Sedimentary Tectonics of the Eastern Fold Belt of the Bengal Basin, Bangladesh

Depositional history and tectonic evolution of the Bengal Basin commenced with the collision between Asia and Greater India in Late Paleocene (55 Ma). Sedimentation within the basin was activated during the Eocene in response to the simultaneous collision of the Indian and Burmese plates in the east and the Indian and Tibetan plate in the north. The dominant compressional force for development of the eastern fold belt resulted from subduction of the Indian plate beneath the Burmese plate in eastern part of the basin. From Miocene onward, the Indian plate has been subducting beneath the Burmese plate with a counter clockwise rotational movement, while the Burmese plate was overriding the ' Indian plate with a clockwise rotation. Consequently, the overall trend of folded structures varies from NNW in the southern part to NNE in the northern part of the eastern fold belt. This variation in structural style along with the presence of longitudinal thrust faults and transverse faults suggests that the Indian plate initially pushed towards east and then turned northeastward during the Cenozoic. Structural style of the eastern fold belt is characterized by the presence of a series of elongated en-Echelon folds, which are tight and steep in the east, while towards the west they tend to be gentler and box shaped. Presence of longitudinal and cross faults along most of the folds suggests their hybrid origin. In seismic sections these faults appear to be heterogeneous in nature; and are believed to represent different time domain of their formation. Analysis of the stratigraphic succession in seismic profiles of selected structures suggests a close relationship between sedimentation style and tectonic framework within the eastern fold belt. Occurrence of overpressure zones in the lower Surma Group is believed to be primarily due to undercompaction caused by rapid accumulation of fine-Grained clastics and not tectonic deformation. Interpretation of seismic data and comparison of subsurface structural depth contour maps of different horizons reveals heterogeneous flexuring rates. The westward shift of the culminations and closures of different horizons from bottom to top reflects the imprints of time variant evolution of the folded structures. Hence it is thought that intensity and direction of the tectonic force responsible for development of compressional deformations within eastern fold belt of the Bengal Basin was time variant and contemporaneous with sedimentation.