Detailed marine geophysical surveys of the inner California Continental Borderland west of northern Baja California show that the region is underlain by two major northwest-trending Quaternary dextral wrench fault systems. The San Clemente fault system lies along the western part of the inner borderland and comprises the San Clemente and San Isidro fault zones. Together, these fault zones connect to form a long (>300 km), narrow (<5 to 10 km), continuous zone of shear similar to the longer San Andreas transform system onshore. The Agua Blanca fault system is a complex northwest-trending zone of dextral shear delineated by three or more subparallel wrench fault zones in the eastern part of the inner borderland. The westernmost, San Diego Trough-Bahia Soledad fault zo e, consists of relatively long (~50 km), continuous main fault traces which cut the Quaternary sediments of the nearshore basin trough. The Coronado Bank-Agua Blanca fault zone is more complicated, with numerous discontinuous, subparallel, right- and-left-stepping, en echelon and anastomosing fault traces which are associated with substantial structural relief. A nearshore zone of faulting, marked by the Estero-Descanso fault zone in the south and the Newport-Inglewood-Rose Canyon fault zone in the north, parallels the coast and defines the eastern boundary of the California Continental Borderland structural province. All of these eastern fault zones merge into the transpeninsular Agua Blanca fault, and their N30°W trend differs

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significantly (>20°) from the trend of the major Peninsular Ranges fault zones. The ridge-and-basin physiography of the inner borderland and transtension evident along the Agua Blanca fault system show that dextral oblique rifting has dominated the late Cenozoic tectonic style of the inner California Continental Borderland. Systematic differences in the post-Miocene style of deformation, with extension in the south, right-slip in the center, and convergence in the north, imply that Transverse Ranges convergence is affecting inner borderland tectonism. Historical earthquake activity shows that the inner borderland is an active part of the present-day Pacific-North American plate boundary, with focal mechanisms generally consistent with the tectonism inferred from the geologic st ucture. Counterclockwise rotation of a semirigid "Southern California Shear Zone," which is the splintered northern end of the long, narrow, Baja California microplate, explains the observed patterns of deformation within the region. Because significant right-slip may occur to the west of Baja California along the San Clemente fault system, and does not cross the peninsula to the Gulf of California transform system, estimates of Pacific-North American relative plate motion based on sea-floor-spreading rates at the mouth of the Gulf of California may be in error.