skip to main content

Title: Influence of Shear Heating and Thermomechanical Coupling on Earthquake Sequences and the Brittle‐Ductile Transition

Localized frictional sliding on faults in the continental crust transitions at depth to distributed deformation in viscous shear zones. This brittle‐ductile transition (BDT), and/or the transition from velocity‐weakening (VW) to velocity‐strengthening (VS) friction, are controlled by the lithospheric thermal structure and composition. Here, we investigate these transitions, and their effect on the depth extent of earthquakes, using 2D antiplane shear simulations of a strike‐slip fault with rate‐and‐state friction. The off‐fault material is viscoelastic, with temperature‐dependent dislocation creep. We solve the heat equation for temperature, accounting for frictional and viscous shear heating that creates a thermal anomaly relative to the ambient geotherm which reduces viscosity and facilitates viscous flow. We explore several geotherms and effective normal stress distributions (by changing pore pressure), quantifying the thermal anomaly, seismic and aseismic slip, and the transition from frictional sliding to viscous flow. The thermal anomaly can reach several hundred degrees below the seismogenic zone in models with hydrostatic pressure but is smaller for higher pressure (and these high‐pressure models are most consistent with San Andreas Fault heat flow constraints). Shear heating raises the BDT, sometimes to where it limits rupture depth rather than the frictional VW‐to‐VS transition. Our thermomechanical modeling framework can be used to evaluate lithospheric rheology and thermal models through predictions of earthquake ruptures, postseismic and interseismic crustal deformation, heat flow, and the geological structures that reflect the complex deformation beneath faults.

more » « less
Award ID(s):
Author(s) / Creator(s):
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Journal of Geophysical Research: Solid Earth
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Deformation experiments on hematite characterize its slip‐rate dependent frictional properties and deformation mechanisms. These data inform interpretations of slip behavior from exhumed hematite‐coated faults and present‐day deformation at depth. We used a rotary‐shear apparatus to conduct single‐velocity and velocity‐step experiments on polycrystalline specular hematite rock (∼17 μm average plate thickness) at slip rates of 0.85 μm/s to 320 mm/s, displacements of primarily 1–3 cm and up to 45 cm, and normal stresses of 5 and 8.5 MPa. The average coefficient of friction is 0.70; velocity‐step experiments indicate velocity‐strengthening to velocity‐neutral behavior at rates <1 mm/s. Scanning electron microscopy showed experimentally generated faults develop in a semi‐continuous, thin layer of red hematite gouge. Angular gouge particles have an average diameter of ∼0.7 μm, and grain size reduction during slip yields a factor of 10–100 increase in surface area. Hematite is amenable to (U‐Th)/He thermochronometry, which can quantify fault‐related thermal and mechanical processes. Comparison of hematite (U‐Th)/He dates from the undeformed material and experimentally produced gouge indicates He loss occurs during comminution at slow deformation rates without an associated temperature rise required for diffusive loss. Our results imply that, in natural fault rocks, deformation localizes within coarse‐grained hematite by stable sliding, and that hematite (U‐Th)/He dates acquired from ultracataclasite or highly comminuted gouge reflect minor He loss unrelated to thermal processes. Consequently, the magnitude of temperature rise and associated thermal resetting in hematite‐bearing fault rocks based on (U‐Th)/He thermochronometry may be overestimated if only diffusive loss of He is considered. 
    more » « less
  2. Abstract

    Establishing a constitutive law for fault friction is a crucial objective of earthquake science. However, the complex frictional behavior of natural and synthetic gouges in laboratory experiments eludes explanations. Here, we present a constitutive framework that elucidates the rate, state, and temperature dependence of fault friction under the relevant sliding velocities and temperatures of the brittle lithosphere during seismic cycles. The competition between healing mechanisms, such as viscoelastic collapse, pressure‐solution creep, and crack sealing, explains the low‐temperature stability transition from steady‐state velocity‐strengthening to velocity‐weakening as a function of slip‐rate and temperature. In addition, capturing the transition from cataclastic flow to semi‐brittle creep accounts for the stabilization of fault slip at elevated temperatures. We calibrate the model using extensive laboratory data on synthetic albite and granite gouge, and on natural samples from the Alpine Fault and the Mugi Mélange in the Shimanto accretionary complex in Japan. The constitutive model consistently explains the evolving frictional response of fault gouge from room temperature to 600°C for sliding velocities ranging from nanometers to millimeters per second. The frictional response of faults can be uniquely determined by the in situ lithology and the prevailing hydrothermal conditions.

    more » « less
  3. Abstract 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. 
    more » « less
  4. null (Ed.)
    Abstract The plate interface undergoes two transitions between seismogenic depths and subarc depths. A brittle-ductile transition at 20–50 km depth is followed by a transition to full viscous coupling to the overlying mantle wedge at ∼80 km depth. We review evidence for both transitions, focusing on heat-flow and seismic-attenuation constraints on the deeper transition. The intervening ductile shear zone likely weakens considerably as temperature increases, such that its rheology exerts a stronger control on subduction-zone thermal structure than does frictional shear heating. We evaluate its role through analytic approximations and two-dimensional finite-element models for both idealized subduction geometries and those resembling real subduction zones. We show that a temperature-buffering process exists in the shear zone that results in temperatures being tightly controlled by the rheological strength of that shear zone’s material for a wide range of shear-heating behaviors of the shallower brittle region. Higher temperatures result in weaker shear zones and hence less heat generation, so temperatures stop increasing and shear zones stop weakening. The net result for many rheologies are temperatures limited to ≤350–420 °C along the plate interface below the cold forearc of most subduction zones until the hot coupled mantle is approached. Very young incoming plates are the exception. This rheological buffering desensitizes subduction-zone thermal structure to many parameters and may help explain the global constancy of the 80 km coupling limit. We recalculate water fluxes to the forearc wedge and deep mantle and find that shear heating has little effect on global water circulation. 
    more » « less
  5. Abstract

    The icy shells of Enceladus and Europa consist from top down of cold (~100 K) ice at low (<0.1 MPa) pressure, cold ice at high pressure up to ~10 MPa, and warm ice near 273 K the base. The pressure ~10 MPa and temperature near 273 K of basal ice within Enceladus and Europa are similar to that within terrestrial glaciers that are known to have seismicity (icequakes). Warm ice easily melts during sliding so its icequakes are qualitatively explained and expected on these satellites if the macroscopic strain rates are comparable to those within terrestrial glaciers subjected to oceanic tides. However, cold ice at high pressures does not readily macroscopically melt during sliding. A dynamic weakening mechanism for crustal faults in rock may be applicable to Enceladus and Europa. Micron‐scale real contacts support ~0.4‐GPa shear tractions and normal tractions on rapidly sliding ice faults. At sliding velocities above ~0.1 m/s, the asperity tips of the contacts become hot and weak in ice. The macroscopic friction depends on the average strength of the asperity tips during the lifetimes of contact. The strength of the asperity tips self‐organizes so that frictional heating balances the heat lost from the asperity tip by conduction. The macroscopic coefficient of friction at coseismic sliding velocities decreases to a modest fraction of the low‐velocity coefficient of friction, but does not approach zero. This velocity‐weakening mechanism likely allows major icequakes within the cold interiors of Europa and Enceladus.

    more » « less