NANO-SCALE EVIDENCE FOR DYNAMIC WEAKENING AND HEALING DURING AN EARTHQUAKE ON A HEMATITE FAULT MIRROR

Friction-generated heat exerts a fundamental control on fault strength and fault rock rheological evolution during the seismic cycle. We apply novel nano-imaging and nano-chemistry tools to document evidence for dynamic weakening and potential re-strengthening of fault surface hematite. We target a hematite fault mirror, or high gloss, light reflective slip surface, which crosscuts the Pleistocene El Laco Fe-ore deposit, northern Chile. Scanning electron microscopy (SEM) imaging and electron back-scattered diffraction (EBSD) show the undeformed sample comprises sub-parallel to randomly-oriented, ~50-100 mm-thick, hematite plates intergrown with minor quartz and magnetite. Adjacent to the slip interface, crystal morphology is characterized by ~0.5-1.5 mm-diameter polygonal, triple junction-forming grains. Scanning/Transmission electron microscope (S/TEM) single crystal diffraction confirms polygonal grains are hematite. Grain diameter and crystallinity decrease over ~20-50 mm perpendicular to the slip surface and imply that morphological changes reflect a thermal process. Slip-surface grains have nm-scale ridges that transect multiple crystal faces and are arranged in a polygonal pattern. SEM and EBSD mapping of the naturally-polished fault mirror surface indicate polygonal grains lack a shape and crystallographic preferred orientation. S/TEM electron dispersive spectroscopy and electron energy loss spectroscopy (Dual-EELS) reveal nm-thick silica between hematite crystal faces that connect to larger domains of silica at hematite grain interstices. TEM lattice resolution imaging indicates this interstitial silica is amorphous. O K-edge and Fe L-edge EELS spectra demonstrate the presence of a nanofilm of Fe²⁺ at the fault surface. These observations collectively imply that frictional heat, generated from a dense network of geometric asperities, resulted in amorphization of hematite and quartz and high-temperature reduction of Fe³⁺ at the slip surface that likely promoted dynamic weakening and earthquake propagation. Interstitial amorphous silica was preserved as hematite rapidly annealed under low stress conditions. Fault surface textures imply hematite crystals interlocked across the slip surface, a process that may aid in strength recovery post-earthquake.