Detrital zircon ages and provenance of Neogene foreland basin sediments of the Karnali River section, Western Nepal Himalaya

Journal of Asian Earth Sciences 138 (2017) 98–109

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Detrital zircon ages and provenance of Neogene foreland basin sediments of the Karnali River section, Western Nepal Himalaya Upendra Baral a,b,⇑, Lin Ding a, Deepak Chamlagain c a

Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China University of Chinese Academy of Sciences, Beijing 100101, China c Department of Geology, Tri-Chandra Multiple College, Tribhuvan University, Nepal b

a r t i c l e

i n f o

Article history: Received 25 April 2016 Received in revised form 28 January 2017 Accepted 2 February 2017 Available online 6 February 2017 Keywords: Nepal Himalaya Siwalik Group Karnali Section U-Pb geochronology Provenance analysis

a b s t r a c t This paper deals with the possible provenance of the middle Miocene to early Pleistocene fluvial sediments of the Siwalik Group along with the Karnali River section (Western Nepal) and adds new insights into the formation and evolution of the Himalayan orogeny by means of detrital zircon U-Pb dating by LA-ICP-MS supplemented with sandstone petrography. The modal composition of the dated sandstone samples shows a ‘recycled orogen’ field in QFL diagram, indicating significant reworking and recycling of the detrital sediments during the mountain building process. The detrital zircon U-Pb ages from the Lower Siwalik cluster around 490–600 Ma and 750–1300 Ma with major peaks at 560 Ma, 927 Ma, and 983 Ma. The result shows that the Tethys Himalaya, Higher Himalaya and possibly the Lesser Himalaya were the predominant sources during the deposition of the Lower Siwalik Group. Contrarily, the detrital zircon U-Pb ages in the Middle Siwalik rocks contains clusters around 299–727 Ma, 750–1200 Ma, 1650–1900 Ma with major peaks at 469 Ma and 905 Ma. However, the increased input of mid-Proterozoic detritus (1600 Ma) points to the possibility of denudation of the lower Lesser Himalaya following the deposition of the lower part of Middle Siwalik (i.e. since 10 Ma). Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Sediments deposited in the foreland basin provide key information of paleo-tectonic events during orogenic development. Careful analysis of these basin sediments allows one to trace them back to their original provenance and reconstruct uplift events (DeCelles and Giles, 1996). Along the entire east-west trending Himalaya, Paleogene-Neogene foreland basin sediments are exposed along the southern margin of the mountain front. This sequence has been used to study the rate of sedimentation and exhumation history of the source region, climatic changes, paleogeography and also for provenance analysis. In Nepal Himalayas, several studies have been conducted, such as detrital white mica 40Ar-39Ar thermochronology (Szulc et al., 2006), fission tack and U-Pb dating (Bernet et al., 2006), to reveals the exhumation of the Himalayan mountain system. The Ar-Ar thermochronology on detrital white mica revealed the increased erosion of the lesser Himalaya since 12 Ma (Szulc et al., 2006). Furthermore, the fission track and U⇑ Corresponding author at: Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China. E-mail address: [email protected] (U. Baral). http://dx.doi.org/10.1016/j.jseaes.2017.02.003 1367-9120/Ó 2017 Elsevier Ltd. All rights reserved.

Pb dating show the widespread cooling age of 16.0 ± 1.4 Ma of the Nepal Himalaya to be associated with the movement of the Main Central Thrust (MCT) and South Tibetan Detachment System (STDS) (Bernet et al., 2006). Similarly, the exhumation rate is constant over last 7 Ma in central Nepal Himalaya, which is 1.8 km Myr 1, whereas, the exhumation rate is lower in Western Nepal Himalaya, which is 1–1.5 km Myr 1 (van der Beek et al., 2006). The isotopic investigation (Robinson et al., 2001; Huyghe et al., 2005) pinpoint the two major changes in sedimentary fill in the Neogene foreland basin at 9.5 Ma by the abrupt increase of the ƐNd (0), possibly due to local exhumation of the Lesser Himalaya by the emplacement of Ramgarh Thrust (RT). The facies analysis classify the Siwalik Group into seven facies association (KFA1– KFA7) in which the fluvial system changes from meandering to gravelly braided river system (Tokuoka et al., 1990; Nakayama and Ulak, 1999; Huyghe et al., 2005; Sigdel, 2013). The magnetostratigraphic dating of the Siwalik group restricts the depositional age between >16.0 and <1 Ma (Gautam and Rösler, 1999; Gautam and Fujiwara, 2000; Ojha et al., 2000, 2009), while the provenance analysis reveals the Lesser, Higher and Tethys Himalayas as the sediment source to the Siwalik foreland basin from north (Critelli and Garzanti, 1994; Garzanti et al., 1996; DeCelles et al., 1998a).

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In the past several decades, U-Pb dating based on detrital zircon (chemically stable and mechanically durable within different depositional environments and weathering condition) has become a precise tool for provenance analysis. Geochronological investigations in Nepal Himalaya have revealed that the foreland basin sediments were derived from the Higher and Lesser Himalayas since late Cenozoic (DeCelles et al., 1998a, 1998b, 2000, 2001, 2004; Najman et al., 2005; Baral et al., 2015). In Nepal Himalaya, the majority of these provenance studies focused on the Paleogene sequence (DeCelles et al., 1998b, 2004, 2014; Najman et al., 2005; Gehrels et al., 2011). However, a few studies were carried out within the Neogene Siwalik Group and most of them are concentrated on some portion of the Siwalik Group (Critelli and Ingersoll, 1994; DeCelles et al., 1998a; Szulc et al., 2006; Baral et al., 2015). Therefore, in the present study, we have selected the Siwalik Group exposed along the Karnali River section, which is magneto-stratigraphically dated in detail (Gautam and Fujiwara, 2000). We applied detrital zircon U-Pb geochronology together with sandstone petrography for better interpretation of the provenance during middle Miocene to early Pleistocene period.

2. Geological setting 2.1. Brief geology of Nepal Himalaya The Himalaya range, a typical continent-continent collision zone between the Indian and Eurasian plates (Gansser, 1964; Patriat and Achache, 1984), represents the youngest collision orogeny of the early Eocene time (55–50 Ma)(Yin and Harrison, 2000). It extends for about 2400 km from the Nanga Parbat in the west to the Namche Barwa in the east. In general, it is divided into four distinct lithotectonic units: (1) the Tethys Himalaya (TH), (2) the Higher Himalaya (HH), (3) the Lesser Himalaya (LH), and (4) the Siwalik domain (Fig. 1). Each of these units is separated by a distinct northward dipping fault/thrust system (Fig. 1) (Le Fort,

99

1975; Yin and Harrison, 2000; Yin, 2006). The Indo-Gangetic Plain is a flat area at the southernmost part of the Himalaya and is covered by Quaternary deposits. The northernmost TH is comprised of Cambrian to Late-Cretaceous fossiliferous sedimentary rocks, although few metamorphic rocks occur near the boundary with the Higher Himalaya and younger Eocene rocks in Tibet. The TH extends from the Indus-Yarlung Suture Zone (IYSZ), the collision zone of the Indian and Eurasian plates (Le Fort, 1975) in the north, to South Tibetan Detachment System (STDS) in the south. The HH consists of high-grade crystalline rocks including various orthogneisses and para-gneisses of kyanite and sillimanite grade, schists and migmatites. The thickness of the HH varies along the entire Himalaya range and extends from STDS to Main Central Thrust (MCT) towards south (Upreti, 1999; Yin, 2006). South of the MCT, a large body of sedimentary and low- to medium-grade metamorphic rocks known as the LH, which extends to the Main Boundary Thrust (MBT) further to the south. Within the Lesser Himalaya, there are several nappes and klippes occurred which were formed by the movements along the major thrusts. Based on the age of the rocks, as well as metamorphic and geochemical criteria, the LH can be further divided into two groups, the Lower and Upper Lesser Himalaya Series, with a major unconformity known as the Great Lesser Himalaya Unconformity (Valdiya, 1998). The Lower Lesser Himalaya (LLH) is Precambrian in age, with rocks ranging from Paleoproterozoic (1800–2000 Ma) to late Neoproterozoic (540 Ma) (Valdiya, 1995, 1998; Parrish and Hodges, 1996). The upper Lesser Himalaya (ULH) comprises Permo-Carboniferous, (Gondwana) sedimentary units below and the early Cretaceous to Eocene marine rocks (Sakai, 1983) above. The Eocene marine sediments of the Bhainskati Formation marked by an oxisol layer, are overlain by the lower to middle Miocene (or Oligocene to early Miocene) fluvial sediments of the Dumri Formation (Sakai, 1983; DeCelles et al., 2004). The Dumri Formation is the youngest strata within the LH, represents the first foreland basin sediment derived from the uprising Himalaya. The Lesser Himalaya duplexes started to raise by 10 Ma and formed an anticlinal structure by folding

Fig. 1. Geological map of Nepal modified after Upreti (1999). LH: Lesser Himalaya, HH: Higher Himalaya, TH: Tethys Himalaya, MBT: Main Boundary Thrust, MCT: Main Central Thrust, MT: Mahabharat Thrust, RT: Ramgarh Thrust, HFT: Himalaya Frontal Thrust, STDS: South Tibetan Detachment System. Thick solid black rectangle shows the study area and oval shape represent the location of Surai Khola section.

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the north limb of the Dadeldhura synform and resulting the erosion of the LH (DeCelles et al., 2001). Within the Lesser Himalaya, several local thrusts emplaced older units above younger units. For example, the Eocene Bhainskati Formation is thrusted over the Miocene Dumri Formation. The LH thrusted over the Siwalik Group, which represents the Neogene foreland basin sequence of the Himalaya. The Siwalik Group consists of about 6 km thick coarsening upward Neogene fluvial sediments that extend from the MBT to Himalayan Frontal Thrust (HFT) towards south (Gansser, 1964; Tokuoka et al., 1990; Gautam, 2008). Further to the south is the Quaternary fluvial deposit called as IndoGangetic Plain sediments. 2.2. Geological setting of the Siwalik Group in Nepal Himalaya The Siwalik Group, deposited during middle Miocene to early Pleistocene, is well-exposed along the southern hills of the Himalaya in various locations particularly along the major river sections (Dhital, 2015). The Siwalik foreland basin is one of the largest foreland basins on Earth (Szulc et al., 2006 and reference therein). Based on the threefold classification, this group is further divided into Lower, Middle and Upper Siwalik from bottom to top (Auden, 1935; Tokuoka et al., 1986, 1988). The Lower Siwalik subgroup is dominated by variegated mudstone with some finegrained sandstone and siltstone beds deposited in fluvial channel environment, in which the size of channel increases towards younger sequence (Ulak and Nakayama, 1998; DeCelles et al., 1998a). The Middle Siwalik subgroup is predominated by sandstone, the medium- to coarse-grained ‘‘salt and pepper” sandstones deposited by a large braided river channel with subordinate mudstone beds. The Upper Siwalik consists of conglomerate, and the clast size ranges from pebble to boulder with patchy sand and mud lenses, and the depositional environment here was gravelly braided river and alluvial fan type (Tokuoka et al., 1988, 1990; Ulak and Nakayama, 1998; DeCelles et al., 1998a). The recent magnetostratigraphic studies constrains the age of the Siwalik Group in Nepal between >16 Ma and <1 Ma (Gautam, 2008) which is in agreement with existing biostratigraphic study (Corvinus, 1988) and geochronological studies (DeCelles et al., 1998a). 2.2.1. The Karnali River section The Karnali River section, approximately 5300 m thick (Gautam and Fujiwara, 2000), is one of the complete exposures of the Siwalik Group in the far western Nepal Himalaya (Huyghe et al., 1998). The lower 3560-m-thick part of this section has been magnetostratigraphically dated at 16–5.2 Ma (Figs. 2 and 3) (Gautam and Fujiwara, 2000) conforming that the oldest age Siwalik Group sediments in the Nepal Himalaya are found here. In this study, we use this age frame as a reference for the depositional age. The paleo-environment of this section is well constrained by the previous studies differentiating the Siwalik Group into seven distinct facies association based on bed forms, nature of contacts, grain size, and sedimentary structures (details is summarized in Table 1) (Huyghe et al., 2005; Sigdel et al., 2011). The Lower Siwalik Group comprises variegated mudstone interbedded with fine-grained sandstone and siltstone with a total thickness of about 2000 m estimated in this study (Fig. 3). The fluvial environment of the Lower Siwalik changes from fine-grained meandering to sandy flood flow dominated type from basal to upper part (Huyghe et al., 2005). At 9.6 Ma, the meandering system changed into the braided system and the boundary between the Lower and Middle Siwalik is somewhere close to this age (Gautam and Fujiwara, 2000; Huyghe et al., 2005). The grain size gradually increases towards up-section. The magnetostratigraphic age varies in between 16 and 9.6 Ma (Gautam and Fujiwara, 2000).

Fig. 2. (a) Generalized Geological Map of Karnali River Section. (b) Geological crosssection along AB.

The Middle Siwalik is dominated by medium- to coarse-grained, grey colored, thin-to thick-bedded sandstone along with the mudstone and siltstone beds (Sigdel et al., 2011). The measured thickness of the Middle Siwalik in the Karnali Section is 2700 m (Fig. 3). This subgroup has a distinctive grain texture appearance, termed as ‘‘salt and pepper”. During deposition of this subgroup, the fluvial environment was changed from sandy braided to deep sandy braided and finally anastomose river system (Huyghe et al., 2005). Gautam and Fujiwara (2000) provided a magnetostratigraphic age of 9.6–5.2 Ma and marked the boundary between the Lower and Middle Siwalik at 9.6 Ma. The change of source materials at 9.5 Ma along both the Karnali River and Surai Khola sections suggest that the tectonic activity of the Ramgarh Thrust started around ca.12 Ma, initiating the exhumation of the LH (Huyghe et al., 2005). Study on oxygen isotope (d18O) of fresh water bivalve shells and mammal teeth shows that seasonal variability in surface water remained unchanged during the late Miocene and Pliocene time (Dettman et al., 2001). The sediment accumulation rate is between 32 and 50 cm kyr 1 for the 10.8 Myr time span (Gautam and Fujiwara, 2000) at Karnali Section and Surai Khola section (230 km eastward, Fig. 1) (Nakayama and Ulak, 1999). The Upper Siwalik comprising pebble-bearing sandstone at the basal part evolving to boulder size conglomerates at the upper most part of the section.

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Fig. 3. Generalized columnar section of the Siwalik Group along with the Karnali River Section with reference of Gautam and Fujiwara (2000). Ms - Mudstone, SS - Sandstone, Pbl.ss - Pebbly sandstone, Cong - Conglomerate, KA-xxx - sample number, 14.5 is age correlated with Gautam and Fujiwara (2000).

3. Material and methods 3.1. Sampling Six samples of medium- to coarse-grained sandstone representative in terms of grain size and composition were collected at various stratigraphic depths (Fig. 2 and Table 1). The same samples used for both petrographic study and detrital zircon U-Pb dating. Three samples (KA-01, KA-03, and KA-04) are from the Lower Siwalik (Fig. 3). Sample KA-01 is from the thick-bedded, medium-grained, grey colored sandstone bed, interbedded with variegated mudstone (Fig. 4a), representing the lower part of the Lower Siwalik. Sample KA-03 representing the middle part of the

Lower Siwalik is from medium- to thick-bedded sandstone bed interbedded with variegated mudstone. Sample KA-04 is from the very thick bedded, fine- to medium-grained, grey colored sandstone bed intercalated with yellowish grey mudstone bed (Fig. 4b), which belongs to the upper most part of the Lower Siwalik. Three samples (KA-05, KA-06, and KA-08) are from the Middle Siwalik represent the lower, middle and upper parts of this sub unit. Sample KA-05 from medium-grained, parallel laminated, thick-bedded bioturbated ‘‘salt and pepper” sandstone bed that is interbedded with thin-bedded yellowish grey mudstone (Fig. 3) belonging to the lower part of the Middle Siwalik. Sand balls were occasionally present in this sub unit (Fig. 4c). Sample KA-06 is from the very thick-bedded, coarse –grained ‘salt and pepper’ sandstone

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Table 1 Depositional age distribution and fluvial environment of the Siwalik Group, along the Karnali River Section. Sample No.

Coordinate

Stratigraphic Depth (meter)

Approx. (Age) (Gautam and Fujiwara, 2000)

Lithology

Facies (Huyghe et al., 2005) (Age in Ma)

Description

KA-01

N28 39.021 E81 17.215

4950

14.5

Variegated mudstone with fine-grained sandstone

KFA1 (15.8–13.1)

KA-03 KA-04

N28 39.802 E81 17.167 N28 40.177 E81 17.057

3800 3400

11.8 11

Interbedding of medium- to coarse-grained sandstone and mudstone

KFA3 (12.6–9.5)

KA-05

N28 40.870 E81 17.055

2900

9.3

KFA4 (9.5–6.4)

KA-06

N28 40.971 E81 17.018

2250

7.2

KA-08

N28 41.873 E81 16.614

1150

3.9

Dominated by medium - to coarse –grained ‘‘salt and pepper” sandstone interdded with mudstone. Dominantly coarse- to very coarse –grained sandstone interdedded with mudstone Coarse- to very coarse –grained sandstone interbedding with mudstone. Presence of some pebbly sandstone bed at the uppermost section

Fine-grained meandering fluvial system Sandy-flood flow dominated the meandering fluvial system Deep sandy braided fluvial system

bed (Fig. 4d), belongs to the middle part of this sub unit. Finally, sample KA-08 is from the upper part within the medium-grained, medium to thick bedded, grey colored sandstone bed with sporadic sand balls. 3.2. Methodology 3.2.1. Petrography The detrital sediments reflect their provenance, the source characteristics and the flow regime of the fluvial system during their deposition that can be deduced through the study of mineralogical composition, texture, grain deformation and cementing material therein. The composition of sandstone holds vital information of the source materials and thus it is useful for provenance analysis even though it is affected by the source rocks’ lithology, nature of the sedimentary processes, sink, and the source distance. The simultaneous tectonic activity also plays a vital role that controls the variation in the composition of sedimentary rocks (Ingersoll and Suczek, 1979; Dickinson et al., 1983). Following the Gazzi-Dickinson point counting method (Dickinson et al., 1983), more than 300 grains per sample were counted under Olympus BX41 polarized microscope. While for petrographic point counting, the detrital grains were differentiated as feldspar (K-feldspar and plagioclase), quartz (monocrystalline and polycrystalline quartz), and lithic grains (rock fragments) under different magnification levels at Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing. 3.2.2. Detrital zircon U-Pb dating The detrital zircons were extracted from the same samples that were used for the petrographic study. Following the traditional method of heavy liquid and magnetic separation technique, all the zircon grains were handpicked under the microscope. Prior to the analysis, more than 300 detrital zircon grains were aligned and randomly mounted in epoxy resin and polished close to onethird of individual grains’ diameters to obtain the smooth and clear surface. The detrital zircons were run through Agilent 7500a ICPMS coupled with a New Wave Research UP193FX Excimer laser (New Wave Instruments, USA) at Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing. The 193 nm ArF excimer laser and the homogenizing imaging optical system were also equipped with the ablation system. Helium gas was used for the carrier of Ions ablated by Laser from roughly 15–20 mm depth of the sample. The gas carrying Ion passes through a 3 mm PVC tube and mixed with high purity argon gas just before entering ICP-MS

KFA5 (6.4–5.5) KFA6 (5.5–3.7)

Shallow-sandy braided fluvial system Anastomosed system

torch, which reduces elemental fractionation of the sample. To get a better result, the spot size of 35 mm was selected at a repetition rate of 8 Hz with 8–10 J/cm2 energy. Considering the contamination of Pb in air, all the gas line were purged for more than an hour prior to each analytical session following the procedure by Wu et al. (2007). During this delay time, the last run sample purged from the system and the peak signal intensity returns to background levels. For U-Pb age determination, GLITTER 4.0 was used and for instrumental mass bias and isotopic fractionation, the program was calibrated against zircon standard Plesovice 337 ± 0.37 Ma (Sláma et al., 2008). Regarding the age probability plots, we used Excel macro program (freely available from http:// www.laserchron.org) that normalizes individual curve based on the number of consistent analysis. Each curve includes the same area and then stacks the probability curves (Gehrels et al., 2011). For the age determination, raw count rates for 29Si, 204Pb, 206Pb, 208Pb, 232Th, 238U were collected. Three integration times were used; 30 ms for all four Pb, 6 ms for 29Si and 15 ms for the rest of the isotopes. The minimum number of grains, required to obtain a good result when dating sedimentary rock is still in debate. Vermeesch (2004) recommended to date at least 114 grains in order to constrain better results by U-Pb dating of sedimentary rocks. However, Dodson et al. (1988) advocate for at least 60 grains to obtain data reliable within 95% confidence limit. In this study, we dated 100 individual grains from each sample. Among the 100 grains, we used those having ±10% concordance for interpretation. We report the ages derived from 206Pb/238U for 61000 Ma and 207 Pb/206Pb for P1000 Ma. 4. Results 4.1. Lower Siwalik In sample KA-01, grains ranging from 0.2 to 0.5 mm are subrounded to angular with calcite as cementing material (Fig. 4e). Monocrystalline quartz (>88% of the quartz) is higher in proportion than polycrystalline one. The basic QFL (Quartz, Feldspar, Lithic Fragments) composition of the sample is 80%, 1%, and 19% respectively with about 4% mica (Table 2). Similarly, in the sample KA-03, the grains are sub-rounded to angular and the calcite is the cementing material. The basic QFL composition is 76%, 3%, 21% and about 6% mica (Table 2, Fig. 4f). Similar to the previous sample KA-01 from Lower Siwalik, the grains in sample KA-04 are subrounded to angular. The basic QFL composition is 66%, 4%, and 30% respectively and 4% mica is the presence (Table 2). Large grains of calcite are also observed as a cementing material with fine gained quartz. Overall, the Lower Siwalik sandstone reveals a

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Fig. 4. Field and micro photographs (thin-section) of samples studied along the Karnali River Section. Field photographs of locations: (a) - KA-01, (b) - KA-04, (c) - KA-05, (d) KA-06. Micro-photos (e) - KA-01, (f) - KA-03, (g) - KA-05, (h) - KA-06.

trend of decreasing percentage of quartz towards upsection, whereas, the percentage of feldspar and lithic grains increases. As shown in the ternary diagram the samples were mostly lithicarenite and all the three samples fall into the ‘‘recycled orogen” field (Fig. 5). Out of 300 grains dated, 259 grains yield reliable ages while the remaining grains yield ages is out of the Concordia range (±10%). The detrital zircons in sample KA-01 are sub-rounded to angular in shape, and depending on orientation during stacking on double side tape the observed size is 10 to 250 mm. The maximum number

of the detrital grains clusters at 450–1100 Ma, with the peak ages at 507 and 984 Ma. Additionally, few subordinate peaks of Paleoand Mesoproterozoic age are also present (Fig. 6). The detrital zircons, in sample KA-03, are rounded to sub-rounded in shape and range from 10 mm to 150 mm in size. Similar to the sample KA-01, maximum numbers of grains are clustered in between 450 and 1100 Ma with two major peaks at 494 and 974 Ma. About half of the remaining grains are clustered within 1500–2000 Ma and 2100–3400 Ma. In sample KA-04, zircon grains are subangular to subrounded, the size ranges from 50 to 250 mm, and almost half

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Table 2 Point Counting result of the sandstone samples from the Karnali River sections. Sample

KA-01 KA-03 KA-04 KA-05 KA-06 KA-08

Composition

Recalculation for QFL%

Qm

Qp

Plag

K

L

Total

Other

Mica

Q

F

L

302 346 250 232 322 329

40 47 33 31 45 17

3 11 6 6 13 12

2 3 10 9 4 5

30 49 80 39 50 45

377 456 379 317 434 408

14 10 87 10 27 62

15 30 20 25 33 35

80.11 75.88 65.96 73.19 74.19 80.64

1.33 3.07 4.22 4.73 3.92 4.17

18.57 21.05 29.82 22.08 21.89 15.20

Qm-monocrystalline quartz, Qp - polycrystlline quartz, Plag - plagioclase, K - potassium feldspar, L - lithic grains, Q - quartz.

of the dated grains are clustered between 500 and 1000 Ma showing a major peak at 805 Ma with subordinate peaks at 558 and 911 Ma. About one-third of the grains peak at 1273 and 1573 Ma and there are some grains that are older than 2400 Ma (Fig. 6, Supplementary dataset S1). 4.2. Middle Siwalik The QFL composition of sample KA-05 is 73%, 4%, 22% and mica ensues about 7% in total. The grains are fine (0.1–0.4 mm) with the dominance of monocrystalline quartz, and calcite occupies more than 60% as cementing material (Fig. 4g). The QFL composition

and the mica content of sample KA-06 are identical to the sample KA-05 (Table 2) but the mica in this sample is in a deformed state (Fig. 4h). The quartz grains are sub-rounded to sub-angular in shape and range in size from 0.2 to 0.6 mm. Sample KA-08 has QFL composition of 81%, 4%, 15%, and 7% of the sample is occupied by mica. The quartz grains are 0.2–0.7 mm in size and the percentage of monocrystalline quartz (95%) is higher than polycrystalline. Sedimentary and metamorphic lithic grains are present and about 7% of the section is occupied by deformed mica (Table 2). In the Middle Siwalik, the proportion of the quartz grains increases up section whereas the feldspar grains seem constant and lithic grains decreases towards up-section. The two samples from lower section (KA-05, KA-06) represent lithiarenites whereas the upper sample (KA-08) is sub-litharenite. The Middle Siwalik samples fall into the ‘‘recycled orogen” field in QFL ternary plot (Fig. 5). Of the 300 grains from three samples used for dating, 255 grains yielded reliable ages. The general morphology of the detrital grains in sample KA-05 is sub-rounded to subangular in shape and 10– 150 mm in size. The younger clustered peak age is at 473 Ma. Half of the grains are clustered in between 500 and 1600 Ma with a peak at 905 Ma and the remaining grains are scattered between 1700 and 3336 Ma (Fig. 6). The detrital grains of sample KA-06 are sub-rounded and elongated in shape, ranging from 10 mm to 150 mm in size. The youngest age obtained is 343 Ma whereas the oldest age is 2953 Ma. More than half of all dated grains are clustered between 340 and 1000 Ma with major peaks at 473, 808 and 897 Ma, and the remaining grains show cluster between 1100 and 2953 Ma (Fig. 6). Likewise, in sample KA-08, the zircon grains are elongated and subangular in shape with size ranging from 10 mm to 200 mm. The samples show the youngest age of 35 Ma and the oldest at 2883 Ma. The major peak is at 501 Ma within a range between 299 and 1400 Ma with subordinate peaks at 889 and 931 Ma. Some minor peaks occur at 1844, 2500 and 2636 Ma (Fig. 6, Supplementary dataset S1). 5. Discussion It is widely accepted that the lithotectonic units in the Himalaya hinterland were the major source of the foreland basin sediments in Nepal Himalaya. Before discussing the probable provenance derived from some past studies on U-Pb dating in the Himalaya, we particularly concentrate on the Nepalese sector. 5.1. Distribution of detrital zircon U-Pb ages in the Himalaya

Fig. 5. Quartz, Felspar, and Lithic diagrams of the Siwalik Group along with the Karnali River section following Gazzi-Dickinsion point counting method.

Foreland basin sediment holds the fundamental information on the paleotectonic setting and nature of the source rock. In this regard, several studies have been carried out in Nepal Himalaya (DeCelles et al., 2004; Najman, 2006; Gehrels et al., 2011). The past studies in northwestern Himalaya showed that intrusive and metavolcanic rocks of the Lesser Himalaya exhibit rock age clustered between 1700 and 1900 Ma, with some minor peaks at 2500 and 2600 Ma (Parrish and Hodges, 1996; Richards et al.,

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Fig. 6. Normalized probability plot of detrital zircons age for the Karnali River section (this study): Lower Siwalik Sub group (KA-01, KA-03, KA-04), Middle Siwalik Sub group (KA-05, KA-06, KA-08). N = number of grain analyzed. Green shade and purple shale indicate the age of igneous rocks on Lesser Himalaya (LH) and Higher Himalaya (HH) respectively, (DeCelles et al., 2004 and reference therein). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

2005; Kohn et al., 2010; McKenzie et al., 2011). The intermediate fine-grained volcanic rocks and gneiss found throughout the LH are 1850 and 2500 Ma (DeCelles et al., 2004; McKenzie et al., 2011). The U-Pb age of HH clustered at 1000 Ma with some subordinate peaks at 1500–1700 and 2500 Ma (DeCelles et al., 2000; Richards et al., 2005; Gehrels et al., 2011) is identical to the age given by Parrish and Hodges (1996). The granitic bodies in the HH are younger, ranging age from 24 to 12 Ma (Carosi et al., 1999, 2013; Godin et al., 2001). The detritus in the Tethys Himalaya also has a peak 1000 Ma, similar to the HH, but there is a strong peak of 500–600 Ma and subordinate peaks at 750–

1200 Ma and 2430–2560 Ma (Gehrels et al., 2011). Both the activation of the MBT, at about 10–12 Ma, and the growth of the Lesser Himalaya, subsequently folded the Dadeldhura synform as early as 10 Ma are contributed to the rapid erosion of Lesser Himalaya rocks (DeCelles et al., 2001; Mishra et al., 2013). Possibly the Higher Himalaya rocks were much more eroded due to activation of MCT at 23–20 Ma and reactivation at 5–3 Ma (Le Fort, 1975; Arita et al., 1997). The detrital sediments in the Paleogene foreland basin are derived from the Lhasa Terrane, Gangdese batholiths, Indus-Tsangpo suture zone, ultra-high pressure gneiss Terrane and Tethys Himalaya until the Early Miocene time (Najman et al.,

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2004, 2009). And after that, the Lesser Himalaya, Higher Himalaya and Tethys Himalaya successively acted as the sources of sediments for the Tertiary foreland basin (DeCelles et al., 1998a, 1998b, 2000, 2004; Ravikant et al., 2011). Similar results have been found in the Subathu sub-basin (Ravikant et al., 2011), northwestern Himalaya (Burbank et al., 1996; Najman et al., 2004, 2009) and eastern Himalaya (Chirouze et al., 2012). 5.2. Detail composition Despite the low amount of petrographic data from both the Lower Siwalik and Middle Siwalik, the available result shows that

there are no up-section trends in mineralogical composition, as seen from the recent study along the Koshi Nadi and Surai Khola section (Baral et al., 2015). Both the Lower and Middle Siwalik sandstones have a lithic to sub-litharenite nature (classification based on Folk, 1974) corresponding to the ‘‘recycled orogen” field. Hence, Siwalik sediments were sourced from the pre-existing relief of the Himalaya, and the sediments removed from the source were reworked, recycled and transported before deposition. In both stratigraphic divisions, lithic grains are of sedimentary/metasedimentary (carbonate, chert, siltstone, mudstone) and metamorphic type (schist and gneiss with biotite and muscovite), suggesting that the Tethys, Higher, and Lesser Himalaya (Yin, 2006 and reference

Fig. 7. Composite normalized probability curve of detrital zircons from the Lower and Middle Siwalik Formations of the Karnali River section. Lower three curves represent detrital zircon ages of the Tethys Himalaya and upper Lesser Himalaya, Higher Himalaya and lower Lesser Himalaya (Gehrels et al., 2011 and references therein). The reference age of the Igneous rocks in the Higher Himalaya and Lesser Himalaya (DeCelles et al., 2004 and reference therein) are marked by the rectangle.

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therein) have contributed the sediments flux continuously. Even though dense minerals (like garnet, staurolite, sillimanite) are specially related to HH, those are absent in our petrographic sections. The count of mica grains (biotite and muscovite) increases (from 4% to 7%) in Middle Siwalik implying possibly that the sediment input from Lesser Himalaya increased in deposition of the Middle Siwalik than the Lower Siwalik. This change of sedimentation was possibly due to the activation of Ramgarh Thrust (RT) (at 10 Ma) that brought the Lesser Himalaya rocks to the surface exposing them to increased erosion (Huyghe et al., 2005). After the deposition of Lower Siwalik and during the time of deposition of Middle Siwalik, there was a change of sedimentation. The optical petrographic results obtained from this study are consistent with the previously published dataset by Guilbaud et al. (2012) along the Surai Khola section and Chirouze et al. (2012) along the Muskar Khola section. The study by Guilbaud et al. (2012) shows that the Higher Himalaya sediments were dominant in the entire Siwalik Group, whereas the Lesser Himalaya detritus increased during the time of deposition of the Middle and Upper Siwalik Group only.

Nepal, but not in the Surai Khola section (Baral et al., 2015). However, there are few such grains in the Karnali River Section. This may imply that the Tethys and upper Lesser Himalaya were rapidly exhumed in eastern and western Nepal during the time of deposition of the Middle Siwalik. The absence of Cenozoic grains suggests that no granitic body was eroded in those areas. The paleocurrent study also shows that the dominant flow direction was southwest and southeast, indicating the northern Himalaya as a source of detritus in Siwalik Group (Huyghe et al., 2005). Due to the similarity in age distribution in the Tethys and Higher Himalaya, U-Pb geochronology is not strong enough to distinguish their age spectra. Additionally, the Upper Lesser Himalaya characterized by age distribution similar to the TH is also difficult to differentiate. According to the Nd isotopic study of the Siwalik Group have shown that the MBT was active from 12 to 10 Ma, implying to the change in the fluvial environment of the foreland basin, from a meandering to braided system, at about 9.5 Ma (Huyghe et al. (2005).

5.3. U-Pb geochronology

6. Conclusions

The detrital zircon age spectra for all three samples (14– 10.3 Ma, Gautam and Fujiwara, 2000 and Table 1 of the present study) from the Lower Siwalik Group reveal that the source of the detrital zircons was a mixture of the three Himalayan lithotectonic units; the Tethys, Higher, and the Lesser Himalaya. In the Lower Siwalik, the youngest age cluster is 490–560 Ma, although the number of grains is low, these ages resemble those of the Tethys and upper Lesser Himalaya (Figs. 6 and 7). The other peaks at 1000 Ma and 2460 Ma reflect the ages of the Higher Himalaya, whereas the subordinate peaks at 1575–1775 Ma reflect the age of the Lower Lesser Himalaya (Fig. 7). The Gangdese batholith, located north of the Himalayan belt, is of Cretaceous–early Tertiary (50–85 Ma) age (Yin and Harrison, 2000). In the current study, none of the grains in the Lower Siwalik Group are younger than the Permian age, which suggests that the Himalayan belt had already formed a relief capable of blocking the sediments of Asian affinity by the time of deposition of Lower Siwalik. The Middle Siwalik (late Miocene to early Pliocene; maximum deposition age 10–8.3 Ma and minimum age 4.5 Ma; Gautam and Fujiwara, 2000, and Table 1 of present study) samples also validate the mixed type of source as the Lower Siwalik. The highest number of grains clusters between Cambrian- Late Carboniferous with a peak age at 470 Ma, pointing the dominant source to be the Tethys and upper Lesser Himalaya. The occurrence of the detrital zircons of <500 Ma is higher in the Middle Siwalik than in Lower Siwalik (Figs. 6 and 7, Supplementary data S1). Other subordinate peaks at 800–900 Ma, 2330 Ma, and 2400–3600 Ma suggest a predominant source was Tethys Himalaya mixed with the Higher Himalaya and Upper Lesser Himalaya (Fig. 7). The Paleoproterozoic detrital grains are more abundant in the Middle than in the Lower Siwalik Group, emphasizing the increase of source materials from the lower Lesser Himalaya by the time of Middle Siwalik deposition. The two thrust MBT and RT in western Nepal were activated at 10 and 11 Ma that causes the exhumation andm denudation of the Lesser Himalayan Duplex in western Nepal and deposited in Middle Siwalik. The recorded youngest grain of 35 Ma is the remnant of the Eocene Himalayan metamorphic/thermal event in the Higher Himalayan Crystalline (Catlos et al., 2002). These age distribution patterns are identical with the age of the Himalaya described by several studies that demonstrate the mixed type of source from the northern Himalaya (Parrish and Hodges, 1996; DeCelles et al., 2000; Richards et al., 2005; Kohn et al., 2010; Gehrels et al., 2011; Baral et al., 2015). Some younger grains (50–200 Ma) were recorded in the Koshi Nadi section in eastern

The detrital zircon U-Pb ages suggest that the sediments in the Siwalik Group were sourced from three lithotectonic units; (1) the Tethys Himalaya, (2) Higher Himalaya, and (3) the Lesser Himalaya. Since 10 Ma (around the beginning of deposition time of Middle Siwalik) the proportion of eroded material from the lower Lesser Himalaya increased, possibly due to the reactivation of the MBT. The uplift and erosion of the Lesser Himalayan duplex were rapid (between 10 Ma and 11 Ma) possibly after the activation of major thrusts MBT and RT (the local thrust) in western Nepal Himalaya. The U-Pb data show that the mid-Proterozoic detrital zircon grains increased during the time of deposition of Middle Siwalik. Both these findings point out that the Lesser Himalaya exhumed rapidly around 10 Ma. Late Mesoproterozoic (1000 Ma) grains are higher in number in the Lower Siwalik than in Middle Siwalik pointing to the possibility that the exhumation of the Higher Himalaya was slow. Such inference is supported by the presence of significant numbers of the detrital zircons grains with dates (1000 to <500 Ma) resembling the age of Tethys and upper Lesser Himalayan rock (Mesoproterozoic to Cambrian). Due to the similar age distribution of the Tethys Himalaya, upper Lesser Himalaya, and Higher Himalaya, it is challenging to pinpoint the particular source region. The finding of the present study suggests that the detrital inputs from the Lesser Himalaya increased since the time of deposition of Middle Siwalik. However, the Tethys, Higher, and Lesser Himalayas together acted as a continuous source during the Neogene. The sandstone composition (litharenite and sub-litharenite) suggests that the sediments were reworked and recycled in the hinterland Himalayan orogen.

Acknowledgments We acknowledge the two reviewers, Dr. Pitambar Gautam and an anonymous reviewer for comments and suggestions, and Dr. Stephen Angsterfor his helpful discussion and suggestions for better improvement of this article and also thankful to the JAES editorial board, especially Miss Diane. We are heartily thankful to Dr. Qasim Muhammad for his help to re-polish the manuscript and figures, Dr. Yahui Yue for her assistance in LA-ICPMS U-Pb zircon dating. This research was financially supported by the grants from the Chinese Academy of Sciences (XDB03010400), the Chinese Ministry of Science and Technology (2011CB403101), additionally the first author is also funded by PIFI fellowship (Grant no. 2016PE023).

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