DECOLLEMENT AT THE BASE OF THE SEISMOGENIC CRUST? [c5, p148]

A theme common to models of crustal convergence along the San Andreas fault system involves low-angle reverse slip on decollement surfaces near the base of the seismogenic crust (Wentworth and others, 1983; Webb and Kanamori, 1985; Namson and Davis, 1988; Eaton and Rymer, 1990). A natural extension of this theme leads to a view of the seismogenic crust as a conglomeration of relatively rigid blocks interacting by frictional slip along weak preexisting faults (block boundaries) in response to regional stresses transmitted through both the brittle crust and quasi-plastic deformation in the underlying lithosphere (Hill, 1982). However, the nature of a decollement surface at the base of the brittle crust and the relation of the seismogenic San Andreas fault system to the aseismic transform boundary in the underlying lithosphere remain speculative. It is not yet clear, for example, whether the San Andreas fault continues below the seismogenic crust as a narrow, near-vertical boundary (possibly offset a substantial distance from the seismogenic fault by slip on the horizontal decollement surface) that slips by quasi-plastic, mylonitic deformation or whether it broadens rapidly with depth into a wide shear zone spanning, say, the entire width of the Coast Ranges (see chap. 7 ; Sibson, 1983).