Nature of the Crust in the Superdeep Bengal Basin Using Teleseismic P Waves

The Bengal Basin is a sedimentary basin in the northeast region of the Indian subcontinent. It lies between the Indian Shield and the Indo-Burma Ranges, where the India plate is obliquely subducting under the Burma microplate. Multiple interpretations of the nature of the crust here have been proposed. Using a compilation of data from 40 regional broadband stations, we determine the crustal structure by waveform modeling receiver functions and autocorrelograms. We obtain useful velocity models for 30 stations with 2–3 sedimentary units overlying the crystalline crust. The sedimentary section is up to 16.4 km thick with depths increasing from northwest to southeast. The first two sedimentary units have mean thicknesses of ∼3.2 and ∼6.5 km and Vp values of ∼2.8 and ∼4.9 km/s, respectively. Below these units, large negative Ps conversions are present, which we interpret as two low-velocity zones in the deepest portion of the Bengal Basin, with average Vp and Vp/Vs values of 4.2 km/s and 1.90. The low seismic velocities could be a result of fluids trapped in the deepest sedimentary unit. Below the sedimentary section the thickness of the crystalline crust varies from 12.9 to 34 km, thinning from northwest to southeast in the opposite general trend of basin depth, with an average Vp of 6.7 km/s. The crystalline crust is thinner and faster than typical continental crust and thicker and slower than typical oceanic crust. We suggest the region has extended continental crust that was altered during the Cretaceous rifting that created the Bengal Basin.