Seismic anisotropy in northern and central Gulf of California region, Mexico, from teleseismic receiver functions and new evidence of possible plate capture

[1] We present anisotropic models for the shallow lithosphere in the northern and central Gulf of California obtained from receiver function (RF) analysis. This region has undergone intense plate reorganization over the past 25 Ma that led to the transfer of the plate boundary east of the modern Baja California Peninsula posterior to the cease of the subduction of the Farallon plate beneath the North American plate. Our main interest in modeling the seismic anisotropy is to characterize rock fabric inherited from past and recent tectonism and to provide new information for understanding the tectonic processes involved in the evolution of the local plate boundary. We calculate RFs from teleseismic P waves recorded by the Network of Autonomously Recording Seismographs(NARS)-Baja array, and we stack them in 20 degrees wide back azimuthal bins to enhance coherent information. These RFs yield significant transverse energy whose azimuthal variations are not consistent with that expected for a crust with dipping Moho. Therefore we interpret the conspicuous RFs energy of the transverse component as convincing evidence of anisotropy. We present forward models obtained by fitting the radial and transverse RFs with synthetic seismograms calculated with a reflectivity code. Our analysis suggests substantial variations in the structure of the shallow lithosphere along the Baja California Peninsula and the presence of stalled fragments of the extinct Farallon plate beneath its central part. The results at station NE81 in Sonora are interpreted as frozen crustal anisotropy inherited from Miocene Basin and Range extension, consistent with previous observations of the upper mantle anisotropy beneath this station.