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La ségrégation et la migration des liquides de fusion lors de la déformation des migmatites : modélisation analogique, numérique et exemples de terrain

Abstract : We have modelled the formation and the horizontal shortening of stromatic migmatites in order to record the evolution of the melt-flow network and of the extraction processes. Thin superposed layers of partially molten wax reproduce the main characteristics of metatexites: anisotropic tensile and shear strength, moderate melt fraction (15-30 vol.%), negative buoyancy of the melt, and very high viscosity contrast between the melt and the solid matrix. The melt moves through the microscopic porosity of the wax and is expelled from compacting regions into dilatant veins. At low strain, the model shows that foliation-parallel leucosomes in flat stromatic migmatites may be veins attesting of a small amount (< 8%) of foliation-parallel shortening. During folding, inflation of melt-filled veins in the limbs (kink bands) or at the hinge (saddle reefs) generates an heterogeneous distribution of the melt and a rapid increase of the permeability in a highly anisotropic manner. With further shortening, the collapse of the macroscopic network of veins causes the melt expulsion, perpendicularly to the shortening direction. Both strain and melt extraction are increasingly localised in the axial planes of folds. The horizontal movement of the melt occurs by pulses, producing periodic extraction. If the shortening stops, the extraction rate decreases rapidly, even if the permeability high. Static extraction occurs only if a sufficiently high pressure gradient exists with a nearby low-pressure site. In the French variscan belt, an outcrop of migmatites has been studied and show structures that are comparable with the experimental ones. It shows also numerous competent enclaves that are surrounded by leucosomes. Thanks to specific analog experiments and numerical modelling, we show that this geometry is due to the pressure shadows that exist around a rigid objet in a soft matrix. This confirms that the perturbations of the pressure field govern the melt movement.
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https://tel.archives-ouvertes.fr/tel-00003225
Contributor : Joseph Barraud <>
Submitted on : Thursday, July 31, 2003 - 4:33:00 PM
Last modification on : Friday, November 6, 2020 - 3:53:05 AM
Long-term archiving on: : Wednesday, November 23, 2016 - 3:41:56 PM

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  • HAL Id : tel-00003225, version 1

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Joseph Barraud. La ségrégation et la migration des liquides de fusion lors de la déformation des migmatites : modélisation analogique, numérique et exemples de terrain. Géologie appliquée. Université Joseph-Fourier - Grenoble I, 2001. Français. ⟨tel-00003225⟩

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