Quaternary Geomorphic Development and Seismic Hazards of Orange County, Southern Los Angeles Basin, California

Gath, Eldon M.¹, Lisa Grant¹ (1) University of California, Irvine, CA

We present a neotectonic model for the geomorphic development of Orange County, including the San Joaquin Hills (SJH), Santa Ana Mountains (SAM), Loma Ridge (LR), Puente Hills (PH). The SAM became emergent 3-4 Ma at a rate of 0.2-0.7 mm/yr. LR formed 2.5 Ma and blocked northern SAM drainages from directly reaching the ocean. The LR is a southwest-vergent Plio-Pleistocene structure, obliquely accommodating north-south crustal shortening when the NIF terminated into a right-stepping horsetail splay (Christianitos, Mission Viejo, Dana Cove faults). In the mid-Quaternary, the NIF stepped westward onto the “Northern NIF” trace, resulting in the compressional step that formed the San Joaquin Hills (SJH), a north-vergent, anticlinally deformed structure, bounded on the east by the Dana Cove fault, driven by strain partitioning off of the NIF, emergent less than 1 Ma, and currently uplifting at 0.25 mm/yr. At 200-300 ka, the NIF straightened into the LA Basin along the “Southern NIF”, producing uplift of Signal Hill, eliminating the source of compressive strain for the LR anticline. The PH became emergent 500-700 ka at 0.6-0.8 mm/yr by uplift along the Puente Hills blind thrust (PHBTF). The PHBTF is accommodating N-S compressional strain, while the right-lateral Whittier fault is accommodating 2-3 mm/yr of N-W vergent lateral strain of the Elsinore fault's (EF) 5-6 mm/yr. The Coyote Hills are an en-echelon series of hanging wall folds similar to LR, caused by thrusting the SAM block into Santa Ana River (SAR) sediments, using the PHBTF as a decollemont. The Chino fault, a right-lateral extension of the EF, accommodates 1-2 mm/yr, which results in an EF strain surplus of 1-3 mm/yr for uplift of the Santa Ana Mountains. The Peralta Hills fault, an EF backthrust, deflects the SAR 3 km westward by uplift of LR. These findings increase the seismic hazard of Orange County.