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

Abstract

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.

Nanoscale Textural and Chemical Evolution of Silica Fault Mirrors in the Wasatch Fault Damage Zone, Utah, USA

“…Ti did not substitute into silica phases in detectable quantities during crystallization, implying a rapid, non-equilibrium process where Ti is sequestered into new Ti-rich phases. The lack of SPO and CPO in basal layer silica/quartz (Figures 5 and 6) suggests crystallization occurs at low dynamic stresses at the end of the coseismic to postseismic periods, consistent with similar interpretations of textures in recrystallized calcite (Siman-Tov et al, 2013), quartz (Kuo et al, 2016), and hematite (Ault et al, 2019;McDermott et al, 2017).…”

“…Our textural and geochemical data, as well as prior observations from other FMs, support high temperatures during the formation of the silica FM volume. First, the polygonal particles observed across the basal layer are analogous to polygonal crystal textures observed in hematite FMs (Ault et al., 2015, 2019; McDermott et al., 2017), and natural and experimentally generated carbonate FMs (Fondriest et al., 2013; Pozzi et al., 2018; Smith et al., 2013). In hematite FMs, this morphology and associated hematite (U‐Th)/He thermochronometry support temperatures >1000 °C at frictional contacts (McDermott et al., 2017) or within the deforming volume (Ault et al., 2019).…”

“…First, the polygonal particles observed across the basal layer are analogous to polygonal crystal textures observed in hematite FMs (Ault et al., 2015, 2019; McDermott et al., 2017), and natural and experimentally generated carbonate FMs (Fondriest et al., 2013; Pozzi et al., 2018; Smith et al., 2013). In hematite FMs, this morphology and associated hematite (U‐Th)/He thermochronometry support temperatures >1000 °C at frictional contacts (McDermott et al., 2017) or within the deforming volume (Ault et al., 2019). In carbonate FMs, these textures are similarly attributed to frictional heating at temperatures >400 °C (Chen et al., 2013; Fondriest et al., 2013; Han et al., 2011; Pozzi et al., 2018; Smith et al., 2013).…”

“…Some dynamic weakening mechanisms require coseismic temperature rise; but others, such as mechanical amorphization, do not (Kaneki et al., 2020; Marti et al., 2020; Pec et al., 2012; Yund et al., 1990). In addition, dynamic weakening may cause rheological changes that ultimately restrengthen faults (Ault et al., 2019; Di Toro et al., 2011; Mitchell et al., 2016; Proctor & Lockner, 2016; Rowe et al., 2019; Yasuhara et al., 2005). Because work done against friction during an earthquake is primarily given off as heat (e.g., Cardwell et al., 1978; Rice et al., 2006), identifying textural and geochemical signatures consistent with elevated temperatures can link fault weakening mechanisms to past seismic slip.…”

“…, and chert and novaculite (Kirkpatrick et al, 2013;Rowe et al, 2019). Some processes invoked for generating FMs include thermal decomposition (Collettini et al, 2013), melting (Ault et al, 2019;Kuo et al, 2016), gel lubrication (Kirkpatrick et al, 2013), brittle-plastic deformation and recrystallization (Fondriest et al, 2013;Pozzi et al, 2018;Smith et al, 2013), and asperity flash heating (Calzolari et al, 2020;McDermott et al, 2017). Many of these processes are dynamic weakening mechanisms that require coseismic heat, although laboratory FMs have also been generated at sub-seismic slip rates (Verberne et al, 2013(Verberne et al, , 2014.…”