Abstract: North-Vergent Thick-Skinned or South-Vergent Thin Skinned Oak Ridge Fault: A View from the Coast

Abstract

Abstract Two contrasting views of the Oak Ridge fault in the Ventura basin have emerged: a thick-skinned view based on onshore subsurface oil-well data and a thin-skinned view based on interpretation of seismic profiles and dipmeter data from the Santa Barbara Channel. The offshore Oak Ridge structure is interpreted as an active axial surface above the ramp of a blind, south-vergent thrust; growth triangles in the kink band south of this surface contain parallel, north-dipping strata in which the kink band tapers toward the surface. In contrast, dips of growth strata onshore near the coast at Montalvo steepen with depth, evidence of progressive limb rotation and a steeply-dipping, large-displacement Oak Ridge fault. The onshore Oak Ridge fault is northward-vergent. West of Oak Ridge, the post-Saugus displacement is transferred via the ductile Rincon Formation to folding of the Ventura-Rincon anticline, also a north-vergent structure onshore and offshore. An onshore seismic line along a cross section documented by electric logs shows a narrow panel of north dips that could be the onshore termination of an offshore kink band mapped in the Santa Barbara Channel. North dips at greater depth in this seismic line do not correspond to well data and are artifacts of processing. Farther east, north dips are possible only if the kink band turns to the northeast, parallel to the Montalvo fault. The onshore-offshore controversy will only be resolved by comparing seismic profiles in the Santa Barbara Channel with subsurface well profiles in the same transect using well-log correlations as well as dipmeter data.