Ductile structures and instabilities: their implication for Variscan tectonics in the Ardennes

Increasing consideration of instabilities may explain the development of structures at all scales in naturally deformed rocks. This address is a review of geometric, scale-free concepts applied to the tectonic study of ductile structures. The notion of instabilities is first given to introduce non-equilibrium dynamics, such as strain localisation and single deformation events producing superimposed structures. These concepts lead to the idea that orogens are regions of accumulated strain in zones dominated by non-coaxial deformation. Consequently, stretching lineations depend on finite strain and do not represent the plate motions. Repeated 'phases' of deformation represent local perturbations during a kinematic and dynamic event rather than being correlatable as regional phases. The fold-and-thrust belt of the northern Variscan belt, in the Ardennes, is taken as a natural example where superposed structures represent the instable response to forces among which plate tectonic forces play a subsidiary role. (C) 1999 Elsevier Science B.V. All rights reserved.