Tectonic Inheritance With Dipping Faults and Deformation Fabric in the Brittle and Ductile Southern California Crust

Abstract

Plate motions in Southern California have undergone a transition from compressional and extensional regimes to a dominantly strike‐slip regime in the Miocene. Strike‐slip motion is most easily accommodated on vertical faults, and major transform fault strands in the region are typically mapped as near vertical on the surface. However, some previous work suggests that these faults have a dipping geometry at depth. We analyze receiver function arrivals that vary harmonically with back azimuth at all available broadband stations in the region. The results show a dominant signal from contrasts in dipping foliation as well as dipping isotropic velocity contrasts from all crustal depths, including from the ductile middle to lower crust. We interpret these receiver function observations as a dipping fault‐parallel structural fabric that is pervasive throughout the region. The strike of these structures and fabrics is parallel to that of nearby fault surface traces. We also plot microseismicity on depth profiles perpendicular to major strike‐slip faults and find consistently NE dipping features in seismicity changing from near vertical (80–85°) on the Elsinore Fault in the Peninsular Ranges to 60–65° slightly further inland on the San Jacinto Fault to 50–55° on the San Andreas Fault. Taken together, the dipping features in seismicity and in rock fabric suggest that preexisting fabrics and faults may have acted as strain guides in the modern slip regime, with reactivation and growth of strike‐slip faults along northeast dipping fabrics both above and below the brittle‐ductile transition.

Crustal-scale listric geometry of the San Andreas Fault driven by lower crustal flow

“…The moderately dipping segment at the southern end was suggested to root in shallowly dipping fabrics in the ductile lower crust which was revealed by seismic anisotropic studies (Schulte-Pelkum et al, 2020). This observation does not conflict with our hypothesis.…”

“…A non-vertical strike-slip fault plane may be an inherited structure from previous moderately dipping fabrics in mid-lower crustal depth (Schulte-Pelkum et al, 2020), or caused by transpressional deformations with a large ratio (>~0.4) of normal to transcurrent displacement that drives fault rotation (Braun and Beaumont, 1995). In a transpressional stress regime, the Anderson theory of faulting predicts conjugate fault planes, but it is hard to discern which one is the master fault.…”

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

“…Receiver function arrivals show strong variations in arrival amplitude and polarity with backazimuth in our study region, and arrivals with a 360° dependence with backazimuth (first azimuthal harmonic) can be interpreted as conversions from contrasts in crustal seismic anisotropy with a plunging symmetry axis, such as dipping foliation (Brownlee et al., 2017; Porter et al., 2011; Schulte‐Pelkum, Ross, et al., 2020). Unlike splitting methods such as local event shear wave, SKS , or receiver function Moho P ‐to‐ S conversion ( Ps ) splitting analyses, this method is not a cumulative measurement of shear wave anisotropy and instead is sensitive to changes in P anisotropy (Bianchi et al., 2010; Levin & Park, 1998; Park & Levin, 2016).…”

“…Some methods are cumulative over the volume sampled (shear wave splitting and tomographic methods), while others are sensitive to the strength of contrasts (receiver function azimuthal conversions). A quantitative amplitude comparison would require forward modeling of specific anisotropic models, since the amplitude response depends on details of not only anisotropy strength, but also geometry and orientation, symmetry type, and layering (e.g., Becker, Schulte‐Pelkum, et al., 2006; Brownlee et al., 2017; Levin & Park, 1998; Schulte‐Pelkum, Ross, et al., 2020; Silver & Savage, 1994b; Xie et al., 2017, 2015). Another difficulty in linking deformation to anisotropic strength lies in the fact that there is no simple scaling of strain to anisotropy.…”

Crustal seismic structure and anisotropy of Madagascar and southeastern Africa using receiver function harmonics: interplay of inherited local heterogeneities and current regional stress

“…For that reason, and to facilitate the interpretation, we plotted the axis parallel to the foliation or the strike in case of a dipping interface, that is α + 90 • for degree-1 components. As argued by Schulte-Pelkum et al (2020), this plotting convention generally yields a better agreement with geological structures.…”