Geological evolution of the Karakoram Ranges - CiteSeerX

02 SEARLE txt 147-160_GEOLOGIA 29/07/11 09.22 Pagina 147

Queste bozze, corrette debbono essere restituite immediatamente alla Segreteria della Società Geologica Italiana c/o Dipartimento di Scienze della Terra Piazzale Aldo Moro, 5 – 00185 ROMA

00 – SEARLE

Ital.J.Geosci. (Boll.Soc.Geol.It.), Vol. 130, No. 2 (2011), pp. 147-159, 5 figs. (DOI: 10.3301/IJG.2011.08) © Società Geologica Italiana, Roma 2011

Geological evolution of the Karakoram Ranges MICHAEL P. SEARLE (*)

ABSTRACT The Karakoram terrane in Northern Pakistan is geologically equivalent to the Qiangtang terrane of central Tibet but unlike Tibet shows extreme topographic relief, high uplift-exhumation and erosion rates and exposes deep crustal metamorphic and igneous rocks of the lower crust. The Karakoram terrane has been above sea-level since Early to Mid-Cretaceous, the age of the youngest marine sedimentary rocks. During the Late Jurassic to Mid-Cretaceous northdipping subduction of Tethyan oceanic lithosphere beneath the Karakoram resulted in widespread Andean I-type granite-granodiorite magmatism (Hunza, Hushe plutonic units) with possible concomitant low-pressure andalusite-sillimanite grade Buchan-type metamorphism. The Kohistan intra-oceanic island arc developed above a second north-dipping subduction zone to the south during the AptianEocene (~120-45 Ma) and was accreted to the southern margin of Asia concomitantly with obduction onto India during the Late Cretaceous. Final closure of Neo-Tethys and collision of India with Asia occurred at ca. 50 Ma. Continuous crustal thickening occurred across the southern Karakoram resulting in widespread regional Barrovian facies kyanite- and sillimanite-grade metamorphism with U-Pb zircon and monazite ages ranging from Latest Cretaceous (~63 Ma) to Pliocene times (4-3 Ma). Crustal melting resulted in intrusion of several sets of leucogranite dykes (Hunza dykes) at ~50 Ma and 35 Ma. The major phase of crustal melting in the Baltoro plutonic unit spanned at least 13 million years from 26-13 Ma (Late Oligocene to Mid-Miocene), and resulted in intrusion of a large-scale biotite monzogranite to garnet two-mica leucogranite intrusion of batholithic proportion. Some extra heat contribution from the upper mantle may have been required to account for the large size of the granite batholith and the lamprophyric dykes intruded around the margins. The youngest metamorphism and partial melting is known from the deep crustal metamorphic-migmatite gneiss domes (Dassu dome) of Pliocene age in the Baltoro region and the Late Miocene Sumayar leucogranite in Hunza. Geological and U-Pb geochronological data show that the Karakoram Range was a dynamic active mountain range with semi-continuous crustal thickening, deformation, metamorphism and partial melting for at least the last 63 million years and continues to this day.

KEY WORDS: Karakoram, Tectonics, U-Pb ages. INTRODUCTION

The Karakoram terrane comprises the southern part of the Asian side of the India-Asia collision zone and is the geological equivalent of the Central Tibet Qiangtang terrane (fig. 1). The Karakoram terrane is bounded to the north by the Palaeo-Tethyan Rushan-Psart suture zone in the central Pamir and along the south by the Neo-Tethyan Shyok suture separating the Karakoram from the Kohistan-Dras island arc and the intrusive I-type granites of the Kohistan-Ladakh-Gangdese continental arc in the Lhasa block and Kohistan arc to the south. The first major geological studies of the Karakoram ranges of

(*) Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK. E-mail: [email protected]

northern Pakistan were made during a series of Italian expeditions during the 1950s including the 1954 first ascent of K2. The geological results of these expeditions were published in a series of papers and memoirs by DESIO (1964), DESIO & ZANETTIN (1970) and DESIO & MARTINA (1972). The geology of the Karakoram terrane can be broadly divided into (a) a northern dominantly sedimentary terrain intruded by diorite-granodiorite intrusions (eg: Broad Peak diorites) with uplifted lower crustal metamorphic core complexes (eg: K2 gneiss), (b) the Karakoram batholith a ~700 km long granite batholith containing both pre-collisional I-type intrusions (eg: Hunza unit) and post-collisional S-type crustal melt monzogranites-leucogranites (eg: Baltoro unit), and (c) the Southern Karakoram metamorphic complex composed of regional Barrovian facies kyanite- and sillimanite- grade gneisses with deep crustal migmatitic core complexes (eg: Dassu gneiss dome). The stratigraphy and structure of the northern Karakoram ranges have been studied in great detail (eg: GAETANI et alii, 1990, 1993, 1995; ZANCHI & GRITTI, 1996). These studies have culminated in the publication of the detailed regional geological map of the Northern Karakoram published in this issue (ZANCHI & GAETANI, 2011). The Southern Karakoram metamorphic complex has been mapped by SEARLE (1991) and studied by REX et alii (1988); SEARLE et alii (1989, 1992, 2010a); ALLEN & CHAMBERLAIN (1991); SEARLE & KHAN (1996); FRASER et alii (2001) and ROLLAND et alii (2001). Here, a structurally complex regional andalusite- kyanite- and sillimanitegrade metamorphic terrane extends across the Southern Karakoram, south of the Karakoram batholith and north of the Shyok suture zone, which has been reactivated by the Main Karakoram Thrust (MKT). Regional mapping of metamorphic isograds and P-T ranges of metamorphic reactions are reasonably well known in the southern Karakoram (SEARLE et alii, 1989; SEARLE & TIRRUL, 1991; ALLEN & CHAMBERLAIN, 1991; FRASER et alii, 2001; ROLLAND et alii, 2001; ROLLAND & PÊCHER, 2001). The Karakoram batholith is a composite granite batholith extending some 700 km along the Afghanistan, Pakistan and Ladakh border regions and includes both pre-collisional subduction-related I-type granites (eg: Hunza granodiorite, Hushe, K2 and Muztagh Tower orthogneisses) many of which have been subsequently deformed and metamorphosed, and the large-scale, post-collision Baltoro S-type granites a series of biotite monzogranites and two-mica leucogranites often also containing garnet and less commonly tourmaline (DEBON et alii, 1986, 1987; REX et alii, 1988; SEARLE et alii, 1989, 1992, 2010a; SEARLE, 1991; CRAWFORD & SEARLE, 1992, 1993; FRASER et alii, 2001; THOW, 2004).


148

M.P. SEARLE

°E

72

74

80

90 °N

lun Oytag suture Ku n e ur sut N.Pamir Muji an sh S.Pamir u R Kongur arte h Psutur Muztagh Alta Karakoram s sh u K hyok suture S K2 AN T S I Ka r a H G1 KO

40

TARIM PLATE

gh n Ta Alty

38

Hin d

u

KUNLUN

kax fault KF NP LA Siachen DA Peshawar KH Nubra Ka sh Indu H m s ir Tangtse su tu re Za ns PAKISTAN ka r Gar

H

A

SUTURES

LA

Kunlun (Triassic) Rushan Pshart-Jinsha (Jurassic)

YA

QIANGTANG TERRANE Bango n

200

300

400

500

g sutu

re

LHASA BLOCK

Kailas Ku m

34

32

ao

nG arh

wa

l

GA NG DE SE

Lhasa

INDIA

km

30

Yarlun g Tsangpo suture

NEPAL

Indus -Yarlung Tsangpo (~50 Ma) 100

36

Jinsha suture

Shyok-Bangong (Cretaceous)

0

Kunlun suture

SONGPAN - GANZE

KF

IM

t faul

BHUTAN Katmandu

Darjeeling

28

Main Boundary Thrust

Fig. 1 - Terrane map of Central Asia showing major plate boundaries, suture zones and faults. Box shows the area covered in fig. 2.

The collision of the Kohistan arc and Indian plate with Asia and the closing of Tethys along the Indus suture zone resulted in formation of the Himalaya and renewed crustal thickening and uplift along the southern margin of Asia (Hindu Kush-Karakoram-Lhasa and Qiangtang terranes of Tibet; SEARLE et alii, 1999). Previous studies have shown that there are important phases of pre-collision metamorphism, magmatism and deformation as well as Tertiary post-collision orogenic effects along the Asian side of the collision zone in the Hindu Kush (HILDEBRAND et alii, 1998, 2000, 2001), Karakoram (SEARLE et alii, 1989, 1990, 2010a; SEARLE, 1991; FRASER et alii, 2001) and the Pamir (ROBINSON et alii, 2004, 2007; SCHWAB et alii, 2004) ranges. Following the closure of Neo-Tethys and collision of the Indian and Asian plates during the Early Eocene (ca. 50 Ma; SEARLE et alii, 1988; ZHU et alii, 2005; GREEN et alii, 2008) regional kyanite and sillimanite-grade metamorphism culminating in Early Miocene partial melting and formation of crustal melt leucogranites occurred along both along the Indian plate in the Himalaya (eg: HODGES, 2000; SEARLE et alii, 1999, 2003, 2006; LAW et alii, 2006) and along the southern margin of the Asian plate in the Karakoram (SEARLE & TIRRUL, 1989; SEARLE, 1991; FRASER et alii, 2001). The

structural, metamorphic and magmatic effects of the India-Asia collision during and following the Early Eocene are well known along the Indian plate margin in the Himalaya but are less well known along the Asian margin. Whereas the south Asian margin in the Lhasa and Qiangtang terranes shows very few deep crustal metamorphic rocks because of the low exhumation and erosion rates, the southern Karakoram metamorphic complex holds the key to interpreting deep crustal structure, the crustal thickening and metamorphic results and the thermal evolution of the collision process. Teleseismic receiver functions have been used to map the Moho which lies at a depth of ~40 km beneath the Indian foreland and reaches depths of 75 km beneath the Karakoram Fault (RAI et alii, 2006), 80 km beneath the Lhasa and Qiangtang terranes (SCHULTE-PELKUM et alii, 2005) and a depth of ~90 km north of the Bangong Suture in western Tibet (WITTLINGER et alii, 2004). Surface wave tomography shows that the whole of Tibet is underlain by high-wave speed upper mantle material to a depth of 225250 km (PREISTLEY et alii, 2008). Fig. 2 shows a lithospheric scale cross-section across the western Himalaya and central Karakoram (SEARLE et alii, 2010a) illustrating the overall crustal structure.

149

Fig.3a

GEOLOGICAL EVOLUTION OF THE KARAKORAM RANGES

25 21

KMC

35°45’

28

Dassu dome 3.5 5.4

Bullah dome

22

. Fig

3c

26.4

35°30’

Ladakh batholith

75°30’

76°

KARAKORAM METAMORPHIC COMPLEX

BALTORO GRANITES

Panmah ultramafic mafic unit

MG

Mango Gusar

CL Chinkiang-la granites

Aghil Fm. (Tr. - Jr.)

Askole amphibolite

Baltoro granites

Shaksgam Fm. (Pm.)

Ganschen paragneiss Bullah orthogneiss

Masherbrum gneiss

Baltoro Fm. (Carb.) black shales

Kande orthogneiss Hushe orthogneiss

Doksam schists Mitre aureole

K

Dumordu marble

H

Muztagh Tower gneiss

Dassu gneiss 28

NORTHERN KARAKORAM

U-Pb age

0

5

10 km

Fig. 2a.

Broad Peak quartzite diorite

M.P. SEARLE

Fig. 3b

150

115 -120

c

.3 Fig

K2

Baltoro granite 13.9 13

Broad Peak 35°45’

13 16.5

21

19.8

17.6

35°30’

KMC

40 21.2 76°15’

Baltoro glacier

a

Trango Towers

76°30’

Paiyu

Nameless Spire

K2 Muztagh

b Fig. 2b.

Broad Peak

G asherbrum IV Lobsang



KARAKORAM METAMORPHIC COMPLEX

The Karakoram Metamorphic Complex (KMC) is composed of interlayered kyanite- or sillimanite-grade pelites with garnet+clinopyroxene amphibolites, impure dolomitic marbles containing diopside, phlogopite, quartz and corundum (ruby), and amphibolites containing hornblende, plagioclase, garnet and biotite (BERTRAND et alii, 1988; SEARLE et alii, 1989; SEARLE & TIRRUL, 1991; SEARLE, 1991; ALLEN & CHAMBERLAIN, 1991; ROLLAND et alii, 2001; ROLLAND & PÊCHER, 2001). In the Baltoro region SEARLE & TIRRUL (1991) originally defined the major metamorphic and deformation phases based on field relationships and preliminary U-Pb age dating. Precollision LP-HT andalusite- staurolite- and garnet- grade metamorphism (M1) was associated with Andean-type intrusions of Hunza granodiorites and other I-type subduction-related plutons (eg: Muztagh Tower gneiss, K2 gneiss, Hushe gneiss; SEARLE et alii, 1989, 1990; SEARLE, 1991). M2 was the major post-collision kyanitesillimanite grade metamorphism associated with crustal thickening. Garnet-kyanite grade metapelites from the Braldu valley south of the Baltoro granite gave a mean 207 Pb/235U age of 28.0±0.5 Ma, interpreted as the age of peak metamorphism (FRASER et alii, 2001). M3 was a high temperature-low-pressure contact metamorphism around the intrusive Miocene Baltoro granites and M4 was a Pliocene-Pleistocene high temperature sillimaniteK-feldspar grade metamorphic-migmatitic event recorded in the Dassu gneiss dome (SEARLE & TIRRUL, 1991). FRASER et alii (2001) refined the phases of metamorphism and magmatism in the Hunza Karakoram using ten new U-Pb (TIMS and LA-ICP-MC-MS) ages. They proposed that M1 metamorphism lasted over 20 million years from ~63 Ma to at least 44 Ma on the basis of U-Pb monazite ages of sillimanite and kyanite-bearing pelitic rocks south of the batholith. SEARLE et alii (2010a) published new U-Pb age data from the Baltoro region and refined the thermal evolution and chronology of the KMC and the Baltoro batholith with more precision (fig. 2). M1 pre-collisional (IndiaAsia) metamorphism is now defined as lasting at least from ~63 Ma to ~50 Ma, prior to the closing of NeoTethys and final India-Asia collision (GREEN et alii, 2008), based on the U-Pb ages of Hunza sillimanite gneisses and monazite ages from the Hunza migmatite (FRASER et alii, 2001). M2 post-collisional metamorphism is defined as the ages of the sillimanite- and kyanite-grade metamorphism in Hunza and the Hushe granite in the east. It is possible that the 40±1 Ma zircon age of the Hushe granite records this phase of high-temperature metamorphism rather than original crystallization of the granite. Deformation fabrics associated with this phase ended with intrusion of the late ‘set 2’ Hunza dykes which cross-cut fabrics and early granite dykes-sills (FRASER et alii, 2001).

151

It is possible that M1 and M2 was one continuous event in Hunza lasting >27 million years. The major kyanite- and sillimanite-grade metamorphism in the Baltoro region was Oligocene-early Miocene age (>28-22 Ma). Metamorphic rocks of the southern Karakoram are cut by numerous garnet two-mica leucogranite dykes of several different ages. M3 metamorphism is associated with intrusion of the massive Baltoro Plutonic unit during the Miocene (~26-13 Ma). A sillimanite- and andalusite-bearing contact metamorphic aureole (M3) has been superimposed onto the Carboniferous black shales along the northern margin of the batholith, whereas an increase in temperature towards the granite contact has been documented in the sillimanite gneisses along the southern margin (SEARLE & TIRRUL, 1991). M4 metamorphism has been dated in the structurally lower levels exposed in the Dassu gneiss dome in the southern Karakoram where sillimanite-K-feldspar migmatites and orthogneisses have Pliocene TIMS monazite and zircon ages. Hunza region In the Hunza region regional Barrovian facies metamorphic rocks occur across the region south of the Karakoram batholith. Deformation patterns are complex with multiple episodes of ductile deformation. FRASER et alii (2001) and SEARLE et alii (2010a) defined the chronology of metamorphism, magmatism and deformation based on U-Pb geochronology. U-Pb monazite dating of sillimanite-kyanite grade gneisses shows both precollision (82.9±6.1 Ma-61.9±4.7 Ma) and post-collision (44.0±2.0 Ma) ages (FRASER et alii, 2001). LA-PIMS dating of monazite from migmatite leucosomes indicated a 5653 Ma partial melting event and together these data suggest perhaps 25-20 million years of continuous high-grade amphibolite facies metamorphism along the southern margin of Asia prior to Indian plate collision (FRASER et alii, 2001). A major deformation phase was associated with pre-collisional metamorphism and crustal thickening concomitant with partial melting of the migmatites in sillimanite grade gneisses immediately south of the batholith in Hunza (D1). Post-collision metamorphism is also present in the Hunza region with highly graphitic garnet-staurolite mica schists giving an U-Pb concordant monazite age of 16.0±1.0 Ma (FRASER et alii, 2001). In general terms the P-T conditions show that higher grade rocks in the north have been progressively thrust southwards over lower-grade rocks creating an apparent inverted thermal gradient with a southward younging of peak metamorphism and deformation. The southern boundary of the KMC is the Main Karakoram Thrust a young (?Late Miocene-Pliocene) south-vergent thrust that places the entire Karakoram terrane over the unmetamorphosed rocks of the Shyok suture zone.

Fig. 2 - a) Geological map of the Baltoro Karakoram, North Pakistan, after SEARLE (1991) showing locations of all the dated U-Pb samples. Also shown are locations of the U-Pb samples of K2 gneiss (SEARLE et alii, 1990) and the previous U-Pb samples from the Baltoro glacier (PARRISH & TIRRUL, 1989; SEARLE et alii, 2010a). MKT-Main Karakoram Thrust. Snow Lake is the large icecap region at the head of the Braldu and Sim Gang glaciers (top left); b) Massive homogeneous leucogranite of the Baltoro batholith exposed in 2.5 km high cliffs on the Great Trango Tower (6452 m), Dunge glacier; c) Baltoro granite batholith, view towards north from Biale peak (6730 m); K2, Muztagh Tower, Broad Peak and the Gasherbrum Range are north of the batholith along the China-Xinjiang border. b) Northern margin of the Baltoro granite at Muztagh Tower (7284 m) on the Baltoro glacier.


152

M.P. SEARLE

Baltoro region In the Baltoro region prograde metamorphic isograds are mostly right way-up and have been folded and domed by later deformation (SEARLE et alii, 1989; SEARLE, 1991). Post-metamorphic recumbent folding and south-vergent thrusting have resulted in local inversion of metamorphic isograds (SEARLE & TIRRUL, 1991; ALLEN & CHAMBERLAIN, 1991). Mapping of sillimanite, kyanite, andalusite and staurolite isograds has defined a central low-grade area around the Chingkiang glacier south of Masherbrum. Major leucogranite dyke swarms intruding the KMC to the south are almost certainly related to the Baltoro batholith. Metamorphic isograds are folded over deeper level crustal domes (eg: Dassu and Bullah domes) with structural ‘basins’ of low-grade andalusite and chloritoid-bearing slates around Mango Gusar peak. These rocks are isoclinally folded on all scales and have undergone a complex and long-lived structural evolution. Kyanitegrade gneisses (8-9 kbars) around Askole occur in higher level thrust sheets above the lower pressure Dassu sillimanite gneisses (5-6 kbars) suggesting an inverted pressure gradient (ALLEN & CHAMBERLAIN, 1991) similar to the thermal structure in the Hunza Karakoram (FRASER et alii, 2001). P-T (THERMOCALC) results suggest metamorphic conditions of 650-800oC and 8-12 kbar for the kyanite grade gneisses from the Braldu and Panmah valleys (THOW, 2004). The Southern Karakoram metamorphic rocks have been intruded by numerous garnet two-mica leucogranite dykes that form a regional network and are geochemically and compositionally similar to the Baltoro batholith (REX et alii, 1988; SEARLE et alii, 1989; SEARLE & TIRRUL, 1991). The age of one dyke from near Bakhor Das has a U-Pb age of 24.7±3.6 Ma and constrains the minimum age of metamorphism and fabric formation in this region (SEARLE et alii, 2010a). Other dykes as yet undated cut the younger kyanite grade rocks in the Korophon region (Braldu river) and even younger set cut the Dassu gneiss dome. The leucogranite dykes cross-cut regional isoclinal folds in the KMC and almost certainly emanate from the Baltoro-K7 granites of similar composition and age (SEARLE et alii, 2010a). Isolated granites related to the main Baltoro granite batholith also outcrop within the KMC, notably the Mango Gusar granite (SEARLE & TIRRUL, 1991; SEARLE, 1991). Many of the surrounding metamorphic rocks have numerous large andalusite crystals in biotite-rich schist. These andalusite grade rocks remain undated, and their relationship to the kyanite grade gneisses along the Braldu-Baltoro valley is also uncertain. Lower crustal gneiss domes composed of ortho- and para-gneisses were subjected to high-temperature metamorphism and intruded by gem-bearing (tourmaline, aquamarine, topaz) pegmatite dykes (eg: Dassu dome). Whereas the Bullah dome is a folded series of KMC paragneisses, the Dassu dome is mostly Precambrian orthogneisses that have been subjected to sillimaniteK-feldspar grade metamorphism, partial melting and leucogranite intrusion during the Pliocene (SEARLE et alii, 1989, 2010a; SEARLE & TIRRUL, 1991; SEARLE, 1991; ALLEN & CHAMBERLAIN, 1991). The southern margin of the KMC is the NNE-dipping Main Karakoram Thrust (MKT), which places high-grade gneisses of the KMC over lowgrade or unmetamorphosed rocks of the Shyok suture

zone. There is evidence of structurally inverted metamorphic isograds immediately above the MKT with decreasing metamorphic grade towards the south (SEARLE et alii, 1989; ALLEN & CHAMBERLAIN, 1991; ROLLAND et alii, 2001). Nubra-Siachen-Pangong area, Ladakh Regional high-grade metamorphic rocks (stauroliteand sillimanite-grade pelites, clinopyroxene- and tremolite-bearing marbles, etc.) also occur in the northeastern Ladakh region of the Karakoram east of the Baltoro and Siachen glaciers. ROLLAND et alii (2008) proposed that granulite (~800oC, 5.5 kbar) – amphibolite (700-750oC, 4-5 kbar) to greenschist facies assemblages were developed within the Karakoram Fault zone during strike-slip shearing and suggested that the Tangtse two-mica garnet leucogranite was emplaced syn-kinematically with respect to shearing along the Karakoram Fault at the contact between a LT and the HT granulite facies. Sillimanite grade metamorphism in graphitic pelites of the Pangong metamorphic complex was superseded by the preserved P-T conditions of a Bt+Ms+St+Grt+Qtz+Fsp assemblage at 585-605°C and 6.05-7.25 kbar, equivalent to ca 20-25 km of burial (STREULE et alii, 2009). Laser ablation monazite U-Pb geochronology reveals that sillimanite grade metamorphism occurred at 108.0±0.6 Ma in rocks immediately adjacent to the Pangong strand of the Karakoram fault, implying that most metamorphic rocks along the Karakoram fault formed earlier than strike-slip shearing (STREULE et alii, 2009) and cannot have formed by shear heating during Miocene strike-slip faulting (LACASSIN et alii, 2004; VALLI et alii, 2007; LELOUP et alii, 2011). ROBINSON (2009a,b) showed by detailed mapping that multiple generations of Quaternary glacial and fluvial deposits at least 150 ka old, and older Pliocene loess deposits overlie all strands of the fault with no offset. These results are consistent with the apparent lack of neotectonic activity along the Nubra valley (BROWN et alii, 2002) and the general lack of seismicity along the fault today. Karakoram granite batholith North of the KMC is the 700 km long Karakoram granite batholith that runs along the Northwest frontier of Pakistan from Afghanistan to SW Tibet (SEARLE, 1991; SEARLE et alii, 1989, 1992; CRAWFORD & SEARLE, 1992; ZANCHI et alii, 2011). The Karakoram batholith includes pre-collision, Andean-type subduction-related granites (Hunza granodiorites, Hushe gneiss, K2 and Muztagh Tower gneisses) and post-collision crustal melt monzogranites and leucogranites (Baltoro Plutonic Unit; BPU). Crustal thickening and regional metamorphism culminated with large-scale melting of the lower crust and intrusion of the Baltoro granites (~25-13 Ma). These granites include the Latok suite (SCHÄRER et alii, 1990), the main Baltoro plutonic suite, the K7 granites and the Masherbrum leucogranites (PARRISH & TIRRUL, 1989; SEARLE et alii, 1992, 2010a). Contact metamorphism along the northern margin resulted in the andalusitecordierite- and sillimanite- grade thermal aureole exposed along the Mitre Peak and further west in the ShimshalHunza region (SEARLE et alii, 1989; SEARLE, 1991). The Baltoro granites intrude and cut folds and fabrics both to the south in the KMC and to the north in the Northern Karakoram terrane implying that much of the deforma-



tion and metamorphism was over and finished by 24-13 Ma. However younger (Late Miocene-Pliocene) deformation and plutonism occurs in the Hunza (Sumayar leucogranite; CRAWFORD & SEARLE, 1993), Baltoro (Dassu migmatites and numerous leucogranite dykes; SEARLE et alii, 1991, 2010a) and in the Pangong (Tangste leucogranites and associated dykes; SEARLE et alii, 1997; PHILLIPS & SEARLE, 2007) regions. Hunza region Most of the Karakoram batholith in the Hunza region is composed of pre-collision biotite- and hornblendebearing granites and granodiorites (LEFORT et alii, 1983; DEBON et alii, 1986, 1987; REX et alii, 1988; SEARLE et alii, 1989, 1992, 2010a; SEARLE, 1991; CRAWFORD & SEARLE, 1992) with a U-Pb zircon age of 105.7±0.5 Ma (FRASER et alii, 2001) intruded by a massive network of post-collision leucogranite dykes. These Hunza dykes have U-Pb zircon ages of ~50-52 Ma (set 1 dykes) and 35.0±1.0 Ma (set 2 dykes; FRASER et alii, 2001). The Late Eocene set 2 Hunza dykes cut all deformation and metamorphic fabrics in the KMC immediately south of the batholith defining an upper age limit of D2 deformation fabrics in the upper Hunza valley. Both peak metamorphism and deformation propagated southward with time to the staurolite-grade zone in the lower Hunza valley, and finally to the very young deformation fabrics associated with culmination along the hanging-wall of the Main Karakoram Thrust. Baltoro region In the Baltoro region of North Pakistan, most of the Karakoram batholith is composed of the Baltoro Plutonic unit (BPU) a co-magmatic suite of biotite monzogranites, K-feldspar megacrystic granites, garnet, two-mica leucogranites with a widespread leucogranite dyke network that intrude the KMC rocks to the south. The Baltoro biotite monzogranites are water-undersaturated, LILEenriched non-minimum melts related by fractional crystallization to the garnet, two-mica bearing leucogranites. The BPU has a mildly peraluminous geochemistry, lower initial 87Sr/86Sr isotope ratios (0.71-0.72) compared to Himalayan leucogranites (0.74-0.82) and is interpreted as derived from a dominantly metapelitic source in the lower crust as a result of biotite dehydration melting at temperatures of ca 850oC (REX et alii, 1988; CRAWFORD & WINDLEY, 1990; SEARLE, 1991; SEARLE et alii, 1992, 2010a). Nd and Sr isotopic ratios are consistent with a crustal origin of the leucogranites. Whereas Himalayan leucogranites have abundant tourmaline and are probably derived from a boron-rich black shale-pelitic protolith in the middle crust, the general lack of tourmaline in the Baltoro granites suggest a vapour-poor melt from a more lower crust source. The sheer volume of granite in the Baltoro and its higher melting temperature compared to the Himalaya suggests that some extra heat source is required. Whereas the Himalayan leucogranites resulted solely from an internal crustal heat source (SEARLE et alii, 2010b), it has been suggested that an input of heat from the mantle may be required to melt the Baltoro granites (REX et alii, 1988; CRAWFORD & WINDLEY, 1990; SEARLE et alii, 1992). The Baltoro granites are of batholithic proportion and crop out for >300 km along strike, at least from the Snow Lake-Biafo glacier region

153

to the Nubra-Siachen and Pangong Ranges in Ladakh (SEARLE, 1991; PHILLIPS, 2008; PHILLIPS et alii, 2004; SEARLE & PHILLIPS, 2007). THERMOCALC P-T calculations for Baltoro garnet two-mica leucogranites indicate emplacement conditions of 762±62oC and 7.1±2.5 kbar (Baltoro granite at Paiyu, Baltoro glacier) and 674±52oC and 6.6±2.6 kbar (Masherbrum leucogranite). KMC rocks south of the Baltoro granites consistently record peak pressures of around 7 kbar (THOW, 2004) suggesting that the Baltoro batholith was derived from depths more than ~25 km. The Baltoro granites must have been either intruded obliquely up through the crust or tectonically tilted to the north after emplacement in order to account for the pressure difference either side of the batholith. Along the northern margin the presence of andalusite in the contact aureole indicates maximum pressures of 3.5 kbar whereas along the southern margin pressures are in excess of 7 kbar across a horizontal width of only 15 km (SEARLE et alii, 2010a). SEARLE et alii (1989, 1992, 2010a), SEARLE (1991), FRASER et alii (2001) and THOW (2004) mapped out different magmatic phases within the Karakoram batholith in the Hispar, Biafo and Baltoro glaciers and Hushe valley areas. The Baltoro and K7 granites intrude through early deformed and metamorphosed orthogneisses (Masherbrum foliated gneisses, Hushe and Kande orthogneisses) and the KMC amphibolites, marbles and pelites. The K7 granite is a sub-horizontal 1-2 km thick sheet of K-feldspar megacrystic granite intruding the older foliated orthogneisses of the Masherbrum complex. The K7 granites form the more internal component of the batholith but are similar to the Baltoro granites in chemistry and age (SEARLE et alii, 1992, 2010a). Both Baltoro and K7 granites intrude sedimentary rocks of the Northern Karakoram terrane to the north. A spectacular contact metamorphic aureole is exposed along the flanks of the Baltoro glacier at Mitre peak where sillimanite and andalusite hornfels show an abrupt temperature gradient decreasing away from the granite contact into unmetamorphosed Carboniferous black slates. Andalusite + cordierite + biotite + muscovite + plagioclase + quartz assemblages are present with fibrolite sillimanite in a static contact metamorphic aureole adjacent to the granite contact. The contact aureole continues to the west along the northern margin of the batholith from the Baltoro glacier to the Muztagh Tower region and further west along the Shimshal valley to Hunza. The more internal central part of the Baltoro batholith is composed of massive monzogranites and two-mica garnet leucogranites exposed around the peaks of the Uli Biaho, Trango Towers and Shipton Spires (SEARLE, 1991; SEARLE et alii, 2010a). The Baltoro granites are mostly undeformed and have intrusive contacts along both northern and southern margins. The dated leucogranites of the Latok-Ogre Range (25-21 Ma; SCHÄRER et alii, 1990) are older than the granites of the Trango Towers region (21-13 Ma; PARRISH & TIRRUL, 1989; SEARLE et alii, 2010a) although they are laterally continuous. Whereas most of the central part of the Baltoro batholith is relatively homogeneous and contains no xenoliths, the southern margin frequently shows large blocks of KMC gneisses wholly enclosed as rafters in the leucogranite. At the highest structural levels subhorizontal layered sheets of leucogranite amalgamated from a sheeted sill complex to form the pale orangecoloured leucogranites around the higher structural levels


154

M.P. SEARLE

Fig. 3 - Schematic profiles approximately 30 km across showing the generalized anatomy of the Baltoro granite batholith and country rocks also showing the U-Th-Pb ages of each unit, after SEARLE et alii (2010a): a) across Latok-PanmahPaiyu are in the west, and b) is across the Mitre Peak-Masherbrum-Hushe area in the east.

in the upper 500-1000 meters of Biale and Masherbrum (SEARLE et alii, 2010a). The upper contact of the Baltoro granite is exposed around the Gondogoro-la where roof pendants of black shales with andalusite hornfels and low-grade gneisses are exposed above and around the granites. The K7 granite also has intrusive contacts with both the Masherbrum complex and the Carboniferous black shales of the Baltoro-Singhie Formation. The Miocene Baltoro granites extend east towards the high peaks surrounding the upper Siachen glacier, an area that is both politically (Kashmir conflict cease-fire line of control) and topographically difficult to access. Nubra-Siachen-Pangong areas, Ladakh In the Siachen glacier region, the right-lateral Karakoram strike-slip fault cuts across unmetamorphosed rocks of the Northern Karakoram terrane in the Teram KangriApsarasas Range, and cuts the Nubra-Siachen leucogranites, rocks which are geochemically and mineralogically the same as the Baltoro leucogranites (SEARLE et alii, 1992; PHILLIPS & SEARLE, 2007; SEARLE & PHILLIPS, 2007). The peaks of the upper Kondus valley and Saltoro

Kangri-K12 range are composed of the same Baltoro granites as around the Trango Towers and the Baltoro glacier region. Detailed field mapping in the upper Baltoro region (Pakistan) and the Nubra and Tangtse valleys (Ladakh) suggest that the Miocene Baltoro-type granites have been offset dextrally by only 17-25 km (SEARLE et alii, 2010a, fig. 9c). Another offset marker is the 120 km dextral offset of the antecedent Indus River (GAUDEMER et alii, 1989; SEARLE, 1996). This suggests that the dextral Karakoram fault, despite being one of the most important strike-slip faults bounding the Tibetan Plateau was responsible only for only a minor amount of eastward extrusion of the thickened crust (SEARLE, 1991, 1996; SEARLE & PHILLIPS, 2007; PHILLIPS & SEARLE, 2007). PHILLIPS (2004), PHILLIPS et alii (2004) and PHILLIPS & SEARLE (2007) carried out detailed mapping combined with numerous U-Pb ID-TIMS age dating of granitoids along the Karakoram shear zone. The Karakoram fault forms a single major strand along the Nubra valley but splits into two branches in the Darbuk-Tangtse region. The major fault is the southwestern branch, the Tangtse fault whereas the northeastern branch, the Pangong fault cuts through Karakoram terrane lithologies. In between


155


Metamorphism

HUNZA

MKT

D4

Middle

Sumayar leucogranite

z

Set 2

Early

z

Lamprophyre dyke

z

Hushe granite

BATHOLITH

D3

kyanite gneiss,Biafo Bullah orthogneiss

M2

Middle

z

Set 1

u

Hunza dykes

D2

EOCENE

Leucogranite dyke, Bakhor Das Latok Mango Gusar granite

Sillimanite, kyanite gneiss, Hasanabad

INDIAASIA COLLISION

z

Sillimanite gneiss Hunza

x u p a

zircon monazite xenotime uraninite pyrochlore allanite

D1

PALEOCENE CRETACEOUS

65

z z a

z Urdukas kyanite gneiss

Hunza migmatite

55

60

Paiyu K7 granite Latok

Hunza dyke

40

50

z z

p

Late

35

45

Masherbrum

BALTORO GRANITES

Staurolite schist Aliabad

contact metísm

Biale, Cathedral Trango Towers

Early

MIOCENE

u

OLIGOCENE

20

Dassu dome

M3

Late

Dassu gneiss

10

30

Pegmatite dyke, Dassu

M4

z

5

25

Deformation

PLEISTOCENE

PLIOCENE

15

Magmatism

BALTORO

regional metamorphism

0

Epoch

M1

Ma

Fig. 4 - Tertiary time chart showing all U-(Th)-Pb age data for the Hunza and Baltoro Karakoram. Age data is from PARRISH & TIRRUL (1989), SCHÄRER et alii (1990), FRASER et alii (2001) and SEARLE et alii (2010a). Inset shows the U-bearing minerals dated. Major phases of metamorphism, magmatism (solid black lines) and deformation are shown in right hand columns.


156

M.P. SEARLE

I N D IA N P L AT E - H IM A L AYA

Kishtwar window

Lesser Himala ya

lower crust

50 mantle lithosphere

MC

GHS

K2

Baltoro granite

Aghil Range Karakoram fault

NNE

Tarim basin

T

MHT

Tarim (Asian) lower crust

Moho

Cumulative erosion level ophiolites Tethyan shelf - upper crust Greater Himalayan sequence - middle crust Lesser Himalaya

Karakoram Range

Indus Shyok suture zone suture

Greater Himalaya

Indian Shield

100

Ladakh Range

MKT

MCT

MBT

Siwalik hills

km 0

Zanskar Range

High Himalaya Range

Lesser Himalaya

SSW

ASI AN PLATE - KAR AKOR AM

Asian plate upper crust ZSZ Zanskar Shear Zone MCT Main Central Thrust MBT Main Boundary Thrust

Karakoram metamorphic complex Baltoro granite Asian plate lower crust

Indian Shield - lower crust

Fig. 5 - Cross-section of the western Himalaya and Karakoram showing the large-scale structures, after SEARLE et alii (2010a). Dark shading represents high-grade metamorphic rocks, migmatites and crustal melt granites. Lighter shading represents mid-crustal Greater Himalayan sequence metamorphic rocks of the Indian plate. Precambrian basement of the Indian plate and Tarim plate is shown in dashed pattern. MHT-Main Himalayan Thrust; MKT-Main Karakoram Thrust.

the two faults the Pangong Range consists of transpressionally uplifted amphibolite facies gneisses, migmatites and several generations of leucogranite dykes (SEARLE, 1991; SEARLE et alii, 1998; PHILLIPS et alii, 2004; PHILLIPS & SEARLE, 2007). These authors concluded that ductile shearing along the Tangtse strand of the fault occurred between 15.68-13.73 Ma the U-Pb ages being determined from both early deformed leucogranites parallel to the mylonite foliation that shows a protomylonite fabric and from late leucogranite dykes that cross-cut the ductile mylonite fabric (PHILLIPS et alii, 2004). All metamorphic host rocks and almost all the leucogranites within the Ladakh sector of the Karakoram Fault mylonite zones have been affected by right-lateral strike-slip shear S-C fabrics imposed after metamorphic peak P-T conditions and after crystallization of the granites. Only a few minor leucogranite dykes cross-cut the ductile fabrics within the shear zone and their ages (13.73 Ma; PHILLIPS et alii, 2004) give an upper age limit of ductile deformation in this sector of the fault. Brittle faulting along the margins of the shear zone cut all rocks and prominent pseudotachylytes within the ductile shear zone indicate palaeoearthquake ruptures. Lamprophyre dykes Undeformed lamprophyre dykes cut all lithologies of the Northern Karakoram terrane including the K2 gneiss, the Gasherbrum diorites, the Hushe gneiss and older foliated granites. Although almost all post-collision granites within the Baltoro batholith are crustal melt granites, a

few isolated lamprophyre dykes intrude the batholith and the Northern Karakoram terrane. The Baltoro lamprophyres are fine-grained clinopyroxene- and biotite phyric minettes and amphibole-bearing vogesites with chilled margins (SEARLE et alii, 1989, 1992, 2010a). The Baltoro lamprophyre dykes are closely related to shoshonites and other potassic- or ultra-potassic dykes that occur regionally across southwestern Tibet (NORIN, 1946; MILLER et alii, 1999; CHAN et alii, 2009), north and central Tibet (CHUNG et alii, 2005) and sporadically across the Northern Karakoram (SEARLE et alii, 1989, 1992; POGNANTE, 1990). Their distribution is of regional extent and is not coincident with the Baltoro granite or the Karakoram Fault. A lamprophyre dyke from the South Masherbrum glacier has a U-Pb age of 32.8±1.3 Ma (SEARLE et alii, 2010a). These dykes cross-cut the older foliated Hushe orthogneiss and the younger Hushe granite (40±1 Ma) to the south, and also cross-cut all sedimentary units to the north, as well as the K2 gneiss. The U-Pb age from the South Masherbrum lamprophyre suggests it was intruded between 7-15 million years before the Baltoro granites (SEARLE et alii, 2010a). However, other lamprophyre dykes cut the Baltoro granites so there must be several phases of lamprophyric dyke intrusion spanning the Oligocene and Early Miocene. The U-Pb age data do not support the linkage between Karakoram high-temperature metamorphism and mantle-related magmatism as proposed by ROLLAND et alii (2001) and MAHÉO et alii (2002), but are compatible with the regional Late EoceneEarly Oligocene ultra-potassic intrusive event (40-30 Ma) recorded across vast tracts of north and central Tibet


157


(HACKER et alii, 2000; CHUNG et alii, 2005, 2009), the Lhasa block (CHUNG et alii, 2005; CHAN et alii, 2009) and the Pamir (DUCEA et alii, 2003). Discussion and Conclusions Geological data from the Karakoram suggests that it is continuous with the southern Pamir and extends east into the Qiangtang terrane of central Tibet. Subductionrelated pre-collision magmatism is known across the Karakoram (Hunza granodiorites, Hushe foliated orthogneiss, Muztagh Tower gneiss, K2 gneiss). These rocks date back to at least Middle Jurassic-Middle Cretaceous time when north-dipping oceanic subduction occurred beneath the Asian active margin (SEARLE et alii, 1990; FRASER et alii, 2001). These gneisses probably originated as subductionrelated pre-collision granites, subsequently metamorphosed during the Early Tertiary and re-melted during the Late Eocene. Final marine sedimentation occurred during the Early Cretaceous since when it has been subjected to active uplift, crustal thickening and erosion. The widespread Urdok conglomerates in the northern Karakoram suggest dominantly fluvial erosion from a major mountain source region. U-Pb age data from the K2 gneiss and the Pangong metamorphic complex suggest major crustal thickening and staurolite- sillimanite-grade metamorphism as old as Cretaceous (SEARLE et alii, 1990, 2010a; STREULE et alii, 2009). Using age data from both the Hunza and Baltoro regions, it is possible that regional high-grade metamorphism could have lasted continuously in these areas for at least 38 million years (from ~50-13 Ma) or as much as 50 million years (from ~63-13 Ma) along the southern margin of the Asian plate in the Karakoram. The Late Oligocene-Middle Miocene Baltoro granite batholith intruded through the thickened Asian crust at least from the Snow Lake region east to the Nubra-Siachen and Pangong Ranges of northern Ladakh, but not apparently to the west in Hunza. Mantle-derived lamprophyre dykes cut the Hushe gneisses south of the Baltoro granites and are also present north of the Baltoro granite batholith across the Northern Karakoram and western Tibet (NORIN, 1946; POGNANTE, 1990). A U-Pb age of 32.8±0.3 Ma from a lamprophyre dyke (SEARLE et alii, 2010a) shows that mantle melting during the Oligocene may have also contributed extra heat input into the crust during that time. The timing of lamprophyre injection in the Baltoro is similar to other potassic and ultra-potassic magmatism across the Northern Karakoram and Tibet (40-30 Ma; CHUNG et alii, 2005). However, there is likely a younger phase of lamprophyre dyke injection because similar lamprophyre dykes also cut the Early Miocene (21.2±0.2 Ma) K7 leucogranites. It is also quite likely than high-grade metamorphism and partial melting is ongoing today at depth, given the very young ages of migmatites and leucogranite dykes in the Dassu dome, and the known extreme thickness of crust under the Karakoram today (~75-85 km; WITTLINGER et alii, 2004; RAI et alii, 2006). It has been proposed that active high-pressure granulite or eclogite facies metamorphism occurs today at depth beneath the Karakoram (SEARLE et alii, 2010a), similar to the Miocene lower crustal xenoliths (felsic and mafic granlites and ultramafic restites) in mantle-derived ultrapotassic shoshonite dykes and lower crust derived adakite dykes and reported from beneath the Pamir (DUCEA et

alii, 2003; HACKER et alii, 2005), north Tibet (HACKER et alii, 2000) and southern Tibet (CHAN et alii, 2009). The main bulk of the Baltoro granite batholith comprises co-magmatic biotite monzogranites and garnet two-mica leucogranites with sheets of K-feldspar megacrystic granites. U-Pb monazite ages from the Paiyu, Trango Towers, Cathedral and K7 peaks range between 21-13 Ma (SEARLE et alii, 2010a). This age range is similar to most of the Himalayan leucogranites along the Indian plate south of the Indus suture zone. The youngest granites within the Baltoro batholith are the Trango Towers and Biale-Cathedral granites in the core of the batholith (~13-14 Ma). All these Baltoro-type granites are mainly undeformed, intrude and cut earlier folds and fabrics, have a contact metamorphic aureole and have been cut by the later dextral Karakoram strike-slip fault. Finally, the geological evolution of the Karakoram Range is likely a good analogue for the structural, metamorphic and magmatic processes operating at depth beneath the Tibetan Plateau along strike to the east (SEARLE et alii, 2011). ACKNOWLEDGEMENTS The analytical and field work was supported by the Natural Environment Research Council grant NER/K/S/2000/951. I would like to thank Brian Windley, Asif Khan, Qasim Jan, Rein Tirrul, Tony Rex, Mark Crawford, Andrew Thow, Randall Parrish and Dave Waters for discussions and comments. Discussions and reviews from Andrea Zanchi and Maurizio Gaetani are greatly appreciated.

REFERENCES ALLEN T. & CHAMBERLAIN C.P. (1991) - Metamorphic evidence for an inverted crustal section with constraints on the Main Karakoram thrust, Baltistan, northern Pakistan. Journal of Metamorphic Geology, 9, 403-418. BERTRAND J., KIENAST J. & PINARDON (1988) - Structure and metamorphism of the Karakoram gneisses in the Braldu-Baltoro valley (north Pakistan). Geodynamica Acta, 2, 135-150. BROWN E.T., BENDICK R., BOURLES D.L., GAUR V., MOLNAR P., RAISBECK G.M. & YIOU F. (2002) - Slip rates of the Karakoram Fault, Ladakh, India, determined using cosmic ray exposure dating of debris flows and moraines. Journal of Geophysical Research, 107(B9) 2192, doi:10.1029/2000JB000100. CHAN G.H.N., WATERS D.J., SEARLE M.P., AITCHISON J.C., HORSTWOOD M.S.A., CROWLEY Q., LO C.H. & CHAN J.S.L. (2009) Probing the basement of southern Tibet: evidence from crustal xenoliths entrained in a Miocene ultrapotassic dyke. Journal of the Geological Society London, 166, 45-52. CHUNG S.L., CHU M.F., ZHANG Y. et alii (2005) - Tibetan tectonic evolution inferred from spatial and temporal variations in post-collisional magmatism. Earth Science Reviews, 68, 173-196. CRAWFORD M.B. & SEARLE M.P. (1992) - Field relationships and geochemistry of pre-collisional (India-Asia) granitoid magmatism in the central Karakoram. Tectonophysics, 206, 171-192. CRAWFORD M.B. & SEARLE M.P. (1993) - Collision-related granitoid magmatism and crustal structure of the Hunza Karakroam, north Pakistan. In: TRELOAR P.J. & SEARLE M.P. (Eds.), Himalayan Tectonics. Geological Society, London Special Publication, 74, 53-68. CRAWFORD M.B. & WINDLEY B.F. (1990) - Leucogranites of the Himalaya/Karakoram: implications for magmatic evolution within collisional belts and the study of collision-related leucogranite petrogenesis. Journal of Volcanology and Geothermal Research, 44, 1-19. DEBON F., LEFORT P., DAUTEL D., SONET J. & ZIMMERMANN J.L. (1987) - Granites of western Karakoram and northern Kohistan (Pakistan): a composite Mid-Cretaceous to Upper Cenozoic magmatism. Lithos, 20, 19-40.


158

M.P. SEARLE

DEBON F., ZIMMERMANN J.L. & BERTRAND J.M. (1986) - Le granite du Baltoro (batholite axial du Karakorum, nord Pakistan): une intrusion subalcaline d’age Miocene Superieur. C.R. Acad. Sci. Paris II, 303, 463-468. DESIO A. (1964) - Geological tentative map of the western Karakoram, 1:500,000 scale. Institute of Geology, University of Milano. DESIO A. & ZANETTIN B. (1970) - Geology of the Baltoro basin. E.J. Brill, Leiden, 2, 308p. DESIO A. & MARTINA E. (1972) - Geology of the upper Hunza valley, Karakoram, West Pakistan. Boll. Gel. Soc. Ital., 91, 11-124. DUCEA M.N., LUTKOV MINAEV V.T. et alii (2003) - Building the Pamirs: the view from the underside. Geology, 31, 849-852. FRASER J.E., SEARLE M.P., PARRISH R.R. & NOBLE S.R. (2001) - Chronology of deformation, metamorphism, and magmatism in the southern Karakoram Mountains. Geological Society of America Bulletin, 113, 1443-1455. GAETANI M., GARZANTI E., JADOUL F., NICORA A., TINTORI A., PASINI M., KHAN K. & SABIN A. (1990) - The North Karakorum side of the Central Asia geopuzzle. Geological Society of America Bulletin, 102, 54-62. GAETANI M., JADOUL F., ERBA E. & GARZANTI E. (1993) - Jurassic and Cretaceous orogenic events in the North Karakoram: age constraints from sedimentary rocks. In: TRELOAR P.J. & SEARLE M.P. (Eds.), Himalayan Tectonics. Geological Society Special Publication, 74, 39-52. G AETANI M., A NGIOLINI L., G ARZANTI E., J ADOUL F., L EVEN E., NICORA A. & SCHUNNACH D. (1995) - Permian stratigraphy in Northern Karakoram, Pakistan. Riv. Ital. Palaeontol. Stratigr., 101 (2), 107-152. GAUDEMER Y., TAPPONNIER P. & TURCOTTE D.L. (1989) - River offsets across active strike-slip faults. Ann. Tecton., 3, 55-76. GREEN O.R., SEARLE M.P., CORFIELD R.I. & CORFIELD R.M. (2008) Cretaceous-Tertiary carbonate platform evolution and the age of the India-Asia collision along the Ladakh Himalaya (Northwest India). Journal of Geology, 116, 331-353. HACKER B., GNOS E., RATSCHBACHER L. et alii (2000) - Hot and dry deep crustal xenoliths from Tibet. Science, 287, 2463-2466. HACKER B., LUFFI R., LUTKOV V. et alii (2005) - Near ultrahigh pressure processing of continental crust: Miocene crustal xenoliths from the Pamir. Journal of Petrology, 46, 1661-1687. HILDEBRAND P.R., NOBLE S.R., SEARLE M.P., PARRISH R.R. & SHAKIRAULLAH (1998) - Tectonic significance of 24 Ma crustal melting in the eastern Hindu Kush, Pakistan. Geology, 26, 871-874. HILDEBRAND P.R., SEARLE M.P., SHAKIRUALLAH KHAN Z. & VAN HEST H.J. (2000) - Geological evolution of the Hindu Kush, NW Frontier Pakistan: active margin to continent-continent collision. In: KHAN M.A. et alii (Eds.), Tectonics of the Nanga Parbat syntaxis and Western Himalaya. Geological Society, London Special Publication, 170, 277-293. HILDEBRAND P.R., NOBLE S.R., SEARLE M.P., WATERS D.J. & PARRISH R.R. (2001) - Old origin for an active mountain range: Geology and geochronology of the eastern Hindu Kush, Pakistan. Geological Society of America Bulletin, 113, 625-639. HODGES K.V. (2000) - Tectonics of the Himalaya and southern Tibet from two perspectives. Geological Society of America Bulletin, 112, 324-350. LACASSIN R., VALLI F., ARNAUD N., LELOUP P.H., PAQUETE J.L., HAIBING L., TAPPONNIER P., CHEVALLIER M.L., GUILLOT S., MAHEO G., & ZHIQIN X. (2004) - Large-scale geometry, offset and kinematic evolution of the Karakoram Fault, Tibet. Earth and Planetary Science Letters, 219, 255-269. LAW R.D., SEARLE M.P. & GODIN L. (2006) - Channel Flow, Ductile Extrusion and Exhumation in Continental Collision Zones. Geological Society, London, Special Publication, 268, 620p. LEFORT P., MICHARD A., SONET J. & ZIMERMANN J.L. (1983) - Petrography, geochemistry and geochronology of some samples from the Karakoram axial batholith (Northern Pakistan). In: Shams F.A. (Ed.), Granites of the Himalaya, Karakoram and Hindu Kush, 377-387. LELOUP P.H., BOUTONNET E., DAVIS W.J. & HATTORI K. (2011) Long-lasting intracontinental strike-slip faulting: new evidence from the Karakoram shear zone in the Himalayas. Terra Nova, 23, 92-99.

MAHÉO G., GUILLOT S., BLICHERT-TOFT J., ROLLAND Y. & PÊCHER A. (2002) - A slab breakoff model for the Neogene thermal evolution of South Karakoram and South Tibet. Earth and Planetary Science Letters, 195, 45-58. NORIN E. (1946) - Geological Exploration in Western Tibet: Report on Sino-Swedish Expedition 29: Sweden Aktiebolaget Thule Stockholm, 214pp. PARRISH R.R. & TIRRUL R. (1989) - U-Pb age of the Baltoro granite, northwest Himalaya, and implications for zircon inheritance and monazite U-Pb systematics. Geology, 17, 1076-1079. PHILLIPS R.J. (2008) - Geological map of the Karakoram fault zone, Eastern Karakoram, Ladakh, NW Himalaya. Journal of Maps, v. 2008, 21-37. PHILLIPS R.J. & SEARLE M.P. (2007) - Macrostructural and microstructural architecture of the Karakoram fault: Relationship between magmatism and strike-slip faulting. Tectonics, 26, TC3017, doi:10.1029/2006TC00946. PHILLIPS R.J., PARRISH R.R. & SEARLE M.P. (2004) - Age constraints on ductile deformation and long-term slip rates along the Karakoram fault zone, Ladakh. Earth Planetary Science Letters, 226, 305-319. POGNANTE U. (1990) - Shoshonitic and ultra-potassic post-collisional dykes from northern Karakoram (Sinkiang, China). Lithos, 26, 305-316. PREISTLEY K., JACKSON J. & MCKENZIE D. (2008) - Lithospheric structure and deep earthquakes beneath India, the Himalaya and southern Tibet. Geophysical Journal International, 172, 345-362. RAI S., PREISTLEY K., GAUR V., MITRA S., SINGH M. & SEARLE M.P. (2006) - Configuration of the Indian Moho beneath the Northwest Himalaya and Ladakh. Geophysical Research Letters, 33, doi:10.1029/2006GL026076. REX A.J., SEARLE M.P., TIRRUL R., CRAWFORD M.B., PRIOR D.J. & REX D.C. (1988) - The geochemical and tectonic evolution of the central Karakoram, north Pakistan. Philosophical Transactions of the Royal Society, London, A326, 229-255. ROBINSON A.C. (2009a) - Geologic offsets across the Karakoram Fault: Implications for its role and terrane correlations in the western Himalayan-Tibetan orogen. Earth and Planetary Science Letters, 279, 123-130. ROBINSON A.C. (2009b) - Evidence against Quaternary slip on the northern Karakoram Fault suggests kinematic reorganization at the western end of the Himalayan-Tibetan orogen. Earth and Planetary Science Letters, 286, 15118-170. ROBINSON A.C., YIN A., MANNING C.E., HARRISON T.M., ZHANG S.H. & WANG X.F. (2004) - Tectonic evolution of the northeastern Pamir: constraints from the northern portion of Cenozoic Kongur extensional system. Geological Society of America Bulletin, 116, 953-974. ROBINSON A.C., YIN A., MANNING C.E., HARRISON T.M., ZHANG S.H. & WANG X.F. (2007) - Cenozoic evolution of the eastern Pamir: implications for strain accommodation mechanisms at the western end of the Himalayan-Tibetan orogen. Geological Society of America Bulletin, 7/8, 882-896. ROLLAND Y. & PÊCHER A. (2001) - The Pangong granulites of the Karakoram Fault (Western Tibet): vertical extrusion within a lithospheric scale fault? C.R. Acad. Sci. Paris, 332, 363-370. ROLLAND Y., MAHEO G., GUILLOT S. & PÊCHER A. (2001) - Tectonometamorphic evolution of the Karakoram Metamorphic Complex (Dassu-Askole area, NE Pakistan): exhumation of mid-crustal HT-MP gneisses in a convergent context. Journal of Metamorphic Geology, 19, 717-737. ROLLAND Y., MAHEO G., PÊCHER A. & VILLA I. (2008) - Syn-kinematic emplacement of the Pangong metamorphic and magmatic complex along the Karakoram Fault (north Ladakh). Journal of Asian Earth Sciences, doi:10.1016/j.aeaes.2008.03.009. SCHÄRER U., COPELAND P., HARRISON T.M. & SEARLE M.P. (1990) Age, cooling history and origin of postcollisional leucogranites in the Karakoram batholith, a multisystem isotope study. Journal of Geology, 98, 233-251. SCHULTE-PELKUM V., MONSALVE G. SHEEHAN A., PANDEY M.R., SAPKOTA S., BILHAM R. & WU F. (2005) - Imaging the Indian subcontinent beneath the Himalaya. Nature, 435, 1222-1225. S CHWAB M., R ATSCHBACHER L., S IEBEL W., M CWILLIAMS M., M INAEV V., L UTKOV V., C HEN F., S TANEK K., N ELSON B.,



FRISCH W. & WOODEN J.L. (2004) - Assembly of the Pamirs: age and origin of magmatic belts from the southern Tien Shan and southern Pamirs and their relation to Tibet. Tectonics, 23, doi:10.1029/2003TC001583. SEARLE M.P. (1991) - Geology and Tectonics of the Karakoram Mountains. Chichester, John Wiley and Sons, pp. 358. SEARLE M.P. (1996) - Geological evidence against large-scale pre-Holocene offsets along the Karakoram Fault: Implications for the limited extrusion of the Tibetan Plateau. Tectonics, 15, 171-186. SEARLE M.P. & KHAN M.A. (1996) - Geological Map of North Pakistan, scale 1:250,000. SEARLE M.P. & PHILLIPS R.J. (2007) - Relationships between rightlateral shear along the Karakoram Fault and metamorphism, magmatism, exhumation and uplift: evidence from the K2-Gasherbrum-Pangong Ranges, north Pakistan and Ladakh. Journal of the Geological Society, London, 164, 439-450. SEARLE M.P. & TIRRUL R. (1991) - Structural and thermal evolution of the Karakoram crust. Journal of the Geological Society, London, 148, 65-82. SEARLE M.P., COOPER D.J.W. & REX A.J. (1988) - Collision tectonics of the Ladakh-Zanskar Himalaya. Philosophical Transactions of the Royal Society of London, A326, 117-150. SEARLE M.P., CRAWFORD M.B. & REX A.J. (1992) - Field relations, geochemistry, origin and emplacement of the Baltoro granite, central Karakoram. Transactions of the Royal Society of Edinburgh, 83, 519-538. SEARLE M.P., PARRISH R.R., TIRRUL R. & REX D.C. (1990) - Age of crystallisation and cooling of the K2 gneiss in the Baltoro Karakoram. Journal of the Geological Society, London, 147, 603-606. SEARLE M.P., REX A.J., TIRRUL R., REX D.C. & BARNICOAT A. (1989) Metamorphic, magmatic and tectonic evolution of the Central Karakoram in the Biafo-Hushe regions of N. Pakistan. Geological Society of America, Special Paper, 232, 47-74. SEARLE M.P., WEINBERG R.F. & DUNLAP W.J. (1998) - Transpressional tectonics along the Karakoram fault zone, northern Ladakh: constraints on Tibetan extrusion, in Holdsworth R.E., Strachan R.A. & Dewey J.F. (eds.), Continental Transpressional and Transtensional Tectonics: Geological Society of London Special Publication, 135, 307-326. SEARLE M.P., KHAN A., FRASER J.E. & GOUGH S.J. (1999) - The tectonic evolution of the Kohistan-Karakoram collision belt along the Karakoram Highway transect, north Pakistan. Tectonics, 18, 929-949. SEARLE M.P., SIMPSON R.L., LAW R.D., PARRISH R.R. & WATERS D.J. (2003) - The structural geometry, metamorphic and magmatic evolution of the Everest massif, High Himalaya of Nepal-South Tibet. Journal of the Geological Society, London, 160, 345-366. SEARLE M.P., LAW R.D. & JESSUP M.J. (2006) - Crustal structure, restoration and evolution of the Greater Himalaya in Nepal-South

159

Tibet: implications for channel flow and ductile extrusion of the middle crust. In: LAW R.D., SEARLE M.P. & GODIN L. (Eds.), Channel Flow, Ductile Extrusion and Exhumation in Continental collision zones. Geological Society, London, Special Publication, 268, 355-378. SEARLE M.P., PARRISH R.R., THOW A.V., NOBLE S.R., PHILLIPS R.J. & WATERS D.J. (2010a) - Anatomy, age and evolution of a collisional mountain belt: the Baltoro granite batholith and Karakoram Metamorphic complex, Pakistani Karakoram. Journal of Geological Society, London, 167, 183-202, doi:10.1144/001676492009-043. SEARLE M.P., COTTLE J.M., STREULE M.J. & WATERS D.J. (2010b) Crustal melt granites and migmatites along the Himalaya: melt source, segregation, transport and granite emplacement mechanisms. Transactions of the Royal Society, Edinburgh, 100, 219-233, doi:10.1017/S175569100901617X. SEARLE M.P., ELLIOTT J.R., PHILLIPS R.J. & CHUNG S.L. (2011) Crustal-lithospheric structure, geological evolution and continental extrusion of Tibet. Journal of Geological Society, London, 168, 633-672, doi:10.1144/0016-76492010-139. STREULE M.J., PHILLIPS R.J., SEARLE M.P., WATERS D.J. & HORSTWOOD M.S.A. (2009) - Evolution and chronology of the Pangong Metamorphic Complex adjacent to the Karakoram fault, Ladakh: constraints form thermobarometry, metamorphic modeling and U-Pb geochronology. Journal of Geological Society, London, 166, 919-932. THOW A. (2004) - Tectonic, Metamorphic and Magmatic Evolution of the Central Karakoram Crust, Northern Pakistan [unpublished D.Phil thesis], University of Oxford. VALLI F., LELOUP P.H., PAQUETTE J-L., ARNAUD N., LI H., TAPPONNIER P., LACASIN R., GUILLOT. S., LIU S., DELOULE E., XU Z. & MAHEO G. (2008) - New U-Th/Pb constraints on timing of shearing and long-term slip rate on the Karakoram Fault. Tectonics, 27, doi:10.1029/2007TC002184. WITTLINGER G., VERGNE J., TAPPONNIER P. et alii (2004) - Teleseismic imaging of subducting lithosphere and Moho offsets beneath western Tibet. Earth and Planetary Science Letters, 221, 117-130. ZANCHI A. & GRITTI D. (1996) - Multistage structural evolution of Northern Karakoram (Hunza region, Pakistan). Tectonophysics, 260, 145-165. ZANCHI A. & GAETANI M. (2011) - The geology of the Karakoram range, Pakistan: the new 1:100,000 geological map of Central-Western Karakoram. Italian Journal of Geosciences, 130, 161-264. ZANCHI A., GAETANI M., SIRONI S., ANGIOLINI L. & DE AMICI M. (2011) - Geological Map of Central Western Karakoram, Pakistan. Italian Journal of Geosciences, 130, 161-262. ZHU B., KIDD W.S.F., ROWLEY D.B., CURRIE, B.S. & SHAIFIQUE N. (2005) - Age of initiation of the India-Asia collision in the eastcentral Himalayas. Journal of Geology, 113, 265-285.

Manuscript received 15 April 2011; accepted 6 June 2011; editorial responsability and handling by C. Faccenna.