Orogen-transverse structures in the eastern Himalaya: Dextral Riedel shear along the Main Boundary Thrust in the Garu-Gensi area (Arunachal Pradesh, India), implication in hydrocarbon geoscience

Structural mapping and analyses reveal deformation in the rocks of the Lesser Himalayan Miri and Gondwana, and Lower Siwalik Dafla successions across the Miri Thrust (MT) and the Main Boundary Thrust (MBT) in the Garu-Gensi area (Arunachal Himalaya, India). The-north-dipping Gondwana sequence is sandwiched between the Miri Group of rocks in the north and the Lower Siwalik Dafla sandstones in the south. The Gondwana rocks folded four times (D-1-D-4). Detail outcrop mapping and structural analyses substantiated by satellite images elucidate that the Miri, the Gondwana and the Lower Siwalik rock sequences in the area are traversed by two sets of NW and NE trending cross-fault zones with similar to 70-100 m thickness mappable for similar to 70 km. The cross-faults trend NW around the locality Daring, while in the western part around Gensi, they trend NE. The westernmost fault slipped dextrally and the easternmost one at Daring sinistrally. These transverse faults offset the MBT: 0.52-0.59 km for the western- and 0.58-1.41 km for the eastern faults. The deformation in the Gondwana sequence may be constrained as syn-Himalayan (i.e., India-Asia collisional event). This was followed by a compression from E/ESE due to the post-Eocene Indo-Burmese collision. The culminating phase of combined India-Asia and Indo-Burmese compression is manifested in field with a strong NE-directed compression with dextral Reidal shear developing in the transverse conjugate fault zones in the area with maximum displacement towards SE. Cluster of the earthquake data indicates record of the events (M >= 6) within 30-80 km depth-both N and S of the MBT in the study area. Further, the southerly continuation of the transverse faults below the alluvium can provide vital clue in studying the potential hydrocarbon trap in the northern fringe of the Brahmaputra river.