Seismic reflection and refraction data from the Coast Ranges of central California west of the San Andreas fault suggest the presence of a low-velocity (4.5–5.0 km/s) wedge at a depth of about 15–22 km that dips east beneath rocks of both the Franciscan assemblage and the Salinian block. We propose that the wedge represents sedimentary materials that were subducted in an offshore trench within the past 23 m.y. The refraction data also suggest the presence of a second low-velocity zone that extends at least to mid-crustal depths beneath the San Andreas fault zone.

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