Controls on the erosion and geomorphic evolution of the San Bernardino and San Gabriel Mountains, southern California

Abstract

We have investigated what controls geomorphic evolution of active mountain belts by comparing the patterns of erosion in the San Gabriel and San Bernardino Mountains of southern California. These siblings in the Central Transverse Ranges are juxtaposed across the San Andreas fault and have been tectonically uplifted since the late Miocene under roughly similar conditions, yet their geomorphic expressions are very different. Because these ranges share numerous boundary conditions and because their syn-uplift exhumation is constrained by thermochronometric and geologic data, they provide a template to explore the role of specific parameters on the erosion of mountains. To address this, existing constraints on long-term exhumation are synthesized and used to construct best-guess models of erosion patterns in each range. These patterns of erosion are then compared to variables that are generally considered to be important influences on erosion rate. Erosion rate is correlated with topographic slope, suggesting that the two are coupled and that the questions of what controls erosion and what controls topography are interchangeable. Erosion patterns are also strongly influenced by the distribution of active structures, suggesting that deformation alone may be responsible for the observed patterns within each range. However, additional correlations with erosion exist in bedrock erodibility and mean annual precipitation. Altogether, broad differences between the two ranges may be explained by either structure, bedrock erodibility, long-term precipitation, or the greater duration of uplift of the San Gabriel Mountains. All of the boundary conditions that are generally considered important in the erosion of mountains are thus found to be related to erosion patterns in the Central Transverse Ranges. Without a means to isolate the relative influence of each parameters, however, we conclude that geomorphic differences between the ranges are the result of coincidental spatial arrangement of independent variables that are important for sculpting mountains.