TECTONIC EVOLUTION OF BENGAL BASIN: A PARADIGM SHIFT OF PRESUMED BLIND MEGATHRUST AND PREDICTED GREAT EARTHQUAKES

Present geotectonic study is focused on the assessment of seismic risk and hazard in the world's youngest and most active delta formed by the Ganges-Brahmaputra-Meghna River system, a major part of the Bengal Basin, consisting of thickest pile of sediments (~25 km) in the planet. Present state of geological and geophysical knowledge points out that the northern stable shelf of Bengal Basin consists of a multi-fractured, undulated basement at shallow depth. During continental convergence, the buried crustal slope suddenly dipped down to 20 km abyssal topographic trench formed of huge grabens and horsts, crushed, pushed, and dumped remnant of transitional and thin continental crust to shape up a deep and uneven to a planer basement in the present central part of Bengal Basin. The geological, geotechnical field investigations, analysis of available seismic and deep core samples of folded belt and deltaic plains revealed the structural pattern as well as differential velocity vectors, those resulting from the dynamics of all the concealed geological processes. The analysis of Chittagong Tripura Fold Belt in the east uncovered the snapshot of the kinematics of the active deformation in the subduction front of plate boundary in the Indo-Burmese wedge. In the north-east edge of Bengal Basin, the north-south compression out of regional torque sets the folds obliquely following the trend of tectonic convergence. The asymmetric sinusoidal folding of Chittagong Tripura Fold Belt diminished in intensity due to release of compressional energy through intense deformation during the active phase of Indo-Burmese plate convergence. Results of field investigations and analysis of gravity and magnetic anomaly of Bengal Basin eliminates the possibilities of existence of oceanic crust in Bengal Foredeep, and absence of large and continuous slips in the subsurface that can release required energy for generating great earthquakes in Bengal Basin.

The assessment of frequency and magnitudes of regional seismicity in comparison to the plate motion vectors and crustal geometry revealed that the decollements are fragmented and inactive due to high angular friction and enormous gravitational load, which is not favorable for the development of any deep megathrust that can generate very high magnitude catastrophic earthquakes in the region.