GEOMETRY OF THE LATE CRETACEOUS SOUTHERN SIERRA NEVADA DETACHMENT SYSTEM: A COMPARISON OF PRE-SAN-ANDREAS-FAULT PALEOGEOGRAPHIC RECONSTRUCTIONS

The Salinian Block consists of Mesozoic granitoids, Paleozoic framework rocks of the Sur Series, Cenozoic sedimentary cover, and accreted schist that is bounded by the San Andreas Fault on the east. Previously-published work has shown the southern Sierra Nevada as the probable origin for these out-of-place rocks, with westward transport occurring as the hanging wall of a Late Cretaceous to Paleocene detachment system that resulted from gravitational collapse of the southern Sierra Nevada. Our recent U-Th/He apatite and thermochronology from the northern Salinian Block at Point Reyes, Ben Lomond, and Montara Mountain suggests that this detachment system may have consisted of multiple detachment surfaces, active at different times.

While the general process of transport of hanging wall slices from east to west is understood, the details are highly dependent on pre-San-Andreas-Fault paleogeography. Here we compare two sets of Late Cretaceous reconstructions that differ in the amount of offset along the Pilarcitos Fault. In the first, which has large offset on this fault, the northern Salinian Block is juxtaposed against the southern San Joaquin Valley. In the second, which has minimal offset along this fault, the San Francisco Bay Block is interposed between the northern Salinian Block and the southern San Joaquin Valley.

If the northern Salinian Block originated in the southern Sierra Nevada, the geometry of the second model would require that the northern Salinian Block (Santa Cruz northward) was transported over the Franciscan Complex rocks that make up most of the San Francisco Bay Block as one or more detachment sheets. This may be similar to the recently proposed interpretation of the Nacimiento Fault as a detachment fault. We explore possible evidence for this history.