UTILIZATION OF A LARGE GEOCHEMICAL – GEOCHRONOLOGIC DATABASE TO DETERMINE THE PATERNITY AND WANDERINGS OF ORPHANED CONGLOMERATE CLASTS IN THE GUALALA BASIN, CALIFORNIA: IMPLICATIONS FOR PROVENANCE STUDIES AND TECTONIC RECONSTRUCTIONS IN THE NORTH AMERICAN CORDILLERA

Late Cretaceous and Paleogene conglomerates currently located in the Gualala Basin of coastal northern California have no local bedrock sources and have been displaced by at least hundreds of kilometers from their site of deposition by the San Andreas fault system. Paleomagnetic and paleontologic studies have suggested an origin as far south as Baja California. Additionally, the timing of conglomerate sedimentation and the basin’s palinspastically restored position prior to Neogene San Andreas system offsets allow for the possibility of conglomerate provenance in the hypothesized “Baja BC” terrane as it migrated northward. In order to discriminate between the wide range of potential conglomerate provenance terranes a large database was compiled, containing published geologic, geochemical, isotopic, and geochronological data for igneous rocks from throughout the Mesozoic Cordilleran magmatic arcs. Comparison of geochemical and isotopic data from Gualala conglomerates with the large Cordilleran dataset enabled definitive exclusion of some hypothesized source terranes. Moreover, although no single clast type in this study was able to be uniquely matched to a source pluton to form a “piercing point”, comparison of the full datasets was instrumental in narrowing the most likely source terrane for the Gualala clasts to the formerly contiguous southwestern Sierra – northern Salinian – western Mojave region. The Mesozoic Cordilleran dataset (mostly plutonic rocks) compiled for this study is currently being added to the NAVDAT database and has potential to be utilized by future provenance studies in the Cordilleran region.