Abstract

The Sur-Nacimiento fault zone extends northwest through the southern and central Coast Ranges of California, and presumably continues offshore on the Continental Shelf. In part, or perhaps in its entirety, it forms the approximate boundary between the Franciscan trench(?) assemblage on the southwest and the granitic and regionally metamorphosed basement rocks of the Salinian block on the northeast. The Sur-Nacimiento fault zone includes the Sur fault zone, the Nacimiento fault, and a number of other faults of various kinds and various ages. There has been a long history of recurrent activity characterized by sequential changes in the types of movement.

The Sur fault zone is here considered to have originated at the former margin of the continent during latest Cretaceous or Early Tertiary time as the culmination of processes that operated during the entire Late Mesozoic. Throughout the latest Jurassic, Early Cretaceous, and mid-Cretaceous, Franciscan deposits were probably carried into a trench and pushed against or partly beneath the crust to the east, by ocean-floor spreading. The Franciscan assemblage has a mélange-like aspect which is appropriate to this kind of disturbance. Some rocks within the assemblage are about the same age (Early or mid-Cretaceous) as the adjacent Salinian granite, but they are unaffected by the granite. The postulated trench containing the Franciscan assemblage was probably far offshore during the time of granitic intrusion, but westward drift of North America narrowed the intervening distance. Conveyor-belt action of eastward-moving oceanic crust impinging against and beneath the edge of the continent may have ripped off and carried downward blocks of sialic crust. Perhaps this was the mechanism that eventually brought the Franciscan assemblage against the Salinian granite, creating the ancestral Sur fault zone.

The Franciscan terrain is locally tectonically covered by allochthonous masses of Great Valley-type Late Mesozoic clastic sedimentary rocks predominantly of two ages: (1) Tithonian-Valanginian, and (2) Campanian-Maestrichtian(?). It is tentatively suggested that these allochthonous rocks were deposited between the continent and the trench in which the Franciscan accumulated, and that they were underthrust by the Franciscan during the same action that produced the Sur fault, in latest Cretaceous or Early Tertiary time.

In the Tertiary, normal faulting occurred along the Sur-Nacimiento zone, following the annihilation of the active trench system. There are indications that strike-slip faulting and reverse faulting ensued at various times.

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