An Anisotropic Contrast in the Lithosphere Across the Central San Andreas Fault

Abstract

Seismic anisotropy of the lithosphere and asthenosphere was investigated with a dense broadband seismic transect nearly orthogonal to the central San Andreas fault (SAF). A contrast in SK(K)S splitting was found across the SAF, with a clockwise rotation of the fast orientation~26°closer to the strike of the SAF and greater delay times for stations located within 35 km to the east. Dense seismograph spacing requires heterogeneous anisotropy east of the SAF in the uppermost mantle or crust. Based on existing station coverage, such a contrast in splitting orientations across the SAF may be unusual along strike and its location coincides with the high-velocity Isabella anomaly in the upper mantle. If the Isabella anomaly is a fossil slab fragment translating with the Pacific plate, the anomalous splitting east of the SAF could indicate a zone of margin-parallel shear beneath the western edge of North America. Plain Language SummaryDirectional dependence of seismic wave speeds, referred to as anisotropy, can illuminate preferred orientations or fabrics in the Earth organized by deformation. Seismic anisotropy near the sharply defined central segment of the San Andreas fault was investigated with a new dense temporary seismic transect. A contrast in uppermost mantle anisotropy across the fault was identified, with nearly fault parallel orientations only on the east side of the fault. We suggest that development of asymmetric anisotropy about the central San Andreas may arise due to fault-parallel movement of a fossil slab beneath the western edge of North America.

Tectonic Inheritance During Plate Boundary Evolution in Southern California Constrained From Seismic Anisotropy

“…Whether and how this mantle circulation, mostly due to sinking of the formerly subducting slab into the mantle, may control the present‐day near‐surface stress state remains an open question (Chamberlain et al., 2014; Ghosh et al., 2013; Humphreys & Coblentz, 2007; Kosarian et al., 2011). Short‐wavelength perturbations to SKS likely reflect influence from shallower, lithospheric structure (e.g., Bonnin et al., 2010; Jiang et al., 2018; Savage et al., 2004), but may also be associated to smaller length scale convection than what was considered by Becker, Schulte‐Pelkum, et al. (2006) and/or reorientation of olivine fabrics (e.g., Wang & Becker, 2019; Zhou et al., 2018).…”

“…Alignment with shear as expected from absolute plate motions does not match the observed orientations well (Bonnin et al., 2010; Silver & Holt, 2002), but modeling of plate motion and density‐driven mantle flow, without any deep shear localization at the plate boundary, captures SKS patterns on scales of ≳200 km (Becker, Schulte‐Pelkum, et al., 2006). Nonetheless, small‐scale variations in teleseismic splitting across the SAF are observed (Jiang et al., 2018; Özalaybey & Savage, 1995; Savage et al., 2004), and the role of the SAF in affecting SKS splitting remains debated (e.g., Bonnin et al., 2010; Savage et al., 2004). Depth‐dependent splitting studies show SAF‐parallel fast orientations in the lithosphere on the northern part of the study area (north of the Garlock fault; Figure 1), although solutions may be ambiguous (Hartog & Schwartz, 2001; Monteiller & Chevrot, 2011).…”

Tectonic inheritance during plate boundary evolution in southern California constrained from seismic anisotropy

“…Alignment with shear as expected from absolute plate motions does not match the observed orientations well (Bonnin et al, 2010;Silver & Holt, 2002), but modeling of plate motion and density-driven mantle flow, without any deep shear localization at the plate boundary, captures SKS patterns on scales of ≳200 km . Nonetheless, small-scale variations in teleseismic splitting across the SAF are observed (Jiang et al, 2018;Özalaybey & Savage, 1995;Savage et al, 2004), and the role of the SAF in affecting SKS splitting remains debated (e.g., Bonnin et al, 2010;Savage et al, 2004). Depth-dependent splitting studies show SAF-parallel fast orientations in the lithosphere on the northern part of the study area (north of the Garlock fault; Figure 1), although solutions may be ambiguous (Hartog & Schwartz, 2001;Monteiller & Chevrot, 2011).…”

“…Whether and how this mantle circulation, mostly due to sinking of the formerly subducting slab into the mantle, may control the present-day near-surface stress state remains an open question (Chamberlain et al, 2014;Ghosh et al, 2013;Humphreys & Coblentz, 2007;Kosarian et al, 2011). Short-wavelength perturbations to SKS likely reflect influence from shallower, lithospheric structure (e.g., Bonnin et al, 2010;Jiang et al, 2018;Savage et al, 2004), but may also be associated to smaller length scale convection than what was considered by and/or reorientation of olivine fabrics (e.g., Wang & Becker, 2019;Zhou et al, 2018).…”

Crustal Deformation in Southern California Constrained by Radial Anisotropy From Ambient Noise Adjoint Tomography

“…Seismic studies conducted in Southern California provide evidence both for and against the block model. On one hand, distinct crustal velocity contrasts are observed across the SAF (Fang et al, 2018; Hauksson, 2000; Lee et al, 2014; Tape et al, 2009; Wang et al, 2018; Zhao et al, 1996; Zigone et al, 2015), which are accompanied by abrupt changes in the Moho depth (Berg et al, 2018; Qiu et al, 2019; Yan & Clayton, 2007; Zhu & Kanamori, 2000), lithospheric thickness (e.g., Ford et al, 2014), and anisotropic signatures (Barak & Klemperer, 2016; Jiang , Schmandt, & Clayton , 2018). The above lines of evidence are consistent with strong localization of deformation along the SAF.…”