Abstract Recent fieldwork in western Bhutan, dedicated to unravelling the tectonic structure of the mid-crustal rocks, indicates a complex deformation pattern in the Greater Himalayan Slab (GHS). A system of normal shear zones, striking NE–SW and steeply to moderately dipping to the SE, has been recognized within this extruding slab or wedge of crystalline rocks. The zones are characterized by well developed shear-sense indicators pointing to a top-down-to-SE sense of shear. The main Barrovian metamorphic minerals are bent and stretched by extensional shear bands and associated deformation mechanisms indicate a range of brittle–ductile deformation conditions. Normal shear zones are concentrated in the middle–upper part of the GHS and indicate a thrust-transport-parallel lengthening of the core itself. Vorticity analysis highlights a non-coaxial flow with pure and simple shear acting together during deformation (mean vorticity number bracketed between 0.63 and 0.76). These data, when compared to available data near the tectonic boundaries of the GHS, indicate an increasing component of pure shear towards the interior of the GHS. The ages of zircon overgrowths and monazites from a slightly deformed granite, 20.5 ± 0.5 Ma, and a mylonitic granite deformed into the shear zones, 17.0 ± 0.2 Ma, bracket the age of shear zone formation at close to 17 Ma. According to our data, the normal shear zones could well accommodate the pure shear component of deformation localized in the inner part of the extruding wedge/slab and is compatible with a channel flow model.

Abstract This chapter examines the along-arc variation in the crustal structure of the Himalayan Mountain Range. Using results from published seismological studies, plus large teleseismic body-wave and surface-wave datasets which we analyse, we illustrate the along-arc variation by comparing the crustal properties beneath four representative areas of the Himalayan Mountain Range: the Western Syntaxis, the Garhwal–Kumaon, the Eastern Nepal–Sikkim, and the Bhutan–Northeastern India regions. The Western Syntaxis and the Bhutan–Northeastern India regions have a complicated structure extending far out in front of the main Range, whereas the Central Himalaya appear to have a much simpler structure. The deformation is more distributed beneath the western and eastern ends of the Range, but in general, the crust gradually thickens from c. 40 km on the southern side of the Foreland Basin to c. 80 km beneath the Tethys Himalaya. While the gross crustal structure of much of the Himalaya is becoming better known, our understanding of the internal structure of the Himalaya is still sketchy. The detailed geometry of the Main Himalayan Thrust and the role of the secondary structures on the underthrusting Indian Plate are yet to be characterized satisfactorily.