QUATERNARY UPLIFT OF THE SANTA ANA MOUNTAIN FOOTHILLS, ORANGE COUNTY, CA: CONSTRAINTS FROM FLUVIAL TERRACE ANALYSIS

The Santa Ana Mountains (SAM; maximum elevation 1,733 m) are located near the southern margin of the transpressive Los Angeles basin, between the seismically active Elsinore and Newport-Inglewood right-lateral fault zones. We infer Quaternary uplift of a portion of the SAM foothills through a tectonic geomorphic evaluation of fluvial terraces along San Juan Creek. San Juan Creek is a major drainage system of the SAM that drains directly into the Pacific Ocean. Along San Juan Creek is a vertical succession of seven flights of unpaired fluvial strath and aggradational terraces that span roughly 100 m in elevation. Possible forcing mechanisms for the development and abandonment of the terrace deposits include: (1) tectonic uplift of the SAM foothills, (2) eustatic sea level fluctuations, or (3) Quaternary climate changes not accompanied by large sea level changes. Our hypothesis is that the SJC terraces formed in response to tectonic uplift of the SAM foothills, which is likely related to transpressive deformation between the Elsinore and Newport-Inglewood right-lateral fault systems. In order to quantify the magnitude and style (i.e. planar, folding, or tilting) of Quaternary uplift of the SAM foothills, we mapped the San Juan Creek terraces using high-resolution topographic base maps (i.e., 1:4,800 scale with 5 ft contour intervals) and digital elevation models. Topographic position was used to correlate terraces and construct paleo-longitudinal river profiles. These profiles indicate up to 105 m of Quaternary incision and also suggest as much as 4 to 10 degrees of eastward tilting. The lowest terrace is aggradational, which we interpret as likely linked to climatic processes. In contrast, the upper six terraces are straths with 2 to 7 m thick capping gravel deposits. Based on these findings, we infer that the upper six terraces formed primarily in response to tectonic uplift of the SAM foothills. By quantifying the magnitude and style of uplift of the southern SAM foothills, this study initiates the process of identifying possible tectonic sources of uplift and seismicity and may ultimately play an important role in future seismic hazard analyses.