Gath, E.M.
Is the Elsinore Fault Responsible for the Uplift of the
Santa Ana Mountains?
Based on Gath, E.M., Runnerstrom, E.E., & Grant, L.B. (2002). Active Deformation and Earthquake Potential of the Southern Los Angeles Basin, Orange County, California. Final Technical Report (USGS Award #01HQGR0117) submitted to USGS, Fall 2002.
Oral Presentation at the Los Angeles Basin Geological Society Monthly Meeting, October 2002, in Long Beach, CA.
Rising to an elevation of nearly 1000 m, the Santa Ana Mountains dominate the skyline of Orange County, California. They are the northernmost extension of the California Peninsular Ranges, bounded by the Elsinore fault on the NE, the Irvine basin on the SW, and the Santa Ana Mountains antecedent Santa Ana River on the north. The 5-6 mm/yr dextral slip Elsinore fault 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 Santa Ana Mountains. The remaining 1.5-2 mm/yr slip is unaccounted for. A suite of six fluvial terraces on the Puente Hills north of the antecedent Santa Ana River indicates an uplift rate of 0.4 mm/yr, possibly due to the Puente Hills blind thrust fault, not strain from the Elsinore fault. In this talk I will present a hypothesis that the Santa Ana Mountains uplift is occurring in response to termination of the Elsinore fault and the consumption of this missing slip. GIS-based geomorphic analysis has indicated the presence of eight probable erosional surfaces on the Santa Ana Mountains and Loma ridge. Recent geomorphic mapping along the SW margin of the Santa Ana Mountains has also revealed a suite of four uplifted fluvial fill terraces on the Peralta Hills and along Santiago Creek. Santiago Creek is trapped within the uplift of the Santa Ana Mountains by Loma Ridge, and these geomorphic surfaces were used to calculate a 0.3 mm/yr uplift rate and a 3.6 Ma emergence age for the Santa Ana Mountains. Santiago Creek formed ~2.4 Ma in conjunction with the initiation of the Loma Ridge structure, a parasitic structure that formed in response to compressional buckling of sedimentary strata on the flanks of the uplifting Santa Ana block. Hanging wall block faulting appears to have deflected Santiago Creek northerly ~1,200 m along five discrete block margin faults. The source of this strain is still undetermined, though it may be from the same north-vergent structures that are generating the San Joaquin Hills uplift in a complex interaction with other north-vergent structures in southern California.