Geology of Nepal and Its Regional Frame

Home , Geology of Nepal, Molasse, Phyllite

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. Regionalaspects 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 sedimentaryhistory of theRange 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

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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 (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, metamorphism 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 crystallinecomplex 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 metamorphicgrade from highly garnetiferous schists at thebase to barelymetamorphosed, fossiliferousPalaeozoic sediments on top. Bandedgneisses and augen-gneisseshave 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 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 based on a review of the peditions (e.g. Ode11 1938). literaturebut includes my own experience from a Withthe opening of Nepalin 1950 this truly recent 2-year mapping programme in Central Nepal, Himalayan country has become a focus of geological carriedout in co-operation with my Nepalesecol- activity. It began with the reconnaissance survey of T. league K. D. Bhattarai under a Mineral Exploration Hagen, which lasted 9 years and covered virtually the project of His Majesty’s Government and the United entire country. Hagen’s (1969) elaborate synthesis, in Nations. Visits to crucial outcrop areas including Dar- which themodel of RudolfStaub’s ‘Bau der Alpen’ jeeling,Khumbu, Thakkhola, Almora, Garhwal, can be perceived, has become a great challenge and Kashmir,Ladakh, Baltistan, Gilgit, Hazara, the Salt source of inspirationfor all hissuccessors; though Range, and parts of Afghan and Soviet Central Asia, strongly criticised, it will remain a unique example of gaveme a personal insight into the more regional pioneerwork in theHimalaya. Simultaneously with problems of Himalayan geology. Hagen’s survey the classical geological expeditions to the Everest group by Lombard (1958) and to Mount b. of Nepal Makalu by Bordet(1961) took place. It was mainly Zonation Himalaya theresults from this initial period of reconnaissance Asimple morpho-tectonic zonation that can be work which provided the material for the chapter on applied to all of the Himalaya is valid also for Nepal Nepal in Ganser’s (1964) masterful ‘Geology of the (Fig. 1). Himalayas’.Subsequently, Bordet launched amost In the S, bordering the Gangetic plain, the Siwalik fruitfulcampaign by theFrench National Scientific belt of Neogenemolasse sediments forms a distinct ResearchCentre (CNRS) in theAnnapurna Range foothill zone, covered by dense tropical jungle. N of and the Tibetan sedimentary zone of the Thakkhola the Siwalik belt and sharply separated from it by the (Bordet et al. 1971),later extended E tothe Nyi- ‘Main BoundaryThrust’ (MBT) is thebroad and Shang region (Bordet et al. 1975). The Tibetan zone geologically complex Lesser Himalayan zone, a rugged was also studied by Bodenhausen et al. (1964) in the andhighly dissected mountain country reaching al- Thakkholaarea and by Fuchs(1964, 19776) and titudes of 4000 m. Morphologically it can be divided Frank & Fuchs (1970) in the Dolpo region furtherW. into the Mahabharat Range, just N of the MBT, and This first wave of geological expeditions concerned themore depressed Midland zone further N. Both mostly the High Range and the Tibetan zone N of it, consist of athick sequence of unmetamorphosedor but the studies were soon extended S to the geologi- weakly metamorphosed sediments supporting rocks of cally morecomplex and less gratifying Lesser highermetamorphic grade, the ‘Lesser Himalayan Himalaya by Fuchs & Frank(1970), by aJapanese crystallines’. Thesediments, though mostly unfos- teamfrom Hokkaido University (Hashimoto et al. siliferous, have yielded scarce organic remains indicat- 1973), and by the CNRS mission, particularly RCrny ing a range from late Precambrian to Tertiary. Deep (1975).Systematic mapping in theLesser Himalaya weathering,thick soil coverage, extensive cultivation was initiated by the Geological Survey of India (Nad- and mountain forests make outcrop conditions gener- gir et al. 1968-73) and is presently being continued in ally poor. connection with mineral surveying by the Department N of the Lesser Himalaya, along a distinct break in of Mines and Geology of His Majesty’s Government slope roughly coinciding with the MCT, rise the tower- of Nepal.Talalov (1972, 1977) made an extensive ing walls of the High Himalaya. Here can be disting- survey of oredeposits and studied the relations of uished the Central Crystalline zone and the overlying magrnatism and metallogeny. Tibetan sedimentary zone which builds up some of the Theimportance which Nepal hasgained in highest peaks and most of the ranges further N in the Himalayanresearch within less than 3 decades has Nepal/Tibet border area. The richly fossiliferous sedi- beenclearly brought out at 3 recent international ments, of Palaeozoic-Mesozoicage, are intruded by

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Map‘ Tert,ar( gr.nltr5 m crnira, Crlltallinr zone tmcr Hmalayan sdment> MCT Natn [tntral ihrusl m Ttbrtan wd#menlary zone m trrrrr Himalayan crystalllnrs m Swllk brlt MBT Main Boundary Thrust FIG. 1. Geological zonation of Nepal Himalaya. After Hagen (1969), with amendments from Bordet (1961), Fuchs & Frank (1970), Hashimoto et al. (1973), Rimy (1975), and others.

large Tertiary granite bodies. Further N in Tibet the the MCT .loses its identity and the Central Crystalline Tsangpoophiolitic zone is takenas a conventional zone merges with the crystalline masses of the Lesser boundary between Himalaya and Trans-himalaya. Himalaya, a clear distinction between the two is not Nepalcomprises the majority of theHimalayan possible. The upper limit, against the unmetamorph- giants exceeding 8000m in altitude. From W to E they osedTibetan sediments, isbroadly transitional. The are:Dhaulagiri, Annapurna, Manaslu, Cho Oyu, the total thickness is in the order of 5-10 km. Everest-Lhotse group, Makalu, and Kanchenjunga at the Nepal-Sikkim border. A number of mighty rivers (i) Composition and stratigraphy originating in Tibet break in deep gorges through the entire range and join the Ganges in the S; the most Littlecan be said about the stratigraphy of the importantones are the Karnali in the W, the Kali Central Crystalline zone. The undefined limits arejust Gandaki in the centre, and the Arun/SaptKosi in the E. one side of theproblem. The extent to which the In spite of its remoteness and high altitude, the High gneisses, themain constituents, are regarded as in- Range of Nepal with its associated Tibetan sedimen- tegral parts of a stratigraphic sequence, as remobilized taryzone is geologicallybetter understood than the old or introduced young material, is another point on Lesser Himalaya. Good outcrop conditions, relatively which opinions differ. A variableproportion of the simple structure, the abundanceof fossils, and, last but crystalline rocks, however, is of indisputable sedimen- not least, the magic of the ‘roof of the world’ which tary origin, represented by phyllites, schists, quartzites, has attracted so many, are the obvious reasons. It is marbles-metasediments of variablemetamorphic here that the mostsignificant data on the history of the grade showing complex associations with the gneisses, Himalayan orogene have been obtained. The follow- as xenoliths, lenses, tongues, thin interbeds or entire ingreview will, therefore, proceed from the High thick zones, themselves in various stages of gneissifica- Range to the more problematic Lesser Himalaya. tion and migmatization. Pyroxene and amphibole calc- gneisses may constitute a considerable portion. Several previousdescriptions suggest that banded, kyanite- andsillimanite-bearing garnet-mica-gneisses with a 2. The High Himalaya high amount of associated metasediments often prevail in the lower parts, whereas augen-gneisses, migmatites a. Central Crystalline zone andgranitic gneisses playa moreimportant role in The Central Crystalline zone forms a distinct area higherparts. In recent French literature (Le Fort betweenthe Tibetan sediments in the N andthe 1975; Bordet 1977) the notion of a tectonically undi- LesserHimalayan metasediments in the S (Fig.1). vided crystalline plate, the ‘dalle du Tibet’ (Lombard Neither the northern (upper) nor the southern (lower) 1958) or ‘Tibetan slab’ has been developed. Le Fort, limits are clearly defined. A major discontinuity, the who from petrographic and chemical studies deduced Main Central Thrust (MCT), is widely accepted as the a sedimentary or volcano-sedimentary derivation for lower boundary, but its definition and accurate posi- most of the gneisses, divided this crystalline slab in the tion are under dispute.In the eastern Himalaya, where Annapurna massif into 3 ‘formations’, with Formation1

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derivedessentially from argillo-arenaceous sedi- (iii) Metamorphism,granitization, and the ments, Formation 2 from an alternation of shale and basement problem limestone, and Formation 3 from either arkosic or acid volcanicdeposits. Such adivision, however, is not Many of the early investigators took an ‘Archaean’ general. age for the enormous gneiss masses in the Himalaya It is obvious from the gradation into the Palaeozoic for granted. The position below the Palaeozoic sedi- sediments of theTibetan zone that a greatdeal of ments of the Tibetan zone and a broad similarity with Precambrianmaterial is contained in thecrystalline Indian shield rocks invited this conclusion and also led complex,or at any rate inits metasedimentary por- directly to the nappe concept. Huge recumbent folds tion. On the other hand, a wholly Precambrian age is or nappe sheets provided a plausible explanation for not necessarily the case: Fuchs (1977a) showed that thesuperposition of the gneisses onthe low-grade, themetamorphism may ascend high into the and apparently younger, metasediments of the Lesser Palaeozoic and even Mesozoic sequence of the Tibe- Himalaya.But most observers also admitted, with tanzone; Powell & Conaghan(1973a) discovered varyingemphasis, an influence on metamorphism by Jurassic fossils in the midst of the crystalline mass and graniteintrusions of possibly Alpineage. Later, the pointed to its complex, multiphase internal deforma- commonlyobserved close relation of granitesand tion, which makes the assumption of a simple stratig- gneisses,the discovery of graniteintrusions insedi- raphic sequence doubtful. mentsas young as Cretaceous, the lack of a clear stratigraphic or metamorphic break between the crystalline rocks and the Tibetan sediments, and the first (ii) Structure results of radiometricdating whichall showedlate Tertiaryages for the crystalline rocks-these and In Hagen’s (1969) view the Central Crystalline zone other facts provided convincing evidence for important forms the root of a number of crystalline nappes, and Alpine metamorphism and granitization. Petrographic he accordingly divided it into a series of thrust sheets. andstructural analyses revealed aclose relationship In western Nepal, Fuchs & Frank (1970) distinguished betweenmetamorphism and Alpine deformation. Le on lithologicalgrounds a thin‘Lower’ and athick Fort (1975) and Brunel & Andrieux (1977) linked the ‘UpperCrystalline nappe’ (Fig. 6).The ‘one-slab’ mainphase of metamorphismwith what they consi- hypothesis of the French school has gained supportby dered as theclimax of the Alpine orogeny, the disloca- theobservation of Frank et al. (1977)that inits tionprocess along the MCT, which Le Fort(1975) westerncontinuation in theupper Kulu Valley explained in terms of continental subduction. (HimachalPradesh) the entire crystalline complex An integral part of Le Fort’s subduction model (Fig. seemsto develop laterally from a largerecumbent 13) is granitic melting under high water pressure in the fold. upper slab. Accordingly, the 28 Ma age obtained by Incentral Nepal, the Central Crystalline zone ap- Rb-Srwhole rock dating of a corresponding granite pearsto have a rathersimple homoclinal structure, (Manaslu) (Hamet & Allkgre 1976) was considered as dipping NNE at low to medium angles. The gneisses reflecting theapproximate age of subductionand of are characterized by tight, isoclinal, recumbent small- the main Alpine metamorphism. From correlation of scalefolds referred by mostauthors to the MCT the firstrecognizable phase of metamorphismwith thrusting event, with axial plane schistosities parallel pre-thrusting but still Alpine (post-Cretaceous) struc- to lithic and metamorphic layering and to the MCT. In tures, Le Fortand hiscolleagues concluded that the lower part of the crystalline complex there appears neitherpre-Alpine (pre-Tertiary) metamorphism nor a strongly discordant NNE-SSW mineral lineation, at pre-Alpine deformation has occurred in the Himalaya. right angles to the general Himalayan trend. Structural Frank et al. (1977a), after studiesin Hirnachal Pradesh, analysisled Hashimoto er al.(1973) to regard this arrived at a similar conclusion, but dated the subduc- discordantlineation as older than the thrust-related tionprocess, which they viewed rather in terms of deformations and to refer it to a pre-Alpine folding obductionand cooling of theobducting plate, as event which, as they inferred from stratigraphic evi- c. 16 Ma, corresponding to thecooling ageof the micas; dence in theLesser Himalaya, they assigned to the they agreed with Le Fort, however, on the 28 Ma age Precambrian.In contrast, the French geologists (Le of thethermal peak of metamorphismand on the Fort1975; Brunel &L Andrieux1977; PCcher 1977) absence of recognizable pre-AIpine metamorphism in regarded these structures as ‘streak lineations’ of the the Himalaya, a view shared also by Powell & Con- main Alpine thrusting event, reflecting the direction of aghan (1973a) and others. transport. This view conflictswith otherradiometric and The problem of the age of these discordant struc- stratigraphicdata. Rb-Sr whole rock dating gave tures is evidentlyrelated tothat of theage of the isochrons corresponding to early Palaeozoic and Pre- earliest metamorphism and to the larger problem of cambrian ages for a number of granites and gneisses in the ‘basement’ in the Himalaya. thePanjab and Kumaon Himalayas. Many of them

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After Hagen’s considered as Alpine, whereas Fuchs (in CNRS, 1977, (1968,1969) initial studies, extensive investigations p. 542) pointed to the gradation of such old granites andmapping were carried out by Fuchs(1964, intosurrounding migmatites and concluded that 1977b), Bodenhausen et al. (1964), and Bordet et al. ‘Caledonian’metamorphism and migmatization has (1971,1975). From the Tibetan side of theJolmo taken place. Sinha Roy (1974), who studied the rela- Lungma(Everest) region, new information has be- tions of mineral parageneses and deformations in the come available from Chinese sources (Mu An-Tze et LesserHimalayan crystallines of easternthe al. 1973;Chang Cheng-fa et al. 1977). A review of Himalaya,showed that a first generationof folds is stratigraphic data was presented by Bassoullet et al. recognizable in thepre-Gondwana, probably Pre- (1977). cambrianDaling schists but not in thewell-dated Permo-CarboniferousGondwana beds, and that an (i) Stratigraphy early, pre-thrusting phase of metamorphism reaching thegarnet grade is relatedto these pre-Gondwana Thesedimentary pile exceeds 10 km in thickness deformations.Many Indian geologists (Ghosh 1956; and represents, in Nepal, a fairly complete sequence Jain et al. 1974; Sinha Roy 1974) have insisted on the rangingin age from Cambro-Ordovician to Aptian fact that the Gondwana beds are not metamorphosed, (Fig. 2);in Tibetan territory, N of Everest,marine andthat metamorphic rocks as well asgranites are deposition continued to the Middle Eocene. found reworked in the basal Gondwana boulder beds The change from the crystalline substratum to the (‘Talchirs’). One mayreasonably claim, as has been non-metamorphicPalaeozoic sediments is perfectly done, that these boulders were derived from the In- gradational. In theDhaulagiri-Annapurna Range it dianshield; if so, onehas, however, to explain why takes place within a predominantly calcareous interval, granitization and metamorphism, which have affected the Larjung-Nilgirilimestone group, which is more the Indian continental plate from at least 2000 Ma to than 3000 mthick. No unconformity orsharp less than 1OOOMa ago (Crawford 1970), should have sedimentary or metamorphic break is seen in these or spared its Himalayan plate margin which in the sub- theunderlying rocks. The Cambrian has not been duction model is supposed to be involved. It appears proved palaeontologically, but is likely to be included difficult,anyhow, to conceive a mechanism of conti- inthe crystalline lower (Larjung-Annapurna) limes- nental subduction without assuming a certain degree tone. The oldest datable fossils are brachiopods and of prior consolidation. echinoderms of MiddleOrdovician age, occurring in While the dominant role of Alpine metamorphism theNilgiri Limestone. The eastern extension of the and granitization can no longer be denied, the obser- Larjung-Nilgiricalcareous unit can be recognized in vations and arguments mentioned above, even if dis- the Chiatsun Limestone Group of the Niyalam-Jolmo putable, certainly do not warrant ruling out the possi- Lungma region, which above an unfossiliferous crys- bility of pre-Alpine processes of folding, magmatism talline lower part has yielded a Lower-Middle Ordovi- and metamorphism in the Himalaya. Considering that cianfauna (Mu An-Tze et al. 1973).The Everest positive evidence of such processes is lacking for the Limestone, which forms the world’s highest peak and Cambrian-Cretaceousperiod in the Tibetan sequ- was formerly considered as Permian or Devonian, is ence (see below), and while it must be admitted that nowcorrelated by theChinese geologists with the little is knownabout Precambrian stratigraphy and same Cambro-Ordovician limestone unit on the basis history in the Himalaya, it is above all in the Precam- of Pb isotope ages of 410-515 Ma. W of Nepal, the brian in which it must be suspected that such processes limestone facies of the Cambro-Ordovician is known tookplace, especially as Precambrian folding, mag- also from Kumaon but contrasts with a shale facies in matismand metamorphism are well documented in Spiti and Kashmir. adjoining parts of the Alpine orogen such as Afghanis- The entire Palaeozoic-Mesozoic sequence indicates tan and Iran. platform-typea epicontinental environment. The essential conformity of the sequence cannot be suffi- b. Tibetan sedimentary zone cientlystressed. Regional sedimentary gaps in the UpperDevonian and Upper Carboniferous, lateral The sediments of the Tibetan zone form someof the andvertical facies changes, and gentle geographic highest peaks of Nepal and extend from the summit unconformities, are perfectlycompatible with an

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Blaini-Infrakrol and corresponding 'Gondwana' beds of the Lesser Himalaya. Theepicontinental facies of thePalaeozoic con- tinues in the Mesozoic, with shallow-water limestones Kagbeni Formation predominating,but also including important detritic 12 shale-sandstonedeposits in theUpper Triassic and krrI Spiti Shale Lower Cretaceous and a regional sedimentary gap in "Lumachellesformation" the Upper Callovian. Jomossom Limestone (ii) Structure 10 The Nepal sector of the Tibetan zone in the Dolpo- ThinigaonFormation Thakkhola region forms a broad synclinorium between the High Range and Hagen's 'Tibetan marginal range'. Anoutstanding feature of thissynclinal zone is the Upper Thini- Chu Formalion N-vergentrecumbent folds in theN flank of the Lower Thini-ChuFormation Dhaulagiri-Annapurna Range, the S Rank of the sync- a Tilichotake Limestone linorium (Fig. 3; see also Hagen 1968, pl. 3; Le Fort Tilicho Pass ,Formation 1975, fig. 5; Fuchs1977b, pl. 4).This conspicuous Muth Quartztte N-vergencecontrasts with the S-thrusting along the MCTand MBT and the general S-vergence of the 'Formation sombre" folds in the Lesser Himalaya. A satisfactory explanation for the N-vergence, attributed by Hagen and the 6 French authors to early Alpine, pre-thrusting movements,has not been found. Northwards, the tight "North Face quartzites" recumbent folds give way to gentle warping with broad openfolds. The sediments are intruded by thelarge Manaslu-Mustang-Mugugranite batholiths to the N 4 Nilgirilimestone of the synclinorium (Fig. 1). The fault-bounded Thakkhola graben transects the folds in a N-S direction and is one of the large young transversestructures whichdivide the longitudinal Annapurna Limestone profile of the High Range into a number of culminations and depressions. As Hagen (1969) pointed out, Pi Formation the highest massifs of the Nepal Himalaya are not a morphologic expression of the tectonic culminations, but occupy the axial depressions.

Larjung Marble c. Indus-Tsangpo suture zone 0 c Mu An-Tze et al.(1973) reported a spectacular facies change, starting with the Upper Triassic, from FIG. 2. 'Tibetan' sedimentary sequence of Thak- the epicontinental ('miogeosynclinal') sediments of the 2Ikhola, Nepal. From Bassoullet et al. (1977). Tibetan Himalaya to thick eugeosynclinal associations of radiolarianshales, pelagic limestones, flysch sediments,and basic and ultrabasic rocks, developed epicontinentalregime subject to epeirogenic pulsa- further N in the Tsangpo Valley. tions, but can in no way be construed as attesting to a Newdata on this'Indus-Tsangpo suture zone' Caledonian or Hercynian orogenic prehistory for the (Gansser1964, 1974) (see below, Fig. 15, symbols Himalaya. 13-14)have become available from the Ladakh sec- Mostimportant for palaeogeographic reconstruc- tor, NW of Nepal (Frank et al. 1977~;Gansser 1977; tions is the presence of distinct Gondwana elements Shah1977; Fuchs 1977a). Pelitic-flyschoid deep-sea appearing with a temporary regression of the sea in sedimentation began here in the Middle Triassic and lateCarboniferous-early Permian times. These com- probablycontinued until the early Tertiary. The prise tillitic boulder beds, thin coal lenses, and spilitic Triassic-Jurassicsediments are free of volcanicma- volcanicrocks in theThakkhola region, and glacial terial,but a siliceous-shaly sequence of Middle-late depositsand a Glossopteris flora N of Everest;they Cretaceous age, several km thick, passes laterally into recall similar occurrences in Kashmir as well as in the avolcanic facies comprising diabases, pillow lavas,

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FIG.3. Recumbent fold in Thakkhola area, northern Annapurna region, seen from NW (Shang). Sketch by author, stratigraphy after P. Bordet (pers. comm.).

agglomerates and volcano-clastic sediments (‘Dras vol- glaucophaneschist. The zone as a whole is thrust canics’).This submarine volcanism seems to have steeply N upon the post-early Eocene ‘Indus molasse’, started in about Aptian time, as suggested by the first which rests transgressively on the Ladakh Granite and appearance of spilitic debris in sandstones of this age containsreworked granitic and ophiolitic material. in the Thakkhola section of Nepal (Bordet et al. 1971; TheLadakh Granite marginallyintrudes the central Egeler in CNRS 1977, p. 526). Ultrabasic rocks are ophiolitezone and separates it fromthe northern foundassociated with theseCretaceous deep-sea sub-zone.It is part of abroad continuous belt of sedimentary and volcanic deposits as olistoliths or as hornblende-richgranites and granodiorites that ac- tectonic slices, and in complex ophiolitic mClange as- company the ophiolites and extend from W of Nanga sociations along thrust faults. The ophiolites are gen- Parbatto Namche Bharwa at the big knee of the erallyaccepted as representing the remnants of a Brahmaputra-the‘backbone of theTrans-himalaya’ formerTethyan oceanic crust which onceseparated running parallel to the Himalayan chain for its whole India from Eurasia; the larger part is thought to have length(Fig. 15). beenconsumed by subductionalong the Indus- A third, southern ophiolitic sub-zone forms a series Tsangpo suture. of outliers some 40 km S of the main suture, within The evidence from Ladakh and western Tibet shows the Tibetan sedimentary zone. The largest one is the that the suture zoneconsists here of 3 roughly parallel Kiogar-AmlangLa occurrence (Gansser 1964; Shah but discontinuous sub-zones (Gansser 1977). 1977). A smallerone, at Spongtang in theZanskar Anorthern sub-zone, about whichvery little is Range,was investigated by Fuchs(1977a). Their known(Norin 1946), is situatedinthe Trans- stratigraphic and structural profile strikingly resembles himalayanrealm and follows amarked lineament the ophiolite nappes of Oman and the Zagros in that alongthe Shyok River. Ophiolitic mClanges seemto they form extensive sheets of peridotite and serpenti- play an important role in it. nite,with which hugeblocks and rafts of Permian- The central, main suture zone follows the Ladakh Lower Mesozoic exotic limestones are associated. The segment of the Indus, extending E to the Kailas area peridotitesheets rest with athin mClange soleon andbeyond. It is astrongly imbricated sub-vertical Cretaceous (and older?) flysch deposits, which include zone of Mesozoic flysch sedimentsand Cretaceous exotic blocks and overlie, in what may or may not be a Dras volcanics,sliced up by steeplyS-dipping thrust normalstratigraphic relationship, the Palaeozoic- faults.The faults are lined with chaotic mClanges of Mesozoic epicontinental sedimentsof the Tibetan zone. flysch sediments,radiolarite, serpentinized basic and Gansserand Fuchs see in theophiolite sheets and ultrabasicrocks, and crystalline schists including flysch rocks tectonic ‘klippen’ derived from the central

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suture, whereas Shah explained the exotic and ophioli- at best, highly disputed), and the ‘reverse metamorph- tic materialtic olistolithsas deposited withan ism’ (the widely observedsuperposition of relatively autochthonous flysch-a controversy that applies in al- high-grade metamorphic rocks on relatively low-grade most every detail also to Oman and the Zagros (Glen- ones).After more than a century of Himalayanre- nie et al. 1973;Wilson 1969; Ricou 1971). Another search, this strange rock pile still presents itself as ‘the explanation was given by Berthelsen (1951), who re- impressivebut unpaged historic manuscript of the ported from another occurrence in Rupshu steep tec- Himalaya’,Valdiyaas (1964) aptly termed it. tonic contacts, and deduced from this a third, south- Nevertheless,therocks show marked lithologic ernmost suture zone. differentiations,their sequence has been studied in Consumption of the Tethyan oceanic crust may thus many places, and significant though sporadic palaeon- have occurred along one, two or even three subduc- tological discoveries have been made both inside and tion zones. Subduction is thought to have come to an outside Nepal. end when India and Asia collided. Collision probably occurred in pre-MiddleEocene time, prior to the b. Palaeontological evidence deposition of theIndus molasse; it may havebeen relatedto a slow-down of thespreading rate in the Amongorganic remains, stromatolites have the Indian Ocean in the early Eocene (McKenzie & Scla- widest distribution (Fig. 4). Many have been compared ter 1971). withRiphean forms (Valdiya 1969; Raha & Sastry 1973; Sinha1973). On theother hand, Egeler (in Valdiya1964, p. 34) mentioned stromatolites from The Lesser Himalaya central W Nepal showing closer resemblance to Lower 3. Palaeozoicforms and actual association with Lower a. ‘The unpaged historic manuscript’ Palaeozoic spores. Agarwal (1974) reported stromatolites associated with Ordovician-Silurian bryozoa and The Lesser Himalaya of Nepal is made up to a large acritarchsfrom Garhwal. In Central Nepal K. D. extent of clastic sediments with locally important car- Bhattarai and I found stromatolites in association with bonatezones. Some of theserocks are practically Lower Palaeozoic algae and echinoderms (see below). unmetamorphosed,but most are converted by low- RCmy (1975) noticed the presence of stromatolites at grademetamorphism to slates, phyllites, quartzites twodifferent stratigraphic levels in westernNepal. and finely crystalline limestones. It is useful to disting- These observations suggest that the Lesser Himalayan uish a narrow outer (southern) and a broad inner or stromatolites do not belong to one short time interval Midlandsedimentary belt. Tectonic deformation is butmay range from early Riphean to at least early intense in the outer belt, where tight folds with steep Palaeozoic. andsometimes overturned dips often disturb the The richlyfossiliferous Lower-Middle Palaeozoic stratigraphicsequence. Seemingly undisturbed sequ- sequence of Phulchauki-Chandragiri in Central Nepal ences are more readily found in the broad open folds (reviewed by Gupta & Stocklin 1978) has long been a that characterize the Midlands. The two sedimentary unique occurrence, but a counterpart has been found belts are separated by a discontinuous zone of schists, in the Tangchu basin of Bhutan (Termier & Gansser gneisses,migmatites, and also minor granites-the 1974).These occurrences, though having important ‘Lesser Himalayan Crystallines’ which seem to overlie implications on the tectonic interpretation of the Les- the sedimentsthe in largesynclinal cores. The serHimalaya, must be considered as allochthonous Mahabharat Range, S of the Midlands, comprises the and belonging to the Tibetan rather than the Lesser outer sedimentary belt and parts of the Crystallines; Himalayanprovince. However, the previously men- however, in eastern Nepal and generally in the eastern tionedLower Palaeozoic fossils associated with Lesser Himalaya, the Crystallines cover also most of stromatolites occur in the Lesser Himalayan sediments the Midlands and merge with the Central Crystalline proper.Moreover, Gupta (1972) described a Lower zone of the High Range (Fig. 1). Palaeozoic or Devonian brachiopod from a quartzite Thismore or less metamorphosed pile of mostly of theLesser Himalaya of Kumaon,and mentioned layered rocks reaches a thickness of 20 km and more. Devonianscolecodonts from the Dogadda area in Whether this enormous thickness resulted from mere Garhwal (Gupta 1977). sedimentary accumulation or from tectonic repetition Permo-Carboniferous plant fossils as well as marine (thrusting) is theforemost geological problem in the faunashave become known from considerablea Lesser Himalaya of Nepal, and a fundamental prob- number of localities in the western and eastern Lesser lem of Himalayan geology at large. Strong arguments Himalaya(Lakhanpal et al. 1958;Acharyya 1969; have been advanced in favour of both solutions, but Ganeshan 1972; Singh 1973; Jain & Das 1973; Gupta geologistscontinue todisagree. Two phenomena 1977;Waterhouse & Gupta1978). The flora has greatly aggravate the problem: the almost total lack of distinctGondwana affinities and is associatedwith fossils (the age of these rocks is simply not known or, carbonaceous shales, ‘agglomeratic slates’ and boulder

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FIG. 4. a. Stromatolites in Dhading Dolomite; 1.5 km S of Dhading,central Nepal. b. Stromatolites in Dhading Dolomite; TrisulgangaValley, 26 km WSW of Dhading,central Nepal. Photographs by author.

beds, in particular the Blaini-Infrakrol sequence in the were mentioned by P&cher (1977) and Bordet (1977) western and the ‘Talchir-Damuda’ equivalents in the from black slates in the lower Burhi Gandaki Valley. eastern Himalaya. Gondwana beds containing Permo- In contrast to the Palaeozoic, the Mesozoic record is Carboniferoustracheids and spores were also disco- extremely poor. In addition to pre-Permian and Per- vered in the Lesser Himalaya of western Nepal (Fuchs mian ages, Triassic, Jurassic and Cretaceous ages have & Frank 1970); and plant fossils, as yet undetermined, beensuggested for the Krol Limestone and alleged

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Krol equivalents, but the palaeontological evidence is The best if not final conclusion which can be drawn is scanty and the Krol identity of the rocks often con- the presence of more than one limestone unit in the troversial.Nannoplankton and microfloras, however, LesserHimalayan sequence; the purely lithological suggesta Middle or late Mesozoic age for the Tal Krol-Shali correlation (West 1939), defendable prior Formation overlying the type Krol (Ghosh & Srivas- tothe discovery of thefossils, can no longer be tava 1962; Tewari 1969; Sinha 1975). Marine Eocene maintained as some writers continue to do. haslong been known from the Simla area. Possible 5.Most of thealmost hopeless confusion that CretaceousTal equivalents and Eocene nummulitic characterizesthe literature onLesser Himalayan beds have also been found in western Nepal (Hagen stratigraphystems precisely from this indiscriminate 1969; Fuchs & Frank 1970; Singh 1973; RCmy 1975; use of formation names. Two rocksof similar lithology Tewari & Gupta 1976). exposed at widelydistant places can and should be The discovery of vertebrate fossils of late Tertiary compared,but they cannot be correlatedunder the age in the Siwaliks of western Nepal (T. Hussein, pers. same name unless the identity is thoroughly demon- comm.) and of early Pleistocene agein the Kathmandu strated (Hedberg 1976, Holland et al. 1978). If, as is Valleysediments (Gupta 19756) may also be men- mostly the casein theLesser Himalaya, lithological tioned. criteriaalone are available, the demonstration of stratigraphic identity can hardly be achieved without c. Stratigraphic implications detailed large-scale mapping of the whole area linking thetwo localities. Without this, the use of different The scantypalaeontological evidence allows some local names along can prevent later confusion. general conclusions: 1. The alleged ‘barrenness’ of the Lesser Himalayan sediments is apparent rather than real. It is partly due d. The Kumaonbackground in western to a high proportion of (late) Precambrian rocks in the Nepal innerbelt (Kuncha Formation, parts of ‘Chails’); Geological exploration in western Nepal has natur- partly perhaps to high salinity in a restricted, lagoon- ally been influenced by concepts developed previously likeKrol-Piuthan basin in the outer belt (gypsum (and simultaneously) in the western Himalaya, above lenses!);largely, however, to metamorphism and to all in the immediately adjoining Kumaon sector. Here, insufficient efforts to find what is still preserved. the zonation of the Lesser Himalaya into an outer and 2. LatePrecambrian and Palaeozoic sediments an inner sedimentary belt and an intervening crystal- seem to predominate, Mesozoic and Tertiary deposits line zone is particularly clear. The synformal crystal- being restricted to local basins in the outer belt. line zone had been interpreted by Auden (1937) as a 3. The fossil record suggests a wide distribution of large tectonic klippe, his ‘Garhwal nappe’, a concept late Precambrian-early Palaeozoic as well as Permo- adopted by Heim & Gansser (1939) for their crystal- Carboniferous,but a very restricted development of line thrust mass of Almora, the eastern continuationof Middle Palaeozoic deposits. The possibility of a wide- Auden’s Garhwal nappe near the Nepal border. In the spread pre-Upper Palaeozoic gap must be taken into outersedimentary belt-the famousKrol Belt- consideration in the interpretation of local successions. Pilgrim & West (1928) and Auden (1934) had worked Evidencefor such a gap in CentralNepal will be out the classical Krol Belt stratigraphy, which came to discussed later. be regarded as a kind of standard stratigraphy for the 4.With the notorious paucity of fossils,stratig- Lesser Himalayan sediments. raphic correlation has to rely largely on lithology. Yet, many of the advantages that could be drawn from the (i) The sedimentary belts rare fossils are lost, eitherby the difficulty or failure to assign the fossiliferous beds to clearly defined forma- Hagen (1969) first pointed out the conspicuous Krol tions, or simply by defyingthe palaeontological evi- affinities of thePiuthan-Thansing zone, the outer dence in order to maintain a preferred but uncertain sedimentary belt of western Nepal, but it remained for lithological‘correlation’. The futile controversy as to Fuchs & Frank(1970) to demonstrate in detail the whether the Tal Formation is Permian or Cretaceous closelithologic and stratigraphic similarities. Asa is a case in point (CNRS 1977, p. 513). Fossils of both consequence,these authors introduced the classical ages have been found in the area, and the conclusion Krol nomenclature in Nepal. Fig. SA shows a typical must be that two formationsof different age have been Krol Belt succession NW of Thansing as worked out erroneously correlated under the name of Tal; the true by Frank & Fuchs (1970), with the thick ‘Krol Limes- Tal Formation is the one defined at its type locality, tone’ as the most prominent formation. As mentioned whatever its age may be. Similarly, Lesser Himalayan previously, in theKrol type area of Kumaon, the limestones have yielded stromatolites of late Precam- Blaini/Infrakrolbeds are palaeontologically dated as brian affinity in some places like Shali, and a meagre Permo-Carboniferousand the Tal Formation as Mesozoicfauna and flora in other placeslike Krol. Middle-lateMesozoic. It must be stressed, however,

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Inner belt v) E Hiunthull a Chail Nappe 1 B

A - Outer belt A NW Thansing Shah “c

Ta l .^ 2 a i: B .-W I m 8 II Krol

Blaini Nagthat Chandpur

\ FIG. 5. Stratigraphiccorrelation between outer and inner sedimentary belts of Lesser Himalaya in

westernNepal as proposed by Frank & Fuchs L ?n L (1970). (Question marks by author; comparison Y with central Nepal suggests that all of A could be younger than B). I B that the identification of the various Krol Belt units in Nepal by Fuchsand Frank is basedprimarily on lithologicalcriteria. Some undiagnostic ostracod and molluscshells inthe ‘Tal Formation’ suggesting Wm a broadly Jurassic-Cretaceous age are the only support- z” ingpalaeontological evidence. A certaincaution in e acceptingthe Krol Belt divisions in Fig. 5A is thus * justified. A parautochthonous schuppen structure has in prin- 2

ciple been accepted by all authors, including Hagen, E for theouter sedimentary belt of westernNepal. In ._ contrast,the sedimentary pile of theinner belt was interpreted by Hagen (1969) as a succession of thin, extensive nappe sheets, thrust over an autochthonous W sedimentary floor which he thought to be exposed in L .-2 the ‘tectonicwindow’ of Pokhara.In a similar way, M L Fuchs & Frank (1970) distinguished three ‘Chail nappes’ in addition to an underlying ‘Rukum nappe’, all 4 0 8 thrustfrom aroot zone below the MCT over a W parautochthonoussedimentary floor and onto the d schuppen structures of the outer belt (Fig. 6). E In concept, the Rukum and Chail nappes of Fuchs & Frank are comparable to the nappes of Hagen, but

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Fuchs & Frank developed stronger stratigraphic argu- Instead, he represented it as a fan-shaped basement mentsto defend their case. A characteristic stratig- uplift simulating a syncline (Hagen 1969, fig. 73). In raphicsection of the inner belt as seen by Frank & itsfurther extension, however, he distinguished the Fuchs(1970) is reproduced in Fig. 5B. The authors semi-crystalline‘Jajarkot nappes’, which helinked correlated this section with the Krol Belt sequence of with the ‘autochthonous’ Dandeldhura massif through theouter belt, admitting only one majorlimestone an intricate systemof parautochthonous basement slices unit(Krol/Shali) and only one major quartzite unit in the Dailekh zone. This area is still very imperfectly (Nagthat)in the succession. Repetitions of similar known. It was re-interpreted by Gansser (1964) and lithologieswere regarded, not as possible facies re- Rtmy(1975) aspart of continuousa Almora- currences, but as tectonic repetitions; this led to the Dandeldhura-Jajarkot crystalline nappe. construction of thenappes. Yet, a glance at Fig. 5 One shouldexpect again that tectonic interpreta- must raise doubts about the proposed correlation, if tions of these crystalline masses be preceded by, and only because of the thickness and facies discrepancies basedupon, anecessary minimum of stratigraphic between the two sections; and more serious objections work. In Nepal, most authors confined themselves to must be raised on the basis of the important, though listing the various crystalline rock types and describing scanty,palaeontological data. Two facts must be their petrography, elaborating mainly on the gneisses recalled in this connection: (1) the postulated identityof to emphasize the contrast in metamorphic grade with the Shali Limestone in the N with the Krol Limestone the (meta-) sedimentary belts. However, RCmy (1975, in the S has never been proved and has become very fig. 3) presented a generalized stratigraphic column of unlikelywith the new age evidence provided by the his ‘nappedu Nepal’ (broadly, the Dandeldhura- probably late Precambrian Shah stromatolites and the Jajarkotcrystalline zone) which, if thegneisses are certainly post-Carboniferous, probably Mesozoic Krol disregardedas ‘non-stratigraphic’ elements, shows a fossils; and (2) the postulated identity of the Chail and surprisingly good correspondence with the Bhimphedi ChandpurFormations of theouter belt, and their Group of the Kathmandu Crystalline in Central Nepal, identity with alleged counterparts in the inner belt, has to be described later. For RCmy, the ‘nappe du NCpal’ neither been proved nor disproved, because continuity is rooted below the ‘dalle du Tibet’ (Central Crystal- of outcropor supporting palaeontological data are line) of theFrench authors. Fuchs & Frank(1970) lacking. Here, the remarks of the previous paragraph distinguished‘Lower’a and an ‘Upper crystalline concerningstratigraphic correlation fully apply. The nappe’,which they linked with corresponding root ‘Chail nappes’, because deduced from hypothetic cor- elements in the Central Crystalline zone. relations,are hypothetic themselves. Instead of 3 But nappe structure is still not universally accepted ‘Chail nappes’ we now risk having in Nepal 3 different forthe Lesser Himalayan crystalline masses. Serious ‘ChailFormations’, more than one ‘Nagthat Forma- objections continue to be raised, in recent years by a tion’, and different ‘Blaini Beds’. growing number of Indian geologists (Kumar & Agar- This in no way diminishes the work of Fuchs and wal 1975;Banerjee & Bisaria 1975;Saxena & Rao Frank in having established a stratigraphic framework 1975; and many others), and also by Talalov (1972) for western Nepal and to have demonstrated the prin- and the Japanese geologists (Hashimoto er al. 1973). cipalcorrespondences with Kumaon. Nor does it The apparent superposition of the crystallinemasses eliminatethe principal possibility of nappestructure. on the low-grade metasediments is explained either by A tectonically unbroken stratigraphic sequence such as sometectonic reversal such as recumbent folding or assumed by Rimy (1975) for the sedimentary pile of steepup-thrusting, or it is referredheatto the same region remains hypothetic as well. No con- metamorphism by in situ graniteintrusions, or to sensus will foreseeably be reached on this point before migmatization of sediments that are either considered a great deal more systematic geological surveying and identicaland contiguous withthose of theadjacent regional large-scale mapping has been done. sedimentaryzones but up-faulted against them, or younger and normally overlying them in the synclines. (ii) The crystalline‘klippen’ The latter proposition (normal stratigraphic superposition)appears to be the least tenable, as it would Thedirect eastern plolongation of theGarhwal- involve sedimentary thicknesses of tens of km with a Almoracrystalline zone of Kumaon,separating the minimalmetamorphism in thedeepest levels. outer from the inner sedimentary belt, is the granite- However,most authors agree in relating the phe- and gneiss-mass of Dandeldhura in westernmost Nepal nomenon to some tectonic cause, and nappe structure (Fig. 1). Curiouslyenough, Hagen (1969), who has remains the most popular explanation. been blamed for an ‘ultra-nappist’ approach, and who tookthe Garhwal-Almora crystalline thrust mass of e. The Sikkim background in eastern Nepal Auden and Heim & Gansser as the classical example of Lesser Himalayan nappe structure, rejected a nappe To accept nappe structure for the Lesser Himalayan interpretationfor the Dandeldhura Crystalline area. crystalline masses has even more serious implications

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ineastern Nepal and Sikkim. From central Nepal asthe gneisses, whether old paragneisses or young eastwardsthe crystalline rocks include an increasing orthogneisses,were considered aspre-dating the amount of gneissesand migmatites, covering ever thrust-movements. In eastern Nepal this was done by larger expanses and finally most of the broad Lesser all investigators, from Lombard (1958), Bordet (1961) Himalayanzone N of anarrow sedimentary border andHagen (1969) to Hashimoto et al. (19731,al- strip (Fig. 1). The gneisses display generally a rather thoughthe tectonic schemes proposed by theseau- simple structure of broad megafolds with gently dip- thors differed widely (Fig. 7). For Hagen, the Midland ping flanks and with both W-E and N-S axial orienta- gneisses constituted his Kathmandu nappes, rooted N tions. In the nappist view they thus form an enormous, of the windows and overlain by Lombard’s Khumbu sub-horizontalcrystalline sheet, thrust over and nappesconstituting the Everest group. For Bordet, largely burying the Lesser Himalayan sediments. This Hagen’s window rocks were only the cover sediments is preciselywhat one is almostinevitably forced to of adeeper migmatite nappe overridden by higher conclude from the reappearance of unmetamorphosed gneissnappes similar to Hagen’s. Hashimoto et al. fossiliferous Gondwana beds deeply below the crystal- conceivedthe structure as a succession of gently N- linemasses in theclassical tectonic window of the dipping ‘schuppen’, a rather modest term for some of RangitValley in Sikkim (Ghosh 1956). In eastern the thrust masses shown. Bordet in addition introduced Nepal,a similar window was discovered by Hagen the notion of ‘zone des Ccailles’, a narrow shear zone (1969) in the core of the large Arun transverse antic- ofmetasediments and augen-gneisses separating the line, and another, larger one in the Okhaldunga region LesserHimalayan nappes from the ‘dalle du Tibet’ farther W (Fig. 1). Although the palaeontological evi- thatforms the High Range (Fig. 7);and the thrust dence is missingin Nepal, equivalents of the Gond- plane separating the ‘zone des Ccailles’ from the ‘dalle wana beds of Sikkim are suspected to be present in the du Tibet’ was to become the ‘Main Central Thrust’ of black carbonaceous slates which I have distinguished recent nomenclature. (this paper) as ‘Benighat Slates’ in the western part of Bordet (1961) extended his ‘zone des Ccailles’ from the Okhaldunga window and which in its eastern part easternNepal to the Annapurna sector of central areunderlain by stromatolitic dolomite (Y. Maruo, Nepal,equating it here withwhat Fuchs & Frank pers. comm.); there is little doubt that this dolomite (1970) were to distinguish in western Nepal as higher corresponds to the stromatolite-bearing Buxa Limes- Chail nappes and lower Crystalline nappe (Fig. 6). But tone in thesub-Gondwana sequence of Sikkimand there is a fundamental difference in Bordet’s ‘zone des Bhutan. Ccailles’ between eastern and western Nepal: in the E, Thephenomenon, common in Sikkim and at the thezone is underlain by theenormous Lesser Darjeelingmountain front, of unmetamorphosed Himalayangneiss masses (Fig. 7);in the W, by the Gondwana beds being overlain by the low-grade Dal- thickLesser Himalayan sediments (Fig. 6).This dis- ingSchists and these by thehigh-grade Darjeeling crepancy poses a difficult and as yet unsolved problem: Gneisses has puzzled geologists for a long time. Nappe shouldthe MCT (the ‘main’ crustal separation) in structurehad first beenenvisaged by Loczy (1907), eastern Nepal be drawn where Bordet placed it, far in who proposed his huge recumbent fold to explain the the N at the foot of the High Range above the Lesser reversed metamorphism. This idea, which implies that Himalayan gneisses, or far to the S below these gneis- the gneisses are the oldest rocks in the sequence, was ses,as Hagen had done originally? For the Lesser adopted by Heim & Gansser (1939) but not defended Himalayan crystalline ‘klippen’ of central and western by Gansser in later publications. Auden (1935) rather Nepaland Kumaon, the question is whetherthey suspected right way up sequences separated by at least should be rooted in a schuppen zone below the MCT twomajor thrust planes which he had actually ob- (Bordet’s version), or above the MCT and, in this case, servedat the mountain front (between Siwaliks and beregarded as asimple extension of theCentral Gondwanas,and between Gondwanas and Dalings). Crystalline zone (Hagen’s version). He stressed the difficulty of tracing a clear boundary Le Fort (1975) gave the Lesser Himalayan gneisses between the Daling Schists and the overlying Darjeel- an entirely ‘new look’. From petrographic and chemi- ingGneisses and suggested that ‘granite under reg- calanalyses heconcluded volcano-sedimentarya ionalstress has invaded the upper part of agreat origin for the so-called ‘Ulleri Gneiss’, a thin band of sedimentary series converting it, where invaded, into a augen-gneissforming the base of Bordet’s‘zone des series of mixedortho- and paragneisses’, an idea Ccailles’ in central Nepal. However, he re-interpreted shared by Heron (1922) and gaining support by much the ‘zone des Ccailles’ of central Nepal as a higher part recent work. of anormal, tectonically undivided, autochthonous Theenormous thickness of thegneisses in these Lesser Himalayan sedimentary sequence; he thus as- eastern parts, amounting to several tens of km, posed signed tothe Ulleri Gneiss a definite place in the another problem. But once the principle of large-scale LesserHimalayan stratigraphy, ‘broadly equiva- thrusting had entered the discussions, the great thick- lent. . . to the Chandpur Formation of Kumaon’. He ness could be explained by tectonic repetitions as long then extended this interpretation to the thick augen-

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S Nya& Sun K05i LamJura pass Cho Oyu N l l I

Makalu

m]Siwaliks CentralCrystalline B Tibetansedimentary zone mu Lesser Hlmalayansedlments m] Lesser Hlmalayan crystalllnes Granites

FIG. 7. Differenttectonic interpretations of easternNepal Himalaya. Upper profile after Hagen (19691, lower profile after Bordet (1961).

gneisses of theLesser Himalaya of easternNepal, f. New studies in Central Nepal suggesting their stratigraphic equivalence with the U1- leriGneiss and the Chandpur Formation. This (i) Generalaspects amounted to nothing less than reverting the bulk of The Central Mahabharat Range and adjoining Mid- the Lesser Himalayan crystalline nappes and ‘klippen’ lands of Central Nepal provide a good example of the to autochthony. In this view, the crystalline schists and complexity of stratigraphic and structural problems in gneisses are part and parcel of the Lesser Himalayan theLesser Himalaya. It is thearea in which Hagen sediments,metamorphosed and including an impor- (1952,1969) began hissurvey and developed his tantvolcanic episode represented by theaugen- much-disputed nappe concept. K. D. Bhattarai and I gneisses. have recently re-studied and mapped the area in detail On purelystratigraphic grounds, I amunable to (Stocklin & Bhattarai, in press). support Le Fort’s correlationof the Ulleri Gneiss with The dominant structural featureis the large, WNW- the thick and extensive augen-gneisses in the E. In the ESE trendingMahabharat synclinorium (Fig. l), a type area of Ulleri, as shown by Le Fort (1975, fig. 6) doublyplunging megafold with steep flanks, awell- and Pccher (1977, fig. 2), the gneiss occupies a level in developedwestern closure (Fig. 8) and a narrow, theargillo-arenaceous lower part of thesequence, elongated eastern wing. The synclinorium is built up of below the limestones characterizing the higher parts. 2 majorrock assemblages, which Hagendisting- In the E, as at Kodari/Tatopani on the Kathmandu- uished as ‘Kathmandu series’ (or ‘Kathmandu nappes’) Lhasaroad (see below, Fig. 12,lower profile), the and ‘Nawakot series’ (or ‘Nawakot nappes’); to avoid augen-gneisses maketheir first appearanceseveral any stratigraphic or tectonic preconceptions, I prefer thousand metres higher in the succession, above these the neutral term ‘Complex’. The Kathmandu Complex limestones,and they keep this position further E in is part of the Lesser Himalayan ‘crystallines’ but in- the ‘Sailung nappe zone’, as shown by Maruo et al. (in cludesfossiliferous sediments of early-Middle Hashimoto et al. 1973). Supposing the sequence to be Palaeozoic age on top, occupying the large core of the stratigraphicallycontinuous, these eastern augen- synclinorium (Fig. 9). The Nawakot Complex belongs gneisses are thus not identical with the Ulleri Gneiss. tothe ‘Lesser Himalayan sediments’ and forms the They are, however, associated with highly garnetifer- peripheralparts of thestructure and much of the ous schists and seem to belong to the basal part of a broadMidland zone to the N. The wholeappears large crystalline thrust mass. thrust steeply S on the Siwalik belt.

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Hagen’s distinction of the two complexes was based tained.Clearly, they had to be treated as different on theconspicuous difference in compositionand stratigraphic entities, whatever their mutual relations metamorphic grade, and on an assumed difference in regarding age and structure might be. Each revealed age. From lithological comparisons with the European ownits characteristic succession of sediments, Alpsheplaced theNawakot Complex in the metamorphosed to varying degrees. The two sections Palaeozoic-Mesozoicandthus considered it as aresummarized inFigs 10 and 11 andare briefly youngerthan the overlying Kathmandu Complex of commented on below.Some formation names have (demonstrably) Precambrian-early Palaeozoic age. He beenadopted from earlier writers, but in eachcase explainedthis apparently abnormal superposition by referstrictly tothe type locality,not necessarily to tectonic emplacement and interpreted the Kathmandu whatthe authors considered as equivalents in other Complexas an erosional relict of a onceextensive places. thrustmass, rooted in theCentral Crystalline and today still linked with it by the ‘tectonic bridge’of the (ii) The Nawakot Complex Gosainkund gneisses N of Kathmandu. Moreover, he TheNawakot Complex (Fig. 10)consists almost claimed to recognizetectonic repetitions within the exclusively of low-grademetasediments. It has been twocomplexes and thus distinguished several thin subdividedinto a Lowerand an Upper Nawakot ‘Nawakotnappes’ succeeded by severalthin Group, the twobeing separated by an erosional un- ‘Kathmandu nappes’. conformity. Nadgir et al. (1968-73) rejected nappe structure and believed in astratigraphically continuous Nawakot- Lower Nawakot Group. This Group is at least 6 kmthick, Kathmandusequence ending in the Palaeozicfossil- thebase not being exposed. The lowerpart forms a beds on top.The discovery of stromatolites of sus- monotonous,flysch-like, entirely non-calcareous sequence, pected Precambrian age in Nawakot rocks seemed to the Kuncha Formafion (‘strie de Kunchha’ of Bordet 1961), supportthis view. The higher metamorphism of composed of phyllites,phyllitic metasandstones, gritstones Kathmandu rocks was considered as heat metamorph- and fine quartz-conglomerates of light green-grey colour. A few layers of ‘diabasic’ volcanic material are locally interbed- ism induced by thegranites that form part of the ded. TheKuncha formation forms large expanses of the Kathmandu Complex. Arita et al. (1973) neither be- Midlands and has been mapped further W by Fuchs & Frank lieved in thefundamental synclinal structure of the (1970) as‘Chail Formation’ of their‘Chail nappe 1’. A MahabharatRange nor did they consider Hagen’s characteristic feature of the KunchaFormation is a pro- distinction between Nawakot and Kathmandu rocks as nounced NNE mineral lineation, which is missing or incon- fundamental; they operated with block tectonics and spicuous in higher units; it suggests a phase of deformation differentstratigraphic groupings. So didTalalov prior to the deposition of the FagfogQuartzite, but this (1972), who relied on a few radiometric (K-Ar) ages question has not been studied in sufficient detail. to develop an elaborate stratigraphic scheme which he The Fagfog Quartzite (Arita et al. 1973) is afine- to coarse-grainedwhite orthoquartzite withseveral phyllite extendedto all of Nepal-a hardlyconvincing intercalations; it forms a good marker. ‘chronostratigraphic’approach in view of the scanti- The Dandagaon Phyllites are distinctly darker than the ness of pertinent data and the material used for dating phyllites of the KunchaFormation. Carbonitic material (mostlymetasediments of detritalorigin). Brunel makes a first, sporadic appearance in the form of laminated (1975), on theother hand, likeHagen, saw in the calc-phyllites and occasional thin bands of dolomite. KathmanduComplex a tectonicklippe derived from The Nourpul Formation, of mixed lithology, has the white theCentral Crystalline zone but considered it asan to pink, strongly ripple-marked Purebesi Quartzite as a basal undividedthrust mass, which he called‘nappe du member. The rest is phyllite with an increasing admixture of Mahabharat’. sandy and dolomiticmaterial, in which colour-banding is typical. As a matter of fact, none of these widely differing The feature-forming Dhading Dolomite (‘Dhadinglimes- and conflicting propositions can be soundly defended tone’ of Arita et al. 1973) is a well-bedded to massive, light or refuted as long as a workable stratigraphic basis is bluish-grey, dense to fine-crystalline carbonate rock contain- lacking.This was clearly realized by several of the ing abundant stromatolites at manylevels (Fig. 4). Alarge above-mentionedauthors, buttheir stratigraphic boulder of this rock in the middle Rigdi Khola has yielded,in schemes were largely built on notions of age that were addition to stromatolites,thealgae Beuocasfria and necessarily hypothetical. Nubecularifes (identified by G. Termier)as well as indeter- K. D. Bhattaraiand I, whensurveying the area, minableechinoderm fragments. The fossilsindicate very tried to work out first of all a consistent lithostratig- early Palaeozoic age, probably early Cambrian. raphy, based primarily on the distinction of a number The well-beddedsandy, dolomitic and phyllitic Hushdi Beds, overlying the Dhading Dolomite with gradational con- of markers that could be reliably identified and traced tact,have been preserved only locally below the Upper throughoutthe area. The systematic use of aerial Nawakot unconformity. photographs, first applied in Nepalfor this purpose, proved an invaluable help. Hagen’s distinction of the Upper Nawakot Group. The Upper Nawakot Group overlies two major rock complexes and their names were re- the Hushdi Beds or the Dhading Dolomite or older units with

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 Nepal and its regional frame 19 Robang Formahn The characteristically chlorite-rich Robang Formation, the wlth Dunga Quartzites (du) highest unit of the Group, is predominantly phyllite in the N, and baslc racks (b) butthe intercalated white Dunga Quartzite Beds become more important in the S, where they attain great thickness. Malekhu Limestone Associatedwith both the phyllites and quartzites occur chloritic and amphibolitic metadiabases showing synsedimen- BenighatSlates taryrelationships, as well asmore massive, gabbroic or with boulder beds (bl) and dioritic bodies showing intrusive contacts. Jhiku carbonate beds (jk) The Robang Formation, andin places older units of theNawakot Complex, are discordantly overlain by highly garnetiferousschistscharacterize that Hushdi Beds everywhere the base of the Kathmandu Complex; the Dhading Dolomite discordance is associated with intense shearing and is interpreted as a thrust plane (Mahabharat Thrust, see Nourpul Formalion below). As regards the age of the Nawakot Complex, the Dandagaon Phyllites early Palaeozoic algae of the Dhading Dolomite are the only palaeontological evidence available. From its FagfogQuartzite positionwith respect tothe Dhading Dolomite, the KunchaFormation can be confidently placed in the (Upper)Precambrian. The Upper Nawakot Group mustbe post-early Palaeozoic. TheBenighat Slates with their carbonaceous material, local boulder beds Kuncha Formation and lenses of carbonate rocks suggest correlation with the Permo-Carboniferous Blaini/Infrakrol units of the Krol Belt, but the palaeontological proof is missing; throughmicrofloral elements (acritarchs) have been recognized, they are too poorly preserved for identification (N. Pantic,pers. comm.). A relationshipbe- tween themetavolcanics in the Robang Formation and the known late Palaeozoic volcanismof the Himalayan belt (Panjal Traps, etc.) is suspected. Itcan besaid with certainty, however, that the FIG.10. Stratigraphic column of Nawakot Complex. section as described does not include major tectonic repetitions.The Upper Nawakot Group is lithologically quite distinct from the Lower Nawakot Group; the sharp rock boundary at the base of the Benighat an erosional unconformity marked by an abrupt lithological Slates, taken by Hagen (1969, fig. 5b) as one of his change and, in the lower Burhi Gandaki, by traces of laterit- nappeseparations, is not a tectonicplane but an ization. No angularity of contact is seen at the outcrop scale, unconformity;itmarks a sedimentaryhiatus and however. The thickness of the group varies strongly with that could, in theory,represent an overlap of the (sus- of the Benighat Slates and may reach as much as 5 km. pected) Permo-Carboniferous on the (proved) Lower The Benighat Slates, the lowest unit, are dark argillaceous Palaeozoic. All other formation boundaries are grada- slates or phyllites containing frequent intercalations of black tional. carbonaceous (graphitic) slates. Two boulder beds, consisting There is thus no reason to split the Nawakot Com- of rounded, unsorted quartzite boulders in a partly quartzitic, partly carbonitic matrix, are intercalated in the upper Hugti plex into two or several nappes. This holds true€or the Khola area. In addition, the slates contain in places tongues tectonically quiet Midland region, but in the ‘Suparitar and lenses of calc-phyllites and strongly argillaceous limes- zone’ (the outer belt of this sector) the Nawakot rocks tones or dolomites of variouscolours (black, brown, buff, are intensely shear-folded and imbricated (e.g., in the orange, yellow, green); the carbonate content of these ‘Jhiku distorted structure of the Malekhu Limestone, Fig. 8). Beds’ is often difficult to recognize, and the rocks have the overall phyllitic appearance of the whole formation. The Malekhu Limestone, an excellent marker, is lithologi- (iii) The Kuthrnandu Complex cally very distinct from the DhadingDolomite (the other The Kathmandu Complex (Fig. 11) has been divided major carbonate unit of the Nawakot Complex). Particularly distinctive are thin-platyyellow, dense, siliceous limestone into the Precambrian Bhimphedi Group (Nadgir et al. beds with pale-green sericitic partings, formingthe lowermost 1968-73), consisting of relatively high-grade metased- and uppermost parts. The middle part is darker, more thickly iments, and the Phulchauki Group of unmetamorphic bedded, dolomitic. Stromatolites are missing. or weakly metamorphosed sediments containingfossils

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“Top limestone”,Devonian quartz-fragments and fist-size rounded quartziteand schist bouldersin a Kalitar-type matrix, is a lens-shaped local ChitlangForm., Silurian member in the hills N of Bhimphedi. The ChisapaniQuartzite (Joshi 1973),animportant marker, is a white, fine-grained, thin- to thick-bedded ortho- Chandragiri Limestone, quartzite showing strong cross-bedding. Cambro -0rdovician The Kulikhani Formation is a well-bedded alternation of fine-grainedbiotitic schist and impure,strongly micaceous Sopyang Formation quartzite, of dark and light green-grey colour. The MarkhuFormation (Joshi 1973) consistsof schists, quartzites and marbles in varying proportions. Marble is the Tistung Formation distinctivelithotype; it is coarse- to medium-grained and makes up about 50% of the total rock volume in the type -? area.Questionable stromatolithic structures were seen loc- Markhu Formation ally. The formation changes from almost pure, massive marble bodies (? stromatolitic bioherms) in the S to schist and quartzite with subordinate thin, impure marble bands in the N. Phyllitic intercalations show clay cracks and worm trails. Kulikhani Formation PhulchaukiGroup. The Phulchauki Group, whichreaches 5-6 km thickness, starts with the Tistung Formation, a fine- clastic sequence of metasandstones, -siltstones, phyllites and ChisapaniQuartzite slates. Very fine biotite is seen in the lower part, but sericite and chlorite are the only metamorphic minerals in the main Kali tar Formation part and in theoverlying units. Distinct colour banding with Jurikhet Conglomerate (ju) (green,pink, yellow, violet) appears, and intense purple and PandrangQuartzite (pa) weathering colour is characteristic. Some sandstones have a distinctly calcareous cement, and impure limestone intercala- BhainsedobhanMarble tions are occasionally found. A few pebble beds, consisting of reworkedlower Tistung material, occur in the lower part. Ripple-marks, cross-bedding, clay cracks and worm trails are common. Raduwa Formafion The Sopyang Formation is a transitional zone between the fine-grainedclastic Tistung deposits and thethick Chan- dragiriLimestone; dark argillaceous and marlyslates and subordinate argillaceous limestones are characteristic. FIG. 11. Stratigraphic column of Kathmandu Complex. The ChandragiriLimestone (Auden 1935) isthe most prominentunit of the Group, over 2000m thick and of massive appearance, thoughwell-bedded, partly flaggy, in of early-Middle Palaeozoic The two are possibly closer view. Argillaceous (micaceous) partings are common, age. especially in the more thinly bedded lowermost and upper- separatedby a slightunconformity. In addition,the most parts. A band of white laminated quartzite is interbed- complexcontains granites and migmatitic gneisses, ded in the upper third of the limestone. Above this, green which will be discussed separately. and pink argillaceous limestone beds displaying wave-marks contain frequent echinodermfragments indicating Middle- BhimphediGroup. The Bhimphedi Group, about 8 km late Ordovician age; a single specimen of Elliptocincrus bar- thick, begins with the Raduwa Formation, a coarsely crystal- randei from a deeper level suggests Middle Cambrian (Gupta line, strongly garnetiferous two-mica schist, with one major & Termier 1978, Stocklin et al. 1977). and severalminor quartzite intercalations.Amphibole and TheChitlang Formation (Hagen 1969) consistsmainly of pyroxeneminerals are frequentlyassociated. Towards the dark violet-coloured slates. A white quartzite is interbedded base (Mahabharat Thrust), the rock changes to chlorite schist. in the lower part, and some beds of argillaceouslimestone The BhainsedobhanMarble (Nadgir et al. 1968-73)is with wave marks in the upper. The unit seems to correlate coarsely crystalline, well-bedded to massive, containing mica withthe thick slate formation of PhulchaukiHill SE of (including phlogopite) in fine dispersion and, in the basal and Kathmandu,which in the upper part also contains 2 or 3 top parts, as partings and thin intercalations. haematitelayers and a richSilurian fauna of trilobites, Dark green-greybiotite- and two-micaschists with ill- brachiopods, echinoderms, etc. (Bordet et al. 1959). differentiatedintercalations of impure,strongly micaceous Avariegated crinoidal limestone follows and a massive, quartzite are the mainlithotypes of the Kalitar Formation. sparry dolomitic limestone that forms the top of Phulchauki Bedding is often obliterated by discordant schistosities. Gar- Hill and the very core of the Mahabharat synclinorium (Fig. net and amphibole minerals are commonin the lower part 9); this top limestone has yielded Devonian conodonts (Gupta but usuallydisappear higher in the section. The Pandrang 1975~). Quartzite Member, a thick intercalation of pale-green orthoquartzite about 200 m above the base of the formation, is a The contact between the Markhu Formation (top of good marker. The Jurikhet Conglomerate, consisting of small Bhimphedi Group) and the Tistung Formation (base

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/1/1/4886597/gsjgs.137.1.0001.pdf by guest on 26 September 2021 G eology of Nepal andNepal Geologyof its regionalframe 21 of Phulchauki Group) was found to be transitional in this interval, however, they are not bound to specific all sections studied. However, Pradhan et al. (in press) horizons but may appear and disappear at all levels. reportedan unconformity, associated with pebble Lateral and vertical changes from schist or quartzite to beds, at(or near ?) thebase of theTistung. This bandedgneiss by theappearance of thinpegmatitic unconformity, if real, cannot be of significant angular- bands,or toaugen-gneiss by theappearance of ity,as the underlying Markhu Formation does not feldsparcrystals, can often be observed. In sections seemto be truncated anywhere (Fig. 9). It could, wherethe proportion of gneiss is small,the various however,explain the change in metamorphicgrade formations of the Bhimphedi Group can still be iden- and the apparently lower intensity of internal defor- tified;where gneiss predominates, the stratigraphic mation in the Phulchauki Group. This question needs distinctions are lost. further study. Inthose parts of theMahabharat synclinorium ThePhulchauki Group shows unmistakable where the gneisses are missing, the metasediments of similaritieswith the Lower Palaeozoic succession of the Kathmandu Complex display a steady increase in the Tibetan sedimentary zone. The coloured, predo- metamorphicgrade from top to bottom. The Devo- minantlysandy Tistung Formation of probably early nian limestone on top is practically unmetamorphosed. Cambrianage is reminiscent of theGarbyang and Extremely finesericite and chlorite appear in the Ralam ‘Series’ described by Heim & Gansser (1939). Silurianslates and are characteristic minerals of the The thick Cambro-Ordovician Chandragiri Limestone Cambrian Tistung Formation. Biotite may make a first can be compared in lithology, thickness and age with appearancein the lower Tistung and is common the Larjung-Nilgirithe Limestone group of the throughout the Bhimphedi Group. Garnet (almandine) Dhaulagiri-Annapurna Range (Fig. 2), with the Chiat- usuallyappears first astiny crystals in themiddle sun Limestone of the Jolmo Lungma-Nyalam region, Kalitar Formation, becomes larger and more abundant andalso with the summit limestone of Everest(Mu in the lower Kalitar, where it is often associated with An-Tze et al. 1973). And the dark Silurian slates of amphibole,and is mostconspicuous in theRaduwa Chitlangand Phulchauki, except for their smaller Formation at the base of the Bhimphedi Group. This amount of associated limestone, can be readily com- metamorphism, which increases with depth and shows paredwith the Silurian-Lower Devonian ‘formation a clear zonation roughly concordant with stratigraphy, sombre’ of the Thakkhola (Fig. 2). is consideredas a primary regional metamorphism. The gneissesseem torepresent second,a high- temperaturephase of metamorphism,superimposed (iv) Metamorphismand granitization on the primary regional one. Almandine seems related Fieldobservations regarding metamorphism and mainlyto the regional primary phase, being a most magmatismwere supported only by aminimum of characteristic mineral of the oldest Bhimphedi schists petrographic laboratory work; they allowed, neverthe- (Raduwa, Kalitar); significantly, red garnet appears in less, some interesting conclusions. appreciable amounts only in those gneisses which de- Associated with the metasediments described above, velop laterally from these older, garnetiferous Bhim- theKathmandu Complex also contains gneisses and phedi schists, whereas the mineral is missing or incon- granites (Fig. 8). The gneisses are mostly of the mig- spicuous in the higher gneisses. matite type, commonly banded gneisses in which dark The granites are equally confined to the Kathmandu bands of biotite-schistalternate with light bands of Complex. They comprise biotite-rich and tourmaline- granitic and tourmaline-rich pegmatitic material, and rich varieties and seem genetically related to the gneis- augen- or porphyroblastic gneisses in which feldspar ses, having a similar mineral composition and, in the crystals may attain enormous size. Minerals indicating easternpart of theMahabharat synclinorium, being higher metamorphic grade such as kyanite, staurolite closelyassociated with them. In these eastern parts, and sillimanite have been noticed only sporadically in aroundSindhuli Garhi, imperceptible changes from association with pegmatite veins. augen-gneiss togranite by disappearance of crystal The lateral and vertical distribution of the gneisses, orientationand schistosity are seen in manyplaces. asbrought out by regionalmapping, is significant. Severalconcordant, tabular granite bodies, up to They are entirely missing throughout the central, west- 200 mthick, occur between the metasediments. ern and southwestern parts of the Mahabharat sync- Augen-gneisses in the upper part of the section pass linorium; in fact,they are limited to parts of theN northwestwardalong strike into alarger body of flank (Fig. 8) and the narrow eastern wing. In the N tourmaline-bearing granite, which occupies the core of flank they link up with the gneisses of Hagen’s ‘tec- theSindhuli Garhi syncline and seems to be a thick tonicbridge’ of Gosainkundand, through this, with layerconcordantly folded with the underlying sedi- the Central Crystalline zone (Fig. 1). ments. Invertical distribution, the gneisses are limited to The Sindhuli Garhi granites are lithologically hardly the interval represented by the Bhimphedi Group and distinguishablefrom the Palung Granite and other, the immediately overlying Tistung Formation. Within larger granite intrusions that occur further W entirely

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/1/1/4886597/gsjgs.137.1.0001.pdf by guest on 26 September 2021 22 J. Stocklin dissociated from the gneisses (Fig. 8). These intrusive Hagenand Nadgir et al. (1968-73) acceptedthe granites are found in contact with formations as old as principal synclinal structure of the Mahabharat Range the Raduwa and as young as the Tistung Formation, (Fig. 12), firstrecognized by Auden (1935).Talalov butnot with the younger, fossiliferous sediments of (1972) and Arita et al. (1973) regarded the fundamen- Phulchauki;they thus affect thesame stratigraphic tal structureof the Rangeas an uplifted autochthonous interval as the gneisses (Fig. 12). Their contacts with block,bounded and dissected by numerousvertical the metasediments are in most places sharply discor- faults. This interpretation was supported by observa- dant, cutting across bedding and schistosity planes and blesteep faulting but ishardly compatible with the truncating folds and many faults. Their contact effects stratigraphy.In the view of theseauthors, the Mid- on the host rocks are insignificant, and it is an impor- lands and the Mahabharat Range are stratigraphically tantfact that the metamorphic grade of thelatter homogeneous,both consisting of anidentical ‘Pre- increases in a direction away from the granites where- cambrian’ sequence which in the Mahabharat Range is ver ‘away’ means down-section. up-faulted,metamorphosed by the granites,and at The field evidence suggests a very young age for the Phulchauki completed by a stratigraphic extension into granites.They show clear intrusive relations with the Palaeozoic. In my view, this is entirely hypotheti- manypre-existing faults and are themselves little cal. The degree of inference can be judged from the affected by faulting. A fewradiometric results ob- generalized stratigraphic column of Arita et al. (1973, tained by the K-Ar method are inconsistent, the ap- fig. 7),inwhich the Palaeozoic Phulchauki beds parent ages ranging from Permian to Miocene (Talalov(‘Kathmandu Group’) are shown as resting directly on 1972; Khan & Tater 1970). More conclusive seem to the Dhading Dolomite. In reality, Pulchauki is more be recent Rb-Sr datings that gave 26-22 Ma for the than 50 km distant from Dhading, its fossiliferous beds Palung Granite (Andrieux et al. 1977), similar to the rest on metamorphic rocks, and the relationship shown agesobtained for the Manaslu Granite of theHigh in the column could at best be suspected but cannot be Range(Hamet & Allkgre1976); more recent work demonstrated at either place; in fact, the newfossil suggests even younger ages (Vidal, in CNRS, 1977, p. evidence from the Dhading Dolomite contradicts it. 539). A crustalanatectic origin of thegranites was But not only does stratigraphy fail to lend support concluded from the high initialS7Srs6Srratio. to autochthonous structure. The structural picture ob- Incontrast theto Kathmandu Complex, the tained by systematic mapping convincinglyillustrates Nawakot Complex in the area studied is entirely void the synclinalnature of theMahabharat Range and, of granites and gneisses, and the metasediments rarely thus,strongly suggests emplacement of the exceedthe chlorite-sericite grade. Fine biotite and Kathmandu Complex by thrusting. Fig. 8 shows how smallgarnets were, however, noticed locallyin the thevarious formations turn as sub-parallel bands deeperparts of thesection and are said to become smoothlyaround thewestern closure of the generally more common further N with approach to Mahabharat synclinorium. The synclinal bend is also the Main Central Thrust, where also the Ulleri Gneiss seen with perfect clarity on satellite pictures and con- is found in Lower Nawakot sediments (Le Fort 1975). ventional air-photographs. Whereas the beds are very steep (70-90”) in thetwo flanks, the dips gradually (v) Autochthony or allochthony? decrease towards this bend and approach the horizon- Progress in stratigraphic, petrographic and structural tal in the axial part of the fold. The map leaves no workled many of Hagen’ssuccessors toreject his doubt that the Kathmandu Complexis not juxtaposed, nappe concept for the Lesser Himalaya, and for the butsuperimposed on the Nawakot Complex in this Mahabharat Range in particular. large synclinal fold, and large-scale thrusting prior to Nadgir et al. (1968-73) for the first time worked out folding remains the only plausible explanation to re- a fairly detailed stratigraphy for this sector; partof the concile this superposition with the stratigraphic facts classification used above has been adopted from them. (Fig. 12). In their view, however, the Nawakot-Kathmandu se- quencewas stratigraphically continuous, and the (vi) Mahabharat Thrust andKathmandu highermetamorphism of theBhimphedi Group in- ducedby the granites. The great thickness of the NaPPe deposits intervening betweeen the stromatolitic Dhad- The map (Fig. 8) also allows the thrust plane to be ing Dolomite and the Lower Palaeozoic beds of Phul- identifiedwith a conspicuous structural discontinuity chauki(about 10 km) appeared compatible with a that coincides with (a) the sharp reversal in the reg- possible Middle or even early Riphean age that could ional metamorphism from the chlorite grade below to be suggested for certainHimalayan stromatolites the garnet-amphibole grade above; (b) a regional dis- (Sinha 1973). With the recent discoveryof early Palaeo- cordance,along which boththe underlying and the zoic fossils in the Dhading Dolomite, this interpretationoverlying stratigraphic successions are truncated to a has become impossible; stratigraphic continuity of the variable degree; and (c) a marked change in structural section can now be considered as disproved. style. The latter is best expressed in the SW flank of

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the synclinorium, where the rocks below the discon- E -z tinuity(Upper Nawakot Group) show intense shear- 0- folding,slicing and imbrication, whereas the rocks immediatelyabove the discontinuity (lower part of BhimphediGroup) appear as a competent, rigid ‘traineau tcraseur’ bulldozing the underlying Nawakot m- rocks to the SW. This discontinuity (the ‘Mahabharat Thrust’) can be tracedwith allits characteristics around the entire Mahabharat synclinorium. 0- Inoutcrop, it is true,the thrust line nowhere ap- pearsas aclear-cut break but rather as a narrow ‘transitional’ zone displayinga reversal of metamorphism. Inthis zone, chloritic phyllites of theUpper Nawakot Group (usually Robang phyllites,in places chloritized Benighat Slates) rapidly pass upwards into lustrous chlorite-schists in which tiny garnets make a first appearance;these in turnpass into the highly garnetiferouscoarse mica-schists (usually Raduwa Formation, in places lower Kalitar Formation) which everywhere initiate the succession of the Kathmandu Complex.The coarse crystallinity is enhanced by a profusion of segregationary quartz and nodular schis- tosityplanes, which often impart to these rocks a gneissic appearance (‘cataclasticgneisses’ of Arita et al. 1973). Upwards the quartz segregations disappear again, and the normal garnet-mica-schists of the lower BhimphediGroup remain. No sharp limits between these rock types can be drawn, and the whole zone, 20-200 m thick, and often displaying intense shearing and mylonitization, must be regarded asa thrust zone. Itmust be stressed that the garnet-mica-schists at thebase of theKathmandu Complex represent the highest grade of regional metamorphism, and that the regularity of upward decrease in metamorphic grade appears distorted only by a locally superimposed migmatization. Migmatization, however, increases in im- portance from S to N andbecomes all-pervading in the Sheopuri gneiss masses N of Kathmandu. Hagen saw in the adjoining gneisses of Gosainkund the ‘tec- tonicbridge’ linking theSheopuri gneisses of Kathmanduwith the Central Crystalline zone of the High Range (Fig. 1). By recognizing this link he came toregard the entire Central Crystalline zone as the root zone of a once much vaster Kathmandu nappe- system covering all of the Nepalese Midlands. As regards Hagen’s ‘tectonic bridge’, Hashimoto et al. (1973)accepted the thrust nature of theGosain- kund gneisses, describing them as a large salient of the CentralCrystalline zone, but they were reluctant to extendthe notion of thrustingfurther S tothe Sheopurigneisses of Kathmandu.Although augen- gneisses associated with numerous dykes and lit-par-lit injections of tourmaline-graniteare an integral con- stituent of both the Gosainkund and Sheopuri gneiss complexes, Hashimoto et al. made a fundamental distinction between the two: they assigned the former to a high-graderegional metamorphism of Barrovian

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type, but described the latter as an independent, fault- gradual(P&cher 1977). Frequent bodies of controlled injection-gneiss zone separating the thrust metamorphosed basic rocks, conspicuously linear and mass of Gosainkundfrom the ‘autochthonous‘ parallel to bedding, were thought to mark the thrust, KathmanduComplex. This petrographic distinction but were found to be repeated and their limits to be doesnot seem to justify the tectonic inference. The unsharp. stratigraphicand structural analysis and systematic Could there be a thrust ‘zone’ instead of a simple mapping led me to interpret the Kathmandu Complex thrust plane? Bordet (1961) noticed lithological repet- as a veritable ‘Kathmandu nappe’-a large thrust mass itionsand grouped them into his‘zone des Ccailles’. asoriginally conceived by Hagen.The Mahabharat Fuchs & Frank(1970) distinguished a thin ‘Lower Thrust is, accordingly, interpreted here as an extension Crystalline Nappe’ of lower metamorphic grade from of the Main Central Thrust (Fig. 12). the main crystalline thrust mass of higher grade. Arita However, there is no evidence to suggest a succes- et al. (1973) conceived a ‘Main Central Thrust Zone’ sion of several nappes such as Hagen inferred from the consisting of 3 thin thrust sheets. But all these devices repetition of gneisses in the profile. Though internally only multiplied the problem of finding clear tectonic complicated by folding, faulting and imbrication, the separations. KathmanduComplex retains the essential continuity Locating the ‘Main’ thrust thus became a matter of of an undivided metasedimentary sequence. personal choice and remained highly subjective. Many chose as MCT the base of the first gneiss layer, which oftenmarks arelatively sharp lithological change. g. Reverse metamorphismand the Main Others placed the MCT along a certain isograd, pref- Central Thrust erablythe base of thekyanite zone, which often Various hypotheses have been proposed to explain conveniently coincided with the first gneiss layer (and thewidespread phenomenon of reversemetamor- some were surprised then to find the isograds running phism-the superposition of high-gradecrystalline remarkably parallel to the MCT). rocks(theCentral Crystalline andtheLesser In Le Fort’s (1975) attractive thermodynamic model Himalayancrystallines) on the low-grade metasedi- of continentalsubduction (Fig. 13),several of the ments of the Lesser Himalaya: contact metamorphism difficulties appear to find a solution. In this model, the by graniteintrusions; ‘selective’ metamorphism by reversemetamorphism reflects an inversion of the magma injection and migmatization at higher stratig- normalthermal gradient resulting from a speed of raphic or tectonic levels; reversalof stratigraphic sequ- thrusting surpassing that of thermal homogenization. ences by recumbent folding or otherwise; and large- Thethrust mechanism is assumedto be aplastic scale thrusting. deformation by intracrystallinegliding distributed Nappestructure as recognized in theAlps and through a broad zone, comparable to gliding in a pack elsewhereusually means emplacement of olderon of cards and consistent with the main schistosity and younger rocks. Transposed to the Himalaya, it means with the mineral ‘streak lineation’ in the direction of that the high-grade crystalline rocks should be older transport.In this way, discontinuities normally as- thanthe low-grade metasediments, and that the sociated with a thrust plane (break in metamorphism, metamorphism of both should pre-date thrusting. The deformationstyle, etc.) can be expected to become proof for thrusting, then, should be a sharp break in spread out through an entire zone. The model has only metamorphic grade at the thrust. In the case of the one fault: it does not help to locate more precisely the MCT, such an abrupt change has nowhere been ob- MCT,the plane of crustalrupture and dislocation served.The inferred tectonic contact everywhere which it would be desirable to find. In the model, the appears as a perfectly transitional zone of upwards in- ‘Main’Central Thrust becomes an arbitrary median creasing metamorphism, from barely metamorphosed plane in a broad, indefinite zone of plastic deformation sedimentsto‘katazonal’ kyanite- and sillimanite- and metamorphic change. gneisses. The thickness of thezone of reversal is at I believe that among the discontinuities associated least hundreds of meters and may be in the km range. with amajor thrust, there is one which in principle Isograds were often found to be remarkably parallel cannot be ‘smoothed out’ by plastic shear, namely the (Le Fort 1975, fig. 9),and successive zones of chlo- break in stratigraphiccontinuity. One fundamental rite,biotite, garnet, kyanite and sillimanite grades geological approach to the problem of the MCT has so were found to be the rule (Ray 1947). far been unduly neglected in Nepal: systematic large- If metamorphism did not reveal the discontinuity, it scale mapping of the MCT region. It has been shown shouldhave been expected from stratigraphy; but previously in this paper that by lithostratigraphic map- stratigraphy, confused as it already is by thelack of pingthe Mahabharat Thrust in CentralNepal has palaeontologicalcontrol, becomes more so within- eventually emerged as a distinct tectonic discordance creasing metamorphism. Changes in deformation style (Fig. 8). It is true that the thrust ‘plane’ has remained across the inferred thrust plane were sought and rec- atransitional zone here too; but this is amatter of ognized,but these changes too were found to be detail,and wherever mapping was detailed enough,

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0 2-3

5.0

7.5 10.0

0 50 km

FIG. 13. Pressure/temperature diagram of Main Central Thrust (in Le Fort 1975).

this zone could be narrowed down to become a real The MBT is deliberately called here Main Boundary 'plane' for all practical purposes. Thrust and not Main Boundary Fault. The main argument against thrusting and in favour of normal block faulting is the steep attitude of the MBT. Its dip is, in h. Main Boundary Tbmst, Siwalik belt, and fact, usually 60-90" to the N and nowhere less than Gangetic plain 45". This argument alone explains nothing except that, TheMain Boundary Thrust (MBT) sharply sepa- if thrusting has taken place, it must have preceded a ratesthe outer sedimentary belt of theLesser process of steepening. Himalaya from the Sub-Himalayan Siwalik belt (Fig. The evidence for thrusting is found in the structure 1). of theSiwaliks. Sedimentary structures in Siwalik InNepal, the 'Siwaliks' forma 20-30 kmwide rocks adjacent to the MBT invariably show that the foothill belt and extend S into the subsurface of the tops of the beds are facing N, i.e., the MBT. This fact Gangetic plain. These terrestrial, largely fluviatile de- is consistent with the normal stratigraphic succession posits reach over 5 km in thickness. The stratigraphy fromMiddle Siwalik sandstonesto Upper Siwalik of the Nepalese Siwaliks is known only in gross out- conglomeratesusually found when approaching the line. A conventional tripartition into the clayey-sandy MBT from the S. The Siwaliks are thus clearly plung- Lower Siwaliks (Nahan), the predominatly sandy Mid- ing below the MBT, and the latter must be a genuine dleSiwaliks, and the predominantly conglomeratic thrust,however steep itmay now be;normal block Upper Siwaliks is used. A MiddleMiocene to early faulting would have had a contrary effect (Siwalik beds Pleistocene age is inferred from palaeontological evi- facing the S). dence outside Nepal. The partly marine older Tertiary Thrustingalong the MBT must have been a very depositsunderlying the Siwaliks in the NW Sub- late event in the structural evolution of the Himalaya, Himalaya are not known in the Siwalik belt of Nepal, involving theyoungest Siwalik rocks of Plioceneor but, as already mentioned, patchy occurrencesof num- early Pleistocene age. As Le Fort (1975) and others mulitic shales and limestones overlying Mesozoic rocks have shown, it must have taken place much later than, have been found N of the MBT in western Nepal. andindependently from, the movements along the Exploratorywells drilled in the Gangetic plain by MCT. theOil and Natural Gas Commission of India What we do not know, however, is the amount of (Karunakaran & Rao 1976) reachedPrecambrian horizontaltransport along the MBT, a question of shieldrocks after passing through strongly reduced considerable concern to petroleum geologists.A meas- Siwalik sectionsand underlying Vindhyan sediments ure of thehorizontal displacement may perhaps be (in the W) or Gondwanabeds (in the E). Ina well gained from an observation in the Siwalik rocks of the locatednear Raxaul at the India/Nepal border, a lower Bagmati Valley, S of Kathmandu. The Siwalik complete Siwaliksection of 4100 mthickness was conglomeratesexposed in thisarea are strikingly found underlain by metasediments and basic igneous different in composition from the recent gravel mater- rocks of the kind exposed just N of the MBT (Upper ial of the Bagmati River. The latter abounds in such Nawakot Group). relativelyhigh-grade metamorphic rocks as garnet- The Siwalikbelt is characterized by broadgentle schists,two-mica schists, biotitic quartzites, coarse- folds dying out towards the Gangetic plain, with minor crystallinemarbles, gneisses, and also granites-the steep thrust faulting nearer to the MBT (see Hagen commonrocks of theKathmandu Complex which 1969, figs. 83-89). today form the bulk of the Mahabharat Range in the

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Late Slwaliktime (Pllocene) d- -S

Recent d’ z 5’ I

K l‘ 40 km 1

FIG. 14. Change in Bagmati drainage resulting from 20-30 km post-Siwalik crustal shortening in Suparitar zone (outer sedimentary belt). Upper profile, late Siwalik time (Pliocene): the drainage area of the ‘Proto-Bagmati’ lies entirely in a broad Suparitar zone composed of Nawakot rocks (N). The Kathmandu Complex (Kathmandu nappe, K) is already thrust on the Nawakotsbut its front lies still beyond the upper reaches of the Proto-Bagmati. Nawakot detritus (n) is forming late Siwalik conglomerates. S = source of Proto-Bagmati; d = distance between source and edge of Siwalik basin; MhT=Mahabharat Thrust. Lower profile, present situation: by thrusting and imbrication along the Main Boundary Thrust (MBT) the Kathmandu Complex has been passively transported farther S and the Suparitarzone narrowed to less than 1 km.Bagmati now receives detritus (k) fromKathmandu Complex. S’ = present source of Bagmati; d‘ = distance from present source to MBT (45 km).

Bagmati sector. None of these rocks are found in the (Karunakaran & Rao1976). As discussedin this Siwalik conglomeratesthat are extensively exposed paper, the most frequent organic remains occurring in along the banks of the Bagmati; the pebbles of these the Lesser Himalaya indicate precisely these two ages: conglomeratesare exclusively chertand low-grade late Precambrian-early Palaeozoic (stromatolites) and quartzites,phyllites and fine-crystalline carbonates Permo-Carboniferous(Gondwana plants and as- suchas characterize the Nawakot Complex. Fig. 14 sociatedmarine fossils). The lithologies of thesedi- gives a possible explanation for this surprising fact. At ments containing these organic remains are still com- the Himalayan scale, the Bagmati is only a small river. parable to those on the shield, but there is a general Ithas itssources inthe Sheopuri Lekh N of thickening to the N, and the absenceof late Palaeozoic Kathmandu, about 45 km from the MBT. If d in the marine beds on the shield, but their appearance in the upper profile of Fig. 14 is estimated to have been only Lesser Himalaya. The evidence suggests continuity of about half of d’ in the lower profile, crustal shortening sedimentation from the shield to the Lesser Himalaya by thrustingand imbrication along the MBT must in late Precambrian-Palaeozoic times, including, how- have been at least 20 km. ever, a significant pre-Carboniferous gap in both reg- Subsequent steepening of the MBT was concomit- ions;palaeontological evidence fortheMiddle antwith the formation of theMahabharat sync- Palaeozoic is entirely missing on the shield and scanty linorium. It may have been produced either by con- in the Lesser Himalayan sediments. tinued horizontal stress or, as may be suggested, by In this connection it is worth recalling that such a crustal down-warping as an isostatic response to up- gap, encompassing all of the Middle and much of the thrustingand erosional isolation of theMahabharat LowerPalaeozoic, characterizes the entire southern Range. marginal belt of the Alpine orogen from Turkeyin the W as far E as Pakistan, i.e. as far as sound palaeon- 4. Regional aspects tological dataare available tosubstantiate the gap: throughout the Zagros Range,in Oman, on the Kabul a. Palaeogeogaphy spur, in the Salt Range, in southern Hazara. Sharma (1977)reported new discoveries of Carboniferous Geophysical and drilling data show a clear inclina- faunas in Jammu, but older fossils are lacking. Further tion of the continental basement and thickening of the E palaeontological control is more meagre. Neverthe- sedimentary cover below the Gangetic plain towards less, thepre-Blaini (pre-Carboniferous) Chandpur- the Himalayan front. The pre-Tertiary sediments in- Nagthatsequence of theKrol Belt islithologically volved arethe late Precambrian-?earlyPalaeozoic much more akin to the dated late Precambrian forma- Vindhyans and the Permo-Carboniferous Gondwanas tions of thewestern Himalaya (Hazara) than to any

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dated Lower or Middle Palaeozoic Himalayan forma- intermittently subsiding Indian continental shelf in the tions. If those ‘Chails’ that underlie the dated Gond- Himalayan belt. The undeniable facies differences be- wanabeds in westernNepal really correlate with tweenLesser and Higher Himalaya should not be Bordet’s Kuncha Formation of central Nepal as post- surprising if anunknown amount of post-deposition ulated by Fuchs & Frank (1970), theirage is not crustal shortening between the two is taken into ac- Devonianbut late Precambrian, as shown for the count. Kuncha Formation in this paper. The possibility of a However,one can only speculate on the original similar gap below the Benighat Slates in Central Nepal extent of the continental basement and its later mod- has already been pointed out; mapping by Stocklin &L ifications, onits present depth below the Lesser Bhattarai (in press) suggests that the scope of this gap Himalaya, and on its possible presence in the crystal- increaseseastwards in the Sun Kosi Valley but de- line thrust masses. creasesnorthwards in theBhote Kosi Valley at the IndigenousPalaeozoic and older deposits are not Tibetanborder. The gap evidentlyreappears in the known in the Indus-Tsangpo eugeosynclinal zone, and outer Lesser Himalaya and Sub-Himalaya of Sikkim this zone may have come into being only in Triassic and Bhutan, where dated Gondwana beds rest discon- time as a result of continental rifting. formablyon stromatolite-bearing formations (Buxa Limestone) and associated beds. b. The Himalaya in the structure of Central Onthe other hand, the occurrence of well- developedLower-Middle Palaeozoic deposits near Asia Kathmanduonlysubstantiates postulatedthe The fascination of the highest mountain range of the allochthonous nature of the Kathmandu Complex and worldhas long diverted the attention of geologists its tectonic derivation from the N. Northwards, in the from the fact that the Himalaya is but a part of the more interior parts of the orogenic belt, the Middle Alpineprofile of Central Asia, and only the smaller Palaeozoicgap everywhere closes:in centralIran, part at that (Fig. 15). The larger portion lies farther N, CentralAfghanistan, the Swabi area of Pakistan, in the vastand stilllargely unexplored expanses of Kashmirand in theTibetan zone of theHimalaya. Tibet, and impressive manifestations of Alpine reacti- Yet, even in this northern zone the Palaeozoic sectionsvation are seen as far N asthe Tienshan Range, still comprise several persistent gaps of smaller scope 1500 km N of the Himalaya. This Alpine tectonism far (pre-Permian,pre-Carboniferous, Lower Devonian, inside Central Asia has clearly emerged from recent and older ones). Contiguity of sedimentation with the exploration work by Chinese geologists in Tibet and Lesser Himalayan zone is, however, clearly indicated Sinkiang, the results of which have been summarized by the Gondwana affinity of the plant. remains and the in a number of publications (particularly: Compilation Tethyan affinity of themarine fauna in thePermo- Groupetc. 1976; Huang Chi-ching 1977;Chang Carboniferous depositsof both the Tibetan and Lesser Cheng-fa et al. 1977). Significantconclusions were Himalayan sedimentary zones. also drawn from the study of earthquakes by Molnar Mesozoic sediments are missing S of the Gangetic & Tapponnier(1975) and from analysis of satellite plain but appear in the outer sedimentary belt of the imagery by the same authors and by Ganser (1977). Lesser Himalaya (Krol belt, Piuthan zone). No exten- In a recent structural comparison between Iran and sion to more northerly and easterly partsof the Lesser Central Asia I proposed (Stocklin 1977) a division of Himalaya is known with certainty, and these deposits the Alpine orogenic belt inthis region into 3 major could represent a restricted foreland basin comparable domains: (a) a SouthernDomain, of Precambrian to the Mesozoic foreland basin of the Lower Indus in basementconsolidation, affected primarily, and in Pakistan.In the N, the Mesozoic section shows a placesexclusively, by lateAlpine (Neogene) tecton- drastic change from thick ‘eugeosynclinal’ deposits in ism; (b) a CentralDomain, N of themain Alpine the Indus-Tsangpo zone to thin, lacunar ‘miogeosync- ophiolitic suture, characterized like the Southern Do- linal’ depositsat the S margin of theTibetan zone, main by Precambrian basement consolidation but dis- which is a purely erosional margin; nothing disproves tinguishedby intense Kimmerian and early Alpine the assumption that this reduction of deposition con- deformation and magmatism prior to late Alpine fold- tinued to the S and led to restriction in a lagoon-like ing; and (c) a Northern Domain, N of the Hindukush- Krol-Piuthan basin, and further to non-deposition on Wanch-Akbaytallineament, characterized by the shield. Hercynian-earlyKimmerian folding and magmatism Taking all palaeontological and stratigraphical evi- and late Alpine reactivation. From this subdivision it dencetogether, the best general conclusion is that wasconcluded that the eastern continuation of the there was continuity of sedimentation from the shield CentralDomain (Central Iran, Central Afghanistan, in the S to the Tibetan zone in the N, withgradual southern and eastern Hindukush, central and southern thickening and completion of the succession, but with Pamirs, Karakorum) has to be sought in the highlands markeddifferentiation in the Mesozoic.This is per- of Tibet, N of the Indus-Tsangpo suture. fectly compatible with the assumption of a steadily or The Chinese studies mentioned above lend support

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to this view. The whole of Tibet is included in a vast The ‘Indosinian’fold belt of northern and eastern ‘Tethys-HimalayanDomain’ and opposed to a ‘Pal- Tibet as described by the Chinese authors reminds one AsiaticDomain’ N of (andincluding) the Kuenlun vividly of apersistent zone of verysimilar Triassic Range. Tibet is described, not in conventional terms as volcano-sedimentaryfacies and intense late Triassic a ‘high plateau’ or a ‘median mass’, but as an intensely deformation extending from the eastern Kopet Dagh folded mountain country in which several E-W trend- in Iran(Aghdarband) to northern Afghanistan, the ingfold belts can be distinguished. The Chinese au- westernHindukush, the northern Pamir, and further thorsstress the wide distribution of Palaeozoic- to the western Kuenlun, where it apparently merges Mesozoicsediments, their predominantly shallow- withthe northern Tibetan zone just described (Fig. marineand partly paralic character, the Tethyan 15). The zone lies immediately N of the Hindukush- affinities of the rich, predominantly neritic faunas, the Wanch-Akbaytallineament, which seemsto join up insignificance of Palaeozoicdeformation and volcan- alongthe S side of thewestern Kuenlun with the ism, but the dominant role of Mesozoic folding and Chinshakiangfault zone of NETibet. InIran- magmatism. There is less emphasis on orogenic events Afghanistan, as in Tibet, this zone reveals also the first of Tertiary age, presumably because Tertiary deposits evidence of Hercyniangeosynclinal deposits, rep- arecontinental and of limitedextent, but cases are resented by thickLower Carboniferous shales and mentioned of metamorphism affecting rocks as young graywackes with abundant submarine volcanic mater- as Tertiary, and of thrusting of Palaeozoic-Mesozoic ial, partly spilitic, and also ultrabasic rocks. A distinct ontoUpper Tertiary and even Quaternary forma- unconforrnityseparates these rocks from the overly- tions.Seismic activity has a scattered distribution all ing, partly continental red clastic deposits of the Per- over Tibet, and active faulting with important strike- mian. The Upper Palaeozoic-Triassic sequence of this slip components is suspected by Molnar & Tapponnier zone contrasts sharply with the entirely non-volcanic, (1975). Moreover, Gansser (1977) showed the impor- platform-type carbonate deposits of thesame age in tance of aNeogene to Recentacid volcanism in Central Iran-Central Afghanistan, as in Central Tibet. Central Tibet. All these facts also characterize Central In the zone of intense late Triassic folding, true Hercy- Iran and the easterly adjoining sectors of the ‘Central nian (late Palaeozoic) deformation seems to have been Alpine Domain’. weak, but it was the dominant folding event further N Animportant newelement distinguished by the in theTienshan, Kuenlun and Tsinling Ranges of Chinese authors is a broad zone of late Triassic (‘In- Sinkiang. The Hindukush-Akbaytal lineament and its dosinian’)folding, comprising the Sungpan-Kantze continuation in the Chinshakiang fault zone of Tibet andSankiang fold systems of northernand eastern appearsthus as the sharp southern border of the Tibet,merging S throughYunnan with the classical Hercynian realm, upon the S margin of which a zone Indosinian foldbelt of SE Asia (Fig. 15). In this zone of intenselate Triassic folding and magmatism is the marine Triassic sequence attains its greatest thick- superimposed. ness; it includes neritic and bathyal deposits containing In Russian literature dealing with Central Asia, the arich Tethyan fauna, followed by paralicsediments pronounced late Triassic tectonism and magmatism is with coal beds in the upper part. Thick flysch deposits ascribed toa ‘lateHercynian’ orogenic phase; the associated with basic and acid volcanic rocks are re- differing designation should, however, not divert from ported to occur in an axial ‘eugeosynclinal’ zone of the the fact that the ‘late Hercynian’ marginal fold belt in Variscan-Indosiniancycle along the Chinshakiang the W is a perfect counterpart and continuation of the (upper Yangtze) fault system. Folding in this ‘Indosi- ‘Indosinian’fold belt in the E. TheChinese authors nian’ belt was most intense in late Triassic time and connect the related Chinshakiang fault zone with the was accompanied and followed by granite intrusions. ophiolitic Red River lineamentof Yunnan-North Viet- Uppermost Triassic or Lower Jurassic beds overlie the nam (Fig. 15),but a southern branch evidently runs fold structures with pronounced unconformity. from Yunnam S through the Chiang Mai-Chiang Rai

FIG. 15. Tectonic sketch map of South-Central Asia. 1 = Palaeo-Asiatic continental cores (Precambrian consolidation); 2 = Gondwana continental cores (Precambrian consolidation) with Deccan traps of India; 3 = Hercynian fold belts; 4 = Turan platform (Hercynian-Indosinian consolidation); 5 = Hercynian-lndosinian granites; 6 = Mesozoic (Indosian-Kimmerian) fold belts; 7 = late Kimmerian-Alpine granites; 8 = Himalayan fold belt (Cenozoic folding, thrusting, metamorphism, granitization); 9 =late Alpine marginal fold belts; 10 = Neogene-Quaternary volcanoes; 11 = Alpine foredeeps and intermontane depressions; 12 = Palaeo-Tethys suture (Hercynian eugeosynclinal deposits withvolcanics and ophiolites;Triassic flysch and volcano-sedimentarydeposits; intense late Triassicfolding); 13 = Neo-Tethys (‘Indus-Tsangpo’) suture, inner belt (Upper Cretaceous-Lower Tertiary ophiolitic melanges and flysch deposits); 14 = Neo-Tethys suture, outer belt (Mesozoic deep-sea sediments, flysch, ophiolites). A = Akbaytal Fault, B =Bindud, Ch = Chinshakiang Fault, H = Hindukush Fault, K = Kashgar, KD = Kopet Dagh, Kk = Karakorum, L = Ladakh, RR = Red River Fault, W = Wakhsh Ranges.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/1/1/4886597/gsjgs.137.1.0001.pdf by guest on 26 September 2021 30 J. Stocklin region of northernThailand (Baum et al. 1970)and 1977). The separation of these continental fragments reappears on the E side of the Central Range of the may have been part of the general break-up of Gond- Malay Peninsula (Gobbett & Hutchison 1973). In this wanaland, usually dated as late Triassic; but it is more classical belt of Indosinian folding and magmatism, the likely tohave taken place at some time in thelate faultzone is linedagain with early Carboniferous Palaeozoic, following the formation of an intracratonic ophiolites and separates an epicontinental faciesof the geosynclinal trough filled by thick Palaeozoic marine Palaeozoic in theW from a geosynclinal facies in theE. shales-the ‘Karakorum black slates’ and their equi- The Kuenlun-Tsinling Range and the mountains of valents in the Wardak-Nawar zone of Afghanistan. In Sinkiang, N of the Indosinian fold belt of Tibet, bear theHimalayan sector, the Middle Triassic Daonella clearly thestamp of theHercynian and of older shales in the Lamayuru Flysch (Frank et al., 1977a) of orogenies. Late Alpine (Himalayan) deformation has the Indus suture zone in Ladakh are the oldest direct remoulded this Palaeozoic orogenic realm far to the N,evidence of oceanic sedimentation in Neo-Tethys, but as manifested by the spectacular young uplift of the pelagic Permian limestones among the exotic blocks of TienshanRange to altitudes above 7000 m (Pic this zone (Ganser 1964) may indicate an earlier com- Pobedy) and by the formation of deep intermontane mencement of rifting. Thelate PalaeozoicPanjal basins such as the Turfan depression, the deepestof its Traps of Kashmir, from which spilitic pillow lava has kind in Asia. This region of broad overlap of Hercy- beenreported (Nakazawa & Kapoor1973), could nianand Alpine folding can be distinguished as a equally have been related to the rifting process. In the northern marginal Domain with respect to the Alpine N, subduction of the oceanic crust of Palaeo-Tethys orogeny. alongthe Binalud-Hindukush-Akbaytal-Kuenlun- Chinshakianglineament could be indicated by the scatteredoccurrence of ophiolitewedges along this c. Eurasia/Gondwana relations lineament,and the Hercynian folding and granite emplacement N of thisline may have been a direct A migration of the centres of orogenic activity from effect of this subduction and related compression. N to S can be clearly recognized and may tentatively The axialtrough of Triassic flysch depositionand be explained by successive northward drift of different submarinevolcanism immediately N of theHindu- Gondwanafragments and their collisionwith, and kush-Akbaytal lineament and along the Chinshakiang accretion to, Eurasia. fracturezone in Tibet outlines the extremely nar- Thenorthern Alpine Domain, in theHimalayan rowed,dying Palaeo-Tethys. Late Triassic (‘Indosi- sector extending N from the S Kuenlun-Chinshakiang nian’,‘early Kimmerian’, ‘lateHercynian’) folding, lineamentto the Tienshan Range, belongs to the which was most intense along this lineament, reflects Hercynian realm of Central Asia, reactivated by Kim- the final closure of the Palaeo-tethyan eugeosyncline, merian and Alpine folding. It forms an integral part of i.e. the collision of the Central Domain with Palaeo- Palaeo-Asia-a mosaic of Precambriancontinental Asia, accompanied and followed by the late Triassic- fragmentssuch as the Tarim block and the Sino- early Jurassic granite intrusions just N of this suture. Koreanand Siberian platforms, welded together by Fromthis time the Central Domain, once a part of various Palaeozoic orogenic belts. The latter contain Gondwanaland,has been a part of Eurasiaand has narrowPalaeozoic ophiolitic sutures within larger shared its geologic and tectonic history. zones of miogeosynclinaland epicontinental deposi- The process of ocean narrowing repeated itself dur- tion, attesting to complex processes of rifting, ocean- ing the later Mesozoic in Neo-Tethys, which had been floorspreading and extensive marine transgressions created by the breakaway and northward drift of the followed by oceannarrowing, suturing, folding, CentralDomain (Tibet). The original width of the metamorphism and granitization during the Baikalian, Neo-tethyan ocean is unknown. Palaeomagnetic and Caledonian and Hercynian orogenic cycles in what is spreading data in the Indian Ocean suggest that India now northern Central Asia and Mongolia (Zonenshain began moving N at least 130 Ma ago. It is awidely 1972). At the dawn of the Mesozoic, Palaeo-Asia was held view that subduction and consumption of oceanic thoroughly cratonized. The former Palaeozoic ocean crustduring the Cretaceous, when ophiolites were of northern Central Asia had retreated to the S of the emplaced in the Indus-Tsangpo zone, led to the clos- newly created Kuenlun Range, the southern margin of ure of Neo-Tethys and collision of India with Eurasia Palaeo-Asiaand northern shore of Palaeo-Tethys in earlyEocene time, reflected in aslow-down of after the Hercynian orogeny. spreading in the Indian Ocean about 55 Ma ago. Post- TheCentral Domain, of which CentralTibet isa collision northwarddrift of India, which spreading part,can be conceived as amosaic of Gondwana data suggest to have amounted to more than 1000 km, continental fragments, which by rifting broke off from resulted in the Himalayan orogeny proper. themother continent and by subsequentnortherly Crustal shortening in post-collision time is now seen drift caused the opening of Neo-Tethys in their rear bymany geologists (Powell & Conaghan 1973b; Le and the narrowing of Palaeo-Tethys in front (Stocklin Fort 1975) as a process of continental subduction in

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/1/1/4886597/gsjgs.137.1.0001.pdf by guest on 26 September 2021 G eo logy of Nepal andNepal Geologyof regionalframe its 31 one form or another. There is, however, no geological recentgravity data (Kono 1974; Huang Chi-ching evidence that continental subduction took place along 1977), which indicate that the greatest crustal thick- the Indus-Tsangpo suture as a sequel to oceanic sub- ness is reached, not below the Himalaya, but N of it duction; it is believedthat buoyancy prevented sub- near the Indus-Tsangpo suture. This rather supports duction of continentalcrust and that major tectonic Dewey & Burke’s(1973) hypothesis of crustalthic- activity along this suture ceased after collision. There kening as a result of basement reactivation related to is hardly any seismic activity along the Indus-Tsangpo continuedplate convergence after collision. Gansser suture at present (Kaila & Narain 1976). Post-collision (1977) estimated the amount of total crustal shorten- subduction apparently developed along a new rupture ing in the Himalaya by thrusting and folding to be in within theIndian continental plate, which wasto theorder of 300km, and Molnar & Tapponnier becomethe Main Central Thrust; the climax of the (1975)believed that most of thenortherly drift of Alpine orogeny in the Himalaya-the main phase of India was accommodated by strike-slip movements in deformation, metamorphism and granitization-can be Tibet, rather than by thrusting in the Himalaya. related to the MCT thrusting event in late Oligocene- Buthere we enter a field of widespeculation. A Miocene times (Le Fort 1975). It was followed by late greatdeal more factual geological and geophysical Alpine large-scale folding and imbrication in the Les- information on the Himalaya and Central Asia will be ser Himalaya and by thrusting along the Main Bound- needed to clarify these problems of crustal structure ary Thrust in Plio-Pleistocene time. and global tectonics. What has clearly emerged from Powell & Conaghan(1973b) suggested that India recent geological studies, however, is that the migrat- may have been much larger than today and may have ing orogenicprocesses, which hadbegun in the underthrust all of the Hirnalaya and all of Tibet; this Palaeozoic in northern Central Asia and had reached would allow for accommodation of the enormous post- northern Tibet by late Triassic and the Indus-Tsangpo collision driftdistance suggested by spreadingdata line by late Cretaceous-early Tertiary time, continued (1000 km) and could account for the excessive thick- to migrate S to the Main Central Thrust in the Middle ness of continental crust (c. 70 km) in Tibet, as indi- Tertiaryand to the Main Boundary Thrust and the cated by gravity. This idea is not supported by more present Himalayan front in the Plio-Pleistocene.

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Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/1/1/4886597/gsjgs.137.1.0001.pdf by guest on 26 September 2021 34 J. Stkklin RecentResearches in Geology, 4, (G. W. Chiplonkar ment in Oman Mountains, southeast Arabia. Bull. Am. Memorial Vol.) 410-37, Hindustan Publ. Corp., Delhi. Assoc. Petr. Geol. 53, 626-71. WEST, W. D. 1939. The structure of the Shali window near ZONENSHAIN, L. P.1972. The geosynclinal theory and its Simla. Rec. geol. Sum. India, 74, 133-67. application to theCentral Asian foldbelt. Nauch.- WILSON, H. H.1969. Late Cretaceous eugeosynclinal issledouat. Lab., Geol. zarubezhnikh stran, 26, 240pp. sedimentation,gravity tectonics, and ophiolite emplace- Moscow(in Russian).

Read and received 25 April 1979. JOVAN ST~CKLIN,Huebstrasse 9a, 9011 St. Gallen, Switzerland.

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