We interpret seismic-reflection profiles to determine the location and offset mode of Quaternary offshore faults beneath the Gulf of Santa Catalina in the inner California Continental Borderland. These faults are primarily northwest-trending, right-lateral, strike-slip faults, and are in the offshore Rose Canyon–Newport-Inglewood, Coronado Bank, Palos Verdes, and San Diego Trough fault zones. In addition we describe a suite of faults imaged at the base of the continental slope between Dana Point and Del Mar, California. Our new interpretations are based on high-resolution, multichannel seismic (MCS), as well as very high resolution Huntec and GeoPulse seismic-reflection profiles collected by the U.S. Geological Survey from 1998 to 2000 and MCS data collected by WesternGeco in 1975 and 1981, which have recently been made publicly available. Between La Jolla and Newport Beach, California, the Rose Canyon and Newport-Inglewood fault zones are multistranded and generally underlie the shelf break. The Rose Canyon fault zone has a more northerly strike; a left bend in the fault zone is required to connect with the Newport-Inglewood fault zone. A prominent active anticline at mid-slope depths (300–400 m) is imaged seaward of where the Rose Canyon fault zone merges with the Newport-Inglewood fault zone. The Coronado Bank fault zone is a steeply dipping, northwest-trending zone consisting of multiple strands that are imaged from south of the U.S.–Mexico border to offshore of San Mateo Point. South of the La Jolla fan valley, the Coronado Bank fault zone is primarily transtensional; this section of the fault zone ends at the La Jolla fan valley in a series of horsetail splays. The northern section of the Coronado Bank fault zone is less well developed. North of the La Jolla fan valley, the Coronado Bank fault zone forms a positive flower structure that can be mapped at least as far north as Oceanside, a distance of ~35 km. However, north of Oceanside, the Coronado Bank fault zone is more discontinuous and in places has no strong physiographic expression. The San Diego Trough fault zone consists of one or two well-defined linear fault strands that cut through the center of the San Diego Trough and strike N30°W. North of the La Jolla fan valley, this fault zone steps to the west and is composed of up to four fault strands. At the base of the continental slope, faults that show recency of movement include the San Onofre fault and reverse, oblique-slip faulting associated with the San Mateo and Carlsbad faults. In addition, the low-angle Oceanside detachment fault is imaged beneath much of the continental slope, although reflectors associated with the detachment are more prominent in the area directly offshore of San Mateo Point. North of San Mateo Point, the Oceanside fault is imaged as a northeast-dipping detachment surface with prominent folds deforming hanging-wall strata. South of San Mateo point, reflectors associated with the Oceanside detachment are often discontinuous with variable dip as imaged in WesternGeco MCS data. Recent motion along the Oceanside detachment as a reactivated thrust fault appears to be limited primarily to the area between Dana and San Mateo Points. Farther south, offshore of Carlsbad, an additional area of folding associated with the Carlsbad fault also is imaged near the base of the slope. These folds coincide with the intersection of a narrow subsurface ridge that trends at a high angle to and intersects the base of the continental slope. The complex pattern of faulting observed along the base of the continental slope associated with the San Mateo, San Onofre, and Carlsbad fault zones may be the result of block rotation. We propose that the clockwise rotation of a small crustal block between the Newport-Inglewood–Rose Canyon and Coronado Bank fault zones accounts for the localized enhanced folding along the Gulf of Santa Catalina margin. Prominent subsurface basement ridges imaged offshore of Dana Point may inhibit along-strike block translation, and thus promote block rotation.

Abstract A series of 16 marine terraces and associated landforms and sedimentary deposits provides a record of Quaternary paleo-geographic and tectonic history of coastal San Diego County, California. The width of individual terrace platforms varies dramatically, from several kilometers in places where the platforms are cut into poorly lithified sedimentary strata, to a few meters or tens of meters where the platforms are cut in more resistant rocks. Beach ridges formed of dune sands lie just landward of most of the terrace shorelines. Radiometric ages and amino-acid racemization ratios, together with shoreline elevations, have been used to calculate average rate of uplift and to construct a chronology of the 16 shorelines, which range in age from 80 ka to perhaps as old as 1.29 ma. Shoreline-angle elevations suggest that nearly the entire length of coastal San Diego County has been uplifted at a rate of 0.13 to 0.14 m/ka during the Quaternary. Both higher and lower rates are recorded in areas deformed by the Rose Canyon fault zone. Changes in configuration of successive shorelines indicate that rocks on one side of the fault have been rising through middle and late Quaternary time and that the Rose Canyon fault has been active for at least the past 1 my.

Abstract 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 zone, 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 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 structure. 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.