FAULT-ADJACENT DAMAGE AT THE BASE OF THE SEISMOGENIC ZONE AND IMPLICATIONS FOR CRUSTAL RHEOLOGY

The base of the seismogenic zone, approximately coincident with the frictional-to-viscous transition (FVT), experiences cycles of coseismic brittle damage followed by longer-term viscous flow. These cycles are accompanied by cycles of grain-size reduction, permeability evolution and mass transfer/reaction, exerting important influence on crustal rheology. Thus, exploring the vertical and lateral extent of coseismic damage is critical for understanding crustal rheology.

Sparse exposures of ancient earthquake faults exhumed from FVT depths have limited studies of earthquake-related cycles in the deeper reaches of the seismogenic zone. From the Sandhill Corner shear zone, a strand of the Norumbega fault system (an ancient seismogenic strike-slip fault), exhumed from FVT depths, we analyze several microstructural features that provide quantitative insights into the earthquake damage cycle: (a) fluid inclusion abundance in quartz, (b) orientation distributions of healed microcracks and cleaveage planes in feldspar, (c) fragment size distributions in highly fractured garnet, and (d) asymmetry of kink bands in muscovite. Our results indicate that coseismic damage exists up to ~200 m from the shear zone core, with some microstructural features not recording the full extent of the damage, and pulverization (extreme brittle damage) may extend ~40 m from the core. To estimate post- or interseismic viscous flow stress and deformation mechanisms in quartz, we also collect quantitative data of quartz microstructures across the shear zone including grain size, crystallographic orientation, misorientation, and fabric intensity through electron backscatter diffraction. Based on the quartz data, the shear zone can be divided into an inner and outer zone at ~40 m from the core. The inner shear zone indicates grain-size-sensitive processes, whereas the outer shear zone was deformed dominantly by grain-size-insensitive processes. This implies that intense coseismic damage affects long-term strength of the fault/shear zone system, thus facilitating strain localization.