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

Abstract

Detailed structural analysis of a large recumbent fold in low-grade Palaeozoic sediments of the Montagne Noire, and the previously established tectonometamorphic history of its crystalline substratum, indicate a two-stage development history. First, relatively homogeneous crustal thickening produced folds with steep axial planes associated with a regional cleavage ( S 1). Subsequently, these structures became rotated and amplified in a subhorizontal flow with pure shear and simple shear components related to low-angle thrusting. The development of an associated horizontal crenulation cleavage ( S 2) with a component of vertical shortening of S 1, plus evidence for a synchronous decompression of autochthonous units allows this flow regime to be placed in the context of a thrust wedge undergoing gravitational spreading above a continuously contracting footwall. Decoupling of an extending thrust wedge from a contracting footwall by a basal thrust with low shear resistance, explains bulk shortening, uplift and exhumation of mid- to lower-crustal autochthonous basement during thrusting and recumbent folding at higher levels. Structural relationships in the southern Montagne Noire demonstrate that traditional tectonic units recognized in this area, the so called ‘nappes’, are in fact unrelated to the recumbent folding as inferred previously. These ‘nappes’ postdate not only the recumbent folding, but also a later 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 footwall domain of a major décollement that formed during a second syn-collisional collapse ( D 4) associated with the formation of intermontane Stephanian basins. The late timing of the ‘nappes’ and simplified tectonic structure of the southern Montagne Noire imply a thinner D 2 recumbent fold structure with only one major anticline–syncline pair, and is in better agreement with the low metamorphic grade of the area.