Dipping fault structures near the brittle-ductile transition and deep foliation fabric in southern California

Vera Schulte-Pelkum, Zachary E. Ross, Karl J. Mueller, & Yehuda Ben-Zion

Published August 14, 2018, SCEC Contribution #8605, 2018 SCEC Annual Meeting Poster #159

We investigate fault structure and shear zone fabric across the brittle-ductile transition zone in Southern California using the earthquake catalog of Hauksson et al. (2012) and seismic anisotropy results from receiver functions. Depth profiles of seismicity across major transform faults in the area of the Elsinore Fault and major branches of the San Jacinto and San Andreas Faults show seismicity on consistently NE-dipping planes with a listric appearance, with dip shallowing from subvertical near the surface down to ~45° near the brittle-ductile transition. A possible regional trend is from steeper dips (80-85°) near the coast (along the Elsinore Fault) to shallower dips going inland towards the Northeast (San Jacinto and San Andreas faults). Another striking observation are dipping planar seismicity features at mid-crustal levels that do not connect to seismicity or mapped fault traces at the surface, appearing as blind dipping faults in a strike-slip regime within the recorded time span. Generally much younger damage zones accompanying the major faults are also located on the northeastern side of each major surface trace.

An independent constraint on fabric near and below the brittle-ductile transition is given by anisotropic receiver functions. We use azimuthally varying conversions from anisotropy contrasts, not splitting of a converted shear phase. First-order observables are strikes of dipping foliation contrasts and dominance of dipping versus vertical or horizontal foliation. In Southern California, the anisotropic signal in the upper seismogenic and deeper ductile crust is from dipping foliation contrasts despite subvertical fault traces mapped at the surface and presumed subhorizontal schists emplaced in the lower crust during the Laramide, and is consistent with the listric patterns seen in seismicity.

Strike-slip motion on dipping faults is not optimally oriented relative to the current state of stress in Southern California, nor are the transform-orthogonal seismicity planes and shear fabric seen in our observations. Our initial interpretation is that inherited structures thus exert control on present-day deformation. This is supported by geological interpretations of the development of transform faults that dip at depth where they have reactivated older east-dipping extensional faults, such as the Western Salton Detachment influencing the present-day geometry of the Elsinore and San Jacinto faults.

Key Words
faults; shear zones; inheritance; seismicity; receiver functions; anisotropy

Citation
Schulte-Pelkum, V., Ross, Z. E., Mueller, K. J., & Ben-Zion, Y. (2018, 08). Dipping fault structures near the brittle-ductile transition and deep foliation fabric in southern California. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)