Numerical modeling of pseudotachylyte injection vein formation [thesis]

Will Sawyer
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 stiness. These studies, however, have generally treated injection vein formation as a purely elastic process and have assumed that processes of melt
more » ... eneration, transport, and solidication have little inuence on the nal vein geometry. Using a modied 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 unaected by the inclusion of these processes. However, some injection veins are ow limited, with a nal geometry reecting cooling of the vein before it reaches an elastic equilibrium with the wall rock. In these cases, numerical results are signicantly dierent 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 signicant coseismic wall rock softening or high transient tensile stress.
doi:10.14418/wes01.1.1251 fatcat:6qzevbufivhrpj6hly2d7hovfu