Morphometric evidences of neotectonic block movement in Yamuna Tear Zone of Outer Himalaya, India

摘要

The Himalayan Foreland fold-thrust belt have developed small to large basins all along its marginal part, which have evolved during the thrust sheet movement. These basins are bound by thrusts in the Outer Himalayan Belt (Siwalik Terrane) and are laterally restricted by a number of thrust splays, which appear as transverse faults on the surface. The present study is related to block movement in Sukh Rao basin which occupies a NW-SE trending synclinal trough south of Paonta Sahib in Sirmaur district of Himachal Pradesh, India. Sukh Rao is a tributary stream of Yamuna River and has a parallel disposition with other tributaries like Bata and Giri rivers, located in the north. All these streams, occupying intermontaine valleys, initially flow longitudinally from NW to SE and take a sharp bend towards south before joining the trunk stream. The southern water divide of Sukh Rao basin is marked by sharp crested Upper Siwalik rocks, which are dominantly conglomeratic in nature. The southern face of this water divide is drained by NE-SW trending Nimbuwala Khala and Chikan Khol transverse streams, which have encroached the adjoining Sukh Rao basin in response to the block movement and tilting. Interpretation of satellite imagery, supported by morphometric analysis, indicates that the tectonic blocks covering the area have moved, tilted and skewed sinistrally forcing the SE flowing Giri, Bata and Sukh Rao streams to take sharp southward trend along the segments of Yamuna Tear. The NE-SW trending syngenetic transverse faults (thrust splays) have induced stream piracy through fast erosion. The Nimbuwala Khala has completely pirated the upper part of Sukh Rao basin while the Chikan Khol has only nibbled part of the basin. With the present rate of erosion (dissection index 0.37), it is likely that Chikan Khol will follow suit of its predecessor Nimbuwala Khala.