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Himalayan Geology

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Structural and Tectonic studies of mountain front of northeastern Himalaya, Arunachal Pradesh, India: implication to active tectonics

S.S. BHAKUNI1, PRADEEP SRIVASTAVA2 AND  KHAYINGSHING LUIREI3 Wadia Institute of Himalayan Geology, 33-GMS Road, Dehra Dun – 248001, India Emails: 1 bhakuniss@rediffmail.com

2 pradeep@wihg.res.in, 3 ashing-_luirei@rediffmail.com  The structural investigation was carried out along the mountain front of the northeastern Himalaya, Arunachal Pradesh. This study has an important implication towards the understanding of the active tectonics along the mountain front of the eastern Himalayan syntaxis (EHS). The reactivation of faults deformed the surface of fan terraces of ca. 15 to 10 ka in age. It indicates that the area has been tectonically active since latest Pleistocene times. Compared to the flat lying Brahmaputra alluvial plains the uplifted mountain front represents the leading edge of the fold- and thrust- belt of the Himalaya. The
Himalayan Frontal Thrust (HFT) delimits the mountain front in the south.

The Main Boundary Thrust (MBT) along which Lesser Himalayan Sequence (LHS) is thrust over the Sub- Himalaya bound the top of the Sub-Himalayan hanging wall (HW) of the HFT. Located between the MBT and HFT the Siwalik rocks of the Sub-Himalaya are characterized by the fault propagation folds (FPF) and regional intraformational thrust (IT). Both the MBT and HFT are the major folded structures. The MBT dips very steep at present level of observation, but becomes gentle in the sub-surface region as evident from fault plane solutions (Holt et al. 1991). The Gondwana rocks and Miri quartzite occur as large lenticular bodies with tapering high-strained shear zones at the basal part of the HW of the MBT. Along river valley section, the Gondwana carbonaceous shales have been suffered by intense ductile deformation recognized at the base of the HW of theMBT. A set of high angle normal and strike-slip faults is recognized near south of the MBT and at basal part of the HW of the HFT. Linear fault related ridges have been uplifted between faults bounding the ridges.

These faults are both parallel and oblique to the surface traces of the MBT and HFT. The Quaternary stratified deposits, lying on eroded crestal part of the asymmetric FPF with very steep inverted limb, are observed in the basal parts of the HWs of the MBT and HFT. Along an IT, the Middle Siwalik rocks have thrust over the Upper Siwaliks including over the fan deposit, and also offset the fan surface. The FPFs of upright geometry have developed at base of the IF. Their hinge lines make acute angle with the trace of the IF. The transverse faults dipping steeply towards NE and E directions have cut the bedrocks and the overlying fan terraces. They also show strike-slip movement along vertical planes characterized by horizontal stretching lineation. The gentle- and moderate-dipping thrust faults are affected and cut by later steep- to vertical- dipping normal faults. Variable style of deformation is distributed along fault lengths. Towards NE direction along mountain front the northeastern limb of the regional-scale NW-SE trending Siang Antiform has become thin along the NW-SE trending Mishmi Thrust (MT), whereas its western limb trends N-S. Towards the eastern Arunachal Himalaya the MT zone separates the LHS from the Brahmapura alluvium; thus the MT, MBT and HFT have merged with each other along the northeastern corner of the Assam syntaxis. Thick zone of fault gauge has developed at base of the MT that separates the Mishmi Crystalines of the LHS from the recent fluvial deposit. The geometry and kinematics of folds and thrusts, and surface deformation suggest that the region has been active since the latest Pleistocene times (Srivastava et al. 2008, in press). Normal and strike-slip faults are straight and define fault-bounded linear ridges, which suggest their younger ages than the folding episode of the Siang Antiform that has folded both the MBT and HFT. These ridges have uplifted the fan deposits.

The reactivation of FPF and intraformational thrusting is relatively young phenomenon as they have deformed and uplifted the Quaternaries. Sandwiched between the LHS and Siwalik rocks, the incompetent Gondwana rocks acted as a zone of accommodation of deformation long the base of the MBT that has  uplifted the LHS along with the overlying recent fan deposit from present riverbed. Top of the FW of the MBT has been imbricated by closely spaced steep dipping faults that have cut the present ground surface, and the strike-slip displacement along them has shifted the Siang river course laterally. Initiated in the uppermost crustal level the brittle normal faults have developed when maximum horizontal stress orientation has changed from horizontal to vertical during late stage of tectonic transportation of rocks along the major thrusts when the displacement along the moderate- to steep-angle MBT and HFT was almost locked. Their later reactivation has further made them much listric in geometry, and as a consequence the fault-block rotations and local subsidence of ground surface are the recently developed tectonic features. Due to recent movements along thrust the active FPF has uplifted the fan terrace deposit to > 40m from riverbed. The recent shear movement along the fan-shaped brittle axial plannar cleavages of the FPF has up-warped and offset the fan terraces. It has also deformed the ground surface and tilted the stratification of fan deposits. The recent movement along the IT has tilted the subhorizontal stratification of young fan deposit at 25 towards NW direction. Blind thrust is inferred within the underthrusting Siwalik FW block beneath the trace of the MBT. This thrust including the in-sequence splays of the MBT has extruded and upthrusted the Lesser Himalayan Gondwana rocks within the top of theSub-Himalaya in the form of tectonic slivers. The strike-slip displacement along the transverse faults has teared the mountain front dextrally. The N-S orientation of western limb of the Siang Antiform coincides with the N-S trending recent normal fault lineament in the mountain front. These transverse faults seem to be the southward continuation of hinterland faults that have later propagated towards the mountain front after the displacement along the foreland propagation steepthrusts has ceased. The recent reactivation of the MT has far traveled the Lesser Himalayan thrust sheet directly over the Brahmaputra alluvium.

The progressive deformation related to the anti-clock wise rotation of the Indian plate resulted therecent underthrusting of the Brahmaputra alluvium along the MT, and the recent deformation along the northern splays of the N-S trending dextral Sagaing strike-slip fault along which the Indian plate has moved towards north relative to the south China. It is inferred that the recent uplift of the high-grade basement rocks along the Namche Barwa syntaxis or the EHS (Burg et al. 1997) has developed the gravity collapse related steep normal faults along the mountain frontal part of the northeastern Himalaya, Arunachal Pradesh. Thus the active tectonics seems to be associated with the later deformation that penetrated within the Indian plate from the tectonic deformation distributed along the margins of the Indian, South China and Indo-Burmese plates, including the recent uplift of the Namche Barwa syntaxis or the EHS.

Burg, J-P., Davy, P., Nievergelt, P., Oberli, F., Seward, D., Diao, Z., Meier, M. 1997. Exhumation during crustal folding in the Namche-Barwa syntaxis. Terra Nova, 9 (2), 53-56.

Holt, W.E., Ni, J.F., Wallace, T.C., Haines, A.J. 1991. The Active Tectonics of the Eastern Himalaya Syntaxis and surrounding regions. Jour. Gepophysical Research, 96 (14), 595-14,632.

Srivastava, P., Bhakuni, S.S., Luirei, K., Misra, D.K. 2008 Morphosedimentary records at the Brahmaputra River exit, NE Himalaya: climate-tectonic interplay during the Late Pleistocene–Holocene. Journal of Quaternary Science (in press).