Extensional tectonic activity in the cratonward parts (peripheral bulge) of the Ganga Plain foreland basin, India

Abstract.

Flexural subsidence of the Indian lithosphere created the foreland basin in front of the emerging Himalayan mountain belt. The continued northward push of the Indian plate and thrust sheet loading in the Himalayan orogen caused an up-warping along its cratonward margin, in the form of a regional gentle bulge. In the cratonward peripheral bulge small-scale to moderate size deformation features, e.g., gentle folds (up-arching of the sediment layers), extensional normal faults and uplifted tilted blocks, and incised river channels with 20–60-m-high cliffs, developed. Cliff sections of many rivers in this cratonward part of the foreland basin expose deposits of latest Pleistocene–Holocene age and show evidences of active tectonics in the last few thousand years: vertical uplift leading to deep incision of the river system, development of prominent fractures cutting through the sedimentary succession, bending and tilting of the strata, and tilted blocks. In the Late Quaternary relaxation phase of the Himalayan orogen-foreland, there is increased vertical tectonic activity in the region of the peripheral bulge. The vertical uplift in this part of the Ganga Plain foreland basin caused the rivers (including the axial rivers) to make further deep incision without shifting from their courses. During periods of increased tectonic activity in the Himalayan region, i.e., the addition of thrust slices more rapidly, probably caused the maximum down-bending in the proximal part of the Ganga plain foreland basin. The high amplitude and asymmetric nature of this foreland basin is partly controlled by extensional tectonism.