CRUSTAL STRUCTURE DURING ACTIVE CONTINENTAL RIFTING IN CENTRAL SALTON TROUGH, CALIFORNIA, CONSTRAINED BY THE SALTON SEISMIC IMAGING PROJECT (SSIP)

Seismic refraction and reflection travel times from the Salton Seismic Imaging Project (SSIP) are being used to constrain crustal structure during active continental rifting in the central Salton Trough, California. SSIP, funded by NSF and USGS, acquired seismic data in and across the Salton Trough in 2011 to investigate rifting processes at the northern end of the Gulf of California extensional province and earthquake hazards at the southern end of the San Andreas Fault system. Seven lines of refraction and low-fold reflection data were acquired onshore, two lines and a grid of airgun and OBS data were acquired in the Salton Sea, and onshore-offshore data were recorded.

Based on prior studies of the central Salton Trough, North American lithosphere appears to have been rifted completely apart and replaced by entirely new crust added by magmatism from below and sedimentation from above. Ongoing active rifting of this new crust is manifested by shallow seismicity in the oblique Brawley Seismic Zone (BSZ; connecting the Imperial and San Andreas faults), the small Salton Buttes volcanoes (aligned perpendicular to the direction of plate motion), and very high heat flow.

Velocity models of a seismic line along the axis of the valley, parallel to the plate motion direction, and a perpendicular line along the south shore of Salton Sea, image a ~40 km wide basin bounded by the San Jacinto fault system on the southwest and a paleo-fault on the northeast. Crystalline basement (~5 km/s) generally occurs at ~4 km depth in the valley, but is at 2-3 km depth in a ~8 x 15 km localized region beneath the Salton Buttes and Salton Sea geothermal field. This crystalline rock is interpreted to be late Pliocene to Quaternary sediment metamorphosed by high heat flow. The shallower basement under the volcanic and geothermal field is due to more intense metamorphism and hydrothermal alteration in this region.

The seismic velocity of basement is slower in the BSZ than under the surrounding valley, which may be due to seismicity-related fracturing. Preliminary analysis of deep seismic reflections indicates a shallow Moho at roughly 20 km depth, but 2-D modeling is still in progress.