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

Renjie Zhou1*, Jonathan C. Aitchison1, Kapesa Lokho2, Edward R. Sobel3,Yuexing Feng1 and Jian-xin Zhao11 School of Earth and Environmental Sciences, The University of Queensland, St Lucia, QLD 4072, Australia2 Wadia Institute of Himalayan Geology, Dehradun, Uttarakhand 248001, India3 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: renjie.zhou@uq.edu.au

Abstract: The IndusMolasse records orogenic sedimentation associated with uplift and erosion of the southern margin of Asiain the course of ongoing India–Eurasia collision. Detailed field investigation clarifies the nature and extent of the depositionalcontact between this molasse and the underlying basement units. We report the first dataset on detrital zircon U–Pb ages, Hfisotopes and apatite U–Pb ages for the autochthonous molasse in the Indus Suture Zone. A latest Oligocene depositional age isproposed 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 thesection. The results provide constraints on the uplift and erosion history of the Ladakh Range following the initial India–Asiacollision.

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, hasbeen a central topic in geology because of the critical role thesemountains play in the evolution of regional and global climate andoceanography (e.g. Raymo and Ruddiman 1992; Richter et al. 1992).Many methods have been suggested for inferring topographicgrowth, including stable-isotope-based palaeoaltimetry (e.g.Huntington and Lechler 2015), fossil leaf morphology (e.g.McElwain 2004), geomorphic features (e.g. Whipple and Tucker1999), and low-temperature thermochronology (e.g. Reiners andBrandon 2006). Geological constraints including the exhumation ofdeep-seated lithological units and the deposition of intermontanesediments also serve to record the growth of high topography. In theHimalaya, continental deposits along the Indus–Yarlung–Tsangposuture zone (IYTS) chronicle a history of collision between India andEurasia and the attendant rise of the Himalaya–Tibetan Plateau (e.g.Searle 1983; Garzanti and Van Haver 1988; Jaeger et al. 1989;Aitchison et al. 2007; van Hinsbergen et al. 2012). Depositional agesand source-to-sink relationships constrain aspects of past oceanclosure and provide information on the uplift of source terrains, yetthere are challenges and complications in studying related rock units.First, these coarse-grained deposits do not always contain suitablefossils or volcanic ash layers that may provide true depositional ages.One common approach is to employ detrital zircon U–Pb or mica40Ar/39Ar dating to constrain the maximum depositional age.However, the utility of results obtained using this methodologycould be strongly influenced by the size of the dataset. The commonpractice of dating c. 100–200 mineral grains from a given lithologicalunit might not be sufficient. Second, commonly used provenanceindicators, including detrital mineral ages and radiogenic isotopiccompositions, geochemistry, analysis of clast compositions andsandstone petrography, do not always provide unique criteria withwhich to discriminate amongst potential sources, some of which maynot have been adequately characterized.

This study focuses on the northern edge of the Indus Basin, wherecontinental deposits (the Indus Molasse) lie in direct contact withthe Ladakh Batholith. Because it provides important informationwith which to constrain uplift history, we investigate the nature ofcontact between the batholith and preserved basin sediments. Wealso provide the first U–Pb age constraints for these deposits. Usingmultiple lines of evidence, we further identify source regions forsediments within the basin and constrain the timing of uplift of theLadakh Range (NW Himalaya).

Background

Ladakh Range and Ladakh Batholith

The Ladakh Range in the western Himalaya extends ∼NW–SE forover 250 km with peak elevations of c. 6100 m (Fig. 1). It isdominated by the Ladakh Batholith, which is part of the Trans-Himalayan magmatic belt that extends for over 2500 km along thesouthern margin of the Eurasian continent. The range lies betweenthe predominantly intra-oceanic arc rocks of the Kohistan arc to thewest and continental margin arc rocks of the Gangdese arc to theeast. Thus it bridges an important transition along-trend from intra-oceanic to continental margin subduction systems (e.g. Hébert et al.2012). The Ladakh Batholith incorporates Cretaceous to earlyCenozoic plutonic rocks intruded by dykes that are as young asc. 45 Ma (Weinberg and Dunlap 2000; White et al. 2011; Heri et al.2015). The Khardung Volcanics are remnants of a volcanic carapacefor parts of the batholith and crop out along the northern flanks ofthe range (e.g. Dunlap et al. 1998; Lakhan et al. 2019). Theiroriginal extent is less well known.

Driven by India–Eurasia collision, the Ladakh Range is thoughtto have been uplifted as early as late Eocene time. Low-temperaturethermochronologic data record differences between unroofing

© 2020 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics

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

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histories along the northern and southern margins (Kirstein et al.2006, 2009). In general, apatite fission-track ages are older (c. 35–25 Ma) towards the southern margin of the range and younger (c.10–5 Ma) to the north, implying later exhumation or uplift for theLadakh Range along its northern margin (Kirstein 2011).

IYTS and Indus Basin

The Indus -Yarlung -Tsangpo suture zone (IYTS) lies at the contactbetween two colliding continental landmasses: India and Eurasia(Fig. 1). In NW India, the suture follows the Indus River,immediately south of the Ladakh Range (Fig. 1). The term ‘IndusBasin’ collectively refers to sedimentary rocks preserved along thissuture. Historically, this name has generally been applied to severallithological units, many of which are in faulted contact with oneanother (Searle 1983; Sinclair and Jaffey 2001; Clift et al. 2002;Henderson et al. 2010, 2011) and are not necessarily geneticallyrelated. Eocene-age nummulitic limestones within the basin areregarded as the last products of marine deposition within theTethyan Ocean (Green et al. 2008) (Fig. 1). Although most unitshave not been directly dated, limited biostratigraphic data areavailable (e.g. Bajpai et al. 2004; Green et al. 2008). Themajority ofage constraints are provided by detrital zircon U–Pb or mica40Ar/39Ar dates, which only give maximum depositional ages (Cliftet al. 2002; Tripathy-Lang et al. 2013; Wu et al. 2007; Hendersonet al. 2010, 2011). Many studies have attempted to propose astratigraphic scheme for the Indus Basin (e.g. Brookfield andAndrews-Speed 1984; Garzanti and Van Haver 1988; Searle et al.1990; Sinclair and Jaffrey 2001; Clift et al. 2002; Henderson et al.2010; Tripathy-Lang et al. 2013; Baxter et al. 2016).

The term ‘Indus Molasse’ was first applied to coarse-grainedclastic deposits that crop out within and flanking the IYTS (e.g.Fuchs 1979). This early work acknowledges the existence of severaldifferent units to which the descriptive term molasse could applyand Fuchs (1979) suggested that two main types could berecognized on the basis of their structural relationships with theLadakh Batholith. Fuchs (1979) referred to one type as an

‘autochthonous molasse’, which is deposited upon an exhumedLadakh Batholith surface. This relationship has been recognized atseveral locations along the southern margin of the Ladakh Range(e.g. Fuchs 1979; Brookfield and Andrews-Speed 1984; Garzantiand Van Haver 1988). Here the molasse commonly involves brightpurple, red or green, and beige banded rocks that were likely derivedfrom, and deposited on, a sloping landscape eroded on the LadakhRange (Fuchs 1979). Another type of molasse occurs in fault-bounded units and is regarded as ‘parautochthonous’. Several unitsof this nature are recognized (Hemis Conglomerate, ChokstiConglomerate and Chogdo Formation, e.g. Fuchs 1979; Cliftet al. 2002; Henderson et al. 2010). The unit on which this studyfocuses exhibits an unambiguously depositional relationship uponan eroded surface of Ladakh Batholith rocks.

Methods and results

Field mapping and sedimentary logging

Field mapping was undertaken from the Basgo to Tia areas (Fig. 2),with a focus on determining the nature of the contact between theLadakh Batholith and sediments of the IndusMolasse.Wewere ableto trace a nonconformable contact along strike for c. 25 km. Graniticclasts ranging from pebble to boulder sizes crop out immediatelyabove an eroded Ladakh Batholith surface with localized palaeosoldevelopment (Supplementary material). Our observations areconsistent with earlier reports by Fuchs (1979), Brookfield andAndrews-Speed (1984) and Garzanti and Van Haver (1988)suggesting that any faulted contacts (Tripathy-Lang et al. 2013)are of restricted local extent. Higher stratigraphic levels in thesection (up to several hundreds of metres thick) are truncated at aregionally extensive north-directed thrust fault.

Six sections were measured along the strike of the autochthonousmolasse basin, from the depositional contact (nonconformity) uponthe Ladakh Batholith to the footwall of the thrust fault at which thesection terminates (Fig. 1d). Four semi-continuous members couldbe discriminated within the preserved Indus Molasse sections

Fig. 1. Overview of geology, NW Himalaya (a) Digital elevation model of India and the southern Tibetan Plateau–Himalayan system. The map wasgenerated 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) Swathtopographic 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 linerepresents 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).

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(Figs 3 and 4). M1 is a crudely stratified, poorly sorted, clast-supported conglomerate with clasts ranging from boulder (c. 1 m) topebble (several centimetres) sizes (Fig. 4). Clasts are dominantlyfelsic and andesitic with subordinate granitic clasts. M2 isdominated by red to grey mudstone. M3 and M4 are laterallycontinuous conglomeratic units. M3 is a poorly sorted, clast-supported monomictic conglomerate dominated by angular to sub-angular andesitic clasts ranging in size from c. 3–4 cm to 10 cm.M4is polymictic, horizontally stratified and cross-bedded and incorpo-rates trough cross-bedded sandstone together with well-sortedpebble layers. Field counts indicate 87% felsic and andesiticvolcanic clasts in M1 (total n = 384) and 80% in M4 (total n = 181)with the remainder comprising granitic lithologies. Clasts in unitM3 are entirely of volcanic origin.

U–Pb geochronology

Detrital zircon from medium- to coarse-grained sandstones of unitsM1, M3 and M4 were dated by laser ablation inductively coupledplasma mass spectrometry (ICP-MS) at the School of Earth andEnvironmental Sciences, The University of Queensland. Zircongrains were extracted by heavy-liquid and magnetic separation,before being hand-picked under a binocular microscope for mountingin epoxy resin. Zircon grains range from c. 50 to 100 µm in width.Most grains are euhedral and few are rounded, implying a short

transportation distance. Analyses of a total of 1885 detrital zircongrains with −5 to 10% discordance are reported (Fig. 5). Detailedmethods are provided in the Supplementary material. All units yieldages that are dominantly <90 Ma, with peaks around 23–24, 50, 57,62 and 78 Ma (Fig. 5). The youngest peak from a single grain in M3and 12 grains in M4 cluster around a latest Oligocene (late Chattian),with a mean age of 23.58 ± 0.11 Ma.

Detrital apatite from M1 were also dated by laser ablation ICP-MS. Grains define an isochron on the Tera–Wasserburg plot thatprovides a value for (207Pb/206Pb)o of 0.8501 ± 0.0030. This initialcommon lead value was used to correct all ages (Fig. 6) with theIsoplotR program (Vermeesch 2018). Detrital apatite U–Pb agesrange from c. 100 to 30 Ma, consistent with major peaks observed indetrital zircon U–Pb ages.

Zircon Hf isotopic analysis

Hafnium isotopic data for 329 zircons (231 from M1, 41 from M3and 57 from M4) were obtained using a Nu Plasma II multi-collector-inductively coupled plasma mass spectrometry (MC-ICP-MS) with laser ablation (Fig. 5) at the School of Earth andEnvironmental Sciences, The University of Queensland. (Analyticaldetails are provided in the data repository). εHf(t) values werecalculated with 206Pb/238U ages and range from −10 to 17. Grainsolder than 45 Ma are characterized by dominantly positive εHf(t)

Fig. 2. Geological map showing the distribution of lithological units and locations of the studied sections. Topographic contours were extracted using SRTM 90-mDEM 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).

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values, clustering around 5–15. All εHf(t) values for grains withinthe youngest peak of c. 24–23 Ma are negative: −2.4 to −9.4(Fig. 5c).

Discussion

Our study indicates a latest Oligocene age for initial sedimentationalong the northern margin of the Indus Molasse. Detrital zircon U–Pb ages reveal a younger c. 24–23 Ma age peak for M4 and providea maximum depositional age for this unit. This radiometric age isconsistent with palaeontological (ostracod) data reported by Bajpaiet al. (2004) from correlative sediments near Basgo (Fig. 2).Sediments in the studied area were entirely sourced from the north

with no evidence for any southerly provenance, from the IndianPlate, for example, which contains older zircon with ages >500 Ma(e.g. Gehrels et al. 2011). Granitic clasts and sandstone with detritalzircon ages of c. 85–45 Ma are consistent with uplift and erosion ofthe Ladakh Batholith, which served as an important source(Brookfield and Andrews-Speed 1984; Searle 1986; Garzanti andVan Haver 1988). Abundant andesitic and felsic volcanic clasts(comprising 100% of clasts in M3 and c. 85% of clasts in M1 andM4) strongly suggest a volcanic source, which we consider mostlikely to be a local equivalent of the Khardung Volcanics. Suchrocks must first have been eroded to expose the Ladakh Batholithabove and upon which they were erupted. Documented ages forthese volcanics include 51.95 ± 0.4 and 56.4 ± 0.4 Ma (whole-rock

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.

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40Ar/39Ar plateau ages, Bhutani et al. 2009), and 60.5 ± 1.3 and67.4 ± 1.1 Ma (zircon U–Pb sensitive high-resolution ion micro-probe ages, Dunlap andWysoczanski 2002) and are consistent withthe dominant peaks identified in our zircon age data (Fig. 5). The

source of the youngest age peak (c. 24–23 Ma) remains lessdefinitive. The Karakorum Batholith potentially represents a sourceof younger zircon grains c. 40–20 Ma (e.g. White et al. 2012;Borneman et al. 2015) and is characterized by negative zircon εHf(t)

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 offthe 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 intermediatevolcanic 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 intermediatevolcanic and granitic clasts. The photo was taken at 34° 18′ 53.36″ N, 77° 4′ 50.44″ E.

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values as low as −10.5 (Ravikant et al. 2009), consistent with ourresults. Dykes that intrude the Oligocene Saltoro Molasse have beenreported from the Shyok suture to the north (Rai 1983). Oligocene–Miocene dykes have also been reported from within the LadakhBatholith with a Rb–Sr isochron age of c. 24 Ma obtained byRavikant (2006). However, we note that no existing studies from theLadakh Batholith report zircon U–Pb ages younger than c. 45 Ma(Weinberg and Dunlap 2000; White et al. 2011; Heri et al. 2015).

Our work refines and builds upon earlier investigations. Garzantiand van Haver (1998) reported a Maastrichtian age for the BasgoFormation based on identifications of an ostracod fauna. However,in a more recent investigation, Bajpai et al. (2004) also reportedostracods that suggest an Oligocene age, which is consistent withour findings. Cenozoic detrital zircon ages further refine this age tothe latest Oligocene. Tripathy-Lang et al. (2013) reported somewhat

different detrital zircon results from sites close to the LadakhBatholith (Figs 2 and 5d), which the authors considered to berepresentative of the Indus Molasse (Basgo Formation). However,our mapping suggests that the samples in question were collectedfrom the hanging wall of a north-directed thrust fault and as suchmay not be related to the Basgo Formation. Moreover, the samplesreported by Tripathy-Lang et al. (2013) did not contain any grainsyounger than 90 Ma, seemingly excluding the Ladakh Batholith as apotential source (e.g. Gehrels et al. 2011).

As sediments documented here overlie a surface eroded into theLadakh Batholith, they place a minimum constraint on the timeneeded to erode the former southern margin of Eurasia. The removalof a thick crustal section (several kilometres or more) must havepreceded deposition of the molasse, earlier than c. 24–23 Ma. Thisis consistent with thermochronological modelling from nearby

Fig. 5. Results of detrital zircon U–Pbdating and Hf isotopic analysis. (a) Kerneldensity estimates of all detrital samples,plotted by unit. (b) Weighted mean age ofthe youngest age cluster (top) andConcordia plot of M4 (bottom). (c) ZirconεHf(t) values (red circles) of analysed grainsbased on 206Pb–238U ages. The black line isthe kernel density estimate of ages that haveaccompanying Hf isotope measurements.(d) Comparison of detrital zircon U–Pbages reported by Tripathy-Lang et al.(2013) and this study. Note that the samplelocation of Tripathy-Lang et al. (2013) islocated in the hanging wall of a north-directed thrust fault and is structurallyseparated from the units in this study.

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locations where cooling c. 30–20 Ma is documented (Kirstein et al.2006, 2009).

The ages we obtained from initial sediments of the IndusMolasseare consistent with those from elsewhere along the length of theIYTS (Gangrinboche facies across Tibet; Aitchison et al. 2002). Insouthern Tibet, Lower Miocene Gangrinboche conglomerates(including the Kailas, Qiuwu, Dazhuqu and Luobusa formations)crop out along the Yarlung Tsangpo suture zone and lie indepositional contact upon rocks of the Lhasa terrane, mostly theGangdese Batholith (Atichison et al. 2002). This observation callsfor the exhumation of the Gangdese Batholith around the same timeas, or prior to, the early Miocene, consistent with previousinvestigations that infer Late Oligocene regional uplift of theLhasa terrane (Harrison et al. 1992). Although the deposition ofsuch a molasse has traditionally been regarded as a key constraint onthe timing of the India–Eurasia collision, our data—which indicatethat sedimentation began in the latest Oligocene—require analternative explanation, as this event significantly post-dates eventhe youngest estimates for the initiation of continental collision(Klootwijk 1984; Garzanti and Van Haver 1988; Najman et al.2001; Clift et al. 2002; Aitchison et al. 2007). The timing of theexhumation of the Ladakh Batholith and deposition of the IndusMolasse may be explained by the slab break-off event proposedc. 25 Ma (Maheo et al. 2002). In the Karakoram Batholith, igneousand metamorphic rocks associated with a high thermal gradient,enrichment of large-ion lithophile elements and depletion of high-field-strength elements have been documented and interpreted asproducts associated with the melting of metasomatized Asianlithospheric mantle (Maheo et al. 2002). Late Oligocene exhum-ation of the batholiths observed in southern Tibet (Gangdesebatholith) and NW India (Ladakh Batholith) is broadly consistentwith alternative mechanisms to the onset of continent–continentcollision that have been proposed to explain exhumation along the

IYTS. These include slab break-off (Maheo et al. 2002; Aitchisonet 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 anderoded surface that developed on the Ladakh Batholith on thesouthern margin of the Eurasian continent are no older than latestOligocene, with a possible depositional age of c. 24–23 Ma.Deposits were derived entirely from the north, with the Ladakh arcand Karakoram terrane being the most likely sources. Thedeposition of the autochthonous molasse of the Indus Basin isanalogous to the formation of the Gangrinboche facies in southernTibet (Aitchison et al. 2002, 2009).

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

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

Author contributions RZ: conceptualization (equal), formal analysis(lead), investigation (lead), methodology (lead), project administration (support-ing), writing - original draft (lead), writing - review & editing (lead); JCA:conceptualization (lead), funding acquisition (lead), investigation (equal), projectadministration (lead), supervision (lead), writing - original draft (equal), writing -review & editing (equal); KL: conceptualization (supporting), project adminis-tration (supporting), writing - original draft (supporting), writing - review &editing (supporting); ERS: formal analysis (supporting), methodology (support-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),writing - review & editing (supporting)

Scientific editing by Linda Kirstein

Correction notice: Figure 1 has been updated.

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