2-D modelling of the anticlinal structures and structural development of the eastern fold belt of the Bengal Basin, Bangladesh

Abstract Structural architecture of the Bengal Basin has been strongly controlled by the collision pattern of the Indian plate with the Burma and Tibetan plates. The eastern fold belt (EFB) of the basin, comprising a series of north–south-trending curvilinear anticlines and synclines, represents a fold-and-thrust belt that constitutes the westward continuation of Arakan–Chin fold system of the Indo–Burman Ranges. The present study is mainly concerned with the 2-D modelling of the anticlinal structures in order to develop an understanding about the process–response relationships between the structural style and tectonic evolution of the eastern fold belt. The dominant fold-generating mechanism is believed to be the east–west-directed compressional force arising from oblique subduction of the Indian plate beneath the Burma plate that resulted in the growth of fault-propagation folds above a detachment or decollement at depth, giving rise to the Neogene accretionary prism complex development. A prominent feature of the region is the major east-dipping thrusts separating successive accretionary wedges. In seismic sections, evidence for several phases of compressional deformation suggests that multiphase stress conditions were responsible for the structural expression of the fold belt. Deep seismic sections reveal that the base of folding is characterized by a low-interval velocity horizon that represents a detachment separating the upper folded zone from the lower, seismically coherent, nearly unfolded zone. This detachment coincides with the undercompacted pressured shale unit, which is thought to have played an important role in the structural development of the eastern fold belt. Clay mineralogical analysis reveals the presence of a low-density shale horizon within the dense and thick shale sequence that is thought to be an undercompacted pressured shale during the geological past, and was responsible for the initiation of decollement and incipient diapirism involving thin-skinned tectonics. It is suggested that the salient structural features of the eastern fold belt, e.g. multiphase fold development, series of east-dipping thrusts and a basal zone of detachment or decollement, are the products of thin-skinned tectonics resulting from the Indo-Burmese plate convergence.