CLIMATIC INFLUENCE ON A TECTONICALLY-DOMINATED LANDSCAPE; WESTERN TRANSVERSE RANGES, CA – INSIGHTS FROM MAPPING, GEOCHRONOLOGY AND MODELING

Southern California landscapes are products of both tectonic and climatic influences. Great progress has been made in our understanding of how these factors influence landscapes, and here we present results from our efforts to distill these influences on present-day landscape configuration in Cuyama Valley, California using multiple techniques. We have generated new 1:24,000-scale geologic mapping of a large portion of the valley. We also have amassed several dozen radiocarbon dates and several preliminary optically stimulated luminescence (OSL) dates from the piedmont and axial-fluvial environments, and continue to expand our geochronological database in order to constrain timing of major Quaternary fluctuation in the fluvial system. In addition, a newly developed quantitative model of integrated hillslope, channel and piedmont evolution was used to test how climate-driven landscape processes have influenced the Cuyama Valley piedmont environment.

Our findings indicate that regional models for climatic control of some surficial processes are applicable to tectonically active areas in southern California, and lead to features such as flights of Pleistocene alluvial-fan terraces having ages that tend to correlate with regional chronologies and also axial-system Holocene arroyos and paleo-arroyos. We find evidence that tectonic influences on the sedimentary basin evolution dominated from the Miocene to sometime in the mid-Pleistocene, at which time a shift towards a more dynamic climate regime began to overprint tectonic signals. This is evidenced by a several-thousand-meter thick conformable and generally similarly-deformed Miocene to Pleistocene sequence of syn-tectonic basin-fills. Deposition of these terrestrial fills ended after a transition to a brief lacustrine episode sometime around ~740 ka. Since then, cyclic erosion and deposition has occurred as the Cuyama Valley transformed from a major sedimentary basin to a more topographically complex basin. Though tectonic activity continues, our application of new landscape evolution modeling indicates that many of the Late Pleistocene landforms can be explained by fluctuation in processes controlled primarily by climate.