Looking beyond kinematics: 3D thermo-mechanical modelling reveals the dynamics of transform margins

Abstract. Transform margins represent ∼ 30 % of non-convergent margins worldwide. Their formation and evolution have traditionally been addressed through kinematic models that do not account for the mechanical behaviour of the lithosphere. In this study, we use high-resolution 3D numerical thermo-mechanical modelling to simulate and investigate the evolution of intra-continental strain localization under oblique extension. The obliquity is set through velocity boundary conditions that range from 15∘ (high obliquity) to 75∘ (low obliquity) every 15∘ for rheologies of strong and weak lower continental crust. Numerical models show that the formation of localized strike-slip shear zones leading to transform continental margins always follows a thinning phase during which the lithosphere is thermally and mechanically weakened. For low- (75∘) to intermediate-obliquity (45∘) cases, the strike-slip faults are not parallel to the extension direction but form an angle of 20∘ to 40∘ with the plate motion vector, while for higher obliquities (30∘ to 15∘) the strike-slip faults develop parallel to the extension direction. Numerical models also show that during the thinning of the lithosphere, the stress and strain re-orient while boundary conditions are kept constant. This evolution, due to the weakening of the lithosphere, leads to a strain localization process in three major phases: (1) initiation of strain in a rigid plate where structures are sub-perpendicular to the extension direction; (2) distributed deformation with local stress field variations and formation of transtensional and strike-slip structures; (3) formation of highly localized plate boundaries stopping the intra-continental deformation. Our results call for a thorough re-evaluation of the kinematic approach to studying transform margins.

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Looking beyond kinematics: 3D thermo-mechanical modelling reveals the dynamic of transform margins

10.5194/se-2021-18 ◽

2021 ◽

Author(s):

Anthony Jourdon ◽

Charlie Kergaravat ◽

Guillaume Duclaux ◽

Caroline Huguen

Keyword(s):

Boundary Conditions ◽

Strain Localization ◽

Numerical Models ◽

Local Stress ◽

Shear Zones ◽

Strike Slip ◽

Convergent Margins ◽

Mechanical Modelling ◽

Kinematic Models ◽

Extension Direction

Abstract. Transform margins represent ~30 % of the non-convergent margins worldwide. Their formation and evolution have long been addressed through kinematic models that do not account for the mechanical behaviour of the lithosphere. In this study, we use high resolution 3D numerical thermo-mechanical modelling to simulate and investigate the evolution of the intra-continental strain localization under oblique extension. The obliquity is set through velocity boundary conditions that range from 15° (high obliquity) to 75° (low obliquity) every 15° for strong and weak lower continental crust rheologies. Numerical models show that the formation of localized strike-slip shear zones leading to transform continental margins always follows a thinning phase during which the lithosphere is thermally and mechanically weakened. For low (75°) to intermediate (45°) obliquity cases, the strike-slip faults are not parallel to the extension direction but form an angle of 20° to 40° with the plates' motion while for higher obliquities (30° to 15°) the strike-slip faults develop parallel to the extension direction. Numerical models also show that during the thinning of the lithosphere, the stress and strain re-orient while boundary conditions are kept constant. This evolution, due to the weakening of the lithosphere, leads to a strain localization process in three major phases: (1) strain initiates in a rigid plate where structures are sub-perpendicular to the extension direction; (2) distributed deformation with local stress field variations and formation of transtensional and strike-slip structures; (3) formation of highly localized plates boundaries stopping the intra-continental deformation. Our results call for a thorough re-evaluation of the kinematic approach to studying transform margins.

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Olivine-induced viscous anisotropy in fossil strike-slip mantle shear zones and associated strain localization in the crust

Geophysical Journal International ◽

10.1093/gji/ggaa400 ◽

2020 ◽

Vol 224 (1) ◽

pp. 608-625

Author(s):

Lucan Mameri ◽

Andréa Tommasi ◽

Javier Signorelli ◽

Riad Hassani

Keyword(s):

Boundary Conditions ◽

Shear Zone ◽

Strain Localization ◽

Strain Distribution ◽

Seismic Anisotropy ◽

Shear Zones ◽

Strike Slip ◽

Intraplate Seismicity ◽

Strain Rates ◽

Mechanical Coupling

SUMMARY We propose that strain localization in plate interiors, such as linear belts of intraplate seismicity, may arise from spatial variations in viscous anisotropy produced by preferred orientation of olivine crystals (CPO or texture) inherited from previous deformation episodes in the lithospheric mantle. To quantify this effect, we model the deformation of a plate containing a fossil strike-slip mantle shear zone at different orientations relative to an imposed horizontal shortening, but no initial heterogeneity in the crust. The fossil shear zone is characterized by different orientation and intensity of the olivine CPO relatively to the surrounding mantle, which is isotropic in most simulations. The anisotropy in viscosity produced by the CPO, which remains fixed throughout the simulations, is described by an anisotropic (Hill) yield function parametrized based on second-order viscoplastic self-consistent (SO-VPSC) models. The results indicate that lateral variations in viscous anisotropy in the mantle affect the strain distribution in the entire lithosphere. Reactivation of the strike-slip mantle shear zone and strain localization in the crust above it occur for horizontal compression at 35–55° to the fossil shear plane, with a maximum at 45°. The magnitude of strain localization depends on (i) the contrast in viscous anisotropy and, hence, on the variations in CPO orientation and intensity in the mantle, (ii) the boundary conditions and (iii) the feedbacks between mantle and crustal deformation. For a strong olivine CPO, when the boundary conditions do not hinder shear parallel to the fossil mantle shear zone, strain rates within it are up to a factor 30 higher than in an isotropic surrounding mantle or up to a factor 200 when the surrounding mantle is anisotropic, which results in strain rates up to a factor 10 or up to a factor 100 higher in the crust right above the fossil shear zone. Frictional weakening in the crust faults increases strain localization in the entire lithospheric column. High strength contrasts between the mantle and the ductile crust result in less efficient mechanical coupling, with strong localization in the mantle and lower crust, but weak in the brittle upper crust. Decrease in the crust–mantle strength contrast enhances the coupling and produces more homogenous strain distribution with depth, as well as a time-dependent evolution of strain localization, which reaches a peak and decreases before attaining steady-state. Comparison of seismic anisotropy, regional stress and focal mechanism data in linear arrays of intraplate seismicity, like the New Madrid and South Armorican seismic zones, to our models' predictions corroborates that olivine CPO preserved in fossil lithospheric-scale shear zones may be key for the development of such structures.

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The structural evolution of pull-apart basins in response to relative plate rotations; A physical analogue modelling case study from the Northern Gulf of California.

10.5194/egusphere-egu2020-5378 ◽

2020 ◽

Author(s):

Georgios-Pavlos Farangitakis ◽

Kenneth J.W. McCaffrey ◽

Ernst Willingshofer ◽

Lara M. Kalnins ◽

Jeroen van Hunen ◽

...

Keyword(s):

Gulf Of California ◽

Motion Vector ◽

Structural Evolution ◽

Strike Slip ◽

Plate Motion ◽

Extension Rate ◽

Sediment Distribution ◽

Pull Apart Basin ◽

Analogue Modelling ◽

Extension Direction

Pull-apart basins are structural features closely linked to the interactions between strike-slip and extensional tectonics. Their morphology and structural evolution are determined by factors such as extension rate, width/length ratio, or changes in the extension direction. In this work, we focus on changes in extension direction during the formation of a pull-apart basin as a basis to further understand the evolution of the northern Gulf of California through a series of physical analogue modelling experiments.We investigate the effect of a variation in the basin extension direction, using a two-layer ductile-brittle configuration to simulate continental crust rheology. Pull-apart basin development is accomplished by displacing a plastic sheet at the bottom of the experiment, with pre-cut geometry resembling interconnected rift and strike-slip segments, orthogonal to the evolving rift axes. Subsequently, we change the relative motion of the base plate by 7o in accordance with the reconstructed plate vector from the Gulf of California. Oblique extension continues on this new plate motion vector to the end of the experiment.To analyse the results, we inserted the model cross-sections in a seismic interpretation software generating 3D interpretations for faulting and sedimentary thickness. Preliminary results show that the shift in the direction of plate motion produces sigmoidal oblique slip faults that become normal when deformation adjusts to the new plate motion vector. Furthermore, it appears that sediment distribution is controlled heavily by the relative plate rotation.Finally, we compare our observations with seismic reflection images, sedimentary package thicknesses and fault interpretations from the pull-apart structure in the Northern Gulf of California transtensional margin, where we find good agreement between model and nature.

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Synconvergent and coherent (ultra)high-pressure crustal rock exhumation

10.5194/egusphere-egu21-11119 ◽

2021 ◽

Author(s):

Lorenzo G. Candioti ◽

Joshua D. Vaughan-Hammon ◽

Thibault Duretz ◽

Stefan M. Schmalholz

Keyword(s):

Numerical Models ◽

A Priori ◽

Western Alps ◽

Ultrahigh Pressure ◽

Plate Motion ◽

Crustal Rock ◽

Plate Boundaries ◽

Crustal Rocks ◽

Natural Data ◽

The Alps

Ultrahigh-pressure (UHP) continental crustal rocks were first discovered in the Western Alps in 1984 and have since then been observed at many convergent plate boundaries worldwide. Unveiling the processes leading to the formation and exhumation of (U)HP metamorphic crustal rocks is key to understand the geodynamic evolution of orogens such as the Alps.&#160;Previous numerical studies investigating (U)HP rock exhumation in the Alps predicted deep (>80 km) subduction of crustal rocks and rapid buoyancy-driven exhumation of mainly incoherent (U)HP units, involving significant tectonic mixing forming so-called m&#233;langes. Furthermore, these predictions often rely on excessive erosion or periods of divergent plate motion as important exhumation mechanism. Inconsistent with field observations and natural data, application of these models to the Western Alps was recently criticised.&#160;Here, we present models with continuous plate convergence, which exhibit local tectonic-driven upper plate extension enabling compressive- and buoyancy-driven exhumation of coherent (U)HP units along the subduction interface, involving feasible erosion.&#160;The two-dimensional petrological-thermo-mechanical numerical models presented here predict both subduction initiation and serpentinite channel formation without any a priori prescription of these two features. The (U)HP units are exhumed coherently, without significant internal deformation. Modelled pressure and temperature trajectories and exhumation velocities of selected crustal units agree with estimates for the Western Alps. The presented models support previous hypotheses of synconvergent exhumation, but do not rely on excessive erosion or divergent plate motion. Thus, our predictions provide new insights into processes leading to the exhumation of coherent (U)HP crustal units consistent with observations and natural data from the Western Alps.

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When did the North Anatolian fault reach southern Marmara, Turkey?

Geology ◽

10.1130/g49726.1 ◽

2021 ◽

Author(s):

Volkan Karabacak ◽

Taylan Sançar ◽

Gökhan Yildirim ◽

I. Tonguç Uysal

Keyword(s):

Shear Zones ◽

Strike Slip ◽

North Anatolian Fault ◽

Plate Boundaries ◽

Marmara Region ◽

Plate Motions ◽

The North ◽

Southern Branch ◽

Timing Of Initiation ◽

Northern Branch

We dated syntectonic calcites on fault planes from the southern branch of the western North Anatolian fault (NAF) in northern Turkey using U-Th geochronology. We selected strike-slip faults that are kinematically related to the current regional strain field. The isotopic ages cluster around different periods during the past ~700 k.y. The most prominent cluster peak of 510.5 ± 9.5 ka (1σ) is consistent with the maximum cumulative strike-slip offset data and tectonic plate motions measured by GPS data, highlighting the fact that the present configuration of the NAF in the southern Marmara region started at ca. 500 ka or earlier. These new isotopic ages, combined with previous considerations of regional tectonics, reveal that faulting along the western NAF initiated primarily in the southern Marmara region at least a few hundred thousand years earlier than the timing suggested for the northern branch of the western NAF. This study presents an innovative approach to constrain the timing of initiation of currently active fault segments along the NAF in southern Marmara. U-Th geochronology of fault-hosted calcite thus has a wide application in determining absolute ages of fault episodes in wider shear zones along plate boundaries.

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Self-Consistent Grain Size Evolution controls Strain Localization during Rifting

10.21203/rs.3.rs-561920/v1 ◽

2021 ◽

Author(s):

Jonas Ruh ◽

Leif Tokle ◽

Whitney Behr

Keyword(s):

Grain Size ◽

Upper Mantle ◽

Numerical Models ◽

Shear Zones ◽

Dislocation Creep ◽

Diffusion Creep ◽

Plate Boundaries ◽

Mantle Dynamics ◽

Size Evolution ◽

Dominant Deformation Mechanism

Abstract Geodynamic numerical models often employ solely grain-size-independent dislocation creep to describe upper mantle dynamics. However, observations from nature and rock deformation experiments suggest that shear zones can transition to a grain-size-dependent creep mechanism due to dynamic grain size evolution, with important implications for the overall strength of plate boundaries. We apply a two-dimensional thermo-mechanical numerical model with a composite diffusion-dislocation creep rheology coupled to a dynamic grain size evolution model based on the paleowattmeter. Results indicate average olivine grain sizes of 3–12 cm for the upper mantle below the LAB, while in the lithosphere grain size ranges from 0.3–3 mm at the Moho to 6–15 cm at the LAB. Such a grain size distribution results in dislocation creep being the dominant deformation mechanism in the upper mantle. However, deformation-related grain size reduction below 100 μm activates diffusion creep along lithospheric-scale shear zones during rifting, affecting the overall strength of tectonic plate boundaries.

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The role of strain dependent rheology on formation of divergent plate boundaries

10.5194/egusphere-egu2020-22326 ◽

2020 ◽

Author(s):

Martina Ulvrova ◽

Taras Gerya

Keyword(s):

Large Scale ◽

Numerical Models ◽

Strike Slip ◽

Plate Boundaries ◽

The Earth ◽

Cartesian Geometry ◽

Formation And Evolution ◽

Narrow Plate ◽

Strain Dependent

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&#160; cartesian geometry to infer the conditions under which realistic divergent plate boundaries develop.

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Processes and deformation rates generating seismicity in metropolitan France and conterminous Western Europe

BSGF - Earth Sciences Bulletin ◽

10.1051/bsgf/2020019 ◽

2020 ◽

Vol 191 ◽

pp. 19 ◽

Cited By ~ 2

Author(s):

Stéphane Mazzotti ◽

Hervé Jomard ◽

Frédéric Masson

Keyword(s):

Plate Tectonics ◽

Western Europe ◽

Numerical Models ◽

Western Alps ◽

Geodetic Data ◽

Plate Motion ◽

Plate Boundaries ◽

Normal Extension ◽

Deformation Localization ◽

Seismicity Rates

Most of metropolitan France and conterminous Western Europe is currently located within the Eurasia intraplate domain, far from major plate boundaries (the Atlantic ridge and Nubia – Eurasia convergence zone). As in other intraplate regions, present-day deformation and seismicity rates are very slow, resulting in limited data and strong uncertainties on the ongoing seismotectonics and seismic hazards. In the last two decades, new geological, seismological and geodetic data and research have brought to light unexpected deformation patterns in metropolitan France, such as orogen-normal extension ca. 0.5 mm yr−1 in the Pyrenees and Western Alps that cannot be associated with their mountain-building history. Elsewhere, present-day deformation and seismicity data provide a partial picture that points to mostly extensive to strike-slip deformation regimes (except in the Western Alps foreland). A review of the numerous studies and observations shows that plate tectonics (plate motion, mantle convection) are not the sole, nor likely the primary driver of present-day deformation and seismicity and that additional processes must be considered, such as topography potential energy, erosion or glacial isostatic adjustment since the last glaciation. The exact role of each process probably varies from one region to another and remains to be characterized. In addition, structural inheritance (crust or mantle weakening from past tectonic events) can play a strong role in deformation localization and amplification up to factors of 5–20, which could explain some of the spatial variability in seismicity. On the basis of this review, we identify three research directions that should be developed to better characterize the seismicity, deformation rates and related processes in metropolitan France: macroseismic and historical seismicity, especially regarding moment magnitude estimations; geodetic deformation, including in regions of low seismicity where the ratio of seismic to aseismic deformation remains a key unknown; an integrated and consistent seismotectonic framework comprising numerical models, geological, seismological and geodetic data. The latter has the potential for significant improvements in the characterization of seismicity and seismic hazard in metropolitan France but also Western Europe.

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Dynamic weakening mechanism in Earth’s mantle - A comparison between damage-dependent weakening and grain-size sensitive rheologies

10.5194/egusphere-egu2020-17979 ◽

2020 ◽

Author(s):

Lukas Fuchs ◽

Thorsten W. Becker

Keyword(s):

Grain Size ◽

Fault Zone ◽

Strain Localization ◽

Effective Viscosity ◽

Shear Zones ◽

Plate Boundaries ◽

Tectonic Inheritance ◽

Parameter Combination ◽

Size Evolution ◽

Viscous Shear

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 &#8220;damage&#8221;, 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.

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DEM investigation on strain localization in a dense periodic granular assembly with high coordination number

Frattura ed Integrità Strutturale ◽

10.3221/igf-esis.59.14 ◽

2021 ◽

Vol 16 (59) ◽

pp. 188-197

Author(s):

Trung-Kien Nguyen ◽

Thanh-Trung Vo ◽

Nhu-Hoang Nguyen

Keyword(s):

Boundary Conditions ◽

Coordination Number ◽

Strain Localization ◽

Biaxial Loading ◽

Shear Zones ◽

Discrete Element Modeling ◽

Perfect Agreement ◽

Granular Assembly ◽

High Coordination Number ◽

High Coordination

Strain localization is one of key phenomena which have been studied extensively in geomaterials and for different kinds of materials including metals and polymers. This well-known phenomenon appears when structure/material is closed to failure. Theoretical, experimental, and numerical research have been dedicated to this subject for a long while. In the numerical aspects, strain localization inside the periodic granular assembly has not been well studied in the literature. In this paper, we investigate the occurrence and development of strain localization within a dense cohesive-frictional granular assembly with high coordination number under bi-periodic boundary conditions by Discrete Element Modeling (DEM). The granular assembly is composed of 2D circular disks and subjected to biaxial loading with constant lateral pressure. The results show that the formation of shear bands is of periodic type, consistent with the boundary conditions. This formation has the origins of the irreversible losing of cohesive contacts, viewed as micro-crackings which strongly concentrated in the periodic shear zones. This micromechanical feature is therefore strongly related to the strain localization observed at the sample scale. Finally, we also show that the strain localization is in perfect agreement with the sample’s displacement fluctuation fields.

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