Tectonique et dynamique du relief : Le socle armoricain au Pléistocène

Abstract : Continental topography is controlled principally by tectonics, eustasy and clirnate through the mechanism of fluvial incision. In order to characterize the way in which relief evolves over time, we must understand how rivers adapt to variations in these parameters. This remains one of the major unsolved problems in geomorphology. The Armorican Massif is a basement complex of moderate relief, whose topography is characterized by sorne plateaus incised by a network of valleys. A study of this incision has been undertaken in order to understand the mechanisms of valley formation and to determine what parameters control this evolution. The approach adopted has been to combine fieldwork with an analysis of the relief using a digital elevation model (DEM). One of the applications of the methods developed is the measurement of vertical displacements of the lithosphere from the consequent incision of the valleys. First of ali, it is shown that the formation of the present relief of the Armorican Massif is due to post-Pliocene deformation. Study of this topography suggests a model of the evolution of the river system, which includes certain features distinct to the region. This evolution is influenced by the heterogeneity of the basement, which in sorne extreme cases produces local base levels that disconnect the topography from regional variations in base level. The alternation of glacial and interglacial periods has induced brief phases of zero incision on the time scale of every 1oS years. On the other hand, glacio-eustatic variations have had no effect on the incision of the bedrock. This may be explained by the higher frequency of these variations, and by the fact that the rivers were disconnected from the extemal base level during episodes of sea level fall. Finally, it is shown that there are regional variations in the amount of erosion, linked to disequilibrium longitudinal river profiles, which can be explained solely by the relative differences in uplift. To fust order, tectonics controls the evolution of topography at ali spatial scales, over the range -1OOm to -1OOlan, and at a temporal scale of 1o5 to 1o6 years. Incision of the Armorican basement complex is thus controlled principally by a Pleistocene lithospheric deformation of low intensity but long wavelength.
Document type :
Géologie appliquée. Université Rennes 1, 1997. Français

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Stéphane Bonnet. Tectonique et dynamique du relief : Le socle armoricain au Pléistocène. Géologie appliquée. Université Rennes 1, 1997. Français. <tel-00619316>



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