Abstract

In 1977, COCORP recorded a short, seismic reflection test line across the San Andreas fault through Parkfield, California, where the Salinian crustal block is juxtaposed against a contrasting terrane of Franciscan and overlying material. Although analysis of this seismic line has been limited by crooked geometry, lack of cross-lines, complex terrain, possible recording noise, and perhaps most of all, its short length, gross crustal features are discernible after extensive reprocessing and reanalysis. The Salinian block southwest of the San Andreas consists of a maximum of 2 km of relatively underformed sediments overlying a reflective and inhomogeneous basement which exhibits deep, subhorizontal layering. At 8 to 10 km depth, density and amplitude of reflections markedly decay, perhaps corresponding to the base of a Salinian “granitic” crust which overlies Franciscan or other contrasting material. Northeast of the San Andreas, subhorizontal reflections suggest that Franciscan material may extend as deeply as 13 km, below which is a nonreflective zone which might represent relict oceanic crust. Reflections from Moho depths differ across the San Andreas fault. Beneath the Salinian block is a thick zone of reflections beginning at about 22 km depth, while less layered and weaker reflections seem to define the Moho discontinuity at 29 km beneath the Franciscan terrane. The San Andreas fault zone appears to be traceable as a distinct feature on the reflection section throughout the entire crust, forming a near-vertical zone about 3 km wide. In the seismogenic upper crust (down to 15 km), the fault zone is marked by sharp structural truncations and short reflection or diffraction segments. At depth, it seems to be distinguished by a nonreflective column. This vertical change in reflection character may correspond to the transition from a brittle to a ductile deformation regime.

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