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ABSTRACT

The Upper 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 and basement are missing. The most likely explanation for this absence is that forearc and western arc assemblages were removed through a combination of surface and tectonic erosion that accompanied Laramide shallow subduction. Petrographic analysis of rounded to subrounded 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 grains derived from these clasts yield generally Late Jurassic to Early Cretaceous U-Pb ages and positive Hf isotopic values, with one sample yielding a Late Cretaceous age and a negative Hf value. These relations strongly suggest that the analyzed clasts experienced subduction zone metamorphism and were derived principally from the western and axial Sierra Nevada batholith, with possible additional input from forearc basement (the Coast Range ophiolite). The presence of western arc–derived detritus in the Las Tablas unit suggests that surface plus tectonic erosion removed a significant amount of these units and incorporated them into the subduction complex. Granitic clasts of the Las Tablas unit were likely introduced into previously subducted and exhumed Franciscan materials by sedimentary rather than tectonic processes.

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