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Geology of Nepal and Its Regional Frame

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Geology of Nepal and Its Regional Frame J. geol. SOC. London, vol. 137, 1980, pp. 1-34, 15 figs. Printed in Northern Ireland. Geology of Nepal and its regional frame J. Stocklin Thirty-third William Smith Lecture CONTENTS 1. Introduction 3 a. General background 3 b. Zonation of Nepal Himalaya 3 2. The High Himalaya 4 a. Central Crystalline zone 4 (i) Composition and stratigraphy 4 (ii) Structure 5 (iii) Metamorphism, granitization, and the basement problem 5 b. Tibetan sedimentary zone 6 (i) Stratigraphy 6 (ii) Structure 7 c. Indus-Tsangpo suture zone 7 3. The Lesser Himalaya 9 a. ‘The unpaged historic manuscript’ 9 b. Palaeontological evidence 9 c. Stratigraphical implications 11 d. The Kumaon background in western Nepal 11 (i) The sedimentary belts 11 (ii) The crystalline ‘klippen’ 13 e. The Sikkim background in eastern Nepal 13 f. New studies in Central Nepal 15 (i) General aspects 15 (ii) The Nawakot Complex 18 (iii) The Kathmandu Complex 19 (iv) Metamorphism and granitization 21 (v) Autochthony or allochthony? 22 (vi) Mahabharat Thrust and Kathmandu nappe 22 g. Reverse metamorphism and Main Central Thrust 24 h. Main Boundary Thrust, Siwalik belt, and Gangetic plain 25 4. Regional aspects 26 a. Palaeogeography 26 b. The Himalaya in the structure of Central Asia 27 c.Eurasia/Gondwana relations 30 S. References 31 SUMMARY: 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 mostreliably recorded in the richlyfossiliferous ‘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. Includedare minor volcanic and glacialdeposits and a Glossopteris flora of Permo-Carboniferous age, suggesting close palaeogeographic links with India and Gondwana- land. The absence of significantunconformities refutes allegations about aHercynian 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 com- menced in Triassic times and continued to the early Tertiary, with emplacementof ophiolites in theCretaceous 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. 1 0016-7649/80/ 0100~001$02.00@ 1980 The Geological Society Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/1/1/4886597/gsjgs.137.1.0001.pdf by guest on 26 September 2021 2 J. Stocklin 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 transi- tional 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 (Mm) 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, metamorph- ism and granitization in the Himalaya, hut 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 predomin- ant 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 problemsremain in the Lesser Himalaya. Stratigraphic work in the thick, slightly metamorphosed argillo-arenaceous and calcareous deposits is hampered by the almost totallack of palaeontologicalcontrol. The sporadic and partlycontroversial discoveries of organic traces point to Tethyan affinities and a range from Precambrian to Tertiary, with a predominance of late Precambrian-earlyPalaeozoic and Permo-Carboniferousdeposits, a widespreadMiddle Palaeozoic gap, and restrictedMesozoic-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 thickeningand completion of the section(closing of theMiddle Palaeozoic gap) but with considerable differentiation in the Mesozoic. The ‘Lesser Himalayan crystallines’, which overlie the low-grade metasediments as klippen- likeisolated masses or, in eastern Nepal, as extensive sheets mergingwith the Central Crystalline zone, pose the difficult problemof ‘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 aright way up sequence of regionallymetamorphosed sediments displaying a metamorphiczonation roughly concordant with stratigraphy and aregular decrease in metamorphic grade from highly garnetiferous schists at the base to barely metamorphosed, fossiliferousPalaeozoic sediments on top. Bandedgneisses and augen-gneisseshave a re- stricted, 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, metamor- phic 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 magmat- ism. 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 epicon- tinentalfacies of theirHimalayan counterparts. The Sungpan-Kantze and Sankiangfold 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-earlyJurassic 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 Tertiaryand to the MainBoundary Thrust and theHimalayan 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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/1/1/4886597/gsjgs.137.1.0001.pdf by guest on 26 September 2021 Geology of Nepalregionalframe and its 3 1. Introduction meetings: the ‘Himaiayan Geology Seminar’ organized by theGovernment of India in NewDelhi in Sep- a. General background temberInternational1976;the Colloquium ‘Himalaya’organized by theCNRS in Paris in De- Fromits earliest beginnings to the middle of this cember1976; and the International Geodynamics century, geological research in the Himalaya had been Conference on ‘Himalayan Region’ organized by the limited tothe westernwing of therange and little Government of Nepal in Kathmandu in March 1978. more than the Darjeeling/Sikkim sector in the E. By Published geological material on Nepal has meanwhile 19.50 allgeological information on Nepal, the large grownto an extent that is already difficult tocom- central sector, consisted of the few notes, dealing with mand. cursoryvisits, of Hooker(1854), Medlicott (1875), The present paper summarizes the most important Auden(1935), and Heim & Gansser(1939). Some results of thesestudies and attempts to place Nepal reconnaissancework had been done on the Tibetan into the wider geological frame of the Himalaya and flank of Mt. Everest during early mountaineering ex- Central Asia. It is primarily
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