Abstract

A continuous 220 km long anticline underlies the Santa Monica Mountains and northern Channel Islands of southern California. This fold has been explained as the result of slip on a ramp in a blind thrust fault. Here we investigate the part of that structure including northwest Santa Rosa Island and southwest Santa Barbara basin. A U.S. Geological Survey (USGS) multichannel seismic reflection profile, USGS-808 was reprocessed and output in migrated depth and time. This profile, an industry multichannel profile, and numerous USGS high resolution reflection profiles were correlated to petroleum test well logs and to published data from Ocean Drilling Program (ODP) Site 893. A continuous fold limb dips north 10°-15° beneath northern Santa Rosa Island and the adjoining shelf and slope. This fold limb is cut by high-angle strike-slip faults on Santa Rosa Island, and by strands of the steeply S-dipping Santa Cruz Island fault at the shelf-break. The base of the fold limb is deformed by Pliocene slip on a blind S-dipping western continuation of the Oak Ridge-deep Mid-Channel fault. Neogene strata beneath central Santa Barbara Channel are nearly undeformed except that slight tilting and normal separation of Miocene strata is preserved across N-dipping faults. Steeply-dipping Pliocene and older strata associated with the North Channel fault are seen in wells beneath the northern margin of Santa Barbara basin, but are not imaged on USGS-808. The gently-dipping strata indicate that little shortening has been accommodated by folding and faulting across Santa Rosa Island or beneath much of Santa Barbara Channel. The observed wide fold limb can be created by less slip above a concave-up (listric) fault than by the more widely applied ramp-flat fault bend fold models. Progressive limb rotation is interpreted beneath the southern margin of Santa Barbara basin, consistent with a listric thrust model.