Geodynamic complexity of the Indo-Burmese Arc region and its interaction with Northeast Himalaya

The Indo-Burmese Arc (IBA) comprises highly populated regions of Northeast India, Bangladesh, and Myanmar. Thus, quantitative estimation of crustal deformation, earthquake occurrence process and seismic vulnerability along this densely populated region is considered to be paramount. However, diverse opinion exists regarding the strain accumulation, status of active convergence, and associated seismic vulnerability across the IBA. In addition, the IBA is surrounded by several seismo-tectonically active geodynamic units such as deformation front of Northeast Himalaya, Eastern Himalayan Syntaxis (EHS), Assam-Brahmaputra valley, Shillong Plateau, and laterally extruding Tibetan lithosphere in adjacent Sundaland block, etc. However, the geodynamic interaction and associated complexities of these regions and their interaction with the IBA remain elusive. In the present review article, these tectonic deformation aspects of the IBA have been explored by constraining geodetic and seismological observations. By constraining geodetic observations, the Euler rotation parameters of the Indian and Sunda plates are estimated. It is observed that the India-Sunda long-term relative plate motion (similar to 37 mm/year) is distributed among three major active fault systems, namely, the Sagaing Fault (SF) (similar to 18 mm/year), Churachandpur-Mao Fault (CMF) (similar to 17 mm/year), and the Blind megathrust (BMT) (similar to 7 mm/year), from east to west across the IBA respectively. The estimated convergence across the BMT is significantly lower than the earlier estimates. Moreover, due to the large scatter in the geodetic data close to the updip edge of the BMT, it is not certain whether the motion across the detachment is accommodated through shallow creep or in a stick-slip manner thus leading to uncertainty in seismic hazard in this densely populated region. Further, in the Northeast Himalaya, the lateral extrusion model of the Tibetan crust appears to contradict with the oblique convergence model of the Himalayan-Tibetan orogeny. Geodetic observations indicate that the overall India-southern Tibet convergence in the Northeast Himalaya (i.e., Bhutan and Arunachal Himalaya) is about 20-25% less than that in the neighboring central Himalaya and EHS. This deficiency of motion is accommodated through distributed deformation along the Dauki Fault and Naga Thrust, which suggests that these two units participate in the Northeast Himalayan strain budget via distributed deformation along the Assam-Brahmaputra valley. Thus, it is proposed that instead of partitioning in the backarc, the Northeast Himalaya has developed an active sliver along the Assam-Brahmaputra valley in the outer deformation front of Main Frontal Thrust in order to accommodate the deficiency in long-term plate convergence. It is argued that the strong eastward extrusion of Tibetan crust in the backarc is the main driving force for such unusual development of sliver in the outer deformation front. From this, it appears that the IBA is a geodynamically and seismo-tectonically complex domain where more data and instrumentations are needed to better characterize the crustal deformation, earthquake occurrence process, and seismic vulnerability in this densely populated region. [GRAPHICS] .