluid-assisted micro-crack healing on fault permeability structure, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22180, https://doi.org/10.5194/egusphere-egu2020-22180, 2020.

Detailed fluid inclusion analyses of fracture cements in tightly cemented hydrocarbon-bearing sandstones and shales reveal that natural fractures tend to form under conditions approaching maximum burial, coinciding with hydrocarbon generation, and during incipient exhumation. Fluid inclusion analyses also reveal that these fractures form under abnormal (above-hydrostatic) pore fluid pressures. While compaction disequilibrium can account for elevated pore fluid pressures that promote fracture growth during early prograde burial, hydrocarbon maturation is likely the primary driver for fracture growth under peak burial conditions. Tectonic processes and thermal stresses provide secondary drivers. Thermal contraction with exhumation and cooling of the rock mass can promote fracture growth depending on the PVT properties of the fluid phase. The possible contribution of hydrocarbon generation after peak burial as a driver for fracture growth during incipient exhumation is discussed.

How to cite: Eichhubl, P.: Fracture growth during exhumation in low-permeability rock formations—the role of fluid PVT properties, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11638, https://doi.org/10.5194/egusphere-egu2020-11638, 2020.

Fault and fracture networks can govern fluid flow patterns in the subsurface and predicting fluid flow on a regional scale is of interest in a variety of fields like groundwater management, mining engineering, energy, and mineral resources. Especially the pore fluid pressure can have a strong impact on the strength of fault zones and might be one of the drivers for fault reactivation. Reliable simulations of the transient changes in fluid pressure need to account for the generic architecture of fault zones that comprises strong permeability contrast between the fault core and damage zone.

Particularly, the distribution and connectivity of large-scale fault zones can have a strong impact on the flow field. Yet, modelling numerically such features in their full complexity remains challenging. Often faults zones are conceptualized as forming exclusively either barriers or conduits to fluid flow. However, a generic architecture of fault zones often comprises a discrete fault core surrounded by a diffuse damage zone and conceptualizing large scale discontinuities simply as a barrier or conduit is unlikely to capture the regional scale fluid flow dynamics. It is known that if the fault zone is hosted in low-permeability strata, such as clays or crystalline rocks, a transversal flow barrier can form along the fault core whereas the fracture-rich fault damage zone represents a longitudinal conduit. In more permeable host-rocks (i.e. sandstones or carbonates) the reverse situation can occur, and the permeability distributions in the damage zones can be governed by the abundance of low-permeability deformation features. A reliable numerical model needs to account for the difference and strong contrasts in fluid flow properties of the core and the damage zone, both transversally and longitudinally, in order to make prediction about the regional fluid flow pattern.

Here, we present a numerical method that accounts for the generic fault zone architecture as lower dimensional interfaces in conforming meshes during fluid flow simulations in fault networks. With this method we aim to decipher the impact of fault zone architecture on subsurface flow pattern and fluid pressure evolution in fractures and faulted porous media. The method is implemented in a finite element framework for Multiphysics simulations. We demonstrate the impact of considering the more generic geological structure of individual faults on the flow field by conceptualizing discontinuities either as barriers, conduits or as a conduit-barrier system and show were these conceptualizations are applicable in natural systems. We further show that a reliable regional scale fluid flow simulation in faulted porous media needs to account for the generic fault zone architecture. The approach is finally used to evaluate the fluid flow response of statistically parameterised faulted media, in order to investigate the impact and sensitivity of each variable parameter.

How to cite: Kelka, U., Poulet, T., and Peeters, L.: Permeability contrasts of fault zones - from conceptual model to numerical simulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6520, https://doi.org/10.5194/egusphere-egu2020-6520, 2020.

In sedimentary basins highly overpressured formations and zones are observed worldwide. The high overpressures have been generated over millions of years due to sedimentation amount and rate, compaction, lateral fluid flow, diagenesis and other processes. The lateral fluid flow is often controlled by the fault pattern and sealing properties of the faults in the area, thus defining what is often termed pressure compartments. When high overpressures builds-up over time in such compartment, eventually natural hydraulic faulting and fracturing will start to develop to cease and relief the overpressure.

In this work we have aimed to simulate fracture generation, how they in an upscaled approach evolve and progress upwards, and how this will influence the water fluid flow and the pore pressure distribution. We use an in-house software (PressureAhead) to simulate three-dimensional water fluid pressure generation and dissipation over millions of years. Interpreted seismic horizons for the whole stratigraphy are back-stripped (decompaction) in order to provide the basin burial history as input to the forward simulator. Uplift and erosion events are included. For each timestep, the effect of pressure generation and dissipation is calculated. For the fault and failure development, the combined Griffith-Coulomb failure criteria are implemented to calculate when failure occurs, secondly, when the fracture has been formed and the cohesion is lost, the frictional sliding criteria is used. The fractures are in this approach working as a pressure valve, that will stay open as long as the pressure support is large enough. Compared to previous approach, the failure criteria is now evaluated for the whole stratigraphic column in 3D Using this approach, the effect of natural fracturing taking place in different parts of the basin at different geological events can be modelled.

The new simulation approach will be presented for a dataset from the deeper part of the Viking Graben, North Sea offshore Norway. The study area covers an NNE-SSW trending graben defined by large faults. Seventeen seismic horizons (resolution 50x50 m) from Middle Jurassic to seafloor have been used to set up the model. The modelling is carried out over the 150 My, with time steps of 250 000 years. Examples of varying key input parameters will be shown. Strength and weakness with such an upscaled modelling framework will be discussed.

How to cite: Lothe, A. E., Grøver, A., and Roli, O.-A.: Upscaled modelling of natural fracturing and leakage due to high overpressures , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22014, https://doi.org/10.5194/egusphere-egu2020-22014, 2020.

It is important to understand the effects of fluid over-pressure in rocks because gradients in over-pressure can lead to failure of rocks and expulsion of fluids. Examples are hydro-fracturing in engineering as well as fluid generation during hydrocarbon maturation, metamorphic reactions or over-pressure below seals in sedimentary basins. In order to have an understanding of the complexity of effective stress fields, fracture, failure and fluid drainage the process was studied with a dynamic hydro-mechanical numerical model. The evolution of fluid pressure build up, fracturing and the dynamic interaction between solid and fluid is modeled. Three scenarios are studied: fluid pressure build up in a sedimentary basin, in a confined zone and in a horizontal layer that is offset by a fault. Results indicate that the geometry of the fluid-overpressure zone has a first order control on the patterns including porosity evolution and fracturing. If the over-pressure develops below a seal in a sedimentary basin, the effective differential and mean stress approach zero and the horizontal and vertical effective stresses flip in orientation leading to horizontal hydro-factures or breccia zones. If the over-pressure zone is confined vertically as well, the standard effective stress model develops with the effective mean stress decreasing while the differential stress remains mainly constant. This leads to semi-vertically aligned extensional and conjugate shear failure at much lower over-pressures than in the sedimentary basin. A perfectly aligned horizontal layer that increases in fluid pressure internally leads to a horizontal hydro-fracture within the layer. A faulted layer develops complex multi-directional failure with the fault itself being a location of early fracturing followed by brecciation of the layer itself. All simulations undergo a phase transition in porosity evolution with an initially random porosity reducing its symmetry and forming a static porosity wave with an internal dilation zone and the development of dynamic porosity channels within this zone that drain the over-pressure. Our results show that patterns of fractures, hence fluid release, that form due to high fluid overpressures can only be successfully predicted if the geometry of the geological system is known, including the fluid overpressure source and the position of seals and faults that offset source layers and seals.

How to cite: Koehn, D., Piazolo, S., Sachau, T., and Toussaint, R.: Porosity channeling and fracturing in fluid over-pressure zones , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8644, https://doi.org/10.5194/egusphere-egu2020-8644, 2020.

The mechanics of fracture propagation and interaction influence the growth and permeability of developing fracture networks. A set of initial flaws grows quasi-statically in response to a remote tensile stress. A finite element, stress intensity factor-based approach grows these flaws into non-planar three-dimensional discrete fracture networks (GDFNs). Their extension and growth angle is a function of local stress intensity factors along a fracture tip. Stress concentration increase when proximal fractures are aligned, and decreases when they are sub-coplanar. These interactions can result in the reactivation of fractures that were initially inactive, and the arrest of fractures that become entrapped by proximal growing fractures. Interaction can cause growth away from an intersection front between two fractures, resulting in evolving fracture patterns that become non-uniform and non-planar, forming dense networks. These GDFNs provide representations of subsurface networks that numerically model the physical process of concurrent fracture growth. Permeability tensors of the geomechanical 3D networks are computed, assuming Darcy flow. Growth influences apertures, and in turn, the hydraulic properties of the network. GDFNs provide a promising way to model subsurface fracture networks, and their related hydro-mechanical processes, where fracture mechanics is the primary influence on the geometric and hydraulic properties of the networks.

How to cite: Paluszny, A., Thomas, R. N., and Zimmerman, R. W.: Permeability tensors of three-dimensional numerically grown geomechanical discrete fracture networks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5538, https://doi.org/10.5194/egusphere-egu2020-5538, 2020.

For understanding the formation of mountain belts it is necessary to gain quantitative insights on fault and fracture mechanics on multiple scales. In particular, for addressing the role of fluids on larger processes, it is inevitable to constrain fault and fracture geometries at depth, as well as gain insights on how fluids influence fault mechanics. At least partly, the future of such analyses lies in exploiting large data sets, as well as in multi- and interdisciplinary research.

In this talk I will present results from variety of geological settings, including dilatant faults at Mid-Ocean Ridges, the Oman Mountains, the Khao Kwang fold-trust belt in Thailand, and the European Alps. I will show how multi-scale studies and the use of large data sets helps constraining fluid migration in mountain belts, fault geometries, as well as possible feedbacks between fluid flow and strain localization. Results are then applied to discuss the role of mechanical stratigraphy on structural style in foreland fold-thrust belts.

How to cite: von Hagke, C.: From Faults and Fluids to Mountain Belt Dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4688, https://doi.org/10.5194/egusphere-egu2020-4688, 2020.

Naturally fractured reservoirs represent one of the most challenging resource in the oil and gas industry. The understanding based on centimeter scale observations is upscaled and modeled at 100-meter scale.

In this paper, we will illustrate with case study examples of conceptual fracture model elaborated using static and dynamic data, the disconnect between the scale of observation and the scale of modelling. We will also discuss the potential disconnect between the detail of fundamental, but necessary, research work in universities against the coarse resolution of the models built in the oil industry, and how we can benefit of the differences in scales and approaches.

The appraisal and development of fractured reservoirs offer challenges due to the variations in reservoir quality and natural fracture distribution. Typically, the presence of open, connected fractures is one of the key elements to achieve a successful development. Fracture modelling studies are carried out routinely to support both appraisal and development strategies of these fractured reservoirs.

Overall fracture modelling workflow consists first of a fracture characterization phase concentrating on the understanding of the deformation history and the evaluation of the nature, type and distribution of the fractures; secondly of a fracture modelling part where fracture properties for the dynamic simulation are generated and calibrated against dynamic data. The pillar of the studies is the creation of 3D conceptual fracture diagrams/concepts which summarize both the understanding and the uncertainty of the fracture network of interest. These conceptual diagrams rely on detailed observations at the scale of the wellbore using core and borehole image data which are on contrasting scale compare to the 10’s of meters to 100’s of meter scale of the grid cells of the dynamic models used for the production history match and forecast. These contrasting scales will be the thread of the presentation.

How to cite: Richard, P. and Bazalgette, L.: Scale discrepancy paradox between observation and modelling in fractured reservoir models in oil and gas industry., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20161, https://doi.org/10.5194/egusphere-egu2020-20161, 2020.

It is of great importance in many fields to be able to forecast the likely propagation paths of fluid-driven factures. These include mineral veins, human-made hydraulic fractures, and dikes/inclined sheets. The physical principles that control the propagation of all fluid-driven fractures are the same. Here the focus is on dikes and inclined sheets where the selected path determines whether, where, and when a particular dike/sheet reaches the surface to erupt. Here we provide analytical and numerical models on dike/sheet paths in crustal segments (including volcanoes) that include layers of various types (lava flows, pyroclastic flows, tuff layers, soil layers, etc) as well as mechanically weak contacts and faults. The modelling results are then compared with, and tested on, actual data of two types. (a) Seismic data on the paths of dikes/sheets as well as human-made hydraulic fractures, and (b) field data on the actual propagation paths of dikes/sheets in layered and faulted rocks

The numerical results show that, particularly in stratovolcanoes, the paths are likely to be complex with common deflections along layer contacts, in agreement with field observations.  Also, some dikes/sheets may use existing faults as parts of their paths, primarily steeply dipping and recently active normal faults. The propagation path is thus not entirely in pure mode I but rather partly in a mixed mode. The energy required to propagate the dike/sheet is mainly the surface energy needed to rupture the rock, to form two new surfaces and move them apart as the fracture propagates. The energy available to drive the fracture is the stored elastic energy in the hosting crustal segment.

From its point of initiation in the magma-chamber roof, a dike/sheet can, theoretically, select any one of an infinite number of paths to follow to its point of arrest or eruption. It is shown that the eventual path selected is the one of least action, that is, the path along which the time integral of the difference between the kinetic and potential energies is an extremum (normally a minimum) relative to all other possible paths with the same endpoints. If the kinetic energy is omitted, and there are no constraints, then least action becomes the minimum potential energy, which was postulated as a basis for understanding dike propagation by Gudmundsson (1986). Here it is shown how this theoretical framework can help us make reliable forecasts of dike/sheet paths and associated volcanic eruptions.

Gudmundsson, A., 1986. Formation of dykes, feeder-dykes, and the intrusion of dykes from magma chambers. Bulletin of Volcanology, 47, 537-550.

Gudmundsson, A., 2020. Volcanotectonics: Understanding the Structure, Deformation, and Dynamics of Volcanoes. Cambridge University Press, Cambridge.

Drymoni, K., Browning, J. Gudmundsson, A., 2020. Dyke-arrest scenarios in extensional regimes: insights from field observations and numerical models, Santorini, Greece. Journal of Volcanology and Geothermal Research (in press).

How to cite: Gudmundsson, A., Drymoni, K., Bazargan, M., and Adeoye-Akinde, K.: Forecasting the propagation paths of fluid-driven fractures, particularly dikes and inclined sheets , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20010, https://doi.org/10.5194/egusphere-egu2020-20010, 2020.

Fracture cementation is an important control on the recovery of prefailure levels of permeability and strength in faults and reservoir rock. The timescales of this process, however, are almost entirely unknown from direct analysis of the rock record. We report U‐Th dating results that quantify rates of fracture cementation in syntectonic calcite veins from the Loma Blanca fault, New Mexico, USA. Measured cementation rates vary from ~0.05 to 0.80 mm/ka and exhibit a power function correlation with minimum fracture apertures. We argue that this correlation is the result of crystal growth in a transport‐limited system, where cementation rates were proportional to rates of fluid flow in individual fractures. We argue that such transport‐limited growth necessarily leads to a heterogeneous distribution of cementation rates as fluids migrate through fracture networks of variable and changing aperture. For this reason, individual fractures are not expected to seal at monotonic rates through time but could instead experience order‐of‐magnitude increases or decreases in sealing rate depending on their geometric properties (e.g., aperture, length/width, and orientation) and position within a continually evolving fracture network. We further argue that such transport‐limited, flux‐dependent cementation necessarily leads to a heterogeneous distribution of permeability and strength recovery as fluids migrate through fault‐zone fracture networks. These heterogeneities may influence rupture propagations pathways and the continual development of fault‐zone architecture/complexity.

How to cite: Williams, R., Mozley, P., Sharp, W., and Goodwin, L.: U-Th Dating of Syntectonic Calcite Veins Reveals the Dynamic Nature of Fracture Cementation and Healing in Faults, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12447, https://doi.org/10.5194/egusphere-egu2020-12447, 2020.

Fractures and faults act as important permeable pathways in the subsurface and are of great significance to the petroleum industry and for future Carbon Capture and Storage. Fractures allow fluid-flow through impermeable units such as mudrocks and can affect how these lithologies act as top seals, source rocks and/or unconventional reservoirs. Natural fractures within mudrocks can strongly influence top seal integrity, primary migration and the performance of unconventional (e.g. shale gas) reservoirs. This project studies the exhumed, early-mature, Jurassic mudrock succession of the Cleveland Basin, NE England, combining structural geology with isotope geochemistry and geochronology. The primary objective is to provide an absolute chronology of faulting and fracturing through novel U-Pb geochronology of fracture-fill calcite. The abundance of well-exposed, natural fractures with different orientations and failure modes provides an opportunity to investigate the properties of these fractures, and provide a basin-wide temporal and spatial framework of evolving deformation. The second objective is to use trace element, stable isotope, and clumped isotope analyses, to constrain fluid composition and temperature. In combination, these objectives will provide an integrated understanding of fracturing, faulting and fluid migration during burial and exhumation of a sedimentary basin.

Current fracture-fill dates from U-Pb geochronology provide intriguing insights into the history of the Cleveland Basin. We have identified and dated three phases of deformation and associated fluid-flow that have contrasting kinematics and fluid-flow regimes. The E-W trending Flamborough Head Fault Zone (FHFZ) bounds the basin to the south, and calcite preserved in one of the major extensional faults provides ages of 64-56 Ma. Calcite from N-S to NNW-SSE trending normal faults and associated fractures in the north of the Cleveland Basin provide ages of 44-25 Ma, revealing a previously unknown phase of Cenozoic faulting, which we speculatively relate to salt-related deformation. Structural and petrographic information suggest that the E-W and N-S trending faults have contrasting fracture-fluid-flow systems. Large (up to 30 cm), chalk hosted, vuggy calcite cements with geopetal sediment-fills in the E-W fault zone suggest it acted as an open fluid conduit with voluminous fluid-flow, linking the shallow sub-surface with deeper levels of the stratigraphy. In contrast, typically thin (<5 mm) vein fills with varying crack-seal-slip type textures in the N-S mudstone-hosted fractures of the Cleveland Basin provide evidence of episodic slip of variable displacement (44-40 Ma); these fracture openings may partly be controlled by pore fluid pressures and pre-date fault movement along the regional Peak Fault and smaller scales N-S faults (40-25 Ma) which are characterised by damage zone calcite mineralisation and extensional jog structures. Initial stable isotopic results are giving indications of fluid temperatures and sourcing which will be built on further by clumped isotope and fluid inclusions work.

How to cite: Lee, J., Roberts, N., Holdworth, R., Aplin, A., Haslam, R., and John, C.: Dating faults, fractures and fluids with U-Pb calcite geochronology: Application to contrasting fracture and fluid-flow modes of the Cleveland Basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5387, https://doi.org/10.5194/egusphere-egu2020-5387, 2020.

The quantification of tectonic forces or, alternatively, stresses represents a significant step towards the understanding of the natural processes governing plate tectonics and deformation at all scales. However, paleostress reconstructions based on the observation and measurement of natural fractures are traditionally limited to the determination of four out of the six parameters of the stress tensor. In the present study, we attempt to reconstruct full paleostress tensors by extending the methodologies advanced by previous authors. We selected Panasqueira Mine, Central Portugal, as natural laboratory, and focused on the measurement of sub-horizontal quartz veins, which are favorably exposed in three dimensions in the underground galleries of the mine. Inversion of the vein data allowed for quantifying the respective orientations of the stress axes and the shape ratio of the stress ellipsoid. In order to reconstruct an additional stress parameter, namely pressure, we extensively sampled vein material and combined fluid inclusion analyses on quartz samples with geothermometric analyses on sulphide minerals. Finally, we adjusted the radius of the obtained Mohr circle with the help of rupture laws, and obtained the six parameters of the paleostress tensor that prevailed during vein formation. Our results suggests a NW-SE reverse stress regime with a shape ratio equal to ~0.6, lithostatic pore pressures of ~300 MPa and differential stress lower than ~20 MPa.

How to cite: Pascal, C., Jaques, L., and Yamaji, A.: Full paleostress tensor determination: case of the Panasqueira Mine, Portugal., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2192, https://doi.org/10.5194/egusphere-egu2020-2192, 2020.

The fact that inherited fault systems show strong variability in their 3D shape provides good reasons to consider the strength of the Earth’s brittle crust as variably anisotropic. In this work we quantify this strength anisotropy as a function of fault system complexity by combining 3D boundary element model, frictional slip theory and fast iterative computation method. This method allows to analyze together a very large number of scenarios of stress and fault mechanical properties variations through space and time. Using both synthetic and real fault system geometries we analyze a very large number of numerical simulations (125,000) to define for the first time macroscopic rupture envelopes for fault systems, referred to as “fault slip envelopes”. Fault slip envelopes are defined using variable friction, cohesion and stress state, and their shape is directly related to the fault system 3D geometry and the friction coefficient on fault surfaces. The obtained fault slip envelopes shows that very complex fault geometry implies low and isotropic strength of the fault system compared to geometry having limited fault orientations relative to the remote stresses, providing strong strength anisotropy. This technique is applied to the realistic geological conditions of the Olkiluoto high-level nuclear waste repository (Finland). The model results suggests that Olkiluoto fault system has a better probability to slip under the present day Andersonian thrust stress regime, than for the strike-slip and normal stress regimes expected in the future due to the probable presence of an ice sheet. This new tool allows to quantify the anisotropy of strength and probability of slip of 3D real fault networks as a function of a wide range of possible geological conditions an mechanical properties. This significantly helps to define the most conservative fault slip hazard case or to account for potential uncertainties in the input data for slip. This technique therefore applies to earthquakes hazard studies, geological storage, geothermal resources along faults and fault leaks/seals in geological reservoirs.

How to cite: Soliva, R., Maerten, F., Maerten, L., and Mattila, J.: Fault slip envelope: A new parametric investigation tool for fault system strength and slip, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8688, https://doi.org/10.5194/egusphere-egu2020-8688, 2020.

The present study provides insights on the segmented nature of normal faults as a function of scale, and attempts to identify whether segmentation is scale invariant, scale dependent or hierarchical. This is a topic of critical importance for studies of fault development and in modelling exercises where one needs to extrapolate observations at one scale to other scales. 

Results are based on data observed in the Blue Lias in Somerset (UK), in Fumanya mine (Spain) and in a 3D seismic reflection survey in the Bonaparte Basin (Australia). Fault segmentation is investigated quantitatively based on previously established methodologies and we focus on neutral relay zones observed between fault segments along the strike of the normal faults.

We found that there are quantitative indications that the shape of the relay zones, the breaching of the relays and the degree of segmentation are all scale independent in Kilve and Fumanya. We propose that this is related to the low variability across scales in the geological parameters controlling segmentation, due to the relative homogeneity of the rock medium across the studied scales, the lack of influence of pre-existing faults or fractures, and the similar deformation histories for all studied faults. By contrast, faults show scale dependency in the Bonaparte Basin where large faults are under the influence of an oblique reactivation of pre-existing faults. Independently of the area, segmentation observed continuously through scale stresses the need to take into account resolution of observation in discussing fault development.

How to cite: Roche, V., Manzocchi, T., Camanni, G., Childs, C., and Papanikolaou, V.: Scale dependency of segmentation along the strike of normal faults., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18032, https://doi.org/10.5194/egusphere-egu2020-18032, 2020.

Fracture systems develop at different stages of progressive deformation and are often genetically associated with folding. The frontal Main Boundary thrust (MBT) sheet is folded in a fault bend fold (Ahmed et al., 2018) and is exposed in Siang window in far eastern Arunachal Himalayan fold-thrust belt (FTB). The Buxa dolomite of the Lesser Himalayan sequence forms part of the MBT sheet and records four different sets of fractures (Basa et al., 2019). We present results from Discrete Fracture Network (DFN) model from the Buxa dolomite. Integrating fold test, cross-cutting, offset and abutting relationships, we have established that the low-angle fracture set (0°-20°) formed as a result of early layer parallel shortening.  These low-angle and the two sets of medium angle fractures (20°-60°) formed prior to the fault-bend folding. The late stage, high-angle fractures (60°-90°) developed synchronous to the fault-bend fold (Basa et al., 2019). We model the fractures formed before and during the folding event using 3D MOVE’s Fracture Modeling module to evaluate how the properties of secondary porosity and permeability, induced by fracture sets, fracture area/unit volume (P32) and overall connectivity are affected by the folding event. The input parameters of fracture orientation, intensity, length and aperture were measured from the field. For the aspect ratio, theoretical value of 1:2 (Olding, 1997; Olson, 2003) was considered.

Results from DFN analysis indicate that the average porosity increases from pre-folding (model-1) (~0.0028) to syn- to post-folding (model-2) (~0.0071). The permeability also increases from ~231 Darcy in model-1 to ~3988 Darcy in model-2. There is also a significant rise in P32 (~2.8m²/m³ to ~4.3m²/m³⁾ value from model-1 to model-2. The late high-angle fracture set led to increase in overall connectivity, including porosity, permeability and fracture intensity. This is also corroborated from the field results that reveal high-angle fractures are more conducive to vein formation (~41%) compared to the lower angle fracture-sets (~15 %).

How to cite: Ahmed, F. and Bhattacharyya, K.: Characterization of fracture networking and connectivity: Insights from Discrete Fracture Network Model and Multi-Scale analysis from the Buxa Dolomite of the Main Boundary thrust sheet, Siang widow, Arunachal Himalayan Fold thrust belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-924, https://doi.org/10.5194/egusphere-egu2020-924, 2020.

Fractures in rocks are sensitive cursers that may enhance porosity and permeability. This is particularly true in carbonates because background fractures might be ubiquitous after embrittlement at early burial (Lavenu & Lamarche, 2018). Barren fractures at depth are susceptible to chemical reactions with underground fluids and cementation that might totally or partially reduce porosity and permeability (Laubach et al., 2019; Aubert et al., 2019). Hence, early background fractures with long lasting tectonic history and structural diagenesis, in addition to fractures neo-formed at any time during burial, tectonic inversion and folding join the game of matrix/fracture permeability and porosity modification. To predict the fractures contribution to flow in Naturally Fractured Reservoirs, it is fundamental to know the fracture sequence and geometry resulting from the geological history in folded carbonates, from the host-rock embrittlement to the present-day situation. At any step, we intent quantifying the fracture geometry and estimating their contribution to the host reservoir properties.

The study is performed in Upper Jurassic to Lower Cretaceous carbonates (Oxfordian, Tithonian, Berriasian) formed in the South-Provençal Basin. From deposition to present-day, the platform carbonates underwent alternating subsidence, uplift, erosion and folding. We sampled a scan-line along a horizontal path across both flanks of the Mirabeau Anticline (SE France). We measured all tectonic and stratigraphic features crossed by the line, checked their nature and position. We deciphered their chronological relationships with respect to each other and to the bed tilting. We compiled all cross-cutting relationships into a coherent sequence of deformation of pre-, syn- and post-fold structures and correlated it to burial, uplift and folding of the host rock. At each brittle stage, the fracture pattern was characterized in terms of architecture, mechanical stratigraphy and reservoir properties in order to draw a time-path in a matrix versus fracture permeability and porosity table (Nelson Reservoir types) during 150My. After embrittlement, the host-rocks bear fractures, pressure-solution, faulting, folding and erosion. If it was a reservoir, its Nelson type would have evolved from IV to III during the burial and initial brittle deformation. The tectonic inversion and onset of multiple-scale brittle structures would have increased and decreased the fracture and matrix contribution respectively and the reservoir evolved to types II and I. During the 150My history, fracture porosity and permeability depends on their geometry (veins versus tension gashes) and cementation. This results in several switches from type II to I as a function of the fracture timing, geometry, connectivity and diagenesis.

Aubert I. et al. (2019). Imbricated structure and hydraulic path induced by strike-slip reactivation of a normal fault in carbonates. Fifth International Conference on Fault and Top Seals, 8-12 September 2019, Palermo, Italy.

Bestani L.et al. (2016) Reconstruction of the Provence Chain evolution, southeastern France., Tectonics Vol: 35, p.1506–1525

Laubach, S. E. et al. (2019) The role of chemistry in fracture pattern development and opportunities to advance interpretations of geological materials. Reviews Geophysics, 57.

Lavenu A.P.C., Lamarche J. (2018) What controls diffuse fractures in platform carbonates? Insights from Provence (France) and Apulia (Italy), JSG 108, p. 94-107

How to cite: Lamarche, J., Espurt, N., Kaci, T., Lionel, M., and P. Pascal, R.: Impact of multiphase fracture sequence in folded carbonates on the evolution of naturally fractured reservoir types. An outcrop case study from Mirabeau Anticline (SE France), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2195, https://doi.org/10.5194/egusphere-egu2020-2195, 2020.

Deformation bands and structurally-related diagenetic heterogeneities, here named Structural Diagenetic Heterogeneities (SDH), have been recognized to affect subsurface fluid flow on a range of scales and potentially promoting reservoir compartmentalization, altering flow paths, influencing flow buffering, and sealing during production. Their impact on reservoir hydraulic properties depends on many factors, such as their permeability contrast with respect to the undeformed reservoir rock, their anisotropy, thickness, geometry as well as their physical connectivity and arrangement in the subsurface. Deformation bands offsets (from a few mm to 20-40 mm) and diagenetic heterogeneities (carbonate nodules) dimensions (from 0.2 to 15 m in length; from 0.1 to 1.0 m in thickness) make them SDH below seismic resolution.

We used Ground Penetrating Radar (GPR) for detection and analysis of the assemblage “deformation bands - carbonate nodules”, in high-porosity arkose sandstone of the Northern Apennines (Italy). Petrophysical (air-permeability) and mechanical (uniaxial compressive strength) properties of host rock, deformation bands, and calcite-cement nodules were evaluated along a 30-meters thick stratigraphic log to characterize the permeability and strength variations of those features. 2D GPR surveys allowed the description of the SDH spatial organization, geometry, and continuity in the subsurface. The assemblage “deformation bands – nodules” decreases porosity and permeability and produces a strengthening effect of the rock volume, inducing a strong mechanical and petrophysical heterogeneity to the pristine rock. Different textural, petrophysical, and geomechanical properties of deformation bands, nodules, and host rock result in different GPR response (dielectric permittivity; instantaneous attributes). We show that GPR can be useful to characterize variations in petrophysical and geomechanical properties other than characterize the geometry and spatial distribution of flow baffles and small-scale flow barriers in the subsurface such as deformation bands and cement-nodules. GPR showed its worth as a high-resolution and non-invasive tool to extend outcrop information (petrophysical and geomechanical data) to 3D subsurface volumes in a way to reconstruct realistic and detailed outcrop analogues. Such potential could be critical in assisting and improving the characterization of SDH networks in the study of faulted aquifers and reservoirs in porous sandstones.

How to cite: Del Sole, L., Calafato, A., and Antonellini, M.: Combining Ground-Penetrating Radar profiles with geomechanical and petrophysical in situ measurements to characterize sub-seismic resolution structural and diagenetic heterogeneities in porous sandstones (Northern Apennines, Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3118, https://doi.org/10.5194/egusphere-egu2020-3118, 2020.

Based on cores, image logs and thin sections, five sets of fractures are developed in the study area, where faults are developed. Most of fractures are open without fillings, and some fractures are filled with calcite, quartz, bitumen, pyrite and mud. Fractures are mainly controlled by lithology, mechanical stratigraphy and faults. Based on mutual crosscutting relationships of fractures, mineral filling sequence of fracture fillings, fluid inclusion and carbon-oxygen isotope analysis of calcite fillings in fractures, and quartz spintronic resonance analysis of quartz fillings in fractures, in combination with thermal and burial history, the formation sequence and time of fractures were analyzed. The results show that fractures mainly formed over three period, that is, the late Triassic, Middle to Late Jurassic, and Late Cretaceous to Paleogene. Then，combined with the paleostress evolution and fracture characteristics of the study area, the formation mechanism of fractures was discussed.

How to cite: Lyu, W., Zeng, L., Chen, S., Tang, L., and Zhang, Y.: Controlling factors and formation mechanism of fractures in the tight-gas sandstones of the Upper Triassic Xujiahe Formation, western Sichuan Basin, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4123, https://doi.org/10.5194/egusphere-egu2020-4123, 2020.

Fracture network characterization is critical for many subsurface engineering problems in petroleum, mining, nuclear waste disposal and Enhanced Geothermal Systems (EGS). Due to limited exposure, direct measurement of fracture network properties at great depth is not possible and geophysical imaging techniques cannot resolve the fractures. Therefore, tomographic imaging techniques have been proposed and applied to reconstruct the structural discontinuities of rock mass. Stress-based tomography is a novel concept aiming at probabilistic imaging of the fracture network using the stress perturbations along deep boreholes. Currently, this approach has only been successfully tested on two-dimensional fracture networks. However, its great potential to unravel the heterogeneous structure of fractured rocks at great depth motivates further scientific effort. Here, we present the potential, open questions, current challenges and necessary future developments in order to apply this methodology to image three-dimensional multiscale structure of the rock mass in the field. Other tomographic approaches such as tracer and hydraulic tomography invert tracer breakthrough curves (BTCs) and pressure response in an observational well. We suggest a joint and comparative tomographic analysis in a Bayesian inversion framework to reconstruct Discrete Fracture Networks (DFN). This is expected to provide a new view of the strengths of each tomographic variant.

How to cite: Bayer, P., Afshari Moein, M. J., Somogyvári, M., Ringel, L., and Jalali, M.: Stress-based tomography: potential, open-questions and future developments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19401, https://doi.org/10.5194/egusphere-egu2020-19401, 2020.

Methane seepage on continental margins derived from shallow gas reserves and gas hydrate stores is significant globally, with initial observations most commonly derived from pockmark expressions on the seafloor. The processes driving fluid flow (liquids and gases) through upper (200m) marine sediments is not well understood. Pockmarks signify present or past seepage events, and are prominent across Vestnesa Ridge. Not all pockmarks are active (venting), suggesting that the mechanism behind fluid flow varies across the ridge.  The main structures observed in the seismic are gas chimneys, faults and fractures. Here we study the characteristics of the observed features through attribute analysis at three significant horizons (age estimates: <0.2Ma, ~0.2Ma and ~1.5Ma). We extract fault orientations through the generation of 3D fault attributes and analysis of fault detect volumes. Attribute extracts at horizons, using amplitude and edge detection methods, together with spectral decomposition and RGB blending, have revealed fine-scale (<10m) faults. High amplitude lineaments at multiple depths match fault trends and radial fracturing is observed around gas chimneys. Small faults propagate outwards from gas chimneys and feed into larger tectonically derived faults, suggesting horizontal inter-connectivity at specific depths.  Enhanced imaging of gas chimneys and small scale features, contribute to our understanding of how fluids migrate through the sediment column. We hypothesize that the connecting fractures, forming between main fault zones may suggest sediment overpressure and restricted flow through tectonically induced faults resulting in horizontal fluid transport.

How to cite: Cooke, F., Plaza-Faverola, A., Bünz, S., and Singhroha, S.: High-resolution 3D seismic investigation of fine scale faults and fractures along the Vestnesa Ridge, western Svalbard Margin , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19487, https://doi.org/10.5194/egusphere-egu2020-19487, 2020.

The present contribution focuses on carbonates fault cores exposed in central and southern Italy, which crosscut Mesozoic limestones and dolostones, pertain to 10’s of m- to 100’s of m-throw extensional fault zones, and include two main domains named as inner and outer fault cores, respectively. The inner fault cores are made up of main slip surfaces (MSS), matrix-supported cataclasites, and fault breccia. Cement-supported cataclasites, if present in the limestone-hosted fault cores, localize around the MSS. The outer fault cores mainly include grain-supported cataclasites, subsidiary slip surfaces, and lithons of fragmented host rocks. In order to assess the fluid flow properties of the carbonate fault cores, the results of microstructural, petrophysical, and ultrasonic studies are first presented, and then discussed in terms of pore type, geometry, textural anisotropy, and poro-perm relationships. Overall, the documented pore distribution is mainly function of both deformation micro-mechanisms and diagenetic processes, which took place in the carbonate fault cores during faulting and fault rock exhumation from depth. In the limestone-hosted fault cores, the experimental results show that the cross-fault fluid flow properties are affected by the the irregular geometry of the cement fronts. These fronts, which depart from the MSS, are due to calcite precipitation in vadose environments from meteoric-derived fault fluids. At depths of about 1 km, these fluids merge with the local freshwater aquifers, and cement the whole fault cores. Overall, these fault cores include a stiff pore networks, and are thought to behave like a granular medium. There, it is proposed that the cross-fault permeability can be computed by applying the Kozeny-Carmen correlation. For any given value of effective porosity, the value of permeability is therefore proportional to the average value of the pore throat, which characterize the aperture of capillary tubes with a geometrical tortuosity of ca. 2.5. On the contrary, the dolostone-hosted fault cores include a soft pore network made up of elongated pores, and are thought to behave like an elastic cracked medium. Accordingly, it is proposed that the cross-fault permeability can be computed by following percolation theory by considering the values of dynamic elastic moduli measured during ultrasonic tests at Pc=30 MPa, and almost isotropic fracture networks. Results of this work could be helpful during appraisal and development operations of hydrocarbon reservoirs, for freshwater aquifer protection, and activities of CO₂ storage in depleted carbonate reservoirs.

How to cite: Agosta, F.: Permeability models for carbonate fault cores, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3360, https://doi.org/10.5194/egusphere-egu2020-3360, 2020.

Quantification of the geometry, distribution, and dimension of fracture networks is key to fully understand the petrophysical properties of outcrop-to-reservoir scales rock volumes. On these regards, Discrete Fracture Network (DFN) modeling is a very useful tool to compute the values of fracture porosity and equivalent permeability of geo-cellular volumes populated with stochastic or deterministic fracture networks. Independently of their size and cell dimensions, the single geocelullar volumes are populated by inputting the following parameters for each fracture set: (i) length; (ii) aspect ratio; (iii) mechanical and hydraulic apertures; (iv) fracture intensity, and (v) attitude. A sensitivity analysis is always carried out in order to test the seeding procedure of the employed software, and to check the validity of the fracture aperture values employed as input data. The latter values, in fact, are the most critical to assess from outcrop and laboratory analyses. The present contribution focuses on the results of recent works performed on the fractured limestone rocks of the Apulian Platform, which are widely exposed along the Italian peninsula. Outcrops are first introduced in order to define the fracture stratigraphy and fault architecture of the Meso-Cenozoic limestone rocks. Then, the criteria behind the construction of DFN models are illustrated. Methods employed for the build of individual fracture units and single fault damage zone domains are illustrated. Finally, the computed values of fracture porosity and equivalent horizontal permeability obtained for multiple DFN models are presented. Discussion of the data focuses on the fluid accumulation and migration properties of the fractured limestone rocks by considering their amount of exhumation experienced during Plio-Quaternary times. Results of DFN modeling could be helpful to optimize the appraisal and development operations of hydrocarbon reservoirs, and minimize the pollution of freshwater aquifer. In fact, the Apulian carbonates host in the underground significant amounts of freshwater of the Mediterranean Region, and the largest oil and gas reserves of continental Europe. Furthermore, the results could shed new lights into the role exerted by faults and fractures on subsurface CO₂ storage in depleted carbonate reservoirs, a practice that envisioned to decrease the greenhouse gas concentration in the atmosphere in the next future.

How to cite: Agosta, F.: Fracture stratigraphy, fault architecture and DFN modeling of both diffuse and localized fracture networks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3370, https://doi.org/10.5194/egusphere-egu2020-3370, 2020.

Finding an adequate bridge between direct and continuum modeling approaches has been the fundamental issue of upscaling fluid flow in rock masses. Typically, numerical simulations of direct fluid flow (e.g. Stokes or Lattice-Boltzmann) in fractured or porous media serve as small-scale building blocks for larger-scale continuum flow simulations (e.g. Darcy). For fractured rock masses, the discrete-fracture-network (DFN) modeling approach is often used as an initial step to upscale flow properties by parameterizing the permeability of each fracture with its hydraulic aperture and solving steady-state flow equations within the fracture system. However, numerical simulations of Stokes flow in small fracture networks (FN) indicate that, depending on the orientation of the applied pressure gradient, fluid flow tends to localize at places where fractures intersect. This effect causes discrepancies between direct and equivalent continuum flow modeling approaches, which ought to be taken into account when modeling flow at the network scale.

In this study, we compare direct flow simulations of small fracture networks to their continuum representation obtained with several techniques in order to find an upscaling approach that takes these intersection effects into account. Direct flow simulations are conducted by solving the Stokes equations in 3D using our open-source finite-difference software LaMEM. Continuum flow simulations are realized with a newly developed parallel finite-element code, which solves fully anisotropic 3D Darcy flow with specific permeability tensors for each voxel. The direct flow simulations serve as benchmarks to optimize the continuum flow models by comparing resulting permeabilities. We tested two different schemes to generate the equivalent continuum representation: 

(1) Fully resolved isotropic permeability discretizations (fracture permeability is obtained from a refined cubic law) where voxel sizes are a fraction of the minimal hydraulic aperture of the FN or

(2) coarse anisotropic permeability discretizations (permeability tensors are rotated according to fracture orientation) with voxel sizes larger than the minimal hydraulic aperture of the FN.

We then assess different scenarios to incorporate the intersection effects by adding, averaging and/or multiplying the permeabilities of the intersecting fractures within intersection voxels. Preliminary results for scheme 1 suggest that a simple addition of both intersecting fracture permeabilities delivers the best fit to the results of the direct flow simulations, if the voxel size is about 68% of the minimal hydraulic aperture. Scheme 2 systematically underestimates the direct flow permeabilities by about 26%.

How to cite: Kottwitz, M. O., Popov, A. A., Abe, S., and Kaus, B. J. P.: Linking direct and continuum fluid flow models for fractured media: The intersection problem, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4902, https://doi.org/10.5194/egusphere-egu2020-4902, 2020.

Hydraulic stimulation is the process of initiating fractures in a target reservoir for subsurface energy resource management with applications in unconventional oil/gas and enhanced geothermal systems. The fracture characteristics (i.e., number, size and orientation with respect to the wellbore) determines the modified permeability field of the host rock and thus, numerical simulations of flow in fractured media are essential for estimating the anticipated change in reservoir productivity. However, numerical modeling of fluid flow in highly fractured media is challenging due to the explosive computational cost imposed by the explicit discretization of fractures at multiple length scales. A common strategy for mitigating this extreme cost is to crudely simplify the geometry of fracture network, thereby neglecting the important contributions made by all elements of the complex fracture system.

The proposed “Hierarchical Finite Element Method” (Hi-FEM; Weiss, Geophysics, 2017) reduces the comparatively insignificant dimensions of planar- and curvilinear-like features by translating them into integrated hydraulic conductivities, thus enabling cost-effective simulations with requisite solutions at material discontinuities without defining ad-hoc, heuristic, or empirically-estimated boundary conditions between fractures and the surrounding formation. By representing geometrical and geostatistical features of a given fracture network through the Hi-FEM computational framework, geometrically- and geomechanically-dependent fluid flow properly can now be modeled economically both within fractures as well as the surrounding medium, with a natural “physics-informed” coupling between the two.

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

How to cite: Chang, K. W., Beskardes, G., and Weiss, C.: Modeling fluid flow through complex fracture network in geological media by using hierarchical hydraulic properties , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5677, https://doi.org/10.5194/egusphere-egu2020-5677, 2020.

The topological and geometrical description of fault and fracture networks is an essential first step in any investigation of fractured or faulted media. The spatial arrangement, density, connectivity, and geometry of the discontinuities strongly impact the physical properties of the media such as resilience and permeability. Obtaining reliable metrics for characterizing fault and fracture networks is of interest for mining engineering, reservoir characterization, groundwater management, and studies on the regional fluid flow history. During large-scale studies, we mostly rely on two-dimensional lineaments obtained through structural mapping, outcrop analysis, or remote sensing. An efficient and widely applicable framework for discontinuity network characterization should therefore be based on the analysis of the frequently available two-dimensional data sets.

Here, we present an automated framework for efficient and robust characterization of the geometric and topologic parameters of discontinuity networks. The geometry of the lineaments is characterised based on orientation, length, and sinuosity. The underlying distribution of these parameters are determined, and representative probability density functions are reported. The connection between the geometric parameters is validated, e.g. correlation between orientation and length. The spatial arrangement is determined by classical line- and window-sampling, by assessing the fractal dimension, and via graph-based topology analysis.

In addition to the statistical analysis of lineament networks, we show how the graph data structure can be utilized for further characterization by linking it to raster data such as magnetic, gravimetric, or elevation. This procedure not only yields an additional means for lineament characterization but also allows users to assess dominant pathways based, for instance, on hydraulic gradients. We demonstrate the applicability of our algorithm on synthetic data sets and real-world case studies on mapped fault and fracture networks.

We finally show how our framework can also be utilized to design detailed numerical studies on the fluid flow properties of analysed networks by conditioning mesh refinement on the type and number of intersections. In addition, due to known scaling relationships our framework can help to determine appropriate parameters for the simulations. We provide examples of statistically parametrized fluid flow simulations in natural discontinuity networks and show the impact of conceptualizing the lineaments as conduits, barriers or conduit-barrier systems.

How to cite: Poulet, T., Kelka, U., Westerlund, S., and Peeters, L.: Computational framework for discontinuity network characterization , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6577, https://doi.org/10.5194/egusphere-egu2020-6577, 2020.

The exploration of the underground is a complex, but common task. Structural characterisation of a given sub-surface volume is of interest, for many purposes including deep geological repositories for radioactive waste, exploitation of mineral resources and geothermal energy production. One major challenge in this regard relates to the identification of sub-seismic scale faults and fractures. Discrete fracture network modelling is one possible technique for this purpose. Ideally, it is supported by borehole data. Even so, the results of stochastic models require critical verification to determine resulting uncertainties and model robustness.

We present a case-study from the village of Schlattingen, located the northernmost Molasse Basin in Switzerland, that is devoted to such a verification aimed at improvement of the DFN modelling workflows. Two boreholes were drilled at this location, a vertical cored borehole reaching into the crystalline basement and a deviated borehole running sub-horizontally for 464 m in the Schinznach Formation (Upper Muschelkalk), a potential geothermal reservoir (Frieg et al. 2015). This borehole layout allows testing the workflow for discrete fracture network modelling from a single borehole and assessment of the the added value of a deviated borehole (and vice versa).

The modelling workflow used borehole data and outcrop descriptions from a range of locations as input data. The spatial distribution of features was simulated using a Poisson distribution. The aims of the study were to investigate the workflow’s ability to account for the different orientation biases in the two boreholes and develop understanding of spatial variability in fracture orientation and frequency.

It was found that reasonable consistency in orientation and overall frequency could be achieved using the borehole orientation distributions but that the spatial variability in fracture frequency and clustering of fractures were significant. It was also necessary to critically evaluate the borehole imagery from the deviated borehole.

Current efforts are focused on better constrain the spatial fracture distribution along the deviated borehole using correlation analysis (Marett et al. 2018, Gale at al. 2018) and assess its influence on the discrete fracture network model. In addition, it is anticipated to integrate observation from nearby outcrops into the modelling strategy.

References:

Frieg, B., Grob, H., Hertrich, M., Madritsch, H., Müller, H., Vietor, T., Vogt, T., and Weber, H.P. (2015). Novel Approach for the Extrapolation of the Muschelkalk Aquifer in Switzerland for the CO₂-free production of vegetables. Proceedings World Geothermal Congress, Melbourne, Australia

Gale, J. F. W., Ukar, E., & Laubach, S. E. (2018). Gaps in DFN models and how to fill them. 2nd International Discrete Fracture Network Engineering Conference, DFNE 2018.

Marrett, R., Gale, J. F. W., Gómez, L. A., & Laubach, S. E. (2018). Correlation analysis of fracture arrangement in space. Journal of Structural Geology, 108, 16–33. https://doi.org/10.1016/j.jsg.2017.06.012

How to cite: Schneeberger, R., Lanyon, B., Herbert, A., Habermüller, M., and Madritsch, H.: Predictive DFN modelling for the Upper Muschelkalk aquifer in the northernmost Swiss Molasse Basin based on vertical and horizontal borehole records , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19664, https://doi.org/10.5194/egusphere-egu2020-19664, 2020.

Fracture networks exist at a wide range of scale in the earth crust and strongly influence the hydraulic behaviour of rocks, providing either pathways or barriers for fluid flow. Many oil, gas, geothermal and water supply reservoirs form in fractured rocks. The main challenge is the development of numerical models that describe adequately the fracture networks and the constitutive equations governing the physical processes in fractured reservoir. The hydraulic properties of fracture networks, derived from Discrete Fracture Network (DFN), models are commonly used to populate continuum equivalent models at reservoir scale, to reduce the computational cost and the numerical complexity. However, the efficiency of fracture networks to fluid flow is strongly tied to their connectivity and spatial distribution, that continuum models are not able to capture explicitly.In this work we used field data and synthetic models to introduce a new parameter to evaluate the efficiency of fracture networks to fluid flow, reflecting a range of variability in fracture network characteristics (e.g. P32, number of fractures, stress field). This alternative method allows to model fractured systems at reservoir scale, in a variety of geological settings, using exclusively a DFN approach.

How to cite: Proietti, G., Romano, V., Conti, A., Tartarello, M. C., and Bigi, S.: An alternative method to evaluate fracture network efficiency to fluid flow , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4799, https://doi.org/10.5194/egusphere-egu2020-4799, 2020.

Fluid transport within the Earth’s crust is predominantly controlled by planar void space like fractures and crack networks. Characterizing the time dependent hydro-mechanical properties of these rock-structural elements therefore is of paramount importance for natural geosystem understanding and geotechnical applications alike.

In this contribution we outline the protocol and results of a long term flow-through experiment of more than 4 months conducted with one single-fractured, pure quartz, and centimeter-sized Fontainebleau sandstone sample displaying very low matrix permeability.

The cylindrical sample was axially split to generate one single and rough tensile fracture and the obtained sample halves were manually offset in axial direction by 200 µm resulting in geometric mismatch of the two fracture faces yielding asperity contacts and high contact stresses upon loading.

The experiment was conducted at constant temperature (333 K) and pore fluid pressure (1 MPa), three different confining pressure levels (2, 18, and 30 MPa), and with two different fluids (deionized water and 0.3 mM SiO₂ solution).

The sample was continuously flown through and the experimental procedure consisted of several successive stages during which confining pressure and fluid type were systematically varied in time intervals of several weeks each.

The experiment yielded results of continuous sample and fracture permeability measurements, the derivation of time dependent changes in hydraulic fracture aperture, a complete ICP-OES chemical analysis of Si concentrations in the effluent in one day time intervals, and a full before/after microstructural investigation of mechanical aperture, contact area ratio, as well as asperity and free fracture face morphology.

Overall, this experiment yields evidenced insights into the low-temperature dynamics of fracture permeability when, concurrently, chemical interactions between fluid and rock are taking place. Moreover, the investigations emphasize the role of pressure solution (creep) in this context as opposed to, e.g., free face dissolution or subcritical crack growth. Finally, conclusions are drawn on the rate-limiting sub-process of pressure solution with possible implications for fluid history matching in quartz-rich fractured rock masses.

How to cite: Milsch, H. and Cheng, C.: Fluid flow through rock fractures undergoing chemical interactions: A look at pressure solution from an experimental perspective, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4712, https://doi.org/10.5194/egusphere-egu2020-4712, 2020.

Dissolution process is a complex phenomenon controlled by several factors such as the nature of chemical dissolution, lithology, porosity, stress orientation, environmental conditions, networks of fractures. In the karst field, however, compressional tectonic structures, as like stylolites, are never been taken into consideration for fluid flow. Stylolites are formed by a pressure solution processes that dissolves the soluble particles and leads to an enrichment in insolvable, non-carbonate particles (NCP) along their surfaces. Potentially they play an important role in fluid circulation during carbonate deformation.

Although they seem macroscopically planar, stylolites have an extremely variable shape from the meso- to microscale, with variable porosity and permeability. Because of this, they have a strong effect on regional fluid flow and the formation of reservoirs since they can act as barriers or conduits for flow.

In this research we investigated the distribution of voids and pores present both within and near the stylolites.This task is challenging because the pore sizes are small and therefore difficult to investigate. To determine which role the NCP and these structures have on fluid circulation, a comparison is herein presented between two different methods used to map the submicroscopic arrangement of pores and voids in and around stylolites. Because the investigated stylolites are relatively narrow, around 30-50 µm, we decided to use a classical micro Computed Tomography (µCT) technique supported by the ¹⁴C-PMMA impregnation method on two marble samples. These two comprelementary methods characterize the spatial distribution of connected voids in and around stylolites. µCT analysis provide adequate information on the 3D distribution of voids even if nanometer scale pores and small fractures are difficult to observe using µCT.

¹⁴C-PMMA method is however able to reveal connected porosities from mineral areas that consist of nanometer scale pores. Methylmethacrylate intrudes into nanometer scale pores, and autoradiography is used to visualize the porosities thanks to ¹⁴C beta emissions. Combining these two techniques, CT tomography and PMMA autoradiography we can visualize the 3D pore structures of the studied samples.

The results show that the micro CT technique supported by the PMMA autoradiography technique provides a useful tool to characterize the voids and pore structures of geomaterials.

The results allowed a more accurate description of the behavior of stylolites in fluid-rock interaction.

Key words: stylolites, permeability, microCT and ¹⁴C-PMMA impregnation

How to cite: Repetto, G., Magni, S., Sardini, P., Sattari, M. S., Sammaljärvi, J., and Mazurier, A.: Stylolites:when they became conduits for fluid pathway ?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5791, https://doi.org/10.5194/egusphere-egu2020-5791, 2020.

A multi-scalar, multi-methodological approach has been used to characterize the deformation mechanisms and fluid-rock interaction processes within the Belluno Thrust (BT), a regional-scale thrust cutting through Mesozoic carbonates of the eastern Southern Alps of Italy. We report the first results of a systematic analysis of the deformation mechanisms that steered strain localization within the BT fault zone during seismogenic faulting. The WSW-ENE-striking BT contributed to development of the south-verging thrust-and-fold belt of the Southern Alps during the Late Oligocene – present time interval.        We studied an outstanding exposure of the BT in the greater Feltre region, where the BT juxtaposes an Early Jurassic oolitic and micritic limestone (the Calcari Grigi Group) in the hanging wall against an Upper Jurassic-Early Cretaceous pelagic and cherty limestone (the Maiolica Fm.). The BT is defined by a 2 m-thick damage zone formed at the expense of both the hanging wall and footwall blocks. Atop the damage zone is a millimetric principal slip surface (PSS) that strikes WSW-ENE and dips 40° to the NNW. Kinematic analysis confirms the top-to-the SSE transport along the BT. Several structural facies have been identified by means of detailed structural mapping and sampled from the damage zone (from within both the hanging- and the footwall blocks) and the PSS. The outcrop structural characterization has revealed a number of physically juxtaposed, yet different, structural facies: i) cohesive, weakly foliated proto- to ultracataclasite; ii) uncohesive, clay-rich gouge; iii) foliated domains with SC-C’ structures. Relatively unstrained host rock lithons are wrapped by these variably strained domains. Petrographic and microstructural analyses show evidence of pervasive pressure solution, with abundant stylolites, slickolites and foliated domains indicating an overall ductile behaviour. Calcite veins are also common in all recognised structural facies showing mutual cross-cutting relationships with the pressure-solution seams. This structural characterization has provided the basis for detailed image analysis of selected cataclastic textures to calculate fractal parameters for the particle size distribution (Ds) and morphology (Dr) of the clasts aiming at better understanding the cataclastic flow active in the BT fault rocks. Results from a range of representative samples suggest corrosive wear to be the main cataclastic process (Ds 1,41 ÷ 2,00; Dr 1,51 ÷ 1,88). Cathodoluminescence imaging revealed multiple generations of cement and permitted discriminating the first-order chemical characteristics of parental fluids and constraining the relationships between calcite veining and cementation. Two syn-tectonic cements have been identified: i) a bright-orange cement, preferentially surrounding carbonate clasts with highly irregular margins, indicative of the involvement of carbonate-reactive fluids; ii) a dull, homogeneous brown/black cement coexisting with a siliceous matrix, mantled clasts and local sigmoidal structures. The latter is at times observed as thin injections and fluidized structures.        Our preliminary results suggest that overall deformation was accommodated by creep and low-T crystal-plastic deformation possibly during inter-seismic phases as indicated by the presence of pressure-solution seams and foliated fabrics. Transient spikes of coseismic rupturing possibly promoted by multiple batches of overpressured fluids were accompanied by significant cataclasis and brittle strain localization.

How to cite: Diamanti, R., Zuccari, C., Bonini, S., Vignaroli, G., and Viola, G.: Textural evolution and fluid-rock interaction during upper crustal, seismic deformation: Insights from the carbonate-dominated fault rock suite of the Belluno Thrust, Italian Southern Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5851, https://doi.org/10.5194/egusphere-egu2020-5851, 2020.

Faults, joints and stylolites are ubiquitous features in fold-and-thrust belts, and have been used for decades to reconstruct the past fluid flow (or plumbing system) at the scale of folded reservoirs/basins. The textural and geochemical study of the minerals filling the fractures makes it possible to unravel the history of fluid flow in an orogen, when combined with a knowledge of the burial history and/or of the paleothermal gradient. In most cases, the latter derives from the former, itself often argued over, limiting the interpretations of past fluid temperatures. Yet, recent methodological developments applied to carbonates and calcite fillings provide new perspectives for a more accurate reconstruction of the temperature, pressure and timing of the fluids that were present in the strata at the time they deformed, at every stage of fold development. Indeed, the temperature at which fluids precipitated can be obtained by Δ⁴⁷CO2 clumped isotopes while the timing of calcite precipitation in veins and faults is given by U-Pb absolute dating. Also, the maximum burial depth of strata before contraction can be estimated using sedimentary stylolite paleopiezometry, hence in a way free of any consideration about the geothermal gradient.

These techniques were jointly applied at the scale of the Umbria-Marches arcuate belt (UMAR, Northen Apennines, Italy). Mesoscale faults and vein sets were measured and sampled in the Cretaceous-Eocene rocks. Focusing on those fractures that developed during Layer Parallel Shortening (LPS, i.e. oriented NE-SW to E-W) and during folding (i.e. oriented parallel to local fold axis), paleofluid sources, temperatures and timing were reconstructed using U-Pb absolute dating, Δ⁴⁷CO2 clumped isotopes as well as δ¹⁸O, δ¹³C, and ^87/86Sr signatures of calcite veins. Results show a regional divide in the fluid system, with most of the belt including the foreland recording a fluid system involving basinal brines resulting at various degree from fluid-rock interactions (FRI) between pristine marine fluids (δ¹⁸O_fluid= 0‰ SMOW) and surrounding limestones (δ¹⁸O_fluid= 10‰ SMOW). Precipitation temperatures (35°C to 75°C) appear consistent with the burial history unraveled by sedimentary stylolite roughness paleopiezometry (600 m to 1500m in the range) and estimated geothermal gradient (23°C/km, Caricchi et al., 2004). As the degree of FRI increases forelandward, we propose a lateral, strata-bound, squeegee-type migration of fluids during folding and thrusting. In the western hinterland however, the fluid system rather involves hydrothermal fluids with a higher degree of FRI, the corresponding precipitation temperatures (100°C to 130°C) of which are inconsistent with local maximum burial (1500m). As the Sr radiogenic signatures preclude any deep origin of the fluids, we propose that the fluid system prevailing in the hinterland during LPS reflects the eastward migration of formational fluids originating from the Tuscan basin, located west from the UMAR, where studied Cretaceous rocks were buried under more than 4 km of sediments during the Miocene.

Beyond being the first combination of paleofluid geochemistry and burial estimates through paleopiezometry, this fluid flow model illustrates how the large scale structures may control the fluid system at the scale of a mountain belt.

How to cite: Beaudoin, N., Labeur, A., Lacombe, O., Hoareau, G., Marchegiano, M., John, C., Koehn, D., Billi, A., Boyce, A., Pecheyran, C., and Callot, J.-P.: Long-term burial history and orogenic-scale fluid flow depicted from stable isotopes and stylolite paleopiezometry in the Umbria-Marches arcuate belt (Northern Apennines, Italy)., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19184, https://doi.org/10.5194/egusphere-egu2020-19184, 2020.

Degassing of volatiles across the Earth crust towards the atmosphere is mainly controlled by long last diffusion. However, it can also be episodic. In fact, tectonic control of solid phase release of radiogenic helium (⁴He) due to, e.g. fracturing, may contribute to explain variance of the continental ⁴He degassing flux over multiple time and space scales. Rock rheology have a controlling influence on a wide range of crustal-scale processes including fluid flow, tectonic deformations and seismicity. Though faults comprise a small volume of the crust, they influence the mechanical and fluid flow properties of the crust, and are mechanisms for accommodating most of the elastic strain in the crust through a variety of slip-behaviours. Helium isotopes (³He,⁴He) are useful tracers for investigating many important geological processes because helium is a stable and conservative nuclide that does not take part in any chemical or biological process. Indeed, ⁴He released from rocks in the porefluid can be used to trace the deformation of rocks in a field of stress [Bauer 2017; Torgersen and O’Donnell 1991]. In fact, a volume of rock starts to be affected by micro-fractures from since  it is subjected to stress conditions exceeding about half its yield strength [Bauer, 2017]. Hence, the network of fractures evolves in a volume of rock progressively increase as a function of the evolution of deformation, improving the release of ⁴He that is trapped since its production. Consequently, ⁴He in natural fluids that outgas in a region of active tectonic can record the evolution of the field of stress and this volatile component could be used to trace changes in stress and deformation field. For the purpose of quantifying the amount of ⁴He present in the geological traps that feed the mud volcanoes of Regnano-Nirano mud volcanoes systems (Bonini et al., 2007), in the north Italy, in our study we have reconstructed the 3D geological model of the reservoirs, and proceeded to estimate the gas contained in them. Fluids emitted from these systems are thermogenic-CH₄ rich, which vertically migrates towards the surface. Helium is in traces and its isotopic signature (≈0.01-0.02Ra, Ra is the ³He/⁴He in air) shows that ⁴He is mainly produced in the crust by U-Th decay. We have found that the present ⁴He is greater than what should be available taking into account only the steady-state crustal production. Therefore, we compared the excess helium present in the reservoirs with the contribution coming from the seismic activity of the area, which is sufficient to explain this excess. Our study highlights that an intense fracturing of a volume of rock, due to the recent seismicity below the studied area, may explain the accumulation of helium in the reservoir higher than the steady-state condition. Therefore, the effective vertical rate of fluid transport in the Earth's continental crust can be characterized by episodic events controlled by fracturing.

Bonini (2007) - JGRes Solid Earth 

Torgersen & O’Donnell (1991) - GRL, vol.18

Bauer (2017) - SAND2017-9438

How to cite: Buttitta, D., Caracausi, A., Favara, R., Chiaraluce, L., Gasparo Morticelli, M., and Sulli, A.: Tectonic control on crustal degassing in continental region: the role of rock fracturation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15407, https://doi.org/10.5194/egusphere-egu2020-15407, 2020.

Karst geothermal systems fluid flow is dominated by structurally controlled porosity, which constrains the paths of aquifer recharge and the upwell of geothermal fluids. In fold-and-thrust belt settings associated with continental collision, geothermal fields occur within basins generally interested by low-enthalpy geothermal systems. Despite that, the deeper and warmer levels of multiply stratified aquifers within the detached sedimentary covers are vertically connected to shallower depths by high-angle faults, thus making of them interesting targets for exploration.

In the frame of the geothermal exploration steered by the Geneva Canton, this work aims at determining how fracture connectivity, orientation and permeability anisotropy has implications on fluid flow within high-angle faults. Recent software development (e.g., FracPaQ) allows to quantify such interconnection providing insights into spatial variation of multiscale fault-controlled porosity in order to have dynamic feedbacks between fluid flow, permeability rise/fall. We use the inner Jura fold-and-thrust belt and the other carbonate relieves surrounding Geneva as an outcrop analogue for the deeper carbonate reservoir, lying at depth beneath the siliciclastic Molasse deposits. Hereby, we present new structural and morphostructural lineament maps and scan box analyses from outcrops that provide a multiscale analysis on fracturing across the study area. The sampling sites are representative of fractured fold hinges constituted of Mesozoic carbonates crossed by high-angle faults.

The map analysis show that the late Oligocene-early Miocene growing carbonate anticlines are shaped by a series of fore- and back-thrusts resulting in salient-and-recess curvy thrusts accommodating different amount of shortening across high-angle tear-faults. With the support of high-resolution LIDAR images, we observe that at the large scale (e.g., five kilometers), as fault zone broadens across transfer zones, the background fracture network is more intense at the salient flanks. Major faults occur as segmented, thus not providing near-surface structure capable of giving any earthquake significantly larger than the already measured ones (e.g., M_L 5.3, Epagny earthquake 1996). Our preliminary results identify the W- and the NNW- striking systems strike-slip faults as the preferred patterns of fluid flow. Cross-cutting relationships vary with their position into the bended belt, thus making them suitable to be multiply reactivated during the Jura arc indentation. At the outcrop scale, the most mature fault zones associated with larger displacement are characterized by high fracture intensity and connectivity. Field evidences show that NNW- and W/NW- striking systems are vein-rich whereas N- and NE-striking systems are accompanied by open fracture sets although they may work with opposite fluid-flow vertical directivity. Mechanical and regional chronological development of the fracture network is also discussed as related to the regional fault evolution.

How to cite: Cardello, G. L. and Meyer, M.: Faults controlling geothermal fluid flow in a karst geothermal system (Western Alpine Molasse Basin, France and Switzerland), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15861, https://doi.org/10.5194/egusphere-egu2020-15861, 2020.

Coupled fluid characterization and absolute dating of fracture networks may provide insights into the understanding of critical stages of evolution of a growing orogen. As a result of the collision between the European and Apulian plates, the Alps have experienced several evolutionary stages comprising continental subduction, nappe stacking, thick- to thin-skin tectonics in relation with the frontal propagation of a fold and thrust belt, and extensional reactivation of the major Penninic Frontal Thrust (PFT). Current evolution of the orogen (Tricart et al., 2001, 2007 and Sue et al., 2007) shows an ongoing extensional seismic activity along PFT while borders of the orogenic system remain in compression. The transition from compression to extension along the PFT remains unconstrained.

This study aims to constrain the time of the PFT inversion and provide a characterization of the tectonic structures through time during the formation of upper Durance normal fault system. For this, we applied several novel dating techniques (in-situ U-Pb calcite and (U-Th)/He hematite dating techniques). In addition, we determined the geochemical signature of the fluids trapped (calcite crystallization) deformation by δ¹³C and δ¹⁸O stable isotope analysis of calcites to constrain the fluid reservoirs, and thus the size of the involved tectonic structures. Stable isotopes show that the fluids associated with the early extensive structures bear isotopic signatures close to those of their host rocks, indicating a fluid at equilibrium and thus a close system in agreement with the small (mm-cm) size of mostly ductile structures. In a second stage, connection of veins and fractures lead to major fault formation (metric to kilometric scale structures) show isotopic signatures in agreement with ascending metamorphic fluids, featuring an open system along the PFT.

U-Pb dating on calcite was successful on several samples despite high common lead concentrations. Two fault gouge samples associated with kilometric scale faults gave ages between 3.5 Ma and 2.5 Ma. These structures are a signature of the paleoseismic activity wich occured some 2.5-3.5 Ma ago when the wall domain of PFT was few km depth. Moreover, (U-Th)/He hematite dating was used on slickensides of the same fault system. Preliminary ages of 2.5, 1.5 and 15 Ma were obtained. The 15 Ma age is interpreted as a minimum age inversion of the PFT, while other ages overlap with the U-Pb calcite ages. This multidisciplinary inverstigation in the Western Alps helps to constrain the exhumation history of the paleo-seismogenic zone related to the inversion of the PFT.

How to cite: Bilau, A., Rolland, Y., Schwartz, S., Dumont, T., Brigaud, B., Gautheron, C., Pinna-Jamme, R., Deschamps, P., Godeau, N., Guihou, A., and Melleton, J.: U-Pb calcite dating  and isotopic fluid signatures of extensional fault gouges affecting the Penninic Frontal Thrust : implications for exhumation of the seismogenic zone., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4762, https://doi.org/10.5194/egusphere-egu2020-4762, 2020.

In the eastern Paris Basin, the Oxfordian (Upper Jurassic) and Bathonian to Bajocian (Middle Jurassic) carbonate platforms have been intensively cemented, despite rather low burial (< 1000 m). These limestone units are separated from each other by a 150 m thick succession of Callovian - Oxfordian clay-rich rocks. These claystones are currently under investigation by the French national radioactive waste management agency (Andra).

Most of the initial porosity in the Middle and Upper Jurassic limestones is now sealed by successive stages of calcite precipitation, which have been thoroughly characterized both petrographically and geochemically over the last fifteen years (Buschaert et al., 2004; Vincent et al., 2007; Brigaud et al., 2009; André et al., 2010; Carpentier et al., 2014). However, despite these research efforts, the timing and temperature of the fluids involved in the cementation of these carbonate rocks were still uncertain.    

Here, we present and discuss newly acquired ∆₄₇ temperatures and U-Pb ages of calcite cements filling the intergranular pore space, as well as vugs and microfractures.

The Middle Jurassic limestones were largely cemented during the Late Jurassic / Early Cretaceous period, as shown by our new LA-ICP-MS U-Pb ages that agree with the previous Isotope Dilution-TIMS U-Pb age of 147.8 ± 3.8 Ma from Pisapia et al. (2017). This event is believed to be associated to the Bay of Biscay rifting. Our data also reveal a second and more discrete crystallization event during the Late Eocene / Oligocene period, related to the European Cenozoic Rift System (ECRIS). In both cases, calcite was precipitated from fluids in thermal disequilibrium with the host rocks. 

By contrast, the Upper Jurassic limestones were largely affected by the successive deformation events that occurred during the Late Mesozoic / Cenozoic period. New LA-ICP-MS U-Pb ages acquired in ca. 200 µm-thick fractures reveal that calcite crystallized during three successive periods corresponding to the Pyrenean compression, the ECRIS extension and, finally, during the Alpine compression. These compression phases generated late stylolitization and subsequent dissolution/recrystallization in the Upper Jurassic limestones, while such tectonic features are rare in the Middle Jurassic.

Therefore, as opposed to the more conventional « burial-induced » model, our study highlights the role of stress propagation in the cementation of carbonate rocks hundreds of kilometers away from the rifting or collisional areas.

References:

Buschaert et al., 2004. Applied Geochemistry 19, 1201 – 1215. Vincent et al., 2007. Sedimentary Geology 197, 267 – 289. Brigaud et al., 2009. Sedimentary Geology 222, 161 – 180. André et al., 2010. Tectonophysics 490, 214 – 228. Carpentier et al., 2014. Marine and Petroleum Geology 53, 44 – 70. Pisapia et al., 2017. Journal of the Geological Society of London 175, 60 – 70.

How to cite: Blaise, T., Brigaud, B., and Carpentier, C.: Large-scale intraplate deformation caused the cementation of Jurassic carbonates in the eastern Paris Basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7405, https://doi.org/10.5194/egusphere-egu2020-7405, 2020.

In order to elucidate the regional variation of stress field in the eastern part of Japan after the 2011 Tohoku earthquake of M=9.3, we tried to analyze focal mechanism data of earthquakes that occurred in 2011, presented by the Japan Meteorological Agency (JMA). Although earthquakes (aftershocks) occurred largely in the offshore area along the subduction zone of the Pacific plate under the North American and Eurasian plates, focal mechanism data presented by JMA are mainly those on land. For fault tectonic analysis, the suggested focal mechanism data are classified into appropriate populations on the basis of clusters and focal depths to reduce the bias and errors of stress tensors resulting from areal stress variation and varying vertical load. According to the results, the stress types of determined stress tensors consist of reverse, wrench and normal faulting ones. As for reverse faulting stresses in which the vertical load is the minimum principal stress axis, those of NW-SE compression prevail, which may be tightly related to northwestward movement of the Pacific plate. Those of E-W compression are determined in the continental crust deeper than about 9 km around Yamagata and in the lower part of subducting oceanic crust. In the Kanagawa and Chiba areas, determined stress tensors display NNW-SSE compression as well as NW-SE and E-W compressions. The NNW-SSE compression seems to be related to the movement of the Philippine Sea plate. Stress tensors of wrench faulting type are found in the continental crust far from the subduction zone of the Pacific plate, displaying NW-SE and E-W compressions in the shallower and deeper parts of crust, respectively. The E-W compression is presumably associated with the Himalayan tectonic domain. Determined stress tensors of normal faulting type show diverse extension directions: NW-SE extension in the coastal area, parallel to the Pacific compression, and E-W or NE-SW extension elsewhere. Especially, numerous focal mechanism data showing normal faulting stresses are present in the coastal area of Fukushima and Ibaraki, from which Poisson’s ratio of shallow crust was determined to be 0.25 to 0.27 using friction lines on Mohr’s circles and focal depths (or corresponding vertical loads). Additional horizontal stress related to the northwestward motion of the Pacific plate was estimated to be 46, 122 and 286 MPa in three groups of 0 to1.5, 1.5 to 4.5 and 3.5 to 11.5 kilometers in depth, respectively.

How to cite: Choi, P.: Fault tectonic analysis of aftershocks of the 2011 Tohoku, Japan, earthquake: interaction between three different tectonic domains and approximation of stress magnitude, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4391, https://doi.org/10.5194/egusphere-egu2020-4391, 2020.

Basement terranes commonly contain complex fault networks developed during repeated episodes of brittle deformation. The Mid-Norwegian margin (from 62 to 63.8 °N) exposes a complexly fractured terrane formed mainly by Caledonian basement rocks. The margin recorded a prolonged brittle deformation history spanning the Devonian to Paleogene time interval. It is characterised by a pervasive NE-SW structural grain due to the ductile-brittle multiphase activity of the Møre-Trøndelag Fault Complex (MTFC).

In order to develop a time-constrained tectonic model of the area, we applied a multidisciplinary approach combining remote sensing, field work, paleostress inversion, microstructural analysis, mineralogical characterization, clumped isotope thermometry on carbonates and K-Ar dating of fault rocks from key representative faults. We present herein the preliminary structural-geochronological data of a still ongoing study of two regions along the Mid-Norwegian margin, the Hitra-Frøya and Kråkenes-Runde areas. These key areas represent the intersection regions between the Mid-Norwegian- and the other sectors of the margin.

The brittle structural record of the entire Mid-Norwegian margin was analysed by remote sensing of lineaments using high resolution LiDAR data followed by ground-truthing of the obtained results during field work. Three main sets of lineaments were identified: i) (E)NE-(W)SW-trending lineaments, parallel to the coastline and to the MTFC; ii) N(NW)-S(SE)-trending lineaments; iii) WNW-ESE-trending lineaments. The main sets of faults and fractures were further characterised by their fault rock association and coating. All generations of faults contain thin coatings of chlorite, variably thick epidote and quartz mineralisations and calcite veins and coatings, locally associated with acicular zeolite. Samples of calcite and related gouges were collected from different sets of faults. Carbonate clumped isotope thermometry constrains the range of temperature of calcite growth between 140 and 30 °C, indicating that calcite precipitated at different thermal conditions during a multiphase structural evolution. K-Ar data collected so far from synkinematic illite separated from fault gouges yield Jurassic-Paleogene ages.

The structural network of the margin is interpreted as reflecting a sequence of different deformation episodes. In order to resolve the orientation of the stress field for each recorded event, we applied paleostress inversion with the Win-Tensor software [1]. The preliminary results suggest that at least three tectonic stages affected the margin. A NE-SW strike-slip dominated transpression possibly reflects the late stages of the Caledonian orogenic cycle. A pure and oblique extensional (E)NE-(W)SW stage is associated with the Jurassic North Sea rifting, followed by a NW-SE Paleogene extensional reactivation observable throughout the margin.

To conclude, a new multidisciplinary database for the reconstruction of the brittle deformation history of the Mid-Norwegian margin is presented. The proposed approach aims to define the temporal and structural characterisation of each single tectonic episode. Such an approach is also pivotal toward the correlation with the deformation history of the corresponding offshore domains, as well as the comparison in time with other segments of the Norwegian margin.

[1] Delvaux, D. and Sperner, B. (2003). Stress tensor inversion from fault kinematic indicators and focal mechanism data: the TENSOR program. Geological Society, London, Special Publications, 212: 75-100

How to cite: Tartaglia, G., Viola, G., Ceccato, A., Bernasconi, S., van der Lelij, R., and Scheiber, T.: Multiphase brittle tectonic evolution of the Mid-Norwegian margin, central Norway, reconstructed by remote sensing, paleostress inversion and K-Ar fault rock dating, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5438, https://doi.org/10.5194/egusphere-egu2020-5438, 2020.

Quartz veins are produced from the crystallization of the last silica enriched hydrothermal phase from granitic magma circulating along the pre-existing fracture of rock. In many instances, these hydrothermal fluid act as a carrier for the ore minerals. The intrusion of quartz veins along fractures depends upon the tectonic stress conditions in the area. Fluid pressure (P_f) of these ascending liquids should be higher than the normal compressive stress (σ_n) to dilate the fractures. We are studying the quartz vein intrusion in the Cu‒Pb‒Zn mineralization belt of Ambaji, South Delhi terrane, Aravalli- Delhi mobile belt, NW India. The host rocks include mica schist, amphibolite, calc schist, talc tremolite schist, and four phases of granite intrusion (G₀‒G₃). The age of G₀, G₁, G₂ and G₃ granite are 960, 860, 800, and 750 Ma respectively. The rocks underwent three phases of folding (F₁‒F₃) and show greenschist to amphibolite facies metamorphism. The quartz vein intrusion is related to syn to post F₃ folding and G₃ granite magmatism. This final phase hydrothermal fluid extremely altered host rock and formed biotite-tourmaline-quartz and tremolite-actinolite-talc-chlorite greisen along the contact. The greisen host chalcopyrite-pyrite-galena-sphalerite mineralization suggesting the ore minerals were transported by the quartz vein. Vein orientation, stress condition, fluid pressure fluctuation, and fluid temperature can decide the fracture dilation and mineralization processes. Therefore, this work concentrates on the geometrical distribution of the vein orientation data. From this we deduced (i) girdle distribution pattern of vein data  (ii) σ₁ = 120º/75º, σ₂ = 052º/07º, σ₃ = 323º/07º indicate maximum extension was NW-SE and σ₁σ₂ plane strikes was N52ºE, (iii) θ₂ =12º, θ₃ = 40º  and (iv) R'(driving pressure ratio) = 0.95, ϕ (tectonic stress ratio) = 0.90 indicates high value for R' leading to dilation of wide range of fractures. Further, the high ϕ value suggests uniaxial extension. Microscopic petrography of fluid inclusions shows three generations of inclusion like primary inclusion, secondary inclusion, and pseudosecondary inclusion. Most of the inclusion has aqueous and vapour phase and some inclusions show solid halite phase. We observed different types of trail bound of inclusion like intragranular inclusion, intergranular inclusion and transgranular inclusion, which suggest deformation and recrystallization in the rock. We are studying microthermometry analysis of fluid inclusion present in the quartz vein and trying to estimate the fluid pressure. With the help of fluid pressure, the 3D Mohr circle will be constructed and paleostress will be quantified. That will help in understanding the stress condition and mineralization in the rock.