Lithosphere, Published Online on 23 March 2011 As Doi:10.1130/L126.1
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Lithosphere, published online on 23 March 2011 as doi:10.1130/L126.1 Lithosphere Pervasive lower-crustal seismic anisotropy in Southern California: Evidence for underplated schists and active tectonics Ryan Porter, George Zandt and Nadine McQuarrie Lithosphere published online 23 March 2011; doi: 10.1130/L126.1 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Lithosphere Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. 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Citations to Advance online articles must include the digital object identifier (DOIs) and date of initial publication. © 2011 Geological Society of America Lithosphere, published online on 23 March 2011 as doi:10.1130/L126.1 RESEARCH Pervasive lower-crustal seismic anisotropy in Southern California: Evidence for underplated schists and active tectonics Ryan Porter1*, George Zandt1*, and Nadine McQuarrie2* 1DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF ARIZONA, GOULD-SIMPSON BUILDING #77, 1040 E. 4TH STREET, TUCSON, ARIZONA 85721, USA 2DEPARTMENT OF GEOSCIENCES, PRINCETON UNIVERSITY, GUYOT HALL, PRINCETON, NEW JERSEY 08544, USA ABSTRACT Understanding lower-crustal deformational processes and the related features that can be imaged by seismic waves is an important goal in active tectonics. We demonstrate that teleseismic receiver functions calculated for broadband seismic stations in Southern California reveal a signature of pervasive seismic anisotropy in the lower crust. The large amplitudes and small move-out of the diagnostic converted phases, as well as the broad similarity of data patterns from widely separated stations, support an origin primarily from a basal crustal layer of hexagonal anisotropy with a dipping symmetry axis. We conducted neighborhood algorithm searches for depth and thickness of the anisotropic layer and the trend and plunge of the anisotropy symmetry (slow) axis for 38 stations. The searches produced a wide range of results, but a dominant SW-NE trend of the symmetry axis emerged. When the results are divided into crustal blocks and restored to their pre–36 Ma locations, the regional-scale SW-NE trend becomes more consistent, although a small subset of the results can be attributed to NW-SE shearing related to San Andreas transform motion. We interpret this dominant trend as a fossilized fabric within schists, created from top-to-the-SW sense of shear that existed along the length of coastal California during pretransform, early Tertiary subduction or from shear that occurred during subsequent extrusion. Comparison of receiver-function common conversion point stacks to seismic models from the active Los Angeles Regional Seismic Experiment shows a strong correlation in the location of anisotropic layers with “bright” refl ectors, further affi rming these results. LITHOSPHERE Data Repository 2011138. doi: 10.1130/L126.1 INTRODUCTION S-wave velocity within layered crust and mantle (e.g., Zandt et al., 1995; Zhu and Kanamori, 2000). More recently, the RF technique has been According to a growing body of geologic and tectonic evidence, applied to the identifi cation of anisotropic zones in the crust, which are the crustal blocks that constitute the crust of southern and central Cali- then used to infer the location and mechanism for previous and current fornia west of the San Andreas fault are largely the scattered remnants deformation (e.g., Levin and Park, 1997; Savage, 1998; Peng and Hum- of a once-continuous magmatic arc lithosphere that was delaminated at phreys, 1997; Sherrington et al., 2004; Ozacar and Zandt, 2004, 2009). mid- to lower-crustal levels by Laramide shallow fl at subduction and The motivation for this study is the observation of consistent move-out subsequently underplated by schists derived from the adjacent accretion- patterns within Southern California indicative of dipping layers or seis- ary trench complex (e.g., Saleeby, 2003; Ducea et al., 2009). The end of mic anisotropy. Southern California is an ideal location for such a study Laramide fl at subduction was marked by oceanward extensional collapse due to the abundance of geologic and geophysical studies in the region, of the delaminated crust and partial exhumation of the underplated schists the availability of numerous long-running (>4 yr) seismic stations with (Saleeby, 2003). Post-Laramide tectonic evolution of the region then led to publicly available data, and well-constrained crustal structure and defor- further crustal fragmentation and the transfer of some crustal blocks to the mational history. This well-constrained geologic history helps to address Pacifi c plate, in some cases accompanied by large-magnitude vertical-axis one of the fundamental diffi culties in any seismic analysis, i.e., the lack of rotation and long-range lateral transport (Nicholson et al., 1994). If this temporal information regarding observations. This is especially important hypothesis is correct, large portions of the crust in southern and west-cen- in anisotropy analyses, which give no indications as to whether obser- tral California should be underlain by schists that were overprinted with a vations refl ect previous deformation or modern tectonics. By interpret- fl attening strain during underplating. In this paper, we describe a seismic ing our new results in conjunction with previous work in the region, we investigation based on the receiver-function (RF) method to search for and expect to better constrain the deformational history of the lower crust and characterize lower-crustal seismic anisotropy that would be expected to improve our understanding of both the regional tectonics and the receiver- delineate such a regional-scale fabric. function anisotropy technique. Teleseismic receiver functions have been used for roughly the past 30 yr (e.g., Langston, 1979) to better understand crustal structure through GEOLOGIC AND TECTONIC SETTING the identifi cation of major impedance contrasts and the ratio of P- to Our study area in Southern California, shown in Figure 1, is composed *E-mails: [email protected]; [email protected]; nmcq@ of two tectonic plates and multiple tectonic terranes with complex geo- princeton.edu. logic histories. Here, we will generally follow the terrane terminology LITHOSPHEREFor permission to copy, contact [email protected] | © 2011 Geological Society of America Lithosphere, published online on 23 March 2011 as doi:10.1130/L126.1 PORTER ET AL. 121°W 120°W 119°W 118°W 117°W 116°W 36°N PKD 36°N Sierra Nevada Garlock fault San Andreas Eastern Mojave PHL fault Desert TUQ Salinian GSC 35°N block Western Mojave RRX HEC 35°N MPP Desert EDW2 Eastern California OSI LKL VCS VTV shear zone San Gabriel block JVA Western Transverse SBPX BBR TOV DEC CHF Ranges MWC BFS 40°N PASC SVD BLA 34° DJJ BEL 34°N PAS San Bernardino Nevada Utah RPV San Jacinto 38°N Continental KNW Ranges California FMP fault Borderland RDM WMC CIU CRY TRO 36°N SND CIA Peninsular SNCC Arizona Range YAQ 34°N 33° 33°N -121° 119°W 118°W 117°W 116°W 32°N Pelona, Orocopia, and Rand schist exposures (locations from Luffi et al., 2009; Grove et al., 2003) 30°N 122°W 118°W 114°W 110°W Cross section (Figure 9) Figure 1. Map of the study area showing station locations (circles) and the approximate boundaries of the crustal blocks used in this study. Colors correspond to assigned crustal block. Station assignments are listed in Table 1. White stations are unassigned. Stations with black X’s are shown in Figures 5A and 5B. Major faults are shown as red lines, and schist outcrops are shaded magenta. Fault locations are from U.S. Geological Survey and Arizona Geological Survey (2008). and tectonic reconstruction of the region compiled by McQuarrie and Arizona (Fig. 1) (Jacobson et al., 1996; Grove et al., 2003; Ducea et al., Wernicke (2005). Because of the importance of the tectonic history to the 2009). Blocks currently attached to the Pacifi c plate include the Salinian interpretation of crustal anisotropy, a more detailed description of each of block, the Western Transverse Ranges block, the San Gabriel block, the these terranes is provided in Appendix A. continental borderlands, and the Peninsular Ranges block. On the North The San Andreas fault is a transform plate boundary separating the American plate, there are the Mojave block, separated into western and Pacifi c and North American plates; it grew from offshore precursors fol- eastern domains by the Eastern California shear zone, the San Bernardino lowing the sequential cessation of subduction near Southern California block, and the Eastern Transverse Ranges block (Fig. 1). starting at ca. 30 Ma (Atwater, 1989). The Sierra Nevada and Peninsular The Salinian block is composed of multiple terranes that, in conjunc- Ranges are pieces of relatively intact batholithic upper crust.