Late Cenozoic Basins of northern California

Abstract

The late Cenozoic basins of northern California developed in response to both convergent tectonics associated with subduction of the Farallon plate (and its modern representatives, the Juan de Fuca and Gorda plates) and transform tectonics associated with northward migration of the Mendocino triple junction and formation of the San Andreas fault system. The modern Eel River basin north of the Mendocino triple junction is an active forearc basin located between the Cascade magmatic arc to the east and the trench at the foot of the continental slope of northern California and Oregon to the west. Five different types of late Cenozoic basins or fragments of basins are preserved in onshore northern California south of the Mendocino triple junction: (1) remnants of a formerly more extensive Neogene forearc basin preserved locally in downdropped blocks within the San Andreas fault system; (2) remnants of slightly older trench‐slope basins that generally developed west of but which may locally structurally underlie the younger forearc basin; (3) younger strike‐slip‐related structural basins that have developed along active right‐lateral faults of the San Andreas fault system; (4) broad shallow embayments, perhaps similar to the modern San Francisco Bay, that were connected to the deeper Pacific Ocean to the west; and (5) structurally emplaced remnants of oceanic crust and its overlying sedimentary cover (the King Range terrane). Our preliminary stratigraphic and sedimentologic studies suggest that much of northern California was covered during the Neogene by a forearc basin that may have extended as far south as the San Francisco Bay region and into the southern San Joaquin Valley. As the Mendocino triple junction migrated northward during the late Cenozoic, the southern margin of the forearc basin was uplifted, basin deposits were stripped off by erosion, and the locus of forearc sedimentation shifted progressively northward through time. Preserved but isolated fragments of the forearc basin provide a partial record of the original paleogeography and tectonic framework of northern California during the late Cenozoic.