TS – Tectonics & Structural Geology

EGU22-13304 | Presentations | TS1.2 | Marie Tharp Medal Lecture

Analogue modelling of subduction: yesterday, today and tomorrow. 

Francesca Funiciello

The use of experimental tectonics (also known as analogue-, laboratory, or physical modelling) to study tectonic processes is not a novelty in Earth Science. Following Sir James Hall’s pioneer work (1815), many modellers squeezed, stretched, pushed and pulled a wide range of materials – e.g., sand, clay, oil, painters’ putties, gelatins, wax, paraffin, syrups, polymers – to unravel a wide range of tectonic processes to determine parameters controlling their geometry, kinematics and dynamics. However, only recently experimental analogue modelling has definitively transformed from a qualitative to a quantitative technique, thanks to appropriate scaling relationships, the improvement in the knowledge of the rheology of both natural and analogue materials and the use of high-resolution monitoring techniques to quantify morphology, kinematics, stress, strain and temperature.

Here, I specifically review the experimental work performed to study one of the most intriguing aspects of plate tectonics: the subduction process. Subduction provides the dominant engine for plate tectonics and mantle dynamics. Moreover, it has also societal importance playing a key role on hazard at short (i.e., earthquakes and mega-earthquakes, tsunami, effusive and explosive volcanic activities with impact on aviation safety) and long time scales (i.e., local and global climate change). Over the last decades, a noteworthy advance in the quality and density of global geological, geophysical and experimental data has allowed us to provide systematic quantitative analyses of global subduction zones and to speculate on their behaviour. These constraints have been integrated into a mechanical framework through modelling.

I will bring you to a journey through the past, the present and the future of analogue modelling and related efforts, results and perspectives for the study of the subduction process. It will be shown how analogue models, with their inherent 3D character and behaviour driven by simple and natural physical laws, contribute to successfully unravelling the subduction process, inspiring new ideas. Challenging ongoing perspectives of analogue models imply the possibility to compare time and space scales, allowing to merge, within the same model, both short- and long-term and shallow and deep processes.

How to cite: Funiciello, F.: Analogue modelling of subduction: yesterday, today and tomorrow., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13304, https://doi.org/10.5194/egusphere-egu22-13304, 2022.

EGU22-12964 | Presentations | TS2.1 | Highlight | TS Division Outstanding ECS Award Lecture

Crustal stress across spatial scales 

Mojtaba Rajabi and Oliver Heidbach

The study of crustal stress examines the causes and consequences of in-situ stress in the Earth’s crust. Stress at any given point has several geological sources, including ‘short-term and local-scale’ and ‘long-term, ongoing and wide-scale’ source. In order to better characterise the crustal stress state, the analyses of both local- and wide-scale sources, and the consequences of their superposition are required. The global compilation of stress data in the World Stress Map database has increased significantly since its first release in 1992 and its analysis revealed large rotations of the stress tensor in several intraplate settings.

Large-scale stress analysis, so called first-order, (> 500 km) provides information on the key drivers of the stress state that result from large density contrasts and plate boundary forces. The analyses of stress at smaller-scales (< 500 km) have numerous applications in reservoir geomechanics, geo-storage sites, civil engineering and mining industry. To date, numerous studies have investigated the stress analysis from different perspectives. However, the stress, in geosciences, is still enigmatic because it is a scale-dependant parameter. It means, stress variations can be studied at both the ‘spatial-scale’ and ‘temporal-scale’. This paper aims to investigate the crustal stress pattern with a particular emphasis on the orientation of maximum horizontal stresses at various spatial-scales, ranging from continental scales down to basin, field and wellbore scales, to better evaluate the role of various stress sources and their applications in the Earth’s crust. The stress analyses conducted in this work shows that stress pattern at large-scales do not necessarily represent the in-situ stress pattern at smaller-scales. Similarly, analysis of just a couple of borehole measurements in one area might not yield a good representation of the regional stress pattern.

How to cite: Rajabi, M. and Heidbach, O.: Crustal stress across spatial scales, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12964, https://doi.org/10.5194/egusphere-egu22-12964, 2022.

TS1 – Topics in Tectonics and Structural Geology

EGU22-854 | Presentations | TS1.2

Crustal evolution and oceanic core complexes at the Ascension fracture zone – MAR 7° S 

Anke Dannowski, Ingo Grevemeyer, Valentin Baehre, Jörg Bialas, and Tim Reston

The Ascension fracture zone (AFZ) is a double transform fault system and offsets the Mid-Atlantic Ridge axis by 230 km at 7 °S. The transform fault system is consisting of two parallel transform faults, the North and South Ascension Fracture Zone, sandwiching a ~20 km long ridge segment, which we name Ascension Fracture Zone segment. The segment shows strong topographical variations and corrugated surfaces typical for detachment faults that form oceanic core complexes. An elongated massif approximately 50 km east of the ridge axis with transform-parallel striations of over 100 km on top, indicate a detachment fault active for several million years. This would be one of the longest transform-parallel corrugated surface observed anywhere in the oceans. The question arises whether the corrugations belong to one OCC, representing a rather stable crustal accretion, or if several OCCs have been developed, representing a rather variable crustal accretion. Changes in melt supply influence the crustal structure, which in turn can be recognised by seismic methods.

RV Meteor (cruise M62-4) set out to acquire seismic refraction and wide-angle reflection data along a 265 km long spreading parallel transect to image the crustal velocity distribution and the crustal thickness of the intervening short AFZ segment. Densely spaced, every ~9.25 km, ocean bottom seismometers recorded P-wave and converted S-wave energy emitted from a 64 l G-gun cluster at a shot interval of 60 s, equal to ~125 m shot distance.

The results reveal P-wave velocities that vary along the profile from 3.5 km/s to 5 km/s at the seafloor and reach 7.2 km/s in ~6 km depth at the ridge axis and at 3 km to 4 km depth under the ridge shoulders. At larger offsets to the ridge axis. S-wave velocities vary from 2 km/s to 2.5 km/s at the seafloor and increase to 3.5 km/s in ~2 km depth east of the ridge axis, while the S-wave velocities west of the ridge axis show a lower velocity gradient and reach 3.5 km/s in 3 km to 4 km depth. A Vp/Vs ratio >1.9 is observed in areas where seafloor corrugations have been observed. These areas are interpreted as serpentinised mantle material. However, the high Vp/Vs ratio seems to be limited to the upper 1.5 km to 2 km of the subsurface, indicating that the hydration of the seafloor is limited to that depth. The eastern ridge flank is dominated by a high Vp/Vs ratio for offsets larger than 40 km from the ridge axis, however, it is interrupted by small stripes of Vp/Vs <1.9. Thus, in the short AFZ segment, detachment faulting seem to occur continuously over a long period with short interruptions when the magmatic budged exceeds a certain upper limit.

How to cite: Dannowski, A., Grevemeyer, I., Baehre, V., Bialas, J., and Reston, T.: Crustal evolution and oceanic core complexes at the Ascension fracture zone – MAR 7° S, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-854, https://doi.org/10.5194/egusphere-egu22-854, 2022.

EGU22-4929 | Presentations | TS1.2

Petrology of Alpine Tethys serpentinites: New insights on serpentinization at passive margins 

Flora Hochscheid, Marc Ulrich, Manuel Muñoz, Damien Lemarchand, and Gianreto Manatschal

Fluid-rock interactions in mantle rocks that turns peridotite into serpentinite has been widely documented during the past two decades, for geological settings such as mid-ocean ridges (MOR) and subduction zones. In contrast, serpentinization at rifted margins has received much less attention, while serpentinites at these settings are largely involved in geochemical and tectonic processes that occur from continental break-up to the establishment of a steady-state MOR. This study presents new petrological and mineralogical investigations on peridotites that were part of the subcontinental mantle exhumed along a former Ocean-Continent Transition (OCT) of the Jurassic Alpine Tethys, nowadays exposed as ophiolitic nappes (Platta, Tasna and Totalp) in the southeastern part of the Swiss Alps. These peridotites experienced various degrees of serpentinization, from moderately to completely serpentinized. At Totalp, initially located close to the continent, serpentinization forms a typical lizardite-bearing mesh texture that surrounds relics of primary minerals. Locally, the association of andradite and polyhedral serpentine occurs as alteration products of clinopyroxene, which may be interpreted in terms of low temperature serpentinization and near-isochemical conditions. At lower Platta, which represents the oceanwards (distal) domain of the OCT, serpentinization is extensive and, similarly to Totalp, predominantly formed by mesh lizardite. For the two previously mentioned sites, the typical mesh texture suggests a fluid-rock interaction with a low water-to-rock ratio. At Tasna and upper Platta, which both correspond to more proximal domains of the OCT (i.e., continentwards), serpentinites are characterized by several superimposed serpentinization events marked by successive generations of serpentine-filling veins with distinct morphologies and textures, forming the following sequence: Mesh texture —> Banded veins (V1) —> Crack seals (V2) —> Lamellar veins (V3). The V1 banded veins are made of several serpentine species including chrysotile, polygonal serpentine, polyhedral serpentine and lizardite. They formed as a result of gradual opening during exhumation of the mantle from a supersaturated solution. The progressive evolution from chrysotile to polygonal serpentine and then lizardite is attributed to more intense fluid-rock interactions and a lower fluid saturation with decreasing depth. V2 crack seals consist of chrysotile veins formed at shallow depth after strain release and under high water/rock ratios. Surprisingly, antigorite was identified as the latest vein generation (V3). Trace element compositions for V3 are comparable to those of earlier vein generations, but strongly differ from those attributed to the Alpine convergence, excluding their formation during prograde subduction metamorphism. Rather, we propose that antigorite veins formed as a result of compressive stresses generated by apparent unbending of the footwall during final exhumation. This result shows that antigorite is not only restricted to convergent domains, and that it may be more common in rifted margins and (ultra-)slow spreading centers than previously thought.

How to cite: Hochscheid, F., Ulrich, M., Muñoz, M., Lemarchand, D., and Manatschal, G.: Petrology of Alpine Tethys serpentinites: New insights on serpentinization at passive margins, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4929, https://doi.org/10.5194/egusphere-egu22-4929, 2022.

The 64°E region of the eastern SWIR is a melt poor end member region of the MOR system. Magma focusing to axial volcanoes leaves >50 km wide along axis corridor, where seafloor spreading occurs almost fully via successive alternate polarity detachment faults (Sauter et al., 2013). The present-day south-dipping young (~300 kyrs; Cannat et al., 2019), active detachment fault cuts through an older north dipping detachment fault. The active detachment emergence can be traced over 32 km from the shipboard bathymetry data (for comparison, the scoped-shaped finely corrugated 13°20’N exhumed detachment surface at the Mid-Atlantic Ridge extends only ~5-6 km along-axis; Escartin et al., 2017). HR micro-bathymetry maps acquired on the west and east sides of this emergence line indicate an along-strike variation of the fault structure and geometry. In the east, the fault emerges at an overall angle of ~26°-30° and the emerging fault surface is irregular, with undulations at hectometer to km scales, close to parallel to spreading direction, and rare occurrences of decameter-scale corrugations, up to 20° oblique to spreading. In the west, the detachment emerges in the form of two distinct fault splays, ~400 m apart, both at an angle ~40°-50°.  This western region receives some magma input, resulting in localized patches of basalt and hummocky ridges.

Near fault deformation structures, documented by Remotely Operated Vehicle (ROV) dives and sampling, also differ between east and west. In the west, sigmoidal blocks (~5-10 m) of moderately fractured serpentinized peridotite, some with gabbro dikes, are observed below the emerging faults, which consist of <1.5 m thick zone of serpentinite breccia, and micro-breccia, with cm-thick intervals of gouge. In the east, the highly strained intervals are thicker (up to 8 m), with a greater proportion of serpentinite gouge and microbreccia. The more coherent rocks below the fault are also more pervasively fractured, with planar decimeter to meter-spaced south-dipping joints. ROV dives near the detachment breakaway offer an opportunity to study the deformation below a more mature region of the previously active detachment. Steep landslide head scarps there expose vertical sections, up to 70 m thick, with several intervals of serpentine gouge and micro-breccia, intercalated with coarser brecciated serpentinized peridotite, and with sigmoidal, meter to decameter sized blocks of serpentinized peridotite. Together, these observations point to a heterogeneous structure and to a variable thickness of the strain localization/damage zone associated with the emerging portions of the 64°40'E SWIR detachment. Based also on the seismic reflection structure of the fault zone at depth (Momoh et al., 2017), we propose that the material that emerges samples distinct regions of a kilometer-thick heterogeneously deformed damage zone, leading to different geometries and structure of the emerged fault surface(s).

How to cite: Mahato, S. and Cannat, M.: Early stages of evolution of an axial detachment fault at the ultraslow spreading mid-ocean ridge (64°40'E SWIR), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5911, https://doi.org/10.5194/egusphere-egu22-5911, 2022.

EGU22-7065 | Presentations | TS1.2

Arctic Mid-Ocean Ridge seismicity: Results from an OBS deployment at Loki’s Castle 

Marie Eide Lien, Matthias Pilot, Vera Schlindwein, Lars Ottemöller, and Thibaut Barreyre

Activity along the Arctic Mid-Ocean Ridge (AMOR) has been progressively explored over the past 20 years. Along the ridge, processes such as dike intrusion and faulting cause earthquakes. We focus on an area around the Loki’s castle hydrothermal vent field (LCVF), located on the Mohn ultra-slow spreading ridge. Ultra-slow spreading systems are strongly controlled by tectonic processes, which provide an opportunity to study almost exclusively the effect of tectonism on a hydrothermal vent field.

In June 2019, we deployed a network of eight broadband ocean bottom seismometers (OBS) in an area of about 20 by 20 km around the LCVF. The OBSs were deployed for a one-year monitoring period until July 2020. We processed the OBS data using an automatic detection routine and machine learning approach to pick phases, and then located the local earthquakes based on a 1D velocity model. This provided an earthquake catalogue that was interpreted to understand the seismicity in terms of spatial and temporal distribution, and to identify fault structures. Within the broader tectonic system we aim to enhance our understanding of the LCVF.

How to cite: Lien, M. E., Pilot, M., Schlindwein, V., Ottemöller, L., and Barreyre, T.: Arctic Mid-Ocean Ridge seismicity: Results from an OBS deployment at Loki’s Castle, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7065, https://doi.org/10.5194/egusphere-egu22-7065, 2022.

EGU22-7217 | Presentations | TS1.2

Seafloor spreading modes across the Charlie Gibbs transform system (52°N, Mid Atlantic Ridge) 

Alessio Sanfilippo, Sergey Skolotnev, Marco Ligi, and Alexander Peyve and the A.N. Strakhov Expedition S50 and A.N. Vavilov Expedition V53 Science Parties

The prominent Charlie Gibbs right-lateral multi-transform system (52°-53°N) offsets the Mid Atlantic Ridge (MAR) by ~340 km. The transform system is formed by two distinct transform faults linked by a short ~40 km-long intra-transform spreading centre (ITR). The two adjacent MAR segments are influenced by both the Azores and the Iceland mantle plume. Recently, high resolution multibeam surveys and a dense sampling program of the entire transform system, including the adjacent southern and northern MAR segments, were carried out during expeditions of R/V Celtic Explorer (2015, 2016 and 2018) [1], R/V A.N. Strakhov (2020) and A.S. Vavilov (2021) [2]. The new surveys show widespread occurrence of large structures with corrugated surfaces and exhumed lower crust and mantle rocks on both sides of the intra-transform spreading axis. Morphological analyses of the intra-transform domain and magnetic data indicate that crustal accretion was driven by flip-flop detachment faulting [3], with minimal ridge melt supply and little axial volcanism. The tectonic spreading persisted for tens of millions of years. Along axis MORB chemistry shows that changes in seafloor accretion styles are mirrored by variations in melt supply, in turn dependent on mantle temperature and by a large-scale mantle heterogeneity. Charlie Gibbs is a key case study of how seafloor accretion modes at a spreading segment is critically dependent on mantle thermal state but also on its intrinsic compositional heterogeneity.

[1] Georgiopoulou A. and CE18008 Scientific Party, 2018. Tectonic Ocean Spreading at the Charlie-Gibbs Fracture Zone (TOSCA): CE18008 Research Survey Report. Marine Institute of Ireland, Dublin, pp 1-24. [2] Skolotnev, S. et al., 2021. Seafloor Spreading and Tectonics at the Charlie Gibbs Transform System (52-53ºN, Mid Atlantic Ridge): Preliminary Results from R/V AN Strakhov Expedition S50. Ofioliti, 46(1). [3] Cannat, Met al., 2019. On spreading modes and magma supply at slow and ultraslow mid-ocean ridges. Earth and Planetary Science Letters, 519, 223-233.

How to cite: Sanfilippo, A., Skolotnev, S., Ligi, M., and Peyve, A. and the A.N. Strakhov Expedition S50 and A.N. Vavilov Expedition V53 Science Parties: Seafloor spreading modes across the Charlie Gibbs transform system (52°N, Mid Atlantic Ridge), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7217, https://doi.org/10.5194/egusphere-egu22-7217, 2022.

EGU22-7720 | Presentations | TS1.2

The thermal regime of mid-ocean ridges: geological perspectives and numerical modelling 

Mathilde Cannat, Jie Chen, and Jean Arthur Olive

The thermal regime of mid-ocean ridges determines their spreading modes (i.e., the combination of mid-ocean ridge tectonic, magmatic and hydrothermal processes that control the composition and structure of the oceanic lithosphere). It is determined by the balance of heat supply and heat loss in the axial region. Most heat is supplied through magma, while hydrothermal energy fluxes depend on the permeability and depth extent of the hydrothermal cooling domain, and on the thickness of the conductive boundary layer at its base. At fast spreading ridges, the flux of melt is high, the thermal regime is hot, and melt resides at depths of only a few kms in a steady state fashion, well within the reach of vigourous axial hydrothermal convection. At slow ridges, the melt flux is lower, the thermal regime is colder, so that melt can only reside durably at depths that are commonly > 10 km, out of the reach of vigourous (high permeability) hydrothermal systems. Melt there, however, is commonly injected higher up in the axial lithosphere, forming transient melt bodies in colder host rocks and triggering high temperature, black smoker, hydrothermal systems.

Here we report on numerical models that explore the thermal effects of varying both the melt flux and the depth of magma emplacement, a parameter that previously published mid-ocean ridge thermal models did not take into account. Our models do predict the large variability in thermal regime that is documented at slow ridges, from cold detachment-dominated settings, to hotter melt-rich segment centers. We discuss these results and the strengths and limitations of the modelling approach. We also explore the potential effects of varying the melt flux and melt emplacement depth with time at a given slow spreading ridge location, on crustal construction processes and on the respective roles of faults and melt intrusions to accommodate plate divergence.  

How to cite: Cannat, M., Chen, J., and Olive, J. A.: The thermal regime of mid-ocean ridges: geological perspectives and numerical modelling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7720, https://doi.org/10.5194/egusphere-egu22-7720, 2022.

Lucky Strike volcano is the central edifice of the Lucky Strike segment, Mid-Atlantic Ridge. Its summit overlies an axial magma chamber (AMC), 3-3.8 km beneath the seafloor, and hosts one of the largest known deep-sea hydrothermal fields. Local seismicity beneath the hydrothermal field has been monitored since 2007 as a part of the EMSO (European Multidisciplinary Seafloor and water column Observatory)-Azores observatory by 5 OBSs with yearly redeployments. From the 2007-2019 earthquake catalog, the primary process for the seismicity observed beneath the volcano region is proposed to be thermal contraction at the base of the hydrothermal circulation. In this interpretation the most seismically active zones represent the domains of maximum heat extraction at the base of the hydrothermal system. Here we present the evolution of the hydrothermal system controlled by magmato-tectonic interactions in the frame of this interpretation.

First, we observe two shifts of the most seismically active zones from ~1.4km North-Northwest of the hydrothermal field as documented in 2007-2009 to ~0.7km to North of the field in between 2010-2013 and then Eastward for about ~0.6km from 2010-2013 to 2015-present. These shifts, of the order ~600-700 meters, occurring at time scales of a few years, might be driven by one or several of the following mechanisms: the relocation of the maximum heat extraction zone to a shallower region of the AMC after significant heat extraction, the relocation to a recent magmatic injection,  and/or a tectonically-driven change in the hydrothermal fluid pathways.

Second, we observe three main Higher Seismic Activity (HSA seismic rate > 18 events/week) periods: April-June 2009, August-September 2015 and April-May 2016. The 2009 HSA period lasted ~13 weeks and the events clustered just above the AMC, while the 2015 and 2016 HSA periods lasted ~4-5 weeks, with events forming a narrow, dike-shaped cluster between the AMC  and just few meters below seafloor. HSA periods are characterized by deeper events and the occurrence of a few higher magnitude events (ML > 1.0). In between HSA periods, the seismicity tends to align along the trace of an inward dipping fault that bounds the narrow axial graben to the west, at the top of the volcano. The HSA periods can thus be interpreted as periods of maximum heat extraction by the hydrothermal circulation, possibly obscuring the background fault-related seismicity that is detected in periods of lesser seismic activity.

How to cite: Bohidar, S., Crawford, W., and Cannat, M.: Seismic constraints on the hydrothermal circulation and magmato-tectonic interactions beneath Lucky Strike volcano, Mid-Atlantic Ridge, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8780, https://doi.org/10.5194/egusphere-egu22-8780, 2022.

Mantle melting at mid-ocean ridges is thought to strengthen residual mantle by extracting water (hydroxyl defects) thereby increasing its viscosity by over two orders of magnitude to create a “compositional” lithosphere. Although water is strongly partitioned into basaltic melt relative to olivine, mantle dehydration also requires that melt extraction be efficient. Otherwise, retained low-degree hydrous melts will rapidly reinfuse surrounding mantle with hydrogen on solidifying owing to its high diffusivity in mantle materials. The pattern of mantle melting at ridges varies strongly both within segments and with spreading rate. We examine these patterns along the northern Mid-Atlantic Ridge and the adjoining Reykjanes Ridge using mantle Bouguer anomalies (MBAs). The Reykjanes Ridge has a linear axis, no transform faults, and a continuous MBA low, indicating continuous axial mantle melting. In contrast the adjoining Mid-Atlantic Ridge is segmented with transform and non-transform discontinuities and has pronounced “bulls-eye” MBA lows indicating focused mantle melting beneath each segment. We hypothesize that the pattern of mantle melting explains the absence or occurrence of transform faults on these systems. Segmented mantle melting results in dry, depleted, and strong mantle beneath ridge segment interiors but at segment ends, low extents of melting and inefficient melt extraction preserve damp and weak mantle.  Since the rheological changes created by segmented melting develop rapidly near the ridge axis and extend from the Moho to the dry solidus depth, a pronounced rheological banding is formed in the mantle. The weak segment ends localize shear zones oriented in the spreading direction where transform faults may form whereas the ridges, flanked by strong compositional lithosphere, will be oriented orthogonally.  Our hypothesis also explains the variation of transform fault spacing with spreading rate or their absence. At ultra-slow ridges, overall melting is limited and irregular and melt extraction is inefficient so that no systematic rheological bands form and transform faults are not favored. At slow spreading rates, mantle melting forms three-dimensional diapiric instabilities at typical spacings of ~40-80 km so that transform faults also have this spacing. As spreading increases to fast rates mantle melting becomes two-dimensional and typical magmatic segment length and corresponding transform spacing increases to >100 km.  At ultra-fast ridges (>145 km/my) mantle melting is ubiquitous and melt extraction is everywhere efficient so that a systematic rheological banding does not form and transform faults are again not favored. Our model implies that beyond cooling and strengthening with age, the pattern of mantle melting shapes the rheological structure of oceanic lithosphere and the geometry of plate tectonics. Reference:  Martinez and Hey, 2022, https://doi.org/10.1016/j.epsl.2021.117351

How to cite: Martinez, F. and Hey, R.: Segmented mantle melting, lithospheric strength, and the origin of transform faults: Insights from the North Atlantic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10702, https://doi.org/10.5194/egusphere-egu22-10702, 2022.

EGU22-11888 | Presentations | TS1.2

An intrusive complex imaged within the roots of an oceanic core complex using 3D full-waveform inversion 

Michele Paulatto, Toby Oxford, and Rory Bardner

3D full waveform inversion (FWI) has been applied to the seismic refraction data of the MARINER (Mid-Atlantic Ridge INtegrated Experiment at Rainbow) experiment to create a robust high-resolution model of the seismic velocity structure of the Rainbow massif. The Rainbow massif is an oceanic core complex located on a non-transform discontinuity (NTD) in a magma-starved region of the mid-Atlantic Ridge. Despite the low magmatic input, the core complex hosts a high-temperature hydrothermal vent field  (>340°C) that requires a long-lived magmatic heat source. The FWI results show that deep within the massif, ∼3-8 km below the seafloor, is a low-velocity body that represents a partially molten sill complex with >20% gabbro intrusions. The complex extends out north to the AMAR Minor N segment suggesting an increased magmatic input into this segment, forcing the NTD to migrate southwards. Extensive magmatic intrusion into the core complex was likely responsible for the termination of slip on the detachment fault. Above the sill complex, we image a channel of lower velocity material that cuts through the main hydration front to the deep sill region. Velocity values and micro-seismicity correlation suggests that this channel consists of 10-30% serpentinized peridotite and fracturing from serpentinization reactions create fluid pathways for fluids to exchange between the deeper partially molten heat source and the fluid network of the hydrothermal vents. A high-velocity chimney below the extinct vent sites of the massif may represent the abandoned stockwork of these extinct hydrothermal systems.

How to cite: Paulatto, M., Oxford, T., and Bardner, R.: An intrusive complex imaged within the roots of an oceanic core complex using 3D full-waveform inversion, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11888, https://doi.org/10.5194/egusphere-egu22-11888, 2022.

EGU22-12802 | Presentations | TS1.2

Dependencies of morphology and lithological variations on tectonics, thermal structure and ocean loading at ultra-slow and slow ridges. 

Leila Mezri, Javier Gracià-Pintado, Marta Pérez-Gussinyé, Zhonglan Liu, and Bach Wolfgang

Why do some mid-ocean ridges have morphology that expresses oceanic core complexes with large gabbro bodies, while others have smooth seafloors with large exposures of serpentinized mantle or rough seafloors, while lavas are observed almost everywhere?

Over the past few decades, numerical models have inferred that the fundamental mechanism controlling the wide diversity of lithology, crustal thickness, and ridge morphology is the balance between magmatism and tectonics. Key controls on this modeled equilibrium are the melt supply rate, which varies to account for the discontinuous volcanism observed on slow ridges, and the thermal structure, which depends on the balance between heat injected during magmatic accretion and heat removed by hydrothermal cooling, modulated by the spreading rate.

Based on this paradigm, it has been established that the fraction of melt that is accreted into the crust controls the formation of large oceanic core-complexes and flip-flop detachments, with the former being formed at fractions corresponding of half of spreading rate and the latter being formed when the melt supply is much smaller.

However, several fundamental questions remain poorly understood or unanswered. Why can slip on oceanic detachment faults continue and why does it stop? How do serpentinization and magmatic intrusions play a role in crustal growth and how do they interact? How and why do mechanisms related to magma supply switch from magmatic to detachment dominated mode during oceanic accretion?

Here we present self-consistent numerical simulations of the development of mid-ocean ridges, starting  from continental rifting and breakup. In our models melt supply varies dynamically with extension velocity and is affected by faulting. We focus on understanding how tectonism, melting, serpentinisation and hydrothermal cooling interact to form smooth-seafloor, core complexes and normal igneous seafloor, and their diverse crustal lithology.



How to cite: Mezri, L., Gracià-Pintado, J., Pérez-Gussinyé, M., Liu, Z., and Wolfgang, B.: Dependencies of morphology and lithological variations on tectonics, thermal structure and ocean loading at ultra-slow and slow ridges., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12802, https://doi.org/10.5194/egusphere-egu22-12802, 2022.

EGU22-13027 | Presentations | TS1.2

How Continents Break-up and New Ocean Ridges are Established 

Marta Pérez-Gussinyé, Javier García-Pintado, Zhonglan Liu, and Leila Mezri

The processes that lead to the transition from continental extension and break-up to steady-state mid-ocean ridge formation are not well understood. Particular unknowns are the paleo-water depths at which the continental lithosphere breaks up, the nature of the crust at the so-called continent-ocean transition and when and how a steady-state mid-ocean ridge is established. To understand these questions we use numerical models that couple tectonic deformation, sedimentation, hydrothermal cooling, serpentinisation and melting, as a virtual laboratory. We present results of models run with different velocities that simulate natural examples observed in nature such as the South China sea and the West Iberia-Newfoundland margins. We focus on the evolution of subsidence, heat-flow and nature of the basement as the rift transforms into a steady-state mid-ocean ridge and show how the interplay between tectonics and hydrothermal cooling lead to the different configurations observed in nature.

 

How to cite: Pérez-Gussinyé, M., García-Pintado, J., Liu, Z., and Mezri, L.: How Continents Break-up and New Ocean Ridges are Established, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13027, https://doi.org/10.5194/egusphere-egu22-13027, 2022.

EGU22-13304 | Presentations | TS1.2 | Marie Tharp Medal Lecture

Analogue modelling of subduction: yesterday, today and tomorrow. 

Francesca Funiciello

The use of experimental tectonics (also known as analogue-, laboratory, or physical modelling) to study tectonic processes is not a novelty in Earth Science. Following Sir James Hall’s pioneer work (1815), many modellers squeezed, stretched, pushed and pulled a wide range of materials – e.g., sand, clay, oil, painters’ putties, gelatins, wax, paraffin, syrups, polymers – to unravel a wide range of tectonic processes to determine parameters controlling their geometry, kinematics and dynamics. However, only recently experimental analogue modelling has definitively transformed from a qualitative to a quantitative technique, thanks to appropriate scaling relationships, the improvement in the knowledge of the rheology of both natural and analogue materials and the use of high-resolution monitoring techniques to quantify morphology, kinematics, stress, strain and temperature.

Here, I specifically review the experimental work performed to study one of the most intriguing aspects of plate tectonics: the subduction process. Subduction provides the dominant engine for plate tectonics and mantle dynamics. Moreover, it has also societal importance playing a key role on hazard at short (i.e., earthquakes and mega-earthquakes, tsunami, effusive and explosive volcanic activities with impact on aviation safety) and long time scales (i.e., local and global climate change). Over the last decades, a noteworthy advance in the quality and density of global geological, geophysical and experimental data has allowed us to provide systematic quantitative analyses of global subduction zones and to speculate on their behaviour. These constraints have been integrated into a mechanical framework through modelling.

I will bring you to a journey through the past, the present and the future of analogue modelling and related efforts, results and perspectives for the study of the subduction process. It will be shown how analogue models, with their inherent 3D character and behaviour driven by simple and natural physical laws, contribute to successfully unravelling the subduction process, inspiring new ideas. Challenging ongoing perspectives of analogue models imply the possibility to compare time and space scales, allowing to merge, within the same model, both short- and long-term and shallow and deep processes.

How to cite: Funiciello, F.: Analogue modelling of subduction: yesterday, today and tomorrow., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13304, https://doi.org/10.5194/egusphere-egu22-13304, 2022.

The Dayi seismic gap of the Longmenshan thrust belt is located between the ruptures of the 2008 Wenchuan Earthquake and the 2013 Lushan Earthquake, with a length of about 40 ~ 60 km. So far, it has been still a heated debate on whether the Dayi seismic gap has the hazard of strong earthquakes in the near future. The occurrence of a strong earthquake in the seismic gap is closely related to the existence of high stress accumulation and the most direct method is to measure the borehole stress in the field. In order to find out the present stress state, in-situ stress measurements were carried out at the hanging wall and footwall of Dachuan-Shuangshi fault zone in Dachuan Town. The results showed that the hanging wall and footwall of Dachuan-Shuangshi fault zone in Dayi seismic gap are in a high-stress state. Based on seismicity parameter b-value, crustal velocity structure, GPS deformation monitoring data and temperature data, etc., it can be learned that there is a positive correlation coupling relationship between near surface shallow stress and deep stress. In this paper, a response model of shallow stress to deep locking was established. It was speculated that Dayi seismic gap has the potential hazard of strong earthquakes. This research result not only deepens the understanding of the relationship between stress and earthquake preparation, but also provides an effective scientific method for identifying seismic hazards in other active fault seismic gaps.

How to cite: li, B., Huang, J., and Xie, F.: In situ stress state and earthquake hazard assessment in Dayi seismic gap of the Longmenshan thrust belt, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-648, https://doi.org/10.5194/egusphere-egu22-648, 2022.

EGU22-799 | Presentations | TS1.4

Machine Learning and Underground Geomechanics – data needs, algorithm development, uncertainty, and engineering verification 

Josephine Morgenroth, Usman T. Khan, and Matthew A. Perras

Machine learning algorithms (MLAs) are emerging as a powerful tool for forecasting complex and nuanced rock mass behaviour, particularly when large, multivariate datasets are available. In engineering practice, it is often difficult for geomechanical professionals to investigate all available data in detail, and simplifications are necessary to streamline the engineering design process. An MLA is capable of processing large volumes of data quickly and may uncover relationships that are not immediately evident when manually processing data. This research compares two algorithms developed for two mines representing end member behaviours of rock failure mechanisms: squeezing ground with high radial convergence, and spalling ground with high in situ stresses and seismicity. For the squeezing ground case study, a Convolutional Neural Network is used to forecast the yield of the tunnel liner elements using tunnel mapping images as the input. For the high stress case study, a Long Short Term Memory network is used to forecast the in-situ stresses that takes time series microseismic events and geomechanical properties as inputs. The two case studies are used to compare input data requirements and pre-processing techniques. Ensemble modelling techniques used to quantify MLA uncertainty for both case studies are presented. The development of the two MLAs is discussed in terms of their complexity, generalizability, performance evaluation, verification, and practical applications to underground rock engineering. Finally, best practices for MLA development are proposed based on the two case studies to ensure model interpretability and use in engineering applications.

How to cite: Morgenroth, J., Khan, U. T., and Perras, M. A.: Machine Learning and Underground Geomechanics – data needs, algorithm development, uncertainty, and engineering verification, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-799, https://doi.org/10.5194/egusphere-egu22-799, 2022.

EGU22-2742 | Presentations | TS1.4 | Highlight

Assessing the effect of mass withdrawal from a surface quarry on the Mw4.9 Le Teil (France) earthquake triggering 

Julie Maury, Théophile Guillon, Hideo Aochi, Behrooz Bazargan, and André Burnol

On November 11th 2019, the Le Teil, France earthquake occurred in the vicinity of a quarry. Immediately, the question was raised about the potential triggering of this earthquake by the quarry. However, another potential triggering source is a hydraulic effect related to heavy rainfall (Burnol et al, 2021). That’s why it is important to quantify precisely the mechanical effect of mass withdrawal. Results from different studies (Ampuero et al, technical report CNRS, 2019; De Novellis et al, Comm. Earth Env., 2021) agrees to a Coulomb stress variation of 0.15 to 0.2 MPa. However, these studies are based on Boussinesq solution supposing a homogeneous half-space that maximize the effect of the quarry. Here we used the distinct element method code 3DEC @Itasca in 3D to take advantage of an improved geological model and assess the impact of discontinuities as well as lithology. Our results show the maximum Coulomb stress change of 0.27 MPa at 1.4 km depth, a value of the same order as what is obtained with Boussinesq solution. A comparison between the location of the earthquake (Delouis et al, 2021) and the maximum Coulomb stress is realized. The maximum value is located at the intersection of the Rouviere fault with another local fault highlighting the interaction between these structures. However, the in situ stress field is not well-known, fault parameters are difficult to assess and there is some uncertainty on the volume of extracted material in the 19th century estimated by the quarry owner. Additionally, the presence of marl in the Hauterivian layer suggests it could have an elasto-plastic behavior. A parametric study has been realized to assess the effect on Coulomb stress change of these uncertainties taking plausible values for each parameter. We show that the uncertainty associated with our calculations affect the results within a range of less than 10%.

How to cite: Maury, J., Guillon, T., Aochi, H., Bazargan, B., and Burnol, A.: Assessing the effect of mass withdrawal from a surface quarry on the Mw4.9 Le Teil (France) earthquake triggering, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2742, https://doi.org/10.5194/egusphere-egu22-2742, 2022.

EGU22-3272 | Presentations | TS1.4

The stress memory in rocks: insight from the deformation rate analysis (DRA) and acoustic emission (AE) 

Zulfiqar Ali, Murat Karakus, Giang D. Nguyen, and Khalid Amrouch

Deformation rate analysis (DRA) and Acoustic Emission (AE) are popular methods of in-situ stress measurements from oriented cored rocks which take advantage of the rock stress memory also known as the Kaiser effect. These methods rely on the accurate measurement of a point of inflection in the characteristic DRA and AE curves, however, due to the complex geological stress history in rocks, locating point of inflection can be problematic. In order to better understand the stress memory experiments were performed on a combination of six different types of soft, and hard crystalline rocks including concrete with no stress history. The effect of loading modes, strain rates, and time delay were studied on preloaded rock specimens to investigate their influence on the stress memory. A fading effect was observed when the number of the cycles in the test were increased which led to the development of a new method of quantifying the preloads. Results show that the type of loading and the loading rate has little to no influence on the Kaiser effect, however, under faster loading rates the Kaiser effect is more distinct. Likewise, no time dependency was observed for time delays up to seven months.

How to cite: Ali, Z., Karakus, M., Nguyen, G. D., and Amrouch, K.: The stress memory in rocks: insight from the deformation rate analysis (DRA) and acoustic emission (AE), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3272, https://doi.org/10.5194/egusphere-egu22-3272, 2022.

A review of works is presented in which new models of continuum mechanics generalizing the classical theories of elasticity are being intensively developed. These models are used to describe composite and statistically inhomogeneous media, new structural materials, as well as complexly constructed massifs in mine and ground conditions; and in the study of phenomena occurring in permafrost under the influence of heating processes. A characteristic feature of the theory of media with a hierarchical structure is the presence of explicit or implicit scale parameters, i.e. explicit or implicit non-locality of the theory. This work focuses on the study of the non-locality effects and internal degrees of freedom reflected in internal stresses that are not described by the classical theory of elasticity, but can be potential precursors of the development of a catastrophic process in a rock mass.

How to cite: Hachay, O. and Khachay, A.: Geophysical research and monitorind within a block-layered model with inclusions of hierrchical structure, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4574, https://doi.org/10.5194/egusphere-egu22-4574, 2022.

EGU22-4738 | Presentations | TS1.4

Modeling principal stress orientations in the Arabian plate using plate velocities 

Santiago Pena Clavijo, Thomas Finkbeiner, and Abdulkader M. Afifi

The Arabian Peninsula is part of a small tectonic plate that is characterized by active and appreciable deformations along its boundaries. Knowledge of the present-day in situ stress field in the Arabian plate and its variability is critical for earth science disciplines that require an understanding of geodynamic processes. In addition, it is essential for a range of practical applications that include the production of hydrocarbons and geothermal energy, mine safety, seismic hazard assessment, underground storage of CO2, and more.

This project aims at modeling the stress orientation field in the Arabian Plate using advanced computational tools together with a plate velocity model. We built a three-layer 3D model of the Arabian crust using digital elevation, basement depth, and Moho depth maps. Based on these data, we built a 3D unstructured finite element mesh for the whole Arabian plate, including the offshore area, with finer resolution at critical locations. The latter is a novel approach to this work.  To capture the deformation caused by the water bodies in the Red Sea, Gulf of Aden, and the Arabian Sea areas, we set a hydrostatic boundary condition as a function of bathymetry. Along the Zagros fold and thrust belt, we pinned the plate boundary to capture continental collision. Finally, the partial differential equation of force equilibrium (a linear static analysis) is solved using plate displacements (inferred from plate velocities) as boundary conditions for several displacement conditions.

The modeling results suggest NE-SW SHmax azimuths in northeastern Saudi Arabia and Kuwait while the Dead Sea transform areas show NW-SE to NNW-SSE azimuths, and the rest of the plate is characterized by predominant N-S SHmax azimuth. Due to pinned boundary conditions at the Zagros Mountains, SHmax azimuth changes from N-S at the Red Sea basin to NE-SW at the Zagros fold and thrust belt. We also notice significant stress concentrations in the transition from the Arabian shield to the sedimentary basins in the Eastern parts of the plate. This is in response to associated changes in rock properties. Hence, the simulated stress orientations corroborate the ongoing tectonic process and deepen our understanding of regional and local in situ stress variation drivers as well as the current elastic deformation in the Arabian plate.

How to cite: Pena Clavijo, S., Finkbeiner, T., and Afifi, A. M.: Modeling principal stress orientations in the Arabian plate using plate velocities, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4738, https://doi.org/10.5194/egusphere-egu22-4738, 2022.

EGU22-5494 | Presentations | TS1.4 | Highlight

Stress characterization in the Canadian Shield: Complexity in stress rotation 

Wenjing Wang and Douglas Schmitt

NE-SW stress compression in the Western Canadian Sedimentary Basin was discovered in the pioneering borehole breakout observations of Bell and Gough (1979). However, all of these and subsequent stress direction indicators are from the Phanerozoic sediment veneer, while the state of stress in the underlying craton remains unexplored. With the emergent demands on geothermal energy and wastewater and CO2 disposal, however, the state of stress in the cratons can no longer be safely ignored. To address this problem, we analyze various vintages of geophysical logs obtained from a serendipitous wellbore-of-opportunity drilled to 2.4 km in NE Alberta.  The profile of breakout orientations inferred from image and caliper logs exhibits a distinct rotation in breakout orientations changing from N100°E at 1650-2000m to N173°E at 2000-2210m and, finally, to N145°E at the bottom from 2210-2315m. The deepest measurement is consistent with the many observations in the overlying sediments. The heterogeneous breakout orientations at different depth intervals possibly indicate a heterogeneous in-situ stress field in the Precambrian craton. In addition, however, there is a strong correlation between the metamorphic textures and the breakout orientations suggesting that anisotropic strength may play an important role.  Using a recently developed algorithm we show that these observations can indeed be explained by foliation-controlled failure patterns in such anisotropic metamorphic rocks (Wang & Schmitt, accepted).  Models demonstrate that the observed breakout rotations can be produced under uniform stress orientations with failure slip planes controlled by the textured metamorphic rocks with anisotropic strength. This modeled stress field indicates that the stress field in the Canadian Shield where the far-field SH azimuth is at N50°E and the region is under normal/strike-slip faulting regime, is coupled with that in the overlying sedimentary basin.

How to cite: Wang, W. and Schmitt, D.: Stress characterization in the Canadian Shield: Complexity in stress rotation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5494, https://doi.org/10.5194/egusphere-egu22-5494, 2022.

EGU22-6453 | Presentations | TS1.4

Can we afford fracture pressure uncertainty? Limit tests as a key calibration for geomechanical models 

Michał Kępiński, David Wiprut, and Pramit Basu

The Leak-Off Test (LOT) is one of the most common fracture pressure/Shmin calibration measurements conducted in wellbores. Well engineers rely on readings from LOTs to design safe drilling plans. The LOT results indicate the maximum mud weight or equivalent circulating density that can be used to drill the next hole section without causing fluid losses to the formation. Losses are one of the most expensive issues to mitigate in drilling operations. In more severe cases, losses may lead to subsequent drilling challenges such as hole collapse or kicks. Oftentimes, drillers choose not to pressurize the well up to the leak-off pressure due to the risk of weakening the rock beneath the casing shoe by creating a fracture. In these cases, a formation integrity test (FIT) is conducted. However, the FIT is inadequate for properly constraining the fracture gradient or for input to geomechanical models because it is possible for the FIT to terminate at pressures that are either above or below the far-field minimum stress.

Geomechanical modelling from several projects in Poland shows that insufficient LOT measurements introduce a wide range of fracture gradient uncertainty, complicating the analysis of optimal ECD values in narrow margin drilling sections. This leads to difficulty in determining the proper mud weight when a loss event occurs. Additionally, without reliable calibration of the minimum horizontal stress, the geomechanical model used to determine the lower bound of the mud window becomes more uncertain. An inadequately constrained mud window can result in further drilling complications such as tight hole, stuck pipe, poor hole condition, and compromised log quality.

How to cite: Kępiński, M., Wiprut, D., and Basu, P.: Can we afford fracture pressure uncertainty? Limit tests as a key calibration for geomechanical models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6453, https://doi.org/10.5194/egusphere-egu22-6453, 2022.

EGU22-8802 | Presentations | TS1.4

Stress state and patterns at the upper plate of Hikurangi Subduction Margin 

Effat Behboudi, David McNamara, and Ivan Lokmer

Quantifying the contemporary stress state of the Earth’s crust is critical for developing a geomechanical understanding of the behavior of brittle deformation (fractures and faults).In this study we characterize the shallow contemporary stress state of the active Hikurangi Subduction Margin (HSM), New Zealand, to better understand how it affects and responds to variable deformation and slip behavior documented along this plate boundary. The HSM is characterized by along-strike variations in megathrust slip behavior, ranging from shallow slow slip events (SSEs) and creep at the northern and central HSM to interseismic locking and stress accumulation in the southern HSM. We estimate the state of stress across the HSM utilizing rock strength estimates from empirical relationships, leak-off test data, wireline logs and borehole geology, and measurement of borehole wall failures such as borehole breakouts and drilling‐induced tensile fractures from eight boreholes. Stress magnitude constraints at depth intervals where BOs are observed indicate that the maximum principal stress (σ1) is horizontal along the shallow (<3 km) HSM and the stress state is predominantly strike-slip or contractional (barring localized areas where an extensional stress state is determined). Our results reveal a NE-SW (margin-parallel) SHmax orientation in the shallow central HSM, which rotates to a WNW- ESE/NW-SE (margin-perpendicular) SHmax orientation in the shallow southern HSM. The central NE-SW SHmax orientation is inconsistent with active, km-scale, NE-SW striking contractional faults observed across the central HSM. Considering both stress magnitude and orientation patterns at the central HSM, we suggest that long-term clockwise rotation of the Hikurangi forearc, over time, may transform motion on these km-scale central HSM faults from contractional dip-slip to a more contemporary strike/oblique-slip. The southern shallow WNW- ESE/NW-SE SHmax orientation is nearly perpendicular to focal-mechanisms derived NE-SW SHmax orientations within the subducting slab. This, combined with observed strike-slip and contractional faulting in the region and the NW-SE convergence direction, implies the overriding plate in the southern HSM is in a contractional stress state, potentially as deep as the plate interface, which is decoupled from that experienced in the subducting slab. Observed localized extensional stress states across the HSM may occur as a result of local extensions or reflect uncertainties in our estimations of SHmax magnitude which are sensitive to the UCS values used (unconstrained by laboratory testing). This UCS uncertainty and the potential errors it can introduce into a stress model highlights the importance of developing robust empirical relationships for UCS in regions where stress is a critical geological consideration for hazard and resource management.

How to cite: Behboudi, E., McNamara, D., and Lokmer, I.: Stress state and patterns at the upper plate of Hikurangi Subduction Margin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8802, https://doi.org/10.5194/egusphere-egu22-8802, 2022.

A series of numerical simulations of mantle convection in 3D spherical-shell geometry were performed to evaluate the intraplate stress regime from numerically obtained velocity and stress fields. The intraplate stress regime was quantitatively classified into nine types by analyzing the principal deviatoric stress axes and the “stress ratio,” which is a continuous parameter accounting for the stress regimes. From the viewpoint of global geodynamics, this study analyzed the depth profile of the stress ratio across the entire depth of the mantle. The results demonstrated that the radial viscosity structure of the mantle interior strongly affected intraplate stress regimes, and the combination of increased viscosity in the lower mantle and the low-viscosity asthenosphere enhanced the pure strike-slip faulting regime within moving plates as indicated using visco-plastic rheology. The temporally averaged toroidal-poloidal ratio (T/P ratio) at the top surface of mantle convection with surface plate-like motion and the mantle’s viscosity stratification may be comparable to the observed T/P ratio of present-day and past Earth. The normal faulting (or strike-slip) regime with a strike-slip (or normal faulting) component, as well as the pure strike-slip faulting regime, were broadly found in the stable parts of the plate interiors. However, the significant dominance of these stress regimes was not observed in the depth profile of the toroidal-poloidal ratio as a remarkable peak magnitude near the top surface of the lithosphere. This result implies that the strike-slip component analyzed in this study does not directly relate to the formation of strike-slip faults that are infinitely narrow plate boundaries compared with the finite low-viscosity boundary obtained from a mantle convection model with visco-plastic rheology. Nonetheless, this first analysis of the stress ratio may contribute to an improved understanding of the intraplate stress reproduced by future numerical studies of mantle convection with further realistic conditions.

How to cite: Yoshida, M.: New analyses of the stress ratio and stress regime in the Earth’s lithosphere from numerical simulation models of global mantle convection, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9132, https://doi.org/10.5194/egusphere-egu22-9132, 2022.

EGU22-11827 | Presentations | TS1.4

A kinematic model for observed surface subsidence above a salt cavern gas storage site in Northern Germany 

Henriette Sudhaus, Alison Larissa Seidel, and Noemi Schulze-Glanert

In nation-wide radar satellite time series data of Germany, a linear subsidence motion of several kilometer spatial wavelength shows up south-east of Kiel, northern Germany. The center region of this signal, showing line-of-sight displacement velocities of about 2 mm/a, coincides with the facilities of a gas storage site managing two in-service and one out-of-service caverns in the salt dome beneath. The three caverns have been water-drilled only a few hundred meters apart in 1971, 1996 and 2014 into a large halite salt dome, which has risen up there to depths of around 1000 m. Their sizes range within a couple of 100.000 m³. Above the salt body thick deposits of mainly chalk, silk and claystone below layers of clays, silts, sands and glacial marls in the upper 200 m form a relatively strong roof layer.

We hypothesize that despite a thick and competent cover layer, the long-term ductile behavior of halite, which evidently causes shrinking of the cavern volumes through time, results in the observed continuous surface subsidence across several square kilometers. We present an attempt to test the hypothesis by optimizing a simple kinematic model to fit the surface subsidence signal. Using equivalent body forces to represent an isotropic volume point source embedded in a viscoelastic host medium below a horizontally layered elastic roof medium, we estimate the horizontal position of a single cavern, its depth and the corresponding volume change at the cavern. The medium properties at the cavern sites are well known from borehole geophysical analyses, but likely vary strongly laterally. We use InSAR time series data from two ascending look directions and two descending.

Our results show that a cavern at about 1200 m depth and in very close proximity to above-ground facilities of the storage site can indeed be associated with the observed ground motion. The best-fit models pin the location to the known positions, also in depth. The estimated volume loss is slightly larger than 20.000 m³ per year and is in the same order of volume loss estimated from volume measurements inside the actual caverns.

The model approach we present, a single kinematic point source for three caverns and a one-dimensional medium model, is simple, the signal-to-noise ratio of the satellite data is rather small and furthermore there are considerable spatial gaps in the InSAR time series data in areas of agriculture and forests. However, with a computationally fast forward calculation of surface displacements we can afford to propagate data error statistics that account for spatially correlated errors to model parameter uncertainty estimates in a Bayesian way through model ensembles. We plan to add modeling errors of the medium to better grasp their potential influence on the volume loss estimations. The optimization code we use, Grond, is part of the seismological open-source software toolbox Pyrocko (pyrocko.org). The data is openly available at bodenbewegungsdienst.bgr.de.

How to cite: Sudhaus, H., Seidel, A. L., and Schulze-Glanert, N.: A kinematic model for observed surface subsidence above a salt cavern gas storage site in Northern Germany, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11827, https://doi.org/10.5194/egusphere-egu22-11827, 2022.

EGU22-11830 | Presentations | TS1.4

Geomechanical explanation of the Enguri power tunnel leakage 

Thomas Niederhuber, Birgit Müller, Thomas Röckel, Mirian Kalabegishvili, Frank Schilling, and Bernd Aberle

The Enguri Dam (Georgia) is one of the highest arch dams in the world, located at Enguri river in the Greater Caucasus. A 15 km long pressure tunnel with a slope of 1.1 % connects the reservoir to the power station. The tunnel was initially flooded in 1978 and takes a flow rate of up to 450 m³/s. Annual water level changes in the reservoir reach 100 m and generate variable internal water pressure, which places a considerable and dynamic strain on the structure. Water losses of more than 10 m³/s required extensive rehabilitation work in 2021.

The pressure tunnel is lined by upper and lower concrete parts separated by longitudinal construction joints. During the rehabilitation in spring 2021, an approximately 40 m long section of a construction joint with a gaping fissure and several smaller cracks were located.

To explain why only one of the construction joints was leaking, we combined field observations with numerical modelling of the stress state around the pressure tunnel. To infer the regional tectonic stress field various stress indicators have been used like borehole observations (borehole televiewer data) in the field, hydraulic fracturing and earthquake focal mechanisms. These different methods provide mean values with standard deviations. This enabled the estimation of uncertainties in the model input data (field data).

Our approach is based on a static linear-elastic 2D model of the tunnel at km 13.7 within a limestone of homogeneous material properties. The orientation of the profile section is parallel to the regional maximum horizontal stress (SH), which corresponds to maximum principal stress in a thrust faulting regime. SV is the vertical stress. To account for uncertainties, the model was calculated for different stress state scenarios e.g. variation of SH/SV-ratio from 2 to 6 and internal pressure from 0 to 1.6 MPa.

The results show a symmetrical distribution of tensile and compressive stresses around the tunnel, with the axis of symmetry tilted by ca. 30° clockwise (in flow direction) for all scenarios. This is due to the high topography. Therefore, in some calculations, tangential tensile stresses are observed on the downslope side in the region of the construction joint, while compressive stresses are expected for the upslope construction joint.

Therefore, it can be concluded:

(A) the initial stress state is an important parameter for the positioning of underground installation like pressure tunnels especially in areas of high topography.

(B) geomechanical numerical modelling can help to design and dimension safe constructions.

These kinds of investigations can help to omit leakage which can lead to a reduction of the capacity of the power plant and to prolongate the integrity of the tunnel statics. Further investigations could consider the hydraulic situation of the karst rock in the surrounding of the tunnel.

How to cite: Niederhuber, T., Müller, B., Röckel, T., Kalabegishvili, M., Schilling, F., and Aberle, B.: Geomechanical explanation of the Enguri power tunnel leakage, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11830, https://doi.org/10.5194/egusphere-egu22-11830, 2022.

EGU22-11879 | Presentations | TS1.4

Optimizing the use of InSAR observations in data assimilation problems to estimate reservoir compaction 

Samantha S.R. Kim, Femke C. Vossepoel, Marius C. Wouters, Rob Govers, Wietske S. Brouwer, and Ramon F. Hanssen

Hydrocarbon production may cause subsidence as a result of the pressure reduction in the gas-producing layer and reservoir compaction. To analyze the process of subsidence and estimate reservoir parameters, we use a particle method to assimilate Interferometric synthetic-aperture radar (InSAR) observations of surface deformation with a conceptual model of reservoir. As example, we use an analytical model of the Groningen gas reservoir based on a geometry representing the compartmentalized structure of the subsurface at the reservoir depth.

The efficacy of the particle method becomes less when the degree of freedom is large compared to the ensemble size. This degree of freedom, in turn, varies because of spatial correlation in the observed field. The resolution of the InSAR data and the number of observations affect the performance of the particle method.

In this study, we quantify the information in a Sentinel-1 SAR dataset using the concept of Shannon entropy from information theory. We investigate how to best capture the level of detail in model resolved by the InSAR data while maximizing their information content for a data assimilation use. We show that incorrect representation of the existing correlations leads to weight collapse when the number of observation increases, unless the ensemble size growths. However, simulations of mutual information show that we could optimize data reduction by choosing an adequate mesh given the spatial correlation in the observed subsidence. Our approach provides a means to achieve a better information use from available InSAR data reducing weight collapse without additional computational cost.

How to cite: Kim, S. S. R., Vossepoel, F. C., Wouters, M. C., Govers, R., Brouwer, W. S., and Hanssen, R. F.: Optimizing the use of InSAR observations in data assimilation problems to estimate reservoir compaction, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11879, https://doi.org/10.5194/egusphere-egu22-11879, 2022.

EGU22-640 | Presentations | SSP1.2

DeepStor-1 exploration well at KIT Campus North (Upper Rhine Graben, Germany) 

Schill Eva, Florian Bauer, Ulrich Steiner, Bernd Frieg, and Thomas Kohl

DeepStor-1 is the exploration well to the Helmholtz research infrastructure "DeepStor". DeepStor focuses on the investigation of high-temperature heat storage at the rim of the fromer oil-field „Leopoldshafen“. It is located about 10 km north of the city of Karlsruhe (Germany). The DeepStor-1 well is planned to reach the Pechelbronn group at 1‘460 m, i.e. it includes nearly the entire Oligocene sediments at the site. Seismic investigation reveal a structurally undisturbed section that below 200 m depth covers the Landau, Bruchsal, Niederrödern and Froidefontaine Formations. Cores will be taken from the entire section below 820 m. In addition to coring, the logging program is planned to include besides technical logging, a caliper-, self-potential-, temperature-, dual latero-, natural gamma spectrometry-, neutron-gamma porosity-, sonic-, elemental capture spectroscopy-, as well as image-logs in the sections 215-820 m as well as 820-1460 m. Drilling of DeepStor-1 is planned between 2022 and 2023.

How to cite: Eva, S., Bauer, F., Steiner, U., Frieg, B., and Kohl, T.: DeepStor-1 exploration well at KIT Campus North (Upper Rhine Graben, Germany), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-640, https://doi.org/10.5194/egusphere-egu22-640, 2022.

EGU22-1019 | Presentations | SSP1.2

Dating the serpentinite mud production of Fantangisña seamount using calcareous nannofossils and planktonic foraminifera biostratigraphy (IODP Expedition 366). 

Arianna Valentina Del Gaudio, Werner E. Piller, Gerald Auer, and Walter Kurz

The Izu-Bonin Mariana (IBM) convergent margin is located in the NW Pacific Ocean (12° N to 35° N) and represents, to the best of our knowledge, the only setting where recent episodes of serpentinite mud volcanism took place. The IBM arc-system started to form around 50-52 Ma when the Pacific Plate began to subside below the Philippine Plate and the eastern Eurasian Margin. On the Mariana forearc system, which constitutes the southward region of the IBM, a high number of large serpentinite mud volcanoes formed between the trench and the Mariana volcanic arc. Their origin is linked to episodic extrusion of serpentinite mud and fluids along with materials from the upper mantle, the Philippine plate, and the subducting Pacific plate to the sea floor, through a system of forearc faults. Among them, Fantangisña seamount was drilled during IODP Expedition 366. Cored material comprises serpentinite mud and ultramafic clasts that are underlain by nannofossil-rich forearc deposits and topped by pelagic sediments.

Integrated calcareous nannofossil and planktonic foraminifera biostratigraphy was performed on Sites U1497 and U1498, which are at the top of the serpentinite seamount and on its most stable southern flank, respectively. A total of nine bioevents were recorded in this study, permitting the establishment of a valid age-depth model for Site U1498A which allows for the definition of the latest phase of activity of Fantangisña serpentinite mud volcano. In particular, the emplacement of the mud production was detected between 6.10 (Late Miocene, Messinian) to 4.20 (Early Pliocene, Zanclean). This time interval is defined by nannofossil bioevents LO Reticulofenestra rotaria and FO of Discoaster asymmetricus. Furthermore, our analyses reveal that the latest stage of the serpentinite mud activity occurred 4 Ma later than the age proposed by a previous study (10.77 Ma) and is coeval with the initiation of the rifting in the Mariana Trough recorded at 7-6 Ma.

The age depth model also shows a rapid shift in sedimentation rates (11.80 to 94.71 m/Myr) during the Middle Pleistocene, which corresponds to a change in deposition of distinct serpentinite mud units, likely associated with the regional tectonic activity (different stages of seamount accretion and subduction and/or changes in the forearc extension related to the slab rollback).

How to cite: Del Gaudio, A. V., Piller, W. E., Auer, G., and Kurz, W.: Dating the serpentinite mud production of Fantangisña seamount using calcareous nannofossils and planktonic foraminifera biostratigraphy (IODP Expedition 366)., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1019, https://doi.org/10.5194/egusphere-egu22-1019, 2022.

EGU22-1277 | Presentations | SSP1.2 | Highlight

The Cenozoic Arctic Climate and Sea Ice History - Scientific objectives, challenges and implementation update of IODP Expedition 377 (ArcOP) 

Ruediger Stein, Kristen St.John, and Jeremy Everest

The Arctic is both a contributor to climate change and a region that is most affected by global warming. Despite this global importance, the Arctic Ocean is the last major region on Earth where the long-term climate history remains poorly known. Major advances in understanding were achieved in 2004 with the successful completion of IODP Expedition 302: Arctic Coring Expedition – ACEX – implemented by ECORD, marking the start of a new era in Arctic climate exploration. Although the ACEX results were unprecedented, key questions related to the Cenozoic Arctic climate history remain unanswered, largely due to a major mid-Cenozoic hiatus (or condensed interval) and partly to the poor recovery of the ACEX record. Building on ACEX and its cutting-edge science, IODP Expedition 377: Arctic Ocean Paleoceanography (ArcOP) has been scheduled for mid-August to mid-October 2022. The overall goal of ArcOP is the recovery of a complete stratigraphic sedimentary record on the southern Lomonosov Ridge to meet the highest priority paleoceanographic objective: the continuous long-term Cenozoic Arctic Ocean climate history with its transition from the early Cenozoic Greenhouse world to the late Cenozoic Icehouse world. Furthermore, sedimentation rates two to four times higher than those of ACEX will permit higher-resolution studies of Arctic climate change in the Neogene and Pleistocene. Key objectives are related to the reconstruction of the history of circum-Arctic ice-sheets, sea-ice cover, Siberian river discharge, and deep-water circulation and ventilation and its significance within the global climate system. Obtaining a geologic record of a 50-60 million year time span will provide opportunities to examine trends, pat­terns, rates, causes, and consequences of climate change that are important and relevant to our future. This goal can be achieved through (i) careful site selection, (ii) the use of appropriate drilling technology and ice management, and (iii) applying multi-proxy approaches to paleoceanographic, paleoclimatic, and age-model reconstructions.

In August 2022, a fleet of three ships, the drilling vessel “Dina Polaris” and the powerful icebreakers “Oden” and “Viktor Chernomyrdin”, will set sail for a location on Lomonosov Ridge in international waters far from shore (81°N, 140°E; 800-900 m of water depth). There, the expedition will complete one primary deep drill site (LR-11B) to 900 meters below seafloor (mbsf) which is twice that of the ACEX drill depth – certainly a challenging approach. Based on detailed site survey data, about 230 m of Plio‐Pleistocene, 460 m of Miocene, and >200 m of Oligocene‐Eocene sedimentary sequences might be recovered at this site. In addition, a short drill site (LR-10B) to 50 mbsf will be supplemented to recover an undisturbed uppermost (Quaternary) sedimentary section to ensure complete recovery for construction of a composite section spanning the full age range through the Cenozoic.

In this talk, background information, scientific objectives and an update of the status of planning and implementation of the ArcOP Expedition will be presented. For further details we refer to the ArcOP Scientific Prospectus (https://doi.org/10.14379/iodp.sp.377.2021).

How to cite: Stein, R., St.John, K., and Everest, J.: The Cenozoic Arctic Climate and Sea Ice History - Scientific objectives, challenges and implementation update of IODP Expedition 377 (ArcOP), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1277, https://doi.org/10.5194/egusphere-egu22-1277, 2022.

EGU22-1509 | Presentations | SSP1.2 | Highlight

A Campaign of Scientific Drilling for Monsoon Exploration in the Asian Marginal Seas 

Peter Clift, Christian Betzler, Steven Clemens, Beth Christensen, Gregor Eberli, Christian France-Lanord, Stephen Gallagher, Ann Holbourn, Wolfgang Kuhnt, Richard Murray, Yair Rosenthal, Ryuji Tada, and Shiming Wan

International Ocean Discovery Program (IODP) conducted a series of expeditions between 2014 and 2016 that were designed to address the development of monsoon climate systems in Asia and Australia. Significant progress was made in recovering Neogene sections spanning the region from the Arabian Sea to the Japan Sea and south to western Australia. High recovery by advanced piston core (APC) technology has provided a host of semi-continuous sections that have been used to examine monsoonal evolution. Use of half APC was successful in sampling sand-rich sediment in Indian Ocean submarine fans. The records show that humidity and seasonality developed diachronously across the region, although most regions show drying since the middle Miocene and especially since ~4 Ma, likely linked to global cooling. The transition from C3 to C4 vegetation often accompanied the drying, but may be more linked to global cooling. Western Australia, and possibly southern China diverge from the general trend in becoming wetter during the late Miocene, with the Australian monsoon being more affected by the Indonesian Throughflow, while the Asian Monsoon is tied more to the rising Himalaya in South Asia and to the Tibetan Plateau in East Asia. The monsoon shows sensitivity to orbital forcing, with many regions having a weaker summer monsoon during times of Northern Hemispheric Glaciation. Stronger monsoons are associated with faster continental erosion, but not weathering intensity, which either shows no trend or decreasing strength since the middle Miocene in Asia. Marine productivity proxies and terrestrial environmental proxies are often seen to diverge. Future work on the almost unknown Paleogene is highlighted, as well as the potential of carbonate platforms as archives of paleoceanographic conditions.

How to cite: Clift, P., Betzler, C., Clemens, S., Christensen, B., Eberli, G., France-Lanord, C., Gallagher, S., Holbourn, A., Kuhnt, W., Murray, R., Rosenthal, Y., Tada, R., and Wan, S.: A Campaign of Scientific Drilling for Monsoon Exploration in the Asian Marginal Seas, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1509, https://doi.org/10.5194/egusphere-egu22-1509, 2022.

EGU22-1679 | Presentations | SSP1.2

Direct evidence of high pore pressure at the toe of the Nankai accretionary prism 

Joshua Pwavodi and Mai-Linh Doan

The Nankai Trough is a locus of slow slip, low frequency earthquakes and Mw>8 classical earthquakes. It is assumed that high pore pressure contributes substantially to earthquake dynamics. Hence, a full understanding of the hydraulic regime of the Nankai accretionary prism is needed to understand this diversity of behaviors. We contribute to this understanding by innovatively integrating the drilling and logging data of the NanTroSEIZE project. We focus on the toe of the accretionary prism by studying data from Hole C0024A drilled and intersected the décollement at 813 mbsf about 3km away from the trench.

Down Hole Annular Pressure was monitored during drilling. We perform a careful quantitative reanalysis of its variation and show localized fluid exchange between the formation and the borehole (excess of 0.05m3/s), especially in the damage zones at the footwall of the décollement.

Pore pressure was estimated using Eaton’s method on both drilling and sonic velocity data. The formation fluids are getting significantly over-pressurized only a few hundred meters from the toe of the accretionary prism near the décollement with excess pore-pressure (P*≈0.04–4.79MPa) and lithostatic load (λ≈88-0.96 & λ*≈0.1-0.62 ) contributing to maximum 62% of the overburden stress.

The hydraulic profile suggests that the plate boundary acts as a barrier inhibiting upward fluid convection, as well as a lateral channel along the damage zone, favouring high pore pressure at the footwall. Such high pressure at the toe of the subsection zone makes high pressure probable further down in the locus of tremors and slow slip events.

How to cite: Pwavodi, J. and Doan, M.-L.: Direct evidence of high pore pressure at the toe of the Nankai accretionary prism, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1679, https://doi.org/10.5194/egusphere-egu22-1679, 2022.

EGU22-1729 | Presentations | SSP1.2

IODP Expedition 386 “Japan Trench Paleoseismology”: Mission Specific Platform Giant Piston Coring to track past megathrust earthquakes and their consequences in a deep-sea subduction trench. 

Michael Strasser, Ken Ikehara, Jeremy Everest, and Lena Maeda and the IODP Expedition 386 Science Party

International Ocean Discovery Program (IODP) Expedition 386, Japan Trench Paleoseismology (offshore period: 13 April to 1 June 2021; Onshore Science Party: 14 February to 14 March 2022) was designed to test the concept of submarine paleoseismology in the Japan Trench, the area where the last, and globally only one out of four instrumentally-recorded, giant (i.e. magnitude 9 class) earthquake occurred back in 2011. “Submarine paleoseismology” is a promising approach to investigate deposits from the deep sea, where earthquakes leave traces preserved in the stratigraphic succession, to reconstruct the long-term history of earthquakes and to deliver observational data that help to reduce uncertainties in seismic hazard assessment for long return periods. This expedition marks the first time, giant piston coring (GPC) was used in IODP, and also the first time, partner IODP implementing organizations cooperated in jointly implementing a mission-specific platform expedition.

We successfully collected 29 GPCs at 15 sites (1 to 3 holes each; total core recovery 831 meters), recovering 20 to 40-meter-long, continuous, upper Pleistocene to Holocene stratigraphic successions of 11 individual trench-fill basins along an axis-parallel transect from 36°N – 40.4°N, at water depth between 7445-8023 m below sea level. These offshore expedition achievements reveal the first high-temporal and high spatial resolution investigation and sampling of a hadal oceanic trench, that form the deepest and least explored environments on our planet.

The cores are currently being examined by multimethod applications to characterize and date hadal trench sediments and extreme event deposits, for which the detailed sedimentological, physical and (bio-)geochemical features, stratigraphic expressions and spatiotemporal distribution will be analyzed for proxy evidence of giant earthquakes and (bio-)geochemical cycling in deep sea sediments. Initial preliminary results presented in this EGU presentation reveal event-stratigraphic successions comprising several 10s of potentially giant-earthquake related event beds, revealing a fascinating record that will unravel the earthquake history of the different along-strike segments that is 10–100 times longer than currently available information. Post-Expedition research projects further analyzing these initial IODP data sets will (i) enable statistically robust assessment of the recurrence patterns of giant earthquakes, there while advancing our understanding of earthquake-induced geohazards along subduction zones and (ii) provide new constraints on sediment and carbon flux of event-triggered sediment mobilization to a deep-sea trench and its influence on the hadal environment.

 

How to cite: Strasser, M., Ikehara, K., Everest, J., and Maeda, L. and the IODP Expedition 386 Science Party: IODP Expedition 386 “Japan Trench Paleoseismology”: Mission Specific Platform Giant Piston Coring to track past megathrust earthquakes and their consequences in a deep-sea subduction trench., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1729, https://doi.org/10.5194/egusphere-egu22-1729, 2022.

EGU22-1917 | Presentations | SSP1.2

Operations and Initial Results from IODP Expedition 396: Mid-Norwegian Continental Margin Magmatism and Paleoclimate 

Sverre Planke, Christian Berndt, Ritske Huismans, Stefan Buenz, Carlos A. Alvarez Zarikian, and Expedition Scientists

The NE Atlantic conjugate volcanic rifted margins are characterized by extensive breakup-related magmatism recorded by basalt flows, volcanogenic sediments, magmatic underplates, and intrusive complexes in sedimentary basins and the crust. Onset of this voluminous magmatism is concomitant with the global hot-house climate in the Paleogene, and the injection of magma into organic-rich sedimentary basins is a proposed mechanism for triggering short-term global warming during the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma).

The aims of IODP Exp. 396 (August-September 2021) were to drill three transects on the mid-Norwegian continental margin to sample 1) hydrothermal vent complexes formed by eruption of hot fluids and sediments above sill intrusions (Modgunn Transect), 2) Paleogene sediments, with particular focus on the Paleocene-Eocene transition (Mimir Transect), and 3) basalt and sub-basalt sequences across the volcanic rifted margin and the initial oceanic crust (Basement Transect). A total of 21 boreholes were drilled, successfully coring all nine primary and one alternate sites. A comprehensive suite of wireline logs was collected in eight boreholes. Most of the sites were located on industry-standard 3D seismic reflection data, whereas additional high-resolution 2D and 3D P-Cable site survey data were acquired across six sites which were highly useful during the Mimir and Modgunn transect drilling. In total, more than 2000 m of core were recovered during 48 days of operations, including more than 350 m of basalt, 15 m of granite, and 900 m of late Paleocene to early Eocene sediments. Drilling was done using a combination of RCB, XCB, and APC drill bits, commonly with half-advances (c. 5 m) to optimize core recovery. Particularly high recovery (almost 100%) was obtained by half-length APC coring of Eocene sediments in two holes on the outer Vøring Margin, whereas basaltic basement recovery was above 60% in seven holes.

Expedition 396 probed the key elements of a typical volcanic rifted margin and the associated sedimentary archive. Of particular importance is the Modgunn Transect, where we drilled five holes through the upper part of a hydrothermal vent complex with a very expanded Paleocene-Eocene Thermal Maximum (PETM) interval dominated by biogenic ooze and volcanic ash deposits. The expedition also recovered an unprecedented suite of basalt cores across a volcanic rifted margin, including both subaerial and deep marine sheet flows with inter-lava sediments and spectacular shallow marine pillow basalts and hyaloclastites, as well as high-resolution interstitial water samples to assess sediment diagenesis and fluid migration in the region. Lastly, we recovered the first cores of sub-basalt granitic igneous rocks and upper Paleocene sediments along the mid-Norwegian continental margin. Collectively, this unique sample archive offers unprecedented insight on tectonomagmatic processes in the NE Atlantic, and links to rapid climate evolution across the Cenozoic.

How to cite: Planke, S., Berndt, C., Huismans, R., Buenz, S., Alvarez Zarikian, C. A., and Scientists, E.: Operations and Initial Results from IODP Expedition 396: Mid-Norwegian Continental Margin Magmatism and Paleoclimate, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1917, https://doi.org/10.5194/egusphere-egu22-1917, 2022.

EGU22-2525 | Presentations | SSP1.2

Biological sulfate reduction in deep subseafloor sediment of Guaymas Basin 

Toshiki Nagakura, Florian Schubert, and Jens Kallmeyer and the IODP Exp. 385 Scientists

Sulfate reduction is the quantitatively most important process to degrade organic matter in anoxic marine sediment and has been studied intensively in a variety of settings. Guaymas Basin, a young marginal ocean basin, offers the unique opportunity to study sulfate reduction in an environment characterized by organic-rich sediment, high sedimentation rates, and high geothermal gradients (100-958°C km-1). We measured sulfate reduction rates (SRR) in samples of the International Ocean Discovery Program (IODP) Expedition 385 using incubation experiments with radiolabeled 35SO42- carried out at in-situ pressure and temperature. Site U1548C, outside of a circular hydrothermal mound above a hot sill intrusion (Ringvent), has the highest geothermal gradient (958°C km-1) of all eight sampling sites. In near-surface sediment from this site, we measured the highest SRR (387 nmol cm-3 d-1) of all samples from this expedition. At Site U1548C SRR were generally over an order of magnitude higher than at similar depths at other sites. Site U1546D also had a sill intrusion, but it had already reached thermal equilibrium and SRR were in the same range as nearby Site U1545C, which is minimally affected by sills. The wide temperature range found in the stratigraphic section at each drill site leads to major shifts in microbial community composition with very different temperature optima. At the transition between the mesophilic and thermophilic range around 40 to 60°C, sulfate-reducing activity appears to be decreased, particularly in more oligotrophic settings but shows a slight recovery at higher temperatures.

How to cite: Nagakura, T., Schubert, F., and Kallmeyer, J. and the IODP Exp. 385 Scientists: Biological sulfate reduction in deep subseafloor sediment of Guaymas Basin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2525, https://doi.org/10.5194/egusphere-egu22-2525, 2022.

EGU22-2909 | Presentations | SSP1.2 | Highlight

Microbial survival through high metabolic rates in a deep and hot subseafloor environment 

Florian Schubert, Felix Beulig, Rishi Ram Adhikari, Clemens Glombitza, Verena Heuer, Kai-Uwe Hinrichs, Kira Homola, Fumio Inagaki, Bo Barker Jørgensen, Jens Kallmeyer, Sebastian Krause, Yuki Morono, Justine Sauvage, Arthur Spivack, and Tina Treude

A fourth of the global seabed sediment volume is buried at depths where temperatures exceed 80 °C, a previously proposed thermal barrier for life in the subsurface. Here, we demonstrate, utilizing an extensive suite of radiotracer experiments, the prevalence of active methanogenic and sulfate-reducing populations in deeply buried marine sediment from the Nankai Trough subduction zone, heated to extreme temperature (up to ~120 °C). Sediment cores were recovered during International Ocean Discovery Program (IODP) Expedition 370 to Nankai Trough, off the cost of Moroto, Japan. The steep geothermal gradient of ~100 °C km-1 allowed for the exploration of most of the known temperature range for life over just 1 km of drill core. Despite the high temperatures, microbial cells were detected almost throughout the entire sediment column, albeit at extremely low concentration of <500 cells per cm³ in sediment above ~50 °C. In millions of years old sediment a small microbial community subsisted with high potential cell-specific rates of energy metabolism, which approach the rates of active surface sediments and laboratory cultures. Even under the most conservative assumptions, potential biomass turnover times for the recovered sediment ranges from days to years and therefore many orders of magnitude faster than in colder deep sediment.

Our discovery is in stark contrast to the extremely low metabolic rates otherwise observed in the deep subseafloor. As cells appear to invest most of their energy to repair thermal cell damage in the hot sediment, they are forced to balance delicately between subsistence near the upper temperature limit for life and a rich supply of substrates and energy from thermally driven reactions of the sediment organic matter.

How to cite: Schubert, F., Beulig, F., Adhikari, R. R., Glombitza, C., Heuer, V., Hinrichs, K.-U., Homola, K., Inagaki, F., Jørgensen, B. B., Kallmeyer, J., Krause, S., Morono, Y., Sauvage, J., Spivack, A., and Treude, T.: Microbial survival through high metabolic rates in a deep and hot subseafloor environment, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2909, https://doi.org/10.5194/egusphere-egu22-2909, 2022.

EGU22-3165 | Presentations | SSP1.2 | Highlight

Drilling Overdeepened Alpine Valleys (ICDP-DOVE): Age, extent and environmental impact of Alpine glaciations 

Flavio Anselmetti and Marius Buechi and the ICDP-DOVE Team

The sedimentary infill of glacially overdeepened valleys (i.e. eroded structures below the fluvial base level) are, together with glacial geomorphology, the best-preserved (yet underexplored) direct archives of extents and ages of past glaciations in and around mountain ranges. ICDP project DOVE (Drilling Overdeepened Alpine Valleys) Phase-1 investigates five drill cores from glacially overdeepened structures at several complementing locations along the northern front of the Alps and their foreland. Two of these drill sites, both in the former reaches of the Rhine Glacier, have been successfully drilled in 2021 with excellent core recovery of 95 %: i) The borehole in Basadingen in Northern Switzerland reached a depth of 253 m, and ii) The Tannwald site in Southern Germany consists of one cored borehole to 165 m and two nearby flush boreholes; all three sites will allow a series of crosshole geophysical experiments. Three previously drilled legacy cores from the Eastern Alps are included in the DOVE Phase-1: iii) a core from Schäftlarn, located in the Isar-Loisach glacier catchment, was drilled in 2017 down to a depth of 199 m; iv) the Neusillersdorf drill site, located in the southern German Salzach Foreland glacier area, recovered a sequence down to 136 m (incl. 116 m of Quaternary strata); and v) the drill site Bad Aussee in Austria is located in the area of the Traun Glacier at an inneralpine location. It recovered almost 900 m of Quaternary sediments.

All the sites will be investigated with regard to several aspects of environmental dynamics during the Quaternary, with focus on the glaciation, vegetation, and landscape history. For example, the geometry of overdeepened structures will be investigated using different geophysical approaches (e.g. seismic surveys) to better understand the process of overdeepening. Sedimentological analyses in combination with downhole logging, investigation of biological remains and state-of-the-art geochronological methods will allow to reconstruct the filling and erosion history of the troughs. We expect significant and novel data relating to the extent and timing of the past Alpine glaciations during the Middle-to-Late Quaternary glacial-interglacial cycles. Besides these basic scientific goals, this proposal also addresses a number of applied objectives such as groundwater resources, geothermal energy production, and seismic hazard assessment.

A successful DOVE Phase-1 will lay the ground for an upcoming Phase-2 that will complete the panalpine approach. This follow-up phase will investigate paleoglacier lobes from the western and southern Alpine margins through drilling sites in France, Italy and Slovenia.

How to cite: Anselmetti, F. and Buechi, M. and the ICDP-DOVE Team: Drilling Overdeepened Alpine Valleys (ICDP-DOVE): Age, extent and environmental impact of Alpine glaciations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3165, https://doi.org/10.5194/egusphere-egu22-3165, 2022.

EGU22-3372 | Presentations | SSP1.2

Re–Os geochemistry of altered dacitic rock at Site U1527, IODP Expedition 376: Implications for the Re cycle in intraoceanic arcs 

Mizuki Ishida, Tatsuo Nozaki, Yutaro Takaya, Junichiro Ohta, Qing Chang, Jun-Ichi Kimura, Kentaro Nakamura, and Yasuhiro Kato

The Re–Os isotopic system is a powerful tool for both geochronology and tracing various geochemical processes. Because the Os isotopic ratio (187Os/188Os) distinctly differs between modern seawater (∼1.06) and hydrothermal fluid (∼0.13), the Re–Os isotopic system is potentially a sensitive tracer of subseafloor fluid flow and the release or uptake of hydrogenous/magmatic Re and Os. The effect of alteration on the Re–Os budget in oceanic crust has been examined for mid-ocean ridge basalt (MORB) and lower oceanic crustal gabbro. In contrast, applications of the Re–Os system in intraoceanic arc settings are limited mainly to fresh igneous rocks; the role of hydrothermal alteration has not yet been examined.

Here, we provide a depth profile of Re–Os geochemistry at Site U1527, located on the NW caldera rim of the Brothers volcano hydrothermal field in the Kermadec arc, which was drilled during International Ocean Discovery Program (IODP) Expedition 376 in 2018. Volcaniclastic rocks from Hole U1527C that had experienced various degrees of high- and low-temperature hydrothermal alteration were analyzed for bulk chemical composition as well as Re–Os concentrations and isotopes. The concentration of Re varied from 0.172 to 18.7 ppb, and that of Os ranges from 9.7 to 147.1 ppt. Hydrothermal alteration usually resulted in the Re uptake by rocks, but a part of Re was released into the ocean by later oxidative weathering. Compared with Re, Os mobility resulting from hydrothermal alteration was limited. Before alteration, our samples likely had homogenous 187Os/188Os of between 0.13 and 0.14, whereas alteration added hydrogenous Os to some drill core sections in two different ways. Elevated 187Os/188Os with Ba enrichment and abundant pyrite occurrence suggests Os precipitation induced by subseafloor mixing of seawater and high-temperature hydrothermal fluid. The highest Re and Os concentrations at Hole U1527C, found in the same interval, were associated with high concentrations of Bi, Sb, and Tl. In contrast, elevated 187Os/188Os without Ba and Os enrichment can be explained by adsorption of seawater-derived radiogenic Os onto Fe hydroxide during seawater ingress into volcaniclastic rocks with a high matrix volume.

Intense Re enrichment at Hole U1527 relative to the high-temperature alteration zone in altered MORB may be related to abundant pyrite precipitation and high Re content in primary arc magmas. We propose that degassed Re from shallow intraoceanic arc magmas may be sequestered by subseafloor high-temperature alteration. Part of the stored Re might also be released into the ocean by later oxidative seawater circulation and seafloor weathering, raising a question about the role of alteration zones in the Re cycle in subduction zones. This study is one of the first attempts to apply the Re–Os system to altered rocks in arc settings, and future research should provide more information about the fate of Re in intraoceanic arcs and the detailed role of hydrothermal alteration in the Re cycle on the Earth.

How to cite: Ishida, M., Nozaki, T., Takaya, Y., Ohta, J., Chang, Q., Kimura, J.-I., Nakamura, K., and Kato, Y.: Re–Os geochemistry of altered dacitic rock at Site U1527, IODP Expedition 376: Implications for the Re cycle in intraoceanic arcs, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3372, https://doi.org/10.5194/egusphere-egu22-3372, 2022.

EGU22-3428 | Presentations | SSP1.2

Hipercorig Hallstatt History (H3) reveals a high-resolution Late Pleistocene to Holocene sediment record at Lake Hallstatt (Salzkammergut, Austria) 

Marcel Ortler, Achim Brauer, Stefano C. Fabbri, Kerstin Kowarik, Jochem Kueck, and Michael Strasser

The innovative, new drilling technique of the Hipercorig platform (Harms et al., 2020, https://doi.org/10.5194/sd-28-29-2020) enables to recover undisturbed long cores of sediment archives, and hence allows us to study past environmental conditions and changes. Here we present initial results from the Hipercorig Hallstatt History (H3) lake drilling campaign 2021, which succeeded to recover two parallel cores (core A: 41m, core B: 51m) from 122 m water depth providing a high-resolution record, within the UNESCO World Heritage Cultural Landscape Hallstatt-Dachstein/Salzkammergut, Austria. The Hallstatt-Dachstein region has a history of over 7,000 years of human salt mining and is one of the oldest documented cultural landscapes worldwide.

We present physical- and litho-stratigraphy based on borehole logging (of hole B), non-destructive core logging data, visual core and lithofacies description, Core-Log-Seismic-Correlation and initial age modelling using 14C dating. The core logging covers (i) x-ray computed tomography, (ii) multi-sensor-core-logger data with Gamma-Ray attenuated bulk density, magnetic susceptibility and visible light photo spectroscopy. The upper ~15 m of the sediment profile can be unambiguously correlated with previous cores (Lauterbach et al., submitted) thus confirming that the sediments are truly representative for Lake Hallstatt. The entire stratigraphic succession comprises two major lithostratigraphic units: The Holocene unit (0-40 m below lake floor (mblf)) and the Late Pleistocene unit (> 40 m). The Holocene unit consists of variably laminated (sub-mm to 5 mm) dark gray clayey-silty carbonate mud interbedded with up to 5.5 m thick mass-movement deposits and thick turbidites. The Late Pleistocene sedimentary succession comprises very thin bedded (1-3 cm) medium gray silty clayey carbonate mud, with some laminated (<1 cm) intervals and multiple cm-thick light gray turbidites. Within the Late Holocene unit, there is a prominent yellowish gray clastic interval of ~4 m with faintly mm- to cm-scale laminated sediments. Another remarkable characteristic of the Holocene unit is the occurrence of at least four major mass-movement deposits containing pebbles (up to 3 cm in diameter) and six thick turbidite deposits >1 m with different sediment colors and compositions.

Detailed multi-proxy analyzes of the Lake Hallstatt cores will provide new insights into the early history of human settlement and salt mining in this Alpine region and their relation to environmental and climatic conditions and meteorological and geological extreme events.

How to cite: Ortler, M., Brauer, A., Fabbri, S. C., Kowarik, K., Kueck, J., and Strasser, M.: Hipercorig Hallstatt History (H3) reveals a high-resolution Late Pleistocene to Holocene sediment record at Lake Hallstatt (Salzkammergut, Austria), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3428, https://doi.org/10.5194/egusphere-egu22-3428, 2022.

EGU22-3534 | Presentations | SSP1.2

Reconstructing the moisture availability of Central Mexico over the past 500,000 years using borehole logging data 

Mehrdad Abadi, Christian Zeeden, Arne Ulfers, and Thomas Wonik

Assessing the moisture history of Central Mexico reveals the responses of tropical areas to variation in past climate. Central Mexico has several long-lived lakes, which are potentially important paleoclimate archives. Lake Chalco in Central Mexico contains a ~300 m lacustrine sequence, which were deposited over a period of ~500,000 years. We conducted Spectral Gamma Ray (SGR) measurements across the lacustrine deposits of Lake Chalco to reconstruct the moisture availability over the past. The SGR data reflect the presence of naturally occurring radioactive elements including potassium (40K) and the equilibrium decay series of uranium (U) and thorium (Th). Natural sources of gamma radiation in lacustrine deposits of Lake Chalco are from volcanic ash deposition and detrital input of eroded sediments containing radioactive elements. However, redox conditions in the lake water influence the mobility of soluble U through conversion to more stable reduced phases. To extract the primary non-volcanic signals, we detected and removed signals from embedded tephra layers in the lacustrine sediments of Lake Chalco. We developed a moisture proxy by calculating the probability of authigenic U distributed across the lake sediments. We expect that an increasing U content in proportion to the content of K and Th indicate redox conditions in lake bottom water as a result of rising lake level. To evaluate this moisture proxy, we examined differences in the percent of the diatom species that are indicative of a deeper lake from literature. Results suggest that Lake Chalco likely formed prior or within MIS13, and the lake level rose gradually over time until the interglacial period of MIS9. Moisture levels are higher during the interglacial than glacial periods and interglacial periods show higher moisture variability. While glacial periods have less moisture, two periods, MIS6 and MIS4, still have a higher likelihood of authigenic U and more moist conditions. In order to determine potential regulators of moisture, we compared models containing the drivers of Earth’s orbital cycles, carbon dioxide and sea surface temperature. Carbon dioxide, eccentricity, and precession are all key drivers of the moisture content of Lake Chalco over the past 500,000 years. High levels of atmospheric CO2 have a positive effect on the moisture in Mexico while eccentricity and precession consistently have negative effects on lake moisture. Obliquity and δ18O have weaker effects on moisture in Mexico, probably due to the equatorial high-altitude region far away from poles, oceans and ice sheets.

How to cite: Abadi, M., Zeeden, C., Ulfers, A., and Wonik, T.: Reconstructing the moisture availability of Central Mexico over the past 500,000 years using borehole logging data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3534, https://doi.org/10.5194/egusphere-egu22-3534, 2022.

EGU22-3538 | Presentations | SSP1.2 | Highlight

Deformation mechanisms along the Main Marmara Fault around the ICDP-site GONAF 

Magdalena Scheck-Wenderoth, Mauro Cacace, Oliver Heidbach, Marco Bohnhoff, Murat Nurlu, Naiara Fernandez Terrones, Judith Bott, and Ershad Gholamrezaie

The Main Marmara Fault (MMF) in NW Turkey south of Istanbul is a segment of the North Anatolian Fault Zone (NAFZ) that constitutes a right-lateral continental transform fault.  Several well-documented strong (M7+) earthquakes indicate that the MMF poses a great risk to the Istanbul metropolitan region. A 150 km long stretch of the MMF has not ruptured since 1766 and the recurrence time of 250 yrs for M7+ events derived from historical records indicate that the fault is overdue. We introduce a new project addressing how the rheological configuration of the lithosphere in concert with active fluid dynamics within the crust and mantle influence the present-day deformation along the MMF in the Marmara Sea region. We test the following hypotheses: (1) the seismic gap is related to the mechanical segmentation along the MMF which originates from the rheological configuration of the crust and lithosphere; (2) variations in deformation mechanisms with depth in response to variations in temperature and (fluid) pressure exert a first-order control on the mode of seismic activity along the MMF, and, (3) stress and strain concentrations due to strength and structural variability along the MMF can be used as an indicator for potential nucleation areas of expected earthquakes. To assess what mechanisms control the deformation along the MMF, we use data from the ICDP GONAF observatory (International Continental Drilling Programme – Geophysical Observatory at the North Anatolian Fault) and a combined work flow of data integration and process modelling to derive a quantitative description of the physical state of the MMF and its surrounding crust and upper mantle. Seismic and strain observations from the ICDP-GONAF site are integrated with regional observations on active seismicity, on the present-day deformation field at the surface, on the deep structure (crust and upper mantle) and on the present-day stress and thermal fields. This will be complemented by numerical forward simulations of coupled thermo-hydraulic-mechanical processes based on the observation-derived 3D models to evaluate the key controlling factors for the present-day mechanical configuration of the MMF and to contribute to a physics-based seismic hazard assessment.

How to cite: Scheck-Wenderoth, M., Cacace, M., Heidbach, O., Bohnhoff, M., Nurlu, M., Fernandez Terrones, N., Bott, J., and Gholamrezaie, E.: Deformation mechanisms along the Main Marmara Fault around the ICDP-site GONAF, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3538, https://doi.org/10.5194/egusphere-egu22-3538, 2022.

EGU22-3793 | Presentations | SSP1.2

Legacy DSDP and ODP data suggest a paradigm shift in methane hydrate stability in the Mediterranean Basin 

Cristina Corradin, Angelo Camerlenghi, Michela Giustiniani, Umberta Tinivella, and Claudia Bertoni

The global reservoir of submarine gas hydrates is favored by the cold temperature of oceanic bottom water and the generally low geothermal gradients along passive continental margins. The continental margins of the land-locked Mediterranean basin are a remarkable exception for the lack of evidence of extensive presence of gas hydrates. Using public data of the physics and chemistry of the subsurface available from 44 Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) wells as lithologic logs, downhole temperature measurements, and pore water salinity values, and observed physical characteristics of bottom waters, we model the theoretical methane hydrate stability zone (MHSZ) below the seafloor and in the water column.

We find important positive pore water salinity anomalies in the subsurface indicating the pervasive presence of concentrated brines up to saturation concentration of halite and gypsum (> 300 ‰). The resulting sub-bottom MHSZ is thinner by up to 90-95% with respect to its thickness calculated assuming constant salinity with depth equal to bottom waters salinity. In the Eastern Mediterranean deep basins the thickness of the subsurface MHSZ is largest (up to ~ 350 m) and the anomaly induced by subsurface brines is highest (~ -300 m), while in the Alboran, Western Mediterranean, Tyrrhenian, Sicily Channel, Adriatic and Aegean basins the MHSZ, where present, thins to less than 100 m with mostly negligible anomaly induced by the presence of subsurface brines.

Modelling results suggest that subsurface brines can produce dramatic reductions of the thickness of the MHSZ only where the geothermal gradient is low (Eastern Mediterranean). We have modelled the same brine-induced limiting effect on the thickness of the MHSZ in synthetic cases of high and low heat flow to simulate Western and Eastern Mediterranean subsurface thermo-haline conditions. The salinity effect is attenuated by the thermal effect in the Western Mediterranean that produces the most relevant thinning of the MHSZ.

The distribution of the MHSZ resulting from the modelling coincides well with the distribution of the Late Miocene salt deposits which limit further the possibility of formation of gas hydrates acting as low permeability seal to the up-ward migration of hydrocarbon gases.

This modelling exercise provides a robust explanation for the lack of evidence of widespread gas hydrates on Mediterranean continental margins, with the exception of areas of local methane upward advection such as mud volcanoes, and it outlines a number of local hydrate-limiting factors that make this basin unfavorable to gas hydrate occurrence.

How to cite: Corradin, C., Camerlenghi, A., Giustiniani, M., Tinivella, U., and Bertoni, C.: Legacy DSDP and ODP data suggest a paradigm shift in methane hydrate stability in the Mediterranean Basin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3793, https://doi.org/10.5194/egusphere-egu22-3793, 2022.

EGU22-4022 | Presentations | SSP1.2 | Highlight

Half-precession signals in marine an terrestrial records – connecting IODP/ICDP sites from the equatorial Atlantic to Greenland 

Arne Ulfers, Christian Zeeden, Silke Voigt, Mehrdad Sardar Abadi, and Thomas Wonik

The characteristics of half-precession (HP) cycles (~9,000 - 12,000 years) is still poorly understood, despite their appearance in numerous records. We analyse HP signals in a variety of different marine and terrestrial proxy records from Europe and the Atlantic Ocean, investigate the temporal evolution of the HP signal from the early/middle Pleistocene to the present, and evaluate the potential of the HP to reflect the connectivity of climate systems over time.

We apply filters on the datasets that remove the classical orbital cycles (eccentricity, obliquity, precession) and high frequency signals, and focus on the bandwidth of HP signals. Wavelet annalysis and correlation techniques are used to study the evolution of specific frequencies through the different records.

In addition to a connection of HP cycles with interglacials, we observe a more pronounced HP signal in the younger part of several proxy records. Besides, we observe a trend of more pronounced HP signals in low latitude records compared to high latitudes. This is in agreement with the assumption that HP is an equatorial signal and can be transmitted northward via various pathways. The appearance of HP signals in mid- and high-latitude records may thus be an indicator for the intensity of the transporting mechanisms. We suggest that the African Monsoon plays a major role in this context, as its magnitude directly influences the climate systems of the Mediterranean and Southern Europe. In order to better understand the African climate variability, both equatorial marine and terrestrial records will be examined with respect to HP.

How to cite: Ulfers, A., Zeeden, C., Voigt, S., Sardar Abadi, M., and Wonik, T.: Half-precession signals in marine an terrestrial records – connecting IODP/ICDP sites from the equatorial Atlantic to Greenland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4022, https://doi.org/10.5194/egusphere-egu22-4022, 2022.

Together with amphibole and garnet, epidote-group minerals are one of the three most important heavy minerals found in orogenic sediments (Garzanti and Andò, 2007). Their chemical composition and optical properties vary markedly with temperature and pressure conditions, and thus provide useful information in provenance analysis on the metamorphic grade of source rocks.

The aim of this study is to devise an efficient and quick method, with micrometric resolution to distinguish among the different species of the epidote group during routine point-counting of heavy-mineral slides, which can be applied on a vast ranges of grain-sizes from fine silt to medium sand.

The geochemical variability of epidote-supergroup minerals from different source rock collected in different sectors of the Alpine orogenic belt was first investigated by coupling Raman Spectroscopy, Scanning Electron Microscopy, and Energy-dispersed X-ray Spectroscopy (SEM-EDS). The geochemical composition, optical properties, and Raman fingerprints of these standard epidote grains were described and in-house database of Raman spectra was created, combining geochemical data and Raman response in the low wavenumbers region and OH stretching bands. A program, written in Matlab® language, has been established which allows to obtain a quick estimate of the amount of iron from the Raman spectra in the clinozoisite-epidote series.

Raman spectra of detrital epidotes contained in turbiditic sediments of the Bengal Fan (IODP Expedition 354) were next compared with Raman spectra of epidote-group standards to determine their composition. The identification and relative amount of detrital epidote, clinozoisite and zoisite in silt- and sand-sized deep-sea sediments contribute to constrain the metamorphic grade of Himalayan source rocks, reconstruct the erosional evolution of the Himalayan orogen, and provide information on climate change and strengthening of the Indian Ocean monsoon throughout the Neogene and Quaternary.

Key words: epidote, provenance, Himalaya, Raman spectroscopy, Microprobe analyses, optical microscope.

Garzanti, E., Andò S., 2007. Plate tectonics and heavy-mineral suites of modern sands. In: Mange, M.A., Wright, D.T. (Eds.), Heavy Minerals in Use, Developments in Sedimentology Series, 58. Elsevier, Amsterdam, pp. 741-763.

How to cite: Limonta, M., Andò, S., Bersani, D., France-Lanord, C., and Garzanti, E.: Raman identification of epidote-group minerals in turbiditic sediments from the Bengal Fan (IODP Exp. 354): a complementary tool to better constrain metamorphic grade of source rocks., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6161, https://doi.org/10.5194/egusphere-egu22-6161, 2022.

A 6-meter drill core from Merensky Reef, Bushveld Complex, South Africa, was scanned in detail with a drill core scanner based on Laser Induced Breakdown Spectroscopy (LIBS). The purpose of the investigation was to visualize variations in the chemical composition along the core, and following a mineral classification of the LIBS data, of variations in the mineral chemical composition, e.g. of Fe/Mg, Cr/Al, and Ca/Na ratios, as well.

The LIBS technology is based on atomic emission spectroscopy, in which the excitation of the atomic species occurs in-situ on the sample surface. The excitation source was a pulsed 50 mJ 1064 nm Nd:YAG laser, and the emitted light was collected with a high-resolution wide-range echelle spectrograph with CCD detector. This approach for measuring mineral chemical ratios such as Mg/Fe, Cr/Al, and Ca/Na, is based on the strength of LIBS in detecting chemical variations using intensity ratios within a single matrix, which in this application is one single particular type of mineral phase. For validation purposes, selected samples were analysed with bulk chemical analysis and electron probe microanalysis as well.

Distinct trends could indeed be extracted from the 6 m core section through the Merensky Reef. From a saw-cut core surface without further preparation, a continuous record could be extracted consisting of Mg/Fe of orthopyroxene, Ca/Na of plagioclase, bulk chemical patterns, modal composition, and direct neighbourhood. The data can be used to highlight the presence of unusual patterns and to relate them to Ni, Cu, PGE or other mineralization. When applied to different core sections, it may become an important tool for comparing lateral variability of diagnostic horizons in vertical sequences in layered intrusions such as Merensky Reef and UG-2.

How to cite: Meima, J., Rammlmair, D., Junge, M., and Nikonow, W.: Continuous measurement of Mg/Fe and Ca/Na ratios with scanning Laser Induced Breakdown Spectroscopy in 6 meter of drill core through Merensky Reef, Bushveld Complex, South Africa, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7513, https://doi.org/10.5194/egusphere-egu22-7513, 2022.

EGU22-8339 | Presentations | SSP1.2

How was the Bushveld Complex assembled? A search for cryptic layering in ICDP drillcores from the Main Zone 

Robert B. Trumbull, Ilya V. Veksler, Wilhelm Nikonov, and Dieter Rammlmair

The Main Zone of the Bushveld Complex in South Africa is the most voluminous but least studied part of the world’s largest igneous intrusion. Modal layering is poorly developed compared with the units above and below (Upper and Critical Zones, resp.), and most of the ca. 3000 meter-thick Main Zone consists of monotonous gabbronorite, occasionally grading into norite and anorthosite. An exception is the ultramafic “Pyroxenite Marker” near the top of the Main Zone, which is present regionally in the complex and represents a major event of magma recharge into the chamber. However, studies of drillcore through the Main Zone in the Bushveld Northern limb (Ashwal et al., 2005; Hayes et al., 2017) found evidence for layering by periodic variations in rock density at vertical length-scales of 40 to 170 m. This implies there were many more episodes of magma recharge than previously thought.

Our study in the Eastern Limb of the complex tests if cryptic layering in the Main Zone is a local phenomenon or is regionally developed like the Pyroxenite Marker. The first step, reported here, was a vertical profile of bulk density data (Archimedes method) for a 1450 m section of the upper Main Zone below the Pyroxenite Marker. Samples were taken at 1 to 5 m intervals and the results show several intervals of density variations at length-scales of 30 to 120 m, comparable to those previously described in the Northern Limb. Periodicity in density changes is not so well developed as in the earlier study, and we identified several 50 to 75 m intervals where density variations are below 0.05 g/cm3. The second step of the study will use multispectral and laser-induced breakdown spectroscopy (LIBS) scanning to provide modal mineralogy profiles of the same drillcore samples used for density measurement. After cryptic modal layering is documented in this way, follow-up petrologic-geochemical studies at the layer boundaries will aim to characterize the composition and temperature of the magmas involved.

For this project the Bushveld Complex Drilling Project (BVDP) provided access to the BH7771 borehole, donated by Impala Platinum’s Marula mine.

References:

Ashwal, L..D., Webb, S.J. and Knoper, M.W. (2005) S. Afr. Jour. Geol., 108, 199-232.

Hayes, B., Ashwal, L.D., Webb, S.J. and Bybee, G.M. (2017) Contrib. Mineral. Petrol., 172, 13.

How to cite: Trumbull, R. B., Veksler, I. V., Nikonov, W., and Rammlmair, D.: How was the Bushveld Complex assembled? A search for cryptic layering in ICDP drillcores from the Main Zone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8339, https://doi.org/10.5194/egusphere-egu22-8339, 2022.

EGU22-8952 | Presentations | SSP1.2

‘SaltGiant’ drilling in the Sorbas Basin: Structural, Petrophysical and Geochemical characterization of the Messinian Salinity Crisis deposits 

Fadl Raad, Philippe Pezard, Cesar Viseras, Francisco J. Sierro, Luis M. Yeste, Javier J. Aguila, Paula Jerez, Andrea Schleifer, Fabio Meneghini, Cinzia Bellezza, Johanna Lofi, Angelo Camerlenghi, and Giovanni Aloisi

The Late Miocene deposits in the Sorbas Basin (Spain) have been of an extreme importance in the understanding of the Messinian Salinity Crisis (MSC) events (5.97-5.33 Ma). They consist of four formations. The pre-crisis Abad marls topped by the evaporitic Yesares gypsum member, followed by two non-evaporitic units known as the Sorbas and Zorreras members. Those deposits have been widely explored and studied thanks to the numerous outcropping sections in the basin.


The ‘SaltGiant’ European Training Network held a training school in October 2021 in the Sorbas Basin, where four boreholes (named SG0, 1, 2 and 3) covering most of the Messinian Salinity Crisis sequence, were drilled, cored and logged in this context along an overall thickness of about 175 m. The drillings took place inside and in the vicinity of the Torralba gypsum mine. It allowed for the first time in the scientific non-industrial domain, access to a continuous and non-outcropping succession of the Messinian deposits in the Sorbas basin. In addition to the recovered cores, borehole geophysical data were obtained from the four holes and digital images of the area were collected with a drone. Prior to the drilling, an OBO (Outcrop / Behind Outcrop) workflow was followed, which will allow integrating the outcrop and subsurface data by combining the 3D geometry of geobodies with geophysical information.


Optical borehole wall images provide mm-scale images of the borehole walls, highlighting the sedimentological and structural characteristics of the deposits. Downhole geophysical measurements included acoustic velocity, electrical resistivity and natural spectral gamma ray, which allowed determining the petrophysical characteristics of the penetrated lithologies. In addition to the petrophysical logs, a Vertical Seismic Profiling was performed in holes SG2 and SG3, including a multi-offset VSP survey in hole SG3.


The petrophysical characterization of the Messinian deposits will provide a reference case study for the lithologic characterization of MSC deposits in the subsurface elsewhere. VSP analysis provided an in-field preliminary seismic velocity evaluation in the encountered formations. Preliminary results confirm the astronomical precession-driven cyclicity observed elsewhere in the Messinian gypsum. Further processing and analyses of the large amount of acquired data will lead to identifying the astronomical and possibly higher-frequency cyclicity in the post-evaporitic deposits in the Sorbas member.

How to cite: Raad, F., Pezard, P., Viseras, C., Sierro, F. J., Yeste, L. M., Aguila, J. J., Jerez, P., Schleifer, A., Meneghini, F., Bellezza, C., Lofi, J., Camerlenghi, A., and Aloisi, G.: ‘SaltGiant’ drilling in the Sorbas Basin: Structural, Petrophysical and Geochemical characterization of the Messinian Salinity Crisis deposits, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8952, https://doi.org/10.5194/egusphere-egu22-8952, 2022.

EGU22-10040 | Presentations | SSP1.2

A profile through fast-spreading oceanic crust in the Oman ophiolite: reference frame for the crustal drillings within the ICDP Oman Drilling Project 

Jürgen Koepke, Dieter Garbe-Schönberg, Dominik Mock, and Samuel Müller

The Oman Ophiolite is the largest and best-investigated piece of ancient oceanic lithosphere on our planet. This ophiolite was target of the Oman Drilling Project (OmanDP) within the frame of ICDP (International Continental Scientific Drilling Program) which aimed to establish a comprehensive drilling program in order to understand essential processes related to the geodynamics of mid-ocean ridges, as magmatic formation, cooling/alteration by seawater-derived fluids, and the weathering with focus on the carbonatisation of peridotites.

Over two drilling seasons, the OmanDP has sampled the Samail Ophiolite sequence from crust to basal thrust. The total cumulative drilled length is 5458 m, with 3221 m of which was at 100% recovery. These cores were logged to IODP standards aboard the Japanese drilling vessel Chikyu during two description campaigns in summer 2017 and 2018. 

Here we present the main results of the working groups of the Universities Hannover and Kiel, focusing on the magmatic accretion of the Oman paleoridge. During 5 field campaigns these groups established a 5 km long profile through the whole crust of the Oman ophiolite by systematic outcrop sampling, providing the reference frame for the 400 m long OmanDP drill cores. The profile contains 463 samples from the mantle, through gabbros up to the dike/gabbro transition. Identical samples have been analyzed by several methods (bulk rock geochemistry, mineral analysis, Isotope geochemistry, EBSD analysis).

The results allow implication on the mechanism of accretion of fast-spreading lower oceanic crust. Depth profiles of mineral compositions combined with petrological modeling reveal insights into the mode of magmatic formation of fast-spreading lower oceanic crust, implying a hybrid accretion mechanism. The lower two thirds of the crust, mainly consisting of layered gabbros, formed via the injection of melt sills and in situ crystallization. Here, upward moving fractionated melts mixed with more primitive melts through melt replenishments, resulting in a slight but distinct upward differentiation trend. The upper third of the gabbroic crust is significantly more differentiated, in accord with a model of downward differentiation of a primitive parental melt originated from the axial melt lens located at the top of the gabbroic crust. Our hybrid model for crustal accretion requires a system to cool the deep crust, which was established by hydrothermal fault zones, initially formed on-axis at very high temperatures.

How to cite: Koepke, J., Garbe-Schönberg, D., Mock, D., and Müller, S.: A profile through fast-spreading oceanic crust in the Oman ophiolite: reference frame for the crustal drillings within the ICDP Oman Drilling Project, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10040, https://doi.org/10.5194/egusphere-egu22-10040, 2022.

EGU22-10406 | Presentations | SSP1.2

Assessing the well logging data from the Lake Bosumtwi (Ghana) 

Christian Zeeden, Mathias Vinnepand, Stefanie Kaboth-Bahr, William Gosling, Jochem Kück, and Thomas Wonik

Insights into the climate variability of western Africa during the Pleistocene epoch have thus far been limited by the lack of well-dated, high-resolution terrestrial climate archives. The missing information on the climate evolution of western African hampers our understanding of the proposed pan-African evolution of our species. The ~294 m lacustrine sedimentary sequence raised from Lake Bosumtwi by the International Continental Drilling program in 2004, encompassing the last ~1.1 Ma, offers the best opportunity provide a climatic benchmark record in western Africa. However, the establishment of a chronology for this record has proven challenging. To try and improve our understanding of the climatic evolution during the last ~1.1 Ma in western Africa, we will use the high-resolution downhole logging data (natural gamma ray, GR) and magnetic susceptibility data from core logging from Site 5, which is situated in the centre of Lake Bosumtwi. To maximise the robustness of this record we will try to correlate data from downhole logs with core data. This approach has help improve interpretation of logging signals and environmental reconstructions for other long lake records, such as e.g. Lake Ohrid.

How to cite: Zeeden, C., Vinnepand, M., Kaboth-Bahr, S., Gosling, W., Kück, J., and Wonik, T.: Assessing the well logging data from the Lake Bosumtwi (Ghana), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10406, https://doi.org/10.5194/egusphere-egu22-10406, 2022.

EGU22-11265 | Presentations | SSP1.2

Heterogeneous deformation across the Papaku fault, Hikurangi accretionary prism 

Rebecca Kühn, Annika Greve, Rüdiger Kilian, Marcel Mizera, and Michael Stipp

At the Hikurangi convergent margin the Pacific plate is subducted westward beneath the Australian plate. This margin has been the location of major earthquakes as well as slow slip events related to the ongoing subduction. Drill site U1518 which was drilled during IODP Expedition 375, 73 km offshore Gisborne (New Zealand), targeted the Papaku fault, a splay fault of the major decollement in sediments of the frontal accretionary prism. We selected samples from the mostly hemipelagic, weakly consolidated mudstones in the fault zone, as well as from hangingwall and footwall. In order to investigate localized and distributed deformation in the fault zone, we analysed composition, microstructure and crystallographic preferred orientation (CPO). For that we applied µXRF measurements and optical microscopy, as well as synchrotron texture analysis at DESY in Hamburg.

The samples from hanging- and footwall sediments show a relatively homogeneous microstructure with local compositional layering. While CPO strength in the hangingwall is slightly increasing with depth for all analysed clay mineral phases, the CPO in the footwall samples is in general lower and does not show a clear trend with depth. This might be interpreted as different deformation histories in hangingwall and footwall which is in accordance with previous studies. Fault zone samples show a variety of microstructures, such as mingling of different sedimentary components, locally overprinted by microfaults. CPO strength in the faulted sediments is also variable, with zones showing strong alignment of phyllosilicates and zones showing weak alignment of phyllosilicates. Variations in CPO and variable distribution of sedimentary components indicate a heterogeneous deformation within the fault zone which might be due to local compositional variations.

How to cite: Kühn, R., Greve, A., Kilian, R., Mizera, M., and Stipp, M.: Heterogeneous deformation across the Papaku fault, Hikurangi accretionary prism, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11265, https://doi.org/10.5194/egusphere-egu22-11265, 2022.

TS2 – Deformation mechanisms and rheology

EGU22-407 | Presentations | TS2.1

Strain localization along a detachment system: Deformation of natural dolomitic and calcitic mylonites (Mt. Hymittos, Attica, Greece) 

Mark Coleman, Bernhard Grasemann, David Schneider, Konstantinos Soukis, and Riccardo Graziani

Carbonate rocks can be thick, mineralogically-homogeneous packages, which accomodate strain in orogenic belts. Despite its contribution to rock strength, the deformation of dolomite as a major rock forming mineral is understudied in comparison to calcite, quartz, and feldspar. We use field, petrographic, and electron back scatter diffraction (EBSD) analyses of dolomitic and calcitic marbles to investigate the response of these rocks to different degrees of strain under greenschist facies. Mt. Hymittos, Attica, Greece, preserves a pair of Miocene top-SSW ductile-then-brittle low-angle normal faults dividing a tripartite tectonostratigraphy. The bedrock of the massif comprises sub-greenschist facies phyllites and marbles in the uppermost hanging wall unit, and high-pressure greenschist facies schists and marbles of the Cycladic Blueschist Unit in the lower two packages. Ductile mylonites in the footwalls of both detachments grade into brittle-ductile mylonites and finally into cataclastic fault cores. The dolomitic and calcitic marbles of the lower units deformed under greenschist facies conditions and their fabrics reflect the relative differences in strengths between these two minerals. In the middle tectonostratigraphic unit, dolomitic rocks are brittlely deformed and calcitic marbles are mylonitic to ultramylonitic with recrystallized grain sizes ranging from 55 to 8 μm. Within the lower package, dolomitic and calcitic rocks are both mylonitic to ultramylonitic with previous P-T data suggesting metamorphism at ~470 °C and 0.8 GPa. EBSD analysis of six dolomitic marbles of the lower unit reveals a progressive fabric evolution from mylonites to ultramylonites reflecting the magnitude of strain and decreasing temperature of deformation. In mylonitic domains, average grain diameters range from 70 to 25 μm. The mylonitic dolomite exhibits low-angle grain boundaries, internal misorientation zones and textures suggestive of subgrain-rotation recrystallization. This mylonitic fabric is crosscut by ultramylonite bands of dolomite with grain diameters of 15 to 5 μm, which overlaps with the dominant grain size of the subgrains formed within the mylonitic domains. In samples closer to the fault core, the ultramylonite fabric is predominant though boudinaged veins, and relict mylonite zones with coarser grains may still be observed. Uniformly ultramylonitic dolomitic marbles exhibit grain diameters of 40 to 5 μm; the majority of grain diameters are less than 15 μm. The ultramylonite bands have low degrees of internal misorientation and an absence of low-angle grain boundaries that, along with correlated misorientation diagrams, suggest the ultramylonitic dolomite grains are randomly oriented and deforming via grain-boundary sliding. Interstitial calcite grains within these samples may reflect creep-cavitation processes interpreted to have occurred syn-kinematically with grain-boundary sliding. The change from subgrain-rotation recrystallization to grain-boundary sliding is interpreted to reflect the interplay of grain-size sensitive and insensitive processes. Following grain size reduction, subsequent deformation was dominantly accommodated by grain boundary sliding. The dolomitic marbles of the lower unit deformed on the retrograde path from the high-pressure, mid-temperature portion of the greenschist facies. The position of the dolomitic ultramylonites immediately below the cataclastic detachment fault suggest these ultramylonites were deforming very close to the brittle-ductile transition suggesting ductile deformation at lower temperatures than might be predicted by deformation experiments.

How to cite: Coleman, M., Grasemann, B., Schneider, D., Soukis, K., and Graziani, R.: Strain localization along a detachment system: Deformation of natural dolomitic and calcitic mylonites (Mt. Hymittos, Attica, Greece), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-407, https://doi.org/10.5194/egusphere-egu22-407, 2022.

EGU22-2627 | Presentations | TS2.1

Constraining transformation weakening in plagioclase-pyroxene mixtures 

Amicia Lee, Holger Stünitz, Mathieu Soret, and Jacques Précigout

Mafic rocks are a key constituent of the oceanic and lower continental crust. Strain localisation and fabric development in these rocks is controlled by the active deformation mechanisms. From studies of natural rocks it has been established that strain localisation and weakening in mafic rocks is directly related to fluid availability and resultant mineral reactions. Understanding the interplay between reactions, fluid availability, and deformation aids in quantifying the stresses and rates of deformation processes. We have conducted an experimental investigation to constrain the weakening mechanisms in gabbro. Shear experiments were performed in a Griggs-type apparatus at 800-900°C, and 1.2-1.5 GPa with a shear strain rate of 10⁻⁵s⁻¹. The starting material consists of mixed powders with <100 µm sized grains of plagioclase and clinopyroxene from an undeformed sample of the Kågen Gabbro in Northern Norway. Experiments have been conducted with ‘as is’ (dried at 110°C) starting material and with 0.1% added water. The experiments at 800°C are very strong with a peak shear stress ~0.8 GPa whilst the 900°C experiments are weaker, reaching peak stresses of ~0.35 GPa. The 800°C experiments show evidence of mineral reactions with newly formed phases making up 10-25% of the sample. In these reaction zones, plagioclase and clinopyroxene have reacted to produce amphibole and garnet. Additionally S-C’ mylonitic fabrics have developed in these samples. The 900°C samples show minimal evidence for mineral reactions (2-5% new material) or crystal-plastic deformation mechanisms. The lack of mineral reactions in the rheologically weak experiments (900°C) and abundance of reaction products in the mechanically strong experiments (800°C) is conflicting to our inferences of natural studies. However, if partial melting takes place in the higher temperature experiments, it may account for the pronounced strength decrease. We plan to conduct EBSD and TEM analysis to determine crystallographic properties and accurate grain size and shape parameters in the fine grained reaction zones. Future experiments will use fully dried natural starting material (dried at 700-800°C) and An60 and end-member diopside, these experiments will be compared with our current experiments and be used to determine the exact weakening properties from impurities in the natural starting material.

How to cite: Lee, A., Stünitz, H., Soret, M., and Précigout, J.: Constraining transformation weakening in plagioclase-pyroxene mixtures, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2627, https://doi.org/10.5194/egusphere-egu22-2627, 2022.

EGU22-2816 | Presentations | TS2.1

Cracking induced by dislocation creep in pure quartz shear bands of granitoids 

Jacques Précigout, Estelle Ledoux, and Laurent Arbaret

The production of micro-pores during viscous creep is a driving mechanism for fluid circulation in deep environments. However, strain-induced cracking in nature is nowadays attributed to grain boundary sliding (GBS), restricting this process to fine-grained ductile shear zones where rocks deform by diffusion creep. Here we give natural evidence of micro-cracking induced by dislocation creep, which is by far the dominant deformation mechanism in lithospheric rocks. Focusing on pure quartz shear bands across the Naxos western granite (Aegean Sea, Greece), we first document sub-micron pores that arise at grain and sub-grain boundaries. Their shape and location emphasize sub-grain rotation as a source of cracking. We then confirm that quartz is dominated by dislocation creep with evidence of a moderate to strong lattice preferred orientation (LPO) and many sub-grain boundaries, including at the margin of the pluton where the brittle/ductile transition was reached. These features coincide with (1) quartz grains located as inclusion into quartz porphyroclasts and (2) a dependency of the LPO strength on grain size. Our findings suggest that creeping cavities act as pumping sites for fluid to penetrate the crystal lattice and nucleate randomly oriented grains along sub-grain boundaries, accounting for (1) shear localization by enhancing hydrolytic weakening and (2) rock embrittlement through growth and interlinkage of cavities where phase nucleation is limited.

How to cite: Précigout, J., Ledoux, E., and Arbaret, L.: Cracking induced by dislocation creep in pure quartz shear bands of granitoids, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2816, https://doi.org/10.5194/egusphere-egu22-2816, 2022.

EGU22-3268 | Presentations | TS2.1

Diffusion creep of a Na-Ca-amphibole-bearing blueschist 

Leif Tokle, Lonnie Hufford, Luiz Morales, Claudio Madonna, and Whitney Behr

Blueschists are a major constituent rock type along the subduction zone interface and therefore critical to our understanding of subduction zone dynamics. Previous experimental work on natural blueschists focus on either seismic anisotropy or on the process of eclogization of a blueschist aggregate; however, little is known about the mechanical properties of blueschist rocks. We have conducted a suite of general shear deformation experiments in the Griggs apparatus to constrain the rheology of a blueschist aggregate. The sample material derives from a natural blueschist that was crushed into a powder. The powder consists of ~55% sodic amphibole, ~30% epidote, ~8% quartz, ~5% titanite, ~2% ilmenite, and <1% mica. Deformation experiments were conducted at 1.0 GPa confining pressure, temperatures of 650, 675, 700, and 750°C, and no water added. All of the deformation experiments were strain rate stepping experiments with either 4 or 5 strain rate steps per experiment with strain rates ranging from ~2.7e-5 to 5.2e-7 s-1. Based on the mechanical data we determine a stress exponent of 1.9 +/- 0.3. Microstructural and EDS analysis shows the initial Na-amphibole grains transform into a fine-grained aggregate of new Na-Ca-amphibole with lower Na and Si and higher Fe and Ca plus albite and ilmenite. The fine-grained aggregates accommodate the majority of the strain while epidote deforms by rigid body rotation or brittle deformation. Based on both the mechanical and microstructural observations, we interpret the fine-grained aggregates to be deforming by diffusion creep. Additional analyses will be conducted to constrain the grain size to develop flow law parameters to estimate the rheology of the subduction zone interface.

How to cite: Tokle, L., Hufford, L., Morales, L., Madonna, C., and Behr, W.: Diffusion creep of a Na-Ca-amphibole-bearing blueschist, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3268, https://doi.org/10.5194/egusphere-egu22-3268, 2022.

The presence of large volumes of eclogite in collision and subduction zones makes their formation and deformation highly relevant for the dynamics of convergent zones. There is however no consensus on the deformation behavior of eclogite. On the one hand, mylonitic eclogite shear zones showing evidence of dominant deformation by dislocation creep have frequently been reported. On the other hand, fluid supported formation and deformation has been recently suggested as a potential mechanism in eclogite whereby the main accommodating mechanism is dissolution-precipitation creep. This raises the question of the factors controlling the deformation behavior of eclogite.

In this contribution, we present microstructural, petrographical and chemical data from a series of eclogite samples derived from low Mg – high Ti gabbro collected at the eclogite type locality (Saualpe-Koralpe Complex, Eastern Alps, Austria). The rocks are characterized by a pronounced foliation defined by the shape preferred orientation of the major minerals (omphacite, amphibole, epidote and garnet). Minor quartz is observed at dilation sites. Overall, grains show rather straight grain boundaries and a uniform extinction. These features are interpreted as evidence of diffusion and dissolution-precipitation dominated formation and strain accommodation. Thermodynamic forward modelling indicates that eclogitization occurred at around 2 GPa and 640–680°C and was supported by fluid. Locally, the eclogite fabric is crosscut by veins showing a similar paragenesis as the host eclogite. However, they are enriched in quartz and epidote, depleted in garnet and show overall a coarser grain size. Depending on their initial orientation, the veins were either reactivated as flanking structures or foliation sub-parallel shear zones. The reactivated veins are characterized by undulatory extinction, twinning and subgrain formation, all being indicative of dislocation creep. The identical paragenesis and similar mineral chemistry indicates that reactivation occurred at conditions close to those of eclogitization. The investigated samples therefore testify that eclogite can deform by two different mechanisms at similar pressure-temperature conditions. Our investigations document that dissolution-reprecipitation is bound to the process of eclogitization and low strain rate whereas post-eclogitization strain localization is accommodated by dislocation creep.

How to cite: Rogowitz, A., Huet, B., and Schorn, S.: How to creep and when? Deformation mechanisms at the eclogite type locality (Saualpe-Koralpe Complex, Eastern Alps, Austria)., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3477, https://doi.org/10.5194/egusphere-egu22-3477, 2022.

EGU22-3889 | Presentations | TS2.1

Feasibility of the mobile-lid regime controlled by grain size evolution 

Antonio Manjón-Cabeza Córdoba, Tobias Rolf, and Maëlis Arnould

One of the most discussed issues of whole-mantle geodynamic models is the need of an 'ad hoc' yield stress which is lower than any strength measurement of natural samples in the brittle or plastic regimes. It is commonly believed that grain size evolution, in particular grains size reduction due to dynamic recrystallization, may decrease the strength of the lithosphere and therefore aid the onset and persistence of the mobile-lid regime. In this work, we carry out an investigation of 2D whole-mantle annulus models with varying yield stress. We compare cases with different grain growth and grain reduction parameters to cases with constant grain size to make inferences on the feasibility of a plate-like convective regime as a function of the yield strength of the lithosphere.

Our results show that viscosity profiles of models with dynamic grain-size evolution are inherently different to those with constant grain size, and that those profiles vary little when changing grain-size evolution parameters. In this context, the lower mantle shows greater variations in viscosity than the upper mantle: with viscosity contrasts between upper and lower mantle and plume widths comparable to those of the Earth, in particular in models with enhanced grain growth. Furthermore, our models show that, while enhancing grain size reduction reduces episodicity and increases mobility up to some point, increasing grain growth favors mobile-lid convection even more. This is at odds with previous conceptions of the grain-size-evolution-induced mobile-lid regime, where grain groth should promote healing of the lithosphere and therefore inhibit subduction. We hypothesize that increased stiffness of the bottom of the lithosphere, together with a more viscous lower mantle, are the main reasons for the grain-grouth-favored mobile-lid regime.

How to cite: Manjón-Cabeza Córdoba, A., Rolf, T., and Arnould, M.: Feasibility of the mobile-lid regime controlled by grain size evolution, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3889, https://doi.org/10.5194/egusphere-egu22-3889, 2022.

EGU22-4606 | Presentations | TS2.1

Strain localization in quartz-rich fault gouge at subseismic slip rates 

Chien-Cheng Hung and André Niemeijer

Understanding strain localization and development of shear fabrics within brittle fault zones at subseismic slip rates is crucial as they have critical implications for the mechanical strength and stability of faults and for earthquake physics. We performed direct shear experiments on ~1 mm thick layers of simulated quartz-rich fault gouge at an effective normal stress of 40 MPa, pore fluid pressure of 15 MPa, and temperature of 100°C. Microstructures were analyzed from strain hardening state (~1.3 mm displacement) to strain softening (~3.3 mm displacement) to steady-state (~5.6 mm) at different imposed shearing velocities of 1 µm/s, 30 µm/s, and 1 mm/s. We performed X-ray Computed Tomography (XCT) on sheared samples with a strain marker to analyze slip partitioning. To analyze and quantify localization from few hundreds to thousands of cross-section images, we used machine learning and developed an automatic boundary detection method to identify the type of shear fabrics and quantify the amount of them. Our results reveal that R1 and Y (or boundary) shears are the two major localization features that developed in a repeatable manner. Slip on R1 shears shows little dependency on both shear displacement and slip velocity and amounts to ~5 to ~30% of slip through the entire frictional sliding. On the other hand, Y and boundary shears show a strong correlation with displacement and velocity where more than 40% of strain was accommodated at steady-state for all velocities. However, Y and boundary shears become less prominent with increasing velocity, suggesting that velocity-weakening and the associated nucleation of unstable sliding are less likely to occur at higher slip rates as the overall friction behavior would be controlled by a thicker gouge layer. In other words, this suggests that Y shear development by grain size reduction is less efficient at high slip velocities which has important implications for the amount of heat generated during accelerating slip.

How to cite: Hung, C.-C. and Niemeijer, A.: Strain localization in quartz-rich fault gouge at subseismic slip rates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4606, https://doi.org/10.5194/egusphere-egu22-4606, 2022.

Seismic rupture in strong, anhydrous lithologies of the lower continental crust requires high failure stress, in the absence of high pore fluid pressure. Several mechanisms proposed to generate high stresses at depth imply transient loading driven by a spectrum of stress changes, ranging from highly localised stress amplifications to crustal-scale stress transfers. High transient stresses up to GPa magnitude are proposed by field and modelling studies, but the evidence for transient pre-seismic stress loading is often difficult to extract from the geological record due to overprinting by coseismic damage and slip. However, the local preservation of deformation microstructures indicative of crystal-plastic and brittle deformation associated with the seismic cycle in the lower crust offers the opportunity to constrain the progression of deformation before, during and after rupture, including stress and temperature evolution.


Here, detailed study of pyroxene microstructures characterises the short-term evolution of high stress deformation and temperature changes experienced prior to, and during, lower crustal earthquake rupture. Pyroxenes are sampled from pseudotachylyte-bearing faults and damage zones of lower crustal earthquakes recorded in the exhumed granulite facies terrane of Lofoten, northern Norway. The progressive sequence of microstructures indicates localised high-stress (at the GPa level) preseismic loading accommodated by low temperature plasticity, followed by coseismic pulverisation-style fragmentation and subsequent grain growth triggered by the short-term heat pulse associated with frictional sliding. Thus, up to GPa-level transient high stress leading to earthquake nucleation in the dry lower crust can occur in nature, and can be preserved in the fault rock microstructure.

How to cite: Menegon, L. and Campbell, L.: High stress deformation and short-term thermal pulse preserved in exhumed lower crustal seismogenic faults (Lofoten, Norway), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4692, https://doi.org/10.5194/egusphere-egu22-4692, 2022.

EGU22-5108 | Presentations | TS2.1

Different mechanical behavior at the same P-T conditions in biotite-quartz assemblage: interconnectivity and composition effect of experimentally deformed mica 

Khadija Alaoui, Laura Airaghi, Holger Stünitz, Hugues Raimbourg, and Jacques Précigout

The effect of composition on microstructural development and mechanical strength was tested using mica-quartz-aggregates during deformation experiments.

This study used two chemically different biotite minerals mixed with quartz: (1) high F-phlogopite and (2) intermediate biotite in order to investigate the role of biotite-bearing systems for the development of shear zones and strain accommodation. Shear experiments (Griggs-type apparatus) were performed using mica (30 vol. %) and quartz (70 vol. %) assemblages at 750 and 800°C, 1000 MPa and a shear strain rate of ~10-5 s-1.

Mechanical results for the F-phlogopite-bearing assemblage indicate strong samples, approximately equivalent to pure quartz samples (Richter et al., 2018), deforming at differential stresses of 764-1097 MPa). F-phlogopite flakes are preferentially oriented parallel to the main shear direction, but poorly interconnected. Most of the strain is accommodated by quartz behaving as an interconnected network. Cathodoluminescence imaging reveals that quartz recrystallizes mainly by local pressure-solution and its strength controls the overall strain accommodation.

In contrast, intermediate biotite assemblages are significantly weaker and deform for lower differential stresses of 290-327 MPa, as expected for natural rocks. Biotite flakes form an interconnected network accommodating most of strain.

The interconnectivity of biotite grains thus plays a major role in weakening quartz-biotite assemblages. However, at similar P-T-strain and grain size conditions, the capacity of biotite grains to interconnect may also depend on its chemical composition, particularly considering the effect of trace elements incorporation (as fluorine) on the strength of the biotite interlayer bounds (Dahl et al., 1996, Figowy et al., 2021). This led us to conclude that different types of mica, behaving differently, strongly affect strength, deformation mechanism, and microstructure of the rock due to their structure, composition and stability fields.

How to cite: Alaoui, K., Airaghi, L., Stünitz, H., Raimbourg, H., and Précigout, J.: Different mechanical behavior at the same P-T conditions in biotite-quartz assemblage: interconnectivity and composition effect of experimentally deformed mica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5108, https://doi.org/10.5194/egusphere-egu22-5108, 2022.

EGU22-5127 | Presentations | TS2.1

Experimental strain localization in granitoid ultramylonites: Pre-fracturing vs. viscous strain localization 

Natalia Nevskaya, Weijia Zhan, Holger Stünitz, Alfons Berger, and Marco Herwegh

Rheological models of Earth’s granitoid mid- to upper crust are commonly based on the physico-chemical properties of the most abundant rock forming minerals quartz and feldspar. However, there is increasing field evidence that deformation in these rocks localizes in ultrafine-grained polymineralic shear zones, which are weaker than any of the end member minerals. Especially at the brittle to viscous transition, the localization and deformation mechanisms, i.e. the role of incipient brittle deformation vs. continuous viscous strain localization, is not yet fully understood.

To fill this gap in knowledge, ultramylonite samples with granitic composition from the Central Aar Granite (Aar Massif, Central Switzerland) were deformed using a Griggs type apparatus. The foliation of the ultramylonitic starting material was oriented 45° to the compression direction, to investigate the influence of grain size and composition on strain localization in the different mylonite bands. Two types of coaxial experiments were conducted at 650°C, and 1.2 GPa confining pressure: A) Discrete fractures were created before the shear deformation starts; B) No fractures were induced during an early stage of the experiment.

All experiments have in common that strain is accommodated in 20-100 µm wide viscous shear zones with elongated grains and minor grain size reduction. In these shear zones, most strain is further localized in 10-20 µm wide zones, showing dramatic grain size reduction down to few tens of nanometres. In the experimentally generated shear zones, both, brittle and viscous processes are active. In terms of overall rock strength, all newly formed ultrafine-grained shear zones are up to three times weaker than comparable experiments on pure quartz or coarser grained granites – which agrees well with field observations. Furthermore, pre-fractured type A) is up to two times weaker than the non-fractured type B), and the orientation and number of shear zones is also fundamentally different between the two experiment types.

This study confirms two weakening factors promoting different types of strain localization at the brittle to viscous transition: 1) The existence of fractures and their interconnectivity – facilitating highly-localized grain size reduction; 2) Initial sample heterogeneity by polymineralic composition and ultrafine grain size – generating grain size reduction along strain gradients by activating viscous processes. Further quantitative microstructural analyses will reveal the role of chemistry and the deformation mechanisms on the localization behaviour.

How to cite: Nevskaya, N., Zhan, W., Stünitz, H., Berger, A., and Herwegh, M.: Experimental strain localization in granitoid ultramylonites: Pre-fracturing vs. viscous strain localization, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5127, https://doi.org/10.5194/egusphere-egu22-5127, 2022.

EGU22-5371 | Presentations | TS2.1

Implementing 3D anisotropic viscosity calculations into ASPECT 

Ágnes Király, Menno Fraters, and Rene Gassmoeller

Olivine, the main rock-forming mineral of Earth's mantle, responds to tectonic stress by deforming viscously over millions of years. This deformation creates an anisotropic (direction-dependent) texture that typically aligns with the mantle flow direction. According to laboratory experiments on olivine, we expect this texture to also exhibit anisotropic viscosity (AV), with deformation occurring more easily when it is parallel to, rather than across, the texture. However, the direction dependency of lithospheric and asthenospheric viscosity is rarely addressed in geodynamic models.

 The open-source modeling package ASPECT can address AV in a 2D setting using the director method, where AV is present due to shape preferred orientation created by dike intrusions (Perry-Houts and Karlstrom, 2019). We have adapted this implementation for current versions of ASPECT and benchmarked it against similar Rayleigh-Taylor instability models by Lev and Hager (2008).

Unfortunately, a 2D method is inappropriate to address AV related to olivine crystallographic preferred orientation (CPO or texture), as, by default, olivine has three independent slip systems on which deformation can occur. Integrating anisotropic viscosity into 3D models would also allow us to use the actual laboratory-based parametrizations of the olivine slip system activities and texture parameters when describing the evolution of CPO and AV. One of the biggest challenges in addressing AV in a 3D setting is to find the full, rank 4, viscosity tensor, which can be done with a method similar to the one for the fluidity tensor in Király et al., (2021).

Here, we present the initial results of simple geodynamic setups (shear box, corner flow), where 3D olivine CPO develops, based on the D-Rex method (Fraters and Billen, 2021), and this CPO creates AV based on the micromechanical model described in Hansen et al., (2016).

Our goal is to create a tool within ASPECT that allows for CPO to develop and affect the asthenospheric or lithospheric mantle’s viscosity to improve modeling a wide range of geodynamic problems.

 

References listed:

Fraters, M.R.T., and Billen, M.I., 2021, On the Implementation and Usability of Crystal Preferred Orientation Evolution in Geodynamic Modeling: Geochemistry, Geophysics, Geosystems, doi:10.1029/2021GC009846.

Hansen, L.N., Conrad, C.P., Boneh, Y., Skemer, P., Warren, J.M., and Kohlstedt, D.L., 2016, Viscous anisotropy of textured olivine aggregates: 2. Micromechanical model: Journal of Geophysical Research: Solid Earth, doi:10.1002/2016JB013304.

Király, Á., Conrad, C.P., and Hansen, L.N., 2020, Evolving Viscous Anisotropy in the Upper Mantle and Its Geodynamic Implications: Geochemistry, Geophysics, Geosystems, v. 21, p. e2020GC009159, doi:10.1029/2020GC009159.

Lev, E., and Hager, B.H., 2008, Rayleigh-Taylor instabilities with anisotropic lithospheric viscosity: Geophysical Journal International, doi:10.1111/j.1365-246X.2008.03731.x.

Perry-Houts, J., and Karlstrom, L., 2019, Anisotropic viscosity and time-evolving lithospheric instabilities due to aligned igneous intrusions: Geophysical Journal International, doi:10.1093/gji/ggy466.

How to cite: Király, Á., Fraters, M., and Gassmoeller, R.: Implementing 3D anisotropic viscosity calculations into ASPECT, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5371, https://doi.org/10.5194/egusphere-egu22-5371, 2022.

EGU22-5979 | Presentations | TS2.1

Quartz grain fabric in shales and sandstones: Some contrasting behaviors 

Charles Aubourg, Hugo Saur, Peter Moonen, and Rebecca Stokes

Many processes are at work when a sedimentary rock deforms. Quartz grains, for example, can rotate rigidly in the matrix, or on the contrary, undergo processes of dissolution and crystallization. Microtomography allows us to image the 3D geometry of minerals at the micron scale and quantify their fabric. Here, we use the quartz shape fabric extracted from microtomography data to evaluate the mechanisms active during burial and deformation of several sedimentary rocks systems.

Our first examples are of shales developing a slaty cleavage oblique to bedding. For shales that have undergone moderate burial (Tburial max ~200°C) (Sigues locality, Pyrenees), we show that the quartz grains rotate very little in the clay matrix. Even with the development of a slaty cleavage, a significant proportion of quartz grains remain parallel to the bedding plane. This surprising result implies that the rigid rotation of quartz grains, even in a ductile clay matrix, is not effective. 

In shales having undergone deeper burial and temperatures approaching the lower greenschist facies (Tburial max ~280°C) (Lehigh Gap locality, Appalachian mountains), we show that the average short-axis of the grains is orthogonal to the cleavage plane.  We suggest that this shape preferred orientation results from preferential dissolution of quartz faces oriented perpendicular to sigma 1, thus resulting in a shape preferred orientation without significant grain rotation.

Our last example concerns fine-grained sandstones, slightly deformed and buried at a shallow depth. If we refer to the example of shales with little burial, we would expect a very strong control of the bedding on the quartz fabric, since at these P-T conditions we expected dissolution-precipitation processes to be sluggish, and grain rotation to be ineffective.  However, surprisingly, the quartz in this rock is well oriented in the fabric which is oriented perpendicular to the bedding.

How the quartz grains were reoriented in the fine-grained sandstone suggests relations still not well understood with the deformation of a porous rock and the cementing processes of the rock. The microtomography approach in fine-grained rocks opens a door to this understanding of the behavior of quartz grains in sedimentary rocks.

How to cite: Aubourg, C., Saur, H., Moonen, P., and Stokes, R.: Quartz grain fabric in shales and sandstones: Some contrasting behaviors, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5979, https://doi.org/10.5194/egusphere-egu22-5979, 2022.

EGU22-6124 | Presentations | TS2.1

Shape Preferred Orientation at scale. From grain-scale aggregates to global mantle convection 

Albert de Montserrat, Manuele Faccenda, and Giorgio Pennacchioni

Earth's mantle rocks are poly-aggregates where different mineral phases coexist. These rocks may often be approximated as two-phase composites with a dominant phase and less abundant one (e.g. bridgmanite-ferropericlase composites in the lower mantle). Severe shearing of these rocks leads to a non-homogeneous partitioning of the strain between the different phases, with the composite developing a laminar fabric of weak and thin material where strain localizes. The resulting bulk rock is a mechanically anisotropic media that is hardened against normal stress, while significantly weakened against fabric-parallel shear stress.

Due to the large scale difference between the laminar gran-scale fabrics and regional-to-global geological processes, Earth’s rocks are idealised as homogeneous materials instead of multi-phase bodies in numerical models. Thus, a characterization of the rheology evolution of the bulk composite is necessary to better understand large-scale geological processes in which anisotropy may play a fundamental role. Recent three-dimensional numerical (de Montserrat et al. 2021) studies have shown that the degree of lateral interconnectivity of the weak and thin layers is rather limited, thus estimating the rheology of a composite with laminar fabrics by the idealized Voigt and Reuss averages for fibres yield a strong underestimation of the strength of the composite. Instead, we use a combination of numerical results and micro-mechanics to develop an empirical framework to estimate the evolution of the (anisotropic) rheology of such composites.

We apply this rheology framework to study the effects of fabric-induced directional-weakening/hardening on global mantle convective patterns. First order effects of extrinsic anisotropy of lower mantle material observed in our two-dimensional models are a decrease of the wavelength of convective cells, and up to a ~50% increase in the average mantle flow velocity caused by the weakening of the flow-parallel component of the viscosity tensor. The latter is particularly evident in mantle plumes, where the ascent and transfer of hot lower mantle material to lower depths is enhanced by the near-alignment of the weak  fabrics with the plume channel.  

How to cite: de Montserrat, A., Faccenda, M., and Pennacchioni, G.: Shape Preferred Orientation at scale. From grain-scale aggregates to global mantle convection, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6124, https://doi.org/10.5194/egusphere-egu22-6124, 2022.

Quaternary deformation in the northern Chile coastal forearc is mainly accommodated by ubiquitous upper-plate faults cataloged as weak fault zones, however, the deformation mechanisms and the internal structure of these reactivated faults remain poorly understood. To solve this problem, we selected seven study sites from reactivated upper-plate faults of the northern Chile forearc (23-25°S). These faults formed during the Early Cretaceous and reactivated during the Quaternary forming conspicuous fault-scarps. Here we present a new characterization of the internal structure at the outcrop and microscopic scale. Samples for thin-sections and XRD were collected in several cross-sections across faults. We define 4 units conforming the internal structure: (1) A decimetric well-defined principal slip zone, referred here as active fault core (AFC), consisting of a gouge layer subunit bounded by a fault breccia subunit, (2) a metric inactive fault core (IFC), surrounding the AFC, composed mostly of cataclasites and in some cases, mylonites, (3) a host-rock unit corresponds mainly to Jurassic-Cretaceous dioritic-granitic intrusives and Jurassic andesites, and (4) a decametric damage zone affecting both the IFC and the host rock. Near the topographic surface, the gouge layer subunit consists of a grey/green ultrafine matrix (40-80%) partially to completely replaced by massive iron oxides. In some sites, the gouge layers are partially foliated or/and exhibit millimetric bands of chaotic microbreccia. Porphyroclasts correspond mainly to (1) highly quartz and plagioclase intracracked individual crystals (<0.4mm), (2) larger fragments (<1mm) generally sigmoidal-like of the IFC (cataclasites) indicating different degrees of cataclastic-flow. Transgranular microfractures are densely propagated through the boundaries of larger porphyroclasts, breaking grains into ever-finer fragments (constrained communition) and generating chaotic microbreccia halos in the boundaries that grade into an ultrafine gouge matrix. (3) Another portion of large porphyroclasts (>1mm) grade from S-C cataclasite at its cores to S-C ultra-cataclasites at its boundaries. Frictional sliding is propagated through this S-C fabric formed by the ultracataclasite boundaries, generating well-defined and smoothened surfaces between large porphyroclasts and gouge layers. Microfractures -commonly filled with quartz>calcite>albite>chlorite-epidote veins- propagate mostly through the gouge layers, which are in turn displaced by microfaults affecting the entire subunit. The IFC composition changes markedly along-strike but multiple-fault cores are ubiquitous. In Jurassic andesites, the IFC is defined by protocataclasites with layers of red gouge, In Jurassic to Cretaceous diorite-metadiorite protoliths the IFC is defined by S-C cataclasites with microstructures showing undulating extinction, subgrains, and bulging recrystallization of quartz, and ultracataclasite bands and green gouge layers developed under low greenschist facies conditions. The IFC formed in mylonitic rocks derived from Jurassic to Cretaceous granitoid includes bands of hydrothermally-altered green and red mylonites. The complex overprinted microtextures indicate a progressive exhumation and shearing of the IFC. The microtexture analysis reflects the evolution of this unit from high temperature-low stain rates formed at deep structural levels to low temperature-high strain rates near-surface. We interpret the highly accumulated strain in S-C ultracataclastic bands and S-C gouge layers of the IFC (constrained communition) reduces the fault frictional strength and promote the frictional slip of the quaternary reactivations of the AFC.

How to cite: González, Y., Jensen, E., and González, G.: Internal Structure and Microtextures of a Quaternary Upper-plate Fault Zone: A Case Study from the Atacama Fault System, Northern Chile., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6742, https://doi.org/10.5194/egusphere-egu22-6742, 2022.

EGU22-6926 | Presentations | TS2.1

Pulverized rock and episodic hydrothermal brecciation along the Median Tectonic Line, Japan 

Geri Agroli, Masaoki Uno, Atsushi Okamoto, and Noriyoshi Tsuchiya

The Median Tectonic Line is a major east-west-trending arc-parallel fault that separates Sanbagawa metamorphic rock and Ryoke granite. We present the novel field observation of possibly pulverized rock and its evolution toward the fault cataclasite/breccia in the Ichinokawa antimony deposit in Central Shikoku. Ichinokawa was considered as largest stibnite mine in the world with a huge stibnite aggregate in which occurs in the brecciated-pelitic schist of the Sanbagawa belt. Based upon the texture in the outcrop and particle size distribution (PSD), this breccia is classified into two types. Breccia-1 (bx-1) is characterized by a centimeter-meter (up to 5m) angular breccia-clast with minimum to no shear displacement and rotational block. This bx-1 subsequently grows to be highly comminuted to produce breccia-2 (bx-2) which appear to have chaotic-polymict clast with matrix-supported texture within the fault zone with variable width and cut the bx-1 by recognizable breccia margin. Both of these breccia are cemented by reddish rock-flour matrix consist of dolomite, quartz, mica, ± pyrite. In addition, bx-2 has a more rounded shape with most of the clast size being less than 50mm and it shows orientation nearly parallel to the fault plane under a thin section. Based on this macro and micro-scale observation breccia in Ichinokawa is more likely to form under different mechanisms. Pulverization is plausible to rupture the pelitic schist and generate bx-1 without rotating the fragment. Hydrothermal activity in this area can’t be neglected which is responsible to create bx-2 as a result of fluid injection and transporting comminuted-fragment of bx-1 into the damage/fault zone. This breccia also underpins the formation of stibnite deposits that mark the latest fluid activity in this area where quartz-stibnite±pyrite±kaolinte vein truncate both pelitic schists of bx-1 as well as bx-2.

How to cite: Agroli, G., Uno, M., Okamoto, A., and Tsuchiya, N.: Pulverized rock and episodic hydrothermal brecciation along the Median Tectonic Line, Japan, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6926, https://doi.org/10.5194/egusphere-egu22-6926, 2022.

EGU22-7175 | Presentations | TS2.1

Weakening effect of grain-size reduction in granitoid shear zones 

Jonas B. Ruh, Leif Tokle, and Behr Whitney

Localization of strain during deformation of crustal rocks to form narrow shear zones requires some form of strain weakening. Bulk weakening of a deforming shear zone may for example result from geometric reorganization and interconnection of weak phases, from concentration of fluids or fluid-rich mineral phases, or from local temperature increase due to shear heating. A further potential weakening effect is work-related grain size reduction driven by dislocation creep, and the consequent activation of grain-size-sensitive diffusion creep in recrystallized zones.

To test the importance of grain size reduction for mechanical weakening of granitoid crustal shear zones, a numerical model of initially undeformed granitoid texture was set up and sheared to a total shear strain of 10. The numerical finite difference code solves for the conservation of momentum (Stokes) and mass with a visco-elasto-plastic rheology. The model setup outlines a naturally constrained multi-phase granitoid texture including quartz, plagioclase, and biotite. The domain measures 5x5 cm with top and bottom velocities describing simple shear, while the left and right prescribe periodic boundaries. For both quartz and plagioclase (anorthite), flow laws for dislocation and diffusion creep are implemented and act in parallel. Grain size evolution is implemented in the form of the paleowattmeter with mineral-specific grain growth laws. The 2D numerical setup of a complex multi-phase initial texture allows us to investigate grain size evolution in a progressively evolving system with a spatially and temporally varying stress field and with simultaneous geometric weakening associated with interconnection of weak phases, neither of which can be analyzed using analytical calculations.

Results show a reduction of grain sizes of quartz and plagioclase during shearing with quartz deforming dominantly under dislocation creep. Plagioclase behaves brittlely at low temperatures, with dominant diffusion creep at intermediate temperatures, switching to dislocation creep at high temperatures. Purely textural weakening of >60% occur at 550 °C. At lower temperatures, anorthite strength reduces given the brittle yield envelope and at higher temperatures, dislocation creep strength of quartz and anorthite converge, resulting in bulk shear and less textural weakening. Additional weakening related to grain size reduction relies on the activation of diffusion creep as the dominant deformation mechanism for anorthite. At 350 °C, anorthite strength is limited by brittle yield and no grain-size-induced weakening is detectable. For higher temperatures, additional grain-size-induced weakening ranges between 12–30 %, and thus represents an important factor for the initiation of granitoid crustal shear zones. The presented numerical study underlines the importance of grain size-related weakening of crustal shear zones, particularly at intermediate temperatures above the brittle-ductile transition (400–450°C) and below the activation of dislocation creep in plagioclase (>650°C).

How to cite: Ruh, J. B., Tokle, L., and Whitney, B.: Weakening effect of grain-size reduction in granitoid shear zones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7175, https://doi.org/10.5194/egusphere-egu22-7175, 2022.

EGU22-7406 | Presentations | TS2.1

Correlative, cross-platform microscopy application reveals deformation mechanisms during seismic slip along wet carbonate faults 

Markus Ohl, Helen King, Andre Niemeijer, Jianye Chen, and Oliver Plümper

Faults in the upper crust are considered major fluid pathways, raising the need for deformation experiments under wet conditions that focus on the nanoscale interaction between gouge material and pore fluid. Friction experiments on calcite at seismic slip velocities show strong dynamic weakening behaviour attributed to a combination of grain-size reduction and nanoscale diffusion. The resulting syn-deformational physico-chemical interactions between fluid and calcite are key in deciphering deformation mechanisms and rheological changes during and after (seismic) faulting in the presence of a fluid phase. We conducted rotary shear deformation experiments (1 m/s, σn = 2 and 4 MPa) on calcite gouge with water enriched in 18O (97 at%) as pore fluid to track and quantify potential fluid – mineral interaction processes. With our correlative, cross-platform workflow approach, we integrate Raman spectroscopy, nanoscale, and Helium-Ion secondary ion mass spectrometry (nanoSIMS, HIMSIMS), focused ion beam – scanning electron microscope (FIB-SEM) and transmission electron microscopy (TEM) to characterise the nanostructure and analyse isotope distribution. Raman analyses confirm the incorporation of 18O into the calcite crystal structure, as well as the presence of amorphous carbon. We identify three new band positions relating to the possible isotopologues of CO32- (reflecting 16O substitution by 18O). In addition, we detect portlandite (Ca(OH)2), pointing to a hydration reaction of lime (CaO) with water. Raman and NanoSIMS maps reveal that 18O is incorporated throughout the deformed volume, implying that calcite isotope exchange affected the entire fault gouge. Based on oxygen self-diffusion rates in calcite we conclude that solid-state 18O – isotope exchange cannot explain the observed incorporation of 18O into the calcite crystals during wet, seismic deformation. Hydration of portlandite and calcite containing 18O, implies breakdown and decarbonation of the starting calcite and the nucleation of new calcite grains. Our results question the state and nature of calcite gouges during seismic deformation and challenge our knowledge of the rheological properties of wet calcite fault gouges at high strain rates. The observations suggest that the physico-chemical changes are a crucial part of hydrous calcite deformation and have implications for the development of microphysical models that allow us to quantitatively predict crustal fault rheology.

How to cite: Ohl, M., King, H., Niemeijer, A., Chen, J., and Plümper, O.: Correlative, cross-platform microscopy application reveals deformation mechanisms during seismic slip along wet carbonate faults, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7406, https://doi.org/10.5194/egusphere-egu22-7406, 2022.

EGU22-8484 | Presentations | TS2.1

Constraining wet quartz rheology from constant-load experiments 

Subhajit Ghosh, Holger Stünitz, Hugues Raimbourg, and Jacques Précigout

Quartz rheology in the presence of H2O is crucial for modelling (numerical and geophysical) the deformation behavior of the continental crust and gives important insights into crustal strength. Experimental studies in the past have determined stress exponent (n) values for flow law between ≤ 2 to 4, while the values for activation energy (Q) vary from ~120 to 242 kJ/mol. Here, we investigated the quartz rheology under high-pressure and high-temperature conditions, using a new generation hydraulically-driven Griggs-type apparatus. In order to develop a robust flow law for quartzite, we performed constant-load coaxial deformation experiments of natural coarse-grained (~ 200 μm) high purity (> 99 % SiO2) quartzite from the Tana quarry (Norway). Our creep tests were carried out at 750 to 900 °C on the as-is (no added H2O) and 0.1 wt.% of H2O added samples under 1 GPa of confining pressure. In contrast to earlier strain rate stepping experiments, the constant-load procedure needs lower strain at each step (≤1−2%) to achieve steady-state conditions. As a consequence, there is a very low amount of recrystallization. Importantly, we can determine the Q-value independently of the stress exponent (n). Microstructures from the deformed samples were characterized using polarized light microscopy (LM), SEM-cathodoluminescence (CL), and Electron backscatter diffraction (EBSD).

Our creep results for both the as-is and 0.1 wt.% H2O-added samples yield Q = 110 kJ/mol and n = 2. Our microstructural analysis suggests that the bulk sample strain is accommodated by grain-scale crystal-plasticity, i.e., dislocation glide (dominantly in prism <a>) with minor recovery by sub-grain rotation, accompanied by grain boundary migration and micro-cracking. It is inferred that strain incompatibilities induced by dislocation glide are accommodated by grain boundary processes, including dissolution precipitation and grain boundary sliding. These intra-grain and grain-boundary processes together resulted in a lower n-value of 2 for the quartzite.

Our new flow law predicts strain rates that are in much better agreement with the inferred natural values than the earlier flow laws. It further suggests that the strength of the continental crust considering quartz rheology is significantly lower than previously predicted.  

How to cite: Ghosh, S., Stünitz, H., Raimbourg, H., and Précigout, J.: Constraining wet quartz rheology from constant-load experiments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8484, https://doi.org/10.5194/egusphere-egu22-8484, 2022.

EGU22-9089 | Presentations | TS2.1

Experimental Investigation of Glaucophane Rheology Through General Shear Deformation Experiments 

Lonnie Hufford, Leif Tokle, Claudio Madonna, and Whitney Behr

Glaucophane is a major constituent mineral associated with subducted mafic oceanic crust at blueschist facies conditions. Viscous deformation of glaucophane has been documented in natural blueschists; however, no experimental study has characterized the specific deformation mechanisms that occur in glaucophane nor the flow law parameters. We are conducting a suite of general shear deformation experiments in a Griggs apparatus to investigate crystal-plastic deformation mechanisms and microstructures of deformed glaucophane over a range of experimental conditions. Experimental samples consist of glaucophane powder separated from natural MORB blueschists  from Syros Island, Greece. Our experimental suite thus far includes temperatures and pressures ranging from 650° to 750°C and 1.0 to 1.5 GPa, strain rates ranging from ~3x10-6/s to ~8x10-5/s (both constant-rate and strain-rate stepping), and different grain size populations from 75-90 µm, 63-125 µm , and 63-355 µm. The lowest temperature and the strain-rate-stepping experiments exhibit evidence for combined frictional-viscous deformation and provide constraints on the brittle-ductile transition in glaucophane at laboratory conditions. The constant-rate experiments conducted at higher temperatures show greater evidence for viscous deformation by dislocation creep, including kinked grains, deformation lamellae, undulose extinction, and bulging via bulge recrystallisation. Mechanical data from the strain-rate stepping experiments allow us to interpret what parameters have the largest effect on peak stress. When comparing experiments conducted at 1 GPa and initial powder grain sizes of 63-355 µm, we find temperature having the largest effect on peak stresses. The 700°C experiment with an initial deformation speed 5 times faster (LH038) than another 700°C strain-rate stepping experiment (LH042) has a ~90 MPa higher peak shear stress, whereas the 750°C strain-rate stepping experiment with an initial deformation speed 4 times faster than LH042 has a ~115 MPa lower peak shear stress. At the time of abstract submission, further constant-rate experiments are planned at slower strain-rates to continue exploring the laboratory conditions necessary to activate glaucophane crystal-plastic deformation mechanisms. These data will be used with further strain-rate stepping experiments to develop flow law parameters from mechanical data.

How to cite: Hufford, L., Tokle, L., Madonna, C., and Behr, W.: Experimental Investigation of Glaucophane Rheology Through General Shear Deformation Experiments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9089, https://doi.org/10.5194/egusphere-egu22-9089, 2022.

   Plate boundary dynamics remain incompletely understood in the context of thermo-chemical convection. Strain-localization is affected by weakening in ductile shear zones, and a change from dislocation to diffusion creep caused by grain-size reduction is one of the mechanisms that has been discussed. However, the causes and consequences of strain localization remain debated, even though tectonic inheritance and strain localization appear to be critical features in plate tectonics.

   Frictional-plastic faults in nature and brittle shear zones in the lithosphere may be weakened by high transient, or static, fluid pressures, or mechanically by gouge, or mineral transformations. Weakening in ductile shear zones in the viscous domain may be governed by a change from dislocation to diffusion creep caused by grain-size reduction. In mechanical models, strain weakening and localization in the shallow parts of the lithosphere has mainly been modeled by an approximation of brittle behavior using a pseudo visco plastic rheology in combination with a linear decrease of the yield strength of the lithosphere with increasing deformation (plastic-strain (PSS) softening). Strain weakening in viscous shear zones, on the other hand, may be described by a linear dependence of the effective viscosity on the accumulated deformation (viscous-strain (VSS) softening). These different types of strain weakening are further explored and compared to the predictions from different laboratory-based models of grain-size evolution for a range of temperatures and a step-like variation of total strain rate with time. Such a parameterized, apparent-strain, or “damage”, dependent weakening (SDW) rheology (mainly PSS) can successfully mimic more complex weakening processes in global mantle convection computations. While we focus on GSS rheology to constrain the parameters of SDW, the analysis is not limited to grain-size evolution as the only possible microphysical mechanism.

   The SDW weakening rheology allows for memory of deformation, which weakens the fault zone as well as the lithosphere for a longer period and allows for a self-consistent formation and reactivation of inherited weak zones. In addition, the memory effect and weakening of the fault zone allows for a more frequent reactivation at smaller strain rates, depending on the strain-weakening parameter combination. Reactivation within the models occurs in two different ways: a), as a guide for laterally propagating convergent and divergent plate boundaries, and b), formation of a new subduction zone by reactivation of weak zones. A longer rheological memory results in a decrease in the dominant period of the reorganization of plates due to less frequently formed new plate boundaries. In addition, the low frequency content of velocity and heat transport spectra decreases with a decreasing dominant period. This indicates a more sluggish reorganization of plates due to the weaker and thus more persistent active plate boundaries. These results show the importance of a rheological memory for the reorganization of plates, potentially even for the Wilson cycle.

How to cite: Fuchs, L.: Plate-boundary maintenance – formation, preservation, and reactivation in global plate-like mantle convection models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9584, https://doi.org/10.5194/egusphere-egu22-9584, 2022.

EGU22-9765 | Presentations | TS2.1

In-situ mechanical testing and characterization of olivine grain boundaries 

Diana Avadanii, Lars Hansen, Ed Darnbrough, Katharina Marquardt, David Armstrong, and Angus Wilkinson

The mechanics of olivine deformation play a key role in long-term planetary processes, such as the response of the lithosphere to tectonic loading or the response of the solid Earth to tidal forces, and in short-term processes, such as the evolution of roughness on oceanic fault surfaces or postseismic creep within the upper mantle. Many previous studies have emphasized the importance of grain-size effects in the deformation of olivine. However, most of our understanding of the role of grain boundaries in the deformation of olivine is inferred from comparison of experiments on single crystals to experiments on polycrystalline samples.

To directly observe and quantify the mechanical properties of olivine grain boundaries, we use high-precision mechanical testing of synthetic forsterite bicrystals with well characterised interfaces. We conduct in-situ micropillar compression tests at high-temperature (700°C) on low-angle (13° tilt about [100] on (015)) and high-angle (60° tilt about [100] on (011)) grain boundaries. In these experiments, the boundary is contained within the micropillar and oriented at 45° to the loading direction to promote shear along the boundary. In these in-situ tests, we observe differences in deformation style between the pillars containing the grain boundary and the pillars in the crystal interior. In-situ observations and analysis of the mechanical data indicate that pillars containing the grain boundary consistently support elastic loading to higher stresses than pillars without a grain boundary. Moreover, only the pillars without a grain boundary display evidence of sustained plasticity and slip-band formation. Post-deformation advanced microstructural characterization (STEM) confirms that under the conditions of these deformation experiments, sliding did not occur along the grain boundary. These observations support the hypothesis that grain boundaries are stronger than the crystal interior. 

These experiments on small deformation volumes allow us to qualitatively explore the differences between the crystal interior and regions containing grain boundaries. Overall, the variation in strain and temperature in our small scale experiments allows fundamental investigation of the response of well characterised forsterite grain boundaries to deformation. 

How to cite: Avadanii, D., Hansen, L., Darnbrough, E., Marquardt, K., Armstrong, D., and Wilkinson, A.: In-situ mechanical testing and characterization of olivine grain boundaries, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9765, https://doi.org/10.5194/egusphere-egu22-9765, 2022.

EGU22-9842 | Presentations | TS2.1

Revisiting stylolites as a gage of overburden pressure – insights from fractal analysis 

Christoph von Hagke, Simon Hirländer, Kevin Frings, and Herfried Madritsch

Stylolites are ubiquitous structures generated by pressure solution primarily found in limestones. They and have been used as indicator for maximum stress a rock has suffered. This is commonly done by characterizing the fractal dimensions of stylolites. The current canon is the expectation from the theory that stylolites form through two physical pressure-driven regimes: low-frequency and higher-energetic - dominated by elastic forces and high-frequency lower-energetic dominated by surface tension. The so-called characteristic length separates both regimes, analytically marked by a kink in the power spectrum, which relates the energy contributions to the frequency.

However, determining this kink is not straightforward, and requires additional assumptions. We present a data set of stylolites recovered from a drill hole in the Alpine foreland basin. We mapped stylolites from different depths at sub-mm resolution semi-automatically and analyzed them using new methods of fractal analysis.

Excitingly, our preliminary studies did not identify the expected kink’s position from several different images of probes of drill cores, despite satisfactory reliability of laboratory preparation. Standard methods such as power spectral density, averaging wavelet coefficients, RMS, min/max, and rescaled range-based approaches revealed variations in their results, generally without evidence for a kink in the corresponding graphs. Implementing more recently developed methods such as adaptive fractal analysis could not improve the results. This finding challenges the classic interpretation of fractal characteristics of stylolites. 

How to cite: von Hagke, C., Hirländer, S., Frings, K., and Madritsch, H.: Revisiting stylolites as a gage of overburden pressure – insights from fractal analysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9842, https://doi.org/10.5194/egusphere-egu22-9842, 2022.

EGU22-10101 | Presentations | TS2.1

The role of grain boundaries for the deformation and grain growth of olivine at upper mantle conditions 

Filippe Ferreira, Marcel Thielmann, and Katharina Marquardt

Crystal defects such as vacancies, dislocations and grain boundaries are central in controlling the rheology of the Earth’s upper mantle. Their presence influences element diffusion, plastic deformation and grain growth, which are the main microphysical processes controlling mass transfer in the Earth’s lithosphere and asthenosphere. Although substantial information exists on these processes, there is a general lack of data on how these defects interact at conditions found in the Earth’s interior. A better understanding of processes occurring at the grain scale is necessary for increased confidence in extrapolating from laboratory length and time scales to those of the Earth. We examined the evolution of olivine grain boundaries during experimental deformation and their impact on deformation in the dislocation-accommodated grain- boundary sliding (disGBS) regime. This may be the main deformation mechanism for olivine in most of Earth’s upper mantle. Our results suggest that grain boundaries play a major role in moderating deformation in the disGBS regime. We present observational evidence that the rate of deformation is controlled by assimilation of dislocations into grain boundaries. We also demonstrate that the ability for dislocations to transmit across olivine grain boundaries evolves with increasing deformation. Lastly, we show that dynamic recrystallization of olivine creates specific grain boundaries, which are modified as deformation progresses. This might affect electrical conductivity and seismic attenuation in the upper mantle. The effective contribution of grain-boundary processes (such as disGBS) on the rheology of the upper mantle is correlated to the amount of grain boundaries in upper mantle rocks, that is, their grain-size distribution and evolution. The grain-size distribution in the Earth’s mantle is controlled by the balance between damage (recrystallization under a stress field) and healing (grain growth) processes. However, grain growth, one of the main processes controlling grain size, is still poorly constrained for olivine at conditions of the upper mantle. To evaluate the effects of pressure on grain growth of olivine, we performed grain growth experiments at pressures ranging from 1 to 12 GPa using piston-cylinder and multi-anvil apparatuses. We found that the olivine grain-growth rate is reduced as pressure increases. Our results suggest that grain-boundary diffusion is the main process of grain growth at high pressure. Based on extrapolation of our experimental results to geological time scales, we suggest that at deep upper-mantle conditions (depths of 200 to 410 km), the effect of pressure on inhibiting grain growth counteracts the effect of increasing temperature. We present estimations of viscosity as a function of depth considering the grain-size evolution predicted here. Our estimations suggest that viscosity is almost constant at the deep upper mantle, which corroborates postglacial-rebound observations.

How to cite: Ferreira, F., Thielmann, M., and Marquardt, K.: The role of grain boundaries for the deformation and grain growth of olivine at upper mantle conditions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10101, https://doi.org/10.5194/egusphere-egu22-10101, 2022.

EGU22-10153 | Presentations | TS2.1

Does porosity really matter? A first model for dissolution-enabled deformation bands in low porosity rocks based on microstructural analysis of calcarenite from Cotiella Basin, Spain. 

Maria Eleni Taxopoulou, Nicolas E. Beaudoin, Charles Aubourg, Elli-Maria Charalampidou, and Stephen Centrella

We report for the first time deformation features functionally described as deformation bands developed in low porosity rocks. This observation contradicts existing knowledge that deformation bands develop only in highly porous rocks. The studied formation is a bioclastic calcarenite of the Upper Cretaceous Maciños Unit in the Cotiella Massif. It is part of a megaflap adjacent to a salt diapir that has experienced extensional tectonics before the Pyrenean contraction. The bands present thickness variations, and in places they imitate the appearance of stylolites. This observation raises the question: how do deformation bands form in low porosity rocks?

To answer the question, we combine field observations with microstructural analysis to identify the occurring processes for the formation of deformation bands within low porosity rocks. After using optical microscopy and cathodoluminescence spectroscopy to conduct a petrographic study, we observe that the rock underwent multiple diagenetic cycles before the deformation stage, confirming that its porosity was significantly reduced before the deformation stage. Subsequently, we characterized the quartz grains inside the host rock and the dissolution-enabled deformation bands, using non-destructive imaging techniques. Three-dimensional image analysis from X-ray microtomography scans shows low grain size variations between the quartz grains in the host rock and in the bands, suggesting minor grain fracturing along the bands. However, grain reorientation has been reported for the quartz grains inside the bands, in relation to those in the host rock. Strain analysis was performed from Electron Backscattered Diffraction measurements, revealing higher strain along the quartz grain contacts inside the deformation band, compared to those in the host rock and stylolites. Our current data suggest that new porosity was created from local dissolution of the matrix, so the less soluble quartz grains were placed in contact. By wrapping-up the above observations, we propose a conceptual model that demonstrates the genesis and evolution of dissolution-enabled deformation bands in low porosity rocks, through local dissolution of the micritic matrix and transient porosity increase.

How to cite: Taxopoulou, M. E., Beaudoin, N. E., Aubourg, C., Charalampidou, E.-M., and Centrella, S.: Does porosity really matter? A first model for dissolution-enabled deformation bands in low porosity rocks based on microstructural analysis of calcarenite from Cotiella Basin, Spain., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10153, https://doi.org/10.5194/egusphere-egu22-10153, 2022.

EGU22-10404 | Presentations | TS2.1

Influence of a yield stress on lower mantle dynamics: filtering and changing morphology of plumes and slabs 

Anne Davaille, Thibaut Chasse, Neil Ribe, Philippe Carrez, and Patrick Cordier

When a fluid can experience a "jammed" state, it will flow only when the local deviatoric stress becomes greater than a critical stress, the so-called  "yield-stress". Jamming can be caused by entangled dislocations in a mineral, or by the existence of a hard skeleton in a two-phase fluid. According to recent numerical modeling, a Bridgmanite lower mantle would predominantly deform by pure dislocation climb; and due to dislocations interactions, it would flow only for local deviatoric stress greater than a critical yield stress which depends on dislocation density. In a first set of fluid mechanics experiments in such a visco-plastic fluid, we showed that hot plumes would develop with a much thicker morphology than in newtonian fluids. Scaling laws derived from the experiments tightly relate the buoyancy and diameter of the hot plumes to the value of the yield-stress, as well as to the mantle microstructure (such as dislocation density and vacancy concentration). Yield stress values between 1 and 10 MPa, implying dislocation densities between 108 and 1010 m−2, would be sufficient to explain the thick plumes morphology observed in seismic tomography images; while low vacancy concentrations could explain the 1000 km-depth horizon also seen in tomography. 

In a second set of experiments, we show that the existence of a yield stress in a Bridgmanite lower mantle will also act as a filter regarding slab penetration in the lower mantle. Given slab buoyancy, a typical slab, 100 km-thick, could not overcome the lower mantle yield stress. So most of single slabs would be expected  to stagnate in the transition zone. However a pile of folded slab with a typical thickness around 400 km would have sufficient buoyancy and would penetrate into the lower mantle. This could explain the seismic tomographic observations regarding slabs in the transition zone and in the lower mantle, without the need to invoke a compositional stratification there.

How to cite: Davaille, A., Chasse, T., Ribe, N., Carrez, P., and Cordier, P.: Influence of a yield stress on lower mantle dynamics: filtering and changing morphology of plumes and slabs, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10404, https://doi.org/10.5194/egusphere-egu22-10404, 2022.

EGU22-11133 | Presentations | TS2.1

Exploring the effect of mantle composite rheology on surface tectonics and topography 

Maelis Arnould, Tobias Rolf, and Antonio Manjón-Cabeza Córdoba

Earth’s surface dynamics and topography are tied to the properties and dynamics of mantle flow. In particular, upper mantle rheology controls the coupling between the lithosphere and the asthenosphere, and therefore partly dictates Earth’s surface tectonic behaviour and topographic response to mantle convection (dynamic topography). The presence of seismic anisotropy in the uppermost mantle suggests the existence of mineral lattice-preferred orientation (LPO) caused by the asthenospheric flow. Together with laboratory experiments of mantle rock deformation, this indicates that Earth’s uppermost mantle can deform in a non-Newtonian way, through dislocation creep. Although several studies suggest the potentially significant effect of upper-mantle non-Newtonian rheology on mantle convection (e.g. Schulz et al., 2020) and topography (e.g. Asaadi et al., 2011, Bodur and Rey, 2019), it is usually not considered in whole-mantle models of mantle convection self-generating plate tectonics.

 

Here, we investigate the effects of using a composite rheology (with both diffusion and dislocation creep) on surface tectonics and dynamic topography in 2D-spherical annulus models of mantle convection with plate-like tectonics and continental drift. We systematically vary the amount of dislocation creep by changing the activation volume for dislocation creep and the reference transition stress between diffusion and dislocation creep. We show that for low yield stresses promoting plate-like behavior in diffusion-creep-only models, modeling a composite rheology in the mantle favors more surface mobility while for large yield stresses which still generate plate-like motions in diffusion-creep-only models, a progressive increase in the amount of dislocation creep leads to stagnant-lid convection. We then compare the amplitudes and spatio-temporal distribution of dynamic topography in models with and without dislocation creep, in light of observed Earth present-day residual topography characteristics.

 

References:

Schulz, F., Tosi, N., Plesa, A. C., & Breuer, D. (2020). Stagnant-lid convection with diffusion and dislocation creep rheology: Influence of a non-evolving grain size. Geophysical Journal International, 220(1), 18-36.

Asaadi, N., Ribe, N. M., & Sobouti, F. (2011). Inferring nonlinear mantle rheology from the shape of the Hawaiian swell. Nature, 473(7348), 501-504.

Bodur, Ö. F., & Rey, P. F. (2019). The impact of rheological uncertainty on dynamic topography predictions. Solid Earth, 10(6), 2167-2178.

How to cite: Arnould, M., Rolf, T., and Manjón-Cabeza Córdoba, A.: Exploring the effect of mantle composite rheology on surface tectonics and topography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11133, https://doi.org/10.5194/egusphere-egu22-11133, 2022.

EGU22-11488 | Presentations | TS2.1

Ilmenite transformations in suevites from the Ries meteorite impact structure, Germany 

Fabian Dellefant, Claudia A. Trepmann, Stuart A. Gilder, Iuliia V. Sleptsova, and Melanie Kaliwoda

Glass fragments (Flädle) in suevites from Zipplingen within the Ries (Germany) meteorite impact structure contain round aggregates of polycrystalline ilmenite with various amounts of rutile, ferropseudobrookite (FeTi2O5), armalcolite ((Fe,Mg)Ti2O5) and titanite (CaTi[OSiO4]). The 10-100s µm sized aggregates often have a thin rim of µm-sized magnetite grains. The ilmenite grains are 5-10 µm in diameter and form an equilibrium fabric with 4-6-sided grains with smoothly curved grain boundaries and 120° angles at triple junctions, i.e. a so-called foam structure. The ilmenite grains have random crystallographic orientations and do not show any internal misorientations. Rutile, typically a few µm in diameter, is associated with similarly fine-grained ilmenite and a high amount of pores. Coarser polygonal ilmenite grains can also show a marked grain boundary porosity. Only rarely in the center of the aggregates, a deformed single ilmenite crystal occurs, indicating that the aggregates originated from shocked coarse ilmenite crystals from the target gneisses. Ferropseudobrookite is intergrown with remnants of original ilmenite grains or secondary ilmenite grains without foam structure. A vermicular intergrowth of ilmenite, rutile, and magnetite can be present at the rim, where armalcolite can be enriched in Mg.

We interpret that ferropseudobrookite formed at high temperatures (>1010°C) and reducing conditions from coarse ilmenite crystals originating from the target gneisses according to the following reaction: 2FeTiO3 → FeO + FeTi2O5. Some FeO migrated towards the rim due to the low oxygen fugacity, resulting in the observed porosity. Upon cooling, FeO migration caused ferropseudobrookite to disintegrate resulting in the formation of rutile and ilmenite: FeTi2O5 → FeTiO3 + TiO2. Silicate melt at the contact of the FeTi-oxides provided magnesium to form armalcolite from ferropseudobrookite and calcium to form titanite within fractures. Rapid cooling resulted in a shift in redox-conditions with the formation of pure Fe magnetite from FeO at the rim of the aggregates. Quenching of the system can explain the local preservation of ferropseudobrookite and armalcolite, whereas the ilmenite foam structure formed during back reaction of ferropseudobrookite at relatively slower cooling rates. The different cooling rates in the aggregates can be explained by the locally varying amount of surrounding superheated melt forming the Flädle-structure.

How to cite: Dellefant, F., Trepmann, C. A., Gilder, S. A., Sleptsova, I. V., and Kaliwoda, M.: Ilmenite transformations in suevites from the Ries meteorite impact structure, Germany, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11488, https://doi.org/10.5194/egusphere-egu22-11488, 2022.

EGU22-12327 | Presentations | TS2.1

Strain localization and weakening during eclogite-facies transformation in experimentally deformed plagioclase-pyroxene mixtures 

Mathieu Soret, Holger Stünitz, Jacques Précigout, Amicia Lee, and Hugues Raimbourg

The rheology of mafic rocks buried at high to ultra-high-pressure conditions remains enigmatic. Minerals stable at these conditions are mechanically very strong (garnet, omphacite, glaucophane, zoisite, kyanite). In the laboratory, they show plastic deformation only at very high temperature (e.g. > 1000°C for pyroxene and garnet). Yet, viscous shear zones in mafic rocks metamorphosed at amphibolite and eclogite-facies conditions are commonly reported in fossil collisional and subduction zones. These shear zones localize and accommodate large amounts of strain by weakening of the host rocks. This weakening is interpreted as being induced by a transition from grain size insensitive to grain size sensitive creep, in particular with the activation of the dissolution–precipitation creep. However, the exact interplay between deformation, mineral reaction and fluid/mass transfer remains poorly-known.

We have conducted a first series of deformation experiments at eclogite-facies conditions on a 2-phase aggregate representative of mafic rocks. Shear experiments were performed in a new generation of Griggs-type apparatus (Univ. Orléans) at 850°C, and 2.1 GPa with a shear strain rate of 10⁻6 s⁻¹. The starting material consists of mixed powders with < 100 µm sized grains of plagioclase and clinopyroxene from an undeformed sample of the Kågen Gabbro in Northern Norway. Experiments have been conducted with ‘as is’ (dried at 110°C) starting material and with 0.2% added water.

The mechanical data indicate that the samples are first very strong with a peak differential stress at 1.4 GPa. Then, a significant weakening is observed with a stress decrease by 0.5 GPa. The high-strain sample is characterized by a strain gradient increasing toward the center of the shear zone. Metamorphic reactions occur throughout the sample, but the high-strain areas contain considerably more reaction products than the low-strain areas. The nucleation of new phases leads to a drastic grain size reduction and phase mixing, whose intensities are positively correlated with the strain intensity. The nature, distribution and fabric of the mineral products vary also progressively with the strain intensity.

  • In the low-strain areas, dissolution-precipitation processes mainly occur along grain boundaries: plagioclase is rimmed by zoisite and a secondary plagioclase more albitic in composition while clinopyroxene is rimmed by amphibole.
  • In the mid-strain areas, dissolution-precipitation processes are more pervasive: amphibole and a secondary more sodic clinopyroxene occurs in pressure shadows of primary clinopyroxene, while primary plagioclase is completely replaced by a fine-grained mixture of zoisite and quartz. Reaction products show a strong shape-preferred orientation parallel to the shear direction.
  • In the high-strain areas, dissolution-precipitation leads to the nucleation of a fine-grained mixture of garnet and secondary clinopyroxene, quartz and kyanite. Most reaction products have subhedral shape with no clear preferred orientation. Hydrous minerals (amphibole and zoisite) are not observed.

Our preliminary results indicate that strain at eclogite-facies conditions is preferentially accommodated and localized by dissolution-precipitation processes. Further micro-structural and geochemical analyses are required to quantify the exact interplay between the physical and chemical processes controlling the dissolution-precipitation creep.

How to cite: Soret, M., Stünitz, H., Précigout, J., Lee, A., and Raimbourg, H.: Strain localization and weakening during eclogite-facies transformation in experimentally deformed plagioclase-pyroxene mixtures, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12327, https://doi.org/10.5194/egusphere-egu22-12327, 2022.

EGU22-12964 | Presentations | TS2.1 | Highlight | TS Division Outstanding ECS Award Lecture

Crustal stress across spatial scales 

Mojtaba Rajabi and Oliver Heidbach

The study of crustal stress examines the causes and consequences of in-situ stress in the Earth’s crust. Stress at any given point has several geological sources, including ‘short-term and local-scale’ and ‘long-term, ongoing and wide-scale’ source. In order to better characterise the crustal stress state, the analyses of both local- and wide-scale sources, and the consequences of their superposition are required. The global compilation of stress data in the World Stress Map database has increased significantly since its first release in 1992 and its analysis revealed large rotations of the stress tensor in several intraplate settings.

Large-scale stress analysis, so called first-order, (> 500 km) provides information on the key drivers of the stress state that result from large density contrasts and plate boundary forces. The analyses of stress at smaller-scales (< 500 km) have numerous applications in reservoir geomechanics, geo-storage sites, civil engineering and mining industry. To date, numerous studies have investigated the stress analysis from different perspectives. However, the stress, in geosciences, is still enigmatic because it is a scale-dependant parameter. It means, stress variations can be studied at both the ‘spatial-scale’ and ‘temporal-scale’. This paper aims to investigate the crustal stress pattern with a particular emphasis on the orientation of maximum horizontal stresses at various spatial-scales, ranging from continental scales down to basin, field and wellbore scales, to better evaluate the role of various stress sources and their applications in the Earth’s crust. The stress analyses conducted in this work shows that stress pattern at large-scales do not necessarily represent the in-situ stress pattern at smaller-scales. Similarly, analysis of just a couple of borehole measurements in one area might not yield a good representation of the regional stress pattern.

How to cite: Rajabi, M. and Heidbach, O.: Crustal stress across spatial scales, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12964, https://doi.org/10.5194/egusphere-egu22-12964, 2022.

The rheology and mechanisms of strain localisation in the middle and lower crust is yet to be fully constrained, but advances in analytical techniques mean we can revisit previously studied areas and build upon understanding already gained.

A strain profile across a Laxfordian-age (2300-1700 Ma) amphibolite-facies shear zone at Upper Badcall, NW Scotland, provides an excellent backdrop to investigate the hydration-strain-deformation mechanism relationship in the granulite-facies garnet-pyroxene quartzofeldspathic gneiss host rock and cross-cutting 25 m wide isotropic dolerite Scourie dyke. Both the granulite faces gneissic banding and mafic dyke are initially oriented at a high angle to the shear zone boundary. With increasing proximity to the shear zone centre the host rocks become progressively rotated, more deformed and hydrated. Increasing strain results in new foliation development, general grain size reduction and full or partial replacement of pre-existing pyroxene and hornblende by lower-temperature hornblende.

Tatham and Casey (2007) showed the 65 m wide shear zone has an estimated maximum shear strain of 15, which drops to ~7 towards the edge of the shear zone, and falls to < 1 at distances ≥ 40 m from the shear zone centre. We present data from four new transects, taken at 50-100 m intervals along the mafic dyke, which detail the change in deformation style and patterns of strain localisation and intensity. Localised anastomosing high strain zones envelop lenses of undeformed dolerite, with 65-70% of protolith undeformed in the dyke 350 and 230 m from shear zone centre. This decreases to 30 and 0% of undeformed protolith 100 m from and within the shear zone, respectively. Mylonite sensu stricto makes up 10% of dyke at distances ≥ 100 m from the shear zone, which increases to 70% within the shear zone, while the remaining dyke forms a weak fabric evidenced by the shape change of mafic grain aggregates.

Microstructural analyses show a switch in dominant deformation mechanisms from dynamic recrystallisation 350 m from the shear zone, to dissolution-precipitation creep inside the shear zone, identified by a change in crystallographic and shape preferred orientation, and distinct microstructural observations. An introduction of ~10 area % quartz and a loss of feldspar in the mafic dyke inside the shear zone accompanies this switch in dominant deformation mechanisms. We outline microstructural observations characteristic of dissolution-precipitation creep within the shear zone, and propose localised infiltration of quartz-rich fluid facilitates a switch from dislocation creep to pervasive dissolution-precipitation creep resulting in rheological weakening and local strain localisation. Our results suggest that strain localisation in the mid crust may be highly dependent on local fluid availability as fluid presence may trigger a switch in deformation mechanism and, with that, significant localised rheological weakening.

Tatham, D.J. and Casey, M., 2007. Inferences from shear zone geometry: an example from the Laxfordian shear zone at Upper Badcall, Lewisian Complex, NW Scotland. Geological Society, London, Special Publications, 272(1), pp.47-57.

How to cite: Carpenter, M., Piazolo, S., Craig, T., and Wright, T.: The link between water infiltration, deformation mechanisms and strain localisation in the mid crust – an example from the Badcall shear zone, NW Scotland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13366, https://doi.org/10.5194/egusphere-egu22-13366, 2022.

EGU22-13371 | Presentations | TS2.1

Glaucophane plasticity and scale-dependent yield strength from nanoindentation experiments 

Alissa Kotowski, James Kirkpatrick, Christopher A. Thom, Sima A. Alidokht, and Richard Chromik

Subduction interface shear zones localize deformation and sustain plate-boundary weakness on million-year timescales, as well as host a variety of enigmatic seismicity and slow slip transients. A physical understanding of the steady-state and transient mechanics of subduction shear zones requires quantitative constraints of the plastic yield strength and deformation mechanism(s) of metamorphic rocks and minerals that occupy the plate interface. However, very little is known about the rheology of many hydrous minerals that occupy the plate interface, such as glaucophane (end-member sodic amphibole). This is partly because conventional deformation experiments meet technical challenges when trying to measure plasticity in the laboratory due to the stability field of glaucophane, the confining pressure needed to suppress fracture, and the limited range of trade-off between temperature and strain rate in experiments.

 

Here, we present preliminary results from room-temperature nanoindentation experiments on thin sections of glaucophane-rich rocks that produced crystal plasticity by dislocation glide under high-stress conditions. Nanoindentation produces in-situ confining pressure that typically inhibits brittle fracture during loading in favor of plastic deformation. Since the volume of deformation beneath the tips is very small compared to the grain size, each indent is essentially a single-grain mechanical test (i.e., effects of grain boundaries can be ignored). We convert load-depth data from two spheroconical tips of different radii to stress-strain curves to quantify the elastic-plastic transition and characterize post-yield behavior. We measure yield stress as a function of grain orientation. Both post-yield weakening and post-yield hardening occur, which likely reflect brittle fracture along micro-faults/cleavage planes, and dislocation bursts and pile-ups, respectively. Glaucophane hardness decreases with increasing length scale of deformation (i.e., indentation radius), capturing a “size effect” that may reflect an effective decrease in dislocation density as the volume of plastic deformation increases beneath the indent tip. This effect is well-constrained for many metals and some geologic materials, including olivine.

 

The mechanical tests provide a basis for interpreting microstructures of naturally-deformed blueschists, which suggest that glaucophane exhibits recovery-limited dislocation glide and dynamic recrystallization. Low-temperature plasticity may provide a micro-physical framework for long-term strain localization and transient brittle shear when meta-mafic rocks are deformed to high strain.

How to cite: Kotowski, A., Kirkpatrick, J., Thom, C. A., Alidokht, S. A., and Chromik, R.: Glaucophane plasticity and scale-dependent yield strength from nanoindentation experiments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13371, https://doi.org/10.5194/egusphere-egu22-13371, 2022.

EGU22-669 | Presentations | TS2.2

Structural analysis of the alpine orogeny in the western High Atlas, Morocco: New insights through a multiscale approach 

Salih Amarir, Mhamed Alaeddine Belfoul, Khalid Amrouch, Yousef Attegue, and Hamza Skikra

The Moroccan Atlas is an intracontinental chain resulted from an aborted rifting during the Mesozoic time, by an uplifting and moderate shortening during the Late Cretaceous-Cenozoic period. Several studies have highlighted the role of tectonic inversion in the evolution of the High Atlas Range, where strike-slip faults are commonly been considered as a main component of the alpine signature within the High Atlas belt. However, more recent works have focused on the geodynamic model of the evolution of the Atlas Range using different approaches. The structural history and chronology of events are still matter of debates. To contribute to the later, a combined meso and microstructural study was conducted in the western part of the chain. It provided an attempt to quantify paleo-stresses from structural analysis of the Permo-Triassic extensional phase to the tectonic reversal phases, acting from Cenozoic to present days.
This work highlighted two major tectonic phases: (1) the first represented by an extensive regime, with a sub-horizontal minimal stress σ3 oriented NE-SW and linked to the Central Atlantic occurrence. This stage is characterized by pull apart basins genesis in horst and graben morphology. (2) the second phase represented by a weakly tilted compression with a maximum stress σ1 oriented in set NNE-SSW to NNW-SSE. This compression began in the Tertiary, contemporary with the Africa and Europe collision. the related inversions are printed at the paleozoic basement/mesozoic cover interface from the Eastern area to the Jurassic-Cretaceous and Cenozoic plateaus in the West, passing through the Triassic detrital formations of the Argana corridor.
Keywords: Paleo-stress, Structural analysis, Tectonic inversion, Western high Atlas, Morocco, Alpine orogeny.

How to cite: Amarir, S., Belfoul, M. A., Amrouch, K., Attegue, Y., and Skikra, H.: Structural analysis of the alpine orogeny in the western High Atlas, Morocco: New insights through a multiscale approach, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-669, https://doi.org/10.5194/egusphere-egu22-669, 2022.

EGU22-1008 | Presentations | TS2.2

Deciphering the tectonic complexity of the Central High Atlas Mountains using brittle deformation mesostructures and calcite mechanical twinning analysis 

Hamza Skikra, Khalid Amrouch, Abderrahmane Soulaimani, Mustapha Hdoufane, and Salih Amarir

Located in the western segment of the intracontinental Atlas system, the Moroccan Central High Atlas is a NE-SW to ENE-WSW-trending Fold-and-Thrust Belt that is formed during the Cenozoic Alpine orogeny by a positive inversion of Triassic-Jurassic basin. It is structurally distinguished from the other segments of the Moroccan High Atlas orogenic belt by the occurrence of S-shaped ENE-WSW oriented tight anticlinal ridges bounding wider synclines. The elongated ridges core disordered association of plutonic rocks, Liassic carbonate and Late Triassic arigilites, whilst the wider synclines are filled by thick Jurassic series with minor magmatic manifestations expressed by mafic and felsic dikes. The origin of these structures has been ascribed to pre-inversion wrench tectonics with significant compressive component whereas they have been attached to post-rift rift block tilting and or salt tectonics in an alternative view. Characterizing the paleostress history is thereby a crucial matter to unravel the structural evolution of these structures. In order to bring new insights into the actual understanding of the Central High Atlas post-rift structural history, we reconstruct the paleostress tensors preserved in the folded Jurassic series of Anemzi and Tirrhist regions based on brittle deformation structures together with calcite twins stress inversion. The preliminary results highlight the presence of pre-folding layer parallel maximum horizontal stress during three stages: E-W to ENE-WSW, NNE-SSW and NW-SE compressions. Local extensional stress features are observed essentially near diapiric structures and the exhumed magmatic intrusions. The latest structural stage is featured by a post-folding NW-SR compression likely related to the recent phases of the Alpine orogeny.

How to cite: Skikra, H., Amrouch, K., Soulaimani, A., Hdoufane, M., and Amarir, S.: Deciphering the tectonic complexity of the Central High Atlas Mountains using brittle deformation mesostructures and calcite mechanical twinning analysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1008, https://doi.org/10.5194/egusphere-egu22-1008, 2022.

EGU22-1324 | Presentations | TS2.2

Stochastic mechanical analysis of the stress field in a 3D thrust fold 

Anthony Adwan, Bertrand Maillot, Pauline Souloumiac, Christophe Barnes, and Pascale Leturmy

Knowledge of the in-situ stress state is a key factor for any subsurface site characterization and for safe underground geotechnical exploitations. Despite the huge progress in estimating the stress field, understanding the state of stress is still a tedious and challenging endeavor due to incomplete data and sparse information. Moreover, the cost of performing stress measurements is quite elevated while the procedure is delicate and time consuming. Thus, the importance of utilizing geomechanical models for a wider stress evaluation.

We conduct a sensitivity analysis of the stress field with respect to rheological parameters in a kilometric scale thrust fold using a 3D numerical implementation of the theory of Limit Analysis (LA). LA searches for the exact loading force at the onset of failure by bounding it through optimization using a kinematic (upper bound) and a static (lower bound) approach. Elastic parameters are not required, and we only adopt the Coulomb failure criterion characterized by a friction angle and a cohesion.

The 3D geological prototype created, is inspired from the north eastern Jura setting, northern Switzerland, and corresponds to the lateral termination of a partially buried fault cored anticline. It is formed by five material layers with different Coulomb parameters and two different décollement levels. We perform a parametric study by varying the friction angle of the bulk materials, the faults and the shallow décollement.

Our simulations, show various stress distribution patterns depending on the uncertainties related to fault and decollement friction angles. This implies different model behaviors and distinct rupture geometries. However, we identify in particular a stress shielded layer presenting low stress values independently of the parametric variations. Comparing our results with a 2D approach consolidates our findings and highlights the importance of 3D modeling. Finally, we perform a stress analysis of several boreholes taken at various locations. We represent each borehole by an average stress profile with its respective standard deviation. In doing so, we are transforming the parametric variations into stress logs reflecting our uncertainties. This process reveals in particular a counter-intuitive vertical stress decrease with depth near activated blind faults. We argue that this observation is related to material uplift in a compression regime and is only possible for a restricted blind fault.

The aim of this study is to evaluate the possibility of performing real stress data inversion in order to both predict stresses away from the measurements, and determine ranges of compatible rock parameters.

How to cite: Adwan, A., Maillot, B., Souloumiac, P., Barnes, C., and Leturmy, P.: Stochastic mechanical analysis of the stress field in a 3D thrust fold, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1324, https://doi.org/10.5194/egusphere-egu22-1324, 2022.

EGU22-1572 | Presentations | TS2.2 | Highlight

Effects of regional and local stresses on fault slip tendency in the southern Taranaki Basin, New Zealand 

Cecile Massiot, Hannu Seebeck, Andrew Nicol, David D. McNamara, Mark J.F. Lawrence, Angela G. Griffin, Glenn P. Thrasher, and G. Paul D. Viskovic

Determining the potential for faults to slip is widely employed for evaluating fault slip potential and associated earthquake hazards, and characterising reservoir properties. Here we use borehole and 3D seismic reflection data to estimate stress orientations and magnitudes, fault geometries and slip tendency in the southern Taranaki Basin, New Zealand. We highlight uncertainties in maximum horizontal stress (SHmax) magnitude calculations from borehole breakout width and rock strength. As in other settings, breakout width is uncertain on resistivity images because one of the breakout edges often lies in-between the resistivity imager pads, so only a subset of borehole breakouts can be used with confidence. The main uncertainty on SHmax magnitude is the rock strength at the borehole depth at which breakouts form. Given the rarity of basin-specific rock mechanical data, we rely on equations used to convert downhole acoustic compressional wave slowness into rock strength defined in sandstone and mudstones. However, lithologies in the southern Taranaki Basin are commonly muddy sandstones and sandy mudstone that can be interlayered. In addition, we show an example where breakouts are confined to moderately cemented carbonate units without change in acoustic compressional wave slowness. Using a range of rock strength equations based on sandstones and mudstones provides a possible SHmax magnitude range. With only one focal mechanism available in the study area, constraints on SHmax magnitudes from borehole data remain valuable and inform on stresses in the shallow crust.

Although the southern Taranaki basin is undergoing active deformation at plate tectonic scales, the stress magnitudes appear insufficiently high to reactivate the faults assuming a classic coefficient of friction. SHmax azimuths and SHmax:Sv magnitude ratios vary locally between boreholes and with depth. A borehole that intersects an inactive seismic-scale fault and borehole-scale faults over a 150-m interval shows SHmax to rotate by up to 30° proximal to the faults, which are favourably orientated for slip in both strike-slip and normal regimes. The small borehole-scale faults may, however, be active within the inactive seismic scale fault's damage zone. We highlight changes of slip tendency along faults resulting from local variations in the stress field and non-planar fault geometries that could not be resolved using only seismic reflection data and regional stress tensor.

How to cite: Massiot, C., Seebeck, H., Nicol, A., McNamara, D. D., Lawrence, M. J. F., Griffin, A. G., Thrasher, G. P., and Viskovic, G. P. D.: Effects of regional and local stresses on fault slip tendency in the southern Taranaki Basin, New Zealand, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1572, https://doi.org/10.5194/egusphere-egu22-1572, 2022.

EGU22-1823 | Presentations | TS2.2

Tectonic evolution of the northern Verkhoyansk fold-and-thrust belt based on paleostress analysis and U-Pb calcite dating 

Elena A. Pavlovskaia, Andrey K. Khudoley, Jonas B. Ruh, Artem N. Moskalenko, Marcel Guillong, and Sergey V. Malyshev

The formation of the Verkhoyansk fold-and-thrust belt (FTB) is traditionally interpreted as a result of Late Mesozoic subduction and consequent closure of the Oimyakon Ocean, followed by the collision of the Kolyma-Omolon microcontinent with the Siberian Craton. In particular, the northern Verkhoyansk FTB reflects the complex tectonic history and interaction of the Arctic and Verkhoyansk orogens. Although previous studies documented several Cretaceous deformation events, the details of the northern Verkhoyansk evolution are still poorly understood.

A combined structural and geochronological study was carried out to identify the tectonic evolution of the northern Verkhoyansk FTB. Fault and fold geometries and kinematics were used for paleostress reconstruction in the central and western parts of the northern Verkhoyansk FTB. The multiple inverse method was used to separate individual stress fields from heterogeneous fault-slip data and three different stress fields (thrust, normal and strike-slip faulting) were identified. Thrust and normal faulting stress fields were found throughout the study area, whereas a strike-slip faulting stress field was only found in Neoproterozoic rocks in the westernmost part of the northern Verkhoyansk FTB. Furthermore, U-Pb LA-ICP-MS dating of calcite fibers on slickensides was performed to obtain a first-order time constraint on fault activity.

The study reveals the following succession of major deformation events across the northern Verkhoyansk: i) The oldest tectonic event corresponding to the strike-slip faulting stress field with NE-SW-trending compression axis is Early Permian (284±7 Ma) and likely represents a far-field response to the Late Palaeozoic collision of the Kara terrane with the northern margin of the Siberian Craton. ii) A slickenfibrous calcite age of 125±4 Ma is attributed to the most intense Early Cretaceous compression event, when the modern fold and thrust structure developed. Dykes in the eastern part of the northern Verkhoyansk FTB cutting N-S trending folds with 90-85 Ma U-Pb zircon ages mark the end of this event. iii) U-Pb slickenfiber calcite ages of 76-60 Ma estimate the age of a Late Cretaceous–Palaeocene compression event, when thrusts were reactivated. Slickensides related to both (ii) and (iii) compressional tectonic events formed by similar stress fields with W-E trending compression axes. iv) From Palaeocene onwards, extensional tectonics with approximately W-E extension predominated. Within the northern Verkhoyansk FTB, extension settings are supported by the formation of a set of grabens and a clearly recognizable normal faulting stress field.

How to cite: Pavlovskaia, E. A., Khudoley, A. K., Ruh, J. B., Moskalenko, A. N., Guillong, M., and Malyshev, S. V.: Tectonic evolution of the northern Verkhoyansk fold-and-thrust belt based on paleostress analysis and U-Pb calcite dating, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1823, https://doi.org/10.5194/egusphere-egu22-1823, 2022.

In November 2017, an Mw 5.4 earthquake with a shallow (~ 4 km) hypocenter occurred in Pohang, South Korea. Seismotectonics of this region is highlighted by ENE-WSW compression, the dominant stress field in the Korean peninsula, belongs to the Himalayan tectonic domain (HTD), and WNW-ESE or NW-SE compression belongs to the Philippine Sea tectonic domain (PSTD). Here we analyzed the aftershocks, involving focal mechanism of 38 events, to understand the characteristics of the earthquake sequence. Our results show that the mainshock sequence occurred on four ruptures showing a NE-SW trend and in February 2018, one another earthquake (or aftershock) occurred on a NNW-SSE trending rupture at 3.5 km west of the epicenter of the mainshock. Note that aftershocks mainly occurred between two NNE-SSW trending faults: Seonggok Fault and Gokgang Fault.

We analyzed the focal mechanism data as done by fault tectonic analysis. We classified them into several clusters following locations and depths and by whether a population shows a strike-slip faulting type or reverse faulting type. They were classified into several different clusters in the central main area, the northeastern area, and the southwestern area. In the central main area, focal mechanism data of strike-slip faulting type show that the WNW-ESE compression prevails in the depth between 2.0 to 4.0 km and 5.6 to 5.8km, while ENE-WSW compression is dominant between 4.3 and 5.0 km. Those of reverse faulting type display the ENE-WSW compression between 4.7 and 5.7 km deep. This implies that the intermediate depth was affected by the HTD and the upper and lower depths by the PSDT, showing a kind of stress layering.

In the northeastern area, roughly E-W compression prevails between 3.7 and 6.5 km deep, and NW-SE compression between 6.0 and 6.7 km deep. In the southwestern area, roughly E-W compressions were induced in the depth of 4.0 to 5.0 km. E-W compression seems to belong to the combinatory stress state of the HTD and PSTD, and NW-SE compression in the lower part might belong to the stress of PSDT.

The phenomenon of stress layering during the Pohang earthquake reveals that the intervention between the HDT and PSDT resulted in the mainshock and aftershocks of 2017 Pohang earthquake, as in the 2016 Kumamoto, Japan, earthquake.

How to cite: Choi, P., Son, M., and Choi, J. H.: Fault tectonic analysis of focal mechanism data of aftershocks of 2017 Pohang, Korea, earthquake of Mw = 5.4: Stress layering phenomenon between Himalayan and Philippine Sea tectonic domains, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2282, https://doi.org/10.5194/egusphere-egu22-2282, 2022.

EGU22-2969 | Presentations | TS2.2

Determination of the six unknowns of the paleostress tensor from vein data 

Christophe Pascal, Luís Jaques, and Atsushi Yamaji

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 favourably 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 mechanical parameters, 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 pressure of ~250 MPa and differential stress between ~40 and ~90 MPa.

How to cite: Pascal, C., Jaques, L., and Yamaji, A.: Determination of the six unknowns of the paleostress tensor from vein data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2969, https://doi.org/10.5194/egusphere-egu22-2969, 2022.

Semi-brittle flow occurs when crystal plasticity and cataclastic mechanisms operate concurrently and may be common in the middle Earth’s crust. To better constrain the characteristics of semi-brittle deformation, we performed triaxial tests up to 12% strain on dry samples of Carrara marble, spanning a wide range of temperature (T = 20 - 800°C), confining pressures (PC = 30 – 300 MPa), and strain rates (ε'= 10-3 - 10­-6 s-1). The (differential) stress (Δσ = σ1 - PC) and the hardening coefficient (h = ∂Δσ/∂ε ) depend on the applied conditions. At most conditions, Δσ increases with strain, whereas h decreases with increasing strain. At 5% strain, stress and the hardening coefficient increase as T decreases and PC increases: Remarkably, both are relatively insensitive to temperature and to rate in the range of ≈ 200 < T < 400°C. At T ≲ 400°C, the mechanical behavior of the marble is very similar to that exhibited by high-strength, high-ductility, hexagonal metals that deform by processes called twinning induced plasticity (TWIP). Qualitative microstructural observations show that twinning, dislocation motion, and inter- and intra-crystalline micro-fractures are abundant in the deformed samples over the entire range of conditions. The interplay of these deformation mechanisms leads to complex relationships of Δσ and h with the applied ε'  - T ‑ PC  conditions. Models for TWIP behavior suggest that hardening increases with decreasing twin spacing and increasing dislocation density. The low sensitivity of Δσ and h to T at 200 to 400°C may be explained by the relatively low temperature sensitivity of the critical resolved shear stress for twinning and dislocation slip in calcite in this range. None of the existing models for the brittle-ductile transition or the brittle-plastic transition are able to fully predict our experimental results, and micro mechanism-based constitutive laws for semi-brittle deformation are missing so far. Nevertheless, our observations suggest that peak strengths for calcite rocks deforming by semi-brittle processes will occur at PC ‑ T conditions of the middle crust, but that the strengths are probably more strongly influenced by total strain rather than by strain rate.

How to cite: Rybacki, E., Niu, L., and Evans, B.: Semi-brittle Deformation of Carrara Marble: A Complex Interplay of Strength, Hardening and Deformation Mechanisms, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3901, https://doi.org/10.5194/egusphere-egu22-3901, 2022.

Crustal scale low-angle normal faults are typical tectonic features in orogenic post-collisional setting driving the exhumation of deep portions of the orogenic wedge. These extensional structures are commonly active at mid to upper crustal levels within quartz- and feldspar-rich rocks. As deformation localizes along these large-scale shear zones, the understanding of mechanisms controlling their development could provide invaluable insights on the rheology of the continental lithosphere. PT ambient conditions, differential stress, pore fluid pressure and time duration of activity are all factors that could significantly operate on how a shear zones develops in space and time.

We investigated by means of a quantitative approach the evolution of the Simplon Fault Zone (Western Alps, N Italy – Switzerland). We took into account: (i) meso- and microstructures distribution across the shear zone, (ii) its time of activity by 40Ar/39Ar dating of syn-shearing micas, (iii) vorticity distribution across the shear zone and its correlation with mylonite ages, (iv) the estimates of magnitude and variation of differential flow stress and strain rates during shear zone evolution obtained through EBSD-assisted quantitative microstructural analysis. All these data have been combined to reconstruct the structural evolution of the shear zone as the result of the rheological response of involved rocks to changing PT and stress conditions.

The Simplon Fault Zone formed as an extensional detachment accommodating E-W directed lateral extrusion after the collision between Adria and Europe. Several tens of kilometres of extension were accommodated by this structure, allowing the exhumation of the deepest portions of the Central Alps. The shear zone evolved from epidote-amphibolite to greenschist facies and then brittle conditions during shearing. A decrease of simple shear component from c. 90% to c. 40% towards the top of the shear zone is observed, with mylonites displaying ages within the 12-8 Ma time interval. Calculated  differential stress (60-80 MPa) and strain rate (10-11-10-12 s-1) estimates are in agreement with values displayed by several others crustal-scale low-angle normal faults developed at medium to shallow crustal levels.

The quantitative approach used at different scales pointed out that the Simplon Fault Zone experienced a complex evolution, with shear strain that was heterogeneously distributed across the fault zone. Despite this heterogeneity, a general decrease of the simple shear component and increase of the differential flow stress toward the top of the shear zone is clearly defined.

How to cite: Montemagni, C. and Zanchetta, S.: How middle and upper continental crust reacts to prolonged extension: some clues from the Simplon Fault Zone (Central Alps), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5040, https://doi.org/10.5194/egusphere-egu22-5040, 2022.

EGU22-5556 | Presentations | TS2.2

Fast stress-loading and -unloading during faulting and shock indicated by recrystallized grains along quartz cleavage cracks 

Lisa Marie Brückner, Fabian Dellefant, and Claudia A. Trepmann

Recrystallized quartz grains are localized along cleavage cracks in coarse original quartz grains within pseudotachylyte-bearing gneisses from the Silvretta basal thrust, Austria, and in shock-vein-bearing gneisses from the Vredefort meteorite impact structure, South Africa.

In the fault rocks of the Silvretta nappe, the recrystallized grains along two sets of {10-11} cleavage cracks at an angle of about 90° occur in rounded quartz clasts with a diameter of several tens of mm to cm embedded within pseudotachylytes. No evidences of shear offset were found in relation to the cleavage cracks. The fine-grained recrystallized grains have diameters of about 10 ± 6 µm or less and are slightly elongated parallel to the cleavage planes. These new grains have similar but also deviating crystallographic orientations to that of the host. As these quartz microstructures occur exclusively in spatial relation to pseudotachylytes, they are interpreted to result from the associated high stress/high strain-rate deformation. Mechanical (-101) twins in amphibole revealed stresses on the order of 400 MPa during formation of the pseudotachylytes. Yet, the new quartz grains do not show evidence of deformation after their growth, i.e., no internal misorientation, no crystallographic preferred orientation related to dislocation glide. Therefore, we suggest that the secondary quartz grains formed during annealing after the pseudotachylyte-forming event localized at the damage zone surrounding the cleavage cracks at quasi-isostatic stress conditions.

Very similar microstructures are found in Archean gneisses of the Vredefort impact structure, South Africa. There, the recrystallized grains with diameters of few µm along {10-11} and (0001) cleavage planes occur in shocked quartz grains related to mm-sized shock veins, characterized by Schlieren-microstructure of secondary feldspar. Also here, no major shear offset of the cleavage cracks is obvious and the secondary quartz grains do not show evidence of deformation. The observation that quartz shock effects are spatially related to both, the shock veins and secondary quartz grains, suggests that they formed during shock loading and subsequent pressure release with high strain rates (ca. 106 s-1) but minor shearing. Analogous to the Silvretta fault rocks, growth of quartz grains is suggested to occur restricted to the damage zone of the cleavage cracks at quasi-isostatic stresses during post-shock annealing.

In both, the Silvretta fault rocks and shocked gneisses from the Vredefort dome, quartz grains fractured without major shearing at high stresses and subsequently recrystallized localized to the damage zone of cleavage cracks at quasi-isostatic stress conditions. Damage in the process zone surrounding the cleavage cracks must have been large enough for effective grain boundary migration, i.e., growth of grains in orientations weakly controlled by the host orientation. Recrystallization ceased because of the missing driving force during subsequent quasi-isostatic stress conditions. These microstructures indicate quasi-instantaneous loading to high differential stresses of a few hundred MPa and fast unloading to quasi-isostatic stress conditions.

How to cite: Brückner, L. M., Dellefant, F., and Trepmann, C. A.: Fast stress-loading and -unloading during faulting and shock indicated by recrystallized grains along quartz cleavage cracks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5556, https://doi.org/10.5194/egusphere-egu22-5556, 2022.

EGU22-6673 | Presentations | TS2.2

Using surface velocities and subsurface stressing rate tensors to resolve interseismic deep slip distribution on closely spaced faults 

Jack Loveless, Hanna Elston, Michele Cooke, and Scott Marshall

Inversions of interseismic surface velocities alone often struggle to uniquely resolve the 3D fault slip rate distribution along systems with branching or closely spaced faults, such as the southern San Andreas Fault (SAF) in California, USA. Local stress states inferred from microseismic focal mechanisms may provide additional constraints on interseismic deep slip because they contain information about stress at depth and closer to the interseismic deep slip than GPS surface velocities. Here, we invert forward-model generated stressing rate tensors and surface velocities, individually and jointly, to assess how well the inverse approach estimates the distribution of slip rates along both simple and complex fault systems. The inverse approach we present can constrain both the interseismic deep slip rates that reveal fault locking depths and the relative activity of faults. 

We assess the inverse approach by inverting forward model-generated stressing rate tensors and surface velocities to recover fault slip distribution for two models. Forward models that include either a single, planar strike-slip fault or the 3D complex geometry of the southern SAF simulate interseismic loading in a two-step back-slip like approach. The forward models produce surface velocities with a 15 km spacing, which is similar to the GPS station density near the southern SAF, or at GPS station locations in southern California. We utilize the planar fault model to determine the smoothing parameters and stressing rate tensor spacing (>10 km) that minimize the misfit. The 10 km minimum spacing samples crustal volumes that are likely to have >39 focal mechanisms needed to robustly determine a stress tensor. The planar fault model inversions and the availability of focal mechanisms along the southern SAF inform the stressing rate tensor locations that we use to assess the complex model inversion performance. Because focal mechanisms provide normalized deviatoric stress tensors, we invert the full forward-model generated stress tensor as well as the deviatoric and normalized deviatoric stress tensors; this allows us to assess the impact of removing stress magnitude from the inversion.  

The inversions of the forward model-generated stressing rate tensors and surface velocities recover the slip rate distribution well with the exception of the normalized deviatoric stressing rate tensor inversion, which struggles to resolve the fault slip rates in both models. The inversions recover the locking depth with a broader transition zone than prescribed in the forward model due to the smoothing-based regularization within the inversion. The full stressing rate tensor inversion resolves slip rates better than the surface velocity inversions. The deviatoric stressing rate tensor inversion resolves slip rates better than the surface velocity inversion in the complex fault model but not in the planar fault model. Inverting stress and surface velocity information jointly improves the fit to the forward model slip distribution for both models. Joint inversions of both surface velocities and local stress states derived from focal mechanisms may improve constraints on the interseismic deep slip rates and locking depths in regions of complex faulting.

How to cite: Loveless, J., Elston, H., Cooke, M., and Marshall, S.: Using surface velocities and subsurface stressing rate tensors to resolve interseismic deep slip distribution on closely spaced faults, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6673, https://doi.org/10.5194/egusphere-egu22-6673, 2022.

EGU22-6855 | Presentations | TS2.2

Can the crustal strength in the brittle-plastic transition zone be estimated from the flow stress of calcite mylonite? 

Hiroaki Yokoyama, Jun Muto, and Hiroyuki Nagahama

Quantifying crustal strength is essential to understanding lithosphere strengths and tectonic processes, such as long-term fault movements caused by plate motions. In this study, we estimated the strength of granitic upper crust using recrystallized grain size piezometer of calcite mylonite intercalated in the Cretaceous granitic Abukuma Mountains. In addition, Raman carbonaceous material thermometer was used to constrain the deformation temperature. Calcite mylonites are originated from Late Carboniferous Tateishi Formation and locate along Shajigami shear zone at eastern margin of Abukuma Mountains, Northeastern Japan. Shajigami shear zone is a strike-slip shear zone active during the Middle Cretaceous. Along Shajigami shear zone, calcite mylonite and granitic cataclasites expose.

Calcite grains are well recrystallized, and the grain size are determined by electron backscattered diffraction (EBSD) mapping with the step sizes of 2-2.5µm. The mean grain sizes are 17-26 µm. The differential stress estimated by recrystallized grain size piezometer of calcite aggregate (Platt and De Bresser, 2017) is 35-80 MPa. The estimated metamorphic temperature using the Raman carbonaceous material thermometer (Kouketsu et al., 2014) is 340-250 ˚C. The difference in estimated metamorphic temperature is attributed to the thermal effects of the Cretaceous granitoids that penetrated along the calcite mylonite. This is because the estimated metamorphic temperature is higher the closer to the granitoid. Because well dynamically recrystallized calcite grains indicate that the deformation temperature exceeding 200˚C, the estimate by Raman carbonaceous material thermometer is the upper bound for the deformation temperature.

The calcite mylonite and the granitic cataclasite are thought to have formed at the same time in the Shajigami shear zone (Watanuki et al., 2020). Although there is a slight temperature gradient near the granite, widespread deformation has occurred in this area. The deformation temperature obtained in this study is the deformation around the brittle-plastic transition zone of the upper crust. Hence, the collecting flow stress estimated from calcite mylonite intercalated in brittle granitic shear zone may be possible to constrain the stress magnitude of the shear zone data near the brittle-plastic transition at 200-300°C.

 

References

Platt and De Bresser, 2017, J. Struct. Geol., 105, 80-87.

Kouketsu et al., 2014, Island arc, 23, 33-50.

Watanuki et al., 2020, J. Struct. Geol., 137, 104046.

How to cite: Yokoyama, H., Muto, J., and Nagahama, H.: Can the crustal strength in the brittle-plastic transition zone be estimated from the flow stress of calcite mylonite?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6855, https://doi.org/10.5194/egusphere-egu22-6855, 2022.

EGU22-7253 | Presentations | TS2.2

Paleostress and paleoburial history of a post-rift, supra-salt, carbonate reservoir offshore Congo (Atlantic): Insights from calcite twinning and stylolite roughness paleopiezometry. 

Anies Zeboudj, Boubacar Bah, Olivier Lacombe, Nicolas E. Beaudoin, Claude Gout, Nicolas Godeau, and Jean-Pierre Girard

Our understanding of the temporal variation of past stress in the crust is usually pictured in relation to tectonic contexts, where it helps decipher the evolution of deformation of rocks at different scales. The paucity of paleostress reconstructions in passive margins makes the knowledge of the origin of stress and of its evolution very incomplete, especially in poorly accessible offshore parts. Moreover, in salt-rich passive margins like the offshore Congo margin, one may question whether the state of stress in supra-salt formations is mainly controlled by salt tectonics, since the salt usually acts as a decoupling level that prevents the transmission and record of far-field crustal stresses. This study focuses on the analysis of an offshore wellbore core of the Albian, post-rift carbonates of the Sendji Fm that directly overlies the salt of the Aptian Loeme Fm in the Lower Congo Basin. Paleopiezometry based on stylolite roughness and mechanical twins in calcite was combined with fracture analysis, laser U-Pb dating of calcite cement, and burial modeling to unravel the tectonic and burial evolution of the Sendji Fm over time. The results of bedding-parallel stylolite roughness inversion constrain the range of depth over which the Sendji Fm strata deformed under a vertical principal stress s1 to 650-2800 m (median ~1100m). Projection of this depth range onto the Sendji burial model derived from TemisFlow™ basin modelling indicates that pressure solution was active from 105 to 12 Ma. Inversion of calcite mechanical twins measured within the early diagenetic cement (U-Pb age = 100 +/- 1Ma) yields two main states of stress: (1) an extensional stress regime with a horizontal σ3 trending ~E-W associated with sub-perpendicular N-S compression, and (2) a strike-slip stress regime with a horizontal σ1 trending ~E-W (changing from pure E-W compression to N-S extension through stress permutations). We interpret the former state of stress as local and related to the complex geometric interactions between moving halokinetic normal faults, while the latter presumably reflects the push effect of the Atlantic ridge, which prevailed from 12 Ma until present-day. Our results highlight that the stress history of the studied part of the offshore Lower Congo Basin passive margin has first been mainly dominated by burial and local normal faulting related to late Cretaceous to Miocene post-rift salt tectonics, then by a regional stress presumably originated from the far-field ridge push from ~12Ma onwards, which would indicate some mechanical re-coupling between the crust and the sedimentary cover during the Miocene.

Keywords: stress, paleopiezometry, calcite twins, stylolites, passive margin, salt.

How to cite: Zeboudj, A., Bah, B., Lacombe, O., Beaudoin, N. E., Gout, C., Godeau, N., and Girard, J.-P.: Paleostress and paleoburial history of a post-rift, supra-salt, carbonate reservoir offshore Congo (Atlantic): Insights from calcite twinning and stylolite roughness paleopiezometry., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7253, https://doi.org/10.5194/egusphere-egu22-7253, 2022.

EGU22-7305 | Presentations | TS2.2

A new free software to reconstruct stress trajectories: the Atmo-stress service 

Sofia Bressan, Olympia Gounari, Valsamis Ntouskos, Noemi Corti, Fabio Luca Bonali, Konstantinos Karantzalos, and Alessandro Tibaldi

The reconstruction of present-day stress and palaeostress trajectories is of paramount importance to study the tectonic regime and its evolution, in a specific area. Its comprehension is crucial also for seismic and volcanic hazard assessment, especially focusing on the shallow crust.

In the framework of the NEANIAS project (https://www.neanias.eu/), EU H2020 RIA, it has been developed the so called ATMO-Stress service (https://docs.neanias.eu/projects/a2-1-service/en/latest/), an open-source cloud service, currently hosted on the GARR Kubernetes platform, which allows to calculate stress trajectories in plain view, based on the concepts from Lee and Angelier (1994). It is designed to run on modern computers for both academics and non-academics purposes, spanning from research activity to oil and gas industries, natural hazard prevention and management.

The service is freely accessible at https://atmo-stress.neanias.eu/ and is designed to calculate the stress trajectories for a specific area, considering as input the same type of stress (e.g. σHmax or σHmin). Data input can be from different sources (e.g. field data, focal mechanism solutions, in-situ geotechnical measures). They must be listed in a homogeneous ASCII text file or Excel file format, including the geographic coordinates, azimuth of the stress and the angular error. The service is capable of processing data from local to regional scale. Following the principles from Lee and Angelier (1994), the trajectory calculation can be done using different parameters and settings. The outputs can be seen directly on the website and can be downloaded with file formats ready to be imported and analyzed in GIS environment and Google Earth.

How to cite: Bressan, S., Gounari, O., Ntouskos, V., Corti, N., Bonali, F. L., Karantzalos, K., and Tibaldi, A.: A new free software to reconstruct stress trajectories: the Atmo-stress service, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7305, https://doi.org/10.5194/egusphere-egu22-7305, 2022.

Reconstructing the brittle structural history of a complex strike-slip fault system remains a challenging process in paleostress reconstructions. Here, we investigated the small-scale brittle structures such as shear fractures and tension joints which are well developed in the Early Paleozoic Inkisi red sandstones in the “Pool” region of Kinshasa and Brazzaville, along the Congo River. The fracture network affects the horizontally bedded sandstones with alternating cross-bedded, horizontally bedded and massive layers. The fractures are particularly dense and of various orientation in the rapids of the Congo River just downstream Kinshasa and Brazzaville. They control the channels of the Congo River in its connection to the Atlantic Coast.

A total of 1150 factures have been measured and assembled into a single data file, processed using the Win-Tensor Program. They contain only a limited number of kinematic indicators for slip sense (displaced pebbles, irregularities on striated surfaces, slickensides) or extension (plume joints). Before interactive fault-slip data separation into subset and stress inversion, a kinematic data analysis evidenced at least three different phases of brittle deformation, each starting by the formation of plume joints and evolving into a strike-slip fault system. We used the principle of progressive saturation of the rock mass by the apparition of new faults or the reactivation of already existing ones during the successive brittle stages. We combined the stress inversion of fault-slip data, fault-slip tendency analysis and data separation in order to obtain well-separated data subsets, each characterized by its own paleostress tensor. The total data set can be explained by the action of 4 different brittle deformation and related paleostress stages, all of strike-slip type. There possible age is estimated from stratigraphic relations and the known geological history of the area.

The oldest stage developed in intact rock under NW-SE horizontal compression, probably before the Jurassic unconformity that affects the entire Congo Basin. It generated dominantly N-160°E striking left-lateral faults. The second stage generated dominantly new N050°E striking right-lateral faults, at a high angle from the ones of the previous stage, under NE-SW horizontal compression. They are estimated to be related to ridge push forces from the opening of the Atlantic Ocean during the Oligocene. The third stage, which corresponds to N-S horizontal compression, generated additional N030°E and N340°E conjugated fractures and reactivated the preceding fracture networks. A fourth and relatively minor system was also identified with WNE-ESE horizontal compression but its chronological relation with the other ones is not clear. 

How to cite: Delvaux, D., Miyouna, T., Boudzoumou, F., and Nkodia, H.: Using combined paleostress reconstruction and slip tendency for reconstructing the brittle structural history of a complex strike-slip fault system: Fault-controlled origin and evolution of the “Pool” area between Kinshasa and Brazzaville, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7401, https://doi.org/10.5194/egusphere-egu22-7401, 2022.

EGU22-8449 | Presentations | TS2.2 | Highlight

Numerical modelling of current state of stress in the Geneva Basin and adjacent Jura fold-and-thrust belt (Switzerland and France). 

Sandra Borderie, Jon Mosar, Louis Hauvette, Adeline Marro, Anna Sommaruga, and Michel Meyer

The Northern Alpine foreland is divided into two domains: the Molasse Basin and the Jura fold-and-thrust belt (FTB). The Mesozoic and Cenozoic sedimentary cover of this area is deformed by thrust-related folds and strike-slip faults. The main structures root in a basal Triassic décollement. The Geneva Basin, located in western Switzerland, is part of the Plateau Molasse (belonging to the Molasse Basin), and is limited to the NW by the Jura FTB, to the SW by the Vuache fault, and to the SE by the Mont Salève ramp related anticline and the Subalpine Molasse.

If current seismicity indicates that the Geneva Basin is tectonically active, few data regarding the state of stress in the area are currently available. The goal of this study is to densify the knowledge of the state of stress in the Geneva Basin and in the adjacent Jura FTB, by using numerical modelling.

The first part of the study is a regional study. In a 2D section, we study the impact of the friction along the basal décollement, on the localisation of deformation and on the associated stress field. Results indicate that depending on the friction, deformation will localise at the rear of the Mont Salève, in the Geneva Basin or at the frontal part of the Jura FTB. In the range of frictions where deformation localises in the Geneva Basin, the distribution of stress varies. Differential stress is higher and more localised for higher basal frictions.

The second part of the study is more local. The prototype section is based on seismic interpretation of a seismic surveys in the Geneva Basin. We study the impact of friction along the inherited faults on incipient deformation. Results indicate that a decrease in the fault’s friction allows forwards propagation of deformation and allows reactivation of inherited faults. If the friction in the faults is too low, deformation will localise at the first inherited fault (i.e. the Salève thrust in this case study). The stress fields vary depending on the localisation of deformation. Stress magnitudes are lower and more distributed when all faults have the same friction. The more deformation is localised on a structure, the more stress concentration is observed.

These results allow to better constrain the mechanical context of these sections and to populate this part of the Northern Alpine foreland with stress data.

How to cite: Borderie, S., Mosar, J., Hauvette, L., Marro, A., Sommaruga, A., and Meyer, M.: Numerical modelling of current state of stress in the Geneva Basin and adjacent Jura fold-and-thrust belt (Switzerland and France)., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8449, https://doi.org/10.5194/egusphere-egu22-8449, 2022.

EGU22-8886 | Presentations | TS2.2

Reconstructing stress magnitude evolution in deformed carbonates: a paleopaleopiezometric study of the Cingoli anticline (North-Central Apennines, Italy) 

Aurélie Labeur, Nicolas E. Beaudoin, Olivier Lacombe, Lorenzo Petraccini, and Jean-Paul Callot

Picturing the distribution of stress, in term of magnitude and orientation, during the development of a fold-and-thrusts belt is key for many fundamental and applied purposes, e.g., crustal rheology, orogen dynamics, fluid dynamics and prediction of reservoir properties. Specific meso- and micro-structures observed in fold-and-thrust belts and related forelands (i.e., faults, stylolites, veins and calcite twins), on top of being good markers of the deformation sequence that affected the rocks before, during and after folding and thrusting, can be used to access the past stress orientation and/or magnitude. This study reports the result of a paleopiezometric analysis of calcite twins and stylolite roughness documented in Mesozoic carbonates cropping out in the Cingoli anticline, an arcuate fold in the Umbria-Marche Apennine Ridge (UMAR), where a complex fracturing sequence was highlighted in a previously published study. The stylolite roughness inversion technique (SRIT) was applied to tectonic stylolites related to early folding layer-parallel shortening (LPS), and the calcite twin inversion technique (CSIT) was applied to cements from veins related to either foreland flexure or LPS. Both inversion processes require somehow the knowledge of the depth at which deformation occurred, as the vertical stress is an input for SRIT in the case of its application to tectonic stylolites, and as the differential stress magnitudes obtained by CSIT combined to vertical stress magnitude provides access to the absolute principal stress magnitudes. Building on a previously published time-burial path valid for the studied strata at the Cingoli anticline that also predicted the timing of each deformation stage, we quantify differential and principal stress magnitudes at the scale of the anticline. Beyond regional implications, our approach helps improve our knowledge of the past stress magnitudes in folded carbonate reservoirs.

How to cite: Labeur, A., Beaudoin, N. E., Lacombe, O., Petraccini, L., and Callot, J.-P.: Reconstructing stress magnitude evolution in deformed carbonates: a paleopaleopiezometric study of the Cingoli anticline (North-Central Apennines, Italy), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8886, https://doi.org/10.5194/egusphere-egu22-8886, 2022.

Shear zones associated with major thrust faults generally record overprinting of deeper crustal deformation signatures by shallower crustal signatures due to faults climbing up-section along the transport direction. In this study, we investigate the deformation signatures related to the shallow crustal conditions on one such major thrust, the Ramgarh thrust (RT) from Sikkim Himalayan Fold Thrust Belt (FTB). RT is an intermediate crustal thrust that has recorded a translation of ~58-65 km and overprinting of deformation structures. RT acts as the roof thrust of Lesser Himalayan duplex, hence got reactivated several times, and records a long deformation history.

In Sikkim Himalaya, the frontal most exposure of the RT is near Setikhola (N26° 56.178’, E88° 26.607’) as ~57m thick shear zone that exposes the lower Lesser Himalayan Daling quartzite and phyllite in the hanging wall over Gondwana sandstone in the footwall. The mean bedding is oriented ~72°, 298°, and the mean dominant cleavage is ~ 70°, 305°. The outcrop forms the overturned forelimb of a fault-bend antiform. The outcrop is strongly fractured. Based on the angular relationship with respect to the bedding, three sets of fractures were identified. Low angle fractures (< 30° to bedding) constitute ~20.23 %, moderate (30° – 60° to bedding) and high angle fractures (60°- 90° to bedding) constitute ~39.88% of the total fracture population. The fractures are uniformly distributed throughout the stretch of the shear zone. Daling quartzites accommodate more number of fractures than the phyllites. Preliminary investigation indicates that the thicker beds have higher fracture intensity than thinner beds. Few of the fractures were identified as opening mode fractures based on their association with the plumose structures. ~ 17.3% of the total measured fractures records slickenline lineations. These shear fractures reveal two clusters on the stereonet (Set 1: ~90°, 098°; Set 2: ~77°, 331°). They have a dihedral angle of ~54⁰ and set 1 and set 2 are oriented ~ 27⁰ and ~ 32⁰ to the bedding respectively. Based on preliminary analysis, the local maximum principal stress (σ1) is oriented sub-horizontally with a SSW trend. Interestingly, this estimate is in agreement with the current global stress orientations from the Eastern Himalaya, where σ1 is near horizontal and trends NNE – SSW (Larson et al., 1999).

How to cite: Jk, A. and Bhattacharyya, K.: Preliminary fracture analysis from the frontal most exposure of a major roof thrust in the Eastern Himalaya: Insights from the Ramgarh thrust, Sikkim Himalaya., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9168, https://doi.org/10.5194/egusphere-egu22-9168, 2022.

We estimate paleostress orientations (σ1, σ2 and σ3), stress ratios (φ) and driving pressure ratios (R′) from the extension veins exposed within the Buxa dolomite of the frontal Main Boundary thrust (MBT) sheet in the Siang valley, Arunanchal Lesser Himalaya. Based on the angular relationship with the bedding, the fractures and veins were divided into low-angle (<30°), moderate-angle (~30°-60°) and high-angle (>60°) sets. Observations in the field as well as at a microscopic level indicate that the high- and moderate-angle veins overprint the low-angle veins implying that the latter are the oldest. The high-angle veins are the most dominant set (~49%; mean orientation: ~23°, ~141°) followed by the moderate- (~31%; mean orientation: ~70°, ~176°) and the low-angle (~20%; mean orientation: ~58°, ~224°) set. The poles to the low- and high-angle veins define a clustered distribution in the stereoplot indicating that the pore fluid pressure (Pf) was less than the intermediate principal stress (σ2) during the formation of these vein sets. In contrast, the poles to the moderate-angle veins mark a girdled pattern in the stereoplot indicating that the pore fluid pressure (Pf) exceeded the intermediate principal stress (σ2) during their formation. On applying the stress inversion method (Yamaji et al., 2010) to the veins, 5 different generations of veins are revealed. Preliminary microstructural study indicates that the low-angle veins are dominantly quartz-rich, whereas the high-angle veins are dominantly calcite-rich indicating the presence of multiple generations of veins. The study also indicates the presence of blocky texture in the veins with the growth direction of the mineral grains at a high angle to the vein wall. Based on the stress ratio (φ), driving pressure ratio (R′) and the orientation of stress axes associated with each generation, the different generations of veins most likely formed under different stress conditions.

How to cite: Behera, S. S. and Bhattacharyya, K.: Characterization of vein-sets and estimation of stress orientations and stress ratios from the Buxa dolomite, Main Boundary thrust (MBT) sheet, Siang Valley, Arunanchal Lesser Himalaya, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9169, https://doi.org/10.5194/egusphere-egu22-9169, 2022.

 

Estimating deformation conditions from shear zone rocks is critical in understanding its complex deformation history. However, often the deformation conditions from mylonite provide information on the finite deformed state conditions. On the contrary, if there are veins preserved, they may record incremental strain stages during progressive deformation. Thus, we used veins as incremental strain markers to evaluate the spatial and temporal variation in deformation conditions along the transport direction of a major shear zone. We estimated vein attributes at the microscopic scale, deformation temperature, flow stress, and strain rate from the Pelling-Munsiari thrust in the Sikkim Himalaya. It is a regionally folded thrust that acts as the roof thrust of a complex Lesser Himalayan duplex. The PT zone is exposed as ~938 m and ~188 m thick quartz-mica mylonite zone at the hinterland-most (Mangan) and the frontal exposures (Suntaley) in eastern Sikkim, respectively. The PT zone is subdivided into three domains where the protomylonite domain is surrounded by mylonite domains on both sides.

We recognize three different vein-sets based on the angular relationship to the mylonitic foliation. At both the locations of the PT zone, the low-angle (0-30°) is the dominant vein-set followed by moderate-angle (30-60°) and high-angle (60-90°). Based on the cross-cutting relationship, we find high-angle vein set is the youngest. The low-angle vein-sets are dominant in both these locations. We observed multiple crack-and-sealed events in Mangan, indicating repeated failure and mineral precipitation. In contrast, we do not observe any such texture in the veins that are preserved in the frontal exposure of the PT zone. At both the PT zones, there are higher distribution of veins near the footwall. In the hinterland, veins record coarser grain sizes in the protomylonite domain than in the mylonite domain. However, we observed a different trend in the frontal exposure, where veins from the mylonite domain record coarser grain sizes. In both locations, quartz grains dominantly exhibit the subgrain rotation recrystallization mechanism. We semi-quantitatively estimate a first-order deformation temperature using the recalibrated quartz recrystallization thermometer (Law, 2014). In the hinterland, the low-angle vein-set records the highest deformation temperature. In contrast, high-angle veins record higher deformation temperature in the foreland. Following Stipp et al. (2003) and Twiss (1977), we estimate flow stress from recrystallized quartz grain-size piezometer. The high-angle (~24.71MPa) vein-set records the highest flow stress in the hinterland. In comparison, moderate-angle (~29.55MPa) veins record the highest flow stress in the foreland exposure. Following Hirth et al. (2001), we estimated similar strain rates (~10-15 sec-1) from both locations. The three sets of veins record different deformation conditions in both locations suggesting different incremental strain stages. Interestingly, the high-angle veins record the fastest strain rate (~6*10-15 sec-1) in the hinterland most exposure, whereas, in the frontal part the moderate-angle veins record the fastest strain rate (~9*10-15 sec-1).

How to cite: Ranjan, R. and Bhattacharyya, K.: Estimation of deformation temperature, flow stress, and strain rate from the veins of an internal shear zone: Insights from Pelling-Munsiari thrust, Sikkim Himalaya, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9170, https://doi.org/10.5194/egusphere-egu22-9170, 2022.

The North Anatolian Fault experienced large earthquakes with 250–400 years recurrence time. In the Marmara Sea region,
the 1999 (Mw=7.4) and the 1912 (Mw =7.4) earthquake ruptures bound the Central Marmara Sea fault segment. Using
historical-instrumental seismicity catalogue and paleoseismic results (≃ 2000-year database), the mapped fault segments, fault
kinematic and GPS data, we compute the paleoseismic-seismic moment rate and geodetic moment rate. A clear discrepancy
appears between the moment rates and implies a signifcant delay in the seismic slip along the fault in the Marmara Sea. The
rich database allows us to identify and model the size of the seismic gap and related fault segment and estimate the moment
rate defcit. Our modelling suggest that the locked Central Marmara Sea fault segment (even including a creeping section)
bears a moment rate defcit 6.4 × 1017 N.m./year that corresponds to Mw ≃ 7.4 for a future earthquake with an average
≃ 3.25 m coseismic slip. Taking into account the uncertainty in the strain accumulation along the 130-km-long Central fault
segment, our estimate of the seismic slip defcit being ≃ 10 mm/year implies that the size of the future earthquake ranges
between Mw=7.4 and 7.5.

Reference:

[1] Meghraoui, M., Toussaint, R. & Aksoy, M.E. The slip deficit on the North Anatolian Fault (Turkey) in the Marmara Sea: insights from paleoseismicity, seismicity and geodetic data. Med. Geosc. Rev. 3, 45–56 (2021). https://doi.org/10.1007/s42990-021-00053-w

How to cite: Meghraoui, M., Toussaint, R., and Aksoy, M. E.: Stress evolution and slip deficit on the North Anatolian Fault (Turkey) in the Marmara Sea: insights from paleoseismicity, seismicity and geodetic data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11676, https://doi.org/10.5194/egusphere-egu22-11676, 2022.

EGU22-13201 | Presentations | TS2.2

Joint inversion of tectonic stress and magma pressures using dyke trajectories 

Frantz Maerten, Laurent Maerten, Romain Plateaux, and Pauline Cornard
    In volcano-tectonic regions, dyke propagation from shallow magmatic chambers are often controlled by ambient perturbed stress field. The variations of the stress field result from combining factors including, but not exclusively, the regional tectonic stress and the pressurized 3D magma chambers. In this contribution, we describe and apply a new multiparametric inversion technique based on geomechanics that can invert for both the far field stress attributes and the pressure of magma intrusions, such as stocks and magma chambers, constrained by observed dyke orientations. This technique is based on a 3D boundary element method (BEM) for homogeneous elastic half-space where magma chambers are modelled as pressurized cavities. To verify this approach, the BEM solution has been validated against the known 3D analytical solution of a pressurized cylindrical cavity. Then, the effectiveness of this technique and its practical use, in terms of mechanical simulation, is demonstrated through natural examples of dyke network development affected by magma intrusions of two different volcanic systems, the Spanish Peaks (USA) and the Galapagos Islands (Ecuador). Results demonstrate that regional stress characteristics as well as pressure of magma chambers can be recovered from observed radial and circumferential dyke patterns.

 

How to cite: Maerten, F., Maerten, L., Plateaux, R., and Cornard, P.: Joint inversion of tectonic stress and magma pressures using dyke trajectories, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13201, https://doi.org/10.5194/egusphere-egu22-13201, 2022.

EGU22-13203 | Presentations | TS2.2

Late Cenozoic faulting deformation of the Fanshi Basin (Northern Shanxi rift, China), inferred from paleostress analysis of mesoscale fault-slip data 

Markos D. Tranos, Konan Roger Assie, Yu Wang, Huimin Ma, Kouamelan Serge Kouamelan, Eric Thompson Brantson, Liyun Zhou, and Yanick Blaise Ketchaya

The Fanshi Basin is one of the NE-SW-striking depocenters formed along the northern segment of the fault-bounded Shanxi rift. In order to understand the crustal driving stresses that led to the basin formation and development, a paleostress analysis of a large number of fault-slip data collected mainly at the boundaries of the basin was accomplished. The stress inversion of these data revealed three stress regimes. The oldest SR1 was a Neogene stress regime giving rise to a strike-slip deformation with NE-SW contraction and NW-SE extension. SR1 activated the large faults trending NNE-NE, i.e., (sub) parallel to the main strike of the Shanxi rift, as right-lateral strike-slip faults. It was subjected to the Shanxi rift before the activation of the Fansi Basin boundary fault, i.e., the Fanshi (or Wutai) fault, as a normal fault. The next is a short-lived NE-SW extensional stress regime SR2 in the Early Pleistocene, which shows the inception of the basin's extension. A strong NW-SE to NNW-SSE extensional stress regime SR3 governed the northern segment of the Shanxi rift since the Late Pleistocene and is the present-day extension. It gives rise to the current half-graben geometry of the Fanshi Basin by activating the Fanshi (or Wutai) fault as a normal fault in the southern part of the graben. Because of the dominance of the NW-SE to NNW-SSE extension, which is perpendicular to the NE-SW extension, mutual permutations between σ3 and σ2 due to inherited fault patterns might occur while the stress regime changed from SR1 to SR3.

How to cite: Tranos, M. D., Assie, K. R., Wang, Y., Ma, H., Kouamelan, K. S., Thompson Brantson, E., Zhou, L., and Blaise Ketchaya, Y.: Late Cenozoic faulting deformation of the Fanshi Basin (Northern Shanxi rift, China), inferred from paleostress analysis of mesoscale fault-slip data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13203, https://doi.org/10.5194/egusphere-egu22-13203, 2022.

EGU22-13406 | Presentations | TS2.2

Paleoburial and paleostress history of a carbonate syn-rift reservoir : constraints from inversion of calcite twins and stylolite roughness in the Toca formation (Lower Congo Basin, South Atlantic) 

Boubacar Bah, Olivier Lacombe, Nicolas Beaudoin, Jean-Pierre Girard, Claude Gout, and Nicolas Godeau

To construct accurate geological models of reservoirs and better predict their properties, it is critical to have a good understanding of the burial and stress history of the host sedimentary basin over time. Stress and strain are important factors influencing the preservation or reduction of reservoir porosity and permeability. One way to access the orientations and magnitudes of paleostresses is to use paleopiezometers. This study aims at reconstructing the stress and burial history of the syn-rift Barremian (130-125 Ma) Toca Fm in the Lower Congo basin (West African passive margin) using stress inversion of calcite mechanical twins and sedimentary and tectonic, bedding-parallel stylolite. This combined approach was applied to two oriented borehole cores drilled in a poorly deformed oil field, offshore Congo, and provided constraints on both paleostress orientations and magnitudes. The timing of the different paleostress regimes documented was derived from a burial-time model reconstructed by use of TemisFlowTM.

The inversion of calcite twins was performed on a widespread early diagenetic cement (dated 127.4 ± 4.9 to 123.1 ± 7.7 Ma by U-Pb LA-ICPMS) and revealed two types of stress regimes. (1) An extensional stress regime with σ1 vertical and σ3 oriented either N50°±20° or N120°±20°, and mean differential stresses of 45 MPa for (σ1-σ3) and 20 MPa for (σ2-σ3). The NE-SW (N50°±20) extensional direction, which restores to N100° after moving back Africa to its position at Barremian times, marks the syn-rift extension that led to the opening of the South Atlantic. The 120° direction (~N-S after restoration) possibly reflects local perturbation and/or σ2-σ3 permutations during rifting in response to tectonic inheritance. (2) A compressional or strike-slip stress regime with horizontal σ1oriented ~E-W (and associated N-S extension) and mean differential stresses of 40 MPa for (σ1-σ3) and 15 MPa for (σ2-σ3). This suggests that the basin underwent a post-rift compressional history during the continuous burial of the Toca formation possibly related to the Atlantic ridge push effects. For the first time, we also reconstructed paleostress orientations from “tectonic” bedding-parallel stylolites, that developed during a tectonic extensional phase. The results point to a NE-SW extension consistent with the direction of the syn-rift extension revealed by calcite twinning. In order to constrain the sequence of stress evolution, we used the results of sedimentary stylolite roughness inversion paleopiezometry, which documents that the burial-related pressure solution in the Toca Fm occurred in the 400-1700m depth range (dissolution along 90% of stylolites halting between 700 and 1000m). Projection of this depth range onto the TemisFlowTM reconstructed burial-time curve of the Toca Fm indicates that vertical pressure solution was active between 122 and 95 Ma, and therefore that σ1 switched from vertical to horizontal around 95 Ma. Our study reveals that the Toca Fm has undergone a complex polyphase stress history during burial, with stress regimes evolving from extensional to compressional/strike-slip. It also illustrates the great usefulness of combining stress inversion of calcite twins and stylolite roughness with a burial-time model to constrain the stress history of a deeply buried reservoir.

How to cite: Bah, B., Lacombe, O., Beaudoin, N., Girard, J.-P., Gout, C., and Godeau, N.: Paleoburial and paleostress history of a carbonate syn-rift reservoir : constraints from inversion of calcite twins and stylolite roughness in the Toca formation (Lower Congo Basin, South Atlantic), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13406, https://doi.org/10.5194/egusphere-egu22-13406, 2022.

The NNW-SSE trending Koziakas-Itamos Mts of Western Thessaly, Central Greece, constitute the innermost part of the External Hellenides, i.e., the Hellenic fold-and-thrust belt, formed from the Tertiary orogenic (alpine) processes due to collision between the Apulia and Eurasia plates. Along these mountains, large ophiolite masses have thrust towards WSW over Mesozoic carbonate and clastic rocks, which in turn thrust over the Tertiary flysch rocks of the Pindos Unit. The mountains bound the NW-SE trending late-alpine Mesohellenic Trough to the east, filled with Late Eocene to Miocene molasse-type sediments, and the younger Thessaly basin filled up with Neogene and Quaternary sediments.

 

A detailed paleostress reconstruction based on the fault-slip analysis and the stress inversion through the TR method (TRM) unravels a multi-stage deformation history for the innermost parts of the Hellenic fold-and-thrust belt. More precisely, the late orogenic faulting deformation temporally constrained in Late Oligocene to Middle Miocene was originally driven by stress regimes that define an ENE-WSW ‘real’ compression normal to the orogenic fabric associated with mainly NE-directed back thrusts. The compression shifted to ‘hybrid’ with the activation of oblique- and strike-slip faults. After that stage, the hybrid compression predominates with counterclockwise changes in the trend of the greatest principal stress axis (σ1) from ENE-WSW to NNE-SSW. The last stage of the late-orogenic faulting deformation is an NW-SE orogen parallel extension segmenting and differentiating the NNW-SSE orogenic fabric along its strike.

 

Post-orogenic faulting deformation is driven by extensional stress regimes that caused the basin-and-range topography and the formation of well-established basins filled up with Late Miocene and younger sediments like the Thessaly basin. In particular, an ENE-WSW pure extension normal to the orogenic fabric has been defined. A general counterclockwise rotation of the least principal stress axis (σ3) occurred, initially giving rise to NE-SW  extension-transtension during Late Miocene-Pliocene and NNE-SSW extension-transtension since the Quaternary.

How to cite: Neofotistos, P. and Tranos, M.: Multi-stage late- and post-orogenic deformation history of the innermost Hellenic fold-and-thrust belt from a detailed paleostress reconstruction (Koziakas-Itamos Mts., Western Thessaly, Central Greece), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13463, https://doi.org/10.5194/egusphere-egu22-13463, 2022.

EGU22-66 | Presentations | ERE5.2

Influence of brittle deformation on the permeability of granite: assessing the geothermal potential of crustal fault zones 

Lucille Carbillet, Michael J. Heap, Hugo Duwiquet, Luke Griffiths, Laurent Guillou-Frottier, Patrick Baud, and Marie Violay

Economically viable geothermal systems rely on the efficiency of fluid circulation and heat transfer. Permeable fault zones are therefore excellent candidates for geothermal exploitation. In crustal fault zones, hot fluids from depths that correspond to the brittle-ductile transition are brought to the surface via crustal-scale permeable fault zones and may therefore constitute a new kind of geothermal system. To assess their geothermal potential, we measured the permeability of reservoir rock during deformation to large strains (up to an axial strain of about 0.1) in the brittle regime - fault formation and sliding on the fault - by performing triaxial experiments on samples of well-characterised Lanhélin granite (France). Prior to deformation, samples were thermally-stressed to 700°C to ensure that their permeability was sufficiently high to measure on reasonable laboratory timescales. All experiments were conducted on water-saturated samples under drained conditions, at a constant pore pressure of 10 MPa and confining pressures of 20, 40, and 60 MPa (corresponding to a maximum depth of about 2 km), and at room temperature. Our data show that permeability decreases by about an order of magnitude prior to macroscopic shear failure. This decrease can be attributed to the closure of pre-existing microcracks which outweigh the formation of new microcracks during loading up to the peak stress. As the macroscopic shear fracture is formed, sample permeability increases by about a factor of two. The permeability of the sample remains almost constant during sliding on the fracture to large strains (corresponding to a fault displacement of ~7 mm), suggesting that the permeability of the fracture does not fall below the permeability of the host-rock. The permeability of the sample at the frictional sliding stress is lower at higher confining pressure (by about an order of magnitude between 20 and 60 MPa) but, overall, the evolution of sample permeability as a function of strain is qualitatively similar for confining pressures of 20−60 MPa. These experimental results will serve to inform numerical modelling designed to explore the influence of macroscopic fractures on fluid flow within a fractured geothermal reservoir.

How to cite: Carbillet, L., Heap, M. J., Duwiquet, H., Griffiths, L., Guillou-Frottier, L., Baud, P., and Violay, M.: Influence of brittle deformation on the permeability of granite: assessing the geothermal potential of crustal fault zones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-66, https://doi.org/10.5194/egusphere-egu22-66, 2022.

EGU22-981 | Presentations | ERE5.2

Friction behavior of gabbro under hydrothermal conditions 

Wei Feng, Lu Yao, Rodrigo Gomila, Shengli Ma, and Giulio Di Toro

Fault friction is one of the most significant parameters controlling fault slip behavior and earthquake mechanics. Great success has been achieved in understanding the stability of fault slip, nucleation of earthquake and dynamic weakening mechanism in the past decades by performing low (~1 μm/s, sub-seismic conditions) to high (~1 m/s, seismic conditions) velocity friction experiments. However, extrapolating these experimental results to nature remains limited. In fact, for low velocity experiments, usually performed with tri-axial machines, though the hydrothermal conditions can be imposed, the shear displacement is limited to several millimeters neglecting the effect of cumulative displacement. For high velocity experiments aiming at reproducing coseismic fault slip, the implementation of hydrothermal conditions has been hindered by technical difficulties leaving high-velocity friction property of faults under realistic crustal conditions still ambiguous.

Here we exploited a Low to High Velocity rotary shear apparatus (LHV) equipped with a dedicated hydrothermal pressure vessel installed at the Institute of Geology, China Earthquake Administration, to investigate the frictional behavior of gabbro under realistic hydrothermal conditions. The samples were sheared at effective normal stresses of 10 MPa and 20 MPa, velocities (V) spanning from 1 μm/s to 0.1 m/s, displacement up to 3 m, under temperature conditions (T) up to 400 ℃ and pore pressure (Pf) up to 30 MPa. Our results showed that at T = 300 ℃ and Pf = 10 MPa (pore fluid as liquid), dramatic slip weakening happened at all tested velocities. At slip initiation the friction coefficient increased sharply to a peak value (~0.7±0.05), then decayed toward a residual value of ~0.35. Instead at T = 400 ℃ and Pf =10 MPa (pore fluid as vapor), we observed that friction remained high (~0.7) at V < 10 mm/s and slip weakening only occurred for V ≥ 10 mm/s. For experiments at T = 400 ℃ and Pf =30 MPa (pore fluid in supercritical conditions), slip weakening behavior occurred in most cases. The evolution of friction coefficient with displacement was complex, e.g., two peaks, large variations. Moreover, comparative experiments conducted at relatively low temperature suggested that mechanisms leading to the dramatic weakening under such a wide velocity range could be closely linked with both fluid-rock interactions and the physical state of the fluid. However, what exact fluid-rock reactions are involved is still an open question, which will be investigated by further microstructural and mineralogical analysis. The unique frictional behavior observed in this study challenges the results obtained from small-displacements experiments in many aspects and improves our understanding on friction behavior of faults in geothermal applications.

How to cite: Feng, W., Yao, L., Gomila, R., Ma, S., and Di Toro, G.: Friction behavior of gabbro under hydrothermal conditions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-981, https://doi.org/10.5194/egusphere-egu22-981, 2022.

Upper Cretaceous (Turonian and Cenomanian) carbonates in the Münsterland Cretaceous Basin, NW Germany, have become a target for geothermal energy production in recent years. These carbonates are present at depths of up to ca. 1,800 m in the region of the city of Münster in the center of the basin (e.g. Münsterland-1 well) and at depths beyond 2,000 m in the so-called Vorosning Depression. They represent the shallowest calcareous strata within the sedimentary succession of the Münsterland Basin and the underlying Rhenish Massif. Previous industrial drilling campaigns mostly focused on potential hydrocarbon gas reservoirs of the Upper Carboniferous. In the context of geothermal reservoir exploration, analog studies in outcrops of the Cretaceous carbonates are a prerequisite for reservoir quality assessment since subsurface/in situ data of these stratigraphic units, and especially petrophysical properties, are very sparse, not accessible or even absent in some areas. Investigations of quarries with Cretaceous carbonates mostly focused on paleontological and facies related research in the past rather than on their petrophysical properties. Three quarries in the Lengerich and Oerlinghausen areas, all at the northern margin of the basin, were now sampled for petrophysical laboratory experiments of Cenomanian and Turonian rocks. Additionally, scanline investigations, which involve collecting information such as length and aperture and others of each fracture along a line intersecting the rock mass, capturing of Unmanned Aerial Vehicle (UAV, commonly called drones) footage and laser scanning was performed at the three Cenomanian outcrops in Lengerich and one Turonian outcrop in Oerlinghausen. Further UAV footage and laser scans were collected for other outcrops within the quarries. The facies of the investigated rocks are expected to be comparable to what can be anticipated in the center of the Münsterland Basin according to the current paleogeographical understanding. Their analysis can thus be helpful in predicting the conditions that may be encountered in the central part of the basin. However, since the data was collected at the northern margin of the basin, the influence of the Osning Fault Zone (Upper Cretaceous inversion tectonics) has to be taken under consideration when further interpreting the data. The drone footage was processed, and Virtual Outcrop Models (VOM) were created using Agisoft Metashape. The point clouds of both, the laser scanning and processed UAV footage, were analyzed using the open-source package CloudCompare with its Facets and Compass plugins. The plugins allowed the detection of differently oriented fracture sets in the point clouds. This allowed to characterize fracture distributions and the comparison between the virtual outcrop data and the scanline data. Subsequently, the parameters of the fracture distributions of these structural features together with the laboratory measurements on bulk petrophysical properties were combined in a discrete fracture network (DFN). This representation of the reservoir, and in particular the 3D distribution of permeability, will be used for reservoir analog modelling to characterize fluid flow in the subsurface.

How to cite: Slama, S., Jüstel, A., Lippert, K., and Kukla, P.: Characterizing fracture networks and petrophysical bulk properties of carbonates from the margin of the Münsterland Cretaceous Basin, NW Germany, from outcrops, virtual outcrop models and laboratory testing, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2503, https://doi.org/10.5194/egusphere-egu22-2503, 2022.

EGU22-3309 | Presentations | ERE5.2

CT scan of a small-scale fault network: 3D fault geometries and their interpretation 

Inbar Vaknin and Andy Nicol

Fault surfaces and networks have been shown to have complex geometries. Outcrop observations are typically two-dimensional and limited in size by the exposure dimensions, while three-dimensional (3D) seismic data lack the resolution to characterize and quantify fault complexities on length scales less than a decameter. Defining the geometries of faults and their networks (high-resolution in 3D) is critical for understanding the interactions between faults and fluids. This presentation will examine the geometries of a network of small-scale normal faults displacing (by <1 cm) well bedded sand and silt layers in the Mount Messenger and Mohakatino formations in Taranaki, New Zealand. A 3D model of faulting was produced from high-resolution multi-band CT scanner (MARS Bioimaging Ltd.) imagery of a 10x8x3 cm rock sample. The digitally sectioned rock contains calcified fault rock that is distinguishable from wall rock and mapped throughout the rock volume at sub-millimeter scale. Fault-rock thicknesses vary by in excess of an order of magnitude, with greatest thicknesses at fault steps and fault bends. Fault zones comprise a series of lenses that have strike lengths greater than dip lengths and lens shapes that are often elongate parallel to bedding. The fault network is highly connected with branch lines, fault steps and fault bends most often sub-parallel to bedding. These observations suggest that mechanical heterogeneity of beds may partly control the geometries of both fault zones and the fault network. At the time of formation, the interconnected fault network likely increased bedding-parallel permeability (at scales from sub-millimeter and above) along fault zones.

How to cite: Vaknin, I. and Nicol, A.: CT scan of a small-scale fault network: 3D fault geometries and their interpretation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3309, https://doi.org/10.5194/egusphere-egu22-3309, 2022.

EGU22-3414 | Presentations | ERE5.2

Fluid pressure diffusion in fractured media: insights from harmonic and non-harmonic periodic pumping tests 

Nicolás D. Barbosa, Nima Gholizadeh Doonechaly, and Jörg Renner

Fractures can significantly impact fluid flow and pore pressure distribution in the subsurface. Understanding the mechanisms and conditions influencing their ability to transport fluids and to promote pore pressure diffusion is key for many activities relying on fracture-controlled flow such as, for example, enhanced geothermal systems. In situ characterization of these properties is typically done by performing hydraulic tests in selected intervals of a borehole and their interpretation relies on the solution of a linear pressure diffusion equation. However, it has been shown that the hydraulic behavior of fractures as well as the associated near borehole flow regimes can be largely affected by the coupling between the solid deformation and fluid pressure upon injection/production. In this work, we explore these effects by performing a series of harmonic injection tests (HIT) as well as non-harmonic production tests (NHPT) in a packed-off interval of a borehole containing multiple natural fractures. The borehole is located in the Bedretto Underground Laboratory for Geosciences and Geoenergies in Switzerland and penetrates granitic rock mass. The two kinds of tests consist of a periodic repetition of the same injection or production protocol. Flow rates, interval pressures as well as pressures above and below the double-packer probe are recorded at the surface. An important advantage of periodic testing is that it permits a continuous tracking of hydraulic changes during the test. For our study, we conducted a so-called injectivity analysis, in which the phase-shift (time delay) and amplitude ratio between flow rate and interval pressure are used to infer effective hydraulic properties. We performed over 200 periodic tests including both HIT and NHPT with a large range of periods (7.5 s to 1800 s) as well as varying mean interval pressures (~1300 kPa to 2100 kPa) and flow oscillation amplitudes. As a result, we obtained a robust constraint of the radial flow regime prevailing in the fractures. Overall, we found that results from HIT and NHPT are in very good agreement despite the remarkably different injection protocols. For all cases, a prominent and consistent period dependence of phase shifts and amplitude ratios of flow rates and interval pressure was observed, in which both increase as the oscillatory period decreases. Amplitude ratios showed almost no variation with mean interval pressure regardless of the injection protocol. In contrast, a prominent pressure dependence of the phase shifts is captured by the HIT but not the NHPT data. Using the pressure-independent NHPT results, we reconstruct the general hydraulic response of the tested fractured section, which can be well represented by an analytical solution of the pressure-diffusion equation. This general trend explains the HIT data as well, although evidence of significant variations that are correlated with the amplitude of the pressure oscillations points to the predominant role of hydromechanical coupling effects on the fluid pressure diffusion process.

How to cite: Barbosa, N. D., Gholizadeh Doonechaly, N., and Renner, J.: Fluid pressure diffusion in fractured media: insights from harmonic and non-harmonic periodic pumping tests, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3414, https://doi.org/10.5194/egusphere-egu22-3414, 2022.

Flow through faults and fractures has been studied extensively in the context of hydrocarbon exploration and production, to understand charge and migration, hydrocarbon column heights and fault transmissibility. Learnings have typically been captured in pragmatic models, such as the Shale-Gouge-Ratio (SGR) concept, providing dimensionless or relative fault permeability definitions based on limited subsurface data.

The resulting coarse predictions are however not suitable for geoenergy applications, including CO2 sequestration (CCS) or underground hydrogen storage (UHS), where injection into a storage reservoir requires assurance that the injected fluids or gases will not leak out of the storage complex via faults or fractured caprocks. The conventional fault seal analyses do not provide this containment assurance.

A new paradigm is required for characterizing faults and fractures in geoenergy projects, focused on derisking leakage of injected fluids and gases along faults. Such approach is not necessarily about accurately predicting the permeability of a fault or fracture, but rather about understanding what geometric properties and mechanical or chemical mechanisms would contribute to either permeable or sealing behaviour of faults. Improved insights in any of these areas would help in screening fault leakage risks in prospective subsurface geoenergy projects.

Analogue data, both from outcrops for geometric fault attributes and from the lab for mechanical and chemical properties, can help gain those fundamental insights into what controls fault leakage. Properties can be quantified and processes studied at a level of detail that cannot be matched by in-situ subsurface datasets, particularly not in the context of geoenergy systems, where operational subsurface projects are still limited. Outcrop studies can help improve our understanding of vertical connectivity, with focus on lower-permeability ductile rocks analogues to typical reservoir seals. Lab studies and in-situ experiments can provide insights into injected fluids such as CO2 or H2 affect the mechanical and chemical integrity of faults and subsequent flow behaviour. For geoenergy systems in particular, experiments should focus on the impact of rapid pressure or temperature cycling. Induced seismicity is another potential threat to containment integrity and requires further research to understand what fault geometries are most prone to reactivation as well as how reactivation affects the sealing behaviour of a fault.

In recent years, integrated studies such as the multi-scale, multiphysics ACT-DETECT project have started to provide some answers to these questions, resulting in novel insights and workflows that provide a first-order fault leakage risk assessment that can be used to identify ideal storage sites. However, with the envisioned increase in the number of geoenergy projects to meet carbon emission reduction targets, the need for more refined screening criteria will increase too as the flexibility in selecting ideal storage locations will decrease.

How to cite: Bisdom, K.: A new paradigm for flow through faults and fractures in the context of geoenergy, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3553, https://doi.org/10.5194/egusphere-egu22-3553, 2022.

At the Äspö hard rock underground laboratory in Sweden, six in situ hydraulic fracturing experiments took place at 410 m depth. A multistage hydraulic fracturing approach is tested with a low environmental impact, e.g., induced seismicity. The idea is to mitigate induced seismicity and preserve the permeability enhancement process under safe conditions. The fractures are initiated by two different injection systems (conventional and progressive). An extensive sensor array is installed at level 410 m, including simultaneous measurements of acoustic emissions, electric self-potential, and electromagnetic radiation sensors. The monitoring catalog includes more than 4300 acoustic emission events with estimated magnitudes from the continuous monitoring setup (in-situ sensors between 1-100 kHz). The experiment borehole F1 is drilled in the direction of Shmin, perpendicular to the expected fracture plane. Two electromagnetic radiation sensors are installed and aligned to (i) Shmin and (ii) the expected fracture plane with a sampling rate of 1 Hz and a frequency range between 35-50 kHz. The self-potential sensors are installed at level 410 with a distance of 50-75 m from the borehole F1, including nine measuring probes and one base probe. A second self-potential setup is deployed at level 280 m in the far-field with a distance of 150-200 m from F1. The self-potential data were measured with a sampling rate of 1 Hz. For the first time (to our knowledge), the results of electric and electromagnetic monitoring of two hydraulic stimulation at meter-scale are presented.

How to cite: Haaf, N. and Schill, E.: Electric self-potential and electro-magnetic monitoring of hydraulic fracturing experiments in the Äspö Hard Rock Laboratoy, Sweden., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3837, https://doi.org/10.5194/egusphere-egu22-3837, 2022.

EGU22-6166 | Presentations | ERE5.2

Assessing damage pattern at depth near the Alpine Fault, New Zealand 

Mai-Linh Doan, Virginia Toy, Rupert Sutherland, and John Townend

 

The Alpine Fault is the principal component of the plate boundary through the South Island of New Zealand, separating the Pacific and Indo-Australian Plates. It is recognised internationally as an important site for studying earthquake physics and tectonic deformation, as it produces large (M7-8) earthquakes approximately every 330 years and last ruptured in 1717. Therefore, the fault is considered to be late in its seismic cycle. It accommodates dextral-slip at a rate of 26 mm/yr with reverse slip at a maximum rate of 10 mm/yr in its central part, thus exhumed a fossil ductile shear zone, that was damaged brittlely during its exhumation.

 

The central Alpine Fault is the focus of the Deep Fault Drilling Project (DFDP), sponsored by the International Continental Drilling Project, which takes advantage of its globally rare tectonic situation to determine what temperatures, fluid pressures, and stresses exist within a plate-boundary fault in advance of an expected large earthquake. During DFDP phase II in 2014, an ~ 900 m drilled well that encountered an exceptionally high geothermal gradient (120 °C/km was measured in the borehole), was extensively characterized by repeated electric and sonic logs. These logs enable detailed study of fracture patterns near a major fault. The more than 19 km of logs run within the borehole gathered datasets covering, among others, thermal resistivity, sonic velocities, acoustic borehole imaging, and electrical resistivity. They show that the hanging wall is extensively fractured, explaining the high geothermal gradient measured in the borehole by lateral flow of hot water deep seated in the mountains.

 

We particularly focus on seven dual laterolog logs that provide a robust and reproducible dataset from which to determine the positions and orientations of conductive fractures. From these, different patterns of damage could be identified within the well. A first pattern consists of an extensive and dense pattern of isolated fractures that could be identified throughout the borehole. A second pattern suggests that decametric  zones of low resistivity localize damage and focus thermal anomalies. This suggests hierarchy of damage zone evolution of the damage zone of the Alpine Fault. A possible explanation is an initial phase of diffuse fracturing (pattern 1) that is followed by subsequent alteration of the major shear zone, which focuses fluid and heat flow (pattern 2).

How to cite: Doan, M.-L., Toy, V., Sutherland, R., and Townend, J.: Assessing damage pattern at depth near the Alpine Fault, New Zealand, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6166, https://doi.org/10.5194/egusphere-egu22-6166, 2022.

The regionalization of hydraulic properties like specific yield/storativity or permeability in fractured crystalline rock is of utmost importance for a variety of applications, such as geothermal and other resources, waste disposal or underground construction. However, accurate predictions for these properties – particularly for undrilled sites – bear a high degree of uncertainty as already direct observations through hydraulic in-situ tests show a variance of about 2 orders of magnitude at any depth (Achtziger-Zupančič et al., 2017).

Permeability-depth relationships using multiple log-log regressions conducted on an extended version of the worldwide permeability compilation of crystalline rocks (roughly 30000 entries in Achtziger-Zupančič et al., 2017; now consisting of about 50000 single in-situ permeability measurements to depths of 2000 mbgs) indicate that depth is generally the most important geological factor, resulting in a permeability decrease of three to four orders of magnitude in the investigated depth range. Specific yield and storativity show a similar but less pronounced depth trend. Beside depth, most influential factors for permeability in crystalline rock are the long-term tectono-geological history described by geological province which locally is overprinted by current seismotectonic activity as determined by peak ground acceleration (Achtziger-Zupančič et al., 2017). Although petrography might be of local importance, only a low impact has been observed for the global dataset, besides lithologies allowing for karstification. Ongoing vertical movements – particularly resulting from glacial isostatic adjustment – alter the permeability trend with depth.

The latter shows distinct trends starting at about logK -14.5 to -14.8 m² at 100 mbgs and showing diversion of about 1.5 orders of magnitude at 1 km depth between areas without significant uplift and areas with uplift of more than 4 mm/y as determined from a probabilistic interpolation of global geodetic measurements (Husson et al., 2018). The difference is attributed either to glacial loading (normal faulting or reactivation) induced destruction preserved during glacial induced rebound and/or uplift-caused horizontal fracture growth which improved connectivity in the rock mass. Areas undergoing subsidence show similar trends like highly uplifting areas which is attributed to efficient normal faulting induced destruction of the rock mass.

References:

Achtziger-Zupančič, P, Loew, S and Mariéthoz, G (2017). A new global database to improve predictions of permeability distribution in crystalline rocks at site scale. JGR: Solid Earth 122(5): 3513-3539.

Husson, L, Bodin, Th, Spada, G, Choblet, G and Kreemer, C (2018). Bayesian surface reconstruction of geodetic uplift rates: Mapping the global fingerprint of Glacial Isostatic Adjustment. J Geodyn 122: 25-40.

How to cite: Achtziger-Zupancic, P.: The influence of glacial induced adjustment and other geological factors on the depth distribution of permeabilities in crystalline rocks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7157, https://doi.org/10.5194/egusphere-egu22-7157, 2022.

EGU22-7835 | Presentations | ERE5.2

Fracture energy variations of rocks: a mechanical investigation 

Antoine Guggisberg, Mathias Lebihain, and Marie Violay

Crack propagation is critical for the assessment of the strength of rocks. Linear Elastic Fracture Mechanics (LEFM) theory is commonly used to describe its propagation. However, the variation of the fracture energy, its key parameter, is generally poorly understood as its experimental measurements are influenced by temperature, stress biaxiality, and rupture velocity. This indicates other dissipative processes may occur in the vicinity of the crack.

We conduct Modified Ring Tests (MRT) on Carrara marble to investigate these mechanisms. MRT provides stable mode I crack propagation under controlled velocity and stress biaxiality conditions. Coupled with a compliance method calibrated through Finite Element Method (FEM), we obtain multiple local measurements of the fracture energy within a single test. FEM also provides estimation of stress biaxiality levels as well as higher order terms of the Williams’ expansion of the stress field.

The method is validated on PMMA through Digital Image Correlation (DIC) techniques. Experiments on Carrara marble show that the stress biaxiality can directly influence the fracture energy measurements. A microscopic investigation on marble is performed to look for micro-mechanisms which may cause observed variations of fracture energy.

How to cite: Guggisberg, A., Lebihain, M., and Violay, M.: Fracture energy variations of rocks: a mechanical investigation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7835, https://doi.org/10.5194/egusphere-egu22-7835, 2022.

Between 2018 and 2021, the STIMTEC and STIMTEC-X hydraulic stimulation experiments were conducted at 130 m depth in the Reiche Zeche underground research laboratory in Freiberg/Germany. The STIMTEC experiment was designed to investigate the rock damage resulting from hydraulic stimulation and to link seismic activity and enhancement of hydraulic properties in anisotropic metamorphic gneiss. The following STIMTEC-X experiment aimed at better constraining the stress field in the rock volume to investigate the mechanisms leading to induced acoustic emission (AE) activity. Here, we present results from focal mechanism analysis of high-frequency (>1 kHz) AE events, associated with brittle deformation at the cm- to dm-scale induced by hydraulic stimulations. Focal mechanisms are calculated using full moment tensor inversion of first P-wave amplitudes using the hybridMT package. We use polarity and amplitude data from a (near) real-time seismic monitoring network, consisting of AE sensors, AE-hydrophones, accelerometers, and one broadband sensor. We observe changes in the predominant type of faulting from reverse faulting focal mechanisms during the frac and refrac cycles to oblique strike-slip focal mechanisms observed during subsequent high-volume fluid-injections performed during periodic pumping test. The observed differences in dominant focal mechanisms are consistent with the activation of less favourably oriented faults at increased pore fluid pressure during extended periodic pumping. We observe a reverse-faulting stress regime from focal mechanism inversion of low-volume injection stages for different boreholes, representative for the rock volume (typically ~5 m radially) surrounding the injection intervals. In contrast, stress field estimates obtained from analysing the instantaneous shut-in pressures of hydraulic stimulations in different boreholes indicate a regime change from thrust to strike-slip faulting in the investigated rock volume. The reservoir complexity seen at the scale of the experiment (30m x 30m x 20m) is large and is reflected by the significant variations in AE event activity in response to stimulation as well as small-scale rock, stress and structural heterogeneities.

How to cite: Boese, C., Kwiatek, G., Renner, J., and Dresen, G.: Stress field observations from hydraulic fracturing and focal mechanism inversion at the STIMTEC underground research lab, Reiche Zeche mine, Germany, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7986, https://doi.org/10.5194/egusphere-egu22-7986, 2022.

Fluid flow in low-porosity/permeability reservoir rocks such as tight carbonates is mostly restricted to structural discontinuities (e.g. faults, fractures, karstified zones). Fault zones, in particular in such rocks, offer both suitable fluid flow pathways, but may also act as impermeable barriers. The heterogeneous permeability structure of fault zones, however, impedes pre-drilling investigations of exploration targets by numerical models. A better understanding of the factors that control the fluid flow and the heterogeneity of permeability distribution along fault zones in tight reservoirs is a pre-requisite for the definition of drilling targets.

In this study, a hydraulic field laboratory with a volume of 30 m x 30 m x 20 m was set up in a quarry in SE Germany to investigate the influence of fault zones on the general permeability structure of tight carbonates. The test field contained three WNW-ESE-striking, repeatedly reactivated normal faults with offsets in the order of <1 m and two roughly perpendicularly oriented NNE-SSW-striking fracture corridors. Fault zones and fracture corridors were targeted by 62 wells. Wells that exhibited a decent hydraulic connection the to the overall conductive fracture network were logged (e.g. borehole image logs, FWS, etcs.) and in selected wells hydraulic tests were conducted. Water levels were measured both during static conditions and during testing. Due to the density of wells we were able to constrain the controlling factors for fluid flow along and across the fault zones. Damage zones were considered as conduits while the fault core was expected to be impermeable. These general assumptions could be confirmed by our tests, however we found some exceptions. While fluid flow in general is restricted to few, well-connected fractures, the majority of the fractures are dead ends, solely serving as storage for fluids. With increasing displacement and complexity of the fault zone, enhanced permeability parallel to the fault zone could be inferred. At larger offsets, where a thicker fault core develops, fhe fault core itself acts as barrier and fractures and fracture corridors do not penetrate the faults. We think that this is related to the presence of the much less competent fault core of a certain thickness which is able to accommodate the brittle deformation. Where the faults offset is less than ~0.4 m, the integrity of the fault seal is breached by fracture corridors, cross cutting the faults. This is clearly shown by the pressure distribution in static and transient conditions. Faulting, hence leads to a compartmentalization of the reservoir, where the compartments do either communicate or interact with significant delay.

The information and data received from the conducted field tests furthermore serve as input parameters and validation for a newly developed numerical approach that aims to simulate fluid flow in this type of geological settings, results of which will be presented in an additional presentation by our project partners.

How to cite: Freitag, S., Bauer, W., Stollhofen, H., and Hähnel, L.: (1)   Transmissivity of fault zones in tight carbonates – results from a reservoir-scale hydraulic field laboratory in the Franconian Alb, SE Germany, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8457, https://doi.org/10.5194/egusphere-egu22-8457, 2022.

In outcrop-based fracture studies, the quantification of fracture intensity is often limited by the limitations of the manual sampling technique, characterized by punctual measurements (e.g. sampling spot, scanline, scanwindow) and moderate biases (e.g. fracture length truncation, technical and personal errors). The proximal remote sensing technologies, as terrestrial or Uncrafted Aerial Vehicle (UAV)-based LiDAR and photogrammetry, can help to overcome these limitations due to the possibility to obtain high-resolution and accurate quantitative data from the digital twin of the outcrop, the so-called Digital Outcrop Model (DOM). The DOMs can be very useful in outcrop-based fracture studies because their analysis allows to obtain several quantitative information with manual and/or automatic methods and with continuity in each position of the outcrop, increasing the accuracy of the fracture intensity estimations. However, due to the novelty of DOM technology and the lack of well-defined DOM-based fracture sampling procedures, these huge fracture datasets are often difficult to study and interpret, and therefore, the benefits of the DOM cannot be fully exploited. 

For this reason we present a complete workflow based on the DICE (Discontinuity Intensity Calculator and Estimator) open-source MATLAB© application that allows to quantitatively characterize the fractures of rocky outcrops from the 3D Digital Outcrop Models (DOMs). The proposed workflow consists in the following steps: (1) fracture mapping onto the 3D DOMs; (2) calculation of the fractures dimension, position and orientation; (iii) determination by DICE algorithm of the discontinuity parameters (persistence/dimension, distribution, spacing and intensity) using different 3D sampling techniques (3D scanline, 3D circular scan window and spherical scan volume). The differences of these sampling techniques and the fracture intensity parameters that can be obtained (p10, p21, p32) are discussed, showing the advantages and limitations of each DICE method.

How to cite: Menegoni, N., Giordan, D., and Perotti, C.: 3D Digital Outcrop Model-based quantification of fracture intensity: the Discontinuity Intensity Calculator and Estimator (DICE) open-source application, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10207, https://doi.org/10.5194/egusphere-egu22-10207, 2022.

EGU22-10259 | Presentations | ERE5.2

CHENILLE: Coupled beHavior undErstaNdIng of fauLts: from the Laboratory to the fiEld 

Audrey Bonnelye, Pierre Dick, Fabrice Cotton, Rüdiger Giese, Yves Guglielmi, Damien Jougnot, Jan Henninges, Grzegorz Kwiatek, and Stefan Lüth

The understanding of the coupled thermo-hydro-mechanical behaviour of fault zones in naturally fractured reservoirs is essential both for fundamental and applied sciences and in particular for the safety assessment of radioactive waste disposal facilities. In this framework, an international research program callled CHENILLE was built to address key questions related to the impact of high temperatures (up to 150°C) on shear zones as well as fault reactivation processes in shale formations. The project includes a thermally controlled in situ fluid injection experiment on a strike-slip fault zone outcropping atIRSN’s Tournemire Underground Research Laboratory (URL) and a series of laboratory experiments to understand the chemical and structural evolution occurring within the fault zones during the thermal and hydraulic loading. The in situ experiment includes a heating system installed around an injection borehole will enable a precise and controlled incremental increase of the thermal load. The injection borehole will be equiped with a Step-Rate Injection Method for Fracture In-Situ Properties (SIMFIP) probe, in order to perform step pressure tests. The probe will not only measure the flow and pressure rate inside the injection borehole but also allow to monitor the borehole’s 3D deformation during the hydraulic and thermal loading steps. In addition, an array of seismicifferent sensors will be implemented around the injection area to measure the seismic and aseismic deformation induced either by thermal or by hydraulic loading. The seismic monitoring system is composed of Acoustic Emission (sensitive between 1kHz and 60kHz) enabling monitoring fracturing processes of sub-decimeter size. Furthermore, a fibre optic network will be installed in the heating boreholes to measure spatially temperature variationsvia Distributed Temperature Sensing technology in the investigation area. Active seismic surveys, using different source types, are scheduled before and after the experiment to determine the structural network but also to detect the appearance of new structures triggered from the hydro-thermal pressurization of the fault by tomography and reflection seismic methods. The overall goal of our work is to present the interaction between the different geophysical methods that we are using as well as some preliminary results. A first part is dedicated to the description of the fault zone through field and core samples observations as well as borehole to borehole correlation, whereas the second is dedicated to preliminary results on the thermal diffusion expected in the fault.

How to cite: Bonnelye, A., Dick, P., Cotton, F., Giese, R., Guglielmi, Y., Jougnot, D., Henninges, J., Kwiatek, G., and Lüth, S.: CHENILLE: Coupled beHavior undErstaNdIng of fauLts: from the Laboratory to the fiEld, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10259, https://doi.org/10.5194/egusphere-egu22-10259, 2022.

EGU22-12748 | Presentations | ERE5.2

Crack healing in salt: time-resolved 3D microtomography 

Yuntao Ji, Christopher Spiers, Suzanne Hangx, Hans de Bresser, and Martyn Drury

Rocksalt caverns are considered or already used as storage for nuclear waste, petroleum, hydrogen, CO2, and compressed air energy because of the low permeability and potential of fracture healing of salt. An important concern is the sealing capacity. Undisturbed rocksalt deposits in nature generally have very low permeability. However, as a result of excavation stress, a network of fractures will be induced within the rocksalt formation, increasing the permeability. At low deviatoric stresses and/or at low effective stresses, a fracture network filled with brine is expected to heal, and the connectivity of the brine-filled network, consisting of grain boundaries, pores, and microcracks, is expected to decrease over time. The driving force for such a healing process is the tendency to reduce the interfacial energy by reducing the total interfacial area. In order to assess the rate of pore reconfiguration and permeability evolution in damaged salt and to capture the key process of crack network evolution during healing, we employ time-resolved 3D microtomography to study the long-term evolution of the fracture network of small-scale polycrystalline rocksalt samples. We found that precipitation prefers to occur in open spaces in the early stage of healing, such as new cracks. As a result, flat cracks evolve into zigzag cracks, which create narrow throats, thereby reducing the permeability of the crack network. Our study also offers a way to testify the thermodynamic models quantitatively.

How to cite: Ji, Y., Spiers, C., Hangx, S., de Bresser, H., and Drury, M.: Crack healing in salt: time-resolved 3D microtomography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12748, https://doi.org/10.5194/egusphere-egu22-12748, 2022.

EGU22-12855 | Presentations | ERE5.2

Direct shear experiments to investigate the effect of chemical alteration on fault frictional behaviour in granitic geothermal systems 

Nick Harpers, Nathaniel Forbes Inskip, Michael John Allen, Daniel Faulkner, Hannes Claes, Andreas Busch, and Sabine den Hartog

Enhanced temperature gradients related to locally elevated heat production in granitic plutons offer the potential for low carbon geothermal energy production. Cornwall in SW England hosts several granitic plutons that are the subject of current geothermal projects (United Downs Deep Geothermal Power [UDDGP] Project and Eden Project). These projects target fault zones in crystalline rock that provide pre-existing pathways for fluid flow. Reinjection of cooler fluids into the reservoir after heat extraction may result in chemical disequilibrium with the host rock, potentially driving precipitation or chemical alteration. Such changes could influence the frictional properties of the fault zones, and hence require modifications to numerical risk-based calculations of the likelihood, or not, of induced seismicity.

In order to study the effects of such alterations, we have conducted a series of direct shear experiments under representative in-situ conditions on Cornish Carnmenellis granite samples which have undergone varying degrees of natural chemical alteration. The direct shear experiments were conducted on gouges (grain size < 125 μm) and at effective normal stresses of 80-105 MPa, pore fluid pressures of 25-50 MPa and temperatures of 16-180 °C. These conditions are relevant for the depths where the UDDGP project injection and production boreholes intercept the Porthtowan Fault zone, the assumed main conduit for fluid flow. In each test, load point velocity was stepped between 0.3 μm/s, 1 μm/s and 3 μm/s, and shear resistance of the sample was measured to determine the stability of sliding and thus the likelihood of induced seismicity as a function of degree of alteration. Initial shear tests at room temperature suggest little difference in the frictional response of altered and unaltered samples.

How to cite: Harpers, N., Forbes Inskip, N., Allen, M. J., Faulkner, D., Claes, H., Busch, A., and den Hartog, S.: Direct shear experiments to investigate the effect of chemical alteration on fault frictional behaviour in granitic geothermal systems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12855, https://doi.org/10.5194/egusphere-egu22-12855, 2022.

EGU22-12888 | Presentations | ERE5.2

A semi-automatic workflow for structural interpretation of large point-cloud Digital Outcrop Models on complex fractured metamorphic rocks (Aosta Valley, Italy) 

Bruno Monopoli, Andrea Bistacchi, Federico Agliardi, Gloria Arienti, Giovanni Dal Piaz, Davide Bertolo, and Stefano Casiraghi

Characterization of fracture networks, both in fault zones and in the less-fractured background, is essential for the analysis and modelling of mechanical and hydraulic properties of the rock mass (i.e. rock plus fractures). Here we present our experience in characterizing fracture networks and other structural features on large outcrops of different basement and metamorphic cover units in the Penninic, Austroalpine and Helvetic units of the Aosta Valley. These units show a variety of lithological, mechanical, and rheological characteristics and were subjected to different ductile and brittle tectonic evolution, resulting in complex combinations of compositional layering, metamorphic schistosity, and fracture networks.

Our methodology is based on a combination of traditional field surveys and remote-sensing techniques such as ground-based and UAS photogrammetric surveys, and terrestrial or helicopter laser scanning. The first task, whose importance is too often overlooked, is represented by selecting outcrops that are representative in terms of structural and lithological properties of a larger rock volume, based on a thorough knowledge of regional structural geology and tectonics. The field survey is carried out with traditional techniques, paying attention to the kinematics, relative chronology, and mineralization (e.g. veins or mineral coatings) of structures. These features, that are often overlooked in fracture studies, are fundamental to frame the evolution of a complex schistosity and fracture network, to separate tectonic fractures with respect to those related to slope dynamics, and to develop predictive models of fracturing at depth (where slope-related fracture will not be present). At the same time, remote-sensing datasets are collected. The choice of the survey technique (terrestrial vs. aerial, photogrammetry vs. laser scanning) depends on various conditions, but in all cases the output is a point cloud DOM, colorized with RGB values, that should have a density (points/area) sufficient to characterize the smallest relevant structural features. From this, also textured surface DOMs and/or DEM plus orthophotos (for almost flat outcrops) can be obtained.

The first step of DOM analysis is carried out “manually”, selecting facets and traces with suitable software tools (e.g. Compass plugin in CloudCompare). This allows selecting different sets of structures, characterizing their orientation statistics, and assigning them to sets defined in the field (with kinematics, chronology, etc.). This step also allows understanding how well the structural features recognized in the field are represented in the DOM. The second step of DOM analysis consists in an automatic segmentation (in case of a point cloud) or tracing (in case of a DEM of triangulated surface textured with images) with algorithms calibrated with results of the manual interpretation. Overall, this results in a supervised semi-automatic workflow, allowing to extract huge structural datasets in a reasonable time, maintaining the connection with kinematic and chronological observations carried out in the field.

The fracture datasets can be eventually characterized with tools allowing to measure statistical distributions of different parameters of the fracture sets using virtual scanlines and/or scanareas, and these distributions can be used to model different properties of the fracture networks or generate stochastic DFN models.

How to cite: Monopoli, B., Bistacchi, A., Agliardi, F., Arienti, G., Dal Piaz, G., Bertolo, D., and Casiraghi, S.: A semi-automatic workflow for structural interpretation of large point-cloud Digital Outcrop Models on complex fractured metamorphic rocks (Aosta Valley, Italy), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12888, https://doi.org/10.5194/egusphere-egu22-12888, 2022.

EGU22-575 | Presentations | GD6.1

Strain relaxation around stressed quartz inclusions in garnet 

Hugo van Schrojenstein Lantman, David Wallis, Mattia Bonazzi, Jay Thomas, Maartje Hamers, Martyn Drury, and Matteo Alvaro

The measurement of residual stresses in exhumed rocks yields valuable information about metamorphic temperature and pressure, deformation and rheology, and stress state. However, the state of elastic strain and stress at the surface of a sample does not necessarily correspond to the state well below the surface. When a sample under elastic strain is cut, polished, or otherwise prepared for analysis, a part of the constraining rock is removed, allowing for the partial relaxation of the elastic strain. To be able to work with residual elastic strain and stress with analytical methods that probe the upper few microns of a sample, the process of strain relaxation must be well understood.

For this work we used high-angular resolution EBSD to analyse stressed quartz inclusions in natural garnet from a range of settings, and in several samples grown in piston-cylinder experiments that were previously analysed with Raman spectroscopy for inclusion pressures. The experimental samples are not expected to have undergone plastic deformation in the garnet during cooling, as the majority of the pressure within the inclusion built up during decompression at room temperature. Additionally, the inclusion pressures in buried inclusions matches what is expected for the experimental conditions, suggesting no plastic yielding. Thus, in these samples we can isolate elastic strain from potential plastic deformation. One of the experimental samples was analysed with TEM to test this expectation.

Forescatter images reveal topographical effects resembling quartz and adjacent garnet “extruding” out of the sample. Furthermore, rotations of the quartz lattice and the garnet lattice immediately around the quartz inclusion are observed. The rotation axis of the misorientation generally lies in the plane of the sample surface. TEM analysis revealed a number of dislocations in experimental garnet where these were not expected. However, a significant degree of bending of a wedge of garnet between the original sample surface and a quartz inclusion is also observed.

The dislocations observed with TEM do not fit with the model of the experiments. Also, the formation of dislocations before sample preparation does not explain the dependence of the rotation axis on the surface orientation. A likely scenario for the deformation measured with EBSD is that the partial relaxation of elastic strains in stressed quartz inclusions in garnet as result of sample preparation induced local distortion of the inclusion and host. Additionally, the persistence of topographical features related to this relaxation despite several steps of polishing suggests that relaxation is not instantaneous but occurs over time.

How to cite: van Schrojenstein Lantman, H., Wallis, D., Bonazzi, M., Thomas, J., Hamers, M., Drury, M., and Alvaro, M.: Strain relaxation around stressed quartz inclusions in garnet, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-575, https://doi.org/10.5194/egusphere-egu22-575, 2022.

EGU22-1247 | Presentations | GD6.1

Fast resetting of zircon in garnet inclusion pressures: implications for elastic geothermobarometry. 

Nicola Campomenosi, Boriana Mihailova, Ross John Angel, Marco Scambelluri, and Matteo Alvaro

The contrast in the thermoelastic properties between one inclusion and its surrounding host is commonly exploited to back-calculate the pressure (P) and temperature (T) conditions of inclusion entrapment. This is elastic thermobarometry and it is based on the elastic properties of minerals rather than chemical equilibrium. The effect of inclusion confinement is the inclusion residual pressure (P-inc), which can be determined via Raman spectroscopy. For a given host-inclusion system, a specific P-inc corresponds a P-T line along which the confinement effects between the two crystals disappear: the isomeke. By definition, this line potentially represents the P-T conditions of inclusion entrapment. Away from the isomeke, the inclusion exhibits over- or under-pressure with respect to the external pressure. The position and slope of the isomeke can be calculated using the equations of state of both the host and the inclusion [1].

In this contribution, we show how zircon-in-garnet isomekes can be partially investigated via in-situ Raman spectroscopy at high T and ambient P by comparing the evolution of the Raman peak position of the inclusion with respect to a free zircon crystal at the same temperature. Several zircon inclusions in pyrope-rich garnets from the Dora-Maira whiteschists (Western Alps) were heated up and brought from the over- to the under-pressure domain across their corresponding isomeke. At temperatures above the isomeke, we found that zircon inclusions in garnet can be reset on the timescale of laboratory experiments: after cooling down the P-inc was different from the original. We interpret this reset as the result of viscous relaxation at the host-inclusion boundary [2] and annealing of submicron dislocations of the garnet host at high temperature. Importantly, for similar heating rate and T range, viscous relaxation occurs more easily when the inclusions are in the under-pressure domain. This suggest that original confinement effects of zircon in a garnet host whose exhumation path mostly occurs within the inclusion under-pressure domain can be easily reset to record P-T conditions on the retrograde path, while those from a garnet host whose exhumation path mostly occurs within the inclusion over-pressure domain can be better preserved. Therefore, since the isomekes of zircon with garnet are steep in P-T, this system may be more reliable for high T and low P terranes for which the exhumation path passes directly or quickly into the over-pressure domain [3]. On the other hand, for UHP domains such as Dora-Maira resetting occurs [4] due to the exhumation path being steep and thus in the under-pressure domain until low pressures.   

[1] Angel et al. 2015 Journal of Metamorphic Geology33(8), 801-813. [2] Zhong et al. 2020 Solid Earth11(1), 223-240.  [3] Gilio et al. 2021 Journal of Metamorphic Geology 10.1111/jmg.12625 [4] Campomenosi et al. 2021 Contributions to Mineralogy and Petrology176(5), 1-17  

This work was supported by the Alexander von Humboldt foundation and the ERC-StG TRUE-DEPTHS grant (number 714936) to M. Alvaro

How to cite: Campomenosi, N., Mihailova, B., Angel, R. J., Scambelluri, M., and Alvaro, M.: Fast resetting of zircon in garnet inclusion pressures: implications for elastic geothermobarometry., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1247, https://doi.org/10.5194/egusphere-egu22-1247, 2022.

EGU22-2449 | Presentations | GD6.1

Hybridization of magmas by break down of partially molten granitic rock and its assimilation 

Pavlina Hasalová, Karel Schulmann, Anne-Sophie Tabaud, and Jitka Míková

During orogenic processes continental crust experiences significant partial melting. Repeated thermal pulses or fluctuation in fluid content can even cause multiple anatectic events that result in complex intrusion suits. The Vosges Mountains (NE France) reveal two chronologically and geochemically distinct tectono-magmatic events. An early major pulse of Mg‒K magmatism was followed ten millions years later by development of a magma-rich detachment zone and intrusion of Central Vosges Granite forming a felsic MASH zone. This MASH zone is characterized by the production of a large quantity of anatectic melts that interacted with the older Mg‒K granites and surrounding granulites and metasedimentary rocks. We aim to understand how such hybridization processes impact on the crustal rocks rheology, deformation as well as its geochemistry and geochronology. Three different granite varieties were distinguished: (i) the older Mg‒K granite end-member that is coarse-grained with a high proportion of feldspar phenocrysts, zircon U-Pb ages of 340 Ma and specific geochemical signature; (ii) Medium-grained type has a smaller amount of phenocrysts and shows advanced brecciation where fine-grained Pl+Kfs+Qtz form discontinuous corridors to an interconnected network surrounding fractured phenocrysts. Its geochemical signature suggests that this represents a mixing of Mg−K and Central Vosges granites, as confirmed by the presence of both inherited (340 Ma) and younger (330‒310 Ma) zircon domains; (iii) Isotropic medium-grained granite that shows geochemical signature typical for the Central Vosges Granite in which younger zircon domains (310‒320 Ma) dominate over inherited xenocrysts (340 Ma). These three granite varieties represent different stages of magma hybridization by the break up of the older Mg‒K granite by the younger Central Vosges Granite magmas. The interaction between new melt and previously crystallized granitoids results in variety of granite textures, fabrics, chemical compositions, isotopic signatures and deformational behavior. In summary, the resulting signature is result of interplay of melt transfer and interaction in the MASH zone.

How to cite: Hasalová, P., Schulmann, K., Tabaud, A.-S., and Míková, J.: Hybridization of magmas by break down of partially molten granitic rock and its assimilation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2449, https://doi.org/10.5194/egusphere-egu22-2449, 2022.

EGU22-3549 | Presentations | GD6.1

Evolution of P-wave velocities during antigorite dehydration at pressures up to 2.5GPa 

Alexandre Schubnel, Arefeh Moarefvand, Julien Gasc, Damien Deldicque, and Loïc Labrousse

Antigorite dehydration is considered as one of the potential triggering mechanisms of intermediate depth earthquakes in subduction zones. Here, the evolution of p-wave velocities were measured during antigorite dehydration experiments at pressure and temperature conditions representative of the upper mantle (1 to 2.5 GPa) for the first time.

Experiments were realized on a natural antigorite serpentinite from Corsica (Gasc et al. 2011), using a 3rdgeneration Griggs-type apparatus equipped with p-wave velocity ultrasonic monitoring (Moarefvand et al. 2021).Velocities were measured maintaining constant hydrostatic pressure conditions at  1, 1.5, 2 and 2.5 GPa, and slowly heating the sample beyond dehydration temperatures. At each pressure conditions, two experiments were carried out at a maximum temperature of 650°C or 700°C respectively, in order to investigate reaction kinetics and equilibrium overstepping. Experiments were quenched once the dehydration was completed, in order to preserve the microstructure.

In all our experiments, P-wave velocity decreased dramatically at the onset of dehydration.  This important drop in elastic properties is related to the fracturing and porous space generated by water release. At 700°C temperature, observed velocity drops were faster, and more pronounced compared to experiments performed at 650°C, indicating that the dehydration reaction progress was faster and more important. The velocity drop also got smaller with increasing pressure, but remained noticeable, even at 2.5GPa, a pressure at which the reaction volume change is negative. This indicates that even in the absence of fluid overpressures, the reaction is accompanied by an important amount of microcracking/softening. Recovered samples were then analyzed using scanning electron microscopy (SEM) and Electron backscatter diffraction (EBSD). With these microstructural data, the final reaction progress/advancement was estimated and we show that in situ measurements of p-wave velocity represent a good proxy for reaction progress and kinetics.

Our study opens up the door to a vast domain, where mineral reactions kinetics could be monitored in situ outside the synchrotron environment, via a direct access to elastic properties. It also reveals our need to apply state of the art effective medium theory modeling of porous and cracked aggregates when computing elastic properties of hydrating/dehydrating mineral assemblages. Finally, the elastic softening observed upon dehydration, even above 2GPa, tends to confirm the dehydration stress transfer model (Ferrand et al. 2017) for intermediate depth earthquake triggering.

 

references:

- Ferrand, Thomas P., et al. "Dehydration-driven stress transfer triggers intermediate-depth earthquakes." Nature communications 8.1 (2017): 1-11.

- Gasc, Julien, et al. "Simultaneous acoustic emissions monitoring and synchrotron X-ray diffraction at high pressure and temperature: Calibration and application to serpentinite dehydration." Physics of the Earth and Planetary Interiors189.3-4 (2011): 121-133.

- Moarefvand, Arefeh, et al. "A new generation Griggs apparatus with active acoustic monitoring." Tectonophysics816 (2021): 229032.

How to cite: Schubnel, A., Moarefvand, A., Gasc, J., Deldicque, D., and Labrousse, L.: Evolution of P-wave velocities during antigorite dehydration at pressures up to 2.5GPa, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3549, https://doi.org/10.5194/egusphere-egu22-3549, 2022.

EGU22-4325 | Presentations | GD6.1

Pervasive melt migration in hot continental crust – numerical models 

Petra Maierová, Pavlína Hasalová, Karel Schulmann, and Pavla Štípská

The common view of melt transport in the continental crust involves an initial stage of percolation along grain boundaries, melt segregation into leucosomes and dykes, coalescence of small melt conduits into larger ones and quick nearly vertical melt flow leading to formation of plutons. An entirely different style of melt migration was described in the Bohemian Massif, eastern European Variscan belt. There, a sequence of metaigneous migmatites was described where veins are lacking, leucosomes are rare and relics of melt are spread along grain boundaries. Textural, geochemical and compositional variations in these rocks show that they formed due to equilibration with melt coming from an external source, and that pervasive flow along grain boundaries was the dominant mechanism of melt transport.

The question arises, at what conditions this style of melt transport can operate and what consequences the different styles of melt transport have on the crustal-scale tectonics. We address this question by means of a 2D crustal-scale model of two-phase flow using the code ASPECT (aspect.geodynamics.org). The system of pores through which the melt flows is not resolved in our model and it is described only by its permeability. A low permeability describes material with pores along grain boundaries while a high permeability corresponds to a system of leucosomes, dykes or cracks

For different material properties and thermal conditions we obtain different styles of melt migration and characteristics of the modeled crust. The melt can form a diffuse zone in the lower–middle crust, km-scale waves of high melt fraction gathering into sub-vertical channels, or a horizontal zone with high melt fraction in the middle crust. The lower crust is depleted and the middle crust is enriched in incompatible elements, and composition of the middle crust typically shows km-scale variations. The compositional variations are obtained even in the models with low permeability that corresponds to the melt percolation along grain boundaries, in agreement with the characteristics of the Bohemian migmatites.

How to cite: Maierová, P., Hasalová, P., Schulmann, K., and Štípská, P.: Pervasive melt migration in hot continental crust – numerical models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4325, https://doi.org/10.5194/egusphere-egu22-4325, 2022.

EGU22-4494 | Presentations | GD6.1

Large-strain elastoplastic formulations for host-inclusion systems with applications to elasto-thermobarometry and geodynamic models 

Evangelos Moulas, Konstantin Zingerman, Anatoly Vershinin, Vladimir Levin, and Yuri Podladchikov

Elastic thermobarometry has been at the forefront of research during the last decade. Using state-of-the-art spectroscopic and diffraction methods it has been possible to assess the residual elastic strain of mineral inclusions in an in-situ manner (Mazzucchelli et al., 2021; Zhong et al., 2019). The interpretation of residual stress/strain and its extrapolation to geological conditions requires mechanical models, that are based on continuum mechanics, which provide the range of pressure-temperature (P-T) conditions where host and inclusion are under homogeneous stress. This set of conditions may correspond to the entrapment conditions if the system is purely elastic. In the case of viscous/plastic relaxation of the host-inclusion system, the inferred P-T conditions represent apparent-entrapment conditions that could lie anywhere between the conditions of the true entrapment and the conditions of viscous/plastic relaxation (Moulas et al., 2020; Zhong et al., 2020). Thus, the interpretation and validity of elastic barometry strongly relies on the purely elastic behavior of the host-inclusion system.

The commonly employed elastic solutions assume a linear-elastic behavior and deal only with small-strain approximations. However, large values of residual stresses/strains may indicate that the range of decompression for such host-inclusion systems requires the incorporation of material/geometric non-linearity. In this work, we provide new numerical and analytical solutions for the non-linear, elasto-plastic behavior of host-inclusion systems. Our analytical solutions are based on new published models that describe the Neo-Hookean behavior of materials and reduce to the Murnaghan equation of state when the deformation is purely volumetric (Levin et al., 2021). We find that for the range of residual pressures that is commonly employed in barometric applications (<1GPa) the incorporation of geometric non-linearity does not influence the results significantly. Nevertheless, the incorporation of plasticity and the combined non-linear elastic and plastic behavior may lead to results that render elasto-thermobarometry inapplicable for very large compression/decompression ranges. Our results can be useful for benchmarking: a) models relevant to elasto-thermobarometry and b) geodynamic models that require the treatment of large volumetric deformations during the exhumation from lithospheric/mantle depths.

References

Levin, V.A., Podladchikov, Y.Y., Zingerman, K.M., 2021. An exact solution to the Lame problem for a hollow sphere for new types of nonlinear elastic materials in the case of large deformations. European Journal of Mechanics - A/Solids 90, 104345. https://doi.org/10.1016/j.euromechsol.2021.104345

Mazzucchelli, M.L., Angel, R.J., Alvaro, M., 2021. EntraPT: An online platform for elastic geothermobarometry. American Mineralogist 106, 830–837. https://doi.org/10.2138/am-2021-7693CCBYNCND

Moulas, E., Kostopoulos, D., Podladchikov, Y., Chatzitheodoridis, E., Schenker, F.L., Zingerman, K.M., Pomonis, P., Tajčmanová, L., 2020. Calculating pressure with elastic geobarometry: A comparison of different elastic solutions with application to a calc-silicate gneiss from the Rhodope Metamorphic Province. Lithos 378–379, 105803. https://doi.org/10.1016/j.lithos.2020.105803

Zhong, X., Andersen, N.H., Dabrowski, M., Jamtveit, B., 2019. Zircon and quartz inclusions in garnet used for complementary Raman thermobarometry: application to the Holsnøy eclogite, Bergen Arcs, Western Norway. Contributions to Mineralogy and Petrology 174, 50. https://doi.org/10.1007/s00410-019-1584-4

Zhong, X., Moulas, E., Tajčmanová, L., 2020. Post-entrapment modification of residual inclusion pressure and its implications for Raman elastic thermobarometry. Solid Earth 11, 223–240. https://doi.org/10.5194/se-11-223-2020

How to cite: Moulas, E., Zingerman, K., Vershinin, A., Levin, V., and Podladchikov, Y.: Large-strain elastoplastic formulations for host-inclusion systems with applications to elasto-thermobarometry and geodynamic models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4494, https://doi.org/10.5194/egusphere-egu22-4494, 2022.

EGU22-6103 | Presentations | GD6.1 | Highlight

The role of mechanics in the modelling of common rock microstructures 

Lucie Tajcmanova, Yury Podladchikov, and Ivan Utkin

Understanding rocks at the microscale is essential to comprehending Earth's history and making reasonable predictions about how planetary processes may change in the future.  

Advanced models for complex rock microstructures, such as symplectites or a development of exsolution lamellae, have been developed (Kuhl & Schmid, 2007; Petrishcheva & Abart, 2009). Despite of this recent valuable progress in our understanding of these microstructures, the mechanisms controlling its evolution especially from slowly cooled rocks are still not complete.

Commonly, such models focus solely on the chemical process. Interestingly, mechanics, i.e. stress and pressure redistribution, may also play an important role on microstructure evolution. In this contribution, we investigate the coupled, chemo-mechanical, effect for representative rock microstructures. We provide a comparison between purely chemical vs. coupled chemo-mechanical systems and provide predictions on the evolution of the given microstructures in 3D.

References:

Kuhl, E., Schmid, D.W. (2007). Computational Modeling of Mineral Unmixing and Growth. Comput Mech 39, 439–451.

Petrishcheva, E., & Abart, R. (2009). Exsolution by Spinodal Decomposition I: Evolution Equation for Binary Mineral Solutions with Anisotropic Interfacial Energy. American Journal of Science, 309(6), 431-449.

 

How to cite: Tajcmanova, L., Podladchikov, Y., and Utkin, I.: The role of mechanics in the modelling of common rock microstructures, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6103, https://doi.org/10.5194/egusphere-egu22-6103, 2022.

EGU22-7325 | Presentations | GD6.1

A multiscale model for coupled chemical reaction and deformation of porous rocks 

Viktoriya Yarushina and Yury Podladchikov

Coupled hydro-mechano-chemical (HMC) modeling is a topic of active ongoing research in various branches of Earth sciences and subsurface engineering. In engineering applications, HMC modeling is used to assess the feasibility of permanent CO2 storage in mafic and ultramafic rocks. The deformation and stresses building during the reaction is believed to induce fracturing, increase permeability and thus promote extensive reactions between CO2 and host rock. CCS in depleted reservoirs faces challenges related to possible CO2 leakage through old plugged and abandoned wells. When CO2 reaches the well, old cement compositions react with cement, compromising well integrity due to chemical degradation. In geology, coupled reactions and deformation are involved in melt extraction and migration, influencing the dynamics of volcanic systems and the evolution of subduction zones.

A large focus of previous studies was whether or not it is possible to achieve 100% of the reaction. Common reactive transport models predict that the reaction product will clog the pores, which will stop the fluid flow and thus further reactions. However, recent developments suggest that reaction progress depends on the assumed reaction kinetics and the constitutive models used in coupled models. Models that account for solid volume change as in mineral replacement reactions have a much higher potential for preserving porosity than the common dissolution-precipitation model, thus predicting the complete reaction. It is often assumed that reaction processes are transport-dominated, i.e., that all dissolved material is carried away by pore fluid. Then it precipitates on the available pore space leading to clogging and permeability reduction. However, recent observations suggest that while some reactions might be associated with dissolution and precipitation at the nano-scale, aqueous species transport is limited, and reaction products do not precipitate in the pores but rather stay attached to the primary mineral. Thus, the overall effect is the same as in mineral replacement reactions.

Using a combination of effective media theory and irreversible thermodynamics approaches, we propose a new model for reaction-driven mineral expansion, which preserves porosity and limits unrealistically high build-up of the force of crystallization by allowing inelastic failure processes at the pore scale. To fully account for the coupling between reaction, deformation, and fluid flow, we derive macroscopic poroviscoelastic stress-strain constitute laws that account for chemical alteration and viscoelastic deformation of porous rocks. These constitutive equations are further used with macroscopic conservation laws to illustrate the mutual impact of reactive transport and mechanical deformation on simple 1D examples of wellbore stability and fluid transport.

How to cite: Yarushina, V. and Podladchikov, Y.: A multiscale model for coupled chemical reaction and deformation of porous rocks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7325, https://doi.org/10.5194/egusphere-egu22-7325, 2022.

EGU22-8033 | Presentations | GD6.1

Modelling focused fluid flow: What matters? 

Lawrence Hongliang Wang, Viktoriya M. Yarushina, and Yury Podladchikov

Two-phase flow equations that couple solid deformation and fluid migration have opened new research trends in geodynamical simulations and modelling of subsurface engineering operations. The physical nonlinearity of fluid-rock systems and strong coupling between flow and deformation in such equations lead to interesting predictions such as the spontaneous formation of focused fluid flow in ductile/plastic rocks. However, numerical implementation of two-phase flow equations and their application to realistic geological environments with complex geometries and multiple stratigraphic layers is challenging. Here, we present an efficient pseudo-transient solver for two-phase flow equations. We first study the focused fluid flow under the viscous regime without considering the elasticity. The roles of material parameters, reservoir topology, geological heterogeneity, and porosity are investigated. We show that focused fluid channels are the natural outcome of the flow instability of the two-phase system with a low ratio (< 0.1) between shear viscosity and bulk viscosity. We also confirm the previous studies that  decompaction weakening is necessary to elongate the porosity profile. The permeability exponents play the dominant role in the speed of wave propagation. The numerical models study fluid leakage from high porosity reservoirs into less porous overlying rocks. Geological layers present in the overburden do not stop the propagation of the localized channels but rather modify their width, permeability, and growth speed. We further validate our conclusions by modelling the full two-phase system with viscoelastic rheology and elastic solid and fluid compressibility (Yarushina et al., 2015). The Deborah number (De), solid (Ks), and fluid (Kf) bulk moduli are thus introduced into the governing equations. We found that the elasticity makes a difference when the Deborah number approaches one by speeding up the channel propagation. At the same time, its effect is rather limited when Deborah's number is small (e.g., 0.1). The effects of compressibility of the solid and fluid, on the other hand, are not found significant within the reasonable ranges of the bulk moduli.

 

How to cite: Wang, L. H., Yarushina, V. M., and Podladchikov, Y.: Modelling focused fluid flow: What matters?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8033, https://doi.org/10.5194/egusphere-egu22-8033, 2022.

EGU22-8422 | Presentations | GD6.1 | Highlight

Chronometry of a nappe-scale thermal event inferred by thermobarometry and viscous relaxation of quartz inclusion pressure (Adula nappe, Alps) 

Xin Zhong, Marisa Germer, Alexandra Pohl, Vincent Könemann, Olga Brunsmann, Philip Groß, Jan Pleuger, and Timm John

The Adula nappe is located at the eastern flank of the Lepontine dome in the Swiss Alps. It consists mainly of orthogneiss and paragneiss with intercalated lenses of eclogite, amphibolite and metasediments. Previous petrological studies on the peak pressure and temperature (P-T) conditions yield somewhat inconsistent results, particularly the pressure in the southern part of the nappe, but in general exhibit an increasing trend in both P-T towards the south. In this work, we applied zirconium-in-rutile thermometer and quartz-in-garnet Raman elastic barometer to constrain the P-T conditions using samples covering most of the nappe with high spatial coverage within the 600 km2 area to obtain an internally consistent dataset. Based on the results of zirconium-in-rutile thermometer, the temperature gradually increases from the north at ca. 540 °C to the south at ca. 680 °C. Using the quartz-in-garnet elastic barometer, the calculated entrapment pressure increases from ca. 2.0 GPa to ca. 2.2 GPa from the north to the middle-south region of the Adula nappe, but rapidly falls to ca. 0.8-1.2 GPa towards the southern region, where the temperature exceeds ca. 650 °C. It is speculated that due to the temperature increase towards the south, viscous relaxation became activated that led to an apparent drop of the recorded residual quartz inclusion pressure. This suggests that by applying a pure elastic model to high temperature conditions, one may potentially underestimate of the formation pressure of garnets. Therefore, this study may provide information on the limit of the quartz-in-garnet (pure) elastic barometry technique. Moreover, it may offer a potential opportunity to constrain the duration of the near-isothermal decompression path if a viscoelastic model can be applied, which requires not only the equation of state of minerals but also the creep behavior of the inclusion-host system.

How to cite: Zhong, X., Germer, M., Pohl, A., Könemann, V., Brunsmann, O., Groß, P., Pleuger, J., and John, T.: Chronometry of a nappe-scale thermal event inferred by thermobarometry and viscous relaxation of quartz inclusion pressure (Adula nappe, Alps), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8422, https://doi.org/10.5194/egusphere-egu22-8422, 2022.

EGU22-8776 | Presentations | GD6.1

Local variations of metamorphic record from compositionally heterogeneous rocks: Inferences on exhumation processes of (U)HP-HT rocks (Cima di Gagnone, Adula-Cima Lunga unit) 

Stefania Corvò, Matteo Maino, Antonio Langone, Filippo Luca Schenker, Leonardo Casini, Sandra Piazolo, and Silvio Seno

The record of metamorphic conditions may be highly heterogeneous in spatially close rocks with different composition and rheology. The Cima di Gagnone area (Central Alps) represents an example of ultrahigh–pressure and high–temperature ultramafic lenses enveloped within amphibolite–facies metasediments. Structural investigations demonstrate that the rheologically strong ultramafics and eclogites and weak metapelites experienced a common Alpine deformation history in a single tectonic unit, excluding their coupling within a tectonic mélange (Maino et al., 2021). New structural, microstructural and petrological analyses and thermodynamic modelling results on the metasediments, confirming that all rocks generally experienced medium pressure and medium temperature conditions of 1.0–1.2 GPa and 640–700 °C, followed by a retrograde stage around 0.6–0.8 GPa and 600–675 °C. However, significantly higher P–T conditions of 1.3–3.0 GPa and 750–850 °C are locally developed close to the rheological boundary depicted by the micaschists-peridotite contact (Corvò et al., 2021; Piccoli et al., 2021). Rock and mineral chemistry changes during growth of new mineral phases indicate a local melt/fluid interaction (i.e., metasomatism) between metasediments and ultramafics during the high temperature deformation. The local occurrence of (U)HP and HT conditions is demonstrated by the absence of significant melting in the unit, although around the peridotite lenses, metapelites show hydrated assemblage at T>800 °C were stable at variable P stage. U-Pb zircon and monazite dating indicate that local HP and HT conditions were accomplished at the early stage of Alpine exhumation (~36 Ma), while the rocks fa form the rheological boundaries records only pre–Alpine ages. Our results documented that, even though weak metasediments share the same structural evolution with the strong UM, large differences in the local metamorphic conditions (ΔP up to 2 GPa; ΔT up to 160 °C) are recorded in relation to the distance from the UM lenses. Fluid–assisted metasomatism is further documented as being strongly localized at the interface between ultramafic lenses and the metapelitic host throughout all part of the metamorphic evolution, including the HP–HT stage. Therefore, in the Cima di Gagnone type–locality, the interplay between metapelites and ultramafic exerts a crucial first–order control to allow assemblage equilibrium during HT metamorphism and amphibolite–facies retrogression. These new findings exclude that the different metamorphic record may be attributed only to differential preservation during the retrograde path. Our new P–T–t–D paths highlight the crucial role of the rheological boundaries in modify the P-T metamorphic records without varying lithostatic pressure and thus depth conditions.

References:

Maino, M., Adamuszek, M., Schenker, F.L., Seno, S., Dabrowski, M., 2021. Sheath fold development around deformable inclusions: Integration of field-analysis (Cima Lunga unit, Central Alps) and 3D numerical models. J. Struct. Geol. 144, 104255.

Corvò, S., Maino, M., Langone, A., Schenker, F. L., Piazolo, S., Casini, L., & Seno, S., 2021. Local variations of metamorphic record from compositionally heterogeneous rocks (Cima di Gagnone, Central Alps): Inferences on exhumation processes of (U) HP–HT rocks. Lithos, 390, 106126.

Piccoli, F., Lanari, P., Hermann, J., & Pettke, T., 2021. Deep subduction, melting, and fast cooling of metapelites from the Cima Lunga Unit, Central Alps. Journal of metamorphic geology

How to cite: Corvò, S., Maino, M., Langone, A., Schenker, F. L., Casini, L., Piazolo, S., and Seno, S.: Local variations of metamorphic record from compositionally heterogeneous rocks: Inferences on exhumation processes of (U)HP-HT rocks (Cima di Gagnone, Adula-Cima Lunga unit), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8776, https://doi.org/10.5194/egusphere-egu22-8776, 2022.

EGU22-9093 | Presentations | GD6.1 | Highlight

Grain-scale equilibrium reactions guide fluid-driven eclogitization of dry crustal rocks 

Timm John, Sascha Zertani, Johannes C. Vrijmoed, Caroline Brachmann, and Oliver Plümper

When a fluid is introduced into dry rocks at high-pressure conditions, it acts as a catalyst and facilitates re-equilibration. This often promotes weakening and subsequent ductile deformation. Here, we present a detailed micro-structural and mineral chemical study of eclogitization of initially dry continental crustal rocks in the absence of ductile deformation. The studied sample features an incomplete (fluid-induced) transition from lower crustal granulite to eclogite, and the transition is fully preserved. None of the mineral phases show any signs of ductile deformation, indicating that the transformation was entirely static. Material transport during the reaction was limited to the availability of fluids. Detailed analysis of the local assemblages along the transect reveals that the reaction occurs in three distinct steps: The plagioclase-plagioclase grain boundaries were the first to re-equilibrate followed by clinopyroxene-plagioclase and garnet-plagioclase grain boundaries. Lastly, the grain boundaries that included only garnet and/or clinopyroxene are involved in the transformation. Thermodynamic modelling of local equilibria at dry conditions and with H2O in excess reveals that this stepwise transformation is caused by the varying reactivity of the local assemblages at the prevailing P-T conditions. Those reactions that result in the largest decrease of the Gibbs free energy from the dry case to the case with H2O in excess occur first. Once the reaction is facilitated, this effect is amplified because the density increase is largest at those grains boundaries that have reacted first, creating new fluid pathways through volume reduction. The calculated stable local mineral assemblages are consistent with those present in the sample indicating that element transport is limited, also supported by the observation that the fabric of the granulite is preserved in the eclogite. Our results demonstrate that reactive fluid flow is guided by the local energy budget along the grain boundaries, and that element transport during static re-equilibration is limited to the extent where it is thermodynamically advantageous.

How to cite: John, T., Zertani, S., Vrijmoed, J. C., Brachmann, C., and Plümper, O.: Grain-scale equilibrium reactions guide fluid-driven eclogitization of dry crustal rocks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9093, https://doi.org/10.5194/egusphere-egu22-9093, 2022.

EGU22-9608 | Presentations | GD6.1 | Highlight

Formation of olivine veins by dehydration during viscously deforming serpentinite: a numerical study 

Stefan Markus Schmalholz, Evangelos Moulas, Ludovic Räss, and Othmar Müntener

The dehydration of serpentinite during subduction and the associated formation of dehydration veins is an important process for the global water cycle and the dynamics of the subducting plate. Field observations suggest that olivine veins can form by dehydration during viscous shear deformation of serpentinite. However, this hypothesis of olivine vein formation, involving the coupling of rock deformation, dehydration reactions and fluid flow, has not been tested and quantified by hydro-mechanical-chemical (HMC) models. Here, we present a new two-dimensional HMC numerical model to test whether olivine veins can form by dehydration during viscous shearing of serpentinite. The applied numerical algorithm is based on the pseudo-transient finite difference method. We consider the simple reaction antigorite + brucite = forsterite + water. Volumetric deformation is viscoelastic and shear deformation is viscous with a shear viscosity that is an exponential function of porosity. In the initial model configuration, total and fluid pressures are homogeneous and in the antigorite stability field. Small, initial perturbations in porosity, and hence in viscosity, cause pressure perturbations during far-field simple shearing. During shearing, the fluid pressure can locally decrease and reach the thermodynamic pressure required for the dehydration reaction, so that dehydration is triggered locally. The simulations show that dehydration veins form during progressive shearing and grow in a direction parallel to the maximum principal stress. During the dehydration the porosity can increase locally from 2% (initial value) to more than 50% inside the dehydration vein. The numerical model allows quantifying the mechanisms and variables that control the evolution of porosity and fluid pressure. We show that the porosity evolution is controlled by three mechanisms: (1) volumetric deformation of the porous solid, (2) temporal variation of the solid density and (3) mass transfer during the dehydration reaction. We quantify the evolution of the fluid pressure that is controlled by five variables and processes: (1) the total pressure of the porous rock, (2) elastic effects of the total volumetric deformation, (3) the temporal variation of porosity, (4) the temporal variation of solid density and (5) mass transfer during the dehydration reaction. This model supports the observation-based hypothesis of the formation of olivine veins due to dehydration during viscous shearing of serpentinite. More generally, our HMC model provides quantitative insights into the evolution of porosity, and hence dynamic permeability, fluid pressure and mass transfer during dehydration reactions in deforming rock.

How to cite: Schmalholz, S. M., Moulas, E., Räss, L., and Müntener, O.: Formation of olivine veins by dehydration during viscously deforming serpentinite: a numerical study, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9608, https://doi.org/10.5194/egusphere-egu22-9608, 2022.

EGU22-9773 | Presentations | GD6.1

The influence of non-hydrostatic stress on mineral equilibria: insights from Molecular Dynamics 

Mattia L. Mazzucchelli, Evangelos Moulas, Boris Kaus, and Thomas Speck

Mountain building, earthquake generation, and volcanic eruptions occur in Earth’s lithosphere and have direct impacts on society. Understanding the mechanism of geodynamic processes relies on the determination of the pressure-temperature history which is recorded by rocks that have been involved in geodynamic processes. In most cases, the interpretation of the conditions attained by rocks is based on the assumption that the stresses in the Earth are hydrostatic. However, non-hydrostatic stresses are observed in the lithosphere, and the significance of the magnitude of the differential stress on phase equilibria is still actively contested among researchers who hold completely incompatible views about the use of various thermodynamic potentials (e.g. [1-3]).

The problem of phase equilibria under non-hydrostatic stress has been explored in several rock-deformation experiments (on mm scale), in which recrystallization of minerals was observed under an applied non-hydrostatic stress [4-6]. However, during experiments, stress and pressure heterogeneities may develop in the sample (e.g. [6]). Therefore, the direct effect of the applied non-hydrostatic stress on the thermodynamics of the reactions cannot be separated from the effect caused by local pressure variations in the sample itself.

Here, we explore the effect of non-hydrostatic stress on the thermodynamics of mineral reactions by investigating a system at the molecular scale. With Molecular Dynamics (MD) we perform coexistence simulations in which two phases are brought in contact and equilibrated at given temperature, pressure, and stress conditions. As expected, the obtained stress component normal to the phase-phase interfaces is homogeneous across the system. Our data suggest that the direct effect of non-hydrostatic stress on the solid-liquid equilibria is rather minor for geological applications, consistent with theoretical predictions [7,8]. However, our analysis does not take into account the indirect effect of stress heterogeneities at the sample scale. Spatial variations of stress can reach GPa level and can therefore indirectly affect phase equilibria.

M.L. Mazzucchelli is supported by an Alexander von Humboldt research fellowship.

References

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

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

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

[4] Hirth, G. et al. J. Geophys. Res. 99, 11731–11747 (1994)

[5] Richter, B. et al. J. Geophys. Res. Solid Earth 121, 8015–8033 (2016)

[6] Cionoiu, S. et al. Sci. Rep. 9, 1–6 (2019)

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

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

How to cite: Mazzucchelli, M. L., Moulas, E., Kaus, B., and Speck, T.: The influence of non-hydrostatic stress on mineral equilibria: insights from Molecular Dynamics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9773, https://doi.org/10.5194/egusphere-egu22-9773, 2022.

EGU22-10147 | Presentations | GD6.1

H2O contents in nominally anhydrous minerals and its effect on the formation of eclogite-facies, hydrous shear zones (Holsnøy, Western Norway) 

Lisa Kaatz, Stefan M. Schmalholz, Julien Reynes, Jörg Hermann, and Timm John

High-grade dry granulites of Holsnøy (Western Norway) were subducted during the Caledonian orogeny and reached eclogite-facies conditions at ~2 GPa and 700° C. However, they stayed in a metastable state until brittle deformation enabled infiltration of an aqueous fluid, which triggered the kinetically delayed eclogitization. Field observations reveal an interconnected network of hydrated eclogite-facies shear zones surrounded by unaltered and pristine granulites. The formation of these features is highly controlled by deformation, fluid infiltration and fluid-rock interaction.

At first, the shear zone evolution was analyzed to better understand the relation between strain localization within the shear zones and the progressive widening of these shear zones from cm- to m-wide thickness. The results showed that widening overcomes the effect of stretching during progressive fluid-rock interaction and strain accumulation, if either a substantial amount of continuously infiltrating fluid and/or numerous repetitive fluid pulses enter the system.

Therefore, investigations have been carried on the H2O contents in nominally anhydrous minerals of the granulite and eclogite. The H2O contents were measured using Fourier transform infrared spectroscopy. Garnet (grt), clinopyroxenes (cpx) and plagioclase (plg) have been measured with a close look on spatial repartition of OH at the grain scale and at the shear zone scale. The aim is to decode the link between fluid infiltration, mineral reaction, and deformation. There are no significant compositional changes between granulite and eclogite, which means that the fluid mainly worked as a catalyst without mass transfer beside H2O. The analyses across a shear zone profile reveal three major observations: (i) average H2O contents of the grt cores increase from granulite towards the shear zone (from 10 to 50 µg/g), (ii) average H2O contents of the cpx increase, too (from 145 to 310 µg/g), (iii) the plg stores limited amounts of H2O until a phase separation leads into an symplectites consisting of albite-rich plg (anhydrous) and clinozoisite (hydrous). The H2O contents of the minerals are interpreted to be a result of two different diffusional mechanisms acting simultaneous at different spatial scales and rates. The H2O increase in grt and cpx cores without mineral reaction is a result of hydrogen diffusion (H+/H2), which is much faster and pervasive than the porous influx of an aqueous fluid (H2O), which, contemporaneously, caused the formation of hydrous phases.

The above findings are combined in a 1D numerical shear zone model to reproduce the measured mineral chemical data and the respective H2O-contents. The results shed light on the dynamic weakening processes caused by the influx of H+/H2 in combination with synkinematic mineral reactions.

How to cite: Kaatz, L., Schmalholz, S. M., Reynes, J., Hermann, J., and John, T.: H2O contents in nominally anhydrous minerals and its effect on the formation of eclogite-facies, hydrous shear zones (Holsnøy, Western Norway), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10147, https://doi.org/10.5194/egusphere-egu22-10147, 2022.

EGU22-10316 | Presentations | GD6.1 | Highlight

Thermolab: a thermodynamics laboratory for non-linear transport processes in open systems 

Johannes C. Vrijmoed and Yury Y. Podladchikov

We developed a numerical thermodynamics laboratory called “Thermolab” to study the effects of the thermodynamic behavior of non-ideal solution models on reactive transport processes in open systems. The equations of state of internally consistent thermodynamic datasets are implemented in MATLAB functions and form the basis for calculating Gibbs energy. A linear algebraic approach is used in Thermolab to compute Gibbs energy of mixing for multi-component phases to study the impact of the non-ideality of solution models on transport processes. The Gibbs energies are benchmarked with experimental data, phase diagrams and other thermodynamic software. Constrained Gibbs minimization is exemplified with MATLAB codes and iterative refinement of composition of mixtures may be used to increase precision and accuracy. All needed transport variables such as densities, phase compositions, and chemical potentials are obtained from Gibbs energy of the stable phases after the minimization in Thermolab. We demonstrate the use of precomputed local equilibrium data obtained with Thermolab in reactive transport models. In reactive fluid flow the shape and the velocity of the reaction front vary depending on the non-linearity of the partitioning of a component in fluid and solid. We argue that non-ideality of solution models has to be taken into account and further explored in reactive transport models. Thermolab Gibbs energies can be used in Cahn-Hilliard models for non-linear diffusion and phase growth. This presents a transient process towards equilibrium and avoids computational problems arising during precomputing of equilibrium data.

How to cite: Vrijmoed, J. C. and Podladchikov, Y. Y.: Thermolab: a thermodynamics laboratory for non-linear transport processes in open systems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10316, https://doi.org/10.5194/egusphere-egu22-10316, 2022.

EGU22-10318 | Presentations | GD6.1 | Highlight

Eclogitization of the Allalin gabbro under heterogeneous stress conditions 

Cindy Luisier, Philippe Yamato, Horst R. Marschall, Evangelos Moulas, and Thibault Duretz

Eclogitization reactions in mafic rocks involve large volume changes, porosity evolution and fluid transfer. They impact many important geological processes such as the localization of deformation and fluid channeling at intermediate depth in subduction zone. The study of exhumed eclogitic bodies in orogens shows that eclogitization of the oceanic crust is heterogeneous from both a structural and metamorphic point of view. For example, in the European Alps, the Allalin metagabbro shows high strain areas, consisting of hydrous metagabbros, fully equilibrated under eclogite-facies conditions during the Alpine orogeny. Conversely, large volumes of low strain, fluid-undersaturated gabbros remained largely unaffected by the high-pressure (HP) metamorphism, locally preserving igneous textures and even, occasionally, relics of their magmatic mineralogy. The intensity of deformation as well as the degree of eclogitization in the metagabbro have been shown to be directly related to the extent of pre-Alpine hydration during high-temperature hydrothermal alteration [1]. However, the influence of this degree of hydration on (1) reaction kinetics and/or (2) enhancing rheological contrasts leading to heterogeneous deformation patterns and metamorphic conditions is still debated.

In order to address this issue, we propose a multidisciplinary study involving petrographic and microtextural observations combined with 2D thermo-mechanical numerical models allowing to discuss the role of pre-Alpine hydrothermal alteration on the development of HP metamorphic assemblages.

We present petrographic and textural data from three different types of rocks from the Allalin metagabbros: i) undeformed and mostly untransformed metagabbros, with relics of igneous augite and plagioclase, ii) coronites, with olivine pseudomorphs showing different levels of hydration, rimmed by a garnet corona, and iii) eclogitized metagabbros, with olivine and plagioclase sites fully replaced by high-pressure assemblages.

The role of protolith hydration on the observed range in metamorphic facies is then tested by using 2D thermo-mechanical models that allow to simulate the deformation of a strong and dry rock with several randomly oriented weak and hydrous zones. Our results show that the shearing of heterogeneous rock can lead to the formation of localized ductile shear zone within a matrix that remains relatively undeformed but where plastic deformation can occur. The associated P field is also highly heterogeneous, with P ranging from 1 to 3 GPa. The deformation patterns and P modelled may suggest that locally hydrated portions of the gabbro acted as rheological perturbations sufficiently efficient in producing the structural and metamorphic record now observed in the field.

 

 

[1] Barnicoat, A. C. & Cartwright, I. (1997) Journal of Metamorphic Geology 15, 93–104

How to cite: Luisier, C., Yamato, P., Marschall, H. R., Moulas, E., and Duretz, T.: Eclogitization of the Allalin gabbro under heterogeneous stress conditions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10318, https://doi.org/10.5194/egusphere-egu22-10318, 2022.

EGU22-10383 | Presentations | GD6.1

Geodynamic constraints on ophiolite emplacement 

Iskander Ibragimov and Evangelos Moulas

Ophiolite complexes are commonly found outcropping along ancient suture zones in continental regions. Many geological studies suggest that, during subduction initiation, a small remnant of the oceanic crust can be thrusted upon continenal regions. This thrusting occurs during a process that is generally termed as “ophiolite obduction”. Despite the relatively small volume of the ophiolite rocks, their occurence provides important geologic/geodynamic constraints for the processes of subduction initiation. 
Following the seminal work of Cloos (1993), oceanic lithosphere that is older than 10 Myrs is dense enough, and as a result, facilitates oceanic subduction in a spontaneous manner. This suggestion is based on the fact that buoyancy is one of the most important forces relevant to large-scale geodynamics. However, old oceanic lithosphere is also expected to be cold and, as a consequence, mechanically strong. The increased strength of the oceanic lithosphere hinders subduction initiation and makes ophiolite obduction difficult.
In this work we perform systematic numerical simulations to investigate the effects of initial geometry and convergence velocity on subduction initiation and ophiolite obduction. We use LaMEM to calculate 2D thermo-mechanical models that include the effects of visco-elasto-plastic rheology. In addition, we have incorporated a thermodynamically-consistent density structure for the crust and mantle. In this way, buoyancy forces are calculated in a consistent manner based on the pressure and temperature fields of the thermo-mechanical models. Our results show that when the oceanic lithosphere is older than 10Myr, subduction is very difficult and does not initiate in a spontaneous manner. Our systematic simulations provide insights for the range of conditions and parameters of oceanic subduction and ophiolite emplacement.

References
Cloos, M. (1993) Lithospheric Buoyancy and Collisional Orogenesis: Subduction of Oceanic Plateaus, Continental Margins, Island Arcs, Spreading Ridges, and Seamounts. Geological Society of America Bulletin, 105, 715-737.
https://doi.org/10.1130/0016-7606(1993)105<0715:LBACOS>2.3.CO;2

How to cite: Ibragimov, I. and Moulas, E.: Geodynamic constraints on ophiolite emplacement, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10383, https://doi.org/10.5194/egusphere-egu22-10383, 2022.

EGU22-10445 | Presentations | GD6.1

Reactive Melt Transport Using Porosity Waves Across the Thermal Boundary Layer. 

Marko Repac, Annelore Bessat, Stefan Schmalholz, Yury Podladchikov, Kurt Panter, and Sebastien Pilet

The lithosphere and the asthenosphere are characterized by different heat transport mechanisms, conductive for the lithosphere, convective for the asthenosphere. The zone associated with the transition between these two distinct mechanisms is known as the "Thermal Boundary Layer" (TBL). How the melt is transported across this zone is an important question regarding intraplate magmatism and for the nature of the seismic Low-Velocity Zone. Numerous studies and models suggest that primary magmas from intraplate volcanos are the product of low degree partial melting in the asthenosphere, while the differentiation process takes place in the crust or shallow lithospheric mantle. The question is how low degree melt ascends through the TBL and the lithospheric mantle. The thermal structure of the lithosphere is characterized by a high geothermal gradient, which questions the ability of melt to cross the lithospheric mantle without cooling and crystallizing. Since the base of the lithosphere is ductile, the possible modes of magma transport are porous flow or porosity waves. For these reasons, we would like to understand how melt is transported and what are the implications on the evolution of primitive melt, going from the convective part of the geotherm to the conductive part of the geotherm and further across the lithosphere.

We present the results of a thermo-hydro-mechanical-chemical (THMC) model1 for reactive melt transport using the finite difference method. This model considers melt migration by porosity waves and a chemical system of forsterite-fayalite-silica. Variables, such as solid and melt densities or MgO and SiO2 mass concentrations, are functions of pressure, temperature, and total silica mass fraction (CtSiO2). These variables are pre-computed with Gibbs energy minimization and their variations with evolving P, T, and CtSiO2 are implemented in the THMC model. We consider P and T conditions relevant across the TBL. With input parameters characteristic for alkaline melt and conditions at the base of the lithosphere, we obtain velocities between 1 to 150 m yr-1,which is a velocity similar to melt rising at mid-ocean ridges2. This implies the inability of primary melts to cross the lithosphere. However, melt addition to the base of the lithosphere is important to understand mantle metasomatism, and could, to some extent, contribute to physical properties of the Lithosphere-Asthenosphere Boundary and Mid Lithosphere Discontinuity observed with geophysical methods. We suggest that the appearance of alkaline magmas at the surface requires multiple stage processes as melts rising in the lithosphere progressively modify the geotherm allowing new melts to propagate to the surface. Our earlier modeling results1 demonstrated that a single porosity wave has a minor impact on chemical evolution. In this study, we search for a mechanism responsible for stabilizing porosity wave motion to some lateral location forcing consecutive waves to follow the same ascent path. The passage of a large number of quickly rising porosity waves over a long time through the same path would accumulate large melt to rock ratios and cause significant chemical evolution.

 

  • Bessat et at., 2022, G3, in press
  • Connolly et al. 2009, Nature 462, 209-212.

How to cite: Repac, M., Bessat, A., Schmalholz, S., Podladchikov, Y., Panter, K., and Pilet, S.: Reactive Melt Transport Using Porosity Waves Across the Thermal Boundary Layer., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10445, https://doi.org/10.5194/egusphere-egu22-10445, 2022.

EGU22-11149 | Presentations | GD6.1

Early reaction of plagioclase : an underrated alteration step during burial of the continental crust 

Loic Labrousse, Marie Baisset, and Alexandre Schubnel

Mutual links between metamorphic reactions and rheological properties of rocks under pressure, temperature and deviatoric stress are a major source of discrepancy of thermo-mechanical models when it comes to predict strain localization for instance. The interactions between metamorphism and strain are also considered as a possible cause for unexpected mechanical instabilities, e.g. mechanical failure, in lithological units buried deep in convergent plate boundaries.

The partially transformed granulite facies anorthosites on the Holsnøy Island, Bergen Arcs, Norwegian Caledonides, constitute one of the few archetypical exposure of crustal rocks deforming and reacting at the same time in the eclogite facies conditions. In these rocks, eclogite-facies paragenesis develops with devitrification patterns in « brittle » pseudotachylyte, and in their damage walls, along a pervasive network of « ductile » shear zones, as well as « statically » along digitations following the preserved granulite facies foliation, with no apparent relation to strain.

The present study, that follows recent advances in the understanding of relationships between crystallization of pyroxene and local scale pressure field, or modeling of the interaction between the eclogitization reactions sequence and strain localization, focuses on the first steps of incipient plagioclase destabilization along eclogite facies « fingers ». 

Granulite facies plagioclase, close to 40 % anorthite in composition, is subject to reactions both in the NASH and CASH subsystems, with contrasted stoechiometries and kinetics. Petrological observations evidence that the lowermost pressure reaction in the CASH system (an + H2O = zo + ky + qz), occurs unbalanced, with high kinetics and reaction volume change and therefore initiates strain within plagioclase grains, that react by twinning and subgrains individualization. This early stage of intra-grain transformation induces an effective grain size reduction, and favors fluid percolation, therefore promoting the eclogitization progression. The reaction occurring inside of plagioclase grains also affects their grain boundaries where kyanite and transient reactions products, such as potential melts, accumulate also altering the overall aggregate properties. 

We claim that this early, fast and pervasive reaction is a significative, yet underrated, step of mechanical alteration of the burying continental rocks.

How to cite: Labrousse, L., Baisset, M., and Schubnel, A.: Early reaction of plagioclase : an underrated alteration step during burial of the continental crust, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11149, https://doi.org/10.5194/egusphere-egu22-11149, 2022.

EGU22-11487 | Presentations | GD6.1 | Highlight

Deformation-facilitated melting of plagioclase 

Sarah Incel, Marie Baisset, Loic Labrousse, and Alexandre Schubnel

Geological processes involving deformation and/or reactions are highly influenced by the rock grain size, especially if diffusion-controlled processes take place such as long-range metamorphic reactions and diffusion creep. Although many processes, inducing grain-size reduction, are documented and understood at relatively high stresses and low temperatures (e.g., cataclasis) as well as at lower stress and higher temperature conditions (e.g., bulging, subgrain rotation), deformation twinning, a plastic deformation mechanism active in various minerals at lower temperatures, has been neglected as cause for grain-size reduction so far. We conducted experiments on natural plagioclase-bearing aggregates at 2.5 to 3 GPa confining pressure and temperatures of 720 to 950 °C using two different deformation apparatus, a DDIA and a Griggs press, as well as a piston-cylinder apparatus. Regardless of the apparatus type, we observe the breakdown of plagioclase into an eclogite-facies paragenesis, which is associated with partial melting in the high pressure, high temperature domain of the eclogite facies. In contrast to the sample that experienced hydrostatic conditions in the piston-cylinder press, the deformed samples reveal melt patches inside of several plagioclase grains. These patches coincide with the occurrence of deformation twins in plagioclase that formed due to differential stress. The ability of plagioclase to form deformation twins and their exploitation for melt initiation significantly lowers the effective grain size of plagioclase-rich rocks and thus impacts their reactivity and deformation behavior.

How to cite: Incel, S., Baisset, M., Labrousse, L., and Schubnel, A.: Deformation-facilitated melting of plagioclase, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11487, https://doi.org/10.5194/egusphere-egu22-11487, 2022.

EGU22-11490 | Presentations | GD6.1

Creep and acoustic emission in Shales from the Barents Sea 

Alina Sabitova, Sergey Stanchits, Viktoriya Yarushina, Georgy Peshkov, Lyudmila Khakimova, and Vladimir Stukachev

Nowadays, environmental awareness has become one of the key directions of humankind development. There are a lot of projects aimed at preserving the environment: ensuring the environmental safety of geothermal energy facilities; study of global geodynamics and its influence on the composition, state, and evolution of the biosphere; geoecological substantiation of safe placement, storage, and disposal of toxic, radioactive and other wastes, etc. An essential role is assigned to the storage of increasing volumes of carbon dioxide gas. This problem requires complex approaches and solutions. Given that both CO2 and radioactive storage are long-term projects, it is necessary to investigate the creep process to monitor the state of the underground environment and assess the risks of leakage. A viscous deformation of the formation accompanies the prolonged loading. Viscosity is an essential parameter in coupling fluid flow and deformation processes occurring on Earth [Sabitova et al., 2021]. At the same time, focused fluid flow is a common phenomenon in sedimentary basins worldwide. Flow structures often penetrate the sandy reservoir rocks and clay-rich caprocks [Peshkov et al., 2021]. The impacts of the viscoelastic deformation of clay-rich materials need to be evaluated from an experimental and modeling perspective to understand better the mechanisms forming such structures. Here, we present multistage triaxial laboratory creep experiments with acoustic emission analysis conducted on samples from the Barents Sea. We performed lithological and geochemical characterization of each sample as a petroleum system element. Bulk and shear viscosities used in numerical models are calculated for all samples. The experimental curves are explained using the theoretical model for porous rock viscoelastoplastic (de)compaction [Yarushina et al., 2020].

References:

Sabitova, A., Yarushina, V. M., Stanchits, S., Stukachev, V., Khakimova, L., & Myasnikov, A. (2021). Experimental compaction and dilation of porous rocks during triaxial creep and stress relaxation. Rock Mechanics and Rock Engineering, 54(11), 5781-5805.

Peshkov, G. A., Khakimova, L. A., Grishko, E. V., Wangen, M., & Yarushina, V. M. (2021). Coupled Basin and Hydro-Mechanical Modeling of Gas Chimney Formation: The SW Barents Sea. Energies, 14(19), 6345.

Yarushina, V. M., Podladchikov, Y. Y., & Wang, L. H. (2020). Model for (de) compaction and porosity waves in porous rocks under shear stresses. Journal of Geophysical Research: Solid Earth, 125(8), e2020JB019683.

How to cite: Sabitova, A., Stanchits, S., Yarushina, V., Peshkov, G., Khakimova, L., and Stukachev, V.: Creep and acoustic emission in Shales from the Barents Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11490, https://doi.org/10.5194/egusphere-egu22-11490, 2022.

EGU22-11811 | Presentations | GD6.1

The simplest visco- or elasto-plastic rheology allowing to spontaneous earthquake nucleation 

Yury Alkhimenkov, Ivan Utkin, Lyudmila Khakimova, Celso Alvizuri, and Yury Podladchikov

Understanding the physical processes governing earthquake nucleation has been a hot topic since the last decade. A lot of research has been done trying to explain the physics of seismic triggering events. However, the exact physics behind seismic events nucleation is still poorly understood. The outcome of our recent research is the new theory of earthquake nucleation (Alkhimenkov et. al., 2021). The simplest visco-plastic or elasto-plastic rheology allows us to model spontaneous earthquake nucleation. We consider pure shear boundary conditions and slowly increase stress in the model reflecting the stress increase e.g., due to tectonic forces in real rocks. Once the stress field reaches the yield surface, the strain localization occurs, resulting in slowly developing fractal shear bands. As time evolves, shear bands grow spontaneously, and stress drops take place in the medium. Such stress drops are caused by the instantaneous development of new shear bands, their intersections, and intersections with the boundaries of the numerical domain. A stress drop corresponds to a particular new strain localization pattern. The new strain localizations act as seismic sources and trigger seismic wave propagation (Minakov and Yarushina, 2021). We suggest that the (seismic) radiation pattern of the focal mechanism might be similar to a particular moment tensor source, typical for realistic earthquakes (Alvizuri et al., 2018). This new modeling approach is based on conservation laws without any experimentally derived constitutive relations.

References

Alkhimenkov Y., Utkin I., Khakimova L., Alvizuri C., Quintal Q., Podladchikov Y. Spontaneous earthquake nucleation in elasto-plastic media. 19th Swiss Geoscience Meeting 2021, Geneva, Switzerland.

Minakov, A. and Yarushina, V., 2021. Elastoplastic source model for microseismicity and acoustic emission. Geophysical Journal International, 227(1), pp.33-53.

Alvizuri, C., Silwal, V., Krischer, L. and Tape, C., 2018. Estimation of full moment tensors, including uncertainties, for nuclear explosions, volcanic events, and earthquakes. Journal of Geophysical Research: Solid Earth, 123(6), pp.5099-5119.

How to cite: Alkhimenkov, Y., Utkin, I., Khakimova, L., Alvizuri, C., and Podladchikov, Y.: The simplest visco- or elasto-plastic rheology allowing to spontaneous earthquake nucleation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11811, https://doi.org/10.5194/egusphere-egu22-11811, 2022.

EGU22-11836 | Presentations | GD6.1

Thermo-chemo-mechanical coupling in Maxwell-Stefan multi-component diffusion 

Lyudmila Khakimova, Evangelos Moulas, Ivan Utkin, and Yury Podladchikov

Classical Fickian linear diffusion of inert or trace-like elements is restricted to ideal solution models of components with equal molar mass. Simultaneous diffusion of multiple concentrations is well-treated by the classical Maxwell-Stefan model. Quantitative predictions of concentrations evolution in real mixtures require careful replacement of concentration gradients by gradients of chemical potentials. Coupling of multi component diffusion to mechanics result in pressure gradients that contribute to Gibbs-Duhem relationship. We aim at developing of thermodynamically admissible multicomponent thermo-chemo-mechanical (TMC) model with ensured non-negative entropy production. We also ensure correct equilibrium limit with zero gradients of chemical potentials of individual components and satisfaction of classical Gibbs-Duhem and Maxwell relationships under pressure gradients. Following recent Tajčmanová et al. (2021) we consider both molar and mass formulations. We present optimal pseudo-transient numerical scheme for multi-diffusional fluxes coupled to visco-elastic bulk deformation.

Tajčmanová, L., Podladchikov, Y., Moulas, E. and L. Khakimova. The choice of a thermodynamic formulation dramatically affects modelled chemical zoning in minerals. Sci Rep 11, 18740 (2021).

How to cite: Khakimova, L., Moulas, E., Utkin, I., and Podladchikov, Y.: Thermo-chemo-mechanical coupling in Maxwell-Stefan multi-component diffusion, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11836, https://doi.org/10.5194/egusphere-egu22-11836, 2022.

EGU22-12215 | Presentations | GD6.1

Channelizing of melt flow by reactive porosity waves and its impact on chemical differentiation 

Andrey Frendak, Yury Alkhimenkov, Lyudmila Khakimova, Ivan Utkin, Yury Podladchikov, and Stefan Schmalholz

Many geodynamic processes are coupled. For example, in the partially molten mantle, the solid and molten mantle phases interact chemically during porous melt flow. For such two-phase reactive melt migration, solid and melt densities are functions of temperature, pressure, and chemical composition. Numerical models of such coupled physical-chemical systems require special treatment of the various couplings and concise numerical implementation. We elaborate a 2-D thermo-hydro-mechanical-chemical (THMC) numerical model for melt migration by porosity waves coupled to chemical reactions (Bessat et. al., 2021). We consider a simple ternary chemical system of forsterite-fayalite-silica to model melt migration within partially molten peridotite around the lithosphere-asthenosphere boundary. Our THMC model can simulate porosity waves of different shapes depending on the ratio of shear to bulk viscosity and the ratio of decompaction to compaction bulk viscosity. For an initial circular (blob-like) porosity perturbation, having a 2-D Gaussian shape, the geometry of the propagating reactive porosity wave remains blob-like if all viscosities are similar. If the decompaction bulk viscosity is smaller than the compaction bulk viscosity, so-called decompaction weakening, then the propagating porosity wave evolves into a channelized form. Our simulations quantify the variation from a blob-like to a channel-like porosity wave as a function of the viscosity ratios. We describe the 2-D THMC numerical algorithm which is based on the pseudo-transient finite difference method. Furthermore, we quantify the impact of channelization on the chemical differentiation during melt flow. Particularly, we quantify the evolution of the total silica concentration during melt migration as a function of the degree of channelization.

References

Bessat, A., Pilet, S., Podladchikov, Y. Y., & Schmalholz, S. M. (2022). Melt migration and chemical differentiation by reactive porosity waves. Geochemistry, Geophysics, Geosystems. In press.  

How to cite: Frendak, A., Alkhimenkov, Y., Khakimova, L., Utkin, I., Podladchikov, Y., and Schmalholz, S.: Channelizing of melt flow by reactive porosity waves and its impact on chemical differentiation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12215, https://doi.org/10.5194/egusphere-egu22-12215, 2022.

EGU22-12337 | Presentations | GD6.1

Experimental and numerical investigation of acoustic emission and its moment tensors in sandstones during failure based on the elastoplastic approach 

Elena Grishko, Viktoriya Yarushina, Maria Bobrova, Sergei Stanchits, Alexander Minakov, and Vladimir Stukachev

Microseismicity and acoustic emission (AE) studies are a part of earthquake science. Compared to ordinary earthquakes, microseismic events are characterized by higher frequencies, lower magnitudes, shorter duration, and more complex source mechanisms. The researchers associate the induced seismicity with different processes: borehole breakouts, tunnel excavations, hydraulic fracturing, wastewater injection, and stimulation of geothermal reservoirs.

Acoustic emission represents elastic waves generated spontaneously due to the formation of microfractures when the rock is undergoing a sufficiently high load. AE can be used to obtain continuous data at various stages of the deformation process: from distributed plastic failure to localized macroscopic failure. The spatial distribution of AE events indicates the location of fractures, and the source mechanism provides information about the failure mode: a tensile fracture, a shear fracture, or a combination of both.

This work shows the results of an experimental study of borehole breakouts in sandstones. We measured AE during the deformation experiments and applied the moment tensor analysis to microseismic waveforms. We used a continuum mechanics model of Minakov and Yarushina [2021] to relate the laboratory AE data to the deformation processes. The comparison of the failure patterns and corresponding seismic responses obtained in laboratory and simulations, allows to classify the deformation regimes in real rocks based on seismic observables.

EG, MB, SS, and VS gratefully acknowledge support from the Ministry of Science and Higher Education of the Russian Federation under agreement No. 075-15-2020-119 within the framework of the development program for a world-class Research Center.

 

References:

  • Minakov, A., Yarushina, V., Elastoplastic source model for microseismicity and acoustic emission, Geophysical Journal International, Volume 227, Issue 1, October 2021, Pages 33–53, https://doi.org/10.1093/gji/gg