1 3 Int J Earth Sci (Geol Rundsch) (2014) 103:1681–1697 DOI 10.1007/s00531-014-1039-0 ORIGINAL PAPER Gravity anomalies, flexure, and deformation of the converging Indian lithosphere in Nepal and Sikkim–Darjeeling Himalayas Md. Afroz Ansari · Prosanta K. Khan · Virendra M. Tiwari · Jayashree Banerjee Received: 6 October 2013 / Accepted: 27 May 2014 / Published online: 17 June 2014 © Springer-Verlag Berlin Heidelberg 2014 topography. Although the highest seismicity concentration is dominant in this zone, the Lesser and the Tethys Hima- layas in Sikkim–Darjeeling area also record relatively fair seismic activity. More compressive stress field in different layers right within the sharp bending zone supports this observation. We thus propose that the sharp bending zone beneath the Greater Himalaya is suffering maximum defor- mation, and the deformation is continued in the mantle too. We also identify both right-lateral shear and radial ver- gence slip, which are presumably associated with the gen- eral dynamics and kinematics of the Himalaya. Keywords Flexing · Deformation · Stress field · Seismicity · Plate obliquity Introduction The Himalayan range is a diffused plate boundary evolved through continental collision between the Indian land mass and the Asia, and began at some 50 Ma before (Besse et al. 1984; Molnar 1984; Patriat and Achache 1984; Gaetani and Garzanti 1991). The convergence rates of Indian plate against the southern margin of the Tibetan plateau vary sig- nificantly between ~4.2 and 5.4 cm/years (DeMets et al. 1990). The convergence led to the tectonic stacking of upper and mid-crustal slabs on the northern margin along the foreland of the Indian plate originating from the basal decollement called the Main Himalayan Thrust (MHT). The compression due to convergence and stacking of crus- tal slabs over the leading edge of the Indian plate results its flexure and progressive thickening of the crust toward the north (Lyon-Caen and Molnar 1985). The weight of the mountains is supported in part by the strength of the plate, and the plate distributes the load by flexing down in front Abstract Researchers ubiquitously noted that the com- mon processes of partitioning oblique convergence in response to drag from the trench-hanging plate simultane- ously produce radial slips, along-strike translation, and extension parallel to the deformation front. Here, we focus on the area between Nepal and Sikkim–Darjeeling Himala- yas, and carry out gravity and finite-element stress mode- ling of the strike-orthogonal converging Indian lithosphere. We delineate the geometries of different layers and their interfaces through gravity modeling. The optimum model parameters along with rheological parameters of different layers are used for finite-element modeling. Finite-element modeling is done with boundary conditions of keeping the upper surface free and rigidly fixing the section of the northern boundary below the Main Himalayan Thrust. We impart on its frontal section an amount of 6 × 10 12 N/m force, equivalent to resistive force of the Himalayan–Tibet system, and analyze the maximum and minimum compres- sive stress fields evolved in the lithosphere. We testify our observations with earthquake database and other geophysi- cal and geological studies. We note that an increasing flex- ing of the Indian lithosphere beyond the Main Boundary Thrust becomes maxima between the Main Central Thrust and South Tibetan Detachment in both the areas; how- ever, more steepening of the Moho boundary is identified in the Sikkim–Darjeeling Himalaya. This abrupt change in lithospheric geometry beneath the Greater Himalaya is likely correlated with the sharp elevation changes in the M. A. Ansari · P. K. Khan (*) · J. Banerjee Department of Applied Geophysics, Indian School of Mines, Dhanbad, India e-mail: pkkhan_india@yahoo.com V. M. Tiwari National Geophysical Research Institute, Hyderabad, India