Accommodating Oblique Plate Motion in the California Continental Borderland: The Interaction Between Rotating and Non-Rotating Domains

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Understanding continental structure and plate boundary deformation is an important problem in earth science. This project is part of a coordinated effort to conduct integrated studies of crustal structure and deformation in the Continental Borderland offshore of southern California. The Borderland was the locus of Pacific-North America plate motion for about 70% of its displacement history, and recent GPS data suggest that up to 20% of current plate motion is still occurring offshore. Much of this plate motion has been carried by predominantly NW-striking right-slip faults that terminate against the southern front of the rotating western Transverse Ranges province. How this dextral motion is transformed into clockwise rotation at fault terminations, and the fault and fold geometry at these transitions are little understood. These active offshore structures also represent a largely as yet unknown hazard to many California coastal communities, and provide important analogs to active buried (less-accessible) onshore structures that may produce large, damaging earthquakes in the Los Angeles basin area.This project is using grids of high-quality industry multichannel seismic reflection data (originally collected for hydrocarbon exploration) to investigate the crustal deformation and tectonic evolution of the offshore portion of the plate boundary. Digital fault surfaces and stratigraphic reference horizons are being produced to define active fault and fold geometry in 3D, and to help quantify the finite strain field with time. A prime objective is to document how the crustal architecture offshore accommodates plate boundary transpression, differential rotation, spatial variations in fault slip and the termination of faults. This includes a better understanding of how active faults interact to partition oblique motion on systems of intersecting high- and low-angle structures. Our results are complementing land-based studies and provide a more synoptic view of the active plate boundary, including how, within such a diffuse plate boundary system, two distinct tectonic regimes (the rotating western Transverse Ranges province and the non-rotating northern Continental Borderland) interact to absorb the same overall plate motion.