Tectonic and polymetamorphic history of the Lesser Himalaya in
central 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 divided tectonically 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 (D1and D2) belong to the pre-Himalayan (pre-Tertiary) orogeny. Petrographic, microprobe and illite crystallinity data show polymetamorphic evolution of the Lesser and Higher Himalayas in central Nepal. The Lesser Himalaya suered 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 suered an Eohimalayan (pre or early-UMCT) kyanite-grade prograde metamorphism (M1) which was, in turn, overprinted by Neohimalayan (syn-early-UMCT) retrograde metamorphism (M2). The isograd inversion from garnet zone in the Lesser Himalaya to kyanite zone in the Higher Himalaya is only apparent due to post-metamorphic thrusting along the UMCT. Both the Lesser and Higher Himalayas have undergone late-stage retrogression (M3) during exhumation.72000 Elsevier Science Ltd. All rights reserved.
1. Introduction
The Himalaya were formed at the northern margin of the Indian sub-continent due to collision of the Indian and Eurasian plates in the Middle Eocene (e.g. Le Fort, 1975; Molnar and Tapponnier, 1975). The Himalaya consists of three main thrust-bounded litho-tectonic units; the Sub-Himalaya (Siwaliks), the Lesser Himalaya, and the Higher Himalaya (including the Central 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 (StoÈcklin, 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
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* Corresponding author. Central Department of Geology, Tribhu-ban University, Kirtipur, Kathmandu, Nepal. Fax: +81 11 706 5305.
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 by Richard 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; PeÃcher, 1989 and many others). The metamorphic grade appears to increase northwards (structurally upwards) from chlorite and biotite zones in the Lesser Himalaya through garnet zone in the MCT zone to kyanite and sillimanite zones in the Higher Himalaya. This feature has received much attention over the last two decades, and several models have been proposed to explain its origin (see a review by Sorkhabi and Arita, 1997). The discussions regarding the inverted metamorphism, however, have been limited to the MCT zone and the Higher Himalaya. In this connec-tion, it is important to constrain the thermal structure of the low-grade metamorphic rocks of the Lesser Himalaya where the inverted metamorphism is exhib-ited.
In an attempt to unravel the structure and meta-morphic history of the Lesser Himalaya in central Nepal, we carried out structural mapping and study on illite crystallinity of low-grade metamorphic rocks,
along with petrographic study and microprobe analysis of rocks along two sections across the Lesser Himalaya and the lower part of the Higher Himalaya. This paper presents the results and discusses the polyphase deformation and metamorphic history of the Lesser and Higher Himalayas on the ground of new data. A conceptual 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 of central Nepal are presented in Fig. 2. Three major north-dipping thrusts occur in central Nepal; the Main Frontal Thrust (MFT), MBT and the MCT (Fig. 2). These thrusts propagated from north to south with time and splays-o an underlying horizontal decolle-ment known as the Main Detachdecolle-ment Fault (MDF, Schelling and Arita, 1991) or the Main Himalayan Thrust (MHT, Zhao et al., 1993). The South Tibetan Detachment System (STDS) marking the boundary between the Higher Himalayan crystallines and the overlying 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).
®el et al., 1992) which do also have a dextral strike-slip component (PeÃcher et al., 1991).
The Lesser Himalaya is divided into several tectonic packages by a series of north-dipping thrusts and faults. Basically it may be divided into the inner (north) and outer (south) belts by the Bari Gad±Kali Gandaki Fault (BKF) (Arita et al., 1982; Sakai, 1985). The outer belt is a parautochthonous unit overlain by the Palpa Klippe and is distributed mainly in the southern part along the MBT. The inner belt consists of the Thrust Sheet I (TS I) and Thrust Sheet II (TS II) divided by the Phalebas Thrust (PT) (Upreti et al., 1980). The northernmost part of the Lesser Himalaya is an intensely sheared and mylonitized MCT zone striking from east to west. It is bounded by the UMCT in the north and by the Lower MCT (LMCT) in the south, which are also named as the MCT II and MCT I, respectively by Arita et al. (1982).
The Lesser Himalaya forms a foreland-propagating duplex structure in most parts of the Nepalese Hima-laya (east Nepal, Schelling and Arita, 1991; west Nepal, Dhital, 1989; far-western Nepal, DeCelles et al., 1998), an interpretation supported by our ®eld ob-servations in central Nepal [Fig. 2(B)]. The rocks of the Higher Himalaya with the overlying Tethys sedi-ments occur as nappe over the Lesser Himalaya in the Kathmandu area [Fig. 2(A)]. Although the crystalline rocks are not present in the Tansen±Pokhara section, they may have existed throughout the Lesser Himalaya in central Nepal, and the lateral continuity was destroyed by erosion (Kizaki, 1994). The Higher Himalayan rocks were thrust a great distance to the south (very close to the MBT) along the UMCT, with the 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 the Kathmandu Nappe and those of the underlying Lesser Himalayan units shows that the UMCT roof thrust was initially horizontal and later was folded, along with the autochthon, during the propagation of horses. The Palpa Klippe, made up of the Nawakot Com-plex, occupies the frontal part of the Lesser Himalaya covering the autochthonous Tansen Group. The basal part of the klippe is a highly sheared and brecciated tectonic melange zone, about 10 m thick along the Tansen±Pokhara motor road. At some places the mel-ange zone is about 200 m thick (Sakai, 1985). The root thrust sheet of the Palpa Klippe has not yet been explained 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 that brought 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 the north at the surface (about 70±808) and is parallel to bedding of both the hangingwall and the footwall. The MBT probably dips more gently at depth and joins the MDF in the north. The MBT is marked by a wide crushed zone, which is expressed as a continuous topo-graphic depression in the study area.
The MCT zone, TS II, TS I, and the Parautochthon are the horses of the southward-propagating duplex bounded by the imbricate faults i.e. the LMCT, PT and the BKF. The LMCT in the Pokhara area is very discordant, and cuts many units at the footwall. To the NW of Pokhara, for example, the MCT zone rocks discordantly override the Fagfog Quartzite of the TS II (Fig. 4). In the Piuthan area, the MCT zone rocks (including the Ulleri-type gneisses) are thrust over the Parautochthon and make a klippe (Jajarkot Klippe) (Arita et al., 1984). The LMCT, however, is often obscured in the eastern parts of central Nepal where the rock units of similar lithology are juxta-posed by the LMCT. In such areas, the LMCT is marked by the dierence in structural style between the MCT zone and the TS II. The MCT zone has homoclinally northward-dipping foliation, whereas the TS II shows foliation folded into a dome and basin structure. The LMCT dips about 10±158 to the NE at the surface and probably steepens at depth as the foli-ation in the MCT zone becomes steeper (30±508) in the north.
The PT is almost parallel to both the UMCT and LMCT. It extends from NW to SE and joins the BKF to the south of Gorkha (Fig. 2). The BKF is steeper (50±708) than the PT, and cuts through the Jajarkot Klippe in the west and the Kathmandu Nappe in the east (Fig. 2). It is thus an out-of-sequence fault (Arita et al., 1997). The Pindi Khola Fault (PKF), which is traced locally in the Syangja area, is a south-dipping fault and joins the BKF in the east and west. It can be interpreted as an antithetic back-thrust developed on the 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 dicult to determine the exact timing of the development of each thrust and fault in the area, it is possible to estimate the approximate timing of activity along major thrusts and the sequence of thrust 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 thrust pile. It is the oldest thrust in the area because it has been folded and faulted by later thrusts and faults. Overthrusting of the Tansen Group (Early Miocene
and older in age) by the UMCT indicates that this thrust reached to the southern part of the Lesser Himalaya later than the Early Miocene, probably in the Middle Miocene. But the UMCT may have been initiated earlier than this time in its root zone. The Dumri 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 of the northern part of the area began at least in the Early Miocene. The uplift may be related to the ramp-ing along the UMCT at depth. Assumramp-ing that the peak 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), an early Miocene age (about 22±15 Ma) has been assigned to movement along the UMCT (Hodges et al., 1996; Macfarlane, 1993). Dextral shearing and north-directed detachment along the STDS was almost synchronous with the UMCT (Guillot et al., 1994; PeÃcher et al., 1991). The movement along the UMCT in the Lesser Himalayan nappe zones was terminated between 14±5 Ma due to the out-of-sequence thrusting in the Lesser Himalaya (Arita et al., 1997). However,
there are many younger isotopic ages (8±3 Ma) from the northern root zone of the UMCT implying either continuous movement at the root zone until the late Pliocene (Arita et al., 1997) or a late Miocene-Pliocene reactivation 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 faulting along the BKF (equivalent to the Trisuli±Likhu Fault in the Kathmandu area) has been constrained to be between 10±7.5 Ma (Arita et al., 1997). The BKF is an out-of-sequence fault and truncates the overlying thrusts, 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 the time of movement along the MBT by the imbrication of 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 Ma in age in central Nepal (Tokuoka et al., 1986). It implies that the MBT reached over the Siwaliks later than the Lower Pleistocene. However, changes in the sedimentation patterns within the Siwaliks after 11 Ma indicates that initial motion along the MBT started in the Late Miocene (Burbank et al., 1996). The MFT places 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 MFT are believed to be still active (Nakata, 1982; Kizaki, 1994).
3. Lithostratigraphy
The Lesser Himalaya consists principally of the late Precambrian to early Paleozoic Nawakot Complex (StoÈcklin, 1980) and the unconformably overlying Gondwana and post-Gondwana sediments (Sakai, 1983) (Fig. 3). The Nawakot Complex has been vary-ingly named as the Midland Metasediment Group by Hashimoto et al. (1973) and Midland Formations by Le Fort (1975), PeÃcher (1977) and Colchen et al. (1980). A full succession of the Nawakot Complex is observed only in TS II, in the Dhading±Malekhu area, where it attains a total thickness of approx. 10 km. It is divided into the Lower and Upper Nawakot Groups by an unconformity (StoÈcklin, 1980).
From the bottom to the top, the Lower Nawakot Group consists of the Kuncha Formation, Fagfog Quartzite, Dandagaon Phyllite, Nourpul Formation, and the Dhading dolomite. The Upper Nawakot Group is divided into the Benighat Slate, Malekhu
Limestone, and the Robang Formation (StoÈcklin, 1980; Fig. 3). These formations can be traced from east to west in central Nepal, and are repeated several times by folding and thrusting (Paudel and Arita, 1998). In the Pokhara area (Fig. 4), the TS II consists only of the approx. 3 km thick lower part of the Nawakot Complex (Kuncha Formation, Fagfog Quartzite and Dandagaon Phyllite). The TS I com-prises the middle part of the Nawakot Complex (Nourpul Formation, Dhading Dolomite and Benighat Slate). The Nourpul Formation occupies the core of an anticline along the Andhi Khola (Khola means river in Nepali). It is also exposed along the Kali Gan-daki River Valley south of Phalebas. The Dhading dolomite is observed at Syangja. The Benighat Slate is exposed just to the north of the BKF (southern part of Fig. 4). The Parautochthon comprises the middle and upper parts of the Nawakot Complex. The Nourpul Formation, Dhading Dolomite and the Benighat Slate are exposed along the motor road between Ramdighat and Tansen and constitute the northern limb of the Tansen Synclinorium while the Malekhu Limestone is exposed just to the north of the MBT and form the southern limb of the Tansen Synclinorium (Fig. 2). The Palpa Klippe, which covers the Parautochthon, is made up of the Nourpul Formation.
The Gondwana and post-Gondwana sediments which unconformably overlie the Nawakot Complex of the Parautochthon were collectively named as the Tansen Group by Sakai (1983) (Fig. 3). The Gond-wana sediments are divided into the Sisne, Taltung, and Amile Formations. The post-Gondwana sediments are 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 lithologically divided into the Lower and Upper Units (Figs. 3 and 4). The Lower Unit consists of interlayered garnetifer-ous pelitic and psammitic schists, with a few bands of chloritic schists and quartzites. Mylonitic augen gneisses (Ulleri augen gneiss of Le Fort, 1975) inter-bedded with psammitic schists, and pegmatite veins cross-cutting the main foliation are found in the lower part (Paudel and Arita, 1998). The Upper Unit is dominated by graphitic schist, calc-schist, and marble. Amphibolite bands are found at dierent levels throughout the MCT zone. The MCT zone rocks are possibly the sheared and metamorphosed equivalents of the Nawakot Complex (Hashimoto et al., 1973; PeÃcher, 1977).
The Higher Himalayan crystalline rocks are observed along the upper part of the Modi Khola and the Seti Khola valleys. These comprise coarse-grained, kyanite-bearing banded gneisses, augen gneisses and schists. The banded gneisses consist of alternating bio-tite rich and feldspar-quartz rich layers. Kyanite blades
Table 1
Deformational events and related structures in the Lesser Himalaya of central Nepal along the Tansen±Pokhara section
Pre-Himalayan phases Himalayan (syn- to post-UMCT) phases
Tectonic units D1 D2 D3 D4 D5
MCT zone Foliation preserved as inclusion trails in garnet (S1) Not seen S±C fabric, NNE±SSW mineral and stretching lineations (L3)
WNW±ESE crenulation and kink folds (F4), NE- or SW-dipping crenulation cleavage (S4)
Small-scale brittle faults Thrust sheet II Bedding-parallel foliation (S1=S0) NNE±SSW trending and west
vergent isoclinal and drag folds (F2)
Bedding-parallel shear planes (S3=S1=S0), NNE±SSW mineral and stretching lineations (L3)
WNW±ESE large scale open folds and minor folds (F4), NE- or SW-dipping crenulation cleavage (S4)
Small-scale brittle faults Thrust sheet I Bedding-parallel foliation (S1=S0) NNE±SSW trending and west
vergent isoclinal and drag folds (F2)
Bedding-parallel shear planes (S3=S1=S0)
WNW±ESE large scale open to tight and overturned folds and minor folds (F4), NE- or SW-dipping crenulation cleavage (S4)
Small-scale brittle faults
Parautochthon Bedding-parallel foliation (S1=S0) NNE±SSW trending and west vergent isoclinal and drag folds (F2)
Bedding-parallel shear planes (S3=S1=S0)
WNW±ESE large scale open to tight and recumbent folds and minor folds (F4), NE- or SW-dipping crenulation cleavage (S4)
Small-scale brittle faults
Tansen Group No No Not seen WNW±ESE large scale open to
tight folds and minor folds (F4), NE- or SW-dipping slaty and fracture cleavages (S4), WNW± ESE pencil lineation (L4)
Small-scale brittle faults
L.P.
Paudel,
K.
Arita
/
Journal
of
Asian
Earth
Sciences
18
(2000)
561±584
Fig. 5. (A) Photograph showing west vergent F2drag fold formed by the deformation of bedding (S0) and S1foliation observed in the Thrust
Sheet I along the Kali Gandaki river valley south of Phalebas. (B) Photograph showing S±C structures related to D3with 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 F4crenulation folds. (D) Photograph
show-ing L3stretching lineation (on S3plane) 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 S4crenulation
clea-vage. (F) Photograph showing F4crenulation folds observed in the Main Central Thrust zone in the Seti valley. Notice D5brittle shear zones
are up to 7 cm in length, and are often fractured and bent. Arita (1983) has also reported the occurrence of needle-like sillimanite from the lower part of the Higher Himalaya in the Modi Khola valley. Sillimanite is usually widespread in the Higher Himalaya in the Buri 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 and structural 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. Structures of the Parautochthon, Palpa Klippe and the Tansen Group were studied along two routes (Fig. 2). The structures of the Lesser Himalaya in the Tansen-Pokhara section display polyphase deformation. At least ®ve deformational phases have been recognized in the area, which are labelled as D1, D2, D3, D4and D5.
Structures having the same geometric style in all the tectonic units are assigned to the same deformational event. However, it does not imply that they were syn-chronous in all tectonic units, and thus the correlation of the deformation events (Table 1) should be regarded as very tentative. The planar structures are labelled as S, linear structures as L and folds as F with a sux referring to the corresponding deformation event. Among the ®ve deformation phases, ®rst two (D1and
D2) are supposed to be of pre-Himalayan
(pre-Ter-tiary) time and the later three (D3, D4and 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 dicult to
distinguish S1 from S3 in most places because of the
D3 bedding-parallel shearing. The S1 is more clearly
observed in the frontal part of the Lesser Himalaya (in the Parautochthon, Palpa Klippe and TS I) where the later shearing events were relatively weak. The in-clusion trails in garnets from the MCT zone may be the traces of S1. The S1is absent in the Tansen Group.
The S1 is probably the result of bedding-parallel
¯at-tening due to syn-sedimentary loading.
D2. The D2event corresponds to the deformation of
the S0 and S1 producing drag and isoclinal folds (F2)
with NNE±SSW trending axes [Fig. 5(A)]. Those drag and isoclinal folds were observed throughout the Nawakot 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 observed in the Tansen Group and the MCT zone along Tan-sen±Pokhara section. The drag folds have consistently WNW vergence throughout the area. The axial trends of those drag and isoclinal folds vary from N108W to N258E with both northern and southern plunges. But the maxima of the axial trend lies toward NNE±SSW (Fig. 6).
The WNW vergence of the drag folds observed in the area is in contrast to the commonly observed southward-vergent shearing and folding due to the Himalayan orogeny. Folds with axes parallel to the tectonic transport (oblique and sheath folds) may be developed in intense ductile shear zones like the MCT zone due to the rotation of fold hinges towards the tectonic transport direction during progressive simple shear deformation (Quinquis et al., 1978; Cobbold and Quinquis, 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 the Nawakot Complex even to the southernmost part of the Lesser Himalaya where the intensity of shearing during the Himalayan orogeny is relatively weak. Due to the above reasons and also due to their absence in the Tansen Group, we argue that D1 and D2are
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 from north to south. Despite this fact, it can be divided into three phases based on the dierence in structural style during dierent stages of deformation.
D3. The D3is characterized by intense ductile
shear-ing more or less parallel to S0and S1. The D3 was the
main deformation event in the MCT zone which pro-duced dominant S3mylonitic foliation (including both
the 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 in some places [Fig. 5(B)] whereas in other places it is represented by anastomoizing shear planes formed by the juxtaposition of the almond-shaped bodies. In places where the S±C fabric is well-recognized, C-planes are more prominent and relatively gentler than the S-planes [Fig. 5(B)]. The dip of the S- and C-planes in the MCT zone varies from 10 to 508NE (Fig. 6). In the TS II, TS I, and the Parautochthon, the S3
foli-ation is represented by shearing more or less parallel to the S0- and S1-planes [Fig. 5(C)]. The intensity of
shear strain gradually vanishes to the south and the shear fabric is less-observed in the southern part of the Parautochthon.
Many F3 isoclinal folds with axes trending in the
NNE±SSW direction have been reported from the MCT zone in central Nepal (PeÃcher, 1977; Brunel et al., 1979; Macfarlane et al., 1992; Vannay and Hodges, 1996). Those folds have been interpreted to have formed at the initial stage of D3 and reoriented
paral-lel to the stretching lineation during the following shearing stages (curved folds). Although such folds
could also be present in the MCT zone of the present area, we did not notice them.
The stretching and mineral lineations (L3) were
reported only from the MCT zone and the TS II. They are de®ned by preferred orientation of the stretched pebbles in metaconglomerates [Fig. 5(D)], elongated quartz and feldspar porphyroclasts in augen gneisses, and preferred orientation of minerals like biotite,
covite and actinolite on the S3 planes. The L3
linea-tions trend to the NE and plunge from 5 to 258 in the MCT zone (Fig. 6). The L3lineations have been folded
by the later events in the TS II. They trend in the NNE±SSW direction and plunge to both the north and south with plunges ranging from very gentle (58) to vertical (908) (Fig. 6).
Shear-sense markers related to D3 are abundant
throughout the Lesser Himalaya. They are represented by S±C structures [Fig. 5(B)] and garnets with spiral inclusions in the MCT zone, and sheared porphyro-clasts of quartz and feldspars with asymmetric pressure shadows in the TS II and TS I [Fig. 5(C)]. All of them consistently show a top-to-the-south sense of shearing during D3. This is in good agreement with the
obser-vations by PeÃcher (1977), Brunel et al. (1979) and Kaneko (1997) in central Nepal. The D3was related to
the thrusting along the UMCT (PeÃcher, 1977; Brunel et al., 1979).
D4. All of the previous planar (S0, S1 and S3) and
linear (L3) structures were deformed during D4due to
the post-UMCT thrust propagation. Most of the major and minor folds with axes trending from WNW to ESE and vergence to the south were formed during D4. The shallow and frontal part of the Lesser
Hima-laya is characterized by S4 axial plane slaty cleavage,
S4fracture cleavage and L4pencil lineaitons. The
dee-per and rear part of the Lesser Himalaya is character-ized by the F4 crenulation folds and S4 crenulation
cleavage.
Major F4 folds are abundant to the south of the
MCT zone. In the TS II, the large-scale F4 folds are
non-cylindrical, doubly plunging, and of the open type. They are arranged in an en-echelon pattern showing a dome and basin structure in the Pokhara area (Fig. 6). An overturned F4 syncline was observed
to the NW of Birethanti (Figs. 3 and 6). The area south of the PT is characterized by tight, overturned and even recumbent F4 folding (Sakai, 1985; Dhital et
al., 1998). The Tansen Synclinorium (Fig. 2) represents a major F4 fold in the Parautochthon. Minor F4
cre-nulation [Fig. 5(F)] and kink folds with WNW±ESE trending axes are well-developed in all the tectonic units. The maxima of the minor fold axes is more or less parallel to the major fold axes (Fig. 6).
Crenulation cleavages (S4) dipping 30±508to the NE
are well-developed in the incompetent pelitic layers of the 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 Tansen Group 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 are
usually widespread in the shales of the Amile and Bhainskati Formations.
D5. The D5 is usually characterized by small-scale
brittle faulting throughout the area. The brittle faults cross-cut all of the previous structures [Fig. 5(F)]. They strike WNW±ESE and dip steeply to the SW or NE. Some brittle shear zones have a normal sense of motion.
5. Metamorphic zonation and petrography
Microscopic observation of samples collected sys-tematically along two parallel sections (Fig. 2) in the Tansen±Pokhara area shows that the metamorphic grade and intensity of deformation increases north-ward to the UMCT. Most parts of the Lesser Hima-laya lie within the chlorite (or lower) zone. Biotite appears north of Pokhara and Kusma, and the biotite zone is distributed as a narrow zone just below the LMCT (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 as 20 km, the garnet isograd crosses the LMCT and passes into the TS II, and kyanite and staurolite are found also in the upper part of the MCT zone (Col-chen et al., 1980). General petrographic features of the rocks 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 UMCT recrystallized under amphibolite facies condition with mineral 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 the Gorkha 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 Modi valleys do not contain sillimanite. Kyanite, however, is widespread in the present area and are elongated par-allel to the foliation and the stretching lineation. They are generally fractured, bent and partially altered into ®ne-grained muscovite. The recrystallized muscovite is also arranged parallel to the foliation. Poikiloblastic euhedral garnets grew up to 5 mm in diameter. They have inclusion-rich cores and inclusion-free rims, and are often fractured, elongated and altered to chlorite. Coarse-grained (2±4 mm long) biotite and muscovite ¯akes are the predominant matrix phases de®ning the foliation. 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 the greenschist facies. The main mineral assemblages are Grt±Bt±Ms±Chl±Ab±Qtz (in pelitic and psammitic schists), Act±Bt±Ms±Chl±Cal±Qtz (in calc-schists) and Hbl±Act±Bt±Ep±Ab±Qtz with relicts of Hbl (in meta-basites). Altered sphene (leucoxene), magnetite, tour-maline and zircon occur as accessories. Chlorite occurs only alteration product. Augen gneisses are mylonitic to protomylonitic, with augens of perthitic microcline and plagioclase up to 1 cm in diameter. Muscovite and biotite are predominant matrix phases in all rocks de®ning foliation. Quartz occurs as granoblastic, pol-ygonal aggregates in the schists and gneisses of the lower part of the MCT zone. In the upper part, it is strongly sheared and shows ribbon texture. Syn-tec-tonic poikiloblastic garnet in the schists is found in dierent shapes (skeletal, elongated, s-shaped and eqi-dimensional) and sizes (0.1±5 mm). Spiral garnets show up to 3608rotations of the inclusions. S±C fabric and rotated garnets show a top-to-the-south sense of shearing in the MCT zone. Many snowball garnets in the mica-rich layers display post-tectonic rim over-growth. Large (up to 2 mm) post-tectonic garnets and muscovites (0.2 mm) occur cutting across the S3
foli-ation.
5.3. TS II
The TS II shows greenschist facies of metamorphism with the biotite zone in the north, and the chlorite zone in the south [Fig. 4(B)]. In the biotite zone, the main 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. Quartz clasts in metasandstones and metaconglomerates are elongated parallel to the foliation, and mark the stretching lineation. The quartz clasts are often poly-gonized. The matrix contains coarse-grained aggregates of polygonal quartz. Ms±Chl±Ab±Qtz is the typical assemblage of phyllites in the chlorite zone. Tourma-line, magnetite, zircon, apatite and sphene occur as accessories. S3 foliation with microfolds and
crenula-tion is common in the pelites. Metasandstone contains large ovoidal clasts of quartz arranged parallel to the foliation, which are accompanied by pressure shadows (showing a top-to-the-south sense of shearing) and mortar structure. However matrix quartz is fully recrystallized into polygonal aggregates.
5.4. TS I and Parautochthon
Rocks of the TS I and the Parautochthon belong to the chlorite and lower zones. Sedimentary features
such as parallel laminae, cross-laminae, graded-bed-ding, mud-cracks and stromatolites are well-preserved in those units [Figs. 5(A) and (C)]. However, phyllites, slates and the matrix of sandstones in the Nawakot Complex contain recrystallized muscovite and chlorite ¯akes arranged parallel to the foliation. Detrital quartz and mica ¯akes (0.05±0.15 mm in length) oblique to the foliation are sometimes observed in the psammitic parts of the phyllites and slates. Sandstones contain detrital muscovite (up to 1 mm), quartz, feldspar, tour-maline, apatite, and zircon. The Nawakot Complex is sheared and recrystallized near the PT and the BKF. Large quartz clasts in sandstones are slightly deformed, and show wavy extinction, whereas the small clasts in the matrix of sandstone and siltstones are polygonized. The sandstones locally contain sheared detrital quartz clasts with well-developed asymmetric pressure shadows showing a top-to-the-south sense of shearing. The Tansen Group contains very 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 minerals
are very ®ne-grained and cannot be identi®ed under the 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
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 the MCT zone seem to be consistent along several sections of central Nepal, e.g. Kali Gandaki valley (Le Fort et al., 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 dierent metamorphic history between those units.
6.2. Muscovite
Detrital and recrystallized muscovites from the Thrust sheets I and II, the MCT zone and the Higher Himalaya were analyzed and plotted on a Miyashiro diagram (Fig. 9). In general, the celadonite component in muscovite decreases with increasing metamorphic grade (Miyashiro, 1973). Recrystallized muscovites from the Lesser Himalaya show a decrease in celado-nite component from south to north (structurally upwards). Recrystallized muscovites in sandstones from the TS I contain up to 8 wt% FeO. This value decreases to 3±6 wt% in the TS II and 1±3 wt% in the MCT zone. However, muscovites from the kyanite-grade Higher Himalayan rocks have greater celadonite components than those from the garnet grade MCT zone samples, and plotted in the biotite±almandine ®eld on the Miyashiro diagram rather than in the staurolite±sillimanite ®eld (Fig. 9). The kyanite and the pyrope-rich cores of garnet do not coexist with celado-nite-rich muscovite, and thus the celadocelado-nite-rich mus-covite was most probably produced by a later event under lower metamorphic conditions as suggested by Arita (1983). Celadonite contents of detrital musco-vites from the Lesser Himalaya vary widely (Fig. 9). Those plotting close to pure muscovite are probably derived 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 (KuÈbler, 1967). The KuÈbler Index (KI), de®ned as the peak width at half height of the 10 AÊ illite peak above the background (KuÈbler, 1967; Dunoyer de Segonzac et al., 1968), decreases with increasing metamorphic grade as illite releases Fe2+, Mg2+, H2O, OHÿ, and
absorbs K+, eventually forming muscovite. On the basis of IC, low-grade metamorphism can be divided into the diagenetic zone, anchizone and epizone which are 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 of metamorphism in the low-grade metamorphic rocks (zeolite facies < 2008C, prehnite±pumpellyite faceis0 200±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. Femeans total Fe as Fe2+.
7.1. Sample preparation and measurement
A total of 200 pelitic rock samples along the Pokhara±Butwal road and the Modi Khola±Kali Gan-daki sections of the Lesser Himalaya were used for IC study. The laboratory procedure followed here is con-sistent with that outlined by the IGCP 294 Working Group (Kisch, 1991a). About 500 g of each sample was broken into small chips and then washed and dried. About 200 g chips were then crushed in a mor-tar and pestle, and passed through 2.38 and 0.59 mm sieves. The ®ne fraction was discarded to reduce any in¯uence from weathered material. About 200 g of the 2.38±0.59 mm fraction was then ground for 3 min in a mortar and pestle, and passed through a 100-mesh (0.149 mm) sieve. The <2 mm fraction was separated from <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 was pipetted on to a 2848 mm microscopic slide produ-cing a slide of thickness ca. 3 mg cmÿ2. Two slides were prepared for each sample, one air-dried and the other ethylene-glycolated.
The diractometer setting was constant for all samples (Rigaku Geiger¯ex diractometer, Cu cath-ode, Ni ®lter, 35 kV tube voltage, 20 mA current, time constant=2 s, scatter slit=18, receiving slit=0.3 mm, divergence slit=18). The KI was measured manually at the precision of 0.2 mm on the chart (=0.005D82y). Minimum peak width measured was 0.095D82y from a muscovite ¯ake in a pegmatite from the MCT zone which has a composition close to that of the ideal muscovite (Fig. 9). One anchizone sample was run at
the beginning and end of analysis each day to check the instrumental drift. Forty-two measurements in 20 days gave an average value of 0.262D82ywith standard deviation (1s) of 0.004D82y. The precision of the ma-chine was checked by ten repeat measurements on
single undisturbed samples from dierent metamorphic grades. This gave mean values of 0.16720.003D82y
(1s) for the epizone sample, 0.26020.005D82y (1s) for the anchizone sample, and 0.99120.036D82y (1s) for the diagenetic zone sample. The total error arising
Fig. 10. Representative X-ray diractograms. Chl, Chlorite; I, Illite-muscovite; Ab, Albite; Qtz, Quartz; 2M1, 2M1 muscovite, KI, Kubler Index. L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
from measurement conditions does not exceed 3% for the anchizone and epizone samples and 5% for the diagenetic samples.
7.2. Mineralogy of <2mm fraction
Both air-dried and ethylene-glycolated samples were scanned from 33 to 58 2y at a scan rate of 28 minÿ1 and chart speed of 2 cm minÿ1 to check mineral assemblages in the <2mm fraction. Representative dif-fractograms are shown in Fig. 10. The Nawakot Com-plex samples from the TS II, TS I and the Parautochthon exhibit epizonal and anchizonal meta-morphism. They are rich in dioctahedral 2M1 K-mica
and chlorite. Illite±muscovite was detected in all samples. Chlorite could not be detected in purple slates of the Nourpul Formation. Albite and quartz are pre-sent in almost all samples. A muscovite/paragonite mixed layer identi®ed by high-angle broadening of the 3.3 AÊ peak is present in the Benighat Slate and black slates of the Nourpul Formation. Samples from the Tansen Group fall into the diagenetic zone, and are rich in illitic muscovite and chlorite. Two samples contained mixed layer illite/smectite and chlorite/ smectite.
7.3. IC results
Samples showing the 10 AÊ illite peak were scanned 5 times from 10±7D82y (scan rate=0.58 minÿ1, chart speed=2 cm minÿ1, TC=2 s), and an average KI was determined on both the air-dried and ethylene-glyco-lated samples. Ethylene-glycoethylene-glyco-lated samples gave better crystallinity (lower KI values) than the air-dried samples, especially for the diagenetic and anchizonal rocks (as observed also by Warr and Rice, 1994). Therefore, the KI from ethylene-glycolated samples are used for discussion. Average KI obtained from ethyl-ene-glycolated sample for each tectonic unit in both sections 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 the sample collections [see Figs. 11(A) and (B)]. The aver-age KI value in the TS II in the Modi Khola±Kali Gandaki section is increased by the greater KI values along the Kusma Fault.
The average KI in the study area decreases north-wards showing that the bulk grade of metamorphism increases from south to north (structurally upwards). The Parautochthon has an average KI (all data) of 0.26620.035D82y (anchizone) for the Nawakot Com-plex and 0.60620.319D82y (diagenetic zone) for the Tansen Group. The Thrust sheets I and II have aver-age KI of 0.23720.057D82y (anchizone) and 0.1952 0.060D82y (epizone), respectively. The Palpa Klippe
which covers the Tansen Group, has lower average KI (0.2582 0.033D82y, anchizone) than the underlying rocks.
The distribution of KI along the two sections are shown in Fig. 11. There is quite a similarity between the distribution patterns of the KI along the Pokhara± Butwal section [Fig. 11(A)] and the Modi Khola±Kali Gandaki section [Fig. 11(B)]. The Tansen Group shows diagenetic grade (zeolite facies) of metamorph-ism in the south of the Palpa Klippe and anchizone grade (prehnite±pumpellyite facies) of metamorphism in the north. There is a sharp break in KI across the unconformity between the Tansen Group and Nawa-kot Complex to the south of the Palpa Klippe, whereas the KI seem rather continuous across the northern Tansen unconformity. A sharp break in KI is
also observed across the thrust boundary between the Tansen Group and the Palpa Klippe (in both sides). The KI values are highly scattered along the anchi-zone±epizone boundary in the Nawakot Complex of the Parautochthon and the TS I. There is no de®nite trend of KI in those units. However, a sharp decline in KI is observed within a narrow zone near the BKF and the PT. In the TS II, the KI values decrease gradually to the north, with a minimum value below the LMCT. Relatively high KI values are observed near the KF and the PKF, which may be due to de-terioration of earlier crystallinity by post-metamorphic deformation (TeichmuÈller et al., 1979 in Kisch, 1991b). Anomalously higher KI values on two samples of the parautochthon (Fig. 11B) are due to highly weathered samples.
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 of central Nepal (Le Fort, 1975; Arita, 1983; Hodges and Silverberg, 1988; PeÃcher, 1989; Inger and Harris, 1992; Hodges et al., 1994). They have reported an earlier high P/high T kyanite-grade Barrovian-type meta-morphism (Eohimalayan, Caby et al., 1983) followed by a later lower P/high T sillimanite-grade metamorph-ism (Neohimalayan). At least three metamorphic events are recognized in the Higher Himalaya of the Tansen±Pokhara section (Table 3). The ®rst meta-morphic event (M1) was a high P/high T amphibolite
facies prograde metamorphism as in the other sections of central Nepal. The kyanite and pyrope-rich garnet cores belong to the M1. Although the M1 is followed
by sillimanite grade metamorphism (lower P/high T) in the Buri Gandaki and Trisuli river valley sections of central Nepal (PeÃcher, 1989; Inger and Harris, 1992), sillimanite is not observed in the Tansen±Pokhara sec-tion. The M2in the later section, however, is shown by
widespread retrogressive metamorphism. The Fe-rich and Al-poor white micas from higher Himalayan gneisses probably do not coexist with the kyanite and pyrope-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 margins of garnets from the Higher Himalaya were formed by resorption due to retrogression. Wide-spread develop-ment of dynamic textures such as fractured and bent kyanite blades and elongated garnet porphyroblasts with pressure shadows shows that the M2 was related
to the thrusting along the UMCT. Therefore, the inversion 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 the UMCT. The replacement of garnet and biotite by chlorite 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 indicate two metamorphic events occurring before and after the deposition of the Tansen Group. The sharp disconti-nuity in the IC values across the unconformity between the Nawakot Complex and the Tansen Group to the south of the Palpa Klippe indicates that the Nawakot Complex had been already heated up to the anchizone (prehnite±pumpellyite facies) prior to deposition of the Permo-Carboniferous formations of the Tansen Group. This heating event can be regarded as the ®rst metamorphic event (M0) in the Lesser Himalaya, and
may be related to D1which produced the
bedding-par-allel foliation in the Nawakot Complex.
The whole Lesser Himalaya suered a strong second metamorphic event (M2) after deposition of the Lower
Miocene formation of the Tansen Group. The grade of metamorphism and the intensity of deformation during the M2 increases gradually from south to
north, reaching a maximum near the UMCT. The M2
in the Tansen Group ranges from a diagenetic zone in the south to the upper anchizone north of the Palpa Klippe. The IC values seem to pass gradually into the lower anchizone and anchizone±epizone boundary in the 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 I indicate that the M2was not strong enough to
comple-tely reset the previous (M1) IC patterns in this part of
the 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 maximum below the LMCT. Metamorphic inversion to the north of the PT has been also supported by the gradual dephengitization of white micas (Fig. 9), and progress-ive change in mineral assemblages to higher grade. The M2 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)
M1or Eohimalayan (pre- or
early-UMCT)
Kyanite grade prograde metamorphism
?
M2or Neohimalayan (syn- to
post-UMCT)
Retrograde metamorphism Diagenetic to garnet grade prograde inverted metamorphism (D3)
fabric and the garnet and muscovite porphyroblasts grown across the foliation. The M2 was probably
re-lated to overthrusting of the Higher Himalayan crys-tallines on the Lesser Himalaya along the UMCT (Le Fort, 1975; PeÃcher, 1975; Arita, 1983; PeÃcher and Le Fort, 1986). Post-kinematic mineral growths in the MCT zone shows that the high temperature conditions must have persisted after the main episodes of shearing (Caby et al., 1983; PeÃcher and Le Fort, 1986). The ®nal phase in the Lesser Himalaya was retrograde metamorphic phase (M3) shown by chloritization of
garnet and biotite, marginal retrograde zoning in the MCT zone garnets, and increase in KI (deterioration of IC) along the younger faults. It occurred during the uplift and erosion.
8.3. Timing of metamorphism
The common occurrence of mineral assemblages such as Grt±Bt±Ms±Chl±Ab±Qtz in the pelitic schists indicate that the metamorphic temperature in the MCT zone was about 500±5508C (Winkler, 1974) during the M2 event. Kaneko (1995) estimated
meta-morphic temperature of about 400±4508C for the MCT zone with a rapid increase up to 600±6508C near the UMCT in the present study area. This is above the argon 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 that there was no signi®cant time gap between peak meta-morphism and uplift and cooling. Thus the K/Ar and
40
Ar/39Ar ages of hornblende from the MCT zone are likely to show the age of the M2 event in the Higher
and Lesser Himalayas. 40Ar/39Ar hornblende ages from the MCT zone in Nepal Himalaya range from 28 23.3 Ma in the Trisuli river valley (Macfarlane, 1993) to 24.621.9 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). Younger ages determined on muscovite (7±14 Ma 40Ar/39Ar ages, Macfarlane et al., 1992, Vannay and Hodges, 1996; Copeland et al., 1991), on biotite (9±13 Ma
40
Ar/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 lower closure temperatures and continuous exhumation and cooling in the MCT zone. The age of the M2 event is
also constrained by the emplacement age of leucogra-nites (24±16 Ma Rb/Sr isochron and U/Pb mineral ages: Vidal, 1978; Harrison et al., 1995; Sorkhabi and Stump, 1993) because according to Le Fort (1975, 1986), the formation of leucogranite was related to the thrusting along the MCT zone.
The M0event in the Lesser Himalaya is
pre-Himala-yan, probably early-Paleozoic (Table 3). Radiometric ages from the low-grade metamorphic rocks of the Lesser Himalaya are relatively scarce compared to those of the higher grade rocks. Khan and Tater (unpublished report) have obtained 550217 Ma and 559218 Ma whole rock K/Ar ages on Benighat Slate from the Kali Gandaki valley. Evidence of pre-colli-sional orogeny and metamorphism in the Lesser Hima-laya has also been documented in the Kumaun Himalaya of India (Johnson and Oliver, 1990; Oliver et al., 1995) and in Pakistan (Baig et al., 1988; Chaudhry and Ghazanfar, 1989). An early Paleozoic thermal event in the Himalaya is also supported by the occurrence of 500 Ma granites in the Lesser Himala-yan thrust sheets (SchaÈrer and AlleÁgre, 1983; Le Fort et al., 1986a).
One of the most outstanding problems in the Hima-layan metamorphism has been the age of the M1event
in the Higher Himalaya. Many geologists consider M1
as having occurred due to the crustal thickening after the India-Asia collision and before or during early period of UMCT activity (Caby et al., 1983; Le Fort, 1986; Hodges et al., 1988; PeÃcher, 1989). Geochrono-metric data do not precisely constrain the age of M1in
the Higher Himalaya because most of the commonly used isotopic geochronometers were completely or par-tially reset during M2. The peak metamorphic
tem-perature in the Higher Himalaya was 600±7008C (PeÃcher, 1989) or up to 7508C (Kaneko, 1995), which is higher than the Ar closure temperature of horn-blende (500±5508C). 40Ar/39Ar ages of hornblende range from 46 to 37 Ma in the north-eastern part of the present study area (Vannay and Hodges, 1996). Based on these ages it is reasonable to consider M1 as
an early Tertiary event as suggested by Arita et al. (1990) and Sorkhabi and Stump (1993). However, the possibility of a pre-Himalayan age for M1 cannot be
ruled out. The Himalaya has been aected by several pre-Himalayan thermal events (see Arita et al., 1990; Sorkhabi and Stump, 1993) and detrital muscovites in the Lesser Himalayan sandstones, which were derived from older high-grade metamorphic rocks (Fig. 9) have been dated at 728 Ma (Krummenacher, 1961).
9. Conclusions
The present study shows that the central Nepal Himalaya has evolved through polyphase deformation and metamorphism (Fig. 12). The Lesser Himalaya has experienced at least ®ve deformational events (D1±
D5), two of which (D1 and D2) are pre-Himalayan.
Bedding-parallel foliation (S1) formed during D1 was
deformed into folds with NNE±SSW trending axes (F2) during D2. The D3 event associated with the
UMCT activity formed dominant bedding-parallel
shear planes (S3), and conspicuous NNE±SSW
trend-ing mineral and stretchtrend-ing lineations (L3) in the Lesser
Himalaya. Shear-sense markers like S±C fabric, asym-metric pressure shadows and spiral garnets show a top-to-the-south sense of thrusting and shearing during D3. Post-UMCT thrust propagation (D4) resulted in
the major and minor folds with WNW±ESE trending axes (F4), NE- or SW-dipping crenulation, slaty and
fracture cleavages (S4), and pencil lineations (L4). The
last phase of deformation (D5) is evidenced by brittle
faulting that cross-cut all of the previous structures. Illite crystallinity as well as muscovite and garnet compositions reveal the polymetamorphic history of the central Nepal Himalaya. The Lesser Himalaya shows a Pre-Himalayan anchizonal metamorphism (M0) and Neohimalayan diagenetic to garnet grade
metamorphim (M2). The M2 was originally inverted
thorughout the Lesser Himalaya. The Higher Himala-yan rocks were recystallized under amphibolite facies condition (kyanite grade) prior to the UMCT activity (M1) and reequillibrated under lower metamorphic
conditions during or after thrusting along the UMCT (M2). Thus the isograd inversion from garnet zone in
the Lesser Himalaya to kyanite zone in the Higher Himalaya is only apparent due to post-metamorphic thrusting. Timing of M1 in the area is not clear.
Although M1 is believed to be post-collisional, it is
equally possible that it was pre-collisional, as the Himalaya had been aected by several pre-Himalayan thermal 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 the research scholarship to L.P.P. The ®eld work for this research was supported by grant-in-aid of 1997 for the promotion of research from the Tokyo Geographical Society. The work was also partly supported by the Sasakawa Scienti®c Research Grant from the Japan Science Society. We thank H. Thapa, K.R. Regmi and D.P. Jaishi of Tribhuvan University, Nepal for their assistance in the ®eld. T. Kuwajima and H. Nomura of Hokkaido University, Japan helped with thin sec-tions. S. Terada helped during microprobe analysis and T. Tajima helped during the X-ray diractometer operation. L.P.P. had useful discussion with K. Oho-mori on IC interpretation. Corrections and comments by J.P. Burg, R.B. Sorkhabi, and B. Roser greatly helped to improve the manuscript. We thank A. PeÃcher for the critical review and fruitful comments on the manuscript.
Appendix A
Equation used to calibrate the centrifuge
t63:0108Z log10 R=S=N2r2 Dÿd
where, t= rotation time (min), Z=viscosity of water (0.01002 poise),R= distance between the rotation axis and water surface in the centrifuge tube (cm), S= dis-tance between the rotation axis and the sedimentation level in the centrifuge tube (cm), N= number of ro-tation minÿ,r= particle size (mm),D= density of the particle (2.50 g mlÿ1), andd= density of water (1.00 g mlÿ1
).
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