Deformation in the Lower Crust of the San Andreas Fault System in Northern California

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Abstract
A continuous seismic velocity and reflectivity cross-section of the San Andreas fault system in northern California shows offsets in the lower crust and the Mohorovic̆ić Discontinuity near the San Andreas and Maacama strike-slip faults. These faults may cut through the crust to the upper mantle in a zone less than 10 kilometers wide. The northern California continental margin to the eastern edge of the Coast Ranges is underlain by a high-velocity lowermost crustal layer that may have been emplaced within 2 million years following the removal of the Gorda plate slab. The rapid emplacement and structure within this layer are difficult to reconcile with existing tectonic models.