DETRITAL ZIRCON GEOCHRONOLOGY AND GEOCHEMISTRY OF NACIMIENTO BLOCK FOREARC STRATA: TRACKING THE EVOLUTION OF CONVERGENT MARGIN LITHOTECTONIC BELTS DURING CORDILLERAN HIGH- FLUX MAGMATISM

Convergent margin lithotectonic belts display non-steady-state processes characterized by periods of variable magmatic flux in the arc, variable depositional rates in the forearc basin, and accretion and non-accretion in the subduction complex, although how events in these lithotectonic belts may be linked is debated. Here, we present detrital zircon geochronology and geochemistry results from the Nacimiento forearc basin to characterize southern California convergent margin events during the Cretaceous high-flux magmatic event.

Our results reveal Mesozoic detrital zircon populations differentiated by age and geochemistry that define two depositional periods with variable provenance from the Albian to Campanian. Lithic-rich Albian–Cenomanian strata yield late Early Cretaceous and Jurassic arc zircon mixed with a distinct population of Jurassic low U/Yb zircon characteristic of juvenile igneous rocks. Arkosic lower Cenomanian–Santonian sandstone is dominated by arc zircon < 102 Ma that is progressively enriched in Hf vs. U/Yb plots.

Our results indicate the end of a regional hiatus in the Nacimiento forearc by 108 Ma that shortly followed increasing magmatic flux rates in Cordilleran batholiths from 120–110 Ma. Although the ages and geochemistry of juvenile Jurassic zircon in Cenomanian sandstone permit derivation from the foothills metamorphic belt in the western Sierra Nevada, the lack of Early Cretaceous zircon in these samples argues against a foothills source. Thus, we suggest that juvenile zircon in Cenomanian sandstone was derived from the Coast Range ophiolite, and indicates a period of forearc contraction, uplift and erosion. Over the same time period within the arc, zircon geochemical trends are consistent with gradual eastward arc migration and its ultimate emplacement in enriched continental crust by 102 Ma. The appearance of more arkosic sand in the Nacimiento forearc is indicative of arc uroofing by 96 Ma, and coincides with the disappearance of juvenile forearc sources and the onset of accretion within the subduction complex. These results highlight differences between the style of deformation in the southern and northern California late Mesozoic forearcs, and have implications for how lithotectonic belts interact with each other during convergent margin deformation.