Displays

The easternmost part of the Southwest Indian Ridge (SWIR) is characterized by a very low melt-supply. Magma is focused along axis at discrete volcanic centers, leaving large portions of the seafloor where plate divergence is accommodated by large offset normal faults, also called detachment faults. These faults exhume mantle-derived samples on the seafloor. Microseismicity indicates a brittle lithosphere up to 25 km thick (Schlindwein & Schmid, 2016). These axial detachments require effective localized weakening in the shallow lithosphere to allow for large displacements along the fault and significant flexure of the footwall plate.

Here, we focus on the strain localization processes that operate in the deep axial lithosphere, in the absence of magma and prior to hydrothermal alteration. Using 99 dredged samples of partially serpentinized peridotites, we show that the primary mineralogy records heterogeneous high stress deformation that is detected in all samples to variable degrees. This deformation combines plastic and brittle mechanisms and is characterized by the development of extensively recrystallized anastomozing microshear zones. Estimates of temperature (800-1000°C) and deviatoric stresses (80-270 MPa) during deformation are derived, respectively, from pyroxene thermometry and olivine grain size piezometry. We show that strain localization is initially controlled by stress concentrations due to the contrast in rheology between orthopyroxene (strong, primarily brittle with microfractures, kinks and local dynamic recrystallization) and olivine (weak, primarily plastic with undulose extinction, subgrains, dynamic recrystallization, but also kinks and localized microfractures). We propose that these microstructures reflect the imprint of an episode of lithospheric deformation that formed the root of the axial detachments and that the resulting grain size reduction helps localize strain at the base of the lithosphere. This weakening mechanism plays an essential role in the development of flip-flop detachments in this area (Bickert et al., 2020). It may also operate in other magma-starved contexts such as ocean-continent transitions, where lithospheric deformation occurs without a significant melt supply.

How to cite: Bickert, M., Cannat, M., Tommasi, A., Jammes, S., and Lavier, L.: Strain localization processes at a magma-starved ridge: from micro-scale to macro-scale, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10244, https://doi.org/10.5194/egusphere-egu2020-10244, 2020.

Oceanic detachment faults are large and long-lived (1-2 Myr), forming at slow- and ultraslow- mid-ocean ridges. They can expose lower crustal gabbroic rocks and mantle peridotite in the seafloor, recognized as oceanic core complexes (OCCs). Mechanical models proposed that detachment faults originate at high angle and, as fault offset increases, are rotated flexurally to an inactive low-angle configuration. Previous studies showed that long-lived detachment faults need a rheological boundary for the offset: (1) an alteration front; (2) the brittle-plastic transition (BPT); (3) the boundary between gabbro intrusions and weakened hydrated peridotite; or (4) low magma supply.  In order to better understand the rheological behavior of oceanic detachments, we investigate numerically potential effects of ductile weakening controlled by grain size reduction on the oceanic detachment faults formation as well as on their subsequent inversion during the Wilson cycle. We employ 3D thermomechanical numerical models with a composite rheology consisting of diffusion and dislocation creep. In our model, oceanic crust deforms in a brittle manner and its strength is controlled by fracture-related strain weakening and healing. In contrast, the lithospheric mantle deforms according to the dry olivine flow law, as a mixture of grain size-dependent diffusion and dislocation creep. Numerical results show that ductile weakening induced by grain size reduction could indeed notably influence both the style of detachment faulting and the fault dipping angles in the depth of the BPT. Grain size has a great effect on the offset of detachment faults and the formation of megamullions and controls the place of new subduction initiation below the BPT. We systematically investigate the influence of the thermal structure, initial grain size and spreading rate on the characteristic oceanic detachment fault pattern. In addition, we also study effects of these parameters on the final inversion of detachment faults during induced intra-oceanic subduction initiation.

How to cite: Liu, M., Gerya, T., and Bercovici, D.: Role of grain size reduction in formation and inversion of oceanic detachment faults, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5771, https://doi.org/10.5194/egusphere-egu2020-5771, 2020.

Mantle earthquakes that occur deeper than the 600 °C isotherm in oceanic transform faults indicate seismic rupturing at conditions where viscous deformation (bulk ductile behavior) is dominant.  However, direct geological evidence of earthquake-related deformation at ambient upper mantle conditions is rare, impeding our understanding of earthquake dynamics in plate-boundary fault systems.  The Bogota Peninsula Shear Zone (BPSZ), New Caledonia, is an ancient oceanic transform fault exhumed from upper mantle depths.  Ductile structures in the BPSZ formed at temperatures > 800 °C and microstructures indicate that differential stress varies spatially and temporally.  Spatial variation is observed as an increase in differential stress with strain toward localized zones of high strain; stress increases from 6–14 MPa in coarse grained tectonites to 11–22 MPa within 1–2 km wide mylonite zones.  Temporal stress variation is observed by the formation of micro-deformation zones that seem to have brittle precursors, are filled with fine-grained recrystallized olivine grains and crosscut the background fabrics in the harzburgites that host them.  The micro-deformation zones are not restricted to the mylonite zones, but rather are located throughout the BPSZ, having affected the protomylonites and the coarse grained tectonites.  The micro-deformation zones record stresses of 22–81 MPa that are 2–6 times higher than the background, steady-state stresses in the surrounding mantle rocks.  We interpret the observed spatial and temporal variations in microstructures and stresses in the upper mantle to demonstrate the influence of seismic events in the upper part of the oceanic transform fault system.  We attribute the increase in stress with strain to be the result of imposed localization induced by downward propagation of the seismic rupture into the underlying mantle.  The micro-deformation zones could result from brittle fractures caused by earthquake-related deformation in the mantle section of the transform fault, which are in turn overprinted by ductile deformation.

Synthesizing the spatial and temporal variations in stresses and microstructures in the Bogota Peninsula Shear Zone we propose a conceptual model where brittle fracturing and shearing take place during coseismic rupture at increased stress, ductile flow at decaying stress is concentrated in the micro-deformation zones during postseismic relaxation, and uniformly distributed creep at low stress occurs in the host-rocks of the micro-deformation zones during interseismic deformation.  The critical result from the studied paleotransform zone is that the fine-grained micro-deformation zones and the mylonites do not represent weak zones.  Instead, they form by dislocation creep at transient high-stress deformation during the seismic cycle.  The spatial distribution of the micro-deformation zones also suggests that repeated stress cycles in oceanic transform faults may not localize strain in pre-existing shear zones but disperse strain across the structure.

How to cite: Chatzaras, V., Tikoff, B., Kruckenberg, S. C., Titus, S. J., Teyssier, C., and Drury, M. R.: Imposed strain localization in the mantle section of an oceanic transform zone revealed by microstructural and stress variations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11897, https://doi.org/10.5194/egusphere-egu2020-11897, 2020.

Surface of the Earth is divided into distinct plates that move relative to each other. However, formation and evolution of new plate boundaries is still challenging to numerically produce and predict. In particular, regional lithospheric models as well as large scale convection models lack realistic strike slip plate boundaries that would arise self-consistently in such models. Here, we investigate the role of different rheologies on the inception and dynamic evolution of the new divergent plate boundaries and their offset by strike-slip faulting. We compare visco-plastic rheology and strain dependent rheology and their capacity to localise deformation into narrow plate limits. We use high-resolution 3D thermo-mechanical numerical models in  cartesian geometry to infer the conditions under which realistic divergent plate boundaries develop.

How to cite: Ulvrova, M. and Gerya, T.: The role of strain dependent rheology on formation of divergent plate boundaries, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22326, https://doi.org/10.5194/egusphere-egu2020-22326, 2020.

Tectonic plate boundaries are shaped by localized and distributed brittle-plastic deformation such as slip on major faults and folding of fault-bounded blocks. However, the microstructural to outcrop-scale mechanisms that enable such deformation and the factors that control the onset of localization remain a matter of debate. Here we combine field-based strain measurements and numerical modeling of a half-graben to investigate patterns of distributed inelastic strain induced by footwall-flexure of the upper crust. We focus on the Sandia Mountains (New Mexico, USA), which have marked the eastern edge of the Rio Grande rift's middle section for the last ~10–25 Myrs. This half-graben is uniquely suited for our study: it consists of a layer of Pennsylvanian limestone which experienced little deformation prior to Cenozoic rifting and lies uncomformably above Proterozoic granite. Furthermore, most of the present-day topography and up-warping of the limestone can be attributed to slip on the Sandia fault system and is well modeled as the deflection of an anomalously weak elastic upper crust. The Sandia limestone thus constitutes a unique record of distributed brittle strain related to inelastic shoulder flexure.

In the field, deformation within the up-warped footwall-block primarily manifests as small faults (<10s of m slip) and sub-mm to cm-scale mode-I calcite-filled fractures. We identified two sets of veins: a N-striking set subparallel to the axis of flexure, and an E-striking set. Fold tests indicate that the veins formed during the onset of flexure and were mostly tilted with bedding. We measured the aperture of thousands of veins sampled by 31 scan lines distributed along an E-W transect through the Sandia footwall. Vein apertures generally follow a power law distribution of slope ~1. Profiles of E-W fracture-borne strain show clear maxima of ~0.1 with 1-mm fracture densities of ~20 cracks/m at outcrops located 12–15 km away from the range bounding fault. This location represents the hinge of the flexure where bending stresses were apparently large enough to exceed the Mohr-Coulomb failure criterion, yet did not result in the localization of a crustal-scale fault.

To test this idea, we designed 2-D numerical simulations of half-graben growth using a visco-elasto-plastic rheology coupled with plastic strain softening to enable spontaneous fault localization. Our models predict spatial patterns of distributed inelastic strain within the footwall block that are consistent with our field-based fracture intensity profiles. We find that the strain softening rate is a key control on (1) the distribution of footwall inelastic strain and (2) whether distributed strain can localize onto a new crustal fault. This enables us to constrain values of weakening rate (~100 MPa/strain) that reproduce the observed pattern of distributed cracking while allowing prolonged slip on a single master fault. Our results demonstrate that numerical geodynamic simulations can be benchmarked against microstructural observations to quantify the strain localization properties of the lithosphere. They also suggest that the low effective rigidity of warped crust stems from the growth and interaction of tensile defects on a range of spatial scales, as is commonly observed in rock deformation experiments.

How to cite: Olive, J.-A., Betka, P., Malatesta, L., Bruhat, L., Petit, L., Oppenheimer, J., Demont, A., and Buck, R.: Zooming in on distributed brittle deformation across the Rio Grande rift shoulder: implications for strain weakening of the upper crust, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5020, https://doi.org/10.5194/egusphere-egu2020-5020, 2020.

Crustal extension is a fundamental process in plate tectonics, and understanding its driving mechanisms is critical to our understanding the role of extensional deformation in the evolution of the Earth’s continents. How and why extension localizes into narrow belts versus being distributed across wide orogens remains enigmatic. Here we investigate extensional strain localization in the North American Cordillera (NAC) and Basin and Range province, where early phases of high magnitude strain (>100%) were fairly localized along a ~2500-km long belt of metamorphic core complexes, and subsequent late-stage low-magnitude strain appears to be fairly distributed across the 500-600-km width of the Great Basin. Various forces compete to drive intracontinental extension in the western United States, and we present field-based case studies of the Central NAC core complexes—the Ruby-East Humboldt, Snake Range, and Albion-Raft River-Grouse Creek—to explore strain localization due to plate-boundary stresses, internal body forces (GPE), and/or crustal rheology including thermal weakening from pervasive magmatism. The studied core complexes consist of significant syn-kinematic intrusions, and we demonstrate how the composition, volume and age (i.e., duration and relative timing) of these intrusions affected strain rates. Through a combination of new and synthesized U-Pb geochronology, ⁴⁰Ar/³⁹Ar thermochronology and electron backscatter diffraction (EBSD) analysis we link transient thermal and rheological evolution of the crust with deformation mechanisms from grain to outcrop to regional scales.  More broadly, we discuss the mechanisms and modes of crustal extension during orogenesis, and whether extension in active orogens is a transient response to modulate GPE gradients, or a precursor to orogenic collapse.

How to cite: Levy, D. and Zuza, A.: Mechanisms of Extensional Strain Localization: An Example from Cordilleran Metamorphic Core Complexes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6221, https://doi.org/10.5194/egusphere-egu2020-6221, 2020.

Understanding the rheological structure of the lithosphere is important for inferring loading rate on faults and their potential downdip extensions. To this end, we compare viscosity estimates from geodynamic models and predictions of lithosphere rheology. At each point on a grid covering Southern California, we first produce deviatoric stress estimates averaged from the surface to 100 km depth obtained by modelling variations of crustal structure (gravity potential energy) and effective viscosity. This geodynamic model is evaluated on the basis of surface geodetic data. In a complementary approach, we generate a strength envelope at each grid point based on various community products provided by the Southern California Earthquake Center (SCEC) and associated researchers. For example, we use the thermal model of Shinevar et al. (2018) and crustal thickness variations from Shen and Ritzwoller 2016). At each depth, the lowest of the stress associated with dislocation or diffusion creep is retained. Eight alternative rheological models are developed, that consider either wet or dry rheologies, a uniform grain size (1mm) or a grain size tied to a piezometer, and a maximum allowed stress of 300 MPa or 30 MPa. We use the flow laws of feldspar from Rybacki et al. (2006) is used in the crust and that of Hirth and Kohlstedt (2004) for olivine in the mantle. At this point, lithological variations in the crust are neglected, although we find evidence in our results that they are probably important. The strength envelop is integrated with depth for various strain rates to produce an effective rheology of the lithosphere. We then determine the strain rate associated with the geodynamically-inferred stress and use it to produce a viscosity estimate from the rheological model

The first result of this analysis is that the effective rheology of the lithosphere is highly non-linear (effective stress exponent between 10 and 30). Therefore only a limited range of stress is expected at any given location. In general, that stress is quite high so that the viscosity estimates from the geodynamic model can be explained only when using the weakest crust and mantle rheologies. Although certain viscosity variations are consistent between the models (strong Sierra Nevada block, weak Salton trough area), in other places they are not. In particular, the Great Basin block appears strong in the geodynamic model but weak in the rheological model due to high temperatures there. This study shows that there are likely important rheological variations due to mineralogy. It is expected that the Great Basin is underplated by gabbroic rocks that are not included here but would increase the effective viscosity of the rheological model. Mineralogical variations would also allow a great variety of accessible stress at each location. Finally, it is also likely that the stress does not reach failure at every depth, which is the basis for considering low saturation values for the strength envelop.

How to cite: Montesi, L., Izquierdo, K., Holt, W., Bahadori, A., and Shinevar, W.: Strength Variations of Southern California from Rheological and Geodynamical Approaches, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10922, https://doi.org/10.5194/egusphere-egu2020-10922, 2020.

In southeastern Tibetan Plateau, the Xianshuihe-Xiaojiang fault system (XXFS) and its neighboring fault systems collectively accommodates the material extrusion of the Tibetan Plateau. However we do not mechanically understand how these faults interact with each other and how the fault interaction impacts strain partitioning, fault slip rates, and seismicity in this region. We develop and use a three-dimensional viscoelastoplastic finite element model to simulate regional deformation, fault slip rates, and fault interaction in the fault system of southeastern Tibetan Plateau. We investigate the effects of inception and activity of faults, fault strength, lithospheric rheology, and topography on partitioning of strain and fault slip rates. Model results show that fault strength, lithospheric rheology, and topography all significantly influence the strain partitioning and slip rates on faults. The initiation of the Daliangshan fault results mainly from the non-smooth fault geometry of the main trace of the XXFS. Our model results support the hypothesis of codependent slip rate between fault systems. For the present fault configuration, our model predicts localized strain in the Daliangshan faults, Yingjing-Mabian faults, and Lianfeng-Zhaotong faults, where numerous earthquakes occurred in recent years.

How to cite: Yin, L.: Fault interaction and strain partitioning in southeastern Tibetan Plateau: from kinematics to geodynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6886, https://doi.org/10.5194/egusphere-egu2020-6886, 2020.

Displacement between tectonic plates is normally partitioned into different tectonic domains accommodating specific components of the bulk strain, such that no single domain can possibly be regarded as representative of the overall kinematics. Eventually, this partitioning can be produced at different scales. Therefore, plate kinematic motion estimations based on the surface geological record should ideally rely on detailed multiscale, structural analyses of all different tectonic domains involved.

The Betic-Rif orogen was formed during the Cenozoic by the convergence and subsequent collision of the Alboran domain and the South Iberian and Maghrebian paleomargins. After the main Miocene event, oblique convergence has been still active up to present times in both branches of the resulting Gibraltar Arc. In this work we analyze how dextral oblique convergence in the northern Betic branch is partitioned into different tectonic domains of the orogen external zones and foreland, where contrasting strain fields are deduced. These domains present distinctive rheologies, thus showing also specific structural styles. As such, we present data of upper Miocene-Present structures from four different tectonic domains along a complete transect of the western Betics (southern Spain), from the internal-external zones boundary outwards. In the inner fold and thrust belt, the detached South Iberian paleomargin and Flysch trough units (mostly limestones and other carbonatic rocks) are deformed mainly by upright and double-verging folds as well as reverse faults, both registering mostly orthogonal shortening. The outer fold and thrust belt progressed toward the foreland incorporating block-in-matrix formations, with evaporite-rich marly matrix, formed ahead the mountain front; its main deformation is resolved at a strike-slip dominated, dextral transpressional zone. The upper Miocene deposits of the foreland basin (calcarenites and marls) are affected by weak deformation combining some shortening and an unconstrained strike-slip component, as deduced from seismic profiles. Finally, Paleozoic structures of the foreland, formerly developed at non- to medium-grade metamorphic conditions, were likely reactivated under a dextral transpressional strain field, which acts in combination with forebulge bending.

The strongly arcuate shape of the Gibraltar Arc likely imposes contrasting kinematics along strike within the same tectonic domain. Indeed, the inner fold and thrust belt shows nearly orthogonal shortening to the west, in a more frontal position, and a strike-slip dominated high-strain zone (the so-called Torcal shear zone) to the east. By contrast, preliminary studies show no significant differences in the kinematics of the foreland eastward from the analyzed transect.

All of our kinematic results from the studied domains are compatible with an overall dextral oblique convergence. However, more accurate strain estimations are needed to constrain the plate displacements responsible for the upper Miocene-Recent deformation in the Gibraltar Arc northern branch. Moreover, detailed analyses of strain partitioning modes will shed light into the relationships between these plate displacements and the resulting strain patterns.

How to cite: Díaz-Azpiroz, M., Expósito, I., Jiménez-Bonilla, A., and Balanyá, J. C.: Relationships between oblique convergence partitioning and plate kinematics. A case study from the western Betics external zones and foreland (southern Spain), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6104, https://doi.org/10.5194/egusphere-egu2020-6104, 2020.

Heat flow data contribute to the imaging the lithospheric thermal structure, which greatly influences tectonic and geological processes and constrains the strength of the lithosphere, the modes of deformation, and the depth distribution of earthquakes. To provide more reliable estimation of the lithospheric thermal structure, some complementary approaches are possible. One of approaches is to update and incorporate the existing thermal data. A new version of database “Thermal Data Collection in and around Japan”, which contains continuously updated of heat flow and geothermal gradient data and adds thermal conductivity data in and around Japan, has been released in March 2019 [https://www.gsj.jp/data/G01M/GSJ_MAP_TDCJ_2019.zip]. This provides an opportunity to revisit the thermal state of the lithosphere along with other geophysical/geochemical constraints and on the lithospheric rheology and deformation, which is sensitive to temperature.

How to cite: Tanaka, A.: Thermal data collection in and around Japan and its implications for lithospheric rheology and deformation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21030, https://doi.org/10.5194/egusphere-egu2020-21030, 2020.

It is now well-established that the Earth’s mantle and lithosphere form an integrated, dynamically self-regulating system. Numerical convection models that self-consistently generate plate-like behavior are a powerful tool to investigate this system, but have only recently reached a level at which they can be linked to the geodynamics of the Earth. Strongly temperature-dependent and viscoplastic rheology is known to be a key ingredient for these models to be successful. Such rheologies, however, are typically time-independent and lack a memory on the previous history of deformation. Yet, it is known that the Earth’s geodynamic evolution is somewhat guided by structures of pre-existing weakness, which was initiated a potentially long time before.

As a step forward we implement a simple form of rheological memory in the mantle convection code StagYY: strain weakening [Fuchs & Becker, 2019, Role of strain-dependent weakening memory on the style of mantle convection and plate boundary stability, Geophys. J. Int., 218, 601-618]. We present calculations in 2D cases with and without continents, and also selected 3D cases. By varying the governing parameters for plate-like behavior as well as the rates of rheological damage and healing, we examine how strain weakening modifies the generation of plate-like behavior and its time dependence as well as the drift of continents.

First results indicate the importance of the balance of weakening (via the critical strain) and thermal healing. The averaged cumulative strain (effectively the degree of lithospheric weakening) is lower when healing is more effective, so that plastic failure of the lithospheric and the formation of new plate boundaries is complicated, as expected. In initial models with strong, long-living continents, accumulated strain is very small within the continents and seems insufficient to induce substantial weakening, even if the memory on previous deformation is infinite (i.e. no healing with continents). Further models with weaker continents and different rheological parameters will be presented.

How to cite: Rolf, T. and Arnould, M.: Effects of strain weakening in self-consistent models of mantle convection with plate-like behavior and continental drift, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5118, https://doi.org/10.5194/egusphere-egu2020-5118, 2020.

The creation and maintenance of narrow plate boundaries and their role in the thermo-chemical evolution of Earth remain one of the major problems in geodynamics. In particular, the cause and consequences of strain localization and weakening within the upper mantle remain debated, even though strain memory and tectonic inheritance, i.e. the ability to preserve and reactivate inherited weak zones over geological time, 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. This has often been implemented by a linear decrease of the yield strength of the lithosphere with increasing deformation. 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.

Here, we analyze how a parameterized, apparent-strain, or “damage”, dependent weakening (SDW) rheology governs strain localization and weakening as well as healing in the lithosphere. The weakening and localization due to the SDW rheology has been related to a grain-size sensitive (GSS) composite rheology (diffusion and dislocation creep). 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. We explore different types of strain weakening (plastic- (PSS) and viscous-strain (VSS) softening) and compare them to the predictions from different models of grain-size evolution for a range of temperatures and a step-like variation of total strain rate with time. PSS leads to a weakening and strengthening of the effective viscosity of about the same order of magnitude as due to a GSS rheology, while the rate depends on the strain-weakening parameter combination. In addition, the SDW weakening rheology allows for memory of deformation, which weakens the fault zone for a longer period. Once activated, the memory effect and weakening of the fault zone allows for a more frequent reactivation of the fault for smaller strain rates, depending on the strain-weakening parameter combination.

How to cite: Fuchs, L. and Becker, T. W.: Dynamic weakening mechanism in Earth’s mantle - A comparison between damage-dependent weakening and grain-size sensitive rheologies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17979, https://doi.org/10.5194/egusphere-egu2020-17979, 2020.