Research article Journal of the Geological Society Published online April 7, 2020 https://doi.org/10.1144/jgs2019-188 | Vol. 177 | 2020 | pp. 818–825

Unroofing the Ladakh Batholith: constraints from autochthonous molasse of the Indus Basin, NW Himalaya

Renjie Zhou1*, Jonathan C. Aitchison1, Kapesa Lokho2, Edward R. Sobel3, Yuexing Feng1 and Jian-xin Zhao1 1 School of Earth and Environmental Sciences, The University of Queensland, St Lucia, QLD 4072, Australia 2 Wadia Institute of Himalayan Geology, Dehradun, Uttarakhand 248001, India 3 Institut für Geowissenschaften, Universität Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam-Golm, Germany RZ, 0000-0001-7232-8820;JCA,0000-0002-3659-5849; ERS, 0000-0001-5030-8773 * Correspondence: [email protected]

Abstract: The Indus Molasse records orogenic sedimentation associated with uplift and erosion of the southern margin of Asia in the course of ongoing India–Eurasia collision. Detailed field investigation clarifies the nature and extent of the depositional contact between this molasse and the underlying basement units. We report the first dataset on detrital zircon U–Pb ages, Hf isotopes and apatite U–Pb ages for the autochthonous molasse in the Indus Suture Zone. A latest Oligocene depositional age is proposed on the basis of the youngest detrital zircon U–Pb age peak and is consistent with published biostratigraphic data. Multiple provenance indicators suggest exclusively northerly derivation with no input from India in the lowermost parts of the section. The results provide constraints on the uplift and erosion history of the Ladakh Range following the initial India–Asia collision. Supplementary material: Method description, Sample locations, Figures S1 and S2 and Tables S1 to S3 are available at: https:// doi.org/10.6084/m9.figshare.c.4858848 Received 15 November 2019; revised 14 February 2020; accepted 14 February 2020

The uplift of the Earth’s highest mountain range, the Himalaya, has This study focuses on the northern edge of the Indus Basin, where been a central topic in geology because of the critical role these continental deposits (the Indus Molasse) lie in direct contact with mountains play in the evolution of regional and global climate and the Ladakh Batholith. Because it provides important information oceanography (e.g. Raymo and Ruddiman 1992; Richter et al. 1992). with which to constrain uplift history, we investigate the nature of Many methods have been suggested for inferring topographic contact between the batholith and preserved basin sediments. We growth, including stable-isotope-based palaeoaltimetry (e.g. also provide the first U–Pb age constraints for these deposits. Using Huntington and Lechler 2015), fossil leaf morphology (e.g. multiple lines of evidence, we further identify source regions for McElwain 2004), geomorphic features (e.g. Whipple and Tucker sediments within the basin and constrain the timing of uplift of the 1999), and low-temperature thermochronology (e.g. Reiners and Ladakh Range (NW Himalaya). Brandon 2006). Geological constraints including the exhumation of deep-seated lithological units and the deposition of intermontane sediments also serve to record the growth of high topography. In the Background Himalaya, continental deposits along the Indus–Yarlung–Tsangpo Ladakh Range and Ladakh Batholith suture zone (IYTS) chronicle a history of collision between India and Eurasia and the attendant rise of the Himalaya–Tibetan Plateau (e.g. The Ladakh Range in the western Himalaya extends ∼NW–SE for Searle 1983; Garzanti and Van Haver 1988; Jaeger et al. 1989; over 250 km with peak elevations of c. 6100 m (Fig. 1). It is Aitchison et al. 2007; van Hinsbergen et al. 2012). Depositional ages dominated by the Ladakh Batholith, which is part of the Trans- and source-to-sink relationships constrain aspects of past ocean Himalayan magmatic belt that extends for over 2500 km along the closure and provide information on the uplift of source terrains, yet southern margin of the Eurasian continent. The range lies between there are challenges and complications in studying related rock units. the predominantly intra-oceanic arc rocks of the Kohistan arc to the First, these coarse-grained deposits do not always contain suitable west and continental margin arc rocks of the Gangdese arc to the fossils or volcanic ash layers that may provide true depositional ages. east. Thus it bridges an important transition along-trend from intra- OnecommonapproachistoemploydetritalzirconU–Pb or mica oceanic to continental margin subduction systems (e.g. Hébert et al. 40Ar/39Ar dating to constrain the maximum depositional age. 2012). The Ladakh Batholith incorporates Cretaceous to early However, the utility of results obtained using this methodology Cenozoic plutonic rocks intruded by dykes that are as young as could be strongly influenced by the size of the dataset. The common c. 45 Ma (Weinberg and Dunlap 2000; White et al. 2011; Heri et al. practice of dating c. 100–200 mineral grains from a given lithological 2015). The Khardung Volcanics are remnants of a volcanic carapace unit might not be sufficient. Second, commonly used provenance for parts of the batholith and crop out along the northern flanks of indicators, including detrital mineral ages and radiogenic isotopic the range (e.g. Dunlap et al. 1998; Lakhan et al. 2019). Their compositions, geochemistry, analysis of clast compositions and original extent is less well known. sandstone petrography, do not always provide unique criteria with Driven by India–Eurasia collision, the Ladakh Range is thought which to discriminate amongst potential sources, some of which may to have been uplifted as early as late Eocene time. Low-temperature not have been adequately characterized. thermochronologic data record differences between unroofing

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Fig. 1. Overview of geology, NW Himalaya (a) Digital elevation model of India and the southern Tibetan Plateau–Himalayan system. The map was generated from SRTM 90 m DEM data. (b) Regional geological map of the NW Himalaya adapted after the compilation by St-Onge et al. (2010).(c)Swath topographic profile across the Ladakh Range. The profile was generated along the black line shown in Figure 1b with a box width of 20 km. The black line represents the average elevation along the swath line, while the grey line represents the maximum and minimum elevation within the swath box. (d) Generalized stratigraphic column of studied autochthonous molasse of the Indus Basin, based on sections logged (Fig. 3).

histories along the northern and southern margins (Kirstein et al. ‘autochthonous molasse’, which is deposited upon an exhumed 2006, 2009). In general, apatite fission-track ages are older (c. 35– Ladakh Batholith surface. This relationship has been recognized at 25 Ma) towards the southern margin of the range and younger (c. several locations along the southern margin of the Ladakh Range 10–5 Ma) to the north, implying later exhumation or uplift for the (e.g. Fuchs 1979; Brookfield and Andrews-Speed 1984; Garzanti Ladakh Range along its northern margin (Kirstein 2011). and Van Haver 1988). Here the molasse commonly involves bright purple, red or green, and beige banded rocks that were likely derived IYTS and Indus Basin from, and deposited on, a sloping landscape eroded on the Ladakh Range (Fuchs 1979). Another type of molasse occurs in fault- The Indus -Yarlung -Tsangpo suture zone (IYTS) lies at the contact bounded units and is regarded as ‘parautochthonous’. Several units between two colliding continental landmasses: India and Eurasia of this nature are recognized (Hemis Conglomerate, Choksti (Fig. 1). In NW India, the suture follows the Indus River, Conglomerate and Chogdo Formation, e.g. Fuchs 1979; Clift immediately south of the Ladakh Range (Fig. 1). The term ‘Indus et al. 2002; Henderson et al. 2010). The unit on which this study Basin’ collectively refers to sedimentary rocks preserved along this focuses exhibits an unambiguously depositional relationship upon suture. Historically, this name has generally been applied to several an eroded surface of Ladakh Batholith rocks. lithological units, many of which are in faulted contact with one another (Searle 1983; Sinclair and Jaffey 2001; Clift et al. 2002; Henderson et al. 2010, 2011) and are not necessarily genetically Methods and results related. Eocene-age nummulitic limestones within the basin are Field mapping and sedimentary logging regarded as the last products of marine deposition within the Tethyan Ocean (Green et al. 2008)(Fig. 1). Although most units Field mapping was undertaken from the Basgo to Tia areas (Fig. 2), have not been directly dated, limited biostratigraphic data are with a focus on determining the nature of the contact between the available (e.g. Bajpai et al. 2004; Green et al. 2008). The majority of Ladakh Batholith and sediments of the Indus Molasse. We were able age constraints are provided by detrital zircon U–Pb or mica to trace a nonconformable contact along strike for c. 25 km. Granitic 40Ar/39Ar dates, which only give maximum depositional ages (Clift clasts ranging from pebble to boulder sizes crop out immediately et al. 2002; Tripathy-Lang et al. 2013; Wu et al. 2007; Henderson above an eroded Ladakh Batholith surface with localized palaeosol et al. 2010, 2011). Many studies have attempted to propose a development (Supplementary material). Our observations are stratigraphic scheme for the Indus Basin (e.g. Brookfield and consistent with earlier reports by Fuchs (1979), Brookfield and Andrews-Speed 1984; Garzanti and Van Haver 1988; Searle et al. Andrews-Speed (1984) and Garzanti and Van Haver (1988) 1990; Sinclair and Jaffrey 2001; Clift et al. 2002; Henderson et al. suggesting that any faulted contacts (Tripathy-Lang et al. 2013) 2010; Tripathy-Lang et al. 2013; Baxter et al. 2016). are of restricted local extent. Higher stratigraphic levels in the The term ‘Indus Molasse’ was first applied to coarse-grained section (up to several hundreds of metres thick) are truncated at a clastic deposits that crop out within and flanking the IYTS (e.g. regionally extensive north-directed thrust fault. Fuchs 1979). This early work acknowledges the existence of several Six sections were measured along the strike of the autochthonous different units to which the descriptive term molasse could apply molasse basin, from the depositional contact (nonconformity) upon and Fuchs (1979) suggested that two main types could be the Ladakh Batholith to the footwall of the thrust fault at which the recognized on the basis of their structural relationships with the section terminates (Fig. 1d). Four semi-continuous members could Ladakh Batholith. Fuchs (1979) referred to one type as an be discriminated within the preserved Indus Molasse sections

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Fig. 2. Geological map showing the distribution of lithological units and locations of the studied sections. Topographic contours were extracted using SRTM 90-m DEM data. Red lines denote the locations of logged sections in the field. The yellow circle marks the sample location shown in Tripathy-Lang et al. (2013).

(Figs 3 and 4). M1 is a crudely stratified, poorly sorted, clast- transportation distance. Analyses of a total of 1885 detrital zircon supported conglomerate with clasts ranging from boulder (c. 1m)to grains with −5 to 10% discordance are reported (Fig. 5). Detailed pebble (several centimetres) sizes (Fig. 4). Clasts are dominantly methods are provided in the Supplementary material. All units yield felsic and andesitic with subordinate granitic clasts. M2 is ages that are dominantly <90 Ma, with peaks around 23–24, 50, 57, dominated by red to grey mudstone. M3 and M4 are laterally 62 and 78 Ma (Fig. 5). The youngest peak from a single grain in M3 continuous conglomeratic units. M3 is a poorly sorted, clast- and 12 grains in M4 cluster around a latest Oligocene (late Chattian), supported monomictic conglomerate dominated by angular to sub- with a mean age of 23.58 ± 0.11 Ma. angular andesitic clasts ranging in size from c. 3–4 cm to 10 cm. M4 Detrital apatite from M1 were also dated by laser ablation ICP- is polymictic, horizontally stratified and cross-bedded and incorpo- MS. Grains define an isochron on the Tera–Wasserburg plot that 207 206 rates trough cross-bedded sandstone together with well-sorted provides a value for ( Pb/ Pb)o of 0.8501 ± 0.0030. This initial pebble layers. Field counts indicate 87% felsic and andesitic common lead value was used to correct all ages (Fig. 6) with the volcanic clasts in M1 (total n = 384) and 80% in M4 (total n = 181) IsoplotR program (Vermeesch 2018). Detrital apatite U–Pb ages with the remainder comprising granitic lithologies. Clasts in unit range from c. 100 to 30 Ma, consistent with major peaks observed in M3 are entirely of volcanic origin. detrital zircon U–Pb ages.

U–Pb geochronology Zircon Hf isotopic analysis Detrital zircon from medium- to coarse-grained sandstones of units Hafnium isotopic data for 329 zircons (231 from M1, 41 from M3 M1, M3 and M4 were dated by laser ablation inductively coupled and 57 from M4) were obtained using a Nu Plasma II multi- plasma mass spectrometry (ICP-MS) at the School of Earth and collector-inductively coupled plasma mass spectrometry (MC-ICP- Environmental Sciences, The University of Queensland. Zircon MS) with laser ablation (Fig. 5) at the School of Earth and grains were extracted by heavy-liquid and magnetic separation, Environmental Sciences, The University of Queensland. (Analytical before being hand-picked under a binocular microscope for mounting details are provided in the data repository). εHf(t) values were in epoxy resin. Zircon grains range from c. 50 to 100 µm in width. calculated with 206Pb/238U ages and range from −10 to 17. Grains Most grains are euhedral and few are rounded, implying a short older than 45 Ma are characterized by dominantly positive εHf(t)

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Fig. 3. Stratigraphic logs of measured sections. Section locations are labelled in Figure 2 and located at the following coordinates. Section 1: 34° 16′ 54.60″ N, 77° 10′ 25.36″ E; Section 2: 34°16′ 57.50″ N, 77° 10′ 12.60″ E; Section 3: 34° 17′ 20.95″ N, 77° 9′ 22.78″ E; Section 4: 34° 17′ 54.50″ N, 77° 7′ 27.83″ E; Section 5: 34° 18′ 50.66″ N, 77° 4′ 44.88″ E; Section 6: 34° 20′ 9.69″ N, 77° 1′ 35.48″ E.

values, clustering around 5–15. All εHf(t) values for grains within with no evidence for any southerly provenance, from the Indian the youngest peak of c. 24–23 Ma are negative: −2.4 to −9.4 Plate, for example, which contains older zircon with ages >500 Ma (Fig. 5c). (e.g. Gehrels et al. 2011). Granitic clasts and sandstone with detrital zircon ages of c. 85–45 Ma are consistent with uplift and erosion of Discussion the Ladakh Batholith, which served as an important source (Brookfield and Andrews-Speed 1984; Searle 1986; Garzanti and Our study indicates a latest Oligocene age for initial sedimentation Van Haver 1988). Abundant andesitic and felsic volcanic clasts along the northern margin of the Indus Molasse. Detrital zircon U– (comprising 100% of clasts in M3 and c. 85% of clasts in M1 and Pb ages reveal a younger c. 24–23 Ma age peak for M4 and provide M4) strongly suggest a volcanic source, which we consider most a maximum depositional age for this unit. This radiometric age is likely to be a local equivalent of the Khardung Volcanics. Such consistent with palaeontological (ostracod) data reported by Bajpai rocks must first have been eroded to expose the Ladakh Batholith et al. (2004) from correlative sediments near Basgo (Fig. 2). above and upon which they were erupted. Documented ages for Sediments in the studied area were entirely sourced from the north these volcanics include 51.95 ± 0.4 and 56.4 ± 0.4 Ma (whole-rock

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Fig. 4. Field photographs. (a and b) Depositional contact between the exhumed Ladakh Batholith and M1 unit. Locations: 34° 18′ 1.72″ N, 77° 8′ 20.67″ E (a) and 34° 18′ 56.80″ N, 77° 5′ 33.78″ E(b). (c) Photograph of M1 unit. Large boulders (c. 1 m) are made of granite and granodiorite, material eroded off the Ladakh Batholith. The photo was taken at 34° 16′ 56.09″ N, 77° 10′ 20.61″ E with the camera facing approximately south. (d) Contact between M1 (conglomerate) and M2 (mudstone). M2 unit is the same unit documented near Basgo (e.g. Bajpai et al. 2004). The photo was taken at 34° 18′ 43.59″ N, 77° 6′ 7.33″ E with the camera facing approximately east. (e) Field photograph of M3, a conglomerate that is entirely made of felsic to intermediate volcanic clasts. The photo was taken at 34° 18′ 49.10″ N, 77° 5′ 24.33″ E. (f) Field photograph of M4. Conglomerate layers contain felsic to intermediate volcanic and granitic clasts. The photo was taken at 34° 18′ 53.36″ N, 77° 4′ 50.44″ E.

40Ar/39Ar plateau ages, Bhutani et al. 2009), and 60.5 ± 1.3 and source of the youngest age peak (c. 24–23 Ma) remains less 67.4 ± 1.1 Ma (zircon U–Pb sensitive high-resolution ion micro- definitive. The Karakorum Batholith potentially represents a source probe ages, Dunlap and Wysoczanski 2002) and are consistent with of younger zircon grains c. 40–20 Ma (e.g. White et al. 2012; the dominant peaks identified in our zircon age data (Fig. 5). The Borneman et al. 2015) and is characterized by negative zircon εHf(t)

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Fig. 5. Results of detrital zircon U–Pb dating and Hf isotopic analysis. (a) Kernel density estimates of all detrital samples, plotted by unit. (b) Weighted mean age of the youngest age cluster (top) and Concordia plot of M4 (bottom). (c) Zircon ε Hf(t) values (red circles) of analysed grains based on 206Pb–238U ages. The black line is the kernel density estimate of ages that have accompanying Hf isotope measurements. (d) Comparison of detrital zircon U–Pb ages reported by Tripathy-Lang et al. (2013) and this study. Note that the sample location of Tripathy-Lang et al. (2013) is located in the hanging wall of a north- directed thrust fault and is structurally separated from the units in this study.

values as low as −10.5 (Ravikant et al. 2009), consistent with our different detrital zircon results from sites close to the Ladakh results. Dykes that intrude the Oligocene Saltoro Molasse have been Batholith (Figs 2 and 5d), which the authors considered to be reported from the Shyok suture to the north (Rai 1983). Oligocene– representative of the Indus Molasse (Basgo Formation). However, Miocene dykes have also been reported from within the Ladakh our mapping suggests that the samples in question were collected Batholith with a Rb–Sr isochron age of c. 24 Ma obtained by from the hanging wall of a north-directed thrust fault and as such Ravikant (2006). However, we note that no existing studies from the may not be related to the Basgo Formation. Moreover, the samples Ladakh Batholith report zircon U–Pb ages younger than c. 45 Ma reported by Tripathy-Lang et al. (2013) did not contain any grains (Weinberg and Dunlap 2000; White et al. 2011; Heri et al. 2015). younger than 90 Ma, seemingly excluding the Ladakh Batholith as a Our work refines and builds upon earlier investigations. Garzanti potential source (e.g. Gehrels et al. 2011). and van Haver (1998) reported a Maastrichtian age for the Basgo As sediments documented here overlie a surface eroded into the Formation based on identifications of an ostracod fauna. However, Ladakh Batholith, they place a minimum constraint on the time in a more recent investigation, Bajpai et al. (2004) also reported needed to erode the former southern margin of Eurasia. The removal ostracods that suggest an Oligocene age, which is consistent with of a thick crustal section (several kilometres or more) must have our findings. Cenozoic detrital zircon ages further refine this age to preceded deposition of the molasse, earlier than c. 24–23 Ma. This the latest Oligocene. Tripathy-Lang et al. (2013) reported somewhat is consistent with thermochronological modelling from nearby

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IYTS. These include slab break-off (Maheo et al. 2002; Aitchison et al. 2009) or foundering of a continental root (Chung et al. 2009; Lee et al. 2009).

Conclusions Initial continental sediments deposited directly upon an uplifted and eroded surface that developed on the Ladakh Batholith on the southern margin of the Eurasian continent are no older than latest Oligocene, with a possible depositional age of c. 24–23 Ma. Deposits were derived entirely from the north, with the Ladakh arc and Karakoram terrane being the most likely sources. The deposition of the autochthonous molasse of the Indus Basin is analogous to the formation of the Gangrinboche facies in southern Tibet (Aitchison et al. 2002, 2009).

Acknowledgments Critical input from L. Kirstein, A. Collins, A. Tripathy-Lang, L. White and anonymous reviewers has greatly improved this work. D. Chew has graciously provided apatite reference material to us. The assistance of Mr. Jigmet Punchok in the field is gratefully acknowledged.

Funding This work is supported by the University of Queensland and the Australia-Germany Joint Research Co-operation Scheme (#2016001932 to Zhou).

Author contributions RZ: conceptualization (equal), formal analysis – (lead), investigation (lead), methodology (lead), project administration (support- Fig. 6. Results of detrital apatite U Pb dating. (a and b) Plots and ing), writing - original draft (lead), writing - review & editing (lead); JCA: calculation were completed using IsoplotR (Vermeesch 2018) assuming conceptualization (lead), funding acquisition (lead), investigation (equal), project all apatite grains have a similar common lead composition. The calculated administration (lead), supervision (lead), writing - original draft (equal), writing - common lead composition (0.8501 ± 0.0030) was then applied to correct review & editing (equal); KL: conceptualization (supporting), project adminis- all analyses to generate common-Pb-corrected ages (b). All errors are tration (supporting), writing - original draft (supporting), writing - review & editing (supporting); ERS: formal analysis (supporting), methodology (support- shown at the 2σ level. ing), writing - original draft (supporting), writing - review & editing (supporting); YF: formal analysis (equal), methodology (equal), writing - original draft (supporting), writing - review & editing (supporting); JZ: formal analysis (supporting), methodology (supporting), writing - original draft (supporting), locations where cooling c. 30–20 Ma is documented (Kirstein et al. writing - review & editing (supporting) 2006, 2009). Scientific editing by Linda Kirstein The ages we obtained from initial sediments of the Indus Molasse are consistent with those from elsewhere along the length of the Correction notice: Figure 1 has been updated. IYTS (Gangrinboche facies across Tibet; Aitchison et al. 2002). 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