Tectonic and polymetamorphic history of the Lesser ...directory.umm.ac.id/Data Elmu/jurnal/J-a/Journal of Asian Earth... · Tectonic and polymetamorphic history of the Lesser Himalaya

Tectonic and polymetamorphic history of the Lesser Himalaya incentral Nepal

Lalu Prasad Paudel*, Kazunori Arita

Department of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Kita 10, Nishi 8, Sapporo, 060-0810, Japan

Received in revised form 22 July 1999; accepted 8 October 1999

Abstract

The Lesser Himalaya in central Nepal consists of Precambrian to early Paleozoic, low- to medium-grade metamorphic rocks

of the Nawakot Complex, unconformably overlain by the Upper Carboniferous to Lower Miocene Tansen Group. It is dividedtectonically into a Parautochthon, two thrust sheets (Thrust sheets I and II), and a wide shear zone (Main Central Thrust zone)from south to north by the Bari Gad±Kali Gandaki Fault, the Phalebas Thrust and the Lower Main Central Thrust,respectively. The Lesser Himalaya is overthrust by the Higher Himalaya along the Upper Main Central Thrust (UMCT). The

Lesser Himalaya forms a foreland-propagating duplex structure, each tectonic unit being a horse bounded by imbricate faults.The UMCT and the Main Boundary Thrust are the roof and ¯oor thrusts, respectively. The duplex is cut-o� by an out-of-sequence fault. At least ®ve phases of deformation (D1±D5) are recognized in the Lesser Himalaya, two of which (D1 and D2)

belong to the pre-Himalayan (pre-Tertiary) orogeny. Petrographic, microprobe and illite crystallinity data showpolymetamorphic evolution of the Lesser and Higher Himalayas in central Nepal. The Lesser Himalaya su�ered a pre-Himalayan (probably early Paleozoic) anchizonal prograde metamorphism (M0) and a Neohimalayan (syn- to post-UMCT)

diagenetic to garnet grade prograde inverted metamorphism (M2). The Higher Himalaya su�ered an Eohimalayan (pre or early-UMCT) kyanite-grade prograde metamorphism (M1) which was, in turn, overprinted by Neohimalayan (syn-UMCT) retrogrademetamorphism (M2). The isograd inversion from garnet zone in the Lesser Himalaya to kyanite zone in the Higher Himalaya isonly apparent due to post-metamorphic thrusting along the UMCT. Both the Lesser and Higher Himalayas have undergone

late-stage retrogression (M3) during exhumation. 7 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction

The Himalaya were formed at the northern marginof the Indian sub-continent due to collision of theIndian and Eurasian plates in the Middle Eocene (e.g.Le Fort, 1975; Molnar and Tapponnier, 1975). TheHimalaya consists of three main thrust-bounded litho-tectonic units; the Sub-Himalaya (Siwaliks), the LesserHimalaya, and the Higher Himalaya (including theCentral Crystallines and the overlying Tethys Hima-laya) (Fig. 1; Gansser, 1964).

The Lesser Himalaya is a fold-and-thrust belt

bounded by the Main Boundary Thrust (MBT) in the

south and the Main Central Thrust (MCT) in the

north. The Lesser Himalaya comprises the low- to

medium-grade metasedimentary rocks of Late Precam-

brian±Early Paleozoic age (StoÈ cklin, 1980), overlain

unconformably by the Gondwana type Late Paleo-

zoic±Early Tertiary sediments (Sakai, 1983). In some

places the Lesser Himalaya is covered by high-grade

crystalline rocks of the Higher Himalaya. The northern

part of the Lesser Himalaya, which is delimited by the

MCT, is a thick ductile shear zone (MCT zone). The

MCT zone is generally supposed to have been most

active at 22±20 Ma (Hubbard and Harrison, 1989),

acting as the locus for at least 140 km of southward

Journal of Asian Earth Sciences 18 (2000) 561±584

1367-9120/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.

PII: S1367-9120(99 )00069 -3

* Corresponding author. Central Department of Geology, Tribhu-

ban University, Kirtipur, Kathmandu, Nepal. Fax: +81 11 706

5305.

E-mail address: [email protected] (L.P. Paudel).

thrusting of the Higher Himalayan crystalline rocks(Schelling and Arita, 1991).

One of the interesting features of the Lesser Hima-laya is the `inverted metamorphism', ®rst noted byRichard Oldham in the Indian Himalaya in 1883(quoted in Gansser, 1964), and subsequently recog-nized by many geologists in other parts of the Hima-laya (Gansser, 1964; Le Fort, 1975; Caby et al., 1983;Arita, 1983; Sinha-Roy, 1982; Hodges et al., 1988;PeÃ cher, 1989 and many others). The metamorphicgrade appears to increase northwards (structurallyupwards) from chlorite and biotite zones in the LesserHimalaya through garnet zone in the MCT zone tokyanite and sillimanite zones in the Higher Himalaya.This feature has received much attention over the lasttwo decades, and several models have been proposedto explain its origin (see a review by Sorkhabi andArita, 1997). The discussions regarding the invertedmetamorphism, however, have been limited to theMCT zone and the Higher Himalaya. In this connec-tion, it is important to constrain the thermal structureof the low-grade metamorphic rocks of the LesserHimalaya where the inverted metamorphism is exhib-ited.

In an attempt to unravel the structure and meta-morphic history of the Lesser Himalaya in centralNepal, we carried out structural mapping and studyon illite crystallinity of low-grade metamorphic rocks,

along with petrographic study and microprobe analysisof rocks along two sections across the Lesser Himalayaand the lower part of the Higher Himalaya. Thispaper presents the results and discusses the polyphasedeformation and metamorphic history of the Lesserand Higher Himalayas on the ground of new data. Aconceptual model for the tectono-metamorphic evol-ution of the central Nepal Himalaya has been also pre-sented.

2. Tectonic outline

2.1. Thrust tectonics

A tectonic map and a geological cross-section ofcentral Nepal are presented in Fig. 2. Three majornorth-dipping thrusts occur in central Nepal; the MainFrontal Thrust (MFT), MBT and the MCT (Fig. 2).These thrusts propagated from north to south withtime and splays-o� an underlying horizontal decolle-ment known as the Main Detachment Fault (MDF,Schelling and Arita, 1991) or the Main HimalayanThrust (MHT, Zhao et al., 1993). The South TibetanDetachment System (STDS) marking the boundarybetween the Higher Himalayan crystallines and theoverlying Tethys sediments, is a normal fault system(Burg et al., 1984; Burch®el and Royden, 1985; Burch-

Fig. 1. Simpli®ed geological map of the Himalaya showing major lithotectonic divisions (modi®ed from Gansser, 1964; Sorkhabi and Arita,

1997).

L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584562

Fig. 2. Tectonic map (A) and geological cross-section (B) of central Nepal. Prolongation of the cross-section to depth is only speculative based

on the assumption that all the faults join to the common decollement (MDF) located about 20±25 km depth from the present outcrop of the

UMCT (Schelling and Arita, 1991). Kt, Kathmandu; Da, Dhading; Ml, Malekhu; Gk, Gorkha; Pk, Pokhara; Rd, Ramdighat; Tn, Tansen; Bt,

Butwal; Pu, Pyuthan.

L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584 563

®el et al., 1992) which do also have a dextral strike-slip component (PeÃ cher et al., 1991).

The Lesser Himalaya is divided into several tectonicpackages by a series of north-dipping thrusts andfaults. Basically it may be divided into the inner(north) and outer (south) belts by the Bari Gad±KaliGandaki Fault (BKF) (Arita et al., 1982; Sakai, 1985).The outer belt is a parautochthonous unit overlain bythe Palpa Klippe and is distributed mainly in thesouthern part along the MBT. The inner belt consistsof the Thrust Sheet I (TS I) and Thrust Sheet II (TSII) divided by the Phalebas Thrust (PT) (Upreti et al.,1980). The northernmost part of the Lesser Himalayais an intensely sheared and mylonitized MCT zonestriking from east to west. It is bounded by theUMCT in the north and by the Lower MCT (LMCT)in the south, which are also named as the MCT II andMCT I, respectively by Arita et al. (1982).

The Lesser Himalaya forms a foreland-propagatingduplex structure in most parts of the Nepalese Hima-laya (east Nepal, Schelling and Arita, 1991; westNepal, Dhital, 1989; far-western Nepal, DeCelles etal., 1998), an interpretation supported by our ®eld ob-servations in central Nepal [Fig. 2(B)]. The rocks ofthe Higher Himalaya with the overlying Tethys sedi-ments occur as nappe over the Lesser Himalaya in theKathmandu area [Fig. 2(A)]. Although the crystallinerocks are not present in the Tansen±Pokhara section,they may have existed throughout the Lesser Himalayain central Nepal, and the lateral continuity wasdestroyed by erosion (Kizaki, 1994). The HigherHimalayan rocks were thrust a great distance to thesouth (very close to the MBT) along the UMCT, withthe latter serving as roof thrust of the Lesser Himala-yan duplex. The Kathmandu Nappe forms a large syn-clinorium. Parallelism of bedding and foliation of theKathmandu Nappe and those of the underlying LesserHimalayan units shows that the UMCT roof thrustwas initially horizontal and later was folded, alongwith the autochthon, during the propagation of horses.

The Palpa Klippe, made up of the Nawakot Com-plex, occupies the frontal part of the Lesser Himalayacovering the autochthonous Tansen Group. The basalpart of the klippe is a highly sheared and brecciatedtectonic melange zone, about 10 m thick along theTansen±Pokhara motor road. At some places the mel-ange zone is about 200 m thick (Sakai, 1985). The rootthrust sheet of the Palpa Klippe has not yet beenexplained in the area and in fact it is very obscured.Fuchs and Frank (1970) have shown it as the south-ward extension of the PT sheet in their cross-section.We suggest it is the leading edge of the UMCT thatbrought a wedge of the Lesser Himalayan rocks [Figs.2(B) and 12]. However, it should be con®rmed by con-structing a balanced cross-section.

The MBT is regarded as the ¯oor thrust of the Les-

ser Himalayan duplex structure. It dips steeply to thenorth at the surface (about 70±808) and is parallel tobedding of both the hangingwall and the footwall. TheMBT probably dips more gently at depth and joins theMDF in the north. The MBT is marked by a widecrushed zone, which is expressed as a continuous topo-graphic depression in the study area.

The MCT zone, TS II, TS I, and the Parautochthonare the horses of the southward-propagating duplexbounded by the imbricate faults i.e. the LMCT, PTand the BKF. The LMCT in the Pokhara area is verydiscordant, and cuts many units at the footwall. Tothe NW of Pokhara, for example, the MCT zonerocks discordantly override the Fagfog Quartzite ofthe TS II (Fig. 4). In the Piuthan area, the MCT zonerocks (including the Ulleri-type gneisses) are thrustover the Parautochthon and make a klippe (JajarkotKlippe) (Arita et al., 1984). The LMCT, however, isoften obscured in the eastern parts of central Nepalwhere the rock units of similar lithology are juxta-posed by the LMCT. In such areas, the LMCT ismarked by the di�erence in structural style betweenthe MCT zone and the TS II. The MCT zone hashomoclinally northward-dipping foliation, whereas theTS II shows foliation folded into a dome and basinstructure. The LMCT dips about 10±158 to the NE atthe surface and probably steepens at depth as the foli-ation in the MCT zone becomes steeper (30±508) inthe north.

The PT is almost parallel to both the UMCT andLMCT. It extends from NW to SE and joins the BKFto the south of Gorkha (Fig. 2). The BKF is steeper(50±708) than the PT, and cuts through the JajarkotKlippe in the west and the Kathmandu Nappe in theeast (Fig. 2). It is thus an out-of-sequence fault (Aritaet al., 1997). The Pindi Khola Fault (PKF), which istraced locally in the Syangja area, is a south-dippingfault and joins the BKF in the east and west. It can beinterpreted as an antithetic back-thrust developed onthe hangingwall of the BKF. The Kusma Fault (KF)is a splay o� of the PT. It is very steep to vertical (70±908) at the surface.

2.2. Sequence and chronology of thrusting

Although it is di�cult to determine the exact timingof the development of each thrust and fault in thearea, it is possible to estimate the approximate timingof activity along major thrusts and the sequence ofthrust development with the help of structural re-lations, geochronological data and the foreland sedi-mentary records.

The UMCT is the highest thrust fault in the thrustpile. It is the oldest thrust in the area because it hasbeen folded and faulted by later thrusts and faults.Overthrusting of the Tansen Group (Early Miocene


and older in age) by the UMCT indicates that thisthrust reached to the southern part of the LesserHimalaya later than the Early Miocene, probably inthe Middle Miocene. But the UMCT may have beeninitiated earlier than this time in its root zone. TheDumri Formation (Early Miocene in age, Fig. 3) con-tains clasts of phyllitic slates derived from the Himala-yan terrain (Sakai, 1985). It indicates that the uplift ofthe northern part of the area began at least in theEarly Miocene. The uplift may be related to the ramp-ing along the UMCT at depth. Assuming that thepeak metamorphism in the MCT zone and the anatexis

and leucogranite emplacement in the Higher Himala-yan Crystallines were the synchronous events associ-ated with the UMCT movement (Le Fort, 1975), anearly Miocene age (about 22±15 Ma) has beenassigned to movement along the UMCT (Hodges etal., 1996; Macfarlane, 1993). Dextral shearing andnorth-directed detachment along the STDS was almostsynchronous with the UMCT (Guillot et al., 1994;PeÃ cher et al., 1991). The movement along the UMCTin the Lesser Himalayan nappe zones was terminatedbetween 14±5 Ma due to the out-of-sequence thrustingin the Lesser Himalaya (Arita et al., 1997). However,

Fig. 3. Tectono-lithostratigraphic subdivision of the Lesser Himalaya in central Nepal. Patterns in column are the same as in Fig. 2. Phy., phyl-

lite; Qzt., quartzite; Amp., amphibolite; Sl., slate; Ss., sandstone; Dol., dolomite; Cgl., conglomerate; Sh., shale.


there are many younger isotopic ages (8±3 Ma) fromthe northern root zone of the UMCT implying eithercontinuous movement at the root zone until the latePliocene (Arita et al., 1997) or a late Miocene-Pliocenereactivation of the UMCT root zone (Copeland et al.,1991, Inger and Harris, 1992; Macfarlane, 1993;Edwards, 1995; Harrison et al., 1997).

The LMCT, PT and the BKF propagated succes-sively from north to south in a piggy-back fashion.Although there are no age constraints on the move-ment along the LMCT and PT, the timing of faultingalong the BKF (equivalent to the Trisuli±Likhu Faultin the Kathmandu area) has been constrained to bebetween 10±7.5 Ma (Arita et al., 1997). The BKF is anout-of-sequence fault and truncates the overlyingthrusts, i.e. the PT, LMCT and the UMCT. Therefore,the LMCT and the PT should be older than Pliocene.The KF and PKF were probably formed during thetime of movement along the MBT by the imbricationof the hangingwalls of the PT and the BKF, respect-ively.

The MBT juxtaposes the Lesser Himalayan metase-diments against the Siwaliks which are about 14±1 Main age in central Nepal (Tokuoka et al., 1986). Itimplies that the MBT reached over the Siwaliks laterthan the Lower Pleistocene. However, changes in thesedimentation patterns within the Siwaliks after 11 Maindicates that initial motion along the MBT started inthe Late Miocene (Burbank et al., 1996). The MFTplaces the Siwaliks over the recent Ganges sediments.It is the latest and structurally lowermost fault pre-sently exposed in the area. The BKF, MBT and MFTare believed to be still active (Nakata, 1982; Kizaki,1994).

3. Lithostratigraphy

The Lesser Himalaya consists principally of the latePrecambrian to early Paleozoic Nawakot Complex(StoÈ cklin, 1980) and the unconformably overlyingGondwana and post-Gondwana sediments (Sakai,1983) (Fig. 3). The Nawakot Complex has been vary-ingly named as the Midland Metasediment Group byHashimoto et al. (1973) and Midland Formations byLe Fort (1975), PeÃ cher (1977) and Colchen et al.(1980). A full succession of the Nawakot Complex isobserved only in TS II, in the Dhading±Malekhu area,where it attains a total thickness of approx. 10 km. Itis divided into the Lower and Upper Nawakot Groupsby an unconformity (StoÈ cklin, 1980).

From the bottom to the top, the Lower NawakotGroup consists of the Kuncha Formation, FagfogQuartzite, Dandagaon Phyllite, Nourpul Formation,and the Dhading dolomite. The Upper NawakotGroup is divided into the Benighat Slate, Malekhu

Limestone, and the Robang Formation (StoÈ cklin,1980; Fig. 3). These formations can be traced fromeast to west in central Nepal, and are repeated severaltimes by folding and thrusting (Paudel and Arita,1998). In the Pokhara area (Fig. 4), the TS II consistsonly of the approx. 3 km thick lower part of theNawakot Complex (Kuncha Formation, FagfogQuartzite and Dandagaon Phyllite). The TS I com-prises the middle part of the Nawakot Complex(Nourpul Formation, Dhading Dolomite and BenighatSlate). The Nourpul Formation occupies the core ofan anticline along the Andhi Khola (Khola meansriver in Nepali). It is also exposed along the Kali Gan-daki River Valley south of Phalebas. The Dhadingdolomite is observed at Syangja. The Benighat Slate isexposed just to the north of the BKF (southern part ofFig. 4). The Parautochthon comprises the middle andupper parts of the Nawakot Complex. The NourpulFormation, Dhading Dolomite and the Benighat Slateare exposed along the motor road between Ramdighatand Tansen and constitute the northern limb of theTansen Synclinorium while the Malekhu Limestone isexposed just to the north of the MBT and form thesouthern limb of the Tansen Synclinorium (Fig. 2).The Palpa Klippe, which covers the Parautochthon, ismade up of the Nourpul Formation.

The Gondwana and post-Gondwana sedimentswhich unconformably overlie the Nawakot Complexof the Parautochthon were collectively named as theTansen Group by Sakai (1983) (Fig. 3). The Gond-wana sediments are divided into the Sisne, Taltung,and Amile Formations. The post-Gondwana sedimentsare divided into the Bhainskati and Dumri For-mations. The Tansen Group contains Upper Carbon-iferous to Early Miocene fossils (Sakai, 1983).

A more than 3 km thick MCT zone is lithologicallydivided into the Lower and Upper Units (Figs. 3 and4). The Lower Unit consists of interlayered garnetifer-ous pelitic and psammitic schists, with a few bands ofchloritic schists and quartzites. Mylonitic augengneisses (Ulleri augen gneiss of Le Fort, 1975) inter-bedded with psammitic schists, and pegmatite veinscross-cutting the main foliation are found in the lowerpart (Paudel and Arita, 1998). The Upper Unit isdominated by graphitic schist, calc-schist, and marble.Amphibolite bands are found at di�erent levelsthroughout the MCT zone. The MCT zone rocks arepossibly the sheared and metamorphosed equivalentsof the Nawakot Complex (Hashimoto et al., 1973;PeÃ cher, 1977).

The Higher Himalayan crystalline rocks areobserved along the upper part of the Modi Khola andthe Seti Khola valleys. These comprise coarse-grained,kyanite-bearing banded gneisses, augen gneisses andschists. The banded gneisses consist of alternating bio-tite rich and feldspar-quartz rich layers. Kyanite blades


Fig. 4. Geological map (A) and cross-section (B) of the Lesser Himalaya in the Pokhara±Kusma area, central Nepal. The biotite isograd is

shown on the map. Garnet and kyanite isograds coincide with the LMCT and UMCT, respectively.


Table

1

Deform

ationalevents

andrelatedstructuresin

theLesserHim

alayaofcentralNepalalongtheTansen±Pokhara

section

Pre-H

imalayanphases

Him

alayan(syn-to

post-U

MCT)phases

Tectonic

units

D1

D2

D3

D4

D5

MCTzone

Foliationpreserved

asinclusiontrailsin

garnet

(S1)

Notseen

S±C

fabric,

NNE±SSW

mineral

andstretchinglineations(L

3)

WNW

±ESEcrenulationandkink

folds(F

4),NE-orSW-dipping

crenulationcleavage(S

4)

Small-scale

brittle

faults

Thrust

sheetII

Bedding-parallel

foliation(S

1=

S0)

NNE±SSW

trendingandwest

vergentisoclinalanddragfolds

(F2)

Bedding-parallel

shearplanes

(S3=

S1=

S0),NNE±SSW

mineral

andstretchinglineations(L

3)

WNW

±ESElargescale

open

folds

andminorfolds(F

4),NE-orSW-

dippingcrenulationcleavage(S

4)

Small-scale

brittle

faults

Thrust

sheetI

Bedding-parallel

foliation(S

1=

S0)

NNE±SSW

trendingandwest


(F2)

Bedding-parallel

shearplanes

(S3=

S1=

S0)

WNW

±ESElargescale

open

to

tightandoverturned

foldsand

minorfolds(F

4),NE-orSW-


4)

Small-scale

brittle

faults

Parautochthon

Bedding-parallel

foliation(S

1=

S0)

NNE±SSW

trendingandwest


(F2)

Bedding-parallel

shearplanes

(S3=

S1=

S0)

WNW

±ESElargescale

open

to

tightandrecumbentfoldsand

minorfolds(F

4),NE-orSW-


4)

Small-scale

brittle

faults

TansenGroup

No

No

Notseen

WNW

±ESElargescale

open

to

tightfoldsandminorfolds(F

4),

NE-orSW-dippingslaty

and

fracture

cleavages

(S4),WNW

±

ESEpencillineation(L

4)

Small-scale

brittle

faults


Fig. 5. (A) Photograph showing west vergent F2 drag fold formed by the deformation of bedding (S0) and S1 foliation observed in the Thrust

Sheet I along the Kali Gandaki river valley south of Phalebas. (B) Photograph showing S±C structures related to D3 with top-to-the-south sense

of shearing in the Main Central Thrust zone to the south of Chhomrong. (C) Photomicrograph showing bedding-parallel shear planes (S3) in the

phyllite from the Thrust Sheet I near Syangja. Notice the well-preserved graded-bedding. Asymmetric pressure shadows with a top-to-the-south

sense of shearing are well-observed in those rocks. The micaceous band has been deformed to form F4 crenulation folds. (D) Photograph show-

ing L3 stretching lineation (on S3 plane) formed by the stretched pebbles in metaconglomerates in the Kuncha Formation from Thrust Sheet II.

(E) Photomicrograph of phyllite from the Kuncha Formation to the south of Pokhara (Thrust Sheet II) with well-developed S4 crenulation clea-

vage. (F) Photograph showing F4 crenulation folds observed in the Main Central Thrust zone in the Seti valley. Notice D5 brittle shear zones

cross-cutting the F4 crenulation folds and L3 stretching/mineral lineations.


are up to 7 cm in length, and are often fractured andbent. Arita (1983) has also reported the occurrence ofneedle-like sillimanite from the lower part of theHigher Himalaya in the Modi Khola valley. Sillimaniteis usually widespread in the Higher Himalaya in theBuri Gandaki river region (Fig. 2) of central Nepal(Hashimoto et al., 1973; Colchen et al., 1980).

4. Geological structures and deformation history

Detailed geological mapping at 1:50,000 scale andstructural analysis were carried out in the Pokhara-Syangja area (Figs. 4 and 6), covering the MCT zone,the TS II and the northern part of the TS I. Structuresof the Parautochthon, Palpa Klippe and the TansenGroup were studied along two routes (Fig. 2). Thestructures of the Lesser Himalaya in the Tansen-Pokhara section display polyphase deformation. Atleast ®ve deformational phases have been recognized inthe area, which are labelled as D1, D2, D3, D4 and D5.Structures having the same geometric style in all thetectonic units are assigned to the same deformationalevent. However, it does not imply that they were syn-chronous in all tectonic units, and thus the correlationof the deformation events (Table 1) should be regardedas very tentative. The planar structures are labelled asS, linear structures as L and folds as F with a su�xreferring to the corresponding deformation event.Among the ®ve deformation phases, ®rst two (D1 andD2) are supposed to be of pre-Himalayan (pre-Ter-tiary) time and the later three (D3, D4 and D5) are re-lated to the Himalayan orogeny.

4.1. Pre-Himalayan phases

D1. Pre-deformational compositional layering (S0)has been preserved throughout the Lesser Himalaya[Figs. 5(A) and (C)]. The ®rst deformational event(D1) is marked by the dominant bedding-parallel foli-ation (S1) in the Nawakot Complex. It is di�cult todistinguish S1 from S3 in most places because of theD3 bedding-parallel shearing. The S1 is more clearlyobserved in the frontal part of the Lesser Himalaya (inthe Parautochthon, Palpa Klippe and TS I) where thelater shearing events were relatively weak. The in-clusion trails in garnets from the MCT zone may bethe traces of S1. The S1 is absent in the Tansen Group.The S1 is probably the result of bedding-parallel ¯at-tening due to syn-sedimentary loading.

D2. The D2 event corresponds to the deformation ofthe S0 and S1 producing drag and isoclinal folds (F2)with NNE±SSW trending axes [Fig. 5(A)]. Those dragand isoclinal folds were observed throughout theNawakot Complex rocks in the TS II, TS I, Parau-tochthon (both in the south and north of the Tansen

Syncline). However, such folds could not be observedin the Tansen Group and the MCT zone along Tan-sen±Pokhara section. The drag folds have consistentlyWNW vergence throughout the area. The axial trendsof those drag and isoclinal folds vary from N108W toN258E with both northern and southern plunges. Butthe maxima of the axial trend lies toward NNE±SSW(Fig. 6).

The WNW vergence of the drag folds observed inthe area is in contrast to the commonly observedsouthward-vergent shearing and folding due to theHimalayan orogeny. Folds with axes parallel to thetectonic transport (oblique and sheath folds) may bedeveloped in intense ductile shear zones like the MCTzone due to the rotation of fold hinges towards thetectonic transport direction during progressive simpleshear deformation (Quinquis et al., 1978; Cobbold andQuinquis, 1980). Oblique and sheath folds have cylind-rical cross-section and they should fade out laterally.It is not the case in the present area [Fig. 5(A)]. More-over, the west-vergent folds are present throughout theNawakot Complex even to the southernmost part ofthe Lesser Himalaya where the intensity of shearingduring the Himalayan orogeny is relatively weak. Dueto the above reasons and also due to their absence inthe Tansen Group, we argue that D1 and D2 are pre-Himalayan (Table 1).

4.2. Himalayan phases

The Himalayan deformation phases can be con-sidered as a single continuous phase of deformation.The structures were gradually evolved with time fromnorth to south. Despite this fact, it can be divided intothree phases based on the di�erence in structural styleduring di�erent stages of deformation.

D3. The D3 is characterized by intense ductile shear-ing more or less parallel to S0 and S1. The D3 was themain deformation event in the MCT zone which pro-duced dominant S3 mylonitic foliation (including boththe S- and C-planes) and NNE±SSW trending L3

stretching and mineral lineations. The mylonitic foli-ation is represented by well-developed S±C fabric insome places [Fig. 5(B)] whereas in other places it isrepresented by anastomoizing shear planes formed bythe juxtaposition of the almond-shaped bodies. Inplaces where the S±C fabric is well-recognized, C-planes are more prominent and relatively gentler thanthe S-planes [Fig. 5(B)]. The dip of the S- and C-planesin the MCT zone varies from 10 to 508 NE (Fig. 6). Inthe TS II, TS I, and the Parautochthon, the S3 foli-ation is represented by shearing more or less parallelto the S0- and S1-planes [Fig. 5(C)]. The intensity ofshear strain gradually vanishes to the south and theshear fabric is less-observed in the southern part of theParautochthon.


Many F3 isoclinal folds with axes trending in theNNE±SSW direction have been reported from theMCT zone in central Nepal (PeÃ cher, 1977; Brunel etal., 1979; Macfarlane et al., 1992; Vannay and Hodges,1996). Those folds have been interpreted to haveformed at the initial stage of D3 and reoriented paral-lel to the stretching lineation during the followingshearing stages (curved folds). Although such folds

could also be present in the MCT zone of the presentarea, we did not notice them.

The stretching and mineral lineations (L3) werereported only from the MCT zone and the TS II. Theyare de®ned by preferred orientation of the stretchedpebbles in metaconglomerates [Fig. 5(D)], elongatedquartz and feldspar porphyroclasts in augen gneisses,and preferred orientation of minerals like biotite, mus-

Fig. 6. Structural map of the Lesser Himalaya in the Pokhara±Kusma area. Foliation and lineations are projected on Schmidt's lower hemi-

sphere.


covite and actinolite on the S3 planes. The L3 linea-tions trend to the NE and plunge from 5 to 258 in theMCT zone (Fig. 6). The L3 lineations have been foldedby the later events in the TS II. They trend in theNNE±SSW direction and plunge to both the northand south with plunges ranging from very gentle (58)to vertical (908) (Fig. 6).

Shear-sense markers related to D3 are abundantthroughout the Lesser Himalaya. They are representedby S±C structures [Fig. 5(B)] and garnets with spiralinclusions in the MCT zone, and sheared porphyro-clasts of quartz and feldspars with asymmetric pressureshadows in the TS II and TS I [Fig. 5(C)]. All of themconsistently show a top-to-the-south sense of shearingduring D3. This is in good agreement with the obser-vations by PeÃ cher (1977), Brunel et al. (1979) andKaneko (1997) in central Nepal. The D3 was related tothe thrusting along the UMCT (PeÃ cher, 1977; Brunelet al., 1979).

D4. All of the previous planar (S0, S1 and S3) andlinear (L3) structures were deformed during D4 due tothe post-UMCT thrust propagation. Most of themajor and minor folds with axes trending from WNWto ESE and vergence to the south were formed duringD4. The shallow and frontal part of the Lesser Hima-laya is characterized by S4 axial plane slaty cleavage,S4 fracture cleavage and L4 pencil lineaitons. The dee-per and rear part of the Lesser Himalaya is character-ized by the F4 crenulation folds and S4 crenulationcleavage.

Major F4 folds are abundant to the south of theMCT zone. In the TS II, the large-scale F4 folds arenon-cylindrical, doubly plunging, and of the opentype. They are arranged in an en-echelon patternshowing a dome and basin structure in the Pokharaarea (Fig. 6). An overturned F4 syncline was observedto the NW of Birethanti (Figs. 3 and 6). The areasouth of the PT is characterized by tight, overturnedand even recumbent F4 folding (Sakai, 1985; Dhital etal., 1998). The Tansen Synclinorium (Fig. 2) representsa major F4 fold in the Parautochthon. Minor F4 cre-nulation [Fig. 5(F)] and kink folds with WNW±ESEtrending axes are well-developed in all the tectonicunits. The maxima of the minor fold axes is more orless parallel to the major fold axes (Fig. 6).

Crenulation cleavages (S4) dipping 30±508 to the NEare well-developed in the incompetent pelitic layers ofthe Kuncha Formation [Fig. 5(E)]. The S4 slaty clea-vage dipping either to the NE or to the SW and cross-cutting the previous planar structures (S0, S1 and S3)are abundant in the Benighat Slate. The TansenGroup shows axial-plane slaty and fracture cleavages(S4) dipping to the NE or SW as well as pencil linea-tions (L4) trending WNW±ESE. Pencil lineations areusually widespread in the shales of the Amile andBhainskati Formations.

D5. The D5 is usually characterized by small-scalebrittle faulting throughout the area. The brittle faultscross-cut all of the previous structures [Fig. 5(F)].They strike WNW±ESE and dip steeply to the SW orNE. Some brittle shear zones have a normal sense ofmotion.

5. Metamorphic zonation and petrography

Microscopic observation of samples collected sys-tematically along two parallel sections (Fig. 2) in theTansen±Pokhara area shows that the metamorphicgrade and intensity of deformation increases north-ward to the UMCT. Most parts of the Lesser Hima-laya lie within the chlorite (or lower) zone. Biotiteappears north of Pokhara and Kusma, and the biotitezone is distributed as a narrow zone just below theLMCT (Fig. 4). The garnet and kyanite isograds co-incide with the LMCT and the UMCT, respectively.However, the isograd distribution patterns are not uni-form throughout central Nepal. In the Gorkha area,for example, the biotite zone becomes as wide as20 km, the garnet isograd crosses the LMCT andpasses into the TS II, and kyanite and staurolite arefound also in the upper part of the MCT zone (Col-chen et al., 1980). General petrographic features of therocks from each tectonic unit along the Tansen±Pokhara section are given below. Mineral abbrevi-ations are after Kretz (1983).

5.1. Higher Himalaya

The Higher Himalayan rocks just above the UMCTrecrystallized under amphibolite facies condition withmineral assemblages of Ky±Grt±Bt±Ms±Pl(An >20%)±Qtz and Grt±Bt±Ms±Pl±Qtz (accessories: Ilm,and Zrn) in metapelites. Despite the widespread occur-rence of sillimanite in the Higher Himalaya of theGorkha area (Hashimoto et al., 1973; Colchen et al.,1980) and sporadic occurrence in the Seti valley (Arita,1983), present samples from both the Seti and Modivalleys do not contain sillimanite. Kyanite, however, iswidespread in the present area and are elongated par-allel to the foliation and the stretching lineation. Theyare generally fractured, bent and partially altered into®ne-grained muscovite. The recrystallized muscovite isalso arranged parallel to the foliation. Poikiloblasticeuhedral garnets grew up to 5 mm in diameter. Theyhave inclusion-rich cores and inclusion-free rims, andare often fractured, elongated and altered to chlorite.Coarse-grained (2±4 mm long) biotite and muscovite¯akes are the predominant matrix phases de®ning thefoliation. Biotite is often masked by phengitic musco-vite. Biotite also occurs as inclusions in kyanite.


5.2. MCT zone

The MCT zone belongs to the garnet zone of thegreenschist facies. The main mineral assemblages areGrt±Bt±Ms±Chl±Ab±Qtz (in pelitic and psammiticschists), Act±Bt±Ms±Chl±Cal±Qtz (in calc-schists) andHbl±Act±Bt±Ep±Ab±Qtz with relicts of Hbl (in meta-basites). Altered sphene (leucoxene), magnetite, tour-maline and zircon occur as accessories. Chlorite occursonly alteration product. Augen gneisses are myloniticto protomylonitic, with augens of perthitic microclineand plagioclase up to 1 cm in diameter. Muscovite andbiotite are predominant matrix phases in all rocksde®ning foliation. Quartz occurs as granoblastic, pol-ygonal aggregates in the schists and gneisses of thelower part of the MCT zone. In the upper part, it isstrongly sheared and shows ribbon texture. Syn-tec-tonic poikiloblastic garnet in the schists is found indi�erent shapes (skeletal, elongated, s-shaped and eqi-dimensional) and sizes (0.1±5 mm). Spiral garnetsshow up to 3608 rotations of the inclusions. S±C fabricand rotated garnets show a top-to-the-south sense ofshearing in the MCT zone. Many snowball garnets inthe mica-rich layers display post-tectonic rim over-growth. Large (up to 2 mm) post-tectonic garnets andmuscovites (0.2 mm) occur cutting across the S3 foli-ation.

5.3. TS II

The TS II shows greenschist facies of metamorphismwith the biotite zone in the north, and the chloritezone in the south [Fig. 4(B)]. In the biotite zone, themain mineral assemblages are Bt±Ms±Chl±Ab±Qtz(pelitic and psammitic rocks) and Act±Bt±Chl±Ep±Cal±Ab±Qtz (basic rocks). Tourmaline, magnetite, zir-con, apatite and sphene occur as accessories. Quartzclasts in metasandstones and metaconglomerates areelongated parallel to the foliation, and mark thestretching lineation. The quartz clasts are often poly-gonized. The matrix contains coarse-grained aggregatesof polygonal quartz. Ms±Chl±Ab±Qtz is the typicalassemblage of phyllites in the chlorite zone. Tourma-line, magnetite, zircon, apatite and sphene occur asaccessories. S3 foliation with microfolds and crenula-tion is common in the pelites. Metasandstone containslarge ovoidal clasts of quartz arranged parallel to thefoliation, which are accompanied by pressure shadows(showing a top-to-the-south sense of shearing) andmortar structure. However matrix quartz is fullyrecrystallized into polygonal aggregates.

5.4. TS I and Parautochthon

Rocks of the TS I and the Parautochthon belong tothe chlorite and lower zones. Sedimentary features

such as parallel laminae, cross-laminae, graded-bed-ding, mud-cracks and stromatolites are well-preservedin those units [Figs. 5(A) and (C)]. However, phyllites,slates and the matrix of sandstones in the NawakotComplex contain recrystallized muscovite and chlorite¯akes arranged parallel to the foliation. Detrital quartzand mica ¯akes (0.05±0.15 mm in length) oblique tothe foliation are sometimes observed in the psammiticparts of the phyllites and slates. Sandstones containdetrital muscovite (up to 1 mm), quartz, feldspar, tour-maline, apatite, and zircon. The Nawakot Complex issheared and recrystallized near the PT and the BKF.Large quartz clasts in sandstones are slightlydeformed, and show wavy extinction, whereas thesmall clasts in the matrix of sandstone and siltstonesare polygonized. The sandstones locally containsheared detrital quartz clasts with well-developedasymmetric pressure shadows showing a top-to-the-south sense of shearing. The Tansen Group containsvery low-grade to non-metamorphosed rocks. In thin-section, ®ne quartz clasts (0.02 mm) are arranged par-allel to the S4 slaty cleavage. Recrystallized mineralsare very ®ne-grained and cannot be identi®ed underthe microscope.

6. Mineral chemistry

Garnets and muscovites were analyzed by EPMA(JEOL Superprobe 733, specimen current 200 mA,accelerating voltage 15 kV, natural and synthetic sili-cates and oxides as standards).

6.1. Garnet

Garnets from the Higher Himalaya and the MCTzone were probed at the cores and rims, and the dataare projected on the Fe�±Mg±(Mn+Ca) triangle(Fig. 7). Garnets from the Higher Himalaya are rich inpyrope (Mg 20±25% core, 15±20% rim) and alman-dine (Fe 65±70% core, 70±75% rim) content. Compo-sitional pro®les across garnets from the HigherHimalaya [Fig. 8(A)] are characterized by a plateau inthe cores. However, the margins of the garnets showreverse zoning, with Fe and Mn increasing and Mgdecreasing towards the rim. Ca is relatively constant.A compositional plateau of this type may be developedby obliteration of growth zoning by later high-tem-perature di�usion process (Spear, 1993). Retrogradezoning pro®les at the margins may be the result ofsubsequent di�usion or resorption due to retrogression(Barker, 1990). Garnets in the MCT zone are spessar-tine rich (Mn 25±45% core, 15±35% rim) (Fig. 7). In-dividual garnets show bell-shaped Mn-pro®lescharacteristic of prograde metamorphism, with Fegradually increasing and Mn decreasing towards the


rims [Fig. 8(B)]. The pro®les are reversed at the outer-most rim, probably due to the late-stage retrogression.

The above patterns of compositional zoning in gar-net porphyroblasts from the Higher Himalaya and theMCT zone seem to be consistent along several sectionsof central Nepal, e.g. Kali Gandaki valley (Le Fort etal., 1986b; Vannay and Hodges, 1996), Modi valley

(Arita, 1983; Kaneko, 1997), Trisuli valley (Macfar-lane, 1995), and Kathmandu area (Rai et al., 1998),suggesting a quite di�erent metamorphic historybetween those units.

6.2. Muscovite

Detrital and recrystallized muscovites from theThrust sheets I and II, the MCT zone and the HigherHimalaya were analyzed and plotted on a Miyashirodiagram (Fig. 9). In general, the celadonite componentin muscovite decreases with increasing metamorphicgrade (Miyashiro, 1973). Recrystallized muscovitesfrom the Lesser Himalaya show a decrease in celado-nite component from south to north (structurallyupwards). Recrystallized muscovites in sandstonesfrom the TS I contain up to 8 wt% FeO. This valuedecreases to 3±6 wt% in the TS II and 1±3 wt% in theMCT zone. However, muscovites from the kyanite-grade Higher Himalayan rocks have greater celadonitecomponents than those from the garnet grade MCTzone samples, and plotted in the biotite±almandine®eld on the Miyashiro diagram rather than in thestaurolite±sillimanite ®eld (Fig. 9). The kyanite and thepyrope-rich cores of garnet do not coexist with celado-nite-rich muscovite, and thus the celadonite-rich mus-covite was most probably produced by a later eventunder lower metamorphic conditions as suggested byArita (1983). Celadonite contents of detrital musco-vites from the Lesser Himalaya vary widely (Fig. 9).Those plotting close to pure muscovite are probablyderived from older high-grade metamorphic rocks.

7. Illite crystallinity

Illite crystallinity (IC) is an important tool in under-standing the thermal structure of low-grade meta-morphic rocks such as slates and phyllites (KuÈ bler,1967). The KuÈ bler Index (KI), de®ned as the peakwidth at half height of the 10 AÊ illite peak above thebackground (KuÈ bler, 1967; Dunoyer de Segonzac etal., 1968), decreases with increasing metamorphicgrade as illite releases Fe2+, Mg2+, H2O, OHÿ, andabsorbs K+, eventually forming muscovite. On thebasis of IC, low-grade metamorphism can be dividedinto the diagenetic zone, anchizone and epizone whichare roughly equivalent to the zeolite facies, prehnite±pumpellyite facies and greenschist facies of meta-morphism in metabasites, respectively (Warr, 1996).Thus IC also helps to estimate the temperature ofmetamorphism in the low-grade metamorphic rocks(zeolite facies < 2008C, prehnite±pumpellyite faceis 0200±3708C, greenschist facies> 3708C, Winkler, 1974).

Fig. 8. Compositional pro®les of garnets from the Higher Himalaya

(sample No. 158) (A) and the Main Central Thrust zone (sample

No. 155) (B) along the Seti Valley. See Fig. 4 for sample localities.

Fig. 7. Chemical composition of garnets from the Higher Himalaya

and the Main Central Thrust zone. Fe� means total Fe as Fe2+.


7.1. Sample preparation and measurement

A total of 200 pelitic rock samples along thePokhara±Butwal road and the Modi Khola±Kali Gan-daki sections of the Lesser Himalaya were used for ICstudy. The laboratory procedure followed here is con-sistent with that outlined by the IGCP 294 WorkingGroup (Kisch, 1991a). About 500 g of each samplewas broken into small chips and then washed anddried. About 200 g chips were then crushed in a mor-tar and pestle, and passed through 2.38 and 0.59 mmsieves. The ®ne fraction was discarded to reduce anyin¯uence from weathered material. About 200 g of the2.38±0.59 mm fraction was then ground for 3 min in amortar and pestle, and passed through a 100-mesh(0.149 mm) sieve. The <2 mm fraction was separatedfrom <0.149 mm powder by centrifuge. The centrifuge

was calibrated using the equation after Wada (1966)(see Appendix A). A 40 mg mlÿ1 suspension was pre-pared from the <2 mm fraction, and 1 ml of this waspipetted on to a 28 � 48 mm microscopic slide produ-cing a slide of thickness ca. 3 mg cmÿ2. Two slideswere prepared for each sample, one air-dried and theother ethylene-glycolated.

The di�ractometer setting was constant for allsamples (Rigaku Geiger¯ex di�ractometer, Cu cath-ode, Ni ®lter, 35 kV tube voltage, 20 mA current, timeconstant=2 s, scatter slit=18, receiving slit=0.3 mm,divergence slit=18). The KI was measured manually atthe precision of 0.2 mm on the chart (=0.005D82y ).Minimum peak width measured was 0.095D82y from amuscovite ¯ake in a pegmatite from the MCT zonewhich has a composition close to that of the idealmuscovite (Fig. 9). One anchizone sample was run at

Fig. 9. (A) Composition of authogenic and detrital muscovites from central Nepal on the Miyashiro diagram (Miyashiro, 1973). (B) Geological

cross-section from the Pokhara area (along line A±B in Fig. 3) showing the approximate structural position of the samples used for microprobe

analysis. See Fig. 4 for sample localities.


the beginning and end of analysis each day to checkthe instrumental drift. Forty-two measurements in 20days gave an average value of 0.262D82y with standarddeviation (1s ) of 0.004D82y. The precision of the ma-chine was checked by ten repeat measurements on

single undisturbed samples from di�erent metamorphicgrades. This gave mean values of 0.167 2 0.003D82y(1s ) for the epizone sample, 0.260 2 0.005D82y (1s )for the anchizone sample, and 0.99120.036D82y (1s )for the diagenetic zone sample. The total error arising

Fig. 10. Representative X-ray di�ractograms. Chl, Chlorite; I, Illite-muscovite; Ab, Albite; Qtz, Quartz; 2M1, 2M1 muscovite, KI, Kubler Index.


from measurement conditions does not exceed 3% forthe anchizone and epizone samples and 5% for thediagenetic samples.

7.2. Mineralogy of <2 mm fraction

Both air-dried and ethylene-glycolated samples werescanned from 33 to 58 2y at a scan rate of 28 minÿ1

and chart speed of 2 cm minÿ1 to check mineralassemblages in the <2 mm fraction. Representative dif-fractograms are shown in Fig. 10. The Nawakot Com-plex samples from the TS II, TS I and theParautochthon exhibit epizonal and anchizonal meta-morphism. They are rich in dioctahedral 2M1 K-micaand chlorite. Illite±muscovite was detected in allsamples. Chlorite could not be detected in purple slatesof the Nourpul Formation. Albite and quartz are pre-sent in almost all samples. A muscovite/paragonitemixed layer identi®ed by high-angle broadening ofthe 3.3 AÊ peak is present in the Benighat Slate andblack slates of the Nourpul Formation. Samples fromthe Tansen Group fall into the diagenetic zone, andare rich in illitic muscovite and chlorite. Two samplescontained mixed layer illite/smectite and chlorite/smectite.

7.3. IC results

Samples showing the 10 AÊ illite peak were scanned 5times from 10±7D82y (scan rate=0.58 minÿ1, chartspeed=2 cm minÿ1, TC=2 s), and an average KI wasdetermined on both the air-dried and ethylene-glyco-lated samples. Ethylene-glycolated samples gave bettercrystallinity (lower KI values) than the air-driedsamples, especially for the diagenetic and anchizonalrocks (as observed also by Warr and Rice, 1994).Therefore, the KI from ethylene-glycolated samples areused for discussion. Average KI obtained from ethyl-ene-glycolated sample for each tectonic unit in bothsections and for the entire dataset is given in Table 2.The average KI values in the Modi Khola±Kali Gan-daki section is greater than those of the Pokhara±But-wal section, due to non-uniform distribution in thesample collections [see Figs. 11(A) and (B)]. The aver-age KI value in the TS II in the Modi Khola±KaliGandaki section is increased by the greater KI valuesalong the Kusma Fault.

The average KI in the study area decreases north-wards showing that the bulk grade of metamorphismincreases from south to north (structurally upwards).The Parautochthon has an average KI (all data) of0.26620.035D82y (anchizone) for the Nawakot Com-plex and 0.606 2 0.319D82y (diagenetic zone) for theTansen Group. The Thrust sheets I and II have aver-age KI of 0.2372 0.057D82y (anchizone) and 0.19520.060D82y (epizone), respectively. The Palpa KlippeT

able

2

AverageKu ÈblerIndex

values

(ethylene-glycatedsamples)

fortheLesserHim

alayantectonic

unitsin

thePokhara±Tansenarea,centralNepal

Pokhara±Butw

alsection

ModiKhola

Kali±Gandakisection

AllData

Tectonic

units

No.ofsamples

AverageKI(D

82y)

SD

(1s)

No.ofsamples

AverageKI(D

82y)

SD

(1s)

No.ofsamples

AverageKI(D

82y)

SD

(1s)

Metamorphic

zone

Thrust

SheetII

15

0.158

0.021

30

0.231

0.066

45

0.195

0.060

Epizone

Thrust

SheetI

39

0.225

0.050

43

0.247

0.061

82

0.237

0.057

Anchizone

Parautochthon

18

0.256

0.037

22

0.278

0.026

40

0.266

0.035

Anchizone

PalpaKlippe

50.230

0.025

70.274

0.024

12

0.258

0.033

Anchizone

TansenGroup

13

0.438

0.205

10

0.856

0.290

23

0.606

0.319

Diagenetic

zone


which covers the Tansen Group, has lower average KI(0.258 2 0.033D82y, anchizone) than the underlyingrocks.

The distribution of KI along the two sections areshown in Fig. 11. There is quite a similarity betweenthe distribution patterns of the KI along the Pokhara±Butwal section [Fig. 11(A)] and the Modi Khola±KaliGandaki section [Fig. 11(B)]. The Tansen Groupshows diagenetic grade (zeolite facies) of metamorph-ism in the south of the Palpa Klippe and anchizonegrade (prehnite±pumpellyite facies) of metamorphismin the north. There is a sharp break in KI across theunconformity between the Tansen Group and Nawa-kot Complex to the south of the Palpa Klippe,whereas the KI seem rather continuous across thenorthern Tansen unconformity. A sharp break in KI is

also observed across the thrust boundary between theTansen Group and the Palpa Klippe (in both sides).The KI values are highly scattered along the anchi-zone±epizone boundary in the Nawakot Complex ofthe Parautochthon and the TS I. There is no de®nitetrend of KI in those units. However, a sharp decline inKI is observed within a narrow zone near the BKFand the PT. In the TS II, the KI values decreasegradually to the north, with a minimum value belowthe LMCT. Relatively high KI values are observednear the KF and the PKF, which may be due to de-terioration of earlier crystallinity by post-metamorphicdeformation (TeichmuÈ ller et al., 1979 in Kisch, 1991b).Anomalously higher KI values on two samples of theparautochthon (Fig. 11B) are due to highly weatheredsamples.

Fig. 11. Distribution of Illite crystallinity (measured on ethylene±glycolated slides) along the Pokhara±Butwal road section (A) and the Modi

Khola±Kali Gandaki section (B) in central Nepal. Abbreviations and patterns in the cross-section (C) as in Fig. 2. Anchizone limits (0.37/

0.21D82y ) after Kisch (1991a).


8. Discussion

8.1. Polyphase metamorphism in the Higher Himalaya

Polymetamorphism of the Higher Himalayan crys-tallines has been documented in various sections ofcentral Nepal (Le Fort, 1975; Arita, 1983; Hodges andSilverberg, 1988; PeÃ cher, 1989; Inger and Harris, 1992;Hodges et al., 1994). They have reported an earlierhigh P/high T kyanite-grade Barrovian-type meta-morphism (Eohimalayan, Caby et al., 1983) followedby a later lower P/high T sillimanite-grade metamorph-ism (Neohimalayan). At least three metamorphicevents are recognized in the Higher Himalaya of theTansen±Pokhara section (Table 3). The ®rst meta-morphic event (M1) was a high P/high T amphibolitefacies prograde metamorphism as in the other sectionsof central Nepal. The kyanite and pyrope-rich garnetcores belong to the M1. Although the M1 is followedby sillimanite grade metamorphism (lower P/high T) inthe Buri Gandaki and Trisuli river valley sections ofcentral Nepal (PeÃ cher, 1989; Inger and Harris, 1992),sillimanite is not observed in the Tansen±Pokhara sec-tion. The M2 in the later section, however, is shown bywidespread retrogressive metamorphism. The Fe-richand Al-poor white micas from higher Himalayangneisses probably do not coexist with the kyanite andpyrope-rich cores of garnet so they must have crystal-lized or re-equilibrated at lower metamorphic con-ditions. The retrograde zoning pro®les at the marginsof garnets from the Higher Himalaya were formed byresorption due to retrogression. Wide-spread develop-ment of dynamic textures such as fractured and bentkyanite blades and elongated garnet porphyroblastswith pressure shadows shows that the M2 was relatedto the thrusting along the UMCT. Therefore, theinversion of isograds from the garnet zone in the Les-ser Himalaya to the kyanite zone in the Higher Hima-laya is only apparent due to thrusting along theUMCT. The replacement of garnet and biotite bychlorite is the third retrogressive metamorphic event(M3), occurred during exhumation.

8.2. Polyphase metamorphism in the Lesser Himalaya

The IC data from the low- to medium-grade meta-pelites of the Lesser Himalayan formations indicatetwo metamorphic events occurring before and after thedeposition of the Tansen Group. The sharp disconti-nuity in the IC values across the unconformity betweenthe Nawakot Complex and the Tansen Group to thesouth of the Palpa Klippe indicates that the NawakotComplex had been already heated up to the anchizone(prehnite±pumpellyite facies) prior to deposition of thePermo-Carboniferous formations of the TansenGroup. This heating event can be regarded as the ®rstmetamorphic event (M0) in the Lesser Himalaya, andmay be related to D1 which produced the bedding-par-allel foliation in the Nawakot Complex.

The whole Lesser Himalaya su�ered a strong secondmetamorphic event (M2) after deposition of the LowerMiocene formation of the Tansen Group. The gradeof metamorphism and the intensity of deformationduring the M2 increases gradually from south tonorth, reaching a maximum near the UMCT. The M2

in the Tansen Group ranges from a diagenetic zone inthe south to the upper anchizone north of the PalpaKlippe. The IC values seem to pass gradually into thelower anchizone and anchizone±epizone boundary inthe Nawakot Complex while crossing the unconfor-mity to the north of Palpa Klippe. The highly-scat-tered IC values in the Parautochthon and the TS Iindicate that the M2 was not strong enough to comple-tely reset the previous (M1) IC patterns in this part ofthe Nawakot Complex.

Inverted metamorphism is rather clear in the TS II,where IC values lie in the epizone and decrease gradu-ally from the PT to the north, reaching a maximumbelow the LMCT. Metamorphic inversion to the northof the PT has been also supported by the gradualdephengitization of white micas (Fig. 9), and progress-ive change in mineral assemblages to higher grade. TheM2 was a syn- to post-tectonic prograde metamorph-ism as suggested by the snowball garnet textures, pro-grade chemical zoning in garnets, well-developed S±C

Table 3

Metamorphic events in the central Nepal Himalaya

Metamorphic events Higher Himalaya Lesser Himalaya

M0 (pre-Himalayan) ? Anchizone grade prograde metamorphism (D1)

M1 or Eohimalayan (pre- or early-

UMCT)

Kyanite grade prograde

metamorphism

?

M2 or Neohimalayan (syn- to post-

UMCT)

Retrograde metamorphism Diagenetic to garnet grade prograde inverted metamorphism

(D3)

M3 (post-UMCT) Late-stage retrogression Late-stage retrogression (D4 and D5)


fabric and the garnet and muscovite porphyroblastsgrown across the foliation. The M2 was probably re-lated to overthrusting of the Higher Himalayan crys-tallines on the Lesser Himalaya along the UMCT (LeFort, 1975; PeÃ cher, 1975; Arita, 1983; PeÃ cher and LeFort, 1986). Post-kinematic mineral growths in theMCT zone shows that the high temperature conditionsmust have persisted after the main episodes of shearing(Caby et al., 1983; PeÃ cher and Le Fort, 1986). The®nal phase in the Lesser Himalaya was retrogrademetamorphic phase (M3) shown by chloritization ofgarnet and biotite, marginal retrograde zoning in theMCT zone garnets, and increase in KI (deteriorationof IC) along the younger faults. It occurred during theuplift and erosion.

8.3. Timing of metamorphism

The common occurrence of mineral assemblagessuch as Grt±Bt±Ms±Chl±Ab±Qtz in the pelitic schistsindicate that the metamorphic temperature in theMCT zone was about 500±5508C (Winkler, 1974)during the M2 event. Kaneko (1995) estimated meta-morphic temperature of about 400±4508C for theMCT zone with a rapid increase up to 600±6508C nearthe UMCT in the present study area. This is above theargon closure temperature of biotite and muscovite(300 and 3508C respectively; Harrison and McDougall,1980), and corresponds to the argon closure tempera-ture of hornblende (5008C; Harrison, 1981). As the M2

event was essentially syn-tectonic, we can assume thatthere was no signi®cant time gap between peak meta-morphism and uplift and cooling. Thus the K/Ar and40Ar/39Ar ages of hornblende from the MCT zone arelikely to show the age of the M2 event in the Higherand Lesser Himalayas. 40Ar/39Ar hornblende agesfrom the MCT zone in Nepal Himalaya range from 2823.3 Ma in the Trisuli river valley (Macfarlane, 1993)to 24.6 2 1.9 Ma in the Buri Gandaki river valley(Copeland et al., 1991) and 20.920.2 Ma in the Ever-est region (Hubbard and Harrison, 1989). Youngerages determined on muscovite (7±14 Ma 40Ar/39Arages, Macfarlane et al., 1992, Vannay and Hodges,1996; Copeland et al., 1991), on biotite (9±13 Ma40Ar/39Ar ages, Edwards, 1995; Copeland et al., 1991)and on zircon (1.2±2.3 Ma F±T ages, Arita and Gan-zawa, 1997) can be explained in terms of their lowerclosure temperatures and continuous exhumation andcooling in the MCT zone. The age of the M2 event isalso constrained by the emplacement age of leucogra-nites (24±16 Ma Rb/Sr isochron and U/Pb mineralages: Vidal, 1978; Harrison et al., 1995; Sorkhabi andStump, 1993) because according to Le Fort (1975,1986), the formation of leucogranite was related to thethrusting along the MCT zone.

The M0 event in the Lesser Himalaya is pre-Himala-

yan, probably early-Paleozoic (Table 3). Radiometricages from the low-grade metamorphic rocks of theLesser Himalaya are relatively scarce compared tothose of the higher grade rocks. Khan and Tater(unpublished report) have obtained 5502 17 Ma and559218 Ma whole rock K/Ar ages on Benighat Slatefrom the Kali Gandaki valley. Evidence of pre-colli-sional orogeny and metamorphism in the Lesser Hima-laya has also been documented in the KumaunHimalaya of India (Johnson and Oliver, 1990; Oliveret al., 1995) and in Pakistan (Baig et al., 1988;Chaudhry and Ghazanfar, 1989). An early Paleozoicthermal event in the Himalaya is also supported by theoccurrence of 500 Ma granites in the Lesser Himala-yan thrust sheets (SchaÈ rer and AlleÁ gre, 1983; Le Fortet al., 1986a).

One of the most outstanding problems in the Hima-layan metamorphism has been the age of the M1 eventin the Higher Himalaya. Many geologists consider M1

as having occurred due to the crustal thickening afterthe India-Asia collision and before or during earlyperiod of UMCT activity (Caby et al., 1983; Le Fort,1986; Hodges et al., 1988; PeÃ cher, 1989). Geochrono-metric data do not precisely constrain the age of M1 inthe Higher Himalaya because most of the commonlyused isotopic geochronometers were completely or par-tially reset during M2. The peak metamorphic tem-perature in the Higher Himalaya was 600±7008C(PeÃ cher, 1989) or up to 7508C (Kaneko, 1995), whichis higher than the Ar closure temperature of horn-blende (500±5508C). 40Ar/39Ar ages of hornblenderange from 46 to 37 Ma in the north-eastern part ofthe present study area (Vannay and Hodges, 1996).Based on these ages it is reasonable to consider M1 asan early Tertiary event as suggested by Arita et al.(1990) and Sorkhabi and Stump (1993). However, thepossibility of a pre-Himalayan age for M1 cannot beruled out. The Himalaya has been a�ected by severalpre-Himalayan thermal events (see Arita et al., 1990;Sorkhabi and Stump, 1993) and detrital muscovites inthe Lesser Himalayan sandstones, which were derivedfrom older high-grade metamorphic rocks (Fig. 9)have been dated at 728 Ma (Krummenacher, 1961).

9. Conclusions

The present study shows that the central NepalHimalaya has evolved through polyphase deformationand metamorphism (Fig. 12). The Lesser Himalayahas experienced at least ®ve deformational events (D1±D5), two of which (D1 and D2) are pre-Himalayan.Bedding-parallel foliation (S1) formed during D1 wasdeformed into folds with NNE±SSW trending axes(F2) during D2. The D3 event associated with theUMCT activity formed dominant bedding-parallel


Fig. 12. Conceptual models (not in scale) showing the tectono-metamorphic evolution of the Lesser Himalaya in central Nepal. Abbreviations as

in Figs. 2 and 4.


shear planes (S3), and conspicuous NNE±SSW trend-ing mineral and stretching lineations (L3) in the LesserHimalaya. Shear-sense markers like S±C fabric, asym-metric pressure shadows and spiral garnets show atop-to-the-south sense of thrusting and shearing duringD3. Post-UMCT thrust propagation (D4) resulted inthe major and minor folds with WNW±ESE trendingaxes (F4), NE- or SW-dipping crenulation, slaty andfracture cleavages (S4), and pencil lineations (L4). Thelast phase of deformation (D5) is evidenced by brittlefaulting that cross-cut all of the previous structures.

Illite crystallinity as well as muscovite and garnetcompositions reveal the polymetamorphic history ofthe central Nepal Himalaya. The Lesser Himalayashows a Pre-Himalayan anchizonal metamorphism(M0) and Neohimalayan diagenetic to garnet grademetamorphim (M2). The M2 was originally invertedthorughout the Lesser Himalaya. The Higher Himala-yan rocks were recystallized under amphibolite faciescondition (kyanite grade) prior to the UMCT activity(M1) and reequillibrated under lower metamorphicconditions during or after thrusting along the UMCT(M2). Thus the isograd inversion from garnet zone inthe Lesser Himalaya to kyanite zone in the HigherHimalaya is only apparent due to post-metamorphicthrusting. Timing of M1 in the area is not clear.Although M1 is believed to be post-collisional, it isequally possible that it was pre-collisional, as theHimalaya had been a�ected by several pre-Himalayanthermal events.

Both the Lesser and Higher Himalayas have experi-enced late-stage retrogression (M3) during exhumation.

Acknowledgements

The authors are indebted to the Ministry of Edu-cation, Science, Sports and Culture, Japan, for theresearch scholarship to L.P.P. The ®eld work for thisresearch was supported by grant-in-aid of 1997 for thepromotion of research from the Tokyo GeographicalSociety. The work was also partly supported by theSasakawa Scienti®c Research Grant from the JapanScience Society. We thank H. Thapa, K.R. Regmi andD.P. Jaishi of Tribhuvan University, Nepal for theirassistance in the ®eld. T. Kuwajima and H. Nomuraof Hokkaido University, Japan helped with thin sec-tions. S. Terada helped during microprobe analysisand T. Tajima helped during the X-ray di�ractometeroperation. L.P.P. had useful discussion with K. Oho-mori on IC interpretation. Corrections and commentsby J.P. Burg, R.B. Sorkhabi, and B. Roser greatlyhelped to improve the manuscript. We thank A.PeÃ cher for the critical review and fruitful comments onthe manuscript.

Appendix A

Equation used to calibrate the centrifuge

t � 63:0� 108 Z log 10 �R=S �=N2r2�Dÿ d�where, t = rotation time (min), Z=viscosity of water(0.01002 poise), R=distance between the rotation axisand water surface in the centrifuge tube (cm), S=dis-tance between the rotation axis and the sedimentationlevel in the centrifuge tube (cm), N = number of ro-tation minÿ, r=particle size (mm), D=density of theparticle (2.50 g mlÿ1), and d=density of water (1.00 gmlÿ1).

References

Arita, K., 1983. Origin of the inverted metamorphism of the lower

Himalayas, central Nepal. Tectonophysics 95, 43±60.

Arita, K., Ganzawa, Y., 1997. Thrust tectonics and uplift process of

the Nepal Himalaya revealed from ®ssion-track ages. Journal of

Geography 106, 156±167 (in Japanese with English abstract).

Arita, K., Dallmeyer, R.D., Takasu, A., 1997. Tectonothermal evol-

ution of the Lesser Himalaya, Nepal: constraints from 40Ar/39Ar

ages from the Kathmandu Nappe. The Island Arc 6, 372±385.

Arita, K., Gautam, P., Ganzawa, Y., 1990. Two metamorphic events

of the Nepal Himalayas prior and posterior to India±Asia col-

lision. Journal of the Faculty of Science, Hokkaido University

(Japan) Series IV 22, 519±528.

Arita, K., Sharma, T., Fujii, Y., 1984. Geology and structure of the

Jajarkot±Piuthan area, central Nepal. Journal of Nepal

Geological Society Special Issue 4, 5±27.

Arita, K., Hayashi, D., Yoshida, M., 1982. Geology and structure of

the Pokhara±Piuthan area, central Nepal. Journal of Nepal

Geological Society Special Issue 2, 5±29.

Baig, M.S.B., Lawrence, R.D., Snee, L.W., 1988. Evidence for late

Precambrian to early Cambrian orogeny in northwest Himalaya,

Pakistan. Geological Magazine 125, 83±86.

Barker, A.J., 1990. Introduction to Metamorphic Textures and

Microstructures. Blackie, New York, p. 162.

Brunel, M., Colchen, M., Le Fort, P., Mascle, G., PeÃ cher, A. 1979.

Structural analysis and tectonic evolution of the central Himalaya

of Nepal. In: Saklani, P.S. (Ed.), Structural Geology of the

Himalaya. Today and Tomorrow's Printers and Publishers, New

Delhi, pp. 247±264.

Burbank, D.W., Beck, R.A., Mulder, T. 1996. The Himalayan fore-

land basin. In: Yin, A., Harrison, T.M. (Eds.), Tectonics of Asia.

Cambridge University Press, New York, pp. 149±188.

Burch®el, B.C., Royden, L.H., 1985. North-south extension within

the convergent Himalayan region. Geology 13, 679±682.

Burch®el, B.C., Zhiliang, C., Hodges, K.V., Yuping, L., Royden,

L.H., Changrong, D., Jiene, X., 1992. The South Tibetan

Detachment System, Himalayan orogen: extension contempora-

neous with and parallel to shortening in a collisional mountain

belt. Geological Society of America Special Paper 269, 1±41.

Burg, J.P., Brunel, M., Gapais, D., Chen, G.M., Liu, G.H., 1984.

Deformation of leucogranites of the crystalline Main Central

sheet in southern Tibet (China). Journal of Structural Geology 6,

536±542.

Caby, R., PeÃ cher, A., Le Fort, P., 1983. Le grand Chevauchement

central himalayen: nouvelles donneÂ es su le meÂ tamorphisme

inverse aÁ la base de la Dalle du Tibet. Revue de GeÂ ographie phy-

sique et de GeÂ ologie Dynamique, Paris 24, 89±100.


Chaudhry, M.N., Ghazanfar, M., 1989. Observations on

Precambrian orogeny and the age of the metamorphism in

Northwest Himalaya, Pakistan. Kashmir Journal of Geology 6,

13±57.

Cobbold, P.R., Quinquis, H., 1980. Development of sheath folds in

shear regimes. Journal of Structural Geology 2, 119±126.

Colchen, M., Le Fort, P., PeÃ cher, A., 1980. Carte geÂ ologique

Annapurnas-Manaslu-Ganesh Himalaya du NeÂ pal au 1/200 000eÁ .

Centre National de la Recherche Scienti®que, Paris.

Copeland, P., Harrison, T.M., Hodges, K.V., MarueÂ jol, P., Le Fort,

P., PeÃ cher, A., 1991. An early Pliocene disturbance of the Main

Central Thrust, central Nepal: implications for Himalayan tec-

tonics. Journal of Geophysical Research 96, 8475±8500.

DeCelles, P.G., Gehrels, G.E., Quade, J., Ojha, T.P., Kapp, P.A.,

Upreti, B.N., 1998. Neogene foreland basin deposits, erosional

unroo®ng, and kinematic history of the Himalayan fold-thrust

belt, western Nepal. Geological Society of America Bulletin 110,

2±21.

Dhital, M.R. 1989. Duplex and imbricate stack in the Dang Valley,

west Nepal. In: International Symposium on Intermontane

Basins: Geology and Resourses, Chiang Mai, Thailand, pp. 386±

398.

Dhital, M.R., Paudel, L.P., Shrestha, R., Thapa, P.B., Oli, C.P.,

Paudel, T.R., Jaisi, D.P., 1998. Geological map of the area

between Kusma, Syangja, and Galyang. Central Department of

Geology, Tribhuvan University, Kirtipur, Kathmandu, Nepal.

Dunoyer de Segonzac, G., Ferrero, J., KuÈ bler, B., 1968. Sur la cris-

talliniteÂ de l'illite dans la diageneÁ se et l'anchimeÂ tamorphisme.

Sedimentology 10, 137±143.

Edwards, R.M., 1995. 40Ar/39Ar geochronology of the Main Central

Thrust (MCT) region: evidence for late Miocene to Pliocene dis-

turbances along the MCT, Marsyangdi River Valley, west-central

Nepal Himalaya. Journal of Nepal Geological Society 10, 41±46.

Fuchs, G., Frank, W., 1970. The geology of west Nepal between the

rivers Kali Gandaki and Thulo Bheri. Jahrbuch der Geologischen

Bundesanstalt 18 (103), 1±103.

Gansser, A., 1964. Geology of the Himalayas. Interscience, London.

Guillot, S., Hodges, K., Le Fort, P., PeÃ cher, A., 1994. New con-

straints on the age of the Manaslu leucogranite: evidence for epi-

sodic tectonic denudation in the central Himalayas. Geology 22,

559±562.

Harrison, T.M., 1981. Di�usion of 40Ar in hornblende.

Contributions to Mineralogy and Petrology 78, 324±331.

Harrison, T.M., McDougall, I., 1980. Investigations of an intrusive

contact, northwest Nelson, New Zealand Ð I. Thermal, chrono-

logical and isotopic constraints. Geochimica Cosmochimica Acta

44, 1985±2003.

Harrison, T.M., McKeegan, K.D., Le Fort, P., 1995. Detection of

inherited monazite in the Manaslu leucogranite by 208Pb/232Th

ion microprobe dating: crystallization age and tectonic impli-

cation. Earth and Planetary Science Letters 133, 271±282.

Harrison, T.M., Ryerson, F.J., Le Fort, P., Yin, A., Lovera, O.M.,

Catlos, E.J., 1997. A late Miocene±Pliocene origin of the central

Himalayan inverted metamorphism. Earth and Planetary Science

Letters 146, E1±E7.

Hashimoto, S., Ohta, Y., Akiba, C., 1973. Geology of the Nepal

Himalayas. Saikon, Japan.

Hodges, K.V., Silverberg, D.S., 1988. Thermal evolution of the

Greater Himalaya, Garhwal, India. Tectonics 7, 583±600.

Hodges, K.V., Hames, W.E., Olszewski, W., Burch®el, B.C.,

Royden, L.H., Chen, Z., 1994. Thermobarometric and 40Ar/39Ar

geochronologic constraints on Eohimalayan metamorphism in the

DinggyeÃ area, southern Tibet. Contributions to Mineralogy and

Petrology 117, 151±163.

Hodges, K.V., Hubbard, M.S., Silverberg, D.S., 1988. Metamorphic

constraints on the thermal evolution of the central Himalayan

Orogen. Philosophical Transactions of the Royal Society of

London A326, 257±280.

Hodges, K.V., Parrish, R.R., Searle, M.P., 1996. Tectonic evolution

of the central Annapurna Range, Nepalese Himalayas. Tectonics

15, 1264±1291.

Hubbard, M.S., Harrison, T.M., 1989. 40Ar/39Ar age constraints on

deformation and metamorphism in the Main Central Thrust zone

and Tibetan Slab, eastern Nepal Himalaya. Tectonics 8, 865±880.

Inger, S., Harris, N.B.W., 1992. Tectonothermal evolution of the

High Himalayan crystalline sequence, Langtang valley, northern

Nepal. Journal of Metamorphic Geology 10, 439±452.

Johnson, M.R.W., Oliver, G.J.H., 1990. Precollison and postcollison

thermal events in the Himalaya. Geology 18, 753±756.

Kaneko, Y., 1995. Thermal structure in the Annapurna region, cen-

tral Nepal Himalaya: implication for the inverted metamorphism.

Journal of Mineralogy, Petrology and Economic Geology 90,

143±154.

Kaneko, Y., 1997. Two-step exhumation model of the Himalayan

metamorphic belt, central Nepal. The Journal of the Geological

Society of Japan 103, 203±226.

Kisch, H.J., 1991a. Illite crystallinity: recommendations on sample

preparation, X-ray di�raction settings, and interlaboratory

samples. Journal of Metamorphic Geology 9, 665±670.

Kisch, H.J., 1991b. Development of slaty cleavage and degree of

very-low-grade metamorphism: a review. Journal of Metamorphic

Geology 9, 735±750.

Kizaki, K. 1994. An outline of the Himalayan upheaval, a case

study of the Nepal Himalaya. Japan International Cooperation

Agency (JICA), Kathmandu, Nepal.

Kretz, R., 1983. Symbols for rock-forming minerals. American

Mineralogist 68, 277±279.

Krummenacher, D., 1961. DeÂ terminations d'age isotopique faites sur

quelques roches de l'Himalaya du NeÂ pal par la meÂ thode potass-

ium±argon. Schweizerische Mineralogische und Petrographische

Mitteilungen 41, 273±282.

KuÈ bler, B., 1967 La dristalliniteÂ de l'illite et les zones tout fait supeÂ r-

ieures du meÂ tamorphisme, Etages Tectoniques A la BaconnieÁ re,

Neuchatel (Suisse) 105±121.

Le Fort, P., 1975. Himalayas: the collided range, present knowledge

of the continental arc. American Journal of Science 275A, 1±44.

Le Fort, P., 1986. Metamorphism and magmatism during the

Himalayan collision. In: Coward, M.P., Ries, A.C. (Eds.),

Collision Tectonics, Geological Society Special Publication, 19,

pp. 159±172.

Le Fort, P., Debon, F., PeÃ cher, A., Sonet, J., Vidal, P., 1986a. The

500 Ma magmatic event in alpine southern Asia, a thermal epi-

sode at Gondwana scale. Sciences de la Terre Memoir 47, 191±

209.

Le Fort, P., PeÃ cher, A., Upreti, B.N., 1986b. A section through the

Tibetan Slab in central Nepal (Kali Gandaki Valley): mineral

chemistry and thermobarometry of the Main Central Thrust

zone. Sciences de la Terre, Nancay, MeÂ moire 47, 211±228.

Macfarlane, A.M., 1993. Chronology of the tectonic events in the

crystalline core of the Himalaya, Langtang National Park, central

Nepal. Tectonics 12, 1004±1025.

Macfarlane, A.M., 1995. An evaluation of the inverted metamorphic

gradient at Langtang National Park, central Nepal Himalaya.

Journal of Metamorphic Geology 13, 595±612.

Macfarlane, A.M., Hodges, K.V., Lux, D., 1992. A structural analy-

sis of the MCT zone, Langtang National Park, central Nepal

Himalaya. Geological Society of America Bulletin 104, 1389±

1402.

Miyashiro, A., 1973. Metamorphism and Metamorphic Belts. Allen

& Unwin, London.

Molnar, P., Tapponnier, P.T., 1975. Cenozoic tectonics of Asia:

e�ects of a continental collision. Science 189, 419±426.

Nakata, T., 1982. A photogrametric study on active faults in the


Nepal Himalayas. Journal of Nepal Geological Society Special

Issue 2, 67±80.

Oliver, G.H.J., Johnson, M.R.W., Fallick, A.E., 1995. Age of meta-

morphism in the Lesser Himalaya and the Main Central Thrust

zone, Garhwal India: results of illite crystallinity, 40Ar/39Ar

fusion and K/Ar studies. Geological Magazine 132, 139±149.

Paudel, L.P., Arita, K., 1998. Geology, structure and metamorphism

of the Lesser Himalayan metasedimentary sequence in Pokhara

region, western Nepal. Journal of Nepal Geological Society 18,

97±112.

PeÃ cher, A., 1975. The Main Central Thrust of the Nepal Himalaya

and the related metamorphism in the Modi Khola cross-section

(Annapurna Range). Himalayan Geology 5, 115±131.

PeÃ cher, A., 1977. Geology of the Nepal Himalaya: deformation and

petrography in the Main Central Thrust Zone. Ecologie et

GeÂ ologie de l'Himalaya, Colloques Internationaux du Centre

National de la Recherche Scienti®que, Paris 268, 301±318.

PeÃ cher, A., 1989. The metamorphism in the central Himalaya.


PeÃ cher, A., Le Fort, P., 1986. The metamorphism in central

Himalaya: its relation with the thrust tectonics. Sciences de la

Terre, Nancy, MeÂ moire 47, 285±309.

PeÃ cher, A., Bouchez, J.L., Le Fort, P., 1991. Miocene dextral shear-

ing between Himalaya and Tibet. Geology 19, 683±685.

Quinquis, H., Audren, C., Brun, J.P., Cobbold, P.R., 1978. Intense

progressive shear in Ile de Groix blueschists and compatibility

with subduction or obduction. Nature 273, 43±45.

Rai, S.M., Guillot, S., Le Fort, P., Upreti, B.N., 1998. Pressure-tem-

perature evolution in the Kathmandu and Gosainkund regions,

central Nepal. Journal of Asian Earth Sciences 16, 283±298.

Sakai, H., 1983. Geology of the Tansen Group of the Lesser

Himalaya in Nepal. Memoirs of the Faculty of Science, Kyushu

University (Japan) 25, 27±74 Series D, Geology.

Sakai, H., 1985. Geology of the Kali Gandaki Supergroup of the

Lesser Himalaya in Nepal. Memoirs of the Faculty of Science,

Kyushu University (Japan) Series D, Geology 25, 337±397.

Schelling, D., Arita, K., 1991. Thrust tectonics, crustal shortening,

and the structure of the far-eastern Nepal Himalaya. Tectonics

10, 851±862.

SchaÈ rer, U., AlleÁ gre, C.J., 1983. The Palung granite (Himalaya):

high-resolution U±Pb systematics in zircon and monazite. Earth

and Planetary Science Letters 63, 423±432.

Sinha-Roy, S., 1982. Himalayan Main Central Thrust and its impli-

cations for Himalayan inverted metamorphism. Tectonophysics

84, 197±224.

Sorkhabi, R.B., Arita, K., 1997. Toward a solution for the

Himalayan puzzle: mechanism of inverted metamorphism con-

strained by the Siwalik sedimentary record. Current Science 72,

862±873.

Sorkhabi, R.B., Stump, E., 1993. Rise of the Himalaya: a geochro-

nologic approach. GSA Today 3, 87±92.

Spear, F.S., 1993 Metamorphic phase equilibria and pressure±tem-

perature±time paths, Mineralogical Society of America

Monograph, Washington.

StoÈ cklin, J., 1980. Geology of Nepal and its regional frame. Journal

of Geological Society of London 137, 1±34.

Tokuoka, T., Takayasu, K., Yoshida, M., Hisatomi, K., 1986. The

Churia (Siwalik) Group of the Arung Khola area, west central

Nepal. Memoirs of the Faculty of Science, Shimane University

20, 135±210.

Upreti, B.N., Sharma, T., Merh, S.S., 1980. Structural geology of

Kusma-Sirkang section of the Kali Gandaki valley and its bear-

ing on the tectonic framework of Nepal Himalaya.

Tectonophysics 62, 155±164.

Vannay, J.C., Hodges, K.V., 1996. Tectonothermal evolution of the

metamorphic core between the Annapurna and Dhaulagiri, cen-

tral Nepal. Journal of Metamorphic Geology 14, 635±656.

Vidal, P., 1978. Rb-Sr systematics in granite from central Nepal

(Manaslu): signi®cance of the Early Miocene age and 87Sr/86Sr

ratio in Himalayan orogeny. Geology 6, 196±197.

Wada, K., 1966. Qualitative and quantitative determinations of clay

minerals. Journal of the Science of Soil and Manure, Japan 37,

9±17.

Warr, L.N., 1996. Standarized clay-mineral crystallinity data from

the very low-grade metamorphic facies rocks of southern New

Zeland. European Journal of Mineralogy 8, 115±127.

Warr, L.N., Rice, A.H.N., 1994. Interlaboratory standardization and

calibration of clay mineral crystallinity and crystallite size data.


Winkler, H.G.F., 1974. Petrogenesis of metamorphic rocks. Springer,

New York.

Zhao, W., Nelson, K.D., project INDEPTH Team, 1993. Deep seis-

mic re¯ection evidence for continental underthrusting beneath

southern Tibet. Nature 366, 557±559.


Documents

Tectonic and polymetamorphic history of the Lesser ...directory.umm.ac.id/Data Elmu/jurnal/J-a/Journal of Asian Earth... · Tectonic and polymetamorphic history of the Lesser Himalaya