La topographie comme marqueur de la déformation de la lithosphère

L’objectif de ma recherche est de contribuer à la compréhension des processus géodynamiques àl’origine des chaînes de montagnes par l’étude des mouvements tectoniques. Ces derniers se décomposenten déplacements horizontaux qui décalent les rivières, et en déplacements verticaux de la topographie enréponse à un amincissement ou un épaississement de la lithosphère, ou encore en raison des mouvementsdans le manteau inférieur à l’origine de la topographie dynamique. Les déplacements verticaux de latopographie correspondent au surface uplift comme l’ont défini England et Molnar (1990), c'est-à-dire lerock uplift moins l’érosion. Dans les chaînes actives, le rock uplift est généré par la tectonique qui force lesrivières à inciser. Même si l’érosion dépend également du flux d’eau et qu’elle est donc modulée par leclimat, elle est surtout contrôlée par la tectonique. Je me suis jusqu’à présent intéressé à la dynamique dusurface uplift dans les chaînes, et à son influence sur le réseau de drainage et le remplissage des bassinssédimentaires

El relieve de los Pirineos

La evolución del relieve de los Pirineos es un buen ejemplo de la interacción entre procesos superficiales y profundos. La elevada topografía media de la cadena es fruto del acortamiento y engrosamiento cortical, hoy inactivo, que se produjo principalmente durante el Paleógeno. Aunque la topografía es antigua, el escarpado relieve local es moderno. La etapa paleógena culminó con la formación de superficies de erosión con bajo relieve, formadas ya desde su inicio a gran altitud, en relación con el aumento de nivel de base fluvial causado por la sedimentación continental en los piedemontes o antepaíses de la cadena. La formación de superficies en altitud comportó la preservación de la raíz cortical y de la topografía elevada a pesar del allanamiento. Los valles que caracterizan la red de drenaje actual y el relieve joven disectan profundamente los restos de la superficie de erosión, y se deben a cambios en la conexión de la red fluvial remota y, sobre todo, a los cambios climáticos del Plio-Cuaternario (ciclos glaciares).The landscape evolution in the Pyrenees is a good example of the interactions between surface and deep earth processes. The high topography is relatively old, resulting from crustal shortening and thickening during Palaeogene times, now inactive. However, the local relief is relatively young. The palaeogene stage produced a high-elevation, low-relief erosional surface in relation to the fluvial base level rise associated to the continental sedimentary aggradation in the forelands. This permitted the preservation of the Pyrenean crustal roots and the high topography, in spite of the lowering of the local relief. The incision of the present drainage network with high relief is due to changes in the connections of the distal fluvial network and the climate changes initiated during the Plio-Quaternary (i.e., the glacial cycles)

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

15 p.International audienceThe 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

The inverted Triassic rift of the Marrakech High Atlas : a reappraisal of basin geometries and faulting histories

The High Atlas of Morocco is an aborted rift developed during the Triassic-Jurassic and moderately inverted during the Cenozoic. The Marrakech High Atlas, with large exposures of basement and Triassic early syn-rift deposits, is ideal to investigate the geometries of the deepest parts of a rift, constituting a good analogue for pre-salt domains. It allows unraveling geometries and kinematics of the extensional and compressional structures and the influence that they exert over one another. A detailed structural study of the main Triassic basins and basin-margin faults of the Marrakech High Atlas shows that only a few rift faults were reactivated during the Cenozoic compressional stage in contrast to previous interpretations, and emphasizes that fault reactivation cannot be taken for granted in inverted rift systems. Preserved extensional features demonstrate a dominant dip-slip opening kinematics with strike-slip playing a minor role, at variance to models proposing a major strike-slip component along the main basin-bounding faults, including faults belonging to the Tizi n'Test fault zone. A new Middle Triassic paleogeographic reconstruction shows that the Marrakech High Atlas was a narrow and segmented orthogonal rift (sub-perpendicular to the main regional extension direction which was ~ NW-SE), in contrast to the central and eastern segments of the Atlas rift which developed obliquely. This difference in orientation is attributed to the indented Ouzellarh Precambrian salient, part of the West African Craton, which deflected the general rift trend in the area evidencing the major role of inherited lithospheric anisotropies in rift direction and evolution. As for the Cenozoic inversion, total orogenic shortening is moderate (~ 16%) and appears accommodated by basement-involved large-scale folding, and by newly formed shortcut and by-pass thrusting, with rare left-lateral strike-slip indicators. Triassic faults commonly acted as buttresses

High elevation of low-relief surfaces in mountain belts: does it equate to post-orogenic surface uplift?

International audienceWe present experiments of upraising and relaxing topographies showing that peneplanation can occur above the ultimate base level (sea level). After active uplift, the erosion of a topography bounded by a piedmont generates a final smooth and highly elevated topography. Smoothing at high elevation is even possible during active uplift if the evolution of topography is disrupted by the deposition of the products of erosion on its piedmont which is the case at the transition from underfilled to overfilled conditions in foreland basins

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

4 p.International audienceGunnell and Calvet [2006] (hereinafter referred to asGC) challenge the recent model that we proposed for theorigin of the highly elevated Pyrenean peneplain by contest-ing our morphometric analysis of this chain and the relationwe made between the morphological evolution and thepiedmont sedimentation. Their reasoning is as follows: (1)According to Calvet [1996] (on which their comment islargely based) the high-elevation, low-relief surfaces in theEastern 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 existin the Central Pyrenees; (3) therefore the relationships wemade between the morphology of the Central Pyrenees andthe pattern of the detrital sedimentation in the adjacent Ebroforeland basin is meaningless; (4) contrary to the initialinterpretation of Calvet [1996], GC recognize that crustalthickening did not develop since the Pliocene in the EasternPyrenees, so they appeal to another geodynamical processsuch as extension or lithosphere delamination to explain thesupposed uplift

Drainage reorganization during mountain building in the river system of the Eastern Cordillera of the Colombian Andes

The Eastern Cordillera of Colombia is a thick-skinned thrust-fold belt that is characterized by two topographic domains: (1) the axial zone, a high altitude plateau (the Sabana de Bogotá, 2500 masl) with low local relief and dominated by longitudinal rivers, and (2) the Cordillera flanks, where local relief exceeds 1000 m and transverse rivers dominate. On the basis of an analysis of digital topography and river parameters combined with a review of paleodrainage data, we show that the accumulation of shortening and crustal thickening during the Andean orogeny triggered a process of fluvial reorganization in the Cordillera. Owing to a progressive increase of the regional slope, the drainage network evolves from longitudinal to transverse-dominated, a process that is still active at present. This study provides the idea of progressive divide migration toward the inner part of the mountain belt, by which the area of the Sabana de Bogotá plateau is decreasing, the flanks increase in area, and ultimately transverse rivers will probably dominate the drainage of the Cordillera

Evidences for a non-steady state and a reorganization of the fluvial network in the Ouarzazate basin (Morocco)

Since it became externally drained, the fluvial dynamics of the Ouarzazate foreland basin has been dominated by erosive processes due to a base level fall. The basin landscape is dominated by extensive Quaternary deposits aggraded by the transverse rivers draining the High Atlas Mountains. These deposits accumulated in valleys and pediments incised in the basin Tertiary infill, characterized mostly by fine-grained and soft sediments. Evidences of fluvial captures are abundant in recent times and throughout the Quaternary, in a scenario where there is more incision efficiency in the interfluves’ secondary streams (characterized by fine-grained or sediment-free pediments) than in the main transverse rivers (characterized by thick gravel deposits). Satellite imagery, elevation data and field observations demonstrate how minor streams increase their catchment areas by headward erosion, creating extensive pediments lying under the main rivers and producing captures of the main rivers. This study demonstrates the unsteady-state of the drainage network of the Ouarzazate basin and its recent reorganization, apparently uncoupled from tectonic or climatic processe

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 Be 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 >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 Bogotá plateau

Geodinámica de las cordilleras del Alto y Medio Atlas : síntesis de los conocimientos actuales

Se presenta una síntesis geodinámica de las cadenas de intraplaca del Alto y Medio Atlas de Marruecos. Estas cadenas derivan de la inversión tectónica de cuencas extensionales Jurásicas, con un acortamiento orogénico modesto (máximo del 24% en el Alto Atlas y del 10% en el Medio Atlas) heterogéneamente distribuido en el tiempo y en el espacio. Las relaciones entre tectónica y sedimentación en el margen sur del Alto Atlas indican que la deformación compresiva tuvo lugar entre el Eoceno medio y el Cuaternario. Consecuentemente con los valores de acortamiento, el espesor de la corteza detectado por gravimetría no supera los 40 km. A pesar de ello, la topografía de las cadenas del Atlas es elevada, y el modelado de campos potenciales sugiere que está causada por un marcado adelgazamiento litosférico, independiente de la tectónica compresiva regional. Un abundante magmatismo alcalino indica que el adelgazamiento litosférico coexistió con el acortamiento en los últimos 15 Ma, aunque los indicadores geomorfológicos sugieren que la mayor parte del levantamiento de origen mantélico es relativamente reciente (últimos 5 Ma). La termocronología indica que la cantidad de erosión es generalmente pequeña en el Atlas, registrándose de manera notable solo en los últimos 20-25 Ma