Progressive changes in strain patterns and fold styles in a deforming ductile orogenic wedge: an experimental study

Abstract

Experiments with scaled, viscous pitch models demonstrate successive stages of growth (Stages I, II and III) of a ductile orogenic wedge with progressive deformation, which are kinematically as well as dynamically distinct from each other. In Stage I, vertical growth of the wedge is significant indicating dominant role of horizontal tectonic forces relative to the gravity forces. With progressive deformation, the wedge attains a stable height when Stage II begins. Migration of the wedge front in the transport direction maintaining constant height during Stage II implies a balance between gravity and horizontal contraction. In the last stage (Stage III), as the action of horizontal tectonic forces tend to weaken gradually, the wedge undergoes collapse under the influence of gravity. The three stages of wedge growth show characteristic strain distributions. In each stage, shape of the strain ellipses (indicator of finite strain at that stage) and their orientation (inclination of the ellipse major axes with respect to the direction of horizontal force) vary from hinterland toward foreland as well as from depth to shallower level. Strain ellipses at a given domain also exhibit changes in shape (aspect ratio) and orientation with advancing stages of deformation (Stages I to III). There is thus a spatial as well as temporal variation in strain observed in the deforming wedge. The experimental analysis is essentially two-dimensional as the flow of pitch in the third direction (parallel to the wedge front) is constrained. Experiments were also performed with layered, anisotropic viscous models to study the spatial and temporal variations of fold styles in a deforming wedge. In Stage I, shallow level folds are upright and symmetrical whereas those at depth are inclined to nearly recumbent and strongly asymmetrical, showing vergence toward foreland. The intermediate stage of wedge growth (Stage II) is characterised by development of 'back folds' and refolded folds, the development of which are consistent with the temporal variations of finite strain within a deforming wedge. New steeply inclined, foreland-vergent asymmetric folds develop in the extreme frontal part of the wedge during the gravity-induced collapse (Stage III).