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

Authors:
Jesse R Zondervan at University College London
Jesse R Zondervan
Martin Stokes at University of Plymouth
Martin Stokes
Sarah J Boulton at University of Plymouth
Sarah J Boulton
Anne E Mather at University of Plymouth
Anne E Mather
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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.
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Palaeo river long profile reconstruction in a fold-and-thrust belt: river terraces as
archives of Quaternary incision and aggradation in the Atlas Mountains
Jesse R. Zondervan1*, Martin Stokes1, Sarah J. Boulton1, Anne E. Mather1, Matt W. Telfer1
1 School of Geography, Earth and Environmental Sciences, Plymouth University, Plymouth PL4 8AA
*Corresponding author: jesse.zondervan@plymouth.ac.uk
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 periods1, 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 capacity2. 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 events3. 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 methods4.
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
years5. 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):
Quaternary Science Reviews v. 166, p. 363-379.
2. Hancock;, G. S. & Anderson, R. S., 2002, Numerical modeling of fluvial strath-terrace formation in response
to oscillating climate: GSA Bulletin, v. 114, p. 1131-1142.
3. Demoulin, A., Mather, A.E. and Whittaker, A.C., 2017, Fluvial archives, a valuable record of vertical crustal
deformation: Quaternary Science Reviews v. 166, p. 10-37.
4. Rixhon, G., Briant, R. M., Cordier, S., Duval, M., Jones, A., & Scholz, D., 2017. Revealing the 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.
ResearchGate has not been able to resolve any citations for this publication.
Transient fluvial incision as an indicator of active faulting and Plio-Quaternary uplift of the Moroccan High Atlas
Article
Full-text available
  • Jun 2014
  • TECTONOPHYSICS
One of the challenges facing Earth Scientists is to determine the extent to which geomorphic features can be used to extract tectonic signals from landscapes. Here, we quantitatively analyse the long profiles of rivers that drain southwards across the South Atlas Fault (SAF), a thrust fault that forms the southern margin of the High Atlas Mountains in Morocco, to derive new data on the Late Cenozoic activity of this fault system. River long profiles were extracted for 32 major rivers flowing southwards into the Ouarzazate Basin. Of these, eleven exhibit concave-up river profiles with a mean concavity of 0.64 and normalized steepness indices in the range 47.5 – 219.0 m0.9. By contrast, 21 rivers exhibit at least one knickpoint upstream of the thrust front. Knickpoint height varies from 100 – 1300 m, with calculated incision at the range bounding fault ranging from 80-900 m, despite the drainage areas upstream of the knickpoint ranging over several orders of magnitude. In map view, knickpoint locations generally plot along sub-parallel lines and there are no obvious relationships with lithological units for knickpoints exhibiting slope-break morphology. Channel reaches below slope-break knickpoints have higher mean concavities (0.76) than above the knickpoint indicative. This observation combined with a lithological or river-capture origin for the knickpoints having been ruled out suggests that an increase in uplift rate along a planar fault zone during the Plio-Quaternary caused the initiation of the transient response (i.e., knickpoint formation) to a change in base-level observed in the river profiles. This uplift event can be correlated to the convective removal of the lithospheric root to the Atlas Mountains resulting in the anomalously high topography at the present day.
View
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)
Article
  • May 2017
  • QUATERNARY SCI REV
This study documents river terraces from upstream reaches of the Dadès River, a major fluvial system draining the south-central High Atlas Mountains. Terraces occur as straths with bedrock bases positioned at 10 m altitudinal intervals up to 40 m (T1-T5) above the valley floor, becoming less common between 50 and 140 m. The rock strength, stratigraphy and structure of the mountain belt influences terrace distribution. Terraces are absent in river gorges of structurally thickened limestone; whilst well-developed, laterally continuous terraces (T1-T4) form along wide valleys occupying syncline structures dominated by weaker interbedded limestone-mudstone. Terrace staircases develop in confined canyons associated with weaker lithologies and influence from structural dip and stratigraphic configuration.
View
View
Numerical modeling of fluvial strath terrace formation in response to oscillating climate
Article
  • Sep 2002
Many river systems in western North America retain a fluvial strath-terrace rec ord of discontinuous downcutting into bedrock through the Quaternary. Their importance lies in their use to interpret climatic events in the headwaters and to determine long-term incision rates. Terrace formation has been ascribed to changes in sediment supply and/or water discharge produced by late Quaternary climatic fluctuations. We use a one-dimensional channel- evolution model to explore whether temporal variations in sediment and water discharge can generate terrace sequences. The model includes sediment transport, vertical bedrock erosion limited by alluvial cover, and lateral valley-wall erosion. We set limits on our modeling by using data collected from the terraced Wind River basin. Two types of experiments were performed: constant- period sinusoidal input histories and variable-period inputs scaled by the marine δ18O rec ord. Our simulations indicate that strath-terrace formation requires input variability that produces a changing ratio of vertical to lateral erosion rates. Straths are cut when the channel floor is protected from erosion by sediment and are abandoned—and terraces formed—when incision can resume following sediment-cover thinning. High sediment supply promotes wide valley floors that are abandoned as sediment supply decreases. In contrast, wide valleys are promoted by low effective water discharge and are abandoned as discharge increases. Widening of the valley floors that become terraces occurs over many thousands of years. The transition from valley widening to downcutting and terrace creation occurs in response to subtle input changes affecting local divergence of sediment-transport capacity. Formation of terraces lags by several thousand years the input changes that cause their formation. Our results suggest that use of terrace ages to set limits on the timing of a specific event must be done with the knowledge that the system can take thousands of years to respond to a perturbation. The incision rate calculated in the field from the lowest terrace in these systems will likely be higher than the rate calculated by using older terraces, because the most recent fluvial response in the field is commonly downcutting associated with declining sediment input since the Last Glacial Maximum. This apparent increase in incision rates is observed in many river systems and should not necessarily be interpreted as a response to an increase in rock-uplift rate.
View
Fluvial archives, a valuable record of vertical crustal deformation: Quaternary Science Reviews v
  • Jan 2017
  • 10-37
  • A Demoulin
  • A E Mather
  • A C Whittaker
Demoulin, A., Mather, A.E. and Whittaker, A.C., 2017, Fluvial archives, a valuable record of vertical crustal deformation: Quaternary Science Reviews v. 166, p. 10-37.