The Geometry and Kinematics of the Darjeeling - Sikkim Himalaya, India

The Darjeeling-Sikkim Himalayan (DSH) fold-thrust belt lies in a zone of arc-perpendicular convergence between the Indian and Eurasian plates. A series of south-vergent folded imbricate thrusts are exposed along the Teesta and Rangit valleys; from N to S these are: (1) the upper Main Central thrust (MCT1) that carries high-grade (sillimanite-bearing) Kanchenjunga-Darjeeling gneisses in its hanging wall. (2) The lower Main Central thrust (MCT2) carries intermediate grade (biotite-garnet) Paro and Lingtse gneisses; together these two sheets form the Greater Himalayan sequence. (3) The Ramgarh thrust (RT) carries Daling phyllites and graywackes. (4) The Main Boundary thrust (MBT) carries the Daling, Buxa and Gondwana units; the RT and MBT sheets together comprise the Lesser Himalayan sequence. (5) The Main Frontal thrust (MFT) carries Sub-Himalayan Siwalik rocks onto the foreland. In southern Sikkim the MCT1, MCT2 and RT sheets are regionally antiformally folded over the Lesser Himalayan Rangit duplex, whose roof and floor are the RT and MBT respectively and which is exposed in the Rangit window. Farther south, the Darjeeling klippe exposes these sheets in a synformally folded stacked sequence.

We have used INDEPTH geophysical data, the template constraint and return to regional dip arguments to constrain the depth to basal detachment, which varies from ~5 km at the mountain front to ~9 km under the Rangit duplex to ~13 km under the MCT1 and MCT2 ramps; the regional detachment dip is ~3°. Using this in combination with surface structures we have constructed a regional balanced cross section across the DSH. Minimum restorations suggest that the MCT1 and MCT2 sheets have each been translated ~100 km southward. In addition, the RT sheet has been translated ~50 km, and the MBT a minimum of 28 km. Including shortening within the Rangit duplex we estimate a total minimum regional shortening of ~350 km.