Numerical stress and fault simulation of Shillong Plateau and its adjoining area in Northeast India, Bangladesh and Myanmar

The Shillong Plateau, NE India is reported to be one of the most seismically active ‘pop-up’ structures lying between the Dauki fault and recently reported Oldham fault in northeast India. Dauki fault lies down faulted Bengal basin to the south and Oldham fault, the Assam valley, the Himalayan foothills region, lesser Himalaya and Butan and Tibet further north. We examined convergent displacement due to northward movement of the Indian plate with respect to the Eurasian plate and also consider uplift of the Plateau simultaneously. Four models with different boundary conditions under both plane strain and plane stress have been selected. The maximum compressive stress shows a preferred orientation that helps us to explain the tectonic environment as well as the fault pattern in plane strain condition. The realistic regional maximum horizontal compressive stress (σ_Hmax) orientation is observed within the best-fit model under plane stress condition. The best-fit model result under plane strain condition shows that the compressive stress regime is dominant in the study area, except for the uppermost part of the crust where the tensional stress is dominant. These tensional stress regimes support normal faulting and structural discontinuities. With increase progressive convergent displacement (under plane strain condition), the principal stresses rotate anticlockwise around the fault zones, and the upper crust of the Bengal basin and the Assam valley, on the other hand, behave thrust faulting. Rock properties (density, Poisson's ratio, Young's modulus, cohesion, angle of internal friction) and Mohr-Coulomb failure criterion are used to calculate failure and faulting pattern. The simulated results have significant explanation on the proposed Oldham fault as well as on the geologically evidenced Dauki fault. Our preferred model explains deformation and seismic activity of the area. Our plane stress simulation results show that the Dauki fault is the major controlling element for overall distribution of regional stress, and it accommodates ～25% horizontal displacement. Our plane strain model further suggests that plate convergence is the driving force for pop-up tectonics of the Shillong Plateau, and also for deformation in the Shillong Plateau and adjoining area within the crust (＜30 km)