IS THE ELSINORE FAULT RESPONSIBLE FOR THE UPLIFT OF THE SANTA ANA MOUNTAINS, ORANGE COUNTY, CALIFORNIA?

Rising to an elevation of 930 m, the Santa Ana Mountains (SAM) dominate the skyline of Orange County (OC), California. They are the northernmost extension of the California Peninsula Ranges, bounded by the Elsinore fault (EF) on the NE, the Irvine basin on the SW, and the antecedent Santa Ana River (SAR) on the north. A suite of six fluvial terraces on the Puente Hills (PH) north of the SAR indicates an uplift rate of 0.5-0.6 mm/yr, possibly due to the Puente Hills blind thrust fault. The 5-6 mm/yr dextral slip EF bifurcates into the 3 mm/yr slip rate Whittier fault, and 1.5-2 mm/yr slip rate Chino fault at the northern end of the SAM. The remaining 1.5-2 mm/yr slip is unaccounted for. We present a hypothesis that the SAM uplift is occurring in response to termination of the Elsinore fault and the consumption of this missing slip.

Recent geomorphic mapping along the SW margin of the SAM has revealed a similar suite of uplifted fluvial terraces on the Peralta Hills and along Santiago Creek (SC). SC is trapped within the uplift of the SAM by Loma Ridge, and the SC terrace suite can be used to calculate the uplift rate of the SAM. Mapping and sampling of these terraces is underway. 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 appear to 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 under the northern SAM. If our hypothesis holds, much larger earthquakes are possible.