Abstract
Numerical model simulations and experiments have suggested that when migration of the main drainage divide occurs in a mountain belt, it can lead to the rearrangement of river catchments, rejuvenation of topography, and changes in erosion rates and sediment flux. We assess the progressive mobility of the drainage divide in three lithologically and structurally distinct groups of bedrock in the High Atlas (NW Africa). The geological age of bedrock and its associated tectonic architecture in the mountain belt increases from east to west in the study area, allowing us to track both variations in rock strength and structural configuration which influence drainage mobility during erosion through an exhuming mountain belt. Collection of field derived measurements of rock strength using a Schmidt hammer and computer based extraction of river channel steepness permit estimations of contrasts in fluvial erodibilities of rock types. The resulting difference in fluvial erodibility between the weakest and the strongest lithological unit is up to two orders of magnitude. Published evidence of geomorphic mobility of the drainage divide indicates that such a range in erodibilities in horizontal stratigraphy of the sedimentary cover may lead to changes in erosion rates as rivers erode through strata, leading to drainage divide migration. In contrast, we show that the faulted and folded metamorphic sedimentary rocks in the centre of the mountain belt coincide with a stable drainage divide. Finally, where the strong igneous rocks of the crystalline basement are exposed after erosion of the covering meta-sediments, there is a decrease in fluvial erodibility of up to a factor of three, where the drainage divide is mobile towards the centre of the exposed crystalline basement. The mobility of the drainage divide in response to erosion through rock-types and their structural configuration in a mountain belt has implications for the perception of autogenic dynamism of drainage networks and fluvial erosion in mountain belts, and the interpretation of the geomorphology and downstream stratigraphy.
Highlights
Collisional mountains form the erosional focus of the Earth’s surface
Fluvial erodibility K values based on Eq 4 and ksn, normalised to the Mesozoic red beds, vary by a factor of four to fifteen (Fig. 4d)
This study shows that in a collisional mountain belt, the drainage divide will be mobile in response to changes in erosion rates of rivers incising into gently dipping and deformed strata of contrasting erodibility in the sedimentary cover, and in response to the exhumation of strong crystalline basement
Summary
Collisional mountains form the erosional focus of the Earth’s surface. The tectonic and climatic interpretation of mountain topography and depositional stratigraphy depends on understanding the dynamics of eroding bedrock rivers. The central drainage divide of a mountain belt is the topographic boundary between river catchments draining either flank. Bedrock erodibility is expected to play a significant role in drainage divide reorganisation since heterogeneous exhumation of weak and strong substrates can enhance and suppress erosion respectively (Giachetta et al, 2014; Gallen, 2018) and cause topographic rejuvenation, for example through the exhumation of a basement palaeosurface (Strong et al, 2019). This is especially the case in postorogenic settings where erosion is dominant over crustal thickening (Gallen, 2018; Bernard et al., 2019). The magnitude of erodibility variation within a mountain belt, and the mobility of the drainage divide as rivers erode through its stratigraphy are still relatively unexplored
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