STRUCTURE OF THE SIWALIKS OF WESTERN NEPAL: AN INTRACONTINENTAL ACCRETIONARY PRISM

Abstract

The Siwaliks are the currently active foothills of the Himalayas, in which erosion, sedimentation, and faulting are proceeding simultaneously. Cross-sections in western Nepal show at least three major north-dipping thrust zones. Displacement is transferred laterally from one thrust to another, and anticlines form at the tips of the propagating faults, although they are destroyed by erosion as the thrusts continue to advance. Sediments are deposited in small piggy-back basins (“the Duns zone”) between highs. Brecciform sediments overridden by the thrusts show that several faults are active simultaneously. Mesostructures close to the major faults are revealing. In the footwall, striae and fractures in pebbles of the Upper Siwalik formations show shearing parallel to the bedding and stretching perpendicular to the displacement plane. In the hanging wall, bedding parallel minor faults and duplexes are seen in the sandstones and mudstones of the Lower Siwalik formations.