clift indus river eps l 2002

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Nd and Pb isotope variability in the Indus River System:implications for sediment provenance and crustal

heterogeneity in the Western Himalaya

Peter D. Clift a;, Jae Il Lee a;1, Peter Hildebrand b, Nobumichi Shimizu a,Graham D. Layne a, Jerzy Blusztajn a, Joel D. Blum c, Eduardo Garzanti d,

Athar Ali Khan

e

a Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USAb Department of Geology, Trinity College, Dublin 2, Ireland

c Department of Geological Sciences, University of Michigan, 425 East University Avenue, Ann Arbor, MI 48109, USAd Dipartimento Scienze Geologiche e Geotecnologie, Universita' di Milano-Bicocca, Piazza della Scienza 4, 20126 Milano, Italy

e National Institute for Oceanography, ST-47-Block 1, Clifton, Karachi 75600, Pakistan

Received 29 October 2001; received in revised form 8 March 2002; accepted 19 March 2002

Abstract

The Indus River system is the only major drainage system in the western Himalaya, and erodes not only the High

Himalaya, but also topographically high regions within and north of the Indus Suture Zone, most notably theKarakoram. Ion microprobe analysis of Pb isotopes in detrital K-feldspar grains taken from the tributaries of the

Indus, together with bulk Nd isotope analysis of those same sediments, is here used to identify distinct sediment

source regions. These span the very radiogenic Nanga Parbat and associated Lesser Himalaya, the relatively

radiogenic-intermediate High Himalaya, the unradiogenic Ladakh and Kohistan Batholiths and intermediate values in

the Hindu Kush, Karakoram and Lhasa Block. The range of compositions reflects differing degrees of recycling of

older continental crust during petrogenesis. K-feldspars from the Ladakh and Kohistan Batholiths are less radiogenic

than the laterally equivalent Gangdese granite of Tibet, interpreted to reflect the preferential recycling of accreted

oceanic arc units within the western Transhimalaya prior to India^Asia collision. Similarly the Zanskar High

Himalaya are less radiogenic than their equivalents in Nepal. Isotope values from Pleistocene Indus Fan sediment are

compatible with a dominant source in the Karakoram, with additional important contributions from the arc

batholiths and High Himalaya, reflecting both the area and modern rates of tectonic uplift within the drainage basin.

In contrast, radiogenic grains are common in the lower reaches of the modern Indus River, possibly as a result of thedamming of the main river channel where it reaches the foreland. 2002 Elsevier Science B.V. All rights reserved.

0012-821X / 02 / $ ^ see front matter 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 01 2 - 8 2 1X ( 02 ) 0 0 6 20 - 9

* Corresponding author. Tel.: +1-508-289-3437; Fax : +1-508-457-2187.

E-mail address: [email protected] (P.D. Clift).

1 Present address: Polar Sciences Laboratory, Korea Ocean Research and Development Institute, Ansan P.O. Box 29, Seoul 425-

600, Korea.

Earth and Planetary Science Letters 200 (2002) 91^106

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Keywords: Himalayas; provenance; ion probe; isotope ratios

1. Introduction

The western Himalaya dier from their equiv-

alents further east in being drained by only one

large river system, the Indus. Understanding the

nature of the sedimentary record that the Indus

leaves in the foreland, or oshore in the Indus

Fan, is important if these deposits are to be

used to reconstruct the growth of topography fol-

lowing India^Asia collision. Throughout the Hi-

malaya erosion has largely removed the thermal

record of Paleogene cooling from the crystalline

orogenic core due to rapid Neogene exhumation

(e.g. [1^7]), although Paleogene cooling ages are

known from parts of the High Himalaya in Paki-

stan [8]. Consequently patterns and rates of early

exhumation can only be reconstructed from the

erosional record.

An understanding of the modern bedload of the

river is also important to attempts at quantifying

the linkage between erosion and climatic varibility

in south Asia. In particular, the relationships be-

tween tectonic evolution of the mountains, ero-

sion and monsoonal strength have been debated

(e.g. [9,10,11]), but remain dicult to correlate indetail. Burbank et al. [12] suggested that decreas-

ing rates of erosion since 8 Ma, immediately fol-

lowing the commonly accepted age of monsoonal

strengthening [13,14] reected a stabilizing of

slopes due to increased vegetation and the retreat

of mountain glaciers. However, measured erosion

rates in the modern Himalaya are faster in regions

where the monsoon is heavier [15]. In addition,

during the last glacial stage erosion rates in the

Himalaya, inferred from sedimentation rates in

the Ganges delta, were higher when the monsoonwas stronger [16]. It suces to say that the rela-

tionship between monsoonal strength and orogen-

ic erosion is presently poorly understood. A rst

stage to clarifying this situation is a good under-

standing of modern erosion and sediment trans-

port in a region where the monsoon is well under-

stood. For this purpose we choose the Indus

River system (Fig. 1).

In this study we have examined the provenance

of sediments in the Indus River system in order

to determine which parts of the mountain sourceregions are providing the bulk of the sediments

to the clastic record of the Arabian Sea, because

this is the most complete repository of eroded

material in the western Himalaya. Understanding

what controls the nature of sediment reaching

this basin is essential if we are to reconstruct the

long-term erosion history of the mountains. We

have opted to use the Pb isotopic system as ap-

plied to K-feldspars because this has an estab-

lished track record as a provenance tool (e.g.

[17]), and moreover has been used to discriminate

evolving provenance within the Indus Suture

Zone [18], as well as to demonstrate the deriva-

tion of suture zone, as opposed to Indian Plate,

material into the Arabian Sea during the Middle

Eocene [19].

2. Sampling strategy

We have examined how the Pb isotopic charac-

ter of detrital K-feldspar grains in the bedload of

the Indus River changes downstream, and howthis relates to the composition of the terrains

that the river is incising. In addition, we have

documented the changes in bulk sediment Nd iso-

tope composition for the same samples. In order

to derive erosion patterns from a marine or delta

sediment comprising grains derived from a num-

ber of dierent sources the range of compositions

of those sources must rst be determined prior to

mixing. Therefore, as well as studying sediments

from dierent locations along the course of the

main Indus River (Figs. 1 and 2), we also col-lected samples from a number of major tributaries

in order to characterize the composition of the

major sediment sources, in particular, the Nanga

Parbat^Harramosh Massif (NPHM; sample S15),

the Karakoram Batholith (sample LA-75), the

Southern Karakoram Metamorphic Belt (sample

W23), and the Ladakh Batholith (sample LA-94).

Samples taken from rivers that originate within a

single tectonic block can be used to characterize

P.D. Clift et al. / Earth and Planetary Science Letters 200 (2002) 91^10692


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the chemical and isotopic variability within that

block over the entire drainage area of that river,

providing a more accurate regional estimate than

is possible through simple analysis of the bedrock

itself.A number of other samples are known to drain

only two major sources, which can then be distin-

guished given the data from the single source riv-

ers and/or basement measurements. The Kabul

River (sample W33) for example is known to

drain the Hindu Kush and the Kohistan Arc ter-

rane (Figs. 2 and 3). The Gilgit River (sample

W2) drains the Southern Karakoram Metamor-

phic Belt (SKMB) and the Kohistan Arc (Fig.

4). The Ravi (sample S1476), Chenab (S1450)

and Sutlej (S1467) rivers drain the High and Less-

er Himalaya, although the Sutlej also drains a

small part of the Indus Suture Zone as well.

The samples taken were ne or medium grainedsands and are from a series of locations shown on

Figs. 1 and 2.

A limited amount of direct basement sampling

was performed in the Hindu Kush, because this

area was the only major basement unit for which

no existing analyses are published. Cross-checking

the detrital Pb isotopic ratios against known val-

ues from the basement helps to provide a high

level of condence in the provenance results.

Fig. 1. Digital topographic map of the Indus drainage basin showing sample locations.

P.D. Clift et al. / Earth and Planetary Science Letters 200 (2002) 91^106 93


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Analyses of K-feldspar grains have been made

from the other major tectonic units that form

sediment sources within the Indus drainage basin.

Distinct isotopic characters of the High Hima-

laya, Transhimalaya, Kohistan/Dras Arc, Kara-

koram and Lhasa Block sources have already

been established, albeit mostly further east in cen-

tral Tibet [21^25]. Determining whether these

same units were isotopically homogenous along

strike into the Indus drainage basin was a keygoal of the study. Four grains from a stream

draining the Ladakh Batholith at Hunder (Fig.

1) were published by Clift et al. [18] and are aug-

mented here with further analyses from the same

river system. For reference we also consider the

composition of the ocean mantle using modern

values for the Pacic and Indian Ocean mid ocean

ridge basalt as a proxy for Tethyan mantle

[26,27].

3. Nd isotopes

Some limitations on the source of the mixed

Indus sediments and the composition of the

source terrains can be gained through examining

the Nd isotopic composition of bulk sediment

samples, from which the single grain Pb isotope

measurements are also made. Because weathering

and the sediment transport process are not ex-

pected to result in isotopic fractionation, the mea-sured isotopic signature of the sediment should

reect the bulk composition of the source. Ten

grams of sediment were powdered from each sam-

ple to ensure a good average composition. Each

sample was then dissolved and the Nd separated

using standard column extraction techniques. Nd

isotopic compositions were determined on VG354

mass spectrometer at Woods Hole Oceanographic

Institution (WHOI). 143Nd/144Nd values are nor-

Fig. 2. Regional geological map of the Indus drainage basin.



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malized to 146Nd/144Nd = 0.7219 and are relativeto 0.511 847 for the La Jolla standard. The results

are shown in Table 1 in the Background Data

Set2. We calculate the parameter ONd [28] using

a 143Nd/144Nd value of 0.512638 for the Chon-

dritic Uniform Reservoir [29].

Fig. 5 shows the range and frequency of ONdvalues noted in the Indus River and marine sedi-

ments and compares these with published values

from the High Himalaya [29,30], Lesser Himalaya

[32,33,34], NPHM [35], the Transhimalaya [36],

Kohistan/Dras Arcs [37,38], the Karakoram [39],and Lhasa Block [40]. In general, sediment sam-

ples taken from rivers draining restricted areas

seem to show good correspondence with the ONdvalues measured from the basement, e.g. S-15 and

W-27 span the range measured from NPHM.

Sediment eroding from the Ladakh Batholith,

LA-94, is slightly lower in ONd than samples mea-sured from Transhimalayan granite samples them-

selves, but this material is still clearly less radio-

genic than other samples in the Indus system. Not

surprisingly, W-2 from the Gilgit River also

shows unradiogenic values reecting the impor-

tance of the Kohistan Batholith, an along-strike

equivalent of the Ladakh Batholith, as a source to

that region. W-2 is more radiogenic than LA-94,

because the Gilgit River also receives sediment

from the Karakoram, which has lower ONd values.

Rivers whose drainage basins mostly comprise theLesser and High Himalaya are seen to lie close to

the peak ONd values seen in the High Himalaya,

i.e. Zanskar (LA-109) and Ravi (S1476) rivers,

consistent with the High Himalaya dominating

the sediment ux to these stream.

In the main Indus itself ONd values fall down-

stream from 38.4 at the Indus^Zanskar conu-

ence (LA-111), to 38.6 at Skardu (W21), to

310.77 at Besham and reaching the lowest values

Fig. 3. Geological map of the Kabul River region. TMF= Tirich Mir Fault; RF= Reshun Fault; CF= Chaman Fault; KF = Ku-

nar Fault; MMT= Main Mantle Thrust; SSZ= Shyok Suture Zone.

2 http://www.elsevier.com/locate.epsl



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shortly before entering the Arabian Sea, 315.0 at

Sukkur (S-1) and 315.4 at Thatta (TH-1). This

trend can be readily understood in terms of a

progressive mixing of radiogenic sediment, mostly

eroded from the Lesser and High Himalayas andNPHM, into the less radiogenic sediment that the

main Indus River derives from the Indus Suture

Zone. The ONd values of the Indus in the Indus

Suture Zone in Ladakh lie closest to the values

measured in the Karakoram basement or in prox-

imal stream draining the Karakoram.

The provenance evolution of the river is notsimple because the ONd values measured from

the Pleistocene Indus Fan (ODP Site 720 [19])

and from the shallow-water shelf (ID-18) are

markedly less radiogenic than the modern sedi-

ment in the lower reaches of the river. Indeed

the ONd values measured from S-1 and TH-1 are

so close to the values measured from the Ravi

River that it implies that by the time the Indus

reaches the Arabian Sea there is almost no con-

tribution from the less radiogenic Indus River

sediments that reached the foreland at Besham(W-32). That this has not always been the case

is shown by the less radiogenic values on the shelf

and on the Indus Fan itself.

4. Pb isotopes of detrital feldspars

In order to better understand the provenance

evolution of the Indus River we employ the newly

Fig. 4. Geological map of the Gilgit River region.

Fig. 5. Nd isotopic discrimination diagrams for the river

sediments sampled from the Indus system, and compared

with previously published values from the basement sources.



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developed technique of measuring Pb in situ [41]

in single sand grains using a high-resolution Ca-

meca 1270 ion microprobe of the Northeast Na-

tional Ion Microprobe Facility (NENIMF) at

WHOI. Although producing analytical uncertain-ties much greater than the conventional thermal

ionization mass spectrometer method, the ion

probe approach allows for the rst time isotopic

determinations on individual sand and silt-sized

particles. In order to exploit the potential of this

method to characterize heterogeneous feldspar

populations several analyses were run from each

sand sample in order to dene the range of iso-

topic ratios in a single sample, and in the case of

the mixed sediment samples to identify small pop-

ulations of grains with distinct isotopic characters

(Table 2 in the Background Data Set2).

The sandstones were sieved, after which the size

fraction 1 mm to 200 Wm, was mounted in epoxy

and polished using aluminum oxide abrasives.

The K-feldspar grains were then identied by

area mapping of Al2O3 and K2O using the

JEOL Superprobe electron microprobe at the

Massachusetts Institute of Technology. This al-

lowed the K-feldspars to be identied for isotopic

analysis. After gold coating the grains were ana-

lyzed using a beam of negatively charged oxygen

ions (O3) focused to a spot as small as 15^20 Wm.Analytical uncertainties are principally a reec-

tion of the counting statistics, typically averaging

2c9 1%. The analytical results are shown in

Table 2 in the Background Data Set2. Analysis

of K-feldspar standards veries that there is no

signicant mass fractionation eect in analyzing

Pb isotopes using the ion microprobe methodol-

ogy compared to conventional mass spectrome-

try.

In order to minimize the risk of secondary Pb

contamination from sources outside the feldspar,analyses were made in the center of the grain,

away from cracks, inclusions or alteration zones.

The ion beam was trained on the spot to be an-

alyzed for 5 min before analysis began, so that

any surface Pb contamination was removed,

thus avoiding any contamination that might

have occurred during preparation of the grains

mount. Through probing grain centers and allow-

ing the beam to remove surface coating of the

sectioned grains we avoid analysis of excess sec-

ondary Pb that is normally removed by leaching

procedures prior to conventional mass spectrom-

etry [23].

K-feldspars from basement samples taken inthe Hindu Kush (Fig. 3) were extracted after

jaw crushing and were analyzed using convention-

al mass spectrometry at the British Geological

Society, Keyworth, UK. Results from this work

are shown in Table 3 in the Background Data

Set2.

5. Results

Figs. 6^8 show the spread of measured isotopic

ratios for detrital Indus River sands compared

with those previously recorded from basement

rocks from the central Himalaya/Tibet area, as

well as Asian mantle sources [20^26]. The elds

dened for the Indian Plate, Transhimalaya, Ka-

rakoram and Lhasa Block are also K-feldspar

analyses, while those from the Kohistan/Dras

Arc represent whole rock analyses from Pakistani

exposures [38], since no K-feldspar data are avail-

able from this unit. It is apparent that there is a

wide spread of Pb isotopic values in Indus River

sands. Streams draining the NPHM are the mostradiogenic noted, exceeding values from the High

Himalaya [23], similar to the pattern seen in the

Nd data. Likewise, grains eroded from the La-

dakh Batholith (LA-94) and some of the grains

in the Gilgit River (W-2), probably derived from

the Kohistan Batholith, represent the least radio-

genic end members. Many grains fall in interme-

diary values overlapping known basement values

for the Lhasa Block, the Karakoram, as well as

the new basement measurements from the Hindu

Kush. In practice there is a general gradation be-tween unradiogenic arc units, intermediate units

representing the southern margin of Asia prior

to India collision and the radiogenic values of

the Indian Plate.

Because K-feldspar contains almost no U the

Pb isotope character of the K-feldspars represents

the whole rock values at the time of crystallization

of the source rocks, and is not aected by subse-

quent ingrowth of radiogenic Pb [42]. Conse-



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quently, the river sediments conrm a broad scale

isotopic heterogeneity within the western Hima-

laya that reects unique geological histories of

the dierent tectonic blocks prior to their present

amalgamation into the Himalaya. The results alsosupport the idea of using Pb as eective source

discriminants for sediments in the western Hima-

laya because of the dierences between sources.

Although there is overlap between arc units and

the Karakoram/Hindu Kush/Lhasa Block crustal

group, a denitive interpretation is possible for

many grains. Similarly, some grains have values

compatible with erosion from either the High Hi-

malaya or from Karakoram/Hindu Kush/Lhasa

Block. However, even in cases of ambiguity it is

normally possible to clearly rule out some sourcesas possible origins for a given grain. For example,

a grain that plots within the overlap between

High Himalaya and Karakoram/Hindu Kush/

Lhasa Block cannot be eroded from the La-

dakh/Kohistan Batholiths or from NPHM. Ear-

lier studies suggest that other variables such as

mineralogy or Nd isotopes can be used to sepa-

rate sources that are not unique in Pb isotopes

[18].

6. Sediment provenance

The range of Pb isotopes recognized in grains

from the proximal streams draining the source

terrains within the Indus drainage basin allowsthe evolution of provenance in the modern river

to be assessed during its ow towards the Arabian

Sea. The Indus River in Ladakh below the con-

uence with the Zanskar (LA-111) has mostly in-

termediate to low isotopic ratios in its detrital

K-feldspars and an ONd value of38.37, indicating

that ux from the High Himalaya draining Zan-

skar River is not a major contributor to the net

ow. The Pb isotopic character of K-feldspars in

the Indus River in Ladakh is compatible with

dominant erosion of the Ladakh Batholith, orthe Paleogene sediments of the Indus Molasse

[18]. However, the bulk sediment Nd isotope

work demonstrates that the Indus sediment here

is too ONd negative to be derived solely from the

Ladakh Batholith. Although the K-feldspars at

the Indus^Zanskar Conuence could be derived

preferentially from other sources compared to

the bulk sediment, we infer that many of the

grains in sample LA-111 were derived upstream,

Fig. 6. Pb isotopic discrimination diagrams for (A) Karakoram Batholith, (B) Ladakh Batholith, and (C) Indus River at Zanskar.

Analyses from detrital grains are compared with previously measured Pb isotope values from basement rocks in the central Hi-

malaya/Tibet region, as well as Indian and Pacic mantle ranges [20^26].



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either directly from the Lhasa Block or reworked

from the Indus Molasse, which itself is largely

eroded from the Lhasa Block in the upper partof its stratigraphy [18].

The Gilgit River contains K-feldspar grains

that span a broad range of Pb isotopic values,

consistent with the Gilgit drainage basin deriving

run-o from both the Kohistan Batholith, with its

low Pb isotopic ratios, and the SKMB, dominated

by intermediate values. Like other rivers entering

the Indus River upstream of NPHM, the Gilgit

River does not have any grains with the very high

ratios typical of the Lesser Himalaya and NPHM,

both reworked parts of the ancient Indian craton.

6.1. Inuence of Nanga Parbat

The inuence of the NPHM in the net sediment

ow to the Arabian Sea can be assessed through

examination of the composition of the Indus up-

and downstream of the massif. The closest sample

upstream is W-21 located just upstream of the

Indus^Shyok Conuence (Fig. 1). The Indus

that ows past the NPHM is diluted by the addi-

tion of the Gilgit River compared to W-21, push-

ing the net ONd value of the Indus to more positivevalues before it passes NPHM. Because the aver-

age ONd values for the Karakoram lie close to the

ONd value of W-21, and this terrain represents the

dominant sediment source upstream of NPHM, it

seems likely that the average ONd value of the In-

dus just upstream of NPHM was close to the

38.64 measured at W-21.

A simple mixing calculation can estimate the

amount of NPHM material needed to push the

net ONd value of the Indus from 38.64 to that

found at W-32, i.e.3

10.77. We use an averageof S-15 and W-27 to represent the ow from

NPHM, and assume similar concentrations of

Nd in the sediment from each source. The modest

gure of 13% from NPHM still allows deep ero-

sion of the massif, but precludes a large propor-

tion of the grains in the Indus at W-32 being

derived from that area. A similar mixing calcula-

tion can be performed for the total sediment

reaching the Indus Fan by using the values of

Fig. 7. Pb isotopic discrimination diagrams for (A) Gilgit River, and (B) Kabul River.



10/16

312.56 and 314.03 measured for Pleistocene In-

dus Fan muds [19], and assuming that the four

major foreland tributaries which mostly drain the

High and Lesser Himalaya have similar ONd val-

ues to the Ravi River sample measured here

(315.0). The average Pleistocene ONd value for

the Indus Fan can be achieved by mixing 41^75% (depending on the value of the Indus Fan)

material derived from the High and Lesser Hima-

laya via the foreland tributaries, and 25^59%

from the main Indus River as sampled at W-32.

In this case NPHM would be the source ofV3^

8% of the total sediment reaching the Arabian

Sea.

Because the streams draining NPHM yield

grains with very high isotopic ratios, the single

grain Pb isotope work allows us to test the hy-

pothesis that the Indus Fan is not strongly af-

fected by erosion of this massif. Analyses of Pb

isotopes from Pleistocene Indus Fan sediments

(Fig. 9B) have been used to infer a weak ux ofsediment from Indian Plate sources [19], including

NPHM. Additional analyses were made from the

Pleistocene fan sample from ODP Site 720 on the

mid fan, which was previously examined, in order

to search for small grain population groups that

may have been missed by the initial work [19].

There is no evidence for the distinctive radiogenic

grains from the NPHM, in the Pleistocene fan

sediment, indicating that this massif provides

only a small amount of the total sediment reach-

ing the Arabian Sea.

The geochemically derived evidence for erosion-

al inux from NPHM can be compared to esti-

mates of eroded rock volumes based on the area

of the source and the degree of Neogene cooling

measured by radiometric methods [19]. By making

such estimates for the entire Indus drainage basin

Clift et al. [19] predicted that as much as V20%

of the modern Indus bedload could be derived

from the NPHM, assuming that a peak rate of

cooling of 7 km/Myr [1,2,43] was applied to the

entire area of the NPHM, and that a temperature

gradient of 30/km was appropriate. However, ex-humation rates based on these simple assumptions

were probably overestimates. Other workers have

proposed peak exhumation rates of 3^7 km/Myr

after considering the eects of topography and

the perturbed geothermal gradient [44,45]. If these

lower rates were correct this would imply 6 10%

of the Indus Fan having been derived from

NPHM. If exhumation was even partly tectonic

in origin (e.g. [46]) then this would reduce the

erosional contribution further, approaching the

3^8% estimate favored by this study. Clearly theerosional record of the Indus Fan is not dominat-

ed by erosion from NPHM.

6.2. Inuence of the High Himalaya

As noted above, the Nd isotope data indicate

that as much as 41^75% of the sediment reaching

the Indus Fan may be derived from the large trib-

utaries of the Indian foreland which principally

Fig. 8. Pb isotopic discrimination diagrams for (A) Nanga

Parbat, and (B) the Chenab, Ravi and Sutlej rivers.



11/16

erode the Lesser and High Himalaya. This hy-

pothesis can be tested using the single grain Pb

isotope data. Although the range of Pb isotope

values in the Chenab, Ravi and Sutlej rivers

does show signicant overlap with the range mea-sured in Karakoram rivers (Fig. 8B), making ab-

solute discrimination of the two sources impossi-

ble, it is noteworthy that each foreland river also

carries a minority of very radiogenic grains that

have no equivalent yet detected in the Karakoram

or the Indus Fan. Grains from the SKMB with

high 206Pb/204Pb values have lower 207Pb/204Pb

values than those from the Lesser Himalaya/

NPHM. The absence of very radiogenic grains

from the Indus Fan sediment argues against a

very large proportion of that sediment in the In-

dus Fan being derived from the foreland tributa-

ries, at least during the Pleistocene.

6.3. Inuence of the Karakoram

The implication of the combined Nd and Pb

data is that much of the sediment in the Indus

Fan is derived from north of the Indus Suture,

mostly from the Karakoram. Exhumation rates

derived from radiometric work in the SKMB

reach 5^7 km/Myr [2^4,6,7], suggesting that this

could be a major sediment source. In contrast, theKarakoram Batholith has much lower recent rates

of exhumation (6 1 km/Myr [47]). The area of the

SKMB is approximately six times that of NPHM

(500U35 km versus 100U30 km), and conse-

quently six times more material may be derived

from this source into the Indus River. The Pb

isotopic character of the K-feldspar grains in the

Indus Fan is consistent with the Karakoram being

their dominant source, as is the bulk sediment ONdvalue. Within the sediment now eroding from the

Karakoram there is a minority population of ra-diogenic grains identied in the Braldu River (W-

23), which does not seem to be reected in the fan

sediment. The High Himalaya show a range of Pb

isotope ratios that overlap with those in the fan,

but this source does not match the ONd values, or

explain the lack of very radiogenic Lesser Hima-

layan feldspar grains that are supplied along with

the High Himalayan material by the foreland trib-

utaries.

The low proportion of High Himalayan mate-

rial in the Indus Fan compared to the Bengal Fan

reects the nature of the drainage basin concen-

trated north of the Indus Suture, and the simple

fact that the western High Himalaya are not topo-graphically very elevated compared to their east-

ern equivalents. Although the High Himalaya in

Zanskar and Lahaul were rapidly exhumed at

V20^23 Ma [5,48,49], metamorphism in the west-

ern Pakistan High Himalaya appears to peak

much earlier, V45 Ma [8,49]. Consequently, the

modern erosional ux from these units into the

Indus is rather less than is the case in the

Ganges^Brahmaputra drainage basin. It is note-

worthy that unlike the central and eastern Hima-

laya, exhumation is strong north of the Indus Su-

ture Zone within the Indus drainage area, an

observation suggestive of an active link between

exhumation and drainage development.

7. Sediment ow rates

Analyses from the Indus River at Sukkur and

Thatta (S1 and TH-1; Fig. 1) provide an addi-

tional dimension to our understanding of sedi-

ment ow within the modern Indus. These sam-

ples were taken downstream of the major forelandtributaries that join the main Indus River (i.e.

Jellum, Chenab, Sutlej and Ravi). The dominant

sources for these tributaries are the High and

Lesser Himalaya, a fact conrmed by the analyses

from the Chenab, Ravi and Sutlej rivers made in

this study (Fig. 8B). In contrast with the Indus

Fan sample (ODP Site 720), the Indus at Sukkur

(S1) has a small number of grains with high iso-

topic ratios, associated with erosion of the Lesser

Himalaya or NPHM. The bulk sediment ONd val-

ues for S1 and TH-1 are more radiogenic than theIndus River upstream or the Indus Fan and Paki-

stan Shelf. No major tributary joins the Indus

River downstream of Sukkur or Thatta, making

its mis-match with the marine dicult to explain.

Both Pb and Nd isotopic data suggest that S1

and TH-1 do not represent the product of steady-

state ow in the Indus River, as they are much

more enriched in material eroded from the Indian

Plate than is normal given the character of the fan



12/16

sediments. The cause for this mis-match is pres-

ently unclear. One possibility is that this repre-

sents seasonal variation in the sediment ux, per-

haps due to climatic variability favoring erosion

of the Himalaya over the Karakoram. For exam-

ple, precipitation during the summer monsoon is

strongest in the frontal Himalayan ranges, while

erosion of the Karakoram may be dominantly

governed by glacial erosion. Alternatively, the ra-

diogenic isotopic character of S1 and TH-1 may

be related to the damming of the Indus close to

where it reaches the foreland at Tarbela (Fig. 1).

Fig. 9. Pb isotopic discrimination diagrams showing a comparison between the range of Pb isotopic characters inferred for the

source regions and detrital K-feldspars in the Indus River at (A) Sukkur, (B) ODP Site 720 on the Indus Fan, and (C) in the In-

dus River at the Indus^Zanskar Conuence.



13/16

The dam there was completed in 1976 and has

drastically cut the amount of sediment from the

upstream parts of the river which reaches the

foreland. The dam must therefore cause sediment

downstream to be enriched in Indian Plate mate-rial relative to normal ow conditions, because

the foreland tributaries would represent a high

proportion of the total sediment ux after dam-

ming. This hypothesis requires that the river be-

tween the dam and Thatta has been largely

washed of sediment eroded from upstream of

the dam, and thus dominated by ow from the

Himalaya-draining tributaries that join the Indus

River in the foreland. This model allows the rate

of sediment ow through the river to be esti-

mated. Thinking of the dam as creating a nega-

tive wave of sediment owing downstream from

the dam since 1976 at an average transport rate of

48 km/yr would be required to cover the V1200

km horizontal distance between Sukkur and the

dam.

Further complexity in the sediment transport is

inferred from the very unradiogenic ONd values

measured from the Pakistan Shelf, less than the

sediments at S1 and TH-1, as well as the Indus

Fan itself. This mis-match may reect along-

shore, eastward transport of material from the

Makran coast, while much of the sediment fromthe Indus itself may bypass the shelf and be trans-

ported directly to the deep fan via the Indus Can-

yon.

8. Crustal evolution

Isotopic dierences between dierent tectonic

blocks in the western Himalaya reect important

dierences in the petrogenesis of each unit. Be-

cause the Pb isotopic character of the K-feldspargrains represents the original Pb isotopic charac-

ter of the whole rock at the time of crystallization,

and before ingrowth of additional radiogenic Pb

after crystallization, the measured values can be

used to infer the relative inuence of ancient ra-

diogenic continental crust versus oceanic mantle

in the melting process. Thus, very high radiogenic

Pb isotopic ratios from NPHM indicate a rework-

ing of very ancient continental crust, in accord

with the correlation of NPHM with the Lesser

Himalaya, based on very radiogenic Nd isotope

data [35]. Similarly, the slightly lower, but still

relatively high Nd isotopic values seen in the

Ravi River reect the radiogenic character ofthe High Himalaya, also derived by melting and

reworking of pre-existing Indian continental crust

[31,36]. Nonetheless, there is a marked dierence

between the range of Pb isotopic ratios measured

in the Indus drainage basin and those recorded

from the High Himalaya further east [23]. The

western High Himalaya do not seem to show

the range to much higher values that the eastern

and central ranges do, implying a generally less

radiogenic (younger) crust, closer in composition

to the Karakoram/Lhasa Block/Hindu Kush.

In contrast, the low, unradiogenic ratios mea-

sured in grains from the Gilgit River, which were

derived from erosion of the Kohistan Batholith,

are in accord with the source being either an in-

tra-oceanic arc intrusion [37,38] or a later intru-

sion into an intra-oceanic arc. The low values

preclude major reworking of ancient radiogenic

continental crust into the Kohistan Batholith. In-

terestingly, the Ladakh Batholith also has domi-

nantly low Pb isotopic ratios, somewhat lower

than its apparent along-strike equivalent in the

Gangdese Batholith [40]. Gangdese, Ladakh andKohistan are all believed to represent the roots of

a continental arc along the southern margin of

Asia active prior to India-Asia collision. The

more continental character of the Gangdese Bath-

olith is typical of continental arc intrusions, and

suggests that either the Ladakh Batholith was

melted from a mantle source and then intruded

into the Asian (Karakoram) margin without

much reworking of the existing crust, or that the

basement into which it was emplaced contained a

large amount of oceanic material, as is clearly thecase in the Kohistan Batholith. Given that Kohi-

stan and Ladakh lie close to one another and are

only separated by the NPHM the latter scenario is

preferred. Chemical and isotopic work show that

intra-oceanic volcanic and volcaniclastic rocks of

the Kohistan/Dras Arc are found as far east as

Ladakh, immediately south of the area sampled

for K-feldspars [50]. The lower structural levels of

that arc sequence may have been reworked during



14/16

the later continental arc magmatism, resulting in a

relatively unradiogenic character to the total

batholith. This conclusion is at odds with the

suggestion that an oceanic arc only existed in

Kohistan and that the arc magmatism in the La-dakh region, east of Kargil, was always continen-

tal [51].

The generally similar Pb isotopic ranges for the

Karakoram, Hindu Kush and Lhasa Block are

consistent with them all being terranes along the

southern margin of Asia prior to Indian collision.

Their generally less radiogenic character than the

Lesser Himalaya and NPHM suggests that the

crust is younger, with less time for radiogenic

Pb to accumulate since extraction from a mantle

source. Alternatively, the crust may have origi-

nally been of the same age, but has been intruded

by younger, less radiogenic melts in an active

margin environment, which reduced the average

radiogenic contents to the values seen. The ap-

pearance of a minor population of radiogenic

grains in the Braldu River suggests that the

SKMB contains a minor amount of older crust,

intercalated within the dominant less radiogenic

material. Clearly the Karakoram, Hindu Kush

and Lhasa Block have been in an active margin

setting for a long time, because intrusions dating

from as early as 195 Ma are known from theHindu Kush [52], and from 150^145 Ma [5] and

115 Ma [53] in the Karakoram. Although much of

this magmatism would involve reworking of the

continental basement on which the arc was sited,

the derivation of some new material from the

upper mantle would necessarily make the crust

less radiogenic than it would otherwise have been.

9. Conclusions

Analysis of the Pb isotopic composition of de-

trital K-feldspar grains and the Nd isotopic char-

acter of the bulk sediment from which they were

taken, at a variety of locations within the Indus

River system has identied signicant isotopic

heterogeneity within the western Himalaya, Kara-

koram and Tibet. Four grain populations can be

distinguished, albeit with some overlap between

groups. A very radiogenic group is associated

with NPHM and the Lesser Himalaya. Another

group with high Pb isotopic ratios is eroded from

the High Himalaya, while an intermediate group

is derived from the Karakoram, Hindu Kush or

Lhasa Block. Finally a group with low Pb iso-topic ratios is eroded from the Ladakh and Kohi-

stan Batholiths. The dierences are inferred to

represent real dierences in bulk age of crustal

accretion, composition and the duration over

which radiogenic Pb has been allowed to accumu-

late. The Ladakh Batholith has low Pb isotopic

ratios similar to Kohistan Batholith, but lower

than the Gangdese Batholith, which is located

further east in the Transhimalaya. This indicates

magmatic recycling of oceanic arc fragments in

the west prior to India^Asia collision.

The bedload of the Indus River in Ladakh is

derived from the Lhasa Block and/or the Indus

Molasse Group, with rather less ux from the

Ladakh Batholith. Downstream large volumes of

material are added from the Karakoram. Pb iso-

topic analyses from deep-sea sands conrm that

the Indian Plate, including NPHM, is not the

main contributor to the Indus Fan, which is dom-

inated instead by the rapidly exhuming terranes of

the SKMB. The High and Lesser Himalaya pro-

vide V40% of the total sediment, mostly via the

large tributaries that join the Indus in the fore-land. Erosional ux to the ocean appears to be

linked to both the area of a given source and the

instantaneous rates of tectonic uplift.

Acknowledgements

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

cial support to perform eldwork in the Indus

Suture and for some analytical support. P.C.

thanks Fida Hussein Mittoo of Leh and Rock-land Tour and Trek for all their logistical help

in the eld, and Nilanjan Chatterjee (MIT) for

technical help using the electron microprobe.

J.L. was supported by the Korea Science and En-

gineering Foundation. J.D.B. thanks C. Gazis for

help in the eld and National Science Foundation

Grant EAR-9418154 for support. The NENIMF

at WHOI is supported by Grant EAR-9904400

from the National Science Foundation.[BARD]



15/16

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clift indus river eps l 2002

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