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