Download Defining the Himalayan Main Central Thrust in Nepal

Journal of the Geological Society, London, Vol. 165, 2008, pp. 523–534. Printed in Great Britain.
Defining the Himalayan Main Central Thrust in Nepal
M I C H A E L P. S E A R L E 1 , R I C H A R D D. L AW 2, L AU R E N T G O D I N 3 , K Y L E P. L A R S O N 3, M I C H A E L J.
S T R E U L E 1 , J O H N M . C OT T L E 1 & M I C A H J. J E S S U P 2
1
Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK (e-mail: [email protected])
2
Department of Geological Science, Virginia Tech, Blacksburg, VA 24061, USA
3
Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canada
Abstract: An inverted metamorphic field gradient associated with a crustal-scale south-vergent thrust fault,
the Main Central Thrust, has been recognized along the Himalaya for over 100 years. A major problem in
Himalayan structural geology is that recent workers have mapped the Main Central Thrust within the Greater
Himalayan Sequence high-grade metamorphic sequence along several different structural levels. Some workers
map the Main Central Thrust as coinciding with a lithological contact, others as coincident with the kyanite
isograd, up to 1–3 km structurally up-section into the Tertiary metamorphic sequence, without supporting
structural data. Some workers recognize a Main Central Thrust zone of high ductile strain up to 2–3 km thick,
bounded by an upper thrust, MCT-2 (¼ Vaikrita thrust), and a lower thrust, MCT-1 (¼ Munsiari thrust). Some
workers define an ‘upper Lesser Himalaya’ thrust sheet that shows similar P–T conditions to the Greater
Himalayan Sequence. Others define the Main Central Thrust either on isotopic (Nd, Sr) differences,
differences in detrital zircon ages, or as being coincident with a zone of young (,5 Ma) Th–Pb monazite
ages. Very few papers incorporate any structural data in justifying the position of the Main Central Thrust.
These studies, combined with recent quantitative strain analyses from the Everest and Annapurna Greater
Himalayan Sequence, show that a wide region of high strain characterizes most of the Greater Himalayan
Sequence with a concentration along the bounding margins of the South Tibetan Detachment along the top,
and the Main Central Thrust along the base. We suggest that the Main Central Thrust has to be defined and
mapped on strain criteria, not on stratigraphic, lithological, isotopic or geochronological criteria. The most
logical place to map the Main Central Thrust is along the high-strain zone that commonly occurs along the
base of the ductile shear zone and inverted metamorphic sequence. Above that horizon, all rocks show some
degree of Tertiary Himalayan metamorphism, and most of the Greater Himalayan Sequence metamorphic or
migmatitic rocks show some degree of pure shear and simple shear ductile strain that occurs throughout the
mid-crustal Greater Himalayan Sequence channel. The Main Central Thrust evolved both in time (early–
middle Miocene) and space from a deep-level ductile shear zone to a shallow brittle thrust fault.
between structural terminology (Greater Himalayan Sequence/
Lesser Himalaya Sequence) and stratigraphic terminology. Since
the original recognition of the Main Central Thrust along the
Himalaya, there has been a great amount of confusion regarding
the structural position and timing of slip along the Main Central
Thrust. This has come about because many different structures
are called the Main Central Thrust by different workers. Clearly
there is an urgent need to find a common definition and location
of the Main Central Thrust, and in this paper we attempt to do
that, based on combined strain and metamorphic criteria.
Previous attempts to map the Main Central Thrust have used
indirect methods such as: (1) a lithological contrast following a
distinctive quartzite unit beneath an orthogneiss unit (e.g.
Gansser 1983; Daniel et al. 2003); (2) following the kyanite
isograd (e.g. Bordet 1961; LeFort 1975; Colchen et al. 1986); (3)
differences in U–Pb detrital zircon ages (e.g. Parrish & Hodges
1996; Ahmad et al. 2000; DeCelles et al. 2000); (4) differences
in Nd isotope compositions (e.g. Robinson et al. 2001; Martin et
al. 2005; Richards et al. 2005, 2006); (5) location of young U–
Pb and Th–Pb monazite ages (e.g. Harrison et al. 1997; Catlos
et al. 2001, 2002;). None of these methods in themselves can be
used independently to define a thrust fault. Lithology, detrital
zircon ages and Nd isotopes give information on stratigraphy, not
structural relationships. Isograds and monazite ages give information on metamorphic reactions, fluids, and timing of mineral
growth, not structure.
The Himalayan Main Central Thrust, with its zone of inverted
metamorphic isograds from sillimanite grade down to biotite
grade, is one of the largest ductile shear zones known from any
collision-related mountain belt. The Main Central Thrust crops
out along c. 2200 km length of the Himalaya from western
Zanskar to Bhutan and Arunachal Pradesh (Fig. 1). It dips north
and places high-grade metamorphic rocks of the Greater Himalaya south over unmetamorphosed rocks of the Lesser Himalaya.
Since the discovery of an inverted metamorphic field gradient
across the Darjeeling–Sikkim Himalaya by Mallet (1874) and
von Loczy (1878), and across the Indian Himalaya by Oldham
(1883), it has been recognized that metamorphic grade increases
up-structural section towards the north from the Lesser Himalaya
to the Greater Himalaya. In Nepal, pioneering geological studies
by Hagen (1954), Hashimoto (1959, 1973) and Bordet (1961)
also recognized the increase of metamorphic grade up-structural
section. The Main Central Thrust zone was defined by Heim &
Gansser (1939) and Gansser (1964) as the thrust fault that places
high-grade metamorphic rocks of the Greater Himalayan Sequence southward over low-grade rocks of the Lesser Himalaya.
Unfortunately, this definition is not useful, because Greater and
Lesser Himalayan rocks refer to thrust-bounded structural
packages. Typically thrusts cut up-stratigraphic section in the
footwall, along ramps placing older rocks over younger rocks.
Thrusts may also follow flats placing similar age or younger
rocks over older rocks. We emphasize here the distinction
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M . P. S E A R L E E T A L .
Fig. 1. Geological map of the Himalaya.
Zones of high strain have been documented across the Main
Central Thrust zone in the Arun Valley by Brunel (1986) and
Brunel & Kienast (1986) using kinematic criteria. Abundant
shear criteria (S–C fabrics, rolled garnets, etc.) and stretching
lineations show southward transport of the Greater Himalayan
Sequence along the Main Central Thrust zone. Quantitative
vorticity studies were first reported from the Sutlej Valley, India
by Grasemann et al. (1999). Law et al. (2004) and Jessup et al.
(2006) showed that mean kinematic vorticity numbers from the
Main Central Thrust zone along the Everest profile in Nepal
yielded 58–44% pure shear component in addition to the
dominant top-to-south simple shear. The fact that metamorphic
isograds across the Main Central Thrust ductile shear zone have
been compressed or telescoped into a 1–2 km thick section
(Searle & Rex 1989; Hubbard 1996) shows that shearing postdates peak metamorphism.
Here we discuss the various previous methods used to map or
define the Main Central Thrust and then we propose a unifying
definition and map location, in the hope that future studies
relating to the Main Central Thrust will refer to one single Main
Central Thrust ductile shear zone and brittle thrust fault.
Metamorphic isograds and the Main Central Thrust
The earliest attempt to map the Main Central Thrust was carried
out by Bordet (1961), who mapped it along a prominent kyanitebearing pelite band within sillimanite gneisses in the Arun valley,
but did not describe any structural criteria to support this
placement (Figs 2 and 3). The same location for the Main
Central Thrust was subsequently used by Lombardo et al. (1993)
and Pognante & Benna (1993), who mapped the Main Central
Thrust further north as far as Kharta in southern Tibet. Above
this kyanite-bearing pelite horizon are some 5–8 km thickness of
sillimanite + garnet + biotite + cordierite orthogneiss (variously
called the Barun gneiss, Black gneiss or Jannu–Kangchenjunga
gneiss) with evidence of abundant partial melting (Brunel &
Kienast 1986; Lombardo et al. 1993; Pognante & Benna 1993;
Searle & Szulc 2005). High-grade calc-silicate gneisses and
H I M A L AYA N M A I N C E N T R A L T H RU S T, N E PA L
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Fig. 2. Map of the Everest–Makalu–
Kangchenjunga Himalaya in east Nepal
showing location of the Main Central
Thrust (MCT) and Greater Himalayan
Series. Shaded area represents the partially
molten channel containing migmatites and
leucogranites. The Bordet (1961) Main
Central Thrust follows a prominent band of
kyanite gneisses at Tashigaon village,
within the sillimanite-grade gneisses. Our
proposed location of the Main Central
Thrust is the ductile shear zone
corresponding to the zone of inverted
metamorphic isograds above Tumlingtar
village along the Arun river, and near
Taplejung in the Tamur river drainage.
LHS, Lesser Himalaya Series; GHS,
Greater Himalaya Series; TSS, Tethyan
sedimentary series.
marbles containing olivine, clinopyroxene, wollastonite and
scapolite are intercalated with the orthogneiss. P–T conditions
reached upper amphibolite facies and even granulite facies at
800–850 8C and 10–12 kbar, with later decompression following
a clockwise P–T–t path to 4–6 kbar during sillimanite-grade
metamorphism (Goscombe & Hand 2000; Dasgupta et al. 2004).
Below the Bordet (1961) Main Central Thrust horizon, the
Num orthogneiss is a 3–4 km thick unit of sillimanite + Kfeldspar orthogneiss with about 15–20% in situ partial melt (Fig.
3). Metamorphic grade is similar above and below this kyanite
gneiss horizon, although protolith rocks are probably from different stratigraphic levels. Internal strain is extremely high across
this entire package with consistent shear criteria indicating
south-directed simple shear with a significant component of
coaxial pure shear. Structurally beneath the Num orthogneiss,
near Tumlingtar, is a telescoped and highly sheared inverted
metamorphic isograd sequence from sillimanite through kyanite
and staurolite to biotite grade, where we map our preferred
location for the Main Central Thrust. Above this, all rocks have
a Tertiary metamorphic imprint on protoliths that range from
Proterozoic to Mesozoic. Below our Main Central Thrust, rocks
have little or no Tertiary metamorphic overprint.
Goscombe et al. (2006) recognized that the Main Central
Thrust had been incorrectly mapped as following a stratigraphic
boundary (their ‘Himalayan unconformity’) and they mapped the
Main Central Thrust beneath the Ulleri–Phaplu augen gneiss in
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M . P. S E A R L E E T A L .
Fig. 3. Simplified schematic section across
the Everest–Makalu Himalaya showing key
features of the structure, stratigraphy and
mineral isograds, together with our
proposed location of the Main Central
Thrust in eastern Nepal. bt, biotite; grt,
garnet; st, staurolite; ky, kyanite; sill,
sillimanite; crd, cordierite; ms, muscovite;
kfs, K-feldspar.
eastern Nepal. However, they also defined a new structure, the
‘High Himal Thrust’, close to, or along the kyanite pelite band
and the Bordet (1961) Main Central Thrust. Sillimanite +
muscovite + K-feldspar grade gneisses and migmatites occur
both above and below this horizon, and we suggest that both are
part of the same Greater Himalayan Sequence metamorphic
package. Goscombe et al. (2006) defined a Main Central Thrust
zone that extends down-section from this horizon south as far as
the garnet isograd beneath the Ulleri–Phaplu orthogneiss (Fig.
3).
In the Annapurna–Manaslu region of central Nepal LeFort
(1975), Colchen et al. (1986) and Pêcher (1989) mapped the
Main Central Thrust as following the kyanite isograd (at Dana in
the Kali Gandaki valley and Bahundanda in the Marsyandi
valley; Figs 4–6). There is no doubt that the kyanite gneisses are
highly strained, but so too are most of the rocks structurally
above and below this horizon. High-strain shear fabrics are
particularly prominent in pelitic schists and K-feldspar augen
gneisses, but less apparent in the more massive homogeneous
marble horizons. The location of the Main Central Thrust as
mapped by LeFort (1975) and Colchen et al. (1986) was followed
by most subsequent workers in the region (e.g. Harrison et al.
1997; Kohn et al. 2001).
Hodges et al. (1996) recognized several shear zones and
thrusts across a ‘Main Central Thrust zone’ and mapped the
southern limit of Main Central Thrust shearing close to the
village of Lamdrung in the Modi khola. Searle & Godin (2003)
mapped the entire inverted metamorphic sequence as being part
of the Greater Himalayan Sequence and placed the Main Central
Thrust brittle fault along the base of the inverted metamorphic
sequence (Fig. 5). This locality marks a sharp break between
highly strained rocks affected by Tertiary metamorphism in the
hanging wall and rocks beneath this that are not highly
metamorphosed or highly strained. Orthogneiss horizons such as
the Proterozoic Ulleri augen gneiss in the Annapurna region, or
the Phaplu augen gneiss in the Everest region, previously
assigned to the ‘upper Lesser Himalaya’ thrust sheet, are now
more logically placed within the Greater Himalayan Sequence
thrust sheet above the Main Central Thrust.
Arita (1983) mapped a Main Central Thrust zone of high
ductile strain up to 2–3 km thick, bounded by an upper thrust,
MCT-2 (Vaikrita thrust), and a lower thrust, MCT-1 (Munsiari
thrust). The earlier, upper MCT-2 corresponds to the Colchen et
al. (1986) Main Central Thrust whereas the lower, later MCT-1
corresponds to our proposed location of the Main Central Thrust.
The rocks between these two thrusts show peak metamorphic
temperatures ranging between 550 8C and less than 330 8C, with
an inverted thermal gradient as deduced from Raman spectroscopy of carbonaceous material by Beyssac et al. (2004) and
Bollinger et al. (2004). Those workers accepted the location of
the Colchen et al. (1986) Main Central Thrust, and so placed
these rocks in the Lesser Himalaya. However, we include all
these metamorphic rocks in the Greater Himalayan Sequence
above our proposed Main Central Thrust.
Within the Main Central Thrust ductile shear zone, Kohn et al.
(2001) showed that garnets from structurally lower locations
grew with increasing P and T (loading), whereas garnets from
structurally higher locations grew with increasing T but decreasing P (exhumation). This records a snapshot in time of the
continuously evolving northward burial (prograde metamorphism) and southward exhumation (decompression and retrograde
metamorphism) particle paths of Greater Himalayan Sequence
metamorphic rocks. Searle et al. (2002) suggested that the
position of the Main Central Thrust along the Darondi valley
should be 15 km south of where it was mapped by Colchen et al.
(1986) and Kohn et al. (2001), along the base of the inverted
metamorphic sequence (‘Location of inferred structure’ in fig. 1
of Kohn et al. 2001).
Dadeldhura and Ramgarh thrusts
DeCelles et al. (2001) and Robinson et al. (2003, 2006) mapped
two thrust sheets structurally beneath the Main Central Thrust in
western Nepal, the higher Dadeldhura and lower Ramgarh thrust
sheets. They failed to locate a discrete thrust at the position of
the Main Central Thrust and inferred its presence from extrapolation along strike in the Karnali Valley. The Dadeldhura thrust
sheet consists of garnet–muscovite–biotite schists, mylonitic
augen gneiss and Cambrian–Ordovician granites. The Ramgarh
thrust sheet consists of greenschist-facies metasedimentary rocks
H I M A L AYA N M A I N C E N T R A L T H RU S T, N E PA L
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Fig. 4. Map of the Annapurna–Manaslu
Himalaya, showing the structure of the
Greater Himalayan Sequence and our
proposed location of the Main Central
Thrust. The Colchen et al. (1986) Main
Central Thrust is coincident with the
kyanite isograd running through Dana and
Bahundanda villages. Our Main Central
Thrust is located along a high-strain zone
further south, south of Gorhka, and
corresponds to the southern limit of Tertiary
metamorphism. The mapped locations of
the South Tibetan Detachment system
(STDS) normal faults are from Searle &
Godin (2003).
Fig. 5. Simplified, schematic section across
the Annapurna Himalaya showing key
features of the structure, stratigraphy and
mineral isograds, and our proposed location
of the Main Central Thrust in central Nepal.
Shaded area represents the migmatites and
leucogranites within the partially molten
channel.
of the Kushma and Ranimata Formations. Both the Dadeldhura
and Ramgarh thrust sheets occur in a synformal klippe that is a
structural equivalent of the Almora klippe to the west in India
and the Kathmandu klippe to the east. The Ramgarh thrust forms
the roof thrust to a series of imbricated thrust slices of
unmetamorphosed Lesser Himalayan rocks of Late Archaean,
Proterozoic and Cambrian age (DeCelles et al. 2001; Robinson
et al. 2006).
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M . P. S E A R L E E T A L .
Fig. 6. Simplified, schematic section across
the Manaslu Himalaya, showing key
features and our proposed location of the
Main Central Thrust. Shaded area
represents the zone of partial melting with
migmatites and leucogranites (crosses). The
Manaslu leuocogranite is wholly within the
Greater Himalayan Sequence, following
Searle & Godin (2003), with the South
Tibetan detachment (STD) wrapping around
the upper level of the granite.
The Ramgarh thrust marks the southern limit of Tertiary
Himalayan metamorphism in western Nepal and we prefer to link
this with the Main Central Thrust. The reported location of the
Ramgarh thrust in central and eastern Nepal (Martin et al. 2005;
Pearson & DeCelles 2005), however, does not coincide with the
Main Central Thrust. In central and eastern Nepal the location of
the Ramgarh thrust is almost entirely interpreted from lithological repetition; a possible fault surface has been observed only
in the Tribeni area of eastern Nepal. A more southerly location
for the Ramgarh thrust is supported by pervasive deformation
documented by quartz c-axis fabrics throughout central Nepal
(Bouchez & Pêcher 1981). Although the recrystallization of
quartz under significantly high strain has been recognized in the
Ramgrah thrust sheet, as mapped by Martin et al. (2005) and
Pearson & DeCelles (2005), it is interpreted to be locally
confined to the immediate hanging wall of the inferred thrust
(Pearson & DeCelles 2005). However, Bouchez & Pêcher (1981)
showed that quartz c-axis fabrics are preserved for more than
6 km farther south than the mapped position of the Ramgarh
thrust. Both lithological and structural data fit better with a
structurally lower, more southerly, Ramgarh thrust where it is
coincident with the Main Central Thrust.
Restored sections show that the Ramgarh, Dadeldhura and
Main Central thrust sheets of DeCelles et al. (2001) all have
Proterozoic sedimentary rocks, Ulleri augen gneiss and Cambrian–Ordovician sedimentary rocks and granites as protoliths.
Hanging-wall–footwall cut-offs can be successfully matched in
restored sections (Fig. 7). We therefore propose that the Main
Fig. 7. Generalized restored section across the Nepal Himalaya showing the pre-thrusting trajectories of the Main Central Thrust and South Tibetan
Detachment shear zones and faults. The shaded horizon represents the Upper Proterozoic sedimentary rocks of the Lesser Himalaya, and Greater
Himalaya. Within the Greater Himalayan Sequence these include the metamorphosed rocks of the Nawakot Group above the Ramgarh thrust (Main
Central Thrust) and the Bhimpedi Group within the Kathmandu nappe, above the Mahabharat thrust. Sillimanite-grade pelitic gneisses within the Greater
Himalayan Sequence are interpreted as metamorphosed Upper Proterozoic sedimentary rocks of the Haimanta–Vaikrita Group. The base of the Tethyan
Himalaya consists of similar Neoproterozoic sedimentary rocks showing that the Lesser, Greater and Tethyan Himalaya were all part of one contiguous
Indian plate.
H I M A L AYA N M A I N C E N T R A L T H RU S T, N E PA L
Central Thrust should be mapped along the Ramgarh thrust, and
all rocks above that should be incorporated into the Greater
Himalayan Sequence. The major thrust systems propagated
southward with time from the Early Miocene motion of the
Greater Himalayan Sequence metamorphic core (Hodges et al.
1996; Godin et al. 2001) to the ,15 Ma motion along the
Ramgarh thrust (DeCelles et al. 2001; Robinson et al. 2006).
During the Late Miocene ductile shearing along the Main Central
Thrust–Ramgarh thrust ceased, and thrusting propagated downsection to the Lesser Himalayan brittle imbricate thrust system.
At least 120 km of southward translation has been estimated
across the Ramgarh thrust sheet (Robinson et al. 2006).
Mahabharat thrust
Several klippen or thrust sheets of high-grade metamorphic rocks
and granites (e.g. Almora klippe; Kathmandu complex) overlie
low-grade or unmetamorphosed rocks of the Lesser Himalaya to
the south of the main Main Central Thrust, as mapped in
Langtang and the Ganesh Himal (Fig. 1). The thrust beneath
these klippen has been variously termed the Almora or Munsiari
thrusts in India (Heim & Gansser 1939; Valdiya 1980), and the
Mahabharat and Dadeldhura thrusts in Nepal (Stöcklin &
Bhattarai 1980; Upreti & LeFort 1999; Johnson et al. 2001). The
Mahabharat thrust beneath the Kathmandu klippe encircles the
southern Kathmandu valley and links up with the Main Central
Thrust in the NW and NE (Upreti & LeFort 1999; Johnson et al.
2001). Rocks above the Mahabharat thrust include Proterozoic
Bhimpedi Group and early–middle Palaeozoic Phulchauki Group
sedimentary rocks, which are intruded by Ordovician granites
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and augen gneisses. Metamorphism reaches kyanite grade at the
base and isograds are right-way-up from kyanite through garnet
and biotite to chlorite grade (Johnson et al. 2001). Along the
Mahabharat thrust dynamic metamorphism has locally inverted
the thermal gradient with formation of garnet–biotite mylonites
and phyllonites. Beneath the Mahabharat thrust carbonaceous
pelites of the Nawakot Group show an inverted thermal gradient
from 468 8C to less than 330 8C, based on Raman spectroscopy
of carbonaceous material (Beyssac et al. 2004; Bollinger et al.
2004).
These crystalline complexes have been termed ‘Lesser Himalayan crystallines’ or ‘Outer Lesser Himalayan crystallines’, but
in reality they are lateral equivalents to the Greater Himalayan
Sequence above the Main Central Thrust. They share similar
upper Proterozoic and lower Palaeozoic protoliths, similar Miocene metamorphism, and similar 21–18 Ma leucogranite pegmatite dykes as the main Greater Himalayan Sequence to the north
(Johnson et al. 2001). We concur with the conclusions of
Johnson et al. (2001) and Johnson (2005) that the Mahabharat
thrust is the same structure as the Main Central Thrust, but along
a more southerly, proximal position in the restoration. The
Mahabharat thrust climbs up-section in the transport direction,
from being along the base of the inverted metamorphic sequence
at Langtang in the north, to along the isograd fold hinge at
Kathmandu (Fig. 8). The youngest thrust splays off beneath the
Kathmandu complex to link with the Ramgarh thrust to the south
and west of the Kathmandu complex (Fig. 1). All these thrusts
are part of the Main Central Thrust but are restored at different
depths, progressively ramping up-section towards the south.
The Darjeeling klippe is another structural outlier of Greater
Fig. 8. Geometry of the Main Central Thrust zone in the Langtang–Kathmandu nappe region of central Nepal showing the relationship of the Mahabharat
and Ramgarh thrusts to the metamorphic isograds. This geometry combines the folded isograd model of Searle & Rex (1989) with the channel flow model
for the Greater Himalayan Sequence (Law et al. 2006; Searle et al. 2006) and with the Johnson (2005) structural model for the Mahabharat thrust and
Kathmandu nappe. It also explains the structural location of the greenschist- and amphibolite-facies metamorphic rocks of the Ramgarh thrust sheet
(Beyssac et al. 2004; Bollinger et al. 2004) structurally beneath the Mahabharat thrust. Shaded area shows the zone of partial melting (sillimanite +
K-feldspar gneisses, migmatites), with the main leucogranites (crosses) concentrated along the north. Metamorphic isograds have been sheared (by a
combination of pure shear and south-directed simple shear), and flattened along the South Tibetan Detachment ductile shear zone above (right-way-up
metamorphic isograds) and along the Main Central Thrust ductile shear zone below (inverted metamorphic isograds).
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M . P. S E A R L E E T A L .
Himalayan Sequence rocks thrust above unmetamorphosed Lesser Himalayan sedimentary rocks in far eastern Nepal and
Sikkim–West Bengal (Fig. 1). The full inverted metamorphic
isograd sequence has been mapped here, but unlike the Kathmandu complex, and like the Greater Himalayan Sequence, the
isograds are structurally inverted from sillimanite down to
biotite–chlorite (Mohan et al. 1989; Dasgupta et al. 2004). The
structures and metamorphic P–T conditions clearly show that the
Darjeeling klippe is linked to the main Main Central Thrust to
the north (Searle & Szulc 2005; Fig. 3).
Detrital zircon ages and the Main Central Thrust
Detrital zircon U–Pb ages provide maximum depositional age
constraints of the metamorphic protolith. Parrish & Hodges
(1996) originally proposed that Greater and Lesser Himalayan
rocks, divided by the Main Central Thrust, had a significant
difference in sedimentary provenance. Zircons from Greater
Himalayan Sequence rocks have mainly late Proterozoic and
early Palaeozoic ages, whereas zircons from the Lesser Himalaya
have late Archaean–early Proterozoic ages. DeCelles et al.
(2000) showed that metasedimentary rocks from the Greater
Himalayan Sequence gave zircon ages of 800–1700 Ma, whereas
quartzites of one unit generally mapped within the Lesser
Himalaya Sequence, the Nawakot Group, yielded zircons
.1.8 Ga old. This reflects the 1866–1833 Ma depositional age
of these units. The upper age limit is given by the Ulleri augen
gneiss, which was intruded into the Nawakot quartzite. However,
a major mylonite zone corresponding to the Main Central Thrust
was mapped beneath the Ulleri augen gneiss in the Annapurna
region (Searle & Godin 2003), so these rocks are now included
within the Greater Himalayan Sequence. The Phaplu augen
gneiss in the Everest profile is a similar age, and at a similar
structural position to the Ulleri augen gneiss. The Phaplu gneiss
is highly sheared and overlain by staurolite- and sillimanite-grade
rocks typical of the Greater Himalayan Sequence, so it has also
been included here within the Greater Himalayan Sequence
(Jessup et al. 2006; Searle et al. 2006).
There seems little doubt that many Lesser Himalayan protoliths, particularly in Nepal, are older than the exposed Greater
Himalayan Sequence protoliths. The upper structural levels of
the Lesser Himalaya in India (Cambrian Krol and Tal Formations, which overlie Proterozoic rocks) are lateral equivalents to
the base of the restored Greater Himalayan Sequence (Steck
2003) It is also widely accepted that the Greater Himalayan
Sequence protoliths were similar in age to the lower levels of the
Tethyan Himalaya, which range in age from Neoproterozoic to
Eocene. Indeed, in the Zanskar Himalaya in India, Searle (1986)
and Walker et al. (2001) were able to correlate thick garnet
amphibolite units in the Greater Himalayan Sequence with
unmetamorphosed Permian Panjal volcanic rocks in Kashmir,
and thick high-grade marbles of the Greater Himalayan Sequence
with unmetamorphosed Triassic (and possible Jurassic) shelf
carbonate units within the Tethyan Himalaya.
Nd isotopes and the Main Central Thrust
Several workers (e.g. DeCelles et al. 2000; Robinson et al. 2001;
Richards et al. 2005) have described the Main Central Thrust as
a ‘discrete ductile shear zone separating isotopically different
protoliths’. Parrish & Hodges (1996) first proposed that there
was no overlap between the ranges of 143 Nd/144 Nd ratios between
Greater Himalayan Sequence and Lesser Himalaya rocks in the
Langtang region. DeCelles et al. (2000) and Robinson et al.
(2001) showed that ENd(0) average values from Lesser Himalayan rocks in Nepal are 21.5, whereas the Greater and Tethyan
Himalaya zones in Nepal have an average ENd(0) value of 16.
They suggested that the Greater Himalayan Sequence was not
Indian basement, but rather a terrane that was accreted onto India
during the Early Palaeozoic, and that the Main Central Thrust
had a large amount of pre-Tertiary displacement. However, there
is no evidence of Palaeozoic suture zone rocks (e.g. ophiolites,
deep-sea sediments, etc.) anywhere along the Main Central
Thrust, and palinspastic reconstructions across the Western
Himalaya (Searle 1986; Steck 2003), the Everest profile in Nepal
(Searle et al. 2006) and the Sikkim–Bhutan Himalaya (Searle &
Szulc 2005) show a continuous sedimentary succession from
proximal to distal across the Lesser, Greater and Tethyan
Himalaya.
With larger Nd isotope datasets, the distinctive differences
between Greater Himalayan Sequence and Lesser Himalaya
protolith start to vanish. Ahmad et al. (2000) and Richards et al.
(2005) recognized a separate zone termed the ‘Outer Lesser
Himalaya’ that had relatively young source rocks, similar to the
Greater Himalayan Sequence. They concluded that Greater
Himalayan Sequence and ‘Outer Lesser Himalaya’ rocks showed
a Meso-Palaeo-Proterozoic source, whereas the rest of the Lesser
Himalaya showed Late Archaean to Early Proterozoic source
rocks. However, Myrow et al. (2003) showed that samples from
the base of the Tethyan Himalaya, north of the Greater Himalayan Sequence, have similar detrital zircon age spectra and Nd
isotopic data to samples from the Kathmandu klippe and Lesser
Himalaya south of the Greater Himalayan Sequence, thus eliminating the need for separate Greater Himalayan Sequence–Lesser
Himalaya ‘terranes’. Lesser, Greater and Tethyan Himalaya
represent a proximal to distal section across a continuous Indian
plate prior to collision with Asia (Searle 1986; Myrow et al.
2003; Steck 2003; Searle et al. 2006). Martin et al. (2005) also
correctly recognized that the Lesser–Greater Himalayan distinction was a protolith designation, fixed at the time of deposition.
Because the Main Central Thrust is a Tertiary thrust fault that
certainly cuts across stratigraphy, detrital zircon ages or Nd
isotopes cannot be used to define the location of the Main
Central Thrust.
Metamorphism, U–Th–Pb monazite ages and the
Main Central Thrust
Inverted metamorphism along the Main Central Thrust zone is
almost certainly related to movement along the Main Central
Thrust. Within the inverted metamorphic field gradient, the rocks
are highly sheared showing ubiquitous C–S–C9 fabrics and
north-plunging lineations that indicate southward transport. Approximately 5–8 km of thickness has been flattened by pure
shear to a section 1–2 km thick along the inverted metamorphic
isograd zone (Searle & Rex 1989). There are no major metamorphic discontinuities within the inverted metamorphic sequence, suggesting post-metamorphic pure shear flattening. The
geometry of the inverted isograds along the Main Central Thrust
zone is similar along the entire Himalayan chain between the
Zanskar–Kishtwar area in the west (Stephenson et al. 2000,
2001) to Nepal, Sikkim and Bhutan in the east (e.g. Bollinger et
al. 2004; Dasgupta et al. 2004; Jessup et al. 2006). Dating of
peak metamorphism has relied on Sm–Nd dating of garnet (e.g.
Vance & Harris 1999), or U–Pb dating of monazites (eg: Walker
et al. 1999; Simpson et al. 2000; Foster et al. 2002), that grew in
equilibrium with kyanite or sillimanite. These are the only
methods that date minerals with high enough closure tempera-
H I M A L AYA N M A I N C E N T R A L T H RU S T, N E PA L
tures. 40 Ar/39 Ar ages of hornblendes or micas record only a point
on the cooling path after peak metamorphism, during exhumation. Within the Greater Himalayan Sequence initiation of garnet
growth and burial metamorphism occurred at 44 Ma, with garnet
rims growing as late as 29 3 Ma. (Prince et al. 2001; Foster et
al. 2002). In the Everest region peak kyanite-grade metamorphism within the Greater Himalayan Sequence has been dated by
U–Pb monazite at 32.2 0.4 Ma with later HT–LP sillimanitegrade metamorphism at 22.7 0.2 Ma (Simpson et al. 2000).
Most leucogranite ages, interpreted as dating peak sillimanitegrade metamorphism and migmatization, along the Nepalese
Himalaya range from c. 24 to 16 Ma (see Searle et al. 1997,
2003, for reviews).
Along the base of the Greater Himalayan Sequence in Nepal,
Harrison et al. (1997) first reported surprisingly young ages of c.
6 Ma from in situ 208 Pb/232 Th dating of monazite inclusions in
garnet. These were interpreted as dating metamorphic recrystallization. Catlos et al. (2001) reported ages as young as
3.3 0.1 Ma from the Marysandi Valley and Kohn et al. (2001)
reported ages of 9–8 Ma from the Lesser Himalaya (reinterpreted here as the lower levels of the Greater Himalayan
Sequence above the Main Central Thrust). Th–Pb monazite ages
from the Main Central Thrust zone in Langtang are c. 16 Ma
(Kohn et al. 2005), and the youngest monazite along the Dudh
Kosi transect south of Everest has an age of 10.3 0.8 Ma
(Catlos et al. 2002). In Sikkim, in situ Th–Pb monazite ages
from the Main Central Thrust zone cluster at c. 22, 15–14 and
12–10 Ma (Catlos et al. 2004). These workers all interpreted the
Th–Pb monazite ages as dating metamorphism along the Main
Central Thrust, and hence timing of slip. However, Bollinger &
Janots (2006) cautioned that some young Himalayan monazites
were a retrograde growth product from the breakdown of allanite
at low temperatures (,370 8C). In this scenario, the Th–Pb
monazite ages only date a retrograde event at low temperature
and may have nothing to do with slip along the Main Central
Thrust. It seems quite likely that the very young monazite ages
along the Main Central Thrust zone may record crystallization
from metasomatic hydrothermal fluids rather than a ‘metamorphic event’. The ubiquitous presence of hot springs along the
Main Central Thrust zone today testifies to the high level of
hydrothermal fluids channelled up along the Main Central Thrust
zone, and to the insignificance of shear heating along the thrust.
The Main Central Thrust system evolved through time and
space. Deep crustal levels show a wide ductile shear zone in
kyanite-grade rocks. Higher-level brittle faults show more discrete fracture planes (e.g. the Ramgarh thrust). Rocks with young
matrix monazite ages would be expected above the youngest,
southernmost thrust planes of the Main Central Thrust.
Restoration of the Main Central Thrust system
A generalized restoration of the Main Central Thrust system in
Nepal is shown in Figure 7. Late Proterozoic rocks extend from
the Lesser Himalaya to the Greater Himalayan Sequence and
across to the base of the Tethyan Himalaya (Haimanta Group–
Cheka Formation). Unmetamorphosed Nawakot Group sedimentary rocks in the Lesser Himalaya pass north into the same
protolith age rocks which have been metamorphosed to greenschist–upper amphibolite facies in the Ramgarh thrust sheet
(Beyssac et al. 2004), up to kyanite grade in the Kathmandu
thrust sheet (Johnson et al. 2001), and finally into high-grade
sillimanite gneisses in the Dadeldhura and Greater Himalayan
Sequence thrust sheets in the high Himalaya. In the internal parts
of the Greater Himalayan Sequence, the sillimanite gneisses
531
(commonly referred to as Greater Himalayan Sequence Formation 1; Colchen et al. 1986) are metamorphosed equivalents of
the same late Proterozoic protoliths. As there are no suture zone
rocks along the Main Central Thrust and the stratigraphy is
continuous across the restored thrusts, there cannot be a Palaeozoic suture zone along the Main Central Thrust (DeCelles et al.
2000; Richards et al. 2005). Instead, the restored Lesser–
Greater–Tethyan Himalaya was a contiguous continental section
(Searle 1986; Myrow et al. 2003; Steck 2003; Searle et al.
2006).
One major implication of the restoration of the Himalaya is
that the Main Central Thrust follows a flat for a long distance
across strike. This flat follows a rheologically weak horizon
along the Neoproterozoic shales. True Indian basement rocks
(Archaean–Lower Proterozoic) are never exposed in the Himalaya. Although impossible to determine accurately because of the
high degree of ductile strain, minimum crustal shortening
estimates from the Himalaya are between 500 and 900 km
(Searle 1986; DeCelles et al. 2001; Robinson et al. 2006).
Because the across-strike width of the basement and cover
(Neoproterozoic–Eocene rocks exposed in the Himalaya) must
balance on the restoration, it must be concluded that Indian
basement has underthrust northwards beneath the Himalaya north
of the Main Central Thrust, and beneath the southern margin of
Asia (Lhasa block) by a similar distance across strike. Very
large-scale subhorizontal detachments such as the Main Central
Thrust therefore can play a major role in the mechanical
decoupling of the crust. In the Himalaya, the Main Central
Thrust and the South Tibetan Detachment effectively decouple
the upper (Tethyan Himalaya), middle (Greater Himalaya) and
lower (Indian basement) crust.
The Main Central Thrust, like all major thrust faults, changes
in style in both the horizontal and vertical planes (Fig. 8), as well
as through time. In deep crustal profiles it is a 1–3 km thick
ductile shear zone in kyanite-grade metamorphic rocks. U–Th–
Pb monazite ages of these rocks range between c. 24 and 18 Ma
(for a review, see Godin et al. 2006). At higher structural levels
ductile shearing passes up into brittle thrust faulting along a
discrete thrust plane. Many of these shallower levels follow flats
placing similar age or younger rocks over similar age or older
rocks (Fig. 7). In more southerly, outboard profiles such as at the
Kathmandu complex, the Main Central Thrust (Mahabharat
thrust) ramps up to higher levels where metamorphism in the
hanging wall is right-way-up (Fig. 8). Here, a new, younger
brittle thrust (Ramgarh thrust) developed in the footwall of the
Mahabharat thrust, placing greenschist-grade rocks over unmetamorphosed Lesser Himalayan rocks (Fig. 8). In the Darjeeling
area, a late out-of-sequence breakback Main Central Thrust
developed behind the Darjeeling klippe (Searle & Szulc 2005).
All these ductile shear zones and brittle thrust faults are part of
the Main Central Thrust system, which developed over a period
.14 Ma (from c. 24 to 10 Ma), from depths equivalent to at least
10–12 kbar (35–40 km) to the surface. Because the inverted
metamorphic gradient resulted from post-metamorphic shearing,
the observed metamorphic gradient should not be directly compared with the geotherm (Searle & Rex 1989; Bollinger et al.
2004).
Conclusions
None of the criteria commonly used for mapping the Main
Central Thrust are valid in themselves for defining the Main
Central Thrust. Lithology, detrital zircon U–Pb ages, and Nd
isotope signatures reveal information about provenance and
532
M . P. S E A R L E E T A L .
stratigraphy, but not structure. Because thrust trajectories cut up
and across stratigraphic section in the transport direction, these
methods are clearly not useful in defining the position of thrust
faults such as the Main Central Thrust. Isograds are metamorphic
reactions that can be mapped (with difficulty) in the field by the
first appearance of key index minerals (sillimanite, kyanite,
staurolite, garnet). Young monazite ages reveal specific information on growth of the garnet that armours them, or the matrix
that contains them, and fluid infiltration, neither of which are
definitively associated with motion along a thrust fault. Only
structural mapping and strain indicators can define the position
of the Main Central Thrust. Following Hanmer & Passchier
(1991) and Passchier & Trouw (2005), the essential criteria to
define a shear zone are the identification of a strain gradient and
the clear localization of strain.
As the metamorphic isograds are always telescoped along the
base of the Greater Himalayan Sequence with up to 50% pure
shear flattening superimposed on the already ‘frozen-in’ isograds
(Jessup et al. 2006), the position of the inverted metamorphism
often correlates closely, or precisely, with the position of the
Main Central Thrust ductile shear zone. In the western Himalaya,
the Main Central Thrust zone inverted metamorphic isograd
sequence along the base of the Greater Himalayan Sequence has
been mapped around a NW-plunging recumbent anticline, and
has been shown to join up with right-way-up isograds along the
footwall of the South Tibetan Detachment low-angle normal fault
at the top of the Greater Himalayan Sequence (Searle & Rex
1989). The map relationship and timing constraints (Hodges et
al. 1996) show that movement along the Main Central Thrust
and South Tibetan Detachment were synchronous, and that the
Greater Himalayan Sequence moved south, bounded by these
shear zones above and below, during southward extrusion of the
ductile partially molten core of the Greater Himalayan Sequence
(Fig. 9; channel flow model).
We suggest that a common unifying definition for the Main
Central Thrust should be ‘the base of the large-scale zone of
high strain and ductile deformation, commonly coinciding with
the base of the zone of inverted metamorphic isograds, which
places Tertiary metamorphic rocks of the Greater Himalayan
Sequence over unmetamorphosed or low-grade rocks of the
Lesser Himalaya’, similar to that suggested for the Kishtwar
Main Central Thrust section by Stephenson et al. (2000, 2001).
Whereas the Kishtwar section shows an exhumed, deeper, more
internal section across the Main Central Thrust zone, the
Kathmandu nappe and Ramgarh thrust sheets show a shallower,
more external section across the Main Central Thrust (Fig. 8).
The Main Central Thrust ductile shear zone is commonly
bounded along the south (base) by a brittle thrust fault, so a
distinction could be made between the Main Central Thrust
ductile shear zone (up to 2 km or more thick) and the brittle
Main Central Thrust fault (sensu stricto) along its base.
We acknowledge NERC, NSF and NSERC grants respectively to M.P.S.,
R.D.L. and L.G., and UK (M.J.S.), New Zealand (J.M.C.), Canadian
(K.P.L.) and US (M.J.J.) PhD studentships grants. We thank the late
Pasang Tamang, and Pradap Tamang and team for excellent trekking
logistics in the Annapurnas, Tashi Sherpa and Sonam Wangdu in the
Everest region, and Suka Ghale and team in the Manaslu region. The
paper benefited greatly from reviews by Paul Myrow and Richard Brown,
and discussions with Mike Johnson, Randy Parrish and Laurent Bollinger.
Fig. 9. Generalized model for the Main
Central Thrust ductile shear zone and thrust
fault, and Greater Himalayan Sequence
channel flow along the Himalaya. The
South Tibetan Detachment and Main
Central Thrust were active simultaneously
during the early to middle Miocene, and the
deeper ductile shear zones pass upward and
outward into brittle faults with time. The
mid-crustal channel of partially molten
crust separates the brittle deforming
seismogenic upper crust from the rigid,
high-pressure granulite lower crust of the
subducted Indian Shield.
H I M A L AYA N M A I N C E N T R A L T H RU S T, N E PA L
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Received 14 May 2007; revised typescript accepted 9 August 2007.
Scientific editing by Rob Strachan