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Geologic correlation of the Himalayan orogen and Indian craton: Part 2. Structural geology, geochronology, and tectonic evolution of the Eastern Himalaya

An Yin, C.S. Dubey, T.K. Kelty, A.A.G. Webb, T.M. Harrison, C.Y. Chou and Julien Célérier

Geological Society of America Bulletin 2010;122;360-395 doi: 10.1130/B26461.1

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Notes

Geologic correlation of the Himalayan orogen and Indian craton: Part 2. Structural geology, geochronology, and tectonic evolution of the Eastern Himalaya

An Yin1,2,†, C.S. Dubey3, T.K. Kelty4, A.A.G. Webb1, T.M. Harrison1, C.Y. Chou1, and Julien Célérier5 1Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-1567, USA 2Structural Geology Group, School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 10083, China 3Department of Geology, Delhi University, Delhi-110007, India 4Department of Geological Sciences, California State University, Long Beach, California 90840-3902, USA 5Research School of Earth Sciences, Building 61, Mills Road, Australian National University, Canberra, ACT 0200, Australia

ABSTRACT etry. Crustal thickening of the Main Central 1985), (2) southward propagation of a thin- thrust hanging wall was expressed by dis- skinned thrust belt (e.g., Schelling and Arita, Despite being the largest active collisional crete ductile thrusting between 12 Ma and 1991; Srivastava and Mitra , 1994; DeCelles orogen on Earth, the growth mechanism of 7 Ma, overlapping in time with motion on et al., 1998, 2001, 2002; Avouac, 2003; Robinson the Himalaya remains uncertain. Current the Main Central thrust below. Restoration et al., 2003, 2006; Robinson and Pearson, 2006; debate has focused on the role of dynamic of two possible geologic cross sections from Kohn, 2008), and (3) southward transport of inter action between tectonics and climate one of our geologic traverses, where one as- high-grade metamorphic rocks via lower-crustal and mass exchanges between the Hima- sumes the existence of pre-Cenozoic defor- channel fl ow or wedge extrusion (Burchfi el and layan and Tibetan crust during Cenozoic mation below the Himalaya and the other Royden, 1985; Chemenda et al., 1995, 2000; India-Asia collision. A major uncertainty in assumes fl at-lying strata prior to the India- Grujic et al., 1996; Nelson et al., 1996; Grase- the debate comes from the lack of geologic Asia collision, leads to estimated shortening mann et al., 1999; Beaumont et al., 2001, 2004, information on the eastern segment of the of 775 km (~76% strain) and 515 km (~70% 2006; Hodges et al., 2001; Grujic et al., 2002; Himalayas from 91°E to 97°E, which makes strain), respectively. We favor the presence of Searle et al., 2003; Klemperer, 2006; Godin up about one-quarter of the mountain belt. signifi cant basement topog raphy below the et al., 2006). The central issue with the afore- To address this issue, we conducted detailed eastern Himalaya based on projections of mentioned models is that they were all estab- fi eld mapping, U-Pb zircon age dating, and early Paleo zoic structures from the Shillong lished from the geology of the central Himalaya 40Ar/39Ar thermo chronology along two geo- Plateau (i.e., the Central Shillong thrust) lo- in Nepal and south-central Tibet (77°E–88°E), logic traverses at longitudes of 92°E and cated ~50 km south of our study area. Since where the classic Himalayan relationships as 94°E across the eastern Himalaya. Our dat- northeastern India and possibly the eastern originally defi ned by Heim and Gansser (1939) ing indicates the region experienced mag- Himalaya both experienced early Paleozoic are exposed (Fig. 1). That is, the Main Bound- matic events at 1745–1760 Ma, 825–878 Ma, contraction, the estimated shortening from ary thrust places the Lesser Himalayan Se- 480–520 Ma, and 28–20 Ma. The fi rst three this study may have resulted from a com- quence over Tertiary strata, the Main Central events also occurred in the northeastern In- bined effect of early Paleozoic and Cenozoic thrust places the Greater Himalayan Crystalline dian craton, while the last is unique to the deformation. Complex over the Lesser Himalayan Sequence, Hima laya. Correlation of magmatic events and the later discovered South Tibet detachment and age-equivalent lithologic units suggests INTRODUCTION places the Tethyan Himalayan Sequence over the that the eastern segment of the Himalaya Greater Himalayan Crystalline Complex (e.g., was constructed in situ by basement-involved The Himalayan orogen was created by the LeFort, 1996; Yin and Harrison, 2000). These thrusting, which is inconsistent with the hy- Ceno zoic India-Asia collision starting at ca. 65– studies generally neglect signifi cant differences pothesis of high-grade Himalaya rocks de- 60 Ma (Yin and Harrison, 2000; Ding et al., in geological relationships along the Himalayan rived from Tibet via channel fl ow. The Main 2005) or earlier (e.g., Zhu et al., 2005; Aitchison strike and have treated Himalayan evolution as Central thrust in the eastern Himalaya forms et al., 2007). Although its plate-tectonic setting a two-dimensional problem in cross-section the roof of a major thrust duplex; its north- is well understood, the growth mechanism of view. As pointed out by DiPietro and Pogue ern part was initiated at ca. 13 Ma, while the orogen remains debated. Competing mod- (2004), Yin (2006), and Webb et al. (2007), such the southern part was initiated at ca. 10 Ma, els emphasizing different controlling factors in- an approach may disguise critical information as indicated by 40Ar/39Ar thermochronom- clude: (1) vertical stacking of basement-involved on the mechanism of the Himalayan develop- thrust sheets (Heim and Gansser, 1939; Gansser, ment when the regional map relationship across †E-mail: [email protected] 1964; LeFort, 1975; Lyon-Caen and Molnar, the whole orogen is not fully considered. For

GSA Bulletin; March/April 2010; v. 122; no. 3/4; p. 360–395; doi: 10.1130/B26461.1; 15 ﬁ gures; 2 tables.

360 For permission to copy, contact [email protected] © 2009 Geological Society of America Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Eastern Himalaya N N N example, the western Himalaya (70°E–77°E) °E 28° 32°N 30°

00 does not preserve the classic Greater Himalayan 1 100°E 10 Crystalline Complex-over-Lesser Himalayan

Yalong R. Sequence relationship across the Main Central

South China block R. thrust (e.g., Yeats and Lawrence, 1984; Fuchs gtze R. Yan Salween R. and Linner, 1995; Frank et al., 1995; Thakur, Jinsha suture - 1992, 1998; Pogue et al., 1999; Yin, 2006; Webb et al., 2007) (Fig. 1). The eastern part of the Himalaya (88°E–98°E) also displays dramati- Irrawaddy R. cally different geology from that in the central

Mekong R. Yin from ed Himalaya: its foreland exhibits a 400-km-long E basement-involved uplift: the Shillong Plateau 96° Indus- Tsangpo suture

Qiangtang terrane . a (Fig. 1) (Bilham and England, 2001; Jade et al., imam

. H s 2007; Biswas et al., 2007; Clark and Bilham, l

n i Indo-Burma- Andaman terrane

Parlung R. K-Cz

a 2008) and the foreland basin is locally absent A N Fig. 13 E. A Arur n (Gansser, 1983). Our work presented here shows Namche Barwa (7782 m) P K-Cz E that (1) the development of major contractional 94°E 94 Bangong- Nujiang suture

Siang Window structures in the eastern Himalaya started at .

i BT r 5–10 Ma after the onset of the equivalent struc-

ma. m

n a

MCT

b tures in the central Himalaya, (2) crustal thick- u .

S ening was accomplished by basement-involved

A Arun TT

92°E 92 9 Pt W. Arun. Hi Arun. W. thick-skinned thrusting rather than thin-skinned

Kameng R . R

thrusting as observed in the western and central

a xln

LHS u p

STD Lhasa terrane

a Himalaya, and (3) the eastern segment of the

m h

r Himalaya has accommodated at least 315 km

B s R s s

anas anas M of Cenozoic shortening derived purely from the KZT GHC Fig. 2

Shillong Plateau map relationships without invoking any assump- E ° an 90

a. tions in constructing balanced cross sections. Bhutan Him Lake THS, Tethyan Himalayan Sequence Tethyan THS, LHS, Lesser Himalayan Sequence and age- equivalent strata in the Shillong Plateau GHC, Greater Himalayan Sequence and age-equivalent rocks in the Shillong Plateau Nam Co

. REGIONAL GEOLOGY R

Teesta Jamuna R. Bhutan Himalaya

t Sikkim Sikki S Hima. H l 88°E e

B The eastern Himalaya consists of the Bhutan

i and Arunachal segments (Fig. 1). In Bhutan, the l THS

MCT work of Jangpangi (1974) and Gansser (1983) h t Arun R. a laid a foundation for the general geology that 86°E B led to proliﬁ c studies across the country in the e Zari Nam Co s past three decades (Ray et al., 1989; Swapp and e

YaluTsangpo a. Cz, Cenozoic foreland basin sediments K-Cz, Cretaceous-Cenozoic sediments Tertiary Jurassic to early Gangdese batholith (150-50 Ma) Permian Gondwana Sequence P,

d MBT Hollister, 1991; Ray, 1995; Bhargava, 1995; ma. 0 100 200 km g E Nepal N Hima. H n 4° Edwards et al., 1996; Grujic et al., 1996, 2002, 8 a STD G 2006; Davidson et al., 1997; Stüwe and Foster, 2001; Wiesmayr et al., 2002; Daniel et al., 2003; Kali Gandaki R. Tangri et al., 2003; Baillie and Norbu, 2004; Bangong- Nujiang suture 40°N 30°N 20°N Carosi et al., 2006; Meyer et al., 2006; Richards A

Indus- Tsangpo suture et al., 2006; Drukpa et al., 2006; Hollister and Grujic, 2006; McQuarrie et al., 2008). Follow- ing the traditional deﬁ nition of major Hima- Mt. Kailas layan structures and lithologic units by Heim Fig. 1B and Gansser (1939), the Bhutan Himalaya is

n r

Indus R. u e imalayaimal t NWN India Plateau Tibetan g u divided into the Lesser Himalayan Sequence,

HimalayaHimHi s o

r o

p O

g Greater Himalayan Crystalline Complex, and

n a

s -T Tethyan Himalayan Sequence units bounded by s n u a d y simpliﬁ (B) Regional geologic map of the Himalayan orogen orogen. sketch map of the Himalayan-Tibetan A 1. (A) Figure faults: MFT—Main Fron major for Abbreviations also shown. 2 and 13 are (2006). Locations of Figures Yin and Harrison (2000) TT—Tipi detachment; BT—Bome thrust; Tibet tal thrust; MBT—Main Boundary MCT—Main Central STD—South the strata of lithologic units: xln—crystalline basement of the northeastern Indian craton; Pt—Proterozoic for Abbreviations thrust. THS— Himalayan Crystalline Complex; Himalayan Sequence; GHC—Greater in northeastern India; LHS—Lesser Shillong Group Himalayan Sequence. Tethyan Tarim Basin Tarim In la a the Main Boundary thrust below, the Main Cen- im H tral thrust in the middle, and the later discov-

Indian Peninsular Highlands Sutlej R. E ered South Tibet detachment at the top (Fig. 2) Bhagirathi R.

78° (Gansser, 1983; Grujic et al., 2002).

MFT n I I I I I The Lesser Himalayan Sequence in Bhutan

70°E 80°E 90°E 100°E consists of the Proterozoic Daling-Shumar Cz B

76°E 7 Group and Proterozoic-Cambrian Baxa Group

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Gangdese thrust Lhasa terrane Gonggar Zedong Grea t Cou nter STD thrust Indus-Tsangpo suture (ITS) NIMS (gn) STD Yalaxianbo NIMS (Tr) gneiss dome (STD window) Lhunze thrust NIMS (gn)

NIMS (Jr-K) Cona normal South Tibet Detachment (STD) fault MCT MCT Mt. Kangto Se La synclinoriumGeevan 28°N (7087 m) MCT Hapoli Bhutan Hima. GHC Zimithang (2) P Ka khtang-Z ng thrust (KZT) GHC imitha Lum La Se La LHSHS LHS BT NIMS (Pt3-Cam) Tawang Fig. 6 Fig. 4 MBT STD Black Mtn. STD klippe STD Kimin MBT (1) BT Sekteng STD klippe Itnagar BT P STD MCT P E-N

N-Q1 Bhalukpong Main frontal Qal MBT Tipi thrust (TT) fold-thrust zone Jamuna Badapani- fault Tyrsad Qal fault xln Brahmaputra trace of xln River y Paleozoic CST Guwahati Inferred gr earl Pt Mikir Hills CST Kapili Oldham thrust NSDD gr 26°N fault Naga E E thrust Guwaharti (3) xln N Shillong Pt Disang E Plateau mf thrust Shillong Dapsi thrust Naga Hills E E thrust belt E N K E N Qal Cheerapunjee N N Dauki thrust

Sylhet Trough

0 50 100 km N

90°E 92°E 94°E

Figure 2 (legend on following page).

362 Geological Society of America Bulletin, March/April 2010 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Eastern Himalaya

Himalayan Units Shillong Plateau-Mikir Hills- Structural Symbols Naga Hills Units

N-Q1, Pliocene Subansiri and Pleistocene Kimin Formations. N, Neogene strata E-N, Late Miocene Dafla Formation with Eocene CST, Central Shillong thrust strata locally present in fault-bounded slivers. E, Mainly Eocene strata P, Permian strata. NSD, North Shillong detachment

K, Cretaceous strata NIMS (Tr) and NIMS (Jr-K), Triassic to Cretaceous North Indian Margin Sequence (also known as the Fold Tethyan Himalayan Sequence). mf, Cretaceous mafic igneous rocks NIMS (gn), gneiss complex, probably metamor- phosed Paleozoic strata and correlative (1) Bhalukpong-Zimithang traverse to Greater Himalayan Crystalline Complex. Pt, Proterozoic Shillong Group, correlative to LHS (this study) GHC, Greater Himalayan Crystalline Complex (2 Kimin-Geevan traverse consisting of high-grade paragneiss, orthogneiss, gr, undifferentiated granites of Proterozoic (this study) and Tertiary leucogranites and migmatite. to Cambrian-Ordovician in age. (3 Guwahati-Cheerapujee traverse LHS, Lesser Himalayan Sequence, mostly (Yin et al., 2009) Proterozoic to early Cambrian strata xln, Precambrian crystalline basement rocks on top of ~1.74 Ga augen gneiss. Figure 2 (legend).

Figure 2. Tectonic map of the eastern Himalayan orogen and the Shillong Plateau between longitude 90°E and 94°E based on Yin et al. (1994, 1999), Harrison et al. (2000), Kumar (1997), Pan et al. (2004), and this study. Numbers in parentheses represent the following geologic traverses: (1) Bhalukpong-Zimithang traverse, (2) Kimin-Geevan traverse, and (3) Guwahati-Cheerapunjee traverse. The geology of traverses 1 and 2 are presented in this study, while traverse 3 across the Shillong Plateau is discussed in Yin et al. (2009). Locations of Figures 4 and 6 are also shown. MBT—Main Boundary thrust; MCT—Main Central thrust; STD—South Tibet detachment; BT—Bome thrust; CST—Central Shillong thrust; TT—Tipi thrust.

(Fig. 3). The Daling-Shumar Group is composed ples from the Baxa Group. One sample from Main Central thrust zone (Stüwe and Foster, of garnet-bearing schist (Jaishidanda Formation), its lower part has a youngest zircon age of 2001; Daniel et al., 2003). Continued cooling quartzite (Shumar Formation), phyllite (Daling ca. 950 Ma, while another sample from its below ~100–60 °C occurred from late Mio- Formation), and tectonically (?) interlayered upper part has a youngest age of ca. 490 Ma. cene to Pliocene time (Grujic et al., 2006). An mylonitized granitic gneisses; the Baxa Group McQuarrie et al. (2008) also showed that sam- 825 Ma orthogneiss intrudes a quartzite unit in above consists of quartzite, phyllite, and carbon- ples from the younger Shumar Formation yield the Greater Himalayan Crystalline Complex, ate (Gansser, 1983; Bhargava, 1995; McQuarrie an age of ca. 1.7 Ga for the youngest zircon. and it yields U-Pb detrital zircon ages between et al., 2008) (Fig. 3). The garnet schist of the As noted in this study, the age distributions 980 and 1820 Ma (Richards et al., 2006). These Jaishidanda Formation below the Main Cen- of detrital zircon from the lower Baxa Group observations suggest that part of the Greater tral thrust experienced peak metamorphism at and Shumar Formation in Bhutan are similar to Hima layan Crystalline Complex must have 650–675 °C and 9–13 kbar during 18–22 Ma those from the middle and lower Rupa Group been deposited between 980 Ma and 825 Ma. (Daniel et al., 2003), while the granitic gneiss in Arunachal (Fig. 3). Carboniferous-Permian The Tethyan Himalayan Sequence in Bhu- units (Jhumo Ri gneiss of Jangpangi, 1974; Gach- strata (also known as the Gondwana Sequence) tan is exposed mostly in the South Tibetan de- hang gneiss of Ray et al., 1989) have yielded a are present in the Main Boundary thrust zone in tachment klippen (Fig. 2), which make up the Rb-Sr age of ca. 1.1 Ga (Bhargava, 1995) and the Bhutan Himalaya, and they are commonly Proterozoic garnet-bearing Chekha Formation, a U-Pb zircon age of ca. 1.76 Ga (Daniel et al., thrust over Tertiary foreland sediments and rhyolite-dacite ﬂ ows of the Singhi Formation, 2003). The Daling-Shumar Group contains a in some places Quaternary deposits (Gansser, and quartz arenite of the Deshichiling For- 1.8–1.9 Ga metarhyolite layer and an arenite 1983; Bhargava, 1995). mation (Bhargava, 1995; Grujic et al., 2002) unit with U-Pb detrital zircon ages between 1.8 The Greater Himalayan Crystalline Complex (Fig. 3). The latter is overlain by Cambrian to and 2.5 Ga (Richards et al., 2006). Although the in Bhutan lies above the Main Central thrust and Jurassic strata that are parts of the North Indian 1.8–1.9 Ga metarhyolite was inferred to be in consists of paragneiss, orthogneiss, migmatite, passive-margin sequence (Yin, 2006) (Fig. 3). depositional contact within the lower Lesser and leucogranite (Gansser, 1983). Kyanite- The Chekha Formation overlying the South Himalayan Sequence (Richards et al., 2006), bearing migmatites experienced peak pressure- Tibetan detachment yielded a detrital zircon given the intense deformation within the Lesser temperature (P-T ) conditions of ~750–800 °C age distribution similar to that obtained from Himalayan Sequence strata that have generally and 10–14 kbar at ca. 18 Ma, followed by the lower Baxa Group, with the youngest zircon obliterated the original contact relationships, it is retro grade metamorphism under conditions of having an age of ca. 950 Ma (McQuarrie et al., possible the metarhyolite is part of the mylonitic 500–600 °C and 5 kbar (Swapp and Hollister , 2008). This relationship suggests that the Main augen lying as a basement to the Lesser Hima- 1991; Davidson et al., 1997; Daniel et al., 2003). Central thrust and South Tibetan detachment to- layan Sequence. Retrograde metamorphism was accompanied by gether may have duplicated the original Hima- McQuarrie et al. (2008) showed two types emplacement of leucogranite at ca. 13 Ma and layan crustal section that was part of the cover of detrital zircon age distributions for sam- cooling below ~350–400 °C at 14–11 Ma in the sequence above the Precambrian Indian craton.

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Eastern Himalayan Orogen (Units in MCT Footwall) NE Indian Craton

Western part of W. Arunachal Shillong Plateau Bhutan

Damudas Sub-Grp. Kimin Fm.: Dominantly Kimin Fm.: Dominantly conglomerate interbedded conglomerate interbedded (Permian): Arenite, shale, coal with thickly bedded with soft sandstone and beds, slate sandstone and thinly bedded claystone claystone Setikhola Fm.: Sandstone, Eastern part of shale, graywacke W. Arunachal Subansiri Fm.: Thickly Subansiri Fm.: Thickly Diuri Fm. (Upper bedded and coarse-grained Siwalik Grp (Lower Miocene- bedded and coarse-grained Carboniferous- Lower sandstone with salt-pepper sandstone with salt-pepper texture Pleistocene): Sandstone, shale, texture Permian?): Conglomerate, claystone Gondwana Units quartzite, phyllite, slate Pliocene Pleistocene Pliocene Pleistocene Dafla Fm.: Resistant thickly Yinkiong Grp (Upper Paleocene- Carboniferous-Permian Lower Eocene): Shale, sandstone bedded sandstone interbedded Cenozoic with shale and clay and locally Dafla Fm.: Resistant conglomerate sandstone interbedded with Miocene shale and clay

Abor Volcanics: Basalt in Siang Miocene Window Quartzite Fm. (Mid. No records

Cambrian): Quartzite with thin Cretaceous limestone beds

Cambrian Limestone, sandstone, basalt

Mesozoic Yamne Fm.: Shale Maneting FM. (Lower Cambrian): Quartz arenite, Bhareli Fm.: sandstone, shale, coal Cretaceous phyllite beds Deshichiling Fm.: Quartz Rangit pebble-slate: Primarily Bichom Fm.: Shale arenite, limestone,

diamictites. Permian conglomerate Miri Fm.: Diamictite, sandstone, shale

Khelong Fm.: Pyrite-bearing Gondwana Units Singhi Fm.: Andesite, carbonate. rhyolite, dacite, thin quartz

arenite Permian Bhareli Fm.: Coal-bearing shale Rangit pebble-slate: Primarily Dirang Fm: Garnet-muscovite schist, diamictites. Chekha Fm.: Garnet-staurolite phyllite, marble, quartzite,

Neoproterozoic schist, phyllite, quartzite conglomerate Khelong Fm.: Pyrite-bearing carbonate. Permian Damuda Group (Gondwana) Pele La Group (Tethyan Himalayan Units) Bhareli Fm.: Coal-bearing Jainti Fm.: shale

Dolomite, phyllite, Diuri Phyllite: Phyllite with Mesoproterozoic slate, quartzite or without sedimentary Dirang Formation

breccia. Damuda Group (Gondwana) Unconformity (?) Manas Fm.: Thungsing Quartzite: Quartz arenite, feldspathic sandstone, Diuri Phyllite: Phyllite with Dolomite, gritty quartzite. Note: this is the U. Baxa Grp or without sedimentary phyllite, structurallly duplicated Rupa Fm.

U-Pb detrital zircon age > 490 Ma breccia. Bichom Group and quartzite Proterozoic (?) based on our field mapping. Phuntsholing Thungsing Quartzite: Quartz Upper Member: Limestone Chilliepam Fm.: Carbonates arenite, feldspathic quartzite, FM.: Phyllite, gritty quartzite. quartzite, containing Neoproterozoic limestone, microstromatolites and filamentous Bichom Group and diorite sills cyanobacteria. Proterozoic (?) Middle Member: Quartzite, phyllite, Pangsari Fm.: rare dolomite, and locally volcanic Dolomite, Sandstone and shale. Upper part L. Baxa Grp rocks (U-Pb detrital zircon > 960 Proterozoic quartzite and Rupa Group Ma) contains > 560-Ma detrital phyllite zircons and lower part contains Lower Member: Metagraywacke, > 1100 Ma detrital zircon. the Daling Fm.: Dominantly quartzite, phyllite (U-Pb detrital sequence is intruded by 520-480 phylite, with minor quartzite. zircon >1.7 Ga) Ma granitoids. Bomdila Group Proterozoic

Lesser Himalayan Units Jangthi gneiss: Mylonitic Tenga Fm.: Quartzite, mafic Shumar Fm.: Dominantly augen gneiss (this is most Shillong Group volcanics, phyllite, orthogneiss quartzite with minor phyllite likely a structural duplication (U-Pb detrital zircon age > 1.7 of the Ani-Uni gneiss). (1640-1910 Ma, Rb-Sr ages) L. Rupa M. Rupa U. Rupa Ga). Paleoproterozoic Highly deformed phyllite and quartzite (most likely part of Rupa Group) Jaishidanda Fm.: Garnet Metamorphic basement: schist, quartzite, carbonate, granitic gneiss, hornblende- Ani-Uni gneiss: mylonitic Khetabari Fm.: Phyllite, garnet calc-silicate schist, mylonitized biotite gneiss, biotite- augen gneiss (1914±23 Ma gneiss schist, quartzite cordierite gneiss. Orthogneisses granitic gneiss (Jhumo Ri and intruded by 1536 ± 60 Ma granite yield U-Pb zircon ages of ~1100 Ma Proterozoic

Gachhang gneiss; Rb-Sr age Bomdila Group dated by Rb-Sr method; U-Pb ziron and 1650 Ma. 1.1 Ga, U-Pb age 1.7 Ga) age of 1750 Ma, this study Gneiss Group Paleoproterozoic Proterozoic Daling-Shumar Group Baxa Group Neoproterozoic-Cambrian

Compiled from Jangpangi (1974), Modified from Acharyya et al. (1975), Kumar (1997) Ghosh et al. (1994), Das Gupta and Gansser (1983), Bhargava (1995), Acharyya (1994), Dikshitulu et al. Biswas (2000), and Yin et al. (2009). Edwards et al. (1996), Grujic et al. (1995), Tewari (2001), Yin et al. (2006), (2002), Tangri et al. (2003), Daniel et and this study. al. (2003), and McQuarrie et al. (2008).

Figure 3. Lithostratigraphy and nomenclature of the eastern Himalayan orogen and northeastern Indian craton. References are listed at the bottom of each lithologic column.

364 Geological Society of America Bulletin, March/April 2010 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Eastern Himalaya

The South Tibetan detachment exhibits two The Arunachal Himalaya may be divided earlier study (Yin et al., 2006), the mylonitic important relationships across Bhutan and south- into the western and eastern domains sepa- augen gneiss and interlayered phyllite are in eastern Tibet. First, it cuts up-section northward rated by the Siang window directly south of tectonic contact. The abundance of augen gneiss by placing Jurassic-Cretaceous strata over the the eastern Himalayan syntaxis (Fig. 1) (Singh and low metamorphic grades of the Arunachal Greater Himalayan Crystalline Complex in and Chowdhary , 1990; Singh, 1993; Acharyya , Lesser Himalayan Sequence contrast sharply to southeast Tibet to the north (Pan et al., 2004) and 1998; Burg et al., 1998; Ding et al., 2001; the equivalent rocks in Nepal of the central by juxtaposing Proterozoic-Cambrian strata over Zeitler et al., 2001). The Siang window is de- Himalaya, which have much higher metamorphic the Greater Himalayan Crystalline Complex in fi ned by a closed trace of the Main Boundary grade up to the amphibolite facies (e.g., LeFort, southern Bhutan to the south (Fig. 2) (Grujic thrust that places Lesser Himalayan Sequence 1996; Kohn et al., 2004). et al., 2002). This relationship can be explained strata over Cretaceous-Paleogene strata (Kumar, The Greater Himalayan Crystalline Complex by northward thrusting or out-of-sequence top- 1997). Regional structural features such as the in Arunachal consists of kyanite-sillimanite- to-the-north normal faulting along the South Main Central thrust, Bome thrust, and the Indus- staurolite schist, paragneiss, augen gneiss, Tibetan detachment. Second, the traces of the Tsangpo suture all make sharp U-turns around and Tertiary leucogranites (Kumar, 1997; Yin South Tibetan detachment and Main Central the window (Fig. 1). et al., 2006). Although the Tethyan Himalayan thrust are located within 1.5 km to 3 km in The western Arunachal Himalaya exposes Sequence is not exposed in the region, the se- southern Bhutan (Gansser, 1983; Grujic et al., six regionally extensive and laterally continuous quence is documented directly to the north in 2002) (Fig. 2). As the Chekha Formation above north-dipping thrusts. From north to south, they southeast Tibet as highly folded Triassic to Cre- the South Tibetan detachment is a garnet-grade are the Zimithang thrust (KZT; correlative to the taceous strata (Pan et al., 2004; Aikman et al., schist (Bhargava, 1995), which must have been Kakthang thrust in Bhutan), the Dirang thrust 2008); there, bedding of the folded Tethyan exhumed from a depth of 10–15 km, restoring (correlative with the Main Central thrust in the Hima layan Sequence strata is mostly transposed this crustal section above the South Tibetan de- central Himalaya), the Bome thrust (BT; also by axial cleavage (Yin et al., 1999). tachment would require the South Tibetan detach- known as the upper Main Boundary thrust in In the eastern Arunachal Himalaya, east of the ment trace to extend 15–20 km southward, our fi eld description), the Main Boundary thrust Siang window, the Cretaceous-Tertiary Gang- assuming the fault dips at 20°–30° to the north. (also known as the lower Main Boundary dese Batholith thrusts over the Greater Hima- This would require the Main Central thrust and thrust in our fi eld description), the Tipi thrust layan Crystalline Complex, omitting the entire South Tibetan detachment to approach each (TT), and the Main Frontal thrust zone (Fig. 2) section of the Tethyan Himalayan Sequence other and eventually merge to the south in cross- (Kumar , 1997; Yin et al., 2006; this study). The (Fig. 1) (Gururajan and Choudhuri, 2003). section view (in other words, the Greater Hima- Dirang thrust places the Greater Himalayan This relationship is in sharp contrast to that in layan Crystalline Complex must thin to the south Crystalline Complex over the Lesser Hima- southeast Tibet, where the Gangdese Batholith in order to place the South Tibetan detachment layan Sequence, the Bome thrust places the thrusts over the Tethyan Himalayan Sequence klippen so close to the Main Central thrust in Lesser Himalayan Sequence over the Permian or mélange complexes in the Indus-Tsangpo su- southern Bhutan; see Fig. 2). The inferred up- Gondwana Sequence, the Main Boundary thrust ture zone (e.g., Yin et al., 1994, 1999; Harrison dip branch line between the Main Central thrust places the Permian Gondwana Sequence over et al., 2000). In places, the Greater Himalayan and South Tibetan detachment is not unique in Tertiary strata, and the Tipi thrust places the Crystalline Complex also thrusts over Quater- the Himalaya: it was established in the western Miocene Dafl a Formation over the Pliocene nary sediments, omitting the Lesser Himalayan and central Himalaya by Webb et al. (2007) and Subansiri and Pleistocene Kimin Formations Sequence that we commonly see in the rest of Webb (2008), indicating that this is a regional (Kumar, 1997). The Main Frontal thrust zone the Himalaya (Gururajan and Choudhuri, 2003; feature along the Himalayan orogen. consists of a series of en echelon folds that Yin, 2006). branch off obliquely from the main Himalayan Arunachal Himalaya Range front toward the Indian craton (Fig. 2). Shillong Plateau and NE Indian Craton The fold arrangement implies broad left-slip Although Godwin-Austin (1875), La Touche shear parallel to and across the Himalayan front. The eastern Himalaya is unique in that its fore- (1885), MaClaren (1904), and Brown (1912) Traditionally, the Arunachal Lesser Himalayan land exposes scattered outcrops of Indian base- explored the Arunachal Himalaya more than Sequence is divided into the Paleoproterozoic ment rocks where the modern foreland basin is 90 years ago, its general stratigraphy, structural Bomdila Group (augen gneiss interlayered with mostly absent (Fig. 2) (Gansser, 1983; Yin et al., framework, and metamorphic conditions were phyllite) and the overlying Mesoproterozoic- 2009). The geology of the NE Indian craton is not established until the 1970s, when Indian Neoproterozoic Rupa Group (quartzite and best exposed in the Shillong Plateau directly geologists fi rst started a systematic survey of phyllite below and carbonate above) (Kumar, south of the eastern Himalaya. There, four phases the region (Thakur and Jain, 1974; Jain et al., 1997) (Fig. 3). The augen gneisses yield Rb-Sr of magmatism at ca. 1600 Ma, ca. 1100 Ma, 1974; Jangpangi, 1974; Acharyya et al., 1975; ages of ca. 1.9 Ga and 1.5 Ga (Dikshitulu et al., ca. 500 Ma, and ca. 105–95 Ma and four episodes Verma and Tandon, 1976). Subsequent work of 1995). The Bomdila and Rupa Groups were of deformation at 1100 Ma, 500 Ma, 100 Ma, Tripathi et al. (1982), Thakur (1986), Kumar correlated with the Daling-Shumar Group in the and 20–0 Ma have been documented (see Yin (1997), Acharyya (1998), and Verma (2002) has Bhutan Himalaya (Kumar, 1997); specifi cally, et al., 2009, and references therein). The fi rst correlated the Arunachal geology with that in the carbonate horizon in the upper Rupa Group two events of deformation were contractional, the central Himalaya using Heim and Gansser’s of Tewari (2001) may be equivalent to the induced by assembly of Rodinia and Eastern (1939) stratigraphic (Greater Himalayan Crys- Baxa limestone in Bhutan, and the 1.5–1.9 Ga Gondwana, while the 100 Ma event was exten- talline Complex, Lesser Himalayan Sequence, Bomdila augen gneiss may be equivalent to the sional, possibly related to breakup of Gondwana and Tethyan Himalayan Sequence) and struc- 1.76 Ga granitic gneiss in the Bhutan Lesser (Yin et al., 2009). Because of its proximity to tural divisions (Main Central thrust and Main Himalayan Sequence (Daniel et al., 2003; the Himalaya and the north- northeast strike, Boundary thrust). Richards et al., 2006). As shown in this and our the 500 Ma contractional structures may extend

Geological Society of America Bulletin, March/April 2010 365 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Yin et al. into the eastern Himalaya. The most prominent Bhalukpong-Zimithang Traverse limb of the Siang window (Fig. 2). Its hanging early Paleozoic structure in the Shillong Plateau wall consists of phyllite, quartzite, metavolcanic is the Central Shillong thrust, which places Pre- The Bhalukpong-Zimithang traverse exposes rocks, carbonate, and augen gneiss (Fig. 4A). cambrian crystalline basement rocks over the the following major structures from south to We divide the upper Main Boundary thrust Proterozoic Shillong Group (Fig. 2), and which north: the active Main Frontal thrust zone, the hanging wall in the Bhalukpong area into created basement relief of >15 km (see Figure 3b Main Boundary thrust, the Main Central thrust, three units: the Bomdila augen gneiss (gn-1) of Yin et al., 2009). This fault can be projected the Se La synclinorium, and the Zimithang duc- below, the middle Rupa Group (PtR2), and the along strike into the eastern Himalaya between tile thrust zone (KZT) (Figs. 4A and 4B). We upper Rupa Group (PtR3). We divide the up- the two structural traverses mapped in this study describe these structures in detail next. per and middle Rupa units by a prominent (Fig. 2). Motion on this early Paleozoic fault medium-bedded (20–30 cm) quartz arenite se- may have created signifi cant structural and strati- Main Frontal Thrust Zone quence (Fig. 5A; MB in Fig. 4A), which shares graphic complexities that affected our estimates The Main Frontal thrust zone is ~30 km wide a similar detrital-zircon age distribution over a of overall Cenozoic crustal shortening across the and consists of an east-plunging anticline and large area (Yin et al., 2006). The unit preserves eastern Himalaya (see Discussion). the north-dipping Bhalukpong and Tipi thrusts cross-bedding that indicates northward sedi- (Figs. 4A and 4B). The east-plunging anticline is ment transport. The upper Rupa Group is char- STRUCTURAL GEOLOGY active and folds Quaternary alluvial and fl uvial acterized by the presence of a gray limestone sediments. The Bhalukpong thrust is also active sequence with an assigned early Cambrian age, We conducted geologic mapping and sam- in the Quaternary, placing the Pliocene Subansiri which is possibly correlative to the upper Baxa ple collections in two areas in the western Formation over Pleistocene Kimin Formation Group in Bhutan (Tewari, 2001) (Fig. 3). Arunachal Himalaya: (1) the Bhalukpong- and Quaternary alluvial deposits (Fig. 4A; see At one location (27°08.991′N, 92°33.419′E), Zimithang traverse in 2003 and 2006, and Fig. 3 for stratigraphic nomenclature). The Tipi we observed the contact between augen gneiss (2) the Kimin-Geevan traverse in 2004. Yin thrust juxtaposes the Miocene Dafl a Formation (gn-1) below and a coarse-grained pebble et al. (2006) reported the initial mapping result over the Pliocene Subansiri Formation. The Tipi quartzite unit above that lies at the base of the from the 2003 fi eld work along the Bhalukpong- thrust zone locally contains Eocene marine strata Rupa Group (Fig. 4A). The augen gneiss below Zimithang traverse. We update that work in this that are not shown in Figure 4 (Acharyya et al., is strongly deformed, as expressed by penetra- paper by presenting additional fi eld data col- 1975; Acharyya, 1998). The Tipi thrust appears tive mylonitic foliation with a downdip stretch- lected in 2006 along the same traverse. to be inactive in the Quaternary. Directly above ing lineation and a top-to-the-south sense of To separate observations from interpretations, the Bhalukpong thrust, there is a south-verging shear (Fig. 5B). The overlying quartzite lay- particularly with respect to regional structural overturned recumbent fold in the Subansiri ers are not deformed and have well-preserved and stratigraphic correlations along Himalayan Formation; the overturned forelimb parallels primary bedding, fi ning-upward sedimentary strike, we group lithologic units with respect to the thrust below. The hanging wall of the Tipi structures, and small channels (7–10 cm across), their underlying structures as the Main Bound- thrust is a homoclinally north-dipping sequence, all indicating a right-way-up depositional con- ary thrust hanging wall, Main Central thrust within which the Dafl a Formation is repeated by tact. These observations suggest that shear hanging wall, and South Tibetan detachment a north-dipping thrust with a S20°E transport di- defor mation in augen gneiss predates deposition hanging wall, respectively. We avoid using rection (Fig. 4A). of the Rupa Group. Greater Himalayan Crystalline Complex, Lesser The upper Main Boundary thrust hanging Himalayan Sequence, and Tethyan Himalayan Main Boundary Thrust and its wall consists of four major north-dipping thrusts Sequence in our fi eld description because they Hanging-Wall Structures that repeat the augen gneiss and Rupa units are defi ned strictly by age range, metamorphic The Main Boundary thrust is a deformation (Fig. 4A). Phyllite and slate units inside each grade, and lithology (Heim and Gansser, 1939; zone consisting of upper and lower faults. The thrust sheets experienced extensive iso clinal LeFort, 1996) and thus preclude the possibil- upper fault places Proterozoic Rupa Group over folding, and their bedding in many places is re- ity that major Himalayan faults may cut up and Permian sandstone and conglomerates (see placed by axial cleavage (Fig. 5C). The trans- down sections laterally if the defi nitions were en- Acharyya et al., 1975), while the lower fault posed bedding in these units in turn is refolded forced strictly (see discussion by Yin, 2006). As places the Permian strata over the Miocene by asymmetric kink folds (Fig. 5D), indicating shown in the western Himalaya, the Main Cen- Dafl a Formation (Fig. 4A). The upper Main a temporal change in folding style, and thus tral thrust juxtaposes lithologic units that depart Boundary thrust is laterally continuous and may defor mation mechanism, as thrust sheets were signifi cantly from the traditional defi nitions by correlate with the Bome thrust along the western progressively cooled as they moved upward. Heim and Gansser (1939) as a result of the fault cutting up-section westward and the merging of the Main Central thrust and South Tibetan detachment in their updip directions (DiPietro and Figure 4 (on following fi ve pages). (A) Geological map of the Bhalukpong-Zimithang traverse Pogue, 2004; Yin, 2006; Webb et al., 2007). Our based on our mapping and a compilation of the existing mapping. See map symbols to dif- description here also strictly separates the use of ferentiate our fi eld measurements from those made by the early workers. Major structures the Main Central thrust and Main Central thrust are defi ned as following: BLT—Bhalukpong thrust; TPT—Tipi thrust; MBT-low—lower zone. The former refers to the fault contact that Main Boundary thrust; MBT-up—upper Main Boundary thrust; BDT—Bomdila thrust; separates different lithologic units, while the MCT—Main Central thrust; ZT—Zimithang thrust; STD—South Tibet detachment. Also latter refers to the extent of deformation related see Yin et al. (2006) for detailed credits of early mapping in the area. Lines A-B, C-D, and to motion of the Main Central thrust, which may E-F represent the locations of the cross section shown in B. See Figure 2 for location of the involve rocks from both the hanging wall and map area. Sample and fi eld photograph locations discussed in the text are also shown. MB— footwall of the Main Central thrust fault. quartz arenite marker bed mapped in the study area.

366 Geological Society of America Bulletin, March/April 2010 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Eastern Himalaya N N N N Qal ′ 27°30 ′ 27°00 ′ 27°45 ′ 27°15 er BLT

E-Tdf Riv

TPT Bhalukpong

R1 Kameng Fig. 5C,D 55

F 57 unconformity Tkm R2 51 gn-1 30 MB 23 20 Pt , Middle Rupa Group, slate, , Upper Rupa Group, limestone, 23 65 54 30 R2 34 R3 55 47 24 Thrust Normal fault Gneissic foliation Cleavage in phyllite Stretching lineation and foliation in mylonitic gneiss Map-scale antiform and synforms Drainage Bedding Outcrop-scale antiform Anticline 45 84 Map Symbols phyllite, and quartz arenite Pt MB, quartzite marker bed gn-1, orthogneiss (Bomdila Group) Tsb, Subansiri Formation Tsb, E-Tdf, Eocene marine sediments and Miocene Dafla Formation Qal, Quaternary deposits Tkm, Kimin Formation P, upper part of Permian strata gn-4, Zimithang augen gneiss gn-3, high-grade gneiss above Zimithang thrust gn-2, high-grade gneiss above MCT and below Zimithang thrust leucogranite gr-lc, Himalayan Sequence Tethyan TH, 60 27 slate, phyllite, and volcanics Pt sl 37 66 20 46 65 80 71 39 Tsb 46 30 50 24 31 49 60 5 67 gn-1

Tkm 80 Wall Units Wall 48 E

Wall Units Wall 44

MCT Footwall Units Footwall MCT MBT Footwall Units Footwall MBT 64

MCT Hanging MCT 63 72 STD Hanging STD 81 24 ′ 92°30 20 R1 Pt 36 32 MB Fig. 5B P E-Tdf

Kangto Kangri (7087 m) a thrust a R1 Teng 54 AY9-13-03-(22) (U-Pb 1.74 Ga) 8 Pt 66 17 gn-1 32 24 gn-2 21

Fig. 5E 65 25

Bomdila Black numbers represent our own measurements Gray numbers are measurements by other workers 35 BT-low = MBT = BT-low

15 MB M 56 R1 Pt gn-1 BDT 35 Dirang Thrust = MCT 45 6.9 Ma) 6.9 13.4 Ma) 13.4 S1 S1 20 MB E 80 E′ 92°30 AY9-17-03-(15) (DZ < 960 Ma) (M AY9-18-03-(10) (M 17 5 R1 Dirang Pt 39 36 ′ 92°15 50 ′ 92°15 43 37 3 30

41 Se La synclinorium La Se 36 38 85 42 5 AY9-13-03-(23) (B 19 Ma) 39 45 62 30 51 50 45 56 AY2-13-06-(9) DZ (900-1900 Ma) Pt-Pz 47 MCT 010km ZT AY9-17-03-(8a) (U-Th 10.0 Ma) (554 ± 105 °C, 7.9 ± 1.6 kbar) 30 30 AY9-17-03-(11) (B 15.5 Ma, M 12.2 Ma) 20 16 23 53 9 Figure 4. Figure 14 gn-3 STD Se La Pass AY2-13-06-(7) (DZ 760-1800 Ma) Fig. 5L E E STD Bomdila Thrust ′ 92°00 ′ 92°00 32 22 AY2-13-06-(8) (DZ 800-2600 Ma) AY9-14-03-(3) (B 13.5 Ma, M 10.7 Ma) Fig. 5K Tawang 27 gr-lc Cona Distance between traces of STD is < 3 km and MCT 33 25 42 STD AY9-17-03-(7) (B 7.8 Ma) Qal AY9-16-03-(1) (B 10.0 Ma) 21 40 44 32 33 61 20 Cona Rift 47 Pt-Pz 50 34 35 AY9-16-03-(6) (B 26 Ma, M 10.3 Ma) Sekteng STD klippe AY9-17-03-(5) (B 16 Ma, M 11 Ma) gr-lc Pt-Pz AY9-16-03-(14b) E 65 AY9-16-03-(15) 61 25 73 ′ 91°45 ZT 6 Lum La gn-4 6.3 Ma, neoblast , Ar muscovite age muscovite Ar S1 50 36 AY9-17-03-(1) (B 12.4 Ma, M 7.7 Ma U-Pb ~878 Ma) 39 51 STD M 10 29 Ar/ 25 21 0 50 50 ′ 91°45 18 MCT 22 21 Ar age of mica. Ar 40 39 27 34 Zimithang Ar/ 18 U-Pb 878 Ma = U-Pb zircon age U-Pb 878 Ma = zircon DZ < 960 Ma, detrital zircon sample with youngest age. B 12.4 Ma = biotite Ar/Ar age Ar/Ar B 12.4 Ma = biotite M 7.7 Ma = 40 U-Th age of monazite inclusion in garnet 14 MB 30 75

Trashigang 45

iver AY9-17-03-(2) (B 12.1 Ma, M 12.3 Ma) R Manas gn-gb 50 45 MCT = Lumla thrust 30 N Figs. 5A, F, G, H, I Figs. 5A, F, N N N 6.5 Ma) S1 6.3 Ma) (A) S1 AY9-16-03-(14) (DZ < 960 Ma) (M AY9-17-03-(1) (B 12.4 Ma, M 7.7 U-Pb 878 Ma) AY9-17-03-(8a) (U-Th 10.0 Ma) AY9-16-03-(14) (DZ <960 Ma, M ′ 27°30 ′ 27°15 ′ 27°45 AY9-16-03-(19) (B 9.2 Ma, M 8.0 Ma)

Geological Society of America Bulletin, March/April 2010 367 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Yin et al. 0 10 10 30 50 20 40 60 Elevation (km) P F Qal Tsb ) Tkm BLT bols E-Tdf Tsb TPT pre-Cenozoic folds in Proterozoic strata P E-Tdf MBT-low E-Tdf Pt3-Camb (THS) R2 R3 Pt Pt gn-1 P P MBT-up E-Tdf R3 R2 Highly folded and sheared Permian strata zone in the MBT Pt R3 Pt Pt R2 Pt Merged STD-MCT fault is shown as an out-of-sequence structure, truncating footwall faults and bedding R3 Pt Contact between basement and cover 3 Tenga Tenga thrust ) 9-13-0 Y A gn-1 U-Pb 1.74 Ga -(22) ( R2 Pt gn-1 ) 23 gn-1 ( ) gn-1 D/E R2 Moho depth from Mitra et al. (2005 9-18-03- Pt Y A B 19.0 Ma ( Bomdila thrust ) ) 10 ( MCT 13.4 Ma 1 s 9-18-03- Y M A ( ) ) a, ) 15 ) ( gn-1 STD 6.9 Ma) 1 9-14-03- s ) About 10-15 km THS About 10-15 km above STD has been eroded away Y A (DZ > 960 Ma, M 8a ( ) 3 ( ) R2 9-14-03- Y 554 ± 105 °C, 7.9 1.6 kb U-Th monazite 10.0 Ma Pt A ( ( 9-14-03- 10.7 Ma Y B 13.5 Ma, A ( M Moho Ar cooling ages of mica and biotite, U-Th age monazite inclusions in Ar AY2-13-06-(7) (DZ 760–1800 Ma) 39 ) 11 ( ) Ar/ AY2-13-06-(8) (DZ 800–2600 Ma) 40 9-17-03- Y B 15.5 Ma, M 12.2 Ma A ( Pt-Camb (THS) gn-2 AY2-13-06-(9) DZ (900–1900 Ma) Folds and ductile shear induced by combined early Paleozoic (?) and Cenozoic contractional deformation Mz-E (THS) ) /C ) 40 50 6 ( ) ) B 7 1 ( ( ) Or-P (THS) ) 9-16-03- Y B 26 Ma, M 10.3 Ma A ( 9-17-03- 9-16-03- Y Y B 7.8 Ma B 10.0 Ma A ( A ( 30 ZT R2 ) gn-1 Pt 14 ( ZT Projected Sekteng STD klippe with garnet schist at the base ) 20 6.5 Ma 9-16-03- S1 DZ > 960 Ma, Y A (( M ) Cona normal fault 10 ). (B) Geologic cross section across the Bhalukpong-Zimithang traverse. The depth to the Moho follows that of Mitra the Bhalukpong-Zimithang traverse. section across ). (B) Geologic cross a) ) ) 1 ( ) 19 ( 9-17-03- -Pb ~878 Ma Y B 12.4 Ma, M 7.7 Ma A ( U 9-16-03- Y A (B 9.2 Ma, M 8.0 Ma) 0 ) ) Ma 1 R2 ) 1

MCT 5 ( Pt M gn-1 ) 2 ( 9-17-03- Y B 16 Ma, gn-2 gn-4 A ( 9-17-03- Y B 12.1 Ma, M 12.3 Ma A ( (B) garnet, and U-Pb detrital zircon ages are from Yin et al. (2006). South Tibetan detachment klippe in the cross section is projected along section is projected detachment klippe in the cross Tibetan et al. (2006). South Yin from ages are garnet, and U-Pb detrital zircon based on mapping by Bhargava (1995) and Grujic (2002). A line in the geology west of cross-section strike from Figure 4 ( continued Figure sim- For parallels the Moho. thrust detachment below the study area section, we assume that the regional et al. (2005), and in the cross by folding and foliation development within individual thrust sheets is not shown in the internal deformation as expressed strong plicity, All unit sym combined early Paleozoic and Cenozoic deformation. from The ductile deformation could have resulted section. cross ages of orthogneiss, The U-Pb zircon A. same as in are Pt-Camb (THS): Proterozoic-Cambrian Tethyan Himalayan Sequence Pt-Camb (THS): Proterozoic-Cambrian Tethyan Himalayan Sequence (THS): Ordovician-Permian Tethyan Or-P Himalayan Sequence Mz-E (THS): Mesozoic-Eocene Tethyan STD ZT gn-3 Jr-K (THS) A 0 10 10 20 30 40 50 60 Elevation (km)

368 Geological Society of America Bulletin, March/April 2010 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Eastern Himalaya 0 10 10 30 50 20 40 60 Elevation (km) P F Qal Tsb ) Tkm BLT 2005 E-Tdf ( Tsb TPT P E-Tdf MBT-low E-Tdf R2 R3 Pt Pt gn-1 e P P MBT-up E-Tdf R3 R2 Pt Pt R2 Pt R2 Pt ault trac Merged STD-MCT is shown as in-sequence roof thrust f R2 Pt R3 R3 Pt Pt gn-1 Tenga Tenga thrust Active gn-1 R2 Moho depth from Mitra et al. Pt Bomdila thrust D/E R2 MCT Pt gn-1 STD MCT

e o

g n i n th i Moho on i 50 Pt-Pz (THS) R2 d exhumat gn-2 i Pt gn-1 Projected Sekteng STD klippe /C r 40 o B i ng and rap i nter i 30 n Frontal thrust zone and produc i malayan pward warp he Ma Active thrust duplex system feeding slip int t u Hi ZT 20 10 ). (C) An alterative geologic cross section of the Bhalukpong-Zimithang traverse, with emphasis on the role of active section of the Bhalukpong-Zimithang traverse, with emphasis on role An alterative geologic cross ). (C) 0 MCT gn-2 gn-4 (C) Figure 4 ( continued Figure La. the Main Central thrust window at Lum of the Himalaya to explain young cooling ages around duplex development below the interior STD gn-3 Jr-K (THS) A 0 10 10 20 30 40 50 60 Elevation (km)

Geological Society of America Bulletin, March/April 2010 369 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Yin et al. km 0 Elevation (km) 10 10 20 30 40 50 60 Qal P F Tkm Qal Tsb Tsb ) Tkm m R3 R2 Pt E-Tdf BLT gn-1 Pt Bhalukpong thrust 2005 E-Tdf P ( R3 Tsb Pt TPT thrust syste R2 E-Tdf g Pt P ue to ) d E-Tdf ? ) ( MBT = MBT-low

g . 2 g enozoic folded Tipi thrust C E-Tdf re- asement arly Paleozoic P entral Shillon hrustin p b e t see Fi Projected trace of early Paleozoic C ( MBT_low P R2 Bome Thrust = MBT-up Pt R2 R3 Pt Pt gn-1 gn-1 R3 Pt gn-1 Merged STD-MCT is shown as in-sequence roof thrust Tenga Tenga thrust R3 Pt P R2 Pt gn-1 E-Tdf MBT_up = Bome thrust Basement of Indian craton Tenga thrust Moho depth from Mitra et al. E R2 / Pt R3 R2 gn-1 Pt Pt gn-1 D R3 Pt MCT Bomdila thrust R3 Pt

STD R2 Pt gn-1 P Bomdila thrust Minimum southernmost position of subducted Permian strata below MCT_up = Bome thrust R2 Pt Minimum southernmost position of subducted Cenozoic strata below MCT_low gn-1 R2 Pt MCT Pt-Pz (THS) gn-2 50 R2 Projected Sekteng STD klippe Pt /C B R2 Pt 40 gn-1 V = H 30 ZT gn-1 20 MCT R2 R2 200 km Pt Pt 10 gn-1 R2 MCT Pt 0 gn-2 gn-4 R2 Pt gn-1 STD gn-1 R2 (D) gn-1 gn-3 Pt gn-1 R2 Jr-K (THS) Pt A ). (D) Geologic cross section with emphasis on long-distance underthrusting of Permian and Tertiary strata below the Tertiary section with emphasis on long-distance underthrusting of Permian and ). (D) Geologic cross 0 10 10 20 30 40 50 60 GHC Elevation (km) THS GHC MCT Figure 4 ( continued Figure The unit below the Himalaya. basement contractional structures Bome and Main Boundary thrusts the existence of pre-Cenozoic section in D, which yields a total shortening of 775 km equivalent to ~76% of geologic cross A. (E) Restored the same as in symbols are shortening strain. STD ZT Original Length = 940 km Final Length: 225 km Shortening = 775 km Total Shortening Strain = 775/940 76% (E)

370 Geological Society of America Bulletin, March/April 2010 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Eastern Himalaya 0 Elevation (km) 10 10 20 30 40 50 60 Tsb Tkm F P Bhalukpong thrust Qal gn-1 R2 Tsb Pt ) Tkm R2 Tipi Tipi thrust BLT gn-1 Pt R3 E-Tdf Pt total Tsb P E-Tdf TPT E-Tdf MBT_low P E-Tdf MBT = MBT-low R3 Pt E-Tdf P R2 Bome thrust = MBT-up Pt Original Length = 740 km Final Length: 225 km Shortening = 515 km Total Shortening Strain = 515/740 70% R2 R3 Pt Pt MBT_up = Bome thrust gn-1 STD shown as a roof thrust R3 Pt gn-1 R3 Pt R2 Pt gn-1 Tenga Tenga thrust MCT oho depth from Mitra et al. (2005 Bomdila thrust M R2 Pt gn-1 D/E R3 Pt P Bomdila thrust V = 2H R3 at-lying and the structural relationship around around at-lying and the structural relationship Pt

STD R2 Pt gn-1 Minimum southernmost position of subducted Cenozoic strata below MCT_low (Bome thrust) Minimum southernmost position of subducted Permian strata below MCT_up R2 Pt MCT Pt-Pz (THS) gn-2 50 R2 Projected Sekteng STD klippe Pt gn-1 /C B 200 km 40 gn-1 R2 30 Pt MCT ZT gn-1 20 GHC R2 Pt R2 Pt 10 gn-1 R2 MCT ). (F) Balanced cross section assuming all pre-Cenozoic strata were ﬂ strata were section assuming all pre-Cenozoic ). (F) Balanced cross Pt 0 gn-2 gn-4 GHC THS STD MCT gn-1 STD (F) gn-1 gn-3 R2 Jr-K (THS) Pt ZT A 0 10 10 20 30 40 50 60 the Siang window can be projected below the Bhalukpong section. (G) Restored geologic cross section shown in F, which yields a section shown in F, geologic cross below the Bhalukpong section. (G) Restored the Siang window can be projected Figure 4 ( continued Figure shortening of ~515 km equivalent to ~70% strain. (G) Elevation (km)

Geological Society of America Bulletin, March/April 2010 371 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Yin et al.

All the augen gneiss units we mapped are mylonitized, and kinematic indicators (S-C fabrics and asymmetric porphyroblasts) con- NE A B sistently indicate a top-to-the-south sense of shear (Fig. 5E). However, at one location, we pebble qtz arenite observed top-to-the-north shear fabrics in an S0 augen gneiss unit. The dominantly top-to-the- unconformity south kine matics in the augen gneiss are consis- S1 tent with the regional top-to-the-south Cenozoic S1 thrust transport direction along the Main Cen- mylonitic tral and Main Boundary thrusts. As shown by augen our newly obtained 40Ar/39Ar cooling-age data, gneiss some of the shear fabrics in augen gneiss may have formed in the Miocene. The trend of stretching mineral lineation in N C N D the augen gneisses varies from place to place. Directly above the upper Main Boundary thrust, the lineation trends northwest, nearly perpendicular to the local north-northeast strike of the nearby thrusts (Fig. 4A). How- ever, higher up in the section, the stretching lineation mostly trends to the north and north- S =S northeast directions, subparallel to the nearby 0 1 S0 northeast-striking thrusts (Fig. 4A). It is not S0=S1 clear whether this discrepancy in lineation S1 trend and sense of shear was induced by local rotation of thrust sheets about vertical axes, E F variable fault kinematics from structure to N MCT E structure (i.e., lower thrust moved southeast- gn ward while the upper thrust moved southward), or superposition of Precambrian and C Cenozoic tectonism. S

Main Central Thrust and its Hanging-Wall Structures gouge zone The Main Central thrust exposed in Arunachal is remarkably sharp, placing gar- qz arenite net schist over quartz arenite or phyllite. The classic site of Main Central thrust exposure is Figure 5 (on this and following page). (A) Quartz arenite at Lum La, immediately below the near Dirang along the Bhalukpong-Zimithang Main Central thrust window. See Figure 4A for location. (B) Depositional contact between traverse, where a major thrust juxtaposes mylonitic augen gneiss below and pebble quartz arenite above. See Figure 4A for location. garnet- and kyanite-bearing gneiss and schist (C) Isoclinal folds transposing original bedding in the lower Rupa Group. See Figure 4A for over phyllite, quartzite, and metavolcanic location. (D) Refolded kink folds of phyllite in the Rupa Group. (E) Mylonitic augen gneiss rocks of the Rupa Group (Fig. 4A) (Verma near Bomdila with a top-to-the-south sense of shear. See Figure 4A for location. (F) Expo- and Tandon, 1976; Kumar, 1997). There, the sure of Main Central thrust fault near Lum La. See Figure 4A for location. (G) The Main Main Central thrust shear zone above the Main Central thrust at Lum La placing garnet-kyanite gneiss over phyllite and quartzite of upper Central thrust fault is ~100–300 m thick and Rupa Group. See Figure 4A for location. (H) Gouge zone of the Main Central thrust near characterized by isoclinally folded calc-schist Lum La. See Figure 4A for location. (I) Cross-bedding in quartz arenite directly below the and garnet-bearing quartzo-feldspathic gneiss. Main Central thrust. See Figure 4A for location. (J) East-dipping normal faults of the Cona The folds in the hanging-wall shear zone have rift zone cutting garnet-kyanite gneiss in the Main Central thrust hanging wall near Lum amplitudes of 3–5 m with fold hinges trend- La. These faults also offset the Main Central thrust. See Figure 4A for location. (K) Greater ing between N5°W and N45°W. As the fold Himalayan Crystalline Complex garnet gneiss interlayered with boudinaged leucogranites hinges are nearly perpendicular to the north- and amphibolite ~5 km east of Tawang. See Figure 4A for location. (L) Leucogranite sills easterly trending eastern Himalaya and sub- interlayered with sillimanite schist at Se La Pass. See Figure 4A for location. (M) Main Cen- parallel to the fault striations in the N10–20°W tral thrust fault gouge zone near Geevan on the Kimin-Geevan traverse. Asymmetric folds direction in the Main Central thrust zone, the indicate top-to-the-south sense of motion. See Figure 6A for location. (N) Mylonitic augen observed fold hinges may have been rotated gneiss (1.74 Ga) immediately above the Main Central thrust zone at the northern end of the about vertical axes nearly 90° from their Kimin-Geevan traverse. See Figure 6A for location. original orientation perpendicular to the thrust transport direction. Shear deformation in the

372 Geological Society of America Bulletin, March/April 2010 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Eastern Himalaya

and bedding above and below (Fig. 5G), and the G E Hh E fault is expressed by a 0.3–0.5-m-thick black gouge zone (Fig. 5E). The quartz arenite is only 2 m qz arenite mildly deformed by small-scale kink folds induced by a minor south-directed ramp-ﬂ at thrust (Fig. 5A). The quartz arenite beds also exhibit gn well-preserved cross-bedding sedimentary phyllite structures (Fig. 5I). In contrast to the little de- qz arenite shear zone formed quartz arenite, a shear zone, ~5 m thick, in phyllite was developed immediately below. It Lum La thrust contains a well-developed stretching lineation = MCT trending N30–50°W (Fig. 5H) and numerous small southeast-verging folds trending N30– SE I gn in MCT J 75°E, perpendicular to the stretching lineation. E hanging These observations suggest that strain distribu- bedding wall tion across the Main Central thrust shear zone cross-bedding is uneven, depending on the mechanical prop- erties of the lithologic units. Thus, using the maximum strain alone as a criterion to deﬁ ne the location of the Main Central thrust can be misleading (cf. Searle et al., 2008). Several north-striking and east-dipping nor- 1 m mal faults offset the Main Central thrust between 5 m and 200 m (Fig. 5J). We interpret these faults to be parts of the north-trending NSK L Cona rift zone extending from southeastern Boudinaged Tibet to the Himalaya (Armijo et al., 1986; amphibolite Yin, 2000; Taylor et al., 2003) (Figs. 2 and 4A). Boudinaged Although the initiation age of the Cona rift zone Tertiary leucogranite folds is unconstrained, the aforementioned relationships suggest that the Main Central thrust is no leucogranite longer active, and east-west extension postdated sills motion on the Main Central thrust. The east-trending Se La synclinorium folds the South Tibet detachment and its hanging-wall strata above and the Main Central thrust and its footwall rocks below (Fig. 4B). The presence of stretching M N the Se La synclinorium allows us to examine a S N lineation change in metamorphic petrology and the preva- lence of Tertiary leucogranites across a tilted section in the Main Central thrust hanging wall. At the base of the Main Central thrust hanging wall near Dirang and Lum La, phyllitic schist immediately above the Main Central thrust contains kyanite and minor Tertiary leucogran- 20 cm ites ranging in size from tens of centimeters to 30 cm a few meters, with total volume less than 1% (Fig. 5K). At higher structural levels, the size of the leucogranites increases to 20–40 m thick and Figure 5 (continued). >100 m long (Fig. 5L), and this increase is associated with the appearance of sillimanite. The total volume of the leucogranite is ~3%–5%. Main Central thrust footwall near Dirang is The Main Central thrust is also exposed as An increase in the size of the Tertiary leuco- heterogeneous . A 40–50-m-thick sequence of a thrust window near Lum La (Fig. 4A) (Yin granites in the Main Central thrust hanging wall quartz arenite lying directly below the fault et al., 2006). There, the fault is knife sharp may be a function of strain, since the size of the is little deformed, while the weaker phyllite (Fig. 5E) and places garnet schist and quartzo- leucogranites increases as the bedding-parallel structurally farther below the arenite displays feldspathic gneiss over a 40-m-thick quartz stretching strain decreases upward. The upward numerous discrete 5–10-m-thick shear zones arenite unit (i.e., the marker bed dividing the decrease in strain is expressed by the highly with faint downdip stretching lineation over a upper and middle Rupa Group) (Fig. 4A). The boudinaged leucogranites at lower structural distance of 200–250 m below the arenite. Main Central thrust lies parallel to the foliation levels (Fig. 5K) and undeformed leucogranites

Geological Society of America Bulletin, March/April 2010 373 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Yin et al. crosscutting gneissic foliation at higher struc- Main Frontal Thrust Zone and the Main Central Thrust tural levels (Fig. 5L). Alternatively, the lack of Hanging-Wall Structures The position of the Main Central thrust along deformation of leucogranites at higher structural The Main Frontal thrust zone consists of the Kimin-Geevan Road has been debated. levels could be related to their younger ages as an eastward-growing and eastward-plunging Kumar (1997) placed the fault in the interior of observed in the Bhutan Himalaya (e.g., Swapp anticlinorium that folds coarse-grained sand- the Himalaya, north of latitude 28°20′N, directly and Hollister, 1991; Daniel et al., 2003; Hol- stone and conglomerate beds of the Pleistocene south of the Himalayan crest line, while Singh lister and Grujic, 2006). This interpretation ap- Kimin Formation along its south limb and the and Chowdhary (1990) interpreted the thrust to lie plied to the Arunachal Himalaya requires the Pliocene Subansiri Formation along its north signifi cantly to the south near Hapoli (~27.20°N) early deformed Tertiary leucogranites to be cut limb (Figs. 2 and 6A). The eastward fold growth (Fig. 6A). Specifi cally, Singh and Chowdhary by the later undeformed leucogranites at higher in the Main Frontal thrust zone is expressed in (1990) envisioned the Main Central thrust to be structural levels, a relationship we did not see the eastern Himalaya by eastward defl ection of a folded low-angle fault with a large half window along our traverse. The presence of the boudi- south-fl owing rivers, a subject that will be dis- opening to the west. We attribute the confusion naged Tertiary leucogranites in the Main Central cussed in detail elsewhere. of locating the Main Central thrust to the diffi - thrust hanging-wall gneisses suggests that the The Tipi thrust is the most dominant struc- culties of assigning the structural positions of the rock experienced signifi cant foliation-parallel ture in the Main Frontal thrust hanging wall, orthogneiss with similar lithology and structural stretching in the Cenozoic. As the foliation is which lies structurally above the active fold fabrics in the Main Central thrust hanging wall parallel to the Main Central thrust at both the belt (Fig. 6A). It places the Miocene Dafl a and footwall (Kumar, 1997). This problem is Dirang and Lum La locations, it suggests that Formation over the Pliocene Subansiri Forma- compounded by the lack of age constraints and the Main Central thrust sheet experienced, at least tion. The latter forms a tight and south-verging detailed mapping in the area. We took three ap- at a local scale, a signifi cant fault-perpendicular syncline directly below the thrust. Two top-to- proaches to overcome these problems. First, we fl attening strain. We relate this strain to meso- the-northwest back thrusts are present in the used the appearance of Tertiary leucogranites as scopic folding widespread in the Main Central hanging wall of the Tipi thrust and bound a pair a proxy for the presence of the high-grade Main thrust hanging wall. of synclines and anticlines (Fig. 6A). The lateral Central thrust hanging-wall rocks; this correla- The Zimithang ductile thrust zone is exposed extent and the magnitude of slip on the two back tion was well established along the well-exposed at the highest structural level in the northern thrusts are unknown. Bhalukpong-Zimithang traverse (Yin et al., 2006; end of our traverse, where it places a mylonitic this study). Second, we used the occurrence of an augen gneiss (U-Pb zircon age of 878 Ma, see Main Boundary Thrust abrupt change in metamorphic grade to indicate following) over garnet-biotite and quartzo- The Main Boundary thrust zone places an the position of the Main Central thrust. In these feldspathic gneisses. This shear zone at the augen gneiss unit over the Miocene Dafl a cases, the Main Central thrust places garnet- Arunachal-Bhutan border lies along the strike of Formation. A thrust sliver, ~200 m thick and bearing schist or quartzo-feldspathic gneiss over the Kakthang thrust mapped by Gansser (1983) consisting of sandstone and siltstone, is pres- low-grade metagraywacke. Third, we examined and Grujic et al. (2002) immediately to the west ent in the fault zone (Fig. 6A). Beds in the shear-zone deformation and variation of strain in Bhutan (Fig. 4A), suggesting that the shear sliver are deformed by isoclinal folds and out- to support the inferred position of the Main Cen- zones are parts of the same structure. Like the crop-scale thrust duplexes. The thrust sliver tral thrust in the fi eld. Using these criteria, we Kakthang shear zone, S-C fabric and asymmetric may be the lower part of the Dafl a Formation found that the Main Central thrust displays a porphyroblasts in the Zimithang zone indicate a that thrusts over the upper part of the same small full klippe and a small half klippe in the top-to-the-south sense of shear. Stretching linea- unit or part of the Permian sequence. We north and a large west-facing half window in tion in the shear zone trends between N10°E and correlate the upper bounding fault with the the south (Fig. 6A). This pattern is quite similar to N45°W perpendicular to the strike of the fault upper fault of the Main Boundary thrust the geom etry of the Main Central thrust in Nepal (Fig. 4A). The 878 Ma augen gneiss intrudes zone in the Bhalukpong area and the Bome (e.g., Brunel, 1986; DeCelles et al., 2001) and into garnet schist and quartzo-feldspathic gneiss fault in the western Siang window area. NW Indian Himalaya (Thakur, 1998; Yin, 2006; in the Greater Himalayan Crystalline Complex. The Main Boundary thrust hanging wall Webb et al., 2007), suggesting that the fault is a This relationship is expressed by: (1) irregu- is composed of isoclinally folded low-grade folded structure along the entire Himalaya. The lar geometry of the contact between the augen metagraywacke strata (Fig. 6A). Quartz are- north-south width of the exposed Main Central gneiss and its surrounding paragneisses, and nite and phyllite are present locally in the thrust requires a minimum of 60 km slip on the (2) the augen gneiss unit, which contains numer- northern part of the traverse, which is simi- Main Central thrust along this traverse (Fig. 6B). ous xenoliths of the intruded garnet schist that lar in lithology and sedimentary structures Near Geevan, the Main Central thrust is a is identical to the country rocks. The intrusive to the marker bed dividing the upper and sharp contact placing garnet-biotite schist over relationship suggests that some of the Greater middle Rupa Group along the Bhalukpong- the metagraywacke unit. The fault zone is com- Himalayan Crystalline Complex metasedi- Zimithang traverse. Despite this correlation, posed of a fi ne-grained gouge zone associated mentary rocks were deposited and metamor- the graywacke unit differs from the middle with southeast-verging folds (Fig. 5M). Directly phosed(?) prior to 870 Ma. Rupa Group in that it is coarse-grained and above the thrust, there is a mylonitic shear zone rich in detrital feldspars and lithic fragments. involving garnet-biotite schist and an orthogneiss Kimin-Geevan Traverse Since the metagraywacke unit lies below unit (Fig. 5N) (U-Pb zircon age of 1752 Ma; the middle Rupa Group observed across see Geochronology section) (Fig. 6A). Stretch- The Kimin-Geevan traverse exposes from the Bhalukpong-Zimithang traverse, we as- ing lineation trends north-northwest in the Main south to north the Main Frontal thrust zone, the sign this unit to be the lower member of the Central thrust zone (Fig. 6A). The footwall meta-

Tipi thrust, the Main Boundary thrust zone, and Rupa Group (PtR1) (Fig. 6A). This part of graywacke unit near Geevan is folded, and hinges the Main Central thrust (Fig. 6A). We describe the Rupa Group is missing in the Bhalukpong- trend northeast and are locally sheared with a the faults and their hanging-wall structures next. Zimithang traverse (Fig. 4). northwest-trending stretching lineation (Fig. 6A).

374 Geological Society of America Bulletin, March/April 2010 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Eastern Himalaya represents represents Rp1 represents the represents Rp2/3 Middle and Upper Members of Middle and Upper not that are the Rupa Group exposed in the Kimin-Geevan traverse. Figure 6 ( on this and follow- Figure ing page ). (A) Geological map of the Kimin-Geevan traverse. the loca- A-B represents Line section shown tion of cross loca- 2 for in B. See Figure Sample tion of the map area. eld photograph locations and ﬁ also discussed in the text are sec- possible cross A shown. (B) tion along the Kimin-Geevan traverse. Unit Pt the Lower Member of the Member the Lower equivalent to unit Rupa Group, mgk . Unit Pt N N ′ ′ B

37°45 27°30 MBT Qal . E E ′ ′ 10 94°00 94°00 Ronga R Tdf-2

. 75 25 61 6 laa

m 53

KamlaKaK R 30 34 6 45 76 Tkm 71 57 Kimin 32 Tsb 38 35 38 gn-2 38 AY 1-1-05-(11A): Leucogranite age of ca. 22 Ma 1-1-05-(11A): AY associated with ca. 500 Ma inherited grains 22 85 44

15 54

MCT 0 2 4 6 8 10 km A Thrust Tipi 55 53 43 25 41 47 AY 12-30-04-(6) AY (U-Pb 500 Ma) 85 brittle thrust 23 85 61 MBT_low 40

44 Hapoli MCT 44

MCT 80 10 88 34 78 MBT_up Tdf-1 or P 24 51 35 36 30 69 85 77 46 30 89 8 50 34

80 30 MBT 88 38 14 29 gn-4 29 mgk 23 gn-4 gn-1 mgk 32 AY 12-31-04 (34A): Leucogranite age of ca. 22 Ma AY with ca. 1750 Ma inherited grain 39 R1 88 AY 12-30-04-(17): ca. 500 Ma with AY (ca. 22 Ma?) evidence of Tertiary metamorphism deformed granite intruded by leucogranite Pt sch 32 N ′ 88 80 62 60 4 gn-1(m) 75 44 4 60 53 mgk 27°30 ′ 70 35 61 50 6 qrzite bed 66 6 66

32 42 MCT 69 37 55 71 gn-2 55 Geevan

34 MCT Bedding Slaty cleavage Gneissic foliation Mylonitic gneiss Brittle thrust with orientations of fault and striation Orientation of mesoscopic fold Macroscopic syncline Macroscopic anticline Sample location and U-Pb zircon age results AY 12-31-04(17): augen gneiss age of ca. 1750 Ma AY 44 AY 12-31-04(1): AY (DZ 1640-2600 Ma) 46 19 gneiss with leucogranites qrzite bed 30 68 30 58 24 gn-2 27 35 32 60 43 38 59 18 E E 80 9 Fig. 5M ′ ′ 46 gn-3 14 93°30 93°30 30 34 53 29 gn-3

27 MCT 43 AY 12-31-04(21): AY augen gneiss age of ca. 1750 Ma 5 45

43 32 MCT 64 4 57 34 Fig. 5N AY 12-31-04(10): AY augen gneiss age of ca. 1750 Ma 4 34 Permian strata (sandstone and pebble conglomerate) Proterozoic metagraywacke as Lower Member of Rupa Grp. Proterozoic augen gneiss Schist Miocene Dafla Formation (sandstone) 61 34 Orthogneiss (ca. 500 Ma) Orthogneiss (ca. 1750 Ma) Paragneiss Augen gneiss, locally mylonitized [gn-1(m)] Pleistocene Kimin Formation (conglomerate) Pliocene Subansiri Formation (sandstone and conglomerate) 36 39 50 35 N 31 N ′ gn-4 gn-3 gn-2 Tkm Tsb 29 gn-1 Td P mgk sch MCT Footwall Units gn-1(m) MCT Hanging Wall Units A AY 12-31-04-(3A): host augen gneiss AY age of ca. 1750 Ma. 12-31-0-(3B): leucogranite intruding AY the host gneiss; with ca. 1750 Ma inherited grains and evidence for ca. thermal event. 500 Ma and/or Tertiary 37°45

Geological Society of America Bulletin, March/April 2010 375 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Yin et al. Elevation (km) 4 2 0 2 4 6 8 14 16 18 20 22 24 10 12 14 B Qal MCT Tkm TPT Tsb MBT gn-1(m) mgk P gn-1 MCT 0 2 4 6 8 10 km gn-1(m) R1 Tdf mgk= Pt gn-1/gn-1(m) gn-1/ gn-1(m) represents the Lower Member of the Rupa Member the Lower represents AY 1-1-05 (11A) 1-1-05 (11A) AY (Leucogr: ~22 Ma with 500 Ma inherited zircons) gn-2 gn-4 Rp1 gn-2 sch mgk gn-1/ gn-1(m) AY 12-30-04-(6) AY (500 Ma) mgk Indian Cratonal Basement AY 12-30-04-(17) AY (500 Ma followed by 22 Ma thermal event) mgk gn-1/ gn-1(m) AY 12-31-04-(34)A AY (Leucogr: ~22 Ma) gn-2 R2 represents the Middle and Upper Members of the Rupa Group that are not exposed in the that are Members of the Rupa Group the Middle and Upper represents Pt gn-1/gn-(m) mgk AY 12-31-04(1) AY (DZ 1640–2600 Ma) Rp2/3 mgk MCT gn-1/ gn-1(m) AY 12-31-04-(21) AY (1750 Ma) mgk ). (B) A possible cross section along the Kimin-Geevan traverse. Unit Pt possible cross A ). (B) gn-1/ gn-1(m) AY 12-31-04-(17) AY (1750 Ma) mgk mgk gn-1/gn-(m) AY 12-31-04-(10) AY (1750 Ma) Group, equivalent to unit mgk . Unit Pt Group, Figure 6 ( continued Figure Kimin-Geevan traverse. gn-2 gn-3 gn-1/ gn-1(m) gn-1/ gn-1(m) gn-3 mgk AY 12-31-04-(3)A AY (1750 Ma) A 4 2 0 2 4 6 8 16 18 20 22 24 10 12 14 B Elevation (km)

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U-Pb ZIRCON DATING Th/U ratios (Table 1) typical for metamorphic concordant, the older one is strongly discor- zircons (e.g., Ding et al., 2001; Mojzsis and dant. We consider these analyses to represent Methods Harrison, 2002). We interpret the 878 Ma age older wall-rock zircons assimilated during to represent the time of crystallization for the emplacement of the granitoid at ca. 1752 Ma. We conducted U-Pb spot dating of zircons pluton and the younger age of 627 Ma to repre- The discordant, low Th/U analysis hints at a from orthogneiss and leucogranite samples sent a later metamorphic event. Phanerozoic metamorphic event. collected from the Bhalukpong-Zimithang and We analyzed six spots of different zircons Kimin-Geevan traverses using the Cameca Orthogneiss from the Kimin-Geevan Traverse from sample AY 12–30–04-(17) collected from 1270 ion microprobe at the University of Sample AY 12–30–04-(6) was from an ortho- an augen gneiss unit directly above the Main California–Los Angeles (UCLA). The analyti- gneiss unit in the Main Central thrust hanging Central thrust (Fig. 6A). The results form a dis- cal procedure follows that of Quidelleur et al. wall near Hapoli (Fig. 6A). We obtained 17 spot cordia line with intercepts on the concordia at (1997). All of the analyses were conducted analyses on 15 zircons (Fig. 8A). Fifteen of the 28 ± 13 Ma (2σ) and 512 ± 14 Ma (MSWD = using an 8–15 nA O– primary beam and an analyses yielded 207Pb/206Pb ages ranging from 1.3). Four spot ages cluster near the upper inter- ~25-μm-diameter spot size. U-Pb ratios were 460.5 Ma to 546.1 Ma, and a weighted mean cept, one plots near the lower intercept with a determined using a calibration curve based on age of 504.9 ± 8.3 Ma (2σ). These 15 analy- low Th/U value, and one plots between the two UO/U versus Pb/U from zircon standard AS3 ses are concordant or reversely discordant on age groups (Figs. 8C). We interpret these results (age 1099.1 Ma; Paces and Miller, 1993). We the U-Pb concordia plot; the increased reverse to indicate crystallization of the augen gneiss at collected our age data during four analytical discordance is associated with higher U con- ca. 512 Ma, which was succeeded by a thermal sessions, each of which has a different calibra- centrations (Table 1). The other two analyses event at ca. 28 Ma. tion curve over distinct ranges in UO/U values yielded 207Pb/206Pb ages of 836.9 ± 13.2 Ma and Sample AY 12–31–04-(17) was from my- (see notes in Table 1 for the range of UO/U 730.2 ± 13.1 Ma (1σ) and plot along the con- lonitic augen gneiss in the Main Central thrust values). We also adjusted isotopic ratios for cordia. Th/U ratios of all of the above analy- footwall (Fig. 6A). We acquired fi ve spot common Pb corrections following Stacey and ses are >0.01, with more than half of them analyses from different zircons. Three analyses Kramers (1975). We calculated concentra- over 0.1 (Table 1). The grain that yielded the cluster together along the concordia, yielding a tions of U by comparison with zircon stan- 826 Ma age is subhedral, and its cathodolumi- weighted mean 207Pb/206Pb age of 1747 ± 7 Ma dard 91500, which has a U concentration of nescence image shows distinct domains with- (2σ) (Fig. 8D). The other two ages are discor- 81.2 ppm (Wiedenbeck et al., 2004). Data out clear defi nition of the core from the rim. A dant, potentially drawn down from ca. 1750 Ma reduction was accomplished via the in-house spot yielding the 836.9 ± 13.2 Ma 207Pb/206Pb toward the Phanerozoic portion of the concordia. program ZIPS 3.0.3 written by Dr. Chris Coath. age corresponds to a high Th/U ratio of 0.419 We interpret these results to indicate crystalliza- and is typical of igneous origin (e.g., Ding et tion of the granitic protolith at ca. 1747 Ma and Results al., 2001; Mojzsis and Harrison, 2002). An- a later Late Proterozoic or Phanerozoic Pb-loss other spot analysis from the same grain yielded event that may correlate with metamorphism. We analyzed 11 samples, among which a reversely discordant result with a 505.8 ± We analyzed fi ve spots of different zircons two are augen gneiss from the Bhalukpong- 8.3 Ma 207Pb/206Pb age; it corresponds to a low from sample AY 12–31–04-(21) collected from Zimithang traverse, six are orthogneiss from Th/U ratio of 0.034 and a UO/U ratio below the a biotite-quartz mylonitic granitoid that lies the Kimin-Geevan traverse, and three are leuco- range of the calibration (Table 1). The dominant directly above the Main Central thrust in the granites from the Kimin-Geevan traverse. We age population of 15 out of 17 analyses and Geevan klippe (Fig. 6A). Four analyses clus- described the results in detail next. moderate-to-high Th/U ratios all indicate that tering on the concordia yielded a weighted the crystallization age of the augen gneiss is mean 207Pb/206Pb age of 1743 ± 7 Ma (2σ) Orthogneiss from the ca. ~505 Ma, with one inherited grain at (Fig. 8E). One spot age was slightly older, Bhalukpong-Zimithang Traverse 836 Ma. Because an 825 Ma pluton exists in the showing a 207Pb/206Pb age of 1939 ± 9 Ma (2σ). Sample AY 09–13–03-(22) was collected Bhutan Himalaya, and an 878 Ma augen gneiss We interpret these results to indicate crystalli- from mylonitic augen gneiss of the Paleo- is present in the Bhalukpong-Zimithang tra- zation of the granitic protolith at ca. 1743 Ma, proterozoic Bomdila Group of Kumar (1997) verse, the 836 Ma zircon may have come from a with the single older age representing an inher- (Fig. 3) in the Main Central thrust footwall pluton emplaced during the same igneous event ited component. (Fig. 4A). We analyzed 13 different zircons and was later intruded by the 505 Ma pluton. Sample AY 12–31–04-(10) was from an and obtained a weighted mean 207Pb/206Pb age Sample AY 12–31–04-(3A) was collected augen gneiss directly above the Main Central of 1743 ± 4 Ma (2σ) by excluding one inher- from a mylonitic augen gneiss unit in the thrust and north of sample AY 12–31–04-(21) ited grain and two very discordant analyses Main Central thrust hanging wall (Fig. 6A). (Fig. 6A). We acquired four spot analyses from (Fig. 7A; Table 1). Sample AY 9–17–03-(1) Of the 15 total analyses from 15 zircons, different zircons. Three analyses cluster on was collected from mylonitic augen gneiss in 13 yielded 207Pb/206Pb ages ranging from the concordia and indicate a weighted mean the Zimithang shear zone above the Main Cen- 1703 Ma to 1780 Ma, with a weighted mean 207Pb/206Pb age of 1772 ± 6 Ma (2σ) (Fig. 8F). tral thrust (Fig. 4A). Fourteen zircons were age of 1752 ± 12 Ma (2σ) (Fig. 8B). Of these We interpret these results to indicate crystalli- analyzed, 10 of which lie on or just above the 13 analyses, the one with the lowest Th/U ratio zation of the granitic protolith at ca. 1772 Ma. concordia and form a cluster with a weighted (0.038) is strongly discordant, plotting along a Based on the similar ages and proximity of mean 206Pb/238U age of 878 ± 12.6 Ma (Fig. 7B; discordia line with a projected intersection of samples AY 12–31–04-(10) and AY 12–31– Table 1). Two zircons yielded younger ages that a Phanerozoic age along the concordia curve. 04-(21), we interpret the mylonitic orthogneiss lie along the concordia at ca. 627.6 Ma (mean The remaining two analyses have 207Pb/206Pb represented by the two samples to have been square of weighted deviates [MSWD] = 0.8) ages of 1921 ± 13 Ma and 2515 ± 12 Ma parts of the same unit defi ning the Main Central (Fig. 7B). The younger ages correspond to low (2σ); while the younger analysis is nearly thrust shear zone (Fig. 6A).

Geological Society of America Bulletin, March/April 2010 377 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Yin et al. ) 23.7 23.7 26.4 26.4 13.2 13.2 10.3 10.3 8.3 8.3 16.5 16.5 8.1 8.1 34.5 34.5 16.6 16.6 14.0 12.8 12.8 11.1 11.1 11.0 11.0 31.1 31.1 9.1 9.1 10.8 10.8 16.4 16.4 Pb* 10.8 10.8 ( Continued 206 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 5.7 5.7 ± ± ± ± ± Pb*/ 207 § ±1 s.e.) ±1 13.9 514.5 27.6 1687.0 14.7 477.3 20.0 836.9 30.9 1743.0 12.2 506.8 29.4 1736.0 19.7 505.8 15.0 517.8 13.6 515.7 37.6 2503.0 29.3 1724.0 13.0 460.5 31.7 1717.0 30.3 1751.0 33.0 1721.0 32.5 1736.0 aM(segA U 235 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Pb*/ 207 55.2 1741.0 34.8 1327.0 15.4 551.9 46.8 1560.0 26.8 664.6 17.0 512.9 17.2 515.2 17.0 547.9 26.7 831.3 53.1 1652.0 51.0 1650.0 18.0 529.4 15.1 491.7 49.4 1628.0 54.8 1587.0 74.6 2332.0 59.8 1723.0 U 238 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 44.1 1392.0 ± 30.2 1668.0 ± ± Pb*/ 206

† U Th/ † / O 9.4 9.10 0.169 1458.0±0.169 9.10 45.5 1584.0 ± 28.3 1756.0 ± 12.8 9.40 0.083490.0 9.40 ± 14.8 487.7 ± 12.2 476.8 ± 9.2 9.28 0.166 1565.0±0.166 9.28 47.3 1665.0 ± 28.5 1792.0 8.5– 9.04 0.174 1739.0 0.174 9.04 9.06 0.145 1602.0 0.145 9.06 9.06 0.138 1554.0 0.138 9.06 9.73 0.021562.9 9.73 8.69 0.034712.4 8.69 9–9.9 / U values of: / U values of: of: values / U U U U 6.68E+02 6.68E+02 5.93E+03 5.93E+03 4.14E+02 4.14E+02 7.24E+02 7.24E+02 8.24E+03 8.24E+03 7.58E+02 7.58E+02 8.40E+03 8.40E+03 8.51E+02 8.51E+02 U-Pb calibration established for U O U O for established U-Pb calibration U-Pb calibration established for U O U O for established U-Pb calibration Pb* (%) (ppm) (%) U s 206 o it a § rc – – – – – – – – – TABLE 1. ION MICROPROBE U-(Th)-PbIONTABLE 1. MICROPROBE ZIRCON DATA i p 04 100.0 2.62E+03 0.137614.8 9.48 ± 18.1 600.4 ± 14.0 546.1 ± 11.2 04 99.47 03 98.79 4.27E+02 1115.0 0.422 9.37 04 99.67 1.00E+03 1433.0 0.137 8.98 04 99.9 04 99.9 1.20E+03 0.429512.5 9.19 04 99.9 04 99.9 9.95E+02 0.400523.7 9.19 04 100.0 04 99.44 04 99.27 04 100.0 04 99.9 04 99.9 5.33E+02 0.483532.1 9.06 04 99.9 04 99.9 1.00E+03 0.419829.2 9.22 04 100.0 5.87E+03 0.073555.7 9.30 04 99.75 1.59E+03 1219.0 0.153 8.66 04 99.54 04 100.0 04 100.0 9.18E+03 0.027650.5 9.00 ± 22.1 617.8 ± 16.5 499.8 ± 7.6 04 99.9 04 99.9 5.10E+02 0.206498.4 9.92 04 99.49 04 99.57 8.90E+02 1572.0 0.112 8.93 04 99.59 1.23E+03 1489.0 0.014 8.67 04 99.38 3.62E+02 1713.0 0.467 8.86 03 99.41 2.95E+02 2141.0 0.207 8.80 ot 1 s.e. 2.68E– 1.33E 7.52E– 4.32E– 2.17E 4.26E– 9.48E 2.36E 1.98E 8.76E– 2.99E– 6.32E– 7.39E– 6.76E 9.48E– 6.47E– 8.17E– 2.12E– 8.15E 5.22E– 3.16E 6.52E 1.05E– osI 1 1 Pb* ± 206 Pb*/ 207 § – – – – – 01 1.03E–01 01 1.03E–01 02 6.70E–02 02 6.70E–02 01 1.05E–01 01 1.05E–01 02 5.66E–02 02 5.66E–02 02 5.76E–02 02 5.76E–02 02 5.84E–02 02 5.84E–02 01 1.06E–01 01 1.06E–01 02 5.66E–02 02 5.66E–02 02 5.74E–02 02 5.74E–02 01 1.06E–01 01 1.06E–01 01 1.07E–0 02 5.74E–02 02 5.74E–02 02 5.77E–02 02 5.77E–02 02 5.76E–02 02 5.76E–02 01 1.07E–01 01 1.07E–01 01 1.10E–01 01 1.10E–01 02 5.72E–02 02 5.72E–02 02 5.62E–02 02 5.62E–02 01 1.65E–01 01 1.65E–01 01 1.05E–01 01 1.05E–01 01 1.02E–01 01 1.02E–01 01 1.07E–01 01 1.07E–01 01 1.06E–01 01 1.06E–01 2.28E– 2.08E– 1.00E 1.33E 3.73E– 2.49E– 4.47E 1.69E– 1.44E– 1.94E– 2.99E– 2.08E– 2.49E– 1.43E– 1.59E– 2.40E– 1.55E– 1.35E 1.51E– 2.30E– 1.75E 1.17E– 3.68E– 1 1 U ±1 s.e. U ±1 235 Pb*/ 207 § – – – – – – – – – – – – – – – – 03 2.69E+00 03 2.69E+00 03 7.22E–01 03 7.22E–01 02 4.09E+00 02 4.09E+00 03 6.57E–01 03 6.57E–01 03 1.27E+00 03 1.27E+00 03 9.24E–0 02 3.78E+00 02 3.78E+00 03 6.17E–01 03 6.17E–01 03 8.38E–01 03 8.38E–01 03 6.23E–01 03 6.23E–01 03 8.06E–01 03 8.06E–01 03 3.97E+00 03 3.97E+00 03 3.65E+00 03 3.65E+00 03 3.76E+00 03 6.61E–01 03 6.61E–01 02 4.55E+00 02 4.55E+00 02 4.46E+00 02 4.46E+00 03 6.84E–01 03 6.84E–01 03 7.15E–01 03 7.15E–01 02 8.94E+00 02 8.94E+00 03 2.94E+00 03 2.94E+00 03 4.15E+00 03 4.15E+00 02 4.08E+00 02 4.08E+00 8.86E– 1.06E– 2.61E– 6.42E 3.04E 2.85E– 4.72E 4.64E 1.12E– 9.36E 2.48E 3.80E 2.53E 3.08E 9.75E 1.07E– 2.89E 9.06E 2.87E– 1.61E 1.21E 1.01E 8.27E U ±1 s.e. U ±1 238 Pb*/ 206 # 7, a 7, a 8.46E–02 10, a 10, a 9.13E–02 8, a 8, a 1.89E–01 7, a 7, a 2.49E–01 8, a 8, a 1.37E–01 5, a 5, a 9.00E–02 1, a 1, a 2.60E–01 13, a 13, a 2.75E–01 12-30-04-(6) (3) AY Spot ID 10, a 10, a 2.82E–01 9, a 9, a 7.90E–02 2, a 2, a 1.06E–01 4, a 4, a 8.04E–02 6, a 6, a 1.00E–01 12, a 12, a 2.73E–01 4, a 4, a 2.08E–01 9, a 9, a 2.54E–01 (1) AY 09-13-03 22 09-13-03 (1) AY 3, a 3, a 8.28E–02 11, a 11, a 3.10E–01 6, a 6, a 2.76E–01 1, a 1, a 8.60E–02 8, b 8, b 1.17E–01 2, a 2, a 3.94E–01 3, a 3, a 3.04E–01

378 Geological Society of America Bulletin, March/April 2010 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Eastern Himalaya ) 12.4 12.4 12.1 12.1 14.8 14.8 5.2 5.2 17.4 17.4 12.2 12.2 10.2 10.2 12.8 12.8 13.1 13.1 19.3 19.3 8.7 8.7 7.0 7.0 4.7 4.7 11.4 11.4 6.4 9.5 9.5 11.6 11.6 Pb* ( Continued 206 ± ± 28.1 28.1 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Pb*/ 207 § 26.5 1720.0 27.0 1703.0 4.1 495.7 4.1 23.8 1769.0 16.0 730.2 32.3 2515.0 24.6 1753.0 27.1 1780.0 32.1 1733.0 16.4 516.7 34.0 1921.0 27.4 1757.0 11.3 499.1 31.6 1734.0 25.1 1732.0 12.8 517.2 12.7 512.1 ±1 s.e.) U (segA 235 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Pb*/ 207 46.6 1711.0 4.7 514.3 4.7 60.5 1823.0 ± 35.5 1772.0 44.4 1790.0 48.7 1716.0 20.7 722.3 64.3 1960.0 47.3 1698.0 60.0 1785.0 64.2 2330.0 43.0 1711.0 14.4 554.2 19.8 559.1 50.4 1749.0 58.4 1783.0 38.0 1474.0 15.8 545.1 16.0 547.3 U 238 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Pb*/ 206 2125.0

) 13 1703.0 20 1868.0 ± 67 1808.0 22 1806.0 ± 51.1 1773.0 ± 29.6 1734.0 ± 19.4 42 1777.0 ± 51.9 1764.0 ± 27.9 1750.0 ± 12.7 00 719.8 57 1830.0 73 1676.0 95 1726.0 70 75 1998.0 51 1633.0 58 567.7 56 1686.0 ± 53.8 1698.0 ± 37.080 1713.0 518.5 ± 42.0 20 569.6 78 1743.0 76 531.4 ± 15.9 524.9 ± 14.0 497.0 ± 23.0 17 1825.0 38 1301.0 50 551.8 31 555.8 0.1 0.2 0.4 0.1 0.0 0.1 0.3 0.1 0.3 0.0 0.2 0.6 0.0 0.4 0.2 0.3 0.3 0.3 0.0 0.7 0.0 0.0 † U Th/ † / U O 9.9 7.8– 9.55 9.55 9.49 9.49 9.71 9.71 9.64 9.64 9.28 9.28 9.81 9.81 9.67 9.67 9.77 9.77 9.40 9.40 9.54 9.54 9.76 9.76 9.99 9.99 9.60 9.60 9.55 9.55 9–9.9 Continued / U values of: of: values / U / U values of: of: / U values U (ppm) 9.30E+02 9.30E+02 6.23E+03 6.23E+03 1.01E+04 3.75E+03 3.75E+03 4.60E+03 4.60E+03 U-Pb calibration established for U O U O for established U-Pb calibration U O for established U-Pb calibration Pb* (%) 206 s oit § – – – – – – – – – – – – ar ci 04 99.7 04 99.7 8.91E+02 04 99.9 04 99.9 1.68E+03 04 99.5 04 99.5 5.70E+02 04 99.7 04 99.7 1.24E+03 9.34 03 98.6 03 98.6 2.87E+02 04 100.0 2.38E+03 8.385 03 99.4 03 99.4 1.82E+02 04 99.9 04 99.9 1.62E+03 03 99.5 03 99.5 6.32E+02 04 99.6 04 99.6 3.03E+02 9.23 04 99.8 04 99.8 8.85E+02 04 99.7 04 99.7 4.45E+02 9.31 04 100.0 04 99.6 04 99.6 4.06E+02 04 100.0 04 100.0 04 99.9 04 99.9 1.72E+03 9.24 04 99.7 04 99.7 3.88E+02 9.27 04 99.9 04 99.9 3.33E+03 9.30 04 100.0 04 100.0 03 99.7 03 99.7 3.46E+02 9.53 pot 3.95E 7.13E 2.40E– 1.67E– 1.12E 5.98E 9.86E 8.37E– 5.18E– 7.45E 5.08E 4.14E 3.71E 5.96E– 7.00E– 3.83E– 3.06E 1.21E– 6.57E– 1.21E 3.04E– 2.48E ±1 s.e. o TABLE 1. ION MICROPROBE U-(Th)-PbION MICROPROBE TABLE 1. ( DATA ZIRCON s I Pb* 206 Pb*/ 207 § – – – – – – – – – – – 01 1.05E–01 01 1.05E–01 01 1.06E–01 01 1.06E–01 01 1.07E–01 01 1.07E–01 01 1.04E–01 01 1.04E–01 02 6.37E–02 02 6.37E–02 01 1.08E–01 01 1.08E–01 01 1.09E–01 01 1.09E–01 02 5.71E–02 02 5.71E–02 01 1.05E–01 01 1.05E–01 03 5.71E–02 03 5.71E–02 01 1.08E–01 01 1.08E–01 01 1.08E–01 01 1.08E–01 02 5.77E–02 02 5.77E–02 01 1.06E–01 01 1.06E–01 01 1.66E–01 01 1.66E–01 01 1.06E–01 01 1.06E–01 02 5.72E–02 02 5.72E–02 01 1.18E–01 01 1.07E–01 01 1.07E–01 02 5.77E–02 02 5.77E–02 02 5.75E–02 02 5.75E–02 01 1.06E–01 01 1.06E–01 1 s.e. 1.41E 1.94E– 6.72E 2.11E 1.37E 1.67E– 3.20E 1.31E– 1.44E– 2.81E 1.56E– 1.80E– 1.84E 2.31E– 1.42E 1.51E– 1.06E– 2.32E 2.15E– 1.93E 3.16E– 2.15E

U ± 235 Pb*/ 207 § – – – – – – – – – – – – – – 03 4.39E+00 03 4.39E+00 02 4.80E+00 02 4.73E+00 02 4.73E+00 02 5.89E+00 02 5.89E+00 03 1.04E+00 03 1.04E+00 03 4.39E+00 03 4.39E+00 03 4.42E+00 03 4.42E+00 03 7.34E–01 03 7.34E–01 03 4.33E+00 03 4.33E+00 02 4.79E+00 02 4.79E+00 02 4.68E+00 02 4.68E+00 02 4.60E+00 02 4.60E+00 03 6.77E–01 03 6.77E–01 02 4.32E+00 02 4.32E+00 04 6.60E–01 04 6.60E–01 02 5.02E+00 02 5.02E+00 03 4.83E+00 03 4.83E+00 02 8.92E+00 02 8.92E+00 03 7.11E–01 03 7.11E–01 03 7.26E–01 03 7.26E–01 03 7.14E–01 03 7.14E–01 03 3.27E+00 03 3.27E+00 1.36E– 3.60E– 1.08E– 7.98E 1.26E 9.11E 1.05E 1.20E 1.24E– 9.41E– 8.65E 9.87E 3.36E 1.06E– 9.45E 1.02E 7.21E 2.67E 2.66E– 1.39E 2.45E– 2.70E ±1 s.e. ±1 U 238 Pb*/ 206 # Spot ID 13, a 13, a 3.02E–01 11, a 11, a 3.07E–01 12, a 12, a 3.23E–01 2, a 2, a 2.88E–01 14, a 14, a 3.28E–01 1, a 1, a 8.38E–02 8, a 8, a 3.36E–01 10, a 10, a 3.24E–01 (5) AY 12-30-04-(17) (5) AY 9, a 9, a 2.97E–01 12, a 12, a 9.24E–02 1, a 3.63E–01 13, a 13, a 9.21E–02 4, a 4, a 3.17E–01 3, b 3, b 8.94E–02 11, a 11, a 1.18E–01 3, a 3, a 3.10E–01 5, a 5, a 3.27E–01 14, a 14, a 8.59E–02 (4) AY 12-31-04 3A 12-31-04 (4) AY 15, a 15, a 2.99E–01 7, a 7, a 3.90E–01 15, a 15, a 9.01E–02 6, a 6, a 2.24E–01

Geological Society of America Bulletin, March/April 2010 379 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Yin et al. ) 15.2 15.2 13.4 13.4 9.6 9.6 10.1 10.1 17.4 17.4 16.8 16.8 13.1 13.1 14.7 14.7 19.9 19.9 17.4 12.5 12.5 10.4 10.4 14.9 19.2 19.2 15.1 15.1 Pb* ( Continued 206 ± ± ± ± ± ± ± 30.4 ± ± ± ± ± ± ± ± ± Pb*/ 207 § 12.8 1734.0 11.8 1742.0 14.6 1743.0 4.9 525.0 4.9 12.1 1751.0 12.6 1756.0 11.6 1630.0 12.2 1651.0 5.0 494.2 5.0 17.4 1752.0 21.4 1748.0 14.1 1804.0 17.4 1746.0 7.9 525.3 7.9 8.6 494.3 8.6 ±1 s.e.) U (segA 235 ± ± ± ± ± ± ± ± ± 14.8 1759.0 13.8 ± ± ± 15.1 1939.0 8.7 ± ± ± ± ± ± ± Pb*/ 207 22.6 1746.0 5.7 488.9 5.7 21.3 1749.0 18.7 1751.0 24.3 1781.0 16.3 1314.0 12.3 1400.0 35.2 1224.0 ± 29.6 1609.0 38.8 1780.0 22.6 1815.0 30.0 1789.0 6.1 381.2 6.1 27.3 1758.0 8.6 515.9 8.6 24.7 1780.0 9.8 482.1 9.8 U 238 ± ± ± ± ± ± ± ± ± 25.4 1827.0 ± ± ± 25.3 1956.0 ± ± ± ± ± Pb*/ 206

) 50 1748.0 24 1759.0 63 1516.0 ± 16.3 1645.0 ± 9.8 1813.0 ± 12.9 78 1807.0 82 481.2 22 1130.0 72 752.9 ± 11.1 894.8 ± 10.4 1264.0 ± 20.1 25 1866.0 38 1241.0 14 1017.0 ± 23 1717.0 ±93 23.5 1761.0 1727.0 ± 12.6 1740.0 ± 14.2 88 1807.0 92 1888.0 71 1973.0 81 362.8 58 1764.0 50 513.8 24 1760.0 27 74.9 ± 1.6 84.6 ± 2.1 366.3 ± 34.8 53 479.6 90 1826.0 0.4 0.3 0.0 0.3 0.2 0.3 0.2 0.4 0.2 0.1 0.4 0.3 0.1 0.2 0.0 0.0 0.2 0.4 0.7 0.3 0.4 0.4 U Th/ † † † † – / U O 9.9 9.9 9.9 9.9 7.8– 7.8– 7.8– 7.8 8.586 8.586 8.721 8.721 8.829 8.829 8.723 8.723 9.312 9.312 Continued / U values of: of: values / U / U values of: of: / U values / U values of: of: values / U / U values of: of: values / U U (ppm) 1.30E+03 1.30E+03 3.01E+03 3.01E+03 5.41E+02 2.70E+03 2.70E+03 2.22E+03 2.22E+03 U-Pb calibration established for U O U O for established U-Pb calibration U-Pb calibration established for U O U O for established U-Pb calibration U O for established U-Pb calibration U-Pb calibration established for U O U O for established U-Pb calibration Pb* (%) 206 s oit § – – – – – – – – – – – ar ci 04 99.8 04 99.8 3.35E+02 8.657 04 99.5 04 99.5 1.05E+03 8.579 04 100.0 04 99.8 04 99.8 2.58E+02 8.692 04 99.4 04 99.4 2.27E+03 8.802 04 99.9 04 99.9 5.26E+02 8.735 04 99.7 04 99.7 3.29E+03 8.752 04 99.8 04 99.8 5.32E+02 8.691 04 99.6 04 99.6 3.26E+03 8.569 03 99.8 03 99.8 1.68E+03 9.443 04 99.7 04 99.7 04 99.7 2.65E+02 2.86E+02 8.374 8.477 03 98.6 03 98.6 2.99E+02 8.535 04 99.8 04 99.8 4.25E+02 8.506 04 99.8 04 99.8 2.99E+02 8.804 04 99.7 04 99.7 1.72E+02 8.739 04 99.8 04 99.8 4.51E+02 8.814 04 100.0 04 100.0 04 100.0 04 99.8 04 99.8 4.65E+02 8.869 04 100.0 pot 8.83E– 2.67E 8.49E 9.37E 7.78E 7.89E 5.63E– 9.21E 8.22E– 8.50E– 7.31E– 8.14E 8.71E 1.62E– 5.15E 5.78E 6.29E– 5.07E– 7.65E 8.32E– 1.02E– 3.90E– ±1 s.e. o TABLE 1. ION MICROPROBE U-(Th)-PbION MICROPROBE TABLE 1. ( DATA ZIRCON s I Pb* 206 Pb*/ 207 § – – – – – – – – – – 02 1.07E–01 02 1.07E–01 02 1.02E–01 02 1.02E–01 02 1.06E–01 02 1.06E–01 02 1.07E–01 02 1.07E–01 02 1.07E–01 02 1.07E–01 02 1.11E–01 02 1.11E–01 02 1.08E–01 02 1.08E–01 03 5.79E–02 03 5.79E–02 01 1.19E–01 01 1.19E–01 02 1.07E–01 02 1.07E–01 02 1.10E–01 02 1.10E–01 02 1.00E–01 02 1.00E–01 02 9.92E–02 02 9.92E–02 03 5.71E–02 03 5.71E–02 03 5.39E–02 03 5.39E–02 02 8.28E–02 02 8.28E–02 02 5.79E–02 02 5.79E–02 02 1.07E–01 02 1.07E–01 02 1.07E–01 01 1.07E–01 01 1.07E–01 02 1.07E–01 02 1.07E–01 02 5.71E–02 02 5.71E–02 1 s.e. 6.86E– 4.15E– 2.48E– 6.51E– 8.02E 7.43E– 7.80E 4.78E 9.74E– 7.18E 6.80E 7.06E 4.87E– 8.79E– 9.99E– 1.02E– 1.22E– 8.00E 1.29E 2.24E 1.37E– 9.67E U ± 235 Pb*/ 207 § – – – – – – – – – – – – – – – 03 4.78E+00 03 4.78E+00 03 4.58E+00 03 4.58E+00 03 1.41E+00 03 1.41E+00 03 2.65E+00 03 2.65E+00 03 4.77E+00 03 4.77E+00 03 4.61E+00 03 4.61E+00 03 4.05E+00 03 4.05E+00 04 6.18E–01 04 6.18E–01 03 2.97E+00 03 2.97E+00 03 4.56E–01 03 4.56E–01 03 4.48E+00 03 4.60E+00 03 4.60E+00 03 4.97E+00 03 4.97E+00 03 5.05E+00 03 5.05E+00 03 4.82E+00 03 4.82E+00 03 2.34E+00 03 2.34E+00 03 5.87E+00 03 5.87E+00 03 4.77E+00 03 4.77E+00 04 8.69E–02 04 8.69E–02 03 6.62E–01 03 6.62E–01 03 4.65E+00 03 4.65E+00 03 6.08E–01 03 6.08E–01 4.60E– 4.98E 9.58E– 3.01E 4.34E– 7.97E 1.93E 2.31E 6.39E– 1.00E 4.75E 3.81E 3.20E 4.68E– 5.27E 6.17E 5.33E 5.03E 2.54E 1.64E– 1.44E– 5.57E ±1 s.e. ±1 U 238 Pb*/ 206 # Spot ID 5, a 5, a 3.24E–01 3, a 3, a 3.12E–01 1, a 1, a 3.40E–01 (7) AY 12-31-04-(21) (7) AY 1, a 1, a 1.92E–01 3, a 3, a 1.71E–01 3, a 3, a 3.24E–01 1, a 1, a 1.24E–01 2, a 2, a 3.36E–01 2, a 2, a 2.12E–01 3, a 3, a 5.79E–02 1, a 3.05E–01 4, a 4, a 8.30E–02 12-31-04-(10) (8) AY 4, a 4, a 3.15E–01 2, a 2, a 3.14E–01 4, a 4, a 2.65E–01 2, a 2, a 7.75E–02 5, a 5, a 3.58E–01 6, a 6, a 7.72E–02 3, a 3, a 3.14E–01 (9) AY 12-31-04-(3B) (9) AY 2, a 2, a 3.14E–01 5, a 5, a 1.17E–02 (6) AY 12-31-04-(17) (6) AY 4, a 4, a 3.27E–01

380 Geological Society of America Bulletin, March/April 2010 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Eastern Himalaya 111.6 13.5 13.5 156.7 94.1 15.4 15.4 27.0 27.0 17.3 17.3 45.4 45.4 65.1 65.1 62.3 62.3 14.1 14.1 157.2 Pb* 206 ± ± ± ± ± ± ± ± ± ± ± ± Pb*/ 207 Pb = 15.35, Pb = 15.35, 204 § Pb/ 207 483.9 23.4 1723.0 0.6 46.7 0.6 15.3 1746.0 1.5 2.8 1.5 1.9 47.2 1.9 6.2 497.7 6.2 7.8 19.9 1454.0 0.9 0.9 negative 0.8 194.8 0.8 272.4 3.0 17.4 1475.0 1.1 29.7 1.1 ±1 s.e.) U (segA 235 ± 1.6 negative ± ± ± 8.9 negative ± ± ± ± ± ± ± ± ± ± ± Pb = 16.2, Pb = 16.2, Pb*/ 204 207 Pb/ 206 0.4 28.0 0.4 35.9 1634.0 0.2 22.3 0.2 0.5 24.9 0.5 0.2 19.7 0.2 0.3 22.7 0.3 0.3 26.2 0.3 492.6 6.6 72.1 1.0 14.8 969.2 23.1 1092.0 28.3 1793.0 6.7 501.4 6.7 0.3 23.6 0.3 U 238 ± ± ± ± ± ± ± ± ± ± 41.7 1.7 ± ± ± ± Pb*/ 206 10, estimated from model of Stacey and Kramers (1975). (1975). Kramers Stacey and of model from estimated 10,

) 07 24.6 65 66.2 21 20.121 ± 20.3 0.3 ± 34.2 1.0 103.6 ± 77 1566.0 52 910.3 26 21.922 ± 0.3 22.1 20.9 ± 0.8 negative 20 20.0 33 22.8 00 491.5 92 28.4 ± 46 769.8 78 1834.0 20 44.7 10 24.4 61 447.7 ±79 10.0 505.3 452.9 ± 9.0 479.2 ± 16.0 50 516.1 ± 6.5 510.0 ± 5.4 482.6 ± 19.5 29 23.6 0.0 0.0 0.0 0.8 0.0 0.0 0.0 0.0 0.0 0.6 0.3 0.0 0.0 0.4 0.0 0.0 0.0 0.6 0.3 06-1, and with composition composition with and 06-1, 11A, AY 12-31-04 34A, AY 12-31-04 21, AY 12-31-04 10], 8.5–9.4 [AY 10], 8.5–9.4 12-31-04 AY 21, 12-31-04 AY 34A, 12-31-04 AY 11A, U Th/ 6 10B, AY 07-02-06 1]. Negative apparent ages are due to reverse to reverse due are ages Negative apparent 1]. 07-02-06 AY 6 10B, † † – / U O 9.9 9.9 7.8– 7.8 Continued / U values of: / U values of: U U U Pb = 38.34 for samples AY12/30/04-6, AY12/31/04-3A, AY1/3/05-1A, AY1/3/05-1B, AY1/3/05-1C, AY1/3/05-1C, AY1/3/05-1B, AY1/3/05-1A, AY12/31/04-3A, AY12/30/04-6, for samples Pb = 38.34 (ppm) 204 Pb/ 208 curves for the four sessions are well determined for U O / U values ranging from 9.0–9.9 [AY 12-30-04 6, AY 12- AY 6, 12-30-04 [AY 9.0–9.9 from / U values ranging O for U determined sessions are well the four curves for U-Pb calibration established for U O U O for established U-Pb calibration U O for established U-Pb calibration Pb = 15.62, Pb = 15.62, 204 Pb* (%) 206 Pb/ s × 1000. 1000. × oit 207 § – – – – – – – – – – – ar ci 04 99.4 04 99.4 2.72E+02 9.172 04 100.0 1.02E+03 8.895 03 99.2 03 99.2 2.54E+03 8.774 03 99.5 03 99.5 1.57E+03 8.648 03 99.6 03 99.6 1.39E+03 8.969 04 100.0 1.19E+03 9.142 03 99.7 03 99.7 2.78E+03 8.523 03 99.7 03 99.7 03 99.8 3.58E+03 3.88E+03 8.79 8.629 03 99.8 03 99.8 3.31E+03 8.674 03 98.1 03 98.1 2.39E+03 8.575 03 94.7 03 94.7 04 99.7 4.21E+03 2.64E+03 9.317 9.269 04 99.8 04 99.8 2.75E+02 8.737 03 99.5 03 99.5 2.72E+03 8.609 03 95.8 03 95.8 4.37E+02 8.674 04 100.0 7.91E+02 04 99.9 8.752 7.00E+02 8.631 03 99.5 03 99.5 2.99E+03 8.507 pot 2.39E 2.02E 1.55E– 1.28E– 9.94E 2.18E 9.00E 1.34E– 3.08E 3.98E 2.12E– 6.84E– 2.19E 5.02E 6.95E– 3.01E 2.17E– 4.09E 7.90E– ±1 s.e. ±1 o 06-4, AY02/05/06-5, AY02/07/06-2, AY06/29/06-10B, and AY07/02/ and AY06/29/06-10B, AY02/07/06-2, AY02/05/06-5, 06-4, TABLE 1. ION MICROPROBE U-(Th)-PbION MICROPROBE TABLE 1. ( DATA ZIRCON s I Pb = 18.86, Pb* 204 206 Pb/ 206 Pb*/ 207 10; E+02—× 100; E+03— 100; E+02—× 10; § – – – – – – – – – × 04 4.67E–02 04 4.67E–02 04 4.70E–02 04 4.70E–02 03 4.36E–02 03 4.36E–02 03 4.61E–02 03 4.61E–02 04 5.00E–02 04 5.00E–02 03 5.72E–02 03 5.72E–02 03 4.59E–02 03 4.59E–02 01 1.06E–01 01 1.06E–01 02 9.14E–02 02 9.14E–02 03 4.70E–02 03 4.70E–02 03 5.17E–02 02 9.24E–02 02 9.24E–02 04 4.43E–02 04 4.43E–02 04 4.56E–02 04 4.56E–02 03 4.66E–02 03 4.66E–02 02 5.67E–02 02 5.67E–02 02 5.68E–02 02 1.07E–01 02 1.07E–01 03 5.68E–02 03 5.68E–02 1 s.e. 1.64E– 1.15E 3.14E 9.57E– 6.27E 5.10E 7.69E 9.14E– 1.50E– 8.51E 8.83E– 1.88E 9.86E– 5.04E 9.42E– 1.38E– 1.26E– 8.83E 1.12E– U ± 235 Pb*/ 207 § – – – – – – – – – – – – 04 7.36E–02 04 7.36E–02 05 2.02E–02 05 2.02E–02 05 2.22E–02 05 2.22E–02 05 1.96E–02 05 1.96E–02 03 6.39E–01 03 6.39E–01 03 4.00E+00 03 4.00E+00 03 1.60E+00 03 1.60E+00 05 2.08E–02 04 4.19E–02 04 4.19E–02 05 2.26E–02 05 2.26E–02 05 2.62E–02 05 2.62E–02 03 6.52E–01 03 6.52E–01 03 6.25E–01 03 6.25E–01 05 2.79E–02 05 2.79E–02 05 2.48E–02 05 2.48E–02 03 5.62E–01 03 1.93E+00 03 1.93E+00 05 2.36E–02 05 2.36E–02 03 4.85E+00 03 4.85E+00 6.91E 3.44E– 8.18E 1.57E– 4.46E 7.09E– 3.62E 2.59E 5.08E 2.60E 5.04E 1.09E– 1.13E 1.11E 4.13E– 5.40E– 3.93E– 1.66E 5.82E ±1 s.e. ±1 × 0.01; E–03—× 0.001; E+01— 0.001; E–03—× 0.01; × U 238 Pb*/ 206 0.1; E–02— # Pb = 36.7 for samples AY12/31/04-3B, AY12/30/04-17, AY12/31/04-17, AY1/1/05-11A, AY12/31/04-34A, AY12/31/04-21, and AY12/31/04- and AY12/31/04-21, AY12/31/04-34A, AY1/1/05-11A, AY12/31/04-17, AY12/30/04-17, samples AY12/31/04-3B, for Pb = 36.7 204 4.42E–03 s.e.—standard error. error. s.e.—standard name. spot number, #—zircon #, ID: Spot These data were collected in four sessions, each with distinct analytical conditions. Calibration conditions. analytical distinct each with sessions, four in collected were data These # † § Pb/ Spot ID *Radiogenic Pb corrected for common Pb with composition composition with Pb common for Pb corrected *Radiogenic (34A) 1, a 1, a 3.83E–03 5, a 5, a 1.03E–02 31-04 3A, AY 01-03-05 1A, AY 01-03-05 1B, AY 01-03-05 1C], 7.8–8.9 [AY 12-31-04 3B, AY 12-30-04 17, AY 12-31-04 17, AY 01-01-05 AY 17, 12-31-04 17, AY 12-30-04 AY 3B, 12-31-04 [AY 1C], 7.8–8.9 AY 01-03-05 1B, 01-03-05 AY 1A, 01-03-05 AY 31-04 3A, 06-29-0 AY 2, 02-07-06 5, AY 02-05-06 4, AY AY 02-05-06 3, 02-04-06 [AY 9.2–10 and 22], 09-13-03 9, AY 02-04-06 AY 6B, 02-05-06 discordance. (11) AY 12-31-04- (11) AY E–01—× 10, a 10, a 3.13E–03 6, a 6, a 1.52E–01 3, a 3, a 3.11E–03 6, a 6, a 3.55E–03 AY02/05/06-6B, AY02/04/06-9, AY9/13/03-22, AY02/04/06-3, AY02/05/ AY02/04/06-3, AY9/13/03-22, AY02/04/06-9, AY02/05/06-6B, 5, a 5, a 1.27E–01 (11A) 1, a 3.41E–03 208 (10) AY 01-01-05- (10) AY 4, a 4, a 3.29E–01 5, a 5, a 6.97E–03 2, a 2, a 3.80E–03 3, a 3, a 8.34E–02 7, a 8.15E–02 8, a 4, a 4, a 2.75E–01 2, a 2, a 3.44E–03 4, a 4, a 7.92E–02 7, a 7, a 3.67E–03 6, a 6, a 7.19E–02

Geological Society of America Bulletin, March/April 2010 381 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Yin et al.

Ages of Orthogneiss from Bhalukpong-Zimithang Traverse

0.3 2600 0.5 AY 09-13-03-(22) (A) AY 09-17-03-(1) (B) 1400 2200 0.4 1200 0.2 1800 U U

0.3 238 238 1000

1400 Pb/

Pb/ 878 Ma 206 800 206 0.2 1000 0.1 600 627.6 Ma 400 0.1

0 0.0 02468101214 0 0.5 1 1.5 2 2.5 3 3.5 207 235 207Pb/235U Pb/ U

Figure 7. Concordia diagrams for augen gneiss samples collected from the Bhalukpong-Zimithang traverse. (A) Con cordia plot for sample AY 09-13-03-(22). (B) Concordia plot for sample AY 09-17-03-(1).

Leucogranites from the Table 1). Additional two spot analyses plot- gas laboratory. Different mineral phases had Kimin-Geevan Traverse ted along a discordia line between the two age specialized step-heating schedules from a mini- Sample AY 12–31–04-(3B) was from a leu- clusters. The ca. 491 Ma zircons may represent mum of 400 °C to a maximum of 1550 °C. Total cogranite that intrudes 1752 Ma augen gneiss inherited zircons from the wall rocks, and the gas ages are reported here for biotite and mus- as represented by sample AY 12–31–04-(3A) younger zircons may result from crystallization covite (Table 2). (Fig. 6A). Of the 15 analyses we obtained, six of the leucogranite at ca. 24 Ma. Because the closure temperature of biotite spot analyses of different zircons were dis- Sample AY 12–31–04-(34A) is from a leu- for retention of 40Ar is 350 ± 50 °C, which cordant, four had UO/U values exceeding the cogranite intruding high-grade gneiss in the is lower than 400 ± 50 °C for muscovite range of calibration, and another four analyses Main Central thrust hanging wall (Fig. 6A). (McDougall and Harrison, 1999), biotite ages had Th/U values below 0.1 (Fig. 9A; Table 1). We analyzed nine spots on different zircons from the same samples should be older than The data plot along a discordia line that inter- (Fig. 9C). One analysis with a high Th/U ratio the muscovite ages. However, for all but one cepts the concordia curve at 373 ± 59 Ma below yielded a 207Pb/206Pb weighted mean age of of our samples from which mica and biotite and 1759 ± 36 Ma above (MSWD = 1.4). The 1746 ± 14 Ma (2σ); the rest yielded moderate to ages were both determined, the biotite ages are upper-intercept age overlaps with the crystal- low Th/U ratios and Cenozoic 238U/206Pb ages, consistently older than the mica ages (Table 2). lization age of the host rock at 1752 ± 12 Ma with a dominant age cluster from ca. 23.5 Ma This implies the existence of excess argon in and likely reﬂ ects inheritance of wall-rock to ca. 20 Ma. We interpret the ca. 1746 Ma age biotite that has caused overestimates of its zircons. The wall-rock zircons may have ex- as reﬂ ecting inheritance from the wall rocks and cooling ages. For this reason, we consider all perienced Phanerozoic metamorphism during the younger ages as indicating crystallization of the biotite ages as maximum age bounds for zircon growth, as indicated by moderate to low the leucogranite at 23–20 Ma. the time of the sample cooled below ~350 °C. Th/U values (Table 1). Considering the large For example, the 19 Ma biotite cooling age uncertainty for this age, it is likely that the met- 40Ar/39Ar THERMOCHRONOLOGY of sample AY9–18–03-(23) indicates that the amorphic event was related to the widespread Tenga thrust sheet where the sample was col- Cambrian-Ordovician plutonism and meta- Determining Cooling History by lected was exhumed to a depth of <14 km morphism across the Himalaya (450–520 Ma; 40Ar/39Ar Thermochronology after 19 Ma (assuming a geothermal gradi- see Gehrels et al., 2006a, 2006b; Martin et al., ent of 25 °C/km). This inference is consistent 2007). This interpretation suggests that some We analyzed biotite and muscovite for with the initiation age of contraction fabrics at Himalayan leucogranite may have been em- 40Ar/39Ar thermochronometry. All mineral sepa- 13 Ma in the Tenga thrust sheet obtained from placed in the early Paleozoic, as suggested by rates, packed in copper foil in quartz tubes or mica in sample AY9–18–03-(10) (Fig. 4B), lo- Gehrels et al. (2006a, 2006b). aluminum-holding containers, were irradiated cated nearby (see Discussion). Sample AY 01–01–05-(11A) is from a within a nuclear reactor. The Fish Canyon Tuff The most robust result of our thermo chrono- leucogranite that intrudes a 500 Ma granitoid standard (FCT) was used to monitor the amount logical study is that the muscovite ages increase as represented by sample AY 12–30–04-(6) of 39Ar produced in the reactor from 39K within with an increase in structural level from the (Fig. 6A). Five analyses plot in two concor- each sample and was packed at regular intervals nearby thrusts (Fig. 4B). For the Zimithang dant clusters, three of which yield a 207Pb/206Pb of 1 cm within the tube of unknowns. Correction thrust, the mica age increases from ca. 7 Ma di- weighted mean age of 491 ± 11 Ma (2σ) and factors were determined for Ca- and K-derived rectly above the fault to about ca. 12 Ma a few the other two of which feature very low Th/U argon by irradiating and measuring salts (CaF2, kilometers higher in its hanging wall (Fig. 4B). 238 206 ratios, yielding U/ Pb weighted mean ages K2SO4) included within the tube of unknowns. For the Main Central thrust near Lum La, the of 24.6 ± 0.5 Ma and 24.4 ± 0.3 Ma (Fig. 9B; Each sample was step-heated at UCLA’s noble muscovite age increases from ca. 8 Ma directly

382 Geological Society of America Bulletin, March/April 2010 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Eastern Himalaya

Ages of Orthogneiss from Kimin-Geevan Traverse

AY 12-30-04-(6) AY 12-31-04-(3A) 2200 0.16 0.4 (A) 900 (B) 1800 0.12 700 0.3 U U 238 238 1400

Pb/ 0.08 500 Pb/ 0.2

206 206 1000 300 0.04 0.1 600 100

0.00 0.0 0.00.40.81.21.6 0246810 207Pb/235U 207Pb/235U 0.10 0.4 AY 12-30-04-(17) 550 AY 12-31-04-(17) 0.08 (C) (D) 1800 450 0.3 U U 1400 0.06 350 238 238 0.2

Pb/ Pb/ 1000 0.04 250 206 206

150 Intercepts at 0.1 600 0.02 28±13 & 512±14 Ma 50 MSWD = 1.3 200 0.00 0.0 0.0 0.2 0.4 0.6 0.8 0123456 207Pb/235U 207Pb/235U 0.4 AY 12-31-04-(21) AY 12-31-04-(10) 1800 (E) 1800 0.3 (F) 0.3 1400 U

1400 U 238 238 0.2 0.2 1000 Pb/ 1000 Pb/ 206 206

0.1 600 0.1 600

200 200 0.0 0.0 0246 012345 207Pb/235U 207Pb/235U

Figure 8. Concordia diagrams for augen gneiss samples from the Kimin-Geevan traverse. (A) Concordia plot for sample AY 12-30-04-(6). (B) Concordia plot for sample AY 12-31-04-(3A). (C) Concordia plot for sample AY 12-30-04-(17). (D) Concordia plot for sample AY 12-31-04-(17). (E) Concordia plot for sample AY 12-31-04-(21). (F) Concordia plot for sample AY 12-31-04-(10). MSWD—mean square of weighted deviates.

above the Main Central thrust to 10–11 Ma 40Ar/39Ar Thermochronology of deﬁ ning white micas from quartz arenite di- ~3–4 km higher and to ca. 12 Ma ~8–10 km White Micas from Main Central Thrust rectly below the Main Central thrust near Lum above the Main Central thrust (Fig. 4B). For the Footwall Quartzite La and Dirang and in the hanging wall of the Main Central thrust near Dirang, the muscovite Tenga thrust below the Main Central thrust (Figs. age near the Main Central thrust is ca. 10 Ma In order to determine the age of contractional 4A and 10). The 40Ar/39Ar thermochrono logic and ca. 12 Ma 8–10 km higher up in the Main fabrics in the Main Central thrust footwall, we analyses of white mica were conducted in the Central thrust hanging wall (Fig. 4B). separated mineral-stretching and lineation- noble gas laboratory of the Australian National

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Ages of Leucogranites from Kimin-Geevan Traverse is 13.4 ± 0.1 Ma near Bomdila, directly above the Tenga thrust in the Main Central thrust foot- AY 12-31-04-(3B) 1800 wall (Figs. 4A and 4B). This age pattern of mica 0.3 may be explained by out-of-sequence thrusting, (A) where the Tenga thrust was active at ca. 13 Ma, 1400 followed by motion on the Bomdila thrust at a higher structural level initiated at 6–7 Ma. U 0.2 238 1000 DISCUSSION Pb/ 206 Our mapping suggests that the Main Central 0.1 600 Intercepts at thrust is a folded low-angle fault bounding a 373±59 & 1759±36 Ma thrust duplex below (Figs. 4B and 6B). Our U-Pb 200 MSWD = 1.4 zircon dating reveals six igneous/metamorphic 0.0 events in the eastern Himalaya: (1) emplace- 012345 ment of orthogneiss at ca. 1750 Ma in both the 207Pb/235U hanging wall and footwall of the Main Central thrust, (2) emplacement of orthogneiss during AY 12-31-04-(34A) 1800 825–878 Ma in the Main Central thrust hanging wall, (3) a thermal event causing metamorphic 0.3 (B) zircon growth at ca. 630 Ma in the Main Central Figure 9. Concordia diagrams 1400 thrust hanging wall, (4) emplacement of ortho- for leucogranite samples from 0.0075 48 gneiss at ca. 500 Ma in the Main Central thrust U the Kimin-Geevan traverse. 0.2 44 hanging wall, (5) emplacement of early Paleo- 238 0.0065 (A) Concordia plot for sample 1000 40 zoic leucogranite (or a metamorphic event)

Pb/ 36 AY 12-31-04-(3B). (B) Concor- 0.0055 at 373 ± 59 Ma, and (6) emplacement of Ter- 206 32 40 39 dia plot for sample AY 12-31- 0.0045 tiary leucogranite at 28–20 Ma. The Ar/ Ar 0.1 600 28 04-(34A). (C) Concordia plot 24 thermo chronology in this study suggests that 0.0035 for sample AY 01-01-05-(11A). 20 the Main Central thrust hanging wall was cooled 0.0025 below ~350–400 °C at ~12 Ma in its upper part 0.015 0.025 0.035 0.045 0.055 0.0 and at ~8 Ma in its lower part; this was probably 02468related to unroofi ng of the Main Central thrust 207Pb/235U sheet. The depositional relationship between the middle Rupa Group and a mylonitic augen unit 0.10 below suggests Precambrian shear-zone devel- AY 01-01-05–(11A) 550 opment. Inclusion of garnet schist in the 870-Ma 0.08 (C) pluton may also imply a possible Precambian 450 metamorphic event in the region. Next, we discuss the implications of our new fi ndings. U 0.06 0.012 350 70 238 0.010 Estimates of Cenozoic Crustal Shortening Pb/ 0.04 250 0.008 50 206 0.006 A fi rst-order issue related to the India-Asia 30 150 0.004 collision is the way in which the convergence of 0.02 0.002 10 the two continents was absorbed by intra conti-

50 0.000 nental deformation (England and Houseman, 0.00 0.02 0.04 0.06 0.08 0.00 1986; Avouac and Tapponnier, 1993). Resolv- 0.0 0.2 0.4 0.6 0.8 ing this question requires knowledge of Ceno- 207 235 zoic strain across the India-Asia collision zone, Pb/ U including the Himalayan-Tibetan orogen. The large magnitude of Cenozoic shortening across the central Himalaya as determined by recon- University. The 40Ar/39Ar white mica ages in ward the Main Central thrust. This age pattern structing balanced cross sections has been used low-grade metasedimentary rocks associated is opposite to those obtained from the Main to infer underplating of Indian lower crust be- with cleavage development may represent crys- Central thrust hanging wall (Fig. 4B). Speciﬁ - neath the Tibetan Plateau (e.g., DeCelles et al., tallization of new mica crystals along contrac- cally, the 40Ar/39Ar weighted mean plateau age 2002). In addition, the magnitude of shortening tional fabrics or cooling of preexisting mica of mica directly below the Main Central thrust estimated from different parts of the 2000-km- (Dunlap et al., 1997). Our results reveal an is 6.5 ± 0.1 Ma at the Lum La window and long Himalayan orogen has been used to infer interesting pattern: the mica ages in the Main 6.9 ± 0.1 Ma near Dirang (Fig. 10). In contrast, possible along-strike variation of strain in re- Central thrust footwall become younger to- the 40Ar/39Ar weighted mean plateau age of mica sponse to the India-Asia convergence boundary

384 Geological Society of America Bulletin, March/April 2010 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Eastern Himalaya

TABLE 2. SUMMARY OF 40Ar/39Ar DATA the Rupa Group experienced isoclinal folding Total gas Weighted mean that transposed the original bedding into slaty age age K2O 40 cleavage. Because phyllite takes up more than Sample number Mineral (Ma, 1σ) (Ma, 1σ) (wt%) Ar* (%) Geology one-third of the total thickness of the exposed AY9-17-03-(2) Bio 12.1 ± 0.2 11.5 ± 0.6 7.6 75.2 GHC Mus 12.3 ± 0.3 9.7 ± 1.4 5.0 64.3 Lesser Himalayan Sequence in Arunachal, it is essential to quantify the effect of folding AY9-17-03-(5) Bio 16.7 ± 0.2 16.2 ± 0.8 7.8 81.4 GHC Mus 11.0 ± 0.2 9.4 ± 1.5 7.7 70.2 on crustal shortening in this type of rocks. To illus trate this, we conducted both area and line AY9-16-03-(19) Bio 9.2 ± 0.2 8.9 ± 0.3 7.2 68.8 GHC balancing of actual folds shown in Figure 5B. Mus 8.0 ± 0.3 7.6 ± 0.7 6.4 38.3 We fi rst used bedding-parallel simple shear to AY9-17-03-(1) Bio 12.4 ± 0.3 11.6 ± 0.8 7.1 73.2 GHC restore folds and then laid the beds horizontally Mus 7.7 ± 0.2 7.5 ± 0.4 19.6 44.0 to calculate the original bed length. Using the AY9-16-03-(1) Bio 10.0 ± 0.2 8.7 ± 2.3 5.0 85.7 GHC current width of the folds in the outcrop, we obtain a shortening strain of 60%–65% (Figs. AY9-17-03-(7A) Bio 7.8 ± 0.2 7.6 ± 0.2 8.5 75.2 GHC 12A and 12B). Because bed thickness does not AY9-16-03-(6) Bio 26.8 ± 0.2 26.0 ± 1.0 8.6 81.7 GHC stay constant during similar folding, we used an Mus 10.3 ± 0.2 9.9 ± 0.7 9.2 46.0 area balancing method to calculate the shortening strain. We selected the thickest part of the AY9-14-03-(3) Bio 13.5 ± 0.2 13.5 ± 0.09 7.6 83.6 GHC Mus 10.7 ± 0.2 10.4 ± 0.2 9.7 52.1 fold limb to represent a mini mal thickness of the original bed. This assumption is justifi ed AY9-17-03-(11) Bio 15.5 ± 0.2 15.2 ± 0.4 7.8 74.2 GHC Mus 12.2 ± 0.2 12.1 ± 0.1 10.0 56.5 because similar folding thins the fold limbs, and thus the observed limb thickness is always AY9-17-03-(11) Bio 19.2 ± 0.6 19.2 ± 0.07 0.5 75.2 LHS a minimum of its original thickness. The area- Note: Bio.—biotite; Mus—muscovite; GHC—Greater Himalayan Crystalline Complex; LHS—Lesser Himalayan Sequence. balancing approach yields a total shortening strain of ~40% (Fig. 12C), which is 20%–25% less than the estimated shortening strain based conditions (Yin et al., 2006; cf. McQuarrie et the Lesser Himalayan Sequence units across on a line-balancing technique. This example al., 2008). Before presenting our estimates of our study areas and the rest of the Himalaya are suggests that even under a perfect situation, Cenozoic crustal shortening in the Arunachal dominantly Precambrian strata; they have been when the geometry of a cross section is known Himalaya, we discuss some of the major uncer- used extensively in the Himalaya for estimating completely, different section-balancing tech- tainties in our calculations. the total crustal shortening across the orogen niques can lead to signifi cantly (>20%) differ- (e.g., Murphy and Yin, 2003). Such an approach ent results on shortening estimates. Pre-Cenozoic Deformation may overestimate the Cenozoic strain because Argles et al. (1999), Catlos et al. (2002), the effect of early Paleozoic deformation is not Nonuniqueness of Balanced Cross Sections Gehrels et al. (2003, 2006a, 2006b), Kohn et al. removed. Accurate estimates of crustal shortening (2004, 2005), and Martin et al. (2007) have pre- also depend on construction of balanced cross sented evidence for the occurrence of Cambrian- Deformation Mechanism sections using surface information and known Ordovician contractional deformation, pluton The existing balanced cross sections across deformation mechanisms. However, surface emplacement, and high-grade metamorphism the Himalaya all assume that folding was ac- geology alone can rarely provide suffi cient across the western and central Himalaya. Our commodated by fl exural slip, so that bed length constraints for making a unique cross section, companion study across the Shillong Plateau and bed thickness can be preserved before and due to the lack of information on (1) the num- and Mikir Hills of northeastern India also indi- after deformation. This may not be the case for ber and depths of detachments below (e.g., Yin cates the occurrence of early Paleozoic contrac- most rocks in the Arunachal Himalaya. In the et al., 2008a, 2008b), (2) spatial variation of tional deformation (Yin et al., 2009). Although Main Central thrust hanging wall, deformation structural style and temporal variation of defor- we do not have direct structural evidence in this is mostly expressed by foliation-parallel stretch- ma tion mechanisms, (3) thickness variation study for early Paleozoic deformation in the ing and widespread mesocopic folding, which of individual units (Yin, 2006), and (4) struc- eastern Himalaya, the 630 Ma zircon growth, thickens the folded crustal section vertically tural framework of the region induced by early 375 Ma thermal disturbance, and widespread while thinning individual fold limbs (Fig. 11). deforma tional events. We use the geology of occurrence of 500 Ma plutonic rocks correlate In this case, it would be misleading to use the the Bhalukpong-Zimithang traverse to illustrate well with a broadly coeval event in the north- state of strain at individual points to infer the the issue of nonuniqueness. We begin by mak- eastern Indian craton (Yin et al., 2009). This overall fl ow fi eld of the Main Central thrust ing a cross section shown in Figure 4B using suggests that early Paleozoic deformation may hanging wall (cf. Law et al., 2004). the standard dip domain method and assume a have affected the eastern Himalayan region. The In the Main Central thrust footwall, phyl- single décollement dipping parallel to the in- correlation raises the possibility that the Protero- lite and slate constitute a major fraction clined Moho obtained from extrapolating the zoic sedimentary strata in the eastern Himalaya of the Lesser Himalayan Sequence through- two-point results of Mitra et al. (2005) below may have been already deformed prior to the out the Himalaya (e.g., Upreti, 1996; DeCelles our study area. However, by allowing multiple Cenozoic India-Asia collision. Ideally, we may et al., 2001; Yin, 2006). They are exclusively de- levels of décollements, we can also construct use the strata deposited after the early Paleozoic formed by intraformational folding associated an alternative section with two levels of du- contractional event (i.e., post-Ordovician strata) with slaty cleavage (e.g., Valdiya, 1980; LeFort, plex systems, as shown in Figure 4C. The two to reconstruct Cenozoic deformation. In reality, 1996). In our study area, the phyllite units in different cross sections imply very different

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30 30 straints on the geometry of the major thrusts AY 9-16-03-(14) (Lum La) and the poor knowledge of the pre-Cenozoic 25 (A) 25 stratigraphic framework of the eastern Hima- laya, we are currently unable to differentiate 20 20 among the possibilities.

Age 15 15 Shortening Estimates (Ma) Around the Siang window (Figs. 1 and 13)

10 10 (Kumar, 1997), the Main Boundary thrust places the Lesser Himalayan Sequence over Creta- ceous and Paleogene strata, and the Bome thrust 5 5 juxtaposes the Lesser Himalayan Sequence units over Permian strata. These relationships 0 0 0.0 0.2 0.4 0.6 0.8 1.0 require the Bome fault to have a minimum Fraction Ar39 released slip of ~95 km and the Main Boundary thrust Figure 10. Argon release spec- 30 30 to have a minimum slip of ~80 km (Fig. 13A). tra and corresponding ages for AY 9-16-03-(14) (Dirange) Finally, we use the northernmost exposure of the Lum La window of Yin et al. (2006; also neoblast micas in quartz arenite 25 (B) 25 samples collected in the footwall see Fig. 4A) and the southernmost exposure of the Main Central thrust mapped across the of the Main Central thrust along 20 20 the Bhalukpong-Zimithang tra- Kimin-Geevan traverse to determine a mini- Age mum slip of 140 km on the Main Central thrust. verse. See Figure 4A for sample 15 15 (Ma) locations. (A) Argon release Assuming that slip on major faults does not vary along strike in the western Arunachal Himalaya, spectrum for sample AY 9-16- 10 10 03-(14) from the Lum La area. we obtain a minimum shortening of ~315 km (B) Argon release spectrum for accommodated solely by the Main Central 5 5 sample AY 9-17-03-(15) from the thrust, Main Boundary thrust, and Bome fault Dirang area. (C) Argon release across the region. As the Main Central thrust 0 0 and Main Boundary thrust are Cenozoic in spectrum for sample AY 9-18- 0.0 0.2 0.4 0.6 0.8 1.0 03-(10) from the Bomdila area. Fraction Ar39 released age and the Bome fault is likely a Cenozoic contractional structure because it cuts Permian 30 30 strata and there was no post-Permian contrac- AY 9-18-03-(10) (Bomdila) tion except the Cenozoic Himalayan event, the 25 (C) 25 above estimated shortening was all induced during the Indian-Asia collision. If we project 20 20 the map relationship around the Siang window (Fig. 13B) and the early Paleo zoic Central Age 15 15 (Ma) Shillong thrust system below the Bhalukpong traverse with ~15 km of basement relief as 10 10 seen in the Shillong region (Fig. 3b in Yin et al. 2009), we obtain an estimated total shortening

5 5 of ~775 km (i.e., ~76% shortening strain) using the line-balancing method (Figs. 4D and 4E). If the Himalayan basement was not deformed and 0 0 0.0 0.2 0.4 0.6 0.8 1.0 all pre-Cenozoic strata were fl at-lying prior to Fraction Ar39 released India-Asia collision, as commonly assumed in balanced cross sections across the central Hima- laya (e.g., Murphy and Yin, 2003) (Fig. 4F), we kine matics. For the cross section in Figure 4B, ing. This may explain widespread seismicity in obtain an estimated shortening of ~515 km (i.e., motion on the Main Central thrust produces a the eastern Himalayan interior (Drukpa et al., ~70% shortening strain) (Fig. 4G) across the duplex system in its footwall and the deforma- 2006; Velasco et al., 2007). eastern Himalaya. The difference in shortening tion front propagates southward. In contrast, the We may also consider two competing situ- estimates from the two cross sections highlights cross section in Figure 4C consists of an upper- ations: one assumes a signifi cant basement the importance of pre-Cenozoic stratigraphic level duplex system associated with motion on topography induced by early Paleozoic con- and structural frameworks below the Himalaya the Main Central thrust and a lower-level duplex traction as seen in the Shillong Plateau (Yin, in estimating crustal shortening strain. Consid- system associated with motion on the Main 2009) (Fig. 4D), and another assumes all strata ering the possible effect of similar folding in the Boundary thrust and Main Frontal thrust zone. were fl at-lying with a thin-skinned style of Lesser Himalayan Sequence and ductile behav- Importantly, structural geometry in Figure 4C deformation as commonly assumed in Hima- ior of the Main Central thrust hanging wall, the requires the Himalayan interior to experience layan research (e.g., Murphy and Yin, 2003) uncertainty of our shortening estimates must be active crustal shortening and thus upward warp- (Fig. 4F). Given the lack of subsurface con- greater than 20%–30%.

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A Original bed length and bed thickness before folding

T 0 Figure 11. Schematic diagram showing the relationship between bedding-perpendicular thickening and bedding-parallel B Final bed length and bed thickness after folding thinning during folding: (A) Bed L = 0.7L before folding. (B) Bed after f o folding. Note that the overall section is thickened by 140%, T1= 0.7To while the marker bed in the fold limbs is thinned by 70%.

Bedding-perpendicular thickening Bedding-parallel stretching

Greater Himalayan Crystalline straints suggest that the Precambrian basement Along-Strike Variation of Complex Provenance and Style of and Proterozoic cover sequences of the north- Himalayan Geology Himalayan Thrusting eastern Indian craton and eastern Himalaya are generally correlative. In Bhutan, the Main Central thrust cuts 16 Ma Our geochronologic results indicate that Correlation of the Precambrian Himalayan leucogranite, whereas the Kakthang thrust the Greater Himalayan Crystalline Com- and Indian cratonal units has three important cuts a leucogranite with an age of 14–15 Ma plex is composed of plutonic rocks with ages implications. First, the Himalayan orogen must (Grujic et al., 2002). Grujic et al. (2002) used at 500 Ma, 880 Ma, and 1745 Ma (Fig. 14). have been constructed in situ by rocks of the these observations to suggest that the Kakthang The presence of 1745 Ma gneiss in the Main Precambrian Indian craton rather than from Ti- thrust is an out-of-sequence structure with re- Central thrust hanging wall suggests that the betan middle crust (e.g., Nelson et al., 1996). spect to the Main Central thrust. However, this Greater Hima layan Crystalline Complex in the This is because our study indicates a lack of crosscutting relationship does not constrain the eastern Himalayan orogen may have originated Cretaceous-Tertiary Gangdese Batholith com- initiation and termination ages of the two struc- from the Indian craton. In northeastern India, ponents in the Main Central thrust hanging wall tures and thus cannot uniquely establish the true magmatic events at 1772–1620 Ma (U-Pb zir- (e.g., Quidelleur et al., 1997; Yin and Harri- sequence of thrusting across the Bhutan Hima- con ages) (Ameen et al., 2007; Yin et al., 2009), son, 2000; Harrison et al., 2000; Yin, 2006). laya. U-Pb dating of monazite and xenotime 1100 Ma (U-Pb zircon ages) (Yin et al., 2009), Second, the style of thrusting in the Himalaya suggests that the Main Central thrust in Bhutan 770–880 Ma (Rb-Sr ages; Ghosh et al., 2005), is not thin-skinned, as is commonly assumed was already active at ca. 22 Ma and continued and 530–480 Ma (Ghosh et al., 2005; Yin et al., throughout the Himalaya (e.g., Steck et al., after 14 Ma (Daniel et al., 2003). The early 2009) have been recorded. There, the sedi- 1993, 1998; Steck, 2003; DeCelles et al., 2001, initiation of the Main Central thrust in Bhutan mentary cover sequence is represented by the 2002; Murphy and Yin, 2003; Robinson et al., is also recorded in the metamorphic history of Protero zoic Shillong Group, which has an initial 2006), but it is thick-skinned, involving verti- the fault zone, which experienced a peak P-T depositional age younger than 1100 Ma (young- cal stacking of Indian basement and sedimen- condition of ~750–800 °C and 10–14 kbar at est zircon age in the lower section of the exposed tary cover sequences as envisioned by Heim ca. 18 Ma, followed by a retrograde metamor- part of the sequence; the contact with the base- and Gansser (1939) followed by LeFort (1975). phic event under conditions of 500–600 °C ment is not exposed in the Shillong Plateau) to a Current crustal thickening in the Shillong and 5 kbar at 14–11 Ma (Stüwe and Foster, terminal depositional age younger than 560 Ma Plateau region may represent the incipient 2001; Daniel et al., 2003). While the retrograde (oldest pluton that intrudes the upper part of stage of this shortening mechanism (e.g., Yin event correlates with the cooling history of the the exposed sequence) (Yin et al., 2009). In the et al., 2009). Third, if the Greater Himalayan Arunachal Himalaya obtained by this study, the eastern Himalaya, igneous crystalline rocks are Crystalline Complex was an exotic terrane ac- prograde metamorphic event and early initia- represented by the 1750 Ma (U-Pb zircon ages) creted in the Cambrian-Ordovician onto Indian tion of the Main Central thrust in Bhutan are (Daniel et al., 2003; Richards et al., 2006; this continent (DeCelles et al., 2000; cf. Cawood not in evidence in our study areas. For ex- study), 1100 Ma (Rb-Sr ages) (Bhargava, 1995), et al., 2007), the inferred terrane must have had ample, our thermochronological data and the 830–880 Ma (Richards et al., 2006; this study) a close geologic tie with India from 1750 Ma to U-Th monazite-inclusion ages (Yin et al., 2006) augen gneisses, and 500 Ma orthogneiss (this ca. 500 Ma. That is, the Greater Hima layan suggest that the Main Central thrust was active study). The age of the middle Rupa Group in the Crystalline Complex could have been a ﬁ rst at 10 Ma. If this age represents the onset time eastern Himalaya is younger than the 1750 Ma continental strip rifted away from the Indian of the Main Central thrust, it implies the thrust augen gneiss, and its terminal deposition may continent after ca. 880–830 Ma and was later in Arunachal is ~10–12 Ma younger than its have occurred in the early Cambrian (Tewari, accreted back to India at 500–480 Ma, as origi- equivalent structure in the Nepal and Bhutan 2001; McQuarrie et al., 2008). These age con- nally suggested by DeCelles et al. (2000). Himalaya (Hubbard and Harrison, 1989; Daniel

Geological Society of America Bulletin, March/April 2010 387 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Yin et al. (1) (1) ine to calculate (2) (3) (4) (2) (2) (1) (c) Left-slip angular shear of 77 degrees (d) Right-slip angular shear of 70 degrees (a) Left-slip angular shear of 81 degrees (b) Right-slip angular shear of 78 degrees (f) Left-slip angular shear of 75 degrees for domains (1) and (2) (B) Bed Length Balancing (3) (3) 15 cm Original bed length = 100 cm Original bed length = 115 cm Original bed length = 115 60% shortening strain 65% shortening strain (A) (1) (4) (4) (3) 63 km (2) 40 cm 3 cm thickness = 3 cm 15 cm (4) the original bed length. (C) Shortening estimate based on area-balancing technique. We assume the thickest part of the fold limb to be a assume the thickest part of fold limb We technique. the original bed length. (C) Shortening estimate based on area-balancing See text bounded by the two fold limbs to calculate original bed length. the original bed thickness and use area minimum value for detailed discussion on comparison of the two methods. for Figure 12. (A) Line drawing of similar folds based on Figure 5B. Numbers in parentheses represent individual dip domains divided by axial individual dip domains divided represent 5B. Numbers in parentheses folds based on Figure 12. (A) Line drawing of similar Figure bedding- of individual fold limbs by applying surfaces of the folds. (B) Bend-length balancing Steps a to d show restoration the combined domains of 1 and 2 3 used to restore Steps e and f show bedding-parallel simple shear parallel simple shear. in two stages, with domains 1 and 2 forming one long limb 3 and produced implies that the folds were This procedure and 4. each limb was itself folded. Step g arbitrarily uses the right end of section as a pin-l long limb before 4 forming another (g) Left-slip angular shear of 67 degrees (e) Right-slip angular shear of 60 degrees for domains (3) and (4) (C) Area Balancing (40% shortening strain) (C)

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92°E 93°E 94°E 95°E95°E 96°E I GT Lhasa terrane MBT Siang window Lhasa terrane MCT GHC STD GCT 29°N GHC MBTM T 29°N THS STD MBT slip > 80 km K-Cz LHS P

GHC LHSS BT slip > 95 km

MCT slip > 140 km MCT MBT BTBT ZT 28°N 28°N LHS GHC 0 50 100 km TT MCT

BT MFT LHS P MBT 272 °N 27°N I′ (A) 92°E922° 93°E 94°E 95°E 96°E

Yala Xiangbo STD I GCT gneiss dome BT I′ Depth Lhasa MBT Depth 44 Ma (Dala granite) (km) terrane N-Q (km) 0 160–40 Ma 870 Ma 0 P LH gn-1 TH LH 500 Ma 10 10 GT GHC MCT LH LH v = 5 H GHC 20 gn-1 20 0 50 100 km gn-1 LH P LH LH 30 K-E GT, Gangdese thrust 30 GCT: Great Counter thrust ZT: Zimithang thrust 40 ZT MCT: Main Central thrust 40 MBT: Main Boundary thrust 50 BT: Bome thrust 50 LH: Lesser Himalayan Sequence gn-1 (1700 Ma) gn: Crystalline basement of the LHS 60 with characteristic age of 1.7 Ga (B) 60

Figure 13. (A) Simpliﬁ ed geologic map of the eastern Himalaya. See Figure 1 for location. The relationship between thrust windows and the frontal thrust trace provides an estimate of minimum slip on the Main Central thrust (MCT), Bome thrust (BT), and Main Boundary thrust (MBT). (B) Schematic cross section of the eastern Himalaya along line II′. The ages of orthogneiss and granites in the eastern Himalaya are also shown. STD—South Tibetan detachment.

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N Shillong Plateau North Himalaya Bhutan Arunachal (Indian craton)

K-E USL Post-Pan-African-orogen (ca. 520–480 Ma) unconformity UR Tr-Jr UB LSL future MCT MR Or-P LB LR Post-Eastern-Ghats-orogen UR (ca. 1150–1100 Ma) 560–490 Ma DS unconformity MR Future basement-involved 950–560 Ma Cenozoic LHS and Shillong thrusts

LR 1500–950 Ma K-E: Postrift sequence with respect to opening of the Neotethys. Tr-Jr: Synrift sequence with respect to opening of the Neotethys. Or-P: Prerift sequence with respect to opening of the Neotethys. UB (Upper Baxa Group), UR (Upper Rupa Group) and USL (Upper Shillong Group) are chronologically correlative and may represent pre- and/or syn-Pan African orogenic deposits. LB (Lower Baxa Group), MR (Middle Rupa Group) and LSL (Lower 1.75-1.55 Ga Shillong Group) are chronologically correlative and may represent a passive-margin sequence deposited during Crystalline basement breakup of Rodinia supercontinent. (A) DS: Daling-Shumar Group, possibly deposited during the eastern Ghats orogen related to the formation of Rodinia supercontinent.

W Bhutan Bhalukpong Kimin

U. Baxa U. Rupa MCT L. Baxa M. Rupa M. Rupa

Daling-Shumar L. Rupa 1.75–1.55 Ga (B) Crystalline Basement

Figure 14. (A) Schematic cross section showing possible lithologic correlations between Indian craton and the eastern Himalaya. MCT— Main Central thrust. (B) Possible along-strike variation of stratigraphic relationships across Bhutan and Arunachal. The lower Rupa Group appears to be missing along the Bhalukpong-Tawang traverse but is present in Bhutan and across the Kimin traverse, suggesting possible existence of paleotopography in the region.

390 Geological Society of America Bulletin, March/April 2010 Downloaded from gsabulletin.gsapubs.org on July 5, 2010 Eastern Himalaya et al., 2003). The differences in the timing of by the Paleogene shortening, the South Tibetan Together, the Lum La and Bomdila duplex sys- the Main Central thrust motion could be ex- detachment and Main Central thrust may have tems produced two antiforms bounding the Se La plained by either progressive eastward initiation variable older-over-younger and younger-over- synclinorium in the middle. In our reconstruc- of the Main Central thrust zone or, more likely, older relationships across the faults. In southern tions, the Greater Himalayan Crystalline Com- the variation of exposure levels of the Main Tibet directly north of Bhutan, the South Tibetan plex, Lesser Himalayan Sequence, and Tethyan Central thrust zone that record different slip his- detachment places Cretaceous strata over Himalayan Sequence were all originated from tory of the complex Main Central thrust zone. Greater Hima layan Crystalline Complex units the northern Indian margin section, including its The chronostratigraphy of the Lesser Hima- (Pan et al., 2004; Dai et al., 2008), whereas in crystalline basement and the Proterozoic to Cre- layan Sequence appears to vary along strike over Bhutan to the south, the South Tibetan detach- taceous cover sequence (Fig. 15E). relatively short distances in the eastern Himalaya. ment places Neoproterozoic and Cambrian In Bhutan, the Daling-Shumar Group, correla- strata over the Greater Himalayan Crystalline CONCLUSIONS tive to the lower Rupa Group (Figs. 3 and 14B), Complex. This relationship suggests that the is present. In contrast, the lower Rupa Group South Tibetan detachment cuts up section of its The eastern Himalaya experienced a series of appears to be missing along the Bhalukpong- hanging-wall strata in its northward transport di- magmatic events at ca. 1750 Ma, 825–878 Ma, Zimithang traverse. Finally, the Kimin- rection, and this relationship is inconsistent with 500 Ma, and 28–20 Ma. The fi rst three events Geevan traverse appears to preserve the lower normal-fault but consistent with thrust-fault are correlative to those in the Indian craton, and middle Rupa Group below the Main geometry. From the observations made along while the last event was associated with the Central thrust but is missing the upper Rupa the Bhalukpong-Zimithang traverse, where fo- Cenozoic development of the Himalaya dur- Group (Fig. 14B). The lack of lower Rupa Group liation development has completely transposed ing the India-Asia collision. Correlation of the along the Bhalukpong-Zimithang traverse may the original bedding of phyllite and slate in the magmatic events suggests that the Himalayan explain the dominance of augen gneiss involved Main Central thrust footwall, one may conclude units were derived from the Indian craton, and in the Cenozoic thrust belt, since the latter rep- that the foliation may not be used as a marker the formation of the eastern Himalaya was ac- resents the Precambrian basement of the Lesser surface for cross-section restoration because it complished by vertical stacking of basement- Himalayan Sequence and Indian craton. The lack was developed during rather than before Ceno- involved thrust sheets of the Indian cratonal of the upper member of the Rupa Group along zoic deformation (cf. Robinson et al., 2006). rocks. This correlation also rules out the pos- the Kimin-Geevan traverse indicates either the Based on these age constraints, we propose sibility that the high-grade rocks of the Hima- unit was eroded away after its deposition or the the following evolutionary history for the de- laya were derived from Tibetan middle crust Main Central thrust cuts down section laterally velopment of the eastern Himalaya (Fig. 15). via channel fl ow. The Main Central thrust in to the east from the Bhalukpong-Zimithang tra- To simplify our illustration, we assume fl at- the eastern Himalaya is broadly warped due to verse to the Kimin-Geevan traverse (Fig. 14B). lying beds in the northern Indian margin prior the presence of two large thrust duplexes in its Our 40Ar/39Ar mica ages between 7 Ma and to the India-Asia collision by neglecting that footwall. The 40Ar/39Ar thermochronology indi- 12 Ma in the Main Central thrust hanging wall Cambrian-Ordovician contraction (Fig. 15A). cates that the northern duplex was initiated at or are signifi cantly younger than those obtained Following Aikman et al. (2008), the north- prior to ca. 13 Ma, while the southern duplex mostly in the western Himalaya between ern Indian margin sequence experienced in- started at or prior to ca. 10 Ma. The differential 15 Ma and 25 Ma (Searle et al., 1999; Dézes tense isoclinal folding in the early Cenozoic cooling ages may result from out-of-sequence et al., 1999; Stephenson et al., 2001), but they (Fig. 15B), which caused crustal thickening and thrusting. The formation of the two duplexes are similar in age range to those from the strong modifi cation of the original pre-Cenozoic lasted until at least 6 Ma in the late Miocene Nepal and Bhutan Himalaya between 0.4 Ma stratigraphic architecture. Because the South and may have continued until the Pliocene. and 14 Ma (e.g., Catlos et al., 2001; Stüwe Tibetan detachment and Main Central thrust are Although the outcrop pattern indicates that the and Foster, 2001). rooted northward into the middle or lower crust minimum Cenozoic shortening is ~315 km, it of the northern Himalaya, these structures must is diffi cult to estimate the total crustal shorten- Cenozoic Evolution of the have cut the isoclinally folded basement and ing strain across the eastern Himalaya due to Eastern Himalaya cover rocks, producing complex juxtaposition great uncertainties in the number, geometry, relationships across the fault. At ca. 20–15 Ma and depths of detachment horizons below the The work of Aikman et al. (2008) suggests in Bhutan, and perhaps later in the Arunachal mountain belt, the original thickness of individ- that the Triassic to Cretaceous Tethyan Hima- Hima laya, motion on the Main Central thrust ual lithologic units and their spatial variation, layan Sequence in southeastern Tibet north of may have caused southward propagation of deformation mechanisms, their variations in our study area experienced intense folding and crustal thickening via ductile folding in its foot- time and space responsible for the development thrusting in the early Tertiary prior to ca. 44 Ma. wall. The presence of a major thrust ramp along of the eastern Himalaya, and fi nally out-of- Folding and the related cleavage development in the Main Central thrust allows transport of its sequence thrusting. Detailed analysis of meso- the fi ne-grained Tethyan Himalayan Sequence hanging-wall rocks from the lower to upper scopic fold geometry in the study area indicates units in the area have completely transposed the crust (Fig. 15C). During 15–10 Ma, the Tenga that the traditional line-balancing methods can original bedding during this early contractional thrust was initiated in the footwall of the Main overestimate as much as 20% of the total Hima- event (Yin et al., 1999). Because the Neogene Central thrust below the frontal part of the layan shortening. Also, because the Himalaya Main Central thrust and South Tibetan detach- Main Central thrust fl at (Fig. 15D). This was and northern Indian craton had experienced a ment were rooted into an already complexly fol lowed by the nearly coeval initiation of the signifi cant crustal shortening event in the early deformed orogen, they must have cut across Bomdila thrust and Lum La thrust duplex in Paleozoic (520–470 Ma), shortening estimated the folded Precambrian and Phanerozoic strata the Main Central thrust footwall and the Zimi- from balancing Precambrian strata represents a in the middle and lower crust. As the normal thang thrust in the Main Central thrust hanging combined effect of early Paleozoic and Ceno- stratigraphic sequence was severely modifi ed wall in an out-of-sequence fashion (Fig. 15E). zoic deformation.

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A 65–55 Ma Cam-K Pt1b-Pt3

Ar-Pt1a

B 55–25 Ma Future STD as an out-of-sequence thrust

Cam-K Pt1b-Pt3

Ar-Pt1a Future MCT as an out-of-sequence thrust

Pt -Pt C 20–15 Ma STD 1b 3 Pt1b-Pt3 Cam-K

Cam-K

Ar-Pt1a

MCTC Ar-Pt1a Pt1b-Pt3

Pt -Pt 1b 3 Ar-Pt 1a Future Tenga thrust

STD changes shear sense from D 15–13 Ma top-south to top-south motion STD Pt -Pt as its hanging wall moves Pt -Pt STD 1b 3 1b 3 northward across MCT-STD branch line Cam-K

Pt1b-Pt3 Ar-Pt1a MCTT

MCT Pt1b-Pt3 Pt -Pt Future Zimithang thrust Future Bomdila Tenga thrust 1b 3 Ar-Pt Future Lum La duplex system 1a thrust

Cam-K Se La synclinorium Gneissic foliation and cleavage that STD have transposed original bedding E 13–7 Ma STD

Zimithang thrust

S Cam-K LHS GHC MCTC GHC LHS MCT

Lum La thrust duplex system LHS Pt -Pt MCT Ar-Pt1a 1b 3

Bomdila thrust Tenga thrust

Figure 15. Cenozoic evolution of the eastern Himalaya. Lithologic units: Ar-Pt1a (older than 1750 Ma), Archean and Lower Paleoprotero- zoic metasedimentary and orthogneiss representing the basement of the Indian craton; Pt1b-Pt3 (1750–540 Ma), Upper Paleoproterozoic to Upper Proterozoic strata; Cam-K, Cambrian to Cretaceous strata. (A) Stage 1 (65–55 Ma): a highly simplifi ed stratigraphic framework of the northern Indian continental margin prior to the India-Asia collision that does not consider the effect of Cambrian-Ordovician contraction and Mesozoic extension on northern Indian margin stratigraphy. (B) Stage 2 (55–20 Ma): Intense Cenozoic contraction possibly involving the crystalline basement of northern Indian craton causing crustal thickening and modifi cation of the pre-Cenozoic crustal architecture. Future South Tibetan detachment (STD) and Main Central thrust (MCT) are rooted in this highly deformed middle crust, cutting isoclinally folded basement and cover rocks. (C) Stage 3 (20–15 Ma): Motion on the Main Central thrust causing southward propagation of crustal thickening in the Himalaya. Ductile folding may have accommodated its footwall deformation. The presence of a major thrust ramp along the Main Central thrust allows transport of middle- and lower-crustal rocks to the upper-crustal levels. (D) Stage 4 (15–13 Ma): Initiation and subsequent development of the Bomdila thrust in the Main Central thrust footwall and a duplex structure producing an antiform over the Bomdila thrust hanging wall. (E) Stage 5 (13–7 Ma): Development of the Lum La thrust duplex due to out- of-sequence thrusting north of the Bomdila thrust, causing the formation of an antiform above. Together, the Lum La and Bomdila duplex systems produced the Se La synclinorium. Following the traditional defi nition, the Greater Himalayan Crystalline Complex (GHC) lies in the Main Central thrust hanging wall, the Lesser Himalayan Sequence (LHS) lies in the Main Central thrust footwall, and the Tethyan Himalayan Sequence (THS) lies in the hanging wall of the South Tibet detachment (STD).

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