Coseismic brecciation at fault stepovers and transient fluid pathways in a mid-crustal San Andreas analogue: The Pofadder Shear Zone, Namibia and South Africa

Abstract:

Fluid transport along faults is important throughout the seismic cycle due to the effects on fault strength. Rheological boundaries in the crust such as the quartz brittle-plastic transition coincide with permeability changes, and play an important role in controlling fluid distribution. Here we present a newly recognized mechanism for fluid migration through the brittle-plastic transition in an ancient San Andreas Fault analogue: The Pofadder Shear Zone in Namibia and South Africa. Breccias formed in elongate pods during the passage of an earthquake rupture through a fault stepover. These breccias form subvertical fluid pathways (perpendicular to the slip direction). Over time, many overprinting or adjacent ruptures could have allowed fluid migration over a large (~ kms) scale, facilitating fluid flow through a low porosity region of the crust. These pathways were subsequently closed during breccia compaction by crystal plastic flow, facilitated by the presence of fluids. Thus, fluid migration within and across the brittle-plastic transitional zone is time and rate dependent and can both cause fault weakening and strengthening. We observed breccias formed in slip events with displacements between ~1-15 cm, consistent with small to moderate magnitude earthquakes and/or tectonic tremor, which occurs at similar depths in the San Andreas Fault. In addition to providing a new way of identifying paleo-seismic slip in the rock record, these observations may help explain co- post-seismic fluid advection in mid-crustal faults. This process of local brecciation in stepovers may be the origin of cryptic geophysical signals such as tremor bursts in continental faults.