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 The Continental Borderland adjacent to southern California and northern Baja California is an exceptionally wide (240 km) region of ridges, islands, and bathymetric basins as deep as 2 km. This continental margin area includes the Nicolas terrane, a relatively intact outer terrane to the west characterized by the presence of Cretaceous and Paleogene forearc sedimentary rocks, and the adjacent Catalina terrane, a highly extended inner terrane to the east characterized by metamorphic basement rocks exhumed during early Miocene oblique rifting. To better understand this continental rifting process, we used regional grids of multichannel seismic reflection data, and stratigraphic information from industry wells and seafloor samples to investigate the nature and tectonic development of the boundary between the Outer Borderland forearc Nicolas terrane and the Inner Borderland denuded, exhumed Catalina terrane. These data show that this terrane boundary is largely defined by an angular unconformity that overlies and helps form an eastward termination or wedge-out of Outer Borderland forearc strata of earliest Miocene through Cretaceous age. These forearc sedimentary rocks were removed along the boundary primarily as a result of early-to-middle Miocene uplift and erosional truncation associated with exhumation of the Catalina basement. The boundary is not predominantly fault controlled. Along the northern part of the boundary, the East Santa Cruz Basin fault system, previously postulated to control the Nicolas–Catalina terrane boundary, is a predominantly east-dipping, Miocene oblique-normal fault system that has been reactivated with blind to partly blind oblique-reverse displacement. It does not align with the terrane boundary, and given its geometry, slip history, and the presence of Nicolas forearc sedimentary rocks on both sides of this fault system, it is unlikely to have had a major influence on terrane boundary development. South of San Nicolas Basin, there is no simple, through-going fault system, and where faults are present, they are often discontinuous segments that strike oblique to the boundary. This implies that major displacements associated with translation of the Outer Borderland were not localized to the eastern Outer Borderland boundary itself, but rather were likely distributed farther east within the evolving Inner Borderland rift.