Clastic Dikes as a Possible Paleo-Earthquake Indicator in the Bengal Basin

Historical earthquakes around the northeast Indian subcontinent have induced significant and widespread damage across the Bengal Basin. Recent work has also shown that the Indian-Burman plate boundary along the basin's eastern margin has built sufficient strain to cause a large magnitude earthquake, but little is known about the style and frequency of rupture. In all, the potential impact of large earthquakes within the region is well recognized, but neither well constrained nor well understood. Here, we report the discovery of large clastic dikes in the central Bengal Basin that appear to record a major paleo-seismic event. The site contains numerous laterally extensive sand dikes that intrude overbank river mud deposits, often reaching the ground surface. The dikes vary in width and comprise a set of at least two main intrusions, ∼10 m apart, largely parallel, and oriented east-west. The surface features have been reworked and are not well preserved. The age of the dikes is not yet known, but the breach appears to have occurred relatively contemporaneously with deposition of the intruded muds. In this case, the muds do not show signs of brittle fracture suggest that they were not yet well consolidated. Furthermore, the complete distortion of laminated bedding in a 30-cm thick layer of very fine sand within the mud section also indicates that the mud unit was relatively young and unconsolidated at the time of sand-dike emplacement. The location of the clastic dikes lies adjacent to a section of large, abandoned river channel that is ∼1.5 km wide and only partially filled with fine-grained muds. The size of the channel suggests that it may be a paleo-Ganges course, and the lack of sandy infill typical of this braided river suggest that it may have been abruptly abandoned. It is not yet known whether there is any correlation between the sand dikes and the channel abandonment. Our team is actively exploring this possibility.