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.

Recent recognition of the existence of a deep fault of regional dimension along the northern boundary of the Great Himalayan (Himadri) lithotectonic subprovince is a repudiation of the time-honored notion of the transition from high-grade metamorphics of the basement complex forming the Great Himalaya into the Phanerozoic fossiliferous sedimentary succession of the Tethyan domain. This plane of dislocation has attenuated and truncated basal Tethyan units (e.g., by the Malari Fault in Kumaun); it has disharmonically deformed or backfolded the lower Paleozoic formations (as discernible north of Nanda Devi in Kumaun and north of Kanjiroba-Annapurna in western Nepal); it has split the lithologic succession into a schuppen zone (as done by the Chomolungma/Main North Himalayan Thrust in the Sagarmatha [Everest] region in northeastern Nepal and by the Trans-Axial Fault in northwestern Sikkim); and it has caused considerable deformation, including mylonitization of the basement metamorphics, migmatites, and mid-Tertiary (28 to 18 Ma) granites that occur as concordant bodies and cross-cutting dikes. The Trans-Himadri Fault (T-HF) may represent the southernmost of the old normal faults in the continental-margin basin in the frontal part of the northward advancing Indian plate. This fault (which becomes a low-angle thrust in northeastern Nepal) was reactivated following the blocking of movements along the Indus-Tsangpo Suture Zone (ITSZ) and slowing down of sliding along the Main Central Thrust (MCT). The T-HF may therefore be accommodating a part of the convergence of the Indian and Asian plates. The domal structural architecture north of Annapurna, the pronounced upwarps of the metamorphic basement immediately to the south of the ITSZ in the Mansarovar region, and the faulted crystalline basement blocks and slices thrust northward in the Tso Morari area in Ladakh bear testimony to the blocking of movements and the consequent upwarping of the Indian crustal plate at its leading edge. The fall and then rise of the Moho in the Kashmir-Nanga Parbat-Pamir section and its abrupt deepening from 55 km under the Sagarmatha to 70 km a few tens of kilometers north, through to the Tibetan country, must be viewed in the context of postcollision, perhaps neotectonic, movements along the deep T-HF.