Wrench faulting in northern California and its tectonic implications

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Abstract
The Plio‐Pleistocene Prairie Creek group of sediments lies unconformably on Mesozoic Franciscan Assemblage rocks in north coastal California, 120 km north of the Mendocino triple junction. The sediments are pervasively fractured and cut by three NNW trending faults, collectively called the Grogan‐Lost Man fault zone. These sediments contain a record of Quaternary deformation in an area of the California coast otherwise devoid of young sedimentary cover rocks. Detailed field mapping and structural analysis reveal a pattern of conjugate fracturing, folding, and outcrop‐scale faulting that strongly suggests the sediments have been deformed by wrench faulting. The spatial relationship of structures is typical both of other known regions with well‐documented wrench faulting and of structural patterns produced by simple shear modeling of wrench faults. The three major faults diverge slightly from expected fault trends based on the wrench fault model because the faults follow the structural grain of underlying Franciscan basement rocks. Because of the orientation of these inherited faults with respect to the Quarternary strain field, wrench faulting is accompanied by convergence. Field exposures, which show that two of the faults have high‐angle reverse separation, confirm that offset includes vertical as well as horizontal components. Net Quaternary right‐lateral displacement cannot be determined because of poor exposure and erosional stripping of the cover sediments, but it is probably less than 5 km. The Grogan‐Lost Man wrench faults are interpreted as the northernmost part of the San Andreas transform system. We propose that northward movement of the San Andreas transform boundary will entail a shift of major San Andreas type fault activity to this northernmost right‐lateral fault system. This shift will coincide with the northward migration of the Mendocino triple junction to a position close to where the Grogan‐Lost Man wrench faults intersect the continental margin.