GEOMORPHIC ANALYSIS OF THE GROWTH OF THE MECCA HILLS: IMPLICATIONS FOR THE EVOLUTION AND KINEMATIC HISTORY OF THE SOUTHERN SAN ANDREAS FAULT SYSTEM

The Mecca Hills formed along the San Andreas Fault in Southern California from transpressional uplift, sediment transport, and erosion. To evaluate the long-term tectonic activity and kinematic history of the fault, we analyzed several geomorphic parameters to investigate potential topographic evolution models. Gray et al. (2014) investigated two models for the region: a radial growth model and a ‘structural knot’ model. The radial growth model suggests a focal point of uplift that spreads outwardly due to the conformity of the highest mean elevation and the highest uplift rate in the central Mecca Hills while the ‘structural knot’ model, due to a knot or bump on the fault, causes a region of uplift that started in the southeast corner on the Pacific Plate and travelled to the northwest corner. They assessed the region by mapping alluvial fan surfaces, using cosmogenic nuclide Beryllium-based geochronology to determine surface exposure ages and landscape development, and examining depth profiles, landscape morphometrics, and catchment-wide denudation rates. Gray et al. concluded that the radial growth model best fits the Mecca Hills region. After re-examining the topographical evolution model of the region through several different geomorphic indices, we find hypsometry integrals, stream profiles, stream-length gradients and basin elongation ratios suggest that the Northwestern region is more tectonically active than the Southeastern region, supporting the structural knot model. Watershed asymmetry factor analysis suggests a strong correlation of northwestwardly tilt near the San Andreas Fault, also supporting the structural knot model. In contrast, an analysis of drainage divides suggests that the Mecca Hills grow radially, not just in the Northwest direction. Overall, the geomorphic parameters investigated support a ‘structural knot’ topographic growth model in the Mecca Hills; these results may be useful for future seismic hazard analysis in the region.