BRITTLE CO-SEISMIC DEFORMATION OF QUARTZ: LOCALIZATION NEAR THE BASE OF THE SEISMOGENIC ZONE

Understanding processes of strain localization at various depths is a key factor to understanding crustal-scale fault evolution and transient rheology of the continental crust associated with earthquake cycles. Near the base of the seismogenic zone (10–20 km), where temperatures exceed the onset of quartz plasticity, quartz deforms viscously by, for example, dislocation creep in the post- and inter-seismic periods, but can also be fractured during co-seismic rupture owing to transiently high shear or differential stress. After the rupture ceases, the fractured quartz can experience neocrystallization (followed by grain growth) and recrystallization and, as a result, exhibit smaller grain size than the parent quartz. This grain-size reduction acts as a strain weakening process contributing to localization in mid-crustal fault zones. However, the recrystallized quartz that experienced co-seismic damage at depth is difficult to distinguish from quartz not affected by co-seismic deformation, based solely on grain-size distributions and crystallographic orientation relationships between the parent and recrystallized grains.

To evaluate whether fluid inclusion concentration can provide clear evidence for brittle co-seismic deformation of quartz at depth, we analyzed the number density (count per unit area) and volume fraction of fluid inclusion in quartz domains, based on secondary electron image analysis and optical observation, from an ancient crustal-scale strike-slip fault zone (~300 m in width) exhumed from the base of the seismogenic zone (deformed at 400–500°C). The quartz domains were recrystallized dominantly by subgrain rotation, and the degree of recrystallization increases from the host quartzofeldspathic rock toward the fault zone core. Fluid inclusions are rare in the host rock well away from the fault zone. The inclusion number density increases abruptly (~15 times the background), by healed cracks generating secondary fluid inclusions, at ~90 m from the core. The inner fault zone with fully recrystallized quartz (~30 m in width) has ~8–9 times more fluid inclusions than the host rock. Our results indicate that coseismic damage can occur tens of meters from the rupture surface at depth and such brittle damage zones may extend from Earth’s surface to the bottom of the seismogenic zone.