STRUCTURAL GEOLOGY AND REGIONAL TECTONICS OF THE CHITTAGONG HILLS FOLD BELT, EASTERN BANGLADESH

The Chittagong Hills fold belt of eastern Bangladesh represents the less intensely deformed foreland of the Indo-Burman Ranges. Exposed stratigraphy consists of Miocene prodeltaic mudstones overlain by tidal-influenced mudstones and sands of Miocene to Pliocene age. The coarsening-upward sequence suggests basinward progradation of deltaic sedimentation. Sediment transport fabrics indicate a northeasterly (southeast Asian) source terrane.

Major structures are box-fold anticlines with broad crests and steeply dipping to overturned limbs, and broad flat-bottomed synclines. Limbs of anticlines are locally thrust-faulted, with thrusts vergent toward both the east and west; features such as the Changohtong - Sitapahar Anticline exhibit thrusts along both flanks. Secondary structural features include minor tear faults indicative of minimal lengthening of the anticlines along strike.

To the east the fold belt is bounded by the Thega Fault and the tightly-imbricated, thrust sheets of the main ranges. To the west the folds die out rapidly along a boundary approximately coincident with the present coastline, and its onshore extension into the Ganges-Brahmaputra Delta. Non-proprietary seismic and well data indicate that west of this line anticlines are broad, low relief structures.

The detachment surface is approximately four kilometers below the present ground surface. Unlike most foreland fold-thrust belts, the regional detachment surface dips toward the foreland at about one degree. This west-dipping detachment is interpreted as the roof thrust of a crustal-scale duplex, above which exposed strata are detached from an underlying west-vergent accretionary prism involving Cretaceous to Oligocene(?) sediments scraped off the Indian plate.

To the north the Tripura - Cachar – Chittagong fold belt plunges into the deep Assam Basin. To the south the detachment rises, and progressively deeper parts of the fold system are exposed in the Bangladesh – Myanmar border regions. The absence of significant strike-slip faults indicates that strain across the plate boundary is partitioned. The fold belt accommodates simple east-west convergence, while the relative southward translation of the major Southeast Asian Plate is accommodated by strike slip along discrete faults like the Sagaing Fault of central Myanmar.