ANALYSIS OF GRAVITY DATA FOR LITHOSPHERIC STRUCTURE IN WESTERN AND CENTRAL BHUTAN

The contemporary understanding of the subsurface structure beneath the Bhutan Himalaya, located within the eastern quarter of the Himalayan orogenic belt, is mostly derived from the balanced geological cross-sections and low resolution geophysical models interpreted from the sparsely distributed broadband seismic data. In order to image a high resolution crustal structure within Bhutan Himalaya, a detailed gravity survey was conducted along the all major roads in western and central Bhutan during 2015-2017. Approximately, 2300 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 (Euler deconvolution and wavelength filtering) which clearly indicate the significant along strike variation in the geology of the area. The residual shows gravity low in the Subhimalayan unit and modern foreland basin, however, the gravity highs are observed mostly within the Lesser Himalayan region that could probably be caused by the quartzitic rocks. Four two-dimensional gravity models across the tectonostratigraphic units were constructed to determine the crustal structure using constrains from the recent seismic studies in the area. The results support the long wavelength anomalies are associated with the variation of Moho depth that 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 variation of Moho depths is not detected beneath the central Bhutan, which is supportive to the seismic results. The modeling also provide constrains on the high resolution geometries and extends of different geologic units including the Lesser Himalayan duplex beneath Greater Himalayan units that exposed as tectonic window (Paro Formation) and major tectonic structures like Main Boundary Thrust, Main Central Thrust and the Shumar Thrust. The combination of forward modeling and a map analysis helps deciphering the lithospheric structures beneath western and central Bhutan.