TECTONIC GEOMORPHOLOGY OF SUBMARINE CANYON SYSTEMS OFFSHORE OF SOUTHERN CASCADIA

The USGS recently acquired a suite of high resolution geophysical data (sparker multichannel seismic, Chirp subbottom, and multibeam bathymetry) across southern Cascadia, designed to examine sedimentary dynamics and upper plate fault structures. This region is heavily influenced by the northward migration of the Mendocino Triple Junction and transpression along NW-oriented, shoreline-crossing faults to the east. The apparent confluence of these tectonic regimes, along with E-W convergence across the subduction zone, results in a broad, bowl-shaped depression that forms the deepest part of the Eel River forearc basin, and is incised by the fan-shaped Trinidad Canyon. N-S trending anticlines along the outer arc high are both deeper and deflected west beneath this depression. A series of relatively straight, shore perpendicular canyons downcut ~150m into the convex, prograded upper slope here, offshore of the Klamath and Mad Rivers. Evidence of seafloor seepage from amplitude anomalies in both water column and subseafloor sonar data is common near canyon heads and several of the shallower gullies appear to have pockmark trains visible in the bathymetry, suggesting fluids may play a role in the canyon development. Although infilling has occurred during the present highstand, many of these canyons appear to be long-lived features, originating >500ka based on the underlying stratigraphy. As much as 500m of basin infill onlaps and buries the base of the northernmost canyons, creating a sharp inflection point at the base of the upper slope. In contrast, canyons to the south coalesce at the center of the depression into a single thalweg of Trinidad Canyon, which then cuts across the deformation front. Seismic profiles seaward of the deformation front show a small, but persistent fan system at the mouth of the Trinidad Canyon. A secondary set of more narrow, shallow, and NW-oriented channels and gullies drain into the southernmost feeder canyon rather than the main Trinidad thalweg, and appear stratigraphically younger than the canyons to the north, suggesting the uplift on the south side is more recent, possibly linked to the arrival of the Mendocino Triple Junction around 500ka. Together, the drainage patterns and regional stratigraphy reflect the complex tectonic history of the actively deforming forearc basin.