Lithological and climatic controls on fluvial landscape evolution of a post-orogenic dryland mountain belt

Abstract

Many mountain ranges in the world are in post-orogenic decay, a stage in which erosion is dominant over crustal thickening. The dominance of regional isostatic and mantle controls on rock uplift over localised fault uplift means climatic changes and intrinsic properties such as drainage capture and lithological variations are principal drivers of landscape evolution. In addition, dryland conditions in low to mid-latitude settings with negligible glacial ice cover result in fluvial processes forming a primary agent of erosion. This thesis investigates the lithological and structural control on long-term fluvial mountain landscape evolution, using a synthesis of detailed field observations and topographic analysis of rivers draining the post-orogenic dryland High Atlas Mountains, and field and geochronological studies on strath terraces in a catchment draining into its foreland basin. In the first part of this thesis, I assess the progressive mobility of the drainage divide in three lithologically and structurally distinct groups of bedrock in the High Atlas (NW Africa). Collection of field derived measurements of rock strength using a Schmidt hammer and extraction of river channel steepness from a digital elevation model allowed me to estimate contrasts in fluvial erodibilities of rock types. I show that fluvial erodibility between the weakest and strongest lithological units in the central High Atlas varies up to two orders of magnitude. The derived range in erodibilities in horizontal stratigraphy of the sedimentary bedrock cover leads to changes in erosion rates as rivers erode through strata, leading to drainage divide migration with a timescale on the order of 106-107 years. In contrast, I 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. These findings demonstrate drainage divide reorganisation can be induced through rock properties alone without the need for tectonic or climatic drivers, which has implications for the perception of autogenic dynamism of catchments and fluvial erosion in mountain belts. I further present geomorphological, sedimentological, and chronological datasets of strath surfaces and bedload sediments preserved in valleys and river reaches of the Mgoun River where morphology is controlled by the high contrast in rock erodibility and passive tectonic structure. To derive a record of terrace aggradation and incision along a 30 km river reach eroding a wedge-top basin and thrust front, over 200 kyr, I: (1) extract terrace surface and river channel elevations from a digital elevation model and field mapping to reconstruct river long profiles; (2) constrain the timing of bedload aggradation within the two latest strath levels using a new approach to OSL dose rate correction of gravels. I show that only one synchronous aggradation event occurred along this reach during the last interglacial maximum MIS 5e (~124 – 119 ka), linked to more frequent penetration of high magnitude storms of subtropical origin, connecting the trunk river with ephemeral tributaries, hillslopes, and alluvial fans. Strath terrace incision nucleates and propagates as a knickpoint connecting reaches of the river at a 105 yr timescale over ~ 20 km river length. In addition, further than 20 km upstream independent nucleation of river incision occurred, asynchronous from the first at a 104 yr timescale. Weak bedrock (10 -14 MPa) allowed the formation of strath terraces through lateral erosion, and three gorges of strong limestone (39 – 90 MPa) slowed down knickpoint propagation and thus erosional connectivity of the trunk river channel. These results demonstrate that studies that constrain river incision by dating terraces in one reach of a river may not constrain the river’s entire history of incision. I also collected sedimentological, grain size and lithology data from these terraces to derive information on sediment source and transport over the last 200 kyr. Terraces in an unconfined valley (3.5 km wide) record abundant gravity flow deposits, whilst terraces in confined valleys (150 – 750 m wide) preserve only completely fluvially reworked deposits. In addition, trends in grain size and clast lithological provenance reflect a low ratio of longitudinal to lateral sediment input. The dominance of lateral sediment flux enhances the effect of valley width on the timescale of buffering between hillslope and river channel response to climatic changes, by varying the accommodation for alluvial and tributary fans. These findings demonstrate lithological and structural controls on valley width influence the sensitivity of river reaches to external perturbations such as climate and internal dynamics spurred on by stochastic events, thus explaining the asynchronous independent incision of terraces along the river channel at a 104 yr timescale. Overall, the findings presented in this thesis demonstrate that lithological and structural properties of mountain belts affect the controls of internal dynamics and climatic changes on geomorphic response, at spatial scales which translate into landscape evolution over 104 – 107 yr timescales.