CHARACTER AND EVOLUTION OF COMPLEX STRIKE-SLIP FAULTING WITHIN THE CALIFORNIA CONTINENTAL BORDERLAND

Evolving geometry of pre-existing faults, structures, and rheology may develop into systems capable of complex rupture as illustrated by recent earthquakes. Simple models of strike-slip faults are insufficient for earthquake hazard assessment of such systems. Active strike-slip fault zones in the submarine Borderland exemplify complex fault structure produced over 20 million years of transform plate boundary evolution. Progressive deformation during plate boundary evolution, from subduction to oblique-rifting and finally transform faulting, involve complex changing boundary conditions. While conjugate shears, folds, and extensional fractures of simple models resemble observed surface features of strike-slip faults, structural evolution may generate more complex fault geometry at depth. Features include flower structures, oblique slip and low-angle strike-slip faults, contractional pop-ups and extensional pull-aparts at fault bends and stepovers. Changing rheology during oblique-extension with increased heat flow and magma injection further complicates the evolution of the Borderland strike-slip fault zones. Plate boundary relative motion changes due to microplate capture and progressive changes in initial fault geometries accommodate the evolving oblique shear in complex ways. New transform faults may form within extensional basins to accommodate the strike-slip while half-graben basins form to accommodate extension. Uneven or jagged plate boundaries formed during triple junction migration and microplate tectonics produced vertical-axis crustal block rotations. Rotating blocks become more obstructive to the dextral shear of the plate boundary to produce transpression; former extensional basins are inverted into fault-bounded anticlinoria. Local oblique-deformation intensifies, and new faults form to bypass these obstacles to smooth transform motion. Progressive north-to-south changes in plate boundary character along the California continental margin have preserved various stages of this deformation in different domains within the Borderland. We present examples which illustrate the complex nature of strike-slip deformation spanning the Borderland from the Western Transverse Ranges to the Southern Borderland offshore northern Baja California.