ABSTRACT
The continental borderland off the Mexican state of Baja California is lithologically diverse, with sedimentary rocks ranging in age from late Paleocene to Holocene predominating. Poway Conglomerate is widespread in this region. Its recognition extends the area over which the formation is known 290 km to the south and 240 km to the southwest. Metagraywacke similar to that of the Franciscan of Isla Cedros also has been dredged, suggesting the possibility of Franciscan basement under at least part of the Baja borderland. Basalts dated as recent as 1 m.y.B.P. are the youngest reported for the borderland. Young dates suggest that volcanic rocks have been superimposed on older sedimentary and crystalline rocks by a late period of volcanism and support a post-Miocene formation for the Baja borderland. Structurally, the borderland off Baja California is dominated by three troughs, each containing two or more basins and separated by ridges. The southern borderland is not downthrown relative to the northern borderland at the Santo Tomás fault as has been suggested by previous investigators; but, the borderland as a whole is a northeast-trending broad synclinorium. Baja Gap, Soledad Gap, the five gaps in the California borderland, and the major east-trending left-lateral Popcorn Ridge fault, which magnetic anomalies show extending into the Pacific plate, may represent a major system of faults conjugate to the predominant northwest, San Andreas trend. Lithology, structures, and timing and style of deformation suggest that accepted plate-tectonic models for the region are incomplete.
