Directory UMM :Data Elmu:jurnal:J-a:Journal of Asian Earth Science:Vol18.Issue5.2000:
Journal of Asian Earth Sciences 18 (2000) 561±584
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-ofsequence 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 show
polymetamorphic evolution of the Lesser and Higher Himalayas in central Nepal. The Lesser Himalaya suered a preHimalayan (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 earlyUMCT) kyanite-grade prograde metamorphism (M1) which was, in turn, overprinted by Neohimalayan (syn-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. 7 2000 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 lithotectonic units; the Sub-Himalaya (Siwaliks), the Lesser
Himalaya, and the Higher Himalaya (including the
Central Crystallines and the overlying Tethys Himalaya) (Fig. 1; Gansser, 1964).
* Corresponding author. Central Department of Geology, Tribhuban University, Kirtipur, Kathmandu, Nepal. Fax: +81 11 706
5305.
E-mail address: [email protected] (L.P. Paudel).
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 Precambrian±Early Paleozoic age (StoÈcklin, 1980), overlain
unconformably by the Gondwana type Late Paleozoic±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
1367-9120/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.
PII: S 1 3 6 7 - 9 1 2 0 ( 9 9 ) 0 0 0 6 9 - 3
562
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
thrusting of the Higher Himalayan crystalline rocks
(Schelling and Arita, 1991).
One of the interesting features of the Lesser Himalaya is the `inverted metamorphism', ®rst noted by
Richard Oldham in the Indian Himalaya in 1883
(quoted in Gansser, 1964), and subsequently recognized by many geologists in other parts of the Himalaya (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 connection, it is important to constrain the thermal structure
of the low-grade metamorphic rocks of the Lesser
Himalaya where the inverted metamorphism is exhibited.
In an attempt to unravel the structure and metamorphic 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 evolution of the central Nepal Himalaya has been also presented.
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 decollement known as the Main Detachment 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).
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
563
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.
564
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
®el et al., 1992) which do also have a dextral strikeslip 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 Himalaya (east Nepal, Schelling and Arita, 1991; west
Nepal, Dhital, 1989; far-western Nepal, DeCelles et
al., 1998), an interpretation supported by our ®eld observations in central Nepal [Fig. 2(B)]. The rocks of
the Higher Himalaya with the overlying Tethys sediments 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 Himalayan duplex. The Kathmandu Nappe forms a large synclinorium. 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 Complex, 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 melange 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 southward 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 constructing 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 topographic 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 juxtaposed 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 foliation 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 relations, geochronological data and the foreland sedimentary 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
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
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) contains clasts of phyllitic slates derived from the Himalayan 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 ramping along the UMCT at depth. Assuming that the
peak metamorphism in the MCT zone and the anatexis
565
and leucogranite emplacement in the Higher Himalayan Crystallines were the synchronous events associated 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,
Fig. 3. Tectono-lithostratigraphic subdivision of the Lesser Himalaya in central Nepal. Patterns in column are the same as in Fig. 2. Phy., phyllite; Qzt., quartzite; Amp., amphibolite; Sl., slate; Ss., sandstone; Dol., dolomite; Cgl., conglomerate; Sh., shale.
566
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
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 successively from north to south in a piggy-back fashion.
Although there are no age constraints on the movement 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, respectively.
The MBT juxtaposes the Lesser Himalayan metasediments 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 presently 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 varyingly 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 comprises 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 Gandaki 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 Gondwana sediments are divided into the Sisne, Taltung,
and Amile Formations. The post-Gondwana sediments
are divided into the Bhainskati and Dumri Formations. The Tansen Group contains Upper Carboniferous 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 garnetiferous pelitic and psammitic schists, with a few bands of
chloritic schists and quartzites. Mylonitic augen
gneisses (Ulleri augen gneiss of Le Fort, 1975) interbedded 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 biotite rich and feldspar-quartz rich layers. Kyanite blades
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
567
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.
568
Pre-Himalayan phases
Himalayan (syn- to post-UMCT) phases
Tectonic units
D1
D2
D3
MCT zone
Foliation preserved as inclusion trails in garnet (S1)
Not seen
S±C fabric, NNE±SSW mineral
and stretching lineations (L3)
Thrust sheet II
Bedding-parallel foliation (S1=S0)
Thrust sheet I
Bedding-parallel foliation (S1=S0)
NNE±SSW trending and west
vergent isoclinal and drag folds
(F2)
NNE±SSW trending and west
vergent isoclinal and drag folds
(F2)
Parautochthon
Bedding-parallel foliation (S1=S0)
NNE±SSW trending and west
vergent isoclinal and drag folds
(F2)
Tansen Group
No
No
D4
WNW±ESE crenulation and kink
folds (F4), NE- or SW-dipping
crenulation cleavage (S4)
Bedding-parallel shear planes
WNW±ESE large scale open folds
(S3=S1=S0), NNE±SSW mineral and minor folds (F4), NE- or SWand stretching lineations (L3)
dipping crenulation cleavage (S4)
Bedding-parallel shear planes
WNW±ESE large scale open to
(S3=S1=S0)
tight and overturned folds and
minor folds (F4), NE- or SWdipping crenulation cleavage (S4)
Bedding-parallel shear planes
WNW±ESE large scale open to
(S3=S1=S0)
tight and recumbent folds and
minor folds (F4), NE- or SWdipping crenulation cleavage (S4)
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)
D5
Small-scale
brittle
faults
Small-scale
brittle
faults
Small-scale
brittle
faults
Small-scale
brittle
faults
Small-scale
brittle
faults
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
Table 1
Deformational events and related structures in the Lesser Himalaya of central Nepal along the Tansen±Pokhara section
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
569
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 showing 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 cleavage. (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.
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L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
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 PokharaSyangja 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 TansenPokhara section display polyphase deformation. At
least ®ve deformational phases have been recognized in
the area, which are labelled as D1, D2, D3, D4 and 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 synchronous 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 (D1 and
D2) are supposed to be of pre-Himalayan (pre-Tertiary) time and the later three (D3, D4 and D5) are related 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 foliation (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 inclusion trails in garnets from the MCT zone may be
the traces of S1. The S1 is absent in the Tansen Group.
The S1 is probably the result of bedding-parallel ¯attening due to syn-sedimentary loading.
D2. The D2 event 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, Parautochthon (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 Tansen±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 cylindrical cross-section and they should fade out laterally.
It is not the case in the present area [Fig. 5(A)]. Moreover, 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 D2 are preHimalayan (Table 1).
4.2. Himalayan phases
The Himalayan deformation phases can be considered 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 D3 is characterized by intense ductile shearing more or less parallel to S0 and S1. The D3 was the
main deformation event in the MCT zone which produced dominant S3 mylonitic foliation (including both
the S- and C-planes) and NNE±SSW trending L3
stretching and mineral lineations. The mylonitic foliation 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, Cplanes 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 508 NE (Fig. 6). In
the TS II, TS I, and the Parautochthon, the S3 foliation 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.
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
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 parallel to the stretching lineation during the following
shearing stages (curved folds). Although such folds
571
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, mus-
Fig. 6. Structural map of the Lesser Himalaya in the Pokhara±Kusma area. Foliation and lineations are projected on Schmidt's lower hemisphere.
572
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
covite and actinolite on the S3 planes. The L3 lineations trend to the NE and plunge from 5 to 258 in the
MCT zone (Fig. 6). The L3 lineations 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 porphyroclasts 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 observations by PeÃcher (1977), Brunel et al. (1979) and
Kaneko (1997) in central Nepal. The D3 was 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 D4 due 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 Himalaya is characterized by S4 axial plane slaty cleavage,
S4 fracture cleavage and L4 pencil lineaitons. The deeper and rear part of the Lesser Himalaya is characterized 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 crenulation [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±508 to the NE
are well-developed in the incompetent pelitic layers of
the Kuncha Formation [Fig. 5(E)]. The S4 slaty cleavage dipping either to the NE or to the SW and crosscutting 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 lineations (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 systematically along two parallel sections (Fig. 2) in the
Tansen±Pokhara area shows that the metamorphic
grade and intensity of deformation increases northward to the UMCT. Most parts of the Lesser Himalaya 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 coincide with the LMCT and the UMCT, respectively.
However, the isograd distribution patterns are not uniform 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 (Colchen et al., 1980). General petrographic features of the
rocks from each tectonic unit along the Tansen±
Pokhara section are given below. Mineral abbreviations 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 occurrence 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 parallel 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 muscovite. Biotite also occurs as inclusions in kyanite.
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
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 metabasites). Altered sphene (leucoxene), magnetite, tourmaline 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, polygonal 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-tectonic poikiloblastic garnet in the schists is found in
dierent shapes (skeletal, elongated, s-shaped and eqidimensional) and sizes (0.1±5 mm). Spiral garnets
show up to 3608 rotations 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 overgrowth. Large (up to 2 mm) post-tectonic garnets and
muscovites (0.2 mm) occur cutting across the S3 foliation.
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, zircon, 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 polygonized. The matrix contains coarse-grained aggregates
of polygonal quartz. Ms±Chl±Ab±Qtz is the typical
assemblage of phyllites in the chlorite zone. Tourmaline, magnetite, zircon, apatite and sphene occur as
accessories. S3 foliation with microfolds and crenulation 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
573
such as parallel laminae, cross-laminae, graded-bedding, 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, tourmaline, 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-thesouth sense of shearing. The Tansen Group contains
very low-grade to non-metamorphosed rocks. In thinsection, ®ne quartz clasts (0.02 mm) are arranged parallel 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 silicates and oxides as standards).
6.1. Garnet
Garnets from the Higher Himalaya and the MCT
zone were probed at the cores and rims, and the data
are projected on the Fe±Mg±(Mn+Ca) triangle
(Fig. 7). Garnets from the Higher Himalaya are rich in
pyrope (Mg 20±25% core, 15±20% rim) and almandine (Fe 65±70% core, 70±75% rim) content. Compositional pro®les across garnets from the Higher
Himalaya [Fig. 8(A)] are characterized by a plateau in
the cores. However, the margins of the garnets show
reverse zoning, with Fe and Mn increasing and Mg
decreasing towards the rim. Ca is relatively constant.
A compositional plateau of this type may be developed
by obliteration of growth zoning by later high-temperature diusion process (Spear, 1993). Retrograde
zoning pro®les at the margins may be the result of
subsequent diusion or resorption due to retrogression
(Barker, 1990). Garnets in the MCT zone are spessartine rich (Mn 25±45% core, 15±35% rim) (Fig. 7). Individual garnets show bell-shaped Mn-pro®les
characteristic of prograde metamorphism, with Fe
gradually increasing and Mn decreasing towards the
574
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
(Arita, 1983; Kaneko, 1997), Trisuli valley (Macfarlane, 1995), and Kathmandu area (Rai et al., 1998),
suggesting a quite dierent metamorphic history
between those units.
6.2. Muscovite
Fig. 7. Chemical composition of garnets from the Higher Himalaya
and the Main Central Thrust zone. Fe means total Fe as Fe2+.
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 celadonite 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 kyanitegrade 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 celadonite-rich muscovite, and thus the celadonite-rich muscovite was most probably produced by a later event
under lower metamorphic conditions as suggested by
Arita (1983). Celadonite contents of detrital muscovites 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
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.
rims [Fig. 8(B)]. The pro®les are reversed at the outermost rim, probably due to the late-stage retrogression.
The above patterns of compositional zoning in garnet 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
Illite crystallinity (IC) is an important tool in understanding the thermal structure of low-grade metamorphic 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 metamorphism 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 faceis 0
200±3708C, greenschist facies > 3708C, Winkler, 1974).
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
7.1. Sample preparation and measurement
A total of 200 pelitic rock samples along the
Pokhara±Butwal road and the Modi Khola±Kali Gandaki sections of the Lesser Himalaya were used for IC
study. The laboratory procedure followed here is consistent 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 mortar 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
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-ofsequence 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 show
polymetamorphic evolution of the Lesser and Higher Himalayas in central Nepal. The Lesser Himalaya suered a preHimalayan (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 earlyUMCT) kyanite-grade prograde metamorphism (M1) which was, in turn, overprinted by Neohimalayan (syn-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. 7 2000 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 lithotectonic units; the Sub-Himalaya (Siwaliks), the Lesser
Himalaya, and the Higher Himalaya (including the
Central Crystallines and the overlying Tethys Himalaya) (Fig. 1; Gansser, 1964).
* Corresponding author. Central Department of Geology, Tribhuban University, Kirtipur, Kathmandu, Nepal. Fax: +81 11 706
5305.
E-mail address: [email protected] (L.P. Paudel).
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 Precambrian±Early Paleozoic age (StoÈcklin, 1980), overlain
unconformably by the Gondwana type Late Paleozoic±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
1367-9120/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.
PII: S 1 3 6 7 - 9 1 2 0 ( 9 9 ) 0 0 0 6 9 - 3
562
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
thrusting of the Higher Himalayan crystalline rocks
(Schelling and Arita, 1991).
One of the interesting features of the Lesser Himalaya is the `inverted metamorphism', ®rst noted by
Richard Oldham in the Indian Himalaya in 1883
(quoted in Gansser, 1964), and subsequently recognized by many geologists in other parts of the Himalaya (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 connection, it is important to constrain the thermal structure
of the low-grade metamorphic rocks of the Lesser
Himalaya where the inverted metamorphism is exhibited.
In an attempt to unravel the structure and metamorphic 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 evolution of the central Nepal Himalaya has been also presented.
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 decollement known as the Main Detachment 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).
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
563
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.
564
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
®el et al., 1992) which do also have a dextral strikeslip 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 Himalaya (east Nepal, Schelling and Arita, 1991; west
Nepal, Dhital, 1989; far-western Nepal, DeCelles et
al., 1998), an interpretation supported by our ®eld observations in central Nepal [Fig. 2(B)]. The rocks of
the Higher Himalaya with the overlying Tethys sediments 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 Himalayan duplex. The Kathmandu Nappe forms a large synclinorium. 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 Complex, 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 melange 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 southward 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 constructing 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 topographic 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 juxtaposed 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 foliation 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 relations, geochronological data and the foreland sedimentary 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
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
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) contains clasts of phyllitic slates derived from the Himalayan 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 ramping along the UMCT at depth. Assuming that the
peak metamorphism in the MCT zone and the anatexis
565
and leucogranite emplacement in the Higher Himalayan Crystallines were the synchronous events associated 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,
Fig. 3. Tectono-lithostratigraphic subdivision of the Lesser Himalaya in central Nepal. Patterns in column are the same as in Fig. 2. Phy., phyllite; Qzt., quartzite; Amp., amphibolite; Sl., slate; Ss., sandstone; Dol., dolomite; Cgl., conglomerate; Sh., shale.
566
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
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 successively from north to south in a piggy-back fashion.
Although there are no age constraints on the movement 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, respectively.
The MBT juxtaposes the Lesser Himalayan metasediments 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 presently 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 varyingly 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 comprises 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 Gandaki 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 Gondwana sediments are divided into the Sisne, Taltung,
and Amile Formations. The post-Gondwana sediments
are divided into the Bhainskati and Dumri Formations. The Tansen Group contains Upper Carboniferous 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 garnetiferous pelitic and psammitic schists, with a few bands of
chloritic schists and quartzites. Mylonitic augen
gneisses (Ulleri augen gneiss of Le Fort, 1975) interbedded 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 biotite rich and feldspar-quartz rich layers. Kyanite blades
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
567
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.
568
Pre-Himalayan phases
Himalayan (syn- to post-UMCT) phases
Tectonic units
D1
D2
D3
MCT zone
Foliation preserved as inclusion trails in garnet (S1)
Not seen
S±C fabric, NNE±SSW mineral
and stretching lineations (L3)
Thrust sheet II
Bedding-parallel foliation (S1=S0)
Thrust sheet I
Bedding-parallel foliation (S1=S0)
NNE±SSW trending and west
vergent isoclinal and drag folds
(F2)
NNE±SSW trending and west
vergent isoclinal and drag folds
(F2)
Parautochthon
Bedding-parallel foliation (S1=S0)
NNE±SSW trending and west
vergent isoclinal and drag folds
(F2)
Tansen Group
No
No
D4
WNW±ESE crenulation and kink
folds (F4), NE- or SW-dipping
crenulation cleavage (S4)
Bedding-parallel shear planes
WNW±ESE large scale open folds
(S3=S1=S0), NNE±SSW mineral and minor folds (F4), NE- or SWand stretching lineations (L3)
dipping crenulation cleavage (S4)
Bedding-parallel shear planes
WNW±ESE large scale open to
(S3=S1=S0)
tight and overturned folds and
minor folds (F4), NE- or SWdipping crenulation cleavage (S4)
Bedding-parallel shear planes
WNW±ESE large scale open to
(S3=S1=S0)
tight and recumbent folds and
minor folds (F4), NE- or SWdipping crenulation cleavage (S4)
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)
D5
Small-scale
brittle
faults
Small-scale
brittle
faults
Small-scale
brittle
faults
Small-scale
brittle
faults
Small-scale
brittle
faults
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
Table 1
Deformational events and related structures in the Lesser Himalaya of central Nepal along the Tansen±Pokhara section
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
569
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 showing 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 cleavage. (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.
570
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
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 PokharaSyangja 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 TansenPokhara section display polyphase deformation. At
least ®ve deformational phases have been recognized in
the area, which are labelled as D1, D2, D3, D4 and 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 synchronous 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 (D1 and
D2) are supposed to be of pre-Himalayan (pre-Tertiary) time and the later three (D3, D4 and D5) are related 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 foliation (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 inclusion trails in garnets from the MCT zone may be
the traces of S1. The S1 is absent in the Tansen Group.
The S1 is probably the result of bedding-parallel ¯attening due to syn-sedimentary loading.
D2. The D2 event 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, Parautochthon (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 Tansen±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 cylindrical cross-section and they should fade out laterally.
It is not the case in the present area [Fig. 5(A)]. Moreover, 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 D2 are preHimalayan (Table 1).
4.2. Himalayan phases
The Himalayan deformation phases can be considered 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 D3 is characterized by intense ductile shearing more or less parallel to S0 and S1. The D3 was the
main deformation event in the MCT zone which produced dominant S3 mylonitic foliation (including both
the S- and C-planes) and NNE±SSW trending L3
stretching and mineral lineations. The mylonitic foliation 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, Cplanes 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 508 NE (Fig. 6). In
the TS II, TS I, and the Parautochthon, the S3 foliation 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.
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
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 parallel to the stretching lineation during the following
shearing stages (curved folds). Although such folds
571
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, mus-
Fig. 6. Structural map of the Lesser Himalaya in the Pokhara±Kusma area. Foliation and lineations are projected on Schmidt's lower hemisphere.
572
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
covite and actinolite on the S3 planes. The L3 lineations trend to the NE and plunge from 5 to 258 in the
MCT zone (Fig. 6). The L3 lineations 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 porphyroclasts 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 observations by PeÃcher (1977), Brunel et al. (1979) and
Kaneko (1997) in central Nepal. The D3 was 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 D4 due 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 Himalaya is characterized by S4 axial plane slaty cleavage,
S4 fracture cleavage and L4 pencil lineaitons. The deeper and rear part of the Lesser Himalaya is characterized 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 crenulation [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±508 to the NE
are well-developed in the incompetent pelitic layers of
the Kuncha Formation [Fig. 5(E)]. The S4 slaty cleavage dipping either to the NE or to the SW and crosscutting 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 lineations (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 systematically along two parallel sections (Fig. 2) in the
Tansen±Pokhara area shows that the metamorphic
grade and intensity of deformation increases northward to the UMCT. Most parts of the Lesser Himalaya 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 coincide with the LMCT and the UMCT, respectively.
However, the isograd distribution patterns are not uniform 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 (Colchen et al., 1980). General petrographic features of the
rocks from each tectonic unit along the Tansen±
Pokhara section are given below. Mineral abbreviations 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 occurrence 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 parallel 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 muscovite. Biotite also occurs as inclusions in kyanite.
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
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 metabasites). Altered sphene (leucoxene), magnetite, tourmaline 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, polygonal 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-tectonic poikiloblastic garnet in the schists is found in
dierent shapes (skeletal, elongated, s-shaped and eqidimensional) and sizes (0.1±5 mm). Spiral garnets
show up to 3608 rotations 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 overgrowth. Large (up to 2 mm) post-tectonic garnets and
muscovites (0.2 mm) occur cutting across the S3 foliation.
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, zircon, 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 polygonized. The matrix contains coarse-grained aggregates
of polygonal quartz. Ms±Chl±Ab±Qtz is the typical
assemblage of phyllites in the chlorite zone. Tourmaline, magnetite, zircon, apatite and sphene occur as
accessories. S3 foliation with microfolds and crenulation 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
573
such as parallel laminae, cross-laminae, graded-bedding, 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, tourmaline, 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-thesouth sense of shearing. The Tansen Group contains
very low-grade to non-metamorphosed rocks. In thinsection, ®ne quartz clasts (0.02 mm) are arranged parallel 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 silicates and oxides as standards).
6.1. Garnet
Garnets from the Higher Himalaya and the MCT
zone were probed at the cores and rims, and the data
are projected on the Fe±Mg±(Mn+Ca) triangle
(Fig. 7). Garnets from the Higher Himalaya are rich in
pyrope (Mg 20±25% core, 15±20% rim) and almandine (Fe 65±70% core, 70±75% rim) content. Compositional pro®les across garnets from the Higher
Himalaya [Fig. 8(A)] are characterized by a plateau in
the cores. However, the margins of the garnets show
reverse zoning, with Fe and Mn increasing and Mg
decreasing towards the rim. Ca is relatively constant.
A compositional plateau of this type may be developed
by obliteration of growth zoning by later high-temperature diusion process (Spear, 1993). Retrograde
zoning pro®les at the margins may be the result of
subsequent diusion or resorption due to retrogression
(Barker, 1990). Garnets in the MCT zone are spessartine rich (Mn 25±45% core, 15±35% rim) (Fig. 7). Individual garnets show bell-shaped Mn-pro®les
characteristic of prograde metamorphism, with Fe
gradually increasing and Mn decreasing towards the
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L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
(Arita, 1983; Kaneko, 1997), Trisuli valley (Macfarlane, 1995), and Kathmandu area (Rai et al., 1998),
suggesting a quite dierent metamorphic history
between those units.
6.2. Muscovite
Fig. 7. Chemical composition of garnets from the Higher Himalaya
and the Main Central Thrust zone. Fe means total Fe as Fe2+.
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 celadonite 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 kyanitegrade 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 celadonite-rich muscovite, and thus the celadonite-rich muscovite was most probably produced by a later event
under lower metamorphic conditions as suggested by
Arita (1983). Celadonite contents of detrital muscovites 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
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.
rims [Fig. 8(B)]. The pro®les are reversed at the outermost rim, probably due to the late-stage retrogression.
The above patterns of compositional zoning in garnet 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
Illite crystallinity (IC) is an important tool in understanding the thermal structure of low-grade metamorphic 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 metamorphism 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 faceis 0
200±3708C, greenschist facies > 3708C, Winkler, 1974).
L.P. Paudel, K. Arita / Journal of Asian Earth Sciences 18 (2000) 561±584
7.1. Sample preparation and measurement
A total of 200 pelitic rock samples along the
Pokhara±Butwal road and the Modi Khola±Kali Gandaki sections of the Lesser Himalaya were used for IC
study. The laboratory procedure followed here is consistent 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 mortar 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