GRAVITY ANALYSIS OF EASTERN BHUTAN

The Himalayan orogeny is the youngest collisional orogenic belt on Earth, and while its lithosphere structure is known in Nepal and India from detailed seismic surveys, in Bhutan the knowledge of the lithosphere is mainly known from the balanced geological cross-sections and low resolution geophysical models interpreted from the sparsely distributed broadband seismic data. The broadband seismic surveys have been in western Bhutan and thus the knowledge of the lithosphere structure in eastern Bhutan is relatively unknown. In order to image the crustal structure within eastern Bhutan, a detailed gravity survey was conducted along the all major roads in western and central Bhutan during 2016-2017. Approximately, 980 gravity stations, with the station interval ranging between 0.5 and 1 km, were acquired using L&R gravimeter and differential GPS methods. The gravity stations were tied to the absolute gravity station at the office of National Land Commission in Thimphu, Bhutan. The data were analyzed using map methods (wavelength filtering) which clearly indicate that there is a significant along strike variation in the density structure as compared to central Bhutan. The residual gravity anomaly map based on wavelength filtering of an isostatic residual gravity anomaly map has gravity minima in the Subhimalayan units and modern foreland basin, however, the gravity maxima are observed within the Lesser Himalayan region that could probably be caused by the quartzitic rocks. The lower amplitude gravity anomalies as compared to central Bhutan is consistent with broadband seismic Rayleigh tomography results which indicates thinner crust in eastern Bhutan. Two-dimensional gravity models will be constructed constrained by the broadband seismic results to determine the density structure of eastern Bhutan.