Structural and isotopic constraints on the location of the Main Central thrust, Langtang National Park, Himalayas of central Nepal

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1 1 Submitted to Geology, 3/22/02 Structural and isotopic constraints on the location of the Main Central thrust, Langtang National Park, Himalayas of central Nepal Ofori N. Pearson*, Mihai N. Ducea, and Peter G. DeCelles Department of Geosciences, The University of Arizona, Tucson, AZ *corresponding author Keywords: Nepal, Himalayas, Neodymium, Main Central thrust Abstract Nd isotopic analyses combined with structural and stratigraphic mapping in Langtang National Park of central Nepal provide new constraints for defining and locating the Main Central thrust (MCT). At Langtang, there is an abrupt transition in steeply dipping metasedimentary rocks within an ~ 200 m thick zone from e Nd values typical of Lesser Himalayan rocks to values typical of Greater Himalayan rocks. Defining the MCT as this tectonostratigraphic boundary avoids problems inherent with definitions based on lithological variation, metamorphism, and structural fabric, all of which vary along the ~ 2500 km length of the fault. We have also mapped the Ramgarh thrust, a far-traveled thrust sheet within Lesser Himalayan rocks, in the proximal footwall of the MCT at Langtang. The presence of a separate thrust sheet within what has previously been called the MCT zone, combined with the recognition that the MCT can be defined as a tectonostratigraphic boundary, suggests that a reevaluation of the MCT zone concept is needed. Introduction Spanning almost the entire width of the Himalaya, the Main Central thrust (MCT) is one of the world s longest faults and the most studied structural element of the Himalayan-Tibetan

2 2 Plateau orogenic system. Most workers view the MCT as a crustal-scale shear zone that places high-grade metamorphic rocks of the Greater Himalaya (GH) on top of low-grade metasedimentary rocks of the Lesser Himalaya (LH). The MCT became active during the early Miocene around 23 Ma (Hubbard and Harrison, 1989; Hodges et al., 1996), and in Nepal has accommodated at least 160 km of shortening (Schelling and Arita, 1991; Robinson, 2001; Pearson et al., 2001a). Heim and Gansser (1939) first described the MCT in Kumaon (India), and since then, much research has been done on the fault, in part because it accommodates a large fraction of the shortening caused by the collision of India with Eurasia. Despite its prominence as a tectonic feature, however, it is difficult to accurately locate the MCT in the field because rocks near the fault have experienced strain and metamorphism that have obscured tectonostratigraphic relationships; no clear stratigraphic, structural, or metamorphic break exists between LH and GH rocks. Most researchers define a zone of distributed deformation associated with the MCT (Hodges, 2000), but no consensus exists on how to define this MCT zone. Models that seek to explain the structural and thermobarometric evolution of the Himalayan fold-thrust belt will benefit from more accurate constraints on the location of the MCT. In this paper, we report on a structural and Nd isotope study from the area near the MCT in the Trisuli Ganga river drainage of Langtang National Park in central Nepal (Fig. 1). The data suggest that the MCT can be defined as a relatively discrete tectonostratigraphic boundary (sensu Heim and Gansser, 1939) that marks the contact between GH and LH rocks. The way this contact manifests itself undoubtedly varies across the Himalaya, but our results argue for a reevaluation of what has become known as the MCT zone. Rather than defining the MCT as a broad shear zone, we propose that it is more appropriate to view the fault as a

3 3 tectonostratigraphic boundary (i.e., a relatively narrow fault zone) that has been overprinted by metamorphism and the development of younger structural elements. Definition and Location of the MCT Heim and Gansser (1939) defined the MCT in Kumaon based on the difference in metamorphic grade between low to medium-grade rocks of the LH and higher-grade rocks of the GH. However, the fault originally defined by Heim and Gansser (1939) is not the MCT, but a fault within LH rocks (Valdiya, 1980; Ahmad et al., 2000). This misidentification symbolizes the challenge that workers have faced in locating the MCT. Whereas most researchers agree that the MCT separates GH from LH rocks, distinctly different ideas exist about how this contact manifests itself. In most places along the MCT, there is a gradual transition from lower to upper greenschist-grade LH rocks to amphibolite-grade GH rocks. This gradual metamorphic transition coupled with the lack of an obvious structural break has led workers to adopt other criteria for defining the MCT and locating the fault in the field. These criteria include a combination of lithological distinctions (e.g., Pêcher, 1989); the appearance of metamorphic index minerals, usually the kyanite-in isograd (e.g. Pêcher, 1977; Reddy et al., 1993); and the presence of highly strained rocks (e.g., Hodges et al., 1996). Based on strained rocks, most workers define the MCT as a broad shear zone (the MCT zone) composed of a tectonic mélange or an imbricated stack of GH and LH rocks (Hodges et al., 2000). Whereas each of these approaches has proved useful, they all have limitations that prevent broad usage. The following examples illustrate the drawbacks inherent in each strategy. Pêcher (1989) advocated locating the MCT in part by recognizing the contact between GH

4 4 gneisses and carbonate-rich rocks from the upper part of the LH sequence. Recent mapping across Nepal (DeCelles et al., 2001; Robinson, 2001; Pearson et al., 2001b) shows, however, that rocks in the proximal footwall of the MCT are quartzites and phyllites from the lower part of the LH sequence that are carried by the Ramgarh thrust (RT) sheet (Fig. 1). The carbonate-rich upper LH rocks crop out in thrust sheets structurally below the RT. The kyanite-in isograd roughly corresponds with the location of the MCT, but provides only a loose constraint. In this study, the first kyanite seen in thin-sections occurs ~1 km above the MCT, as we define it. Other studies have found kyanite in LH rocks well below the MCT (Kohn et al., 2001). Locating the MCT by recognizing a zone of highly strained rocks is also problematic. Recent studies (DeCelles et al., 2001; Robinson, 2001; Pearson et al., 2001b) have identified a separate thrust sheet, the above-mentioned RT, in the midst of what many workers call the MCT zone. The RT carries highly sheared but unequivocal LH rocks, and although it shares a structural history with the MCT (as the footwall-flat upon which the MCT was emplaced), it also has an independent history manifested by the ~120 km of displacement accommodated by the fault (DeCelles et al., 2001; Robinson, 2001; Pearson et al., 2001a). Some of the strain seen in rocks proximal to the MCT may therefore be genetically related to deformation along the RT, rather than the MCT. An example of the use of highly strained fabrics in locating the MCT involves the enigmatic Ulleri augen gneiss, which crops out in the lower part of the LH sequence. Several workers (e.g., Arita et al., 1973; Arita et al., 1982; Hubbard and Harrison, 1989; Hodges et al., 1996) define the southern extent of the MCT shear zone in part by the presence of the mylonitic Ulleri augen gneiss. The development of a mylonitic fabric in the Ulleri, however, may not be related to displacement on the MCT. In western Nepal, the northernmost occurrence of the Ulleri (which in places is strongly mylonitic) is five thrust sheets

5 5 below the MCT in LH rocks (DeCelles et al., 2001; Robinson, 2001). In the Modi Khola drainage of central Nepal, the location of the Arita et al. (1982) and Hodges et al. (1996) studies, our mapping suggests that the northernmost exposures of Ulleri are two thrust sheets below the MCT. Thus, it is probable that much of the strongly deformed fabric in footwall MCT rocks is genetically related to thrusting on faults structurally below the MCT, rather than to displacement on or above the MCT. Recent isotopic work in the Himalaya suggests another method of locating the MCT. Nd isotope studies (Parrish and Hodges, 1996; Whittington et al., 1999; Ahmad et al., 2000; Robinson et al., 2001a; Huyghe et al., 2001) and U-Pb ages of zircons (Parrish and Hodges, 1996; DeCelles et al., 2000) show that rocks from the LH are significantly older and have an Nd isotopic signature distinct from that of GH rocks. e Nd (0) values of LH rocks range from 15.9 to 25.5 with an average of 21.5 whereas GH rocks range from 7.6 to 19.9 with an average of 16 (Robinson et al., 2001a). U-Pb ages of detrital zircons in Nepal show that rocks from the lower LH sequence were deposited during Paleoproterozoic to Mesoproterozoic time, and GH protoliths were deposited after ~750 Ma and before ~470 Ma (Parrish and Hodges, 1996; DeCelles et al., 2000). The different provenance and resulting isotopic distinction between LH and GH rocks in Nepal may allow geologists to unequivocally constrain the location of the MCT and to recognize the MCT as a tectonostratigraphic boundary. In this study, we combine detailed structural mapping with petrographic and Nd isotope analyses to locate the MCT in the Langtang area of central Nepal. We also contrast the results of this study with those of previous workers who located the MCT in Langtang by using the criteria described above.

6 6 Geology of the MCT in Langtang National Park Langtang National Park is ~50 km north of Kathmandu (Fig. 1) and has been the subject of much research (Arita et al., 1973; Bogacz and Krokowski, 1986; Macfarlane et al., 1992; Inger and Harris, 1992,1993; Reddy et al., 1993; Macfarlane, 1993,1995; Parrish and Hodges, 1996; Fraser et al., 2000). The park is an ideal location for an Nd isotope study of rocks in the vicinity of the MCT, as a well-exposed panel of GH and LH rock crops out near the small village of Syaphru at the confluence of the Trisuli Ganga and Langtang rivers (Fig. 2). Our work builds on the previous Nd isotope study done in the area by Parrish and Hodges (1996). Stratigraphy Macfarlane et al. (1992) devised a stratigraphic nomenclature specific to the Langtang area for rocks that they placed within an ~4.3 km thick MCT shear zone. We have not used their formation names as all LH rocks we observed fit within the more broadly used stratigraphic scheme of Stöcklin (1980). In accordance with Stöcklin (1980), the LH sequence in the study area is made up almost entirely of the >6 km thick Kuncha Formation, which primarily consists of greenish-gray phyllites and impure (in places phyllitic) quartzites. Toward the top of the Kuncha Formation, thin carbonates (impure marble and calcareous phyllite and quartzite) are interbedded with greenish quartzite and phyllite that become increasingly graphitic near the MCT. North of Syaphru, at the confluence of the Langtang and Trisuli Ganga rivers, rocks of Stöcklin s (1980) Robang Formation crop out. The Robang Formation is a thin sheet of garnet-rich chloritic phyllite with interbedded clean white quartzite, the Dunga quartzite beds. An ~40 m thick lenticular body of the Ulleri Formation, a mylonitic augen orthogneiss, occurs within the Robang

7 7 Formation. The GH rocks exposed in the study area are pelitic schists and gneisses of Formation I (Le Fort, 1975) with subordinate calc-silicates, granitic augen gneisses, and rare quartzites. In addition, a small leucogranite intrusion crops out in the northeastern part of the study area. Structure At Syaphru, all workers agree that the trace of the MCT is roughly N-S, but begins to strike NW-SE in the NW corner of the map area (Fig. 2). Macfarlane et al. (1992) suggested that this bend is due to the presence of a lateral ramp in the MCT. We note however, that bedding and foliation/schistosity in both the hangingwall and footwall of the MCT show this strike rotation, which suggests that the lateral ramp is at a structural level below the MCT. The orientation of bedding and foliation/schistosity in LH rocks is sub-parallel to that in GH rocks, which implies a hangingwall-flat on footwall-flat thrust relationship for the MCT, a geometry that appears throughout the Nepal Himalaya (Robinson et al., 2001b). Previous workers have placed the MCT within a broad shear zone at Syaphru. Arita et al. (1973) defined the shear zone on the basis of lithology and structural discordances, but did not observe any discrete fault surfaces. Macfarlane et al. (1992) defined the shear zone on the basis of lithology, structural fabric, and the first appearance of migmatites. Based on our experience mapping LH rocks across Nepal (DeCelles et al., 2000; Robinson et al. 2001b; Pearson et al., 2001a and 2001b), we suggest that the lithological differences cited by Macfarlane et al. (1992) are part of the normal lithological variation within the Kuncha Formation. Additionally, we note that small-scale brittle faults such as those described by Macfarlane et al. (1992) are ubiquitous across the fold-thrust belt, and we did not observe a marked increase in fault density in the study area. In the study area, both GH and LH rocks appear to be highly strained, but we did not

8 8 observe any distinguishable zones of more highly strained rocks. This qualitative observation is in accordance with quantitative work elsewhere that found only minor variations in strain magnitude from MCT zone and GH rocks (Grujic et al., 1996). The recognition of the Robang Formation at the confluence of the Trisuli Ganga and Langtang rivers is important from a structural standpoint. Stöcklin (1980) assigned the Robang Formation (and associated Dunga quartzite beds) to the uppermost part of the LH sequence. However, U-Pb ages of detrital zircons and lithological characteristics suggest these rocks are at the stratigraphic base of the LH sequence (DeCelles et al., 2001). The Robang Formation and associated Dunga quartzite beds are probably the distal facies of the lowermost part of the Kuncha Formation, and likely correspond to the Kushma and Ranimata Formations in western Nepal and the Tumlingtar Group in eastern Nepal. The presence of the Robang Formation in the footwall of the MCT suggests that the RT, mapped in India (Valdiya, 1980) and in western Nepal (DeCelles et al., 2001; Robinson, 2001; Pearson et al., 2001b), is present in the study area. The RT sheet is also present to the south of the study area, where it is synformally folded underneath the Kathmandu klippe (Pearson et al., 2001a). Our interpretation of the structural geology at Syaphru is shown in Figure 2. For the sake of clarity in the figure, we have not shown the Ulleri augen gneiss (which crops out at station 473 and extends northwards to Chilime Khola and southwards to at least the south side of Langtang Khola) as a separate formation within the RT sheet. It is important to note that recognition of the RT did not enable us to precisely locate the MCT in the field. To pin down the location of the fault, we used results from Nd isotopic analyses.

9 9 Nd Isotope Results Twelve samples collected in the study area were analyzed for their Nd isotopic signature. Sample locations are shown in Figure 2, and results are summarized in Table 1. Analytical procedures for Nd isotopic analyses follow the techniques described in Ducea et al. (2002). Nd isotopic ratios were normalized to 143 Nd/ 144 Nd = , and estimated analytical ±2s uncertainties are ~ 143 Nd/ 144 Nd= 0.002%. Multiple runs of the LaJolla standard measured during the course of this study yielded an average of ±2. We determined present day Nd isotopic ratios without performing age corrections because the effect of the ~ Ma age of metamorphism on the Nd isotopic system is negligible compared to the large isotopic differences between LH and GH rocks. Additionally, Robinson et al. (2001) use present day e Nd values to differentiate LH and GH rocks. Figure 2 also shows results from the Parrish and Hodges (1996) study of the Langtang area, recalculated to show e Nd at 0 Ma. Because no samples show evidence of partial melting, we assume that the Nd isotopic characteristics are representative of the sedimentary protoliths. The range of e Nd (0) values obtained in this study (-25.0 to 21.1 for LH rocks and 18.6 to 5.3 for GH rocks) is similar to that seen by Parrish and Hodges (1996) as well as other Himalayan Nd isotope studies (see Robinson et al., 2001a and references therein). Sample 471 has a value of 5.3, which is more positive than that usually seen in GH rocks, and may reflect the addition of younger components in the sedimentary protoliths of GH rocks. At the confluence of the Trisuli Ganga and Langtang rivers, there is an abrupt transition from values typical of LH rocks to those typical of GH rocks. We interpret the location of this transition to correspond to the tectonostratigraphic contact between LH and GH rocks the MCT.

10 10 It is unclear whether sample 467 has a LH or GH affinity as its e Nd (0) value of 18.6 falls between average values (Robinson et al., 2001a) for GH and LH rocks. The sample is an impure quartzite, and may correspond to the Mangol quartzite of Macfarlane et al. (1992) from which Parrish and Hodges (1996) obtained 980 Ma zircons. If so, the sample is most likely of GH affinity. The Nd dataset suggests that there is a discrete tectonostratigraphic boundary between LH and GH rocks in the study area. Our sampling places this boundary within an ~200 m wide zone, and it is possible that denser sampling could narrow this zone even further. Figure 3 shows a compilation of MCT zone locations from previous studies in the Syaphru area. The discrepancy in interpretations of the MCT s location illustrates the powerful impact that Nd isotopic studies will have, when used in conjunction with detailed stratigraphic and structural mapping. Discussion The lack of a clear structural break and a gradual increase in metamorphic grade from LH to GH rocks has led most geologists to map the MCT as a broad shear zone. In the Langtang area of central Nepal, however, there is a discrete tectonostratigraphic boundary between LH and GH rocks. Therefore, it may be possible to define the MCT as this tectonostratigraphic boundary. This simple and testable definition honors Heim and Gansser s (1939) original concept of the fault and avoids potential problems created by defining the MCT on the basis of lithological variation, metamorphic index minerals, and structural fabrics, all of which may vary along the length of the Himalaya.

11 11 At Langtang, the MCT lies within a broad zone of highly strained rocks, a relationship that exists across the Himalaya (Hodges et al., 2000 and references therein). The recognition that the MCT can be defined as a tectonostratigraphic boundary combined with the recent realization that rocks in both the proximal footwall and hangingwall do not represent structurally simple and continuous sections, suggests that a re-evaluation of the MCT zone concept is needed. Based on thermobarometric data, Fraser et al. (2000) suggested that the GH sequence at Langtang consists of two distinct, possibly duplicated sequences. Therefore, some of the fabric in GH rocks that has previously been linked to displacement on the MCT may be genetically related to deformation along unmapped faults and/or shear zones within GH rocks. Similarly, the presence of the RT in the proximal footwall at Langtang, as well as at other locations across Nepal and Kumaon (Valdiya, 1980; DeCelles et al., 2000; Robinson, 2001; Pearson et al., 2001a and 2001b), suggests that the development of a highly strained fabric in footwall MCT rocks may be genetically related to displacement along faults other than the MCT. In order to better refine petrologic and structural models of the Himalayan fold-thrust belt, it will be essential to locate the MCT as precisely as possible. The results of this study suggest that combining Nd analyses with detailed stratigraphic and structural mapping in the vicinity of the MCT may provide a useful way to both define and locate the fault. Acknowledgements We thank Tank Ojha and the staff of Himalayan Experience for logistical support in Nepal, and Jonathan Patchett and Clark Isachsen for their generous support in the mass spectrometry laboratory at The University of Arizona. ExxonMobil, Conoco, and the Geostructure Partnership at the University of Arizona provided funding for this study.

12 12 References Cited Ahmad, T., Harris, N.B.W., Bickle, M., Chapman, H., Bunbury, J., and Prince, C.I., 2000, Isotopic constraints on the structural relationships between the Lesser Himalayan Series and the High Himalayan Crystalline Series, Garhwal Himalaya: Geological Society of America Bulletin, v. 112, p Arita, K., Ohta, Y., Akiba, C., and Maruo, Y., 1973, Kathmandu Region in Hashimoto, S., Ohta, Y., and Akiba, C., eds., Geology of the Nepal Himalayas, Saikon, Tokyo, 286 pp. Arita, K., Hayashi, D., and Yoshida, M., 1982, Geology and structure of the Pokhara-Piuthan area, central Nepal: Journal of the Nepal Geological Society, v. 2, p Bogacz, W., and Krokowski, J., 1986, Mesoscopic structural studies of post-metamorphic deformations and tectonic evolution of the central Nepal Himalaya, in Saklani, P.S., Himalayan Thrusts and Associated Rocks, Current Trends in Geology, v. IX, p DeCelles, P.G., Gehrels, G.E., Quade, J., LaReau, B., and Spurlin, M., 2000, Tectonic implications of the U-Pb zircon ages of the Himalayan orogenic belt in Nepal: Science, v. 288, p DeCelles, P.G., Robinson, D.M., Quade, J., Ojha, T.P., Garzione, C.N., Copeland, P., and Upreti, B.N., 2001, Stratigraphy, structure, and tectonic evolution of the Himalayan fold-thrust belt in western Nepal: Tectonics, v. 20, p Ducea M.N., Sen, G., Eiler, J., and Fimbres, J., 2002, Melt depletion and subsequent metasomatism in the shallow mantle beneath Koolau volcano, Oahu (Hawaii): Geochemistry, Geophysics, Geosystems, v. 10 p. 1029/2001GC Fraser, G., Worley, B., and Sandiford, M., 2000, High-precision geothermobarometry across the High Himalayan metamorphic sequence, Langtang Valley, Nepal: Journal of Metamorphic Geology, v. 18, p Grujic, D.E., Casey, M., Davidson, C., Hollister, L.S., Kundig, R., Pavlis, T.L., and Schmid, S.M., 1996, Ductile extrusion of the Higher Himalayan Crystalline in Bhutan; evidence from quartz microfabrics: Tectonophysics, v. 260, p Heim, A., and Gansser, A., 1939, Central Himalaya Geological observations of the Swiss expedition, 1936: Mémoire, Société Helvetique Science Naturelle, v. 73, p Hodges, K.V., 2000, Tectonics of the Himalaya and southern Tibet from two perspectives: Geological Society of America Bulletin, v. 112, p Hodges, K.V., Parrish, R.R., and Searle, M.P., 1996, Tectonic evolution of the central Annapurna Range, Nepalese Himalayas: Tectonics, v. 15, p

13 13 Hubbard, M.S., and Harrison, T.M., 1989, 40 Ar/ 39 Ar age constraints on deformation and metamorphism in the Main Central Thrust zone and Tibetan Slab, eastern Nepal Himalaya: Tectonics, v. 8, p Huyghe, P., Galy, A., Mugnier, J.-L., and France-Lanord, C., 2001, Propagation of the thrust system and erosion in the Lesser Himalaya; geochemical and sedimentological evidence: Geology, v. 29, p Inger, S., and Harris, N.B.W., 1992, Tectonothermal evolution of the High Himalayan crystalline sequence, Langtang Valley, northern Nepal: Journal of Metamorphic Geology, v. 10, p Inger, S., and Harris, N.B.W., 1993, Geochemical constraints on leucogranite magmatism in the Langtang Valley, Nepal Himalaya: Journal of Petrology, v. 34, p Le Fort, P., 1975, Himalayas; the collided range; present knowledge of the continental arc: American Journal of Science, v. 275, p Macfarlane, A.M., Hodges, K.V., and Lux, D., 1992, A structural analysis of the Main Central Thrust zone, Langtang National Park, central Nepal Himalaya: Geological Society of America Bulletin, v. 104, p Macfarlane, A.M., 1993, Chronology of tectonic events in the crystalline core of the Himalaya, Langtang National Park, central Nepal: Tectonics, v. 12, p Macfarlane, A.M., 1995, An evaluation of the inverted metamorphic gradient at Langtang National Park, central Nepal Himalaya: Journal of Metamorphic Geology, v. 13, p Parrish, R.R., and Hodges, K.V., 1996, Isotopic constraints on the age and provenance of the Lesser and Greater Himalayan sequences, Nepalese Himalaya: Geological Society of America Bulletin, v. 108, p Pearson, O.N., DeCelles, P.G., Ducea, M.N., and Ojha, T.P., 2001a, Structural evolution of the Himalayan fold-thrust belt in central Nepal: GSA Abstracts with Programs, Boston, v. 82. Pearson, O.N., DeCelles, P.G., Robinson, D.M., and Gillis, R., 2001b, Structural and microstructural geology of the Ramgarh thrust sheet, far-western Nepal: EOS, Transactions of the American Geophysical Union, abstracts with programs, v. 82, p. F1125. Pêcher, A., 1977, Geology of the Nepal Himalaya: deformation and petrography in the Main Central Thrust zone: Colloques international, C.N.R.S., v. 268, p Pêcher, A., 1989, The metamorphism in the central Himalaya: Journal of Metamorphic Geology, v. 7, p

14 14 Reddy, S.M., Searle, M.P., and Massey, J.A., 1993, Structural evolution of the High Himalayan gneiss sequence, Langtang Valley, Nepal, in Treloar, P.J., and Searle, P.P., eds., Himalayan tectonics, Geological Society [London] Special Publication 74, p Robinson, D.M., 2001, Structural and ND-isotopic evidence for the tectonic evolution of the Himalayan fold-thrust belt, western Nepal and the northern Tibetan Plateau [Ph.D. thesis]: The University of Arizona, 224 p. Robinson, D.M., DeCelles, P.G., Patchett, P.J., and Garzione, C.N., 2001a, The kinematic evolution of the Nepalese Himalaya interpreted from Nd isotopes: Earth and Planetary Science Letters, v. 192, p Robinson, D.M., DeCelles, P.G., Garzione, C.N., and Pearson, O.N., 2001b, Kinematic Alternative to Reactivation of the Main Central thrust in Nepal: Eos (Transactions, American Geophysical Union), v. 82, p. F1124. Schelling, D., and Arita, K., 1991, Thrust tectonics, crustal shortening, and the structure of the far-eastern Nepal Himalaya: Tectonics, v. 10, p Stöcklin J., 1980, Geology of Nepal and its regional frame: Journal of the Geological Society of London, v. 137, p Valdiya, K.S., 1980, The two intracrustal boundary thrusts of the Himalaya: Tectonophysics, v. 66, p Whittington, A., Foster, G., Harris, N., Vance, D., and Ayers, M., 1999, Lithostratigraphic correlations in the western Himalaya An isotopic approach: Geology, v. 27, p

15 15 Figure Captions Figure 1: Geologic map of Nepal. Note the location of the Langtang area and Figure 2. Figure 2: Figure 3: Geologic map of Langtang National Park near Syaphru. Solid barbed lines show approximate fault locations, as no discrete fault surfaces were observed in the field. MCT = Main Central thrust, RT = Ramgarh thrust. Compilation of MCT and MCT zone locations from previous studies. Map area is the same as Figure 2. Rivers are shown in gray for reference. Solid barbed lines are the MCT and RT from this study. Shaded areas are MCT zones from A) Macfarlane et al., 1992; B) Parrish and Hodges, 1996; C) Bogacz and Krokowski, 1986; D) Reddy et al., Dashed barbed line in D is the MCT location from Inger and Harris, 1993 and Fraser et al., 2000.

16 80 30 RT MCT km 80 Nepal 90 China (Tibet) Bhutan 30 India Bangladesh RT MFT Subimalaya Miocene Granites Tethyan Himalaya Greater Himalaya Lesser Himalaya STDS RT Pokhara MBT MCT Kathmandu Bay of Bengal MFT = Main Frontal thrust; MBT = Main Boundary Thrust RT = Ramgarh thrust; MCT = Main Central thrust; STDS = South Tibetan Detachment System Figure 1 Langtang (Figure 2) MFT 88 MCT 28

17 85 20'00" 85 22'30" 28 07'30" 28 10'00" 28 12'30" LESSER HIMALAYA Trisuli Ganga Nadi RT Chilime Khola LT4;-25.0 MCT LT18;-26.2 LT19;-23.7 LT10;-25.5 LT20;-21.6 LT6;-25.2 LT7; ; ; '00" 525; ; ; Syaphru RT Bhotekoshi Nadi 524; ; ; ;-18.6 LT34;-17.5 MCT Figure 2 469; LT33; ;-5.3 GREATER HIMALAYA LT21;-15.8 LT22;-14.8 Langtang Khola LT24; km 2 3 scale 1:100, '30" Bedding/Foliation Station;e Nd (0) (Parrish & Hodges, 1996) Station;e Nd (0) (This Study) Thrust Fault Village Road Greater Himalayan Rocks (Formation I) Lesser Himalayan Rocks (Robang, Kuncha, and Ulleri Fms.) North contours 100m 28 12'30" 28 10'00" 28 07'30"

18 A B C D Figure 3

19 TABLE 1. STRATIGRAPHIC, MINERALOGIC, AND Nd DATA Sample ID Formation Lithology Mineral Assemblage * 143 Nd/ 144 Nd e Nd at 0 Ma Lesser Himalayan 465 Kuncha Phyllitic quartzite pg Kuncha Quartz-rich phyllite chl+pg Robang (Dunga) Quartzite K-fsp+pg N.D. N.D. 473 Robang Chloritic phyllite gt+chl Robang (Dunga) Quartzite N.D. N.D. 527 Ulleri Augen orthogneiss K-fsp+pg Kuncha Marble calc+chl N.D. N.D. 529 Kuncha Quartz-rich phyllite gt+pg Greater Himalayan 467 Formation I Garnet-mica schist chl+gt+pg Formation I Micaceous quartzite gt+k-fsp+pg N.D. N.D. 469 Formation I Garnet-mica schist gt+pg Formation I Gneiss gt+pg N.D. N.D. 471 Formation I Gneiss chl+gt+ky+pg Formation I Garnet-mica schist chl+gt+pg Formation I Gneiss chl+gt+gl+pg Formation I Gneiss gt+pg * All samples except 528 contain quartz, biotite, and muscovite. 528 does not contain quartz. chl=chlorite; K-fsp=K-feldspar; pg=plagioclase; gt=garnet; ky=kyanite; calc=calcite; gl=glaucophane. 2s errors of 143 Nd/ 144 Nd ratios are N.D.=Not determined.