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.

Abstract The subduction of Indian plate lithosphere during its collision with Asian plate in the Eocene resulted in a regional metamorphic belt along the strike of the Himalayan orogen. High-/ultrahigh-pressure (HP/UHP) metamorphic rocks (eclogites and host gneisses) confirm the metamorphic event in western Himalaya (Kaghan c. 46 Ma and Tso Morari at c. 47 Ma) at mantle depths (>90 km: coesite-stable). In contrast, HP/UHP rocks have not been reported from central and eastern Himalaya and only highly retrogressed eclogites and granulites ( c. 25 to 13 Ma) occur. The presence of UHP rocks in western Himalaya and highly retrogressed eclogites and granulites in central and eastern Himalaya was regarded as evidence for a diachronous India–Asia collision. Despite the along-strike regional homogeneity in major lithotectonic units of the Himalayan orogen, metamorphic diachroneity is enigmatic. It is unlikely to have a subduction-related prolonged progressive metamorphic event. In contrast, the age difference and preservation of UHP phases in the west and their transformation into granulites in central and eastern Himalaya could be associated with their prolonged residence times at crustal levels in the central and eastern Himalaya whereas the rocks exhumed rapidly in the west. The higher thermal events relating to melting of the subducting Indian lithosphere in central and eastern Himalaya evidenced from ultra-potasic volcanics in southern Tibet probably decompressed the early metabasites into granulitized eclogites, even resetting their geological clock, which is why eclogites and granulites in the east show younger ages compared with their UHP counterparts in the west.