Geometry and kinematics of recumbent folding and low-angle detachment in the Pardailhan nappe (Montagne Noire, Southern French Massif Central)

Abstract

The Pardailhan nappe is the uppermost unit of the superimposed Variscan recumbent fold nappes in the Southern Montagne Noire thrust belt. Detailed mapping and structural investigation demonstrates the presence of a welldevel-oped upper normal limb, a frontal ENE-WSW oriented major hinge and a predominant overturned limb many kilometres in size in lower Palaeozoic non-metamorphic sediments. Finite strain and microstructures related to recumbent folding show maximum elongation and stretching perpendicular to the fold axes consistent with tangential shortening and general south-southwestward overthrust tectonics. Minor internal deformation throughout the fold structure suggests a rigid overturning mechanism and only small amounts of progressive shear during the Variscan recumbent folding (late Early Carboniferous). Post-shortening deformation in the nappe is accounted for by subsequent heterogeneous deformation in extensional shear and fault zones. The earlier folded and schistose structures were displaced along E-W trending low-angle normal faults. Microstructures of the superimposed brittle and ductile deformation in gently S-dipping detachment zones indicate southwest to southward hanging wall movements in the uppermost units. Throughout the lower limb, the extensional features show a progressive rotation into a W-SW direction and a dextral wrench component along the tectonic and metamorphic discontinuity with underlying metamorphic polyphase fold structures. Mechanisms and timing of recumbent fold development and subsequent detachment tectonics are discussed in relation to Variscan orogenic convergence and late orogenic (Late Carboniferous) extension induced by the uplift of metamorphic basement. The compressional and subsequent extensional tectonics in the Southern Montagne Noire nappe belt are associated with the evolution of a late Variscan metamorphic core complex.