The northwestern margin of the Basin-and-Range Province, part 1: Reflection profiling of the moderate-angle (~ 30°) Surprise Valley Fault

Seismic reflection profiling demonstrates that the active, significant-offset Surprise Valley Fault that marks the western boundary of the Basin-and-Range Province in northernmost California dips at a moderate angle, only ~ 30° to ~ 2 km depth. A nearby seismic refraction/wide-angle reflection profile, albeit of lower-resolution, shows the fault-plane remains approximately planar to at least 7 km depth. Variably-tilted volcanic strata of known ages within the Surprise Valley basin demonstrate that ≥ 7 km of normal slip occurred along the Surprise Valley Fault within the past 8-4 Myr at a time-averaged slip rate of ~ 1-2 mm/yr, and that the fault rotated from an initial dip of as much as 60° to its present dip of 30°. The rotation of this isolated fault was not accomplished by domino-style rotation as seen elsewhere in the Basin-and-Range province, but rather by flexural rift-shoulder uplift aided by mid-crustal flow.