Ramesh Bashyal
Department of Mines and Geology
Lainchaur, Kathmandu, Nepal
Bernard Delcaillau and Gerard Herail
CIMA-UA 366- CNRS, Institut de Geographie D. Faucher,
109 Bis rue Vauquelin – 31058 Toulouse Cedex, France
George Mascle
Institut Dolomieu – Geologie Alpine (US 69 CNRS),
15 rue Maurice Gignoux – 38031 Grenoble Cedex, France
In central Nepal, the Siwalik front represents the southernmost and younger thrust zone of the Himalayas. Here the detrital Mio-Pleistocene Siwalik Formations overthrust the Gangetic environment. Two main types are characterized. Escarped fronts (400 to 600 m of relative altitude) coincide with steeply dipping structures, strong morphology resulting from a thrust. The smoothed fronts (100 to 300 m) correspond to a flat lying thrust.
The regional example allows to propose a geodynamic evolutionary model for a foreland thrust front. During the flat thrust motion, thickening and erosion are balanced (smoothed front). In the following time, the southward propagation of the deformation with addition of new units, results in the formation of a frontal ramp which induces a steepening of the structures. The rate of uplift is not compensated by erosion (escarped front).