THE ROLE OF MECHANICAL AND THERMAL FRACTURE IN STRAIN LOCALIZATION IN UPPER- AND MID-CRUSTAL SEISMOGENIC FAULT/SHEAR-ZONE SYSTEMS (Invited Presentation)

Large-displacement, steeply-dipping, strike-slip, seismogenic fault zones transition to middle crustal shear zones across the frictional to viscous transition (FVT). These coupled fault/shear-zone systems localize and stabilize as long-term structures through a variety of well-known processes, including grain size reduction via dynamic recrystallization, changes in the mineral assemblage, and thermal perturbations that can increase reaction and strain rates. Two additional, yet underappreciated factors affect rocks within and up to several kilometers below the seismogenic zone through transient cracking and dilatancy. These are (1) passage of high-energy seismic waves, and (2) thermal shock near frictional slip surfaces. The GPa-level moduli in mineral elastic stiffness tensors combined with the anisotropy of elasticity and thermal expansion in the major rock-forming minerals ensures that large earthquake ruptures will lead to grain-scale principal stresses well in excess of the tensile strengths of individual minerals. The result is pervasive tensile cracking, pulverization and transient dilatancy in “damage zone” rocks surrounding the fault/shear-zone core. We suggest that these processes are major contributors to both frictional and viscous localization in large fault/shear-zone systems by driving grain size reduction, temperature increases through fracture-related energy dissipation, and fluid pressure changes and reaction rates associated with transient dilatancy. These processes may also be major contributors to stabilization of the fault/shear-zone core through time in long-lived systems, though their effect is probably largest above the FVT. In this presentation we show field and microstructural evidence interpreted as mechanical and thermal damage associated with the earthquake cycle, and present novel numerical modeling results of damage evolution in polycrystalline aggregates at various depths applied to both computer-derived and EBSD-derived rock microstructures.