Fault welding by pseudotachylyte generation

Abstract:

During earthquakes, frictional melts can localize on slip surfaces and dramatically weaken faults by melt lubrication. Once seismic slip is arrested, the melt cools and solidifies to form pseudotachylyte (PST), the presence of which is commonly used to infer earthquake slip on ancient exhumed faults. Little is known about the effect of solidified melt on the strength of faults directly preceding a subsequent earthquake. We performed triaxial deformation experiments on cores of tonalite (Gole Larghe fault zone, N. Italy) and mylonite (Alpine fault, New Zealand) in order to assess the strength of PST bearing faults in the lab. Three types of sample were prepared for each rock type; intact, sawcut and PST bearing, and were cored so that the sawcut, PST and foliation planes were orientated at 35° to the length of the core and direction of σ₁, i.e., a favorable orientation for reactivation. This choice of samples allowed us to compare the strength of ‘pre-earthquake’ fault (sawcut) to a ‘post-earthquake’ fault with solidified frictional melt, and assess their strength relative to intact samples. Our results show that PST veins effectively weld fault surfaces together, allowing previously faulted rocks to regain cohesive strengths comparable to that of an intact rock. Shearing of the PST is not favored, but subsequent failure and slip is accommodated on new faults nucleating at other zones of weakness. Thus, the mechanism of coseismic weakening by melt lubrication does not necessarily facilitate long-term interseismic deformation localization, at least at the scale of these experiments. In natural fault zones, PSTs are often found distributed over multiple adjacent fault planes or other zones of weakness such as foliation planes. We also modeled the temperature distribution in and around a PST using an approximation for cooling of a thin, infinite sheet by conduction perpendicular to its margins at ambient temperatures commensurate with the depth of PST formation. Results indicate that such PSTs would have cooled below their solidus in tens of seconds, leading to fault welding in under a minute. Cooled solidified melt patches can potentially act as asperities on faults, where faults can cease to be zones of weakness.