3-D Tomography of the Gulf of California Extensional Province by Rayleigh Wave Inversion

The Gulf of California, separating the Pacific and North American plates, is an excellent area to study the continental rifting process, including magmatism, lithosphere deformation and crustal thickness distribution. To create a 3-D image of this rift zone, we carry out a Rayleigh wave dispersion study, which can sense velocity structure at different depths using different period surface waves. We use fundamental modes waves ranging in period from 20 to 140 s generated by teleseismic sources and recorded by the NARS-Baja seismic array and stations in southern California. Surface waves traveling along the Gulf or Baja California have typically undergone waveform distortion before they reach the study area by propagating along continental margins and subduction zones that induce scattering and multipathing. Further waveform distortion is caused by heterogeneities in the extensional province. In other words, the incoming waves are no longer well represented by a simple plane wave. Instead, we employ an array-processing method (Forsyth and Li, 2005), which describes the incoming wave field as the interference of two plane waves which travel along slightly different paths from the great circle paths and employs finite frequency response kernels (Zhou et al. 2004) to represent scattering within the study area. In southernmost California, there is a low-velocity anomaly in the shallow upper mantle dipping eastward from beneath the Peninsular Range toward the Salton Sea. At depths greater than 150 km, there is a high velocity anomaly beneath the Peninsular Range. Together, this pattern resembles the anomaly pattern beneath the southern Sierra Nevada, where the lower lithosphere is thought to have delaminated and been replaced by upwelling asthenosphere. One of the primary goals of this study is to determine how far south this pattern continues. Another goal is to detect remnants of the Farallon plate that may still be attached to the lithosphere beneath Baja California, left over from the stalling of spreading before the ridge subducted. Results show: 1) Peninsular Range anomalies do not continue into Baja California ; 2) there is a consistent high velocity anomaly in short periods in Guadalupe plate area, which indicates one of the remnants of Farallon plate.