Fault rock injections record paleo-earthquakes

Abstract

Fault rocks such as pseudotachylyte melt or granular gouge are sometimes injected into cracks off a fault surface. The width to height ratio of the injection veins is a direct measure of the shear strain in the wall rock required to accommodate the injection, so the aspect ratios preserve a record of the overpressure opening the cracks and injecting melt or fluidized granular material from the fault zone. We measured the aspect ratios for 201 pseudotachylyte injections and 29 granular injections. They have aspect ratios of ∼0.2 (0.17±0.025 and 0.22±0.077, respectively, within 99% confidence). These are significantly proportionally wider than the aspect ratios of ordinary dikes (from 10−4 to 10−3). The injection aspect ratios exceed the usual limit of elastic strain in the wallrock (∼1%) so we infer that the injection veins were accommodated by permanent deformation by microcracking and slip on foliation surfaces in the host rock, which is difficult to detect observationally. The fully elastic model therefore offers an upper bound on the stress required to open these injection veins (∼108–1010Pa, with higher pressures in stiffer host rock lithologies.) These overpressures cannot be achieved by expansion of the fault rock fluids during melting or gouge fluidization mechanisms, as the pressure along the fault is limited to ∼107Pa by the wall rock compliance. Dynamic tension parallel to the fault is required to explain the large apparent overpressures in the injection veins. Therefore, both pseudotachylyte and gouge injections of this aspect ratio must be considered seismic signatures.