Palaeo river long profile reconstruction in a fold-and-thrust belt: river terraces as archives of Quaternary incision and aggradation in the Atlas Mountains

Abstract

Geomorphological analysis of rivers is a popular tool to decode the tectonic evolution and erosional history of actively uplifting orogens. In particular, river longitudinal profiles contain information on crustal deformation in the geological present, and transient response to Quaternary tectonic perturbation is recorded in knickpoints propagating through catchments. The catchments in the southern Atlas Mountains in Morocco exhibit knickpoints and an abundance of river terraces recording glacial-interglacial river incisional periods 1 , enabling the reconstruction of paleo river long profiles. River strath terraces are formed by transitions between valley widening and downcutting of terraces in response to local divergence of sediment-transport capacity 2. Consequently, they record changes in catchments due to climate or tectonics. Given a chronology, river strath terraces can constrain rates of fluvial erosion and so the speed of knickpoint propagation and vertical incision. They can also show that terraces might not have formed synchronously throughout the catchment. The evolution of a paleo-river long profile may in fact show propagation of knickpoints and/or river capture events 3. Getting the paleo-river long profile right is integral to obtaining the right incisional and aggradational histories of mountainous rivers responding to tectonic as well as climatic perturbations. Unlocking the river terrace archive in a fold-and-thrust belt requires mapping, characterisation and strong age control of terraces. The latter is helped significantly by recent advances in numerical dating methods 4. A combination of remote sensing and field mapping was completed in May 2018, followed by Optically Stimulated Luminescence chronological work over the summer. River terraces have been mapped with newly released high-resolution DEM data in the southern High Atlas in Morocco, and additional surveying of these terraces and their overlying fluvial conglomerates was done in the field. River profiles suggest catchment reorganisation is an important process in the southern Atlas, while the presence of knickpoints suggest tectonic controls have influenced landscape development over the last few million years 5. A pilot study of material collected in the field highlights both the opportunities and challenges of dating glacial-interglacial conglomeratic river strath terraces using established and new OSL techniques. A combination of geomorphological remote sensing, field and OSL chronologies helps us to unlock the terrace archive of incision and aggradation in an active fold-and-thrust belt. References 1. Stokes, M. et al., 2017, Controls on dryland mountain landscape development along the NW Saharan desert margin: Insights from Quaternary river terrace sequences (Dadès River, south-central High Atlas, Morocco): pace of river landscape evolution during the Quaternary-recent developments in numerical dating methods: Quaternary Science Reviews, v. 166, p. 91-113. 5. Boulton, S. J., Stokes, M., and Mather, A. E., 2014, Transient fluvial incision as an indicator of active faulting and Plio-Quaternary uplift of the Moroccan High Atlas: Tectonophysics, v. 633, p. 16-33.