T036-02
Inherited tectonic grain in southern California inferred from receiver functions and seismicity and possible effects on present-day deformation

Friday, 11 December 2020: 10:36
Virtual
Vera Schulte-Pelkum, University of Colorado at Boulder, Cooperative Institute for Research in Environmental Sciences, Boulder, CO, United States, Zachary E. Ross, California Institute of Technology, Seismological Laboratory, Pasadena, CA, United States, Karl J Mueller, University of Colorado Boulder, Geological Sciences, Boulder, CO, United States and Yehuda Ben-Zion, University of Southern California, Department of Earth Sciences and Southern California Earthquake Center, Los Angeles, CA, United States
Recorded Presentation
Abstract:
We present heterogeneity and tectonic fabric at a regional lithospheric scale observed using receiver functions and microseismicity. Our findings have implications for brittle faulting and deformation at plastic depths.

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 these faults have a dipping geometry at depth. We analyze receiver function arrivals that vary harmonically with backazimuth at all available broadband stations in the region. The results show a dominant signal from contrasts in dipping foliation as well as dipping contrasts in isotropic velocity from all crustal depths, including from the ductile middle to lower crust. We interpret these receiver function observations as a dipping fault-parallel structural grain 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, with damage zones and seismicity asymmetric to the NE of the surface trace of the fault. Some of the dipping planar features in microseismicity coincide with arrivals from dipping contrasts in receiver functions from the same location and depth.

We interpret these observations as indicating that southern California contains a regional fabric. The regional fabric includes aligned shear zones and contrasts between isotropic bodies, i.e., a regional-scale tectonic grain that is likely formed by older dipping fabrics and interfaces related to earlier episodes of compressional or extensional tectonics as well as the present-day region-wide deformation regime. The pervasive nature of such fabric in southern California implies that rather than considering individual shear zones and rheological boundaries, a useful approach may be to impose a preferential orientation (e.g., anisotropy of viscosity) when modeling deformation processes on a lithospheric scale.

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