Structural and Thermal Evolution of the Karakoram Crust

Journal of the Geological Society, London, Vol. 148, 1991, pp. 65-82, 11 figs, 3 tables. Printed in Northern Ireland

Structural and thermal evolution of the Karakoram crust

M.P. SEARLE' & R.TIRRUL2 (deceased) 'Department of Geology, University of Leicester, Leicester LE1 7RH, UK. Present address: Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK %eological Survey of Canada, 588 Booth Street, Ottawa, Ontario, Canada, KIA OE4

Abstract: Prior to the Eocene (c.50 Ma) collision of the Indian and Asian plates, the southern margin of Asia along the Karakoram plate was an Andean-type margin dominated by tonalitic-granodioritic magmatism of Jurassic-Lower Cretaceous age (Hushe gneiss, Muztagh Tower gneiss and K2 orthogneiss)and associated low pressure andalusite, staurolite and garnet grade metamorphism (Ml). Following India-Asia collision, crustal shortening, thickening and regional Barrovian metamorphism (M2) occurred between 50-37 Ma. Thermobarometry of kyanite-grade metapelites indicate burial to depths of around 30-35 km. Simultaneous solution of the garnet-biotite geothermometer with the garnet-muscovite-biotite-plagioclase and garnet-AI,SiO,-quartz-plagioclase geobarometersindi- cates peak M2 P-T conditions of 696 f 20 "C at 8.6 f 0.7 kbar (860 MPa). Temperatures may have exceeded 700°C in sillimanite-grade metapelites to produce in situ partial melting and leucogranitic meltpods. Peak M2 metamorphism occurred prior to 37f0.8Ma, the crystallization age of the Mango Gusar two-mica granite pluton which cross-cuts syn-metamorphic deformation fabrics. Post-M2 thermal relaxation followed from 37-25 Ma, after which localized high heat concentra- tions at the baseof the thickened crust caused widespread crustal melting and intrusionof the Baltoro granitebatholith at 25-21 Ma. A high temperature-low pressure thermal aureole (M3) along the northern contact is synchronous with the 21 f 0.5 Ma zircon age of the Baltoro granite. Andalusite hornfelsalong the northern contact of thebatholith (Mitre thermal aureole) indicates maximum pressures of 3.75kbar (375 MPa). A 75 "C increase of temperaturein kyanite-sillimanite grade gneisses approaching the southern granite contact of the Baltoro granite is interpreted as the thermal upwarping of pre-37 Ma Barrovian metamorphic M2 isograds around the 21 Ma contact aureole M3 isotherms.

The Karakoram Range (Fig. 1) is a linear belt of extremely which haveclosing temperaturesabove 500°C ataround high mountains, resulting from recent rapid uplift of thick 21 Ma in Nepal (Hubbard & Harrison 1989), synchronous (c. 65-70 km) continental crust (Marussi 1964; Desio 1964; with leucogranite emplacement (LeFort et al. 1987). Searle Desio & Zanettin 1970; Searle et al.1986, 1988; Zeitler et al. (1988, 1989) demonstratedthat crustal thickening, 1985; Molnar 1988). High grade Barrovian facies metamor- regionalmetamorphism andanatexis was happening phic rocks are exposed within the Karakoram metamorphic simultaneously bothnorth (Karakoram) and south (High complex in the south, and the Karakoram batholith includes Himalaya) of theIndus suture zone (ISZ) approximately the massive (at least 100km X 1-20 km) Miocene Baltoro 20-30Ma after initial collision. In the western Himalaya of plutonicunit, which consists of monzogranitesand Pakistan, regional metamorphism appears to be somewhat leucogranites with late-stage leucogranitic dyke swarms. The older (prior to 40Ma; Treloar et al. 1989b) and affected by regional metamorphic rocks aredominantly Oligocene syn- to post-metamorphic folding of isograds and thrusting (Searle et al. 1989), and the Baltoro granite has yielded a (Treloar et al.1989a; Searle & Rex1989). Zeitler (1985) precise U-Pb zircon Miocene age of 21 f 0.5 Ma (Parrish & and Chamberlain et al. (1989) have shown that the Nanga Tirrul 1989). Parbat-HaramoshRange has an extremelyrapid uplift- Incontinental collision mountain belts, recent studies tectonic denudation rate (up to 7 mm per year) during the have shown that the thermal evolution has involved several last 10 Ma, and has been exhumed or eroded by 5-10 km 'phases' of metamorphismat different P-T conditions.In relative to the Kohistan arc rocks adjacent to it. Preliminary thecentral Nepal Himalaya and Garhwal region of India fission track and Ar-Ar data from the Hunza Karakoram, thereare two distinct metamorphicevents: anearly summarized by Rex et al. (1988), also indicate rapid recent Barrovianevent (Ml), anda later high temperaturebut uplift-exhumation rates along the Karakoram. lower pressure Buchan event (M2) (Hodges et al. 1988a,b; The N-S striking Nanga Parbat-Haramosh Range of the Hodges & Silverberg1988). Following Eocene Barrovian HighHimalaya and the ENE-WSW striking Karakoram metamorphism, the P-T evolution of this part of the High (Fig. 2) therefore are both characterized by young cooling Himalayarecords 'erosion-controlled'an uplift path ages andare structurally controlled by youngbreakback (England & Richardson 1977) whichwas followed by a thrust faults. The Liacharthrust (Butler & Prior 1988) second phase of burial with heating of the upper part of the bounds the western margin of the Nanga Parbat culmination slabdue tothe intrusion of leucogranites(Hodges et al. andthrusts deep crustal gneisses over Quaternary river 1988a,b) . gravels of theIndus River (Owen 1988; Butler & Prior Timing of metamorphism in the High Himalaya (Fig. 1) 1988). The Main KarakoramThrust (MKT) is a late has been constrained by 40Ar-39Ar dating of hornblendes, Tertiary breakback thrust whichis responsible for the recent 65

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INDIA

Fig. 1. Geological sketch mapof central Asia showing the western Himalaya, Karakoram, Hindu Kush, Pamirs, Tien Shan and Kun Lun mountain ranges. The stippled areas show the approximate extentof post-collisional regional metamorphic rocks north (Karakoram) and south (High Himalaya) of the main collision zone, the Indus suture zone(ISZ). The Shyok suture zone(SSZ) marks the southern limit of Asian plate rocks in the Karakoram. Kohistan and Ladakhare dominantly accreted island-arc and batholith terranes separating Indian plate gneissesof Nanga Parbat (NP)-Haramosh (H) from the Karakoram plate. PS is the Peshawar Basin. K is the Kashmir intermontane basin. MCT is the Main Central thrust; MBT is the Main Boundary thrust.

uplift of the central Karakoram (Searle et al. 1987, 1989), the India-Asia collision. Four field seasonswere spent although it mayhave also been active during late mappingand sampling during expeditions tothe Biafo Cretaceous-Palaeogene times. glaciersystem (1984), Baltoro glaciersystem upto K2, Thispaper summarizes the geologicsetting and Masherbrum and the Gasherbrum Range (1985), the Hushe petrography of the Karakoram metamorphic complex north valley (andAling, S. Masherbrum,Gondoro, Charakusa of the two major Tethyan suture zones in northern Pakistan glaciers) (1986), and the Biale-Trango Towers area (1988) (Figs 1 & 2). Pressure, temperature and time constraints on (Fig. 2). The tragic death of Rein Tirrul in 1987 has meant metamorphismare summarized, and the cooling-uplift- that this paper was written by the first author. Much of the tectonicexhumation history of differentcrustal blocks fieldwork and many of the ideas in this contribution were ascertained. A tectono-thermal evolution of the Karakorarn accomplished or discussed jointly by both authors. crust is thenpresented based on four distinct phases of metamorphism. Finally we discuss how these metamorphic data,combined withgeochronological constraints can be used to constrain thestructure and evolution of theThe Karakoram mountains are situated north of the Shyok Karakoramcrust in thecontext of thewider implications of suturezone, the Ladakh-Kohistan arc-batholith andthe

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/148/1/65/4891155/gsjgs.148.1.0065.pdf by guest on 30 September 2021 -KEY KARAKORAMMETAMORPHIC COMPLEX Hushe gneiss NORTHERNKARAKORAM ZONE m

,.,.,,:,.>:,:,, . , UndifferentiatedMetamorphlc rocks a K2 gnelss CA’?. (Ganschen.Dumordu.Askole unlts) m Dassu gneiss GasherbrumGroup (carbonates) SHYOKSUTURE ZONE Ealtoro Fm (shales1 m 0PalaeozoIc-L.Mesozoic sediments KARAKORAMBATHOLITH - Rakaposhl volcanic Group MuztaghTower unlt KOHISTAN-LADAKHBATHOLITH Ealtoro Plutonlcunlt 0 m Granodiorlte,tonaIlte with leucogranitedykes. + Masherbrumcomplex m Metamorphlcscreens Hunza Plutonicunlt HIGHHIMALAYA ZONE 0Nanga Parbat gnelss

2. Geological map of the central Karakoram (box on Fig. 1) from the Hispar glacier In the west to the Baltoro glacier, Hushe valley in the . MGG is the Mango Gusar granite; CLG is the Chingkang-la granite; MKT is the Main Karakoram thrust.

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Indussuture zone (Fig. 1).The Indus suture zone is the assemblages are present throughout the area. Both kyanite- major suture dividing the Indian plate from the Karakoram andsillimanite-bearing pelites arecommon, and around andLhasa blocks. It closed at c. 50 Ma, according to Chakpo, 10 km west of Askole, the assemblage sillimanite- sedimentological and structural data in Ladakh (Searleet al. garnet-muscovite-biotite-plagioclase-quartz is present. 1987, 1988). The Shyok suture zone (also previously termed The muscovite-outreaction can bedemonstrated by the theNorthern suture) divides the Kohistan-Ladakh arc- replacement of muscovite by sillimanite-K-feldspar without batholith tothe south from the Karakoram plate to the partialmelting (reaction M2c on Fig. 5). Thisreaction northand is interpretedas asmall back-arc basin which occurs at high temperatures (620-650°C) but relatively low closedduring thelate Cretaceous (Pudsey et al. 1985; pressures (<3.5 kbar) depending on PHzo(Fig. 5). Coward et al. 1987, 1988). Around Askole (Figs 3 & 4) kyanite-staurolite-garnet- The central Karakoram, between the Hunzavalley in the biotite-muscovite-plagioclase-quartz assemblages are wide- west and the Indian-Pakistan cease-fire line (approximately spread. Garnet and clinopyroxene-bearing amphibolites and along the eastern edge of Fig. 2) in the east, can be divided high-grade impuredolomitic marble containing diopside, intothree tectono-stratigraphic domains from south to olivine andphlogopite are interlayered with the kyanite- north:the Karakoram metamorphic complex, the Kara- staurolite pelites. Justeast of thePanmah-Braldu river koram batholith and the northern Karakoram zone (Fig. 2). confluence, the staurolite-out isograd (reaction M2a on Figs Thesouthern margin of thiscomplex is areactivated 4 & 5) is presentand pelitic assemblagesshow breakbackthrust. It is theMain Karakoram thrust which kyanite-biotite-garnet-muscovite-plagioclase-qua~ (M2b follows closely the line of the older Shyok suture zone in on Fig. 5). Sillimanite-garnet-biotite-muscovite as- northernPakistan. It is a steepnorth-dipping thrust fault semblages appear east of Bardumal (Figs 3 & 4) and along zone whichplaces Karakorammetamorphic rocks south- thesouthern margin of theBaltoro granite. These rocks wards over rocks of the Shyok suture zone (Carboniferous indicatepressures of metamorphismbetween 5-6 kbar to Albian-Aptian) and the Ladakh batholith (Cretaceous to (5-600 MPa) and temperatures around 550-650 "C (Fig. 5). Eocene). The relationswere described by Searle et al. Whereaskyanite and sillimanite-bearing pelites are (1986, 1989) and Rex et al. (1988). A U-Pb zircon age of common along the Biafo, Panmah and Braldu valleys in the 21 f 0.5 Ma on biotite monzogranite and two mica f garnet north,south andeast of Mango Gusar (Fig. 2) leucogranite of the Baltoro Plutonic unit (BPU)which forms andalusite-bearingassemblages are widespread. Sillimanite the bulk of theKarakoram batholith in thisarea was is rarely present in this area although a few samples show obtained by Parrish & Tirrul (1989) and Scharer et al. (1990) kyaniteapparently replacing andalusite indicating an have published a more detailed study. Rb-Sr and K-Ar age increase in pressure with time. Andalusite-bearing slates are determinations were presented in Searle et al. (1989). common boulders in float from the Chingkang and Mango valleys draining the Skoro-la and Mango Gusar range south of theBraldu river (Fig. 3). Inthis area pressures and temperatures increase towards the WNW and also increase Petrography of the Karakoram metamorphic rocks with greater structural depth. TheKarakoram metamorphic complex, south of the In the central part of the area, a low-grade metamorphic Karakorambatholith consists of varietya of meta- zone is centredaround the Chingkang valley and upper sedimentaryandmeta-igneous rocks with fold and Alingglacier (Fig. 4). Biotite-muscovite-quartz- thrust-relatedculminations of sillimanite-bearingtonalitic plagioclase-chlorite-chloritoid assemblages characterize the orthogneisses(Dassu gneiss). Earlier studies have divided pelites,whereas epidote-actinolite-chlorite-albite as- these rocks into poorly defined stratigraphic formations with semblagescharacterize the meta-basalts. Inthis area little or no detailed mappingin the area southof the Baltoro metamorphicisograds also appear tobe the right way-up Basin (Desio 1964; Desio & Zanettin 1970). The rocks show with grade increasing with structural depth. acomplex polyphase history of deformationand meta- In the eastern part of the area, the Hushe gneiss (Figs 2 morphism,and it is notyetpossible to definea & 3) is composeddominantly of orthogneissesincluding pre-metamorphicstratigraphy (Bertrand & Debon 1986; metamorphosed and deformed hornblende diorites, biotite Searle et al. 1986). Two main metasedimentary units have granodioritesand K-feldspar megacrystic monzogranites. been loosely defined and used during regional mapping: the Uncommon pelitic rocksclosely associated with these Dumordounit which is dominantlycomposed of marble orthogneisses always show low pressure assemblages, with with minor pelite, orthoquartzite and graphitic schist; and andalusite and garnet-bearing schists the most frequent. theGanschen unit which is composeddominantly of Mafic amphibolitesarecommon around Askole, high-gradepelite interbedded withmafic amphibolites but theupper Biafo glacier and Panmah valley. These are with little marble (Fig. 3). composed of hornblende-plagioclase-biotite-garnet f Figure 4 shows thedistribution of metamorphicgrade clinopyroxene f epidote assemblages andare frequently within theKarakoram metamorphic complex andthe interbandedwith sillimanite and kyanite-bearing gneisses mapped positions of key isograds as well as locations of the and high-grademarbles, for example nearthe Biafo radiometrically datedigneous samples. Figure 4 is keyed glacier-Braldu river confluence. with the P-T diagram(Fig. 5) wherethe locations of Marble forms over 50% of the Karakoram metamorphic metamorphic reactions and boundaries in the field are also complex in someareas and is relatively pure,although it locatedin P-T space.Metapelites arewidespread may contain minor amounts of clinopyroxene, phlogopite, throughout theKarakoram metamorphic complex, usually quartz and corundum. In places the marbles are interbedded with abundantaluminosilicate phases. Along the Braldu with garnetamphibolites, pelites, graphitic schists and riverleading upthetoBaltoro glacier, high-grade calc-silicate gneisses.

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Panmah ultramafic-mafic unit At the junction of the Biafo glacierand Braldu River Aprominent belt of tectonicmelange occurs immediately (Figs 3 & 4) high-grade impuremarbles of the Dumordu south of the Karakoram batholith and includes lenses and unitcontain garnet,diopside, hornblende calcite, epidote, blocks of gabbros,meta-basalts and ultramafic rocks in a plagioclase and titanite. The garnets are grossular-rich with slaty matrix. The ultramaficrocks include harzburgite Cacontents up to 50%. Theircompositional ranges are comprisingserpentinized forsterite and hypersthene with Alm41-4sPy5-6 Gr44--50Sp,_,. The metamorphic complex in Mg-chlorite, talc andspinel. Lherzolite consisting of relict theHunza valley section contains over 50% marble,and clinopyroxene,orthopyroxene and olivine is also present, garnets from staurolite schists interbanded with the marbles althoughmost blocks are strongly alteredto serpentinite contain upto 13% grossular compositions.Leucogranitic (talc-antigorite)and magnesite. Blocks of amphibolite- garnetsfrom the Baltoro pluton and the dykes in the gradegabbro, meta-basalt and rarelychert are associated Masherbrum complex have variable compositions but are all with ultramaficlenses contained within pelite and marble spessartine-rich, with MnO contents up to 42%. In common bands. Thetectonic ultramafic-gabbroicmelange zone with other magmatic garnets whichhave garnet composi- occursboth in low-grade(chlorite-biotite) regional tions with more than 10% spessartine (Miller & Stoddard metamorphicrocks as well ashigh-grade rocks (kyanite- 1981),they are clearly of magmaticorigin, and arenot garnet-biotite-muscovite-quartz-plagioclase pelites),and xenocrystsderived from themetamorphic country rock. appears to form a discontinuous band south and southwest Garnetsfrom leucogranites at the Baltoro glacier-Mundu of theKarakoram batholith (Searle et al. 1989). The glacierjunction (Fig. 3) havecompositional ranges of ultramafic-gabbroic association may once have been part of A1m74-77Py7-11 Gr2 Sp9-17 and Alm65--66PY5 Grl-z SPz7. an ophiolite complex although original emplacement-related These arecomparable togarnets analysed fromgarnet- structureshave been totally obliterated by thelater tourmaline-muscoviteleucogranites of the HighHimalaya post-collision ductile shearing and folding. from the Zanskar Himalaya (Searle &c Fryer 1986) although Mn-contents of Baltorogarnets are significantly higher. Garnets from theMasherbrum leucogranite from the Dassu gneiss Yermanandu glacier (N Face Masherbrum) are even more Structural culminations of mid-crustal gneisses and granite spessartine-rich with compositions in therange Alms3-59 gneisses occur around Dassu and the Biafo glacier-Panmah Pyz_, Gr,-, Spy-42. The difference in garnet compositions valley area (Fig. 3).The Dassu felsic gneissconsists of between those of the Baltoro leucogranites and those from biotite-K-feldspar-plagioclase-quartz-sillimanite-garnet f the metamorphic complex exclude a xenocrystic origin for muscovite gneisses formed at temperatures at or above the theformer. The low mode of garnet in theBaltoro muscovitebreakdown reaction. Partial melt podsand leucograniteand its propensity to retainMn may well veinsas well as dykes of garnet - biotite - muscovite f account for the high spessartine content of the garnets. tourmaline leucogranite are present. Numerouscross-cutting pegmatite-aplite dykes rich in tourmaline, topaz, aquama- Thermobarometry rine and beryl are compositionally unlike, and unrelated to, the dykes of the Baltoro plutonic unit or the Masherbrum Pressure-temperatureconditions of metamorphismhave complex tothe north. Granodioritic and quartz dioritic beencalculated from samples of high-grademetamorphic gneisses are also present, and migmatites form a small but assemblages from the Braldu River-Panmah River transect, important part of the complex. Foliation in the surrounding as well asfrom samples of garnet-biotite-muscovite-K- pelites andmarbles wraps around Dassu gneiss domes or feldspar-plagioclase-quartz leucogranites from the Baltoro double-plungingculminations, the uplift of whichwas pluton and Masherbrum injection complex. P-T estimates late-stage and post-metamorphic. wereobtained using simultaneous solution of thegarnet- biotite geothermometer(GB; Ferry & Spear (1978) modified by Hodges & Spear (1982) for non-ideal garnet Mineral chemistry solution)with the garnet-muscovite-biotite-plagioclase Representativeanalyses of garnetsfrom the Karakoram geobarometer(GMBP; Hodges & Crowley1985). Where metamorphic complex are given in Table 1. Garnets from kyanite or sillimanite is present,pressures have been the kyanite gradepelites along the Braldu River are estimatedusing the garnet-aluminiumsilicate-quartz- euhedral to subhedral and can reach up to 3 cm diameter. plagioclase geobarometer (GAQP; Ghent 1976; Newton & Theyare all almandine-richgarnets although the Fe0 Haselton1981). Equilibrium reactions controlling these contentsvary considerably. Spessartine contents of the geothermometer and geobarometers are shown in Table 2. metamorphic garnets are always less than 5% and frequently Preliminary thermobarometicresults from the Karakoram around 1% or less. Microprobetraverses across garnet are shown in Table 3 with the actual analyses given in Table porphyroblastsfrom the kyanite-grade gneisses reveal 1. The high Mncontents of garnetsfrom the Baltoro practically no significant compositional zoning from core to leucogranitesmean thatthey cannot be used in these rim (Fig. 6).We interpret this uniformity to indicate thermobarometersandthe P-T results,which are homogenization by diffusion in garnet at high metamorphic unrealistically low, must therefore be discarded. grade, withonly minor rim gradients indicating re- In orderto constrain the pressures and temperatures equilibration in response to changing P-T conditions during of metamorphism it wouldideally be necessary to obtain final crystallization. Typical pelitic garnetsfrom the thermobarometry results over a wide area. Single transects kyanite-grade rocks at the Panmah-Braldu River confluence maywell notberepresentative due to alongstrike have compositions Alm,,~,,Py12~,sGr2~sSp2~~.Garnets from differences in metamorphicgrade. As a preliminary study the kyanite-staurolite-bearing gneisses atBardumal have we haveselected samples from thekyanite-garnet- similar compositions A1m77-79py13-14 Gr5-7 Spa-,. muscovite-biotite f stauroliteassemblage gneisses to give

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CLG Chngkang-lagranlte (36-34 Mal 0 m Dassu orthogneoss k Follatlon k Thrust Fault Glaclers A Mountam Ilneatlon Stretchng

01 5 10 11II1--I kms

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A Broad . Peak

35' 30'

Fig. 3. Geological map of the Karakoram metamorphic complex southof the Baltoro glacier system (SE quadrant of Fig. 2). MKT is the Main Karakoram Thrust which includes severalsplays.

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Fig. 4. Map showing metamorphic grade in the Karakoram metamorphic complex southwestof the Baltoro glacier.MGG is the Mango Gusar granite, CLG is the Chinglang-la granite. 1, Very low-grade sediments and volcanic rocksof Shyok suture, chlorite + white mica. 2, Low-grade muscovite + chlorite f chloritoid f biotite f garnet. 3, Staurolite grade. 4, Kyanite grade. 5, Sillimanite grade. 6, Sillimanite gradewith granitic partial melt pods and migmatite textures. M1, M2 etc. refers to localities of metamorphic reactions keyed to Fig.5.

someindication of thepeak pressures and temperatures attainedduring M2 regionalmetamorphism. We do not have sufficient thermobarometricdata to coverthis very large and complex area. Field and petrographic data have been used to compile the metamorphic map (Fig. 4) and we use geochronological constraints (Searle et al. 1989; Rex et a1 1988;Parrish & Tirrul 1989) withlimited P-T data to establish approximate P-T-t paths.

Tectonic interpretation A Tertiary time chart showing all the known and reliable radiometricdata from the central Karakoram magmatic rocks, as well as the inferred time spansof metamorphic and deformationalevents, is shown in Fig. 7. Combining our studies of field relations,structure, metamorphism and therrnobarometry we candefine four temporally distinct ‘phases’ of metamorphism. 1 I I I I I 450 550 650 750

TEMPERATURE (OC) M1: low pressure (andaiusite-garnet grade) metamorphism (Jurassic-? L. Cretaceous) Fig. 5. Pressure-temperature grid showing key metamorphic assemblages and reactions in the region shown in Fig.4. All The low pressure regional metamorphism around the Aling assemblages contain biotite and quartz.A, Andalusite, kyanite or glacier and Chingkang valley area, west of Hushe (Figs 2 &L sillimanite; C, Cordierite; Ch, chlorite; G, garnet; L, granitic liquid; 3) appears from our mapping to be spatially associated with M,muscovite; 0, orthoclase, S, staurolite; V, vapour.M1, M2 and the foliated biotite-hornblende granodiorites and diorites of M3 refer to the metamorphic phases, and the locationof the Hushe complex. The hornblende diorites appear to be assemblages is shownin Fig. 4. Granite wet melting curve isafter the oldest igneous components of the Karakoram batholith Fyfe (1970). (sensu lato), one sample of which has a U-Pb zircon age of

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TaMe 1. Selected garnet, biotite, muscovite and plagioclase analysis from the Karakoram metamorphic complex

1 2 1 3 6 4 5 7 8 9 10 G T B10 GT MUSC PLAG GT B10 MUSC PLAG GT B10 SiO, 38.75 36.05 45.29 62.52 38.38 37.47 46.58 61.30 38.03 40.72 Ti02 0.05 2.07 0.71 - - 2.13 0.52 - - I .76 AI203 21.84 18.63 32.99 22.59 21.44 19.08 34.29 23.27 21.94 17.71 Cr203 - 0.05 0.04 - - - 0.06 - 0.04 0.04 Fe203 - - - 0.10 - - - 0.02 - - Fe0 34.47 20.06 1.19 - 35.24 16.69 1.24 - 36.25 17.21 MnO 0.23 - 0.05 - 0.12 - 0.04 - 0.08 - MgO 3.38 8.64 0.69 - 4.09 9.81 0.76 - 4.05 7.79 CaO 2.42 - - 5.12 1.88 0.05 - 5.35 0.98 0.37 Na,O - 0.33 1.16 9.50 - 0.26 1.26 8.70 - 0.24 K@ - 9.03 9.76 0.10 - 8.16 9.67 0.09 - 6.14 Total 101.14 94.86 91.88 99.93 101.15 93.65 94.42 98.73 101.37 91.98

0 12.000 22.000 22.000 32.000 12.000 22.000 22.000 32.000 12.000 22.000 Si 3.042 5.510 6.249 11.124 3.023 5.647 6.240 11.021 2.995 6.136 TI 0.003 0.238 0.074 - - 0.241 0.052 - - 0.199 A1 2.021 3.356 5 ,366 4.738 1.990 3.390 5.414 4.931 2.037 3.146 Cr - 0.006 0.004 - - - 0.006 - 0.002 0.005 Fe3+ - - - 0.013 - - - 0.003 - - Fe2+ 2.263 2.564 0.137 - 2.321 2.104 0.139 - 2.388 2.169 Mn 0.015 - 0.006 - 0.008 - 0.005 - 0.005 - Mg 0.396 1.968 0.142 - 0.480 2.204 0.152 - 0.475 1.750 Ca 0.204 - - 0.976 0.159 0.008 - 1.031 0.083 0.060 Na - 0.098 0.310 3.277 - 0.076 0.327 3.033 - 0.070 K - 1.761 1.718 0.023 - 1.569 1.653 0.021 - 1.180 Sum 7.944 15.501 14.006 20.151 7.982 15.239 13.988 20.039 7.985 14.715

XFe 0.7865 0.7821 0.8091 XMg 0.1375 0.1618 0.161 1 XCa 0.0707 0.0535 0.0280 XMn 0.0053 0.0027 0.0018 An 22.8 25.2 Ab 76.6 74.3 Or 0.5 0.5 XNa 0.1530 0.1653 Mg No. 14.9 43.4 50.8 17.1 51.2 52.2 16.6 44.7

1, K146 garnet 1/38 (RIM) Kyanite-garnet-biotite-muscovite-staurolite gneiss, Bardumal, Braldu River 2, K146 biotite 1/1 3, K146 muscovite 1/1 4, K146 plagioclase 1/1 5, K158 garnet 1/41 (RIM) Kyanite-garnet-biotite-muscovite gneiss, Panmah River 6, K158 biotite 6/1 7, K158 muscovite 1 8, K158 plagioclase 2/2 9, K159 garnet 1/38 (RIM) Kyanite-garnet-biotite-muscovite gneiss, Panmah River 10, K159 biotite 12/2 Analyses were done at the University of Leicester on a JEOL JXA-8600s Superprobe using an operating voltage of 15 kV, probe current of 3 X 10-'A and a beam diameter of 20 pm.

145 f 5 Ma (R. Parrish, pers. comm. 1988). 40Ar-39Arstep Metapelites with andalusite-staurolite-garnet-biotite- heating analyses on two amphiboles from different samples quartz-muscovite-plagioclase assemblages (M1 in Figs 4 & of biotite-hornblende diorite of the Hushe complex show 5) are regionally associated with these hornblende-bearing plateauages of 150 and 160 Maand integrated ages of granodiorites and monzogranites of the Hushe complex, and 205 * 1.4 Ma and 203 f 0.6 Ma (D. C. Rex, pers. comm., we interprettheir age of metamorphism tobe roughly 1988). Theseages, together with twoK-Ar hornblende synchronous with the Jurassicmagmatism. This is the (T "C-500 "C) ages of 208 f 8 Ma and 163 f 7 Ma previously earliestmetamorphic phase in theKarakoram, and the reported by Searle et al. (1989) support the evidence for a ubiquitous presence of andalusite infers a widespread, low Jurassic age of metamorphism. pressure (<3.75 kbar) thermal event, prior to collision of

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10 Indiawith Asia. M1 metamorphicassemblages are overprinted by higher grade sillimanite belonging to the M2 09 stage.

08 L M2: high-P, high-T Barrovian metamorphism (50- 07 37 c Mu)

c0 The collision of India with theKarakoram-Lhasa blocks o 06 I marked the closing of Tethys along the Indus suture zone, 05 and is relatively well constrained in the Ladakh-South Tibet -a, region, east of Nanga Parbat at approximately 50 Ma, based 2 0.4 onpalaeomagnetic, sedimentological and structural evidence (Searle 1983, 1986; Patriat & Achache 1984; Searle et 03 al. 1987,1988). Following collision, crustalthickening and regionalmetamorphism occurred both south of theIndus 02 suturezone along theHigh Himalaya, and north of that 1’1 zone in the Karakoram(Fig. 1). 01 The dominant regional Barrovian metamorphism in the centralKarakoram is constrained in agebeingas 00 500 1500 250015000 500 3500 4500 5500 pre-37 f 0.8Ma,the age of theMango Gusar two-mica granite (U-Pb zircon data; R. Parrish, pers. comm. 1988), which cross-cuts the syn-metamorphic foliation (D2, Fig. 7) 10 south of the Braldu River(Figs 2 & 3). There is no accurate 09 lower (older) constraint on the age of M2 metamorphism. It could be as old as late Cretaceous-Palaeogene, although we 08 believe it is more likely to havefollowed afterthe main India-Asia collision at c. 50 Ma. An earlierphase of c o7 deformation,D1, is seen along theBraldu gorge near .-0 c Chakpo, west of Askole, where the D2 fabric, defined by 2 0.6 L aligned sillimanite needles and micas, cuts obliquely across LL 05 earliersmall scale folds. In this area sillimanite-garnet- -a, muscovite-biotite-plagioclase-quartz assemblages are pre- 0 > 04 sent withsillimanite-K-feldspar replacing muscovite at temperatures above 620-650 “C,but without partial melting, 0.3 indicating thatpressures were lower than the 3.5 kbar (350 MPa). 02 The M2 metamorphism is clearlysynchronous in time with D2 deformationas aluminosilicates and mica have 01 grownalong foliation planes. Stretching lineations consis- tently plunge to the east or ESE, away from the high-grade 00 area around the Braldu River (Fig. 8). Foliation, folds and k0 1000 20003000 4000 5000 6000 7000 8000 stretchinglineations in theMango Gusar valley south of Askole (Fig. 3) are cut by the unaltered, isotropic, two-mica granite of Mango Gusar. The Chingkang-la granite pluton (Fig. 3) alsocuts thesurrounding syn-metamorphic foliation,and has a K-Ar hornblende age of 34 f 1Ma (Searle et al. 1989) providing a minimum age constraint on M2 and D2. 07- C An area of low-grade metamorphism is centred around .-0 c the upper Chingkang valley(Fig. 4). Chloritoid-chlorite and o 06- I chlorite-biotiteassemblages are associatedwith relatively 05- unmetamorphosedconglomerates, orthoquartzites and -a, shales. Metamorphicisograds areright-way-up, with 2 04- assemblages showing increasing P-T with structural depth. Mapping of isogradsalso establishes that they are folded 03 - and that deepercrustal levels are exposed in the NW around the Braldu gorge, the Biafo glacier, and Panmah valley than in the SE aroundthe Chingkang and Alingglacier areas (Fig. 3).

Fig. 6. Garnet zoning profiles from the three samples of kyanite-grade gneisses used in the thermobarometric calculations in Table 1 and 3.

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Table 2. Equilibrium reactions used for geothermobarometry

GB Ferry GB &(1978),Hodges Spear & Spear (1982) Mg,AI2Si,O,, + KFe3AISi30,,(OH), = Fe,AI,Si,O,, + KMg,AlSi,O,,(OH), garnet biotite garnet garnet biotite GAQP Newton & Haselton (1981) Ca,AI,Si,O,, + 2Al,SiO, + SiO, = SCaAI,SiO, garnetsillimanite quartz plagioclase GM PB Hodges GMPB (1985) & Crowley Fe,AI,Si,O,, + Ca,AI,Si,O,, + KAl,Si,O,~,(OH),= 3CaA12Si,0, + KFe,AlSi30,,(0H), garnet garnet muscovite plagioclase biotiteplagioclase muscovite garnet garnet

Table 3. Geothermometry and barometry : selected data with averages and standard deuiatiom for each determination

Sample T1(H+S) pl(H+C) n P2(N+H) T2(H+S) n T3(H+S) n

K146 69 K146 1 8.2 8 696 8.6 6 f19 f0.6 f19 f 20 f0.7 K158 646 7.0 8 644 6.6 6 f2 fO.l f2 fO.l K159 701 4 f 28

Tin "C, P in kbar Tl,,,,,, Pl(,+,):simultaneous solution of Hodges & Spear (1982) gt/biotite thermometer with Hodges & Crowley(1985) gt/biotite/muscovite/plagioclase barometer. T2(,+s), P2(N+H): simultaneoussolution of Hodges & Spear(1982) gt/biotitethermometer with Newton & Haselton(1981) gt/plagioclase/kyanite/qtz barometer. T3(H+s):Hodges & Spear (1982) gt/biotite thermometer at 7.5 kbar. n, number of determinations.

themetamorphic rocks tothe south. One of thesedykes M3:high-']: contact metamorphism aroundthe Baltoro from the Braldu river east of the Biafo glacier junction (Fig. plutonic unit. (25-21 Mu) 3) has a precise U-Pb zircon age of 21.5 f 0.8 Ma which is Thisunit forms the bulk of thegranitic rocks along the contemporaneous with the main Baltoro plutonic emplace- Baltoro glaciertransect through the Karakoram batholith ment age (Parrish & Tirrul 1989). Field relationships show betweenPaiyu and Biange (Figs 3 & 4) and consists of that the majorityof leucogranitic dykes were emplaced after biotitemonzogranite, two-mica-garnet leucogranite and regional deformation and not before as previously thought swarms of leucograniticpegmatite-aplite dykes. Samples (Bertrand et al. 1988). collected nearUrdukas on the Baltoro glacieryielded a The contactmetamorphic aureole superimposed on precise U-Pb zircon age of 21 f 0.5 Ma and monazite ages Carboniferous black slates along the northern contact of the of 19-17 Ma (Parrish & Tirrul 1989). U-Pb monazite ages Baltoro granite at Mitre peak-Chogolisa (Fig. 2) was related of 25.5 f 0.8 Ma and 21.4 f 0.6 Ma have also been obtained to emplacement of the biotite monzogranite and two-mica- from garnet two-mica leucogranite samples from the Latok garnet leucogranite of the BPU. Along the Vigne-Baltoro peaks north of the Biafo glacier (Scharer et al. 1990). glaciers theaureole assemblageincludes andalusire- TheBaltoro granite has intrusive contacts along both cordierite-biotite-muscovite-chlorite-plagioclase-quartz, southand north margins and is undeformed, havingbeen indicating pressures of less than 3.5 kbar (M3b on Figs 4 & emplacedafter M2 regionalmetamorphism and D3 5). Adjacent to the granite contact, small knots of fibrolite deformation (Fig. 7). Theseobservations clearly disprove and biotite replace cordierite and muscovite, indicating that the claim by Bertrand et al. (1989) thatthe high-grade the highest aureole temperatures occurred at the contact. metamorphic event was 20-5 Ma in age. There is now no Along its southern margin at Paiyu, the Baltoro granite doubtthat the mainhigh-grade metamorphism occurred (Figs. 3 & 4) is intrusiveinto vertically foliatedgneisses prior to the 37 f 0.8 Ma Mango Gusar granite intrusion, and which haveassemblagean garnet-biotite-muscovite- that a later totally separate M3 contact metamorphic (low quartz-plagioclase-sillimanite andgranitic melt pods. pressure, high temperature) event around the margin of the Northwards from Bardumal the successive disappearance of Baltorogranite was approximately 25-18 Ma witha staurolite, then kyanite, and the appearance of sillimanite relativelylong, slow initial cooling lasting around 7 Ma andgranitic melt pods (M3a on Figs4 & 5) towards the during the earlyand middle Miocene (Fig. 7). Numerous southernmargin of thebatholith at Paiyu,indicates an garnet-biotiteleucogranitic dykes emanate from the increase in temperature of c. 75 "C (Fig. 5). Although this batholith, cross-cutting foliation and deformation fabrics in temperature increase approaching the Baltoro granite might

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CENTRAL KARAKORAM

IETAMORPHISh IEFORMATION

M41 Low P-T MKT culmination

Hlgh T M3 LOW P m

0 i

m m0 X

High T D2

D1 l- INDUS SUTURE ZONE COLLISION

Jurasslc

60 Fig. 7. Tertiary time chart for the METHOD CLOSING TEMPERATURE ('C) Baltoro Karakoram summarizing all the n 0 K-Ar bl. rnusc 300 - 350 radiometric data obtained on this project - Rb - Sr W.R. 500 (Searle et al. 1989; Parrish & Tirrul 1989; Scharer et al. 1990). Phases of 040Ar - 39Ar hbl. 600 metamorphism (M) and deformation + U - Pb Monazlte 650 (D) are tied in to the magmatic episodes X U - Pb Zircon 700 in the central Karakoram.

becoincidental andpart of the regionalBarrovian M2 metamorphism, it seems more likely that this temperature increase is a thermal effect of granite emplacment and thus part of M3 contact metamorphism, but superimposed on an alreadyhigh-grade kyanite-sillimanite grade terrain.We

W geometry the interpret therefore of thethe isograds on Baltoro cross-section (Fig. 9) as showing the superposition of 21 Ma M3 aureole isotherms on pre-37 Ma M2 isograds along the southern margin of the Baltoro granite.

M4: retrograde metamorphism and the Main Karakoram thrust Q. 8 Lower hemisphere stereographic projections showing orientationand plunge of stretchinglineations (a) south of the Localretrogressive metamorphism occurs in thesouthern Karakorambatholith along the Braldu and Chingkang River and part of theKarakoram metamorphic complex and is thought (b) north of thebatholith along the north bank of theBaltoro to berelated to the post-Miocenethrusting along the Main glacierbetween Biange and Concordia to BroadPeak Base Camp. Karakoram thrust zone. Relict kyanite and staurolite of the

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SW NE MKT KBL

\ MAJORTHRUST \ \\ \\\DUCTILESHEAR ZONE

\ LAMPROPHYREDYKE

HORIZONTAL SCALE = VERTICAL

Fig. 9. Thermal model for the Baltoro Karakoramat c. 20 Ma. We assume approximately 10 km of erosion along the southern Karakoram since 20 Ma at approximately 0.5 mm a-' uplift + erosion = exhumation rate. Erosion levels are schematic. Major ductile shear zonesare shown bounding Indian, Kohistan and Karakoram crustat depth. M2 metamorphic isograds, are folded and offsetby the late movements on the MKT. KBL is the inferred Karakoram batholith lineament or culmination collapse normal fault which could explain P-Tconditions either side of the 21 Ma Baltoro granite.M3 thermal metamorphic aureole is shownas small circles surrounding the BPU. MMC is the Masherbrum complex which is partly migmatitic, partly leucogranitic injectioninto older mafic gneisses. Lamprophyre dykesare shown schematically around the Baltoro granite and must have originated from a sub-continental mantlewedge below Karakoram crust. The normal fault northeast of K2 is inferred from the structural positionof sediments of the Aghil Rangeto the north.

regional M2 metamorphism is preservedin muscovite- chloritoid gradepelites which have a pervasive foliation Cooling paths and temperature-time history parallel to thisthrust. Actinolite, chlorite, epidote and Figure10 compares empirical cooling histories forthe secondary muscovite are retrogressive mineral assemblages. Karakoram with theHigh Himalayan Nanga Parbat- The thrust is interpreted nowas a late-stage crustal-scale Haramosh massif on temperature-timea diagram. M2 faultwith several hangingwall splays (Fig. 3)which offset prograderegional metamorphism in theKarakoram regional pre-37 Ma M2 isograds. occurredprior to the 37 f 0.8 Ma cross-cuttingMango The thrust may also have been an important thrust fault Gusar granite (Fig. 3) and reached peak P-T conditions of earlier in the structural evolutionof the Karakoram. A small around 650-700 "C and 8-9 kbar (800-900 MPa.). 36-34Maepizonal pluton, the Chingkang-lagranite (Fig. Sillimanite-bearingassemblages along theBraldu River 3),crops out along the hangingwall of thethrust in the transect may haveexceeded 700 "C at lowerpressures Chingkang glacier area. This granite is interpreted to have without having crossed the biotite dehydration reaction. The beenemplaced atvery shallow depths, based on the M2 metamorphic rocks were cooled between 37 and 21 Ma presence of miaroliticcavities and a narrow low-pressure probably along 'erosion-controlled' uplift paths (England & metamorphicaureole. The thrustcould have been active Richardson 1977; England & Thompson 1984, 1985). priorto 36 Ma during initial crustalthickening along the U-Pb zircon and monazite systematics (Parrish & Tirrul southern margin of the Karakoram plate. Hanson (1986) has 1989; Scharerer al. 1990) indicate that melting temperatures mappedBarrovian metamorphic isograds within meta- for the Baltoro granite were higher than 7OO0C, probably pelites of theLadakh-Kohistan rocks on the footwall of, close to 750-800"C, andthat these temperatures were and SW of,the Main Karakoram thrust. The age of maintained for about 5-7 Ma. These time constraints show metamorphismin the Ladakh Range is debateablebut is thatthe Baltoro granite had relativelylong slow initial presumably also related to early crustal thickeningfollowing cooling(above 650"C), followed by faster, later cooling collision at c. 50 Ma. during late Miocene-Recent exhumation. 40Ar-39Ar biotite

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marginwere re-heated totemperatures around 650°C causing localized partial melting in the sillimanite-garnet- metapelites around Paiyu (Figs 3, 4, & 5). The subsequent and final part of the cooling curve must have mimicked the cooling curve of the Baltoro granite both terrains were 700 - as exhumed by simultaneousthrust-related uplift andrapid erosion. 600 - - Figure 10b shows a summary of the Ar-Ar and fission 0 track dating of Zeitler (1985) and Chamberlain et al. (1989) % 500 - forthe High Himalaya in the Nanga Parbat-Haramosh 2 massif (Fig. 2), together with their inferred cooling curve. 2 400 - Herethepeak of metamorphismoccurred around a, 25-20 Ma, coincident in timewith the generation of many of the High Himalayan crustal melt leucogranites. Fission track E 300 - l- 200 - data suggest extremely rapid recent uplift-exhumation rates (5-10 km relative to the Kohistan arc in the last 10 Ma) for 100 - the Nanga Parbat-Haramosh massif.

Of I I I I 1 Discussion on crustal structure 0 10 20 30 40 50 The present-day crustal structure of the western Himalaya NANGA PARBAT - HARAMOSH MASSIF east of Nanga Parbat, across the Kashmir-Zanskar-Ladakh region(Searle et al. 1988;Searle & Rex1989) andthe central Karakoram is shown in Fig. 11. Crustal thickness of 30-38 kmbeneath the Indian foreland (Kaila 1982) 6007001 increases to c. 65 km beneaththe HighHimalaya and .P Karakoram(Molnar & Chen 1983;Molnar 1984; 1988). v 500 Gravity profiles usinginformation from deep seismic soundings indicates that continental crustal thickness in the 400 2 KashmirHimalaya is approximately 58 kmincreasing to a, 71 km belowthe Karakoram and apoorly constrained 55 km Q 300 E, below the Pamir and Trans-Alai Ranges (Misra 1982). Fault c planesolutions andearthquakes focal depthsdemonstrate 200 thatthe Indian plate underthrusts the LesserHimalaya along the MainBoundary thrust (Seeber et al. 1981). 100 Gravity datafrom Kohistan seem to showarelatively shallowdetachment below theKohistan arc-batholith 01 I I I I 1 0 10 20 30 40 50 (Malinconico 1986). Searle et al. (1988) estimatedatotal shortening of Cooling Age (Ma.) 550 km across thewestern Himalaya in the Kashmir- Zanskar-Ladakhsegment, which has been taken up by Fig. 10. Temperature-time chart showing empirical cooling crustalstacking in theHimalaya and by underthrusting histories for (a) the Baltoro Karakoram and(b) High Himalaya in lower Indian crust northwards beneath the Ladakh Ranges the Nanga Parbat-Haramosh Range. M2, M3 and M4 refer to the as far as the southern partof the Karakoram. Our minimum metamorphic phases described in the text. Crosses are the U-Pb shortening estimates were 200 km for the High Himalaya zircon (T,= 700°C) and monazite(T, = 650°C) ages for the Baltoro granites (Parrish & Tirrul 1989), circles are the 40Ar-39Ar and K-Ar and 150 km forthe Zanskar shelf sediments,while the hornblende ages (Searle et al. 1989) and box at 300-350°C is the remaining 200 kmshortening could be accounted for by range of K-Ar ages for the Baltoro granite. Nanga Parbat data thrusting in the Lesser Himalaya and in the Indo-Pakistan (40Ar-39Ar and fission track) is from Zeitler (1985) and foreland. Chamberlain et al. (1989). Alongthe northern side of theKarakoram, gravity anomalies suggest that the Tarim Basin continental plate is underthrustingsouthwards beneath the Kun Lun and the ages with closing temperatures around 300°C are as young northern margin of the Karakoram (Lyon-Caen & Molnar as 8.9 and 8.4 Ma. 1984). Thus the Karakoram is an active convergent zone on The solid arrows and line on Fig. 10a shows the cooling both sides with intense crustal shortening occurring on both curvefor the Baltoro granite with a relatively long slow marginsand at depth. A negative isostatic anomaly exists cooling as it was emplaced into already hot country rocks, overthe Karakoram Range which hasbeen attributed to and later, rapid cooling-uplift-tectonicerosion rates from massive low-density granitic intrusions (Marussi 1964) or the late Miocene to the present day. The dashedline on Fig. 10a sinking of coldmaterial beneath the Karakoram dragging is the cooling curve of the Karakoram metamorphic rocks theMoho down(Molnar 1988). The Karakoram is also along thesouthern margin of theBaltoro granite. Initial underlain by azone of deepearthquakes at 70-300 km burial,heating and metamorphism from 50-37Ma was depths (Chen & Molnar 1983). Earthquakes at these depths followed by steadycooling until intrusion of theBaltoro imply very low geothermal gradients and a relatively cold plutonaround 21 Ma when the rocks along the southern upper mantle (Molnar 1988).

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ssw NNE

L...., n1m.t.w Ht#h Htm.l.y. Kar.koram *- *- < B.Itor0 K2 XarakQ0.m T'kl. str.tt* HIII. Pi, P.n,*l .?."'h., *h.lt #,."l,. V.I. Of Ladakh D~lhDIllh Broad Peak 'WA' Addt Ran#.' Y.k*n Kashmtr YC T ZSZ 1. 7 I 7 /L/UF

INDIANSHIELD

7o TARIH PLATE ?

Moho YET Main Boundan ThrYIt PPT P,, P."l.l Thrvrl /-

VKTVat. 01 Kashmir ThrYSl X, ,IoLv*ds UCT Y.," C."t,.l Tnrvrl 1 zsz 2.nrl.r sn..r zone MKT Mall K.r.kOr*m Thrust --.l / KBL K.rak0r.m 8.tholilh Lm.am.nl I s.dtmWlS K~T 12 Thrust g' gr..IsChlll 1.CI.I I/ a amphlbolll. taCi.S g D,.IWli,.S

IS2 Indus SulYr. Zone \\\ mrgm.m.r

ss2 Sh"0k sutur. zone m.ttin9 ZO". 0 10 50 100 9 I.Ycoqr."il., l::*;:"1 v/'' LcriII I Km Fig. 11. Schematic but scaled (vertical= horizontal) cross-section of the western Himalaya-central Karakoram orogenic belt depicting the overall crustal structure. Metamorphic isogradsin the High Himalaya and southern Karakoram are schematically shown after Searleet al. (1988,1989) and Searle& Rex (1989). Constraints on the depth to the Moho and lithospheric subduction, shown by the large arrows, are after Molnar (1984, 1988 and referencestherein). Geological constraints from the western Himalaya including the Kashmir, Zanskarand Ladakh Ranges are from Searle (1983, 1986) and Searleet al. (1988). High Himalayan leucogranites are shownin black, and the collision-related granites of the Karakoram are shownin small crosses.

It appearsthat in the western Himalaya-Karakoram (3) Identation of India; lateral extrustion of Tibet. This collision belt the convergence between India and Asia was model of horizontal plane strain and the eastward expulsion accommodated by three major processes. of Tibet along giant strike-slip fault systems was proposed (1) Crustal underthrusting. South of the Indus suture this by Molnar & Tapponnier (1975,1977) and Tapponnier & process was the dominant factor. Over 470 km of shortening Molnar (1977). The amount of eastward extrusion of Tibet was estimated by Coward & Butler (1985) in Indian plate remainsdebateable but at least some of it hasbeen uppercrustal rocks south of thesuture (or MainMantle accommodated by thrusting in theLongmen Shan thrust) in Pakistan. Coward & Butler (1985) and Coward et mountains along the eastern margin of Tibet (Dewey et al. al. (1987) arguedfor wholescale underthrusting of lower 1988). Indian crust northwards as far as the Pamirs. Geochemical, The Karakoram crustal plate is bounded along the west strontiumand oxygen isotopic studies of post-collisional by left-laterala strike-slip or transpressionalzone of granites on the Karakoram plate show that they have not distributed shear along the Hindu Kush and Chaman fault been derived from melted Indian plate gneisses (Rex et al. systems (Fig. 1). To the east it is bounded (or offset) by the 1988), but have resulted from melting of Karakoram plate right-lateral Karakoramstrike slip fault.This fault runs lowercrust gneisses as a result of homogeneouscrustal NW-SE immediately north of K2-Skiang Kangri (Fig. 2), shortening and thickening. also has a component of normal (down to the NE) motion. (2) Crustalshortening and thickening. Thiswas the It abruptly separates an areaof extremely rapid recent uplift dominantprocess operating north of the Indus-Shyok ratesalong the high Karakoram to the SW (including the sutures after collision both in the Karakoram (Searle et al. =-Broad Peak-Gasherbrum-Rimo-Saser Kangri Ranges) 1989) and south Tibet (Dewey et al. 1988). Approximately from an area of steady state gentle recentuplift in the Aghil doublenormal crustal thicknesses in bothregions must Rangeand Depsang Plains, more akin tothe Tibetan indicate 50% horizontal shortening: a figure which can easily plateau rates of uplift. The latter region, NE of the Siachen be accommodated by the deformation within the Karakoram glacier in India, is a flat high plateau similar to the Tibetan metamorphiccomplex. A model of viscousvertical plane plateau. strain shortening and thickening of Tibetan lithosphere was Theamount of offset along theKarakoram fault is proposed by Dewey & Burke (1973) andEngland & difficult tojudge because it is oblique tothe regional Houseman (1986, 1988). England & Searle (1986) proposed structuraltrend, but it couldonlybe 50-100 km that following collision, crustal thickening and deformation right-lateral. The left-lateral motion on the Altyn Tagh-Kun propagated southwards across the Himalaya and northwards Lun faults (Molnar et al. 1987) appears to support a certain across Tibetand the Karakoram simultaneously. This amount of eastwardsextrusion of Tibet away from the impliesa breakback thrust propagation sequencetowards indentor of the NW Himalaya(Nanga Parbat-Haramosh) thenorth in the region north of thesuture in the Indiancrust promontory afterits collision with the Karakoram. Karakoram plate.

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regional metamorphic isograds along 21 Ma contact aureole Summary and conclusions M3 isotherms adjacent to the Baltoro granite (Fig. 9). Extensive field studies and reconnaissance mapping of the The difference inmetamorphic conditions north and centralKarakoram mountains in northern Pakistan have south of the Baltoro granitesuggests either northward tilting shown thatthe Karakoram metamorphic complexhas of the Karakoram crust exposing deeper crustal levels in the undergoneapolyphase metamorphic and deformational south, or northward-directed culmination collapse along an history.Relating field structuraldata withradiometric inferrednormal fault along the present outcrop of the dating has enabled us to constrain the timing of deformation batholith, or a combination of these. The entire southern andmetamorphism related to magmatism. The proposed Karakoram was further uplifted along the Main Karakoram chronology is summarized on the time chart in Figure 7. thrustzone, which offsets regional M2 isograds,and is The pre-collision thermal history of thesouthern related to alate retrogressive M4 metamorphism.Major Karakoram is dominated low-pressure, aby static differences in thestructural and thermal history of the metamorphismreaching andalusite, staurolite and garnet Karakoramcrust are partly caused by the east-west gradewith little fabric development. The mostobvious differencesin the MainKarakoram thrust footwall rocks, source of heat for this M1 metamorphismis that provided by andrelative differences in theamount of underplating the melting andintrusion of widespread,Andean-type Indian crust either side of the Nanga Parbat syntaxis. tonalitic-granodioritic magmas. These pre-collision granites Recent uplift andtectonic erosion rates along the are widespreadin the northern Karakoram terrain (K2 Karakoram are extremely high and are as yet unquantified. orthogneiss), along the main Karakoram batholith (Hunza Present-day relief of the southern Karakoram ranges from plutonicunit), and south of the batholith(Hushe gneiss 3000-8000 m with approximately 50% of the surface area of complex). The geochemistry(Crawford 1988) and age the high Karakoramabove 5000m. This spectacular (Searle et al. 1989) of thesegranites suggest that they present-day relief andcontinuing high uplift ratesare formed in the Jurassic-Lower Cretaceous by partial melting maintained by activeconvergence and underplating along of the crust above a northward-dipping oceanic subduction both northand south margins.Subduction of Indian zonealong thesouthern continental margin of the continental crust and Indian mantle lithosphere northwards Karakoram plate. is occurringalong the southern margin of the Karakoram The post-collision thermal history of the Karakoram is (Lyon-Caen & Molnar 1983; Molnar 1988). Subduction of related to complex a deformation historyinvolving the Tarim Basin continental plate southwards beneath the polyphase deformation, intense crustal shorteningby ductile KunLun andnorthern marginof theKarakoram shearing,crustal thickening producing an increasedcon- (Lyon-Caen & Molnar 1984) hascreated a lithospheric- tinentalthermal gradient, uplift andtectonic erosion- scale'pop-up' structure, withits axis of maximum uplift exhumation.Peak metamorphic temperatures around aligned along the high Karakoram. 650-700 "C and pressures around 8.5 f 1 kbar (850 MPa) are recorded by preliminary thermobarometric studies of pelitic Four Karakoram expeditions have been funded by NERC research rocks from south of the Karakoram batholith. The age of grantGR3/4242, at theUniversity of Leicesterand in 1989 by this regional M2 metamorphism is constrained by a U-Pb NERC research grant GT5/F/89/GS6 at the University of Oxford, zirconcrystallization age of 37 f 0.8Maon the Mango theRoyal Society, British Mountaineering Council and the Mt. Gusar two-mica granitepluton, whichcross-cuts syn- Everest Foundation. We thank B. F. Windley, P. Hoffman, M. St metamorphic deformation fabrics. Partial melting is evident Onge, M. B. Crawford, A. J. Rex and M. Q. Jan for many fruitful along the Braldu river-Baltoroglacier area in sillimanite- discussions, and R. Wilson for help with the microprobe. The paper gradegneisses aroundPaiyu. The uplift-exhumation of benefitted greatly from commentsby P. Hoffman, P. Treloar and P. these deep crustalgneisses andtheir presence at the Molnar. present-day erosion surface above approximately 70 km of continental crust can only be explained by major disruption References of the thermal regime and structurally-controlled uplift. The BERTRAND,J. M. & DEBON,F. 1986. Evolution tectonique polyphase de la most obvious processes which could explain the uplift and chaine du Karakoram (Baltoro, Nord Pakistan). ComptesRendw des exhumation of these high grade gneisses are continued seances de I'Academie des Sciences, Park 303, 1611-1614. crustalthickening after final India-Asia collision, -, KIENAST,J. R. & PINAROON,J. L. 1988. Structure and metamorphism of the Karakoram gneisses in the Braldu-Baltoro valley (North Pakistan). northward-directed continental subduction, and southward- Geodinamica Acta, 2, 135-150. directedoverthrusting of continentalcrustal thrust sheets. BUTLER,R. W. H. & PRIOR, D. J. 1988. Tectonic controls on the uplift of the Ductilefabrics throughout the Karakoram metamorphic Nanga Parbat massif, Pakistan Himalaya. Nalure, 333, 247-250. complexindicate that south-directed thrusting was con- CHAMBERLAIN,C. P., ZEITLER,P. K. & JAN.M. Q. 1989. The dynamics of the comitantwith regional M2 metamorphism, and uplift was suture between the Kohistanisland arc and the Indian plate in the Himalaya of Pakistan. Journal of Metamorphic Geology, 7, 135-149. probably caused both by thrust ramping and underplating of CHENWANGPING & MOLNAR, P. 1983. Focal depths of intracontinental and subducting Kohistan-Ladakh or Indian crust. intraplate earthquakes and their implicationsfor the thermal and EarlyMiocene thermal metamorphism was associated mechanical properties of the lithosphere. Journal of Geophysical with large-scale crustal melting at 25-21 f 0.5 Ma around Research, 88, 4183-4214. COWARD,M. P. & BUTLER,R. H. W. 1985. Thrust tectonicsand the deep theBaltoro granite. Ahigh-temperature, low-pressure structure of the Pakistan Himalaya. Geology, 13, 417-420. contact metamorphic aureole showingmaximum pressures --, , ASIFKHAN, M. & KNIPE, R. J. 1987. The tectonic history of of 3.75 kbar (375 MPa) is present along the northern margin Kohistanand its implications for Himalayan structure. Journal of the Geological Society, London, 144, 317-391. of the Baltoro granite. A 75 "C increase in temperature in -- kyanite-sillimanite grade gneisses approaching the southern , , CHAMBERS,A. F., GRAHAM,R. H., IZATI,C. N., KHAN, M. A., KNIPE, R. J., PRIOR,D. J., TRELOAR,P. J. & WILLIAMS,M. P. 1988. margin of the Baltoro granite atPaiyu may be fortuitous but Folding and imbrication of the Indian crust during Himalayan collision. is interpreted as the thermal upwarping of pre-37 Ma M2 Philosophical Transactions Royal Sociery of London, A326, 89-116.

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CRAWFORD,M. B. 1988. Leucogranites of the NW Himalaya. PhD thesis, implications of geophyiscal data. Annual Reviews of Earth and Planetary University of Leicester. Science U,489-518. DESIO,A. 1964. Geological tentative map of the western Karakoram. Institute - 1988. A review of geophysical constraints on the deep structure of the of Geology, University of Milan. Tibetan Plateau, the Himalaya and the Karakoram, and their tectonic -& ZANEITIN,B. 1970. Geology of the Baltoro Basin E. J. BRILL, LEIDEN. implications. Philosophical Transactions of the Royal Society, London, DEWEY,J. F. & BURKE.K. C. A. 1973. Tibetan, Variscan and Precambrian A326,33-88. basement reactivation: products of a continental collision. Journal of - & CHENWANPING. 1983. Focal depths andfault plane solutions of Geology, 81, 683-692. earthquakes under the Tibetan Plateau. Journal of Geophysical -, SHACKLETON,R. M,, CHANG CHENGFA& SUN YIYIN.1988. The tectonic Research, 88, 1180-1%. evolution of the Tibetan Plateau. Philosophical Transactions of the Royal - & TAPPONNIER,P. 1975.Cenozoic tectonics of Asia:effects of a Society, London, A327,379-413. continental collision. Science, 189, 419-426. ENGLAND,P. C. & HOUSEMAN,G. A. 1986.Finite strain calculations of - & - 1977. Relation of the tectonics of eastern China to the continental deformation. 11: Application to the India-Asia plate collision. India-Eurasia collision: application of slip-line field theory to large-scale Journal of Geophysical Research, 91, 3664-3676. continental tectonics. Geology, 5, 212-216. - & - 1988. The mechanics of the Tibetan Plateau. Philosophical -, BURCHFIEL,B. C., LIANGKUANGYI & ZHAO ZIYUN.1987. Geomorphic Transactions of the Royal Society, London, A326, 301-320. evidence for active faulting in the AItyn Tagh and northern Tibet and -& RICHARDSON,S. W. 1977. The influence of erosion upon the mineral qualitative estimates of its contribution to the convergence of India and facies of rocks from different metamorphic environments. Journal of the Eurasia. Geology, 15, 249-253. Geological Society, London, W, 201-213. NEWTON, R. C. & HASELTON,H. T. 1981.Thermodynamics of the - & SEARLE,M. P. 1986. The Cretaceous-Tertiary deformation of the garnet-plagioclasc-AI, Si0,-quartz geobarometer In: NEWTON,R. C. et LhasaBlock and its implications for the crustal thickening of Tibet. al. (eds) Thermodynamics of minerak and meh. NewYork Tectonics, 5, 1-14. Springer-Verlag 131-147. - & THOMPSON,A.B. 1984. Pressure-Temperature-Time Paths of OWEN,L. A. 1988. Terraces, uplift and climate, the Karakorammountains, RegionalMetamorphism 1. Heat transfer during the evolution of Northern Pakistan. PhD thesis University of Leicester. Regions of Thickened Continental Crust. Journal of Petrology, 25, PARRISH,R. R. & TIRRUL,R. 1989.U-Pb age of the Baltoro granite, 894-928. northwestHimalaya, and implications for zircon inheritance and -& - 1986. Some thermal and tectonic models for crustal melting in monazite U-Pb systematics. Geology, 17, 107&1079. continental collision zones. In: COWARD,M. P. & RIES, A. (eds) PATRIAT,P. & ACHACHE,J. 1984. The chronology of the India-Eurasia Collision Tectonics. Geological Society, London, Special Publication, 19, collisionand implications for crustal shortening and the driving 83-94. mechanism of plates. Nature, 311, 615-621. FERRY,J. M. & SPEAR,F. S. 1978. Experimental calibration of the PUDSEY,C. J., COWARD,M. P,, LUFF,I. W., SHACKLETON,R. M., WINDLEY, partitioning of Fe and Mg between biotite and garnet. Contributions to B. F. & JAN.M. Q. 1985. Collision zone between the Kohistan arc and Mineralogy and Petrology, 66, 113-117. the Asian plate inNW Pakistan. Transactions of the Royal Society of FYFE,W. S. 1970.Some thoughts on granitic magmas. In: NEWALL,G. & Edinburgh, Earth Sciences, 76, 463-479. RAST,N. (eds) Mechanics of igneous intrusion. Geological Journal special REX, A. J., SEARLE,M. P,,TIRRUL, R.,CRAWFORD, M. B., PRIOR,D. J., issue, 2, 201-216. REX, D. C. & BARNICOAT,A. 1988. The geochemicaland tectonic GHENT,E. D. 1976. Plagioclase-garnet-A12Si0,-quartz: a potential evolution of the central Karakoram, NorthPakistan. Philosophical geobarometer-geothermometer. American Mineralogist. 61, 710-714. Transactions of the Royal Society, London, A326,229-255. HANSON,C. R. 1986. Bedrock geology of the Shigar valley area Skardu, SCHARER,U,, COPELAND, P., HARRISON, M.T. & SEARLE,M. P. 1990. Age, Northern Pakistan. MSC thesis. Dartmouth College, New Hampshire. origin and cooling history of post-collisional leucogranites in the Biafo HODGES,K. V. & CROWLEY,P. D. 1985. Error estimation andempirical glacier region, Karakoram Batholith, N. Pakistan; a U-Pb, 4Ar-39Ar, geothermobarometry forpelitic systems. American Mineralogist, 70, Sm-Nd,Rb-Sr and Pb-Pb isotope study. Journal of Geology, 98, 702-709. 233-251. -, & SILVERBERG,S. D. 1988. Thermal evolution of the Greater Himalaya, SEARLE,M. 1983.P. Stratigraphy structure and evolution of the Garhwal, India. Tectonics, 7, 583-600. Tibetan-Tethys zone in Zanskar and the Indus suture zone in the Ladakh - & SPEAR,F. S. 1982. Geothermometry, geobarometry, garnet closure Himalaya. Royal Society of Edinburgh Transactions, Earth Sciences, 73, temperatures, and the AI,SiO, triple point, Mt.Moosilauke N. H. 205-219. American Mineralogist, 67. 1118-1134. - 1986. Structural evolutionand sequence of thrusting in the High -, HUBBARD,M. S. & SILVERBERG,S. D. 1988a. Metamorphic constraints Himalayan, Tibetan-Tethys and Indus suture zones of Zanskar and on the thermal evolution of the central Himalayan orogen. Philosophical Ladakh, western Himalaya. Journal of Structural Geology, 8, 923-936. Transactions of the Royal Society, London, A326, 257-280. SEARLE,P.M. & FRYER, B. J. 1986. Garnet, tourmaline and -, LEFORT,P. & PECHERA. 1988b. Possible thermal buffering by crustal muscovite-bearing leucogranites, gneisses and migmatites of the Higher anatexisin collisional orogens: Thermobarometric evidencefrom the Himalaya from Zanskar, Kulu, Lahoul and Kashmir, In: COWARD,M. P. Nepalese Himalaya. Geology, 16, 707-710.4 & RIES,A. C. (eds) Collision Tectonics. GeologicalSociety, London, HUBBARD,M. S. & HARRISON,T. M. 1989. Ar-39Ar age constraints on Special Publication, 19, 185-201. deformation and metamorphism in the MCT zone and Tibetan Slab, -& REX,A. J. 1989. Thermal Model for the Zanskar Himalaya. Journal eastern Nepal Himalaya. Tectonics, 8, 865-880. of Metamorphic Geology, 7, 127-134. KAILA, K.L. 1982. Deep seismicsounding studies in India. Geophysical -, COOPER,D. J. W. & REX, A. J. 1988.Collision Tectonics of the Research Bulletin, U)! 309-328. Ladakh-Zanskar Himalaya. Philosophical Transactions of the Royal LEFORT,P., CUNEY, M,, DENIEL, FRANCE-LANORD, C., C., SHEPPARD,S. M. Society of London, A326, 117-150. F., UPRETI,B. N. & VIDAL,P. 1987. Generation of the Himalayan -, REX, A. J., TIRRUL,R., REX, D. C. & BARNICOAT,A. 1989. leucogranites. Tectonophysics, W, 39-57. Metamorphic,magmatic and tectonic evolution of the Central LYON-CAEN,H. & MOLNAR,P. 1983. Constraints on the structure of the Karakoram in the Biafo-Baltoro-Husheregions of N. Pakistan. Himalaya from an analysis of gravity anomalies and a flexural model of Geological Society of America, special paper, 232, 47-73. the lithosphere. Journal of Geophysical Research, 88, 8171-91. ---, , , WINDLEY, B.F., ST ONGE, M,,& HOFFMAN,P. 1986. A -& -1984. Gravity anomalies and the structure of western Tibet and geologicalprofile across the Baltoro Karakoram Range, N. Pakistan. the southern Tarim Basin. Geophysical Research Letters, 11, 1251-1254. University of Peshawar Geological Bulletin, 19, 1-12. MALINCONICO,JR, L.L. 1986. The structure of the Kohistan terrane in -, WINDLEY,B. F, COWARD,M. P,, COOPER,D. J. W., REX,A. J., REX, Northern Pakistan, as inferred fromgravity data. Tectonophysics, W, D. C., LI TINGDONG,XUCHANG, X., JAN. M. Q., THAKUR,V. C. & 297-307. KUMAR,S. 1987. The closing of Tethys and the tectonics of the MARUSSI,A. 1964. Geophysics of the Karakoram. Italian expeditions to the Himalayas. Geological Society of America Bulletin, 98, 678-701. Karakoram (KZ) and Hindu Kush, Scientific Report 2,l. SEEBER,L., AWRUSTER,J. G. & QUI~TMEYER,R. C. 1981. Seismicity and MILLER,C. F. & STODDARD,E. F. 1981. The role of manganesein the continental subduction in the Himalayan arc. In: GUPTA,H. & DELANY, paragenesis of magmatic garnet: an example from the Old Woman-Piute F. (eds) Zagros-HinduKush-Himalaya, Geodynamic Evolution. Range, California. Journal of Geology, 89,233-246. American Geophysical Union Geodynamin series, 3,215-242. MISRA,D. C. 1982. Crustal structure anddynamics under Himalayaand TAPPONNIER,P. & MOLNAR,P. 1977. Active faulting and tectonics in China. Pamir ranges. Earth and Planetary Science Letters, 57, 415-420. Journal of Geophysical Research, 82, 2095-2930. MOLNAR,P. 1984. Structure and tectonics of the Himalaya: constraints and TRELOAR,P. J., BROUGHTON,R. D., WILLIAMS,M. P,, COWARD,M. P. &

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/148/1/65/4891155/gsjgs.148.1.0065.pdf by guest on 30 September 2021 82 M. P. SEARLE & R. TIRRUL

WINDLEY,B. F. 1989a. Deformation, metamorphismand imbrication of Ar-Ar geochronologyof the Himalayancollision in NW Pakistan: the Indian plate south of the Main Mantle Thrust, north Pakistan. constraints on the timing of suturing, deformation, metamorphism and Journal of Metamorphic Geology, l, 111-125. uplift. Tectonics, 8, 881-909. -, REX,D. C., GUISE, P. G., COWARD,M. P., SEARLE,M. P.,WINDLEY, ZEITLER,P. K., 1985, Coolinghistory of the NW Himalaya, Pakistan. B. F., PET~ERSON,M. G., JAN,M. Q. & LUFF,I. W. 1989b. K-AI and Tectonics, 4, 127-151.

Received 12 June 1989; revised typescript accepted 4 April 1990.

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