SEARCH FOR THE LOST ARC: A U-PB ZIRCON GEOCHRONOLOGIC AND ISOTOPIC STUDY OF THE LAS TABLAS UNIT, FRANCISCAN COMPLEX OF CENTRAL CALIFORNIA

The Late Cretaceous Las Tablas unit of the Franciscan Complex, a conglomerate-breccia containing a diverse array of clasts, is located in the central California Coast Ranges. The Las Tablas unit was originally deposited in southern California, where significant amounts of the western half of the Sierra Nevada batholith and coeval Great Valley forearc basin are missing. This absence has prompted two possible explanations: (1) forearc and western arc assemblages were displaced by sinistral strike slip motion along the Sur-Nacimiento fault and then relocated to central California by dextral strike slip motion along the San Andreas and related faults or (2) a combination of surface and tectonic erosion removed the forearc and western arc during Laramide shallow subduction. Petrographic analysis of gabbro, quartz diorite, tonalite, granodiorite, and andesite clasts from the Las Tablas unit reveals a prehnite-pumpellyite grade overprint of primary igneous textures. Furthermore, zircon derived from these clasts yield Late Jurassic to Early Cretaceous U-Pb ages and positive Hf isotopic values. These relations strongly suggest that the analyzed clasts experienced subduction zone metamorphism and were derived from both the forearc basement (the Coast Range ophiolite) and the western Sierra Nevada batholith. Recognition of forearc and western arc-derived detritus in the Las Tablas unit suggests that surface plus tectonic erosion removed significant amounts of these materials and incorporated them in the subduction complex. The new petrographic, geochronologic, and isotopic data presented here support the surface plus tectonic erosion hypothesis for removal of western arc domains in southern California and is at odds with the strike slip displacement model.