Geology of Nepal and its regional frame: Thirty-third William Smith Lecture

Abstract

Since the opening of Nepal in 1950, a wealth of new information on the geology of the Himalaya has emanated from this country. The sedimentary history of the Range is most reliably recorded in the richly fossiliferous ‘Tethyan’ or ‘Tibetan’ zone, which extends to the N from the summit region and has revealed an epicontinental to miogeosynclinal sequence, over 10 km thick, ranging from Cambrian to Cretaceous. Included are minor volcanic and glacial deposits and a Glossopteris flora of Permo-Carboniferous age, suggesting close palaeogeographic links with India and Gondwana-land. The absence of significant unconformities refutes allegations about a Hercynian or Caledonian orogenic prehistory for the Himalaya. The Mesozoic portion of the sequence passes northwards into the Indus-Tsangpo eugeosynclinal zone, where deep-sea sedimentation commenced in Triassic times and continued to the early Tertiary, with emplacement of ophiolites in the Cretaceous and thick flysch deposits in the Cretaceous-Eocene. Subduction of Tethyan oceanic crust and collision of India with Eurasia along the Indus-Tsangpo ophiolitic suture is a current hypothesis. The Central crystalline zone, which forms the High Range, appears at first sight to be the Precambrian crystalline basement of the Tibetan sediments. But the crystalline rocks (which in addition to gneisses and granites contain high- and low-grade metasediments) show a transitional relationship with the sediments of the Tibetan zone. There is an intimate relationship of metamorphism and granitization with late Tertiary deformation and thrusting, particularly with the Main Central Thrust (MCT) which separates the High Range from the Lesser Himalaya. Thrusting along the MCT is equated by many with continental subduction along a rupture in the Indian continental plate which was primarily responsible for the deformation, metamorphism and granitization in the Himalaya, but this is hardly conceivable without assuming prior consolidation of the plate fragments involved. Radiometric dating of gneisses and granites from the Central Crystalline zone has indicated Precambrian-Cambrian in addition to the predominant late Tertiary ages; together with stratigraphic data from the Lesser Himalaya they suggest that Indian shield elements are present in the crystalline masses of the Himalaya but have been largely obliterated by the Himalayan orogeny. Fundamental problems remain in the Lesser Himalaya. Stratigraphic work in the thick, slightly metamorphosed argillo-arenaceous and calcareous deposits is hampered by the almost total lack of palaeontological control. The sporadic and partly controversial discoveries of organic traces point to Tethyan affinities and a range from Precambrian to Tertiary, with a predominance of late Precambrian-early Palaeozoic and Permo-Carboniferous deposits, a widespread Middle Palaeozoic gap, and restricted Mesozoic-early Tertiary deposition in marginal basins in the S. The facies suggest continuity of shelf sedimentation from the Indian platform in the S across the Lesser Himalayan zone to the Tibetan zone in the N, with gradual thickening and completion of the section (closing of the Middle Palaeozoic gap) but with considerable differentiation in the Mesozoic. The ‘Lesser Himalayan crystallines’, which overlie the low-grade metasediments as klippen-like isolated masses or, in eastern Nepal, as extensive sheets merging with the Central Crystalline zone, pose the difficult problem of ‘reverse metamorphism’. Heat metamorphism by in situ granite intrusions, selected metamorphism and migmatization, inversion of stratigraphy by recumbent folding, block faulting, nappe structure and other explanations have been offered. The crystalline complex of Kathmandu in central Nepal, recently mapped in detail, consists primarily of a right way up sequence of regionally metamorphosed sediments displaying a metamorphic zonation roughly concordant with stratigraphy and a regular decrease in metamorphic grade from highly garnetiferous schists at the base to barely metamorphosed, fossiliferous Palaeozoic sediments on top. Banded gneisses and augen-gneisses have a restricted, laterally and vertically irregular distribution in this sequence, reflecting a superimposed migmatization that disrupts the primary (regional) metamorphic zonation. Small granite bodies are genetically related to the migmatites. The contact of the Kathmandu Crystalline zone with the underlying metasediments is marked by intense shearing and by a stratigraphic, metamorphic and structural discontinuity indicating a thrust plane. The Kathmandu Crystalline zone is interpreted as the remnant of a nappe, rooted in the Central Crystalline zone. The Himalayan orogeny also involved vast expanses of Trans-himalayan Tibet and Sinkiang. Studies by Chinese geologists show Tibet to be an intensely folded mountain country, forming part of a vast ‘Tethys-Himalayan Domain’ affected by Mesozoic-Tertiary folding and magmatism. It displays striking similarities with central Iran and appears linked with it through the Hindukush-Pamir-Karakorum system, a continuous orogenic belt to the N of the main Alpine-Himalayan ophiolitic suture. The Palaeozoic deposits of Central Tibet have the epicontinental facies of their Himalayan counterparts. The Sungpan-Kantze and Sankiang fold systems of northern and eastern Tibet are distinguished as a broad ‘Indosinian’ belt of intense late Triassic folding. Its axial zone, the Chinshakiang fault zone, is characterized by thick flysch deposits associated with basic and acid volcanic material of the Variscan-Indosinian cycle and accompanied by late Triassic-early Jurassic granite intrusions. This fold belt links the late Triassic (‘late Hercynian’) fold belt of northern Afghanistan and the northern Pamir with the classical Indosinian (late Triassic) fold belt of Yunnan and SE Asia. The mountains of Sinkiang, part of the ‘Pal-Asiatic Domain’ N of the Chinshakiang fault zone, bear the stamp of the Caledonian and Hercynian orogenies; however, the late Tertiary Himalayan movements strongly remoulded them as far N as the Tienshan Range, 1500 km N of the Himalaya. A southward migration of the centres of orogenic activity from the mountains of Sinkiang in Palaeozoic time to northern Tibet in late Triassic and to the Indus-Tsangpo line in Cretaceous-early Tertiary time, and further to the Himalayan Main Central Thrust in Middle Tertiary and to the Main Boundary Thrust and the Himalayan front in Pliocene-Pleistocene time, can be clearly recognized. It is tentatively explained in terms of continental drift by the breakaway of two large continental fragments—Tibet and India—from Gondwanaland and their successive collision with, and accretion to, Eurasia.