Himalayan Geology Vol.22 (2), 2001, pp.83-98, Printed in India

Stratigraphy and structure of low-grade metasedimentaries in eastern Bhutan and western Arunachal Pradesh

V. SRINIVASAN
Geological Survey of India (NER), Publication Division, Kumud Villa, Complex, Nongrim Hills, Shillong -793 003, India.

The low-grade metasedimentaries exposed between sedimentary belt in the south and Central Crystalline in the north can be broadly subdivided into five groups namely Bomdila Group, Rupa Group, Thungsing (Bichom) Group, Shumar Group and Buxa Group.

The oldest Bomdila Group comprising gneisses of granitic composition (Bomdila gneiss) and a highly tectonised sequence of quartzite and phyllites (Tenga Formation) is exposed in the core of a westerly plunging regional antiform, named as Narphung antiform. Rupa Group, which structurally overlies Bomdila Group with a tectonic contact, comprises of quartzites, phyllits, dolomites and pebble bed. It is well exposed in the western closure and northern limb of Narphung antiform and also in the core part of Rupa syaform, which is a mega digitations in the northern limb of Narphung antiform.

Thungsing Group comprising massive white gritty and feldspathic quartzites (Thungsing quartzite) and phyllites with or without rock fragments (Diuri phyllite), structurally overlies Rupa Group with a tectonic contact in the closure part of Narphung antiform. In the southern overturned limb of Narphung antiform, this group structurally underlies Bomdila group as an invertd sequence, whereas in the northern limb, it is missing due to Main Central Thrust (MCT). Further west of the closure part of Narphung antiform (i.e. in down the plunge direction), the outcrop pattern of Thungsing Group reveals refolding of this earlier antiform to generate two NE plunging later mega folds, namely Shumar synform and its complimentary Kenkhar antiform. Shumar group comprising gray quartzites, phyllites, impure limestone (mostly in rhythmic alternation). Structurally overlies Thungsing Group with a tectonic contact and it is well exposed in the core of Shumar Synform. Thus Bomdila group is in the lower part of tectono-stratigraphy and Shumar group is in the upper level. Buxa Group comprising quartzits, phylluites, limestones and dolomites is exposed in the southwestern part of the study area, structurally underlying Thungsing Group, in the inner core of Kenkhar antiform.

The Main Central Thrust, which separates this pile of low-grade metasedimentaries from the overlying high-grade metamorphites, is not co-folded with the autochthonous mass. This tectonic contact is affected by N-S trending broad warps.