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Poster #176, SCEC Community Models (CXM)

Oblique-rifting and evolution of southern California fault systems

Mark R. Legg, & Marc J. Kamerling
Poster Image: 

Poster Presentation

2020 SCEC Annual Meeting, Poster #176, SCEC Contribution #10634 VIEW PDF
Oblique-rifting has produced complex faulting and ridge and basin systems along the southwestern North America continental margin. Predominant NW-trending structure parallels the relative motion of the Pacific Plate away from the former North America subduction margin. Oblique rift systems evolved along the continental margin, from low-angle detachment and core complexes to high-angle pull-apart rhombochasms and eventual seafloor spreading. Evidence of this process is manifest in the California Continental Borderland, Owens Valley-Death Valley, and Gulf of California regions. Similar morphology of these oblique extensional regions may represent self-similarity of a chaotic process which may ...be used to predict deep structure relevant to earthquake potential. Major features include NW-trending right-slip faults that bound major crustal domains. Within these domains are zones of NE-trending conjugate faults, typically left-slip with oblique components that may originate in extension and reactivate in convergence during basin inversion. Other geophysical data provide deep crustal and upper mantle lithospheric information. A major difference between the three regions is the crustal thickness, which varies based on the amount of extension achieved during late Cenozoic activity. Within all three regions, oblique-rifting persists on active transtensional fault systems, while in areas adjacent to the Transverse Ranges and San Andreas big bend, post-Miocene transpression and microplate collision overprint the Neogene extensional structure. Uplift of former deep extensional basins enables direct examination in outcrop, and exploration wells and boreholes provide samples to confirm interpretations from geophysical data and tectonic models. Subsequent deformation often uses the major boundary faults for basin inversion and crustal block rotation above detachments. New faults may form if existing structures are unfavorably oriented for the new stress and strain fields. Existing transtensional boundary faults may proceed through phases of parallel slip into transpressional zones inducing structural inversion of Miocene basins. New transform faults may accommodate the strike-slip while older faults may accommodate oblique slip or get bypassed and abandoned. Many faults remain active at reduced strain rates. Existing detachments at crustal depths link arrays of steep strike-slip faults to produce large, complex, multi-fault earthquake rupture scenarios.