Julien Babault

Influence of piedmont sedimentation on erosion dynamics of an uplifting landscape: An experimental approach

Models of relief development generally assume that eroded products are evacuated far from the landscape, whereas in nature they are often deposited at the foot of mountain belts, within continental environments. Because piedmont aggradation can modify the base level for erosion, we investigate the influence of piedmont sedimentation on the dynamics of an upstream relief. We developed an experimental study of relief dynamics using laboratory-scale models submitted to uplift under runoff-driven erosion. We compare the dynamics of topographies surrounded, or not, by a depositional belt made of eroded products coming from upstream. Piedmont aggradation acts on the dynamics of the upstream relief by modifying the relative uplift rate (applied uplift rate minus aggradation rate) that denudation tends to balance. Relief denudes at a lower rate than the applied uplift rate, so the mean elevation of the uplifting topography rises. When the time scale of aggradation is higher than the time scale of relief development, the topography cannot reach a steady state between denudation and the applied uplift rate as long as aggradation occurs. However, in this case denudation balances a continuously varying relative uplift rate during a dynamic equilibrium phase of the topography.

Timing and nature of Quaternary fluvial incision in the Ouarzazate foreland basin, Morocco

The history of alluvial fan and terrace formation within a stretch of the Ouarzazate basin along the southern margin of the Central High Atlas is reconstructed using geomorphological and 10Be terrestrial cosmogenic nuclide (TCN) methods. Alluvial fan and terrace incision was controlled partially by a drop in base level during the Pliocene or early Pleistocene as the outlet channel, the Draa river, progressively cut through the Anti-Atlas to the south of the Ouarzazate foreland basin, the drainage of which started to become external after a long period of internal drained conditions. The alluvial fans and terrace surfaces have abandonment ages that date to at least the past four glacial cycles. Their formation was strongly modulated by climate on glacial–interglacial time scales as base level dropped. This demonstrates a strong climatic control on sediment transfer and landscape denudation during the Quaternary and provides a model for understanding sediment transfer in other intracontinental mountain belts. Furthermore, these data show that mean rates of fluvial incision in this region range between 0.3 and 1.0 mm a−1 for the latter part of the Quaternary. This study provides the first comprehensive TCN chronology for the Atlas Mountains, and it illustrates the applicability and limitations of TCN methods.

Longitudinal to transverse drainage network evolution in the High Atlas (Morocco): The role of tectonics

The High Atlas of Morocco is a still-active, linear intracontinental mountain chain in the NW African plate, which results from weak crustal thickening associated with rift inversion during the Cenozoic and from uplift related to mantle thermal doming. A striking morphological feature of the High Atlas is the occurrence of both transverse and longitudinal (i.e., strike-parallel) drainage characterized by deep fluvial incision of more than 1000 m in low-relief topography of the axial zone of the chain. Most of the transverse component of the drainage appears to postdate the longitudinal component as indicated by recent or incipient captures and wind gaps. The longitudinal drainage is inherited from an early stage of fluvial organization controlled by the tectonic structures developed during upper crustal folding and thrusting in the post-Paleozoic cover. Amplification of N-S regional slope in the western High Atlas by continued crustal shortening and thickening triggered: (i) higher erosion rates in transverse than in longitudinal catchments and (ii) captures of longitudinal streams by transverse ones, creating a new organization of the drainage system toward the regional slope. Such evolution from a longitudinal to a transverse-dominated drainage may represent a common mechanism of fluvial network development in mountain belts where the amplification of the regional slope results from long-lived lithospheric convergence.

Shortening, structural relief and drainage evolution in inverted rifts: insights from the Atlas Mountains, the Eastern Cordillera of Colombia and the Pyrenees

Abstract The Atlas, Eastern Cordillera and Pyrenees are thick-skinned thrust-fold belts formed by tectonic inversion of rift basins in continental settings. A comparison of shortening between them shows a gradation from 20–25% in the central High Atlas, to 25–30% in the Eastern Cordillera, and c. 40% in the Pyrenees. Accordingly, there is a structural variation from interior zones with low structural relief and isolated basement massifs in the first two cases, to an axial culmination of stacked basement thrust sheets in the Pyrenees. This results in marked topographic and drainage variation: the High Atlas and Eastern Cordillera contain axial plateaus dominated by structure-controlled longitudinal rivers and orogen flanks with slope-controlled transverse rivers, whereas the Pyrenees show a two-sided wedge profile dominated by transverse rivers. In spite of singularities exhibited by each orogen, we propose that this spatial variation can be understood as reflecting different degrees of evolution in mountain building. Rapidly incising, transverse rivers are capturing earlier longitudinal streams of the Atlas and Eastern Cordillera, thus reducing their axial plateaux, which will eventually disappear into a transverse-dominated drainage. This pattern of landscape evolution may be characteristic of inversion orogens as they develop from initial stages of inversion to full accretion.

5.6 Plateau Uplift, Regional Warping, and Subsidence

The authors present the physiognomic and deep characteristics of the seven largest plateaus on Earth. In the second part, the evidence used by the Earth sciences community to infer rock and/or surface uplifts of plateaus are reviewed. Finally, the authors briefly present the debate between the feedbacks between plateaus uplift and global climate change. Some of the results contained in this chapter are still under debate and ongoing research is still needed to confirm them because different studies invoke for a same object contradictory processes, in that they result in conflicting uplift history.

Reply to comment by Yanni Gunnell and Marc Calvet on “Origin of the highly elevated Pyrenean peneplain”

Gunnell and Calvet [2006] (hereinafter referred to as GC) challenge the recent model that we proposed for the origin of the highly elevated Pyrenean peneplain by contest- ing our morphometric analysis of this chain and the relation we made between the morphological evolution and the piedmont sedimentation. Their reasoning is as follows: (1) According to Calvet [1996] (on which their comment is largely based) the high-elevation, low-relief surfaces in the Eastern Pyrenees are remnants of a peneplain that devel- oped before the Pliocene from applanation near to sea level, and which was later uplifted by 2000 m during the Plio- Quaternary (in other words, GC belong to the ‘‘applanation’’ school, whereas we woul d belong to the ‘‘altiplanation’’ school); (2) high-elevation, low-relief surfaces do not exist in the Central Pyrenees; (3) therefore the relationships we made between the morphology of the Central Pyrenees and the pattern of the detrital sedimentation in the adjacent Ebro foreland basin is meaningless; (4) contrary to the initial interpretation of Calvet [1996], GC recognize that crustal thickening did not develop since the Pliocene in the Eastern Pyrenees, so they appeal to another geodynamical process such as extension or lithosphere delamination to explain the supposed uplift.

Plateau reduction by drainage divide migration in the Eastern Cordillera of Colombia defined by morphometry and 10Be terrestrial cosmogenic nuclides

Catchment-wide erosion rates were defined using 10Be terrestrial cosmogenic nuclides for the Eastern Cordillera of the Colombian Andes to help determine the nature of drainage development and landscape evolution. The Eastern Cordillera, characterized by a smooth axial plateau bordered by steep flanks, has a mean erosion rate of 11 ± 1 mm/ka across the plateau and 70 ± 10 mm/ka on its flanks, with local high rates of > 400 mm/ka. The erosional contrast between the plateau and its flanks was produced by the increase in the orogen regional slope, derived from the progressive shortening and thickening of the Eastern Cordillera. The erosion rates together with digital topographic analysis show that the drainage network is dynamic and confirms the view that drainage divides in the Eastern Cordillera are migrating towards the interior of the mountain belt resulting in progressive drainage reorganization from longitudinal to transverse-dominated rivers and areal reduction of the Sabana de Bogota plateau. This article is protected by copyright. All rights reserved.

Tectonics from topography: two examples from the Pyrenees and the High Atlas

The Subandean Basins of South America extending from Trinidad to Tierra del Fuego have been the object of intensive exploratory activities (Fig. 1). The largest amount of hydrocarbons discovered during the last 30 years in these basins was found in complex structural terrains. A total of 59 Billion Barrels of Oil Equivalent (BBOE) have been discovered in areas affected by compressional tectonics. Of these basins, the largest discoveries are in the Furrial Trend of Venezuela (24 BBOE), followed by the Chaco area in Bolivia and Argentina (13 BBOE), the Llanos Foothills of Colombia (4.4 BBOE), and the Madre de Dios Basin of Peru (4.2 BBOE).