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Himalayan
Geology, Vol. 30 (1), 2009, pp.17-34 Printed in India
Geology and sructural evolution of the eastern Himalayan Syntaxis
B.K. CHOUDHURI*, N.S. GURURAJAN, R.K. BIKRAMADITYA SINGH
Wadia Institute of Himalayan Geology, Dehradun - 248001, India
Abstract: This work deals with the geology and structural evolution
of the eastern Himalayan syntaxial bend in eastern Arunachal Pradesh, which
is a large scale antiform, known as the Siang Antiform. The crestal portion
of the antiform exposed in the Namche Barwa peak area, is occupied by Higher
Himalayan Crystallines. The area comprises of the following major
litho-tectonic units and from south to north they are: the Sub-Himalayan
sediments, Lesser Himalayan Sedimentary Sequence, Lesser Himalayan
Crystallines, Higher Himalayan Crystallines (HHC), Trans-Himalayan tectonic
units, namely the lower Tsangpo Suture Zone and the upper subduction related
Lohit Plutonic Complex (LPC). Field investigations have been carried out
along four major valleys viz, Siyom, Siang, Dibang and Lohit Valleys.The
structural analysis indicates that the area has undergone four phases of
deformation, out of which the first two phases are coaxial and the third
phase of deformation has produced a regional scale NNW plunging Siang
Antiform, which cross-folded the earlier structures. The variation in the
pattern of foliation and lineation on both the limbs is interpreted as due
to passive rotation during F3 folding and the western limb is more rotated
than than the eastern limb.The last phase of deformation has affected the
Trans-Himalayan units exposed in the eastern limb and reactivated the early
thrusts, into dextral strike-slip faults. Both the Lesser and Higher
Himalayan Crystallines exhibit inverted metamorphic character. The HHC
exposed in the eastern limb of the Siang Antiform show an increase in both
temperature and pressure towards higher structural levels and the inverted
metamorphism is related to post metamorphic shearing. The high grade
gneisses, exposed along the Walong Thrust that divide the LPC into western
and eastern belts, forms the basement for the LPC and these crystalline
rocks can be correlated with the Mogok Gneissic Belt of Central Burma in the
south and the Nyainqentangiha Crystallines in the north that tectonically
overlie the Tsangpo Suture Zone in eastern Tibet. Our field investigations
in the syntaxial region suggest that the thrust bound litho-stratigraphic
units have been translated towards south during the main phase of
deformation (D2). Later, these tectonic units have been folded by the
overall north trending Siang Antiform. The folding has tilted the early
thrusts particularly in the eastern limb that were reactivated into dextral
strike-slip faults. Subsequent indentation of the folded Indian plate rocks
have folded the suture zone and the Gangdese belt and the northward
migration have been accomodated by the strike-slip faults on both sides of
the antiformal syntaxis. |
