ASSEMBLING THE WORLD’S TYPE SHALLOW SUBDUCTION COMPLEX: DETRITAL ZIRCON GEOCHRONOLOGIC CONSTRAINTS ON THE ORIGIN OF THE NACIMIENTO BLOCK, CENTRAL CALIFORNIA COAST RANGES

Temporal and spatial patterns in the architecture of the Franciscan Complex provide valuable insights into the subduction processes through which such patterns arise. The Nacimiento Franciscan belt represents a sliver of subduction assemblages displaced either from southern California by San Andreas-related motion alone or from central California by >500 km of trench-linked sinistral motion followed by Neogene shuffling. New U-Pb detrital zircon data from 19 (meta)clastic samples indicate that the Nacimiento Franciscan section was assembled between ca. 95 and 80 Ma. Abundant Cretaceous (particularly Late Cretaceous) and diminishing amounts of Jurassic and Proterozoic zircon grains point to a southern California origin for Nacimiento Franciscan protoliths, precluding significant strike-slip along the Sur-Nacimiento fault. Furthermore, the suite of detrital zircon ages reported here bears a strong resemblance to new and existing data from subduction complexes in southern California that were emplaced during Laramide shallow subduction (i.e., Sierra de Salinas, Portal Ridge, Quartz Hill, Rand, San Emigdio, and Tehachapi schists). Hence, the Nacimiento Franciscan is distinct from “classic” Franciscan rocks and more likely represents an outboard element of the Late Cretaceous southern California low-angle subduction system. Upon restoring the Nacimiento block to its Late Cretaceous position, an inboard younging trend is apparent in the composite Nacimiento-schist belt, suggesting that progressively younger accretionary materials were underplated further inboard by tectonic erosion. We posit that arc and forearc elements absent from southern California were removed by a combination of physical and tectonic erosion attending shallow subduction, interleaved in the subduction complex, and recycled into the mantle.