THE STRUCTURE OF THE SIANG WINDOW, ITS EVOLUTION AND BEARING ON THE NATURE OF EASTERN SYNTAXIS OF THE HIMALAYA

The allochthonous low grade pre-Tertiary metamorphic rocks underlying the high grade crystalline nappe, override the frontal belt Neogene mo lasse sediments along Main Boundary Thrust (MBT). In eastern Himalaya, marine Paleogene sediments are only exposed as narrow thrust slivers over wide lateral extent close to and beneath MBT and within the imbri cated early Neogene sediments from the northern parts of the frontal b elt. Several domal window structures in eastern Himalaya expose low gr ade pre-Tertiary metasediments occurring beneath the crystalline nappe . The largest Slang window, located close to eastern syntaxis of the H imalaya, is unique and exposes a duplex of Paleogene rocks which have arched, folded and breached MET. The overlying Himalayan nappe rocks a re passively folded. Several nappes in the eastern limb of the dome is greatly attenuated in width and finally overridden by the Trans-Himal ayan granitoid nappe. The thrust systems bordering the western limb of the complimentary synform west of the Slang window are obliquely trun cated by a N-S trending dextral tear-fault. Further south it merges wi th the western end of the floor thrust which delineates the southern m argin of the Slang half-window. The Slang and other half-windows of ea stern Himalaya have many structural similarities and some distinctions . They have been inferred to have evolved in a similar way. The Slang half-window, located close to eastern syntaxis of the Himalaya, was pr oduced by the NE projecting indenter of the Indian continent which act ed as an oblique crustal ramp over which the Himalayan and Trans-Himal ayan nappes climbed. Convergence of tectonic movements at the eastern syntaxis produced duplex arch in the sub-thrust Paleogene rocks which breached MBT and passively folded overlying Himalayan nappes.