ABSTRACT: Active Strike-Slip Faulting within the Peralta Hills Anticline, Orange County, California -- a Transverse Range-Peninsular Range Boundary Effect
WHITNEY, ROBERT A, Leighton and Associates, Inc., Diamond Bar, CA, and DAVID C. SEYMOUR, Leighton and Associates, Inc., Irvine, CA
The Peralta Hills are a small, southerly overthrust asymmetric anticline that has been proposed as lying along the south-central Transverse Range Province boundary in Orange County, California. They abut the northwest corner of the Santa Ana Mountains block, which is the most northwesterly Peninsular Range Province structure, and which is moving northwest along the Elsinore fault.
Recent trenching in the eastern parts of the Peralta Hills exposed within the anticlinal structure a high-angle fault, with subsurface dips of about 60 degrees north, crosscutting 30 degrees north-dipping bedding. Near the ground surface, the fault flowers and decreases in dip to subhorizontal. Slickensides have a maximum rake of 22 degrees east-northeast. Absence of a sidehill trough or bench indicate the vertical component is hanging wall up, and the horizontal component is sinistral. Absence of reverse drag on bedding indicates cumulative reverse movement is minimal.
Soil development within a faulted mudflow/alluvium sequence indicates a late Holocene age for the latest displacement. Carbon samples from the flow are being dated to determine the maximum age of the last fault activity.
The southerly movement of the Peralta Hills, in response to Transverse Range Province shortening, encounters interference along the southern boundary from the northwesterly moving Santa Ana Mountains block. In response to this interference, the Peralta Hills slide westerly, with slight overthrusting, on this east-west-trending, left-lateral, strike-slip fault. Extensive gouge development along the fault (to 8 in. thick) indicates it is a major structure accommodating extensive boundary deformation in this area.
AAPG Search and Discovery Article #91016©1992 AAPG-SEPM-SEG-EMD Pacific Section Meeting, Sacramento, California, April 27-May 1, 1992 (2009)