Structure and Stratigraphy
Published:January 01, 1998
Several papers in this volume deal with the structure beneath the Santa Barbara Channel from opposing view points. These are the thickskinned structural interpretations versus the thin-skinned ones. In the thick-skin interpretations, high angle reverse faults cut the crust to the base of the seismic zone (see Yeats, this volume). In the thin-skinned model these reverse faults are actually active axial surfaces of fault-bend-folds above detachment ramps in the lower crust (see Novoa, this volume). The detachments are necessarily blind, and they do not crop out.
A closely spaced data set of analog and digital (CDP) seismic reflection lines was used to map the structure of the offshore northwestern Santa Barbara basin and the offshore southern Santa Maria basin. East-west structures of the Santa Barbara margin between the Molino gas field and to the west of Point Conception are linear fold and fault trends offset by minor tear faults and, west of Gaviota, by the South Santa Ynez fault zone. The structures in both mapped areas are Plio-Pleistocene in age and most are active today.
Offshore of Point Conception in the northwest corner of Santa Barbara basin the change from the east-west tectonic grain of the western Transverse Ranges and the northwest grain of the offshore southern Santa Maria basin occurs across a narrow, structurally complex zone. This “Disrupted Zone” begins near the State Lands/Federal OCS boundary west of Point Conception and extends to the west for several kilometers. Bordered on the east by the thrust faults of the Point Conception fault zone and on the west by a zone of right-lateral shear, the zone is 4 to 7.5 km in width. Orientations of folds, monoclinal flexures and faults within the zone are highly variable (east-west to north to northwest). Widest west of Point Conception, the zone narrows to the north of Point Arguello and becomes less well defined, It also lies on trend with the southern Hosgri fault zone.
An older uplift, the Point Arguello-Conception-Amberjack (?) (PACA) high that trends to the southwest of the headland formed by the points, is overprinted by the Plio-Pleistocene structures. Today the tectonic border of the Western Transverse Ranges, including the Santa Barbara basin, is the “Disrupted Zone”. The older “PACA” high is the Paleogene boundary of the Santa Barbara and offshore southern Santa Maria basins.