SPATIAL PROVENANCE TRENDS IN THE LATE CRETACEOUS-EOCENE CALIFORNIA FOREARC: IMPLICATIONS FOR MARGIN TECTONISM AND PALEOGEOGRAPHY

Upper Cretaceous-Eocene forearc strata along the California continental margin record a complex history of plate convergence that shaped the tectonic development of the U.S. Cordillera. Synthesis of existing and new detrital zircon U-Pb age distributions over a 1,800 km length of the southern Oregon-California-northern Baja forearc clearly demonstrates spatial and temporal changes in sedimentary provenance and drainage networks that accompanied the extinction of continental margin arc magmatism and transfer of deformation to the continental interior during latest Cretaceous-early Cenozoic Laramide flat-subduction. Measured age distributions from Cenomanian through Campanian forearc strata reflect the existence of a drainage divide formed by a high-standing mid-Cretaceous Cordilleran arc that effectively shielded the forearc from interior sediment source regions. Details of the age distributions were primarily controlled by the inherited crustal structure of the margin and the relative level of dissection and erosion of the adjacent volcano-plutonic arc segments. Maastrichtian through middle Eocene strata preserved south of the Sierra Nevada record a shift from local to extraregional provenance caused by the development of drainages that extended across the breached mid-Cretaceous continental margin batholith to tap the continental interior. As a result, measured age distributions in this region reflect the distribution of sediment sources interior to the mid-Cretaceous arc. Comparison with time-equivalent backarc strata indicates a transition from distinct to shared source regions following topographic destruction of the mid-Cretaceous arc. We infer that regional-scale spatial and temporal trends in forearc provenance reflect variability in subduction regime, particularly the angle of subduction and depth to the subducting slab, which resulted in the margin undergoing net accretion or subduction erosion/tectonic underplating. Thus the character of convergent-margin tectonism profoundly influenced margin topography and sediment dispersal patterns within the forearc.