GEOLOGY | Volume 44 | Number 7 | www.gsapubs.org 575 Rapid variation in upper-mantle rheology across the San Andreas fault system and Salton Trough, southernmost California, USA Shahar Barak* and Simon L. Klemperer Department of Geophysics, Stanford University, 397 Panama Mall, Stanford, California 94305, USA ABSTRACT We present new shear-wave splitting data showing systematic lat- eral variations in upper-mantle anisotropy across the plate boundary in southernmost California (USA). Beneath the Peninsular Ranges batholith, fast polarization directions parallel the direction of former Farallon subduction, suggestive of a slab remnant. Near the eastern edge of the batholith, across the Elsinore fault, fast polarization direc- tions change rapidly to align with the direction of San Andreas fault shear. We infer that the Elsinore fault penetrates the entire lithosphere and may represent a future localization of the plate boundary that is migrating west from the San Andreas fault. Beneath the Salton Trough and the Chocolate Mountains region, large splitting times, despite a very thin lithosphere, imply vertical melt pockets in the uppermost mantle aligned in the shear direction. Largest splitting times, ~1.2 s, are seen closest to the Sand Hills fault that projects southeast from the San Andreas fault. Further east, in the southern Basin and Range prov- ince, fast directions align with North America absolute plate motion. INTRODUCTION Analysis of teleseismic shear-wave splitting is a standard tool for study- ing upper-mantle anisotropy created by strain-induced lattice-preferred orientation of minerals or by preferentially oriented melt-filled inclusions, and hence also for studying changes in rheology (e.g., Savage, 1999, and references therein). A shear wave passing through an anisotropic medium splits into slow and fast waves with orthogonal polarizations. Two splitting parameters (polarization, ϕ, and delay time, δt ) provide a direct estimate of the axis and magnitude of the anisotropy for simple cases and show systematic variations with back-azimuth to the source earthquake in more complex scenarios. Despite Southern California’s (USA) complex tectonic history and active plate boundary oriented northwest-southeast (e.g., Dickinson, 2008, 2009; Barak et al., 2015) (Fig. 1A), previous studies of shear- wave splitting in Southern California (e.g., Polet and Kanamori, 2002; Kosarian et al., 2011) show a nearly uniform fast axis of anisotropy ori- ented approximately west-east (Fig. 1C). This has been interpreted as due to inherited North America plate motion (Kosarian et al., 2011), pre–late Cenozoic compression (Polet and Kanamori, 2002), or mantle flow around the southern edge of the subducting Gorda slab (Zandt and Humphreys, 2008). The same general west-east pattern continues to the southern tip of Baja California (Mexico), west and east of the Gulf of California rift margins (Long, 2010; Fig. 1A). Recent three-dimensional (3-D) tomo- graphic inversion of shear-wave splitting measurements, despite the pre- vious scarcity of data in southernmost California, has begun to suggest complicated structure, including a northwest fast axis of anisotropy in the Salton Trough (ST) (Monteiller and Chevrot, 2011) where Gulf of California ocean spreading propagates into continental crust along the San Andreas fault (Elders et al., 1972). Here we analyze shear-wave splitting measurements on our array of 48 seismometers that spanned southernmost California from A.D. 2011 to GEOLOGY, July 2016; v. 44; no. 7; p. 575–578 | Data Repository item 2016190 | doi:10.1130/G37847.1 | Published online 15 June 2016 © 2016 Geological Society of America. For permission to copy, contact editing@geosociety.org. 32°N 33°N 34°N PAC(HS) NAM(HS) PAC(NAM) 10 mm/yr 1.0 s 0 117°W 116°W 115°W 114°W -113˚ 32°N 33°N 34°N SHF AF IF SJF SAF LSF EF CPF SAFs USA Mexico { USA Mexico AZ CA PRB CMR ST BRP Pacific Ocean ECSZ PAC Gulf of California A B C 20 40 Diff. from SAFs [°] M7.2 N N N Figure 1. A: Pacific (PAC)–North America (NAM) plate boundary (black—transform; orange—spreading center), showing location of B and C (black rectangle), and shear-wave splitting (SWS) fast polari- zations further south in Mexico (Long, 2010) using length scale and color scale as in C. B: Shaded-relief topography overlain with our seismic stations (stars), other permanent stations shown in Figure 2 (triangles), and line of section in Figure 2 (white line). BRP—south- ern Basin and Range province; CMR—Chocolate Mountains region; PRB—Peninsular Ranges batholith; ST—Salton Trough. Dot-patterned orange boxes show active spreading centers (Elders et al., 1972). Strike-slip focal mechanism: 4 April 2010 El Mayor–Cucapah earth- quake. Thick red lines show major faults of San Andreas fault system (SAFs) (AF—Algodones; CPF—Cerro Prieto; EF—Elsinore; IF—Impe- rial; LSF—Laguna Salada; SAF—San Andreas; SHF—Sand Hills; SJF—San Jacinto). Thin red lines show southeast extension of Eastern California Shear Zone (ECSZ) (Darin and Dorsey, 2013). C: SWS fast polarizations with lengths proportional to delay time (colored bars). This study: white circles (open circles if no “good” or “fair” results were available); other SWS results: Wüstefeld et al. (2009); Liu et al. (2014; only “A” quality data, weighted by their standard deviation). Color of bars indicates difference in degrees from trend of SAFs. Black arrows are plate-motion vectors relative to fixed-hotspot (HS) frame (Gripp and Gordon, 2002). Red line is 55 km depth contour of lithosphere-asthenosphere boundary (Lekic et al., 2011). Thin black lines are fault traces (Plesch et al., 2007). *Current address: Noble Energy, Houston, Texas 77070, USA.