Geometry and crustal shortening of the Himalayan fold-thrust belt ...
Transcript of Geometry and crustal shortening of the Himalayan fold-thrust belt ...
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ABSTRACT
We present a new geologic map of eastern and central Bhutan and four balanced cross sections through the Himalayan fold-thrust belt. Major structural features, from south to north, include: (1) a single thrust sheet of Sub himalayan rocks above the Main Fron-tal thrust; (2) the upper Lesser Himalayan duplex system, which repeats horses of the Neoproterozoic–Cambrian(?) Baxa Group below a roof thrust (Shumar thrust) carrying the Paleo protero zoic Daling-Shumar Group; (3) the lower Lesser Himalayan duplex system, which repeats horses of the Daling-Shumar Group and Neoproterozoic–Ordovician(?) Jaishidanda Formation, with the Main Cen-tral thrust (MCT) acting as the roof thrust; (4) the structurally lower Greater Hima layan section above the MCT with overlying Tethyan Himalayan rock in stratigraphic contact in central Bhutan and structural contact above the South Tibetan detachment in eastern Bhu-tan; and (5) the structurally higher Greater Himalayan section above the Kakhtang thrust. Cross sections show 164–267 km short-ening in Subhimalayan and Lesser Himalayan rocks, 97–156 km structural overlap across the MCT, and 31–53 km structural overlap across the Kakhtang thrust, indicating a total of 344–405 km of minimum crustal shorten-ing (70%–75%). Our data show an eastward continuation of Lesser Himalayan duplex-ing identifi ed in northwest India, Nepal, and Sikkim, which passively folded the overlying Greater Himalayan and Tethyan Himalayan sections. Shortening and percent shorten-ing estimates across the orogen, although minima, do not show an overall eastward increase, which may suggest that shortening variations are controlled more by the origi-nal width and geometry of the margin than by external parameters such as erosion and convergence rates.
INTRODUCTION
Collision between the Indian and Eurasian plates, which started in the Eocene (Yin and Harrison, 2000; Leech et al., 2005; Guillot et al., 2008) and continues today, produced the Tibetan-Himalayan orogenic system, our best example of modern continent-continent colli-sion. Global positioning system (GPS) motion rates and neotectonic deformation rates show that approximately one-half (~20 mm/yr) of the convergence between these two plates may be accommodated through crustal shortening in the Himalayan fold-thrust belt (DeMets et al., 1994; Bilham et al., 1997; Larson et al., 1999; Lave and Avouac, 2000; Mugnier et al., 2003). This contraction of the northern Indian margin is ac-commodated by a series of south-vergent thrust faults, which detach and repeat the Proterozoic to Paleocene sedimentary cover (Gansser, 1964; Powell and Conaghan, 1973; LeFort, 1975; Mattauer, 1986; Hauck et al., 1998; Hodges, 2000; DeCelles et al., 2002; Murphy and Yin, 2003; Yin, 2006; Yin et al., 2009).
Determining the range of shortening magni-tudes along the length of the Himalayan fold-thrust belt is essential for testing predictions of how convergence is accommodated through-out the orogen, and for estimating the budget of crustal material into the orogenic system. Predictions of fi rst-order shortening variations along-strike include: (1) shortening and percent shortening should increase from west to east, as a result of postcollisional, counterclockwise rotation of India (Patriat and Achache, 1984; Dewey et al., 1989) or an eastward increase in convergence rates (Guillot et al., 1999) and ero-sion rates (Grujic et al., 2006; Yin et al., 2006); (2) shortening magnitude should mimic the width of the Tibetan Plateau measured in an arc-normal direction (DeCelles et al., 2002), or (3) shortening should be the greatest at the center of the orogen and decrease to the east and west (classic “bow-and-arrow” model of Elliott [1976]). To date, several studies have estimated Himalayan shortening with regional-
scale balanced cross sections. The majority of shortening estimates come from the central and western portions of the orogen, in Paki-stan, northwest India, and central and western Nepal (Coward and Butler, 1985; Srivastava and Mitra , 1994; DeCelles et al., 1998, 2001; Robinson et al., 2006), where much of the previous work on Himalayan stratigraphy and structure has been focused (e.g., Srivastava and Mitra, 1994; Hodges et al., 1996; Upreti, 1996; Vannay and Hodges, 2003; Searle et al., 1997; DeCelles et al., 2000, 2001; Vannay and Grase-mann, 2001; Richards et al., 2005; Robinson et al., 2006). These shortening estimates range between ~400 and 900 km (DeCelles et al., 2002; Robinson et al., 2006) and show a sys-tematic increase from the western syntaxis to the midpoint of the Himalayan arc.
However, comparable shortening estimates for the eastern quarter of the Himalayan oro-gen are either lacking in key areas or based on preliminary efforts. The eastern Himalaya occu-pies a key position along the arc for testing the predictions of systematic shortening variation listed above, giving shortening estimates here a greater impact. However, in Sikkim, Bhutan, and Arunachal Pradesh (Fig. 1), accessibility for fi eld research has only come recently, and mini-mal geologic mapping and stratigraphic data are available, particularly for the frontal Lesser Himalayan portion of the fold-thrust belt (e.g., Acharyya, 1980; Raina and Srivastava, 1980; Gansser, 1983; Bhargava, 1995; Kumar, 1997; Yin et al., 2006), inhibiting a rigorous evaluation of fold-thrust belt geometry. As a result, only preliminary studies illustrating geometry and estimating shortening are available from Sikkim (Mitra et al., 2010), Bhutan (McQuarrie et al., 2008), and Arunachal Pradesh (Yin et al., 2010).
In Bhutan (Fig. 1), most recent work has fo-cused on determining the metamorphic and defor ma tional history of the Greater Himalayan section, above the Main Central thrust (MCT) (Swapp and Hollister, 1991; Grujic et al., 1996, 2002; Davidson et al., 1997; Daniel et al., 2003; Hollister and Grujic, 2006; Long and McQuarrie,
For permission to copy, contact [email protected]© 2010 Geological Society of America
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GSA Bulletin; Month/Month 2010; v. 1xx; no. X/X; p. 1–21; doi: 10.1130/B30203.1; 8 fi gures; 3 tables.
†E-mail: [email protected]
Geometry and crustal shortening of the Himalayan fold-thrust belt, eastern and central Bhutan
Sean Long1†, Nadine McQuarrie1, Tobgay Tobgay1, and Djordje Grujic2
1Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA2Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada
Long et al.
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2010). However, recent studies that present new geologic mapping between the MCT and Main Frontal thrust (MFT) in eastern Bhutan, and an extensive U-Pb detrital zircon data set (McQuarrie et al., 2008; Long et al., 2010), have built a detailed stratigraphy for Subhimalayan and Lesser Himalayan rocks. The combined re-sults of these foreland- and hinterland-focused studies facilitate, for the fi rst time in Bhutan, a de-tailed study focused on the geometry, kine matics, and shortening of the Bhutan fold-thrust belt.
The main objective of this paper is to provide accurate estimates of shortening through the Himalayan fold-thrust belt in Bhutan. To ac-complish this, we introduce a new, detailed geo-logic map of eastern and central Bhutan, based on mapping carried out in three separate fi eld seasons, and four regional-scale, deformed and
retrodeformed, balanced cross sections, which illustrate the geometry of the fold-thrust belt, and provide minimum shortening estimates. These new shortening estimates fi ll a signifi cant data gap for the eastern Himalaya, and allow for the fi rst along-strike comparison of shortening estimates across the full length of the orogen. The second objective of this paper is to present an updated compilation of shortening and per-cent shortening estimates along the Himalayan arc, in order to test the predictions listed above of systematic variation along the orogen.
GEOLOGIC BACKGROUND
Heim and Gansser (1939) and Gansser (1964) originally divided the Himalayan fold-thrust belt into four tectonostratigraphic zones
(Fig. 1), which represent distinct structural packages that have been imbricated and thrust to the south since the collision of India and Asia (e.g., Gansser, 1964; Powell and Conaghan, 1973; LeFort, 1975; Mattauer, 1986; Hodges, 2000; DeCelles et al., 2002; Murphy and Yin, 2003; Yin, 2006). From south to north, these are the Subhimalayan, Lesser Himalayan, Greater Himalayan, and Tethyan (or Tibetan) Hima-layan zones. The Subhimalayan zone represents synorogenic rocks, and the Lesser Himalayan, Greater Himalayan, and Tethyan Himalayan zones represent packages of pre-Himalayan sedi-mentary and igneous rocks of Greater India.
The Lesser Himalayan zone consists of clas-tic and carbonate sedimentary rocks originally deposited on the northern margin of the Indian craton (Gansser, 1964; Schelling and Arita,
STD
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Paro Formation
Quaternary sediment
Lesser Himalaya
Greater Himalaya:Higher structural level, leucogranite
Chekha Formation
Tethyan (Tibetan) Himalaya:
Subhimalaya (Siwalik Group)
Higher structural level, undifferentiated
Lower structural level, orthogneiss unit
Lower structural level, metasedimentary unit
Paleozoic and Mesozoic, undiff.
Maneting Formation
Lesser and Subhimalaya:
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China (Tibet)
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20°N
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Figure 1. Simplifi ed geologic map of Bhutan and surrounding region, after Gansser (1983), Bhargava (1995), Grujic et al. (2002), and our own mapping. Locations of the four tectonostratigraphic zones shown, along with bounding structures. Area of detailed geologic map (Fig. 2) outlined, along with locations of lines of section for four balanced cross sections (Fig. 3). Upper left inset shows generalized geo-logic map of central and eastern Himalayan orogen (modifi ed from Gansser, 1983). Structural detail left out of Tethyan Himalayan section north of Bhutan; map patterns of NE-striking, crosscutting normal faults northwest of Lingshi syncline (Yadong cross structure on Fig. 1 of Grujic et al. [2002]) are simplifi ed. Abbreviations: (1) inset: GH—Greater Himalaya; LH—Lesser Himalaya; TH—Tethyan Himalaya: (2) structures from north to south: STD—South Tibetan detachment; KT—Kakhtang thrust; MCT—Main Central thrust; MBT—Main Boundary thrust; MFT—Main Frontal thrust; (3) windows and klippen from west to east: LS—Lingshi syncline; PW—Paro window; TCK—Tang Chu klippe; UK—Ura klippe; SK—Sakteng klippe; LLW—Lum La window (location from Yin et al., 2009).
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1991; Upreti, 1999; Yin, 2006). The Lesser Himalayan zone contains units as old as Paleo-proterozoic (Parrish and Hodges, 1996; Upreti, 1999; DeCelles et al., 2000; Richards et al., 2005; McQuarrie et al., 2008; Long et al., 2010) that are exposed across the entire orogen (Kohn et al., 2010). Younger Lesser Himalayan strata include Mesoproterozoic rocks local to Nepal (Martin et al., 2005; Robinson et al., 2006), Neoproterozoic to Cambrian and locally Ordo-vician rocks in northwest India, Nepal, Sikkim, Bhutan, and Arunachal Pradesh (Valdiya, 1995; Myrow et al., 2003; Hughes et al., 2005; Rich-ards et al., 2005; Shukla et al., 2006; McQuarrie et al., 2008; Schopf et al., 2008; Long et al., 2010) and Permian rocks across most of the length of the orogen (Brookfi eld, 1993; DeCelles et al., 2001; Najman et al., 2006; Robinson et al., 2006; Yin, 2006; Long et al., 2010, in press). During Himalayan orogenesis, much of the Lesser Himalayan section was metamor-phosed to greenschist facies (Gansser, 1964; Schelling and Arita, 1991; Robinson et al., 2003), and Lesser Himalayan rocks were inter-nally deformed and thrust southward over the Subhimalayan zone across the Main Boundary thrust (MBT) (Gansser, 1964).
The Greater Himalayan zone consists of upper amphibolite facies (Gansser, 1964; LeFort , 1975; Harrison et al., 1997) meta igneous and metasedi mentary rocks, and synorogenic intrusive igneous rocks, which structurally overlie the Lesser Himalayan zone across the Main Central thrust (MCT) (Heim and Gansser, 1939; Gansser, 1964). Sedimentary protoliths of Greater Himalayan metamorphic rocks range between Neoproterozoic and early Paleozoic in age (Parrish and Hodges, 1996; DeCelles et al., 2000; Gehrels et al., 2003; Martin et al., 2005; Myrow et al., 2009; Long and McQuarrie , 2010), and Cambrian–Ordovician orthogneiss units are present in the Greater Himalayan section throughout the orogen (Stocklin and Bhattarai , 1977; Stocklin, 1980; Parrish and Hodges, 1996; DeCelles et al., 2000; Gehrels et al., 2003; Martin et al., 2005; Cawood et al., 2007; Long and McQuarrie, 2010). Evidence for widespread Cambrian–Ordovician metamor-phism and igneous activity has been interpreted as early Paleozoic orogenic activity that affected the Greater Himalayan section (DeCelles et al., 2000; Gehrels et al., 2003; Cawood et al., 2007). Since the contact between the Greater Himala-yan and Lesser Himalayan sections is always the MCT, the original paleogeographic relation-ship between these two tectonostratigraphic zones is in question.
The Tethyan Himalayan zone consists of Neoproterozoic to Eocene sedimentary rocks that in most places sit structurally above the
Greater Himalayan zone across a top-to-the-north–sense shear zone and/or one or mul-tiple top-to-the-north–sense detachment faults called the South Tibetan detachment system (Burg, 1983; Burchfi el et al., 1992). However, several studies throughout the Himalaya have interpreted a stratigraphic contact at the base of the Tethyan Himalayan section, where these rocks are preserved above lower-grade Greater Himalayan rocks in the frontal, southern por-tions of the orogen (Stocklin, 1980; Gehrels et al., 2003; Robinson et al., 2003; Long and McQuarrie, 2010). Stratigraphic evidence for Cambrian–Ordovician uplift, exhumation, and coarse-clastic deposition in northwest India and Nepal (Gehrels et al., 2003, and refer-ences therein) indicates that early Paleozoic orogenic activity also affected the Tethyan (or Tibetan) Himalayan section. This tectonic ac-tivity was succeeded by a passive margin set-ting on northern Greater India, represented by an Ordovician to Carboniferous shelf sequence that accumulated on the southern margin of the Paleotethys Ocean (Gaetani and Garzanti, 1991; Brookfi eld, 1993; Garzanti, 1999), and a Permian to Mesozoic passive margin sequence that accumulated on the southern margin of the Neotethys Ocean, which postdates the late Paleo zoic breakup of Greater India and the northward migration of crustal fragments toward Asia (Yin and Harrison, 2000). South-vergent deformation of the Tethyan Himalayan zone commenced during the Eocene with the initiation of northward subduction of the In-dian plate under Asia (Powell and Conaghan, 1973; Coward and Butler, 1985; Mattauer, 1986; Ratschbacher et al., 1994; Yin and Harri-son, 2000; Ding et al., 2005; Leech et al., 2005; Aikman et al., 2008; Guillot et al., 2008).
The Subhimalayan zone consists of the Mio-cene to Pliocene Siwalik Group (Gansser, 1964; Tukuoka et al., 1986; Harrison et al., 1993; Quade et al., 1995; Burbank et al., 1996; DeCelles et al. 1998, 2001, 2004; Ojha et al., 2000; Huyghe et al., 2005; Robinson et al., 2006), and repre-sents foreland basin deposits shed off of the ac-tively growing Himalayan orogen. The Siwaliks are bound at their base by the MFT, which coin-cides with the present Hima layan topographic front, and bound at their top by the MBT. South of the MFT, modern Hima layan foreland basin sediments onlap onto cratonic rocks of north-ern India. While synorogenic units as old as Eocene have been recognized in Nepal , north-west India, and Arunachal Pradesh, they are structurally above the MBT and are mapped as part of the Lesser Himalayan zone (Acharyya, 1980; DeCelles et al., 1998, 2001, 2004; Rich-ards et al., 2005; Robin son et al., 2006; Yin et al., 2006, 2009).
MAPPING METHODS
This study presents new geologic mapping, which was undertaken in the fi eld at a scale of 1:50,000, and is shown at 1:250,000 on Figure 23. Mapping was focused between the MFT and the Kakhtang thrust. Map data from the structurally higher Greater Himalayan section above the Kakhtang thrust are projected from the geologic maps of Gansser (1983), Gokul (1983), and Bhargava (1995) (Fig. 2*). Map data were collected in four ~100-km-long, north-south traverses (Fig. 2), which provide data for four balanced cross sections (Fig. 3 [see footnote 3]. The four traverses, listed from east to west, are: (1) along roads south and north of the town of Trashigang; (2) along the Kuru Chu (note: chu means river), which was along roads north of 27°10′N and trails south of this latitude; (3) between the Kuru Chu and Bhumtang Chu, along a road be-tween 27°40′N and 27°20′N, and along a trail that meets the Manas Chu at its southern end; and (4) on roads, north and south of the town of Trongsa, which pass through the town of Shem gang, and end in the south at the town of Geylegphug. Map data from these traverses were projected onto the Trashigang (A–A′), Kuru Chu (B–B′), Bhumtang Chu (C–C′), and Mangde Chu (D–D′) cross sec-tions, respectively (Fig. 3).
In addition, we collected map data in two along-strike transects, one on a trail between the Kuru Chu and the town of Pemagatshel, and another on the road connecting the towns of Trashigang, Mongar, Ura, Jakar, and Trongsa (Figs. 1 and 2). Our mapping was integrated with published geologic maps of Bhutan (Gansser, 1983; Gokul, 1983; Bhar-gava, 1995; Grujic et al., 2002), to help trace contacts and structures between traverses (see Fig. 2 caption).
The level of rock exposure in eastern Bhu-tan varies signifi cantly with elevation and the presence or absence of road or trail cuts. In general, south of ~27°00′N, lower eleva-tions and wetter climates resulted in much heavier vegetation cover, and discontinu-ous exposures of ~5- to 20-m-thick sections spaced apart ~250–500 m on average. Two exceptions to this were on the road north of Samdrup Jongkhar and the road north of Geyleg phug, where roadcuts permitted ex-posures spaced every ~100–200 m. In gen-eral, north of 27°00′N, vegetation cover was much more sparse, and exposures were more closely spaced (~100–200 m).
*Figures 2 and 3 are on a separate sheet accompa-nying this issue.
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BHUTAN TECTONOSTRATIGRAPHY
Subhimalayan Zone
The Siwalik Group coarsens upward from siltstone and claystone to sandstone and con-glomerate, and has been divided into lower, middle, and upper members (Nautiyal et al., 1964; Jangpangi, 1974; Gansser, 1983; Lakshminarayana and Singh, 1995; McQuarrie et al., 2008; Long et al., 2010). Near Samdrup Jongkhar (Figs. 1 and 2), all three members are exposed, with a combined thickness of 5.6 km (Long et al., 2010) (Fig. 4). Along the Manas Chu (Fig. 2), only the lower and middle members are exposed, with a total thickness of 2.3 km.
Lesser Himalayan Zone
In eastern and central Bhutan, the Lesser Hima layan zone consists of six map units, with a combined thickness between 8 and 19 km (Fig. 4). Lesser Himalayan units can be divided into two stratigraphic successions: (1) the Paleo protero zoic lower Lesser Himalayan sec-tion, and (2) the Neoproterozoic–Paleozoic upper Lesser Himalayan section. Stratigraphy and deposition age constraints of Lesser Hima-layan map units in Bhutan are discussed in de-tail in Long et al. (2010.)
Lower Lesser Himalayan SectionThe lower Lesser Himalayan section consists
of the Paleoproterozoic Daling-Shumar Group, which displays a consistent two-part stratig-raphy of quartzite of the Shumar Formation be-low schist, phyllite, and quartzite of the Daling Formation (McQuarrie et al., 2008; Long et al., 2010). Both units are metamorphosed to lower greenschist facies (Gansser, 1983). The lower contact is always the Shumar thrust, which places the Daling-Shumar Group over the Baxa Group (Ray et al., 1989; Ray, 1995; McQuarrie et al., 2008). The upper contact is an uncon-formity with the Jaishidanda Formation (Long et al., 2010). An upper stratigraphic contact with the Baxa Group is not observed, although this contact is documented in Sikkim (Bhattacharyya and Mitra, 2009; Mitra et al., 2010).
The Shumar Formation consists of thick-bedded quartzite, with schist and phyllite inter beds. The Shumar Formation is generally 1–2 km thick, but a 6-km-thick section is local to the Kuru Chu valley (Long et al., 2010). The Daling Formation overlies the Shumar Forma-tion across a gradational contact, consists of green phyllite and schist with quartzite interbeds, and is 2.2–3.2 km thick. Granitic orthogneiss bodies are observed in variable stratigraphic
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1998; Lin et
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Figure 4. Column showing tec-tonostratigraphy of central and eastern Bhutan. The four Hima layan tectonostratigraphic zones and positions of major bounding structures are shown. Lesser Himalayan unit ages from McQuarrie et al. (2008) and Long et al., (2010) , unit ages for structurally lower Greater Himalayan section and Tethyan (or Tibetan) Himalayan units below Kakhtang thrust from Long and McQuarrie (2010), unit age range for Tethyan (or Tibetan) Himalayan units above Kakhtang thrust from Gansser (1983). Unit thickness range in km shown on right-hand side, from this study, McQuarrie et al. (2008), and Long et al. (2010), or as cited. See Figure 1 for structure abbreviations.
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positions within the Daling-Shumar Group (Fig. 2). Based on intrusive contact relation-ships, the orthogneiss bodies are interpreted as granite intrusions originally emplaced in the Daling-Shumar Group (Long et al., 2010).
Upper Lesser Himalayan SectionThe Neoproterozoic–Paleozoic upper Lesser
Himalayan section consists of the four map units, the Baxa Group, Jaishidanda Formation, Diuri Formation, and Gondwana succession (Fig. 4).
The Neoproterozoic–Cambrian(?) Baxa Group consists of coarse-grained to conglomer-atic quartzite, with common lenticular bedding and trough cross-bedding, interbedded with dark-gray phyllite and dolomite (McQuarrie et al., 2008; (Long et al., 2010). Upper strati-graphic contacts with the Gondwana succession and Diuri Formation are observed on the Manas Chu and Kuru Chu transects, respectively (Fig. 2). The Baxa Group is 1.5–2.6 km thick (Along-Strike Variability of Lesser Himalayan Duplexing section).
The Neoproterozoic–Ordovician(?) Jaishi-danda Formation unconformably overlies the Daling-Shumar Group under the MCT, and is not exposed within the upper Lesser Himalayan section below the Shumar thrust (Figs. 2 and 4) (Long et al., 2000; Bhargava, 1995). The Jaishi-danda Formation consists of biotite-rich, locally garnet-bearing schist interbedded with biotite-rich quartzite, and ranges in thickness from 500 to 1700 m.
The Diuri Formation consists of pebble-clast diamictite (Jangpangi, 1974; Gansser, 1983; Tangri, 1995; McQuarrie et al., 2008; Long et al., 2000). The Diuri Formation is in strati-graphic contact above the Baxa Group in the southern Kuru Chu valley (Fig. 2), but all other contacts are tectonic. The Diuri Formation is 2.4–3.0 km thick, and pinches out east of the Manas Chu (Fig. 2). A ca. 390 Ma youngest detrital zircon (DZ) peak indicates a Devonian maximum deposition age (Long et al., 2010).
The Gondwana succession consists of sand-stone, carbonaceous siltstone and shale, and coal (Gansser, 1983; Joshi, 1995; Lakshminarayana, 1995; McQuarrie et al., 2008; Long et al., 2010). The unit is 1.2–2.5 km thick in southeast Bhutan (Long et al., 2010), and a 500-m-thick section is exposed along the Manas Chu, in stratigraphic contact above the Baxa Group (Fig. 2). The Gondwana succession yields Permian fossils (Joshi, 1989, 1995; Lakshminarayana, 1995).
Greater Himalaya
The Greater Himalayan zone is divided into a lower structural level above the MCT and be-low the Kakhtang thrust, and a higher structural
level above the Kakhtang thrust and below the South Tibetan detachment (Figs. 1, 2, and 4) (Gansser, 1983; Grujic et al., 2002). The struc-turally higher section is at least 13 km thick, and consists of migmatitic orthogneiss and metased-imentary rocks and Miocene leucogranite (Figs. 2 and 4) (Gansser, 1983; Swapp and Hollister, 1991; Davidson et al., 1997; Grujic et al., 2002).
The structurally lower Greater Himalayan section consists of a lower orthogneiss unit and an upper metasedimentary unit (Long and McQuarrie , 2010) (Figs. 1 and 2). Together they are between 5.3 and 10.5 km thick (Figs. 3 and 4). In eastern Bhutan, both units display partial melt textures (granite-composition leuco somes) throughout the entire section (Grujic et al., 1996, 2002; Davidson et al., 1997; Daniel et al., 2003). However, south of Shemgang in central Bhutan (Figs. 1 and 2), beneath an erosional remnant of Tethyan Himalayan rocks, partial melt textures are only observed within the lower part of the orthogneiss unit, and much of the Greater Hima-layan section was deformed at temperatures be-tween ~450° and 500 °C (Long and McQuarrie, 2010). Throughout Bhutan, Greater Himalayan rocks just above the MCT are distinguished from Lesser Himalayan rocks below by the presence of kyanite, sillimanite, and granitic leucosomes (Grujic et al., 2002; Daniel et al., 2003; Long and McQuarrie, 2010).
The Greater Himalayan orthogneiss unit is 1.5–8.0 km thick, and consists of granitic ortho-gneiss with metasedimentary intervals <200 m thick. The Greater Himalayan metasedimentary unit is 0.5–6.7 km thick, and consists of quartz-ite, schist, and paragneiss. Circa 500 and 460 Ma youngest detrital zircon (DZ) peaks obtained from Greater Himalayan quartzite near Shem-gang indicate that much of the Greater Hima-layan metasedimentary unit in central Bhutan can only be as old as Cambrian–Ordovician (Long and McQuarrie, 2010). A ca. 900 Ma youngest detrital zircon (DZ) peak obtained from Greater Himalayan quartzite near Lhuentse (Figs. 1 and 2) indicates a Neoproterozoic lower deposition age for part of the section (Long and McQuarrie, 2010).
Tethyan Himalaya
Five isolated exposures of Tethyan Himala-yan metasedimentary rocks are mapped on top of Greater Himalayan rocks in the axes of syn-clines (Fig. 1) (Gansser, 1983; Bhargava, 1995; Kellett et al., 2009). The two westernmost ex-posures contain Paleozoic to Mesozoic rocks above a basal quartzite unit called the Chekha Formation (Gansser, 1983; Bhargava, 1995; Tangri and Pande, 1995). The Chekha Forma-tion lacks fossils, and is mapped stratigraphi-
cally below fossiliferous Cambrian units in the Tang Chu exposure (Bhargava, 1995; Tangri and Pande, 1995; Myrow, 2005; McKenzie et al., 2007), which has led to an inferred Neoprotero-zoic deposition age.
The three Tethyan Himalayan exposures in central and eastern Bhutan (Shemgang, Ura, and Sakteng) contain the Chekha Formation, and the Shemgang exposure contains the over-lying Maneting Formation (Figs. 1 and 2). The Chekha Formation is 2.2–4.0 km thick (Figs. 3 and 4), and consists of thick-bedded, locally conglomeratic quartzite interbedded with biotite-muscovite-garnet schist. The Maneting Formation is at least 1.0 km thick, and consists of graphitic, biotite-garnet phyllite (Tangri and Pande, 1995; Long and McQuarrie, 2010). A ca. 460 Ma youngest DZ peak obtained from Chekha quartzite near Shemgang indicates an Ordovician maximum deposition age (Long and McQuarrie, 2010), and suggests along-strike variation in the age of the oldest Tethyan Himalayan strata. Differing age estimates be-tween west-central and central Bhutan could indicate either: (1) structural complication that has telescoped the Tethyan Himalayan section, or (2) discrepancies in Tethyan Himalayan stra-tigraphy as currently defi ned, which have given the same name (Chekha Formation) and strati-graphic description to both Precambrian and Ordovician (or younger) quartzite.
Four of the fi ve erosional remnants of Tethyan Himalayan rock have been interpreted as klip-pen above the South Tibetan detachment (Grujic et al., 2002). However, because no fi eld observa-tions were available at that time, a fault contact at the base of the Shemgang exposure was queried on the map of Grujic et al. (2002). Recent map-ping in the Shemgang region indicates that the Chekha Formation is in interfi ngering depo-sitional contact above the Greater Himalayan metasedimentary unit (Long and McQuarrie, 2010), similar to original mapping by Gansser (1983) and Bhargava (1995) (Figs. 1 and 2).
BALANCED CROSS SECTIONS
Methods
Four balanced cross sections were constructed based on fi eld data projected from four north-south traverses (Figs. 2 and 3). From east to west, these are the Trashigang (A–A′), Kuru Chu (B–B′), Bhumtang Chu (C–C′), and Mangde Chu (D–D′) cross sections. Deformed sections are shown at 1:300,000 scale, and restored sec-tions are shown at 1:600,000 scale on Figure 3 (note different scale than Fig. 2). The lines of section are oriented N-S, which is parallel to the principal direction of Himalayan shortening
Long et al.
6 Geological Society of America Bulletin, Month/Month 2010
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in Bhutan, as indicated by the regional strike of structures and N- and S-trending mineral stretch-ing lineation (Fig. 5, stereonet O). Cross sections are constrained by surface data and earthquake seismology (Ni and Barazangi, 1984; Pandey et al., 1999; Mitra et al., 2005; Schulte-Pelkum et al., 2005) and seismic-refl ection constraints (Hauck et al., 1998) that suggest an average dip of 4°N for the basal décollement, the Main Hima layan thrust (Fig. 3, #I).
Line lengths of thrust sheets measured on deformed sections were matched on restored sections (e.g., Dahlstrom, 1969). Apparent dips were calculated from surface data and pro-jected along-strike to their position on the cross section. The orientations of fold axial planes were determined by bisecting the interlimb angle at fold hinges, and most axial planes were modeled as kink surfaces (e.g., Suppe, 1983). Areas of cross sections were divided into dip domains, based on the average appar-ent dips of surface data, and dividing lines be-tween adjacent dip domains were also treated as kink-type fold hinges. No attempt was made to incorporate outcrop and smaller-scale fold-ing, or to account for ductile deformation in Greater Himalayan rocks.
The main unknowns in the balancing pro-cess are the positions of hanging-wall cutoffs of Subhimalayan, Lesser Himalayan, and Greater Himalayan units that have passed through the erosion surface. We used conservative geome-tries that minimize shortening in all cases, which involved placing hanging-wall cutoffs just above the erosion surface in the case of Subhimalayan and most Lesser Himalayan units, or just be-yond a unit’s southernmost exposure in the case of the structurally lower Greater Himalayan sec-tion. At the north end of the restored sections, we interpret that the northernmost footwall ramp through the Daling-Shumar Group marks the northern extent of Lesser Himalayan units and the permissible southern extent of the lower Greater Himalayan section (Fig. 3, #XIV). Jus-tifi cations for individual decisions on all cross sections are annotated on Figure 3.
Structural Zones
Main Frontal Thrust SheetThe map pattern of the Siwalik Group varies
signifi cantly from east to west, from an ~8 km N-S exposure in southeast Bhutan, to a ~4 km N-S exposure at and west of the Manas Chu, and no exposure near Geylegphug (Fig. 2). The southern contact of the Siwaliks with Quater-nary sediment, which covers the MFT, was drawn at the slope break between the foothills and the fl at Brahmaputra plain. The location of the MFT is interpreted just to the south of
this slope break, except where located by off-set terraces near Geylegphug (Gansser, 1983) (Fig. 2). In general, the Siwalik Group exhibits dips aver aging between 35° and 50°N. We ob-served no structural repetition of the three-part Siwalik Group stratigraphy, and thus interpret the Sub himalayan zone as one thrust sheet, up-lifted along the MFT (Fig. 3). The thickness of this thrust sheet varies between 2.3 and 6.7 km, and thins from east to west, in accord with the westward-narrowing map pattern. A lack of southward dips at the southern end of Siwalik Group exposures indicates that the hanging-wall cutoff has passed through the erosion surface on all four sections (Fig. 3, #III).
Diuri Formation and Gondwana Succession Thrust Sheets
In southeast Bhutan, a 2- to 10-km-wide (N-S) exposure of the Gondwana succession is observed in thrust contact above the Siwalik Group across the MBT. The Gondwana succes-sion is in thrust contact beneath the Diuri For-mation, which is observed in a 4- to 8-km-wide (N-S) exposure, in thrust contact beneath the Baxa Group (Fig. 2). In map pattern, the surface exposures of both the Gondwana succession and Diuri Formation merge to the west, pinch-ing out between the Kuru Chu and Bhumtang Chu transects (Fig. 2). The Gondwana succes-sion carried in the MBT hanging wall is steeply north dipping (40°–50° average), and is 2.5 km thick on the Trashigang cross section (Fig. 3A) and 1.2 km thick on the Kuru Chu cross section (Fig. 3B). The extra length of the Gondwana succession shown above the erosion surface is necessary to align the northern end of the thrust sheet with footwall ramps through the Diuri Formation and Baxa Group on restored sections (Fig. 3, #1 and #10).
On the Trashigang cross section (Fig. 3A), the Diuri Formation is folded into a syncline with 40°N and 40°S average limb dips, and is at least 2.4 km thick. One horse of the Gond-wana succession and one horse of the Baxa Group are interpreted to fi ll space under the Diuri Formation, and provide a mechanism for passively folding the overlying thrust sheet. On the Kuru Chu cross section (Fig. 3B), the Diuri Formation dips 40°N on average, and is 3.0 km thick. On both the Trashigang and Kuru Chu cross sections, we interpret that the length of eroded Diuri Formation section that has passed through the erosion surface must equal the total restored length of duplexed Baxa Group horses (see Along-Strike Variability of Lesser Hima-layan Duplexing section below), indicating that the majority of the original length of the Diuri thrust sheet has been removed by erosion (Fig. 3, #2 and #12).
Upper Lesser Himalayan DuplexAcross southeastern and south-central Bhu-
tan, the Baxa Group is exposed over a 16- to 29-km-wide N-S region that displays signifi cant along-strike variation in width. At the southern extent of exposure, the Baxa Group is in thrust contact over the Diuri Formation in southeastern Bhutan, and over the Siwaliks across the MBT west of the Manas Chu (Fig. 2). At the northern extent, lower Lesser Himalayan rocks are thrust over the Baxa Group across the Shumar thrust (Figs. 6A and 6B) (Ray, 1989).
Klippen of the Daling Formation in fl at-on-fl at thrust contact over the Baxa Group are present on high ridgetops in the Kuru Chu val-ley (Figs. 2 and 6B). These klippen require that the lower Lesser Himalayan thrust sheet in the hanging wall of the Shumar thrust extends at least as far south as the southern limit of Baxa Group exposure. Observations of thrust contacts within the Baxa Group (Fig. 6D) and faults that place the Baxa Group over lowermost Diuri Formation on the southern Kuru Chu transect indicate that the same Baxa Group section is being structurally repeated (Figs. 2 and 3B). The Shumar thrust extends over multiple Baxa Group thrust sheets in the southern Kuru Chu valley (Fig. 2), which indicates that the structur-ally repeated Baxa Group sections are a duplex system that feeds slip into the Shumar thrust, defi ning it as a roof thrust. We call this duplex system the upper Lesser Himalayan duplex. Kinematic indicators observed in Baxa Group rocks include shear cleavage in phyllite inter-beds (Figs. 6C and 6D), and consistently show a top-to-the-south sense of motion. We defi ne shear cleavage as a C-type, shear-band cleavage (Passchier and Trouw, 1998) with secondary tectonic foliations (S surfaces) gently inclined relative to bedding planes or primary tectonic foliation (C surfaces).
From east to west, the Trashigang, Kuru Chu, Bhumtang Chu, and Mangde Chu tran-sects expose continuous 11-, 6-, 7-, and 12-km-thick Baxa Group sections. However, based on the following stratigraphic, struc-tural, and geomorphic observations, we argue that these thick sections represent the same 1.5- to 2.6-km-thick Baxa Group section be-ing structurally repeated as multiple horses. On the Kuru Chu transect, a thickness of 2.6 km is calculated for the Baxa Group between lower thrust contacts and upper stratigraphic con-tacts with the Diuri Formation in two separate thrust sheets (Figs. 2 and 3B), and a minimum thickness of 2.5 km is exposed in the core of an anticline just below the Shumar thrust (Figs. 3B and 6B). The spacing of localized zones of deformation, which we interpret as the sites of intraformational thrust faults, provides further
Shortening and geometry of Bhutan fold-thrust belt
Geological Society of America Bulletin, Month/Month 2010 7
30203 1st pages / 7 of 21
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Long et al.
8 Geological Society of America Bulletin, Month/Month 2010
30203 1st pages / 8 of 21
support for a 2.6-km-thick Baxa Group section on the Kuru Chu transect, and constrains the thickness of the Baxa Group section to 1.5 km on the Bhumtang Chu transect (Fig. 3C). These deformation zones include ~10- to 25-m-wide zones of highly sheared phyllite (Figs. 3 and 6D, #15), intensely folded dolomite with hot springs and tufa precipitation (Figs. 3 and 6E, #20), brecciated quartzite, brecciated dolo mite, intensely sheared and folded phyllite, and a
sulfur-rich spring (Fig. 3, #21), iso clinally folded quartzite exhibiting outcrop-scale thrust faulting with an abrupt change in attitude (Fig. 3, #22), and folded, brecciated quartzite and intensely sheared phyllite with an abrupt change in attitude (Fig. 3C, #23). The abrupt attitude changes observed at these localized zones are in contrast to the generally homo-geneously dipping Baxa Group strata observed between structures. On the Trashigang tran-
sect, prominent ENE-trending valleys and sad-dles spaced ~3 km apart north to south support dividing the exposed part of the Baxa Group into fi ve 2.1-km-thick thrust-repeated sections (McQuarrie et al., 2008) (Fig. 3A, #5). Finally, on the Mangde Chu transect, the placement of fi ve intraformational Baxa Group faults be-tween the Shumar thrust and MBT was deter-mined by interpreting six structural repetitions of a 2.1-km-thick Baxa Group section, which
Figure 6 (on this and following page). (A) Picture facing NE of Shumar Formation (Fm.) thrust over Baxa Group (Gp.) across Shumar thrust on Bhum-tang Chu transect. Baxa Group quartzite cliffs are ~200 m high. (B) Picture facing NW of Shumar Formation thrust over folded Baxa Group across Shumar thrust, on Kuru Chu transect. Note klippe of Shu-mar Formation on south side. At least 2.5 km of Baxa Group strata are exposed in the cen-ter of the anticline; anticline axis marks northern extent of foreland-dipping part of up-per Lesser Himalayan duplex. (C) Top-to-south–sense shear cleavage in phyllite interbed between quartzite layers of Baxa Group; location is just be-neath Shumar thrust on Trashi-gang transect (30-cm hammer for scale). (D) Top-down-to-south–sense, σ-shaped shear cleavage in phyllite interval in Baxa Group. This is part of a ~10-m-thick interval of highly sheared and deformed phyllite interpreted as the site of an in-traformational thrust on the Kuru Chu transect (Fig. 3, #15). Note that this is in the foreland-dipping part of the upper Lesser Himalayan duplex, and is inter-preted to have been rotated to its top-down-to-south position by emplacement of subsurface Baxa Group horses (Fig. 7). (E) Picture of intensely folded Baxa Group dolomite, which is part of a ~25-m-wide zone that also exhibited hot springs and tufa precipitation, which we inter pret as the location of an intraformational Baxa Group thrust fault on the Bhumtang Chu transect (Fig. 3, #20). (F) Top-to-south–sense shear cleavage in Shumar Formation quartzite in Kuru Chu valley. Outlined zone between bedding planes is ~2 m thick.
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Shortening and geometry of Bhutan fold-thrust belt
Geological Society of America Bulletin, Month/Month 2010 9
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is the average of the thicknesses observed on the other three transects (Fig. 3D, #28).
The geometry of the upper Lesser Himalayan duplex is predominantly hinterland-dipping. On the Trashigang transect, the fi ve exposed Baxa Group horses dip 30° to 40° north on average (Fig. 5, stereonet D). On the Kuru Chu transect, the four southernmost horses (#1, #6, #7, and #8) dip 40° to 50° north on average (Fig. 3B), and the hanging-wall cutoffs of horses #1 and #7 pass through the erosion surface (Fig. 3, #V), which together with the Daling Formation klip-pen, constrain the position of the Shumar thrust. On the Bhumtang Chu transect, two syncline and two anticline axial traces are mapped in horse #5, which is attributed to passive folding above two horses (#6 and #7) interpreted in the subsurface. North of this folding (note that map data are projected from the transect east of the section line [Fig. 3, #23]), Baxa horses #1–#4 dip between 20° and 35° north on average (Fig. 5, stereonet G), and hanging-wall cutoffs for horses #1 and #2 pass through the erosion surface (Fig. 3C, #V), constraining the position of the Shumar thrust. On the Mangde Chu tran-sect, the six Baxa Group horses dip 50° north on
average (Fig. 5, stereonet I) but are interpreted to fl atten signifi cantly in the subsurface, based on average foliation dips of 20°N in the overly-ing Greater Himalayan section (Fig. 3D).
An east-trending syncline-anticline pair mapped in Baxa Group rocks immediately south of the Shumar thrust along the Kuru Chu tran-sect can be traced along-strike to folded Daling-Shumar Group sections on the Trashigang and Bhumtang Chu transects (Fig. 2). These two folds are separated by a southward-dipping and southward-verging intraformational Baxa Group thrust mapped in the fi eld (Fig. 6D), and require a foreland-dipping section of the upper Lesser Hima layan duplex in the Kuru Chu section (Fig. 3). These two fold traces are present east of the Kuru Chu section line, based on the geologic maps of Gokul (1983) and Bhargava (1995), and we connect them with an anticline-syncline pair that we map in the Daling-Shumar Group (Fig. 2). Here, two Baxa horses (#1 and #2) are inferred in the subsurface (Fig. 3, #6), with an interpreted antiformal stack geometry, which explains the folding observed in the overlying Daling-Shumar Group. This is consistent with map patterns showing the Baxa Group exposed
in the core of an anticline beneath the folded Shu-mar thrust just west of the section line (Gokul, 1983; Bhargava, 1995) (Fig. 2). We interpret the folding of the Shumar thrust and its hanging-wall section as the result of duplex geom etries of Baxa Group horses that vary in size and displacement both along and across strike.
Lower Lesser Himalayan DuplexAcross eastern and central Bhutan, an expo-
sure of the Daling-Shumar Group overlain by the Jaishidanda Formation is present above the Shu-mar thrust and below the MCT (Figs. 2, 6A, and 6B). The map pattern varies signifi cantly along strike, and the N-S distance of exposure varies between ~15 km on the Trashigang transect, ~50 km in the Kuru Chu valley, ~11 km west of the Kuru Chu valley, and ~1–2 km west of the Mangde Chu. In the Kuru Chu valley, where the trace of the MCT is shifted ~45 km to the north relative to the east and west, we map two sections of the Daling-Shumar Group, which we interpret as structural repetition (McQuarrie et al., 2008) (Fig. 2). Map relationships showing only one lower Lesser Himalayan section under the MCT to the east and west of the Kuru Chu
Figure 6 (continued). (G) Top-to-south–sense feldspar augen σ-clast in orthogneiss body intruded into Daling Forma-tion section in northern Kuru Chu valley (Fig. 2). (H) Top-to-south–sense sheared quartz vein boudin in Daling Forma-tion phyllite just above Shumar thrust on Trashigang tran-sect. Hammer handle is 20 cm long. (I) Picture facing NW of structurally lower Greater Hima layan section thrust over Jaishidanda Formation across Main Central thrust (MCT) in upper Kuru Chu valley, near Lhuentse (Fig. 2). Thickness of Jaishidanda section between highlighted bedding plane and MCT is ~25 m.
NS
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MCT
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Long et al.
10 Geological Society of America Bulletin, Month/Month 2010
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valley (Fig. 2) suggest that the Greater Hima-layan section overlaps thrusts that repeat the lower Lesser Himalayan section. Further sup-port for thrust repetition of the lower Lesser Himalayan strata is found in regional-scale, long-wavelength, low-amplitude, east-west–trending folds defi ned by tectonic foliation and bedding measurements in Greater Himalayan and Tethyan Himalayan rocks (Gansser, 1983; Bhargava, 1995; our mapping) (Figs. 2 and 3). We suggest that these relationships indicate structural repetition of lower Lesser Himalayan thrust sheets in a thrust duplex system at depth (which we name the lower Lesser Himalayan duplex), with the MCT acting as the roof thrust (Fig. 3, #X). While Greater Himalayan and Tethyan Himalayan tectonic foliation and bed-ding locally display variable attitudes indicative of outcrop-scale folding, we used the average dip direction of the majority of measurements to defi ne dip domains separated by fold axial traces on the map and cross sections (Figs. 2 and 3). Duplexing of lower Lesser Himalayan
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Figure 7. Schematic cross sections showing sequential development of the upper Lesser Himalayan duplex in the Kuru Chu cross section (Fig. 3B). Active thrust faults for each increment shown in bold. (A) Lower Lesser Himalayan thrust sheet with thick Shumar Formation section begins to move over Daling-Baxa footwall ramp; (B) Baxa horse #1 translated farther than its length; (C) Baxa horse #2 translated less than its length, rotating horse #1 to foreland-dipping geometry; (D) Baxa horse #3 translated much farther than its length, further rotat-ing horses #1 and #2; results in a hybrid anti-formal stack and foreland-dipping geometry. The long translation relative to horse length in this increment is necessary for develop-ment of the fi nal duplex geometry in Figure 3B. Rapid forward propagation may have been facilitated by an increase in taper due to emplacement of the thick lower Lesser Himalayan thrust sheet; (E) Baxa horse #4 translated over what will become horse #5 and part of horse #6; (F) Baxa horse #5 translated less than its length, further rotat-ing parts of older horses to foreland-dipping geometries; (G) limited translation of Baxa horses #6–#8 produces hinterland-dipping geometries, progressively rotating older horses. Geometry of schematic Baxa duplex contains both foreland- and hinterland-dipping geometries, and is similar to fi nal geometry on Figure 3B.
Shortening and geometry of Bhutan fold-thrust belt
Geological Society of America Bulletin, Month/Month 2010 11
30203 1st pages / 11 of 21
thrust sheets in the subsurface is interpreted as the mechanism that forms the structural lows and highs in the folded Greater Himalayan and Tethyan Himalayan sections, and duplexed lower Lesser Himalayan thrust sheets are shown fi lling space under the Greater Himalayan sec-tion on all cross sections (Fig. 3).
The lower Lesser Himalayan thrust sheets display signifi cant along-strike thickness varia-tions. On the Trashigang section, the lower Lesser Himalayan thrust sheet is 5.4 km thick (Fig. 3A). On the Kuru Chu transect, the south-ern lower Lesser Himalayan thrust sheet is 9.2 km thick, with a 6.0-km-thick Shumar For-mation section, and does not include the Jaishi-danda Formation in the line of section (Figs. 2 and 3B). The northern lower Lesser Himalayan thrust sheet on the Kuru Chu transect is 7.0 km thick, with a 2.0-km-thick Shumar Formation section, which is attributed to a décollement high within a 6.0-km-thick Shumar Formation section, rather than a thinner Shumar Formation to the north. A 6.0-km-thick Shumar section that rapidly thins to the north under Greater Hima-layan rocks near Lhuntse is another possible interpretation (Fig. 3, #18). On the Bhumtang Chu section, the lower Lesser Himalayan thrust sheet is 4.0 km thick (Fig. 3C). The thickness variations described above are interpreted as along-strike changes in depositional thickness (Himalayan river anticlines section). On the Mangde Chu section, the Shumar Formation and lower part of the Daling Formation are not exposed, and the thickness of the lower Lesser Himalayan thrust sheet is only 1.2 km (Fig. 3D). Since the Daling, Shumar, and Jaishidanda For-mations with a combined thickness of 4.0 km are exposed 25 km along-strike to the east (Fig. 2), we interpret the missing stratigraphy and thin section observed along the Mangde Chu as the result of a lateral ramp of the Shumar thrust that cuts upsection to the west as well as in the direc-tion of transport. The associated hanging-wall ramp through the lower Daling and Shumar sec-tions is interpreted in the subsurface, making the northern extent of horse #5 3.3 km thick, which is similar to the exposed thickness on the Bhum-tang Chu section (Fig. 3D). The location of the hanging-wall ramp corresponds to the syn-cline axial trace in the center of the Shemgang Tethyan Himalayan exposure (Fig. 2). Finally, since the Jaishidanda Formation is not present in the upper Lesser Himalayan section south of the trace of the Shumar thrust, it is interpreted to pinch out to the south, and its southern extent is queried above the erosion surface on all cross sections (Fig. 3, #VIII).
Kinematic indicators from lower Lesser Hima layan units consistently display top-to-the-south motion senses, and include shear cleavage
in quartzite (Fig. 6F), feldspar augen σ-clasts in orthogneiss bodies (Fig. 6G), and sheared and rotated quartz vein boudins in Daling Forma-tion schist and phyllite (Fig. 6H) and Jaishi-danda schist. In thin section, Shumar, Daling, and Jaishidanda samples display dynamically recrystallized quartz microstructure (Grujic et al., 1996; Long et al., 2010), with quartz crystallographic–preferred orientation (Grujic et al., 1996) and mineral stretching lineation (Figs. 2, 5O) indicating a N-S transport direc-tion. These observations argue against thick-ening via E-W–oriented ductile fl ow of Lesser Himalayan rocks, and support our interpretation for differences in original depositional thickness across eastern Bhutan.
Throughout eastern Bhutan, the lower Lesser Himalayan duplex is generally hinterland-dip-ping, but the geometry and number of horses varies along-strike. On the Trashigang section, the exposed lower Lesser Himalayan thrust sheet dips 30°N (Figs. 3B and 5C), except for a 25°S-dipping section that is interpreted as folding above subsurface Baxa Group horses (Along-Strike Variability of Lesser Himalayan Duplexing section above). Two additional lower Lesser Himalayan horses are interpreted under the Greater Himalayan section (Fig. 3A), and co-incide with fold axial traces observed in Greater Himalayan tectonic foliations (Fig. 2), includ-ing a synclinal trace that projects along-strike to the Sakteng klippe (Grujic et al., 2002), and an anticlinal trace that projects along-strike to the Lum La window (Yin et al., 2010). On the Kuru Chu transect, the southern lower Lesser Himalayan thrust sheet decreases in dip from 30°N to 4°N from south to north (Figs. 3B and 5F). The 4°N-dipping section corresponds to a décollement at the top of the Diuri Formation at depth (Fig. 3, #16). The northern lower Lesser Himalayan thrust sheet on the Kuru Chu transect steepens in dip from 4°N to 35°N from south to north (Figs. 3B and 5F), which constrains the position of a footwall ramp through the Diuri Formation and Baxa Group at depth (Fig. 3, #16 and #17). On the Bhumtang Chu section, the exposed lower Lesser Himalayan thrust sheet (#5) has a 20°N average dip (Figs. 2, 3C, and 5H), and four additional lower Lesser Hima-layan horses (#1, #2, #3, and #4) are inferred in the subsurface (Fig. 3, #X). These horses defi ne a hinterland-dipping duplex that coincides with a broad synform in the center of the Ura klippe and an antiform observed to the north in the Greater Himalayan section. We interpret a similar geom-etry for the Mangde Chu cross section, with four lower Lesser Himalayan horses (#1, #2, #3, and #4) that defi ne a hinterland-dipping duplex co-incident with a broad antiform in the Greater Hima layan and Tethyan Himalayan sections.
Bedding and tectonic foliation from both lower Lesser Himalayan thrust sheets in the Kuru Chu valley defi ne a north-trending–, north-plunging anticline (Fig. 5, stereonet F), which we name the Kuru Chu anticline. This structure is responsible for the ~45 km northward shift of the MCT trace centered on the Kuru Chu valley.
Main Central Thrust SheetThe structurally lower Greater Himalayan
section is exposed above the MCT (Fig. 6I), and sits either structurally below (across the South Tibetan detachment) or stratigraphically below isolated Tethyan Himalayan exposures (Long and McQuarrie, 2010), and structurally beneath the Kakhtang thrust on its northern end (Grujic et al., 2002). North to south (across-strike) surface exposure varies between a maximum of ~90 km in central Bhutan and a minimum of ~14 km in the Kuru Chu valley (Fig. 2).
The structurally lower Greater Himalayan section displays signifi cant across-strike gradi-ents in pressure and temperature conditions, as recorded by metamorphic mineral assemblages, the presence or absence of partial melt textures, and quartz deformation microstructure (Long and McQuarrie , 2010). In eastern Bhutan, on the Trashigang, Kuru Chu, and Bhumtang Chu tran-sects, kyanite and partial melt textures (deformed granitic leucosomes) are present throughout the section, and peak pressure and temperature conditions are estimated at 8–12 kbar and 750–800 °C (Daniel et al., 2003). In contrast, in cen-tral Bhutan, on the Mangde Chu transect, partial melt textures are absent or only present near the base of the Greater Himalayan section, a biotite-muscovite-garnet mineral assemblage dominates the majority of the Greater Himalayan section and entire Tethyan Himalayan section, with no observed change across the Greater Hima-layan–Tethyan Himalayan contact, and quartz and feldspar microstructure indicates deforma-tion temperatures between 450° and 500 °C (Long and McQuarrie, 2010). In the lower-grade Greater Himalayan section observed in central Bhutan, quartzite preserves original sedimen-tary structures, including bedding, compositional laminations, and upright cross-bedding. Quartz-ite bedding and schist, phyllite, and paragneiss foliation are approximately parallel, and low-strain magnitudes show that bedding has not been transposed (Long and McQuarrie, 2010) (Fig. 5, stereonets B, J, M, and N).
The structurally lower Greater Himalayan section is shown in cross section as a 5.3- to 10.5-km-thick thrust sheet. However, since this section is bound by ductile shear zones at its base and top, and pervasive fabrics throughout the section indicate penetrative ductile defor-ma tion (Grujic et al., 1996, 2002; Daniel et al.,
Long et al.
12 Geological Society of America Bulletin, Month/Month 2010
30203 1st pages / 12 of 21
2003; Hollister and Grujic, 2006; Long and McQuarrie, 2010), we emphasize that interpret-ing these rocks as a thrust sheet with a discrete thrust at the base provides a minimum estimate of displacement. On all transects, tectonic folia-tion in Greater Himalayan rocks just above the MCT is parallel to bedding and tectonic folia-tion of Lesser Himalayan rocks just below. This relationship can be traced at the surface for an across-strike distance of ~45 km in the Kuru Chu valley (Figs. 2 and 3). This regional-scale, fl at-on-fl at relationship is shown in cross sec-tion as a hanging-wall fl at over a footwall fl at (Fig. 3). Parallel dips above and below the MCT also indicate that the hanging-wall cutoff for the Greater Himalayan section has passed through the erosion surface on all transects. On the cross sections, the MCT trace is projected from its southernmost extent on either side of the Kuru Chu valley (Fig. 3, #IX), and the hanging-wall cutoff through the Greater Himalayan section is positioned just to the south, to minimize struc-tural overlap across the MCT (Fig. 3, #IX). Our cross sections show a westward-increasing mini mum structural overlap between 97 and 156 km across the MCT (Table 1).
Based on our mapping, the structurally lower Greater Himalayan section is ~8 km thick across eastern Bhutan. On the Trashigang section, at least 7.0 km of lower Greater Himalayan rocks are exposed, and the complete lower Greater Himalayan section is shown as 8.5 km thick be-tween the MCT and South Tibetan detachment based on projection of the Sakteng klippe above the erosion surface (Fig. 3A). On the Kuru Chu section, the Greater Himalayan section is 8.1 km thick between the MCT and Kakhtang thrust (Fig. 3B). On the Bhumtang Chu transect, the structurally lower Greater Himalayan section is 8.2 km thick south of the Ura klippe, and the Greater Himalayan metasedimentary unit thick-ens signifi cantly to the north between the Ura klippe and the Kakhtang thrust (Fig. 3, #25). In cross section, the Greater Himalayan ortho-gneiss unit is interpreted as thinning to the north in the subsurface, which keeps the total thick-ness of the Greater Himalayan section nearly constant (Fig. 3C). In central Bhutan, on the Mandge Chu transect, the lower Greater Hima-layan section thickens to the north, from 5.3 km between the MCT and the base of the Shem-gang Tethyan (or Tibetan) Himalayan exposure to 10.5 km between Trongsa and the Kakhtang thrust (Fig. 3D). In cross section, most of the thickness change is attributed to northward thickening of the metasedimentary unit (Fig. 3, #29). Note that the thinnest Greater Hima layan section, between the MCT and Shemgang, also corresponds to the coolest temperature and high-est viscosity Greater Himalayan section studied
thus far in Bhutan (Long and McQuarrie , 2010). In this region the Tethyan Himalayan section at Shemgang is in depositional contact above the structurally lower Greater Himalayan sec-tion (Long and McQuarrie, 2010), making the Greater Himalayan and Tethyan Himalayan sec-tions part of the same thrust sheet (Fig. 3D).
Tethyan Himalayan KlippenGrujic et al. (2002) provided fi eld evidence for
top-to-the-north–sense shear zones correlated with the South Tibetan detachment at the base of the Chekha Formation. This interpretation was based on structures observed at the base of the Ura and Sakteng Tethyan Himalayan exposures in eastern Bhutan (Fig. 2). Because no fi eld obser-vations were available at that time, the southern-most Tethyan Himalayan exposure at Shemgang (Fig. 2) was inferred to be the Black Mountain klippe (Grujic et al., 2002). New mapping in the Shemgang region shows an interfi ngering depo-sitional contact between the Chekha Formation and Greater Himalayan metasedimentary rocks
indicating the original stratigraphic relationship between the Chekha Formation and the structur-ally lower Greater Hima layan section (Long and McQuarrie, 2010).
The Sakteng klippe is exposed in the core of a syncline with ~30°N- and S-dipping limbs (Fig. 5, stereonet A), and although Chekha Formation bedding and tectonic foliation are varia ble, the majority of measurements are sub-parallel to the tectonic foliation of Greater Hima-layan rocks below (Fig. 3A), indicating that hanging-wall and footwall cutoffs of the South Tibetan detachment are above the erosion sur-face to the south. The Ura klippe displays two synclinal traces and an anticlinal trace, with ~20°N- and S-dipping limbs (Fig. 5, stereo net L). The southern contact of the Chekha Formation with the Greater Himalayan metasedimentary unit shows a fl at-on-fl at relationship across the South Tibetan detachment, while the northern contact displays a distinct change in bedding orientation from NE-dipping strata below to SE-dipping strata above (Long and McQuarrie,
TABLE 1. SHORTENING AND FAULT DISPLACEMENT ESTIMATES FOR EASTERN AND CENTRAL BHUTAN
(A–A′)Trashigang
(B–B′)Kuru Chu
(C–C′)Bhumtang Chu
(D–D′)Mangde Chu
LH length deformed (km) 135 139 148 164LH length restored (km)* 401 406 312 342
871461762662*)mk(gninetrohsHL25356666*)%(gninetrohsHL
Minimum MCT overlap (km) 97 107 140 156Minimum KT overlap (km) 42 31 40 53Total length restored (km)** 540 544 492 551
783443504504)mk(gninetrohslatoT07074757)%(gninetrohslatoT
Subhimalayan zone5575)mk(gninetrohS3332)%(gninetrohsHL1121)%(gninetrohslatoT
Diuri-Gondwana thrust sheets––1292)mk(gninetrohS––811)%(gninetrohsHL––57)%(gninetrohslatoT
Upper LH duplex76701731661)mk(gninetrohS83561526)%(gninetrohsHL71134314)%(gninetrohslatoT
Lower LH duplex6012520166)mk(gninetrohS06238352)%(gninetrohsHL72515261)%(gninetrohslatoT
GH lower structural levelMinimum MCT overlap (km) 97 107 140 156
04146242)%(gninetrohslatoTGH higher structural levelMinimum KT overlap (km) 42 31 40 53
4121801)%(gninetrohslatoT6699)mk(tnemecalpsidTFM3157643)mk(tnemecalpsidTBM04049794)mk(tnemecalpsidTS3102––)mk(tnemecalpsidDTS
Note: Abbreviations: GH—Greater Himalaya; KT—Kakhtung thrust; LH—Lesser Himalaya; MBT—Main Boundary thrust; MCT—Main Central thrust; MFT—Main Frontal thrust; SH—Subhimalaya; ST—Shumar thrust; STD—South Tibetan detachment.
*LH length and shortening estimates include SH zone.**Includes restored length from LH-SH, MCT overlap, and KT overlap.
Shortening and geometry of Bhutan fold-thrust belt
Geological Society of America Bulletin, Month/Month 2010 13
30203 1st pages / 13 of 21
2010). Approximate locations for footwall and hanging-wall ramps through the Chekha Forma-tion are shown on the Bhumtang Chu section (Fig. 3C). These features are shown at their max-imum permissible southern and northern extents, respectively, which are constrained between the along-strike projection of the Shemgang Tethyan Himalayan exposure, where the Chekha Forma-tion is in depositional contact above the Greater Himalayan section (Long and McQuarrie, 2010), and the southern extent of the South Tibetan de-tachment at the Ura klippe, where a fl at-on-fl at relationship shows that the hanging-wall cutoff must be above the erosion surface to the south (Fig. 3, #26). These geometries constrain dis-placement on the South Tibetan detachment to ~20 km (Long and McQuarrie, 2010) (Table 1). On the Mandge Chu section, a similar geometry for hanging-wall and footwall cutoffs of Tethyan Himalayan units is shown, with the along-strike position of the Ura klippe projected above the erosion surface (Fig. 3, #30).
Our mapping in central Bhutan has implica-tions for tectonic models that interpret the South Tibetan detachment as a passive roof thrust (Webb et al., 2007). Our interpreted geometry for Greater Himalayan and Tethyan Himalayan units in central Bhutan (Fig. 3D), with detailed observations presented in Long and McQuarrie (2010), shows the South Tibetan detachment cutting downsection to the north through the Maneting Formation and Chekha Formation. This geometry is compatible with a top-to-the-north–sense normal fault, and is not compatible with a top-to-the-north–sense thrust fault. For the South Tibetan detachment to be a passive roof thrust above a Greater Himalayan wedge, rocks we map as Greater Himalayan south of Shemgang (Fig. 2) would have to be re inter-preted as Tethyan (or Tibetan) Himalayan rocks, requiring the South Tibetan detachment to merge with the MCT between the Bhumtang Chu (high-grade Greater Himalayan rocks dis-playing kyanite and partial melt textures) and the Mangde Chu (lower-grade Greater Himalayan rocks), and requiring the MCT to cut upsection from Greater Himalayan strata to Tethyan (or Tibetan) Himalayan strata south of Shemgang. The passive roof duplex model would also re-quire the trace of the South Tibetan detachment to be exposed between Trongsa and Shemgang. We have mapped and sampled a transect south of Trongsa in detail (Fig. 2), and observe only top-to-the-south-sense structures in outcrop and thin section (Long and McQuarrie, 2010).
Kakhtang Thrust SheetThe structurally higher Greater Himalayan
section is exposed above the Kakhtang thrust and below the South Tibetan detachment across
Bhutan (Figs. 1 and 2), and is shown on the cross sections as a thrust sheet (Fig. 3), similar to the structurally lower Greater Himalayan section. Note that surface data and cross-section lines terminate within the structurally higher Greater Himalayan section, so only minimum thick-nesses are shown on the cross sections. On the Kuru Chu and Trashigang sections, the higher Greater Himalayan section is at least 13 km thick (Fig. 4). Map data from the higher Greater Himalayan section are projected from the maps of Gansser (1983), Gokul, (1983), Bhargava (1995) (Fig. 3, #XI), and show that tectonic fo-liation dips 20°–40°N on average (Fig. 2), sub-parallel to dips of the structurally lower Greater Himalayan section beneath the Kakhtang thrust, indicating that the hanging-wall cutoff has passed through the erosion surface. Assuming that the hanging-wall cutoff is just above the erosion surface, and that the Kakhtang thrust roots into the Main Himalayan thrust at depth (e.g., Nelson et al., 1996), we measure structural overlap across the Kakhtang thrust from its trace to the footwall cutoff of the structurally lower Greater Himalayan section (Fig. 3, #XIII). This provides minimum shortening estimates for the Kakhtang thrust, which vary between 31 and 53 km (Table 1). Just like the lower Greater Himalayan section, fabrics indicative of penetrative ductile deformation are observed throughout the higher Greater Himalayan sec-tion (Gansser, 1983; Swapp and Hollister, 1991; Davidson et al., 1997; Daniel et al., 2003; Hol-lister and Grujic, 2006), so we emphasize that structural overlap across the Kakhtang thrust is only a minimum estimate for the amount of strain that these rocks have accommodated.
Crustal Shortening Estimates and Along-Strike Variation
Shortening Estimates by Tectonostratigraphic Zone
Our four deformed and restored, balanced cross sections (Fig. 3) allow estimation of the minimum shortening accommodated by the Lesser Himalayan and Subhimalayan zones in Bhutan to be 164–267 km, or 52%–66% ( Table 1). At the northern end of the restored cross sections, the footwall ramp through the northernmost lower Lesser Himalayan thrust sheet is interpreted to mark the northernmost extent of Lesser Himalayan rocks, and the per-missible southernmost ramp for the structur-ally lower Greater Himalayan section (Fig. 3, #XIV). By adding in the structural overlap across the MCT (97–156 km) and across the Kakhtang thrust (31–53 km), the minimum contributions of shortening of the structurally lower and higher Greater Himalayan sections
can be included in our shortening estimates. Our estimate for the total minimum shortening accommodated by the Subhimalayan, Lesser Himalayan, and Greater Himalayan zones is 344–405 km, or 70%–75% (Table 1).
Shortening data for each structural zone are also listed on Table 1. The Subhimalayan zone accommodates only 5–7 km of shortening, 1%–2% of the total. The upper Lesser Hima-layan duplex accommodates 67–166 km, or 17%–41% of the total on individual sections, and the lower Lesser Himalayan duplex ac-commodates 52–106 km, or 15%–27% of the total on individual sections. Together, duplex-ing of Lesser Himalayan units accommodates 159–239 km, or 44%–59% of the total short-ening estimated on individual sections. Finally, structural overlap across the MCT on individ-ual sections accounts for 24%–41%, and across the Kakhtang thrust accounts for 8%–14% of the total shortening, although these estimates do not account for internal strain within the Greater Himalayan zone.
Along-Strike Variability of Lesser Himalayan Duplexing
At the scale of Bhutan, there are signifi cant along-strike variations observed in the geom-etry, number of horses, overall shortening, and relative shortening contributions of Lesser Hima layan duplexes. This variation indicates that along-strike horse width is on the order of the distance between adjacent cross sections (25 km average). The amount of shortening in the upper Lesser Himalayan duplex decreases signifi cantly from east to west (Table 1). Par-tially, this is taken up by an increase in short-ening in the lower Lesser Himalayan duplex. However, the total shortening in the lower Lesser Himalayan duplex does not show such a clear along-strike trend. The relative shorten-ing contributions of the upper and lower Lesser Himalayan duplexes could be controlled by the ratio of décollement strength to taper angle through time. As an example, the presence of a foreland-dipping duplex on the Kuru Chu sec-tion, which contains a locally thick lower Lesser Himalayan section (Fig. 3B), may illustrate the control of original basin geometry on later defor-mation. The sequential development of this duplex is illustrated in Figure 7. The foreland-dipping geometry results from a translation of three Baxa horses (#1–#3) that is greater than their individual lengths, most importantly the very long translation of horse #3 shown in incre-ment D. We suggest that the signifi cant forward propagation during this increment was facili-tated by an increase in taper due to emplacement of the locally thick lower Lesser Himalayan thrust sheet (Lower Lesser Himalayan duplex
Long et al.
14 Geological Society of America Bulletin, Month/Month 2010
30203 1st pages / 14 of 21
section) over a footwall ramp, highlighting the control that original basin geometry can exert on fi nal deformation geometry.
At the scale of the Himalayan orogen, du-plexing of Lesser Himalayan units accommo-dates signifi cant shortening across the majority of the arc. Lesser Himalayan duplexing most likely represents a hinterland taper-building mechanism in response to either rapid forward propagation of the deformation front or rapid removal of material by erosion. An along-strike comparison of Lesser Himalayan duplexing is discussed in detail in Mitra et al. (2010). Not surprisingly, the thickness and relative transla-tion of individual horses involved in building the Lesser Himalayan duplex, as well as the area that needs to be fi lled between the décolle-ment and the roof thrust, have the largest effect on shortening magnitude. The former two are a response to original basin geometry, while the latter is dictated by taper, and may be a func-tion of basin geometry or external processes such as erosion. In northwest India and west-ern Nepal , the Lesser Himalayan duplex is pri-marily hinterland-dipping, and repeats multiple horses below the roof thrust, the Ramgarh thrust (RT) (Srivastava and Mitra, 1994; Robinson et al., 2006). In this region, from west to east,
the thickness of duplexed strata decreases and the vertical distance between the décollement and the roof thrust increases, resulting in a three- to four-fold increase in percent shorten-ing. In Sikkim, Paleoproterozoic and Paleozoic units are repeated in a duplex with a hybrid hinterland-dipping and antiformal stack geom-etry beneath the Ramgarh thrust, and thicker Paleoproterozoic thrust sheets are repeated in a hinterland-dipping duplex above the Ramgarh thrust and below the MCT (Bhattacharyya and Mitra, 2009; Mitra et al., 2010). The two-duplex system in Sikkim is very similar to what we ob-serve in eastern and central Bhutan. The Shumar thrust occupies a similar role as roof thrust that the Ramgarh thrust performs in duplexes fur-ther to the west. Note that offset on the Shumar thrust varies between 40 and 79 km across Bhu-tan, but the Ramgarh thrust in Sikkim has much more offset (164 km) (Mitra et al., 2010).
DISCUSSION
Shortening along the Himalayan Arc
A compilation of shortening estimates ob-tained across the ~2500 km arc-length of the Hima layan orogen is shown in Figure 8A, with
the data from individual studies listed in Table 2. Figure 8A and Table 2 are modeled after the compilation originally presented in DeCelles et al. (2002), but are updated to include more recent studies, in particular new data from the eastern Himalaya.
Other than Coward and Butler (1985), who presented a balanced cross section across the entire fold-thrust belt in Pakistan, the remain-ing studies compiled on Figure 8A present cross sections and shortening estimates for the Subhimalayan, Lesser Himalayan, and Greater Himalayan zones only, and not the Tethyan Hima-layan zone. These studies include, from west to east, Srivastava and Mitra (1994) in north-west India, Robinson et al. (2006) in western Nepal, Schelling (1992) in east-central Nepal, Schelling and Arita (1991) in eastern Nepal, Mitra et al. (2010) in Sikkim, this study in cen-tral and eastern Bhutan, and Yin et al. (2009) in western Arunachal Pradesh. Note that shorten-ing estimates from DeCelles et al. (1998, 2001) are replaced by updated estimates from Robin-son et al. (2006), and the preliminary estimate from McQuarrie et al. (2008) is replaced by the data from this study.
For shortening estimates across the entire fold-thrust belt, the results from the studies listed
TABLE 2. COMPILATION OF SHORTENING ESTIMATES FOR THE HIMALAYAN FOLD-THRUST BELT
Referencein Figure 8A Reference Location
Structuralboundaries
Tectonostratigraphiczones
Shortening(km)
074HS,HL,HGTFM-TMMnatsikaP)5891(reltuBdnadrawoC12 Srivastava and Mitra (1994) India, Kumaon and Garhwal STD-MCT GH (includes Almora thrust sheet) 193–260
161HS,HLTFM-TCMlawhraGdnanoamuK,aidnI)4991(artiMdnaavatsavirS3822HS,HLTFM-TCMlapeNnretseW)8991(.lateselleCeD
DeCelles et al. (2001) Western Nepal STD-MCT GH (includes Dadeldhura thrust sheet) 131–206 782HS,HLTFM-TCMlapeNnretseW)1002(.lateselleCeD
4 Robinson et al. (2006) Western Nepal STD-MCT GH (includes Dadeldhura thrust sheet) 111–221 225–473HS,HLTFM-TCMlapeNnretseW)6002(.latenosniboR5
mk002HGTCM-DTSlapeNlartnec-tsaE)4002(.latenhoK012–041HGTCM-DTSlapeNlartnec-tsaE)2991(gnillehcS6
07HS,HLTFM-TCMlapeNlartneC*)2991(gnillehcS7571–041HGTCM-DTSlapeNnretsaE)1991(atirAdnagnillehcS8
07–54HS,HLTFM-TCMlapeNnretsaE*)1991(atirAdnagnillehcS9942HGTCM-DTSmikkiS)0102(.lateartiM01352HS,HLTBM-TCMmikkiS)0102(.lateartiM11
12 This study 902–831HGTCM-DTSnatuhBnretsaednalartneC13 This study 762–461HS,HLTFM-TCMnatuhBnretsaednalartneC
331HGTCM-DTSnatuhBnretsaE)8002(.lateeirrauQcM622HS,HLTFM-TCMnatuhBnretsaE)8002(.lateeirrauQcM702HGTCM-DTShsedarPlahcanurAnretseW)0102(.lateniY41803HS,HLTFM-TCMhsedarPlahcanurAnretseW**)0102(.lateniY51621HTTMM-ZSIhkadaLdnaraksnaZ,aidnI)6891(elraeS
071–051HTTCM-ZSIhkadaLdnaraksnaZ,aidnI)7991(.lateelraeS6117 Corfi 58HTDTS-ZSIhkadaLdnaraksnaZ,aidnI)0002(elraeSdnadle
78HTDTS-ZSIhkadaLdnaraksnaZ,aidnI)3991(.latekcetS211HTDTS-ZSInoigersaliaKtnuoM,tebiT)3002(niYdnayhpruM81671ZS+HTDTS-ZSInoigersaliaKtnuoM,tebiT)3002(niYdnayhpruM91331HTTCM-ZSIreviRnurAfohtron,tebiT)4991(.laterehcabhcstaR02931HTTCM-ZSImikkiSfohtron,tebiT)4991(.laterehcabhcstaR12
Note: Compilation of shortening estimates for the Himalayan fold-thrust belt. Data accompany Figure 8A. Individual estimates are listed for each tectonostratigraphic zone (note that LH and SH zones are grouped together). Note that not all studies are referenced on Figure 8A. Augmented estimates marked with asterisk are discussed in the section “Shortening along the Himalayan arc”. Abbreviations: GH—Greater Himalaya; LH—Lesser Himalaya; SH—Subhimalaya; ISZ—Indus-Yalu suture zone; MBT—Main Boundary thrust; MCT—Main Central thrust; MFT—Main Frontal thrust; MMT—Main Mantle thrust; STD—South Tibetan detachment; TH—Tethyan Himalaya.
*Projection of Ramgarh thrust into eastern Nepal would increase LH shortening by 122–193 km (DeCelles et al., 2001; Pearson, 2002).Note that these offsets are similar to the 164 km slip on the Ramgarh thrust reported from Sikkim (Mitra et al., 2010).**50 km northward shift of northernmost ramp on Yin et al. (2010). Figures 4F–4G would decrease LH shortening by 75 km.
Shortening and geometry of Bhutan fold-thrust belt
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above must be combined with shortening esti-mates for the Tethyan Himalayan zone. Several Tethyan Himalayan estimates are available from northwest India (Searle, 1986; Steck et al., 1993; Searle et al., 1997; Corfi eld and Searle, 2000). The minimum (Corfi eld and Searle, 2000) and maximum (Searle et al., 1997) of these estimates are added to the estimates of Srivastava and Mitra (1994). Minimum and maximum estimates from Murphy and Yin (2003), obtained north of west-ern Nepal, are added to the estimates of Robin-son et al. (2006). Data from Ratschbacher et al. (1994) from a transect north of east-central Nepal are added to the estimates of Schelling (1992). Data from Ratschbacher et al. (1994) from a transect north of Sikkim represent the easternmost available shortening estimate for the Tethyan Himalayan zone, and are added to the shortening estimates from eastern Nepal, Sik-kim, Bhutan, and Arunachal Pradesh. The data listed in Table 2 split out the range of shortening estimates individually by tectonostratigraphic zone. For each study site, the column on the left represents a total of the minimum estimates for each zone, and the column on the right repre-sents a total of the maximum estimates for each zone, after DeCelles et al. (2002). For this rea-son, the totals may be different from shortening estimates listed for individual cross sections in these studies.
To be able to directly compare shortening magnitudes, there are three places where we calculate shortening estimates that are different than published values (marked with asterisk). It is important to mention that the estimates we obtain in this manner are approximate. In east-ern Nepal, the shortening estimates of Schelling (1992) and Schelling and Arita (1991) are sig-nifi cantly smaller than those reported farther west in Nepal and to the east in Bhutan, particu-larly for the Lesser Himalayan zone. DeCelles et al. (2002) attributed this to a lack of iden-tifi cation of important structures in these cross sections, most notably the Ramgarh thrust. Sub-sequent studies have identifi ed an along-strike equivalent of the Ramgarh thrust in eastern Nepal (Pearson, 2002; Pearson and DeCelles, 2005), and this structure is also identifi ed to the east in Sikkim (Bhattacharyya and Mitra, 2009; Mitra et al., 2010). Projecting maximum and minimum displacements estimated for the Ramgarh thrust would add 122 km (DeCelles et al., 2001) to 193 km (Pearson, 2002) shorten-ing to the Lesser Himalayan zone (Table 2; Fig. 8A). Note that these offset estimates are simi-lar to the 164 km estimate obtained in Sikkim (Mitra et al., 2010).
Estimates from western Arunachal Pradesh, presented in Yin et al. (2009), are accompanied by deformed and restored cross sections that
include (1) an estimate with ~18 km of pre-Himalayan topography on the basal décolle-ment (Figs. 4D and 4E of Yin et al., 2009), and (2) an estimate with no pre-Himalayan defor-ma tion (fl at-lying strata) (Figs. 4F and 4G of Yin et al., 2009). Since the space between the erosion surface and the décollement exerts one of the largest controls on shortening magnitude, we use the latter estimate of Yin et al. (2010); this estimate minimizes shortening (515 km). In addition, for cross sections to balance, hang-ing-wall ramps and décollements must match footwall ramps and décollements in both the deformed and restored sections (e.g., Wood-ward et al., 1985). This constraint requires a ~50 km northward shift of the northernmost décollement ramp on Yin et al. (2010) Figure 4F, and suggests that the minimum estimate of Lesser Himalayan shortening in Arunachal Pradesh is 440 km (Fig. 8A; Table 2).
With new data from western Nepal, our augmented estimates for eastern Nepal and Arunachal Pradesh, and the addition of new data from Sikkim and Bhutan, for the fi rst time we can compare along-strike shortening varia-tions across the entire Himalayan arc (Fig. 8A). This allows us to evaluate the validity of the predictions of systematic shortening variation across the orogen listed above in the Introduc-tion. The data compiled on Figure 8A suggest that the greatest amount of shortening is accom-modated in the central part of the orogen, as fi rst discussed in DeCelles et al. (2002). Compared to western Nepal, shortening estimates from eastern Nepal, Sikkim, Bhutan, and Arunachal Pradesh are incompatible with an overall east-ward increase in shortening as suggested by the convergence history or increase in erosion (Guillot et al., 1999; Yin et al., 2006). Maximum shortening estimates for the eastern half of the orogen are very similar (ca. 580–640 km), and fall 280–340 km short of the maximum estimate in western Nepal. While not a perfect match for either, the data shown in Figure 8A are more compatible with the “bow-and-arrow” model (Elliott, 1976), or the hypothesis that shortening should mimic the width of the Tibetan Plateau (DeCelles et al., 2002), which is indicated by the dashed black line. The maximum shortening estimates from the majority of compiled studies fall very close to this line. However, exceptions that stand out are (1) the maximum estimate from western Nepal (Robinson et al., 2006), which exceeds the Tibetan Plateau width by ~250 km, and (2) the maximum augmented esti-mates from eastern Nepal (Schelling and Arita , 1991; Schelling, 1992), which fall short of the Tibetan Plateau width by ~100 km. The esti-mates from eastern Nepal are ~30–60 km less than estimates just to the east, falling short of
the estimates predicted by a simple “bow-and-arrow” model where shortening is greatest at the center of the orogen (Elliott, 1976). However, since our augmented estimates are not based on new cross sections that incorporate the Ramgarh thrust, future work in eastern Nepal could sig-nifi cantly refi ne and update these estimates al-lowing for a more robust comparison.
It is important to re-emphasize that our short-ening estimates from Bhutan, as with the rest of the shortening estimates compiled above, are minima. Several factors could increase shortening estimates, including: (1) increasing the depth of the basal décollement (e.g., Figs. 4D and 4E of Yin et al., 2010); (2) replacing thicker strata with thrust repetition of thinner stratigraphic units (e.g., addition of Ramgarh thrust to Schelling [1992] and Schelling and Arita [1991]); (3) greater displacement on thrust sheets where hanging-wall cutoffs have been eroded; or (4) incorporating penetrative strain or small-scale folding and faulting. For factor 1, earthquake seismology (Ni and Barazangi, 1984; Pandey et al., 1999; Mitra et al., 2005) and seismic refl ection (Hauck et al., 1998) have imaged a consistent, fl at, gently north-dipping basal décollement across the Himalayan orogen, removing signifi cant changes in the décolle-ment as a mechanism for increasing shortening. Factor 2 depends on the level of detail of map-ping, and the ability to delineate individual thrust-repeated units. In the case of our Bhutan cross sections, we see stratigraphic and struc-tural evidence for repeated Baxa Group horses (Along-Strike Variability of Lesser Himalayan Duplexing section) and a consistent two-part stratigraphy of the Daling-Shumar Group al-lows mapping of separate thrust sheets (Lower Lesser Himalayan duplex section). Factors 3 and 4 pose the biggest unknown for Subhimala-yan, Lesser Himalayan, and Greater Himalayan shortening estimates across the Himalaya, be-cause hanging-wall cutoffs are rarely exposed, and because of the degree of ductile deforma-tion, particularly in Greater Himalayan rocks. In Bhutan, klippen of lower Lesser Himalayan rock in the Kuru Chu valley allow constraint of the amount of erosion of Baxa Group horses. However, fortuitous map relationships such as this are not present in every study site across the orogen. Finally, one of the largest uncertainties in the composite estimates we compile above may be shortening in the Tethyan Himalayan zone, particularly east of Sikkim.
Percent Shortening along the Himalayan Arc
Precipitation from the Indian monsoon has been documented to increase significantly from the western to the eastern Himalaya (e.g.,
Long et al.
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?
MMT
MCT
MBT
STD*
MCT
RT & MBT
MCT*
STDISZ
ISZ*
STD*
MCT*
ISZ*
MCT
MBT
STD
ISZ
Sho
rten
ing
(km
)
0
200
400
600
800
1000
0
200
400
600
800
1000
0 200 400 600 800 1000 220020001800160014001200 2400Arc distance (km)
Hazarasyntaxis
NamcheBarwa
syntaxisPakistan Northwest India Nepal
Sikk
im
BhutanArunachal
Pradesh
Perc
ent
sho
rten
ing
(%)
40
60
80
30
50
70
90
40
60
80
30
50
70
90
0 200 400 600 800 1000 220020001800160014001200 2400Arc distance (km)
A
B
Tethyan Himalayan zoneGreater Himalayan zone
Lesser Himalayan zoneSubhimalayan zone
Structure abbreviations:ISZ = Indus-Yalu Suture zoneMMT = Main Mantle thrustSTD = South Tibetan detachment
MCT = Main Central thrustRT = Ramgarh thrustMBT = Main Boundary thrust
470
1
1
439
2
3
2
3
591
17
16
597
4
5
18
919
19
4
5
6
7
6
7
343
20
413
20
465*
606*
89
324
21
446*
21
8
9
384
577*
21
12
13
441
21
12
13
615654
21
14
15
1
23
4
4*
5
5*
87
109
1211
14
13 1615
Tethyan Himalaya (TH)
Subhimalaya + Lesser Himalaya (SH+LH)Subhimalaya + Lesser Himalaya + Greater Himalaya (SH+LH+GH)Subhimalaya + Lesser Himalaya + Greater Himalaya + Tethyan Himalaya (SH+LH+GH+TH)
641
21
10
11
6
579*
8*
Figure 8. (A) Compilation of shortening estimates from west to east across the Himalayan fold-thrust belt after DeCelles et al. (2002) but modifi ed and updated to incorporate recent work. Black dots connected with dashed line correspond to arc-normal width of Tibetan Plateau on fi ve transects shown on Figure 1 of DeCelles et al. (2002). Small numbers within colored boxes are referenced to data sources listed on Table 2. Left column shows minimum estimates, and right column shows maximum estimates for all tectonostratigraphic zones, except where only one estimate is available. Numbers with asterisk refer to augmented shortening estimates in eastern Nepal and Arunachal Pradesh (connected with adjacent estimates by dashed gray lines); changes listed in Table 2 and discussed in the Shortening along the Hima-layan arc. (B) Compilation of percent shortening ranges across the Himalayan arc. Data are totals for tectonostratigraphic zones listed below fi gure. Numbers are referenced to data sources listed on Table 3. Gray line connects maximum percent shortening from Greater Himalayan, Lesser Himalayan, and Subhimalayan zones. Numbers with asterisk (connected with dashed gray line) refer to augmented estimates in eastern Nepal and Arunachal Pradesh, as listed in Figure 8A and Table 2.
Shortening and geometry of Bhutan fold-thrust belt
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Finlayson et al., 2002; Bookhagen et al., 2005). If the amount of precipitation can be directly related to erosion magnitude, as has been sug-gested by studies in the eastern Himalaya ( Grujic et al., 2006; Yin et al., 2010) and northwest India (Thiede et al., 2005), and if erosion exerts a fun-damental control on the width of orogens (e.g., Beaumont et al., 1992, 2001; Zeitler et al., 1993, 2001; Willett, 1999; Whipple and Meade, 2004), then the documented precipitation gradient should be refl ected in an overall increase in per-cent shortening from the western to the eastern Himalaya (e.g., McQuarrie et al., 2008).
Oxygen isotope records of sediments in Tibet (Dettman et al., 2003) and Nepal (Dettman et al., 2001) and foraminifera (Kroon et al., 1991) and diatom (Burckle, 1989) records from the north-ern Indian Ocean have been used to document a late Miocene (ca. 10–12 Ma) onset age for the Indian monsoon. Paleoelevation studies indicate that the High Himalaya and southern Tibet had achieved similar elevations to the present by approximately the same time (ca. 10–12 Ma) (Garzione et al. 2000a, 2000b; Rowley et al., 2001), indicating an orographic barrier had been established by the late Miocene. Since the ma-jority of deformation in the Tethyan Himalayan zone occurred between Paleocene and Oligocene time (Ratschbacher et al., 1994; Harrison et al., 2000; Wiesmayr and Grasemann, 2002; Murphy and Yin, 2003; Ding et al., 2005; Aikman et al., 2008), percent shortening estimates for the Miocene Himalayas (shortening in the Greater Hima layan, Lesser Himalayan, and Sub hima-
layan zones from the Miocene to the present) (e.g., Hodges et al., 1996; DeCelles et al., 2001; Grujic et al., 2002; Daniel et al., 2003; Searle et al., 2003; Kohn et al., 2004; Vannay et al., 2004; Robinson et al., 2006; Yin et al., 2009) would be the most representative standard for along-strike comparison of percent shortening.
Table 3 is a compilation of available percent shortening estimates for the Himalayan fold-thrust belt, which are shown graphically in Figure 8B. These data were either originally pre-sented in their source study, or were calculated from the balanced cross sections accompanying those studies. Data for the total percent shorten-ing in the Greater Himalayan, Lesser Himalayan, and Subhimalayan zones are shown by red boxes and connected with a gray line on Figure 8B. Since the shortening estimates that generate per-cent shortening ranges are most likely minima, the gray line connects the maximum estimates. Augmented estimates for eastern Nepal and for Arunachal Pradesh, calculated as discussed above (Shortening along the Himalayan arc), are shown as transparent boxes, and are marked with an asterisk and connected with a dashed gray line. Figure 8B shows that percent shortening in the Greater Himalayan, Lesser Himalayan, and Subhimalayan zones varies between 64% near the western syntaxis, increases to a maximum of 84% near the center of the orogen in western Nepal, and decreases to 76%–77% at the aug-mented estimates in eastern Nepal, increases to 82% in Sikkim, and decreases to 75% in Bhutan and 66% in Arunachal Pradesh.
Although this is only based on eight studies distributed across the ~2500 km arc-length of the orogen, our compilation does not support the prediction of an overall west-to-east increase in percent shortening, indicating a lack of fi rst-order correlation between precipitation magni-tude, shortening, and width in the Himalayan orogen. However, since the timing of defor-mation initiation of Greater Himalayan rocks extends back to the early Miocene (e.g., Daniel et al., 2003; Robinson et al., 2006), which pre-dates the late Miocene onset of the Indian mon-soon, this along-strike comparison of percent shortening should be viewed as a preliminary effort. A more robust test of the relationship of percent shortening to along-strike precipitation variations would require more precise timing constraints of late Miocene and younger short-ening magnitudes in the Lesser Himalayan and Subhimalayan zones in multiple along-strike locations. Finally, note that percent shortening across the arc smooths out the large variations in shortening amount, which may suggest that shortening variations may be controlled more by the original width and geometry of the north-ern Indian margin than by external features such as precipitation and convergence rate.
Himalayan River Anticlines
The observation that many major Himalayan rivers, including the Sutlej River in northwest India, the Arun River in eastern Nepal, and the Kuru Chu in Bhutan, fl ow parallel to the hinges
TABLE 3. COMPILATION OF PERCENT SHORTENING ESTIMATES FOR THE HIMALAYAN FOLD-THRUST BELT
ReferencenoitacoLecnerefeRB8erugiFni
SH + LH(%)
SH + LH + GH(%)
Total:SH + LH + GH + TH
(%)–4646natsikaP)5891(reltuBdnadrawoC1
2 Srivastava and Mitra (1994) India, Kumaon and Garhwal 65 76–79 721–742
3 Robinson et al. (2006) Western Nepal 74–77 79–84 723–774
4 Schelling (1992) East-central Nepal 32 58–65 56–615
4* Schelling (1992)* East-central Nepal 57–64 69–76 63–695
5 Schelling and Arita (1991) Eastern Nepal 26–36 59–65 55–596
5* Schelling and Arita (1991)* Eastern Nepal 56–67 70–77 63–696
672867mikkiS)0102(.lateartiM6 6
76–3657–0766–25natuhBnretsaednalartneCydutssihT7 6
8 Yin et al. (2010) Western Arunachal Pradesh 58 70 656
8** Yin et al. (2010) Western Arunachal Pradesh 51 66 626
Referencein Figure 8A
HTnoitacoLecnerefeR(%)
9 Searle (1986) India, Zanskar and Ladakh 5610 1Searle et al. (1997) India, Zanskar and Ladakh 6011 2Corfi eld and Searle (2000) India, Zanskar and Ladakh 6512 Steck et al. (1993) India, Zanskar and Ladakh 5913 3Murphy and Yin (2003) Tibet, Mount Kailas region 4914 4Murphy and Yin (2003) Tibet, Mount Kailas region 60 (w/ISZ)15 5Ratschbacher et al. (1994) Tibet, north of Arun River 5216 6Ratschbacher et al. (1994) Tibet, north of Sikkim 51
Note: Abbreviations: GH—Greater Himalaya; ISZ—Indus suture zone; LH—Lesser Himalaya; SH—Subhimalaya; TH—Tethyan Himalaya. Compilation of percent shortening estimates for the Himalayan fold-thrust belt. Data accompany Figure 8B. Maximum and minimum percent shortening are listed as totals for the indicated tectonostratigraphic zones. Superscripts in Total (SH + LH + GH + TH) column are referenced to the specifi c TH study used. Augmented estimates marked with asterisk are discussed in sections “Shortening along the Himalayan arc” and “Percent shortening along the Himalayan arc’’ and are matched with superscripts in “Reference” column.
*Calculated by adding 122–193 km shortening to LH zone (see Table 2).**Calculated by subtracting 75 km shortening from LH zone (see Table 2).
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of orogen-perpendicular anticlines, has led to a number of studies searching for a folding mech-anism (Bordet, 1955; Krishnaswamy, 1981; Oberlander, 1985; Meier and Hiltner, 1993; Johnson, 1994; Burg et al., 1997; DiPietro et al., 1999; Montgomery and Stolar, 2006). However, no general consensus has been reached. Lateral ramps in major Himalayan thrusts and accretion of lenticular horses are two possible explana-tions (Johnson, 1994). In a recent study of the Arun River valley in eastern Nepal, Montgom-ery and Stolar (2006) argued that the observed spatial correlation between higher rainfall and the river valley itself supports the interpretation that Himalayan river anticlines are the result of “focused rock uplift in response to signifi cant differences between net erosion along major riv-ers and surrounding regions”(Montgomery and Stolar, 2006).
An alternative explanation is that preexist-ing structures and accompanying sedimentary thickness variations (i.e., paleogeography) may control the development of the north-trending Himalayan anticlines. The 9-km-thick section of the Daling-Shumar Group that we observe is local to a ~35-km-wide (east-west) area centered on the Kuru Chu valley (Fig. 2), and the thick-ness of the group decreases to ~4 km to the east and west. Since the lower contact is the Shumar thrust, these are minimum thicknesses. However, we assume that the faults detach the Shumar Formation at the base of the section. Displac-ing this locally thick section southward over the Baxa Group adds an extra ~5 km of structural elevation compared to the east and west (Figs. 3A–3C), creates two lateral ramps to the east and west, and as a result creates a structural high localized on the Kuru Chu valley. Note that the axis of the anticline does not continue below the Shumar thrust in the Baxa Group exposure to the south (Fig. 2), which provides further support for association with the thick Daling-Shumar Group section in the Shumar thrust hanging wall. Along strike through the eastern Himalaya, several north-trending folds are present, includ-ing the Arun antiform centered on the Arun River valley in eastern Nepal. This area also contains a thick (12–13 km) Lesser Himalayan section (Schelling and Arita, 1991; Schelling, 1992) when compared to the 3–4.5 km thickness of correlative Lesser Himalayan units in central and western Nepal (Robinson et al., 2006).
Locally thick Lesser Himalayan sections trending perpendicular to the Greater Indian continental margin could have been gener-ated from preexisting or syntectonic irregulari-ties, such as at the intersections of transform systems with the continental margin. While signifi cant along-strike synrift sediment thick-ness changes at the scale of salients and re-
entrants in continental margins have been identifi ed (e.g., Thomas, 1977), thick, smaller-scale, margin-perpendicular, basin-fi ll sections are also documented. Thomas (1991) observed abrupt along-strike thickness changes of synrift rocks in the Appalachian-Ouachita continen-tal margin, including a 65-km-wide, margin-perpendicular graben, localized along a synrift transform fault interpreted to have propagated onto the continent. Francheteau and Le Pichon (1972) also document similar deep, margin-perpendicular coastal basins interpreted to have formed where transform fracture zones inter-sected the east coast of Argentina.
CONCLUSIONS
A new 1:250,000-scale geologic map and four balanced cross sections through the Hima-layan fold-thrust belt in eastern and central Bhu-tan allow us to conclude the following:
(1) Major structural features of the fold-thrust belt include: (a) the ~2- to 7-km-thick Main Frontal thrust sheet; (b) the upper Lesser Hima-layan duplex, which structurally repeats ~2- to 3-km-thick horses of the Baxa Group, with the Shumar thrust acting as the roof thrust; (c) the lower Lesser Himalayan duplex, which repeats ~4- to 9-km-thick thrust sheets of the Daling-Shumar Group and Jaishidanda Formation, which passively folds the roof thrust, the MCT, and the structurally lower Greater Himalayan section; (d) the ~5- to 11-km-thick, structurally lower Greater Himalayan thrust sheet above the MCT and below the South Tibetan detachment; (e) Tethyan Himalayan rock in stratigraphic contact above the lower Greater Himalayan sec-tion in central Bhutan and in structural contact above the South Tibetan detachment in eastern Bhutan; and (f) the >13-km-thick, structurally higher Greater Himalayan thrust sheet above the Kakhtang thrust.
(2) In the Subhimalayan and Lesser Hima-layan zones, 164–267 km of minimum short-ening, or 52%–66%, is recorded. Structural overlap across the MCT and Kakhtang thrust varies between 97 and 156 km and 31–53 km, respectively. Total minimum shortening of the Himalayan fold-thrust belt between the MFT and the South Tibetan detachment ranges be-tween 344 and 405 km, or 70% to 75%. The Subhimalayan zone only accounts for 5–7 km, or 1%–2% of the total. The upper and lower Lesser Himalayan duplexes together account for 159–239 km of shortening, or 44%–59% of the total. When combined with observations from northwest India, Nepal, and Sikkim, this indicates that Lesser Himalayan duplexing ac-commodates signifi cant shortening across much of the Himalayan orogen.
(3) A compilation of shortening estimates across the length of the orogen does not indicate a systematic west-to-east increase, as predicted by the obliquity of Indian-Asian convergence, or increased erosion in the east. Although not a perfect fi t to either, the shortening data are more compatible with classic “bow-and-arrow” models (e.g., Elliott, 1976), and the hypothesis that shortening magnitude should mimic the arc-normal width of the Tibetan Plateau, as predicted by DeCelles et al. (2002). Finally, al-though precipitation increases from west to east across the Himalayan front, a compilation of the percent shortening across the length of the orogen does not indicate a systematic west-to-east increase, which would be predicted if pre-cipitation, erosion magnitude, and shortening were all positively correlated. We suggest that the original width and geometry of sedimentary basins on the northern Indian margin may exert a stronger control on shortening across the arc than external features such as precipitation and convergence rate.
ACKNOWLEDGMENTS
The authors would like to thank the government of Bhutan for their assistance and support, particularly Director General D. Wangda and the Department of Geology and Mines in the Ministry of Economic Af-fairs. This work was supported by National Science Foundation grant EAR-0738522 to N. McQuarrie. We would also like to thank reviewers Paul Kapp and Gautam Mitra and Associate Editor Matt Kohn for constructive comments. Finally, we are also grateful to Lincoln Hollister for discussions and comments that greatly improved the manuscript.
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MANUSCRIPT RECEIVED 29 OCTOBER 2009REVISED MANUSCRIPT RECEIVED 21 JANUARY 2010MANUSCRIPT ACCEPTED 25 JANUARY 2010
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BhutanIn
dia
BhutanC
hina
ChinaIndia
Bhutan
India
Bhutan
India
91°00′E
91°00′E 91°15′E
91°15′E
91°30′E
91°30′E
26°4
5′N
27°0
0′N
27°1
5′N
27°3
0′N
27°4
5′N
26°4
5′N
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0′N
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90°30′E 90°45′E
B′
B A
A′
CD
C′D′
Stratigraphy
Symbols
40 31
2612
Stratigraphic contact
Thrust fault (teeth on upper plate)
Detachment fault (ticks on upper plate)
Anticline
Syncline
Strike and dip of bedding and foliation
Structural data from Gokul (1983)
Vertical bedding and foliation
Horizontal bedding
Road
River
Contour line (elevation in meters)
Town
Structures (North to South)
STD
MCT
MBT
MFT
ST
South Tibetan detachment
KT Kakhtang thrust
Main Central thrust
Shumar thrust
Main Boundary thrust
Main Frontal thrust
Qs
Ts
Pzg
Pzd
Pzb
pCd
pCs
Pzc
GHlo
GHhm
Modern sediment (Quaternary)
pCg
Siwalik Group (Miocene Pliocene) - lower (Tsl), middle (Tsm), upper (Tsu)
Subhimalaya
Lesser Himalaya
Gondwana succession (Permian)
Diuri Formation (Permian)
Baxa Group (Neoproterozoic Cambrian[?])
Dolomite
Daling Formation (Paleoproterozoic)
GHhg
Shumar Formation (Paleoproterozoic)
Orthogneiss (Paleoproterozoic)
Tibetan (Tethyan) Himalaya
Chekha Formation (age uncertain; Neoproterozoic to Ordovician[?])
Greater Himalaya
Scale: 1:250,000
Structurally lower orthogneiss unit (Cambrian Ordovician)
GHlm Structurally lower metasedimentary unit (Neoproterozoic Ordocivian[?])
GHho Structurally higher orthogneiss unit
Structurally higher metasedimentary unit
Structurally higher leucogranite (Miocene)
7
Mineral stretching lineation
Crenulation fold axis
Pzj Jaishidanda Formation (Neoproterozoic Ordovician[?])
71
Pzm Maneting Formation (Ordovician[?])
Structural data from Gansser (1983)
Structural data from Bhargava (1995)2612
4721
Geologic Map of Eastern and Central Bhutan
0 5 10 15 20 25
kilometers
S. Long, N. McQuarrie, T. Tobgay, and D. Grujic
Ura
Figure 2. Geologic map of eastern and central Bhutan (scale 1:250,000). Location shown on Figure 1. Stratigraphy, unit abbre-via tions, structure abbreviations, sources of map data, and symbols shown in legend. The following data sources were used to trace contacts between our traverses: locations of Main Boundary thrust (MBT), Kakhtang thrust, Main Central thrust (MCT) east of Trashigang traverse, Tang Chu klippe, and all structurally higher Greater Himalayan level contacts from Gansser (1983); locations of Shumar thrust (ST; folded Shumar thrust in eastern Kuru Chu valley from Gokul [1983]), most Lesser Himalayan unit contacts, and MCT from Bhargava (1995); location of Shumar thrust east of Trashigang traverse matches along-strike of location of Bomdila thrust from Yin et al. (2010); location of Lum La window from Yin et al. (2010).
Geometry and crustal shortening of the Himalayan fold-thrust belt, eastern and central BhutanLong et al.
Plate 1Supplement to: Geological Society of America Bulletin, v. XXX; no. X/X; doi: 10.1130/B30203.S1
© Copyright 2011 Geological Society of America
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rth
) A
′26°45′N
IndiaBhutan
Samdrup Jongkhar
27°00′N
27°15′N
Trashigang
27°30′N
Trashi Yangtse
27°45’ N
GH
ho
GH
loPz
gPzd
Pzb
Pzb
Pzb
Pzb
Pzb
Pzb
Pzb
Pzd
pC
d
pC
d
pC
s
pC
s
Qs
KT
MC
TST
MBT
MFT
Tsu
Tsm
Tsu
Tsm
Tsl
Tsl
Pzg
Pzg
pC
dST
MC
T
GH
loPz
cST
D
A. T
rash
igan
g C
ross
-Sec
tio
n
?
?
53
46
78
Pzb
Pzb
12
?
Pzj
pC
d
pC
s
Pzj
GH
lm
GH
lm Drangme Chu
Kulong Chu
Kulong Chu
Sakt
eng
klip
pe
pC
d
pC
s
Pzj
GH
lm
–100 –4 Thickness (km) –12–8–2 –6 –14
Pzg
pC
d
pC
s
Tsm
Pzd Pz
b
Tsu
Tsl
Rest
ore
d T
rash
igan
g C
ross
-Sec
tio
n
5
34
67
8
12
?
pC
sp
Cd
Pzj
Perm
issi
ble
so
uth
ern
exte
nt
of G
H s
ecti
on
2
–6 –84 0 –2 –4 –10Elevation (km) –12
–14
–16
2 –6 –84 0 –2 –4 –10
Elevation (km)
–12
–14
–16
–18
–20
–22
–24
–26
–28
Kuru Chu
Kuru Chu
Manas Chu
26°45′N
IndiaBhutan
27°00′N
27°15′N
27°30′N
27°45′N
B (
Sou
th)
(No
rth
) B
′
Pzc
GH
ho
GH
lo
Pzg
Pzg
Pzd
Pzb
Pzb
Pzb
Pzb
Pzb
Pzb
Pzb
Pzb
Pzb
Pzd
Pzd
Pzd
pC
d
pC
d
pC
dPzj
pC
d
pC
s
pC
s
pC
s
Ts
TsQ
s
KT
MC
T
MC
T
STD
ST
ST
MBT
MFT
?
pC
gp
Cg
B. K
uru
Ch
u C
ross
-Sec
tio
n
?
?
5
34
67
8
2
1G
Hh
m
GH
lm
GH
lm
GH
lm
Mongar
Kuru Chu
Kuru Chu
Kuru Chu
Kuru Chu
Kuru ChuLheuntse
ST
GH
loGH
lm
–80 –4 Thickness (km) –12
–16–6–2 –10
–14
Pzb
Pzd
pC
d
pC
s
Ts
Pzg
Rest
ore
d K
uru
Ch
u C
ross
-Sec
tio
n
5
34
67
8
21
Pzj
pC
s
pC
d?
Perm
issi
ble
so
uth
ern
exte
nt
of G
H s
ecti
on
pC
gp
Cg
2
–64 0 –2 –4Elevation (km)
2 –6 –84 0 –2 –4 –10
Elevation (km)
–12
–14
–16
Manas Chu
26°45′N
IndiaBhutan
27°00′N
27°15′N
27°30’ N
27°45’ N
Manas Chu
Manas Chu
C (
Sou
th)
(No
rth
) C
′
Pzc
GH
ho
GH
lo
Pzg
Pzg
Pzb
Pzb
Pzb
Pzb
Pzb
Pzb
Pzb
Pzj p
Cd
pC
d
pC
dp
Cd
Pzj
pC
s
pC
sp
Cs
TslTsm
Qs
KT
MC
TM
CT
STD
ST
ST
MBT M
FT
?
?
GH
lo
Tsl
Tsm
Pzg
Pzg
?
Pzc
Pzj
pC
d
Pzj p
Cs
GH
lm?
pC
d
pC
s
42
3
5
6
7
1
MC
T
GH
ho
GH
hm
GH
hg
GH
lm
GH
lo
GH
lm
GH
lm
GH
lm
Pzb
pC
d
Pzj
pC
s
Ura
21
34
5
C. B
hu
mta
ng
Ch
u C
ross
-Sec
tio
n
Ura
klip
pe
0 –4 Thickness (km) –6–2Pz
g
Pzb pC
d
pC
s
Tsl
Tsm
4
23
56
7
1
Rest
ore
d B
hu
mta
ng
Ch
u C
ross
-Sec
tio
n
?Pz
j
pC
s
pC
d
Perm
issi
ble
so
uth
ern
exte
nt
of G
H s
ecti
on
21
34
5
2
–64 0 –2 –4Elevation (km)
2 –6 –84 0 –2 –4 –10
Elevation (km)
–12
–14
–16
–18
Mangde Chu
26°45′N
IndiaBhutan
27°00′N
27°15′N
27°30′N
27°45′N
D (
Sou
th)
(No
rth
) D
′
Pzc
GH
loPz
b
Pzb
Pzb
Pzb
Pzb
Pzb
Pzb
Pzj
pC
d
pC
d
pC
d
pC
dp
CdPz
j
pC
sp
Cs
pC
s
Ts
TsQs
KT
MC
T
MC
T
STD
ST
ST
MBT
MFT
?
?
GH
lo
Pzj
pC
dPz
jp
Cs
?
pC
d
pC
s
3
12
45
6M
CT
GH
ho
GH
lmG
Hlo
GH
lm
GH
lm
Pzm
?
?
Pzc
Pzm
pC
dPz
j pC
sPz
j
STD
21
34
5
?
GH
lm
GH
lm
D. M
ang
de
Ch
u C
ross
-Sec
tio
n
Geylegphug
Shemgang
Trongsa
0 –4 Thickness (km) –2 –6
pC
d pC
s
Ts
Pzb
3
12
45
6
Rest
ore
d M
ang
de
Ch
u C
ross
-Sec
tio
n
?
Pzj
pC
s
pC
d
Perm
issi
ble
so
uth
ern
exte
nt
of G
H s
ecti
on
2
1
34
5
Cro
ss-S
ecti
on
s o
f Eas
tern
and
Cen
tral
Bh
uta
n
Def
orm
ed s
ecti
on
s: 1:
300,
000-
scal
e0
510
1520
25
kilo
met
ers
Rest
ore
d s
ecti
on
s: 1:
600,
000-
scal
e0
1020
3040
50
kilo
met
ers
bed
din
g
folia
tio
n
Ap
par
ent
dip
sym
bo
ls:
map
dat
a fr
om
th
is s
tud
ym
ap d
ata
fro
m G
oku
l (19
83)
map
dat
a fr
om
Gan
sser
(198
3)m
ap d
ata
fro
m B
har
gav
a (1
995)
No
ver
tica
l exa
gg
erat
ion
No
ver
tica
l exa
gg
erat
ion
Qs
Ts
Pzg
Pzd
Pzb
pC
d
pC
s
Pzc
GH
lo
GH
hm
Mo
der
n s
edim
ent
(Qu
ater
nar
y)
pC
g
Siw
alik
Gro
up
(Mio
cen
e-Pl
ioce
ne)
Sub
him
alay
a
Less
er H
imal
aya
Go
nd
wan
a su
cces
sio
n (P
erm
ian
)
Diu
ri F
orm
atio
n (P
erm
ian
)
Bax
a G
rou
p (N
eop
rote
rozo
ic–C
amb
rian
[?])
Dal
ing
Fo
rmat
ion
(Pal
eop
rote
rozo
ic)
GH
hg
Shu
mar
Fo
rmat
ion
(Pal
eop
rote
rozo
ic)
ort
ho
gn
eiss
(Pal
eop
rote
rozo
ic)
Tib
etan
(Te
thya
n) H
imal
aya
Ch
ekh
a Fo
rmat
ion
(ag
e u
nce
rtai
n; N
eop
rot.
to
Ord
ovic
ian
[?])
Gre
ater
Him
alay
a
Stru
ctu
rally
low
er o
rth
og
nei
ss u
nit
(C
amb
rian
–Ord
ovic
ian
)
GH
lmSt
ruct
ura
lly lo
wer
met
ased
imen
tary
un
it
(Neo
pro
tero
zoic
-Ord
oci
vian
[?])
GH
ho
Stru
ctu
rally
hig
her
ort
ho
gn
eiss
un
it
Stru
ctu
rally
hig
her
met
ased
imen
tary
un
it
Stru
ctu
rally
hig
her
leu
cog
ran
ite
(Mio
cen
e)
Pzj
Jais
hid
and
a Fo
rmat
ion
(Neo
pro
tero
zoic
–Ord
ovic
ian
[?])
Pzm
Man
etin
g F
orm
atio
n (O
rdov
icia
n[?
])
low
er (
Tsl),
mid
dle
(Ts
m),
up
per
(Ts
u)
STD
MC
T
MBT
MFT
ST
Sou
th T
ibet
an d
etac
hm
ent
KT
Kak
hta
ng
th
rust
Mai
n C
entr
al t
hru
stSh
um
ar t
hru
stM
ain
Bo
un
dar
y th
rust
Mai
n F
ron
tal t
hru
st
No
tes
for
Man
gd
e C
hu
sec
tio
n:
27
. Po
siti
on
of M
BT fr
om
Gan
sser
(198
3); s
urf
ace
dip
s o
f Siw
alik
s fr
om
Go
kul (
1983
).2
8. I
ntr
afo
rmat
ion
al B
axa
Gro
up
fau
lts
loca
ted
by
inte
rpre
tin
g 6
du
ple
xed
Bax
a G
rou
p h
ors
es e
ach
2.1
km
th
ick
(ave
rag
e th
ickn
ess
of t
he
Bax
a G
rou
p o
n o
ther
th
ree
cro
ss-s
ecti
on
s) b
etw
een
MBT
an
d S
T. I
ntr
afo
r-m
atio
nal
fau
lts
wer
e n
ot
map
ped
in t
he
field
. Su
rfac
e d
ips
in s
ou
ther
n t
hre
e B
axa
ho
rses
fro
m G
oku
l (19
83).
29
. GH
low
er s
tru
ctu
ral l
evel
incr
ease
s in
th
ickn
ess
to t
he
no
rth
fro
m 5
.3 k
m, m
app
ed b
etw
een
th
e M
CT
and
th
e C
hek
ha
Form
atio
n in
th
e so
uth
, to
10.
5 km
, map
ped
bet
wee
n a
syn
clin
e ax
is n
ort
h o
f Tro
ng
sa a
nd
th
e K
T. M
ost
of t
hic
knes
s in
crea
se t
akes
pla
ce in
GH
met
ased
imen
tary
un
it.
30
. Fo
otw
all a
nd
han
gin
g w
all r
amp
s fo
r TH
un
its
are
app
roxi
mat
ely
loca
ted,
sh
ow
n a
t th
eir p
erm
issi
ble
max
imu
m s
ou
ther
n a
nd
no
rth
ern
po
siti
on
s, re
spec
tive
ly.
Loca
tio
ns
con
stra
ined
bet
wee
n n
ort
her
n e
xten
t o
f TH
exp
osu
re, w
her
e C
hek
ha
Form
atio
n is
in d
epo
siti
on
al c
on
tact
ab
ove
GH
sec
tio
n (L
on
g a
nd
McQ
uar
rie,
201
0), a
nd
alo
ng
-str
ike
pro
ject
ion
of s
ou
ther
n e
xten
t o
f STD
at
Ura
klip
pe,
wh
ere
bed
din
g a
nd
folia
tio
n
dat
a sh
ow
th
at h
ang
ing
wal
l cu
toff
mu
st b
e ab
ove
ero
sio
n s
urf
ace
to s
ou
th.
31
. Fo
otw
all r
amp
th
rou
gh
up
per
par
t o
f Dal
ing
Fo
rmat
ion
co
rres
po
nd
s to
rest
ore
d le
ng
th o
f all
Bax
a h
ors
es.
No
tes
for
Bh
um
tan
g C
hu
sec
tio
n:
19
. Ap
par
ent
dip
dat
a is
pro
ject
ed fr
om
map
tra
nse
ct to
th
e ea
st o
f sec
tio
n li
ne.
20
. In
traf
orm
atio
nal
Bax
a G
rou
p fa
ult
loca
ted
at
zon
e o
f in
ten
sely
-fo
lded
do
lom
ite,
ho
t sp
rin
gs,
and
tu
fa p
reci
pit
atio
n.
21
. In
traf
orm
atio
nal
Bax
a G
rou
p fa
ult
loca
ted
at
~20
m t
hic
k zo
ne
of b
recc
iate
d q
uar
tzit
e, b
recc
iate
d d
olo
mit
e, a
nd
inte
nse
ly-s
hea
red
an
d fo
lded
ph
yllit
e. S
ite
also
incl
ud
ed s
ulfu
r-ri
ch s
pri
ng.
22
. In
traf
orm
atio
nal
Bax
a G
rou
p fa
ult
loca
ted
at
zon
e o
f iso
clin
ally
-fo
lded
qu
artz
ite
exh
ibit
ing
ou
tcro
p-s
cale
th
rust
fau
ltin
g, a
nd
ab
rup
t ch
ang
e in
att
itu
de
fro
m s
ou
th-d
ipp
ing
rock
s ab
ove
to n
ort
h-d
ipp
ing
rock
s b
elo
w.
23
. In
traf
orm
atio
nal
Bax
a G
rou
p fa
ult
loca
ted
at
~10
m t
hic
k zo
ne
of f
old
ed, b
recc
iate
d q
uar
tzit
e an
d in
ten
sely
-sh
eard
ph
yllit
e, a
nd
ab
rup
t ch
ang
e in
att
itu
de
fro
m s
ou
th-d
ipp
ing
rock
s ab
ove
to n
ort
h-d
ipp
ing
ro
cks
bel
ow
.2
4. N
ort
her
n e
xten
t o
f Bax
a G
rou
p d
up
lex
in s
ub
surf
ace
corr
esp
on
ds
to s
ign
ifica
nt
shal
low
ing
of f
olia
tio
n d
ips
in G
H s
ecti
on
. 2
5. S
ign
ifica
nt
thic
knes
s ch
ang
es o
f GH
met
ased
imen
tary
un
it a
re o
bse
rved
bet
wee
n s
ou
ther
n e
nd
of U
ra k
lipp
e an
d a
rea
ben
eath
KT.
In
terp
rete
d s
ub
surf
ace
geo
met
ry s
ho
ws
GH
met
ased
imen
tary
un
it t
hic
ken
-in
g a
nd
GH
ort
ho
gn
eiss
th
inn
ing
sig
nifi
can
tly
to n
ort
h, w
hile
kee
pin
g to
tal l
ow
er G
H s
ecti
on
th
ickn
ess
sim
ilar.
Th
is is
just
ified
by
thic
knes
s ch
ang
es o
bse
rved
bet
wee
n t
hes
e tw
o u
nit
s ac
ross
cen
tral
an
d e
aste
rn
Bh
uta
n (F
ig. 2
). N
ote
th
at lo
wer
co
nta
ct b
etw
een
GH
met
ased
imen
tary
un
it a
nd
GH
ort
ho
gn
eiss
un
it m
atch
es a
pp
aren
t d
ips
of b
edd
ing
an
d fo
liati
on
dat
a. L
ow
er L
H h
ors
e #1
co
uld
be
GH
ort
ho
gn
eiss
, bu
t th
is
wo
uld
resu
lt in
sig
nifi
can
t an
d a
bru
pt
thic
ken
ing
of t
he
low
er G
H s
ecti
on
.2
6. F
oo
twal
l an
d h
ang
ing
wal
l ram
ps
for C
hek
ha
Form
atio
n a
re a
pp
roxi
mat
ely
loca
ted,
sh
ow
n a
t th
eir p
erm
issi
ble
max
imu
m s
ou
ther
n a
nd
no
rth
ern
po
siti
on
s, re
spec
tive
ly.
Loca
tio
ns
are
con
stra
ined
bet
wee
n
alo
ng
-str
ike
pro
ject
ion
of n
ort
her
n e
xten
t o
f TH
exp
osu
re n
ear S
hem
gan
g, w
her
e C
hek
ha
Form
atio
n is
in d
epo
siti
on
al c
on
tact
ab
ove
GH
sec
tio
n (L
on
g a
nd
McQ
uar
rie,
201
0), a
nd
so
uth
ern
ext
ent
of S
TD e
xpo
sure
at
Ura
klip
pe,
wh
ere
bed
din
g a
nd
tect
on
ic fo
liati
on
dat
a sh
ow
th
at h
ang
ing
wal
l cu
toff
mu
st b
e ab
ove
ero
sio
n s
urf
ace
to s
ou
th.
Co
nst
rain
s sl
ip o
n S
TD to
~20
km
(Lo
ng
an
d M
cQu
arri
e, 2
010)
.
No
tes
for
Ku
ru C
hu
cro
ss-s
ecti
on
:9
. Lo
cati
on
of M
BT t
aken
fro
m B
har
gav
a (1
995)
. Su
rfac
e d
ips
of S
iwal
iks
pro
ject
ed fr
om
eas
t an
d w
est,
and
tak
en fr
om
Go
kul (
1983
) an
d B
har
gav
a (1
995)
.1
0. L
eng
th o
f ero
ded
Go
nd
wan
a su
cces
sio
n s
ecti
on
th
at h
as p
asse
d a
bov
e er
osi
on
su
rfac
e m
ust
eq
ual
len
gth
of d
eco
llem
on
t o
n to
p o
f Diu
ri F
orm
atio
n b
etw
een
th
e G
on
dw
ana
succ
essi
on
foo
twal
l ram
p a
nd
Diu
ri
Form
atio
n/B
axa
Gro
up
foo
twal
l ram
p.
11
. Fo
otw
all r
amp
of G
on
dw
ana
succ
essi
on
is a
lign
ed w
ith
no
rth
en
d o
f Bax
a h
ors
e #8
bec
ause
if a
ny
furt
her
so
uth
, dis
pla
cem
ent
on
MFT
wo
uld
be
too
litt
le to
pas
s Si
wal
ik G
rou
p h
ang
ing
wal
l cu
toff
th
rou
gh
er
osi
on
su
rfac
e, a
nd
if a
ny
furt
her
no
rth
, dis
pla
cem
ent
on
MFT
wo
uld
n’t
be
min
imiz
ed.
12
. Len
gth
of e
rod
ed D
iuri
Fo
rmat
ion
sec
tio
n t
hat
has
pas
sed
ab
ove
ero
sio
n s
urf
ace
mu
st e
qu
al to
tal r
esto
red
len
gth
of d
up
lexe
d B
axa
Gro
up
ho
rses
.1
3. B
axa
Gro
up
ove
r Diu
ri F
orm
atio
n t
hru
st fa
ult
infe
rred
in s
ub
surf
ace
un
der
Dal
ing
Fo
rmat
ion
klip
pe;
loca
tio
n b
ased
on
2.6
km
th
ickn
ess
calc
ula
ted
for B
axa
Gro
up
sec
tio
n b
etw
een
low
er t
hru
st c
on
tact
s an
d
up
per
str
atig
rap
hic
co
nta
cts
wit
h D
iuri
Fo
rmat
ion
. N
ote
th
at fa
ult
is q
uer
ied
on
eas
t an
d w
est
sid
es o
f Dal
ing
Fo
rmat
ion
klip
pe
on
Fig
. 2.
14
. In
traf
orm
atio
nal
Bax
a G
rou
p fa
ult
loca
ted
at
abru
pt
chan
ge
in a
ttit
ud
e an
d li
tho
log
y fr
om
S-d
ipp
ing
qu
artz
ite
abov
e to
N-d
ipp
ing
do
lom
ite
bel
ow
; in
terp
rete
d a
s st
ruct
ura
l rep
etit
ion
of d
olo
mit
e se
ctio
n
exp
ose
d s
trat
igra
ph
ical
ly a
bov
e q
uar
tzit
e ju
st to
no
rth
(Fig
. 2).
15
. In
traf
orm
atio
nal
Bax
a G
rou
p fa
ult
loca
ted
at
~10
m t
hic
k zo
ne
of h
igh
ly-s
hea
red
ph
yllit
e w
ith
top
-to
-so
uth
sen
se o
f sh
ear i
nd
icat
ors
.1
6. D
éco
llem
ent
on
top
of D
iuri
Fo
rmat
ion
co
rres
po
nd
s to
ver
y sh
allo
w d
ips
mea
sure
d in
Dal
ing
-Sh
um
ar G
rou
p a
t su
rfac
e.1
7. F
oo
twal
l ram
p t
hro
ug
h D
iuri
Fo
rmat
ion
an
d B
axa
Gro
up
co
nst
rain
ed b
y si
gn
ifica
nt
incr
ease
in d
ip o
f Dal
ing
-Sh
um
ar G
rou
p a
t su
rfac
e.1
8. M
easu
red
th
ickn
ess
of S
hu
mar
Fo
rmat
ion
in n
ort
her
n lo
wer
LH
th
rust
sh
eet
is m
uch
less
th
an t
hic
knes
s o
bse
rved
in s
ou
ther
n t
hru
st s
hee
t; su
bsu
rfac
e g
eom
etry
inte
rpre
ted
as
a d
éco
llem
ent
wit
hin
a t
hic
k Sh
um
ar s
ecti
on
, rat
her
th
an a
th
inn
ed S
hu
mar
sec
tio
n.
No
tes
for T
rash
igan
g c
ross
-sec
tio
n:
1. E
xtra
len
gth
of G
on
dw
ana
succ
essi
on
ab
ove
ero
sio
n s
urf
ace
is n
eces
sary
to a
lign
wit
h B
axa
Gro
up
/Diu
ri F
orm
atio
n fo
otw
all r
amp
on
rest
ore
d s
ecti
on
.2
. Len
gth
of e
rod
ed D
iuri
Fo
rmat
ion
sec
tio
n t
hat
has
pas
sed
ab
ove
ero
sio
n s
urf
ace
mu
st e
qu
al to
tal r
esto
red
len
gth
of d
up
lexe
d B
axa
Gro
up
ho
rses
.3
. Bax
a G
rou
p h
ors
e #8
co
uld
be
Go
nd
wan
a su
cces
sio
n, b
ut
this
wo
uld
sh
ift fo
otw
all r
amp
th
rou
gh
Diu
ri F
orm
atio
n a
nd
Bax
a G
rou
p fa
rth
er n
ort
h; c
urr
ent
con
figu
rati
on
min
imiz
es s
ho
rten
ing.
4. B
axa
Gro
up
ho
rse
#7 h
as e
no
ug
h o
ut-
of-
seq
uen
ce d
efo
rmat
ion
to b
reak
th
rou
gh
roo
f th
rust
an
d p
lace
Bax
a G
rou
p o
ver D
iuri
Fo
rmat
ion
.5
. Pla
cem
ent
of i
ntr
afo
rmat
ion
al t
hru
st fa
ult
s b
etw
een
Bax
a G
rou
p h
ors
es b
ased
on
reg
ula
r sp
acin
g o
f EN
E-tr
end
ing
sad
dle
s an
d v
alle
ys (M
cQu
arri
e et
al.,
200
8).
6. S
ub
surf
ace
Bax
a G
rou
p h
ors
es #
1 an
d #
2 ar
e co
nsi
sten
t w
ith
map
dat
a sh
ow
ing
Bax
a G
rou
p e
xpo
sed
ben
eath
fold
ed S
T ju
st w
est
of s
ecti
on
lin
e. S
ou
th-d
ipp
ing
sec
tio
n o
f Dal
ing
-Sh
um
ar G
rou
p o
verl
ies
fore
lan
d-d
ipp
ing
par
t o
f Bax
a G
rou
p d
up
lex,
co
nsi
sten
t w
ith
ob
serv
atio
ns
on
Ku
ru C
hu
sec
tio
n.
7. D
iuri
Fo
rmat
ion
an
d B
axa
Gro
up
foo
twal
l ram
p c
ou
ld b
e p
osi
tio
ned
fart
her
no
rth
, wh
ich
wo
uld
dec
reas
e sh
ort
enin
g in
Dal
ing
-Sh
um
ar s
ecti
on
, bu
t ad
d le
ng
th to
rest
ore
d B
axa-
Diu
ri s
ecti
on
, kee
pin
g s
ho
rten
ing
ap
pro
xim
atel
y th
e sa
me.
8. M
ap d
ata
abov
e er
osi
on
su
rfac
e p
roje
cted
fro
m s
urf
ace
dat
a co
llect
ed e
ast
of c
ross
-sec
tio
n li
ne.
Pro
ject
ed to
ap
pro
xim
ate
stru
ctu
ral p
osi
tio
n.
No
tes
com
mo
n t
o a
ll cr
oss
-sec
tio
ns:
I. 4º
N d
ip o
f bas
emen
t-co
ver c
on
tact
bas
ed o
n e
arth
qu
ake
seis
mo
log
y (N
i an
d B
araz
ang
i, 19
84; P
and
ey e
t al
., 19
99; M
itra
et
al.,
2005
) an
d IN
DEP
TH
refle
ctio
n s
eism
olo
gy
(Hau
ck e
t al
., 19
98).
II. L
igh
ter s
had
ing
on
def
orm
ed a
nd
rest
ore
d c
ross
-sec
tio
ns
rep
rese
nts
mat
eria
l th
at h
as p
asse
d a
bov
e er
osi
on
su
rfac
e.II
I. M
FT d
isp
lace
men
t is
en
ou
gh
to m
ove
Siw
alik
Gro
up
han
gin
g w
all c
uto
ff t
hro
ug
h e
rosi
on
su
rfac
e an
d to
alig
n S
iwal
ik G
rou
p fo
otw
all c
uto
ff w
ith
G
on
dw
ana
succ
essi
on
foo
twal
l ram
p o
n B
hu
mta
ng
Ch
u s
ecti
on
.IV
. Bax
a G
rou
p h
ors
es in
up
per
LH
du
ple
x fe
ed s
lip in
to S
T. B
axa
Gro
up
ho
rses
are
nu
mb
ered
seq
uen
tial
ly fr
om
low
(old
est
dis
pla
cem
ent)
to h
igh
(y
ou
ng
est
dis
pla
cem
ent)
.V.
ST
is p
osi
tio
ned
so
han
gin
g w
all c
uto
ffs
of B
axa
Gro
up
ho
rses
pro
ject
just
ab
ove
ero
sio
n s
urf
ace,
to m
inim
ize
sho
rten
ing.
No
te p
lace
s o
n K
uru
Ch
u
sect
ion
wh
ere
Dal
ing
Fo
rmat
ion
klip
pen
co
nst
rain
po
siti
on
of S
T, a
nd
pla
ces
on
Ku
ru C
hu
an
d B
hu
mta
ng
Ch
u s
ecti
on
s w
her
e p
osi
tio
ns
of h
ang
ing
wal
l cu
toff
s o
f Bax
a G
rou
p h
ors
es a
re c
on
stra
ined
by
surf
ace
dat
a (*
).V
I. M
inim
um
dis
pla
cem
ent
on
ST
con
stra
ined
by:
1) f
lat-
on
-fla
t re
lati
on
ship
wit
h B
axa
Gro
up
ho
rse
#8 in
so
uth
ern
Ku
ru C
hu
val
ley,
ind
icat
ing
th
at
han
gin
g w
all r
amp
th
rou
gh
Dal
ing
Fo
rmat
ion
sta
rts
at s
ou
ther
n e
dg
e o
f klip
pe
(th
is g
eom
etry
is p
roje
cted
on
to T
rash
igan
g s
ecti
on
); 2)
no
rth
en
d o
f G
on
dw
ana
succ
essi
on
exp
osu
re o
n B
hu
mta
ng
Ch
u s
ecti
on
, bas
ed o
n lo
w-s
trai
n m
agn
itu
des
ob
serv
ed in
th
in s
ecti
on
s o
f Go
nd
wan
a su
cces
sio
n
com
par
ed to
hig
h-s
trai
n m
agn
itu
des
ob
serv
ed in
Bax
a G
rou
p (t
his
geo
met
ry is
pro
ject
ed o
nto
Man
gd
e C
hu
sec
tio
n).
VII
. Lo
cati
on
of h
ang
ing
wal
l ram
p t
hro
ug
h S
hu
mar
Fo
rmat
ion
co
nst
rain
ed b
y fie
ld re
lati
on
ship
s ju
st e
ast
of T
rash
igan
g s
ecti
on
lin
e an
d ju
st e
ast
of
Kuru
Ch
u s
ecti
on
lin
e. L
oca
tio
n o
n B
hu
mta
ng
Ch
u s
ecti
on
fro
m a
lon
g-s
trik
e p
roje
ctio
n fr
om
Ku
ru C
hu
sec
tio
n.
Sub
surf
ace
loca
tio
n o
n M
and
ge
Ch
u
sect
ion
co
nst
rain
ed b
y ch
ang
e in
bed
din
g a
nd
tect
on
ic fo
liati
on
dip
dir
ecti
on
at
surf
ace.
VII
I. Ja
ish
idan
da
Form
atio
n is
sh
ow
n p
inch
ing
ou
t to
so
uth
bec
ause
it is
no
t o
bse
rved
in u
pp
er L
H s
ecti
on
in fo
rela
nd
; th
is re
lati
on
ship
is s
ho
wn
as
qu
erie
d o
n re
sto
red
an
d d
efo
rmed
sec
tio
ns.
No
te t
hat
on
Ku
ru C
hu
sec
tio
n t
he
Jais
hid
and
a Fo
rmat
ion
is n
ot
exp
ose
d o
n t
he
sou
ther
n lo
wer
LH
th
rust
sh
eet,
and
is in
terp
rete
d a
s p
inch
ing
ou
t to
th
e so
uth
on
th
e n
ort
her
n lo
wer
LH
th
rust
sh
eet.
No
tes
com
mo
n t
o a
ll cr
oss
-sec
tio
ns,
co
nti
nu
ed:
IX. S
ou
ther
n e
xten
t o
f MC
T an
d s
tru
ctu
rally
-lo
wer
GH
sec
tio
n h
ang
ing
wal
l cu
toff
bas
ed o
n p
roje
ctio
n o
f so
uth
ern
mo
st p
osi
tio
n o
f MC
T tr
ace
bet
wee
n
east
(~5
km e
ast
of T
rash
igan
g s
ecti
on
lin
e) a
nd
wes
t (~
10 k
m e
ast
of M
ang
de
Ch
u s
ecti
on
lin
e) e
nd
s o
f Ku
ru C
hu
val
ley.
Tec
ton
ic fo
liati
on
dip
dat
a fr
om
str
uct
ura
lly-l
ow
er G
H s
ecti
on
ind
icat
es t
hat
th
e h
ang
ing
wal
l cu
toff
has
pas
sed
th
rou
gh
ero
sio
n s
urf
ace
on
all
cro
ss-s
ecti
on
s, so
GH
cu
toff
as
dra
wn
min
imiz
es s
tru
ctu
ral o
verl
ap a
cro
ss t
he
MC
T.X
. Ho
rses
co
nsi
stin
g o
f Sh
um
ar, D
alin
g, a
nd
Jai
shid
and
a Fo
rmat
ion
s ar
e re
pea
ted
in t
he
low
er L
H d
up
lex,
an
d fe
ed s
lip in
to t
he
MC
T. L
ow
er L
H h
ors
es
are
nu
mb
ered
seq
uen
tial
ly o
n B
hu
mta
ng
Ch
u a
nd
Man
gd
e C
hu
sec
tio
ns,
fro
m lo
w (o
ldes
t d
isp
lace
men
t) to
hig
h (y
ou
ng
est
dis
pla
cem
ent)
. We
sug
ges
t th
at d
up
lexi
ng
of l
ow
er L
H s
ecti
on
fold
s th
e st
ruct
ura
lly-l
ow
er G
H s
ecti
on
an
d T
H s
ecti
on
, an
d u
se te
cto
nic
folia
tio
n a
nd
bed
din
g d
ata
in G
H
and
TH
rock
s to
def
ine
geo
met
ry o
f un
der
lyin
g d
up
lex.
X
I. Su
rfac
e d
ata
for s
tru
ctu
rally
-hig
her
GH
sec
tio
n o
n a
ll cr
oss
-sec
tio
ns,
and
par
ts o
f str
uct
ura
lly-l
ow
er G
H s
ecti
on
on
Man
gd
e C
hu
an
d B
hu
mta
ng
Ch
u
cro
ss-s
ecti
on
s, ar
e ta
ken
fro
m G
anss
er (1
983)
, Bh
arg
ava
(199
5), a
nd
Go
kul (
1983
).X
II. Q
uer
ied
ed
ges
of C
hek
ha
Form
atio
n b
ased
on
alo
ng
-str
ike
pro
ject
ion
of n
ort
her
n a
nd
so
uth
ern
ext
ents
of S
TD t
race
of S
akte
ng
klip
pe
on
to
Tras
hig
ang
sec
tio
n a
nd
Ura
klip
pe
on
to K
uru
Ch
u s
ecti
on
. B
oth
th
e ex
ten
t o
f STD
tra
ce fo
r Ura
klip
pe
and
ext
ent
of d
epo
sito
nal
TH
ove
r GH
co
nta
ct o
f Sh
emg
ang
TH
exp
osu
re w
ere
pro
ject
ed o
nto
Bh
um
tan
g C
hu
an
d M
ang
de
Ch
u c
ross
-sec
tio
ns.
Bed
din
g a
nd
tect
on
ic fo
liati
on
dat
a sh
ow
th
at h
ang
ing
w
all c
uto
ffs
thro
ug
h C
hek
ha
Form
atio
n in
Sak
ten
g a
nd
Ura
klip
pen
are
ab
ove
ero
sio
n s
urf
ace.
X
III.
Min
imu
m o
verl
ap o
f KT
mea
sure
d a
lon
g fa
ult
fro
m p
oin
t o
f su
rfac
e ex
po
sure
to s
ub
surf
ace
inte
rsec
tio
n w
ith
MC
T.
Stru
ctu
ral g
eom
etry
of K
T h
ang
ing
wal
l pas
t n
ort
her
nm
ost
su
rfac
e d
ata
is s
chem
atic
.X
IV. A
t n
ort
h e
nd
of r
esto
red
sec
tio
ns,
foo
twal
l ram
p t
hro
ug
h n
ort
her
nm
ost
low
er L
H t
hru
st s
hee
t is
ass
um
ed to
mar
k n
ort
her
n e
xten
t o
f LH
un
its
and
p
erm
issi
ble
so
uth
ern
ext
ent
of s
tru
ctu
rally
-lo
wer
GH
sec
tio
n.
I
I
I I
II
II
II
II
II
II
II
II
III
III
III
III
IV, V
IV, V
IV, V
IV, V
**
*
VI
VI
VI V
I
VII
VII
VII
VII
VIII
VIII
VIII
VIII
IX
IX
IX
IX
X
X
X
X
XI
XI
XI
XI
XII
XII
XII
XII
XIII
XIII
XIII
XIII
XIV
XIV
XIV
XIV
12
3
4
56
7
8
8
9
10
11
12
1314
15
16
17
18
2021
2223
24
25
2626
2728
29
30
30
31
Fig
ure
3. B
alan
ced
cros
s se
ctio
ns o
f ea
ster
n an
d ce
ntra
l B
huta
n fo
ld-t
hrus
t be
lt:
(A)
Tra
shig
ang;
(B
) K
uru
Chu
; (C
) B
hum
tang
Chu
; an
d (D
) M
angd
e C
hu. L
ines
of
sect
ion
show
n on
Fig
ures
1 a
nd 2
. D
efor
med
sec
tion
s sh
own
at 1
:300
,000
sca
le;
rest
ored
sec
tion
s sh
own
at 1
:600
,000
sca
le (
note
dif
fere
nt
scal
e th
an F
ig. 2
). D
ata
sour
ces
for
appa
rent
dip
sym
bols
, str
atig
raph
y, u
nit a
bbre
viat
ions
, and
str
uctu
re
abbr
evia
tion
s sh
own.
Rom
an n
umer
als
on c
ross
sec
tion
s co
rres
pond
to
note
s co
mm
on t
o al
l sec
tion
s on
ri
ght-
hand
sid
e; n
umbe
rs o
n cr
oss
sect
ions
cor
resp
ond
to n
otes
ben
eath
indi
vidu
al s
ecti
ons.