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

Abstract

[1] We calculated SKS splitting parameters for the California Integrated Seismic Network. In southern California, we also estimated splitting in the upper 100 km using azimuthal anisotropy determined from surface waves. The inferred splitting from surface waves in the mantle lithosphere is small (on average < 0.2 s) compared with SKS splitting (1.5 s) and obtains a maximum value (0.5 s) in the transpressive region of the Big Bend, south of, and aligned with, the San Andreas Fault (SAF). In contrast, the SKS splitting is approximately E-W and is relatively uniform spatially either side of the Big Bend of the SAF. These differences suggest that most of the SKS splitting is generated much deeper (down to 300-400 km) than previously thought, probably in the asthenosphere. Fast directions align with absolute plate motions (APM) in northern and southeastern California but not in southwestern California. We interpret the parallelism with APM as indicating the SKS anisotropy is caused by cumulative drag of the asthenosphere by the overlying plates. The discrepancy in southwestern California arises from the diffuse boundary there compared to the north, where relative plate motion has concentrated near the SAF system. In southern California the relative motion originated offshore in the Borderlands and gradually transitioned onshore to the SAF system. This has given rise to smaller displacement across the SAF (160-180 km) compared with central and northern California (400-500 km). Thus, in southwestern California the inherited anisotropy, from prior North American APM, has not yet been overprinted by Pacific APM.Citation: Kosarian, M., P. M. Davis, T. Tanimoto, and R. W. Clayton (2011), The relationship between upper mantle anisotropic structures beneath California, transpression, and absolute plate motions,

Anisotropy from SKS splitting across the Pacific-North America plate boundary offshore southern California

“…This has led to the belief that SKS splitting is associated with regions shallower than ∼400 km (Becker et al 2006). Previous SKS analyses in southern California by Kosarian et al (2011) andBecker et al (2012), however, found that SKS splitting directions align with the North America APM (WSW-ENE), including offshore where one might expect a transition to Pacific APM (Becker et al 2006;Kosarian et al 2011;Becker et al 2012). This suggests that anisotropy is not due entirely to lithospheric drag on the asthenosphere (Kosarian et al 2011).…”

“…When more recent studies are considered, however, this explanation seems unlikely. Kosarian et al (2011) found EW fast direction SKS splitting results on both sides of the big bend of the San Andreas fault, whereas their lithosphere surface wave analysis produced azimuthal anisotropy aligned with the San Andreas fault. These differences suggest that most of the SKS splitting is generated deeper than the lithosphere.…”

“…It has been proposed that the Pacific APM west of the San Andreas fault simply has not yet had time to overprint the effects of North America APM. In central California, a tendency to align with Pacific plate motion west of the San Andreas fault has been observed (Kosarian et al 2011), suggesting that if APM is the cause, more finite strain has developed north of the big bend of the San Andreas fault than in southern California where the strain has been more diffuse. In that case the effective plate boundary may be offshore, beyond which splitting would align with Pacific APM.…”

“…In onshore southern California, the majority of SKS splitting measurements show fast polarization directions that are approximately EW across a wide zone in southern California (e.g. Savage et al 1990; Silver & Savage 1994;Liu et al 1995;Ö zalaybey & Savage 1995;Polet & Kanamori 2002;Kosarian et al 2011;Monteiller & Chevrot 2011), even west of the San Andreas fault where recent geodetic (GPS) monitoring shows relative Pacific plate surface movement to the northwest.…”

“…SKS splitting has no depth resolution in and of itself, and therefore the depth of generation of anisotropy is uncertain since Rayleigh wave anisotropic studies generally see smaller effects than those detected by splitting (West et al 2009;Kosarian et al 2011;Yuan & Beghein 2013). The ALBACORE project presented a new opportunity to constrain the source of anisotropy since it is the first SKS splitting study to extend across the plate boundary between the North America and Pacific plates.…”