Defining the Himalayan Main Central Thrust in Nepal

Transcription

1 Journal of the Geological Society, London, Vol. 165, 2008, pp Printed in Great Britain. Defining the Himalayan Main Central Thrust in Nepal MICHAEL P. SEARLE 1,RICHARDD.LAW 2, LAURENT GODIN 3, KYLE P. LARSON 3, MICHAEL J. STREULE 1,JOHNM.COTTLE 1 & MICAH J. JESSUP 2 1 Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK ( mikes@earth.ox.ac.uk) 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. 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 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 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. 523

2 524 M. P. SEARLE ET AL. 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

3 HIMALAYAN MAIN CENTRAL THRUST, NEPAL 525 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 C and 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 + K- feldspar 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

4 526 M. P. SEARLE ET AL. 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

5 HIMALAYAN MAIN CENTRAL THRUST, NEPAL 527 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).

6 528 M. P. SEARLE ET AL. 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.

7 HIMALAYAN MAIN CENTRAL THRUST, NEPAL 529 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 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 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).

8 530 M. P. SEARLE ET AL. 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 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 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-

9 HIMALAYAN MAIN CENTRAL THRUST, NEPAL 531 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 Ma with later HT LP sillimanitegrade metamorphism at 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 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. 16Ma (Kohn et al. 2005), and the youngest monazite along the Dudh Kosi transect south of Everest has an age of Ma (Catlos et al. 2002). In Sikkim, in situ Th Pb monazite ages from the Main Central Thrust zone cluster at c. 22, and 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 (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 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

10 532 M. P. SEARLE ET AL. 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.

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