CONTEMPORARY TECTONICS IN THE DARJILING - SIKKIM HIMALAYA : INSIGHTS FROM HIGH PRECISION GLOBAL POSITIONING SYSTEM (GPS) MEASUREMENTS

The Darjiling-Sikkim Himalaya (DSH) extends for ~ 130 km from the Indian foreland in the south to the Tibet Plateau in the north. The frontal part of the DSH is characterized by a zone of imbricate thrusts that repeat the Siwalik section in the footwall of the Main Boundary thrust (MBT). The MBT sheet lies below the Ramgarh thrust that is involved in the Lesser Himalayan Duplex (LHD). The LHD is exposed in the Tista half-window that erodes through the overlying Main Central thrust (MCT1 & 2) sheets. The MCT sheets are exposed north of the LHD, extending to the northern end of the Himalaya defined by the South Tibet Detachment (STD); the Tibet Plateau begins in the immediate hanging wall of the STD. High precision GPS measurements were made at stations located on in-situ rock over the period 2006-2008 to work out the convergence being accommodated in the DSH. The Pakur (24.563 N, 87.828 E) station was set up in the Rajmahal Hills to serve as the reference station outside the DSH. This station recorded an ITRF 2005 velocity of 53.86 ± 0.6 mm/yr, N 47.39 ± 0.62 E. Relative velocities of the frontal DSH stations (Nim, Mungpu, Kurseong and Gayabari) with respect to Pakur are of the order of 2-3 mm/yr (S to SW) indicating accommodation of ~1- 4 mm/yr of convergence in the frontal part of the DSH. The relative velocities of stations located near the northern part of the LHD (Kyongnosla, Panthang and Peling) with Pakur are ~ 7-9 mm/yr (S) pointing to the accommodation of ~ 3.5-5 mm/yr in the LHD. Similarly, relative velocity of the northern most station in the Himalaya at Gova (27.978 N, 88.585 E) with Pakur is 13.86 ± 0.79 mm/yr, S 6.92 ± 4.08 E. This indicates that ~ 4-5 mm/yr of convergence is being accommodated in the Higher Himalayan MCT sheets north of the LHD and a total of 12.32 ± 0.82 mm/yr N-S arc-perpendicular convergence is being accommodated in the DSH. In comparison, the N-S arc-perpendicular convergence measured east of DSH in the Bhutan Himalaya is 16.57 ± 0.13 mm/yr. Thus, there is a differential of 4.25 ± 0.83 mm/yr in the convergence between the DSH and the Bhutan Himalaya. This either points to lateral variation in the amount of convergence accommodated in the eastern Himalaya or suggests that there are mechanisms other than N to S thrusting that can accommodate the convergence in the DSH.