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The study of brittle deformation of the collisional mountains can explain its shallow crustal tectonic evolution and the palaeostress regime. The Main Boundary Thrust (MBT) zone in the western Arunachal Himalaya displays imbrication in the Permian Gondwana sequence between the MBT‐1 (/Bome Thrust/MBT‐Upper) in the north and MBT‐2 (/MBT‐Lower) in the south with consistent northerly dip. The Lower Gondwana rocks occur in the footwall of the MBT‐1 with the Proterozoic Bomdila Group in the hangingwall. The upper Gondwana rocks constitute the hangingwall sequence for the MBT‐2 with Neogene Siwalik rocks in the footwall. This article analyses palaeostress using brittle fractures in the Gondwana rocks that crop out for ~120 km2 in the study area. The fault‐bounded imbricate zone depicts eight brittle shear indicators and four sets of joints (J1 and J2: inclined and J3 and J4: subvertical). The signatures of the inherited pre‐Himalayan extensional deformation are preserved in the Lower Gondwana Miri Formation. The Bichom and Bhareli rocks exhibit brittle deformation features of the Himalayan Orogeny under strong ~N‐S compression. The palaeostress analysis of all joint sets indicates three phases of brittle deformation in the Gondwana and Siwalik rocks of the area. The subvertical joint sets and normal faults in the Miri Formation indicate a north‐northwest (NNW)‐directed extensional phase of the pre‐Himalayan origin. The inclined joint sets of the Bichom and Bhareli formations of the Gondwana sequence depict Himalayan orogeny with ~N‐S compressional phases. The third phase of brittle deformation in the Siwalik sequence depicts an east‐west (~E‐W) extension. The arc‐parallel extension in the frontal fold belt of the Arunachal Himalaya may be due to oblique India‐Asia collisional tectonics.

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Structural mapping and fieldwork in the Lesser and Higher Himalayan sequences in the western Arunachal Himalaya reveal crucial deformation fabrics in the Main Central Thrust (MCT) and Dirang Thrust (DT) zones. The top‐to‐SW ductile shear in the MCT and DT zones is correlated with the swing in the trend of MCT and DT from NE to NNW. The curved MCT and DT as traced by previous authors on the regional map of Arunachal Himalaya are studied. It is found that at places where these faults swing, shear senses developed at the meso‐scale. These shear senses are studied in meso‐ and micro‐scales. Seismicity in the western Arunachal Himalaya is influenced by basement cross‐strike crustal‐scale NW‐trending buried Bomdila Fault (BF). Landslides occur frequently along the Bhalukpong‐Bomdila–Sela traverse are also linked with the transverse BF. In its southern part, the BF coincides with the Dhansiri Lineament and is the basin margin fault in the upper Assam shelf. The Gondwana sediments extended further south below the Brahmaputra alluvium along the fault is to be explored for hydrocarbon exploration.

Carbonaceous shales of the Upper Disang Formation of the Inner Paleogene Fold Belt of Assam-Arakan Basin, exposed along the Dimapur-Senapati road section of Nagaland and Manipur states of India have been studied for interpreting hydrocarbon potential. The Upper Disang Formation is mostly composed of alteration of shales, sandstones and siltstones. The shales are black coloured and splintery in nature. The shales and associated sandstones show sedimentary structures like bioturbation, cross bedding and ripple marks indicating shallow water origin. The shales contain appreciable amount of pyrites indicating marine influence in the depositional basin. The average TOC of the carbonaceous shale samples is 0.53% indicating poor generative potential. The poor generative potential of the formation is also supported by low S1 (average 0.04 mg/gTOC), S2 (average 0.16 mg/gTOC) and HI (average 36 mg/gTOC). The organic matters are represented dominantly by Type IV kerogens (inertinite) with subordinate amount of Type III kerogens (vitrinite) as shown by both Rock Eval parameters and organic petrography. The dispersed organic matters (DOM) are identified as semifusinite and collotelinite under the microscope. The organic matters are interpreted to be inert/post mature kerogens from the maturity parameters like Rock Eval Tmax (average 563°C), and mean vitrinite reflectance (average 2.0% in oil). The study indicates very poor hydrocarbon potential of the Upper Disang Formation of the Dimapur-Senapati road section.

The Oligocene Laisong Formation in the Naga Schuppen Belt is coal bearing along the Dimapur-Kohima Road section, Nagaland. The coals are sulphurous and classified as lignite to sub-bituminous 'C' based on various rank parameters. The coals are dominantly composed of vitrinite followed by inertinite and liptinite in order of abundance. The coals have high-quality Type III kerogens derived mainly from angiosperm plants. The richness and quality of organic matter in the coal is excellent to act as a petroleum source rock. Most of the maturity parameters indicate that maturity ranges from immature to early mature. The biomarker parameters and facies critical maceral associations point to deposition of the coal-bearing strata in non-marine/ terrestrial depositional environment under moist hydrodynamic conditions, possibly in an upper delta plain environment. However, the high sulphur content of the coal points to deposition in a lower delta plain environment. Comparison with other Palaeogene coal deposits of the same sedimentary basin shows that the Laisong coals contain lesser amount of liptinite macerals and much higher amount of sulphur than these deposits. All these deposits are of similar rank, however, the Laisong coals show marginally low thermal maturity from the viewpoint of hydrocarbon generation.

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