Origin of a large-scale fold nappe in the Montagne Noire, Variscan belt, France

Detailed structural analysis of a large recumbent fold in low-grade Palaeoz oic sediments of the Montagne Noire, and the previously established tectono metamorphic history of its crystalline substratum, indicate a two-stage dev elopment history. First, relatively homogeneous crustal thickening produced folds with steep axial planes associated with a regional cleavage (S-1). S ubsequently, these structures became rotated and amplified in a subhorizont al flow with pure shear and simple shear components related to low-angle th rusting. The development of an associated horizontal crenulation cleavage ( S-2) with a component of vertical shortening of S-1, plus evidence for a sy nchronous decompression of autochthonous units allows this flow regime to b e placed in the context of a thrust wedge undergoing gravitational spreadin g above a continuously contracting footwall. Decoupling of an extending thr ust wedge from a contracting footwall by a basal thrust with low shear resi stance, explains bulk shortening, uplift and exhumation of mid- to lower-cr ustal autochthonous basement during thrusting and recumbent folding at high er levels. Structural relationships in the southern Montagne Noire demonstr ate that traditional tectonic units recognized in this area, the so called 'nappes', are in fact unrelated to the recumbent folding as inferred previo usly. These 'nappes' postdate not only the recumbent folding, but also a la ter phase of refolding (D-3) associated with the formation of a gneiss dome . The traditional nappe division can be simplified to a hanging wall and fo otwall domain of a major decollement that formed during a second syn-collis ional collapse (D-4) associated with the formation of intermontane Stephani an basins. The late timing of the 'nappes' and simplified tectonic structur e of the southern Montagne Noire imply a thinner D-2 recumbent fold structu re with only one major anticline-syncline pair, and is in better agreement with the low metamorphic grade of the area. (C) 1999 Elsevier Science Ltd. All rights reserved.