LITHOLOGY, ROCK STRENGTH AND FRACTURE CONTROLS ON FLUVIAL ERODIBILITY

Lithology and its influence on rock mass strength exert a first order control on landscape evolution by affecting the ability of rivers to erode into bedrock, determined by fluvial erodibility. However, fractures complicate the relationship between rock strength and erodibility, as tectonic history affects erodibility through its control on fracture development. Here, we assess the extent to which lithology controls fluvial erodibility through its control on rock strength in the High Atlas Mountains, where the geological age of bedrock and the associated duration of tectonic history in the mountain belt increases from east to west. In this setting bedrock contains discontinuities along lithological boundaries, bedding, and secondary fabrics such as fractures and faults. We quantify the effect of fractures and other discontinuities on erodibility by: (i) collecting mechanical measurements of intact rock strength in the field; and (ii) extracting the normalised river channel steepness of rock units from a digital elevation model. We estimate fluvial erodibilities using empirical relationships and intact rock strength data, resulting in a range of values across two orders of magnitude. The landscape expresses only one order of magnitude range in erodibility through river channel steepness. Consequently, we estimate that fractures and other discontinuities effectively reduce the range of erodibilities between lithological units by up to an order of magnitude. Using data from across the mountain range we show that the tectonic history of bedrock influences the range of erodibilities between lithologies through its control on the development of fractures, with implications for long-term landscape evolution.