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Mécanismes d'éclogitisation et conséquences mécaniques pour l'exhumation des roches métamorphiques de haute pression

Abstract : The granulite-facies anorthositic unit on northwestern Holsnøy, within the Bergen Arcs, Norway, was buried and partially eclogitized at depth during the caledonian orogenesis. The protolith and the high-pressure rocks and structures were remarkably preserved throughout exhumation, enabling us to study in detail eclogitization processes and their consequences for exhumation and deep crustal movements. Inherited garnets, although mostly preserved through eclogite-facies transformations, were fractured and reequilibrated by diffusion from their rims. Back-scattered electron (BSE) and electron microprobe analysis of the different garnet generations involved highlights the very slow diffusion of Ca compared to Fe and Mg, impeding the reequilibration of inherited garnets in grossular content during the eclogite-facies event. The systematic distribution of the orientations of the fractures cutting through inherited garnets shows that fracturing results from the volume decrease associated with eclogite facies reactions. Small-scale propagation of eclogitized domains involves combination of fluid diffusion, metamorphic reactions and mineral fracturing. Eclogite-facies deformation is very consistent at the scale of the studied zone (5*5 km), and results therefore from the action of large-scale tectonic stresses, rather than local stresses related to metamorphic reactions. When the unit is restored to its original position at depth within the caledonian slab, the top-to-the-east sense of shear is interpreted as resulting from the the thrusting of kilometer-scale crustal slivers. The eclogite-facies shear zones, which enable the mechanical decoupling between the light crustal slivers and the dense underlying mantle, play therefore a crucial role in the initiation of exhumation. The relevance of such a conceptual model is assessed through the unidimensional channel-flow model. Within this model, all material properties are integrated into a single adimensional parameter, the exhumation number, which describes the competition between reactions-related density increase (impeding exhumation) and viscosity decrease (promoting exhumation). The evolution of the exhumation number through the partially eclogitized domain controls the circulation over the entire channel, and the burial of high-exhumation-number crust leads to the formation of a backward flow starting in the partially eclogitized zone. We show that exhumation rates of the order of the plate convergence velocities are only reached in the transitional regime when the nature of the buried crust drastically change.
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https://tel.archives-ouvertes.fr/tel-00010888
Contributor : Hugues Raimbourg <>
Submitted on : Monday, November 7, 2005 - 7:11:25 PM
Last modification on : Monday, December 14, 2020 - 9:53:10 AM
Long-term archiving on: : Friday, April 2, 2010 - 10:30:54 PM

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  • HAL Id : tel-00010888, version 1

Citation

Hugues Raimbourg. Mécanismes d'éclogitisation et conséquences mécaniques pour l'exhumation des roches métamorphiques de haute pression. Géologie appliquée. Université Pierre et Marie Curie - Paris VI, 2005. Français. ⟨tel-00010888⟩

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