DETRITAL ZIRCON GEOCHRONOLOGY OF THE TORO FORMATION, NACIMIENTO BLOCK, CENTRAL CALIFORNIA COAST

The California Mesozoic arc is commonly cited as the archetypal example of a convergent margin tectonic setting, although the juxtaposition of Nacimiento Block subduction mélanges and Salinian Block arc rocks across the enigmatic Sur–Nacimiento Fault in the central California coast serves as a reminder of the complexity of the Cordilleran Mesozoic forearc. Prior to Cenozoic slip along the San Andreas Fault, these central California coast terranes restore to a position near the present-day latitude of San Diego, although earlier slip along the Sur–Nacimiento Fault is poorly understood, and alternate models for its evolution call for either sinistral strike-slip, dextral strike-slip or thrust displacement. Here, we present detrital zircon ages from the Toro Formation as part of a broader study investigating the provenance of Nacimiento Block sediments that is designed to place new paleogeographic constraints on the evolution of the Sur–Nacimiento Fault.

The Late Jurassic–Early Cretaceous Toro Formation is dominated by mudstone interlayered with sandstone and rare conglomerate that rests unconformably above Jurassic ophiolite, and 4 sandstone samples were selected for detrital zircon geochronology by LA-ICPMS at the UC Santa Barbara. The youngest zircons from each sample indicate Tithonian and Valanginian maximum depositional ages that are consistent with fossil ages in adjacent outcrops, while Cordilleran-aged grains cumulatively define probability maxima at 140–150, and ~168 Ma, and a lesser peak at ~258 Ma. Pre-cordilleran zircons comprise ~60% of the analyzed grains, and broad probability peaks occur at 1.0–1.2, ~1.45, and 1.6–1.8 Ga. Toro Formation conglomerate clasts are dominated by chert, although quartzite and sandstone clasts are not uncommon; detrital zircon results from 7 individual conglomerate clasts reveal a suite of Triassic sandstones, and quartzites that are dominated by pre-Cordilleran ages with 1.0–1.2, 1.4 and 1.6–1.8 Ga ages. These initial results suggest that Late Jurassic and Early Cretaceous Nacimiento Block sediments were dominated by input from miogeoclinal source terranes in the continental interior and from the growing Middle–Late Jurassic Sierran/Salinian arc, and mixed with a minor, but measureable component derived from early Cordilleran arc terranes.