Post-Miocene Compressional Tectonics Along the Central California Margin

Abstract

High-resolution, 36-fold seismic reflection data from offshore Santa Maria Basin, northern Santa Barbara Channel, and off Pt. Conception reveal major post-Miocene thrust faulting in offshore central California. These thrusts can be recognized on lines both normal and parallel to the regional structural grain. On lines normal to the grain they are commonly imbricate and curve asymptotically downward to a basal sole thrust. On lines parallel to the grain they appear as a band of nearly horizontal reflectors that truncate tightly-folded strata above the thrust. The northern Santa Barbara Channel and Pt. Conception area are part of the Transverse Ranges where reverse and thrust faults are common. The offshore Santa Maria Basin, however, is generally regarded as a wrench-style basin. The Hosgri and other major northwest-trending faults within the basin, however, appear to be predominately thrusts rather than strike-slip faults. Detailed mapping of the offshore basin indicates that the overall structural pattern is unlike typical wrench-style tectonics. Folds, for example, have a preferred asymmetry and their axes closely parallel faults rather than being en echelon to them. We conclude that whereas some strike-slip is probable, the folds and faults, as well as the present morphology of the offshore Santa Maria Basin, chiefly reflect postMiocene northeast-southwest-directed compression. Similar conclusions can be drawn from the onshore Santa Maria Basin on the basis of field relations and well data.