Paper No. 14
Presentation Time: 11:30 AM
DETRITAL ZIRCON GEOCHRONOLOGY OF FRANCISCAN METACLASTIC ROCKS FROM THE NACIMIENTO BLOCK, CENTRAL COASTAL CALIFORNIA
The Salinian block, principally underlain by Late Cretaceous granitoids of Sierra Nevada batholith affinity, and the Nacimiento block, consisting mainly of Mesozoic Franciscan Complex, are separated by the Nacimiento fault, one of the most puzzling features of California geology. By comparison with the Franciscan-Great Valley-Sierran triad east of the San Andreas fault, the juxtaposition of the Salinian and Nacimiento blocks along the Nacimiento fault implies the removal of a width of 150 km or greater of formerly intervening arc and forearc basin assemblages. While there is general agreement that major displacements along this structure were driven by Laramide shallow subduction and took place between 75 and 56 Ma, with late Cenozoic strike-slip remobilization linked to the San Andreas fault system, there is no consensus on the nature of Late Cretaceous to Paleocene slip along the fault. Existing hypotheses include: 1) the Nacimiento fault is a thrust with ~ 150 km offset that placed Cretaceous batholithic rocks above the Cretaceous forearc basin and subduction complex and 2) the Nacimiento fault is a sinistral tectonic escape structure with ~ 500 to 900 km offset that formed in response to collision of an aseismic ridge.
Detrital zircon geochronology of twelve Franciscan metasandstone samples from the Nacimiento block permits testing of these hypotheses as each predicts widely different provenance patterns. New U-Pb detrital zircon data show maximum depositional ages of 80 to 95 Ma for all Nacimiento block samples and prominent age groups of 80 – 120 Ma (55% of analyzed grains), 120 – 200 Ma (34%), 200 – 300 Ma (3%), 300 – 1000 Ma (1%), and 1000 – 2700 Ma (6%). Abundant Cretaceous (particularly Late Cretaceous) and diminishing amounts of Jurassic, Permian-Triassic, and Proterozoic zircon grains point to a Mojave Desert origin for Nacimiento block Franciscan protoliths. Furthermore, the suite of detrital zircon ages reported here bears a strong resemblance to new and existing data from schist exposures of the Mojave Desert region (Sierra de Salinas, Portal Ridge, Rand, San Emigdio, and Tehachapi localities). Deposition of Franciscan clastic metasediments outboard of the Mojave Desert region is most consistent with a dip slip origin for the Nacimiento fault.