Crustal Anisotropy in Southern California: Evidence for a Fossilized Detachment?

Receiver functions were calculated for sixteen stations in southwestern California and are currently being calculated for eight more stations in the Mojave Desert and surrounding area to explore a proposed fossilized mid/lower crustal regional detachment related to past subduction in the region (Ozacar and Zandt, 2008). All of the stations exhibit large azimuthal variations in receiver function polarity and amplitude on both the tangential and radial components. A trend seen regionally on radial components is an arrival immediately before the Moho which appears as a high amplitude positive arrival. This pre-Moho arrival peaks in amplitude as a negative arrival at ~200 ° and is a low amplitude positive at ~40°. On the tangential component this arrival appears as a negative arrival from ~0° to ~200° where it becomes a positive arrival to ~360°. The move out pattern and consistency of this signal over a large geographic area suggests that it stems from an anisotropic lower-crustal low velocity zone. While dipping interfaces may contribute to the complexity of the signal seen in individual stations, it is unlikely that similar dipping interfaces could be present throughout the region. A neighborhood algorithm search was run on stations RPV, MWC, CHF, PHL, CIA, VTV and PKD to invert for interface depth, strike and dip, percent anisotropy and orientation of anisotropy. The inversion for all of the stations (with the exception of VTV and CIA) resulted in an anisotropic low velocity layer that range in depth from ~14 to 20 km with anisotropy ranging from -14% to -20% with a unique axis oriented at ~240 °. We are currently calculating receiver functions and running inversions for the Mojave stations. These initial results support the idea of a regionally pervasive subduction related detachment underlying the Salinian, San Gabriel and related terranes. Further work will help to confirm the existence of this detachment and determine its extent.