NEW CONSTRAINTS ON MODELS FOR FOREARC SHORTENING ALONG THE SUR–NACIMIENTO FAULT, CENTRAL CALIFORNIA COAST

The Sur–Nacimiento fault accommodated at least 150 km of latest Cretaceous–Paleocene forearc shortening along the central California coast, and represents an unexplained mode of forearc deformation. Here, we present U-Pb zircon ages from the Nacimiento block and the Orocopia Schist that place new constraints on existing sinistral slip and thrust models for displacement along the Sur–Nacimiento fault.

Detrital zircon spectra from Upper Jurassic–Lower Cretaceous Nacimiento forearc and trench strata display Jurassic–Early Cretaceous arc zircon mixed with zircon recycled from Neoproterozoic–Paleozoic miogeoclinal shelf sediments and overlying Mesozoic deposits. Detrital zircon spectra from Albian–Santonian Nacimiento strata are dominated by zircon from the Late Cretaceous arc. These results are most consistent with a Nacimiento block paleogeographic origin south of the San Joaquin Valley, in southern California or Baja California. These new paleogeographic data require that existing sinistral slip models for the Sur–Nacimiento fault be revised to orientations more oblique to the margin, with displacements <300 km. This revised model, however, lacks a clear tectonic mechanism for creating post-75 Ma sinistral displacement along the dextral-oblique North American convergent margin.

A southerly origin for the Nacimiento block is also compatible with thrust models for the Sur–Nacimiento fault, although the timing and fate of the underthrust Nacimiento block are unresolved. Zircon ages from the Orocopia Schist at Cemetery Ridge, Arizona, interpreted as trench sediments underplated beneath the Late Cretaceous arc, indicate that the schist protolith may be no younger than 70–65 Ma. Given the lack of forearc basin-derived protoliths within the Orocopia Schist, and related Pelona and Rand Schists, we suggest that thrusting on the Sur–Nacimiento fault could have post-dated schist emplacement, and buried the Nacimiento block beneath the previously underplated schists. Paleocene forearc thrusting could have occurred in response to Late Cretaceous low-angle subduction along the southern California margin, although uncertainties remain regarding the nature of potential structures that could have accommodated or reduced this shortening strain farther south within the Peninsular Ranges.