Abstract

We have investigated the fabric and the deformational processes of an exhumed subduction zone thrust active at seismogenic depths. The Rodeo Cove thrust zone, which outcrops north of the Golden Gate Bridge of San Francisco, imbricates two basalt-chert-sandstone sequences belonging to the Marin Headlands terrane (Franciscan Complex). The thrust outcrop is a 200-m-thick complex zone that displays a range of stratal disruption from incipient deformation to a broken formation in the central part of the outcrop, dominated by basaltic lithologies, where zones of concentration of deformation have been mapped. Disruption is made by variably dense discrete fault systems synthetic to the main thrust (R and P fractures). These faults are marked by cataclasites with a shaly matrix that shows a scaly foliation defined by chlorite and pumpellyite, which also constrain the depth of faulting (8–10 km, T = 200–250 °C) within the seismogenic zone.

The central part of the fault also features the densest system of carbonate-filled veins. Veins occur in the broken formation matrix and fragments, in both cases parallel to the foliation. The veins are either folded, truncated, or pressure-solved along the cleavage. Cementation and hardening of shear surfaces of the fault core may have caused the distribution, as opposed to localization, of subsequent slip events. The fault core may have developed in basaltic rocks because of their inherently high permeability and propensity to transmit overpressure from deeper levels of the subduction zone.

Our analysis has shown that accretionary deformation is strongly controlled by injection of overpressured fluids occurring through systems of multiple dilatant fractures grossly parallel to the décollement zone. The crosscutting relationships between veining and foliation suggest that fluid injection is cyclic and, consequently, that large transient variations in permeability and cohesion may occur. The repeated injection of veins parallel to the fault zone may be explained by cyclic changes of the stress, or by difference in tensional strength parallel to and perpendicular to the foliation, both of which would require extremely high fluid pressure.

We interpret the features of the Rodeo Cove thrust zone as evidence of the seismic cycle and hypothesize a compressional stress field in the interseismic phase and an extensional stress field in the immediately postseismic phase.

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