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Research Projects Active deformation at the northern end of the Elsinore fault zone: implications for the Quaternary evolution of Santa Ana River (Eastern Los Angeles Basin, Southern California) PI: Tom Bullard and S.G. Wells Project Period: 1995 - 1999 Funding: National Science Foundation & USGS Earthquake Hazards Reduction Program |
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Keywords: Quaternary, geomorphology, active tectonics, soil-geomorphology, earthquakes, National Science Foundation - USGS Earthquake Hazards Reduction Program Project Description Quantifying the pattern, magnitudes, and rates of deformation in areas where active faulting is not expressed by surface fault rupture (e.g., blind-thrust faulting) requires an approach that integrates Quaternary geologic mapping, soil geomorphology, and tectonic geomorphic analyses. For example, tectonic influence on drainage network development has been demonstrated using geomorphic analyses (Pazzaglia, et al., 1998; Keller and Pinter, 1996; Bullard and Lettis, 1993; Keller and Rockwell, 1984; Gardner et al., 1987). Moreover, abrupt facies changes in the stratigraphy of unconsolidated sediments (e.g., fluvial, colluvial, and alluvial fan deposits), and the presence of diagnostic horizons (for example peat layers), that may be associated with recent tectonic deformation. In the Santa Ana River basin along the northern Elsinore fault zone in the southeastern Los Angeles Basin accumulations of organic-rich deposits offer the opportunity to evaluate the possibility that organic layers represent ponding by local stream blockages caused by local coseismic uplift. The organic-rich layers also offer the possibility of associating the layered organic-rich sediments to seismically independent flood-related ponding in response to extreme climate events. In this study an approach utilizing geomorphic analyses, stratigraphy, and soil-geomorphology was used to assess the control of Quaternary active tectonics on the geomorphic evolution of the Santa Ana River along the Northern Elsinore fault zone in the southeastern Los Angeles Basin in southern California. Mapping of Quaternary deposits and soil profile descriptions indicate well-developed soils (maximum reddening 6.75YR and Bt horizon thickness >2m) on older alluvial fan deposits (Upper Pliocene-Lower Pleistocene) and thin, weakly developed soils (maximum reddening 7.5YR and Bt < 50cm) on Holocene fluvial deposits. Morphometric analyses in the stream basins of the Puente-Chino Hills area and along the Santa Ana River included detailed longitudinal and cross valley profiles and the evaluation of geomorphic indices, such as steam gradient index and drainage asymmetry. These studies revealed spatial variation in the relative degree and magnitude of tectonic deformation across major structural zones, indicating a strong asymmetric distribution of relief (uplift) across the Puente-Chino Hills with the greatest amount of uplift immediate northwest of the Elsinore-Whittier fault where it passes through Santa Ana Canyon. The longitudinal profile and spatial distribution of the Santa Ana River and its terraces through the canyon also reflect strike-slip motion along the Elsinore-Whittier fault. The right-lateral Chino fault has exerted some influence on eastward draining streams, with slight southwestward deflection of channels and valley floors. Immediately north and east (upstream) from the Santa Ana Canyon, cores contain peat deposits indicating numerous periods of marshy conditions. Available data do not allow direct correlation of these organic-rich deposits to earthquake-induced uplift along the Chino fault; rather, the age of these deposits suggest that ponding events may have occurred in response to changes in surface and groundwater hydrology. |
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