Timing, quanti¢cation and tectonic modelling ofPliocene^Quaternary movements in the NW Himalaya:

evidence from ¢ssion track dating

A.K. Jain a;*, Devender Kumar b, Sandeep Singh a, Ashok Kumar c, Nand Lal c

a Department of Earth Sciences, University of Roorkee, Roorkee 247 667, Indiab National Geophysical Research Institute, Uppal Road, Hyderabad 500007, India

c Department of Earth Sciences, Kurukshetra University, Kurukshetra 136 119, India

Received 4 February 2000; accepted 20 April 2000

Abstract

Variable exhumation rates, deduced from the Pliocene^Quaternary FT zircon^apatite ages from the HimalayanMetamorphic Belt (HMB) of the NW Himalaya along the Sutlej Valley in Himachal Pradesh, have been modelled in thetectonic framework of fast exhumed Lesser Himalayan windows, which caused lateral extensional sliding of themetamorphic nappe cover along the well-known Main Central Thrust (MCT) and differential movements along thrustzones as well. In the northern belt of the Higher Himalayan Crystallines (HHC), two distinct clusters of the FT apatiteages have been deciphered: apatite ages having a weighted mean of 4.9 þ 0.2 Ma (1 c) in basal parts on the hanging wallof the MCT, and 1.49 þ 0.07 Ma (1 c) in the hanging wall of a newly, recognized NE, dipping Chaura thrust furthernorth. Fast exhumation of the Chaura thrust hanging wall has been inferred at a rate of 4.82 þ 0.55 mm/yr from thezircon^apatite cogenetic pairs during 1.54 Ma and 0.97 Ma, and 2.01 þ 0.35 mm/yr since 1.49 Ma. In comparison, itsfoot wall has been exhumed at a much slower rate of 0.61 þ 0.10 mm/yr since 4.9 Ma. The overlying Vaikrita Thrustzone rocks reveal an exhumation rate of 1.98 þ 0.34 mm/yr from 2.70 þ 0.40 Ma to 1.31 þ 0.22 Ma and 2.29 þ 0.66 mm/yr since 1.31 þ 0.22 Ma. Using these data, a vertical displacement of ca. 2.08 þ 0.68 km has been calculated along theChaura thrust between 4.9 and 1.50 Ma on an average rate of 0.6 mm/yr. It is of the order of 1.18 km from 2.70 Ma to1.54 Ma along the Vaikrita Thrust, and 0.78 mm/yr from 1.31 Ma to 0.97 Ma, and has behaved as an extensionalnormal fault during these periods. Tectonic modelling of the exhumation rates in the NW Himalaya reveals fastestuplifting Himalayan domes and windows like the Nanga Parbat in Pakistan, Suru and Chisoti domes in Zanskar andKishwar^Kulu^Rampur Window axis in SE Kashmir and Himachal Pradesh during Pliocene^Quaternary. Thesewindows appear to have caused lateral extensional sliding of the Himalayan metamorphic nappes in the lower parts.The middle parts of the HHC belt have witnessed both overthrusting and extensional faulting due to complex andvariable exhumation patterns within the hanging and foot walls of the MCT and Vaikrita Thrust along the SutlejValley, thus causing movement of upthrust crustal wedge between the extensional ones. Thus, FT zircon^apatite agesprovide evidence for the presence of a number of crustal wedges having distinct tectonothermal history within theHHC. ß 2000 Elsevier Science B.V. All rights reserved.

0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 1 3 3 - 3

* Corresponding author. E-mail: himalfes@rurkiu.ernet.in

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Earth and Planetary Science Letters 179 (2000) 437^451

www.elsevier.com/locate/epsl

Keywords: thermochronology; ¢ssion-track dating; exhumation; Himalayas

1. Introduction

Northern parts of the extensively remobilizedHimalayan Metamorphic Belt (HMB) due to con-tinental collision tectonics between the northerlymoving Indian Plate and the Eurasian Plate sinceEarly Cenozoic (V55 Ma) constitute the highestuplifted and fastest eroded terrain of the Hima-laya [1^5]. It has been modelled to have evolvedwithin a 15^20 km thick, northeast-dipping intra-continental ductile shear zone due to intense crus-tal shortening [3,6,7], and has been thrust south-westward over the Lesser Himalayan sedimentarysequence along the Main Central Thrust (MCT)and its various splays, e.g. the Jutogh/Vaikritathrusts (Fig. 1).

Geochronological data from the NW Himalayaduring the past two decades have demonstratedthat the Higher Himalayan Crystallines (HHC)have undergone fastest and episodic exhumationduring the Cenozoic after the collision tectonics ;the main Neo-Himalayan phase (V25^18 Ma)has caused rapid uplift, tectonic denudation anddeposition of colossal molassic sediments in theSub-Himalayan foreland basin and the magafans within the Indian Ocean [8]. Another majoruplift phase during the Late Pliocene^Quaternaryappears to be mainly responsible for the present-day Himalayan relief [9], and links the ongoingcrustal deformation of the Himalaya with theEarly Cenozoic geodynamics.

Fission track dating of zircon and apatite hasrecently been applied to numerous geologicalproblems covering low-temperature tectonother-mal evolution, and calculation of variable coolingand exhumation rates across numerous faultzones, e.g. Tectonic Median Line [10], the Insub-ric fault line in Alps [11], the Main Mantle Thrust[12,13], and many others. However, the applica-tion of FT technique to calculate the amount ofdisplacement along such fault zones is limited[14]. The main objective of this paper is to docu-ment and interpret accelerated exhumation of theHHC from the FT ages along the Sutlej Valley,Himachal Pradesh as well as to estimate the dis-

placements along the thrust zone, and to providetectonothermal models for similar phenomenaelsewhere in the NW Himalaya.

2. Geological framework

In the NW Himalayan segment of Himachal

Fig. 1. Simpli¢ed tectonic framework of the NW Himalaya.1: Eurasian Plate margin^Karakoram Metamorphic Complex(KMC). 2: Indian Plate Subduction Zone: (a) Shyok SutureZone (SSZ), (b) Ladakh Batholith Complex (LBC), (c) IndusTsangpo Suture Zone (ITSZ). 3: Himalayan Collision Zone:(a) Tso Morari Crystallines (TMC), (b) Tethyan SedimentaryZone (TSZ), (c) HMB of the Proterozic remobilized base-ment with the Lesser Himalayan granitoid belt (c1) and LateCenozoic collisional leucogranite (c2), (d) Lesser Himalayan(LH) Proterozoic sedimentary sequence, (e) Sub-Himalayan(SH) Cenozoic sedimentary foreland. Abbreviations: MBT:Main Boundary Thrust, MCT: Main Central Thrust, JT: Ju-togh Thrust, MF: Martoli Fault, ZSZ: Zanskar Shear Zone,K: Karchham, Ki: Kishtwar, M: Mandi, SA: Sangla, S:Shimla, UK: Uttarkashi, G: Gangotri, DD: Dehradun. Boxshows the location of Fig. 2.

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Pradesh along the Sutlej^Baspa^Beas Valleys, theoutermost and frontal para-autochthonous LesserHimalayan Krol^Shali Belt of the Proterozoicquartzite^dolomite^slate^volcanic sequence isthrust southwestward and imbricated with theSub-Himalayan Cenozoic belt of the Punjab re-entrant along the Main Boundary Thrust (Fig.1) [15,16]. Overriding the frontal tectonic units,the HMB is thrust southwestward along the Ju-togh Thrust as the Jutogh nappe. This belt skirtsaround the closure of the long Kulu^RampurWindow along the folded MCT and its splayswithin the Lesser Himalaya. This window is anorthwest^southeast trending belt of the RampurVolcanics, Manikaran Quartzite, carbonaceousphyllite, and concordant 1800 Ma Bandal gran-itoid, all belonging to the Rampur Group [15].The window sequence is overthrust by a thin dis-membered Kulu^Bajura nappe [17] of highly my-lonitized augen gneiss. Above the MCT, the JeoriFormation of the HHC contains pelite and psam-mite alternations and a few amphibolite bands,and is separated from the overlying Wangtu

gneissic complex (WGC) by a newly recognizedChaura thrust on the basis of ¢nite strain deter-mination along the margin (Fig. 2) [18,19]. Thelatter is comprised predominantly of highlysheared augen gneiss, porphyroclastic granitegneiss, mica schist intercalations and undeformedgranitoids. Rb/Sr whole rock [20] and U^Pb zir-con dates [21] cluster around 1870 Ma and2070 Ma, indicating the presence of distinct Mid-dle Proterozoic basement elements within theHHC. An imbricated zone of quartz schist sepa-rates this sequence from predominantly garnetif-erous mica schist/gneiss by the Vakrita Thrust;the upper metamorphic sequence also containsstaurolite, kyanite and sillimanite-bearing schist/gneiss, granitoids, migmatite and leucogranite.The uppermost contact of the HHC is markedby a distinct complex tectonic zone of regionalsigni¢cance along which the Phanerozoic TibetanSedimentary Zone has been thrust southwestwardinitially, and subsequently moved northeastwards[22].

The deformation pattern within the HHC can

Fig. 2. Geological map of the HHC along the Sutlej^Baspa Valleys, Himachal Pradesh. 1: Lesser Himalayan (LH) RampurGroup: (a) Manikaran Quartzite, (b) Rampur Volcanics, (c) Carbonaceous phyllite. 2: Kulu^Bajura Nappe^augen mylonite.3: HHC: (a) Jeori Group: staurolite/garnetiferous schist, banded biotite gneiss, augen gneiss (a1), amphibolite (a2), quartz micaschist (a3) and Wangtu granite gneiss/granite (a4), (b) Karcham Group: garnet/staurolite/kyanite/sillimanite schist/gneiss, calc^sili-cate, augen gneiss and migmatite (b1), Akpa leucogranite (b2). 4: Tibetan Sedimentary Zone (TSZ). Abbreviations: KT: KuluThrust, MCT: Main Central Thrust, VT: Vaikrita Thrust. R: Rampur, K: Karchham. The LH and TSZ are within biotitegrade. Locations of samples used for FT data are also indicated serially.

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best be explained due to intense ductile shearingwithin an intracontinent shear zone, where pene-trative NE-dipping composite S^C planar fabric ischaracterized by down-dip plunging stretching/mineral lineation [3,4]. Flattening-type ¢nitestrain appears to be typical of such zones, wherepiling of the Himalayan nappes reveals an over-thickened continental crust. Such a pattern hasbeen extremely useful in the delineation of theChaura thrust within the HHC [19]. Further, duc-tile shear fabric and strain patterns are cut by theMCT-controlled shear zones, having brittle^duc-tile/brittle cataclastic fault gouge, thus revealing acontinuation of late stage deformation along nu-merous shear zones, including the Chaura thrust.

3. Sample preparation and analytical procedure

Conventional crushing, magnetic and heavy liq-uid separation techniques were used to recoverapatite and zircon fractions from the rock sam-ples. Apatite crop was available in all the samples,while zircons could only be recovered in su¤cientquantity from ¢ve samples on the hanging wall ofthe Chaura thrust and one sample from within theVaikrita Thrust zone. Araldite epoxy was used for¢xing the apatite grains at room temperature,while zircon grains were ¢xed in PFA te£on at320³C. The grains were ¢xed in a regular arrayof 10U10 to facilitate the polishing and countingof tracks. The samples were grounded and pol-ished to reveal internal surfaces with 4Z geometry.Spontaneous ¢ssion tracks in apatite were etchedin 0.6% HNO3 by volume at 30³C for 60^70 s.The zircon mounts were optimally etched in aneutectic KOH^NaOH melt at 230³C. After etch-ing, the mounts were thoroughly washed, and amuscovite detector of extremely low uranium con-tent was attached ¢rmly to each apatite and zir-con mount for the purpose of calculating the FTages by an external detector method [23,24]. Thesamples were then stacked between two thermalneutron glass dosimeter mounts and encapsulatedin an aluminum can of 3.5 cm length and 1.5 cmdiameter. The present work was done in twophases. Sample numbers 1^10 were analyzed dur-ing 1989^1991 by A.K., and sample numbers 11^

17 during July 1998 by D.K., using respectivelythe thermal irradiation facilities of CIRUS andAPSARA reactors at Bhabha Atomic ResearchCentre Bombay, India. The monitoring of thermalneutron £uence and degree of thermalization inthe reactors was done using SRM-612 glass, pro-duced by the National Institute of Standards andTechnology, Gaithersburg (Washington, DC,USA), and CN1 and CN5 glasses prepared byJ. Scheurs at Corning Inc., Corning (NewYork), having a known chemical compositionand traces of ¢ssioning elements [25]. CN1 andCN5 glasses have a uranium content of V40ppm and V12 ppm, respectively, while SRM-612 has V37 ppm of uranium traces. The ura-nium in Corning glasses has a natural composi-tion, while it is depleted in SRM-612. The tho-rium concentration in Corning glasses is verysmall in comparison to that of SRM-612. Its val-ue in CN5 is quoted to be negligible (6 0.019ppm), while CN1 and SRM-612 glasses have tho-rium concentrations of V0.4 ppm and V37 ppm,respectively.

The degree of thermalization of the thermalcolumn of the CIRUS reactor was investigatedby measuring the ratio of the induced track den-sities of SRM-612 and CN1, as suggested by Ta-gami and Nishimura [26]. In case of reactor witha lower degree of thermalization, besides the in-duced tracks due to ¢ssion of 235U, some of the¢ssion tracks will also be contributed due to theinduced ¢ssion of 238U and 232Th with epithermaland fast neutrons and the contribution will belarger in glasses having higher 238U/235U and232Th/235U ratios. As a consequence, the ratiosof induced track densities of two simultaneouslyirradiated glass dosimeters shall be di¡erent forreactors of varying degrees of thermalization(and, thus, with di¡erent Cd ratios). The meanvalue of this ratio, as determined from 47 pairsof dosimeters in 23 di¡erent irradiations in theCIRUS reactor during the period when thepresent work was done, comes out to be2.86 þ 0.02 (1 c). This value for the CIRUS reac-tor is comparable to that of the KUR Pneumatictube-2 reactor facility (with Cd ratio for Au as5.8) [26], indicating thereby the comparability ofthe Cd ratio for Au for both the reactors. This

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A.K. Jain et al. / Earth and Planetary Science Letters 179 (2000) 437^451440

value is above the minimum permissible value ofthree for the FTD work [27]. Although such stud-ies have not been performed for the APSARAreactor, the ratios of induced track densities oftwo simultaneously irradiated CN1 and CN5glasses (bCN1/bCN5) have been determined forboth the reactors. The value of this ratio obtainedthrough irradiations of 11 pairs of CN1 and CN5glasses in ¢ve di¡erent capsules in the thermalcolumn of CIRUS reactor comes out to be2.79 þ 0.04. The value of this ratio estimatedthrough four di¡erent irradiations of eight pairsof such glasses for the APSARA reactor is3.28 þ 0.09. There is a signi¢cant di¡erence inthe two values. Because of a higher concentrationof thorium in CN1 in comparison to CN5 glass,the chances of creating non-thermal ¢ssion inCN5 are comparatively lower than that in CN1glass. The higher value of the ratio bCN1/bCN5 inAPSARA reactor, thus, surely points towards itshigher degree of thermalization than that of CI-RUS and, thus, making it also suitable for FTDwork.

After irradiation, muscovite detectors were sep-arated from the samples and etched in 48% HF at30³C for 6 min. Counting of tracks was doneunder a Nikon Optiphot microscope using 100Udry objective and a pair of 15U eye-pieces. Incase of zircon samples, grains with surfaces hav-ing sharp polished scratches were used for count-ing so as to ensure the high etching e¤ciency oftracks [23]. The e¡ect of £ux gradient in the cap-sule was considered by taking the average of themuscovite counts of the two glass dosimeters.

Fission track ages were calculated following thezeta calibration approach [28,29], and as recom-mended to I.U.G.S. Subcommission on Geochro-nology by Fission Track Working Group [30].The ages were calculated by using the followingrelation [30] :

T � 1=V d ln �1� Gj V d�b sb d�=b i�

where Vd = total decay constant of 238U( = 1.55125U10310/yr) ; bs = spontaneous trackdensity; bi, bd = induced track density in mica de-tector placed against the mineral sample and glassdosimeter, respectively; G = geometry factor (the

value used in the present study is 0.5) ; j= zetacalibration factor.

4. Results

4.1. Zeta values

The ages, reported in the present work, weredetermined by two analysts using two di¡erentreactors. Hence, it will be apposite to mentionabout the zeta values employed to calculate theages. For the CIRUS reactor, a zeta value of110.6 þ 1.3 (1 c) against CN1 glass was deter-mined by A.K. through multiple irradiations ofage standards consisting of two apatites (FishCanyon Tu¡ and Mt. Dromedery Complex), threezircons (Fish Canyon Tu¡, Buluk Member andMt. Warning Complex) and one sphene (Mt. Dro-medery Complex). For the APSARA reactor usedby D.K., zeta values for CN1 glass have beendetermined using multiple irradiations (thoughlimited in number in comparison to those usedfor CIRUS reactor) of apatites and zircons. Thezeta values come out to be 115.59 þ 2.86 (1 c).

4.2. FT ages

The FT analytical data of apatite and zirconsamples are presented in Table 1. Statistical un-certainties quoted with the FT ages follow Green[31]. Track counts for apatites are rather smallbecause of low track densities due to youngages. Some samples even contained many zero-track grains, which were taken into account toobtain a realistic age [32]. The subjectivity in theselection of grains was minimized by counting allsuitable grains with or without tracks. Due to lowtrack density, there was hardly any con¢ned trackavailable for length measurements, and hence thisaccepted diagnostic tool, used in FT apatite ageinterpretation to unravel the nature of cooling,could not be utilized. Five FT zircon ages fromthe WGC vary from 1.42 þ 0.10 Ma to 1.74 þ 0.19Ma with a weighted mean as 1.54 þ 0.06 Ma, andone sample from the Vaikrita Thrust zone yields2.70 þ 0.40 Ma (Fig. 3). The FT ages of the cor-responding ¢ve cogenetic apatite pairs yield a

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A.K. Jain et al. / Earth and Planetary Science Letters 179 (2000) 437^451 441

Tab

le1

Fis

sion

trac

kzi

rcon

^apa

tite

anal

ytic

alda

taal

ong

the

Sutl

ejva

lley,

Him

acha

lP

rade

sh

Sr.

No.

Sam

ple

No.

Dis

tanc

efr

omM

CT

Ele

vati

onM

iner

alN

umbe

rof

crys

tals

Spon

tane

ous

Indu

ced

b(M2

)G

lass

dosi

met

erA

geþ

1c

(km

)(m

)b s

Ns

b iN

i(%

)b D

Nd

(Ma)

(U10

6

cm3

2)

(U10

6

cm3

2)

(U10

6

cm3

2)

1J

252.

001

320

Apa

tite

260.

2321

342

8.44

612

441

303.

3288

416

15.

05þ

0.28

2R

R17

/21

3.70

120

0A

pati

te19

0.05

0986

2.05

573

468

993.

3288

416

14.

56þ

0.50

3J

276.

501

600

Apa

tite

140.

0495

403.

1325

252

530

4.87

686

069

4.24

þ0.

6720

0.04

6759

3.33

24

205

s99

6.26

327

829

4.85

þ0.

63W

eigh

ted

mea

n4.

61þ

0.47

4C

78.

701

600

Apa

tite

200.

0258

313.

0843

369

580

4.87

686

096

2.26

þ0.

4113

0.02

0819

3.54

933

237

856.

2632

782

92.

03þ

0.46

Wei

ghte

dm

ean

2.17

þ0.

315

C2

10.0

160

0A

pati

te10

0.04

6620

6.75

292

897

604.

876

609

61.

86þ

0.42

6C

410

.21

600

Apa

tite

340.

0441

809.

1931

1666

715

4.87

686

096

1.29

þ0.

147

J59

14.1

160

0A

pati

te44

0.03

5596

5.74

4615

499

804.

876

609

61.

66þ

0.17

8J

5615

.11

280

Apa

tite

230.

0241

233.

8682

369

840

6.26

327

829

2.15

þ0.

459

J64

15.1

51

360

Apa

tite

260.

0232

375.

8988

939

118

6.26

327

829

1.36

þ0.

2210

RR

7/6

15.2

136

0A

pati

te14

0.01

7586

3.76

424

280

144.

8768

609

61.

25þ

0.28

Zir

con

390.

3187

349

1.42

281

558

180.

1145

630

1.42

þ0.

1011

SH37

/62

15.4

158

0A

pati

te4

0.00

421

0.55

0013

270

1.69

353

387

0.74

þ0.

74Z

irco

n13

0.32

4517

110

.774

25

678

920.

895

179

01.

56þ

0.13

70.

3447

101

11.9

522

350

265

0.89

51

790

1.49

þ0.

15W

eigh

ted

mea

n1.

53þ

0.10

12SH

38/6

515

.71

580

Apa

tite

140.

0178

231.

7066

221

080

1.69

353

387

1.02

þ0.

21Z

irco

n27

0.24

4039

68.

0018

1298

7s

950.

895

179

01.

58þ

0.09

13SH

40/6

619

.21

600

Apa

tite

110.

0116

91.

5187

117

760

1.65

13

302

0.73

þ0.

2417

0.01

3518

1.41

731

885

671.

651

330

20.

91þ

0.22

Wei

ghte

dm

ean

0.85

þ0.

16Z

irco

n3

0.28

2177

8.84

982

416

s95

0.89

51

790

1.65

þ0.

1914

SH42

/68

22.0

165

0A

pati

te15

0.00

182

0.92

971

032

61

1.65

13

302

0.18

þ0.

0315

SH34

/58

25.5

167

0A

pati

te8

0.04

5316

1.77

6262

78

1.84

61

846

2.72

þ0.

6913

0.05

2729

1.74

0095

76

11.

846

184

63.

23þ

0.81

Wei

ghte

dm

ean

2.98

þ0.

5316

SH36

/61

27.2

172

0A

pati

te21

0.01

2525

1.35

152

703

s99

1.69

353

387

0.90

þ0.

187

0.00

362

1.30

3673

040

1.65

13

302

0.26

þ0.

18W

eigh

ted

mea

n0.

86þ

0.17

Zir

con

60.

2067

926.

1348

273

020

0.89

51

790

1.74

þ0.

1917

KA

132

.65

178

0A

pati

te19

0.02

0736

1.54

942

696

551.

6935

338

71.

31þ

0.22

Zir

con

30.

2821

778.

8498

241

6s

950.

895

179

02.

70þ

0.40

b s=

spon

tane

ous

¢ssi

ontr

ack

dens

ity

;N

s=

tota

lnu

mbe

rof

spon

tane

ous

trac

ksco

unte

d;b i

=in

duce

dtr

ack

dens

ity

;N

i=

tota

lnu

mbe

rof

indu

ced

trac

ksco

unte

d;

b d=

indu

ced

trac

kde

nsit

yin

the

dosi

met

ergl

ass;

Nd

=to

tal

num

ber

ofin

duce

dtr

acks

mea

sure

din

the

dosi

met

ergl

ass;

b(M2

)=pr

obab

ility

ofob

tain

ing

the

ob-

serv

edM2

valu

efo

r(n

umbe

rof

crys

tals3

1)de

gree

sof

free

dom

,w

hich

isqu

oted

tone

ares

t5%

.Sr

.N

o.=

the

sam

ple

num

bers

inF

ig.

2.

EPSL 5490 16-6-00

A.K. Jain et al. / Earth and Planetary Science Letters 179 (2000) 437^451442

weighted mean of 0.97 þ 0.1 Ma (Fig. 4). Seven-teen FT apatite ages, estimated along the SutlejValley from the MCT zone to the Vaikrita Thrustappear to fall in two distinct groups (Table 1 andFig. 3). In the hanging wall of the MCT, the FTapatite ages from three samples of the Jeori For-mation are statistically indistinct, and have aweighted mean of 4.9 þ 0.2 Ma (1 c). On thehanging wall of the Chaura thrust, the FT apatiteages of 13 samples of the WGC form anotherdistrict group with a weighted mean as low as1.49 þ 0.07 Ma (1 c). One FT apatite age fromthe Vaikrita Thrust zone is 1.31 þ 0.22 Ma (1 c).

For the purpose of useful interpretation of theFT ages from this region, FT data of nine samplesfrom the Bandal granitoid of the Lesser Hima-layan Kulu^Rampur Window have been incorpo-rated in this study [33,34]. Apatite ages from eightsamples range from 2.0 þ 0.44 to 3.74 þ 0.87 Mawith a weighted mean of 2.68 þ 0.1 Ma (1 c). Nine

samples of zircon yield an age of 3.89 þ 0.4 Ma to5.35 þ 1.24 Ma with a weighted mean of4.53 þ 0.07 Ma (1 c).

4.3. Chemical analysis of apatites

To investigate the e¡ects of chlorine and £uo-rine contents on the ages of apatites, the chemicalcomposition of four apatite samples, two eachfrom the hanging and foot walls of the Chaurathrust, was determined. These measurements weremade using JEOL JXA-8600M EPMA (electronprobe micro analyzer) with a 15 kV acceleratingvoltage and 2U1038 A sample current, having abeam size of 1 Wm. Natural mineral standards(SPI Standards, Canada) were used for the anal-ysis. ZAF corrections were applied to the data. Ineach sample, 4^5 apatite grains were analyzed tocalculate the mean values. The Cl content in foursamples, namely C7, J59, J25 and J27, was foundto be 0.012 þ 0.007, 0.062 þ 0.012, 0.126 þ 0.014and 0.008 þ 0.007 wt% (1 c), respectively, whileF varies from 0.335 þ 0.040 to 0.423 þ 0.055.These values fall between end-member £uorapa-tite and Cl content of Durango apatite (Mexico),and are much less than the 0.4 wt% of Cl contentof the latter [35]. Cl/Cl+F ratio varies from 0.016to 0.273 from samples located on either side ofthe Chaura thrust, hence the chemical composi-tion of apatite appears to have no e¡ects on theannealing properties of ¢ssion tracks, and conse-quently the FT ages.

5. Discussion

5.1. Cooling and exhumation rates

The HHC are characterized by numerous in-tensely deformed bodies of granite gneiss, whichappear to have been emplaced as granitoids likethe WGC. U^Pb dates on zircons, extracted fromthe undeformed portions of this body, are2066 þ 9 Ma from the lower parts of the hangingwall of MCT and 1866 þ 10 Ma from the mainbody above the Chaura thrust [21]. This is sup-ported by a 12-point Rb^Sr whole rock isochronage of 1895 þ 64 Ma from the Wangtu body [20].

Fig. 3. Age vs. distance curve for the HHC from the MCTabove the Lesser Himalayan Kulu^Rampur Window.Weighted mean ages are shown by horizontal stripes acrossdi¡erent segments of the HHC (+: FT zircon age, hatured:FT apatite age, dots: FT apatite age of cogenetic samples).Exhumation rates are calculated in mm/yr for each segment.Lower portion of the diagram depicts relative movementsand exhumation of individual segments of the HHC. Thickarrows indicate main movements using FT apatite age only,while arrow pairs along individual thrusts reveal relativemovements. Circles: FT zircon ages. Rectangles: FT apatiteages.

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These ages strongly suggest the involvement ofthe Middle Proterozoic basement of the northernmargin of the Indian Plate into the Himalayandeformation and metamorphism. Though theage of peak metamorphism could not be deter-mined so far along this valley, cooling ages deter-mined on muscovite and biotite from the SutlejValley fall between 30 and 20 Ma (Rb^Sr^musco-vite), 20 Ma (40Ar/39Ar^muscovite) and 17^16 Ma(40Ar/39Ar^biotite) [36]. It is, therefore, apparentthat the FT zircon and apatite ages also representthe cooling ages along this valley.

Uranium ¢ssion tracks in apatite and zircon aremetastable, because recombination of defectscauses the tracks to fade and anneal at elevatedtemperature. The annealing behavior of ¢ssiontracks in apatite and closure temperatures for re-tention of tracks therein have been estimated byboth extrapolation of laboratory annealing datato geological time [37^40], as well as from thebore hole samples of reasonably well-known ther-mal history, where increase in depth and temper-ature produces a decrease in age [41,42]. Over thegeological time between 1 and 100 Ma, tracks inapatite begin to anneal around 70³C with com-plete track loss over a range of V105^150³C.These low-closure temperatures are sensitive tothe time spent in the track annealing zone, andthus, to the rate of cooling. These have beenfound to range from 90³C at relatively lowcooling rates to 140³C at fast cooling rates [14].Because of the young apatite ages V1 Ma toV5 Ma, and hence rapid cooling rates of s 20and 6 100³C/Ma, the assumed closure tempera-ture of apatite in the present study is 130 þ 20³C[14].

For ¢ssion tracks in zircon, similar extrapo-lations of laboratory annealing experimentswere performed with freshly produced inducedtracks suggesting closure temperatures as highas s 300³C [42^44], whilst limited geological evi-dence favors a lower temperature of 175^250³C[11,45^47]. Using the annealing laboratory dataof spontaneous tracks in zircon [48] and borehole data [49], closure temperatures between 200and 250³C have been estimated for di¡erent cool-ing rates [50]. For the purpose of calculating cool-ing and exhumation rates, the assumed closure

temperature of zircon is 240 þ 25³C in the presentstudy. Due to non-availability of con¢ned sponta-neous ¢ssion tracks in apatite, it was not possibleto infer the cooling pattern. However, consideringthe very young ages and fast cooling history, asindicated by a very small di¡erence in ages ofcogenetic mineral pairs of apatite and zircon hav-ing di¡erent closure temperatures, it is unlikelythat the samples resided for any length of timein the partial annealing zone of apatite and/orzircon. The ages, therefore, may be taken as rep-resenting the closure temperatures of respectiveminerals, and are used in calculating the coolingand exhumation rates (Fig. 4).

There are hardly any data reported in literatureon the estimation of geothermal gradients in theHimalayan region especially the Higher Hima-layan rocks. However, for the Sub-HimalayanCenozoic foreland molassic basin, it has been es-timated using a geothermal gradient of 30³C/km[51]. However, where apatite ages are very youngand the zircon to apatite age di¡erence is 6 5 Ma,a higher geothermal gradient of 40³C/km has beensuggested to accommodate the increased near-sur-face gradient resulting from the rapid uplift, andthe same has been used in the present study toestimate the exhumation rates [12]. The value as-sumed for the mean annual surface temperature is10 þ 5³C, which has been adopted by severalworkers in the Himalaya as well as the Alps.Clear distinction between the FT apatite agesfrom three samples of the Jeori Formation(weighted mean of 4.9 þ 0.2 Ma) and 13 samplesof the WGC (weighted mean of 1.49 þ 0.07 Ma)indicates that these two groups of rocks have ex-humed di¡erentially across the Chaura thrust(Fig. 3). The hanging wall of the MCT has a slow-er exhumation rate of 0.61 þ 0.10 mm/yr (Fig. 4a)in comparison to the hanging wall of the Chaurathrust, which has a mean exhumation rate of2.01 þ 0.35 mm/yr since 1.49 Ma (Fig. 4b). How-ever, exhumation rates from cogenitic zircon^apa-tite pairs are much faster and range from4.82 þ 0.55 mm/yr during 1.54 Ma and 0.97 Ma,and 3.09 þ 0.71 mm/yr since then (Fig. 4c). Onesample from the Vaikrita Thrust zone reveals anexhumation rate of 1.98 þ 0.34 mm/yr from2.70 þ 0.40 Ma to 1.31 þ 0.22 Ma and 2.29 þ 0.66

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mm/yr since then (Fig. 4d). In comparison to thefoot wall, these rates are much slower when com-pared with those derived from cogenetic mineralpairs, but lie within error limits when decipheredfrom the weighted FT apatite ages. In comparisonto the HHC, the adjoining Lesser Himalayan win-dow has exhumed faster at a rate of 1.49 þ 0.09mm/yr during 4.53 Ma and 1.12 Ma (cogeneticzircon^apatite pairs), and 1.12 þ 0.18 mm/yr sincethen.

The ages from this section do not indicate anycorrelation with elevation, possibly due to rela-tively high exhumation rates across the MCT,Chaura and Vaikrita thrusts. For the hangingwall of the MCT, the exhumation rate of 0.61mm/yr would show only 0.6 Ma of FT age di¡er-ence for 400 m of vertical relief. This age di¡er-ence would be obscured by our analytical error.Similar arguments hold good for the hangingwalls of the Chaura and Vaikrita thrusts. TheChaura thrust, therefore, appears to be responsi-ble for separating two distinct groups of ¢ssiontrack data, and thus, exhumation of its hangingwall is much faster in comparison to the foot wall.The FT data, therefore, also con¢rm the structur-al discontinuity along this thrust.

5.2. Estimation of timing and displacement alongfaults

Quanti¢cation of vertical tectonic movements issigni¢cant for geodynamics for any orogenic belt,hence the FT technique has occasionally been ap-plied to investigate sense, amount and timing ofmovement along faults [10,52,53]. The principleunderlying such estimation can be understoodby a thermotectonic model of an area having noage variation with horizontal distance and subse-quently a¡ected by faulting, which will in£uencethe regular age pattern on either wall of the faultplane [14]. Assuming constant uplift rates on ei-ther side of the blocks a¡ected by constant tem-poral and regional geothermal gradient, amountsof vertical displacement can be calculated fromthe (i) age di¡erence t23t1 on either side of theblocks, (ii) di¡erence between e¡ective retentiontemperature, Teff:, (iii) ambient surface tempera-ture, Tamb: and (iv) geothermal gradient (Fig. 5).

Amount of displacement [14] :

d � �T eff :3Tamb:�U�t23t1�Geothermal gradientUt2

Considering the closure temperature of apatite

Fig. 4. T^t curves from FT zircon^apatite age data of theHHC, Sutlej Valley. Exhumation rates are given in bracketsas mm/yr. (a) Hanging wall of the MCT (Jeori Formation),(b) hanging wall of the Chaura thrust (WGC), (c) same ascurve (b) from cogenetic zircon^apatite pairs after calculatingtheir weighted mean, and (d) Vakrita Thrust Zone. Insertshows T^t curves of individual cogenetic zircon^apatite pairsfrom the WGC.

Fig. 5. Calculation of the amount of displacement d on afault from cooling ages t1 and t2 (see text for details) [14].

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(Teff:) as 130 þ 20³C, and the ambient surface tem-perature (Tamb:) as 10 þ 5³C, a vertical displace-ment of 2.08 þ 0.68 km appears to have takenplace along the Chaura thrust between 4.9 and1.49 Ma at an average rate of 0.6 mm/yr. How-ever, displacement is of the order of 2.34 þ 0.45km along this thrust, when calculated with thecogenetic age of 0.97 þ 0.1 Ma of apatite grains.The error has been estimated by taking into ac-count the uncertainties associated with di¡erentparameters in the equation. The FT apatite dataalso reveal that the Chaura thrust has behavedlike an overthrust between 4.9 and 0.97 Ma ofits history within the brittle phase of deformation.This led us to visualize the Chaura thrust as acomplex ductile shear zone, which appears tohave evolved at a higher temperature and contin-ued displacements in the brittle regime of lowtemperature (130 þ 20³C). This is also con¢rmedby the presence of discrete brittle thrusts havingcm thick fault gouge zones, which cut across thelate stage folds in the gneiss. On the other hand,the Vaikrita Thrust interestingly behaves like anormal fault having vertical displacement of theorder of 1.18 þ 0.09 km during 2.70 Ma and 1.54Ma using FT zircon ages on both sides of thefault and 0.78 þ 0.17 km during 1.31 Ma and0.97 Ma from the FT apatite ages on either sideof the fault zone.

6. Tectonic exhumation of the NW Himalaya

Systematic isotopic data from a few segmentsof the HHC from the NW Himalaya reveal accel-erated exhumation pulses after the initial India^Asia collision V55 Ma, which has caused region-al overthickening of the continental crust, its de-formation and regional metamorphism. Tectoni-cally, these appear to be controlled by (i) large-scale domes and/or windows, which have beenactive either independently or in combinationwith (ii) major thrusts as well as extensional nor-mal faults, which have caused tectonic denudationand rapid erosion of the uplifted mountains. Oth-er factors like accelerated erosion during the Qua-ternary without tectonism may also have beenresponsible for the exhumation.

6.1. Domes/windows

Following the India^Asia collision in the NWPakistan V55 Ma, deformed and metamorphosedleading edge of the Indian Plate appears to havebeen exhumed along northerly-dipping MainMantle Thrust, a tectonic boundary between theHigher Himalaya and the island-type Kohistanarc, between 35 and 16 Ma during the coolingof hot crust through 500³C to 140³C. Cessationof tectonic movements has been recorded V20Ma using FT apatite age [54]. However, distinctdi¡erential tectonothermal histories prior to Mid-dle Miocene are decipherable from the FT zir-cons, indicating the extensional character of theMain Mantle Thrust V20 Ma [13].

Nearer the Nanga Parbat^Harmosh massif,more rapid and recent exhumation was causedby its syntaxial growth by northerly plunging an-ticlinal folds and superposed faulting [55], as isevident from systematic domal variation and de-crease in 40Ar/39Ar biotite ages in south^centralcore to as low as 0.9 þ 0.1 Ma from V8.0 Ma.FT zircon^apatite ages during the interval 2.0^0.5Ma indicate an enhanced exhumation rate of5 mm/yr and its further acceleration to 9 mm/yrsince 0.5 Ma in the south.

In the southeastern parts of Jammu and Kash-mir across the Kishtwar^Padar^Zanskar section,the HHC exhibits a mean exhumation rate ofabout 0.27 mm/yr during Middle to Late Mio-cence from its base along the MCT to the north-ernmost margin along the Zanskar Shear Zoneand having a small spatial variation between0.22 mm/yr and 0.31 mm/yr [56]. In contrast,three large domal structures/windows, namelythe Suru dome, the Chisoti dome and the Kisht-war antiformal window, have remarkably indi-cated very young and fast exhumation rates of0.80 mm/yr, 1.4 mm/yr and 2.97 mm/yr, respec-tively, during 1^5 Ma, which have been estimatedfrom the already published ages (Fig. 6b^d) [56].Exhumation rates due to the Chisoti dome haveundergone acceleration from 0.31 mm/yr to 2.03mm/yr during 13.7 Ma to 6.2 Ma and to 4.4 Ma,respectively. Since then it has been somewhat lessto 0.87 mm/yr (Fig. 6b). Further to the southeastand away from the Suru dome, exhumation rates

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range between 0.32 mm/yr and 0.65 mm/yr sinceabout 20 Ma and do not reveal any Pliocene^Quaternary acceleration (Fig. 6c,d). In fact, theKishtwar Window has yielded apatite ages of

1 Ma along the crest of the antiform, indicatingthe evidences of even the Quaternary foldingevents on large scale like the Nanga Parbat [55];in both the regions where di¡erential exhumation

Fig. 6. Miocene^Quaternary exhumation rates from the NW Himalaya (Kashmir, Himachal and Garhwal), using new FT zircon^apatite data and published work on Th^Pb (monazite), Rb^Sr, K^Ar and 40Ar/39Ar muscovite^biotite and FT zircon^apatitedata. (a) Simpli¢ed map of the NW Himalaya showing location of stations for exhumation rates. Symbols as in Fig. 1. Exhuma-tion rates calculated from (b) Chisoti dome (Ch: Padar, SE Kashmir), (c) Safat area (S: Doda Valley, Zanskar), (d) MulungTokpo (M: Zanskar), (e) Bandal (B: Beas Valley), (f) Kulu (Beas Valley), (g) Rohtang (R: Beas Valley), and (h) Gangotri(G: Bhagirathi Valley). Locations b^d, g^h belong to the HHC (e: Lesser Himalayan granitoid from the Kulu Rampur Window,f: Jutogh nappe of the Lesser Himalaya). Source of published data: Chisoti dome [56,61], Safat [56,61,62], Mulung Tokpo[56,62], Bandal [34,36], Kulu [34,63], Rohtang [17,34] and Gangotri [58^60]. For uniformity in exhumation rates, uncertainties arenot shown.

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rates from the crest to the £anks [55,56] appear tohave been controlled by the presence of extension-al fault boundaries during faster exhumed win-dows.

Another zone of tectonic exhumation within theHHC has been deciphered along the Beas^SutlejValleys, where the Kulu^Rampur Window ap-pears to have exhumed relatively faster than itsmetamorphic cover (Fig. 6e^g). Late Miocene^Pliocene exhumation rates within the window, asdeciphered from 40Ar/39Ar sericite, Rb^Sr biotiteand FT zircon ages, accelerate from 0.42 mm/yrto 1.33 mm/yr (Fig. 6e) [33]. Further acceleratedexhumation within the window to 1.49 þ 0.09 mm/yr has been deciphered from FT zircon^apatiteages during 4.5 Ma and 2.68 Ma before slightlyslowing down to 1.12 þ 0.18 mm/yr since then.This contrasts to much slower exhumation of0.61 þ 0.10 mm/yr on its northern limb along theSutlej Valley since 4.9 Ma (Fig. 4). However,these are much faster on the southwestern £ankof the window around Kulu to 15.38 mm/yr for avery short span from 13.7 Ma to 13.5 Ma (Fig. 6f)and much slower (0.55 mm/yr) along the north-western closure during 28.2 Ma and 16.12 Ma(Fig. 6g). Pliocene^Pleistocene accelerated exhu-mation within window zone at a rate between1.49 mm/yr and 1.12 mm/yr surpasses all the ex-humation rates, measured from the FT zircon^apatite and FT apatite surface, respectively, onthe £anks and along the strike of the metamor-phic cover towards the northwest (Fig. 6e^g).These range from 0.39 mm/yr to 0.78 mm/yr onthe southwestern £ank (Fig. 6f), between 0.88mm/yr and 1.05 mm/yr towards northwest (Fig.6g) and 0.61 mm/yr on the northeastern £ank(Fig. 4) along the Sutlej Valley on the hangingwall of the MCT. All these rates reveal that theMCT has distinctly been active as extensionalnormal fault after the nappe emplacement onboth £anks of the window and possibly facilitatedits faster exhumation, which has resulted into thelateral sliding of the metamorphic cover.

6.2. Thrust faults

A distinct example of variable FT ages across atectonic boundary in the NW Himalaya is now

available from the Sutlej Valley, where thrustshave exclusively controlled the exhumation ratesacross a section of the HHC also. As has beenelaborated earlier, the newly recognized Chaurathrust has mainly been responsible for distincttectonothermal evolution of its foot and hangingwalls within the HHC, which has been di¡eren-tially exhumed at a rate of 0.61 þ 0.10 mm/yr and2.01 þ 0.35 mm/yr from 4.9 þ 0.2 Ma and1.49 þ 0.07 Ma to present, respectively. However,considering the FT apatite from cogenetic pairson its hanging wall, the exhumation rate is3.09 þ 0.71 mm/yr since 0.97 Ma. The characterof the Vaikrita Thrust is uncertain and obscuredwithin error limits of average FT apatite ages of1.49 þ 0.07 Ma and 1.31 þ 0.22 Ma on its foot andhanging walls, respectively. On the other hand,this thrust has a complex exhumation history asextensional normal fault during 2.70 þ 0.40 Maand 1.31 þ 0.22 Ma with an exhumation rate of1.98 þ 0.34 mm/yr using FT zircon^apatite ages.Further, its extensional character is strengthenedif the FT apatite age of 0.97 þ 0.1 Ma of cogeneticsamples from the foot wall is compared with thatof the hanging wall of the Vaikrita Thrust. Theoverall tectonics of the HHC along the Sutlej Val-ley, thus, appear to be an interplay of thrust andextensional faulting, which has produced smallercrustal wedges, out of which the middle Wangtuwedge is upthrust between downward movingwedges at the base and top of the HHC due tothe extensional character of the MCT and theVaikrita Thrust during Pliocene^Pleistocene. In-stead of the HHC constituting single crust wedgeextruding southwestward [57], FT zircon^apatiteages from the Sutlej Valley reveal an interplay ofa number of crustal wedges, moving di¡erentiallyboth southwestward and northeastwards duringlater tectonothermal evolution of this belt inPlio^Pleistocene.

6.3. Extensional faults and erosion

An early Miocene rapid exhumation pulsearound 18 Ma has been deciphered using 40Ar/39Ar muscovite^biotite and systematic FT zir-con^apatite dating of the core of the HHC fromthe Gangotri leucogranite [9,58], possibly due to

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extensional detachment along the Martoli Fault.When combined with the Th^Pb monazite age ofV22.5 Ma for the emplacement of the GangotriGranite [59,60], it appears to have exhumed at arate of 2.65 mm/yr during 23 Ma and 18 Mabefore slowing down between 0.36 and 0.70 mm/yr, as has been calculated from the available FTzircon^apatite data from adjoining granite [61].Another accelerated Late Pliocene^Quaternaryexhumation followed at a rate of 2.60 mm/yr since1.67 Ma, due to rapid erosion (Fig. 6h) [9].

Acknowledgements

The present investigations were undertaken as apart of the Thrust Area Programme of the De-partment of Science and Technology (DST) enti-tled `Evolution of the Metamorphic Belts in Pha-nerozoic Collision Boundary'. N.L. thanksGunther Wagner for fruitful discussion duringthe early stage of this work. Constructive criticismon a version of the paper by Anthony Hurfordand three anonymous reviewers, and by MauricePagel at various stages substantially improved themanuscript. J.W.H. Schreurs is thankfully ac-knowledged for the supply of CN glasses. KailashChandra and T.K. Ghosh are thankfully ac-knowledged for their help during the EPMA ana-lysis.[AC]

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