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Joint Meeting Pacific Section, AAPG & Cordilleran Section GSA April 29–May 1, 2005, San José, California

Transpression and Crustal Shortening in the Epicentral Region of the 2003 San Simeon Earthquake, West-Central California

William R. Lettis and Jeffrey Unruh
William Lettis & Associates, Inc, 1777 Botelho Dr., Suite 262, Walnut Creek, CA 94596, [email protected]

The 2003 San Simeon earthquake occurred on a northeast-dipping reverse fault that forms the northeastern margin of the “Los Osos domain”, a region of transpressional deformation in the southern Coast Ranges that is part of the boundary between the Pacific plate and the Sierra Nevada-Central Valley (“Sierran”) microplate. Several models have been proposed to explain patterns of strike-slip faulting and crustal shortening in the Los Osos domain (PG&E, 1988; 1991; Lettis and others, 2004). One model postulates that clockwise rotation of the Transverse Ranges drives the southern margin of the Los Osos domain northward, causing north-northeast- directed crustal shortening and reverse faulting on a series of northwest-striking reverse faults within the domain, such as the Los Osos fault. Crustal shortening along the northeastern margin of the Los Osos domain and within the Southern Coast Ranges domain also may be driven, in part, by a left-restraining transfer of slip between prominent strike slip faults in the region. A possible example of this process is transfer of dextral slip from the northwest-striking Rinconada fault zone to the San Simeon fault zone across the west-northwest-striking Oceanic-West Huasna fault along the northeastern boundary of the Los Osos domain. Crustal shortening and uplift of the Santa Lucia Range and Piedras Blancas anticlinorium in the San Simeon region also appears to be generated by a broad left-restraining bend in the Hosgri-San Simeon-San Gregorio fault system, centered on the San Simeon fault. The scale of this restraining bend is comparable to the restraining bend along the San Andreas fault in the Santa Cruz Mountains area, which has produced uplift and crustal shortening in the southwestern San Francisco Bay area. Quaternary deformation in the San Simeon region likely results from a combination of these three processes.

Posted with permission of The Geological Society of America; abstract also online ( © Copyright 2005 The Geological Society of America (GSA).