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Abstract: Tectonic Geomorphic Development of the Southeastern Los Angeles and Orange County Basins: A Hypothesis

GATH, ELDON, Earth Consultants International, Orange, CA

The southeastern Los Angeles and southern Orange County (OC) basins lie between the strike-slip dominated tectonics of the Whittier and Newport-Inglewood faults, and the blind thrust-dominated tectonics of the SW-vergent Puente Hills (PH) and NE-vergent San Joaquin Hills (SJH) fault systems. Geomorphic analysis of the SJH (emergent >1Ma) indicates an uplift rate of 0.2-0.3 m/ky, and for the PH (0.5-0.6 m/ky; emergent ~ 0.7 Ma). Alluvial fan sediments shed from the uplifting PH are anticlinally deformed by south vergent compression along the PH thrust into a subparallel en-echelon fold belt lying south of the Whittier fault. Compression of the Transverse Ranges, created the PH thrust upon which the Whittier fault accommodates strike-slip strain. The Elsinore fault's 6 m/ky strain is apportioned between the Whittier fault (3 m/ky), the Chino fault (2 m/ky) and uplift of the Santa Ana Mountains SAM (1 m/ky). The SAM are uplifting in response to the termination of the Elsinore fault. Loma Ridge is forming in response to compressional tectonics of the SJH thrust forcing the sedimentary section against and around the western end of the SAM batholith. Loma Ridge is a northwesterly trending, anticlinally deformed structure parallel to the (SAM) and cross-cut by several N-S trending strike slip faults that consistently deflect Santiago Creek 2000 m northerly (in an upstream direction), but do not affect the dissected range front. The activity of the N-S faults is uncertain, but M 4.0 earthquakes are occurring. The southernmost part of OC is dominated by flat vertical uplift, dissected by an older NE-SW drainage system that is now increasingly captured by a N-S drainage system forming along the structural grain.


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