A structural analysis of the Main Central Thrust zone, , central Himalaya

A. M. MACF ARLANE* 1 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, K. V. HODGES J Massachusetts 02139 D. LUX Department of Geological Sciences, University of Maine at Orono, Orono, Maine 04469

ABSTRACT within the MCT zone may have initiated as orogen has experienced multiple regional meta- the MCT zone was transported over a ramp morphic events (see Hodges and others, 1988, The Main Central Thrust (MCT) is one of in the MBT. and Pêcher, 1989, for reviews), but the latest the most tectonically significant structures in and most widespread in the central Himalaya the Himalayan orogen. Detailed geologic INTRODUCTION was a medium- to low-pressure amphibolite- mapping and structural analysis of the MCT facies episode that included anatectic melting re- in the Langtang National Park region of cen- The Main Central Thrust (MCT) is a major sponsible for leucogranite plutons like the tral Nepal reveals that this segment of the north-dipping fault zone in the Himalayan oro- Manaslu granite of central Nepal (Le Fort, fault zone experienced multiple episodes of gen that is estimated to have accommodated at 1975). Reliable U-Pb zircon and monazite crys- south-directed movement, under both brittle least 100 km of crustal shortening in Tertiary tallization ages for the leucogranites in the cen- and ductile conditions, during the Tertiary time (Gansser, 1966; Brunei, 1975; Pêcher, tral Himalaya range from late Oligocene to period. Early (mid-Miocene) movement re- 1978). One of the most controversial issues as- Miocene (Schârer, 1984; Schârer and others, sulted in the development of mylonitic fabrics sociated with the MCT is its temporal relation- 1986; Deniel and others, 1987; Copeland and synchronous with amphibolite-facies meta- ship with regional metamorphism. In the Garh- others, 1988; Parrish, 1992). On the basis of morphism. The mean orientation of the wal Himalaya of India (Fig. 1), the MCT geochronologic data for the latest metamorphic dominant mylonitic foliation is N28°W, corresponds to a sharp discontinuity between event from the Everest region, Hubbard and 38°NE. An associated mineral/stretching lin- middle to upper amphibolite-facies rocks to the Harrison (1989) established that some move- eation plunges 40° to N40°E. Kinematic indi- north and greenschist-facies units to the south ment on the MCT occurred at approximately 21 cators suggest hanging-wall movement to the (Heim and Gansser, 1939; Gansser, 1964; Val- Ma. In contrast, if it is assumed that most of the southwest relative to the footwall along the diya, 1979; Valdiya, 1980; Hodges and Silver- movement on the MCT was post-metamorphic north-dipping fault. It is not possible to con- berg, 1988). In the Manaslu region of central (Gansser, 1964; Valdiya, 1979; Valdiya, 1980; strain the magnitude of high-temperature dis- Nepal (Fig. 1 ), the MCT has been described as a Brunei, 1986; Brunei and Kienast, 1986; Schell- placement on the MCT at the longitude of ductile shear zone across which there is no sig- ing, 1987), then the implication is that this Langtang. Late-stage structures in the MCT nificant metamorphic break (Bordet, 1961; Le faulting occurred more recently than mid- zone at Langtang include a series of imbri- Fort, 1975; Bouchez and Pêcher, 1976; Pêcher, Miocene time. cate, brittle thrust faults that separate differ- 1977). More-recent studies have produced evi- There are three possible explanations for dis- ent lithostratigraphic units and correspond to dence for syn-metamorphic thrusting in the agreements among Himalayan geologists about metamorphic discontinuities. We interpret Everest region of eastern Nepal (Hubbard, the relative ages of metamorphism and thrusting this fault system as a duplex structure. Mus- 1988; Hubbard, 1989; Fig. 1), post-metamor- 40 39 on the MCT. First, it is possible that the MCT covite Ar/ Ar cooling ages from the MCT phic thrusting in the Makalu region of eastern has become a generic term for any fault zone zone range from 8.9-6.9 Ma. Because the Nepal (Brunei and Kienast, 1986; Fig. 1), and that marks that physiographic break between nominal closure temperature of Ar diffusion diachronous movement spanning regional meta- the higher Himalaya and the lower Himalaya, in muscovite (approximately 62S K) is higher morphic events in the Rowaling region of east- such that the MCT in one region may have a than the apparent temperature conditions central Nepal (Schelling, 1987; Fig. 1) and the different age than the MCT in another region under which late brittle deformation oc- Annapurna region of central Nepal (Caby and (Hodges and others, 1988; Hodges and others, curred, we suggest that brittle deformation others, 1983; Fig. 1). 1989). Second, the MCT may be the same struc- was a latest Miocene-Pliocene phenomenon. Inconsistencies in the tectonic histories of dif- ture in all areas, but the age and/or southward Another major Himalayan fault, the Main ferent parts of the orogen, such as Garhwal and extent of regional metamorphism may vary Boundary Thrust (MBT), was developing to central Nepal, have led to disagreements about along the range. Third, the relationship between the south of Langtang at approximately the the age of the MCT. If it is assumed that move- faulting and metamorphism may be difficult to same time. We speculate that brittle faulting ments on the MCT are syn-metamorphic, then it establish with certainty in some areas due to follows that geochronologic measurements of poor exposure. In many parts of the central the age of metamorphism in the hanging wall Himalaya, the MCT lies in thickly wooded or *Present address: Center for Materials Research in Archaeology and Ethnology, Massachusetts Institute provide constraints on the absolute age of fault- extensively cultivated terrain where outcrops are of Technology, Cambridge, Massachusetts 02139. ing. The metamorphic core of the Himalayan relatively sparse. This is not the case in the Lang-

Geological Society of America Bulletin, v. 104, p. 1389-1402, 14 figs., 6 tables, November 1992.

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^SJ \ Garhwal N \ T region • 30°-j

100 km

Annapurna'y region Langtang National Manaslu\^ Park (Fig. 2) region V Everest ' Kathmandu 1 Everest

Tibetan Sedimentary Series

Greater Himalayan Sequence

; : ; |Lesser Himalayan Sequence

[subhimalayan Sequence

85° 88°

Figure 1. General geologic map of Nepal. Location map of south Asia is in box on top right. The Main Central Thrust (MCT) is the barbed thrust contact between the Greater Himalayan sequence and the Lesser Himalayan sequence. The Main Boundary Thrust (MBT) is the barbed thrust contact between the Lesser Himalayan sequence and the Subhimalayan sequence.

tang National Park region of central Nepal (Fig. sediments of Cambro-Ordovician to Eocene (?) Himalayan sequence and the Lesser Himalayan 1), where extensive ridgetop exposures and a age. The base of the Tibetan Sedimentary Se- sequence is the Main Central Thrust. As mapped remarkable recent roadcut provide an unusual quence is marked by a family of Miocene, post- by various workers (for example, Gansser, 1964; opportunity for a detailed study of the MCT. metamorphic normal faults collectively referred Le Fort, 1975; Brunei, 1975), the MCT zone Our investigation shows that the MCT in the to as the South Tibetan detachment system ranges in thickness from <50 m to > 10 km and Langtang area had a complex movement history (Burchfiel and others, 1992; Burg and others, dips moderately northward. The MCT zone is involving at least two distinct periods of defor- 1984; Burchfiel and Roy den, 1985; Herren, characterized by well-developed mylonitic fab- mation, with initial thrusting during Miocene 1987). The underlying Greater Himalayan se- rics that indicate hanging-wall movement to regional metamorphism followed by a major quence consists primarily of amphibolite-facies the south relative to the footwall (Pêcher, 1978; displacement after approximately 9-7 Ma. gneisses and schists of Precambrian to Cambrian Brunei, 1986). The Main Boundary Thrust (?) age that are locally migmatitic. Large leu- forms the lower contact of the Lesser Himalayan TECTONIC SETTING cogranite plutons, commonly containing tour- sequence. It is an active north-dipping structure maline, occur near the top of the Greater that carries the Lesser Himalayan sequence over From western India to eastern Nepal, the core Himalayan sequence. The Greater Himalayan unmetamorphosed molasse units of the Sub- of the Himalaya is composed of three major sequence rocks overlie the Lesser Himalayan himalayan sequence (Molnar, 1984). tectonostratigraphic units separated by north- sequence (the "Midlands Formations" of Le Langtang National Park is located approxi- dipping structural discontinuities (Fig. 1). North- Fort, 1975), a package of upper Precambrian (?) mately 50 km north of Kathmandu, at approxi- ernmost and structurally highest is the Tibetan to lower Eocene (?), greenschist- to lower- mately 28°10'N and 85°25'E. Figure 2 is a Sedimentary Sequence, dominated by a series of amphibolite-facies, metasedimentary and meta- geologic map of part of this area, based on shallow-marine continental and miogeoclinal volcanic rocks. The contact between the Greater 1:25,000 mapping conducted in 1989 and 1990

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presence of syn- to post-kinematic porphyro- blasts in these rocks. Quantitative thermoba- rometry provided estimates of the conditions of M2 in Lesser Himalayan sequence rocks of 767-787 K and 626-665 MPa; in lower MCT zone rocks, 700-785 K and 510-634 MPa; in upper MCT rocks, 794-849 K and 522-899 MPa (Macfarlane, 1992).

TECTONIC STRATIGRAPHY

In the Langtang region, the Greater Hima- layan sequence is characterized by pelitic gneisses and migmatites, with thin units of am- phibolite and pyroxene gneisses in the upper- most part of the section. The Lesser Himalayan sequence consists primarily of pelitic schists (commonly containing folded quartz lenses), with less-abundant calcareous metapsammites, siliceous metalimestones, and amphibolites. The Lesser Himalayan sequence is separated from the Greater Himalayan sequence by a 3.7-km- wide MCT zone. The MCT zone is character- ized by slices of rocks of both Greater Hima- layan sequence and Lesser Himalayan sequence affinity that were juxtaposed by brittle faults. The alternation of lithologies in this zone is in stark contrast to the general lithologic monotony of the coherent Greater and Lesser Himalayan sequences. Lower units within the MCT zone have affinities with the Lesser Himalayan se- quence, and upper units within the zone are as- sociated with the Greater Himalayan sequence. Assignment of these units to the Lesser or the Greater Himalayan sequences was based primar- Figure 2. Geologic map of the MCT zone in the Langtang region. Units are patterned ily on lithology, mineralogy, and petrologic according to Figure 3. All heavy barbed lines are post-metamorphic brittle thrust faults. A-A' analysis (Macfarlane, unpub. data). In order to is the line of section for Figure 10. facilitate mapping, the Greater Himalayan, Lesser Himalayan, and MCT zones were subdi- vided into a number of different tectonostrati- along the Trisuli-Langtang river transect, the (Macfarlane, unpub. data), but a brief summary graphic units, which are described in Table 1. Trisuli-Chilime river transect, the Barabal trail will be useful in this paper. Evidence from mi- The unit nomenclature was devised by the au- transect, and the Gosainkunda ridge transect crofabric analysis and Gibb's Method modeling thors and is pertinent to the Langtang region. (Macfarlane, unpub. mapping). The MCT zone suggests that the core of the Himalaya in the was best exposed along a road in the western- Langtang region appears to have undergone two Two units in the Langtang region appear to most portion of the Trisuli-Langtang river tran- periods of metamorphism. The earlier meta- have correlatives in other areas of Nepal. The sect and the southernmost portion of the morphic event was high-pressure, moderate- Lesser Himalayan sequence is dominated by the Trisuli-Chilime river transect. In the northwest- temperature (Mj), and the later event was chlorite- to garnet-grade Dhunche schists and ern part of the area, the MCT zone and litho- moderate-pressure, high-temperature (M2). The metapsammites (Table 1; Figs. 2 and 3). This logic contacts in the hanging wall and footwall second event appears to be related to the em- unit is probably correlative with the Midlands strike generally east-west and dip northward, placement of two-mica leucogranites located in "formations" of Le Fort (1975) and the Ok- consistent with regional trends. In the central the upper part of the Greater Himalayan se- handhunga unit of Maruo and others (1979). In and southern parts of the Langtang area, how- quence. Mi is best displayed in the Lesser Hima- the MCT zone, the light color and large ever, the general strike of the units changes to layan sequence, where rotated garnet porphyro- K-feldspar augen of a mylonitized augen ortho- north-south with an eastward dip. This varia- blasts predate the dominant foliation. Kyanite gneiss, the Syabru Bensi gneiss, make it a dis- tion, interpreted below as the consequence of inclusions in garnet porphyroblasts in upper tinctive unit that may be correlative with the ramping subsequent to ductile deformation, MCT zone and Greater Himalayan sequence Ulleri augen gneiss of Le Fort (1975) and the provides an important perspective on the rocks are the only evidence of Mi in these units. Phaplu augen gneiss of Maruo and Kizaki geometry of thrust imbrication slices. Maximum temperatures associated with M2 ap- (1981). In central Nepal, the Ulleri augen gneiss The metamorphic history of the Langtang pear to have occurred during and after the latest is located in the middle of the Lower Midland area has been discussed in detail elsewhere ductile motion on the MCT, on the basis of the formations of the Lesser Himalayan sequence.

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TABLE 1. TECTONOSTRATIGRAPHIC DATA

Structural unit Unit name Lithology Mineral assemblage Grain size Thickness Weathering characteristics

Lesser Himalayan Dhunche schist Pelitic and graphitic schists, calcareous Pelitic schist: qz + bi + mu + pg Fine to 5=3,900 m Dark brown/black ledge-former sequence and metapsammite metaspammite, siliceous metalimestone, ± chi ± gt; calc. metapsammites: medium amphibolite qz + calc + mu ± bi ± pg ± chi >phl

Thangjet gneiss Ortho-augen gneiss qz + pg + mu + bi + sphene + apatite Fine «60 m Dark gray ledge-former

Goljhong unit Quartzite, marble, phyllite. pelitic schist, Quartzite: qz + bi + mu + pg + K-fsp Fine to carbonaceous schist Phyllite: calc + qz + mu medium «130 m White to gray slope-former

Gompagong calc- Calcareous psammite alternating with calc + qz +• phi + pg ± mu Fine to «320 m Dark gray ledge-former quartzite pelitic schist medium

MCT-Lesser Chili me calc-schist Calcareous graphitic schist with qz + mu + bi + chi + graphite Medium «170 m Dark gray slope-former Himalayan sequence interbeds of calcareous quartzite (5-10 m) affinities Trisuii gneiss and schist Pelitic schists and gneisses qz + bi + mu + pg + chi + g ± st ± ky Medium «245 m Dark gray ledge-former ±sill

Bhote marble Siliceous marble calc + qz +• phi + pg ± mu ± chi ± zois Medium «250 m Tan ledge-former ± clinozois

Barabal schist and gneiss Calcareous pelitic schists and gneisses qz + mu + bi + calc + chi + pg + clinozois Medium «15 m Dark gray ledge-former

Syabru Bensi gneiss Augen orthogneiss qz + pg + K-fsp + mu + bi Medium to «300 m Gray/brown ledge-former coarse

Phulung gneiss Pelitic augen gneiss qz + pg + bi + mu + gt + clinozois Medium «55 m Dark gray slope-former

Mangol quartzite "Dirty" quartzite qz + mu + gt Fine to «60 m Dark brown ledge-former medium

MCT-Greater Syabru gneiss Pelitic gneiss qz + pg + bi + mu + gl ± ky Coarse «2,100 m Gray/brown ledge-former Himalayan sequence affinities

Greater Himalyan Gosainkund gneiss Migmatitic pelitic gneiss qz +• pg + bi + mu ; gt ± ky Fine to 3,600 m Dark brown ledge-former sequence coarse

Note: qz, quartz; bi, biotite; mu, muscovite; pg, plagioclase; chi, chlorite; gt, garnet; ky, kyanite; sill, sillimanite; phi, phlogopite; calc, calcite; K-fsp, K-feldspar; st, staurolite; zois, zoisite; clinozois, clinozoisite.

This is not the situation in Langtang, where only Deformation in the Lesser Himalayan Zone characterized by a crenulation cleavage, S3LH>

two units having Lesser Himalayan sequence af- and a crenulation lineation, L3LH (Fig. 5D), that finities lie above the Syabru Bensi gneiss. It is Four deformational events have affected the were found only in pelitic lithologies. Meso-

possible that higher units were cut out along Lesser Himalayan sequence in the Langtang re- scopic-scale concentric folds (F3Lh) arc less faults within the MCT zone. gion (Table 2). In addition, a pre-deformation common than the crenulation cleavage. Finally,

compositional layering (S0LH) was identified in the D4LH event is marked by a few late brittle DEFORMATION HISTORY some of the more psammitic zones. The first faults, which strike to the northwest and dip deformation event in the Lesser Himalayan se- steeply to the northeast and are of indistinguish- The tectonic evolution of the Langtang region quence, DILH, predated development of the able movement sense. s is complex, involving at least two deformational dominant foliation in the rocks (S2LH)- ^ i events and two or more metamorphic events. To manifested only in thin section as snowball Deformation in the Main Central aid in the understanding of the sequence of these inclusion trails in large Mi garnets that predate Thrust Zone events, a specific notation for each event in each S2LH (Fig- 4A) and as foliation that is transposed major tectonostratigraphic unit will be used. The into the dominant foliation, S2LH- The second The MCT zone in the Langtang area is char- nomenclature used to discuss different character- event, D2LH> is the main ductile event in the acterized by five periods of deformation (Table istics of deformation will follow the traditional Lesser Himalayan sequence. Megascopically, it 3). Figure 4B shows the earliest foliation,

format of Dj, D2, D3 . . ., but the authors do is manifested by the dominant foliation, S2LH S1MCT, transposed into the main foliation, not intend to imply that all "deformation (Figs. 5A and 5B), which strikes to the north- S2MCT. which is parallel to the bottom of the

events" are completely separate in time and west and dips to the northeast, and an associated photograph. The S|MCT fabric, in addition to space and relate in some way to a significant stretching/mineral lineation, L2LH (Fig- 5C), a pre-S2MCT phase of isoclinal folding present tectonic event. DnLH will be used for deforma- that plunges gently and trends to the north- in the field, provides evidence for an early tional events (n = 1-5) in the Lesser Himalayan northeast. The dominant foliation, S2lh> is axial ductile event, DJMCT- Helicitic inclusions in M| sequence; DnMCT will be used for events in the planar to mesoscopic, F2LH concentric folds. garnet indicate that synkinematic growth of MCT zone, and DnGH will represent events in Smaller M2 garnets grew synkinematically with garnet occurred during this early deformation the Greater Himalayan sequence. Note that respect to S2LH- These garnets display asymmet- event (Fig. 4C). DILH is not necessarily correlative with D|(;iH, ric pressure shadows that indicate top-to-the- The main ductile event in the MCT zone is and so on. south sense of shear. The D3LH event is D2MCT- Megascopically, it produced the domi-

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GREATER HIMALAYAN main foliation runs parallel to the bottom of the gos- Gosainkund gneiss SEQUENCE page. Note that sillimanite is not aligned in the

foliation; therefore, it grew post-D2MCT- Sim- ilarly, kyanite also grew post-D2MCT- The Figure 3. Tectonostrati- E>3MCT event is represented by a crenulation graphic section of the MCT cleavage, S3MCT. and lineation, L3MCT (Fig. zone in the Langtang region. 6D). Some of the units within the MCT zone are Unit descriptions are in affected by a retrograde metamorphic event that Table 1. Widths of units are may have occurred during, or just after, the actual relative thicknesses D3MCT event. measured near the Trisuli The multiple faults in the map pattern of the sy Kosi. MCT zone (Fig. 2) result from imbrication of Syabru gneiss & schist lithostratigraphic units along post-metamorphic

brittle thrust faults associated with D4MCT de- formation. Figures 7A and 7B are photographs of these faults, which are subparallel to the main foliation. Drag folds that post-date the mylonitic fabrics and earlier folding events and are adja- cent to these faults suggest a reverse sense of

motion. The D4MCT brittle faults are 10-cm- to 10-m-wide zones composed of graphitic fault gouge (Figs. 7A and 7B). The gouge is reces- Mangol metapsammite sively weathered, structurally incoherent mate- Phulung augen gneiss rial. The brittle faults are generally subparallel to S2MCT but demonstrably cross-cut the fabric in sb* Syabru Bensi gneiss some places (Fig. 7A). The brittle faults strike northwest and dip approximately 35°NE. No Barabal schist & gneiss slickensides were observed. The D4MCT event Bhote carbonate juxtaposed units within the MCT zone, resulting in the present tectonostratigraphy. Brittle listric faulting, possibly of normal sense, constitutes the Trisuli gneiss & schist final phase of deformation, D5Mcr- Where the DSMCT faults were encountered, they strike Chilime calc-schist northwest and dip steeply to the southwest.

Only a few D5MCT faults were encountered. Gompagong calc-quartzite

Goljong Unit Deformation in the Greater Himalayan Zone Thangjet gneiss The Greater Himalayan sequence in the LESSER HIMALAYAN Dhunche schist Langtang region also experienced five deforma- SEQUENCE tional events (Table 4). Diqh produced meso-

scopic, F1GH tight to isoclinal folds that have been overprinted by the dominant foliation, nant foliation, S2MCT. and a stretching/mineral S2GH- During and after DKJH> there was growth was lineation, L2MCT. S2MCT strikes to the northwest of both M[ garnet and M| sillimanite. D2GH and dips to the northeast, similar to S2LH (Figs. the main ductile event in the Greater Himalayan

6A and 6B). L2MCT trends to the northeast, sub- parallel to L2LH (Fig. 6C). In some units (augen gneisses, quartzites, and marbles), mylonitic s-c TABLE 2. LESSER HIMALAYA fabrics developed during D2MCT (Fig- 4D). In the mylonitic quartzite lithologies (Gompagong unit and Mangol unit), sense of shear from mica D1 D2 D3 D4

fish indicate movement to the southwest (Fig. Deformation Folded Main schistosity, Crenulation Brittle faults style 4E). Asymmetric pressure shadows associated foliation, SLhi cleavage, SLH3 a with D2MCT lso give top-to-the-southwest Pressure shadows Crenulation lineation, 1^3

sense of movement. Staurolite grew synkinemat- Mineral/stretching n some lineation, L^h2 ically with respect to D2MCT > lithologies. Figure 4F shows a staurolite porphyroblast sur- Metamorphic relations Syn-kinematic Syn-kinematic garnet garnet rounded by a corona of M2 sillimanite. The

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sequence. The dominant foliation, S2GH. AND a plunges moderately to the northeast. In the the northeast. The vergence reversal is consistent

mineral/stretching lineation, L2GH> characterize lower part of the Greater Himalayan sequence, with contemporaneity between ductile thrusting

D2GH (Figs. 8A, 8B, and 8C). The dominant D2GH asymmetric pressure shadows suggest top- on the MCT and normal displacement on the foliation, S2GH> strikes toward the northwest to-the-southwest sense of shear, but in the upper structurally higher South Tibetan detachment

and dips toward the northeast, and L2QH 3-5 km of the section, the movement sense is to system (compare with Burchfiel and Royden,

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Figure 4. Photomicrographs of MCT zone microfabrics. (A) Lesser Himalaya pelitic schist of S2GH's indicative of D3GH- During this event, under plane light; garnet has "snowball" quartz inclusions (associated with an earlier deforma- intrusion of granite and pegmatite associated

tional event—Dim) and pressure shadows (associated with the main foliation—D2LH)- Bot- with M2 was probably still occurring. The D4GH tom of photo is 1.5 mm across; northeast is to the left side of photo. (B) MCT zone pelitic event is characterized by chlorite-grade 1- to 5- schist under crossed polars with main foliation (S2) diagonal, folding an earlier foliation (Si). mm-wide shear zones of indeterminate dis- Bottom of photo is l.S mm across. (C) MCT pelitic schist under plane light; helicitic inclusion placement sense. Brittle faulting of probably

trails in garnet are associated with S]MCT; pressure shadows associated with S2MCT- Garnet is small displacements occurred during the last de- 5 mm in diameter. (D) MCT mylonitic augen gneisses, outcrop view. (E) "Mica fish" in MCT formational event, D5GH, of unknown move- mylonitic quartzite, under plane light. Bottom of photo is 1.5 mm across; southwest is on left ment sense. side of photo. (F) MCT pelitic schist under plane light. Here prismatic sillimanite (si) forms a

corona around staurolite (st). Sillimanite is not aligned in the main foliation (S2MCT; parallel to Correlation among Deformation Events bottom of photo) and therefore grew after D2MCT- Bottom of photo is 1 mm across. Some correlation between deformational events in the three different tectonostratigraphic units is possible. Based on helicitic inclusion trails in M[ garnets from both the Lesser Hima-

layan sequence and MCT zone, DJLH and 1985; Burchfiel and others, 1992). At the bot- post-D2GH> based on the crosscutting relation- tom of the section, migmatitic horizons are elon- ship of the granites to foliation and the unfo- DIMCT are probably correlative events, al- gate subparallel to S2GH and are themselves liated appearance of the granites. During and though it is not clear whether this deformation is the result of a Himalayan or older event. If the foliated; thus, melting associated with M2 oc- after D2GH> both M2 garnet and M2 sillimanite curred during D2GH- The majority of the gran- grew syn- to post-kinematically. deformation is Himalayan in age, it may even ites near the top of the section intruded Open to closed meso- to macroscopic folding then be part of a single continuous phase of

Lesser Himalaya sequence

S2LH data, n = 53 Mean vector: 56, S40°W Lesser Himalaya sequence Great circle: N49°W, 34°NE 5>2LH contour interval: area

Lesser Himalaya sequence Lesser Himalaya sequence

L2LH data, n = 23 L3lh data, n = 34 Mean vector: 29, N6.5°E Mean vector: 41,N52°E

Figure 5. Lower-hemisphere, equal-area stereographic representations of fabric data from Lesser Himalayan sequence D2LH features; n is the

number of data points. (A) S2LH scatter plot with mean great circle. (B) S2LH contour plot; contour interval = 1% per % area. (C) L2LH mineral/stretching lineation plot. (D) L3LH crenulation lineation plot.

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MCT zone S2MCT data, n = 106 Mean vector: 52, S62°W MCT zone Great circle: N28°W, 38°NE S2MCT contour interval: 1%/% area

MCT zone LwMCT data, n = 29 Mean vector: 34.5, N55°E

Figure 6. Lower-hemisphere, equal-area stereographic representations of fabric data from Main Central Thrust zone D2MCT features; n is the

number of data points. (A) S2MCT scatter plot with mean great circle. (B) S2MCT contour plot; contour interval = 1% per % area. (C) L2MCT mineral/stretching lineation plot. (D) Ljmct crenulation lineation plot.

Tibet (Burchfiel and others, 1992). Figures 5, 6, ductile deformation that actually includes D], graphic units are probably correlative. This and 8 show stereonet plots of the main foliation

D2, and D3. It is not possible to ascertain event is interpreted to be associated with ductile in the Lesser Himalayan sequence, the MCT whether D^n is the same as the first events in movement on the MCT and the Giyrong de- zone, and the Greater Himalayan sequence. the Lesser Himalayan sequence and MCT zone. tachment, part of the South Tibetan detachment Note that the foliations for all three units are The D2 events within the three tectonostrati- system exposed north of Langtang in southern very similar. The stereonet plots of the mineral/ stretching lineations (Figs. 5, 6, and 8) show similarities between those from the MCT zone

TABLE 3. MAIN CENTRAL THRUST ZONE and the Greater Himalayan sequence. Although the Lesser Himalayan sequence L2 lineation data D1 D2 D3 D4 D5 are somewhat limited, the mean vector for the Lesser Himalayan sequence seems to be appre- Deformation Schistosity, Main schistosity, Crenulation Brittle faults/ Brittle listric style SMCTI SMCT2 cleavage, imbrication normal ciably different from that of the MCT zone. This SMCT3 of the MCT faults situation suggests that rotation on faults that zone ^MCTl 'sod'113' folds Mineral/stretching Crenulation were subparallel to the main foliation occurred lineation, LMCT2 lineation, LMCT3 between the Lesser Himalayan sequence and the MCT zone subsequent to the formation of the Pressure shadows principal macroscopic fabric associated with D2. Mylonitic fabrics The rotation may have been caused by the cren- Metamorphic Syn-kinematic garnet Syn-kinematic ulation cleavage event (D CT) or brittle relations garnet staurolite 3M imbrication of tectonostratigraphic units within Post-kinematic kyanite and the MCT zone (D4MCT). sillimanite The D3 event is characterized by develop-

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A B Figure 7. Photographs of the post-metamorphic brittle faults. (A) Fault zone here is approximately 5 cm wide and places the Thangjet gneiss unit (th) on top of the Dhunche schist (dh) of the Lesser Himalayan sequence; photo looking east. (B) fault zone (fz) here is made up of two discrete faults, indicated by the arrows, both approximately 10 cm wide. This fault zone places the Chilime unit (ch) on top of the

Gompagong unit (gom); photo is looking west. In both photos, the fault zones are subparallel to the main foliation S2 and are composed of graphitic gouge.

ment of a crenulation cleavage in both the later brittle. It appears that the ductile phase of south portion of the zone. The two regions Lesser Himalayan sequence and the MCT zone; deformation may have affected a larger area within the MCT zone yield two distinct groups thus, it is probably correlative in both sequences. than the brittle phase. The extent of crenulation of data on the stereonet. A girdle through the The crenulation cleavage may likely be part of a cleavage into the Lesser Himalayan sequence data results in an axis of 42° to N60°E. In addi- continuous sequence of deformation related to suggests that the upper portion of what is now tion, Figure 9B shows a distinction between the

D2 and possibly Dj, if Dj is of Himalayan age. defined as the Lesser Himalayan sequence was L2MCT stretching or mineral lineations for the The crenulation lineation in the MCT zone (Fig. affected by ductile motion on the MCT and two structural regions within the MCT zone. 6C) is subparallel to the stretching/mineral lin- therefore was part of the "ductile" MCT zone. Thus, it appears that bending of the MCT zone

eation in the MCT zone. This suggests that D3 This portion of the Lesser Himalayan sequence occurred some time after D2MCT- Because no

and D2 in the Lesser Himalayan sequence and was not affected by the later brittle deformation. mesoscopic evidence for another geometrically the MCT zone may be part of a continuum of Due to the fact that there is no metamorphic appropriate folding event exists, the bend is in- deformation. Again, it is difficult to determine break across the MCT zone in Langtang, most of terpreted as the result of lateral thrust ramping

whether D3GH was coeval with D3LH and the ductile movement on the MCT must have during D4. D-iMCT- Because D4LH brittle faults are of inde- occurred prior to or during the D2MCT event. Figure 10 is a cross section of the MCT zone terminate movement direction, D4LH may corre- In the Langtang area, the MCT zone changes in the Langtang region. Due to N60°E ramping or late with either D4MCT DSMCT - D5MCT and from the regional strike of west-northwest/east- and the average dip of 34°NE on the fault, the D5GH faults are also brittle and appeared to be southeast to north/south (Fig. 2). Figure 9A is a map pattern is essentially a cross section looking of normal sense and similar in style; thus, they stereonet plot of two groups of the main folia- down-dip on the fault. Down-plunge projections may have formed during the same event. D4GH. tion: those points that lie in the "bend" area of the area along the N60°E axis are reminiscent Dsgh, D5MCI . and D4m may all be correlative, of the MCT zone and those that lie in the north/ of a duplex structure (Boyer and Elliott, 1982), perhaps part of a single, continuous brittle de- formation event. TABLE 4. GREATER HIMALAYA

STRUCTURAL INTERPRETATION

Deformation F 1 Main schislosity. F 10 Chlorite-grade Brittle faulting GHI "S " GH3 OP™ It is importang to clarify the definition of the style shear zones to isoclinal closed folds MCT zone in the Langtang region. The base of folds the MCT zone in the Langtang area is defined Mineral/stretching Mortar texture in on the basis of the first appearance of a rapid lineation, L quartz fabric succession of different lithologic units. The top Metamorphic Syn- and post- Syn- and post- relations kinematic kinematic of the MCT zone is defined on the basis of the garnet and garnet and sillimanite sillimanite last appearance of crenulation cleavage and the growth first appearance of migmatites. In the Langtang Syn- and post- Himalaya, the MCT zone experienced two dis- kinematic migmatites tinct periods of movement: earlier ductile and

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Greater Himalaya sequence Greater Himalaya sequence Greater Himalaya sequence S2GH data, n = 117 S2GH contour interval area LJGH data, n = 64 Mean vector: 62, S39°W Mean vector: 36, N49°E Great circle: N52°W, 28°NE

Figure 8. Lower-hemisphere, equal-area stereographic representations of fabric data from Greater Himalayan D2GH features; n is the number

of data points. (A) S2GH scatter plot. (B) S2GH contour plot; contour interval = 1% per % area. (C) L2GH mineral/stretching lineation plot.

and we have adopted such an interpretation for (assuming the thickness listed earlier) above that Mitra, 1988). Roof fault height is inferred from the MCT zone at Langtang. In addition, there horizon, extending the unfolded length of the the size of horses in the down-plunge projection. are outcrop-scale duplexes related to the D4MCT horse, and using the difference between short- Cut-off angles of 15°-25° for the floor fault event faults in the field area. Depth to detach- ened and restored length as one side of a were determined assuming that the depth to de- ment for the cross section is 1,900 m and was rectangle of area equal to the excess determined tachment was 1,900 m and that bedding dips calculated by assuming an arbitrary horizon, earlier. The other side of the rectangle of excess were subparallel to the brittle faults (as seen in calculating the excess area for a particular horse area is the depth to detachment (Marshak and outcrop). Roof fault cut-off angles of 35°-45° are based on roof fault position and the assump- tion that compositional layering is subparallel to the faults.

AGE OF MAJOR THRUSTING EVENTS

On the basis of the structural and microfabric

data, at least some of the D2 ductile movement on the MCT in the Langtang area occurred syn- chronously with amphibolite-facies metamor- phism. U-Pb ages for monazite and zircon from both gneisses and leucogranites in the Greater Himalayan sequence of the Langtang region suggest that metamorphism occurred between 20 and 15 Ma (Parrish and others, 1992). These data are consistent with the conclusion of Hub- bard and Harrison (1989) that ductile motion on

N/S S2 foliations N/S L2 lineations the MCT in the Everest area was occurring at 20 "Bend" S2 foliations "Bend" L2 lineations Ma. The absolute age of D4 in the Langtang area Great circle: N25°W, 46°SW N/S mean vector: 31, N57°E is difficult to ascertain. Experimental deforma- Pole (fold axis): 44, N60°E "Bend" mean vector: 34, N24°E tion of quartz from granite suggests that brittle deformation at low pressure and low to moder- Figure 9. Lower-hemisphere, equal-area stereographic projection of data for MCT "bend" ate strain rates occurs below 573-673 K (Tullis and north-south regions of the MCT zone in the Langtang area; n is the number of data points. and Yund, 1977). These temperatures are lower than, or in the range of, the closure temperature (A) Plot of the foliation (S2) data. (B) Plot of the lineation (L2) data. On A, the data are interpreted to represent a girdle with a pole representing the fold axis of the MCT zone. for Ar in muscovite (625 K; McDougall and

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Harrison, 1988). To further constrain the age of

brittle deformation (D4MCT in particular) in the MCT zone, 40Ar/39Ar geochronology of mus- covites from MCT zone rocks was attempted. Three samples from the MCT zone were chosen for 40Ar/39Ar analysis: L51, an augen orthogneiss from the Syabru Bensi gneiss unit; L66, a pelitic gneiss from the Phulung augen gneiss unit; and L20a, a pelitic gneiss from the Syabru gneiss (Table 5). L51 and L66 have af- finities with the Lesser Himalayan sequence, whereas L20a has affinities with the Greater Himalayan sequence. Standard mineral-separa- tion procedures using heavy liquids and mag- netic separation were followed. A detailed discussion of analytical procedures can be found in Lux (1986). Samples were irradiated at the H5 reactor at the University of Michigan for 72 hours. An in-house flux monitor, SB-51, was used to determine the J-values. Plateaus were established by using the critical value test of Dalrymple and Lanphere (1969) to assess Figure 10. Cross section across the northern part of the MCT zone in the Langtang region; whether age increments were different. A York A-A' as in Figure 2. Unit names as in Figure 3. There is no vertical exaggeration. The MCT 2 least-squares regression technique was used in zone in Langtang is interpreted to be a duplex structure. All faults on this diagram are brittle determining isochron ages. All muscovites ana- and post-metamorphic. lyzed were estimated to be 99.9% pure. Musco- vite ranged in composition from 9.3 to 10.6% K2O. Plateau and total gas ages are given with two-sigma uncertainties; the J-value has a 0.25% uncertainty (Table 5; Figs. 11-13).

40 39 m The results from Ar/ Ar analysis are listed TABLE 5. h

lyzed. Sample L51 occupies the lowest structural L51m Syabru Bensi gneiss augen orthogneiss muscovite 306.7 * 6.8 8.86 - 0.16 none level within the MCT zone. Muscovite from L51 L66m Phulung augen gneiss pelitic augen gneiss muscovite 304.4 -4.1 8.49 i 0.16 8.34 ; 0.08 L20am Syabru gneiss pelitic gneiss muscovite 298.5 - 3.0 6.87 - 0.01 6.86 = 0.16 gave no plateau, but an isochron age of 8.86 ± 0.16 Ma was determined when the first and last increment points were dropped (Fig. 11). Mus- covite from L66 gives a plateau age of 8.34 + 0.08 Ma and an isochron age of 8.49 ± 0.16 Ma (Fig. 12). Sample L20a is structurally highest in the MCT zone. L20a muscovite gave a plateau age of 6.86 ±0.16 Ma and an isochron age of 6.87 ± 0.01 Ma (Fig. 13). Muscovite 40Ar/39Ar age data for samples

L51 Muscovite Intercept age = 8.86 ± 0.16 Ma 40Ar/36Ar = 307 ± 7 MSWD = 3

Figure 11. Isotope correlation plot for sample L51. All two-sigma errors are less than the diameter of the circles that represent the increments. The intercept age for this sample was calculated after the first and last increments were omitted. 36 Ar/40 Ar

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TABLE 6. ^Ar/^Ar DATA DISCUSSION

Temp. (K) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar Moles 39Ar 39Ar 39Ar Apparent age 3 14 (x IO ) (* IO ) (S total) (?) K/Ca (Ma) In the Langtang region of central Nepal, there is abundant evidence for significant ductile and L5!-m J = .006232 brittle movement on the MCT. In the Annapur-

923 18.830 0.0073 0.056 70.4 0.6 12.1 66.80 25.35 i 5.31 na/Manaslu region to the west and the Everest 1.023 5.648 0.0020 0.0155 211.2 1.8 18.3 244.79 11.61 ±0.77 area to the east of Langtang, ductile movement 1.093 2.623 0.0022 0.0060 361.6 3.1 31.3 220.59 9.19 ±0.24 1.153 1.624 0.0003 0.0026 857.6 7.5 49.5 1,549.55 9.01 =0.17 appears to have been dominant, with little or no 1.213 1.351 0.0008 0.0017 1,555.2 13.5 60.7 594.00 9.20 ± 0.15 1.273 1.177 0.0008 0.0012 2.116.8 18.4 69.1 628.47 9.12 ± 0.16 evidence for brittle deformation described (Le 1.343 1.088 0.0006 0.0009 2,841.6 24.7 73.2 796.78 8.93 ± 0.13 Fort, 1975; Pêcher, 1978; Le Fort, 1981; Pêcher FUSE 1.070 0.0026 0.0007 3,472.0 30.2 79.2 188.32 9.50 s 0.13 and Le Fort, 1986; Hubbard, 1988; Hubbard,

L66-m J = .006104 1989). Thus, on the scale of approximately 90 km, the distance from Manaslu to Langtang, 783 26.623 0.0169 0.0851 32.0 0.3 5.5 28.97 16.01 I 5.17 923 9.960 0.0799 0.0303 76.8 0.8 10.0 6.13 10.98 ± 2.28 movement on the MCT appears to be diachro- 1,023 4.715 0.0030 0.0130 285.6 2.9 17.9 165.07 9.29 ± 0.40 nous in nature, provided that the metamorphism 1,093 2.740 0.0010 0.0062 896.8 9.1 31.9 481.52 9.61 I 0.43 1.153 1.903 0.0005 0.0035 1,768.0 18.0 44.2 957.07 9.25 s 0.24 is of the same age in both areas. 1,213 1.589 0.0003 0.0026 2,013.6 20.5 50.8 1,511.07 8.87 ±0.18 1,273 1.485 0.0005 0.0023 1,546.4 15.8 52.4 1,073.69 8.55 ±0.14 Of major importance is the relative displace- 1,343 1.595 0.0004 0.0027 1,345.6 13.7 49.2 1,259.32 8.63 ± 0.23 FUSE 1.372 0.0084 0.0019 1,852.8 18.9 56.6 58.05 8.53 ± 0.24 ment on the MCT during ductile and brittle de- formation. It was not possible to estimate the L20a-m J = .006183 amount of displacement on the MCT during D2MCT- Due to the possibility that more than 923 17.373 0.0237 0.0566 56.0 0.5 3.6 20.72 6.91 ± 2.97 1,023 4.648 0.0044 0.0133 156.0 1.3 14.9 111.67 7.73 ± 0.56 one episode of ductile motion occurred on the 1.093 1.994 0.0015 0.0045 1,201.6 10.2 32.0 325.54 7.11 ± 0.27 1.153 1.414 0.0008 0.0026 1,714.4 14.5 44.6 586.66 7.02 ± 0.21 MCT with shear fabrics continuously over- 1.213 1.189 0.0008 0.0018 2,313.6 19.6 53.1 643.66 7.03 ±0.19 printed, strain estimates on the mylonites in the 1.273 1.067 0.0009 0.0015 2,004.8 17.0 57.5 558.04 6.83 ±0.11 1.343 1.053 0.0012 0.0014 1,752.0 14.8 58.1 425.04 8.81 ± 0.13 MCT as exposed in Langtang would probably FUSE 0.907 0.0018 0.0009 2,622.4 22.2 68.7 278.70 6.93 ± 0.09 vastly underestimate the amount of shortening that occurred during ductile deformation. On the basis of field mapping and thermobarome- from the MCT zone in Langtang suggest that the 6.9-8.9 Ma ages to reflect a maximum time con- try, no obvious large-scale metamorphic breaks zone was still at the Ar closure temperature for straint for brittle deformation (D4) of the MCT occur across the MCT zone (Macfarlane, unpub. muscovite (nominally 625 K; McDougall and zone. In addition, the ages decrease upsection, data). The barometry data do show a slight per- Harrison, 1988) as late as 6.9 Ma. D4 movement indicating that the upper MCT zone cooled after turbation across the MCT; at most, 130 MPa on the MCT at Langtang occurred at tempera- the lower MCT zone. This age sequence leads us (3.5 km). Thus, the maximum amount of brittle tures that were sufficiently low enough that to speculate that either deformation proceeded displacement on the MCT according to the ba- quartz was deforming in the brittle regime, at in the MCT zone from structurally lower levels rometry data is 3.5 km. temperatures probably lower than the muscovite upward, or the upper MCT zone was reset by a Figure 14 shows our interpretation of the tec- closure temperature for Ar. We interpret the later event. tonic evolution of the MCT in the Langtang region. The D2MCT event is related to ductile movement on the MCT between 15 and 20 Ma (Fig. 14A). During this event, initial develop-

ment of S2MCT WITH the folding of SJMCT and the growth of staurolite porphyroblasts oc- curred. Further movement on the MCT may have resulted in the crenulation cleavage. Note

that it is not altogether clear that D3MCT is a separate event from D2MCT- 'n the middle or

L66 Muscovite Intercept age = 8.5 ± 0.2 Ma 40Ar/36Ar = 304 ± 4 MS WD = 2.6

Figure 12. Isotope correlation plot for sample L66. All two-sigma errors are less than the diameter of the circles that represent the increments.

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Figure 13. Isotope correlation plot for sample L20a. All two-sigma errors are less than the diameter of the circles that represent the increments. 1.5 L20a Muscovite Intercept age = 6.9 ±0.1 Ma 40Ar/36Ar = 298 ± 3 MSWD = 2.7

<

MBT detachment approximately below the sur- face expression of the MCT. Brittle movement 0.5 began on the MCT in Pliocene time (Fig. 14C). The MCT, as now exposed in Langtang, may have been unroofed by further normal faulting in the hanging wall, bringing the section above the brittle-ductile transition. Movement over a ramp on the MBT may have caused out-of- sequence thrusting (or back-breaking thrusting) 36Ar/40Ar and reactivation of the MCT (Fig. 14C). Thus,

latter part of D2MCT. ^ South Tibetan detach- phism and anatexis associated with M2 out- we suggest that the brittle deformation in the ment system developed at structurally higher lasted D2. MCT zone may have resulted from movement levels, accommodating gravitational collapse Some time after the middle Miocene epoch, on the Main Boundary Thrust. At this time, the n of the growing orogen (Burchfiel and Royden, movement began on the Main Boundary Thrust present duplex structure of D4MCT ¡ the MCT 1985; Burchfiel and others, 1992). Metamor- (MBT, Fig. 14B). A large ramp formed on the zone formed. This hypothesis is strengthened by the situation in central Nepal. There is no evi- dence for a ramp structure in the MBT in central Miocene Figure 14. Our inter- Nepal because there are no tectonic windows pretation of the tectonic south of the MCT. If the out-of-sequence thrust- evolution of the MCT ing hypothesis for the cause of brittle movement zone in the Langtang re- on the Langtang MCT is correct, then there is no gion. In all frames the ac- need for significant brittle displacement on the tive faults have arrows. MCT in central Nepal. According to many (A) Formation and ductile workers (Bordet, 1961; Le Fort, 1975; Bouchez and Pêcher, 1976; Pêcher, 1978; Colchen and Miocene-Pliocene (?) movement on the MCT coeval with normal move- others, 1986), little evidence for brittle deforma- ment on the South Tibet- tion in the MCT zone exists in central Nepal. an detachment system The evidence for out-of-sequence thrusting is (STDS) during Miocene equivocal in eastern Nepal. The Mahabarat time. (B) Motion begins window lies in southeastern Nepal and may on MBT in Mio-Pliocene again be the result of ramping on the MBT. B (?) time; exact age of initi- Hubbard (1988) found evidence of syn-meta- morphic thrusting with no sign of later brittle Pliocene ation of movement on the MBT is unknown. (C) Re- movement on the MCT south of Everest; there- activation of the MCT fore, Hubbard's (1988) conclusions contradict (now designated as MCT'; the out-of-sequence-thrusting hypothesis. On the other hand, Brunei and Kienast (1986), who initiation of D4MCT event) in the brittle regime due worked south of Makalu (just east of Everest), to "back breaking" from found abundant evidence for significant post- the MBT. (D) Further metamorphic movement. Thus, the process that Modern movement over a ramp created significant post-metamorphic movement on a structurally lower on the MCT may be more complex than we fault associated with the suggest. MBT (now designated The later brittle listric extensional faulting MBT') causes folding of (d5MCT) may have in part accommodated con- the MCT. Thus, the Kath- tinuing gravitational collapse of the range mandu window is created. (Burchfiel and Royden, 1985; Royden and

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Hubbard, M. S., and Harrison, T. M., 1989, ^Ar/^Ar constraints on deforma- Burchfiel, 1987). Further ramping on a lower to A. M. Macfarlane. The majority of graduate tion and metamorphism in the Main Central Thrust zone and Tibetan fault of the MBT in modern time may have support for Macfarlane came from a National Slab, eastern Nepal Himalaya: Tectonics, v. 8, p. 865-880. Le Fort, P., 1975, Himalaya: The collided range. Present knowledge of the resulted in the creation of the Kathmandu win- Science Foundation Fellowship. continental arc: American Journal of Science, v. 275A, p. 1-44. Le Fort, P., 1981, Manaslu leucogranite: A collision signature of the Himalaya, dow, as previously suggested (Lyon-Caen and a model for its genesis and emplacement: Journal of Geophysical Re- Molnar, 1983; Fig. 14D). search, v. 86, p. 10545-10568. REFERENCES CITED Lombard, A., 1958, Un intineraire geologique dans l'Est du Nepal (massif du Mont Everest): Mémoires de la Société Helvetique des Sciences Natu- CONCLUSIONS Bordet, P., 1961, Recherches géologiques dans l'Himalaya du Nepal, region du relles, v. 82, 107 p. Makalu: Paris, France, Centre National de Recherches Scientifiques, Lux, D. R., 1986, ^Ar/^Ar ages for minerals from the amphibolite dyna- mothermal aureole, Mont Albert, Gaspe, Quebec: Canadian Journal of 275 p. Earth Science, v. 23, p. 21-26. Bouchez, J. L., and Pêcher, A., 1976. Microstructures and quartz preferred Lyon-Caen, H., and Molnar, P., 1983, Constraints on the structure of the The data presented in this paper demonstrate orientations in quartzites of the Annapurna area (Central Nepal), in the Himalaya from an analysis of gravity anomalies and a flexura! model of proximity of the Main Central Thrust: Himalayan Geology, v. 6, that the MCT in the Langtang region had at least the lithosphère: Journal of Geophysical Research, v. 88, p. 8171 -8191. p. 118-132. Macfarlane, A. M., 1992, The tectonic evolution of the core of the Himalaya, two major periods of movement: an early duc- Boyer, S., and Elliott, D., 1982, Thrust systems: American Association of Langtang National Park, Nepal [abs.]: Himalaya-Karakorum-Tibet Petroleum Geologists Bulletin, v. 66, p. 1196-1230. Workshop, 7th, Oxford, England, April 1992. tile phase and a later brittle phase. Ductile Brunei, M., 1975, La nappe du Mahabharat, Himalaya du Nepal central: Marshak, S., and Mitra, G., 1988, Basic methods of structural geology: Engle- Academie des Sciences Comptes Rendus, v. 280, p. 551-554. movement on the MCT is constrained to have wood Cliffs, New Jersey, Prentice-Hall, 446 p. Brunei, M., 1986, Ductile thrusting in the : Shear sense criteria and Maruo, Y., and Kizaki, K. 1981, Structure and metamorphism in eastern begun sometime before 20-15 Ma. Brittle mo- stretching lineations:' Tectonics, v. 5, p. 247-265. Nepal, in Saklani, P. S., ed„ Metamorphic tectonites of the Himalaya: Brunei, M., and Kienast, J. R., 1986, Etude petro-structurale des chevauche- New Dehli, India, Today and Tomorrow's Printers & Publishers, tion took place after 9-7 Ma and continued for ments ductiles himaiayens sur la transversale de l'Everest-Makalu (Nepal p. 175-230. oriental): Canadian Journal of Earth Sciences, v. 23, p. 1117-1137. an unknown period of time. Maruo, Y., Pradhan, B. M., and Kizaki, K., 1979, Geology of eastern Nepal: Burchfiel, B. C., and Royden, L., 1985, North-south extension within the con- Between Dudh Kosi and Arun: Bulletin of the College of Science, vergent Himalayan region: Geology, v. 13, p. 679-682. On the basis of previous work (Bordet, 1961; University of Ryukus, v. 28, p. 155-191. Burchfiel, B. C.. Zhiliang, C., Hodges, K. V., Yuping, L„ Royden, L. H., McDougall, I., and Harrison, T. M., 1988, Geochronology and thermo- Le Fort, 1975; Bouchez and Pêcher, 1976; Changrong, D„ and Jiene, X., 1992, The South Tibetan detachment chronology by the ^Ar/^Ar method: New York, Oxford University system, Himalayan orogen: Extension contemporaneous with and paral- Press, 212 p. Pêcher, 1978), the MCT less than 100 km west lel to shortening in a collisional mountain belt: Geological Society of Molnar, P., 1984, Structure and tectonics of the Himalaya: Constraints and America Special Paper 269, 41 p. of the Langtang region appears to have evolved implications of geophysical data: Annual Review of Earth and Plane- Burg, J. P., Brunei. M., Gapais, D„ Chen, G. M„ and Liu, G. H„ 1984, tary Science, v. 12, p. 489-518. quite differently, having only a strong ductile Deformation of leucogranites of the crystalline Main Central Sheet in Parrish, R. R., Hodges, K. V.,and Macfarlane, A.M., 1992, U-Pb geochronol- southern Tibet (): Journal of Structural Geology, v. 6, imprint, with little evidence for extensive brittle ogy of igneous and metamorphic rocks near the Main Central Thrust in p. 535-542. the Langtang area, central Nepal Himalaya [abs.]: Himalaya-Kara- Caby, R.. Pêcher. A., and Le Fort, P., 1983, Le M.C.T. Himalayen: Nouvelles deformation. Initially, we remarked that there korum-Tibet Workshop, 7th, Oxford, England, April 1992. donnees sur la métamorphisme inverse a la base de la Dalle du Tibet: Pêcher, A., 1977, Geology of the Nepal Himalaya: Deformation and petrog- were three sources for the controversy over the Revue de Geographie Physique et de Geologie Dynamique, v. 24, raphy in the Main Central Thrust zone [abs.]: Colloque internationale p. 89-100. timing relationship between deformation and 268, Paris, Ecologie et geologie de l'Himalaya, p. 301-318. Colchen, M., Le Fort, P., and Pêcher, A., 1986, Notice explicative de la carte Pêcher, A., 1978, Déformations et métamorphisme associés à une zone de geologique Annapurna-Manaslu-Ganesh (Himalaya du Nepal) au metamorphism within the MCT zone. It appears cisaillement: Exemple du grand chevauchement central Himalayen 1/2000.000: (bilingue: francais-english): Paris, France, Centre National (MCT) [D.Sc. thesis]: Grenoble, France, Grenoble University, 354 p. that the extent of metamorphism and later tec- de Recherches Scientifiques, 136 p. Pêcher, A., 1989, The metamorphism in the Central Himalaya: Journal of Copeland, P., Harrison, T. M., Parrish, R., Burchfiel, B. C., Hodges, K. V., and tonic developments such as the ramp in the Metamorphic Geology, v. 7, p. 31-41. Kidd, W.S.F., 1987, Constraints on the age of normal faulting, north Pêcher, A., and Le Fort, P., 1986, The metamorphism in central Himalaya, its face of Mt. Everest: Implications for Oligo-Miocene uplift: Eos, v. 68, MBT are the major factors in the noncorrelative relation with the thrust tectonic: Sciences de la Terre, v. 47, p. 285-309. p. 1444. Royden, L. H., and Burchfiel, B. C., 1987, Thin-skinned N-S extension within timing associations on the MCT. Thus, it seems Copeland, P., Parrish, R., and Harrison, T. M„ 1988, Identification of inherited the convergent Himalayan region: Gravitational collapse of a Miocene radiogenic Pb in monazite and its implications for U-Pb systematica: that the application of one overall hypothesis for topographic front, in Coward, M. P., Dewey, J. F., and Hancock, P. L., Nature, v. 333, p. 760-763. eds., Continental extensional tectonics: Geological Society Special Publi- Dalrymple, R. D., and Lanphere, M. A., 1969, Potassium-argon dating: San the tectonic history of the MCT is inappropriate cation, v. 28, p. 611-619. Francisco, California, W. H. Freeman, 258 p. Schàrer, U., 1984, The effect of initial 230Th disequilibrium on young U-Pb in light of the present data. Deniel, C., Vidal. P. F., and Fernandez, A.. Le Fort, P., and Peucat, J.-J., 1987, ages: The Makalu case, Himalaya: Earth and Planetary Science Letters, Isotopic study of the Manaslu granite (Himalaya, Nepal): Inferences on v. 67, p. 191-204. the age and source of the Himalayan leucogranites: Contributions to Schàrer, U., Xu, R., and Allegre, C., 1986, U-(Th)-Pb systematica and ages of Mineralogy and Petrology, v. 96, p. 78-92. Himalayan leucogranites. South Tibet: Earth and Planetary Science ACKNOWLEDGMENTS Gansser, A., 1964, Geology of the Himalayas: London, England, Interscience Utters, v. 77, p. 35-48. Publishers, 289 p. Schelling, D., 1987, The geology of the Rolwaling-Lapchi Kang Himalayas, Gansser, A., 1966, The Indian Ocean and the Himalaya: A geologic interpreta- east-central Nepal: Preliminary findings: Journal of the Nepal Geologi- A. M. Macfarlane would like to thank Bar- tion: Ecologae Geologicae Helveticae, v. 59, p. 832-848. cal Society, v. 4, p- 1-19. Heim, A., and Gansser, A., 1939, Central Himalaya: Geologic observations of Stôcklin, J., ! 980, Geology of Nepal and its regional frame: Geological Society bara Sheffels, Sharon Carr, Randy Parrish, the Swiss expedition, 1936, Zurich: Mémoires de la Société Helvetique of London Journal, v. 137, p. 1-34. des Sciences Naturelles, 245 p. Lhakpa Therky Sherpa, Carolyn Ruppel, Meg Tullis, J., and Yund, R. A., 1977, Experimental deformation of dry Westerly Herren, E., 1987, Zanskar shear zone: Northeast-southwest extension within the granite: Journal of Geophysical Research, v. 82, p. 5705-5718. Coleman, Mangale Sherpa, and Tika Magar Higher Himalayas (Ladakh, India): Geology, v. 15, p. 409-413. Valdiya, K. S., 1979, An outline of the structural set-up of the Kumaun Hima- Hodges, K. V., and Silverberg, D. S., 1988, Thermal evolution of the Greater laya: Journal of the Geological Society of India, v. 20, p. 145-157. (Peanut Bhai) for support in the field; the Nepali Himalaya, Garhwal, India: Tectonics, v. 7, p. 583-600. Valdiya, K. S., 1980, Geology of Kumaun Lesser Himalaya, Dehra Dun. India: Hodges, K. V.. Hubbard, M. S., and Silverberg, D. S., 1988, Metamorphic government for research permission; and Wadia Institute of Himalayan Geology, 291 p. 40 39 constraints on the thermal evolution of the central Himalayan orogen: D. West for help with Ar/ Ar analyses. Royal Society of London Philosophical Transactions, v. A326, p. 257-280. D. Silverberg, D. Applegate, M. Coleman, Hodges, K. V., Le Fort, P., and Pêcher, A., 1989, Reply to comment on P. Chamberlain, and an anonymous reviewer "Possible thermal buffering by crustal anatexis in collisional orogens: Thermobarometric evidence from the Nepalese Himalaya": Geology, provided helpful reviews of the manuscript. This v. 17, p. 575-576. Hubbard. M. S., 1988, Thermobarometry, ^Ar/^Ar geochronology, and research has been supported by National Sci- structure of the Main Central Thrust zone and Tibetan Slab, eastern ence Foundation Grant No. EAR8721403, Nepal Himalaya [Ph.D. thesis]: Cambridge, Massachusetts, Massachu- setts Institute of Technology, 169 p. awarded to K. V. Hodges, and a Geological So- Hubbard, M. S., 1989, Thermobarometric constraints on the thermal history of MANUSCRIPT RECEIVED BY THE SOCIETY MAY 24,1991 the Main Central Thrust Zone and Tibetan Slab, eastern Nepal Hima- REVISED MANUSCRIPT RECEIVED MARCH 17,1992 ciety of America student research grant awarded laya: Journal of Metamorphic Geology, v. 7, p. 19-30. MANUSCRIPT ACCEPTED MARCH 19,1992

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