Abstract

Geologic, geophysical, and paleomagnetic data suggest that major lithologic belts which now make up the outer California Continental Borderland and western Transverse Ranges were disrupted from their original geographic and tectonic setting off northern Baja California. Disruption began in the early Miocene and is closely linked to a change in relative plate motions from subduction to right-lateral strike-slip (transform) motion along the southern California—Baja California continental margin.

A model is proposed which restores these lithologic belts to their original tectonic setting within an earlier arc-trench system. The model is based on the following assumptions: (1) major lithologic belts in southern California, northwestern Baja California, and the California Continental Borderland are equivalent in age, lithology, and structure to the Franciscan and Great Valley lithologic belt of northern and central California; (2) the Franciscan and Great Valley belts and their southern equivalents, respectively, represent the subduction complex and forearc basin deposits of a Late Jurassic to mid-Cenozoic arc-trench system; (3) the borderland off southern California contains paired Franciscan and Great Valley belts that are repeated in map view, and the western Transverse Ranges are made up largely of Great Valley or forearc basin deposits which have undergone large-scale clockwise rotation. The model is supported by areal distributions of lithofacies, paleocurrent trends, structural trends, and paleomagnetic data from Miocene volcanic rocks. If valid, the model proposed here will require major reconsiderations of earlier lithologic correlations and Paleogene and Neogene paleogeographic and paleotectonic models of southern California and the offshore borderlands.

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