Strain distribution within the Pardailhan Nappe (Montagne Noire, France) and structure of its basal thrust zone: implications for events associated with nappe emplacement

A strain study carried out within the overturned limb of the Pardailhan Nappe (Montagne Noire, France), using deformed trilobites, shows that a general agreement exists between the two-dimensional strain ellipse and the corresponding fabric ellipse determined from the measurement of the preferred orientation of phyllosilicates. The principal extension direction X parallels ENE-trending, first generation F₁ folds, contemporaneous with early stages of nappe emplacement and both are approximately perpendicular to the transport direction. Strain values are low ( X/ Y = 1.3-2.2) and strain gradients within the nappe are small. Similar strain values are measured, using deformed fossils, along the nappe's basal thrust zone. In this zone F₁ folds are seen to have been flattened and passively rotated towards parallelism with the transport direction within the cleavage plane, by penetrative movement during thrusting. The deformation here is heterogeneous and concentrated within small, discontinuous shear zones. The different microstructures are described and their superposition is interpreted in terms of a progressive tectonic evolution associated with emplacement of the Pardailhan Nappe.