Distinguishing between climatic and tectonic influences on topography: Best practices and application to the Northern Bolivian Andes

Abstract:

The hypothesis that high rainfall rates have the power to drive erosion and shape landscapes is tantalizing. However, definitive support for this hypothesis has been somewhat elusive, in part because high rainfall rates often occur in areas of high rock uplift rates. Here we present a number of best practices, developed from theory and landscape evolution modeling, that can be used to decipher whether climate, tectonics, or both importantly affect landscape morphology and evolution. For illustration we assume that there is a single, dominant gradient in both rainfall and rock uplift rates across the landscape. We quantify the morphology of rivers using the normalized channel steepness index, k_sn, or channel slope normalized by drainage area. If rainfall influences landscape evolution solely through the production of runoff and fluvial discharge, then k_sn is inversely proportional to the upstream averaged rainfall rate. In contrast, k_sn is directly proportional to the local rock uplift rate. Exploring patterns in channels that primarily flow perpendicular to the gradient in forcing and channels that flow parallel to the gradient in forcing, illustrates the influence of different drivers. In channels that flow in the direction of the gradient, the rock uplift forcing should have a greater impact on channel form. However, when comparing among channels that flow perpendicular to the gradient, k_sn should vary with the local rainfall rate. We apply these ideas in the Eastern Cordillera of the Northern Bolivian Andes, where modern rainfall rates and hypothesized rock uplift rates vary in the same dominant direction. We explore the morphology of channels that are both parallel and perpendicular to the gradient in forcings and find that the pattern of rock uplift dominates the morphology of the landscape, but that climate likely has a secondary effect. Using published erosion rate data from detrital ¹⁰Be, we find no correlation between rainfall and erosion rates.