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The Siang Dome located at the Eastern Himalayan Syntaxis is a large structure involving entire Himalayan packet from the Paleogene rocks to the south at the core of the Siang Window to the ophiolitic suture rocks and the Trans-Himalayan granitoids around Tuting at its northern periphery. About a kilometer thick shear zone with intense granitic injection is exposed in the Siang River section located at the tectonic base of the Yang Sang Chu Fm., overriding the Central Crystallines. This zone is tentatively equated to the South Tibetan Detachment. Marcoscopically the Himalayan thrust pile at the Siang Dome has a prominent orogen-transverse antiformal structure trending NW-NNW. It is truncated to the west by the N to NE trending Tuting-Basar dextral fault. The Siang Window at the core exposes a duplex antiform of Paleogene rocks that are placed beneath the low grade Proterozoic Himalayan rocks tectonically separated by the arched splay of the MBT roof-thrust. Two structurally discordant units frame the Siang Window. The crystalline thrust packet and the subjacent Lesser Himalayan thrust sheets of low-grade Proterozoic and locally present Permian metasediments on the hanging wall of the arched MBT to the north constitute one congruous unit and preserve identical structure. The duplex antiform of the Paleogene rocks beneath the arched MBT, which are further truncated to the south against the Neogene Siwalik Sub-Himalayas by the ENE-SSW trending and north dipping frontal MBT and North Pashighat Trust (NPT) represent the other tectonic unit. The antiform of Paleogene rocks are dissected by subsidiary imbricate faults that are oblique to both MBT at the roof and NPT at the floor, truncating and dislocating MBT, but remaining asymptotic to NPT. These are responsible for generating the Siang duplex antiform and its northern fault bend antiformal closure (Figure 1, 2). Tuting-Basar tear fault also breeches folded thrusts, and off sets the Siang and the Namche Barwa Domes at its northeastern end, whereas, to the south the frontal Gondwana thrust belt, frontal belt MBT and MFT remain unaffected, and therefore represent the youngest thrust structures. The lower unit of the Paleogene sequence at the Siang Window, the base of which is not yet well defined, is mainly composed of white to pink colored, skolitho-bearing and mature Miri like quartzite. It is also frequently intruded by dykes and sills feeding the overlying Abor Volcanics. The stratigraphic base of this quartzite with type Miri Quartzite is yet to be defined. Calcareous bands or limestones occurring at the upper part of this quartzite and at the lower parts of the Abor Volcanics have yielded rich larger foraminifera record characterized by Assilina depressa, A. regularia, Orbitosiphon cf. tibetica, Nummulites

Geological setting of the Siang Dome located at the Eastern Himalayan Syntaxis