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High-resolution, 36-fold seismic reflection data with penetration to 3 sec have been collected recently in the northeastern offshore Santa Maria basin, the northern Santa Barbara Channel, and off Point Conception, California. These profiles reveal major east-over-west thrust faults in areas previously interpreted as being characterized by strike-slip faults and/or high-angle normal or reverse faults. Like those in well known foreland thrust belts, these faults typically form an imbricate system in which they curve asymptotically downward to a common basal sole thrust. "Soling out" generally occurs at depths of 1.5-3 km (5,000-10,000 ft).
Detailed mapping of faults and folds associated with these thrust systems coupled with fault-plane solutions suggest that (1) these thrusts formed within the last 5 m.y., (2) many have modern activity, and (3) compressive forces causing them are normal to the strike of the San Andreas fault. These observations agree with present-day plate motion studies which require that Pacific-North American relative plate motion include a component of compression orthogonal to the San Andreas fault.
These overthrust regions are all sites of recent major petroleum discoveries. However, these discoveries have all been made on obvious anticlinal structures that generally are attributed to wrench tectonics. Recognition of thrust faulting in these areas may lead to additional discoveries from more subtle geologic traps associated with overthrusting.
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