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Abstract: Development and Growth of Large Transpressional Stepovers in Strike-Slip Faults--Seismotectonics of Major Restraining Bends along Strike-Slip Faults

LEGG, MARK, Legg Geophysical, Huntington Beach, CA

Major restraining bends along strike-slip faults tend to become locked, releasing accumulated tectonic strain during large earthquakes. Seafloor uplift and deformed late Quaternary turbidites along the offshore San Clemente fault reveal the geometry and deformation history of major restraining bends. The 15 degree left bend in the N40W-trending fault creates a broad asymmetrical uplift, higher to the northeast and subparallel to the fault, but also bent and locally uneven in elevation. Individual peaks of the anticlinorium separated by saddles represent right-stepping en echelon anticlines typical of dextral wrench faulting. The principal displacement zone of the active San Clemente fault mirrors the complex uplift pattern with right-stepping en echelon fault segments that are connected via local pull-apart basins. On a broad scale, the fault zone cuts straight through the oblique segment, consistent with a high-angle subsurface fault; local trace irregularity likely represents the surface expression of palm tree structure. Low-angle thrust faults parallel the principal right-slip fault traces, whereas north-trending normal faults obliquely cross the transpressional uplift. Buried anticlinal structures of major Los Angeles basin oil fields show similar structural patterns. Major southern California restraining bends include the San Andreas big bend, the Palos Verdes Hills and anticlinorium, the Whittier-Elsinore fault zone, the San Joaquin Hills and Oceanside segments of the Newport-Inglewood fault zone, and possibly the Catalina escarpment.  Large earthquakes (M>7) along major restraining bends include strike-slip events with epicenters outside the oblique fault segment and rupture propagation into the bend (1857 Fort Tejon, 1906 San Francisco, and 1999 Izmit, Turkey) and oblique-reverse events that rupture within the bend region (1989 Loma Prieta). Regional detachment faults add the potential for complex interaction between high-angle strike-slip and low-angle oblique(?)-thrust faulting during earthquakes (1986 Oceanside). Also, large restraining bend earthquakes on submarine faults are likely to generate local tsunamis with destructive effects on adjacent coastal areas.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California