scholarly journals From slow to fast faulting: recent challenges in earthquake fault mechanics

2017 ◽  
Vol 375 (2103) ◽  
pp. 20160016 ◽  
Author(s):  
S. Nielsen
Keyword(s):  
Frictional Heating ◽  
Shear Zones ◽  
Thin Layers ◽  
Plate Boundaries ◽  
Fault Mechanics ◽  
State Laws ◽  
Crystalline Materials ◽  
Dynamic Stress Drop ◽  
Localized Shear Deformation ◽  
Anelastic Deformation

Faults—thin zones of highly localized shear deformation in the Earth—accommodate strain on a momentous range of dimensions (millimetres to hundreds of kilometres for major plate boundaries) and of time intervals (from fractions of seconds during earthquake slip, to years of slow, aseismic slip and millions of years of intermittent activity). Traditionally, brittle faults have been distinguished from shear zones which deform by crystal plasticity (e.g. mylonites). However such brittle/plastic distinction becomes blurred when considering (i) deep earthquakes that happen under conditions of pressure and temperature where minerals are clearly in the plastic deformation regime (a clue for seismologists over several decades) and (ii) the extreme dynamic stress drop occurring during seismic slip acceleration on faults, requiring efficient weakening mechanisms. High strain rates (more than 10 4  s −1 ) are accommodated within paper-thin layers (principal slip zone), where co-seismic frictional heating triggers non-brittle weakening mechanisms. In addition, (iii) pervasive off-fault damage is observed, introducing energy sinks which are not accounted for by traditional frictional models. These observations challenge our traditional understanding of friction (rate-and-state laws), anelastic deformation (creep and flow of crystalline materials) and the scientific consensus on fault operation. This article is part of the themed issue ‘Faulting, friction and weakening: from slow to fast motion’.

Unravelling heterogeneities in magnetic fabric record of the strain: a combined AMS and ApARM data analysis applied to intraplate shear zones.

2021 ◽  
Author(s):  
Claudio Robustelli Test ◽  
Elena Zanella ◽  
Andrea Festa ◽  
Francesca Remitti
Keyword(s):  
Cluster Analysis ◽  
Hanging Wall ◽  
Shear Zones ◽  
Magnetic Fabric ◽  
Plate Boundaries ◽  
Thrust Faults ◽  
Localized Deformation ◽  
Main Thrust ◽  
Shear Sense ◽  
And Cluster Analysis

<p>Deciphering the stress and strain distribution across plate boundary shear zones is critical to understanding the physical processes involved in the nucleation of megathrust faults and its behaviour. Plate boundaries at shallow depth represent complex and highly deformed zones showing structures from both distributed and localized deformation.</p><p>As magnetic minerals are sensitive to stress regime, the investigation of the magnetic fabric has proven to be an effective tool in studying faulting processes at intraplate shear zones.</p><p>Anisotropy of magnetic susceptibility (AMS) provides insights into the preferred orientation of mineral grains and the qualitative relationships between petrofabrics and deformation intensity.</p><p>We present an approach of combined Contoured Diagram and Cluster Analysis to isolate the contribution of coexisting petrofabrics to the total AMS and evaluating the significance of magnetic fabric clusters.</p><p>Our results reveal distinct subfabrics with reasonably straightforward correlations with structural data. Specific AMS pattern may be associated to the intensity of the reworking related to tectonic shearing and the structural position within the shear zone (i.e., the proximity to the main thrust faults).</p><p>Close to the main thrust the magnetic fabric is dominantly oblate with magnetic foliation consistent to the S-C fabric and/or mélange foliation and the magnetic lineation parallel to the shear sense.</p><p>Away from the thrust faults the degree of anisotropy as well as the ellipsoids oblateness gradually diminishes. Thus, the presence of subfabrics related to previous tectonic events or less intense deformation (i.e. intersection lineation fabric) became dominant. The discrimination of subfabrics also allowed to unravel the presence of minor thrust plane and qualitatively evaluate the heterogeneous registration of strain (i.e. distributed versus localized deformation).</p><p>An abrupt change in magnetic ellipsoid shape and parameters is also observed below the basal décollements showing purely sedimentary magnetic fabric or previous deformation history with minor to absent evidences of shearing in the hanging wall.</p><p>Then, the integration with anisotropy of magnetic remanence experiments in different coercivity windows (ApARM) allow to separate the contribution of different ferromagnetic subpopulation of grains, constraining the significance of the different magnetic pattern/clusters detected through the AMS analysis.</p><p>In conclusion, our results show the potential of a combination of density diagrams and cluster analysis validated by ApARM experiments in distinguishing the superposition of deformation events, unravelling strain partitioning/concentration and thus to better understand the geodynamic evolution of subduction-accretion complexes.</p>

Localized Shear Deformation of Amorphous Metals

1981 ◽  
Vol 8 ◽  
Author(s):  
J.C.M. Li
Keyword(s):  
Stress Concentration ◽  
Shear Deformation ◽  
Narrow Band ◽  
Shear Localization ◽  
Amorphous Metals ◽  
Localized Deformation ◽  
Crystalline Materials ◽  
Localized Shear Deformation

ABSTRACTThe characteristics of localized deformation in amorphous metals are reviewed. All the available evidences seem to suggest that dislocations are responsible for shear localization. These dislocations seem to require a stress concentration for their nucleation. Once nucleated, they are capable of producing both forward and reverse shear in a narrow band, much like the dislocations in crystalline materials.

scholarly journals Electric measurements of PV heterojunction structures a-SiC/c-Si

2018 ◽  
Vol 69 (1) ◽  
pp. 52-57 ◽  
Author(s):  
Milan Perný ◽  
Vladimír Šály ◽  
František Janíček ◽  
Miroslav Mikolášek ◽  
Michal Váry ◽  
...  
Keyword(s):  
Silicon Carbide ◽  
Amorphous Silicon ◽  
Electrical Transport ◽  
Cell Structure ◽  
Transport Processes ◽  
Thin Layers ◽  
Amorphous Silicon Carbide ◽  
Crystalline Materials ◽  
Photovoltaic Energy Conversion ◽  
Energy Conversion Devices

Abstract Due to the particular advantages of amorphous silicon or its alloys with carbon in comparison to conventional crystalline materials makes such a material still interesting for study. The amorphous silicon carbide may be used in a number of micro-mechanical and micro-electronics applications and also for photovoltaic energy conversion devices. Boron doped thin layers of amorphous silicon carbide, presented in this paper, were prepared due to the optimization process for preparation of heterojunction solar cell structure. DC and AC measurement and subsequent evaluation were carried out in order to comprehensively assess the electrical transport processes in the prepared a-SiC/c-Si structures. We have investigated the influence of methane content in deposition gas mixture and different electrode configuration.

scholarly journals A safer future with clues from earthquakes past

Impact ◽  
2019 ◽  
Vol 2019 (9) ◽  
pp. 6-8
Author(s):  
Luca Menegon ◽  
Iain Stewart
Keyword(s):  
Mechanical Behaviour ◽  
Lower Crust ◽  
Shear Zones ◽  
Plate Boundaries ◽  
Potential Hazard ◽  
Earthquake Cycle ◽  
Seismogenic Layer ◽  
Seismic Slip ◽  
Viscous Shear ◽  
Continental Lower Crust

Understanding the short- and long-term mechanical behaviour of the lower crust is of fundamental importance when trying to understand the earthquake cycle and related hazard along active fault zones. In some regions some 20% of intracontinental earthquakes of magnitude > 5 nucleates in the lower crust at depth of 30-40 km. For example, a significant proportion of seismicity in the Himalaya, as well as aftershocks associated with the destructive 2001 Bhuj earthquake in India, nucleated in the granulitic lower crust of the Indian shield. Earthquakes in the continental interiors are often devastating and, over the past century, have killed significantly more people than earthquakes that occurred at plate boundaries. Thus, a thorough understanding of the earthquake cycle in intracontinental settings is essential. This requires knowledge of the mechanical behaviour and of the strength (by which Earth scientists commonly mean the maximum stress that rocks can sustain before deforming) of the lower crust. The most common conceptual model of the strength of the continental crust predicts a strong, seismogenic brittle upper crust (where the base of the seismogenic layer is typically considered to be at depth of 10-15 km), and a weak, viscous, aseismic lower crust. This model has been recently questioned by the finding that the lower crust can be seismic and, therefore, mechanically strong. The question arises, how thick is the seismogenic layer in the crust? Answering this question is crucial to determine the potential hazard caused by large earthquakes, which are also generally the deepest.<br/> Our limited knowledge of the mechanical behaviour of the lower crust is largely due to the lower crust itself being poorly accessible for direct geological observations, so that most of our knowledge derives from indirect geophysical measurements (like the distribution of earthquakes). There are only a few well-exposed large sections of exhumed continental lower crust in the world. One of these is located in the Lofoten islands (northern Norway), which were exhumed from their original deep crustal position during the opening of the North Atlantic Ocean.<br/> We propose an integrated, multi-disciplinary study of a network of brittle-viscous shear zones (i.e. zones of localized intense deformation of geological materials) from Lofoten, which records episodic rapid slip events (earthquakes) alternating with long-lasting aseismic creep. The study will link structural geology (analysis of geological faults and shear zones), petrology (analysis of the composition and textures of rocks), geochemistry (detailed chemical characterization of rocks and minerals) and experimental rock deformation (to reproduce in the lab under controlled conditions the deformation processes operative in the deep Earth's crust). This integrated dataset will provide a novel, clear picture of the mechanical behaviour of the continental lower crust during the earthquake cycle. Our direct geological and experimental observations will be tested against geophysical observations of currently active seismic deformation. The cumulative results of the projects will shed light on the currently poorly constrained mechanical behaviour of the lower crust during the earthquake cycle, and therefore on the sequence of inter-seismic slip (the period of slow accumulation of elastic deformation along a fault), co-seismic slip (the sudden rupture along a fault that is the earthquake) and post-seismic slip (the immediate period after an earthquake when the crust and the fault adjust to the modified state of crustal stress caused by an earthquake). This will greatly extend and complement existing efforts by the scientific community to understand and interpret the mechanical behaviour of rocks during the earthquake cycle recorded in the lower crust and the related hazard, and will provide key input for numerical models of continental dynamics.

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

Geology ◽  
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.

scholarly journals Self-Consistent Grain Size Evolution controls Strain Localization during Rifting

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.

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

Solid Earth ◽  
2021 ◽  
Vol 12 (5) ◽  
pp. 1211-1232
Author(s):  
Anthony Jourdon ◽  
Charlie Kergaravat ◽  
Guillaume Duclaux ◽  
Caroline Huguen
Keyword(s):  
Boundary Conditions ◽  
Strain Localization ◽  
Numerical Models ◽  
Motion Vector ◽  
Shear Zones ◽  
Strike Slip ◽  
Plate Motion ◽  
Plate Boundaries ◽  
Mechanical Modelling ◽  
Extension Direction

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.

Up-scaling eclogitization : from experimental and natural aggregates behaviours to seismological signatures

2020 ◽  
Author(s):  
Loic Labrousse ◽  
Incel Sarah ◽  
Zertani Sascha ◽  
Baisset Marie ◽  
Kaatz Lisa ◽  
...  
Keyword(s):  
Reaction Kinetics ◽  
Time Scales ◽  
Fluid Pressure ◽  
Experimental Studies ◽  
Shear Zones ◽  
Receiver Functions ◽  
Plate Boundaries ◽  
Large Set ◽  
Eclogite Facies ◽  
Geophysical Imaging

&lt;p&gt;Eclogite formation in the subducting crust was the first metamorphic transformation to be acknowledged as important in the dynamics of convergent plate boundaries. It is indeed expected to affect the mass distribution via density change, but it also influence the fluid content of crustal and possibly lithospheric wedges; both density and fluids being first order in values measured by passive geophysical imaging such as tomography of receiver functions. Recent high accuracy focal mechanism solutions showing singular signatures in deep orogens actually imply that eclogitization could also have a signature in the seismological source signals, and hence have an impact at much shorter time-scales. This presentation aims at bridging what we know from the field and the lab at smaller time and space scales, to what we observe at larger scales in collision zones. Field-based studies show the ways a pristine rock can evolve from metastable to fully eclogitized from the thin section to the kilometre scale. More than the contrast between eclogitized and non-eclogitized domains, the eclogitization front itself is expected to be detected in the geophysics, especially when driven by strain. Indeed strain-assisted eclogitization develops a characteristic shear zone network pattern with a significant anisotropy. This network itself evolves with the eclogitization progress. The observed progressive widening and increasing connectivity of eclogite-facies shear zones with increasing fluid availability could actually be controlled by the transient properties of the newly formed assemblages, inducing fluid pressure gradients for instance. In this context it appears that the competition between reaction kinetics and strain-rate is a key factor. This is also the case at shorter time scales. Experimental studies show that strain of metastable assemblages in the eclogite facies is more likely to lead to mechanical instabilities for intermediate reaction kinetics, implying again that not the eclogite but the eclogitization rate is the smoking gun. Eclogitization of plagioclase-bearing rocks is the finite result of a large set of reactions involving different chemical subsystem (Na or Ca end-members, with or without fluid available), not reacting at the same pace. Further work is therefore needed on the kinetics of the different reactions and their interactions to distinguish the one(s) that controls the eclogitization front signature, and hence improve the seismological imaging acuity.&lt;/p&gt;

scholarly journals Fluid-Present Partial Melting of Paleoproterozoic Okbang Amphibolite in the Yeongnam Massif, Korea

Lithosphere ◽  
2020 ◽  
Vol 2020 (1) ◽  
pp. 1-26
Author(s):  
Yuyoung Lee ◽  
Moonsup Cho
Keyword(s):  
Partial Melting ◽  
Shear Zones ◽  
Thin Layers ◽  
The North ◽  
Ductile Shear Zones ◽  
Ductile Shearing ◽  
Basaltic Melt ◽  
Yeongnam Massif ◽  
Migmatitic Gneiss ◽  
The Times

Abstract The waning stage of a long-lived collisional orogeny is commonly governed by an extensional regime in association with high-temperature metamorphism, anatexis, and magmatism. Such a late-orogenic process is well-recorded in the Okbang amphibolite, Yeongnam Massif, Korea, where thin layers or irregular patches of tonalitic leucosomes are widespread particularly in association with ductile shear zones. Various microstructures including interstitial felsic phases and former melt patches indicate that leucosomes are the product of partial melting. These leucosomes are aligned en echelon and contain large (up to ~2 cm) grains of peritectic hornblende, suggesting synkinematic fluid-present anatexis. The leucosomes are enriched in Na2O and Sr contents compared to the amphibolite but depleted in rare earth and high field-strength elements. P-T conditions of the anatexis were estimated at 4.6–5.2 kbar and 650–730°C, respectively, based on hornblende-plagioclase geothermobarometry. Sensitive high-resolution ion microprobe U-Pb analyses of zircon from an amphibolite and a leucosome sample yielded weighted mean 207Pb/206Pb ages of 1866±4 Ma and 1862±2 Ma, which are interpreted as the times for magmatic crystallization and subsequent anatexis of mafic protolith, respectively. The latter is consistent with the time of partial melting determined from a migmatitic gneiss and a biotite-sillimanite gneiss at 1861±4 Ma and 1860±9 Ma, respectively. The leucosomes are transected by an undeformed pegmatitic dyke dated at 1852±3 Ma; by this time, extensional ductile shearing has ceased. Initial εHft values of zircon from the amphibolite range from 4.2 to 6.0, suggesting juvenile derivation of basaltic melt from the mantle. In contrast, lower εHft values (–0.1 to 3.5) in leucosome zircons indicate a mixing of crust-derived melt. Taken together, the Okbang amphibolite has experienced synkinematic fluid-present melting during the waning stage of Paleoproterozoic hot orogenesis prevalent in the Yeongnam Massif as well as the North China Craton.

scholarly journals Laboratory investigation of mechanisms for phase mixing in olivine + ferropericlase aggregates

2018 ◽  
Vol 376 (2132) ◽  
pp. 20170417 ◽  
Author(s):  
Harison S. Wiesman ◽  
Mark E. Zimmerman ◽  
David L. Kohlstedt
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Plate Tectonics ◽  
Shear Zones ◽  
Grain Boundary Sliding ◽  
Outer Radius ◽  
Thin Layers ◽  
Two Phase ◽  
Phase Mixing ◽  
Grain Boundary Pinning

To investigate the role of grain boundary pinning and the mechanisms by which phase mixing occurs during deformation of polymineralic rocks, we conducted high-strain torsion experiments on samples consisting of olivine plus 30 vol% ferropericlase. Experiments were performed in a gas-medium deformation apparatus at 1524 K and 300 MPa. Samples were deformed to outer radius shear strains of up to γ ( R ) = 14.1. The value of the stress exponent and the small grain sizes of our samples indicate that our two-phase material deformed by dislocation-accommodated grain boundary sliding. In samples deformed to 1 <  γ  < 7, elongated clusters of ferropericlase grains form thin layers in the olivine matrix, and small grains of ferropericlase appear at olivine grain boundaries and three- and four-grain junctions. By γ  ≈ 14, a well-distributed mixture of small ferropericlase grains among the olivine grains developed. Microstructures exhibit similarities to both mechanical and chemical models proposed to describe the processes leading to phase mixing. Our results provide evidence for grain size reduction during phase mixing that results in a grain size significantly smaller than the value predicted by the single-phase recrystallization piezometer for olivine. Thus, phase mixing provides a mechanism for the persistent weakening of rocks that is important for developing and maintaining shear zones necessary for plate tectonics. This article is part of a discussion meeting issue ‘Earth dynamics and the development of plate tectonics'.

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