Folding in orogens: a case study in the northern Iberian Variscan Belt

Abstract

Folding during the Variscan deformation in NW Iberia has been analysed from the foreland to the hinterland of the orogen using several geometrical techniques complemented by the numerical simulation of kinematical folding mechanisms with the aid of a computer program (‘FoldModeler’). In the foreland, folds are related to thrusts; in the inner zones, folds can be attributed to three deformation phases. D₁ and D₂ involved deformation with horizontal foreland-directed displacements. D₁ gave rise to closed or tight folds (F₁) and cleavage (S₁); development of large recumbent F₁ folds in the hinterland required a simple shear regime and a final coaxial strain component with sub-vertical maximum shortening. Strain incompatibilities at deeper levels, together with high temperatures, favoured the concentration of ductile deformation in shear zones and development of mylonites during D₂. Local flow instabilities generated F₂ folds that were passively amplified by a combination of simple shear, coaxial strain and area change. D₃ involved a change to a regime with dominant coaxial deformation and a sub-horizontal maximum shortening; it gave rise to upright or steeply inclined folds (F₃). Development of D₃ structures was heterogeneous and depended on the previous dip of the bedding and S₁, the presence or emplacement of granitoids, the stacking of thrust sheets or the previous development of large faults bringing into contact rocks with different competence. D₃ structures are mainly concentrated in metapelitic areas and appear distributed in bands on several scales. On a small scale, tectonic banding appears associated with small folds as a result of pressure-solution processes. In areas with sub-vertical S₁, later sub-horizontal kink bands were formed by a vertical compression. Copyright © 2010 John Wiley & Sons, Ltd.