An overview of the stratigraphy and tectonics of the Nepal Himalaya

An overview of the stratigraphy and tectonics of the NepalHimalaya

B.N. Upreti

Department of Geology, Tri-Chandra Campus, Tribhuvan University, Ghantaghar, Kathmandu, Nepal

Abstract

Nepal can be divided into the following five eastwest trending major tectonic zones. (i) The Terai Tectonic Zone which

consists of over one km of Recent alluvium concealing the Churia Group (Siwalik equivalents) and underlying rocks of northernPeninsular India. Recently active southward-propagating thrusts and folds beneath the Terai have aected both the underlyingChuria and the younger sediments. (ii) The Churia Zone, which consists of Neogene to Quaternary foreland basin deposits and

forms the Himalayan mountain front. The Churia Zone represents the most tectonically active part of the Himalaya. Recentsedimentologic, geochronologic and paleomagnetic studies have yielded a much better understanding of the provenance,paleoenvironment of deposition and the ages of these sediments. The Churia Group was deposited between014 Ma and01 Ma.Sedimentary rocks of the Churia Group form an archive of the final drama of Himalayan uplift. Involvement of the underlying

northern Peninsular Indian rocks in the active tectonics of the Churia Zone has also been recognised. UnmetamorphosedPhanerozoic rocks of Peninsular India underlying the Churia Zone that are involved in the Himalayan orogeny may represent atransitional environment between the Peninsula and the Tethyan margin of the continent. (iii) The Lesser Himalayan Zone, in

which mainly Precambrian rocks are involved, consists of sedimentary rocks that were deposited on the Indian continentalmargin and represent the southernmost facies of the Tethyan sea. Panafrican diastrophism interrupted the sedimentation in theLesser Himalayan Zone during terminal Precambrian time causing a widespread unconformity. That unconformity separates

over 12 km of unfossiliferous sedimentary rocks in the Lesser Himalaya from overlying fossiliferous rocks which are >3 kmthick and range in age from Permo-Carboniferous to Lower to Middle Eocene. The deposition of the Upper OligoceneLowerMiocene fluvial Dumri Formation records the emergence of the Himalayan mountains from under the sea. The Dumri

represents the earliest foreland basin deposit of the Himalayan orogen in Nepal. Lesser Himalayan rocks are lessmetamorphosed than the rocks of the overlying Bhimphedis nappes and the crystalline rocks of the Higher Himalayan Zone. Abroad anticline in the north and a corresponding syncline in the south along the Mahabharat range, as well as a number ofthrusts and faults are the major structures of the Lesser Himalayan Zone which is thrust over the Churia Group along the Main

Boundary Thrust (MBT). (iv) The crystalline high-grade metamorphic rocks of the Higher Himalayan Zone form the backboneof the Himalaya and give rise to its formidable high ranges. The Main Central Thrust (MCT) marks the base of this zone.Understanding the origin, timing of movement and associated metamorphism along the MCT holds the key to many questions

about the evolution of the Himalaya. For example: the question of whether there is only one or whether there are two MCTshas been a subject of prolonged discussion without any conclusion having been reached. The well-known invertedmetamorphism of the Himalaya and the late orogenic magmatism are generally attributed to movement along the MCT that

brought a hot slab of High Himalayan Zone rocks over the cold Lesser Himalayan sequence. Harrison and his co-workers, asdescribed in a paper in this volume, have lately proposed a detailed model of how this process operated. The rocks of theHigher Himalayan Zone are generally considered to be Middle Cambrian to Late Proterozoic in age. (v) The Tibetan TethysZone is represented by Cambrian to Cretaceous-Eocene fossiliferous sedimentary rocks overlying the crystalline rocks of the

Higher Himalaya along the Southern Tibetan Detachment Fault System (STDFS) which is a north dipping normal fault system.The fault has dragged down to the north a huge pile of the Tethyan sedimentary rocks forming some of the largest folds on theEarth. Those sediments are generally considered to have been deposited in a more distal part of the Tethys than were the Lesser

Himalayan sediments.The present tectonic architecture of the Himalaya is dominated by three master thrusts: the Main Central Thrust (MCT), the

Main Boundary Thrust (MBT) and the Main Frontal Thrust (MFT). The age of initiation of these thrusts becomes younger

from north to south, with the MCT as the oldest and the MFT as the youngest. All these thrusts are considered to come

Journal of Asian Earth Sciences 17 (1999) 577606

1367-9120/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.PII: S1367-9120(99 )00047 -4

together at depth in a flat-lying decollement called the Main Himalayan Thrust (MHT). The Mahabharat Thrust (MT), an

intermediate thrust between the MCT and the MBT is interpreted as having brought the Bhimphedi Group out over the LesserHimalayan rocks giving rise to Lesser Himalayan nappes containing crystalline rocks. The position of roots of these nappes isstill debated. The Southern Tibetan Detachment Fault System (STDFS) has played an important role in unroofing the higher

Himalayan crystalline rocks. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction

It is generally agreed that the Himalaya were gener-ated as a result of the collision of the northward mov-ing Indian continent with the Asian landmass. Thatorogenic process continues, and mountains are stillbeing formed. Continued activity is manifest in thepresent day northward movement of India at a rate of05 cm a year and in the occurrence of frequent seis-mic events along the mountain range and in its sur-roundings (Seeber and Armbruster, 1981; Jackson andBilham, 1994; Pandey et al., 1995; Bilham et al., 1997,1998). Most of the convergence is accommodatedwithin the Himalaya by movement on various thrustsand folds.The wide acceptance of plate tectonic theory by the

early seventies paved the way for a fresh start inintense research on most parts of the Himalaya.Quickly, the Himalaya became a resource-full labora-tory for international scientists in testing many of thenew ideas opened up by the theory of plate tectonics.The past two decades in particular have been a veryproductive period in Himalayan research. TheHimalaya is one of the few mountain ranges on ourEarth in which an over 40 km thick cross-section,from the mountain roots to the mountain tops, can beobserved in a single traverse. Excellent exposures ran-ging from deep-seated metamorphic rocks to unmeta-morphosed fossiliferous rocks are fairly well accessible.Nepal occupies the central sector of the southwardly

convex Himalayan mountain arc and 800 km of the2800 km long Himalayan range lies within Nepal.Following Hagen (1969) from south to north, Nepalcan be divided into eight EW trending physiographicunits (Fig. 1, Table 1).

2. Brief history of geological investigations in Nepal

Systematic geological investigations in Nepal startedvery late in comparison with the start of studies inother countries of the region. Hooker, who visited theTamur valley in eastern Nepal in 1848 (cf. Auden,1935), provided the earliest note on the geology ofNepal. Subsequently Medlicott (1875) took a traversethrough Kathmandu in central Nepal. This was fol-lowed by brief visits by a number of other ocers ofthe Geological Survey of India during the period of

British rule; viz. Jones (1889), Oldham (1899), Diener(1912), Heron (1922) and Sutton (1933).The first account of the geology of Nepal that con-

tains any detail was the work of Auden (1935) whovisited eastern and central Nepal after the greatNepalBihar earthquake of 1934. Auden was one ofthe earliest workers to attempt the correlation of the,then very little known, Nepalese rocks with betterknown formations in the Indian Himalaya. An earlyaccount of the geology of north-western Nepal can befound in the work of Heim and Gansser (1939) whoattempted to extend the established structural units ofKumaon into the western part of Nepal.

Hagen conducted the first comprehensive regionalstudy of the geology of Nepal during the fifties of thiscentury. He worked for nearly a decade in Nepalunder the United Nations Programme of TechnicalAssistance and covered most parts of the country.Hagen summarized his work in two publications inEnglish (Hagen, 1968, 1969). Those works became thefoundation for further geological investigations inNepal. Hagen identified a number of nappe groupseach of them consisting of several nappes of similarlithology and structures. Three of his higher nappesviz. from top to bottom: the KangchendozongaLumbasumba nappes, the Khumbu nappes and theKathmandu nappes consisted predominantly of crys-talline rocks. The rest of the nappes were mainly com-posed of metasediments. The value of Hagenspioneering work in Nepal proved limited because hehad an inadequate stratigraphic base which renderedhis tectonic interpretations rather complicated and notalways readily understandable.Bordet with his colleagues also undertook pioneer-

ing work in Nepal. His early works (e.g. Bordet 1955,1961) related to eastern Nepal. He broadly agreed withHagens concepts but diered in detail. Later, heextended his work into central Nepal (Bordet et al.,1960). He also described the geology of the Phulchaukiarea south of Kathmandu and confirmed the Silurianage of the rocks at the base of Phulchauki hill basedon trilobite fossils. Bordet further extended his work incentral-western Nepal into the PokharaThakkholaregion (Bordet et al., 1964, 1972). The contribution ofhis team to the understanding of the geology of thatregion, and especially of the Tethys Himalaya in thearea is represented by a large number of publications.Among the earlier workers in Nepal, Lombard

B.N. Upreti / Journal of Asian Earth Sciences 17 (1999) 577606578

Fig.1.PhysiographicsubdivisionsofNepal.1:Terai(N

orthernGangeticPlain)andDuneValleys(100200m)2:ChuriaRange(Siwalikequivalent)(200700m)3:MahabharatRange(1000

2500m)4:LesserHimalaya(M

idlands)(3002000m)5:Fore

Himalaya(20004500m)6:Higher

Himalaya(>

4000m)7:Inner

Valleys(25004000m).

B.N. Upreti / Journal of Asian Earth Sciences 17 (1999) 577606 579

(1958) contributed to the understanding of the geologyof eastern Nepal in the Everest area. Gansser (1964),mainly on the basis of the works of Hagen, Bordet,Lombard and others, incorporated a discussion of thegeology of Nepal into his book on the Geology of theHimalaya and attempted a unified tectonic map.Bodenhausen (Bodenhausen et al., 1964) visited someparts of central western Nepal and broadly describedthe geology of that region. His main work consisted ofproviding a provisional stratigraphic scheme for theTibetan Tethys Zone and correlation with other well-known sections of the Himalaya. He observed that theTibetan zone in Nepal together with the crystallinerocks at its base forms a huge overthrust unit, over-lying the complex of the Lesser Himalaya to the south.Fuchs (Fuchs, 1967; Frank and Fuchs, 1970; Fuchs

and Frank, 1970) has contributed substantially to geo-logical studies in western Nepal. He attempted to cor-relate the Nepalese rocks of that region with theKumaon and Garhwal Himalaya in India. He hastried to show that the stratigraphy established byAuden (1935) for the western Himalaya is applicablevery well to Nepal, and that the tectonics described byPilgrim and West (1928), and by West (1939) havesimilarity to those observed in Nepal. Fuchs con-sidered that some Lesser Himalayan nappes (makingup his Chail Nappe) are represented throughout theHimalaya (Fuchs, 1977).A team of Japanese geologists led by Hashimoto

started geological investigations in Nepal in the late fif-

ties and continued their work for well over a decade.They summarized their findings in a book (Hashimotoet al., 1973) which embodied the idea that the Alpinenappe concept made use of by Hagen could not beapplied to Nepal. They suggested that the essentialstructures of the Nepalese mountain range resultedfrom longitudinal block movements with vertical dis-placement and not from Alpine folding. Following thework of Sharma (1973), they also subdivided the thicksequence of Midland metasedimentary rocks (consti-tuting the Lesser Himalayan unit) into four divisionsbased on lithologic characters. Talalov (1977) after histwo years of studies in Nepal also tried to interpret thegeology in a way that did not consider thrustssuggesting that the Himalaya formed as a result ofblock faulting. Besides these major early works a largenumber of published and unpublished studies by bothNepalese and foreign scientists were completed es-pecially during the mid-sixties to late-seventies.When Nepal opened its borders in 1950 after seclu-

sion from the rest of the world for over 100 years itattracted, in the following decades, a great number ofmountaineering and scientific expeditions from westerncountries. Because of the unrestricted access of theseexpeditions and the involvement of foreign scientistsNepal became the focus of intensive geological study.As a result most knowledge that has been acquired tothis day on the tectonics and the magmatic and meta-morphic evolution of the Himalaya has essentiallycome from the study of the Nepal Himalaya.

Table 1

Geomorphic units of Nepal

Geomorphic unit Width

(km)

Altitudes

(m)

Main rock types Age

1. Terai (Northern

edge of the

Gangetic Plain)

2050 100200 Alluvium: coarse gravels in the north near the foot

of the mountains, gradually becoming finer

southward. Forelan basin deposits

Recent

2. Churia Hills

(Siwalik

equivalent rocks)

1050 2001000 Sandstone, mudstone, shale and conglomerate.

Mollase deposites of the Himalaya

Mid-Miocene to

Pleistocene

3. Dun Valleys 530 200300 Valleys within the Churia Hills fille up by coarse to

fine alluvial sediments

Recent

4. Mahabharat

Range

1035 10002500 Schist, phyllite, gneiss, quartzite, granite and

limestone belonging to the Lesser Himalayan Zone

Precambrian and

Paleozoic,

occasionally also

Cenozoic

5. Midland 4060 2002000 Schist, phyllite, gneiss, quartzite, granite, limestone

geologically belonging to the Lesser Himalayan

Zone

Precambrian and

Paleozoic to

Mesozoic

6. Fore Himalaya 2070 20005000 Gneisses, schists and marbles mostly belonging to

the northern edge of the Lesser Himalayan Zone

Precambrian

7. Higher Himalaya 1060 > 5000 Gneisses schists and marbles belonging to the

Higher Himalayan Zone

Precambrian

8. Inner and Trans

Himalayan

Valleys

25004000 Gneisses schists and marbles of the Higher

Himalayan Zone and Tethyan sediments

(limestones, shale, sandstone etc.) belonging to the

Tibetan-Tethys Zone

Precambrian and

Cambrian to

Cretaceous


3. Major tectonic zones of Nepal

Nepal can be divided, from south to north, as canother regions of the Himalaya, into the following fivemajor tectonic zones:

1. Terai Zone2. Churia (Siwalik equivalent) Zone3. Lesser Himalayan Zone4. Higher Himalayan Zone5. Tibetan-Tethys Zone

Each of the zones is characterized by its own lithology,tectonics, structure and geologic history. A generalisedgeological map showing these features is presented asFig. 2.

4. Terai Zone

This zone represents the northern edge of the vastalluvial Indo-Gangetic foreland basin and belongs tothe southernmost tectonic division of Nepal. In thenorth it is bounded by the Main Frontal Thrust(MFT) whose outcrops are exposed at many places

along the southern front of the Churia Range. WithinNepal, the Terai plain gradually rises from about100 m in the south to 200 m in the north.

The Terai plain is made up of Pleistocene to Recentalluvium with an average thickness of 01500 m. Thetopography of the Ganges basin below the alluvium isnot uniform and the depth to the basement beneathalluvial sediments also varies greatly (Valdiya, 1988).The sediments of the Terai (PleistoceneRecent) inNepal rest on the Churia Group (Middle MiocenePliocene) which in turn rests on rocks belonging to theGondwanas (rocks of Peninsular India that are ofPermo-Carboniferous age) or younger rocks (EoceneOligocene). Coarser Terai sediments lie close to themountain front, which is known as the Bhabhar zonein Nepal, where they form large alluvial fans. Finersediments are found farther to the south. Rocks underthe alluvium far south of the mountain front areexperiencing tectonic activity and a number of thrustsand thrust propagated folds have been recognised(Bashyal, 1998; Mugnier et al., 1999).

The Recent Terai sediments represent the latest fore-land basin deposit of the Himalaya, its precursorsbeing the Neogene to Quaternary Churia (Siwalik

Fig. 2. Geological map of Nepal (after Upreti and Le Fort, 1999). LH: Lesser Himalaya, HH: Higher Himalaya, TTS: Tibetan-Tethys sediments,

MBT: Main Boundary Thrust, MCT: Main Central Thrust, MT: Mahabharat Thrust, STDS: South Tibetan Detachment System. AA : Cross-section lines (see Fig. 9).


equivalent) and the still older Late OligoceneEarlyMiocene Dumri Formation which has been tectonicallyincorporated into the Lesser Himalaya. The TeraiZone is already sharing a significant proportion of cur-rent Himalayan stress accumulation as is manifest inthe development of thrusts and thrust-propagatedfolds beneath the sediments. The northern part of theTerai Zone represents a mountain in the making. TheHimalayan mountain front is propagating to the souththrough the zone.

5. Churia (Siwalik equivalent) Zone

The Churia (Siwalik equivalent) tectonic Zone isoccupied by Himalayan foreland basin deposits. Thezone, which forms one of the largest foreland basin ac-cumulations on the Earth, consists of Neogene toQuaternary fluvial sediments that form the southern-most hill range of Nepal, the Churia hills. The ChuriaZone is bounded to the north by the Main BoundaryThrust (MBT) and to the south by the Main FrontalThrust (MFT). Lesser Himalayan metasedimentaryrocks have been thrust southward over the Churiarocks along the MBT and a large part of the ChuriaZone rocks must be buried beneath the cover of theoverthrust Lesser Himalayan rocks to the north. TheChuria Group, itself, is thrust over the Recent allu-vium of the Terai Zone along the MFT. The sedimen-tary rocks of the Churia Group, outcropping betweenthe two thrusts, form an archive of the final drama ofthe Himalayan uplift.Until recently systematic study of the Churia Zone

in Nepal received very low priority. The only earlywork that covered the greater part of the Churia Zonewas that of Hagen (1969). Recently, however, becauseof petroleum exploration in the Terai plains and theadjacent Churia hills by the Department of Mines andGeology (DMG) of the Government of Nepal, sys-tematic mapping of this region has attained a high pri-ority. A great deal of detailed subsurface and surfacedata have been accumulated, though most is unpub-lished and restricted (packages of data are on sale).Renewed interest shown by researchers in these fore-land basin deposits, especially involving tools such aspaleomagnetic dating and isotope geochemistry, hashelped to generate a good database. Recent work hasadvanced understanding of the biostratigraphy, corre-lation and environments of deposition of the Churiarocks (Munthe et al., 1983; West et al., 1975, 1978;Tokuoka et al., 1986; Appel et al., 1991; Corvinus,1993; Corvinus and Nanda, 1994; Harrison et al.,1993; Rosler et al., 1997; DeCelles et al., 1998; Ojha etal., 1999; Gautam and Rosler, 1999; Appel et al.,1991). The foreland basin deposits of the Churia Zone

record the most recent phases in the history ofHimalayan evolution spanning the past014 My.

5.1. Stratigraphy and age

The well-established three-fold classification of theSiwaliks into lower, middle and upper units that hadbeen developed in the Potwar region of Pakistan andin the western Indian Himalaya was freely applied tothe Churia Group from the beginning of the geologicalstudies in Nepal (Auden, 1935; Bordet, 1961; Hagen,1969; Itihara et al., 1972; Sharma, 1973; Herail andMascle, 1980; Mascle and Herail, 1982; Yoshida andArita, 1982; Herail et al., 1986). Those early attemptsat subdivision and correlation of the Churia Groupwere mainly based on lithofacies. Unfortunately theNeogene to Quaternary rocks of Nepal, unlike thoseof the type localities in the Potwar region, are poor invertebrate fossils. This paucity of characteristic fossilsand considerable lateral changes in facies have madethe earlier tripartite division of the Churia Group andcorrelation with the type sections only provisional andinformal. Despite this fact, for all practical purposes,the Churia rocks of Nepal are still generally being sub-divided on the basis of lithology. Attempts have alsobeen made to give local names to the lithological sub-divisions of the Churia Group (Corvinus, 1988; Dhitalet al., 1995; Sah, 1998) but this proliferation of nameswithout age constraints will hardly help in correlation.The Lower Churia Group is essentially composed of

alternations of fine-grained sediments such as varie-gated mudstone, siltstone and shale with subordinateamounts of fine-grained sandstone. Occurrences ofpaleosols are locally common (DeCelles et al., 1998).The base of a sequence that is informally correlatedwith the Middle Siwalik beds is marked by the firstappearance of multistoried thick sandstone bedsmeasuring several meters to tens of meters in thicknessand alternating with subordinate beds of mudstone.Because of the abundance of biotite and the presenceof light-coloured quartz and feldspars these sandstoneshave acquired the nickname salt and pepper sand-stones. Cycles of finingupward sequences are com-monly observed in which a bed begins with a coarsesandstone, gradually fines upward and ends up in thinclay layers or paleosols. The clay layers normally pre-serve good plant fossils and locally freshwater mol-luscs. Upper Siwalik correlatives are characterized bythe presence of very coarse sediments such as boulderconglomerates with minor proportions of mudstoneintercalations.

5.2. Magnetostratigraphic study

The first paleomagnetic study of the Churia (Siwalikequivalent) rocks of Nepal was carried out in the


Tinau Khola section in western Nepal by Munthe etal. (1983). Tokuoka et al. (1986) extended that workeast of the Tinau Khola into the Arung and BinaiKhola areas. They divided the Churia Group in thoseareas into four formations on the basis of lithofacies,namely from bottom to top: The Arung Khola, BinaiKhola, Chitwan and Deorali formations. On the basisof magnetic polarity stratigraphy they correlated theserocks with the type localities in Pakistan. Although thepaleomagnetic work of Tokuoka et al., may needrefinement, it is one of the very earliest succesfulattempts at a correlation. The work showed that theoldest rock in the area had an age of about 14.515Ma. The lower and middle members of the ArungKhola Formation are correlated with the ChinjiFormation and the upper member with Nagri for-mation in the type area. The Binai Khola Formationcorresponds to the Dhokpathan and TatrotFormations, and Chitwan Formation is equivalent toPinjor Formation. The Deorali Formation of Nepalmay thus be correlatable with the BoulderConglomerate of Lower Pleistocene age in Pakistan.The rate of sedimentation in the sequence was foundto vary from 0.2 to 0.5 mm/yr, younger units showinga higher depositional rate than older units. Thesevalues correspond well with the values obtained in thePotwar region where depositional rates varied from0.12 to 0.30 mm/yr (Johnson et al., 1985). On the basisof paleocurrent analysis Tokuoka et al. suggested thatalmost all the terrigenous sediments were suppliedfrom the northern side of the foreland basin and par-ticularly from the NNE.Based on paleomagnetic studies, the age of the

exposed Churia Group in Nepal ranges from 014 Mato

5.4. Structures

The Churia Zone is tectonically less complicatedthan the overlying Lesser Himalayan Zone.Nevertheless Churia group rocks are faulted, foldedand thrust to the south over the Recent alluvium ofthe Terai Zone. Intra-Churia thrusts called the MainDun Thrust (Herail et al., 1986; Mugnier et al., thisissue) or the Central Churia Thrust (Tokuoka et al.,1986) have developed resulting in the repetition of for-mations. Thrust-propagated broad asymmetric foldshave given rise to many wide valleys in the ChuriaZone of Nepal (Dun Valleys) which are generally filledwith the Recent sediments (Bashyal, 1998; Mugnier etal., this issue, Fig. 3). The southward propagation ofthe Himalayan thrusts and resulting folds has reachedfar south into the plains. As a result tilting and foldingof post-Siwalik sediments south of the HFT (e.g.Ratukhola section, central Nepal) and uplift of partsof southern Terai, marked by radial drainage patterns(for example, south of Bhadrapur, eastern Nepal) areobserved. The tectonics of the Churia rocks in westernNepal is discussed by Mugnier et al. in this issue.

6. The Lesser Himalayan Zone

6.1. General

The Lesser Himalayan Zone lies between the MainBoundary Thrust (MBT) in the south and the MainCentral Thrust (MCT) in the north (Fig. 2). The MBT

is well seen in the field and clearly identifiable in air-photos and satellite images. UnmetamorphosedNeogene to Quaternary Churia Group rocks, essen-tially composed of sandstones, mudstones and con-glomerates, are overlain tectonically along the MBTby the much older Precambrian metasedimentarysequences and associated younger sedimentary rocksof the Lesser Himalaya. The thrust plane ranges frombeing sharp, with only a thin and well-defined shearzone, to diuse with a shear zone extending over sev-eral tens of meters. The MBT is essentially a continu-ous structure all along the Himalayan range but it isoset at many places by0NS transverse faults.The Lesser Himalayan Zone constitutes a relatively

broad tectonic zone, especially in western Nepal.Sandwiched between the Churia Range in the southand the High Himalaya in the north, the LesserHimalayan Zone displays a relatively subdued andmature geomorphology. Tectonically, the zone is madeup of low-grade metasedimentary rock units, withoverriding crystalline nappes and klippen. Therefore, adistinction can be made between the non-metamor-phosed to low-grade metamorphic Lesser Himalayanrock units and the rocks of the crystalline nappes ofthe Lesser Himalayan Zone which are discussed later.The unfossiliferous nature of the low-grade metasedi-mentary rocks and the complicated structure of thezone in Nepal have contributed to many problems re-lated to the age, stratigraphy, tectonics and correlationof the Lesser Himalayan sedimentary units.Correlation outside Nepal suers equally from uncer-tainties. It has only been very recently that work invol-

Fig. 3. Structural cross-section across Terai, Siwaliks, Lesser Himalaya (Bashyal, 1998, with permission from Nepal Geol. Soc.).


Fig.4.Geologicalmapofwestern

NepalLesserHimalaya(compiled

from

FuchsandFrank,1970;Sakai,1983,1985;Sharm

aet

al.,1984;Hayashiet

al.,1984;Tateret

al.,1984;Dhitaland

Kizaki,1987a,b;Shresthaetal.,1986,1987;Hirayamaetal.,1988;Kansakar,1991;ESCAP/D

MG,1993;AmatyaandJnawali,1996;JnawaliandTuladhar,1996).


ving more detailed mapping in central and westernNepal has begun to provide some sound stratigraphicfoundation for the study of the Lesser Himalayan low-grade metasedimentary rocks (Stocklin and Bhattarai,1977; Stocklin, 1980; Sakai, 1983, 1985; Sharma et al.,1984; Dhital and Kizaki, 1987a). Extension of thisdetailed work will undoubtedly bring a better under-standing of this important tectonic zone.

6.2. Stratigraphy

The stratigraphy of the Lesser Himalaya of centralNepal including that of the sedimentary rocks and thatof the rocks of the overlying crystalline nappes hasbeen established and mapped continuously for about250 km along the strike from the Dudh Kosi River inthe east to the Marshyangdi River in the west(Stocklin and Bhattarai, 1977; Stocklin, 1980).Presently, the area between the Marshyangdi Riverand the Tansen area has not been investigated in detailso that lithounits have not yet been well traced fromcentral to western Nepal. This has created some uncer-tainties in the correlation of the formations. A com-piled geological map of the western Nepal LesserHimalaya is presented in Fig. 4 and stratigraphic col-umns along various measured sections that exclude thecrystalline nappes are given in Figs. 5 and 6. A syn-thesis of the stratigraphy of western Nepal and its cor-relations is given in Table 2.In central Nepal Stocklin and Bhattarai (1977) and

Stocklin (1980) have used the term Nawakot Complexto include all the formations of the Lesser Himalayaand the term Kathmandu Complex for all the rockformations lying within the Kathmandu Nappe includ-ing both crystalline rocks and overlying fossiliferoussedimentary rocks. Sakai (1983) used the term KaliGandaki Supergroup for the older sequence (LatePrecambrian-Early Paleozoic) and Tansen Group forthe younger sequence (Late-Carbeniferous-Permian toOligocene-Early Miocene) for Lesser Himalayan rocksin western Nepal.Except for the crystalline rocks of the Jajarkot

Nappe, the entire area of western Nepal between theMBT and the MCT is underlain by rocks of NawakotComplex (Stocklin and Bhattarai, 1977; Stocklin, 1980,Fig. 4) and the Tansen Group (Sakai, 1983). The top-most unit of Tansen Group is a thick sequence ofunmetamorphosed sandstones and mudstones calledthe Dumri Formation. This fluvial unit, which overliesLower-Middle Eocene marine rocks unconformably, isgenerally considered to represent the first forelandbasin sedimentary deposits to be derived from theHimalaya in Nepal. The section south and southwestof Pokhara around Tansen was investigated in detailby Sakai (1983, 1985) and most of the western part ofthe outcrop as far as the Bheri River was investigated

by Fuchs and Frank (1970), Sharma et al. (1984) andDhital and Kizaki (1987a,b). Farther west in the Dangand Surkhet Surkhet areas, the stratigraphy of theLower Tertiary rocks has been described by Kayastha(1992). Though the area between the MarsyangdiRiver and Tansen is not mapped in detail, most of therocks outcropping in that area are broadly correlatablewith the Nawakot Complex of Stocklin and Bhattarai(1977).The eastern Nepal Lesser Himalayan Zone east of

Kathmandu has not been mapped in detail and itsstratigraphy remains poorly understood. LesserHimalayan rocks are mainly exposed in tectonic win-dows cut through thrusts that carry crystalline rockssouthward from the Higher Himalayan Zone. ExposedLesser Himalayan Zone sedimentary rocks can be gen-erally recognised as belonging to the NawakotComplex but detailed correlation with central and wes-tern Nepal is dicult.

6.3. Age

The age of the Lesser Himalayan formations hasremained a matter of discussion and controversy for along time. Early investigators with Alpine backgroundswere tempted to assign Paleozoic and Mesozoic agesto most Lesser Himalayan rocks (Hagen, 1969; Bordet,1961) but the general lack of fossils has rendered ageassignment dicult. The stratigraphy of the LesserHimalayan rocks is therefore presently based mainlyon lithology. In Nepal the Lesser Himalayan rocksmay be divided into two groups:

1. The Older Lesser Himalayan Formations; and2. The Younger Lesser Himalayan formations.

The two groups are separated by a major unconfor-mity contemporary with Late Panafrican diastrophism(Valdiya, 1995, 1998). This unconformity may becalled the Great Lesser Himalayan Unconformity(Pan-African Unconformity of Valdiya, 1998). Theolder formations beneath the unconformity arePrecambrian in age, probably ranging from thePaleoproterozoic (approx. 18002000 Ma) (Parrish andHodges, 1996; Valdiya, 1995, 1998; Sakai, 1983, 1985;Sakai et al., in preparation) to the uppermostPrecambrian (0570 Ma). The 1800 Ma LesserHimalayan augen gneiss associated with the KunchaFormation also gives an age constraint to these rocks(Le Fort and Rai, this issue). The older group of for-mations is succeeded above the Great unconformity bycontinental facies Gondwana sedimentary rocks ofPermo-Carboniferous age. The Gondwana rocks inturn are unconformably overlain by marine rocks ofEarly Cretaceous to Eocene age (Sakai, 1983, 1985)and the sequence of the Lesser Himalaya is finally


Fig.5.StratigraphiccolumnsoftheLesserHimalayanrocksin

centralandwestern

Nepalandtheircorrelations.Fm:Form

ation,Dol:Dolomite,lms:limestone,metasst:metasandstone,phyll:

phyllite,sst:sandstone,Qtz:Quartzite.


capped unconformably by the fluvial DumriFormation of Late OligoceneEarly Miocene age.The younger Lesser Himalayan rocks were originally

thought to be largely confined to outcrops in theTansenSallyanJajarkot area of western Nepal butrecently they have been found to occur over a much

wider area. The younger rocks seem to be present intectonically isolated patches throughout the LesserHimalayan domain. It is not yet very clear whetherthese younger rocks originally covered the entireLesser Himalayan region or were deposited in isolatedshallow basins. Sakai et al. (this issue) has mapped the

Fig. 6. Stratigraphy of younger lesser Himalayan sediments above the unconformity (Tansen Group, Sakai, 1983, Sakai et al., this issue).


younger Lesser Himalayan sedimentary rocks as far asnorth of Jajarkot near the MCT. On a country-widebasis the distribution of the younger Lesser HimalayanZone rocks from areas close to the MBT in the southto the base of the MCT in the north, and from theeastern to the western boundary of Nepal, indicatesthat probably these younger Lesser Himalayan conti-nental to marine fossiliferous rocks originally coveredan area corresponding to a significant part of theLesser Himalaya of Nepal.

6.4. Tectonics and structure

From east to west, the tectonics of the LesserHimalayan Zone of Nepal show great variety. In east-ern Nepal the tectonics are characterized by the devel-opment of an extensive crystalline thrust sheet that hastraveled southward for at least 100 km to reach closeto the outcrop of the MBT. Almost the entire LesserHimalayan Zone in this eastern area is occupied bythis single thrust sheet. Lesser Himalayan sedimentsare exposed only in a few large tectonic windowsthrough the sheet such those as Taplejung, Arun andChautara-Okhaldhunga (Fig. 2). The LesserHimalayan Zone in Central Nepal is similarly occupiedby the Kathmandu nappe which outcrops over a widearea around Kathmandu and extends in a narrow arm

south-eastward along the Mahabharat Range to join

with the great thrust sheet of eastern Nepal. West of

Kathmandu between the Burhi Gandaki and Bheri riv-

ers, crystalline rocks are restricted to the Higher

Himalayan Zone north of the MCT except for out-

crops in a narrow klippe in the Jajarkot area. Between

the two rivers a large tract of Lesser Himalayan Zone

outcrop is free from crystalline rock cover and exposes

only metasedimentary rocks. It is dicult to guess how

extensively crystalline thrust sheets covered the Lesser

Himalaya in the past. In addition to the larger nappes

very small and scattered crystalline klippen in many

areas (Jnawali and Tuladhar, 1996) point to the former

presence of much more extensive crystalline outcrops.

West of Bheri and extending as far as the western bor-

der of Nepal, which abuts the Kumaon Himalaya of

India, crystalline nappes again reappear and cover

much of the Lesser Himalaya.

In the east, from the meridian of Kathmandu to

Bhutan and the Arunanchal Himalaya of India,

Himalayan tectonics are dominated by large thrust

sheets of Higher Himalayan crystalline rocks. To the

west comparable crystalline nappes are much less

extensive. The Himalaya to the west of Kathmandu

exposes a large tract of Lesser Himalayan rocks in an

area that extends from the MBT to the MCT. The

topography of the western Lesser Himalaya is also

Table 2

Stratigraphic correlation of the Lesser Himalayan formations of western Nepal


much more subdued and mature than that of easternareas, where the topography is more rugged. I inter-pret this dierence as indicating that thrust tectonicsand exhumation were initiated earlier in the west thanin the east so that most of the thrust sheets of theHigher Himalayan crystalline rocks in the westernregion have been eroded away.The Lesser Himalayan Zone contains many major

thrusts as well as other types of fault. Older LesserHimalayan metasediments have, for example, beenbrought to override younger rock sequences alongthrusts such as the Phalebas thrust (Upreti et al., 1980;Sakai, 1983, 1985; Sharma et al, 1984; Poudel andArita, 1998). The Barigad Fault (Fig. 4) in westernNepal which extends for over 150 km in a NWSEdirection is considered to be one of the more import-ant faults of the Lesser Himalayan Zone. It is inter-preted to be an active fault with a right-lateral slipcomponent (Nakata, 1989). In addition to smallerfolds, Lesser Himalayan rocks are folded into an eastwest trending mega-anticline (TrisuliGorkhaPokharaanticline). The core of this anticline has exposed theoldest rocks of the Lesser Himalaya, in the KunchaFormation. However, no basement rocks of the LesserHimalayan sediments are exposed anywhere. TheLesser Himalayan rocks are also folded into a corre-sponding syncline to the south known as theMabharat Syncline which runs almost throughout theentire frontal part of the Nepal Himalaya.

7. Higher Himalayan Zone

7.1. General

Since Heim and Gansser (1939) first identified anddescribed the Central Crystalline zone of the KumaonHimalaya, that zone has been mapped and tracedthroughout the entire Himalayan region, although ithas been given dierent names in dierent areas.Geologically, the Higher Himalayan Zone in Nepalcorresponds to the Central Crystalline Zone ofKumaon and includes the tectono-stratigraphic unitthat lies to the north of, and above, the Main CentralThrust (MCT) and below the fossiliferous Tibetan-Tethys Zone of sedimentary rocks. The HigherHimalayan Zone is essentially composed of variouskinds of gneisses that form a basement to the Tibetan-Tethys sedimentary pile. Topographically, in contrastto the geologically defined Higher Himalayan Zone,most of the highest peaks of the High Himalaya inNepal including Everest, Manaslu, Annapurna,Dhaulagiri, Kanjiroba, Saipal and Api, are cappedwith Tibetan Tethyan Zone sediments and the Tertiaryleucogranites (Fig. 2).Whereas the upper boundary of the Higher

Himalayan Zone is comparatively easily discerniblelocating the lower boundary of the unit is problematicand complicated. This diculty is directly linked todiculty in defining the precise location of the MCTand its associated metamorphism and deformation. Adetailed discussion of the problems of position, num-ber and correlation of MCT and of its associated de-formation, metamorphism and thermal history ispresented below. First the nature of the simpler upperboundary of the Higher Himalayan Zone is discussed.Along the Himalayan range in many areas the HigherHimalayan Zone is separated from the Tethyan sedi-mentary rocks by intervening bodies of Tertiary leuco-granite forming the Higher Himalayan Granites. Thesegranitic bodies obscure the nature of the contactbetween the rocks of the Higher Himalayan Zone andthe Tethys sedimentary rock zone. In the past it wasgenerally considered that the contact between theHigher Himalayan gneisses and the Tethyan rocks wasgradational and showed no structural break (Gansser,1964; Fuchs and Frank, 1970). More recently a normalfault system, the Southern Tibetan Detachment FaultSystem (STDFS) has been recognised. That fault sep-arates the rocks of the two zones (Pecher, 1991;Burchfiel et al., 1992; Hodges et al, 1992; Brown andNazarchuk, 1993; Coleman, 1996).In Nepal, the Higher Himalayan Zone consisting of

high-grade crystalline rocks including various kinds ofgneisses, schists and migmatites extends continuouslyalong the entire length of the country. The northsouth width of the unit varies from place to place. Atplaces where the unit is confined to its root zone, thewidth is restricted to a few kilometers. On the otherhand, where the Higher Himalayan crystalline rocksoccupy thrust sheets, as in eastern Nepal, the widthreaches to tens of kilometers (Fig. 2).

7.2. Nomenclature

The crystalline rocks of the Higher Himalaya ofNepal have been variously named. In eastern Nepal,Lombard (1953, 1958) used the term Dalle du Tibet(Tibetan Slab) for this unit and later French workersretained that term extending its use along the entireHimalayan Range (Le Fort, 1975a,b, 1989; Bordet etal., 1972, Pecher, 1975, 1977; Brunel et al., 1979;Colchen et al., 1980). The Higher Himalayan Zone ofNepal falls within Hagens (1969) Khumbu andLumbasumba nappes as well as partly within both theupper units of his Kathmandu nappe, and the rootzones of his Kathmandu nappe. Bodenhausen et al.(1964) called the Higher Himalayan crystalline rocksthe Annapurna Gneissic Complex from outcrops in theKali Gandaki valley. American authors, and someothers, commonly call the rocks of this zone theGreater Himalayan Sequence or the Greater


Himalayan Crystallines (Hodges et al., 1988;Macfarlane et al., 1992; Harrison et al., 1998). In wes-tern Nepal, Frank and Fuchs (1970), Fuchs and Frank(1970); Fuchs (1977) describe this unit as the uppercrystalline nappes. Japanese workers (Hashimoto etal., 1973; Arita, 1983) have included this unit in theirHimalayan Gneiss Zone or Himalayan Gneiss Group(Kano, 1984) which lies above and to the north oftheir MCT II or Upper MCT, and below the Tethyansediments. Schelling (1989) in eastern Nepal includedthis unit in his Junbesi Group that embraced hisRolwaling (Khumbu) migmatites and Rolwaling para-gneisses. The rocks of this zone are designated as theHimal Group in the geological maps published byDMG (Shrestha et al., 1987). I prefer to call the rockunits of the Higher Himalayan Zone the HigherHimalayan Gneisses, or the Higher HimalayanCrystalline rocks to make a clear distinction betweenthese rocks and the gneisses of the Lesser Himalayasuch as the Ulleri augen gneiss (Le Fort, 1975a).

7.3. Geology

The crystalline rocks of the Higher Himalayan Zoneof Nepal have been studied in detail in outcrops withinthe deep gorges of the major rivers that cut throughthe High Himalaya. Those gorges expose the entire>12 km thick Higher Himalayan crystalline rocks aswell as an overlying 1012 km thick section ofTethyan zone sedimentary rocks. Exposure of thisdeep-seated core of the mountain belt is one of themost distinctive characteristics of the Himalaya. Theother sections of the Higher Himalayan Zone areextremely remote and lie at very high altitudes, makingthem highly inaccessible. The best-studied sections ofthe Higher Himalayan Zone in Nepal are along theriver sections of Upper Karnali, Kali Gandaki, Modi,Marshyangdi, Burhi Gandaki, and Trisuli (Langtangsection). Some work has also been done along the sec-tions in the Tama Kosi, Dudh Kosi and Arun valleys.

7.3.1. The Karnali and Upper Thulo Bheri SectionsThe area between the Mugu Karnali and the Kali

Gandaki valleys has been studied almost solely byFuchs (Fuchs, 1967, 1977, 1980; Fuchs and Frank1970; Frank and Fuchs, 1970) who places the HigherHimalayan Zone in his upper crystalline nappe. In theMugu Karnali section Fuchs (1977) placed the MCTabove his Lower Crystalline nappe near KhumpaVillage. The exposed base of the gneiss zone consistsof well-layered, fine to medium grained, banded two-mica gneisses. Above these gneisses lie thicker layers oflight to grey quartzites and quartzite gneisses followedby garnet-mica-schists to paragneisses. Farther up thesection, graybrowngreenwhite banded alternationsof quartzite, quartzite gneiss, garnet-kyanite-two-mica

gneiss, calc-silicate bearing paragneisses and amphibo-lite occur. Migmatites and augen gneisses, graniticgneiss and migmatites follow up in the section. Thegneisses generally contain kyanite and sillimanite. Thisunit which is exposed over 5000 to 10,000 m consistsof coarse-grained garnet-kyanite gneisses (2stauroliteor sillimanite) passing upward into augen granite-gneiss, ortho-to migmatitic gneisses, calc-gneisses,marbles, and calc mica schists.

7.3.2. Central and western NepalIn central and western Nepal, this zone has been

studied by many workers e.g. Fuchs (1977); Fuchs andFrank (1970); Frank and Fuchs (1970); Bordet et al.(1972); Le Fort (1975a,b); Pecher (1975, 1977, 1989);Pecher and Le Fort (1986); Le Fort et al. (1986, 1987);Brunel et al. (1979); Bouchez and Pecher (1981); Arita(1984a,b); Kano (1982, 1984) and Hodges et al. (1988).

7.3.2.1. The Kali Gandaki section. This area oers anexcellent cross-section of the entire unit where the riverKali Gandaki cuts through the Higher Himalaya mak-ing one of the deepest gorges of the world. The HigherHimalayan gneiss Zone of this area has been studiedby Le Fort (1971), Le Fort et al. (1986); Bordet et al(1972); Sako et al. (in: Hashimoto et al., 1973); Kano(1982). The section of Modi Khola, one of the Easterntributaries of Kali Gandaki has been thoroughly stu-died by Pecher (1975, 1977) and Arita, (1983).Le Fort (1975a) grouped the Higher Himalayan

crystalline rocks into three formations: Formation I, IIand III (from bottom to top). He interpreted this zoneas showing a homoclinal structure dipping to the NEor NNE at angles ranging from 30 to 758. The totalthickness of the zone in Kali Gandaki section isbetween 6000 and 7000 m. Formation I, which is1400 m thick, is the lowest unit consisting of Kyanite(to sillimanite)-garnet two-mica banded gneisses ofpelitic to arenaceous composition. Formation I con-tinuously increases in thickness eastward reaching upto 5000 m in Marshyangdi section. In the KaliGandaki section, where the thickness is less, only kya-nite has developed, in thicker sections, sillimanite hasdeveloped in the upper part of the formation. The pre-sence of augen gneisses, remobilization (migmatization)and intercalations of lime silicate rocks and quartzitescharacterize the upper part of the formation (Le Fort,1975a). Formation II commonly begins with a coarsequartzite bed several tens of meters thick. It is mainlycomposed of an alternation of pyroxene and amphi-bole bearing calc-gneisses and marbles. The total thick-ness of Formation II is 3500 m which remains fairlyconstant along the EW extension. The contactbetween Formation II and III is arbitrary butFormation III is characterized by a more pelitic tograywacke character. The top of the formation is


made up of a 3000 m thick coarse augen gneiss. The

boundary between the Formation III and the lime-

stone of the Tibetan-Tethys Zone (Larjung Formation)

is marked by faults of the South Tibetan Detachment

System (STDS) (Brown and Nazarchuk, 1993; Pecher,

1991; Coleman, 1996; Godin et al., 1999).

7.3.2.2. The Marshyandi, Burhi Gandaki and Trisuli sec-

tions. The Higher Himalayan Zone of these sections

has been studied by Hashimoto, 1973 (in Hashimoto

et al., 1973); Le Fort (1975a,b, 1981), Caby et al.

(1983), Pecher and Le Fort (1986) and Hodges et al.

(1988). Some of the most detailed petrographic and

geochemical studies are those of the Marshyangdi sec-

tion by Le Fort (1975a,b, 1981, 1988), Le Fort et al.

(1987), Vidal et al. (1982) and Deniel et al. (1986,

1987). This section constitutes one of the thickest sec-

tions of the Higher Himalayan Zone of Nepal exceed-

ing 10,000 m in thickness. The lithologic succession is

quite similar to the Kali Gandaki section. The sections

along Burhi Gandaki and Trisuli rivers show a more

of less similar stratigraphy and mineralogy to that of

the Marshyangdi except that there is a thick unit of

quartzite in formation I.

7.3.2.3. KathmanduLangtang section. This zone has

been particularly studied by Rai et al. (1998), Macfar-

lane (1995), Inger and Harris (1992) and Arita et al.

(1973). In the Kathmandu area defining the limit of

southward extension of the Higher Himalayan Zone

has been a problem. Most earlier maps show the rocks

of the Higher Himalayan Zone extending to the south

of Kathmandu with the MCT reaching very close to

the MBT. But Upreti and Le Fort (1999) located the

MCT to the north of the Kathmandu valley placing

the rock units to the south in a separate tectonic unit

which is discussed later.

Discussions of the PT conditions and ages of meta-

morphism of the Higher Himalayan Zone rocks and

the associated MCT can be found in the works of

Maruo and Kizaki (1983), Le Fort et al. (1986),

Hodges et al. (1988), Pecher (1989), Hubbard, (1989),

Hubbard and Harrison (1989), Copeland et al. (1991),

Inger and Harris (1992), Macfarlane (1995), Coleman

and Parrish (1995), Yoneshiro and Kizaki (1996),

Arita et al. (1997, Rai et al. (1998), Harrison et al.

(1998), Harrison et al. (this issue), Guillot (this issue).

A great deal of the petrography and geochemistry of

these rocks and also that of the Manaslu granite have

been worked out by Le Fort (1981, 1987), Vidal et al.

(1982), Deniel et al. (1986, 1987), Guillot et al. (1995,

this issue) and other workers. In general the rocks of

the Higher Himalayan Zone have been metamor-

phosed in the upper amphibolite to granulite facies

(Pecher, 1989; Rai et al., 1998).

8. Tibetan-Tethys zone

This northernmost tectonic zone of the Himalaya

occupies a wide belt consisting of sedimentary rocks

known as the Tethyan Sedimentary Series (TSS). It

has undergone very little metamorphism except at its

base where it is close to the Higher Himalayan

Crystalline rocks of the Higher Himalayan Zone. The

Tibetan-Tethys Zone lies between the South Tibetan

Detachment Fault System (STDFS), a north dipping

normal fault system, and the Indus-Tsangpo Suture

Zone (ITS). Ages within the fossiliferous sequence

making up this zone range from Cambrian to Late

Cretaceous-Lower Retiary (Colchen et al., 1980). In

Nepal, the Tibetan-Tethys tectonic zone has a very

limited aereal extent with the best sections being found

in western Nepal in the Annapurna-Dhaulagiri and

Dolpa regions (Fig. 2). Most high peaks of the Nepal

Himalaya including the Everest are made up of the

rocks of this zone (Fig. 2).

Subsequent to the Hagens (1968) pioneering work

in the Thakkhola area in the upper section of the Kali

Gandaki River a great deal of further work on the

Tethys zone of the Nepal Himalaya was carried out

(Bodenhausen et al., 1964; Fuchs, 1964, 1974, 1977;

Waterhouse 1966, 1978; Bordet et al., 1971a,b, 1975;

Bassoullet and Mouterde, 1977; Fort et al., 1980;

Colchen et al., 1986; Fuchs et al., 1988; Garzanti and

Frette, 1991, and this issue; Ogg and Rad 1994; Rad et

al., 1994; Fuchs and Poudel, 1998). The rocks of the

Tibetan-Tethys Zone (TSS) consist of a thick and

nearly continuous lower Paleozoic to lower Tertiary

marine sedimentary succession. Stratigraphic columns

and correlations from dierent sections of Nepal are

presented in Fig. 7. The rocks are considered to have

been deposited in a part of the Indian passive conti-

nental margin (Liu and Einsele, 1994). Exposures are

excellent and easily accessible in the Manang area in

the Marsyangdi River, in the Thakkhola, in the Kali

Gandaki River, and in the Dolpa area of western

Nepal that lies to the north of the high ranges of

Annapurna, Dhaulagiri and Kanjiroba Himal (Fig. 2).

Tibetan-Tethys Zone rocks also cover a small area in

the upper reaches of Burhi Gandaki north of the

Manaslu range. In far western Nepal these rocks are

exposed in remote areas along the upper reaches of

Kali and Seti rivers lying to the north of the Api and

Saipal ranges respectively. Detailed description of

these rocks and fuller references are given Garzantis

paper in this volume.


Fig.7.CorrelationofTibetan-Tethyssedimentary

form

ationsin

dierentsectionsofNepal.


9. Tectonics of the Nepal Himalaya

9.1. Master thrusts of the Himalaya

The basic framework, or architecture, of theHimalaya is controlled by three master thrusts thatextend all along the Himalayan range: namely theMain Central Thrust (MCT), the Main BoundaryThrust (MBT) and the Main Frontal Thrust (MFT).The three thrusts are considered to come together in alow angle decollement known as the Main HimalayanThrust (MHT) to the north, deep below the Himalaya.Harrison et al. in this volume illustrate this relation-ship. The decollement is a major crustal break separ-ating the upper and the lower continental crust of theIndian plate. The lower crust underthrusts to the northbelow the Tibetan continental crust resulting in thedoubling the crust below Tibet but the crust above thedecollement is being pushed out to the south along thethree master thrusts. Initiation of the three thrusts wasyounger in age from north to south. The MCT wasthe first thrust to break the Indian crust. It carriedsouthward on its back a pile of rocks 2530 km thickfor 200 km or more (Schelling, 1992; Srivastava andMitra, 1994; Parrish and Hodges, 1996; Harrison etal., this issue). Excellent reviews on the nature, ageand evolution of MCT are also given by Le Fort(1975a,b, 1996), Hodges et al. (1988), Copeland et al.(1991), Searle (1995) and by other authors.

9.2. The Mystic MCT

For long, one of the most controversial topics in theHimalayan geology has been the nature, position andthe role of MCT in the evolution of the mountainrange. A great deal of recent research in the Himalayacontinues to revolve around the MCT and studies ofthat thrust system continue to attract high priority forresearch funding. The long ago recognised problems ofthe inverted metamorphism and the origin of HigherHimalayan leuco-granites are generally considered tobe intimately related to movements along the MCT. Agreat variety of models have been proposed to explainthe abnormal bottom to top increase of metamorphismand these are well reviewed by Harrison et al. (1998),as well as in a paper in this volume.

9.2.1. The MCT in India and Nepal

9.2.1.1. The original MCT. Heim and Gansser (1939)while working in the eastern Kumaon Himalaya alongthe Kali River section adjacent to the Nepal borderrecognised a thrust at the base of a gneissic unit justnorth of the border town of Darchula. The gneissiczone essentially consisted of coarse biotite-alkali-feld-spar gneiss with the general aspect of a granitic gneiss

that passed into a muscovite-biotite gneiss and a bio-tite-granite gneiss with local porphyroblastic augen.According to Gansser (1964, p.104), metamorphismseemed to decrease upward and a sequence of sedimen-tary rocks called the Sirdang Sedimentary Zoneappeared. A thick snow-white sericite quartzite formedthe base of the next higher crystalline rock unit whichbegan with garnetiferous biotite-psammite gneisses andgave way upward to an intercalated rock sequencewith a predominance of staurolite-garnet-biotiteschists. Farther upward still, augen gneisses appearedwhich were again followed by biotite-psammiticgneisses. Higher up in the sequence, metamorphismdecreased and a conspicuous schist zone occurred,which was given the local name of the Budhi schist.Heim and Gansser (1939) considered that the inter-

calation of the Sirdang Sedimentary section dividedthe crystalline rocks of the Higher Himalayan Zoneinto lower and upper crystalline units. This thrust atthe base of the lower gneisses was originally defined asthe Main Central Thrust but no thrust was placed atthe bottom of the upper crystalline unit. Farther westin areas in India where the lower gneissic units aremissing, the thrust, locally called the Jutogh Thrustwas placed at a horizon where the sedimentary rocksof the Lesser Himalaya gave way to low to mediumgrade metamorphosed rock units such as biotite togarnet bearing schists and marbles. For long, and eventoday in India, this is the true MCT to most research-ers.

9.2.1.2. MCT in Nepal. In Nepal, Hagen (1969) wasworking independently and mapping a multitude ofthrusts at many levels both in the sedimentary rocksand in the crystalline rocks. Because Hagen envisageda structure with many nappes no single thrust equival-ent to the original MCT was recognisable althoughsome of Hagens thrusts could be considered as poss-ibly correlatable with the MCT in India. On the otherhand French researchers, working particularly in cen-tral and western Nepal (Bordet, 1961; Bordet et al.,1972; Le Fort, 1975a,b; Pecher, 1975, 1977), recognisedan MCT which was dierent from the MCT of Heimand Gansser (1939) and Gansser (1964). That MCT isat a higher structural level than the MCT in India andis characterised by a thick ductile shear zone that wasgiven the name: the MCT zone. Though in most casesthe precise location of a thrust plane was dicult tomap, it was realised that a thrust in this position is themost significant and fundamental of the thrusts thathave contributed to the geodynamic evolution of theHimalaya. The thrust separates two fundamentallydierent domains of the Himalaya: the Lesser Himala-yan zone with unmetamrphosed and low grade meta-morphic rocks and the high grade metamorphic rocksof the Higher Himalaya. Following Lombard (1958),


the thick pile of the kyanite to sillimanite bearinghigh-grade metamorphic rocks which are essentiallycomposed of gneisses, calcgneisses, schists and migma-tites lying above the MCT, was called the Tibetan Slab(Le Fort, 1975a,b). In Nepal, French workers did notrecognise a thrust equivalent to the MCT of Heim andGansser (1939).After a decade of research nearly covering the whole

country, the Japanese research group (Hashimoto etal., 1973) came up with the idea of two thrusts whichthey called MCT I (Lower MCT) and the MCT II(Upper MCT). The MCT I is equivalent to the MCTof Heim and Gansser (1939) and the MCT II corre-sponds with the MCT of the French workers (Le Fort,1975a,b; Pecher, 1989). The intervening zone betweenthe MCT I and II was called the Main Central Thrustzone. In most areas, the MCT I is placed at the baseof the augen gneiss (Ulleri augen gneiss, Le Fort,1975a,b) or arbitrarily along the base of a rock unitwith medium grade metamorphism not far below fromthe base of the Higher Himalayan crystallines. Thecontact between the rocks of MCT zone and theLesser Himalayan metasediments is not sharp andlooks gradational everywhere. The famous invertedmetamorphism of Himalaya is actually observed inthis zone. Interestingly, the French researchers do notrecognise this lower thrust anywhere in the NepalHimalaya.In western Nepal, Fuchs and Frank (1970) also

recognised two distinct thrusts lying at the bases ofUpper (Tibetan slab) and Lower (MCT zone) crystal-line nappes in similar positions to those of theJapanese workers. Along the Kali Gandaki section, atTatopani they described quartzites and basic rocks ofthe Chail Nappe dipping beneath the phyllitic micaschists, which contain layers of carbonate rocks, quart-zites and gneisses. These latter rocks constitute theLower Crystalline Nappe of Fuchs and Frank (1970)and the gneiss probably represents the LesserHimalayan augen gneiss. Perhaps this led theseworkers to place a thrust at the base of the gneiss.Surprisingly, most authors have not reported thisaugen gneiss in Kali Gandaki section. Their nextthrust (equivalent to the MCT of French) begins justnorth of Dana at the base of their Upper CrystallineNappe.Subsequent workers in Nepal (Stocklin, 1980;

Hodges et al., 1988; Hubbard and Harrison, 1989;Copeland et al., 1991; Macfarlane et al., 1992;Schelling, 1992; Harrison et al., 1993) have all recog-nised the Tibetan Slab as the distinctive tectonic unitof the Higher Himalayan Zone and put the MCT atits base. Equally importantly, all these later workershave also recognised the MCT zone which lies at thebase of the Tibetan Slab.Until 1980 when Valdiya (1980) put the idea of the

existence of two thrusts, his Munsiari Thrust whichwas the original MCT of Heim and Gansser (1939)lying at a lower level and the Vaikrita Thrust, theequivalent of the French MCT at the base of theTibetan Slab, most authors in India were content withthe original MCT of Heim and Gansser (1939) and nothrust was put at the higher level at the base of thehigh grade crystalline unit. Valdiya also proposed thatthe Lesser Himalayan crystalline nappes root to thenorth in the Munshiari Group of low to mediumgrade metamorphic rock units lying between his intra-crustal boundary thrusts the Vaikrita and Munsiarithrusts.

9.3. The Lesser Himalayan augen gneisses (the Ullerigneisses) and the MCT I

Le Fort (1975a,b) and Pecher and Le Fort (1977)had early pointed out the nature and origin of theaugen gneisses of the Lesser Himalaya, which theycalled the Ulleri augen gneisses. They indicated thatthey belong to the Lesser Himalayan zone and do notrequire any thrust contact with the surrounding rocks.These Ulleri-type augen gneisses, which I prefer to callthe Lesser Himalayan augen gneisses to distinguishthem from the augen gneisses of the HigherHimalayan Zone and the augen gneisses of the LesserHimalayan crystalline nappes, are very widely devel-oped in the Nepal Himalaya extending intermittentlyfor a long distance from west to east. They consistentlyoccur more or less at the same stratigraphic level inthe Lesser Himalayan sequence. In most cases they arefound in the upper part of the Kuncha Formation(Stocklin, 1980), the oldest rock unit of the LesserHimalaya in Nepal. Large bodies are mapped in theChainpur area of the far west, in the Lower Karnalisection, north-west of Surkhet, north-west of Pokhara,at Ulleri, which is the type locality of the Ulleri augengneisses, and north-west of Kathmandu in the upperTrisuli River section. In the Trisuli section, interest-ingly, the augen gneiss is found in two horizons: oneat the lower level within the Kuncha formation andthe other at the top of the Nawakot Complex withinthe Benighat Slate (Rai, 1998). In the east, the augengneisses exposed to the north of Ramechap andOkhaldhunga are over 2 km thick and extend fornearly 100 km (Fig. 2). That is the largest body ofLesser Himalayan augen gneiss in Nepal. I here pro-pose calling it the Melung-Salleri augen gneiss (thePhaplu augen gneisss of Maruo and Kizaki, 1983).Farther east, in the Arun and Taplejung widows, theaugen gneiss again appears in the upper part of theLesser Himalayan Zone. In addition to the occurrencesdescribed here there are many thin lenticular smallerbodies of augen gneiss in other areas within the


Kuncha Formation for example in Daraundi Kholasection, north of Gorkha.The presence of bodies of augen gneiss amidst the

low-grade rocks of Lesser Himalaya led most research-ers to interpret them as thrust sheets. The gneisseswere considered as high-grade metamorphic rockslying side by side with low-grade rocks such as theKuncha phyllites. Regarding the augen gneisses ashigh-grade metamorphic rocks has been the criticalreason for putting thrust contacts between the gneissesand adjacent rocks with low grade metamorphism(Heim and Gansser, 1939; Gansser, 1964; Arita, 1983;Maruo and Kizaki, 1983). But interestingly, in mostcases thrusts were put only at the base of the gneissesand not at their upper limits.Field relationships between the gneisses and the sur-

rounding rocks show that the gneisses imperceptiblygrade into low-grade country rocks such as theKuncha phyllites. There are excellent outcrops showinggradations and interlayered relationships with thecountry rocks along the Chaku Khola, at 102 km fromKathmandu, on the KathmanduKodari highway,north of Okhaldhunga near the village of Necha, and,by my personal observation, along the DaraundiKhola, north of Gorkha and in the Ulleri area (LeFort, 1975a,b). In the Chaku Khola area, the augengneisses gradually grade into the Upper NawakotGroup of Stocklin (1980) without any structural break.Near the contacts the augens gradually become finerand stretched and the rocks merge into the surround-ing phyllites. In the Daraundi Khola section, there aretwo augen gneiss sheets intercalated within theKuncha Formation, each over 50 m thick. The inter-vening rocks between the two gneiss bodies are alsophyllites of Kuncha Formation. This field evidenceclearly indicates that the augen gneisses are not thrustbound. The gneisses and the host rocks have under-gone metamorphism essentially under the same con-ditions.Many authors have described the gneissic rocks as

cataclastic gneisses or blastomylonitic or myloniticaugen gneiss (Ishida and Ohta, 1973; Arita, 1983).This textural interpretation of the nature of thegneisses helped to promote the idea of thrust contactsbetween the gneisses and the country rock. It is truethat the Lesser Himalayan augen gneisses in generalare characteristically strongly foliated and show astreaky appearance as a result of a mineral lineationobservable on foliation planes. The individual grainsof mica (0.10.2 mm) are hardly identifiable in handspecimen. The streaks are produced by micas, es-pecially black mica. Other minerals such as quartz andfeldspars too show stretching. This texture is so perva-sive that it is found throughout the over 2 km thickgneiss section and is not limited to the contact zone. Iinterpret this texture of the augen gneiss to be the

result of strong regional deformation of the protolith,which probably consisted of either granite or siliceousvolcanic rocks, at a low temperature, around 300 to3508C, but not at a temperature dierent from thatobtaining during deformation of the protolith of thesurrounding country rocks. Because of the dieringphysical properties of the protoliths of the gneissesand the county rocks, dierent kinds of textures wereproduced by the same degree of deformation.Therefore, at least in Nepal, the contact between the

Lesser Himalayan augen gneiss and the host rock isnot marked by a regional thrust the MCT I of Arita(1983) lying at the base of the Ulleri augen gneiss orthe Phaplu Thrust of Maruo and Kizaki (1983).However, it may be pointed out that in places theexistence of faults of local significance cannot be ruledout.

9.4. The Lower Boundary of the MCT zone and theexistence of MCT I

Japanese researchers (Hashimoto et al., 1973; Arita,1983) have placed the MCT I or the Lower MCT notonly at the base of the Lesser Himalayan augengneisses but also close to the upper units of the LessserHimalayan metasediments where the gneisses are notdeveloped. This thrust is placed at a level where thelow grade metasedimentary rocks of the LesserHimalaya are supposed to give way to higher grademetamorphic rocks. A break in metamorphism hasbeen envisaged at this level. Fuchs and Frank (1970)also placed a thrust at the base of their LowerCrystalline Nappe in a similar position to that postu-lated by the Japanese researchers.Field observations in several sections in central and

western Nepal indicate that no sharp metamorphicbreak exists (Le Fort, 1975a,b; Pecher, 1975, 1989;Pecher and Le Fort, 1986; Schelling, 1992; Rai et al.,1998) at the place where the MCT I has been placed.In all these sections, the upper part of the metasedi-ments of the Lesser Himalaya shows a gradual increasein metamorphism in an upward direction towards thebase of the Higher Himalayan Zone. Rocks at chloritegrade in the lower part of the Lesser Himalayan Zonegradually pass upward to garnet staurolite and rarelyto kyanite grade at the base of the Higher HimalayanZone. This is the well-known inverted metamorphismof the Himalaya. For example: in western Nepal,along the Marsyangdi River section the KunchaFormation starts showing developments of biotite andgarnet south of Besisahar. Biotites and garnets startappearing almost imperceptibly and become more pro-minent and larger as one approaches to the MCT.Near Khudi, the garnets enlarge to as big as severalmm in diameter and become very prominent. No realbreak in metamorphism can be observed along the sec-


tion. Similar observations can be made in other sec-tions such as along SunkosiBhotekosi River on theKathmanduKodari Highway, in the Trisuli River, theBurhi Gandaki, the Kali Gandaki, the Modi Kholaand elsewhere. These gradational changes make it verydicult to accept the existence of the MCT I or theLower MCT of Japanese workers and Fuchs andFrank (1970).However, apart from the boundary thrusts, there

exist many thrusts within the major tectonic zones ofthe Himalaya. Macfarlane et al. (1992) have recognisednot one but a number of brittle thrusts in the MCTzone in the Langtang area of central Nepal. They aregenerally of local importance and do not extendthroughout the length of the orogen. I would thereforestress that, though there may be locally importantthrusts at the levels of an MCT I or within the MCTzone, they are not regionally significant thrusts com-parable to the MCT or the MBT. This is contrary tothe interpretations given by Arita (1983) and Fuchsand Frank (1970).The use of the name of MCT for proliferating

thrusts at many levels whether within the Lesser orHigher Himalayan zones renders the meaning of theterm MCT vague and diuse. I therefore restrict theuse of the term only to the intracrustal boundarythrust recognised by the French workers that separatesthe distinct Higher Himalayan crystalline rocks, theTibetan Slab of some authors, from the LesserHimalayan rocks.Recently Harrison et al. (1998, and this issue) have

proposed a new model for the origin of the invertedmetamorphism and the anatexis that gave rise to thehigher Himalayan and north Himalayan granites. Theyexplain the inverted metamorphism in the MCT zoneas a result of the juxtaposition of the two normalmetamorphic sequences by out-of-sequence thrusting.They consider that a 69 Ma old out-of-sequencethrust, equivalent to the MCT I of Arita, developedon the Lesser Himalayan unit which was metamor-phosed to the biotite-garnet grade during the eohima-layan metamorphism. These rocks were thus welded tothe Higher Himalayan crystalline rocks giving rise tothe apparent inverted metamorphic zonation.Although the model better explains the origin of thetwo contrasting granite belts delineating a thrust in thefield is very dicult. Moreover, the postulated thrustdoes not seem to coincide with the visible metamorphicbreak.

9.5. Age of MCT thrusting

Constraining the ages of both the initiation of theMCT and the cessation of its activity has been a di-cult task. Though in most cases the MCT is seen tooverthrust rocks no younger than the Eocene (Le Fort,

1996), Sakai et al. (this issue) reports overthrusting ofthe Dumri Formation (Late Eocene to Early Miocenein age) by the MCT. Part of the metamorphism of theDumri Formation (2617 Ma) occurring close to theMCT is interpreted to have resulted from heat trans-ferred from the crystalline rocks of the HigherHimalayan Zone brought over these rocks by theMCT. This constrains the upper age limit of the MCT.Parrish and Hodges (1993) put the MCT initiation incentral Nepal at after 22 Ma. In the Everest sectionmovement on the MCT was already active at 21 Ma(Hubbard and Harrison, 1989). Broadly speaking, theMCT may have been active between 25 and 15 Ma(Copeland et al., 1991; Harrison et al., 1998) andHarrison et al. (1998, this issue) indicate that the MCTwas reactivated during 68 Ma.

9.6. The Lesser Himalayan crystalline nappes and theirroot zones

A large number of crystalline nappes and klippenhave been recognised throughout the Himalaya (inIndia: Auden, 1935; Heim and Gansser, 1939; Gansser,1964; Valdiya, 1981, 1988; Thakur, 1981; Sinha, 1989;in Nepal: Hagen, 1969; Le Fort, 1975a,b; Brunel,1975; Fuchs, 1977; Stocklin, 1980; Arita et al., 1984;Schelling, 1992; Upreti and Le Fort, 1999).In Nepal the following crystalline nappes have been

recognized: 1. Kathmandu nappe, 2. Jajarkot nappe, 3.Dadeldhura nappe, 4. Parchauni nappe (north ofBaitadi), and 5. Karnali nappe (Upreti and Le Fort,1999, Fig. 2). In addition to these nappes, there is aflat-lying large thrust sheet of crystalline rocks in east-ern Nepal that covers most of eastern Nepal. Thatthrust reaches close to the MBT in the south.The crystalline nappes of the Lesser Himalaya can

be divided into two groups depending upon the strati-graphy and the metamorphism of their contained rockunits: 1. Nappes composed of upper amphibolite togranulite facies rocks, similar to the rocks of HigherHimalayan Zone or Tibetan Slab. 2. Nappes of theBhimphedi Group with Lower Paleozoic cover. TheBhimphedi Group (Stocklin, 1980) is composed of lowto medium grade metamorphic rocks such as biotite-garnet schists and marbles. The stratigraphy and themetamorphic grades of rocks in these two groups ofnappes are quite distinct and dierent.

1. Nappes with high grade rocks: The Karnali nappebelongs to this category (Figs. 2 and 7). This nappeoutcrops along the Karnali and Tila rivers and cov-ers an area of over 5,000 sq. km. The rock units ofthis nappe consist of kyanite-sillimanite bearinggneisses, calc-silicate gneiss, migmatitic gneiss andaugen gneiss (Hayashi et al., 1984). These rocksshow very close similarity to the rock units of the


Higher Himalayan Zone. Near Jumla, the Karnali

nappe is separated from its root zone by only a few

km of intervening Lesser Himalayan rocks (Fig. 2).

Therefore, the root zone of this nappe is very clearly

located in the Higher Himalayan Zone or Tibetan

Slab. At the core of the Karnali nappe lies a 1500 m

thick carbonate sequence composed of calcareous

biotite schist with psamitic and pelitic schist. The

rocks in the core carbonate sequence show typical

brick-red weathering products similar to those com-

monly seen in the Annapurna (Dhaulagiri)

Limestone of the Tibetan-Tethyan Sedimentary

Series. This core zone may thus represent the basal

part of the Tibetan-Tethys sedimentary series

(Hayashi et al., 1984).

The large thrust sheet covering most of eastern

Nepal has been very well mapped (Schelling, 1992,

Fig. 1). The deep erosion of this sheet has produced

large tectonic windows along the major rivers: the

Dudh Kosi, the Arun and the Tamor (Fig. 2).

Lesser Himalayan low-grade metamorphic rocks

such as phyllites, slates and limestones are exposed

in these windows. As the crystalline thrust sheet

dips at a very low angle to the north, the base of

the unit is exposed only in the river gorges. Because

the thrust sheets can be traced northward continu-

ously from their frontal region to their root zone in

the Higher Himalayan zone and because the rocks

are same in both areas (kyanite+sillimanite bearing

gneisses, schists and quartzites), the problem of link-

ing a root zone to frontal nappes does not arise.

2. Nappes of the Bhimphedi Group: Rocks of the

Kathmandu nappe, the Jajarkot nappe, the

Dadeldhura nappe and the Parchauni nappe belong

to the Bhimphedi Group or equivalent (Stocklin,

1980; Shrestha et al., 1993, Fig. 8). The Bhimphedi

Group has been described by Stocklin and

Bhattarai (1977) and Stocklin (1980) from central

Nepal and it also forms the base of the Kathmandu

nappe. This nearly 8 km thick rock sequence (Fig.

Fig. 8. Stratigraphic columns of the Lesser Himalayan nappes (nappes of Bhimphedi Group with Paleozoic cover).


8) consists of formations with various lithologiesmetamorphosed to biotite-garnet grade and onlylocally reaching as high as kyanite grade, e.g. inKathmandu nappe. The biotite-garnet bearing rocksof the Bhimphedi group have been thrust over verylow grade to unmetamorphosed Lesser Himalayanrocks. The contacts in most cases are sharp. Thegrade of metamorphism decreases gradually towardsthe top giving way to a sequence of fossiliferousrocks. The Kathmandu, Jajarkot and Dadeldhuranappes contain unmetamorphosed sedimentaryrocks at the top of their sections (Fuchs and Frank,1970; Stocklin, 1980; Sharma et al., 1984; Shresthaet al., 1993; Upreti, 1996, Fig. 8). In theKathmandu nappe these rocks are described asPhulchauki Group (Fig. 6) and make up a 56 kmthick unit of Early Middle Paleozoic age. In theJajarkot nappe, Fuchs and Frank (1970) havereported crinoids in the carbonate rocks at the topof the section and have correlated them with theChandragiri Limestone of the Phulchauki Groupand the Annapurna (Dhaulagiri) Limestone of theTibetan Sedimentary Series. No good description isavailable of the young unmetamorphosed rocks ofthe Dadeldhura nappe.

9.7. Root zone of the nappes of Bhimphedi Group

Two interpretations have been proposed.

1. The root zone is placed in the Higher Himalayanzone (in Kumaon, India: Heim and Gansser, 1939,in Nepal: Hagen, 1969; Le Fort, 1975a,b; Stocklin,1980). This interpretation is based on the obser-vations (a) that the rock units in both the nappesand the root zones are crystalline, although showinga dierence in grade of metamorphism and (b) thatno rocks exposed between the nappes and theHigher Himalayan crystalline rocks are comparableto the rocks within the nappes. The presence of theLower Paleozoic fossiliferous rocks at the top of thecrystalline rock sections exposed in the nappes hasbeen taken as further support for this interpretation.The sedimentary rocks of Tethyan anity at the topof the crystalline sections in both the postulatedroot zone and in the nappes is compatible with theidea that the rock packages in both areas belong tothe same tectonic unit.

2. The root is placed at the MCT zone (In Kumaon,India: Valdiya, 1980 and Srivastava and Mitra,1994; In Nepal: Fuchs and Frank, 1970 and Aritaet al., 1984). The rock packages in the MCT zonei.e. the area lying between the MCT I (equivilant tothe Munsiari Thrust of Valdiya) and MCT II(equivalent to the Vaikrita Thrust of Valdiya) have

comparable metamorphic grades with the rocks ofthe Bhimphedi Group (Munshiari Group ofValdiya, 1980). That similarity has prompted identi-fication of the MCT zone as the root zone for thenappes.

Upreti and Le Fort (1999) have adopted a dierent in-terpretation pointing out that the roots of the nappesare not exposed in the north. Neither the rocks of theMCT zone nor those of the Higher Himalayan Zonecan be considered as the root zones of the nappes ofBhimphedi Group. The arguments put forward infavour of this view may be summarised as follows:

1. The rocks of MCT zone belong to the upper LesserHimalayan unit (Upper Nawakot Group (Le Fort,1975a,b; Pecher and Le Fort, 1986). They showhigher grade metamorphism because of their proxi-mity to the MCT along which the hot slab of theHigher Himalayan Zone rocks were thrust over thecold Lesser Himalayan rocks and produced theinverted metamorphioc gradient (Le Fort, 1975). Infact, in most sections, the stratigraphic sequenceand the typical sedimentary features of these rocksare identical with their southern counterparts. Thenappes on the other hand, do not contain rockswith lithology and stratigraphy similar to that ofthe rocks the MCT zone. The thickness of the rocksin the nappes (over 13 km in the Kathmandunappe) far exceeds the thickness in the root zone.

2. The metamorphism in the MCT zone is clearlyinverted. In the nappes, on the other hand, themetamorphism is quite normal. This can not beexplained if the MCT zone forms the root to thenappes.

3. The nappes have clearly preserved the younger sedi-mentary sequences (Lower Paleozoic in age) at theirtops with well recognised fossils, such as those inthe Phulchauki Group in Kathmandu nappe(Stocklin, 1980) and in the Jaljala Fomation inJajarkot nappe (Fuchs and Frank, 1970; Sharma etal., 1984; Upreti, 1996, Fig. 8). No fossiliferousrocks are recognised of the MCT zone. One mayargue that the fossils have been obliterated in strongdeformation and metamorphism but on the basis ofthe well-preserved sedimentary features in rocks ofthe MCT zone, this argument is not tenable.Moreover, the stratigraphic sequences of the sedi-mentary rocks that are found in the nappes are nowhere discernible in the MCT zone.

4. No large granitic bodies are seen within the MCTzone that are comparable to those found in thenappes such as the several large cordierite-bearingtwo mica granites of the Kathmandu nappes,Dadeldhura nappe and granites in Almora nappe inKumaon (Gansser, 1964).

5. Crystalline rocks of the the Higher Himalayan Zone


Fig. 9. Northsouth geological crosssection through the Nepal Himalaya showing positions of the Lesser Himalayan nappes and their root zones

(redrawn after Upreti and Le Fort, 1999). (a) Section through eastern Nepal (Everest section). (b) Secton through LangtangKathmandu. The

MCT seperates the Gosainkund nappe and the Kathmandu nappe. (c) Section through western Nepal. The rocks of Karnali nappe shows its

root in the Higher Himalayan zones. Legend: 1. Tibetan-Tethys sediments, 2. Phulchauki Group, 3. Higher Himalayan crystallines, 4. Bhimphedi

Group, 5. Lesser Himalayan rocks (Nawakot Complex), 6. Higher Himalayan leucogranites. Lesser Himalayan granites (within Bhimphedi

Group), 8. Churia Group (Siwalik equivalents).


cannot be considered as the root zone of theBhimphedi Group nappes for the simple reason thatthere is a distinct dierence in the grade of meta-morphism between the rocks of the regions. Therocks in the Higher Himalayan Zone were metamor-phosed at upper amphibolite to granulite facies con-ditions under 8902260 MPa and 7542408C, whilethe rocks of the nappes of the Bhimphedi Groupwere metamorphosed at 755251 MPa and5122228C (Rai et al., 1998). It may be argued thatthe nappes may represent a far-travelled frontal andshallow part of the Higher Himalayan Zone crystal-line rocks that could be expected to show a meta-morphic grade lower than its northern counter part.But this argument becomes weak when one finds, ineastern Nepal, that the Higher Himalayan Zonecrystalline rocks reach almost as far as the MBTwithout showing any change in their grade of meta-morphism. The Karnali nappe, which is composedof crystalline rocks of the Higher Himalayan Zone,does not show lower metamorphic grades. In manycases the rock units of the Bhimphedi Group andthe Higher Himalayan Zone occur side by side. It isquite unlikely that the same rock units will exhibitsuch contrasting behavior in neighboring areas.

Based on the above arguments, Upreti and Le Fort(1999) divided the large Kathmandu nappe of earlierworkers (Le Fort, 1975a,b; Stocklin, 1980) into a smal-ler Kathmandu nappe in the south and a newly definedGosainkund nappe in the north. The two nappes areseparated by the MCT which is placed at the northernedge of the Kathmandu valley (Fig. 9b). The southernboundary of the Kathmandu nappe is placed along theMahabharat Thrust (MT) which was previously con-sidered to be the southward continuation of the MCTwhere it formed the base of the Kathmandu Nappe(Stocklin, 1980). The newly defined Kathmandu nappeis made up of rocks of the Bhimphedi Group withLower Paleozoic cover, the Phulchauki Group(Stocklin, 1980). To the north, the Gosainkund nappeis represented by the high-grade metamorphic rocks ofthe Higher Himalayan Zone. Among the LesserHimalayan nappes, the Kathmandu nappe has a pos-ition that is unique in the entire Himalayan range.Here a nappe of the Bhimphedi group has come intocontact with a nappe made up of Higher Himalayanzone crystalline rocks along the MCT. The boundarybetween the two nappes is complex because of thedevelopment of a long and narrow zone of pegmatitein the northern hills of the Kathmandu Valley (Rai,1998; Rai et al., 1998) that obscures the position of theMCT plane. Copeland et al. (1997) have carried outAr/Ar dating on a large number of muscovite mineralsacross the Kathmandu and the Gosainkund nappes.The results show that there is a gradual decrease in the

age of the muscovites from south to the north. Theolder ages obtained from the Kathmandu nappe indi-cate that the rocks making up this nappe must havebeen exhumed earlier than those of the Gosainkundnappe. A U/Pb age obtained on a mineral from one ofthe pegmatites that mark the the boundary betweenthe two nappes indicates that ductile movement alongthe MCT ended 025 Ma. Movement on the MCT hassince been transferred to the level of MT to the south(Rai et al., 1998).

9.8. The lost roots of the nappes

The foregoing discussion suggests that neither theMCT zone nor the Higher Himalayan Zone crystallinerocks form the root zone of the nappes of theBhimphedi Group. There is no possibility either forthese nappes to have roots farther north. The questiontherefore arises: Where are the roots of the outlyingnappes that contain rocks of the Bhimphedi Group?Upreti and Le Fort (1999) suggested that the rootzones of these nappes are not exposed anywhere in thenorth. The rocks are truly exotic (Fig. 9a,c). The ques-tion also arises: Under what conditions were the rocksof the Bhimphedi Group metamorphosed to a gradeintermediate between that of the crystalline rocks ofthe Higher Himalayan Zone and that of the LesserHimalayan Zone sedimentary rocks?In answer to this question Upreti and Le Fort

(1999) further suggested that the rocks of theBhimphedi Group originally belonged to an intermedi-ate zone between the location of deposition of the sedi-mentary units deposited in the north which, have thePhanerozoic Tethyan sedimentary sequence at theirtop,and the largely Precambrian sedimentary unitswhich outcrop in the Lesser Himalaya farther southwhich were originally deposited closer to the continent.The stacking of nappes from the Indus Suture zoneduring the Himalayan collision has produced theupper amphibolite to granulite facies metamorphism ofthe northern unit (eohimalayan metamorphism, LeFort, 1996). Similar tectonics farther south, with areduced overburden, may have given rise to the loweramphibolite facies metamorphism of the basal part ofthe newly recognised intermediate zone. TheBhimphedi Group rocks of that zone have at their topoverlying Paleozoic rocks that show little metamorph-ism. Harrison et al. (1998, and this issue) proposedsimilar eohimalayan metamorphism of the rocks at thenorthern edge of the Lesser Himalayan unit. The rockslying farther south in the present day LesserHimalayan domain were possibly unaected by thiseohi

An overview of the stratigraphy and tectonics of the Nepal Himalaya

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