DETRITAL ZIRCON GEOCHRONOLOGY AND GEOCHEMISTRY OF NACIMIENTO BLOCK FOREARC STRATA: IMPLICATIONS FOR THE PALEOGEOGRAPHY OF THE LATE MESOZOIC SOUTHERN CALIFORNIA MARGIN

The Nacimiento block was located in southern California prior to San Andreas slip, and accurate descriptions of its tectonic evolution and paleogeographic origin are central to reconstructions of the southern California margin. We present coupled detrital zircon geochronology and geochemistry measured by LA-ICPMS from 18 samples of Nacimiento forearc basin sandstone to characterize its depositional and provenance evolution.

Our results reveal Mesozoic detrital zircon populations differentiated by age and geochemistry that define three depositional periods with variable provenance. Lithic-rich Valanginian sandstone yields zircon derived from geochemically diverse Jurassic (175–145 Ma) and Early Cretaceous (135–144 Ma) arc rocks. Lithic-rich Albian and Cenomanian strata display bimodal age distributions with late Early Cretaceous zircon mixed with increasingly abundant Jurassic zircon. In the Cenomanian when Jurassic zircon was most abundant, sandstone yields 45% 97–110 Ma arc zircon, 16% 110–130 Ma arc zircon, 2% Early Cretaceous arc zircon, 18% Jurassic arc zircon and 19% Jurassic low U/Yb zircon characteristic of juvenile igneous rocks. Arkosic sandstone with late Cenomanian to Santonian maximum depositional ages is dominated by enriched continental arc zircon < 102 Ma.

These results lead us to two conclusions. First, although the ages and geochemistry of Jurassic low U/Yb zircon in Cenomanian sandstone permit derivation from juvenile rocks in the foothills metamorphic belt in the western Sierra Nevada, the negative correlation between Early Cretaceous arc zircon and low U/Yb zircon in Valanginian and Cenomanian sandstones argues against a foothills source. Thus, we tentatively suggest that low U/Yb zircon was derived from the Coast Range ophiolite, and that uplift and exhumation of the Nacimiento outer forearc may have occurred as early as the Albian. Second, the appearance of more arkosic sands and younger, enriched arc zircon in the Nacimiento forearc by 96–85 Ma is indicative of rapid exhumation of eastern arc rocks adjacent to the Nacimiento block at this time. This pattern of arc exhumation is more consistent with thermal histories in the Peninsular Ranges than in the Sierra Nevada, and supports a paleogeographic origin for the Nacimiento block south of the southern San Joaquin basin.