RESEARCH FOCUS: How Dynamic Weakening Makes Faults Stronger: The Role Of Melting In Post-Seismic Healing

Griffith, W. A.

doi:10.1130/focus122016.1

Cited by 3 publications

(1 citation statement)

References 28 publications

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“…We suggest that temperature-induced weakening seeds fault restrengthening due to thermal decay and associated rheological changes on some FMs. Processes inferred here complement fault welding and postseismic strength recovery mechanisms documented in experimentally generated pseudotachylytes (Griffith, 2016;Mitchell et al, 2016;Proctor and Lockner, 2016). Our results highlight how thermally induced nanoscale transformations may promote and inhibit earthquake behavior on a broader range of fault surfaces.…”

Section: Implications For the Earthquake Cyclesupporting

confidence: 61%

Nanoscale evidence for temperature-induced transient rheology and postseismic fault healing

Ault

Jensen

McDermott

et al. 2019

Geology

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Friction-generated heat and the subsequent thermal evolution control fault material properties and thus strength during the earthquake cycle. We document evidence for transient, nanoscale fault rheology on a high-gloss, light-reflective hematite fault mirror (FM). The FM cuts specularite with minor quartz from the Pleistocene El Laco Fe-ore deposit, northern Chile. Scanning and transmission electron microscopy data reveal that the FM volume comprises a <50-μm-thick zone of polygonal hematite nanocrystals with spherical silica inclusions, rhombohedral twins, no shape or crystallographic preferred orientation, decreasing grain size away from the FM surface, and FM surface magnetite nanoparticles and Fe2+ suboxides. Sub–5-nm-thick silica films encase hematite grains and connect to amorphous interstitial silica. Observations imply that coseismic shear heating (temperature >1000 °C) generated transiently amorphous, intermixed but immiscible, and rheologically weak Fe-oxide and silica. Hematite regrowth in a fault-perpendicular thermal gradient, sintering, twinning, and a topographic network of nanometer-scale ridges from crystals interlocking across the FM surface collectively restrengthened fault material. Results reveal how temperature-induced weakening preconditions fault healing. Nanoscale transformations may promote subsequent strain delocalization and development of off-fault damage.

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Section: Implications For the Earthquake Cyclesupporting

confidence: 61%

Nanoscale evidence for temperature-induced transient rheology and postseismic fault healing

Ault

Jensen

McDermott

et al. 2019

Geology

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Origin of multiple principal slip zones in a fault gouge zone within granitoids

Park

Kim

Ree

2022

Journal of Structural Geology

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How a strong low-angle normal fault formed: The Whipple detachment, southeastern California

Axen

2019

GSA Bulletin

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Many low-angle normal faults (dip ≤30°) accommodate tens of kilometers of crustal extension, but their mechanics remain contentious. Most models for low-angle normal fault slip assume vertical maximum principal stress σ1, leading many authors to conclude that low-angle normal faults are poorly oriented in the stress field (≥60° from σ1) and weak (low friction). In contrast, models for low-angle normal fault formation in isotropic rocks typically assume Coulomb failure and require inclined σ1 (no misorientation). Here, a data-based, mechanical-tectonic model is presented for formation of the Whipple detachment fault, southeastern California. The model honors local and regional geologic and tectonic history and laboratory friction measurements. The Whipple detachment fault formed progressively in the brittle-plastic transition by linking of “minidetachments,” which are small-scale analogs (meters to kilometers in length) in the upper footwall. Minidetachments followed mylonitic anisotropy along planes of maximum shear stress (45° from the maximum principal stress), not Coulomb fractures. They evolved from mylonitic flow to cataclasis and frictional slip at 300−400 °C and ∼9.5 km depth, while fluid pressure fell from lithostatic to hydrostatic levels. Minidetachment friction was presumably high (0.6−0.85), based upon formation of quartzofeldspathic cataclasite and pseudotachylyte. Similar mechanics are inferred for both the minidetachments and the Whipple detachment fault, driven by high differential stress (∼150−160 MPa). A Mohr construction is presented with the fault dip as the main free parameter. Using “Byerlee friction” (0.6−0.85) on the minidetachments and the Whipple detachment fault, and internal friction (1.0−1.7) on newly formed Reidel shears, the initial fault dips are calculated at 16°−26°, with σ1 plunging ∼61°−71° northeast. Linked minidetachments probably were not well aligned, and slip on the evolving Whipple detachment fault probably contributed to fault smoothing, by off-fault fracturing and cataclasis, and to formation of the fault core and fractured damage zone. Stress rotation may have occurred only within the mylonitic shear zone, but asymmetric tectonic forces applied to the brittle crust probably caused gradual rotation of σ1 above it as a result of: (1) the upward force applied to the base of marginal North America by buoyant asthenosphere upwelling into an opening slab-free window and/or (2) basal, top-to-the-NE shear traction due to midcrustal mylonitic flow during tectonic exhumation of the Orocopia Schist. The mechanical-tectonic model probably applies directly to low-angle normal faults of the lower Colorado River extensional corridor, and aspects of the model (e.g., significance of anisotropy, stress rotation) likely apply to formation of other strong low-angle normal faults.

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RESEARCH FOCUS: How Dynamic Weakening Makes Faults Stronger: The Role Of Melting In Post-Seismic Healing

Cited by 3 publications

References 28 publications

Nanoscale evidence for temperature-induced transient rheology and postseismic fault healing

Nanoscale evidence for temperature-induced transient rheology and postseismic fault healing

Origin of multiple principal slip zones in a fault gouge zone within granitoids

How a strong low-angle normal fault formed: The Whipple detachment, southeastern California

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