EVALUATING THE RELATIVE INFLUENCE OF ROCK STRENGTH AND UPLIFTRATE ON THE GEOMORPHIC EXPRESSION OF A MOUNTAIN BELT: CASE STUDY FROM THE WESTERN TRANSVERSE RANGES OF CALIFORNIA

The Western Transverse Ranges are an east-west trending tectonically active mountain belt in Southern California. The lower elevation coastal regions have some of the fastest uplift rates in North America; up to 7 mm/yr. However, the higher-elevation mountain belt interior is rising much slower at rates of 0.5 to 2mm/yr. We combined uplift rates, lithology, rock strength, stream profiles and normalized channel steepness to investigate the disconnect between topography and uplift rates and to determine the dominant factors that control the overall topography of the Western Transverse Ranges.

Geomorphic, tectonic and fluvial data was aggregated in ArcGIS Pro for spatial comparison. Normalized channel steepness allowed us to identify areas of imbalance in stream gradients and compare it to lithology, faulting, and uplift rates.

Higher elevations correlate with increased rock strength, older lithologies and elevated normalized channel steepness. Conversely, uplift rates did not correlate with elevation and normalized channel steepness. Profiles of major N-S trending streams show apparent lithologic knickpoints and higher normalized channel steepness in older more resistant lithologies. In the lower elevations and weaker lithologies, high uplift rates do not result in tectonic stream channel perturbations.

The inverse relationship between uplift rate and elevation is a result of lithologic differences between the high and low elevations. Cretaceous to Oligocene sandstone dominated units comprise most of the high elevations, and knickpoints/ knickzones are observed where large sandstone beds interact with fluvial channels. We hypothesize that long-term uplift along interior faults and deeper regional detachment structures exhumed the higher topography of the Western Transverse Ranges. Though, more resistant lithologies are keeping the interior of the Western Transverse Ranges high. While Pliocene to Pleistocene lithologies at the margins of the mountain belt are uplifting at high rates, they are less resistant and erode faster than the interior. This study suggests that even in areas of high uplift rates, the geomorphic expression of an active mountain belt is largely controlled by lithology.