Formation of Longitudinal River Valleys and the Fixing of Drainage Divides in Response to Exhumation of Crystalline Basement

Variations in rock strength act as a first-order control on mountain landscapes. However, the transient topographic signal of basement exhumation has not been explored. We use model outputs to demonstrate the mobility of drainage divides in mountain ranges in response to the exhumation of basement rocks and the implications for the morphology of river catchments. The exhumation of harder rocks within a catchment reduces upstream channel steepness and erosion rates in contrast to neighboring catchments. The results are a shift in the orogen-scale drainage divide toward the harder rocks, and the formation of range parallel longitudinal valleys as neighboring river networks capture the headwaters of catchments impacted by the harder lithology. Our model outputs provide a process explanation for the initiation of many longitudinal valleys in mountain ranges, and for the pinning of drainage divides on rocks of higher strength as seen the Central Pyrenees, Western Alps, or High Atlas. Plain Language Summary River networks that drain mountain ranges tend to have a fairly regular spacing along the range, and develop similar plan view shapes with a drainage divide at the crest of the range. However, when rocks of different hardness are exhumed to the surface, this simple pattern can be heavily modified. Large exposures of crystalline basement rocks often form the highest mountains and hence define the main drainage divide in many mountain chains such as the European Alps and the Pyrenees. Additionally, these massifs are often associated with river valleys that run parallel (longitudinal) to the mountain range. Our experiments use a numerical model that simulates the growth of mountain topography to demonstrate the processes responsible for the pinning of drainage divides and the formation of longitudinal valleys.

Automated summary

Variations in rock strength play a crucial role in shaping mountain landscapes, with the exhumation of harder rocks leading to changes in drainage divide positions and the formation of longitudinal valleys in mountain ranges.