Pull-apart emplacement of the Margeride granitic complex (French Massif Central). Implications for the late evolution of the Variscan orogen

摘要

A microstructural, magnetic fabric and gravity study is performed on the Carboniferous Margeride granitic complex that crops out in the central part of the Variscan French Massif Central. This complex consists of three facies, namely, a main porphyritic monzogranite, a two-mica granite, and late leucogranite dykes and stocks. In spite of local variations, the magnetic lineation mainly trends NW-SE with a shallow plunge throughout the complex. The magnetic foliation pattern is more complex with various strikes and generally a moderate dip. This fabric pattern complies with the late-orogenic NW-SE regional extensional deformation already recognised in this part of the Variscan Belt. New gravity measurements complete available data and are used to build up a gravity map and a cross-section with which four gravity minima are identified. Three gravity minima are interpreted as extensional fractures consistent with a NW-SE maximum stretching. The last gravity low corresponds to an extensional jog between the two westernmost fractures. The structural and gravity features of the complex are used to propose a feeding and emplacement model controlled by the regional late-orogenic extensional tectonics. The Margeride complex is interpreted as a kilometre-scale laccolith-like pluton emplaced in a transtensional setting controlled by a NW-SE opening direction. Such a model strengthens the relationships between pluton emplacement and late-orogenic collapse of the Variscan Belt.