Himalayan Geology, Vol. 21 (1& 2), 2000, pp. 1-16, Printed in India
Foreland Palaeogene Rocks of the Eastern Himalaya: their basin extension, Magmatism and Tectonics
S.K. ACHARYYA Geological Survey of India, Calcutta, 700 016, India
The narrow frontal belt of Neogene Siwalik molasses sediments in the eastern Himalaya, overridden by the pre-Tertiary rocks along the Main Boundary Thrust (MBT), comprises two tectonic domains: an autochthonous zone to the south exposing homoclinally northerly dipping Neogene sequence and a northern narrower parautochthonous zone of steeply dipping, deformed and imbricated early Neogene sediments. The Papaeogene sediments are restricted in their distribution in the Eastern Himalaya, being exposed mainly as thin thrust slivers of marine sediments over wide lateral extent along the frontal belt. They occur close to and beneath the MBT and as wedges within the imbricated early Neogene unit or in the homoclinally dipping Neogene Sub- Himalayan unit. They also occur at the core of the Siang Window beneath an up-arched MBT close to the eastern Himalayan syntaxis.
Sub-surface data from the Ganga-Brahmaputra basin reveal that the foreland basin axis of older Neogene sediments progressively shifted northward. Marine Palaeogene rocks are developed close to the Himalayan front and have been intercepted at sub-surface only from the eastern sector. These foreland Tertiary sediments unconformably overlie the Precambrian rocks of the Indian shield in general, but usually their stratigraphic continuity with the adjacent pre-Tertiary (nappe) rocks of the Himalaya, which are extra-basinal and override them along the MBT, cannot be established.
The narrow belt of the low grade pre-Tertiary Himalayan nappe occurring to the north of MBT is overridden along the Main Central Thrust (MCT) by the high grade crystalline nappe. However, at the cores of several window structures, low grade Proterozoic metasediments similar to those exposed in the frontal belt with or without Late Palaeozoic sediments are seen beneath the up-arched crystalline nappe. The Siang Window exposes a duplex arch of Palaeocene-Eocene marine sediments interbanded with the Abor volcanic rocks beneath the up-arched and breached MBT, which represents the folded roof-thrust of thw window. The Palaeogene rocks of the window are truncated against the Neogene sediments of the frontal belt along a rectilinear floor-thrust. In Central Nepal, the Tansen synform exposes Cretaceous to early Neogene foreland basin sediments unconformably overlying the Himalayan Palaeozoic and Proterozoic nappe rocks. The basal early Cretaceous cover rocks here are associated with basaltic flows and Gondwana plant beds. The Palaeogene sequence is similar in facies to that recorded from the Sub-Himalaya and the Siang Window.
The Palaeogene foreland basin located in the marginal parts of the Himalayan fold-thrust belt and in front of the underformed Indian shield had extensive flood basalt and some acid volcanism contemporaneous to terminal collision in the north. The foreland volcanism might have been linked to collision induced deep dislocation structures. Northern parts of the foreland basin often overlapped the Himalayan nappes and rode over them passively, whereas major parts of the basin are tectonically concealed beneath the pre-Tertiary MBT nappe. Thrust movements involving the pre-Tertiary Himalayan nappes, the Tertiary sub-thrust rocks, and their architecture influenced the structure of the Himalayan nappes. Duplexes and windows were often developed at their external fringes. Convergence of tectonic movements at the eastern syntaxis produced imbricate thrusts and duplex arch in the sub-thrust Palaeogene rocks which arched-up the MBT and passively folded overlying Himalayan nappes. In other sections of the Himalayan windows, the sub-thrust Tertiary foreland rocks appear to have played more passive role.
