Abstract

Onshore and offshore geologic evidence suggests a two-stage evolution of the California borderland that began near the end of the Oligocene Epoch. Both stages are inferred to have been caused by northwestward movement of the Pacific plate relative to the American plate; during the first, the plate boundary in southern California trended more northerly than the relative motion; during the second, more westerly. During the first stage, the Pacific plate extended to a line projected from the Vizcaino Peninsula of Baja California, through the California borderland, along the Salinian block, and thence to the east side of the Sierra Nevada block. In the borderland and in the Basin and Range province, dilation led to basin formation and Miocene volcanism. During the second stage, which still continues, the Pacific plate extends to the San Andreas fault zone, carrying batholithic rocks of the wedgelike Salinian block northwestward to their present position. Basement rock of the southern half of the Salinian block was partly derived from the central and eastern part of the Sierra Nevada block; Franciscan rocks from north of the Vizcaino Peninsula were juxtaposed against batholithic rocks both of the Salinian block and in the source area of the San Onofre Breccia; and Upper Cretaceous strata of the central valley of California, the northern California coast, and the Vizcaino Peninsula were originally continuous. The anomalous orientation of the Transverse Ranges resulted from the wide divergence in the trend of the plate boundary in southern California between the two stages.