Coseismic damage runs deep in continental strike-slip faults

Abstract

Coseismic off-fault damage and pulverization significantly influence the mechanical and transport properties, and in turn the rupture dynamics of faults. Although field-based, laboratory, and numerical studies help elucidate the structure of damage zones adjacent to modern strike-slip faults, the vertical extent of these zones remains an open question. To address this question, we analyzed particle size distribution and microfracture density of fragmented garnets from the Sandhill Corner shear zone, a strand of an ancient, seismogenic, strike-slip fault system exhumed from frictional-to-viscous transition depths (400–500°C). The shear zone has mutually overprinting pseudotachylyte and mylonite, and juxtaposes quartzofeldspathic and schist units. The inner parts of the quartzofeldspathic and schist units (∼63 m and ∼5 m wide from the lithologic contact, respectively) have two-dimensional D-values ≥1.5, which indicates dynamic fragmentation during rupture propagation. Similar to the particle size distribution analysis, microfracture density data from the garnets show progressive but asymmetric increase toward the core in each unit. Our results suggest that coseismic damage extends down to the base of the seismogenic zone in mature strike-slip faults, and the asymmetric distribution of damage may indicate preferred rupture directivity as proposed for some modern strike-slip faults.