Les massifs cristallins externes des Alpes occidentales françaises, un fragment de la zone interne varisque / The external crystalline massifs of the French western Alps, a part of the internal variscan zone

Abstract

The external crystalline massifs of the French western Alps, a part of the internal variscan zone. The external crystalline massifs (ECM) have undergone a polycyclic orogenic evolution. After Liassic rifting which mainly concerns the Haut-Dauphiné (Pelvoux) and the southern part of Belledonne, the ECM, external basement of the alpine chain, underwent several periods of deformations as well as a weak metamorphism. The main phase of deformation begins at the limit between the Late Oligocene and the Early Miocene. During this period, the ECM have been thrust towards the external part of the chain, but their general arcuate disposition was not fundamentally modified. This arcuate structure is inherited from the Variscan orogeny, the evolution of which is still decipherable, notwithstanding the Alpine orogeny. The Variscan evolution of the ECM, after a rifting stage in Cambrian times marked by both acid and basic intrusions, is characterized by several stages. The earliest stage is an high pressure-high temperature (HP/HT) stage, represented by an eclogitic and a less frequent granulitic facies metamorphism. Ordovician to Silurian in age, this metamorphism mainly affects supracrustal material. This was followed by crustal thrusting intra-Devonian in age, synchronous with a Barrovian synmigmatitic metamorphism which was followed by a partial to complete verticalization of the thrust pile, the metamorphic grade being unchanged. The evolution up to this stage, concerns the whole of the ECM with the exception of the south-western part of the Belledonne massif, in which the first thrusts are Dinantian in age. They include a fragment of Cambrian oceanic crust and are linked to an epizonal to mesozonal metamorphic event. In the other massifs, epizonal strike-slip faults are synchronous with these thrusts. Following this last stage and before the intrusion of late orogenic granites, at about 300-320 Ma, the majority of the massifs have been locally reworked by a high temperature-low pressure (HT-LP) mobilization, essentially postkinematic. The tectonic and metamorphic evolution of the ECM closely resembles that of the French Massif Central. In consequence the ECM form can be considered as a part of the French variscan chain.