Morphology, structure and evolution of California Continental Borderland restraining bends

Mark R. Legg, Chris Goldfinger, Marc J. Kamerling, Jason Chaytor, & Debra Einstein

Published 2007, SCEC Contribution #954

Exceptional examples of restraining and releasing bend structures along major strike-slip fault zones are found in the California Continental Borderland. Erosion in the deep sea is diminished, thereby preserving the morphology of active oblique fault deformation. Long-lived deposition of turbidites and other marine sediments preserve a high-resolution geological record of fault zone deformation and regional tectonic evolution. Two large restraining bends with varied structural styles are compared to derive a typical morphology of Borderland restraining bends. A 60-km long, 15-degree left bend in the dextral San Clemente Fault creates two primary deformation zones. The southeastern uplift involves "soft" turbidite sediments and is expressed as a broad asymmetrical ridge with right-stepping en echelon anticlines and local pull-apart basins at minor releasing stepovers along the fault. The northwest uplift involves more rigid sedimentary and possibly igneous or metamorphic basement rocks creating a steep-sided, narrow, and more symmetrical pop-up. The restraining bend terminates in a releasing stepover basin at the northwest end, but curves gently into a transtensional releasing bend to the southeast. Seismic stratigraphy indicates that the uplift and transpression along this bend occurred within Quaternary time. The 80-km long, 30-40 degree left bend in the San Diego Trough-Catalina fault zone creates a large pop-up structure that emerges to form Santa Catalina Island. This ridge of igneous and metamorphic basement rocks has steep flanks and a classic "rhomboid" shape. For both major restraining bends, and most others in the Borderland, the uplift is asymmetrical with the principal displacement zone lying along one flank of the pop-up. Faults within the pop-up structure are very steep dipping and subvertical for the principal displacement zone. In most cases, a Miocene basin has been structurally inverted by the transpression. Development of major restraining bends offshore southern California appears to result from reactivation of major transform faults associated with middle Miocene oblique rifting during the evolution of the Pacific-North America plate boundary. Seismicity offshore southern California demonstrates that deformation along these major strike-slip fault systems continues today.

Legg, M. R., Goldfinger, C., Kamerling, M. J., Chaytor, J., & Einstein, D. (2007). Morphology, structure and evolution of California Continental Borderland restraining bends. In Legg, M. R., & Goldfinger, C. (Eds.), Tectonics of Strike-Slip Restraining and Releasing Bends, (, pp. 143-168) London, : Geological Society of London doi: 10.1144/SP290.3.