Hyperobliquity of Changlang Block of eastern Arunachal Pradesh, India and the role of Mishmi Block, India and Hukawng Block, Myanmar in its development

Plate tectonic history of northeast Indian subcontinent can be viewed as a window to the
evolution of Southeast Asia. One such important tectonic feature is the northern most part of
Indo-Burmese Ranges where this research work has been carried out. Here we propose an
evolutionary model that shows northward moving ‘horse-tail’ feature of the Hukawng Block
from the Burma basin, pushed this region towards the rigid Mishmi Block and Upper Assam
shelf, that caused the hyperoblique pattern of the ranges. It is the juxtaposition of the three
continental blocks: India-Asia-Burma, where there are tectonic and geomorphic influences in
the Block from both the Himalayan and Indo-Burmese orogeny. Stress distribution among
north-easterly moving Indian plate and comparatively stiff Eurasian and Burma plates, within
the India specific reference frame, is resulting in further changes. The study area mainly falls
under Changlang district of Arunachal Pradesh, India; and the regional study has been done in
the quadrangle from 26° to 28°N in latitudes and 95° to 97°E in longitudes. Morphotectonic
study, lineament analysis, fault system characterisation, focal plane mechanism along with
dynamic topography, seismic tomography and gravity anomaly have been incorporated in the
field evidences. Morphotectonic study for Noa-Dihing River basin has resulted in a value of
56.59 for Asymmetric Factor, which shows similar asymmetry result like in the Chi (χ)
analysis. This SW-ward tilted basin is moderately asymmetrical with Transverse Topographic
Symmetric Factor value of 0.42. This indicates that the major river basin along with other subbasins
are under the influence of active oblique rotational component. The regional lineaments
are showing mean orientations of N11°E-S11°W, N70°W-S70°E and EW whereas some local
trends of minor lineaments, in some places have mean orientations of N40°W-S40°E, N82°WS82°
E and N42°E-S42°W. In Mishmi block the major regional trends are N35°W-S35°E and
N40°E-S40°W comprising of probable cross-faults. In Hukawng Block, the lineament
orientation changes from N50°W-S50°E in the west to N30°W-S30°E, N-S and N15°E-S15°W
in the central valley region (north of Jade mines) and then to N50°E-S50°W in the eastern side.
Major fault systems are mostly thrust, with some showing very low angle slip component,
along with some oblique slip faults (e.g. Noa-Dihing River). The dynamic topography and
seismic tomographic studies indicate presence of a high seismic velocity zone beneath Mishmi
block indicating the crystalline rock materials. The block is still actively exhuming. Moreover,
Changlang and Hukawng blocks have undergone uplift and then phases of subsidence during
the last 20Ma. This indicates that the Low Velocity materials that are present underneath were
subjected to some crustal deformations. This tectonic process has also resulted in gravity
anomalies. The role of massive and rigid Mishmi block, comprising older crystalline rocks and,
later forming Burma basins formed the oblique rotation of the Changlang block which is
observed from all stated methods. Hukawng Block, which is controlled by the motion of
Sagaing Fault, have influenced the Changlang Block by its varied strike-slip stress components.
Moreover, Indo-Burmese Ranges also has an influence on this block and vice-versa.