Spatial variations of ductile strain in fold-and-thrust belts: From model to nature

Abstract

Fold and thrust belts (FTBs) accommodate tectonic convergence through folding and faulting of crustal rocks during a collisional event between two continental plates. Although evidence of distributed deformation is common in FTBs that usually leads to continuous foliations and regionally occurring ductile structures of multiple orders, it has rarely been given much attention assuming that zones of localized deformation, like shear zones and brittle faults, accommodate the entire amount of tectonic convergence. This study presents 3D laboratory-scale models, using viscous thin sheet as crustal layers, to understand the evolution of ductile strain in a tectonic wedge. We varied the degree of mechanical coupling at the basal decollement (i.e., weak versus strong) to investigate this issue at constant convergence velocity in all experiments to avoid the influence of rate-dependence on viscous rheology. Our results reveal that the strength of basal decollement controls the mode of wedge growth and hence, the strain pattern particularly towards the hinterland. The weak decollement models yield a zone of constriction towards the central part of hinterland, explaining the occurrence of isolated patches of L-tectonites and cross-folds in FTBs; while the strong decollement condition allows gravity driven flow to be active in the hinterland, leading to orogen-parallel recumbent folds. In contrast, both weak and strong decollement models produce deformation that characterises the commonness of pervasive, hinterland dipping ductile fabrics towards the mountain front. We correlate our findings to show that spatio-temporal variations in basal coupling are responsible for varying occurrence of ductile structures in FTBs.