TS – Tectonics & Structural Geology

TS1.2 – Exploring frontiers in ocean-plate tectonics and marine geosciences

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.

EGU22-1791 | Presentations | TS1.2

The role of crustal accretion variations in determining slab hydration at an Atlantic subduction zone 

Robert Allen, Jenny Collier, and Tim Henstock

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.

EGU22-7065 | 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.

EGU22-7720 | 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.

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.

EGU22-6115 | Presentations | TS1.2

A global quantification of submarine mass-wasting at fault-bound abyssal hills

Alex Hughes, Javier Escartin, Jean-Arthur Olive, and Malatesta Luca

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.

EGU22-8070 | Presentations | TS1.2

Sulfide trace element signatures as a tracer for low- and high-temperature processes in serpentinites

Malte Kalter, Esther M. Schwarzenbach, Falco Menne, Marcus Oelze, Martin J. Whitehouse, and Timm John

EGU22-8493 | Presentations | TS1.2

How Hydrothermal Cooling and Magmatic Sill Intrusions Control Flip-Flop Faulting at Ultraslow-Spreading Mid-Ocean Ridges

Arne Glink and Jörg Hasenclever

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.

TS1.4 – Geomechanics – From field data to models and uncertainties

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.

EGU22-11827 | 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.

EGU22-4738 | 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.

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.

EGU22-11387 | Presentations | TS1.4 | Highlight

On the recoverability of geomechanical model complexities from surface deformation data above the Groningen gas field

Marius Wouters, Samantha S. R. Kim, Femke C. Vossepoel, and Rob Govers

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.

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.

EGU22-6453 | 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.

EGU22-5494 | 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.

EGU22-3272 | 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.

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.

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.

EGU22-8802 | 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.

TS2.1 – Deformation processes from grain- to planetary-scales: experiments, observations, and models

   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.

EGU22-11133 | 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.

EGU22-10101 | 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.

EGU22-9765 | 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.

EGU22-5371 | 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.

EGU22-3889 | 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.

EGU22-6124 | 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.

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.

EGU22-12964 | 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.

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.

EGU22-12327 | 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.

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.

EGU22-3268 | 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.

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.

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.

EGU22-8484 | 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.

EGU22-5108 | 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.

EGU22-2816 | 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.

EGU22-5127 | 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.

EGU22-7175 | 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.

EGU22-9842 | 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.

EGU22-5979 | 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.

EGU22-4606 | 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.

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.

EGU22-6926 | 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.

EGU22-10153 | 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.

EGU22-11618 | Presentations | TS2.1

Variations in Dissolution-Precipitation Creep Along an Active Plate Boundary

Jack McGrath and Sandra Piazolo

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.

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.

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.

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.

EGU22-11488 | 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.

TS2.2 – Current and past stress in the crust: quantitative techniques, case studies and rheological implications ?

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.

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.

EGU22-2969 | 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.

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.

EGU22-5556 | 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.

 

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.

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.

EGU22-7305 | 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.

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.

EGU22-1324 | 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.

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.

EGU22-1572 | 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.

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.

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.

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.

EGU22-1823 | 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.

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.

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.

EGU22-6001 | Presentations | TS2.2

Kinematics and geochronological evolution of the Vinschgau Shear Zone (N Italy): Large-scale thrusting within the Austroalpine domain of the central-eastern Alps

Stefano Zanchetta, Chiara Montemagni, Martina Rocca, Igor Villa, Corrado Morelli, Volkmar Mair, and Andrea Zanchi

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.

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.

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.

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.

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.

TS3.1 – Seismic imaging and characterization of crustal faults

EGU22-1403 | Presentations | TS3.1

Seismic anisotropy structure of the northern Hikurangi margin, New Zealand, and its significance for subduction fault systems

Ryuta Arai, Shuichi Kodaira, Stuart Henrys, Nathan Bangs, Koichiro Obana, Gou Fujie, Seiichi Miura, Daniel Barker, Dan Bassett, Rebecca Bell, Kimihiro Mochizuki, Richard Kellett, Valerie Stucker, and Bill Fry

The NZ3D OBS experiment performed in 2017-2018 in the northern Hikurangi margin off the east coast of North Island, New Zealand, provided the highest-resolution seismic refraction/wide‐angle reflection data with multi-azimuth ray coverage in subduction zones to date (Arai et al., 2020). The study area extending 60 km in the trench-normal direction and 14 km in the trench-parallel direction covers source regions of a variety of slow earthquake phenomena, such as shallow slow slip events and tectonic tremor (e.g., Wallace, 2020), and thus offers an ideal location to link our understanding of structural and hydrogeologic properties at subduction faults to slip behavior. We applied an anisotropic traveltime tomography analysis to this active-source dataset from 97 ocean bottom seismographs deployed with an average spacing of 2 km on four parallel lines and dense air gun shooting with a 25 m interval, and succeeded in quantitatively constraining the P-wave velocities (Vp) of the upper plate forearc and the subducting slab and their azimuthal anisotropy in three dimensions. The velocity models revealed some locations with significant Vp azimuthal anisotropy over 5 % near the splay faults in the low-velocity accretionary wedge and the deformation front. This finding suggests that the anisotropy is not ubiquitous and homogeneous within the upper plate, but more localized in the vicinity of active thrust faults. While the fast axes of Vp are mostly oriented in the trench-normal direction in the accretionary wedge, which is interpreted as results of preferentially oriented cracks in a compressional stress regime associated with the plate convergence, they are rotated to the trench-parallel direction on the seaward side of the trench and in the landward backstop. This regional variation is consistent with the results of shear-wave splitting analysis (Zal et al., 2020) and the directions of maximum horizontal stress inferred from the borehole breakouts at two IODP drilling sites (Wallace et al., 2019). The significant magnitudes of anisotropy may indicate that in addition to the crack orientation, clay-rich sedimentary sequences that stack and form coherent strata along the accretionary wedge also contribute to seismic anisotropy in the subduction margin.

 

How to cite: Arai, R., Kodaira, S., Henrys, S., Bangs, N., Obana, K., Fujie, G., Miura, S., Barker, D., Bassett, D., Bell, R., Mochizuki, K., Kellett, R., Stucker, V., and Fry, B.: Seismic anisotropy structure of the northern Hikurangi margin, New Zealand, and its significance for subduction fault systems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1403, https://doi.org/10.5194/egusphere-egu22-1403, 2022.

EGU22-4047 | Presentations | TS3.1

Complex fault growth controls 3-D rift geometry: Insights from deep learning of seismic reflection data from the entire northern North Sea rift

Thilo Wrona, Indranil Pan, Rebecca Bell, Christopher Jackson, Robert Gawthorpe, Haakon Fossen, and Sascha Brune

Understanding how normal faults grow is critical to an accurate assessment of seismic hazards, for successful exploration of natural (including low-carbon) resources and for safe subsurface carbon storage. Our current knowledge of fault growth is, in large parts, derived from seismic reflection data of continental rifts and margins. These seismic datasets do however suffer from limited data coverage and resolution. In addition, detailed fault mapping in increasingly large seismic reflection data requires a large amount of expertise and time from interpreters. Here we map faults across the entire northern North Sea rift using a combination of supervised deep learning and broadband 3-D seismic reflection data. This approach requires us to interpret <0.1% of the data for training and allows us to extract almost 8000 individual normal faults across a 161 km wide (E-W), 266 km long (N-S) and 20 km deep volume. We find that rift faults form incredibly complex networks revealing a previously-unrecognised variability in terms of fault length, density and strike. For instance, while we observe up to 75.9 km long faults extending from the Stord Basin and Bjørgvin Arch in the south into the Uer and Lomre Terrace to the north, most faults (>90%) are closely spaced (< 5 km) and relatively short (<10 km long). Moreover, these faults show a large range of strikes varying from NW-SE to NE-SW with two dominant fault strikes (NE-SW & NW-SE) almost perpendicular to each other. This observation is difficult to reconcile with previous studies on the extension directions during rifting of the northern North Sea. While previous studies suggest that pre-existing shear zones control faulting in the northern North Sea, we only observe faults aligning with the southern parts of the Lomre shear zone and the eastern parts of the Ninian shear zones, but none of the other eight previously mapped shear zones. Instead we think that these variations in fault strike could occur naturally through the complex evolution of fault networks. As such our innovative approach allows us to map faults across the entire northern North Sea revealing complex networks, which challenge many conventional views of fault growth during continental rifting.

How to cite: Wrona, T., Pan, I., Bell, R., Jackson, C., Gawthorpe, R., Fossen, H., and Brune, S.: Complex fault growth controls 3-D rift geometry: Insights from deep learning of seismic reflection data from the entire northern North Sea rift, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4047, https://doi.org/10.5194/egusphere-egu22-4047, 2022.