Shear‐wave splitting observations at the regions of northern Baja California and southern Basin and Range in Mexico

Abstract

Recent installation of 3‐component broadband digital stations around the Gulf of California allowed us to make shear wave splitting observation from records of 73 SKS and SKKS phases from 30 events recorded at 12 stations since January of 2001. Stations in the southern Basin and Range province yield upper mantle fast shear wave direction from NE‐SW to E‐W, consistent with both local direction of Miocene extension and North American absolute plate motion. The shear at the Pacific‐North American plate limit seems to control tightly the anisotropy pattern of the closest station NE70. The anisotropic pattern along the Peninsular Range, east of the former trench between the North American plate and the now subducted Farallon plate, is almost uniform and is interpreted as asthenospheric flow induced by the sinking fragment of the Farallon plate although frozen anisotropy associated with in situ captured fragment of this plate cannot be ruled out.

The relationship between upper mantle anisotropic structures beneath California, transpression, and absolute plate motions

“…However, in west southern California the effect of the Big Bend causes the plate margin to be much more diffuse than further north. This contrast south to north, across the Big Bend, extends to Baja California where splitting analyses have obtained similar E-W fast directions to those in southern California [Obrebski et al, 2006;Obrebski and Castro, 2008]. We suggest that the mantle flow models [e.g., Becker et al, 2007aBecker et al, , 2007bSilver and Holt, 2002] are unlikely to have a sudden change across the Big Bend, and that the difference is due to the history of the plate tectonic interactions.…”

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

“…Upper crust anisotropy in the GoC region was first characterized in the Mexicali‐Imperial Valley by Zúñiga et al [1995] and González and Munguía [2003], who analyzed the birefringence that affects local S waves. Recently, the installation of broad band stations of the NARS‐Baja and RESBAN arrays have enabled the first studies of the anisotropy of the crust and upper mantle throughout the entire region of GoC using surface waves [ Zhang et al , 2006; Markee and Gaherty , 2006; Zhang et al , 2007] and also SKS waves [ Obrebski et al , 2006; Van Benthem et al , 2006].…”

Rayleigh wave dispersion measurements reveal low‐velocity zones beneath the new crust in the Gulf of California

“…Shear wave splitting measurements at high [ Obrebski et al ., ; van Benthem et al ., ] and low frequencies [ Long , ] show broadly consistent results. The influence of present‐day flow on the anisotropic fabric of the underlying mantle can be grouped into three categories, horizontal return flow away from the upwelling in the northern portion of the study region (NE70–NE72 in Figure ), near‐vertical mantle upwelling or nearly transform parallel flow in central Baja California (NE74–NE77), and more complex or insufficient data south of 25°N [ Long , ; Obrebski et al ., ; van Benthem et al ., ]. The overall pattern reveals complex upper mantle anisotropy with a considerable degree of both lateral and vertical heterogeneity, which appears to be controlled by localized processes like upwelling associated with partial melting [ Long , ].…”