Physiography of an active transpressive margin basin: high-resolution bathymetry of the Santa Barbara basin, Southern California continental borderland.

Monterey Bay Aquarium Research Institute, Moss Landing, CA 95039
1 now at: Dept. of Geological and Environmental Sciences, Stanford University, Stanford CA 94305
2 also at: Moss Landing Marine Laboratories, Moss Landing, CA 95039

Abstract. High-resolution swath bathymetry and backscatter data from the Santa Barbara basin reveal a distinct assemblage of morphologic and sedimentologic features characteristic of the tectonically active setting of the basin. Such characteristics include the north-south asymmetry of the basin, the presence of an intra-basinal structural and morphologic high, extensive mass wasting, and fields of mounds and pockmarks. The north-south asymmetry of the basin and the increased abundance of slope failure along the northern basin slope likely reflect higher rates of tectonic shortening and sediment accumulation along the northern basin margin. High sedimentation rates lead to high pore water content of the sediment which, in combination with seismic shaking, may explain the abundance of mass wasting features even on shallowly inclined lower basin slopes.
Mounds and pockmarks are the surface expression of locally extensive fluid seepage which we interpret to be driven by a combination of high sedimentation rates, a high organic content of sediment, and anoxic bottom water conditions during extended periods in Neogene and Quaternary times. During ROV dives we observed active fluid seepage to have occurred preferentially on the shelf and on a structural and topographic high, presumably due to structural focusing of migrating formation fluids. The preferred alignment of some pockmarks along faults is consistent with structurally controlled expulsion of formation fluids from underlying Neogene units.
Despite the high tectonic activity of the basin, fault scarps are rare and only observed in areas of low sediment accumulation on the shelf and on topographic highs. Mass balance considerations suggest that sedimentation rates averaged over interseismic time scales are too high and scarp growth rates too low for scarps to be characteristic features of the basin floor. Long-term sediment accumulation rates that account for the effect of sediment compaction are low enough, however, to lead to long-wavelength surface expressions of the most active faults.