TRANSVERSE STRUCTURES DEVELOPED DURING INDIA – ASIA COLLISION IN THE GANGA FORELAND BASIN, NEPAL

Foreland basin stratigraphy can be used to investigate the behaviour of basement features during continent-continent collision, and to test links between basement structure, along-strike segmentation of deformation, and lithospheric flexure in a developing orogen. The Himalayan orogen has been subdivided from east to west into along-strike segments with differing thicknesses, deformation styles, thermal evolution, and seismicity. Heterogeneities in the Indian plate, such as crustal scale basement faults and ridges, are possible controls on this lateral segmentation.

The seismic stratigraphy within and below the Ganga foreland basin across Nepal is imaged through 2D seismic reflection data. The data have been depth-converted using time-depth relationships derived from wells. Regionally interpreted surfaces include: two horizons internal to the Cenozoic succession; an angular unconformity at the base of the Cenozoic succession where older stratified units are truncated; and the nonconformity that separates sedimentary strata from acoustic basement representing Archean granitoids and Proterozoic gneisses. Because much of the foreland basin fill was deposited in fluvial environments close to sea level, thickness can be used as a proxy for subsidence rate. The horizons have been used to produce isopach maps for each of the main stratigraphic intervals. In addition, faults and deformation zones are identified in both the foreland basin strata and the underlying basement.

Our seismic interpretation shows that basement depth fluctuates dramatically, ranging from > 12 km to < 3 km. These variations define two sets of transverse depressions and ridges, and several large graben. The Cenozoic succession thins and thickens in step with the basement below, indicating that the basement features affected the rigidity of the Indian lithosphere and thereby controlled accommodation in the foreland basin.

Within the foreland basin succession, steep, strike-slip tear faults indicate that a new frontal thrust has propagated south from the main frontal thrust above a basement ridge and continues to be active beneath the Ganga Basin.