Journal of Asian Earth Sciences

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1 Journal of Asian Earth Sciences () 9 Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: Detrital thermochronology and sediment petrology of the middle Siwaliks along the Muksar Khola section in eastern Nepal François Chirouze a, Matthias Bernet a,, Pascale Huyghe a, Véronique Erens b, Guillaume Dupont-Nivet b, François Senebier a a ISTerre, Université Joseph Fourier, rue de la piscine, BP, Grenoble, France b Faculteit Geowetenschappen, Universiteit Utrecht, The Netherlands article info abstract Article history: Available online January Keywords: Exhumation Fission-track thermochronology Provenance Siwaliks Himalaya Sediment derived from erosion of the Himalaya during the Miocene Pliocene was deposited in the Himalayan foreland basin and is now exposed in the Siwalik Formation between the Main Boundary Thrust and Main Frontal Thrust. These sediments hold important information on orogenic exhumation during this time. In this preliminary study we sampled the middle Siwalik Formation in eastern Nepal along the Muksar Khola section for thermochronologic and sediment petrologic analysis. Detrital zircon fission-track thermochronology shows that the age signal of the middle Siwalik Formation of eastern Nepal has some similarities but also differences with published data for western-central Nepal. The Muksar Khola section samples show two static peaks at about and > Ma, which is similar to the Siwalik Formation of western-central Nepal. In contrast, a signal of consistently fast exhumation was not observed in the upper middle Siwaliks in eastern Nepal. This could be related to a position of the Main Central Thrust further south, to overall less erosional exhumation of Higher Himalayan rocks, or to a reduced exposure of the underlying Lesser Himalayan units in comparison with western-central Nepal. Based on our apatite fission-track data we propose that above the section dated by Ojha et al. (9) with magnetostratigraphy about m of overlying upper Siwalik Formation exist. Ó Elsevier Ltd. All rights reserved.. Introduction Synorogenic clastic sedimentary rocks in foreland basins adjacent to convergent mountain belts preserve the exhumation record of their sediment source areas. Petrologic, thermochronologic, and geochemical analyses can provide a great deal of information on sediment provenance, source rock lithology, and the rate and timing of exhumation (e.g. Dickinson, 9; McLennan et al., 99; Garver et al., 999). In the case of the Himalaya, the Miocene Pliocene foreland basin sediments of the Siwalik Formation hold important information on the late Tertiary exhumation history of this mountain belt. For the central part of the Himalaya, most research in recent years was focused on sections of exposed Siwalik Formation rocks in western and central Nepal, resulting in large datasets on the sediment petrology and zircon U Pb geochronology (DeCelles et al., 99,, ), apatite and zircon fissiontrack and white mica Ar 9 Ar thermochronology (Bernet et al., ; Szulc et al., ; van der Beek et al., ), and e Nd isotope geochemistry (Robinson et al., ; Huyghe et al., ). The stratigraphy and depositional age of the different Siwalik units in the Mio Pliocene Himalayan foreland basin are fairly well constrained through magneto- and biostratigraphy (Appel et al., 99; Gautam and Rösler, 999; Gautam and Fujiwara, ; Corvinus and Rimal, ; Ojha et al., 9). Because collision and exhumation were not synchronous from west to east along the Himalayan chain (e.g. Yin, ), it is of interest to follow the exhumation signal toward the east to improve our understanding of sediment provenance and timing of exhumation in this part of the Himalaya. So far, the geological data available for the Siwalik Formation in eastern Nepal are relatively limited. Only some petrologic, magnetostratigraphic and geochemical information are currently available (Robinson et al., ; Ojha et al., 9). Therefore, we present in this study a preliminary dataset of detrital apatite and zircon fission-track ages and new data on sediment petrology and on clay mineralogy of the middle Siwalik Formation from samples collected along the Muksar Khola section in eastern Nepal (Figs. and ).. Geology of the Nepalese Himalaya Corresponding author. address: matthias.bernet@ujf-grenoble.fr (M. Bernet). The collision between India and Asia started at Ma, causing intense shortening and crustal thickening, and resulting in the -9/$ - see front matter Ó Elsevier Ltd. All rights reserved. doi:./j.jseaes...9

2 F. Chirouze et al. / Journal of Asian Earth Sciences () 9 9 Simplified Geological Map of Nepal Darchula LHCN Karnali river MCT Jumla STDS Brahmaputra river T I B E T Nepalganj MBT LHCN Pokhara Mount Everest Arun N km Narayani kali Kathmandu Bagmati MFT MU LHCN Saptakoshi Dhankulj MU Sample location Muksar Khola MCT Main Central Thrust MBT Main Boundary Thrust MFT Main Frontal Thrust STDS South Tibetan Detachment System LHCN Lesser Himalayan Crystalline Nappes Quaternary Sub-Himalaya Miocene Granites Lesser Himalaya Higher Himalaya Tethyan Himalaya Fig.. Simplified geologic map of Nepal (modified from Amatya and Jnawali (99), DeCelles et al. (99), and Bernet et al. ()). The location of the Muksar Khola section is indicated with a star. formation of the Himalayan orogen and the Tibetan plateau (Le Fort, 9). The Himalayan belt is generally divided into four main litho-tectonic units, separated by major northward-dipping fault systems (e.g. Hodges, ). From north to south these litho-tectonic units have the following arrangement (Fig. ). The Tethyan Himalaya is characterized by Cambrian to Eocene sedimentary and low-grade metamorphic rocks, and is separated from the Higher Himalaya to the south by the South Tibetan Detachment System (STDS), which is a large normal fault system. The Higher Himalaya consists of Late Proterozoic to early Cambrian high-grade metamorphic rocks, over-thrusting the Lesser Himalaya along the Main Central Thrust (MCT). The Lesser Himalaya is composed of Proterozoic to Palaeozoic sedimentary and meta-sedimentary rocks and thrusts over the Sub-Himalaya (or Siwalik Formation) along the Main Boundary Thrust (MBT). The Siwalik Formation consists of siliciclastic sedimentary rocks and represents the Miocene Pliocene fill of the Himalayan foreland basin (e.g. DeCelles et al., 99). The Siwaliks are limited southward by the currently active Main Frontal Thrust (MFT). The STDS and the MCT seem to have been active simultaneously before Ma (e.g. Hodges et al., 99; DeCelles et al., ) while the other south-propagating Himalayan thrust systems have been active at different times. In western Nepal, thrusting on the MBT started at Ma (e.g., Robinson et al.,, ; Huyghe et al., ). Duplex formation and exhumation of Lesser Himalayan rocks seem to have started earlier, between and Ma (Robinson et al.,,, ; Huyghe et al., ; Herman et al., ). Activity on the MFT started at.. Ma (Mugnier et al., ). In contrast to the Himalaya in western-central Nepal, erosion has not removed most of the Higher Himalayan cover, here called the Lesser Himalayan Crystalline Nappe (LHCN) (Yin, ) forming the Klippe of Kathmandu, resulting in less exposure of Lesser Himalayan units and location of the MCT much further south (Fig. )... The Siwalik Formation along the Muksar Khola section Outcrops exposed along the Muksar Khola section allow reasonably good access to the middle Siwalik Formation, which was sampled for this study. A stratigraphic log, modified after Ojha et al. (9), is presented in Fig.. The section dated by Ojha et al. (9) is about m thick and contains a thickening upward succession of mudrock, sandstone and conglomerates. Sedimentary facies successions are very similar to those described in western and central Nepal (Gautam and Appel, 99; Gautam and

3 9 F. Chirouze et al. / Journal of Asian Earth Sciences () 9 Fig.. Schematic stratigraphic log of the Muksar Khola section modified from Ojha et al. (9), indicating the lower, middle and upper members of the Siwalik Formation, as used in this study. Sampling for detrital thermochronology was focused on the middle Siwalik Formation (samples MU). Fujiwara, ; Ojha et al., 9). The lower Siwalik Formation is composed of thin beds ( m thick) of fine sandstone inter-layered with reddish silt layers ( cm thick). Well-developed paleo-soil horizons are common. This type of deposit may be attributed to a meandering river system. The middle Siwalik Formation, about m thick, presents thick beds of medium to coarse grained salt and pepper sandstone. The thickness of these layers ranges between m and m. The beds are composed of numerous amalgamated channels with pebble lag deposits. Such a facies may be attributed to a large braided river system. The upper Siwalik Formation is characterized by coarse sand and conglomerate deposits of a gravely braided river system. The stratigraphic section dated by Ojha et al. (9) is here used as the reference section for the stratigraphic depths of our samples. The exact thickness of the overlying upper Siwalik Formation is not known, because the rocks are poorly consolidated and the outcrop conditions are less than ideal. The magnetostratigraphic analysis of Ojha et al. (9) indicates that the transitions from the lower to the middle Siwalik

4 F. Chirouze et al. / Journal of Asian Earth Sciences () 9 9 Lesser Himalaya Higher Himalaya Depositional age (Ma) Robinson et al. () Bedrock εnd Tibetan Himalaya Higher Himalaya Lesser Himalaya Robinson et al. () Khutia Kohla Surai Kohla Muskar Khola Huyghe et al. () Surai Khola Karnali River ε Nd Fig.. Neodymium isotopic data from the Siwalik Formation in Nepal and from bedrock samples of the Lesser and Higher Himalaya. All data from Huyghe et al. () and Robinson et al. (). Formation and from the middle to the upper Siwalik Formation was more or less coeval with the transitions observed in western and central Nepal. The sedimentation rate of the units in the Muksar Khola section was estimated at about. mm/year between and. Ma, increasing to about. mm/year after. Ma, overall being somewhat slower than the sedimentation rates determined for western and central Nepal (Ojha et al., 9). Neodynium isotopic analysis (Robinson et al., ) indicates that the sediments of the Muksar Khola section (ranging from. to. e Nd ) were mainly derived from Higher Himalayan rocks or Tethyan Himalayan rocks. In contrast to the stratigraphic sections in western Nepal (Huyghe et al., ; Robinson et al., ), the e Nd values of the Muksar Khola section do not shift toward more negative values around Ma (Fig. ), indicating that Lesser Himalayan rocks in eastern Nepal were not at the surface and eroded at that time.. Methods.. Fission-track analysis of detrital apatite and zircon Over the past twenty years, fission-track analysis of detrital apatite and zircon has developed into a standard technique for studying sediment provenance and exhumation of orogenic source areas (e.g. Hurford and Carter, 99; Carter, 999; Garver et al., 999; Bernet and Garver, ). For this study, apatite and zircon grains were separated from overall fine to medium grained sandstone samples using heavy liquid and magnetic separation techniques. Apatite aliquots were mounted in epoxy, polished to expose internal crystal surfaces, and etched with. M HNO for s at C. Zircons were mounted in Teflon Ò sheets, polished, and etched at C in a laboratory oven in a eutectic NaOH KOH melt. Two mounts were assembled per sample and etched for different lengths of time, with etching times ranging between and h. The etching progress and the quality of the etched tracks were controlled between subsequent etching steps to obtain countable fission tracks in the majority of the grains (e.g. Naeser et al., 9; Bernet et al., ). All samples were covered with mica sheets as external detectors and sent for neutron irradiation to the FRM II Research Reactor at the Technische Universität München. Apatite samples were irradiated together with IRMM glass standards and Durango and Fish Canyon Tuff age standards. Zircon samples were irradiated together with CN glass standards and Fish Canyon Tuff and Buluk Tuff age standards. After irradiation the mica sheets of all samples and standards were etched for min at C in % HF. The samples and standards were counted dry at magnification, using an Olympus BH optical microscope and the FTStage. system of Trevor Dimitru. The objective was to date between and grains per sample, when possible. Observed grain-age Table Detrital apatite fission-track results, Muksar Khola section, eastern Nepal. Sample Depositional age (Ma) n Age range (Ma) P (Ma) P (Ma) P (Ma) Central age (Ma) MU. ±... ± MU. ±... ±.. ±.. ±. 9.%.% MU. ± 9... ±..9 ±.. ±. 9.%.% MU. ±... ±.9. ±.. ±. 9.% 9.% MU. ± ±..9 ±.. ± ±..%.%.% Note: n = total number of grains counted; binomial peak-fit ages are given ± SE. Also given is the percentage of grains in a specific peak. No peaks were fitted for sample MU because of the low number of dated grains. All samples were counted at dry ( objective,. tube factor, oculars) by F. Chirouze using a zeta (IRM ) of. ± 9.9 (± SE).

5 9 F. Chirouze et al. / Journal of Asian Earth Sciences () 9 (A) Probability density (%/Δ z=.) Probability density (%/Δ z=.) Probability density (%/Δ z=.) Probability density (%/Δ z=.) Probability density (%/Δ z=.) MU N = MU N = 9 MU N = 9 MU N = 9 Muksar Kohla detrital apatite FT data MU N = Apatite fission-track grain age (Ma) (B) Probability density (%/Δ z=.) Probability density (%/Δ z=.) Probability density (%/Δ z=.) Probability density (%/Δ z=.) Probability density (%/Δ z=.) Muksar Kohla detrital zircon FT data 9 MU N = 9 MU N = MU N = 9 MU N = MU N = Zircon fission-track grain age (Ma) Fig.. Detrital fission-track results for the Muksar Khola section. (A) Probability density plots with histograms, observed grain-age distribution (black curve) and best-fit peaks of apatite samples. (B) Probability density plots with histograms, observed grain-age distribution (black curve) and with best-fit peaks of zircon samples.

6 F. Chirouze et al. / Journal of Asian Earth Sciences () 9 99 Fission-track peak age (Ma) static peak Muksar Khola Depositional age (Ma) 9 depositional age Myr lag time reset AFT ages lag-time contours P AFT P ZFT Fig.. Lag-time plot of the youngest apatite and zircon age peaks (P) in each sample. Samples MU MU show signs of annealing of fission-tracks in apatite but none of the zircon grains seem to be reset. Table Detrital zircon fission-track results, Muksar Khola section, eastern Nepal. Sample Depositional age (Ma) n Age range (Ma) P (Ma) P (Ma) P (Ma) P (Ma) MU. ±... ±.. ±.. ±. 9.%.%.% MU. ±... ±.. ±. 9.%.% MU. ±... ±.. ±.9. ±..%.%.% MU. ±... ±.. ±.. ±..%.% 9.% MU. ± 9... ±.. ±.. ±.. ±..%.%.%.% Note: n = total number of grains counted; binomial peak-fit ages are given ± SE. Also given is the percentage of grains in a specific peak. All samples were counted at dry ( objective,. tube factor, oculars) by M. Bernet using a zeta (IRM ) of. ±. (± SE). Table Point counting results Muksar Khola section. Sample Qm Qp K-feldspar Plagioclase Mudrock Carbonate Chert Lv Lm Mica Chlorite Opaque Other Total MU MU MU MU MU Note: Data are given in percent values based on point counts per sample by V.E. Qm = monoquartz, Qp = polyquartz, Lv = volcanic lithic grains, Lm = metamorphic lithic grains. distributions were decomposed into major grain-age components or peaks, using the BINOMFIT software of Brandon (Stewart and Brandon, ; Ehlers et al., )... Thin section analysis Petrographic thin sections of the same five samples used for thermochronology were analyzed using an Olympus BX polarizing microscope. Three hundred points were counted per thin section, differentiating monocrystalline and polycrystalline (> subgrains) quartz, K-feldspar and plagioclase, sedimentary lithic grains (mudrock, carbonate and chert), volcanic and metamorphic lithic grains, micas, chlorite, opaque minerals, and other (unidentifiable) grains. Because all five samples were obtained from very poorly consolidated sandstone, the samples were imbedded in epoxy before thin-section preparation. For this reason no quantitative information was obtained concerning cementation or porosity... Clay mineral analysis A suite of seven samples (NP - to NP -9) were collected from mudrock and siltstone layers of the lower and middle Siwalik

7 F. Chirouze et al. / Journal of Asian Earth Sciences () 9 (A) Quartz 9 MU MU MU MU MU Lv MU MU MU MU MU Feldspar 9 Lithic grains Ls 9 (B) Lm Fig.. Point counting results of the Muksar Khola section samples. (A) QFL diagram after Dickinson et al. (9). All samples lie in the recycled orogenic field. Q = total quartz (monoquartz + polyquartz + chert), F = K feldspar + plagioclase, L = lithic grains. Percentages are based on normalized results from point counts per sample. (B) Lithic grain diagram, differentiating sedimentary (Ls), volcanic (Lv) and metamorphic (Lm) lithic grains. Ls includes mudrock, carbonate and chert lithic grains. Note the overall stratigraphic trend with increasing metamorphic and decreasing sedimentary lithic grains up-section. Fig.. Sandstone petrology of middle Siwalik Formation rocks along the Muksar Khola section. (A) Mudrock lithic grain in sample MU. (B) Deformed muscovite grain in sample MU. (C) Gneiss and micaschist lithic grains in sample MU. (D) Gneiss lithic grain in sample MU. Formation (Fig. ) for X-ray diffraction analysis. Samples were gently crushed to obtain a < lm grain size fraction. The samples were decarbonated in. N HCl. Excess acid was removed by repeated water washing and centrifuging followed by homogenization. The < lm fractions were collected by decantation, applying Stoke s Law for determining the settling time and depth to isolate the <-lm size fraction, and oriented aggregates were mounted on glass slides. All samples were scanned three times: () air-dried, () glycolated ( h under vacuum in ethylene glycol at room temperature), and () heated ( h at 9 C); using a Philips PW

8 F. Chirouze et al. / Journal of Asian Earth Sciences () 9 Stratigraphic depth (m) Illite (%) Chlorite (%) Kaolinite (%) Smectite (%) IS and CS mixed-layers (%) Fig.. XRD analysis results of samples from the lower and middle Siwaliks along the Muksar Khola section. A change in clay mineralogy is noticeable at about m burial depth, corresponding to a Ma depositional age. diffractometer at the University of Lille. Semi-quantitative estimation of the clay mineral content (illite, chlorite, kaolinite, smectite and chlorite smectite and illite smectite mixed-layers) is based on the heights and areas of basal reflections on the ethylene glycol XRD diffractograms, assuming that these weighted amounts add up to % (Biscaye, 9; Capet et al., 99; MacDiff software of Petschick (99)).. Results.. Apatite fission-track results Apatite fission-track ages of five samples from the middle Siwaliks in the Muksar Khola section are summarized in Table and Fig.. The ages range from about Ma to Ma. For sample MU only apatite grains could be dated. Therefore, no peaks were fitted for this sample and only the central age of. Ma is given in Table. Sample MU has a central age and peak ages that are clearly older than the depositional age estimated from magnetostratigraphic correlation. The largest fraction of dated grains in this sample (%) falls into the second age peak (P). In samples MU, MU and MU the youngest age peak (P) is either close to or much younger than the estimated depositional age, and contains the majority of grains in samples MU and MU (Table, Fig. ). The older age peaks in all four samples range between Ma and Ma... Zircon fission-track results The observed zircon fission-track ages of five Muksar Khola section samples (see Table ) range from about Ma to over Ma. All determined peak ages are clearly older than the estimated depositional ages of the individual samples (see Fig. ). Each sample has a young age peak (P) of about Ma and an old age peak of over Ma. With exception of sample MU, all samples also have an intermediate age peak of about Ma, somewhat similar to the second peak of the detrital apatite fission-track age distributions... Sediment petrology results Normalized point counting results of the fine to medium grained sandstone samples MU to MU are summarized in Table. All five samples fall into to the recycled orogenic field of Dickinson et al. (9), as shown in Fig. A, with total quartz including monocrystalline, polycrystalline and chert grains. In all samples, monocrystalline quartz grains are the dominant fraction, while feldspar grains are very rare to absent. For the majority, lithic grains consist of either mudrock sedimentary lithic grains or gneiss and micaschist metamorphic lithic grains (Figs. B and ). Carbonate lithic grains are somewhat more frequent in the stratigraphically older samples toward the base of the middle Siwaliks. All samples contain detrital chlorite and mica (mainly muscovite) grains, with the highest mica content being detected in sample MU. No quantitative information was obtained for cementation and porosity, as mentioned above. Nonetheless, qualitative observations include traces of calcite cementation in all samples, and considerable iron-oxide formation in the stratigraphically lower samples, which makes up the opaque fraction in Table... Clay mineral analysis The X-ray diffraction analysis of the < lm fraction of the middle Siwalik sediments mainly consists of illite, chlorite, kaolinite, smectite and mixed-layer minerals (illite smectite/chlorite smectite). These clayey minerals show a relatively constant content through the deposition of the Siwalik with about % illite, % chlorite and kaolinite, % smectite and % mixed-layers (Fig. ).. Discussion.. Thermal evolution and exhumation of the Siwaliks At first glance the apatite and zircon fission-track peak ages of the middle Siwaliks along the Muksar Khola section seem to be compatible with published apatite and zircon fission-track data from western and central Nepal (Bernet et al., ; van der Beek

9 F. Chirouze et al. / Journal of Asian Earth Sciences () 9 Fig. 9. Estimated depth of the upper limit of the exhumed apatite fission-track partial annealing zone in the middle Siwaliks of the Muksar Khola section, based on a C paleo-surface temperature and an average paleo-geothermal gradient of C/km in the Himalayan foreland basin (van der Beek et al., ).

10 F. Chirouze et al. / Journal of Asian Earth Sciences () 9 Depositional age (Ma) Karnali Surai Khola Tinau Khola Muksar Khola lag-time contours m.y. Zircon fission-track peak age (Ma) Fig.. Comparison of western and eastern Nepalese detrital zircon fission-track data. Shown are all grain-age components of samples from the Karnali, Surai Khola, Tinau Khola and Muksar Khola sections (Bernet et al.,, and this study). et al., ). The section we studied is the same as the one dated by Ojha et al. (9) and has a total stratigraphic thickness of about m. It is not well known how much of the overlying upper Siwalik formation is not represented in this section or may have been lost because of erosion and internal thrusting, which superposes lesser Siwaliks over upper Siwaliks rocks (Amatya and Jnawali, 99). The apatite fission-track ages presented in this study provide a first-order estimate on the not represented upper Siwaliks section. Sample MU was collected at about m stratigraphic depth of the Ojha et al. (9) section (Fig. ). The age distribution and age peaks of this sample show that the fission tracks in these apatites were unaffected by thermal resetting due to post-depositional burial. Samples MU, MU and MU on the other hand, collected at 9 m, m and 9 m stratigraphic depth respectively, show signs of partial resetting, with the youngest age peak being the same or younger than the depositional age estimated from magnetostratigraphy. In addition, the central ages of these three samples are much younger than the central ages of samples MU and MU (Table ). However, even in the most deeply buried sample (MU) fission tracks in apatite have not been fully reset. This means burial temperatures were not exceeding C, given that the partial annealing zone (PAZ) of average fluorine apatite ranges from about C to C, and given Myr of heating (Brandon et al., 99; see Fig. 9). Unfortunately, it was not possible to perform statistically meaningful track-lengths measurements for thermal history modelling on our samples due to the lack of sufficient horizontally confided tracks and the limited sample material available for this study. Nonetheless, from these relationships between depths and fission-track age, we can make a first-order estimate on the thickness of the poorly documented overlying upper Siwalik Formation on top of the section dated by Ojha et al. (9). Assuming a C surface temperature and an average thermal gradient of about C/km for the Siwaliks (van der Beek et al., ), km of burial depth are needed to reach a temperature of C and the upper limit of the apatite fission-track PAZ. As mentioned above, apatite fission-track ages of sample MU at m stratigraphic depth of the Ojha et al. (9) section show no signs of annealing, while fission tracks in sample MU at 9 m have been partially annealed. The upper limit of the former apatite fission-track PAZ falls between about 9 and m stratigraphic depth of the Ojha et al. (9) section, which is about m too shallow to allow partial annealing of fission tracks in apatite. Therefore, we can estimate that roughly m of the overlying upper Siwalik Formation are not documented by Ojha et al. (9), and a minor part of the stratigraphy may have been removed by thrusting and/or erosion. Taking into account the youngest sedimentation rate (. mm/ year) calculated along the Muksar section and the last deposit dated by Ojha et al. (9), we think that sedimentation ended at about Ma. The rate of subsidence should generally increase through time as the elastic flexural wave migrates in the foreland (Angevine et al., 99), and therefore this age could be older. We can interpret the end of sedimentation as the beginning of thrusting over the MFT ramp, which means that the MFT has been at minimum active since Ma. In Western Nepal, apatite fission-track results and field observations indicate that the MFT activity began around Ma (Mugnier et al., ; van der Beek et al., ). Taking into account the uncertainties on sedimentation rate and geothermal gradient, our result suggests that MFT was active at the same time in eastern Nepal... Exhumation of the Eastern Himalaya Apatite fission tracks in samples MU and MU were not reset, and therefore provide information on source exhumation. Due to the low number of grains available, theses results should not be over-interpreted. Nevertheless, the central ages of these samples (. Ma and. Ma) are older than those calculated for the same time interval (from Ma to Ma) in western and central Nepal, of 9. Ma and. Ma (van der Beek et al., ). The detrital zircon fission-track ages of samples MU to MU are all clearly older than the depositional age and show no signs of resetting. This is in agreement with the apatite fission-track data discussed above. The zircons show in general three age peaks at about Ma, Ma and > Ma. Compared to the available detrital zircon fission-track data from western-central Nepal (Bernet et al., ), there are similarities but also two important differences in the datasets (Fig. ).

11 F. Chirouze et al. / Journal of Asian Earth Sciences () 9 Regarding similarities, both in eastern and western-central Nepal, two static age peaks were observed. A static age peak is a peak age that can be observed in several samples, independent of depositional age. For a static peak the lag time increases up-section. In contrast to the static peak is the moving peak. The age of this peak becomes younger up-section but the lag time either remains constant or becomes shorter (e.g. Bernet and Garver, ). The old static age peak concerns zircons with pre-himalayan, mainly Cretaceous and Jurassic to Triassic cooling ages. As suggested by Bernet et al. () for western-central Nepal, we interpret zircons with such old cooling ages as being derived from non-reset cover units, most likely the Tethyan Himalayan series. The young static peak of about Ma is just slightly younger than the young static peak observed in western-central Nepal of Ma (Bernet et al., ). Zircons in this age peak can be derived either from Higher Himalayan or Lesser Himalayan rocks. Additional U/Pb dating of the same zircon grains would be necessary for better constraining provenance (DeCelles et al., 99; Bernet et al., ). Besides these similarities there are also differences. The first zircon fission-track age peaks (P) of the stratigraphically older MU, MU and MU samples, belong either to the static Ma peak, or the Myr lag-time moving peak, because the peaks plot close to the Myr lag-time curve (Fig. ). Unfortunately, this cannot be clearly distinguished with the available data. However, the stratigraphically younger MU and MU Muksar Khola samples do not show a young moving peak with a steady lag time of about Myr. They plot far from the Myr lag-time curve. The second difference is that the second age peak (P) in the Muksar Khola samples, ranges from to Ma. Such an age peak was not detected in western-central Nepal (Bernet et al., ). How can these differences be explained? In western-central Nepal, zircons belonging to the Myr lag-time peak were attributed to being derived from Lesser Himalayan and Higher Himalayan metamorphic rocks in the vicinity of the MCT. The apparent absence of the moving peak, at least in the upper middle Siwaliks and the presence of the Ma peak throughout the middle Siwalik Formation must therefore be related to either differences between eastern Nepal and western Nepal exhumation dynamics or development of a local drainage network, only draining the LHCN and the overlying THS. In contrast to western-central Nepal, the MCT is located much further south in eastern Nepal and large areas are still covered by Higher Himalayan units (Fig. ). This means that during deposition of the middle Siwaliks in eastern Nepal between and. Ma, erosional exhumation had not exposed rocks with young cooling ages. This difference may explain the ZFT population of Ma observed in the Muksar Khola section. These results are in agreement with the tectonic structure of eastern Nepal, with rare duplex structures within the Lesser Himalaya in comparison to western Nepal (Schelling and Arita, 99; Schelling, 99; Robinson et al., ). The apparent lack of a young moving peak in the upper middle Siwaliks is more difficult to explain. Acknowledging our rather limited dataset, the apparent lack suggests that Higher Himalayan rocks were possibly not rapidly exhumed on a large spatial scale, at least not during deposition of the upper middle Siwaliks in eastern Nepal. However, this interpretation has to remain rather speculative given that lack of more middle and upper Siwaliks samples. Nowadays, despite the MCT being located far to the south in eastern Nepal, the presence of a topographic break at km north of the MFT (Yin, ) and the localized incision of the Arun River in the vicinity of the Higher Himalayan topographic front suggest that Higher Himalayan rocks are being rapidly exhumed during the Holocene, as in western-central Nepal (Lavé and Avouac, ). In agreement with this interpretation, Ar 9 Ar data along the Arun River present young cooling ages of about Ma close to the topographic front (Haviv et al., 9). These ages are quite similar to those calculated for central Nepal (Herman et al., ). The apparent absence of a Myr lag-time peak in sample MU and MU of the upper middle Siwaliks, but relatively young white mica Ar 9 Ar ages in the Arun River drainage today point to an increase in exhumation rate in some parts of the Higher Himalaya in eastern Nepal since the late Miocene/early Pliocene. Such a potential late Neogene increase in exhumation rate was not detected in western-central Nepal (van der Beek et al., ; Bernet et al., ; Szulc et al., ). Unfortunately, we do not have samples from the upper Siwaliks to document or disprove an increase in exhumation rate. As in western and central Nepal (Robert et al., 9; Herman et al., ), thermochronologic studies along the Arun River do not suggest MCT reactivation close to the topographic front of the range (Haviv et al., 9). Therefore, a possible explanation for the increase in exhumation could be the localized formation of duplex structures in Lesser Himalayan rocks close to the topographic front after Ma (Schelling and Arita, 99; Robinson et al., ). Higher Himalayan rocks located in the hanging wall of this duplex would be rapidly uplifted and form high topography, coupled with rapid river incision. These results hint at a decrease in the age of the formation of the duplex structures towards eastern Nepal, because duplex formation began at Ma in western Nepal (Huyghe et al., ; Robinson et al., ), at 9. ±. Ma in central Nepal (Herman et al., ) and after Ma in eastern Nepal. This interpretation is strongly supported by the increase of sedimentation recorded in the Muksar section at Ma, from. mm/year to. mm/year. This acceleration of sedimentation started at about Ma in western Nepal (Ojha et al., 9). A second possible explanation for the absence of the Myr lagtime peak in the upper middle Siwalik samples MU and MU in the Muksar Khola section lies in the organization of the drainage system in eastern Nepal, which is mainly drained by the Sunkoshi River and the Arun River. The Sunkoshi River flows along the MBT for km and reaches the foreland basin km east of the Muksar Khola section. The rivers flowing between the Narayani River and the Saptakoshi River mainly drain the LHCN and the Siwaliks hills (Fig. ). The organisation of the drainage system may be linked to MBT activity around Ma (DeCelles et al., ; Huyghe et al., ; Robinson et al., ). Currently, rivers draining the foothills of the Himalaya along the MBT and MFT are sandy and gravely braided rivers, but they are much smaller than the major antecedent rivers that cut across the mountain range, such as the Arun River (Fig. ). All the middle Siwaliks sediments of the Muksar Khola section seem to have been deposited by large rivers such as the antecedent rivers today, but this is difficult to judge on the outcrop scale. Therefore, based on sedimentary facies alone, it is not possible to clearly determine whether the middle Siwaliks sedimentary rocks along the Muksar Khola section were deposited by a smaller local drainage or are part of a mega fan of larger river drainages. We need to look at the available petrologic and thermochronologic sediment information... Provenance The sediment petrology of our samples indicates that more sedimentary lithic grains are present in the lower part of the studied section, and an increase in high-grade metamorphic lithic grains is observed up-section. This at least is consistent with a classic orogenic exhumation signal, with removal of sedimentary cover rocks and erosion of crystalline basement up-section. Our dataset is very limited and should not be over-interpreted, because the abundance of mudrock clasts in fluvial sandstone could be simply the results of sediment recycling within the fluvial system itself. Nonetheless,

12 F. Chirouze et al. / Journal of Asian Earth Sciences () 9 the occurrence of carbonate lithic grains at the base of the middle Siwaliks suggests that sedimentary clasts are provided by the THS and the presence of metamorphic lithic grains, detrital mica and chlorite in all samples indicates the importance of high-grade crystalline rock source areas, in this case, Higher Himalayan crystalline rocks. As suggested by the AFT data, the middle Siwalik sediments of the Muksar section did not reach temperatures exceeding 9 C, which is required for smectite to illite or mixed-layers transformation (Dunoyer de Segonzac, 9). Therefore, no significant burial diagenetic clay mineral transformation may have occurred and the signal shown in the clay mineral fraction could in that case to be considered reflecting provenance. At any rate, the illite and chlorite contents of the lm fraction of the middle Siwaliks along the Muksar section is very similar to that found in the central and western Nepalese Siwaliks sections (Huyghe et al.,, ), the content in smectite is slightly lower (% instead %) considering that our semi-quantitative analyses present a ±% error. If we accept that the clay mineral composition was not heavily affected by diagenesis because of the relatively cool temperatures, the data could suggest that the proportions of TSH, HHS and LHS source domains that compose the catchment basins of the main Himalayan rivers slightly differed between central/ western Nepal and eastern Nepal during the time of deposition of the middle Siwaliks regarding the proportion of smectite and kaolinite. Nonetheless, this remains highly speculative in lieu of more precise data. For the LHCN, close to Kathmandu, ZFT ages of 9 Ma were determined (Arita and Ganzawa, 99). In this klippe Ar 9 Ar muscovite ages from the MBT to the Higher Himalayan topographic front range between and Ma respectively (Arita et al., 99; Herman et al., ). Lesser Himalayan rocks close to the MBT along the Arun River have Ma old Ar 9 Ar ages (Haviv et al., 9). Acknowledging the scarcity of the available data, none of these can explain the old > Ma ZFT population that we find in all of our samples and the > Ma AFT central ages in the upper two samples. Either proximal Tethyan cover rocks with unreset ZFT ages were present during the time of deposition of the middle Siwaliks and locally drained or the Muksar Khola section rocks were sourced by an antecedent river drainage that reached the Tethyan series to the north. Considering the sedimentary facies and the increase in sedimentation rate from about Ma on, sediment petrology and detrital thermochronologic data together, it seems more likely that the middle Siwaliks sedimentary rocks of the Muksar Khola section were sourced by a large drainage system, rather than a small local river. Nonetheless, more detailed work including the upper Siwaliks is necessary to further explore this question.. Conclusions Our preliminary study of the thermochronology and sediment petrology of the middle Siwalik Formation along the Muksar Khola section provides three conclusions or observations. First, based on the partial resetting of fission tracks in apatite and the present day stratigraphic depth of our samples, we can estimate that about m of the overlying upper Siwalik Formation are not documented in the section dated by Ojha et al. (9). One thousand meters of additional burial are needed to explain the exhumed apatite fission-track PAZ that we observe in the middle Siwalik Formation of the Muksar Khola section today. Secondly, detrital zircon fission-track thermochronology shows two static age peaks at about Ma and > Ma in all samples, which are similar to the static age peaks observed in Siwalik Formation sandstone in western-central Nepal. In contrast, the signal of fast exhumation with Myr lag-time age peaks is not clearly developed, at least not in the upper part of the middle Siwaliks of the Muksar Khola section. Acknowledging our limited dataset, we attribute this to a lesser amount of total Miocene exhumation in the Himalaya of eastern Nepal, as already indicated by a position of the MCT further to the south, and the much less exposed Lesser Himalayan units beneath the Higher Himalayan crystalline rocks. This is consistent with the slightly slower sedimentation rate in eastern Nepal during that time, in comparison with western-central Nepal as proposed by Ojha et al. (9). Nonetheless, young thermochronologic ages from the present-day Arun River drainage hint at an increase in exhumation rate in eastern Nepal since the late Miocene/early Pliocene. Thirdly, the combination of information on sedimentary facies, sediment petrology and detrital thermochronologic data does not allow an unequivocal interpretation of the size of the drainage that sourced the middle Siwaliks sedimentary rocks of the Muksar Khola section. A large drainage system is more likely than a small local river, given the old detrital zircon fission-track ages observed in all our samples. Acknowledgements We gratefully acknowledge critical yet constructive reviews by Sam van Laningham and Massimillano Zattin which helped to improve the manuscript. 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