Abstract The Queen Charlotte Fault defines the Pacific–North America transform plate boundary in western Canada and southeastern Alaska for c. 900 km. The entire length of the fault is submerged along a continental margin dominated by Quaternary glacial processes, yet the geomorphology along the margin has never been systematically examined due to the absence of high-resolution seafloor mapping data. Hence the geological processes that influence the distribution, character and timing of mass transport events and their associated hazards remain poorly understood. Here we develop a classification of the first-order shape of the continental shelf, slope and rise to examine potential relationships between form and process dominance. We found that the margin can be split into six geomorphic groups that vary smoothly from north to south between two basic end-members. The northernmost group (west of Chichagof Island, Alaska) is characterized by concave-upwards slope profiles, gentle slope gradients (<6°) and relatively low along-strike variance, all features characteristic of sediment-dominated siliciclastic margins. Dendritic submarine canyon/channel networks and retrogressive failure complexes along relatively gentle slope gradients are observed throughout the region, suggesting that high rates of Quaternary sediment delivery and accumulation played a fundamental part in mass transport processes. Individual failures range in area from 0.02 to 70 km 2 and display scarp heights between 10 and 250 m. Transpression along the Queen Charlotte Fault increases southwards and the slope physiography is thus progressively more influenced by regional-scale tectonic deformation. The southernmost group (west of Haida Gwaii, British Columbia) defines the tectonically dominated end-member: the continental slope is characterized by steep gradients (>20°) along the flanks of broad, margin-parallel ridges and valleys. Mass transport features in the tectonically dominated areas are mostly observed along steep escarpments and the larger slides (up to 10 km 2 ) appear to be failures of consolidated material along the flanks of tectonic features. Overall, these observations highlight the role of first-order margin physiography on the distribution and type of submarine landslides expected to occur in particular morphological settings. The sediment-dominated end-member allows for the accumulation of under-consolidated Quaternary sediments and shows larger, more frequent slides; the rugged physiography of the tectonically dominated end-member leads to sediment bypass and the collapse of uplifted tectonic features. The maximum and average dimensions of slides are an order of magnitude smaller than those of slides observed along other (passive) glaciated margins. We propose that the general patterns observed in slide distribution are caused by the interplay between tectonic activity (long- and short-term) and sediment delivery. The recurrence (<100 years) of M > 7 earthquakes along the Queen Charlotte Fault may generate small, but frequent, failures of under-consolidated Quaternary sediments within the sediment-dominated regions. By contrast, the tectonically dominated regions are characterized by the bypass of Quaternary sediments to the continental rise and the less frequent collapse of steep, uplifted and consolidated sediments.

Abstract Multibeam echosounder (MBES) images, 3.5 kHz seismic-reflection profiles and piston cores obtained along the southern Queen Charlotte Fault Zone are used to map and date mass-wasting events at this transform margin – a seismically active boundary that separates the Pacific Plate from the North American Plate. Whereas the upper continental slope adjacent to and east (upslope) of the fault zone offshore of the Haida Gwaii is heavily gullied, few large-sized submarine landslides in this area are observed in the MBES images. However, smaller submarine seafloor slides exist locally in areas where fluid flow appears to be occurring and large seafloor slides have recently been detected at the base of the steep continental slope just above its contact with the abyssal plain on the Queen Charlotte Terrace. In addition, along the subtle slope re-entrant area offshore of the Dixon Entrance shelf bathymetric data suggest that extensive mass wasting has occurred in the vicinity of an active mud volcano venting gas. We surmise that the relative lack of submarine slides along the upper slope in close proximity to the Queen Charlotte Fault Zone may be the result of seismic strengthening (compaction and cohesion) of a sediment-starved shelf and slope through multiple seismic events.

Abstract The Santa Cruz Basin (SCB) is one of several fault-bounded basins within the California Continental Borderland that has drawn interest over the years for its role in the tectonic evolution of the region, but also because it contains a record of a variety of modes of sedimentary mass transport (i.e., open slope vs. canyon-confined systems). Here, we present a suite of new high-resolution marine geophysical data that demonstrate the extent and significance of the SCB submarine landslide complex in terms of late Miocene to present basin evolution and regional geohazard assessment. The new data reveal that submarine landslides cover an area of ~160 km 2 along the eastern flank of the Santa Rosa–Cortes Ridge and have emplaced a minimum of 9 to 16 km 3 of mass transport deposits along the floor of the SCB during the Quaternary. The failures occur along an onlapping wedge of Pliocene sediment that was uplifted and tilted during the later stages of basin development. The uplifted and steepened Pliocene strata were preconditioned for failure so that parts of the section failed episodically throughout the Quaternary—most likely during large earthquakes. Once failed, the material initially translated as a block glide along a defined failure surface. As transport continued several kilometers across a steep section of the lower slope, the material separated into distinctive proximal and distal components. The failed masses mobilized into debris flows that show evidence for dynamic separation into less and more mobile components that disturbed and eroded underlying stratigraphy in areas most proximal to the source area. The most highly mobilized components and those with the lowest viscosity and yield strength produced flows that blanket the underlying stratigraphy along the distal reaches of deposition. The estimated volumes of individual landslides within the complex (0.1–2.6 km 3 ), the runout distance measured from the headwalls (>20 km), and evidence for relatively high velocity during initial mobilization all suggest that slides in the SCB may have been tsunamigenic. Because many slopes in the California Continental Borderland are either sediment starved or have experienced sediment bypass during the Quaternary, we propose that uplift and rotation of Pliocene deposits are important preconditioning factors for slope failure that need to be systematically evaluated as potential tsunami initiators.

Abstract An active fault system carrying a significant component of right-lateral strike-slip motion extends for ~60 km along the slope–basin transition, ~10 to 20 km offshore of the southern California coast from La Jolla to Dana Point. From south to north, this fault system includes the Carlsbad, San Onofre, and San Mateo fault zones. High-resolution single channel minisparker and chirp seismic reflection data gathered from 2006 to 2011 reveal complex and variable fault zones that are generally characterized by nearly vertical to steeply east-dipping faults with a reverse slip component. The Carlsbad fault zone shows evidence of reverse motion followed by normal separation and probably also includes a component of strike-slip offset. The San Onofre fault zone shows clear evidence of right-lateral slip, offsetting submarine gullies near the base of the slope by approximately 60 m. North of these offset gullies, the base of the slope bends about 30° to the west, following the trend of the San Mateo fault zone, but strands of the San Onofre fault zone trend obliquely up slope, appearing to merge with the Newport–Inglewood fault zone at the shelf edge. These San Onofre fault strands consist of several en echelon left-stepping segments separated by “pop-up” structures, which imply a significant component of right-lateral offset that may serve to transfer right-lateral slip from faults along the base of the slope to the Newport–Inglewood fault zone. Using approximate base Quaternary and base Holocene reflections, segments of the Carlsbad and San Onofre fault zones appear to have experienced right-lateral motion in the Holocene, whereas deformation along the San Mateo fault zone appears to represent a period of mostly pre-Quaternary transpression.

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.

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.