GEOMORPHIC RESPONSE TO TECTONIC ACTIVITY WITHIN THE MECCA HILLS, CALIFORNIA

The Coachella Valley segment of the San Andreas fault in southern California has a “sawtooth” geometry in map view that consists of plate motion-parallel regions separated by regions of transpressive character. It has been hypothesized that these transpressive regions may no longer be tectonically active coincident with the opening of the Salton Trough. One of these transpressive regions, the Mecca Hills, has undergone significant deformation resulting in folded and faulted Plio-Pleistocene stratigraphy, strong structural controls on basin geomorphology, spatially variable topographic relief, and apparently young tectonic features. To examine this hypothesis and to contribute towards developing and testing models of geomorphic evolution of strike-slip restraining bends, a series of geomorphic indices were calculated to define areas of increased relative tectonic activity using digital elevation models and GIS. Indices (stream length index, basin volume-to-area ratio, mountain front sinuosity, drainage density, channel sinuosity, first order stream gradients, and basin hypsometric integrals) measured in the Mecca Hills show that while the entire range has experienced deformation, the most recent loci of tectonic activity seem to occur in discrete zones that frequently (but not always) correspond with locations of highest relief. While the topographic growth of strike slip restraining bends is known to be often structurally controlled by preexisting basement weaknesses, analysis of geomorphic indices may help to determine if if faults accommodate the majority of the deformation as opposed to active folding. In addition, our analysis combined with prior geologic mapping provides the focus for applying geochronology and determining deformation rates throughout the region.