Tracing patterns of erosion and drainage in the Paleogene Himalaya through ion probe Pb isotope...

Tracing patterns of erosion and drainage in the PaleogeneHimalaya through ion probe Pb isotope analysis of detrital

K-feldspars in the Indus Molasse, India

Peter D. Clift *, Nobumichi Shimizu, Graham D. Layne, Jerzy BlusztajnDepartment of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

Received 23 October 2000; received in revised form 2 March 2001; accepted 12 April 2001

Abstract

The Indus Molasse is a pre- and syn-tectonic sedimentary sequence situated in the Indus Suture Zone of the westernHimalaya. Spanning in time the collision of India and Asia, this deposit is well placed to record the evolving uplift anderosion history of the early Himalayan orogen. Nd isotope analyses from clay extracted from shales interbedded withinthe dominantly alluvial sequence indicate a low negative ONd (31.64 to 0.72), in the basal Paleocene Chogdo Formation,slightly more negative than measured values from the Transhimalaya and Kohistan/Dras Arc. Up-section ONd becomesmore negative, as low as 310.05, indicating influence of a different, more enriched source. Ion microprobe Pb isotopicanalyses of single K-feldspars help constrain this source as being either the Lhasa or Karakoram Blocks, with westwardpaleo-current flow favoring the former. 207Pb/204Pb ratios are too low to be consistent with known Indian plate sources,a conclusion supported by the lack of muscovite or garnet that would be indicative of a High Himalayan contribution.Given the known age of rapid cooling of the High Himalaya at V20 Ma, and the lack of exposure of suitable lithologiesprior to that time, an age of sedimentation prior to V20 Ma is inferred. The post-collisional change in paleo-flow andprovenance is suggested to reflect the initiation of the Indus River during the Early Eocene. This study demonstrates thepower of combined bulk sediment and single grain analyses in resolving provenance in tectonically complexsettings. ß 2001 Elsevier Science B.V. All rights reserved.

Keywords: India; Asia; Himalayas; provenance; ion probe data; isotopes

1. Introduction

The Early Cenozoic collision of India with Eur-asia and the consequent uplift of the Himalayaand Tibetan Plateau have created the most dra-

matic relief on earth. Understanding the growthof this system is not only important to under-standing the processes of orogeny, but is also cru-cial to testing models of climate^tectonic couplingin South Asia. The height and extent of the Tibe-tan Plateau and High Himalaya disrupts atmos-pheric circulation on a global scale [1] and hencethe Cenozoic growth of the plateau, coupled withchemical weathering in the Himalaya, may ulti-mately be responsible for the global cooling thatled to the Plio^Pleistocene Ice Age [2,3]. More-

0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 3 4 6 - 6

* Corresponding author. Tel. : +1-508-457-2000/3437;Fax: +1-508-457-2187; E-mail: [email protected]

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Earth and Planetary Science Letters 188 (2001) 475^491

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over, the Tibetan Plateau appears to play an im-portant role in driving a strong summer monsoon[4^6]. Determining the uplift history of the systemis crucial to testing such models. Knowledge ofthe Miocene to Recent uplift and erosion historyis now constrained in outline, due to work onsediments from the foreland basins [7], the BengalFan [8,9], and from direct measurements on thecrystalline basement of the High Himalaya (e.g.[10]). However, the early development of the sys-tem has remained obscure. This is because thoserocks exposed at or close to the surface at thattime have been eroded away leaving only the rel-atively insensitive, high temperature paleother-mometers to record the cooling history at thattime. Peak metamorphism in the Pakistan Hima-laya postdates collision by V10 Myr [11] so thatno record of Himalayan orogenesis precedingV45 Ma can be found in this tectonic unit. Fur-ther east, in the High Himalaya of Zanskar andLahaul, rapid cooling dates from 20^25 Ma

[12,13], limiting the record of earlier orogenesiseven further. Uplift history is therefore bestcharted through study of the detrital sedimentaryrecord that spans this period. Unfortunately ac-cess to Eocene^Oligocene sediments is limited dueto the di¤culty in drilling the great thicknesses ofthe Indus or Bengal Fans, and due to the frag-mentary nature of the sedimentary record of thisage in the Indian foreland [14], which in any casewas located far from the zone of active collisionduring the Eocene.

In this study we present data on the erosionhistory of the early Himalaya recorded in the In-dus Molasse Basin, located in the Indus SutureZone in the Indian Himalaya, which help con-strain the nature of early Cenozoic erosion andthe development of a post-collisional drainagesystem. To do this we investigate the source ofthe detrital minerals that comprise this successionusing a combination of bulk sediment and singlegrain isotopic analyses.

Fig. 1. (A) Schematic tectonic map of the Indus Suture showing the main tectonic units discussed in this paper, as well as princi-ple localities. Insert shows the location of Ladakh with the Himalaya^Tibet system. (B) Detailed map of the Zanskar Gorgeshowing the relationships of the Indus Molasse with the underlying units of the Asian and Indian margins.

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P.D. Clift et al. / Earth and Planetary Science Letters 188 (2001) 475^491476

2. Geologic setting

The Indus Molasse is a folded and thrusted se-quence of dominantly clastic formations which isobserved locally to rest unconformably over theLadakh Batholith, where this contact is not re-thrusted. The Indus Molasse also unconformablyoverlies Indian passive margin units (LamayuruGroup) south of Upsi [11], ophiolitic melangeswest of Chilling [15], and the Cretaceous forearcof the Kohistan/Dras Arc (Nindam Formation),also west of Chilling [15,16]. However, the IndusMolasse is observed to conformably overlie theUpper Cretaceous^Paleogene Jurutze Formation[16,17] within the Zanskar Gorge, south of Sum-da-Do (Fig. 1). The Jurutze Formation representsthe forearc basin to the Asian margin prior toIndian collision.

The Ladakh Batholith represents the pre-collisional active margin of Asia, and is thuscorrelative to the Gandese Batholith and otherparts of the Transhimalayan igneous belt exposedfurther east. The Ladakh Batholith is dated asyoung as 60 Ma near Leh by U^Pb methods[18]. Most recently di¡erent phases of intrusionspanning 49 to 61 Ma have been mapped byWeinberg [19] through ion probe dating of singlezircon crystals. The position of the Indus Molassewithin the suture between Indian and Asianplates led earlier workers to postulate a forearcposition for its deposition prior to India^Asiacollision [16,17,20]. In this scenario sedimentationin what was a continental arc forearc basin con-tinued in an early intra-montane setting after col-lision [17]. Garzanti and van Haver [17] subdi-vided the molasse section into a series offormations, the older of which are dated by theirmarine fauna, but which become continental andbarren of datable fauna above the NummuliticLimestone on the south side of the basin (Fig.2). Recent re-dating of the fauna in the Nummu-litic Limestone [21] as latest Paleocene, and therecognition that the molasse oversteps both Indi-an and Asian tectonic units [16] allows India^Asia collision in Ladakh to be constrained aspre-Eocene. Since that time the entire sequencehas been deformed by Neogene north-vergent tec-tonism, related to motion along the main Zanskar

backthrust, located just south of the molasse out-crop [22].

3. Stratigraphy

We choose to follow the de¢ned stratigraphy ofSearle et al. [22], based on the Indus Molassesection exposed in the Zanskar Gorge (Figs. 1and 2), because this section is the focus of thisstudy. In this scheme the base of the section ismarked by a well-cleaved, light, bu¡-coloredshalely carbonate sequence, the Sumda Forma-tion, in practice the upper part of the JurutzeFormation [17,23]. The Sumda Formation isclearly marine, having a foraminifer fauna thatconstrains the Maastrichtian age [21]. It is con-formably overlain by the Chogdo Formation, adark, red-purple weathering series of interbeddedsandstones, siltstones and mudstones, as well asminor conglomerates. This formation is sharplyoverlain by dark, thick-bedded, Nummulite-bear-ing limestone of latest Paleocene age [21]. Searleet al. [22] map the Chogdo Formation as uncon-formably overlying deformed turbidites close toChilling Village, which they assigned to the Nin-dam Formation, part of the Kohistan/Dras intra-oceanic volcanic arc. However, these have beenre-assigned to the enigmatic, but presumablyAsian, Khalsi Flysch of Brook¢eld and An-drews-Speed [23] by Clift et al. [16] (Fig. 1B).The Chogdo Formation further overlies La-mayuru Group Indian slope turbidites and ocean-ic serpentinized harzburgite in the same area.Consequently the Chogdo Formation is the ¢rstunit to overstep both terrains of Indian and Asianorigin and so constrains the timing of collision tobeing prior to its deposition, i.e. s 54.6 Ma [16].

Above the Nummulitic Limestone the section isdominated by coarse-grained siliciclastic sedi-ments of typical braided alluvial facies. Threeupper formations are recognized (Fig. 2). Thelowermost Nurla Formation is topped by ashalely lacustrine section containing plant fossils,and is overlain by thick conglomerates of theChoksti Conglomerate, in turn succeeded bycoarse alluvial sediments of the Choksti Forma-tion (Nimmu Formation of Garzanti and van Ha-

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ver [17]). The Choksti Formation onlaps the edgeof the Ladakh Batholith, although the originaldepositional contact is often rethrust. The HemisConglomerate, exposed at Hemis, located east ofthe Zanskar Gorge (Fig. 1), is a particularly thicksequence seen on both sides of the basin, and mayre£ect the ¢rst in£ux of coarse material followingcollision. In this study we equate the ChokstiConglomerate with the Hemis Conglomerate be-cause along the Hemis and Zanskar sections bothunits represent the oldest thick conglomerate bedin the stratigraphy. Although it is conceivablethat a second higher conglomerate in the sectionat Hemis might be the equivalent of the ChokstiConglomerates [17], this is not possible to demon-strate without precise dating or long-distancestratigraphic correlation, neither of which is pres-ently possible. We therefore de¢ne the ChokstiConglomerate as the ¢rst major granite-bearingconglomerate to be derived into the basin, asseen in the Zanskar Gorge. The Choksti Con-glomerate contains palm leaves that have beententatively dated as early Oligocene [24], but oth-erwise the Molasse is undated above the Nummu-litic Limestone.

Clift et al. [16] report apatite ¢ssion track agesof 35 Ma for the Ladakh Batholith underlying theupper part of the Choksti Formation just north ofKhalsi (Fig. 1). This date constrains the top of theformation to be younger than 35 Ma. As dis-cussed below, the lack of minerals derived fromthe High Himalayan crystalline units means thatsedimentation of the Indus Molasse pre-dates ex-posure of these terrains at V20 Ma [12,13]. Itseems likely that the northward thrusting of theZanskar Himalayas synchronous with the unroof-ing of the High Himalaya at V20^25 Ma invertedthe basin and halted sedimentation [22]. If so thensedimentation may have continued until V20^25 Ma.

Only modest data presently exist to constrainthe provenance of the Indus Molasse clastic sedi-ments. Sandstones below the Choksti Conglomer-ate on the northern side of the Indus MolasseBasin (Temesgam Formation of van Haver [24],Nurla Formation of Searle et al. [22]) comprisequartzose alluvial and deltaic sandstones andshales, with intercalations of marine facies. Gar-

zanti and van Haver [17] interpreted these to havebeen derived by erosion of the Ladakh Batholithduring active Andean-type subduction prior toIndia collision, based on petrographic work. Sim-ilarly other studies [22,23] have interpreted theMolasse as principally being derived from erosionof the Ladakh Batholith, in part because of theunconformable relationship between the Molasseand the Ladakh Batholith noted along the north-ern boundary of the basin. Along this boundarythe similar composition of boulders in the basalconglomerate and the basement is clear.

4. Detrital mineral compositions

The provenance of the Indus Molasse can beconstrained in part through its mineralogy. Whileepidote is common throughout the section, horn-blende is only found in signi¢cant amounts in theupper parts of the Choksti Formation (Fig. 2).There is also apparent decrease in the dominanceof biotite up-section. What is most noticeable inbackscattered electron probe images is how sand-stone located above the Nummulitic Limestonecontains grains of large, single K-feldspar andquartz minerals, whereas the proportion of micro-crystalline granites is high in the Chogdo Forma-tion. Alteration is also higher for feldspars in theChogdo Formation, as shown by electron probeanalytical totals signi¢cantly less than 100% inthis unit. Chogdo Formation feldspar crystalsare often cloudy in thin section, due to their con-version to clays. However, it is not clear whetherthis change from more altered to less weatheredfeldspar re£ects a change in the source character,or if this might be in part due to the greater burialtemperatures experienced by the Chogdo Forma-tion. Illite crystallinity data indicate that theChogdo Formation approached anchizonal condi-tions (V250³C, [16]) compared to low diageneticgrade for the northernmost Choksti Formation(110^200³C). The most remarkable aspect of thedetrital mineralogy is its apparent lack of mineralscharacteristic of the High Himalaya, i.e., musco-vite and garnets (cf., [25]).

At outcrop there are clear di¡erences betweenthe Chogdo and younger formations, aside from

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the weathering color. Speci¢cally, in the ChogdoFormation the proportion of granitic clasts is less,whereas degraded volcanic rocks, serpentinizedharzburgite and red chert are more dominant. Inthe upper part of the Indus Molasse graniteboulders are the greatest single contributors, butunlike the altered appearance of those boulders inthe Chogdo Formation, these are often quite freshand show well developed biotite and hornblendephenocrysts, in addition to quartz, plagioclaseand potassium feldspars.

5. Paleo-current indicators

Paleo-£ow directions can be useful in constrain-ing possible sediment sources. Since most of thesediments that comprise the Indus Molasse arebraided river facies sandstones [24], this meansthat cross bedding is the most common form ofrecognizable paleo-current indicator. Brief turbi-dite intervals in the Choksti Formation, presum-ably of lacustrine origin, show scour structureswithin channel complexes. The anastamosing na-ture of braided streams means that there is aninherent spread of current indicators, due to thelateral growth of sand or gravel bars. In addition,because the Indus Molasse was deposited in atectonically active setting, one might expect thepaleo-rivers that carried the sediment to have ne-gotiated irregular topography and not to have aperfectly linear geometry. Further disruption to asimple current pattern may be introduced duringthe rethrusting of the Indus Molasse towards thenorth during the Neogene, when individual thrustsheets may rotate relative to one another alongthe strike of the suture zone. Despite these com-plications the paleo-current indicators from theIndus Molasse show a clear evolution from apre-collisional trend towards the SW in the Juru-tze Formation, followed by dominant NNE-di-rected £ow in the Chogdo Formation, switchingto £ow from NE to SW above the NummuliticLimestone in the Nurla and Choksti Formations(Fig. 3). This latter trend appears to be constantfrom east to west across the studied area andmatches earlier observations [22].

The SW-directed £ow in the Jurutze Formationis consistent with these sediments being erodedfrom the Transhimalayan Arc, located to the

Fig. 3. Measured paleo-current indicators for the IndusMolasse showing evolving provenance during India^Eurasiacollision and onset of Indus River £ow.

Fig. 2. Simpli¢ed stratigraphic log of the Indus Molasseshowing relative abundance of di¡erent mineral groups up-section.

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north. The ophiolitic character of many of thegrains (red cherts, basalts, with rarer gabbrosand peridotites) and the NNE-directed £ow inthe Chogdo Formation argues in favor of amarked change of provenance during the earlieststages of collision, dominated by erosion fromophiolites and associated units on the ZanskarPlatform [17]. The presence of some granitic clastsin the Chogdo Formation is more di¤cult to rec-oncile with an origin from the SW, because thereare no appropriate sources known of that age inthat area. Although a proto-arc complex has beenidenti¢ed associated with the Spontang Ophiolite(Spong Arc [26]) this comprises only volcaniclasticmaterial and cannot be considered a source of thegranite cobbles. Most likely these clasts were de-rived from the Ladakh Batholith and were mixed

with the ophiolitic and platform material in anintermontane basin setting. The correlation of achange to a westward £ow with a more graniticclast population in the Nurla and Choksti Forma-tions is noteworthy. It is not clear what the sourceof this in£ux is, although the Transhimalaya andLhasa Block are the most likely candidates.

6. Nd isotopes

The source of the Indus Molasse can be furtherconstrained using the Sm^Nd isotopic system.The technique is based on the assumption thatthe ¢nest fraction of detrital material representsa good average composition of the source areadrained. Since weathering and the sediment trans-

Table 1Nd isotopic values measured from the Indus Molasse

Sample 143Nd/144Nd ONd Stratigraphic position Location

LA98-27 0.512290 þ 5 36.83 Choksti Formation StokLA98-29 0.512682 þ 5 0.82 Choksti Formation StokLA98-59 0.512125 þ 5 310.05 Choksti Formation LikirLA98-41 0.512406 þ 5 34.56 Choksti Formation UpsiLA98-34 0.512140 þ 5 39.75 Choksti Formation Zanskar GorgeLA98-61 0.512316 þ 5 36.32 Choksti Formation Zanskar GorgeLA-00-1 0.512722 þ 5 1.60 Choksti Formation Zanskar GorgeLA-00-5 0.512467 þ 5 33.37 Choksti Formation Zanskar GorgeLA-00-3 0.512311 þ 5 36.41 Choksti Formation Zanskar GorgeLA-00-4 0.512209 þ 5 38.40 Choksti Formation Zanskar GorgeLA-00-6 0.512347 þ 5 35.71 Choksti Formation Zanskar GorgeLA98-38 0.512348 þ 4 35.69 Choksti Conglomerate Zanskar GorgeLA-00-11 0.512466 þ 6 33.39 Choksti Conglomerate Zanskar GorgeLA-00-13 0.512413 þ 5 3-4.43 Nurla Formation Zanskar GorgeLA-00-10 0.512476 þ 5 33.19 Nurla Formation Zanskar GorgeLA-00-15 0.512522 þ 5 32.30 Nurla Formation Zanskar GorgeLA-00-16 0.512566 þ 8 31.44 Nurla Formation Zanskar GorgeLA-00-17 0.512477 þ 5 33.18 Nurla Formation Zanskar GorgeLA-00-9 0.512516 þ 5 32.42 Nurla Formation Zanskar GorgeLA98-15 0.512631 þ 5 30.18 Chogdo Formation Zanskar GorgeLA98-19 0.512556 þ 5 31.64 Chogdo Formation Zanskar GorgeLA98-23 0.512573 þ 5 31.31 Chogdo Formation Zanskar GorgeLA98-24 0.512569 þ 6 31.38 Chogdo Formation Zanskar GorgeLA-00-18 0.512629 þ 4 30.21 Chogdo Formation Zanskar GorgeLA-00-21 0.512603 þ 5 30.72 Chogdo Formation Zanskar GorgeLA-00-23 0.512590 þ 5 30.98 Chogdo Formation Zanskar GorgeLA-00-25 0.512641 þ 8 0.02 Chogdo Formation Zanskar GorgeLA-00-27 0.512607 þ 4 30.64 Chogdo Formation Zanskar GorgeLA-00-22 0.512677 þ 5 0.72 Chogdo Formation Zanskar GorgeLA-00-20 0.512650 þ 6 0.19 Chogdo Formation Zanskar Gorge

Results corrected for La Jolla standard = 0.511847.

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port process are not expected to result in isotopicfractionation, the measured isotopic signature ofthe shale fraction should re£ect the bulk compo-sition of the source.

We compare modern Nd isotopic character ofthe sedimentary rocks and the sources with no agecorrection. Because the concentration of the par-ent 146Sm is di¡erent in the sources and sedi-ments, 146Nd/144Nd values in the two will gradu-ally diverge after erosion. However, the half life of146Sm is 1.03U108 yr, while the sediments were alldeposited after 55 Ma, insu¤cient time for anisotopic di¡erence of signi¢cant magnitude com-pared to the isotopic di¡erences between sourcesto emerge.

Twenty nine samples were analyzed, 25 takenalong the Zanskar River section, and four fromthe Choksti Formation along strike at Upsi, Stokand Likir (Fig. 1). The clay fraction was separatedfrom shales by simple crushing, sieving and thencentrifuging to concentrate the ¢nest fraction(6 2 Wm). The clay was then dissolved and theNd separated using standard column extractiontechniques. Nd isotopic compositions were deter-mined on VG354 mass spectrometer at WoodsHole Oceanographic Institution (WHOI). 143Nd/

144Nd values are normalized to 146Nd/144Nd =0.7219 and are relative to 0.511847 for the LaJolla standard. The results are shown in Table1. We calculate the parameter ONd [27] using a143Nd/144Nd value of 0.512638 for the ChondriticUniform Reservoir (CHUR; [28]).

On the Bengal Fan comparison of clay mineralNd characteristics with those of High Himalayanmetamorphic rocks [29] provided a good ¢t andindicated that these were the principal sedimentsources to the fan [9,30^32]. Despite the sugges-tion that Nd isotopes are sensitive to input fromoceanic sources [33], values measured from BengalFan clays are consistent with other measurementsof £uvial and aeolian particles, and are similar tothose measured in modern sediments from theGanges River [34]. This probably re£ects thevery low concentration of rare earth elements inseawater compared to the source terrains. In thecase of the £uvial Indus Molasse contaminationfrom the oceans is not a factor and therefore ourcon¢dence in the Nd characteristics re£ectingsource is even greater.

Fig. 4 shows the range and frequency of ONd

values noted in the source and Indus Molasseand compares these with published values from

Fig. 4. Comparison of ONd values measured (A) in the Indus Molasse, and (B) from the potential source terranes of the Himalayaand Tibet.

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Table 2Pb isotopic values measured on detrital feldspars from the Indus Molasse

Sample Formation 206Pb/204Pb 206Pb/204Pb c 207Pb/206Pb 207Pb/206Pb c 208Pb/206Pb 208Pb/206Pb c(%) (%) (%)

LA-98-45 Choksti Formation 18.7336 0.28 0.8363 0.13 2.0830 0.12LA-98-45 Choksti Formation 19.2864 0.21 0.8081 0.11 1.9990 0.12LA-98-45 Choksti Formation 18.4128 0.55 0.8380 0.23 2.0710 0.28LA-98-45 Choksti Formation 18.4638 0.48 0.8397 0.25 2.0720 0.26LA-98-45 Choksti Formation 19.2976 0.21 0.7968 0.14 2.0544 0.21LA-98-45 Choksti Formation 18.7614 0.22 0.8343 0.10 2.0844 0.09LA-98-45 Choksti Formation 18.7238 0.21 0.8344 0.10 2.0865 0.11LA-98-45 Choksti Formation 18.6668 0.42 0.8326 0.32 2.0824 0.41LA-98-45 Choksti Formation 18.9380 0.30 0.8227 0.15 2.0548 0.19LA-98-45 Choksti Formation 18.4094 0.49 0.8332 0.34 2.0663 0.48LA-98-45 Choksti Formation 18.7210 0.23 0.8364 0.23 2.0913 0.29LA-98-45 Choksti Formation 18.6518 0.51 0.8386 0.28 2.0759 0.33LA-98-45 Choksti Formation 18.6150 0.33 0.8367 0.16 2.0744 0.15LA-98-45 Choksti Formation 18.7913 0.35 0.8302 0.18 2.0768 0.20LA-98-45 Choksti Formation 18.3318 0.31 0.8406 0.14 2.0865 0.15LA-98-45 Choksti Formation 18.8437 0.42 0.8346 0.27 2.0832 0.32LA-98-45 Choksti Formation 18.4379 0.61 0.8359 0.26 2.0755 0.27LA-98-45 Choksti Formation 18.8126 0.25 0.8333 0.15 2.0862 0.19LA-98-45 Choksti Formation 18.5401 0.22 0.8341 0.11 2.0856 0.12LA-98-45 Choksti Formation 19.0042 0.22 0.8214 0.11 2.0558 0.14LA-98-45 Choksti Formation 18.7981 0.35 0.8275 0.17 2.0671 0.19LA-98-45 Choksti Formation 18.5995 0.33 0.8338 0.17 2.0782 0.16LA-98-45 Choksti Formation 17.6907 0.76 0.8360 0.31 2.0867 0.35LA98-63 Choksti Formation 18.4366 0.50 0.8356 0.35 2.0790 0.39LA98-63 Choksti Formation 18.6150 0.22 0.8399 0.13 2.0850 0.12LA98-63 Choksti Formation 18.4809 0.61 0.8390 0.50 2.0843 0.86LA98-63 Choksti Formation 18.4638 0.41 0.8365 0.29 2.0870 0.44LA98-63 Choksti Formation 18.6289 0.27 0.8396 0.14 2.0870 0.14LA98-63 Choksti Formation 18.4997 0.28 0.8419 0.13 2.1010 0.19LA98-36 Choksti Conglomerate 18.6289 0.56 0.8433 0.10 2.1070 0.11LA98-36 Choksti Conglomerate 18.7935 0.74 0.8399 0.25 2.0770 0.26LA98-36 Choksti Conglomerate 18.7441 0.29 0.8315 0.16 2.0730 0.23LA98-36 Choksti Conglomerate 18.5151 0.47 0.8335 0.23 2.0820 0.24LA98-36 Choksti Conglomerate 17.3883 2.70 0.8491 2.60 2.1052 3.30LA98-36 Choksti Conglomerate 18.7021 0.26 0.8332 0.18 2.0740 0.20LA98-36 Choksti Conglomerate 18.3925 0.78 0.8372 0.46 2.0740 0.64LA98-36 Choksti Conglomerate 18.6220 0.22 0.8354 0.12 2.0840 0.11LA98-36 Choksti Conglomerate 18.5908 0.29 0.8324 0.15 2.0750 0.17LA98-36 Choksti Conglomerate 18.6741 0.18 0.8385 0.09 2.0850 0.10LA98-36 Choksti Conglomerate 19.0186 0.58 0.8201 0.48 2.0380 0.79LA98-22 Chogdo Formation 18.3352 0.28 0.8471 0.11 2.0860 0.12LA98-22 Chogdo Formation 16.8067 5.00 0.8464 1.00 2.1060 0.94LA98-25 Chogdo Formation 18.3150 0.36 0.8389 0.16 2.0708 0.21LA-98-12 Chogdo Formation 18.3918 0.45 0.8393 0.20 2.0765 0.18LA-98-12 Chogdo Formation 18.4807 0.39 0.8406 0.17 2.0835 0.14LA-98-12 Chogdo Formation 18.5551 0.34 0.8412 0.17 2.0808 0.13LA-98-12 Chogdo Formation 18.4673 0.36 0.8404 0.15 2.0870 0.14LA-98-12 Chogdo Formation 18.4679 0.33 0.8428 0.17 2.0913 0.16LA-98-12 Chogdo Formation 18.4981 0.29 0.8405 0.15 2.0883 0.12LA-98-12 Chogdo Formation 18.4720 0.22 0.8407 0.13 2.0847 0.11LA-98-12 Chogdo Formation 18.4550 0.32 0.8402 0.15 2.8699 0.13LA-98-12 Chogdo Formation 18.5901 0.35 0.8413 0.18 2.0891 0.15LA-98-12 Chogdo Formation 18.5154 0.28 0.8403 0.17 2.0903 0.17

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the High Himalaya [32,35], Lesser Himalaya[9,30,36], the Transhimalaya [37], Kohistan/DrasArcs [23,38,39], the Karakoram [40] and LhasaBlock [41]. It is noteworthy that the range of val-ues from the Transhimalayan and Kohistan/DrasArcs are similar, as are those from the Karakor-am and Lhasa Block, meaning that this isotopicsystem cannot resolve between these sources. Thesimilarity of the Karakoram and Lhasa Block re-£ects the fact that they both represent parts of theAsian margin on which the Transhimalaya Arcwas built. Di¡erences between these terranes to-day principally re£ect greater degree of exhuma-tion in the Karakoram [42].

The Chogdo Formation has the most positivevalues (0.72 to 31.64), the Nurla Formation isintermediate (31.44 to 34.43), and the ChokstiFormation has the least radiogenic values (1.60to 310.05). The range of ONd values from theChogdo Formation requires that this unit bedominated by sources with positive ONd values,i.e., the Transhimalaya and Kohistan/Dras Arc.Although no Nd data are known from the Spon-tang Ophiolite, its similar age and oceanic originwould imply that it too would have a stronglypositive ONd value. Some values from the LhasaBlock also overlap the measured range. The Nddata are thus consistent with the Spontang Ophio-lite, or some equivalent Cretaceous ophiolite unit,together with the Ladakh Batholith being thedominant sources to the Chogdo Formation.These sources are evidenced by the south to northpaleo-current data (indicating the SpontangOphiolite) and the presence of granitic clasts (in-dicating the Ladakh Batholith). The ONd valuespermit additional limited mixing with an isotopi-cally more negative source. In the Nurla and

Choksti Formations ONd becomes progressivelymore negative up-section. The range of ONd valuesis similar to measured values from the LhasaBlock, but could represent mixing of both moreisotopically positive and negative sources. Twosamples from the Choksti Formation di¡er fromthe general pattern in showing positive ONd values,more similar to the Chogdo Formation. Boththese samples come from exposures directlyadjoining the Ladakh Batholith and represent lo-cal derivation from that block in the form ofalluvial fans. This observation con¢rms a rela-tively positive ONd value for the Transhimalayain Ladakh.

On the basis of the Nd data alone it is notpossible to resolve which of the possible endmembers may be contributing to the values seenin the Nurla and Choksti Formations, althoughthe petrographic data does not favor involvementof the High Himalaya as the isotopically negativesource. Consequently we employ single grainanalyses to resolve which sources were mixingwith Transhimalayan and/or ophiolitic sources inproviding sediment to the basin in post-Chogdotimes.

7. Pb isotopes of detrital feldspars

Although the mineral assemblage observed inthe Indus Molasse and the positive ONd valuesargue against the modern High Himalaya as aplausible source, further discrimination is di¤cultbecause the mineralogy is insu¤cient to distin-guish between possible Transhimalayan, LhasaBlock and Kohistani sources. It is also possiblethat the isotopically negative Nd source needed

Table 2 (continued)

Sample Formation 206Pb/204Pb 206Pb/204Pb c 207Pb/206Pb 207Pb/206Pb c 208Pb/206Pb 208Pb/206Pb c(%) (%) (%)

LA-98-12 Chogdo Formation 18.5727 0.34 0.8380 0.17 2.0774 0.14LA-98-12 Chogdo Formation 18.3785 0.32 0.8410 0.13 2.0883 0.10LA-00-94 Ladakh Granite 18.6162 0.63 0.8398 0.37 2.0832 0.36LA-00-94 Ladakh Granite 18.3877 0.59 0.8435 0.28 2.0934 0.72LA-00-94 Ladakh Granite 18.4810 0.38 0.8442 0.27 2.0880 0.21LA-00-94 Ladakh Granite 18.3036 0.48 0.8435 0.26 2.0936 0.25

EPSL 5856 18-5-01


to account for the evolving Nd compositions wasin part from the Indian Zanskar Shelf, if not theHigh Himalaya. We employ the Pb isotope systemapplied to detrital K-feldspars in order to addressthese problems. This is done with the understand-ing that the Nd isotopic character is measured onvery ¢ne clay, while the single grain Pb isotopework requires sand grade material. It is possiblefor the two grain sizes to have di¡erent sources,and this needs to be considered in the case ofcon£icting results.

The Pb isotope character of detrital K-feldsparshas previously been used as a provenance toolusing conventional mass spectrometry methods[43]. In this study our data were obtained fromindividual K-feldspar grains and is compared withthe distinct isotopic character of the High Hima-laya, Transhimalaya, Kohistan/Dras Arc, Kara-koram and Lhasa Block sources (e.g., [40,41,44^46]). In addition, we analyzed four grains fromthe Ladakh Batholith at Hunder (Fig. 1) in orderto check that the range of Pb isotopic values mea-sured further east on the Gandese Batholith wasapplicable to this region. For reference we alsoconsider the composition of the ocean mantle us-ing modern values for the Paci¢c and IndianOcean mid ocean ridge basalt (MORB) as a proxyfor Tethyan mantle [47,48].

We employ the newly developed technique ofmeasuring Pb in situ [49] using a high-resolutionCameca 1270 ion microprobe of the NortheastNational Ion Microprobe Facility (NENIMF) atWHOI. In order to exploit the potential of thismethod to characterize heterogeneous feldsparpopulations several analyses were run on di¡erentfeldspars from a small number of samples, threefrom the Chogdo Formation, one from the Chok-sti Conglomerate and three from the upper Chok-sti Formation, including 23 analyses from a singlesandstone (Table 2; Fig. 5).

The sandstones were disaggregated and sieved,after which the size fraction 1 mm to 200 Wm, wasmounted in epoxy and polished using aluminumoxide abrasives. The K-feldspar grains were thenidenti¢ed by area mapping of Al2O3 and K2Ousing the JEOL Superprobe electron microprobeat the Massachusetts Institute of Technology.This allowed the K-feldspars to be identi¢ed for

isotopic analysis. After gold coating the grainswere analyzed using a beam of negatively chargedoxygen ions (O3) focused to a spot as small as15^20 Wm. Analytical uncertainties are principallya re£ection of the counting statistics, typicallyaveraging 2c= 1%. The analytical results areshown in Table 2. Analysis of K-feldspar stan-dards verify that there is no signi¢cant mass frac-tionation e¡ect in analyzing Pb isotopes using theion microprobe methodology compared to con-ventional mass spectrometry.

In order to minimize the risk of secondary Pbcontamination from sources outside the feldspar,analyses were made in the center of the grain,away from cracks, inclusions or alteration zones.In order to avoid any contamination that mighthave occurred during preparation of the grainmounts the beam was trained on the spot to beanalyzed for 5 min before analysis began, so thatany surface Pb contamination was removed.Through probing grain centers and allowing thebeam to remove surface coating of the sectionedgrains we avoid analysis of excess secondary Pbthat is normally removed by leaching proceduresprior to conventional mass spectrometry [44].

Fig. 5 shows the spread of measured isotopicratios compared with those previously recordedfrom the central Himalaya^Tibet area [40,41,44^46]. The ¢elds de¢ned for the Indian Plate, Trans-himalaya, Karakoram and Lhasa Block are alsoK-feldspar analyses, while those from the Kohi-stan/Dras Arc represent whole rock analyses fromPakistani exposures [39], since no K-feldspar dataare available from this unit. The Spontang Ophio-lite is not considered because there are no K-feld-spar-rich lithologies in this unit. Although analyt-ical uncertainties are higher for 204Pb than otherPb isotopes, use of this isotope is crucial becauseit provides signi¢cant separation between the dif-ferent sources considered, without which no dis-crimination can be achieved.

Despite the di¤culty in analyzing the feldsparswithin the dominant microcrystalline graniteclasts of the Chogdo Formation several reliablemeasurements were made. Several analyses werepossible in samples from the Choksti Conglomer-ate and Choksti Formation, where abundant,large K-feldspar grains were identi¢ed. Some

EPSL 5856 18-5-01


Fig. 5. Pb isotopic discrimination diagrams showing the current measured range of Pb compositions for K-feldspars from theLhasa Block, Transhimalaya and Indian Plate [44^46], and from whole rock values from the Kohistan Arc section of Pakistan[39]. Indian and Paci¢c MORB ¢elds from Sun [47] and Ben Othman et al. [48]. All analyses are shown with 1c uncertainties.

EPSL 5856 18-5-01


K-feldspars were too Pb-poor to allow analysis. Itis possible that our analyses provide a biased im-age of the feldspar population by only consideringthose with high Pb content. Alteration of thegrains is not considered to be an important sourceof error, since the major element analytical totalswere typically high (s 98%), suggesting little al-teration.

7.1. Interpretation of Pb data

One noteworthy aspect to the data is that mostof the analyses do not plot in any of the prede-¢ned ¢elds, even when the 1c uncertainty is ac-counted for. Given the depositional setting andthe constraints derived from the Nd isotopework it seems most likely that these data representparts of the compositional range of the di¡erentsources that have not yet been identi¢ed by earlierstudies. This is perhaps not surprising given thatmuch of the earlier work was located in the cen-tral part of the Himalaya, remote from the Mo-lasse basin in Ladakh. In addition, those studiesonly analyzed a modest number of grains fromeach block. The de¢ned range for the Karakoramis based on only four analyses [40]. Similarly, theplutonic parts of the Kohistan/Dras Arc are onlyexposed in Pakistan, and are only analyzed aswhole rock data. In any case, given the paleo-current data these exposures are most unlikelyto be the source of the Indus Molasse sediments.The analyzed grains from the Ladakh Batholith atHunder (Figs. 1 and 5D) show a range of valuesoverlapping with, but trending to lower isotopicratios than, the previously known Transhima-layan values [44]. These data demonstrate isotopicheterogeneity within the Transhimalaya and alsoshows that a local source is possible for thosegrains with low Pb isotopic ratios.

The low Pb isotopic ratios seen in the ChogdoFormation rule out Indian plate rocks as impor-tant sources for these sandstones. The most likelysource of the Chogdo Formation K-feldspars isthe Ladakh Batholith. Although the paleo-currentinformation and abundance of ophiolitic litholo-gies within the Chogdo Formation argues forsediment derivation from the SW, this does notrule out further input from the batholith. The

lack of sources towards the SW that containabundant K-feldspars also makes sole derivationof sediment from that direction unlikely.

With the exception of a few grains, it is clearthat the Indian Plate ¢eld is remote from nearlyall the measured values from all formations, espe-cially with respect to 207Pb/204Pb. This con¢rmsother lines of evidence that suggest that this wasnot a major source of K-feldspar. Only a few ofthe highest ratios from the Choksti Formationand Conglomerate may be of Indian Plate origin,and even those are within error of the Karakoramand Lhasa Block. This conclusion is in accordwith the relatively positive ONd values seenthroughout the Indus Molasse.

The Choksti Formation and Conglomerate ap-pear to be mixtures of material from the Transhi-malaya and a source with a more negative rangeof ONd, which cannot be Indian. The only possiblesuch sources known are the Karakoram or LhasaBlock. The dominant NE to SW paleo-currentdirections favor involvement of the Lhasa Blockrather than the Karakoram. Consequently, weconclude that those Pb analyses lying close tothe Lhasa Block ¢eld de¢ned by Gariepy et al.[44] were derived from this terrane, albeit fromareas with slightly di¡erent Pb isotopic valuesthan those previously analyzed. Likewise, thosefeldspar analyses within the Choksti Formationthat trend to lower 207Pb/204Pb values are takento represent erosion from the Ladakh Batholithor other parts of the Transhimalaya exposed to-wards the east. For those grains whose analysesfall between the higher 207Pb/204Pb (Lhasa Block)and lower 207Pb/204Pb (Transhimalaya) grains thesources are less clear. Indeed, the overlap betweenmeasured basement values from these areas prob-ably means that a unique de¢nition of the sourcemay well be impossible for every grain even iftruly comprehensive source compositional rangescould be de¢ned. Despite these shortcomings theion probe data provide evidence that above theChogdo Formation the clastic sediments enteringthe basin formed a mixture of material from theTranshimalaya and the Lhasa Block. Paleo-cur-rent data argue against signi¢cant involvementfrom the Karakoram or the Kohistan/Dras Arc.This means that while the lower Indus Molasse

EPSL 5856 18-5-01


represents local drainage in an intermontane set-ting, the upper part of the section is in£uenced bysediment transport over a regional scale, becausethe Lhasa Block is not exposed close to the studyregion.

8. Discussion

The ion probe isotopic data combined with thegeologic constraints and the clay Nd data providean image of the erosion history of the early Hi-malaya. It is important to remember that erosionis not the same as tectonic uplift, since this can betriggered by other factors, such as climate change.For the detrital history to be used to track ratesof denudation thermochronological work wouldneed to be performed on individual grains. Thisapproach is most e¡ective when depositional agecan be compared with cooling age (e.g., [50]) toprovide a minimum estimate of cooling rate, andthis in turn can be used to constrain erosion rates.In the poorly dated Indus Molasse this approachmay only be applicable in the Paleocene ChogdoFormation.

8.1. Uplift of the Lhasa Block

It is noteworthy that there is a change in sedi-ment provenance from the Paleocene ChogdoFormation to the upper parts of the section. Weinfer that this re£ects increased input from theLhasa Block. The pre-collisional location of theIndus Molasse Basin on the active margin of Asiameans that if the Lhasa Block had been a signi¢-cant sediment source prior to collision this shouldhave been recorded in the Chogdo Formation.Instead the continental and accreted arcs thatlay along the active margin dominate the record.We thus tentatively suggest that the reason therewas no early erosion of Lhasa Block was that itwas not a signi¢cant topographic feature untilafter the collision, i.e. at least the Eocene. Thisconclusion ¢ts reconstructions that have proposedmuch of Tibet close to sealevel during the Creta-ceous but with an elevated continental volcanicarc (Transhimalaya) along its south margin[51,52]. The Lhasa Block would then be progres-

sively uplifted as crustal thickening propagatednorthwards after collision. It is important to real-ize that topography does not automatically implyrapid erosion, although it is rare to ¢nd rapiderosion without elevated topography. However,a close correspondence between exhumation ratesderived from thermochronology and estimates ofmodern surface uplift derived from stream inci-sion rates indicate a strong relationship betweenuplift and erosion in the modern Himalayas andKarakoram [53]. In the absence of other data wefollow this logic in interpreting the erosion datapresented here. The provenance data require in-creasing erosion of the Lhasa Block and the £owof this material into the Indus Molasse Basin dur-ing the Eocene.

This conclusion is at odds with alternative re-constructions that consider the Lhasa Block tohave been highly elevated since the Late Creta-ceous [54] because these do not provide a tectonicmechanism to preferentially increase the erosionof the Lhasa Block after collision. Provenancework on Cretaceous sequences from centralsouthern Tibet, interpreted as forearc basin de-posits (Xigaze Group), has identi¢ed the northernportion of the Lhasa Block as their source, imply-ing that the Transhimalaya arc was subdued topo-graphically at this time [55]. Although our studycannot clearly resolve this argument for areas lo-cated much further east, the new data can elimi-nate this possibility in Ladakh.

There is strong evidence to support continuouserosion of the Transhimalaya throughout the col-lision, and with increasing in£uence up-section.This scenario is supported by reconstructions ofthe cooling history of the Ladakh Batholith basedon thermochronology work [16,56], that indicateaccelerated cooling during the Eocene (V45^52 Ma). This is presumably driven in part by tec-tonic uplift and erosion following collision. Asimilar accelerated uplift and unroo¢ng for theLhasa Block would be consistent with this pat-tern.

The implications of the Lhasa Block being up-lifted early in the collision process are signi¢cantfor models of strain accommodation and for cli-mate^tectonic interactions. Our result is consis-tent with crustal thickening, i.e., horizontal short-

EPSL 5856 18-5-01


ening, being the ¢rst reaction of the Asia marginfollowing collision with India. Unless large-scaleregional £exure or magmatic underplating can beinvoked, crustal thickening and the following iso-static reaction is the only common mechanism forgenerating elevated topography of the type in-ferred. Although early uplift need not precludetectonic extrusion of Indochina or Tibet [57],many reconstructions that emphasize extrusionas an important method of strain accommodationhave suggested that little crustal thickening occursin Tibet and the Himalaya during the Eocene andOligocene (e.g., [58]). Such models do not predictthe increasing uplift and erosion of the LhasaBlock recorded by the Indus Molasse.

8.2. Origin of the Indus River

The evolving provenance and paleo-currentdata suggest that the Indus River may have ini-tiated soon after collision, in practice during Nur-la Formation times (post-latest Paleocene). Thechange in paleo-£ow towards the west, and im-portantly the in£ux of detritus from the LhasaBlock is suggestive of an axial river, drainingthe western part of the plateau. In practice thisis what the modern Indus does today. Our resultis incompatible with the model of Sinclair andJa¡ey [59], who suggested an internally drainedIndus Molasse Basin, pre-dating Indus River ini-tiation. However, the absence of an appropriatesource with the necessary isotopic characteristicsand the observed change in paleo-£ow within thebasin now make this model untenable. We suggestthat the change between Chogdo and Nurla For-mations represents the change from a basin dom-inated by local drainage to one of regional drain-age. In this scenario the Indus has remainedstationary since at least the Middle Eocene, de-spite the subsequent horizontal compression thatinverted the Indus Molasse Basin. Such a timingfor Indus River initiation is consistent with theapparent start of Indus deltaic sedimentation inthe Katawaz Basin of Pakistan close to the Pale-ocene^Eocene boundary [60], as well as the evi-dence of Indus Suture Zone material reaching theArabian Sea at least by the Middle Eocene [61].The presence of such grains in the earliest parts of

the Indus Fan requires a long river system, anal-ogous to the modern Indus.

9. Conclusions

This study demonstrates that despite signi¢cantanalytical uncertainties the in situ analysis of Pbisotopes within single K-feldspar grains is e¡ectiveat constraining provenance in tectonically com-plex areas when used in conjunction with bulkmineral analyses, such as the Nd work on clayspresented here. The approach allows end mem-bers to mixed sedimentary sequences to be con-strained. Whole rock analyses only provide anaverage measurement of source composition.Without such single grain work it is not possibleto know how many sources are involved, and insettings like the Indus Molasse it may not be pos-sible to single out which source is acting as an endmember if data from only one isotopic system isused. This type of data are important to recon-structing erosion patterns and as an importantprecursor before attempting thermochronologywork on detrital minerals. Only single grainwork is able to provide the resolution requiredto provide unambiguous identi¢cation of sedi-ment sources.

In the case of the Indus Molasse the isotopicprovenance work shows the lack of input fromthe Indian Plate and demonstrates an evolutionfrom a Paleocene (Chogdo Formation) basindominated by erosion of Tethyan ophiolites (in-cluding Spontang) and the Transhimalaya to oneincorporating signi¢cant detritus from the LhasaBlock (Nurla and Choksti Formations). The onsetof erosion of the Lhasa Block is suggestive of itsuplift following India^Asia collision and that thisarea was not signi¢cantly elevated before the Eo-cene. The uplift accompanies, or may even cause,the initiation of the Indus River, following thesuture, a location it continues to occupy to thepresent day.

Acknowledgements

P.C. thanks JOI/USSAC and WHOI for ¢nan-

EPSL 5856 18-5-01


cial support to perform ¢eldwork in the IndusSuture and for some analytical support. P.C. isindebted to M.P. Searle for his introduction tothe geology of the suture zone and to Fida Hus-sein Mittoo of Leh and Rockland Tour and Trekfor all their help. M.P. Searle and Y.M.R. Naj-man are thanked for their advice on Himalayanerosion. J.P. Burg, C. France-Lanord and ananonymous reviewer provided helpful reviewsthat improved the quality of the work. TheNENIMF at WHOI is supported by GrantEAR-9904400 from the National Science Founda-tion. This is WHOI contribution 10457.[AH]

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Tracing patterns of erosion and drainage in the Paleogene Himalaya through ion probe Pb isotope...

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