Numerical modeling of pseudotachylyte injection vein formation

Pseudotachylyte, a fault rock formed through coseismic frictional melting, provides an important record of coseismic mechanics. In particular, injection veins formed at a high angle to the fault surface have been used to estimate rupture directivity, velocity, pulse length, stress and strength drop, as well as slip weakening distance and wall rock sti ness. These studies, however, have generally treated injection vein formation as a purely elastic process and have assumed that processes of melt generation, transport, and solidi cation have little in uence on the nal vein geometry. Using a modi ed analytical approximation of injection vein formation based on a dike intrusion model we nd that the timescales of quenching and ow propagation are similar for a suite injection veins in the Fort Foster Brittle Zone, indicating a complex, dynamic process whose behavior is not fully captured by the current approach. To assess the applicability of the simplifying assumptions of the dike model when applied to injection veins we employ a nite-element time-dependent model of injection vein formation. This model couples elastic deformation of the wall rock with the uid dynamics and heat transfer of the frictional melt. The nal geometry of a majority of injection veins is una ected by the inclusion of these processes. However, some injection veins are ow limited, with a nal geometry re ecting cooling of the vein before it reaches an elastic equilibrium with the wall rock. In these cases, numerical results are signi cantly di erent from the dike model, and two basic assumptions of the dike model, self-similar growth and a uniform pressure gradient, are shown to be false. Additionally, we provide two new constrains on the Fort Foster coseismic environment: a lower limit on the initial melt temperature of 1400 ◦C, and either signi cant coseismic wall rock softening or high transient tensile stress.