Post-Oligocene evolution of Indo-Burma wedge: Insights from deformation structures of Tripura Mizoram fold belt

Abstract

The Tripura Mizoram fold belt (TMFB) belongs to the widest segment of the outer wedge of Indo-Burma Wedge (IBW). The structure of the outer wedge is characterized by a series of north-south trending anticlines-and-synclines of varying tightness, which progressively decreases towards the foreland direction. To explain their development, earlier workers proposed the model of fault-related folding, assuming that crustal shortening between India and Burma occurred dominantly by brittle deformation. However, our field investigations in TMFB reveal imprints of unequivocal ductile deformation, including interference of folds. Based on new structural map coupled with outcrop-scale structures, we recognize the existence of multi-ordered folds in the Neogene sequence of TMFB. However, the complexity of folding increases toward the hinterland part of wedge. We substantiate the spatial variations of penetrative ductile structures of TMFB with laboratory experiments under oblique convergence. Model results show consistency with the existence of along-strike variations of deformation intensity in the outer wedge of IBW, which gradually increases southward with narrowing the width of the wedge. Combining our field observations and laboratory experiments with earthquake focal depth distributions around TMFB, we propose that stratigraphic horizons of the entire outer wedge are mechanically weak to have deformed dominantly by brittle faulting.