SIGNIFICANCE OF FRICTIONAL MELTS AND THEIR KINEMATICS DURING SEISMIC RUPTURE: EXAMPLE FROM EXTENSIVE PSEUDOTACHYLYTE NETWORKS IN THE SANTA ROSA MOUNTAINS, CALIFORNIA

Fault pseudotachylytes, commonly regarded as "earthquake fossils," are critical indicators of deformation at seismic velocities (>0.1 m/s) and are typically associated with large-magnitude earthquakes (Mw >6). These materials often form extensive, anastomosing vein networks, which provide valuable insights into the rupture processes within earthquake nucleation zones. However, the kinematics of frictional melts during the incipient stages of seismic rupture remain poorly understood, particularly whether the melt is continuously smeared along the slip surface or laterally expelled obliquely. Additionally, the role of restraining and releasing bends in fault planes in controlling frictional melt kinematics is unclear. In this study, we focus on two >120 m long, continuous pseudotachylyte veins in the Santa Rosa Mountains, California, a region where pseudotachylytes have been exhumed from depths of 5-10 km. This locality, with its well-exposed veins hosted in Asbestos Mountain tonalite, offers an ideal field laboratory for testing the hypothesis that pseudotachylytes represent seismic slip. Using the Anisotropy of Magnetic Susceptibility (mini-AMS) method on 3.5 mm-oriented samples, we aim to infer viscous flow direction and slip direction. Preliminary results indicate that in both releasing and restraining bends, the melt’s viscous flow deviates from the main slip direction, challenging the assumption that pseudotachylytes directly reflect seismic slip. This research advances our understanding of fault friction, slip partitioning, and the implications for seismic hazard assessments, particularly at moderate depths (~5-15 km).