CENOZOIC TECTONIC HISTORY OF THE NORTHERN SALINIAN BLOCK FROM LOW-TEMPERATURE THERMOCHRONOLOGY

The Salinian block of California consists of Mesozoic granitoids intruding into Paleozoic miogeoclinal sedimentary rocks overlain by Cenozoic sedimentary rocks. Traditionally the Salinian basement rocks have been considered to have been displaced from the southern Sierra Nevada batholith by simply northward transport along the San Andreas Fault. Recently, it has been proposed that the Salinian block underwent a more complex history and was first removed toward the southwest from the southern Sierra Nevada as the upper plate of a detachment complex before being offset along the San Andreas Fault.

Our recent work in the Salinian block north of Monterey Bay, at Point Reyes, Montara Mountain, and Ben Lomond, has yielded zircon U/Th-He ages of 70-77 Ma and apatite U/Th-He ages spanning a broad range from 10-58 Ma. Zircon U/Th-He ages of these units are about 10 Myr younger than corresponding ages from the southern Sierra Nevada, testifying to a late exhumation event, while the large distribution of apatite U/Th-He ages may suggest burial and exhumation events linked to northward transport along the San Andreas Fault.

Here, we speculate on the mechanisms that may have led to a tectonic history that diverges from that of southern Sierra Nevada. The late exhumation event in the southern Sierra Nevada was likely driven by a different tectonic process from the Late Cretaceous exhumation event that took place in the southern Sierra Nevada. While several processes may explain the range of apatite U/Th-He ages, we speculate that evolution of releasing-restraining bends and step-overs may have led to repeated uplift and erosion and subsidence and burial cycles during northward transport along the San Andreas Fault.