Earth and Planetary Science Letters 366 (2013) 59–70

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Earth and Planetary Science Letters

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382007

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Mantle fluids in the Karakoram fault: Helium isotope evidence

Simon L. Klemperer a,n, B. Mack Kennedy b, Siva R. Sastry c, Yizhaq Makovsky d,T. Harinarayana c,1, Mary L. Leech e

a Department of Geophysics, Stanford University, CA 94305-2215, USAb Center for Isotope Geochemistry, Lawrence Berkeley National Laboratory, CA 94720, USAc National Geophysical Research Institute, Hyderabad 500007, Indiad Department of Marine Geosciences, University of Haifa, Haifa 31905, Israele Department of Geosciences, San Francisco State University, CA 94132, USA

a r t i c l e i n f o

Article history:

Received 19 June 2012

Received in revised form

11 January 2013

Accepted 12 January 2013

Editor: B. Martydepth. Here we show 3He/4He ratios in geothermal springs along a 500-km segment of the KKF are 3–

Keywords:

Karakoram fault

Tibet

Himalaya

helium isotopes

geothermal activity

India–Asia collision

1X/$ - see front matter & 2013 Elsevier B.V.

x.doi.org/10.1016/j.epsl.2013.01.013

esponding author. Tel.: þ1 650 723 8214.

ail address: sklemp@stanford.edu (S.L. Klemp

ow at: Gujarat Energy Research and Manage

, India.

a b s t r a c t

The Karakoram fault (KKF) is the 1000 km-long strike-slip fault separating the western Himalaya from

the Tibetan Plateau. From geologic and geodetic data, the KKF is argued either to be a lithospheric-scale

fault with hundreds of km of offset at several cm/a, or to be almost inactive with cumulative offset of

only a few tens of kilometers and to be just the upper-crustal localization of distributed deformation at

100 times the normal ratio in continental crust, providing unequivocal evidence that a component of

these hydrologic systems is derived from tectonically active mantle. Mantle enrichment is absent along

the Indus–Yarlung suture zone (ISZ) just 35 km southwest of the KKF, suggesting that the mantle fluids

flow only within the KKF. Within the last few Ma, the KKF must have accessed tectonically active

Tibetan mantle northeast of the ‘‘mantle suture’’ which we therefore locate vertically beneath the KKF,

very close to the surface trace of the ISZ. Hence, in southwestern Tibet, Indian crust may not now be

underthrusting substantially north of the ISZ, even though Miocene underthrusting may have placed

Indian crust north of the ISZ in the lower half of the Tibetan Plateau crust. This is in significant contrast

to central and eastern Tibet where underthrust Indian material not only forms the lower half of the

Tibetan crust but is also currently underthrusting for �200 km north of the ISZ. Our new constraint on

KKF penetration to the mantle allows an improved description of the continuously evolving Karakoram

fault, as a tectonically significant yet perhaps geologically ephemeral lithospheric structure.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Observation of mantle contributions to 3He at the surfacerequires both a source of helium in the mantle, and a path to thesurface. In Tibet the mantle source is believed to be Tibetan mantlethat is tectonically active (i.e., with incipient melting and likelydeforming), as opposed to cratonic Indian mantle (Hoke et al., 2000).We propose that elevated 3He/4He ratios in hot springs along theKarakoram fault (KKF) (Fig. 1) demonstrate that within the last fewMa the KKF has channeled fluids to the surface from tectonicallyactive Tibetan mantle. The KKF therefore marked the ‘‘mantlesuture’’, or northern limit, at the Tibetan Moho, of actively subduct-ing Indian lower lithosphere. Our new data showing no mantlecontamination of hot springs on the Indus–Yarlung suture (ISZ)complement our new and existing data showing clear mantle

All rights reserved.

erer).

ment Institute, Gandhinagar

enrichment along the KKF and together imply focusing of the mantlefluids by the strike-slip fault system.

1.1. The Karakoram fault controversy and the northern limit of India

The structure of Tibet as a geodynamic response to thecollision of India with Asia, and the distribution of deformation,are particularly contentious in west Tibet (for the purpose of thispaper, west of about 821E). Two continental-scale strike-slipfaults—the dextral KKF and the sinistral Altyn Tagh fault (ATF)(Fig. 1)—play a disputed role in the eastward extrusion of Tibetsince the onset of the India–Asia collision at �57 Ma (Leech et al.,2005). Two extreme and opposing views exist for themechanism(s) responsible for crustal accommodation of short-ening across Tibet: (1) discrete tectonic blocks, internally rela-tively undeformed, are extruded eastward between lithosphericstrike-slip faults (e.g. Tapponnier et al., 2001; Thatcher, 2007) and(2) deformation is continuously distributed within the litho-sphere via ductile flow of the lower crust and upper mantlematched by brittle failure throughout a ubiquitously faulted

30°N

35°N

KKF77°E 82°E

JSZ

BSZ

ISZ

MFT

BSZ

KKF

Himalaya

TM

ZB

ATF

LhasaBlock

LGF

Qiangtang Block

Tarim Basin

ISZ

MCT

30°N

35°N

MCT

MFT

ATF

JSZ

BSZ

ISZ

KKF

Fig.

4a

Figs. 1b,c

Fig. 4b

30°N

35°N

YT

D

Songpan-Ganzi

Qiangtang

Lhasa

Himalaya

N

Changlung

Panamik

ShiquanheLangjiu

TirthapuriMengshi

GaikChumathang

Puga

ManikaranKasolKullu

CP

S

L

TM

GCh

Pu

MaKa

Ku

80°E 90°E

S.L. Klemperer et al. / Earth and Planetary Science Letters 366 (2013) 59–7060

upper crust (e.g. England and Molnar, 1997; cf. Beaumont et al.,2006).

The relative merit of these viewpoints in western Tibet hasbeen assessed in part from studies of slip rate on the KKF and ATF.Geologic inferences of rapid slip up to 32 mm/a (Valli et al., 2008)and large offsets up to 555 km (Replumaz and Tapponnier, 2003)or �300 km (Rolland et al., 2009) on the KKF seem to supportplate-like behavior, and an important role for the KKF as alithospheric structure. In contrast, geodetic measurements ofslow or zero modern slip on the KKF (375 mm/a: Jade et al.,2004; 0–6 mm/a: Wang and Wright, 2012) seem to supportcontinuous deformation of Tibet. Geologic claims for small totaloffset compatible with slow slip (66–150 km: Murphy et al.,2000; 40–150 km: Phillips et al., 2004) also suggest only a minorupper-crustal role for the KKF.

At a regional scale the depth of penetration of the KKF definesthe tectonic interaction between India and Tibet. Hypothesizedactive channel flow (e.g. Beaumont et al., 2006) of mid-to-lowercrust from Tibet into the northwest Himalaya driven by gravita-tional potential energy of the Tibetan Plateau would require thatthe KKF is limited to the shallow crust (Phillips et al., 2004),terminating downwards at a partial melt/channel flow zone at ca.20 km depth above underthrusting Indian crust. Alternatively theKKF may reach into the middle crust to form a southern bound tothis channel flow (Leech, 2008) or penetrate the whole litho-sphere and constrain the northern limit of the underthrust Indianplate below the Tibetan Moho (Rolland et al., 2009).

Seismic imaging of western Tibet has not yet imaged the KKF, asopposed to the ATF that is widely accepted from teleseismic imagingto be of lithospheric scale (Wittlinger et al., 2004), in part becausethe KKF is located close to disputed international boundaries.However, the northern limit of Indian crust beneath Tibet mayconstrain the depth of penetration of the KKF, now or in the past.Some tomographic images of west Tibet (Li et al., 2008) allowTibetan mantle as far south as the KKF, consistent with but notrequiring a crustal-penetrating KKF. However, most seismologicestimates of the northern limit of India (Nabelek et al., 2009;Priestley et al., 2008; Rai et al., 2006; Zhao et al., 2010) (Fig. 1b)nominally preclude penetration of the KKF into the mantle becausethey interpret an unbroken subthrust Indian crust, and infer thisIndian crust is being subducted almost horizontally at the presentday. A receiver-function ‘‘doublet’’ of Ps converters at �801E(Wittlinger et al., 2004) may mark the top and bottom of under-thrust Indian crust forming the lower half of the Tibetan crustnorthward to about the Banggong–Nujiang suture (Nabelek et al.,2009). Similar Moho depths immediately north (Wittlinger et al.,2004) and south (Rai et al., 2006) of the KKF may imply

Fig. 1. 3He/4He sample locations and faults in Tibet, and previous estimates of the

northern limit of India beneath western Tibet. (a) Shaded relief index map of Tibet

and major faults: KKF—Karakoram fault, MFT—Main Frontal thrust, MCT—Main

Central thrust, ISZ—Indus-Yarlung suture zone, BSZ—Banggong–Nujiang suture zone,

JSZ—Jinsha suture zone, ATF—Altyn Tagh fault (Karakax fault). N: Nagqu; Y, T, D:

Yangbajain, Tingri, and Daggyai Co graben systems; Y is location of mantle suture of

Hoke et al. (2000). (b) 3He data locations (diamonds—this study; circles—previous

studies in Tibet) (Hoke et al., 2000) and the Himalaya (Walia et al., 2005) super-

imposed on tectonic map (Murphy et al., 2000). Letters identifying geothermal sites

(C, P, G, Ch, Pu, S, L, M, T, Ku, Ka, Ma) are spelled out in Fig. 1c and used in Figs. 2 and

3. Yellow shading (C, P, S, L, M, T): RC/RA40.05/RA—all on or immediately north of the

KKF. White symbols: no mantle signature. Bold red lines: previous interpretations of

northern limit of Indian lithosphere at the Moho from S-receiver functions (Zhao

et al., 2010), from P-receiver functions (Nabelek et al., 2009; Wittlinger et al., 2004),

from northern limit of deep earthquakes (at 85–95 km depth—black stars) (Priestley

et al., 2008), and from body-wave tomography (Li et al., 2008). LGF—Longmu

Co-Gozha Co fault system. TM and ZB: Tso Morari and Zada basin (Leo Pargil) normal

faults. (c) Satellite image of KKF (same area and scale as (b)). (For interpretation of the

references to color in this figure legend, the reader is referred to the web version of

this article.)

S.L. Klemperer et al. / Earth and Planetary Science Letters 366 (2013) 59–70 61

underthrusting of Indian crust and mantle as far north as the ATF(Rai et al., 2006), and Sp receiver-function images at �821E havealso been interpreted as imaging Indian mantle lithosphere directlybeneath Tibetan crust across the full north–south extent of westTibet (Zhao et al., 2010) (Fig. 1b). Finally, the distribution of ‘lowercrustal’ earthquakes, including those at �771E, has been suggestedto show ‘‘underthrust Indian Shield y underlies virtually the wholeof NW Tibet as far as the ATF’’ (Priestley et al., 2008). Most of thesestudies do not separately delimit the northern edges of Indian crustand Indian lithospheric mantle; do not have the resolution todistinguish a possible thin layer of Tibetan mantle separating Indianlower lithosphere from Indian upper lithosphere; and do not havethe ability to distinguish whether Indian lithosphere beneath centralTibet is currently subducting or alternatively has been stranded inplace north of the active subduction system. However, theseinterpretations of the northern limit of Indian material at Mohodepths (bold red lines in Fig. 1b) span the entire north–south widthof Tibet showing that current seismic studies are not yet a reliableguide to the northern limit of India in west Tibet.

1.2. 3He as a tracer of mantle fluids

Noble gases, being inert, are excellent natural tracers for fluidmigration and see through complex chemical processes affectingother more reactive species (e.g. Ballentine et al., 2002). Distinctmantle, crustal, and atmospheric sources are characterized byunique noble gas compositions, so their contributions can beidentified and used to constrain the fluid’s history, and heliumisotopes can provide unequivocal evidence for the presence ofmantle-derived volatiles. Atmospheric helium has a 3He/4He ratioRA¼1.4�10�6, whereas crustal helium (dominated by radiogenic4He), is characterized by a 3He/4He ratio conventionally taken tobe 0.02RA, and upper-mantle helium (enriched in primordial 3He)has a value most often taken as 8RA (Ballentine et al., 2002).

Observation of mantle helium at the surface requires both a wayto extract helium from the mantle, and a path to the surface. Mantlehelium enters the crustal hydrologic system either by intrusion anddegassing of mantle-derived magmas or by invasion of fluidsdegassed from mantle melts. In order to release its 3He, thelithospheric mantle must be undergoing at least incipient meltingbecause solid-state processes are too slow to enable extraction ofnoble gases (Ballentine et al., 2002). Upward transport within thecrust may be accomplished by focused flow along major faults(Dogan et al., 2009; Kennedy et al., 1997), disseminated fluxthrough a deforming ductile lower crust (Kennedy and van Soest,2007; Newell et al., 2005), or shallow emplacement of deep-sourcedmagmas (Hilton, 2007) into which helium is strongly partitioned.

Observations of 3He/4He ratios 40.02RA can be due to at leastfour effects other than mantle fluids:

(1)

Air contamination: Helium in crustal fluids is not typicallyimpacted by contamination by air or air-saturated waterbecause helium has a very low atmospheric abundance(�5 ppm). Nonetheless, atmospheric contributions need tobe considered, particularly for samples with low heliumabundance. However, because both 22Ne and 36Ar in crustalfluids are dominated by atmospheric sources one can correctfor atmospheric contributions to helium using either 4He/36Aror 4He/22Ne ratios (Ballentine et al., 2002). The resultingcorrected 3He/4He ratios are conventionally denoted RC.

(2)

Radiogenic 3He produced by the nuclear reactions 6Li(n,a)3H(b�)3He: The conventionally assumed continental crustal3He/4He ratio of 0.02RA is based on zero input of mantle fluidsand a Li concentration in the rock matrix of 40–60 ppm(Andrews, 1985). Recent estimates of 24 ppm as the averageLi composition of the upper crust (Rudnick and Gao, 2005)

might imply a better value of 0.01RA so that any measuredvalues 40.01RA would imply some mantle influx. In thispaper we conservatively use the conventional value of 0.02RA.

(3)

3He produced by the decay of 3H that is a residual product of

atmospheric hydrogen-bomb tests: The abundance of tritium inthe environment has rapidly decreased since the 1963 PartialTest Ban Treaty due to the short (12-yr) half-life of 3H, so thatrecent samples are scarcely affected.

(4)

Re-mobilized helium from older mafic intrusions within the

crust: To dominate over a direct mantle source suchintrusions would have to be mafic or ultramafic (deriveddirectly from the mantle), relatively young (such that post-eruptive decay of U and Th has not diluted the initial elevated3He/4He ratio), and volumetrically huge (�104 km3) to pro-vide sufficient 3He to influence the helium signature ofgeothermal systems (Hoke et al., 2000).

During transport from the mantle source with 3He/4He �8RA,the degree to which the 3He/4He ratio is lowered (diluted) by theaddition of radiogenic 4He depends on the mantle helium fluxinto the crust, the production rate of 4He in the crust, and theresidence age or fluid flow rate within the crust. The integratedupward flow rate for steady-state one-dimensional flow scaleswith the crustal thickness and can be calculated from themeasured 3He/4He ratio and 4He concentration in the sampledfluids, using conventional values for helium isotopic ratios in thecrust (0.02RA) and mantle (8RA), and the present-day crustalproduction rate of 4He from Th and U (Kennedy et al., 1997).Conventionally, any corrected 3He/4He ratio RCRA40.1 is consid-ered to have an unequivocal mantle component (Ballentine et al.,2002) reflecting recent transport from the mantle.

1.3. Association of spatial patterns of 3He/4He ratios with different

tectonic styles

1.3.1. Continental strike-slip faults

In our study of the KKF, the helium isotopic signature ofcontinental strike-slip faults is of particular interest. Plate-boundary strike-slip faults that penetrate from Earth’s surface tothe mantle are obvious pathways for migration of mantle fluids,and both the San Andreas fault (SAF; Kennedy et al., 1997;Kulongoski et al., 2013) and the North Anatolian fault (NAF;Dogan et al., 2009; Gulec- et al., 2002) have RC/RA�4 in somesprings along these faults or their active splays (Fig. 2a). However,even closely spaced springs can exhibit very different degrees ofcrust–mantle interaction suggesting input from different hydro-logic systems. Some groundwater samples separated along theSAF by o10 km have RC/RA values varying by a factor 420(Kulongoski et al., 2013), so it is customary to focus on the most-enriched samples that likely have the most geodynamic signifi-cance (envelopes of data points in Fig. 2). The localization ofincreased 3He/4He to within one crustal thickness (�30 km) ofboth the SAF and NAF, and the ratio of the highest 3He/4He valuesclose to the SAF and NAF to the lowest values away from thesefaults (�40) is a strong argument for fault-focused fluid flowpenetrating the whole crust along the SAF and NAF. Simple 1Dsteady-state flow models imply time-averaged upward flow ratesof the 3He-enriched fluid in the SAF of up to 11 mm/a (Kennedyet al., 1997) or even �147 mm/a (Kulongoski et al., 2013) and ofup to 9 mm/a in the NAF (de Leeuw et al., 2010), suggesting higheffective permeability in these intra-continental transforms.

1.3.2. Distributed intra-continental deformation

In both the western USA (Kennedy and van Soest, 2007; Newellet al., 2005) and Turkey (Gulec- et al., 2002; Dogan et al., 2009; de

R /R

km NE of SAFkm N of NAFkm NE of KKFkm W of 118°WKKF

SAFNAF

mantle helium domain

BRP

mm/yrN35°W

NAFSAF

km NE of KKF

southeast Tibet

mm/yrN110°E

westTibet

+

••••••••••••••

+

+ ++

ISZ

R /R

+

crustal Hedomain

KKF

mantle sutures, west & southeast Tibet

km N of ISZ

M

+••

mantle He domain

Y+

KKFKKF

Fig. 2. Characteristic 3He/4He patterns in different tectonic environments, and 3He/4He measurements from Tibet. (a) Data from geothermal areas across intra-continental

plate boundary strike-slip faults and the Basin and Range extensional province (BRP) at the same scale. Orange line: envelope of highest 3He/4He values (orange dots)

measured at each distance relative to the San Andreas fault (SAF) (Kennedy et al., 1997; Kulongoski et al., 2013), i.e., those springs that presumably penetrated most deeply

and scavenged most mantle helium (note, broad 3He/4He peak is due to high mantle helium component also present on the Holocene Ortigalita splay of the SAF system).

Pink line: envelope of 3He/4He values measured close to the North Anatolian fault (NAF) (individual data points not shown for clarity) (Dogan et al., 2009). Rapid spatial

gradients of 3He/4He across SAF and NAF suggest these plate boundary strike-slip faults tap mantle sources. Red line: envelope of 3He/4He values measured across the KKF

(from Fig. 2b), at same scale. Turquoise line: baseline trend of 3He/4He ratios across BRP showing a long wavelength increase from east to west inferred to represent mantle

fluids penetrating the ductile lithosphere via deformation-enhanced permeability (Kennedy and van Soest, 2007), related to westward-increasing strain and NW shear

component shown by dashed turquoise line: GPS velocity in N351W direction with respect to North America (Hammond and Thatcher, 2004). BRP values are shown from

east to west, to more easily compare to southeast Tibet (Fig. 2b). (b) Red line: highest 3He/4He values in west Tibet measured at each distance southwest/northeast of the

KKF (upper scale). Data points labeled as in Fig. 1; circles—from Hoke et al. (2000) and Walia et al. (2005) (no error bars available) and diamonds—this work, analytic

errors smaller than symbols, except as shown for Changlung and Langjiu. Yellow shading (M, C, T, T, P, S, L): samples enriched in mantle helium; white shading: samples

lacking any mantle helium. Blue line: envelope of highest 3He/4He values in southeast Tibet measured at each distance south/north of the ISZ (lower scale) and blue dots:

data points from Hoke et al. (2000), and Yokoyama et al. (1999) (no error bars available). Y: Yangbajain geothermal area. Dashed blue line: GPS velocity of southeast Tibet

relative to Eurasia in the direction 1101, orthogonal to the India–Asia convergence direction (Zhang et al., 2004) plotted against distance north of the ISZ to show increasing

eastward lateral escape velocity of Tibet. Broad grey vertical bars: inferred mantle suture (the southern limit of Tibetan mantle in contact with subducting India), 50–

100 km north of the ISZ in southeast Tibet (Hoke et al., 2000), and at the KKF in west Tibet (this paper). Upper and lower distance scales in (b) are offset 35 km to align our

samples from ISZ in NW India (Gaik, Chumathang) with ISZ in southeast Tibet. In (a) and (b), definition of crustal and mantle helium domains from Hoke et al. (2000);

distance scale and GPS velocity scale are identical, but 3He/4He RC/RA scale changes by a factor of 2.5. (For interpretation of the references to color in this figure legend, the

reader is referred to the web version of this article.)

S.L. Klemperer et al. / Earth and Planetary Science Letters 366 (2013) 59–7062

Leeuw et al., 2010) even the lowest 3He/4He ratios measuredrequire that some mantle fluids are traversing the crust that isboth thinner and hotter than global averages (e.g. Gilbert, 2012;Sass et al., 1997; Mutlu and Karabulut, 2011; _Ilkıs-ık, 1995). Acrossthe western USA, regions of highest 3He/4He correspond to regionsof lowest seismic wavespeed in the mantle, reflecting tectonicallyactive and partially molten mantle (Newell et al., 2005). Far fromthe SAF and NAF, smaller quantities of mantle volatiles measuredat the surface have traversed a ductile lower crust, without the aidof crust-penetrating faults or shallow emplacement of recentmelts. This upward transport requires at least transient perme-ability in the lower crust. In the Basin and Range Province, westernUSA, Kennedy and van Soest (2007) show that baseline 3He/4He

ratios increase systematically to the west, correlating to higherrates of crustal deformation (Fig. 2a), and they hypothesize that theincrease in total strain and specifically the northwest-orienteddextral shear component, greatly enhance average fluid flow rates.This correlation of enhanced 3He/4He with increased (shear) strainoffers a possible model for interpreting other intra-continentalzones of distributed deformation.

1.4. Previous helium isotope observations from Tibet and inferences

on Tibetan tectonics

Observations of mantle helium at Earth’s surface require atleast incipient mantle melting to either degas 3He or to produce

mantleheliumR/RA>0.1

crustal heliumR/RA<0.05

Values reported here, with 2σ uncertainties;yellow indicates mantle enrichment

Values from Hoke et al. (2000) (no uncertainties reported)

KKF

west Tibet,NE of KKF

SW of KKF

Air & air-saturated water (ASW)

10 100 1,000 10,00010.001

0.01

0.1

1

( He/ Ne) /( He/ Ne)

5% mantle fluid

10% mantle fluid

1% mantle fluid

Air

R/RA( He/ He)

( He/ He)

20% mantle fluid

10

ASW

Mixing curves: 100% mantle fluid

mantle-enriched fluids from southeast Tibet

crustal He domain, southeast Tibet

no intermediate fluids reportedcompositional gap:

from Tibet or Himalaya

west Tibet,

crustalfluids only

mantle)0% (

Fig. 3. Helium isotope signature of all Karakoram fault, western Tibet and western

Himalaya geothermal springs with published He and Ne data (this study—-

diamonds, Hoke et al. (2000)—circles) (see Fig. 2 and Table 1 for additional data

for which only He are available (this study; Walia et al., 2005)). Data are

superimposed on mixing curves of three end-member components (mantle R/

RA¼8 and 4He/22Ne41000, crust R/RA¼0.02 and 4He/22Ne41000, and air R/RA¼1

and 4He/22NeE3). 2s uncertainty ellipses are shown where available (this study

(ellispse is smaller than symbol for Mengshi); not Hoke et al. (2000)). The

displacement of the samples with mantle enrichment (yellow: M, C, T, T, S, L)

from the mixing line between the crustal end-member and air or air-saturated

water requires excess mantle 3He. Yellow field: mantle-enriched samples from

southeastern Tibet (Yokoyama et al., 1999; Hoke et al., 2000). Blue field: samples

from ‘‘crustal helium domain’’ in southeastern Tibet (Hoke et al., 2000; Newell

et al., 2008). Grey field: compositional gap between pure crustal and mantle-

enriched fluids in which no data are reported from Tibet or the Himalaya (this

study; Hoke et al., 2000; Newell et al., 2008; Walia et al., 2005; Yokoyama et al.,

1999). Definition of ‘‘crustal’’ and ‘‘mantle’’ helium domains from Hoke et al.

(2000). (For interpretation of the references to color in this figure legend, the

reader is referred to the web version of this article.)

S.L. Klemperer et al. / Earth and Planetary Science Letters 366 (2013) 59–70 63

melts that transfer 3He directly into the crust (Ballentine et al.,2002). Hence helium data in Tibet (Hoke et al., 2000) have beenused to probe the boundary between Indian cratonic mantlethat remains far below the solidus and largely undeformed duringits rapid northward subduction (e.g. Klemperer, 2006; Priestleyet al., 2008), and the orogenic mantle further north knownfrom seismic studies to be strongly deformed, highly attenuating,and close to its melting point (e.g. Barron and Priestley,2009; Klemperer, 2006; Zhao et al., 2010). The large south-to-north increase in seismic wave speed across the northern edge ofthe high Tibetan Plateau both immediately beneath the Moho(Liang and Song, 2006) and in the upper mantle (Liang et al.,2012) suggests there are separate Tibetan and Asian mantlelithospheres with distinct tectonothermal histories, and so werefer to the tectonically active mantle north of subducting Indiancratonic mantle and south of the ATF/Kunlun fault systems as‘‘Tibetan’’.

Previous helium isotope studies in Tibet focused on a regionfrom �86–921E south of the Banggong–Nujiang suture zone (forthe purposes of this paper, hereafter called southeast Tibet) (Hokeet al., 2000; Newell et al., 2008; Yokoyama et al., 1999; Zhao et al.,2001), including the Nagqu, Yangbajain, Tingri and Daggyai Co

regions (Fig. 1). Additional papers in the Chinese literaturecontain helium data but lack details of other noble gases orcorrections for air-contamination (e.g., Hou and Li, 2004; Zhaoet al., 2002). In west Tibet, 80–811E, helium isotopes have beenreported from just three hot spring systems (Hoke et al., 2000;Zhao et al., 2002). Hoke et al. (2000) suggested the association ofmantle helium with the KKF, but their Tirthapuri samples (Fig. 1)were collected from a location within 1 km of both the KKF andthe Indus–Yarlung suture zone (ISZ), making it impossible toseparate the effects of these structures. In this paper we useadditional samples from NW India where the KKF and ISZ areseparated by 35 km to show the mantle helium is associated withthe KKF and is not present in the ISZ.

In southeast Tibet Hoke et al. (2000) used all the available data(Fig. 2b, blue line) to show gradual spatial variation between a‘‘crustal helium domain’’ in the Himalaya in which RC/RAo0.05(blue field, Fig. 3) and a ‘‘mantle helium domain’’ in the northernLhasa block in which RC/RA40.1 (yellow field, Fig. 3). Because inTibet even the highest corrected 3He/4He ratios were {RA, Hokeet al. (2000) suggested that between these two unambiguousdomains, the ‘‘transition zone’’ in which 0.05oRC/RAo0.1(3He/4He ratios between 2.5 and 5 times the canonicalcrustal ratio) represents a gradually increasing mantle influx of3He. The transition, 50–100 km north of the ISZ (near Y in Figs. 1aand 2b), is interpreted as the southern limit of tectonicallyactive Tibetan mantle in contact with subducting India (the‘‘mantle suture’’: Hoke et al., 2000). Although existing datafrom the relatively few available geothermal sites could permita very rapid transition from ‘‘crustal’’ to ‘‘mantle’’ domains,such a rapid transition has not been inferred because it wouldoccur in the middle of the Gangdese (‘‘Andean’’) batholith in aregion with no obvious Cenozoic west–east structural or mag-matic features.

The majority of the 3He-enriched samples studied byYokoyama et al. (1999), Hoke et al. (2000) and Zhao et al.(2001) come from the Yangbajain graben of the Yadong–Gulu riftsystem. The Yangbajain graben is also the locus of well-studiedseismic brightspots (Makovsky and Klemperer, 1999) and elec-trical conductivity anomalies (Li et al., 2003) interpreted asaqueous fluids above magma chambers at 15 km depth withinthe rift. Hence Hoke et al. (2000) interpreted the mantle-derived3He to represent degassing of volatiles from young (Quaternary)mantle-derived melts intruded into the crust, and the southernlimit of the mantle helium domain as defining the southern limitof recent mantle melt extraction beneath the Tibetan Plateau.

2. New observations of 3He/4He in western Tibet

We present five new helium isotope measurements made innorthwest India on and just southwest of the KKF at �781E, andtwo new isotopic analyses from west Tibet on and just northeastof the KKF at �801E (Fig. 1). Our data complement the onlyprevious helium isotope data from western Tibet, three samplesfrom two locations close to the eastern end of the KKF (Hoke et al.,2000). Our data collection and analysis followed standard proce-dures (Kennedy and van Soest, 2006) (Appendix A). Table 1presents our field observations and noble gas isotope data,including new R/RA values of 0.01, 0.02, and 0.02 close to theISZ, 35–50 km southwest of the KKF; values of 0.1, 0.7, and 2.2 onthe KKF; and a value of 0.19 45 km northeast of the KKF.

The sample with R/RA¼2.2 has unambiguous 3He enrichmentirrespective of any atmospheric contamination. However, correc-tions to the measured helium isotopic ratios for contamination byair or air-saturated water are important for our samples with lowabsolute helium abundance and with RoRA. We corrected all

Table 1Sample locations, characteristics, and noble gas isotope analyses of geothermal occurrences in west Tibet and northwest Himalaya.

Location Type of sample Latitude Longitude Elevation Distance Temp. pH Chloride Conductivity Geologic setting(north) (east) (m) (km to KKF) (1C) (ppm) (S/m)

New Field Measurements, 2008Karakoram fault

Panamik Water 341 46.8270 771 32.5760 3213 0 76 7.53 22 0.079 Miocene Karakoram leucogranite, formed by crustal melting

(Phillips et al., 2004)

Changlung Water 341 56.2240 771 28.4210 3407 0 75 7.25 74 0.264 ibid.

Indus suture zone

Gaik Water 331 33.5220 781 08.4470 3784 35 79 8.36 23 0.065 Paleocene-Eocene Ladakh batholith, granite-granodioritic

magmatic arc (Sharma and Choubey, 1983)

Chumatang Fumarole 331 21.5910 781 19.4490 4023 40 n.d. n.d. n.d. ibid.

Puga valley / Tso Morari normal fault

Puga Gas/boiling

spring

331 13.3590 781 19.6320 4416 50 81 7.68 381 0.282 Paleozoic Puga paragneisses, Greater Himalaya (Tso Morari

Complex) (Harinarayana et al., 2006) with overlying river/spring

sediments and borax deposits (Chowdhury et al., 1974)

Notes: Locations and elevations measured with handheld Garmin GPS.

Temperature, conductivity and pH measured with a handheld Hanna Instruments 98130 digital meter.

Chloride measured with Hach chloride Quantab test strips.

Location [36Ar] 7 R/Ra 7 Rc/Ra 7 Rc/Ra 7 F(4He) 7 F(22Ne) 7 F(84Kr) 7 F(132Xe) 7 40Ar/36Ar 7 40*Ar/4He 7(ccSTP/cc-gas or

ccSTP/ml-water)

(F(He)method)

(F(Ne)method)

New Mass Spectrometer DataNW India: Karakoram fault

Panamik 2.599E�06 1.001E�08 0.1008 0.0143 0.0683 0.0148 n.d. 28.672 0.364 n.d n.d n.d 293.68 1.16

Changlung 4.643E�07 7.727E�10 0.7250 0.2045 0.5309 0.3492 0.6625 0.3213 2.417 0.033 0.4474 0.0077 1.8116 0.0143 4.5714 0.1076

NW India: Indus suture zone

Gaik 2.392E�06 4.253E�09 0.0196 0.0037 0.0138 0.0037 0.0173 0.0036 169.458 2.111 0.4046 0.0046 1.9927 0.0204 4.3145 0.0877 302.76 2.19 0.257 0.080

Chumatang 1.322E�07 1.156E�09 0.0233 0.0030 0.0230 0.0030 0.0232 0.0030 3498.989 48.723 0.3471 0.0147 2.0247 0.0251 4.8839 0.2062 371.78 2.50 0.131 0.012

NW India: Puga valley / Tso Morari normal fault

Puga 4.434E�06 4.387E�08 0.0087 0.0031 0.0086 0.0031 0.0087 0.0031 8581.946 113.529 0.4607 0.0484 2.0109 0.0284 5.0910 0.4709 475.38 2.16 0.126 0.010

West China: Karakoram fault

Mengshi 4.348E�07 3.515E�10 2.2251 0.0833 2.2651 0.0861 2.2426 0.0845 31.652 0.125 0.4462 0.0115 2.4075 0.0090 7.2332 0.0667 294.85 2.45 0.000 0.000

West China: Shiquanhe area

Langjiu well 911 8.161E�06 2.880E�08 0.1922 0.1112 0.1911 0.1114 0.1917 0.1112 736.023 10.653 0.4526 0.0059 1.7906 0.0088 3.9409 0.0413 312.62 0.92 0.139 0.014

Air 1 cc-atm 3.161E�05 1 1 1 1 1 295.50 0.000

ASW 01C–1 atm 1.754E�06 0.988 0.177 0.2345 2.0715 4.2444 295.50 0.000

ASW 101C-1 atm 1.338E�06 0.987 0.217 0.2722 1.9412 3.6767 295.50 0.000

Notes: All analyses reported here were performed at Lawrence Berkeley National Laboratory’s Center for Isotope Geochemistry.

R/Ra¼the measured (3He/4He) ratio normalized to the ratio in air (1.39E�06).

Rc/Ra¼(3He/4He) ratio normalized to the ratio in air (1.39E�06) after correction for atmospheric contamination as measured by

(1) the F(4He) method: assumes all 36Ar is from air contamination and R corrected accordingly, resulting in a maximum correction and

(2) the F(22Ne) method: uses the F(Ne)/F(He) ratio and assumes air contamination from both air and air saturated water. All 22Ne is assumed atmospheric.

F(i)¼air normalized relative abundances: F(i)¼[(i/36Ar)s/(i/36Ar)air]

(40*Ar/4He)¼radiogenic 40Ar to 4He ratio¼f[K/(UþTh)] and relative Ar and 4He extraction efficiency from source minerals,

calculated assuming (1) all of the 4He is radiogenic and (2) all of the measured 36Ar is atmospheric with a 40Ar/36Ar ratio of 295.5

n.d.¼not determined: an op-amp that controls the ion accelerating voltage died in the middle of the Panamik analysis to determine relative noble gas abundances and the Ne data was lost, hence we

cannot plot this sample relative to the mixing line in Fig. 2.

Uncertainties: All reported uncertainties are analytical errors (1 standard deviation). Errors associated with the reproducibility of a given measurement are folded in as part of normalization to air values.

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S.L. Klemperer et al. / Earth and Planetary Science Letters 366 (2013) 59–70 65

samples for atmospheric contributions using two methods(Table 1). The first method assumed all of the measured 36Arwas from air contamination (i.e. no contribution from air satu-rated water) and the helium isotope ratios were corrected usingthe measured 4He/36Ar ratios and the known 4He/36Ar ratio in air.This approach results in a maximum correction and, therefore,lowest corrected helium isotope ratios (Table 1). The secondmethod used a more conventional procedure as followed byHoke et al. (2000), Newell et al. (2008), Walia et al. (2005), andYokoyama et al. (1999). This method uses the measured 4He/22Neratios, assumes all of the 22Ne was derived from air and/or airsaturated water and presumes that little or no He/Ne elementalsolubility fractionation has occurred due to boiling or gas effer-vescence, relying on the similar solubility of He and Ne in water.This approach results in corrected helium isotope ratios inter-mediate to the He/Ar method and the measured ratios (Table 1).

Elemental fractionation of the heavier noble gases (Ne, Ar, Kr,Xe) is known in multi-phase geothermal systems due to boilingand steam separation (e.g. Pinti et al., 2013), and is expected foreffervescence of gas (e.g. CO2) saturated fluids. Since neither the36Ar nor the 22Ne method for correcting the helium isotope ratiosfully accounts for potential elemental fractionation, both methodsare approximations. However, in the case of our samples, therehas been only minimal noble gas fractionation as shown byfractionation factors (F values) that are all close to the anticipatedvalues for air-saturated water (ASW) at 10 1C (e.g. F(132Xe) valuesof 3.9–7.2 compared to the expected value of 3.7) (Table 1;Appendix A). Indeed, the fact that the F(132Xe) values are allslightly above the F-value for 10 1C ASW suggests that oursampled fluids have lost a small amount of noble gases, so thatthe F(4He) values are lower limits (He being less soluble thanAr, Kr and Xe), hence our air corrections to R/RA are over-estimated, and our estimated RC/RA values are minimum values(Appendix A). These minimum RC/RA values are 0.01, 0.02, and0.02 35–50 km southwest of the KKF; 0.07, 0.66, and 2.24 on theKKF; and 0.19 45 km northeast of the KKF (Fig. 2b; Table 1).

The Mengshi sample with RC/RA¼2.24, like the Hoke et al.(2000) Tirthapuri samples with RC/RA¼0.30–0.38, is unequivocalevidence for mantle fluids in the KKF and/or the ISZ where theKKF intersects the ISZ at �811E (Murphy et al., 2000) (Fig. 1). Ourclaim that the mantle fluids are associated with the KKF ratherthan the ISZ relies on our low observed RC/RA values (no mantlecomponent) on the ISZ 350 km NW of Mengshi, and an unambig-uous albeit small mantle contribution to fluids on the KKF,500 km NW of Mengshi. The high RC/RA for Changlung makes thissample most critical in our discussion of mantle contributions tothe surface fluids at 77.51E. The very low helium abundance in ourChanglung sample resulted in relatively large analytical uncer-tainty in the measured 3He/4He ratio (large error bar in Fig. 2b;Table 1) and a low He enrichment factor with respect to airleading to a relatively large air correction. However, plotting R/RA

against the 4He/22Ne ratio (Fig. 3) demonstrates that this Chan-glung sample lies above the mixing line between canonical crustaland air end-members at the 3s confidence level, and records aclear mantle-derived He contribution with 33 times the 3He/4Heratio nominally expected for continental crust. Thus Changlung,Tirthapuri (Hoke et al., 2000) and Mengshi (collected by Zhaoet al. (2002); isotopic data reported here) samples that were allcollected on the KKF, all show a mantle signature. This contrastswith our samples from the ISZ that have o0.2% mantle contam-ination (Fig. 3).

As discussed in the Introduction, elevated 3He/4He ratios couldin unusual circumstances be due to radiogenic 3He produced from6Li or by decay of tritium from atmospheric nuclear testing, ordue to re-mobilization of 3He from older mantle-sourced intru-sions in the crust. We further assess these potential contributions

S.L. Klemperer et al. / Earth and Planetary Science Letters 366 (2013) 59–7066

based on the geologic setting of our samples and comparison withprevious 3He studies in Tibet. One of our sampled springs (Puga)rises through up to 65 m of unconsolidated sediments(Harinarayana et al., 2006) in which borax deposits have alkalienrichment up to 4500 ppm Li derived from subjacent magmabodies (Chowdhury et al., 1974). Nonetheless this sample has thelowest 3He/4He ratio yet reported from the Himalaya, only 0.01RA

(Fig. 3). We conclude that radiogenic 3He produced from 6Li is nota likely source of our observed elevated 3He/4He ratios. Our Pugasample also has the highest F(4He) value, perhaps suggesting anassociation of the geothermal fluids with the inferred magmabody at �2 km depth (Harinarayana et al., 2006) and that thisintrusion may be unusually radiogenic—Miocene Himalayanleucogranites are characterized by bulk uranium contents morethan twice that of average granitic rocks (Cuney et al., 1984) andextraordinarily high uranium content of their zircons (e.g., Leech,2008). The high F(4He) in the Chumathang sample suggests asimilarly uraniferous granitic magma at shallow depth could beresponsible for the Chumathang geothermal field. Yokoyama et al.(1999) analyzed their 1991 geothermal samples from southeast-ern Tibet for tritium but found no 3H at their detection limit of1 tritium unit (T.U.; 3H/H¼10–18), and showed that this max-imum level was not a significant source of 3He. Using the sameargument as Yokoyama et al. (1999), and assuming 1 T.U. in our2008 Changlung fluids, less than �10–15% of the measured 3Hecould be derived from tritium.

Re-mobilization of 3He from older intrusions is also animplausible source of the observed mantle helium anomaly alongthe KKF. Simple calculations (Hoke et al., 2000) show the size ofsuch intrusions would be larger than the small distances acrosswhich we report large changes in 3He/4He ratios. Although fluidinclusions in an olivine xenocryst from a �18 Ma ultrapotassicdyke in southwestern Tibet yielded an elevated 3He/4He ratio of0.17RA (Hoke et al., 2000), such dykes are too volumetricallyminor, too old, and too little enriched in 3He to be the source ofthe 3He we measured. The samples from which we reportelevated 3He/4He ratios (mantle helium domain: Changlung,intermediate domain: Panamik) were collected from the marginsof the Karakoram leucogranite, a Miocene intrusion formed bycrustal melting (Phillips et al., 2004). In summary, the enrichmentof 3He/4He above the canonical crustal value of 0.02RA in theMengshi, Changlung, and Panamik samples reported here, and theTirthapuri samples of Hoke et al. (2000), can only plausiblyrepresent a small but real and geologically recent flux ofmantle-derived fluids along the KKF.

3. Interpretation of 3He/4He ratios along and across the KKF

Hoke et al. (2000) and Zhao et al. (2002) sampled just twolocations, on and north of the KKF, and Walia et al. (2005)sampled three closely spaced locations all structurally below/south of the Main Central Thrust. Our new samples, coupled withthe older data, let us test for the first time the close spatialrelationship of 3He/4He anomalies to the KKF (Fig. 2b).

All four spring systems yet sampled on the KKF (two by us inthe Nubra Valley and two by Hoke et al. (2000) and Zhao et al.(2002) 4500 km to the southeast at Tirthapuri) exhibit a mea-surable mantle helium signal. In contrast, all three samples just35–50 km southwest of the KKF completely lack evidence of thismantle enrichment (Fig. 2b). The three samples 35–50 km south-west of the KKF were obtained close to the ISZ and to the activeTso Morari normal fault (Fig. 1b). Hence mantle fluids are notwidely disseminated through a fractured, continuously deformingupper crust in this region, but are apparently transported fromthe mantle to the surface by focused flow within the KKF.

The ratio of the maximum 3He/4He values on the KKF to themaximum 3He/4He values SW of the KKF is �100, and to 3He/4Hevalues NE of the KKF is �8 (red line, Fig. 2b), comparable to theequivalent ratios (�40) seen across the SAF and NAF (Dogan et al.,2009; Kennedy et al., 1997) (Fig. 2a). This enrichment in mantle3He occurs over o35 km on the southwest side of the KKF ando45 km on the northeast side (far less than the crustal thicknessof ca. 70 km: Rai et al., 2006). Similarly, major enrichment inmantle 3He is only present within �25–50 km of the SAF and NAF(Dogan et al., 2009; Kennedy et al., 1997), or 1–1.5 times therespective crustal thicknesses of 25–30 km (Gilbert, 2012) and30–35 km (Mutlu and Karabulut, 2011). The lower RC values inTibet than in California and Turkey may correspond to the farthicker crust in Tibet (Rai et al., 2006) allowing for more dilutionof the mantle helium component by radiogenic crustal helium.The 10-fold change in RC/RA between the Panamik and Changlunghot springs that are separated by 16 km, and the similar changebetween the Mengshi and Tirthapuri locations that are separatedby 18 km, no doubt representing different degrees of interactionof crustal and mantle fluids in separate hydrologic systems, is lessthan the largest changes between adjacent measured springs onthe SAF, where springs separated by o10 km can differ in RC/RA

by a factor 420 (Kulongoski et al., 2013). Time-averaged upwardflow rates of helium have been calculated for the SAF in centralCalifornia (up to 11 mm/a: Kennedy et al., 1997) and southernCalifornia (up to 147 mm/a: Kulongoski et al., 2013), and for theNAF (up to 9 mm/a: de Leeuw et al., 2010). Using the same modelcorrected for the much thicker crust, we derive time-averagedupward flow rates of 12 mm/a and 19 mm/a (for Changlung andPanamik respectively), implying a 3.5–6 Ma transit time frommantle to surface through the ca. 70 km-thick crust. The high flowrate estimated for the KKF suggests near-vertical transport, aslateral transport would increase the distance traveled and hencethe inferred transport rate. We conclude that an active near-vertical KKF accessed fluids from a tectonically active mantleat least until the latest Miocene (ca. 6 Ma) or early Pliocene(ca. 3.5 Ma), and possibly to the present day.

In summary the comparison of spatial gradients of 3He/4Heratios from the ISZ to the KKF with spatial gradients across theSAF and NAF (Fig. 2), and of upward flow rates within all thesefaults, suggests similar processes of fault-controlled fluid flowfrom mantle to the surface. Northeast of the KKF only two(essentially collocated) data points exist, suggesting that theKKF is a singular feature within Tibet but insufficient to refutethe possibility that the KKF is merely the southwestern edge of abroader anomaly. Either hypothesis is consistent with our con-clusion that the KKF is a dominant pathway for mantle fluids toreach the surface.

4. Comparison of west (77–811E) and southeast Tibet(86–921E) and implications for India–Asia collision

Comparison of datasets from west to east across Tibet (Fig. 2b)shows the highest 3He/4He ratio on the KKF is nine times thehighest value reported by Yokoyama et al. (1999), Hoke et al.,(2000) or Newell et al. (2008) from southeast Tibet, and that thehorizontal transition from ‘‘crustal’’ to ‘‘mantle’’ helium domainslikely occurs over a far shorter distance at the KKF than insoutheast Tibet. This short spatial transition for an order ofmagnitude increase in RC/RA is evident both at Mengshi/Tirthapuriand 500 km northwest at Changlung/Panamik (solid line, Fig. 2b).Upward transport of mantle fluids is focused in the NW Himalayaby the KKF but unfocused in southeast Tibet.

The low spatial variation in 3He/4He ratios across southeastTibet, when compared to results from the SAF, NAF, and KKF

MHT

MCT STD ISZMFT

Indian lithospheric mantle

Indian lower crust

Tibetanmantle

Lhasa Block

magma chambers

Kosarev et al., 1999

Kind et al., 2002

Makovsky et al., 1999

Makovsky & Klemperer, 1999

Kind et al., 2002

Zhao et al., 2011

Kind et al., 2002

Zhao et al., 1993Zhao et al., 2011

Jaupar tet al.,1985Schulte-Pelkum et al., 2005

Priestley et al., 2008

BNS

Indian middle crust

Schulte-Pelkum et al., 2005

Makovsky et al., 1999

Indian lower crust

Indian upper crust

Caldwell et al., 2013

MHT

MCT STD ISZMFT KKF

Indian lithospheric mantle

Indian lower crust

Tibetanmantle

Lhasa Block

?Zhao et al., 2010

Zhao et al., 2010

Wittlinger et al., 2004

Caldwell et al., 2013

Caldwell et al., 2009

Rai et al., 2006

BNS

Zhao et al., 2010

?

Arora etal., 2007

Harinarayana et al., 2006

QiangtangBlock

strandedIndian crust

Li et al.,2008

Arora et al., 2007

Indian lower crust

magma chambers

200 100 0 100 200 km

0 km

50 km

100 km

150 km

200 km

0 km

50 km

100 km

150 km200 100 0 100

southwest northeast

south north }

He/ He

3 4

Mantle sutures inferred from } data

cross- sectionsaligned on ISZ

He flux from mantle3

Fig. 4. Comparative cross-sections of southeastern and western Tibet at present day. Both sections are true scale, from MFT across the Himalayan orogenic wedge (MHT:

Main Himalayan Thrust, STD: South Tibet Detachment), to Tibetan terranes (Lhasa and Qiangtang blocks) and aligned on the ISZ. (a) South–north at 901E (INDEPTH transect

(Klemperer, 2006, 2008); also representative of Hi-CLIMB transect at 851E (Nabelek et al., 2009)); (b) SW–NE crossing the KKF at �791E. Wiggly red arrows are mantle

helium flux; solid grey lines are major faults, crustal boundaries and magma chambers taken directly from geophysical studies referenced in figure, projected along strike

where necessary (Arora et al., 2007; Caldwell et al., 2009, 2013; Jaupart et al.,1985; Makovsky et al., 1999; Schulte-Pelkum et al., 2005; Zhao et al., 1993, Zhao et al., 2011;

and other references cited within the figure); dashed grey lines and question marks identify boundaries and magma chambers inferred but not yet imaged. Green shading

denotes Indian crust and its sedimentary cover; receiver-function ‘‘doublet’’ corresponds to northern tongue of ‘‘Indian middle crust’’ in (a) and ‘‘stranded Indian crust’’ in

(b). Black arrows schematically mark presumed material transfer, of Indian upper crust upwards across the MHT as anticipated by channel flow models, and eclogitised

Indian lower crust downwards across the Moho to be subducted into the deep mantle. Stars are deep earthquakes close to the crust–mantle boundary. (For interpretation of

the references to color in this figure legend, the reader is referred to the web version of this article.)

S.L. Klemperer et al. / Earth and Planetary Science Letters 366 (2013) 59–70 67

(Fig. 2), implies that no single fault cuts through the crust ofsoutheastern Tibet across the helium transect. The occurrences ofmantle helium in southeastern Tibet are not uniquely associatedwith active faults. Although several enriched samples come fromthe Yadong–Gulu rift, the most 3He-enriched samples fromsoutheast Tibet come from Nagqu (Hoke et al., 2000; Yokoyamaet al., 1999), outside the Yadong–Gulu rift and north of the Jialistrike-slip fault (Fig. 1) (Klemperer, 2006).

Hoke et al. (2000) interpreted the 3He enrichment in south-eastern Tibet to represent degassing of volatiles from Quaternarymantle-derived melts emplaced at shallow levels in the crust, basedon the geophysical evidence for magma chambers at 15 km depth(Li et al., 2003; Makovsky and Klemperer, 1999) beneath some oftheir samples (Fig. 4a). However, more recent observations demon-strate that other crustal magma chambers in Tibet are not asso-ciated with elevated 3He/4He ratios. A seismic low-velocity anomalyindicative of partial melt at 15–20 km depth (Hetenyi et al., 2011) islocated vertically beneath four hot springs and fumaroles that lackany mantle 3He signature at Daggyai Co just north of the ISZ (Hokeet al., 2000). Similarly, we found only crustal helium in the Puga hotsprings sampled directly above a magma body attested to by heat

flow, seismicity, and magnetotelluric data (Harinarayana et al.,2006) (Fig. 4b). The absence of a mantle helium signature suggeststhe Puga and Daggyai Co inferred magma bodies are intra-crustalmelts, and strongly suggests shallow anatexis, not mantle melting, isthe origin of these shallow magma chambers.

As an alternative to the Hoke et al. interpretation, the slowspatial variation of 3He/4He ratios south-to-north across south-east Tibet (Fig. 2b) may be understood via comparison with3He/4He ratios that vary east-to-west across the western USAwith similar spatial gradients (Fig. 2a). In the Basin and Rangeprovince, mantle-derived helium passes through a lithospherethat appears to lack crust-penetrating normal faults and also lackswidespread active mantle-sourced volcanism (Hilton, 2007;Kennedy and van Soest, 2007). The westward increase of thebaseline trend of 3He/4He across the Basin and Range province hasbeen correlated with the westward increase in dextral shearstrain (Kennedy and van Soest, 2007). Similarly, the northwardincrease of 3He/4He across southeast Tibet at �86–921E corre-lates with the northward increase in dextral shear (eastwardescape) of Tibet with respect to India and Eurasia (Zhang et al.,2004). Thus 3He/4He ratios up to 0.25RA in the northern Lhasa

S.L. Klemperer et al. / Earth and Planetary Science Letters 366 (2013) 59–7068

block may represent diffuse passage of fluids from Tibetan mantleupwards through continuously deforming, very weak, albeit verythick crust (Klemperer, 2006) (Fig. 4a).

In southeastern Tibet, metamorphosed Indian crust likelyforms the lower half of Tibet’s thick crust for �200 km north ofthe Indus suture zone (Fig. 4a), as shown by the receiver-function‘‘doublet’’ of positive converters (seismic wavespeed increasingdownwards), of which the deeper is the Moho, on transectscrossing the ISZ at 851E and 901E (Kind et al., 2002; Nabeleket al., 2009). The shallower converter, interpreted as the top of theunderthrusting Indian lower crust (Kind et al., 2002; Nabeleket al., 2009), extends up to 100 km north of the mantle suture at901E as located by 3He/4He ratios and receiver-function images(Hoke et al., 2000; Kosarev et al., 1999) (Fig. 4a). If underthrustingIndian lower crust has occupied the same region over the timescales of upward transport of helium from the mantle (a fewmillion years) then the observed 3He-enriched fluids observed insoutheastern Tibet must have passed through the underthrustIndian crust, which must therefore be permeable hence likelyweak and actively deforming, as opposed to cold, rigid (Priestleyet al., 2008), and impermeable. This in turn suggests that thenorthern prolongation of underthrust Indian crust is in a physicalstate consistent with its participation in channel flow (Beaumontet al., 2006) (upper black arrow in Fig. 4a). A stronger componentof eclogitized Indian lower crust may be subducting with theIndian lower lithosphere (Beaumont et al., 2006) (lower blackarrow, Fig. 4a) where rare sub-Moho (Chen and Yang, 2004) orlower-crustal (Priestley et al., 2008) earthquakes occur.

5. Implications for the evolution of the Karakoram fault

The unequivocal observation (Hoke et al., 2000; Zhao et al.,2002; this work) of mantle-derived fluids along the KKF (Fig. 2b)strongly suggests the KKF reaches the Moho. These mantlefluids are not widely disseminated through a fractured, continu-ously deforming crust, because—based on our current dataset—neither the ISZ nor the active Tso Morari normal fault,show any evidence of mantle helium (Figs. 2b and 3). Hence theKKF must reach significantly deeper in the crust than theseother faults, presumably to the Moho or beyond, to accesstectonically active mantle lithosphere north of the India–Tibetmantle suture (Fig. 4b). The KKF may well broaden in the lowercrust to 420 km width, as suggested by studies of exhumedductile shear zones that acted as pathways for mantle fluids (e.g.Pili et al., 1997).

In southeastern Tibet, metamorphosed Indian crust likelyforms the lower half of Tibet’s thick crust for �200 km north ofthe Indus suture zone (e.g., Klemperer, 2008) (Fig. 4a). The samelithospheric configuration may have been present in westernTibet prior to the initiation of the KKF, and may have left Indiancrust stranded north of the KKF, cut off from subducting India bya subsequently active lithospheric-scale KKF (Leech, 2008)(hatched region in Fig. 4b). Seismic interpretations of Indiancrust forming the lower crust of Tibet far north of the KKF(Nabelek et al., 2009; Priestley et al., 2008; Rai et al., 2006; Zhaoet al., 2010) can be understood only if this crust is not presentlybeing underthrust to the north but is instead stranded north ofthe active KKF, and/or if this Indian crust is permeable hencelikely weak and actively deforming. In addition, tectonicallyactive Tibetan mantle must be present immediately below thecrust of the Tibetan Plateau, hence above subducting Indiancratonic lower lithosphere (Fig. 4b). Our helium data imply themantle suture is close to the KKF as hinted at by some previousstudies (dashed mantle boundary in Fig. 4b: see Zhao et al.(2010), their Fig. 2).

The 3He-enriched fluids at the surface today require that theKKF accessed the mantle at 6–3.5 Ma (estimated transit time ofmantle fluids to surface), and possibly also earlier and later. Thisage is consistent with the most recent assessments of KKFinitiation at 25–15 Ma (Leech, 2008; Phillips et al., 2004; Styronet al., 2011; Valli et al., 2008) in the central segment near the BNS,and southward propagation of the KKF to its southern tip after13 Ma (Styron et al., 2011; Valli et al., 2008). Our estimatedupward flow rates are also consistent with geologic observationsthat the northern KKF became inactive in the Late Pliocene,following southwest propagation of the ATF along the LongmuCo-Gozha Co fault system to meet the KKF at �341N (Robinson,2009) (Fig. 1b). The 3He-enriched fluids we collected in the NubraValley at 351N originated in the mantle between 6 and 3.5 Ma buttraveled up a segment of the KKF that may no longer be active norform a continuing barrier to northward subduction of India. Theearthquakes 30–80 km northeast of the KKF at 85–95 km depththat have very diverse focal mechanisms and are only recordednorth of 361N (Priestley et al., 2008) (Fig. 1b), may mark a returnto the underthrusting of Indian crust (Fig. 4a) following a limitedperiod of strike-slip truncation of the MHT (Fig. 4b). In contrast,the 3He-enriched fluids collected at Mengshi (Zhao et al., 2002)and Tirthapuri (Hoke et al., 2000) are on two separate strands ofthe KKF (separated by 20 km in a northeast direction) both stillactive based on satellite imagery (Murphy et al., 2000; Murphyand Burgess, 2006).

The most significant difference between our cross-sectionsin Fig. 4 is in the northern limit of active underthrusting ofIndian crust beneath Tibet. It is unclear whether this representscontrol by the KKF, or whether along-strike changes in theHimalayan belt give rise to these differences in crustal struc-ture that in turn help control evolution of the KKF. In numericalthermo-mechanical models of channel flow extrusion of theHimalaya from beneath the Tibetan Plateau, the distance thatIndian crust underthrusts Asian crust north of the ISZ issensitive to small changes in vertical crustal strength profiles(Jamieson et al., 2006) and to cross-strike changes in crustalstrength (Beaumont et al., 2006). For a range of models thatshow well-developed channel flow, Indian crust may under-thrust 200 km north of the ISZ (cf. Fig. 4a), or not underthrust atall (cf. Fig. 4b) (Beaumont et al., 2006; Jamieson et al., 2006).Because numerical models with modestly different startingscenarios simulate very different channel flow behaviors, it ishard to test the channel flow hypothesis from simple observa-tions of crustal structure, and a large range of scenarios fortectonic development is possible. Thus even if the KKF nowlimits (or until recently limited) active northward underthrust-ing of Indian crust into the lower crust of southwestern Tibet(Fig. 4b), channel flow extrusion of the Himalaya from beneaththe southernmost Tibetan Plateau (Fig. 4a) could still be viablesouthwest of the KKF. Initiation of the KKF, perhaps as aresponse to oblique plate convergence in the NW Himalaya(Styron et al., 2011), must have been contemporaneous withmodification of, but not necessarily termination of, channelflow extrusion of the Himalaya.

6. Summary

Our measurements of 3He/4He levels in geothermal springslets us map the configuration of mantle lithospheric typesbeneath the Tibet Plateau in the geologically recent past andsheds light on deep crustal processes within Earth’s largestcollisional orogen. Despite the relatively few hot springs yetsampled, we see the close association of the KKF with a mantlecomponent of geothermal fluids that requires the KKF to access

S.L. Klemperer et al. / Earth and Planetary Science Letters 366 (2013) 59–70 69

tectonically active Tibetan mantle. Our cross-section, consis-tent with all available geophysical and geological data, suggeststhe KKF may have acted as a plate-boundary transcurrent faultin latest Miocene and early Pliocene time. Because this orogenicsystem is rapidly evolving, our geochemical data cannotshow whether the KKF is still acting as a crust-penetratingfault today.

Acknowledgments

Fieldwork was supported by NSF grant EAR-0409939 andNGRI. Laboratory measurements at Lawrence BerkeleyNational Laboratory were supported by the Director, Office ofEnergy Research, Basic Energy Sciences, Chemical SciencesDivision under Contract no. DE-AC02-05CH11231. Reviews byJ.P. Avouac, D. Grujic, P. Kapp, and an anonymous reviewer,and editorial comments from B. Marty, greatly improvedthis paper.

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.epsl.2013.01.013.

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Klemperer, S.L., et al., Mantle fluids in the Karakoram fault: Helium isotope evidence. Earth and Planetary Science Letters (2013), http://dx.doi.org/10.1016/j.epsl.2013.01.013i Appendix A. Supporting information Data collection and analysis Sampling protocols Samples of non-condensable gas or water were collected from springs and fumaroles. For springs that were either non-boiling or without a free gas phase, a tube was submerged in the spring, as close to the spring inlet as possible, and water was pulled through the sampling apparatus using a peristaltic hand pump. After several volumes of water had passed through the system a sample of the liquid was collected for analysis of the dissolved gases. When sampling boiling springs or springs with a free gas phase, water displacement method was used to capture the gas phase. First water was pulled through the sampling apparatus using the hand pump and then the vapor/gas phase was captured, using an inverted funnel submerged in the spring, and forced through the water-filled sampling apparatus. Along the flow line, the total fluid (liquid and gas) encounters an inverted “Y-shaped” tube that separates the liquid and gas phases, allowing for collection of the non-condensable gases at ambient conditions. Fumarole gases were captured and forced through the sampling apparatus using an inverted funnel sealed around the funnel-ground contact. The gas and vapor are allowed to flow through the sampling apparatus in order to adequately purge the sampling lines of air before collecting a sample. While purging the line, the gas is discharged into a water reservoir to minimize back diffusion of air gases. Analytical method In all cases, a 9.8 cc sample of gas or liquid was collected at ambient conditions in a Cu-tube cold-welded at each end using bolt-

driven clamps. To insure sample integrity, the clamps remained in place until the sample was ready for analysis and attached to the sample preparation vacuum line that is in series with the noble-gas mass spectrometer. Sample preparation and the noble gas analyses were conducted in the RARGA (Roving Automated Rare Gas Analysis) laboratory at the Lawrence Berkeley National Laboratory. Sample preparation, analytical techniques and the instrumentation employed in the RARGA laboratory, are identical to those described by Kennedy and van Soest (2006). The Cu-tube is opened by re-rounding a cold weld at one end of the tube, allowing the sample gas to expand into the sample preparation line for processing. First, water vapor is condensed in a flow-through trap cooled externally using a methanol-liquid N2 slurry. Following removal of water vapor, the line pressure was measured to calculate the amount of non-condensable gases in the sample. Then CO2 and other reactive gases (N2, H2, CO, etc.) are chemically removed by exposure to a stream of evaporating Ti-metal. After removal of the reactive gases, an aliquot of the remaining purified noble gas fraction is isolated for determination of absolute and relative abundances. The rest of the noble gas fraction (~95%) is trapped on activated coconut charcoal cooled to ~35 K, from which each noble gas can be thermally separated and analyzed individually for its isotopic composition. The sample preparation line and mass spectrometer performance are calibrated using aliquots of air and a Berkeley helium standard with an isotopic composition of 2.4 RA (where RA is the 3He/4He ratio in air: 1.39•10-6).

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Corrections for contamination by air Helium in crustal fluids is not typically impacted by contamination by air or air-saturated water because helium has a very low atmospheric abundance (~5 ppm). Nonetheless, atmospheric contributions need to be considered whether they occur during spring recharge or during sampling, particularly for samples with low helium abundance. These corrections are most relevant to sample Changlung, which has the highest measured helium isotopic composition (R/RA = 0.725) and lowest F(4He) (air-normalized relative abundance, 2.417 compared to values of ~30–8500 for the other samples, see Table 1). Furthermore, the high R/RA for Changlung makes this sample most critical in our discussion of mantle contributions to the surface fluids. In the main text, we discuss correction of R/RA values for air contamination using two methods. In the first air-correction method we assumed that all 36Ar was from air contamination and corrected the measured R/RA accordingly. This results in a maximum correction to the measured 3He/4He ratio so gives a minimum RC/RA (Changlung R/RA = 0.725±0.205 becomes RC/RA = 0.531±0.349). In the second method we used the He/Ne ratio and assumed air contributions came from both air and air-saturated water. The only other assumption is that the He/Ne ratio is not significantly altered by gas loss (e.g. boiling, etc.) due to the similar solubility of He and Ne. Using this neon method, Changlung R/RA = 0.725±0.205 is corrected to RC/RA = 0.663±0.321. Given these large analytical uncertainties, to further strengthen our case for the presence of mantle (or excess) 3He in the Changlung sample, we provide a third correction method that relies on all of the air-derived noble gases (22Ne, 36Ar, 84Kr and 132Xe).

The Supplementary Figure plots of F(132Xe) vs. F(22Ne) and F(84Kr) vs. F(22Ne) show that all our samples (including Changlung) have experienced a combination of gas loss and consequent elemental fractionation (due to boiling or bubble formation) and contamination with air. Our model for fluid evolution presumes that our samples have initial F values for the air-derived noble gases equal to air-saturated water (ASW). The geothermal fluids undergo a small amount of elemental fractionation with a decrease in F(22Ne) and increase in F(132Xe) and F(84Kr), either by batch gas loss (black dashed line) or Rayleigh distillation (black dotted line) or some combination of the two processes. The resulting residual liquids are then contaminated by some fraction of air (solid red arrows) to reach their final F values (diamonds) close to but just above the ASW-Air mixing line (red dotted line). Assuming the sample compositions reflect mixing between two components (air, and liquid residual to gas loss) and using the interpolated end-member compositions for the residual liquid, the fraction of 36Ar from air can be readily calculated. Once the fraction of 36Ar from air is calculated, we calculate the corresponding fraction of 4He from air and correct the measured R/RA values for air contamination. The advantage of this approach is that a more realistic value for F(22Ne) in the water end-member is used, as opposed to assuming the value for air-saturated water, resulting in a more accurate estimate of the fraction of 36Ar from air. For Changlung the corrected RC/RA value by this method is 0.678 (using an F(22Ne) value for the residual water of ~0.150). In all three cases, we are left with the conclusion that Changlung has excess 3He with respect to anticipated radiogenic (or crustal) values. This conclusion is consistent with and evident from our plot of R/RA (measured) vs. 4He/22Ne (Figure 3, in the main text).

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Supplementary Figure: Top: plot of F(132Xe) vs. F(22Ne). Bottom: plot of F(84Kr) vs. F(22Ne). Diamonds: observed values for Changlung–C, Chumathang–Ch, Gaik–G, L–Langjiu, M–Mengshi, and Puga–Pu. (Panamik not shown, as heavy noble-gas data were not measured for this sample.) Red crosses: compositions of air, and air-saturated water at 0°C and 10°C. Black dashed line: batch gas loss; dotted line: a Rayleigh process, both representing liquid compositions residual to gas loss from the 10°C ASW composition. Red solid lines: mixing lines between residual liquid and air. Red dashed line: mixing line between ASW and air.