Abstract Study of shear zones and associated basins within an oblique rift can shed light on the development of a young transform continental margin. San Pedro Basin lies within the Inner Borderland Rift offshore southern California, where it is bisected by the San Pedro Basin Fault (SPBF). Based on seismic reflection and multibeam bathymetry data, we show that the SPBF attained continuity with the San Diego Trough Fault (SDTF) between 1 Ma and 800 ka, to form a 350-km-long shear zone. Prior to that time, the SDTF was linked to the Catalina Fault, forming a restraining bend that contributed to the uplift of Catalina Ridge. Seismically defined depositional sequences in San Pedro Basin record a multistage history of uplift and subsidence for the basin. Young, flat-lying sequences filling a sigmoidal depocenter indicate that subsidence has been occurring since about 1 Ma. This date is corroborated by a series of submarine lowstand depositional terraces surrounding Santa Catalina Island. A 5 Ma to 1 Ma progressively tilted sequence, onlapped by the flat-lying strata, is confined to the present basin. Folded sequences older than ca. 5 Ma extend beyond the present basin onto Catalina Ridge and are correlated to Mohnian and Luisian strata on Santa Catalina Island and Palos Verdes Peninsula. From these data, we interpret the growth history of San Pedro Basin to involve at least three successive, nested basins. The first, which we call the “San Pedro (SP) protobasin,” formed before 5 Ma and was of indeterminate size, including within its boundaries areas flanking the current basin that were subsequently uplifted, Catalina Ridge and Palos Verdes Anticlinorium. Between 5 Ma and 1 Ma, approximately, a second basin, nested within the first, formed as the two flanking structural highs initiated. Finally, a third basin, nested within the first two, began to form when the SPBF–SDTF link was established and rapid local subsidence began; this is the depocenter of the current San Pedro Basin, and its southwestern boundary is occupied by the trace of the SPBF. Our model of basin formation begins with the initial oblique Inner Borderland (IB) Rift, which formed during rotation and translation of crustal blocks away from the continental margin (about 20 Ma). The IB Rift was segmented due to preexisting structural configurations. Published reconstructions show that the SP protobasin was originally in a narrow zone flanked by active volcanoes. Continued extension widened and deepened the rift, while volcanism continued along the flanks of the rift until about 10 Ma. As the rift widened, nested basins formed within the original protobasins along the axis of the rift. These basins were later fragmented (after 5 Ma for the SP protobasin) by transpressive processes associated with the shift of the transform plate boundary to the southern San Andreas Fault. New nested basins also formed during this time as shear zones reorganized to shortcut restraining geometries.

Abstract Much about the offshore California Continental Borderland remains unknown, despite its location off the most populated area of the west coast of North America, vital importance to US interests, and potential for seismic and tsunamigenic hazards. In 2015 and 2016, Exploration Vessel (E/V) Nautilus of the Ocean Exploration Trust undertook a series of seven expeditions in that region. During those cruises, we acquired 13,075 km 2 of multibeam bathymetric data, performed 58 dives with remotely operated vehicles (ROV) Hercules and Argus , totaling 500+ hours underwater (1000+ hours of video), and collected 532 geological and biological samples across this 200-km-wide, tectonically active region. Because Nautilus is equipped with state-of-the-art telepresence technology, numerous scientists based on shore actively participated in these expeditions in real time. Here, we describe the data acquired during these expeditions and present some very preliminary results. The digital data and physical samples are accessible to the scientific community via online request systems.

Abstract Continental transform plate boundaries are broad, composed of numerous active and subparallel strike-slip fault zones. Irregular geometry along the major transform structure creates convergence and divergence zones within the plate boundary where other strike-slip faults terminate. Some prominent irregularities result from microplate interactions. Relative fault displacement, diminishing to zero at fault terminations, must be accommodated or transferred to other structures, laterally or vertically, away from the fault end-point. Distinct styles of strike-slip fault termination may represent different degrees of vertical strain partitioning within the plate boundary. The Western Transverse Ranges (WTR) of California mark a major structural discontinuity that cuts at high angle across the Pacific–North America transform boundary. Within the California Continental Borderland, two end-member classes of right-slip fault termination against the WTR are apparent. (1) Several major faults, including the San Clemente, San Pedro Basin, Ferrelo and Newport–Inglewood, intersect the southern boundary of the WTR at high angles, with negligible to minor local deflection and minor dissipation of right shear at the Earth’s surface. These faults are inferred to cut through the entire borderland crust and continue in the lower crust beneath the WTR, as evident in geophysical data. These ‘blind’ near vertical faults may control segmentation and earthquake activity on the overlying west-trending WTR structures. (2) In contrast, the Palos Verdes and possibly Whittier faults appear broadly deflected westward to merge at low angle with WTR structure. NW-trending faults rotate counterclockwise, away from the axis of principal shortening as observed in pure shear models, and slip is dissipated through folding, thrust transfer and rotation. Deflected faults are inferred to be predominately upper crustal features, detached from the lower crust and unable to underthrust the WTR. These two distinct right-slip fault termination styles, and associated convergent structures, suggest that basal shear drives vertical-axis rotation of the WTR block over the underthrust Inner Borderland plate. Furthermore, the lower plate, slivered by these right-slip faults, is incompletely coupled with the Pacific plate.

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.