Geodynamics of the Indo-Burmese Arc: Northwest Sunda Arc

Abstract

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 Indo-Burmese Wedge (IBW) in the forearc, deformation front of Northeast Himalaya, Eastern Himalayan Syntaxis (EHS), Assam-Brahmaputra valley, Shillong Plateau, and laterally extruding Tibetan lithosphere in adjacent Sundaland block, etc. The geodynamic interaction and associated complexities of these regions and their interaction with the IBA remain elusive. Apart from these tectonic complexities, the second-largest seasonal hydrological load lies over the Southeast Asia covering the IBA plate boundary region. However, the exact interaction between the seasonally induced non-tectonic and tectonic deformation process along this plate boundary remains debated. In the present Ph.D. work, these tectonic and non-tectonic deformation aspects of the IBA have been characterized by constraining combined geodetic, seismic, and satellite based observations.
By constraining geodetic observations, the Euler rotation parameters of the Indian and Sunda plates are estimated. The India‐Sunda long-term relative plate motion (~37 mm/year) is distributed among three major active fault systems, namely, the Sagaing Fault (SF) (~18 mm/year), Churachandpur‐Mao Fault (CMF) (~17 mm/year), and the Blind megathrust (BMT) (~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 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 stick‐slip manner, thus leading to uncertainty in the 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 has 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 has 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. Further, by considering geodetic observations from Southeast Asia along with satellite data from Gravity Recovery and Climate Experiment (GRACE), the seasonal deformation over the IBA is characterized. In fact, the relationship between hydrological mass oscillation and seismicity modulation over the IBW and SF indicates no such prominent annual or semi-annual periodicity in the seismicity catalogue, which may be attributed to the higher magnitude completeness of the available seismicity catalogue (Mc>4.0). It appears that the IBA is a geodynamically and seismo-tectonically complex domain where both tectonic and non-tectonic deformation process operates.