EXTENSIONAL DUCTILE TECTONICS OF THE SIOULE METAMORPHIC SERIES (VARISCAN FRENCH MASSIF-CENTRAL)

In the Northern part of the Variscan French Massif Central, the Sioule series, from top to bottom, consists of a pre-Visean granite, migmati te, gneiss and mica schist. Two ductile deformations have been recogni zed. The earlier phase is characterized by a north-east - south-west t rending stretching lineation; the second phase, characterized by a nor th-west - south-east trending mineral, stretching and crenulation line ation, is better marked in the lower mica schist part than in the uppe r granito-gneissic part. This second phase occurred during retrogressi on of the metamorphic rocks; related shear criteria indicate a top to the south-west shear. The Namurian - Westphalian magmatic bodies such as the Echassieres leucogranite, Pouzol-Servant microgranite and numer ous north-east - southwest trending microgranite dykes are emplaced in extensional fractures related to the same north-west - south-east max imum stretching direction. The asymmetrical shapes of the two granitic massifs indicate that they intruded towards the south-east. The synki nematic retrogression of the metamorphic rocks, the shape of the magma tic bodies and a re-examination of the numerous available data support the interpretation that the deformation is due to the extensional tec tonic regime related to the Variscan crustal re-quilibration. This int erpretation is in agreement with the correlation of the Sioule series with the Chavanon series. The two series belong to a unique tectono-me tamorphic unit left-laterally offset by the Stephanian motion of the S illon Houiller fault. This study also shows that the Sillon Houiller d id not play a significant part during the Namurian-Westphalian extensi onal tectonics of the Massif Central.