Métamorphismes et déformations des séries cristallophylliennes du Chavanon, de la Sioule et d'Ussel (Massif Central français). Discussion du modèle de nappes du Massif Central.

The Chavanon, Sioule and Ussel metamorphic sequences are located in the center part of the French Massif Central and are thus a key area for the understanding of this inner part of the variscan belt of western Europe. Their inverted metamorphism is usually explained by a single Devonian nappe stacking event. Afterwards, a set of strike-slip regional shear-zones affect this stack of nappes. This work presents a comparison between the different sequences thanks to numerous new field and geochronological data. The succession of the deformation events is as follows : 1) a D1 event which is characterised by a S1 strongly dipping anatectic foliation, 2) a D2 event during which the S1 foliation is reworked by a flat amphibolite-facies S2 foliation which is more and more penetrative towards the basis of the sequences, 3) the sequences are affected by large-scale strike-slip shear zones as the la Courtine one for example. The new geochronological data argue for a polyphased succession of metamorphism and deformations. Viewed in a more global geodynamical model, those data are in agreement with an extrusive structure within 2 awls. The discussion points out the problems in the nappe model that prevailed.