Wilmington Structure, Long Beach, California: a Strike-Slip Restraining Bend Popup?

Legg, Mark R.¹, Scott W. Prior² (1) Legg Geophysical, Huntington Beach, CA (2) THUMS Long Beach Company, Long Beach, CA

The Wilmington oil field occupies a large northwest-trending anticlinorium bounded to the southwest by the THUMS-Huntington Beach fault. The overall morphology of the Wilmington Structure resembles a large restraining bend uplift or "popup" along a strike-slip fault, such as the San Clemente fault offshore southern California. Both structures consist of a broad, northwest-trending, asymmetrical, uplift that trends subparallel to a major fault and are cross-cut by numerous dip-slip or oblique-slip faults. However, the right-slip San Clemente fault is sub-vertical throughout the restraining bend popup whereas the oblique-reverse THUMS-Huntington Beach fault has moderate dip. The north-trending cross faults in the Wilmington Structure show normal separation, moderate to steep dips (about 65 degrees) and local components of strike-slip. In contrast, the north-trending oblique-slip faults of the San Clemente bend region are very steep, 70 to 80 degree dip, with reverse separation. Although transpression is prominent for both structures, we conclude that unlike the San Clemente fault restraining bend, the Wilmington Structure is a transpressional fold above a possibly reactivated THUMS-Huntington Beach fault that accommodates strain partitioning adjacent to major transpressional right-slip faults, the Palos Verdes and Newport-Inglewood fault zones. Numerous structures with similar character exist adjacent to the major right-slip faults of the Pacific-North America transform fault system of California. Many hold actively producing oil fields like Wilmington, but others remain to be explored and drilled.