The geology of the Tama Kosi and Rolwaling valley region, East-Central Nepal

Kyle P. Larson* Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan, S7N 5E2, Canada

ABSTRACT Tama Kosi valley in east-central Nepal (Fig. 1). which were in turn assigned to either the Hima- In order to properly evaluate the evolution of the layan gneiss group or the Midlands metasedi- The Tama Kosi/Rolwaling area of east- Himalaya and understand the processes respon- ment group (Fig. 2). The units of Ishida (1969) central Nepal is underlain by the exhumed sible for its formation, it is critical that all areas and Ishida and Ohta (1973) are quite similar in mid-crustal core of the Himalaya. The geol- along the length of the mountain chain be inves- description to the tectonostratigraphy reported ogy of the area consists of Greater Hima- tigated, at least at a reconnaissance scale. by Schelling (1992) who revisited and expanded layan sequence phyllitic schist, paragneiss, The Tama Kosi valley is situated between the the scope of their early reconnaissance work. and orthogneiss that generally increase in Cho Oyu/Everest/Makalu massifs to the east Schelling (1992) separated the geology of metamorphic grade from biotite ± garnet and the Kathmandu klippe/nappe to the west the lower and middle portion of the Tama Kosi assemblages to sillimanite-grade migmatite (Fig. 1). Recent work in these areas serves to into the more traditional Greater Himalayan up structural section. All metamorphic rocks highlight stark differences between them. In the sequence (Higher Himalayan Crystallines) and are pervasively deformed and commonly Kathmandu region, the extruded midcrustal core Lesser Himalayan sequence lithotectonic assem- record top-to-the-south sense shear. The top is folded and preserved far into the orogenic blages. His “Higher Himalayan Crystallines” of the Greater Himalayan sequence in the foreland in the form of a klippe or nappe (e.g., approximately correspond to Ishida and Ohta’s mapped area is marked by an undeformed, Johnson et al., 2000). Furthermore, there may be (1973) Himalayan gneisses (Fig. 2) and include pegmatitic leucogranite stock. Relation- evidence for a merger of the two major, antithetic a series of sillimanite-bearing paragneiss and ships in adjacent areas constrain the age of fault systems that bound the mid-crustal core orthogneiss units that display varying degrees the leucogranite and the deformation struc- (Webb et al., 2011). In contrast, the geology of of partial melting, granitic intrusion, and mig- tures it crosscuts, including the top-to-the- the Everest region refl ects a deeply eroded, for- matization. The rocks within Schelling’s (1992) south sense deformation, to be older than merly ductily extruded mid-crustal channel and Lesser Himalayan sequence comprise much of middle Miocene. The lower portion of the associated leucogranitic bodies (e.g., Searle et al., Ishida and Ohta’s (1973) Midland metasedimen- exhumed midcrustal package has been sub- 2006; Jessup et al., 2006; Cottle et al., 2009; tary group (Fig. 2). The lithologies mapped by ject to late-stage folding during the forma- Streule et al., 2010). This makes the Tama Kosi Schelling as the Lesser Himalaya include locally tion of the Tama Kosi window, a structural valley area important not only to help complete graphitic-rich, garnet ± staurolite ± kyanite culmination that may refl ect out-of-sequence the geologic map of the Himalaya, but also for schist, and orthogneiss, commonly K-feldspar adjustment of the orogenic wedge. The geol- potential assessments of lateral variation during augen-bearing. ogy of the mapped area appears similar to the evolution of the mountain belt. This prelimi- that observed in the adjacent, better-studied nary study presents the basic lithology, structure, Structure Everest region. and metamorphism recorded in the Tama Kosi area and interprets those fi ndings within the cur- Differences in the structural interpretation INTRODUCTION rent conceptual framework of the orogen. between previous studies of the lower and middle Tama Kosi valley and area are more sig- Much has changed in our understanding of GEOLOGIC SETTING— nifi cant than those for the tectonostratig raphy. orogenesis since the fi rst reconnaissance map- PREVIOUS WORK Ishida (1969) mapped thrust faults between ping of the Himalaya. The ideas put forth to almost every “formation” or “zone” (Fig. 2), explain the varied evolution of the orogen, such Lithology separating them into tectonically bound rock as midcrustal fl ow (e.g., Bird, 1991; Grujic et al., units. Each of these faults is considered to 1996; Beaumont et al., 2001), critical taper (e.g., The geology of the lower and middle por- be an originally north-dipping structure that DeCelles et al., 1998a, 1998b), and plateau col- tions of the Tama (also written as Tamba) Kosi accommodated top-to-the-south sense dis- lapse (e.g., England and Houseman, 1988), have valley (Fig. 1) was fi rst reported on in the late placement. Subsequent folding of some of the all been based on geologic map interpretation. 1960s (Ishida, 1969). This early work outlined faults has since modifi ed their dip direction There are still some areas along the mountain the basic lithologic framework of the area and and, paired with erosion, favored the develop- belt, however, that have not been mapped. One how it is related to the adjacent Everest/Makalu ment of tectonic windows (Ishida, 1969). In of those areas is the uppermost portion of the region. Ishida subdivided the area into a series contrast, Schelling (1992) considered the con- of tectonic units termed “formations” (Ishida, tacts between most lithotectonic units to be *[email protected]. 1969) or “zones” (Ishida and Ohta, 1973), gradational, at least in the Higher Himalayan

Geosphere; April 2012; v. 8; no. 2; p. 507–517; doi:10.1130/GES00711.1; 8 fi gures.

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70°E 80°E 90°E 40°N

Sub-himalaya Lesser Himalaya Miocene Granites MFT=Main Frontal thrust Tethyan Himalaya MBT=Main Boundary thrust 30°N RT=Ramgarh thrust Greater Himalaya MCT=Main Central thrust Lower Greater/ STDS=South Tibetan detachment system NEPAL Upper Lesser MHT=Main Himalayan thrust 20°N (A) Figure 1. Regional-scale geologic 80° map of Nepal after McQuarrie et al. (2008). The present study 30° MCT Tama Kosi area is outlined by a thin black INDIA 88° S map-area line in B. 28° RT

MCT

MFT Pokhara NEPAL (B) MBT KathmanduKathmandu MCT 28° 0 100 MFT 80° km INDIA

Crystallines . He mapped a major thrust disconti- Ishida (1969) and Ishida Schelling (1992) nuity, the Main Central thrust, at the base of the and Ohta (1973)

Higher Himalayan Crystallines that juxtaposed H IMALAYAN them on top of the Lesser Himalayan sequence Rolwaling-Khumbu (Fig. 2). While Schelling (1992) considered his granites Main Central thrust to be the major structural

Rolwaling-Khumbu Khumbu GNEISSES discontinuity in the region, he also recognized IMALYAN Khumbu formation thrust intense shearing of his mapped Lesser Hima- H paragneiss

layan sequence rocks below the structure. He RYSTALLINES Rolwaling-Khumbu Solo C considered these rocks, which include all units IGHER Solo formation thrust

H Main migmatite below the Main Central thrust outlined in Fig- Central ure 2, to be part of the Lesser Himalayan Shear thrust Junbesi paragneiss M Jiri formation Jiri IDLAND Zone (Schelling, 1992). He notes that it is akin Khare phyllite thrust to the Zone des Ecailles of Bordet (1961), the GROUP

Melung-Salleri augen METASEDIMENT “MCT zone” of Arita (1983), and the “Nappes Melung augen gneiss Midland gneiss thrust

Inferieurs” of Brunel (1986) and Brunel and IMALAYAN

Kienast (1986). The rocks that comprise this H Dolakha phyllite Dolakha formation zone are characterized by mylonitic deforma- SERIES Suri Dhoban augen Tam(b)a Kosi window

tion structures (Schelling, 1992) and record ESSER inverse metamorphism with low-grade rocks at L gneiss formation low structural levels and higher-grade rocks at higher structural levels (Ishida and Ohta, 1973; Figure 2. Lithotectonic correlations between previous studies that have examined a portion Schelling, 1992). of the present study area.

TECTONOSTRATIGRAPHY OF THE UPPER TAMA KOSI AND Kosi river from Dolhaka in the south to the of the Everest region (e.g., Jessup et al., 2006; ROLWALING VALLEYS Nepal-Tibet border and the river’s headwaters Goscombe et al., 2006) and the Kathmandu in the north (previous studies did not include klippe/nappe (e.g., Johnson et al., 2000; Webb The present study builds on the previous areas north of Lamabagar; Fig. 3). This map- et al., 2011), which is signifi cantly different work by Schelling (1992) and Ishida and Ohta ping was also extended into the tributary Rol- from the interpretations of either Ishida (1969) (1973) and extends into areas not yet reported waling and Khare valleys (Fig. 3). These new or Schelling (1992). on geologically. This study presents the results observations allow for direct comparison of All rocks in the study area have been meta- of detailed geologic mapping along the Tama the Tama Kosi region to the present knowledge morphosed; in general, metamorphic grade

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86° 10′ H

30 Cenozoic leucogranite 20 74 49 75 quartz + feldspar + biotite gneiss

35 16 Lapche 31 granitic augen orthogneiss; augen >10 cm in diameter 36 72 37 12 38 24 quartz + feldspar + biotite gneiss, locally migmatitic 61 kyanite or sillimanite-bearing migmatitic paragneiss

26 quartzite G 34 graphitic schist Thanchhemu

36 Greater Himalayan sequence granitic augen orthogneiss 36 biotite + garnet schist

Lumnan granitic orthogneiss, commonly augeniferous Ta

ma Kosi 86° 20′ 86° 30′

25 Gaurishankar F 7135 30 3000 4500 4000 5000 35 Ghodchadi 6000 70 350010 80 5093 33 Melagpe Shira Lamabagar 64 30 450 54 0 53 50 41 6253 4 11 58 2 000 50 E 48 37 39 Beding 68 54 Rolwaling Chekigo 5000 44 56 50 12 45 30 40 54 30 48 35 6689 40 5555 Na Tsoboje Tabayabyum 34 35 4 42 Dr Simigaon 5636 56 3500 Figure 3. Geologic map of the 2500 Dorje Phagmo 51 33 60 Yalung Ri Tama Kosi-Rolwaling Hima- 5630 2 000 44 laya. The section depicted in 50 33 31 6259 Figure 4 is shown along the main 27° 50′ 39 55 D 24 27° 50′ 26 32 Chyklma Go 44 40 Khare river valley between lettered Jagat 46 32 5930 15 30 control points “A” through “H.” 6 Ramdung Go 86° 30′ 48 40 40 35 40 35 22 32 41 45 15 36 12 26 Geology of the upper Tama 42 40 60 35 5293 3 30 15 58 000 2500 Jatta-Go Tibba 36 26 Kosi and Rolwaling valleys 27Suri 50 11 C 18 Dhoban 34 37 east-central Nepal 3000 N 23 33 16 9 24 32 2000 5 10 Ramdung Go Shigati 10 km 10 12 86° 20′ 4629 1500 4569 Ramdung Go Piguti B 10 30 geological contact (approximate, inferred) summit 20 1 village 500 tectonic foliation 31 stream, river 200 s-plane foliation 000 0

A 47 400 trail, road 29 Dolakha 25 ′

16 75 c -plane foliation 0 55 3690 topographic contour 14 48 Chordung Peak foliation measured in country 4000 (in meters) 10 1000 rock within granite 45 51 3500 27° 40′ 27° 40′ crenulation lineation 3 mineral shape lineation fold axis 12 3 2500 antiform axial surface Jiri 3000 fault plane, normal 86° 10′

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increases northward. The structurally low- granitic laccoliths (Le Fort, 1989). The contact A thin quartzite layer with subordinate musco- est unit encountered in the mapped area is a between the units now is tectonic. vite structurally overlies the pelitic schist (equiv- locally augeniferous orthogneiss (equivalent A second orthogneiss unit occurs structur- alent to the lower Solo formation; Fig. 2). This to the Tama Kosi window formation, Fig. 2; ally above the phyllitic schist (equivalent to quartzite unit is at most 300 m thick along the for brevity only the equivalent unit of Ishida the Melung augen gneiss, Fig. 2). This upper river valley and can be followed laterally across [1969] is noted; readers are referred to Figure orthogneiss is also augeniferous (Fig. 5C) and adjacent ridges and into other valleys (Fig. 3). 2 for the equivalent unit of Schelling [1992]). has a very similar mineralogy to the lower The quartzite unit marks a distinct change in The quartz + feldspar + biotite + muscovite ± unit, though it is typically coarser grained. It is character of the rocks in the mapped area with garnet orthogneiss crops out along the Tama ~800 m thick along the river valley and folds rocks that contain a signifi cant volume (>20%) Kosi River just south of the town of Shi- over the top of the Tama Kosi window (Fig. 4) proportion of anatextite above it and rocks that gati and extends northwards to Suri Dhoban to crop out in the town of Dolakha to the south do not below it. (Fig. 3). It is at least 300 m thick; however, (Fig. 3). It is not known if this rock unit is The next structurally higher rock unit in its total thickness is unconstrained. The ortho- related to the orthogneiss found structurally the mapped area is an ~5200-m-thick quartz gneiss forms the core of the Tama Kosi window lower. It may represent a thrust repeated section, + biotite + muscovite + feldspar + garnet + (Fig. 4), a late-stage structural culmination multiple original intrusions or layers, or have a sillimanite ± chlorite migmatitic paragneiss that will be discussed in a later section of this completely different progeny. (equivalent to part of the Solo formation, Figs. study. The unit contains garnet-bearing ana- The upper orthogneiss is structurally overlain 2 and 5E) with subordinate intercalated quartz- texite locally (Fig. 5A); however, it does not by a pelitic schist unit consisting of quartz + ite and calc-silicate gneiss (Fig. 5F). The mig- typically make up more than 5% of the rock muscovite + biotite + feldspar + garnet ± stauro- matite neosome consists of quartz + feldspar ± unit by volume. lite ± kyanite ± chlorite (equivalent to the Jiri muscovite ± tourmaline leucosome layers and The orthogneiss is overlain by a quartz + feld- formation, Fig. 2). It also contains a thin, later- associated biotite-rich melanosome halos that spar + muscovite + biotite ± garnet ± chlorite ally continuous graphitic schist layer. This layer generally increase in volume up structural sec- phyllitic schist (equivalent to the Dolakha for- serves as a marker horizon and was observed tion (up to 50% leucosome toward the top of mation, Fig. 2). The unit also contains local at four different locations at approximately the the unit). In addition to stratiform leucogranite, coarser-grained intercalations of metasandstone same structural position (Fig. 3). The pelitic these rocks also contain evidence of later in (Fig. 5B). The thickness of the phyllitic schist schist is at least 800 m thick along the river val- situ development of pegmatitic anatexite pods, along the southern side of the Tama Kosi win- ley (Figs. 3 and 4); however, its interpreted map which will be discussed in more detail later in dow is unconstrained (Fig. 3); however, along pattern indicates that it may be signifi cantly this study. the northern side it is ~800 m (Fig. 4). While thicker in adjacent areas. It contains a small The migmatitic paragneiss transitions up the original relationship between the underly- volume (5%–10%) of leucogranitic anatexite, structural section into a medium- to fi nely crys- ing orthogneiss and the phyllitic schist is not which typically occurs as thin discontinuous talline quartz + feldspar + biotite ± muscovite known, orthogneiss units in similar structural deformed layers (Fig. 5D). It is not known if this ± sillimanite ± garnet gneiss (equivalent to the positions along the Himalayan front (e.g., Ulleri pelitic schist is related to the structurally lower upper portion of the Solo formation; Fig. 2). The augen gneiss) have been interpreted as inter- phyllitic schist; however, there is a conspicuous transition between the two units is not exposed calated volcanoclastics (Le Fort, 1975), meta- absence of a graphitic layer within the lower and as such the nature of it is not known. The somatized granitic basement (Arita, 1983), or rock unit. unit is ~1.5 km thick along the line of section

ACDEFGHB

5 5

4 4

3 3

2 2

1 ? 1

0 0 muscovite biotite garnet staurolite kyanite sillimanite

Figure 4. Vertical geologic section along the Tama Kosi River. Approximate distribution of metamorphic index minerals is shown. See Figure 3 for lithologic legend. Dashed lines indicate interpreted eroded unit contacts. Dotted lines indicate interpreted structural fabric orientation.

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Figure 5. (A) Leucogranite boudin within orthogneiss at low structural level. Note the surrounding melanosome residuum. (B) Intercalated phyllite and metasandstone (the hammer, ~36 cm in length, is resting on the metasandstone). (C) K-feldspar augen within the upper ortho- gneiss at lower structural levels (see text for discussion). (D) Phyllitic schist with discontinuous stratiform anatexite lenses. (E) Sillimanite- grade paragneiss with signifi cant leucosome. Hammer head is ~13 cm long. (F) Calc-silicate gneiss at upper structural levels displaying typical erosional characteristics. (G) Migmatitic quartz + feldspar + biotite gneiss near the village of Bedding. (H) Augen orthogneiss at high structural level near the village of Na. (I) Isoclinally folded quartz + feldspar + biotite gneiss.

(Fig. 4), has a well-developed gneissic foliation, eter (Fig. 5H). It is 1.5–2 km thick along the line positional bedding or gneissic layering. Unlike and is variably migmatitic with leucosome for- of section (Fig. 4). the rocks below it, this unit does not contain a mation localized within more fertile, mica-rich The augen orthogneiss unit is structurally typical high-grade mineral assemblage, which layers (Fig. 5G). In one location, ~2–4 km to overlain by quartz + feldspar + biotite gneiss may be a refl ection of its protolith. the west of the village of Bedding (Fig. 3), sil- that is locally migmatitic (part of the Khumbu The contact with the structurally higher limanite and quartz occur together as 1–2-cm- formation, Fig. 2). The thickness of this unit is intrusion is characterized by a network of diameter fl attened nodules. not constrained as it is intruded by a large gra- dykes and sills that increase in density toward The next structurally higher unit is a distinc- nitic complex higher up in the structural section the main granite body (Fig. 4). The intrusion tive feldspar + quartz + biotite ± muscovite (Fig. 4). This rock unit is characterized by a [equivalent to the Rolwaling-Khumbu granites granitic augen orthogneiss (equivalent to part well-defi ned foliation that is commonly isocli- of Schelling (1992); Ishida (1969) reported no of the Khumbu formation; Fig. 2) that contains nally folded, at least at the outcrop scale (Fig. equivalent units; Fig. 2] consists of at least two K-feldspar augen that are up to 15 cm in diam- 5I). The foliation may refl ect transposed com- distinct phases. A subordinate older medium- to

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coarsely crystalline quartz + feldspar + biotite STRUCTURE AND METAMORPHISM Rocks at lower structural levels are typically + muscovite granite is only observed locally OF THE UPPER TAMA KOSI AND more phyllitic or schistose in nature (Fig. 3; see and is most commonly associated with con- ROLWALING VALLEYS above descriptions) than those at higher struc- tact with the underlying metasediments. The tural levels and commonly record the develop- much more voluminous phase is a pegmatitic Structure ment of secondary foliations, including both S feldspar + quartz + muscovite ± sillimanite and C′ planes (Fig. 6A). These secondary folia- leucogranite. The ages of these phases are The rocks exposed in the upper Tama Kosi tions consistently indicate a top-to-the-south interpreted to be Cenozoic due to the structural and Rolwaling valleys are pervasively, ductily sense of shear. Orthogneiss at lower structural position of the granite body, its relationship deformed; all planar and linear features have levels also preserves secondary foliations that with surrounding rocks, and the ages of gran- been transposed into parallelism. The transposi- record pervasive top-to-the-south sense defor- ite bodies in similar positions in adjacent areas tion foliation that resulted from that deformation mation (Figs. 6B and 6C). In addition, local (e.g., Searle et al., 2003; Viskupic et al., 2005; is the dominant structural fabric preserved in the anatexite pods within these orthogneiss units Streule et al., 2010). mapped area. commonly act as sigma-type porphyroclasts,

Figure 6. (A) C-C′-S fabrics indicating top-to-the-south sense shear. (B) Boudinaged leucogranite within ortho gneiss. Arrows indicate shear direction. Hammer is ~36 cm long. (C) C-S fabrics developed within ortho- gneiss at lower structural levels. Arrows indicate shear direction. (D) Sigma-type clast leucogranite pod indicating top-to-the-south sense shear. (E) Well-layered migmatitic gneiss. (F) Asym- metric leucogranite pod indicat- ing top-to-the-south sense shear. (G) K-feldspar augen ortho- gneiss indicating top-to-the- west sense shear. (H) Isoclinally folded quartzite. Dashed black lines approximate the folded foliation. Cliff face is ~25 m high.

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which record a similar shear sense (Fig. 6D). structural culminations observed elsewhere in rivers (Fig. 3). The fi rst occurrence is within a Lineations are rarely observed at lower struc- Nepal (e.g., Godin et al., 2006b). It has been migmatitic quartz + feldspar + garnet + biotite tural levels (Fig. 3), but become more common interpreted to refl ect interference folding of one + sillimanite + muscovite paragneiss (Fig. 7D). at the structural level of the village of Suri Dho- local fold event with a vertical axial plane par- Sillimanite is found throughout the rest of the ban and above (Fig. 3). There, they are defi ned allel to the NNE trend of the river and another upper portion of the exhumed metamorphic by the alignment of micaceous minerals and regional event with a vertical axial plane per- core when the protolith allows, including locally mineral aggregates and typically plunge moder- pendicular to the fi rst (Ishida and Ohta, 1973). within the leucogranite at the structurally high- ately toward the north. est levels observed (Fig. 7E). The presence of Rock units in the structurally higher portion Metamorphism and Crustal Melting sillimanite (Figs. 7E and 7F) within the leuco- of the exhumed metamorphic core (i.e., north of granite indicates that it was either intruded under Jagat, Fig. 3) display well-developed gneissic All rocks exposed in the study area have been sillimanite-grade conditions or it was metamor- foliations defi ned by mineral segregation, plasti- metamorphosed to at least greenschist facies. In phosed at sillimanite grade after its intrusion. cally deformed quartz, and aligned mica grains. general, metamorphic grade increases structur- Two signifi cant anatexite phases are recognized These rocks do not typically exhibit sec ondary ally up section from south to north and from within the sillimanite-bearing metamorphic foliations, though they are present locally in lower to higher structural levels until just north core of the study area. Continuous, foliation- more mica-rich lenses. Lineations are well of the town of Jagat (Fig. 3), where sillimanite parallel leucosome here comprises at least 40% developed within the paragneiss and ortho gneiss grade is reached. This inverted metamorphic of the rock by volume (Fig. 8C). The stratiform at this structural level. They are commonly sequence is typical of the Himalayan meta- leucogranite and surrounding paragneiss are defi ned by elongate, plastically deformed quartz morphic front and has been variably attributed intruded locally by pegmatitic pods (Figs. 8C and aligned micaceous minerals and typically to shear heating (Arita, 1983; Harrison et al., and 8D). The crosscutting quartz + feldspar + plunge moderately toward the north (Fig. 3). 1998), tectonic assembly/ductile shear (Jamie- muscovite ± tourmaline pegmatites are entirely Leucosome in these rocks generally occurs in son et al., 1996; Hubbard, 1996; Stephenson undeformed and appear to be the result of in situ layers parallel to the dominant foliation (Fig. et al., 2001), or post–over thrusting conduc- melt formation (Fig. 8D). The nature of those 6E) without distinct asymmetry, however, local- tive heating (Le Fort, 1975). Anatexite within pegmatitic pods is similar to that exhibited by ized asymmetric lenses record top-to-the-south the mapped area follows a similar pattern with the granitic stock observed at the highest struc- sense deformation (Fig. 6F). Local top-to-the- volume increasing up structural section, perhaps tural levels in the mapped area where it intrudes west sense of shear may be recorded by large indicating a change in peak temperatures. migmatitic paragneiss (Fig. 8E). K-feldspar augen in deformed orthogneiss (Fig. In the southernmost part of the map area, The stock is a multiphase intrusion with an 6G) just to the east of the village of Na (Fig. 3), near the town of Dolakha, the metamorphic earlier medium- to coarsely crystalline granite however, this sense of shear was not observed assemblage typically includes garnet + biotite + phase and a later, more voluminous, very coarse elsewhere in the map area (Fig. 3). The South muscovite. Evidence of crustal melting in this to pegmatitic phase (Fig. 8F). The stock appears Tibetan detachment system, a major top-to-the- portion of the map area occurs in the form of to be largely undeformed. A well-developed sill north sense structure that marks the top of the local leucogranite lenses that are commonly and feeder dyke network can be followed from exhumed metamorphic core along the length of boudinaged (Figs. 5A and 6B). The leucogranite below the stock into the main body (Fig. 8G). the orogen (Burchfi el et al., 1992; Yin, 2006; that forms the lenses typically contains quartz + This feeder network cuts across foliation, often Godin et al., 2006a), occurs to the north of the feldspar + muscovite ± tourmaline; however, at at a high angle, and does not record evidence of present study area (Jessup and Cottle, 2010); no one location, ~2.5 km north of Shigati (Fig. 3), signifi cant deformation (Fig. 8H). top-to-the-north sense shear related to that struc- garnet is also observed as part of the leuco- ture was observed. granite (Fig. 7A). Melt products at this struc- DISCUSSION Local isoclinal folding is preserved at the tural level do not comprise a signifi cant portion outcrop and cliff-side scale at upper structural of the total rock volume (~5%). The main goals of this preliminary study were levels in the mapped area (Fig. 5I). In most The same metamorphic mineral assemblage is to examine the geology of the Tama Kosi val- cases the limbs of the isocline and the fold axial maintained northward until approximately half- ley and adjacent areas and interpret those fi nd- plane are parallel to the dominant foliation (Fig. way between the villages of Suri Dhoban and ings within the framework of our contemporary 6H), which indicates layer perpendicular short- Jagat (Fig. 3), where staurolite is observed (Fig. understanding of the region. This understanding ening and layer parallel elongation. 7B). Anatexite at this structural level is more now includes the recognition of channel fl ow All rocks south of approximately Jagat voluminous (10%–15%) than at lower levels. It as a potential major process that has accom- (Fig. 3) in the mapped area are affected by late, typically forms discontinuous stratiform lenses modated convergence at mid-crustal levels km-scale open folding (Fig. 4). This folding intercalated with the country rock (Fig. 8A). (Beaumont et al., 2001, 2004), which has been postdates all ductile shear and the development Kyanite is fi rst observed approximately at the interpreted to explain the geology in the Everest of the transposition foliation. The folding is structural level of the village of Jagat in kya- region (e.g., Searle et al., 2006) as well as the most apparent just south of Suri Dhoban (Fig. 3) nite + garnet + biotite + muscovite migmatitic potential recognition of the “tip” of the extruded where the tectonic foliation to the north of the gneiss (Fig. 7C). The volume of leucogranite in mid-crustal core of the orogen observed within village dips to the north and the foliation imme- the rocks at this level is markedly higher than at the Kathmandu nappe structure (e.g., Webb diately to the south of the village dips away from lower levels, comprising up to 30%. The leuco- et al., 2011). Neither of these signifi cant contri- the river valley on either side. Farther south, granite here forms semicontinuous layers within butions had been made at the time of the last near Shigati (Fig. 3), the foliation changes again the host paragneiss and is often rimmed by a study of the areas examined in the present paper. and dips southward. The change in foliation thin biotite-rich layer in the host rock (Fig. 8B). This investigation confi rms that the rocks outlines the Tama Kosi window of Ishida (1969) Sillimanite is recognized ~1 km south of the within the upper Tama Kosi and Rolwaling and Ishida and Ohta (1973), which is similar to confl uence of the Tama Kosi and Rolwaling valley are part of the exhumed mid-crustal

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500 μm 500 μm

Figure 7. Photomicrographs depicting the metamorphic character of rocks within the mapped area. (A) Garnet-bear- ing leucogranite from within the structurally lowest orthogneiss in the mapped area. (Crossed polarized light—xpl.) (B) Syn- 500 μm tectonic Staurolite porphyro- blasts within a quartz + mus- covite + biotite schist. (Plane polarized light—ppl.) (C) Kya- nite and garnet bearing para- gneiss (xpl). Inset is the same photomicrograph in ppl. (D) Sil- limanite and garnet bearing paragneiss (xpl). Inset is the same photomicrograph in ppl. (E) Sillimanite-bearing leuco- 500 μm granite (ppl). (F) Sillimanite- bearing leuco granite (ppl). qtz— quartz, ms—muscovite, tr— tourmaline, fl d—feldspar, grt— garnet, st—staurolite, bi—bio- tite, ky—kyanite, sl—sillimanite.

500 μm 500 μm

core of the orogen. Like other portions of the at the highest structural levels by a two-phase maps the Main Central thrust, the base of the Himalayan metamorphic front (e.g., Larson leucogranite stock. Greater Himalayan sequence (the exhumed and Godin, 2009; Searle et al., 2008; Larson The previous studies that examined portions metamorphic core), approximately at the vil- et al., 2010a), the lower part is characterized of the present mapped area generated sig- lage of Jagat (Fig. 3), this study does not rec- by pervasive top-to-the-south sense shearing nifi cantly different interpretations of its struc- ognize the structure in the mapped area. All defi ned by secondary foliations and inverted tural history (Fig. 2; Ishida and Ohta, 1973; rocks in the mapped area are interpreted to be metamorphism, ranging from garnet + bio- Schelling, 1992). The fi ndings of this study do in the hanging wall of the Main Central thrust tite assemblages at lowest levels to kyanite + not support the initial structural interpretations as defi ned by Searle et al. (2008), who state that garnet + biotite ± muscovite at higher levels. of Ishida (1969) and Ishida and Ohta (1973), the structure separates rocks that record meta- There is a possibility of structural duplication who mapped the metamorphic core as a series morphism and cooling related to Hima layan of two orthogneiss units and vertical thicken- of discrete thrust-separated lithotectonic units. orogenesis in its hanging wall (the Greater ing of the lower part of the metamorphic core; The metamorphic core exposed in the study Himalayan sequence) from those that do not in however, confi rming or refuting that will be the area records evidence of pervasive ductile strain its footwall (the Lesser Himalayan sequence). subject of future work. The upper portion of distributed throughout the entire exhumed Therefore, all rocks within the mapped area are the exhumed metamorphic core in the map area mid-crust. There is no evidence of signifi cant part of the Greater Himalayan sequence. The is dominated by sillimanite-grade migmatitic localized high-strain zones. Furthermore, in change in geology that occurs near Jagat does paragneiss and orthogneiss, which is intruded contrast to the work of Schelling (1992), which not correspond to an abrupt structural or meta-

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Figure 8. (A) Discontinuous leuco some within phyllitic schist. (B) Stratiform migmatite with minor melanosome development. (C) Migmatitic sillimanite para- gneiss crosscut by an undeformed pegmatitic anatexite pod. Ham- mer is ~36 cm long. (D) Anatexite pod with large muscovite book- lets (dark color due to organic growth) crosscutting paragneiss. Relationship between the leuco- granite and host rock appears to indicate in situ melting. (E) Migmatitic gneiss intruded by pegmatitic leucogranite at highest structural levels within the mapped area. Hammer is ~36 cm long. (F) Two phases of intrusion observed within the granitic stock. Pegmatitic leuco- granite cuts across an older medium to coarse-grained gran- ite. The approximate bound- ary of the pegmatite is shown in the photo on the right side by dashed black lines. (G) Dyke and sill network outlined by thin dashed lines crosscutting country rock below the main intrusion. (H) Granitic dyke crosscutting gneissic foliation below the main stock. Book is ~15 cm tall.

morphic break, but instead refl ects a change in thickening and horizontal shortening (Price, The evolution of the Tama Kosi window post- lithology perhaps across a structure akin to the 1972)—consistent with the possibility of struc- dates the pervasive deformation associated with Himalayan Unconformity of Goscombe et al. tural repetition of units (Larson et al., 2010a). the development of the dominant foliations in (2006) mapped in the adjacent Everest region. At a fi rst order, these deformational character- the mapped area. Similar culminations in Nepal The change in geology near Jagat also may istics are also consistent with the spatial and and Tibet have been interpreted to refl ect out- refl ect a change in displacement and distortion, temporal evolution of channel fl ow models of-sequence rebuilding of the orogenic wedge or structural style, similar to that observed in (e.g., Jamieson et al., 2004; Larson et al., 2011). after the early to middle Miocene southward the Manaslu-Himal Chuli Himalaya, where the While more investigative work is necessary to extrusion of the mid-crust (Godin et al., 2006b; upper part of the Greater Himalayan sequence fully explore and evaluate this possibility, this Larson et al., 2010b). In this scenario, the Tama records extending fl ow (Price, 1972)—vertical is consistent with the original observations of Kosi window would have been developed to thinning and horizontal extension—consistent Ishida (1969) and Schelling (1992) that the facilitate the foreland migration of deforma- with foliation parallel isoclinal folds and the geology of the Tama Kosi area is similar to that tion from the Main Central thrust to the Main lower part records compressing fl ow—vertical of the Everest region. Boundary thrust after the metamorphic core

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was extruded (e.g., Larson et al., 2010b). Alter- structures are mapped in the area; all rocks DeCelles, P.G., Gehrels, G.E., Quade, J., and Ojha, T.P., natively, the Tama Kosi window may be due mapped are interpreted to be in the hanging wall 1998b, Eocene early Miocene foreland basin develop- ment and the history of Himalayan thrusting, western to tectonic inversion of local, structurally con- of the Main Central thrust. The geometry of the and central Nepal: Tectonics, v. 17, no. 5, p. 741–765, trolled sedimentary thickness variations (Long Greater Himalayan sequence was altered by doi:10.1029/98TC02598. England, P., and Houseman, G.A., 1988, The mechanics et al., 2011), lateral ramp structures (Johnson, later large-scale folding as refl ected in the devel- of the Tibetan Plateau: Philosophical Transactions of 1994), or localized increased erosion along river opment of the Tama Kosi window. The relation- the Royal Society of London. Series A: Mathematical valley bottoms (Montgomery and Stolar, 2006). ship between a leucogranite stock at high struc- and Physical Sciences, v. 326, no. 1589, p. 301–320, doi:10.1098/rsta.1988.0089. Detailed geochronologic constraints will help tural levels and deformation related to the South Godin, L., Grujic, D., Law, R.D., and Searle, M.P., 2006a, discriminate between these models. Tibetan detachment in adjacent areas indicates Channel fl ow, ductile extrusion and exhumation in con- Inferences about timing constraints on the the age of that intrusion is likely older than ca. tinental collision zones: An introduction, in Law, R.D., and Searle, M.P., eds., Channel fl ow, ductile extrusion tectonometamorphic evolution of the Greater 13–16 Ma. That constraint also limits the age of and exhumation in continental collision zones: Geologi- Himalaya series in the mapped area can be south-directed ductile deformation crosscut by cal Society [London] Special Publication 268, p. 1–23. Godin, L., Gleeson, T.P., Searle, M.P., Ullrich, T.D., and made based on work in adjacent areas. The the leucogranite to pre–mid-Miocene and later Parrish, R.R., 2006b, Locking of southward extrusion South Tibetan detachment system exposed on folding to be younger than ca. 13 Ma. in favour of rapid crustal-scale buckling of the Greater the northern fl ank of the Lapche range just Himalayan sequence, Nar valley, central Nepal, in ACKNOWLEDGMENTS Law, L.D., Searle, M.P., and Godin, L.. eds., Channel north of the study area in Tibet (the study area fl ow, ductile extrusion and exhumation in continental includes the southern fl ank of the same range) This study was funded by a University of Saskatch- collision zones: Geological Society [London] Special is characterized by pervasively deformed ewan Faculty Start-up Grant. Alicia, Josh, and Dale Publication 268, p. 269–292. Goscombe, B., Grey, D., and Hand, M., 2006, Crustal archi- marble , paragneiss, and leucogranite that Larson are thanked for their assistance and company in the fi eld. This paper benefi ted from a discussion tecture of the Himalayan metamorphic front in east- records ductile top-to-the-north sense shear; ern Nepal: Gondwana Research, v. 10, p. 232–255, with K. Ansdell, reviews by C.J. Warren and an doi:10.1016/j.gr.2006.05.003. the leucogranite commonly occurs as ultra- anonymous reviewer, and editorial direction from Grujic, D., Casey, M., Davidson, C., Hollister, L.S., Kundig, mylonite layers with feldspar porphyroclasts M. Williams. Logistical support was provided by R., Pavlis, T., and Schmid, S., 1996, Ductile extru- (Jessup and Cottle, 2010). Movement across Teke, Pradap, Kajiman, Lakpa, Some, Sete, Dawa, sion of the Higher Himalayan Crystalline in Bhutan: Karma, and Teke Bahadur Tamang; Phum, Beg, and Evidence from quartz microfabrics: Tectonophysics, the South Tibetan detachment system in that Gajindra Shresta; Dorha Dahal; and Forba Sherpa. v. 260, p. 21–43. part of the Himalaya ceased between 16 and Harrison, T.M., Grove, M., Lovera, O., and Catlos, E., 1998, A 13 Ma (Jessup and Cottle, 2010). If the leuco- REFERENCES CITED model for the origin of Himalayan anatexis and inverted metamorphism: Journal of Geophysical Research, v. 103, granite deformed in the detachment system Arita, K., 1983, Origin of the inverted metamorphism of no. B11, p. 27,017–27,032, doi:10.1029/98JB02468. on the northern fl ank of the Lapche range is the Lower Himalayas, central Nepal: Tectonophysics, Hubbard, M., 1996, Ductile shear as a cause of inverted related to similar intrusive rocks observed in v. 95, p. 43–60, doi:10.1016/0040-1951(83)90258-5. metamorphism: Example from the Nepal Himalaya: Beaumont, C., Jamieson, R., Nguyen, M., and Lee, B., 2001, The Journal of Geology, v. 104, no. 4, p. 493–499, the present study on the southern fl ank of the Himalayan tectonics explained by extrusion of a low-vis- doi:10.1086/629842. range, then the intrusive stock mapped in this cosity crustal channel coupled to focused surface denu- Ishida, T., 1969, Petrography and structure of the area study must be older than ca. 16–13 Ma. This dation: Nature, v. 414, p. 738–742, doi:10.1038/414738a. between the Dudh Kosi and Tamba Kosi, East Nepal: Beaumont, C., Jamieson, R., Nguyen, M., and Medvedev, S., Journal of the Geological Society of Japan, v. 75, no. 3, provides an important time constraint for the 2004, Crustal channel fl ows: 1. Numerical models with p. 115–125, doi:10.5575/geosoc.75.115. history of the Greater Himalayan sequence in applications to the tectonics of the Himalayan-Tibetan Ishida, T., and Ohta, Y., 1973, Ramechhap-Okhaldhunga orogen: Journal of Geophysical Research, v. 109, region, in Hashimoto, S. et al., Geology of the Nepal the mapped area. The top-to-the-south sense no. B6, doi:10.1029/2003JB002809. Himalayas: Sapporo, Japan, Saikon Publishing Com- shear deformation migmatite development Bird, P., 1991, Lateral extrusion of lower crust from under pany, p. 39–68. in the rocks crosscut by the stock and related high topography in the isostatic limit: Journal of Jamieson, R., Beaumont, C., Hamilton, J., and Fullsack, Geophysical Research, v. 96, B6, p. 10275–10286, P., 1996, Tectonic assembly of inverted metamorphic anatexite pods must be older than middle Mio- doi:10.1029/91JB00370. sequences: Geology, v. 24, p. 839–842, doi:10.1130 cene. This is consistent with deformational and Bordet, P., 1961, Recherches geologiques dan L’Himalaya /0091-7613(1996)024<0839:TAOIMS>2.3.CO;2. metamorphic ages from the Everest region that du Nepal, Region du Makalu: Paris, Centre National de Jamieson, R., Beaumont, C., and Medvedev, S., 2004, Crustal la Recherche Scientifi que, 275 p. channel fl ows: 2. Numerical models with implications outline a protracted history from ca. 39 Ma Brunel, M., 1986, Ductile thrusting in the Himalayas: Shear for metamorphism in the Himalayan-Tibetan orogen: to ca. 16 Ma (Cottle et al., 2009). Moreover, sense criteria and stretching lineations: Tectonics, v. 5, Journal of Geophysical Research, v. 109, p. B06407, p. 247–265, doi:10.1029/TC005i002p00247. doi:10.1029/2003JB002811. regional scale folding and development of Brunel, M., and Kienast, J.R., 1986, Etude petrostructurale des Jessup, M.J., Law, R.D., Searle, M.P., and Hubbard, M., the Tama Kosi window, which is interpreted chevauchements ductiles Himalayens sur la transverse 2006, Structural evolution and vorticity of fl ow during to be younger than the last movement across de l’Everest-Makalu (Nepal oriental): Canadian Journal extrusion and exhumation of the Greater Himalayan of Earth Sciences, v. 23, p. 1117–1137, doi:10.1139 Slab, Mount Everest Massif, Tibet/Nepal: Implica- the South Tibetan detachment system, likely /e86-111. tions for orogen-scale fl ow partitioning, in Law, L.D., occurred during the middle to late Miocene. Burchfi el, B.C., Chen Zhiliang, Hodges, K.V., Liu Yuping, Searle, M.P., and Godin, L., eds., Channel fl ow, duc- Royden, L.H., Deng Changrong, and Xu Jiene, 1992, tile extrusion and exhumation in continental collision The South Tibetan detachment system, Himalayan oro- zones: Geological Society [London] Special Publica- CONCLUSIONS gen: Extension contemporaneous with and parallel to tion 268, p. 379–413. shortening in a collisional mountain belt: Geological Jessup, M.J., and Cottle, J.M., 2010, Progression from south- Society of America Special Paper 269, 41 p. directed extrusion to orogen-parallel extension in the The upper Tama Kosi/Rolwaling valley Cottle, J.M., Searle, M.P., Horstwood, M.S.A., and Waters, Southern Margin of the Tibetan Plateau, Mount Everest region of Nepal is characterized by pervasively D.J., 2009, Timing of midcrustal metamorphism, melt- Region, Tibet: The Journal of Geology, v. 118, no. 5, deformed, exhumed, mid-crustal rocks that ing and deformation in the Mount Everest Region of p. 467–486, doi:10.1086/655011. Southern Tibet revealed by U(-Th)-Pb geochronology: Johnson, M.R.W., 1994, Culminations and domal uplifts commonly preserve a top-to-the-south shear The Journal of Geology, v. 117, no. 6, p. 643–664, in the Himalaya: Tectonophysics, v. 239, p. 139–147, sense. The lower part of the metamorphic core doi:10.1086/605994. doi:10.1016/0040-1951(94)90111-2. records an inverted metamorphic sequence from DeCelles, P.G., Gehrels, G.E., Quade, J., Ojha, T.P., Kapp, Johnson, M., Oliver, G., Parrish, R.R., and Johnson, S., P.A., and Upreti, B.N., 1998a, Neogene foreland basin 2000, Synthrusting metamorphism, cooling, and ero- biotite-garnet to kyanite grade, while the upper deposits, erosional unroofi ng, and the kinematic his- sion of the Himalayan Kathmandu Complex, Nepal: portion is typically at sillimanite grade and con- tory of the Himalayan fold-thrust belt, western Nepal: Tectonics, v. 20, no. 2, p. 394–415. Geological Society of America Bulletin, v. 110, Larson, K., and Godin, L., 2009, Kinematics of the Greater tains a signifi cant proportion of anatexite. In p. 2–21, doi:10.1130/0016-7606(1998)110<0002: Himalayan sequence, Dhaulagiri Himal: Implications contrast to previous studies, no major discrete NFBDEU>2.3.CO;2. for the structural framework of central Nepal: Jour-

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nal of the Geological Society, v. 166, no. 1, p. 25–43, ence Letters, v. 272, p. 105–117, doi:10.1016/j.epsl Stephenson, B., Searle, M.P., Waters, D.J., and Rex, D., doi:10.1144/0016-76492007-180. .2008.04.030. 2001, Structure of the Main Central Thrust zone and Larson, K., Godin, L., and Price, R.A., 2010a, Relationships Montgomery, D.R., and Stolar, D.B., 2006, Reconsidering extrusion of the High Himalayan deep crustal wedge, between displacement and distortion in orogens: Link- Himalayan river anticlines: Geomorphology, v. 82, Kishtwar-Zanskar Himalaya: Journal of the Geological ing the Himalayan foreland and hinterland in central p. 4–15, doi:10.1016/j.geomorph.2005.08.021. Society, v. 158, p. 637–652, doi:10.1144/jgs.158.4.637. Nepal: Geological Society of America Bulletin, v. 122, Price, R.A., 1972, The distinction between displacement and Streule, M.J., Searle, M.P., Waters, D.J., and Horstwood, no. 7–8, p. 1116–1134, doi:10.1130/B30073.1. distortion in fl ow, and the origin of diachronism in tec- M.S.A., 2010, Metamorphism, melting, and chan- Larson, K., Godin, L., Davis, W.J., and Davis, D.W., 2010b, tonic overprinting in orogenic belts: Proceedings, 24th nel flow in the Greater Himalayan Sequence and Out-of-sequence deformation and expansion of the International Geological Congress, Montreal, Canada, Makalu leucogranite: Constraints from thermobarom- Himalayan orogenic wedge: Insight from the Changgo Section 3, p. 545–551. etry, metamorphic modeling, and U-Pb geochronol- culmination, south central Tibet: Tectonics, v. 29, no. 4, Schelling, D., 1992, The tectonostratigraphy and structure ogy: Tectonics, v. 29, no. 5, p. TC5011, doi:10.1029 p. TC4013, doi:10.1029/2008TC002393. of the Eastern Nepal Himalaya: Tectonics, v. 11, no. 5, /2009TC002533. Larson, K.P., Cottle, J.M., and Godin, L., 2011, Petro chrono- p. 925–943, doi:10.1029/92TC00213. Viskupic, K., Hodges, K., and Bowring, S., 2005, Time- logic record of metamorphism and melting in the upper Searle, M.P., Simpson, R.L., Law, R.D., Parrish, R.R., scales of melt generation and the thermal evolution Greater Himalayan sequence, Manaslu-Himal Chuli and Waters, D.J., 2003, The structural geometry, of the Himalayan metamorphic core, Everest region, Himalaya, west-central Nepal: Lithosphere, v. 3, no. 6, metamorphic and magmatic evolution of the Everest eastern Nepal: Contributions to Mineralogy and Petrol- p. 379–392, doi:10.1130/L149.1. massif, High Himalaya of Nepal-South Tibet: Jour- ogy, v. 149, p. 1–21, doi:10.1007/s00410-004-0628-5. Le Fort, P., 1975, Himalaya: The collided range: American nal of the Geological Society, v. 160, p. 345–366, Webb, A.A.G., Schmitt, A.K., He, D., and Weigand, E.L., Journal of Science, v. 275(A), p. 1–44. doi:10.1144/0016-764902-126. 2011, Structural and geochronological evidence for Le Fort, P., 1989, The Himalayan orogenic segment, in Segnor, Searle, M.P., Law, R.D., and Jessup, M.J., 2006, Crustal the leading edge of the Greater Himalayan Crystal- A.M.C., ed., Tectonic evolution of the Tethyan region: structure, restoration and evolution of the Greater line complex in the central Nepal Himalaya: Earth and Dordrecht, Kluwer Academic Press, p. 289–386. Himalaya in Nepal-South Tibet: Implications for chan- Planetary Science Letters, v. 304, no. 3–4, p. 483–495, Long, S., McQuarrie, N., Tobgay, T., and Grujic, D., 2011, nel fl ow and ductile extrusion of the middle crust, in doi:10.1016/j.epsl.2011.02.024. Geometry and crustal shortening of the Himalayan Law, L.D., Searle, M.P., and Godin, L., eds., Channel Yin, A., 2006, Cenozoic tectonic evolution of the Hima layan fold-thrust belt, eastern and central Bhutan: Geologi- fl ow, ductile extrusion and exhumation in continental orogen as constrained by along-strike variation of cal Society of America Bulletin, v. 123, p. 1427–1447, collision zones: Geological Society [London] Special structural geometry, exhumation history, and foreland doi:10.1130/B30203.1. Publication 268, p. 355–378. sedimentation: Earth-Science Reviews, v. 76, p. 1–131, McQuarrie, N., Robinson, D.M., Long, S., Tobgay, T., Searle, M.P., Law, R.D., Godin, L., Larson, K., Streule, doi:10.1016/j.earscirev.2005.05.004. Grujic, D., Gehrels, G., and Ducea, M.N., 2008, Pre- M.J., Cottle, J.M., and Jessup, M.J., 2008, Defi ning liminary stratigraphic and structural architecture of the Himalayan Main Central Thrust in Nepal: Jour- MANUSCRIPT RECEIVED 13 APRIL 2011 Bhutan: Implications for the along strike architecture nal of the Geological Society, v. 165, p. 523–534, REVISED MANUSCRIPT RECEIVED 6 DECEMBER 2011 of the Himalayan system: Earth and Planetary Sci- doi:10.1144/0016-76492007-081. MANUSCRIPT ACCEPTED 22 DECEMBER 2011

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