ACTIVE DEFORMATION OF THE NORTHERN ELSINORE FAULT ZONE: IMPLICATIONS FOR THE QUATERNARY EVOLUTION OF SANTA ANA RIVER, SOUTHERN CALIFORNIA

Quaternary geologic mapping, soil geomorphology, morphometric analysis, and borehole stratigraphy are used to assess the influence of Quaternary active tectonics on the geomorphic evolution of the Santa Ana River (SAR) in Santa Ana Canyon (SAC) in the southeastern Los Angeles Basin. The reach is situated between the Puente-Chino Hills (PCH) and Prado Dam where the Elsinore fault splits into the Whittier and Chino faults. Soil B horizons on mapped units are well developed, > 2 m thick and reddened to 6.75 YR on Upper Pliocene-Lower Pleistocene fan units and <50 cm thick and reddened <7.5YR on the oldest Holocene fluvial deposits. Geomorphic analyses included detailed longitudinal and cross-valley profiles and the evaluation of geomorphic indices, such as stream length-gradient and transverse topographic symmetry. Results indicate a strong asymmetric distribution of relief (uplift) across the PCH and spatial variation in the relative degree and magnitude of deformation across major structural zones. The right-lateral Chino fault results in slight southwest deflection of stream channels and valleys on the east flank of the PCH. Boreholes containing prominent dark, organic-rich layers suggest episodic ponding upstream of SAC. Available data do not directly support coseismic ponding-events along the Chino fault, however, deposit age suggests possible ponding in response to regional flooding events in the SAR drainage. Greatest long-term uplift occurs northwest of the Elsinore-Whittier faults (EWF) where it passes through SAC and is reflected in the magnitude of SAR terrace uplift. The SAR long profile and distribution of terraces through SAC also reflect strike-slip motion along the EWF. Unlike other large fluvial systems in the same area, the Santa Ana River has persisted in its present course since the PCH uplift began, principally controlled by the tectonic boundary between the maximum uplift in the PCH to the north and uplift of the Santa Ana Mountains along the restraining bend south of the canyon.