GRAVITY ANALYSIS OF THE BHUTAN HIMALAYAN OROGENY

The Bhutan Himalayan orogenic belt, located within the eastern quarter of the Himalayan orogenic belt, is mostly known from the balanced geological cross-sections derived along roads and models derived from the sparsely distributed broadband seismic data. In order to obtain more higher resolved lithospheric model of the Bhutan Himalayan orogenic belt, detailed gravity surveys have been conducted along the all available roads in Bhutan since 2015. Approximately, 5300 gravity stations, with the station interval ranging between 0.5 and 1 km, have been acquired along with differential GPS data to obtain submeter elevation data. The gravity data were processed into complete Bouguer gravity anomalies using a 10 meter DEM obtained from the Bhutan Geological Survey. In addition, using the 10m DEM and lithospheric thicknesses derived from the broadband seismic models, isostatic residual gravity data were obtained. The complete Bouguer and isostatic residual gravity anomalies were analyzed using a variety of map methods including Euler deconvolution, wavelength filtering, and derivative methods. Residual gravity anomaly maps derived using wavelength filtering clearly indicate significant along strike variations in the density structures of the orogenic belt. The residual gravity anomaly maps indicate gravity minima over the Subhimalayan units and the modern foreland basin, however, gravity maxima are observed mostly over the Lesser Himalayan region which may be explained by the presence of quartzitic rocks. Two-dimensional roughly north-south trending gravity models were constructed to determine the crustal structure using constraints from the recent seismic models. The modeling indicate that the long wavelength anomalies are associated with variations of the depth to the Moho and are caused by the flexure of Indian plate beneath Bhutan Himalaya. The modelling shows that the Moho depths increases towards the north from 50 km beneath Main Frontal Thrust to 75 km beneath the Greater Himalayan sequence in the western Bhutan. Conversely, such substantial variations of Moho depths is not detected beneath the central Bhutan, which is supportive to the seismic results.