Kohistan Island Arc Research Articles

Coronas and high‐P veins in metagabbros of the Kohistan island arc, northern Pakistan: evidence for crustal thickening during cooling

Abstract The E‐W‐trending Kohistan terrane in the NW Himalaya is a sandwich of a magmatic arc between the collided Karakoram (Asian) and Indian plates. The southern part of the Kohistan arc is principally made up of amphibolites derived from volcanic and plutonic rocks of Early Cretaceous age. Gabbroic relics in the amphibolites display calc‐alkaline character, and their mineralogy is similar to low‐P plutonic rocks reported from modern and ancient island arcs. The largest of these relics, occurring along the southern margin of the amphibolite belt near Khwaza Khela, is subcircular in outline and is about 1 km across. It consists of cumulate gabbros and related rocks displaying a record of cooling and crustal thickening. Primary olivine and anorthite reacted to produce coronas consisting of two pyroxenes +Mg‐Fe2+‐Al spinel ± tschermakitic hornblende at about 800° C, 5.5–7.5 kbar. This thermotectonic event is of regional extent and may be related to the overthrusting of the Karakoram plate onto the Kohistan arc some 85 Ma ago, or even earlier. Later the gabbros were locally traversed by veins containing high‐P assemblages: garnet, kyanite, zoisite, paragonite, oligoclase, calcite, scapolite and quartz ° Chlorite ° Corundum ± diopside. Formed in the range 510–600° C, and 10–12 kbar, these suggest further thickening and cooling of the crust before its uplift during the Tertiary. This paper presents microprobe data on the minerals, and discusses the tectonic implications of the coronitic and vein assemblages in the gabbros.

Differential Unroofing and Inherited Radiogenic Strontium in the Western Himalaya

In the western Himalaya, the Nanga ParbatHaramosh massif (NPHM; Indian continental crust) has been differentially exhumed relative to the adjacent Kohistan island arc terrane (Zeitler 1985). This study aims to constrain the detailed cooling history of the region by using both Rb-Sr and laser probe 4~ dating of muscovite and biotite from closely-spaced samples. Such a study should also help to evaluate the resolution of mineral ages in determining cooling rates in regions of rapid exhumation. Samples dated include granite sheets and their metamorphic country rocks, and come from the NPHM, the Kohistan arc, and from the intervening steeplydipping, high-grade shear zone.

Different P-T-t paths and leucogranite occurrences along the High Himalayan Crystallines : Implications for subduction and collision along the northern Indian margin

Different P-T-t paths and a heterogeneous distribution of Tertiary leucogranites are recorded along the High Himalayan Crystallines (HHC), in the collided Indian plate. Rapid subduction with eclogite formation during the early-Eocene is apparently restricted to part of the HHC of northern Pakistan, that is the zone of the first collision between the narrow NW Indian spur and the Kohistan island arc. In northern Pakistan rapid exhumation occurred along a cooling path and the crustally derived leucogranites of Tertiary age are rare or absent. East of the spur the delayed India-Eurasia collision occurred at lower rates of convergence and determined less rapid and deep continental subduction. Here exhumation of the HHC occurred at lower rates and at increasing T, giving a relatively warm thickened crust which underwent extensive anatexis during uplift in Oligocene-Miocene times. In eastern Himalayas, anatexis at medium- and low-P produced numerous cordierite-andalusite-bearing leucogranites which emplaced along extensional shear zones at high structural levels during the Miocene.

The tectonic implications of contrasting granite magmatism between the Kohistan island arc and the Nanga Parbat-Haramosh Massif, Pakistan Himalaya

Abstract In northern Pakistan, an integrated structural and geochemical study across the contact between the Cretaceous-Tertiary Kohistan island arc and Indian crust of the Nanga Parbat-Haramosh massif (NPHM) has identified strongly contrasting source regions contributing to post-50 Ma magmatism, which constrains the tectonic evolution of the region. Nd-model ages of Tertiary intrusives increase dramatically across the contact from c. 700 Ma in the Kohistan terrane to c. 2500 Ma in the NPHM. Following collision between the Indian continent and the Kohistan island arc terrane at c. 50 Ma, biotite granite sheets (Confluence granites) were emplaced into the Kohistan batholith at 50–30 Ma, followed by muscovite granite sheets (Parri granites) at c. 26 Ma. These are geochemically distinct, with the Confluence granites comprising a range of granitic compositions with high Sr and Ba abundances and the Parri granites forming granite sheets enriched in Rb. Undeformed granites in both suites have ( 87 Sr/ 86 Sr) i in the range 0.7045–0.7054 and ε Nd ( T ) of + 0.1 to +2.7, suggesting that both groups may be derived from juvenile arc sources. Detailed structural studies verify that the Kohistan granite sheets postdate the initial collision of the Kohistan terrane with the Indian continent, but are preor syntectonic with respect to continued underthrusting of the Indian continent below the Kohistan arc. These results, together with isotopic evidence from the undeformed granite sheets, suggest that significant underthrusting of northern Kohistan by the Indian continental crust could not have occurred until after 26 Ma. Along the western margin of the NPHM, deformed granite sheets show a marked increase in ( 87 Sr/ 86 Sr) i (0.7075–0.7784), with decreased ε Nd ( T ) (−13 to −26). These trends are thought to be due to a combination of sub-solidus fluid infiltration and assimilation of crustal material, with fluids or material derived from the adjacent, isotopically evolved NPHM crust. The NPHM (Indian continent) has been intruded by a series of tourmaline leucogranite dykes and plutons at 2–12 Ma. These intrusives have trace-element signatures consistent with generation by vapour-absent melting of a pelitic source. Sr-Nd isotope systematics indicate derivation from metasediments such as are currently exposed in the basement rocks of the massif, although melting at the present exposure level is generally precluded by subsolidus metamorphic grades in the country rock. Extreme heterogeneities in radiogenic Sr(( 87 Sr/ 86 Sr) i > 0.9) are observed in leucogranite dykes from the Liachar thrust zone, probably indicating subsolidus fluid infiltration. In general the NPHM leucogranites result from rapid exhumation of metasediments characterised by unusually high heat productivity.

Chemistry of chromite and associated phases from the Shangla ultramafic body in the Indus suture zone of Pakistan

Abstract The partly to wholly serpentinized ultramafic rocks of the Shangla area constitute the northern part of the Mingora ophiolitic melange which lies along the Main Mantle Thrust (MMT) zone between the Kohistan island arc and the Indo-Pakistan plate. These rocks contain disseminated grains and massive bodies of segregated chromite. Field, textural, and chemical characteristics of the segregated chromite closely resemble those of podiform chromite deposits. The chemical composition of the chromite varies considerably from sample to sample with modal proportion, from grain to grain in the domain of a thin section, and from core to margin within individual grains. In general, the segregated chromites are richer in Cr and Mg, but poorer in Fe, Al and Mn than the accessory chromites. The inter- and intragranular chemical variations are much greater and more common in the case of accessory chromite due probably to a greater degree of subsolidus re-equilibration with the associated abundant silicate matrix. The intragranular variation, probably a result of alteration, mostly involves enrichment in Fe, Mn (and Ti), and depletion in Al, Mg and Cr, leading to the development of ferritchromit and, rarely, magnetite along fractures and margins of individual grains. The studied rocks from the Shangla area display cumulus textures but contain abnormally magnesian olivine (Fo 96.0–98.5 ). The 100Cr/(Cr + Al) ratios of their chromite suggest a complex origin. These rocks may have formed under conditions transitional between those of arc and oceanic settings. The Shangla rocks underwent a low- to ?medium-grade metamorphism as indicated by the development of talc and antigorite. This prograde metamorphic process probably also accounted for the extreme chemical zoning of the Shangla chromite.

Evolution of the lower arc crust in Kohistan, N. Pakistan: temporal arc magmatism through early, mature and intra-arc rift stages

Abstract The middle to lower island-arc crust in southern Kohistan represents a 40 Ma life-span of subduction-related magmatism in an intraoceanic setting. The Kamila amphibolite belt comprises two varieties of metavolcanic amphibolites. One enriched in high-field strength (HFS) and heavy rare-earth (HRE) elements is transitional in character between N- and E-MORB with a minor subduction-related component, and represents the earliest arc basement in Kohistan. The other variety of the metavolcanic amphibolites, together with deformed and amphibolitised intrusive basic plutons, has a transitional tholeiitic to calc-alkaline nature marked by depleted HFS and HRE elements and a distinct negative anomaly for Nb, suggesting emplacement in the early to mature stages of arc growth. The subsequent magmatism in southern Kohistan took place in two stages. The extensive calc-alkaline gabbronorites of the Chilas complex were derived from partial melting of a mantle diapir emplaced into a mature arc or, more probably, during the initial stages of sub-arc splitting. This intra-arc rifting, at its advanced stages, generated tholeiitic picrite to high-Mg basalts which crystallized the ultramafic-mafic-anorthosite (UMA) association and basic dykes of the Chilas complex. The Late Cretaceous accretion of the Kohistan island arc with the Karakoram plate in the north ceased the intraoceanic history of magmatism in southern Kohistan.

The Sapat mafic-ultramafic complex, Kohistan arc, North Pakistan

Abstract The Sapat layered complex, occupying the tectonic base of the Cretaceous Kohistan island arc, occurs in the immediate hanging wall of the Indus suture zone north of the Kaghan valley. Previously undescribed, much of the complex consists of gabbroic rocks, but in the basal part there are ultramafic cumulates (15 × < 2 km) in direct tectonic contact with the Indian plate. In the central part, the complex can be divided into five zones. From the base these are: (1) homogeneous serpentinized dunites (350 m thick), (2) dunites with thin chromite layers (100 m), (3) thinly layered dunites-pyroxenites (30 m), (4) layered dunites-pyroxenites-gabbros (170 m) with chromite layers in the lower part, and (5) gabbros. The gabbroic rocks are amphibolitized and consist of a variety of lithologies ranging from troctolites to anorthosites, with local pyroxenite (± olivine) layers and horizons of olivine-rich ultramafics. These rocks are locally cut by pyroxene-rich pegmatites containing up to 45 cm long pyroxene crystals. Preliminary microprobe studies on the ultramafic rocks show that the clinopyroxene is Mg-Ca-rich diopside, and olivine has a forsterite content of 92 to 78, but in rare cases > 95. The chromite displays high Cr No. (75–67) and low Fe 3+ /R 3+ (<10). These data, especially the chromite analyses, are akin to those of ultramafic rocks of island arcs, including the Jijal complex, 80 km to the west. We propose that the Sapat complex formed the basal part of the Kohistan terrain and is related to arc magmatism. It has been metamorphosed under epidote amphibolite facies conditions.

Geology of the northern part of the Nanga Parbat massif, northern Pakistan, and its implications for Himalayan tectonics

The northern outcrop termination of Indian continental crust lies at the Nanga Parbat massif, from where this contribution presents new field data. The tectonic contact with the structurally overlying Kohistan island arc is concordant and ductile, is associated with interleaving of Nanga Parbat and Kohistan lithologies, and may be correlated with the Main Mantle thrust found elsewhere in the NW Himalayas. This ductile shear zone is locally overprinted by cataclastic faults associated with exhumation of the massif but overall, the northern outcrop termination of the massif is controlled by erosion through a gently, northward-plunging antiformal structure. This folds both the Main Mantle thrust and the underlying, concordant 'Layered Unit' of the Nanga Parbat basement. Thus there is no indication that the massif acted as a promontory to the Indian continent during collision nor that it is a structure entirely bound by neotectonic faults. Ductile shear fabrics associated with the 'Main Mantle thrust' are cross-cut by leucogranite sheets and pegmatites. These may represent the stockwork to a significant crustal-melt granite body described here in the northern massif. This Jutial granite shows many geochemical characteristics in common with similar bodies in the High Himalayas which are consistent with anatexis of the buried Indian continental basement rocks. However, the granite is enriched in heat producing elements (particularly Th: 22 ppm) and shows extremely high 87 Sr/ 86 Sr ratios (>0.88). The high concentrations of radiogenic Sr are also an attribute of a suite of hitherto enigmatic leucogranitic pegmatites that laces the Nanga Parbat massif, suggesting that these may represent a stockwork to a largely buried body of which the Jutial granite is a small exposure. The enrichment in heat-producing elements within the granite may reflect similarly high heat production in the source Indian continental crust requiring in turn a fundamental re-examination of the thermal evolution of this crust during Himalayan metamorphism and exhumation.

Constraints on the Tectonic Evolution of the Northwestern Himalaya from Geochronologic and Petrologic Studies of Babusar Pass, Pakistan

Petrologic, geochronologic, and structural studies of Indian-plate rocks in the footwall of the Main Mantle Thrust indicate that peak metamorphism occurred in thrust imbricates well before the final assembly of the Kohistan island-arc with the Indian plate. As the northern margin of the Indian plate was thickened by this early thrusting, the metamorphic rocks evolved along increasing pressure-temperature paths and, in some regions, reached temperatures sufficient for partial melting. It is difficult to constrain the timing of this early deformation and metamorphism, but $$^{40}Ar/^{39}Ar$$ hornblende ages and U/Pb ages of crosscutting leucogranite dikes suggest that in most areas peak metamorphism was earlier than ~40 Ma and in some areas earlier than 50 Ma. After peak metamorphism, in the interval from ~40 to 25 Ma, rocks of the Indian plate continued to be thrust southward, creating sharp metamorphic discontinuities at thrust boundaries. From 25 to 20 ma rocks of the Indian plate underwent rapid cooling, and data suggest that this rapid cooling occurred over large regions of the northern margin of the Indian plate, along as much as 300 km of the Main Mantle Trust. We tentatively suggest that the rapid cooling resulted from tectonic denudation of the Indian plate as Kohistan slid northward along later normal faults. Between 15 and 20 Ma, faulting ceased and the terranes were locked into their present structural position; since then these terranes have simply undergone uplift and erosion. North of the Babusar region, in the vicinity of Nanga Parbat, tectonic activity along the Main Mantle Thrust continued to the present. In this region, the Indian plate rocks are presently being thrust over the adjacent Kohistan terrane.

Structural evolution and asymmetric uplift of the Nanga Parbat syntaxis, Pakistan Himalya

The Nanga Parbat syntaxis, in the NW Himalaya, is a still growing crustal-scale north-trending antiformal structure in the core of which Indian Plate gneisses have been uplifted from beneath the overthrust rocks of the Kohistan island arc. Isotopic and fission track geochronology show that uplift rates within the syntaxis have increased to present day rates of > 6 mm/yr. Uplift has been accommodated by a combination of initial northwest verging thrusting on the western margin of the syntaxis, followed by crustal scale folding within the syntaxis and latterly by dextral reverse faulting on the western margin. This thrusting, folding and faulting is the effect of deformation at the north-western lateral tips of the main Himalayan thrusts where they interfere with the south-southeast verging thrusts of the northwest Himalaya.

Changing source regions of magmas and crustal growth in the Trans-Himalayas: evidence from the Chalt volcanics and Kohistan batholith, Kohistan, northern Pakistan

The Kohistan batholith and Chalt volcanics form a major part of the Kohistan island arc terrane in northern Pakistan. They record some 70–80 Ma of magmatism within the northwestern Himalayan orogen; the Chalt volcanics are Albian-Aptian (mid-Cretaceous), and currently available Rb Sr age data for the batholith indicate an intrusive age span of 102 to 30 Ma. The volcanics are composed of medium-K, calc-alkaline andesites and low-K, high-Mg, tholeiites, some of which have boninitic and basaltic komatiitic chemistry. Three intrusive stages are recognised in the batholith: (1) (110–90 Ma) comprises low-K trondhjemites and medium-high-K calc-alkaline gabbro-diorites with associated hornblendite cumulates; (2) (85–40 Ma) comprises low-high-K, calc-alkaline gabbro-diorites (with hornblendite cumulates, granodiorites and granites; and (3) (circa 30 Ma) comprises biotite ± muscovite ± garnet leucogranites. 87Sr/ 86Sr 0 for the batholith range between 0.7039 and 0.7052. Four magmatic source regions (Sce 1–Sce 4) have been identified. Sce 1 is a variably metasomatised mantle wedge situated above an active subduction zone during stages 1 and 2. Sce 2 is a harzburgite depleted with respect to major elements and incompatibles, whilst retaining “subduction-related” trace element ratios. Sce 2 melted during stage 1 within a fore-arc position during the subduction of young, hot oceanic crust to provide the necessary extra thermal input for harzburgite anatexis. Melts from Sce 2 produced the high-Mg tholeiitic volcanics. Sce 3 melted during stage 1 to produce the low-K trondhjemites, and it is envisaged as incompatible element-depleted primitive arc crust with a similar composition to the depleted Chalt volcanics. Sce 4 melted at 30 Ma during stage 3 as India underthrust Eurasia: frictional heating and dehydration of the Indian plate caused crustal anatexis within the deep Kohistan arc and these melts formed the stage 3 leucogranites. The low 87Sr/ 86Sr initial ratios for the great bulk ( > 95%) of the batholith indicate that it represents a major juvenile addition to the continental crust. This conclusion has important implications for crustal growth theories of island arc accretion and batholith underplating.

K‐Ar and Ar‐Ar geochronology of the Himalayan collision in NW Pakistan: Constraints on the timing of suturing, deformation, metamorphism and uplift

Mica and hornblende K‐Ar and Ar‐Ar data are presented from each of the three crustal components of the Himalayan collision zone in North Pakistan: the Asian plate, the Kohistan Island Arc, and the Indian plate. Together with U‐Pb and Rb‐Sr data published elsewhere these new data (1) date the age of suturing along the Northern Suture, which separates Kohistan from Asia, at 102–85 Ma; (2) establish that the basic magmatism in Kohistan, which postdates collision along the Northern Suture, predates 60 Ma, and that the later granite magmatism spanned a range of 60–25 Ma; (3) show that uplift amounts within Kohistan are greater toward the Nanga Parbat syntaxis than away from it and that rate of uplift near the syntaxis increased over the last 20 Ma to a current figure of about 5.5 mm a year; (4) show that much of southern Kohistan had cooled to below 500°C by 80 Ma and that the major deformation which imbricated Kohistan probably predated 80 Ma and certainly predated 60 Ma and was related to the Kohistan‐Asia collision rather than the Kohistan‐India one; (5) imply that uplift along the Hunza Shear in the Asian plate together with imbrication of the metamorphics in its hanging wall took place at about 10 Ma and was associated with breakback thrusting in the hanging wall of the Main Mantle Thrust; (6) suggest that the Indian plate has a lengthy pre‐Himalayan history with an early metamorphism at about 1900 Ma, major magmatism at 500–550 Ma and early Jurassic lithospheric extension or inversion; and (7) show that the Indian plate rocks were metamorphosed shortly after the collision within Kohistan, which occurred at circa 50 Ma, and subsequently cooled back through 500°C at circa 38 Ma and 300°C at 30–35 Ma with ages of cooling through 200° and 100°C (as determined by fission track data) locally controlled by Nanga Parbat related uplift tectonics.

The dynamics of the suture between the Kohistan island arc and the Indian plate in the Himalaya of Pakistan

ABSTRACT The pressure‐temperature and temperature‐time paths derived for rocks in the Kohistan arc and adjacent Nanga Parbat‐Haramosh massif record the dynamics of the collision between the island arc and the Indian plate. Studies of P‐T‐t paths show that the Kohistan arc was thrust over the Nanga Parbat‐Haramosh massif at least 25 Ma ago, but not more than 30–35 Ma ago. Rocks in the Kohistan arc followed decreasing pressure paths, with the early metamorphism beginning at high pressures (9.5 kbar) and later metamorphism occurring at 8.0 kbar. In contrast, rocks in the Nanga Parbat‐Haramosh massif (Indian plate) experienced increasing pressure and temperature paths. Prior to thrusting, the massif was at low pressures (4.0 kbar) and low temperatures (450°c). Later, the pressure and temperature increased to 8 kbar and 580°c. The authors interpret the convergence (to approximately the same pressure and temperature) of the P‐T paths in the two terranes as being the result of thrusting and thermal equilibration between the thrust sheets. 40Ar/39Ar cooling ages of hornblendes and other geochronological data suggest that the time of peak metamorphism and hence the completion of thickening was approximately 30–35 Ma ago.Temperature‐time paths show that after thrusting, during the period 25–10 Ma, the Kohistan arc and Nanga Parbat‐Haramosh massif were uplifted at similar rates (0.5 km Ma). However, in the past 10 Ma the Nanga Parbat‐Haramosh massif has been uplifted more rapidly than the adjacent Kohistan arc. Rapid uplift has been accommodated by late faults along the edge of the massif.

Geochemistry of amphibolites from the southern part of the Kohistan arc, N. Pakistan

AbstractThe southern part of the Cretaceous Kohistan island arc is occupied by an extensive belt dominantly comprised of amphibolites. These include banded amphibolites of partly meta-volcanic parentage, and non-banded amphibolites derived from intrusive rock. In addition to being relict, banding has also been produced by shear deformation, metamorphic/metasomatic segregation and, possibly, by lit-par-lit injection of plagiogranitic material. Non-banded amphibolites also occur as retrograde products of noritic granulites forming the lopolithic Chilas complex. The chemistry of 37 rocks has been compared with those of known tectonic environments. The amphibolites have chemical characteristics similar to volcanic rocks found in island arcs and most of the analyses apparently support affinity with the calc-alkaline series. The amphibolites consist essentially of hornblende, plagioclase and/or epidote. Garnet and clinopyroxene have developed locally in rocks of appropriate bulk composition. Metamorphism may have taken place during the mid-Cretaceous under conditions of 550 to 680°C and 4.5 to 6.5 kbar PH2O. The metamorphic grade appears to increase from the centre of the southern belt toward the Chilas complex to the north and Indus-Zangbo suture (IZS) to the south. In the vicinity of the IZS, garnet-clinopyroxene ± amphibole assemblage developed locally in response to high P-T.

The tectonic history of Kohistan and its implications for Himalayan structure

The tectonic history of Kohistan, northern Pakistan, involves two collisional events. Cleavage and folding developed at 90-100 Ma along the northern suture between the Kohistan island arc and the Asian plate. At the same time there was major folding and shearing of the lower part of the Kohistan arc, approximately 100 km south of the suture. This deformation was followed by ocean subduction south of the Kohistan arc, generating the Kohistan calc-alkaline batholith, with subsequent ocean closure during the Eocene and obduction of the Kohistan arc, together with the adjacent part of the Asian plate, over the Indian continental crust. The construction of balanced cross-sections through the imbricated upper part of the Indian continental crust, in the footwall to this southern suture indicates a minimum displacement of 470 km, requiring the western Himalayan hinterland to be underlain by a large wedge of Indian middle to lower crust. There is some shortening of the overriding Kohistan and Asian plates by thrusts and shear zones, but it is insufficient to satisfy the palaeomagnetic data; there must be major crustal shortening, involving thrusts, in the Hindu Kush and Pamirs north of Kohistan. The post-Eocene thrust direction, which for most of Pakistan is towards 160°, is almost perpendicular to that immediately to the east in the Himalayan belt, generating complex refolded thrust patterns in the Hazara syntaxis and large scale folding and rapid uplift with associated brittle faulting and seismic activity adjacent to the Nanga Parbat syntaxis. These different thrust trends indicate that major thrust movement as well as the folds and deformation fabrics, cannot always be related to plate movement vectors, but are modified by, or develop from, complex rotations during place collision or from the gravitational spreading of a thickened crust. A regional approach is required to recognize and correctly attribute the various components in thrust displacements.

The closing of Tethys and the tectonics of the Himalaya

Recent geological and geophysical data from southern Tibet allow refinement of models for the closing of southern (Neo-) Tethys and formation of the Himalaya. Shelf sediments of the Indian passive continental margin which pass northward into deep-sea Tethyan sediments of the Indus-Tsangpo suture zone were deposited in the Late Cretaceous. An Andean-type margin with a 2,500-km-long Trans-Himalayan (Kohistan-Ladakh-Gangdese) granitoid batholith formed parallel to the southern margin of the Lhasa block, together with extensive andesites, rhyolites, and ignimbrites (Lingzizong Formation). The southern part of the Lhasa block was uplifted, deformed, and eroded between the Cenomanian and the Eocene. In the western Himalaya, the Kohistan island arc became accreted to the northern plate at this time. The northern part of the Lhasa block was affected by Jurassic metamorphism and plutonism associated with the mid-Jurassic closure of the Bangong-Nujiang suture zone to the north. The timing of collision between the two continental plates (ca. 50-40 Ma) marking the closing of Tethys is shown by (1) the change from marine (flysch-like) to continental (molasse-like) sedimentation in the Indus-Tsangpo suture zone, (2) the end of Gangdese I-type granitoid injection, (3) Eocene S-type anatectic granites and migmatites in the Lhasa block, and (4) the start of compressional tectonics in the Tibetan-Tethys and Indus-Tsangpo suture zone (south-facing folds, south-directed thrusts). After the Eocene closure of Tethys, deformation spread southward across the Tibetan-Tethys zone to the High Himalaya. Deep crustal thrusting, Barrovian metamorphism, migmatization, and generation of Oligocene-Miocene leucogranites were accompanied by south-verging recumbent nappes inverting metamorphic isograds and by south-directed intracontinental shear zones associated with the Main Central thrust. Continued convergence in the late Tertiary resulted in large-scale north-directed backthrusting along the Indus-Tsangpo suture zone. More than 500 km shortening is recorded in the foreland thrust zones of the Indian plate, south of the suture, and > 150 km shortening is recorded across the Indian shelf (Zanskar Range) and the Indus suture in Ladakh. There was also large-scale shortening of the Karakoram and Tibetan microplates north of the suture; as much as 1,000 km shortening occurred in Tibet. The more recent deformation, however, involved the spreading of this thickened crust and the lateral motion of the Tibetan block along major approximately east-west–trending strike-slip fault zones.

The Northern Suture, Pakistan: margin of a Cretaceous island arc

AbstractThe Northern Suture is a fault separating the Cretaceous Kohistan island arc terrain (northwest Himalayas) from Palaeozoic sediments of the Asian Plate to the north. The Kohistan arc includes volcanic and sedimentary rocks (andesitic lavas, tuffs, volcaniclastics, slates and limestones), metamorphosed to greenschist facies and intruded by the two-phase Kohistan Batholith. Asian continental margin sediments are mainly of shelf type, are variably metamorphosed and intruded by the Karakoram Batholith. The Northern Suture is a zone of melange from 150 m to 4 km wide, and contains blocks of volcanic greenstone, limestone, red shale, conglomerate, quartzite and serpentinite in a slate matrix. It has a strong planar fabric; but in many places bedding is preserved in blocks and matrix, and depositional rather than tectonic contacts are seen between the two. The melange is inferred to be an olistostrome largely derived from the Kohistan arc, formed in a small back-arc basin between Kohistan and Asia. Limestone blocks in the melange are dated as Aptian–Albian; post-tectonic intrusions yield radiometric ages from 111 to 62 Ma. The Northern Suture therefore probably formed in the early Late Cretaceous during closure of the back-arc basin. The Tethys ocean lay south of Kohistan, where the Main Mantle Thrust represents the westward continuation of the Indus–Tsangpo Suture.

The structure of the Kohistan-Arc terrane in northern Pakistan as inferred from gravity data

Modelling of gravity data taken across the Kohistan Island-Arc terrane in northern Pakistan can be used to constrain the shape and thickness of the Arc. Over 600 new gravity measurements were made across the Kohistan Island-Arc terrane in northern Pakistan. These data were taken along traverses normal to the structures bounding the Arc and were reduced to terrain-corrected Bouguer values. The reduced data were then modelled using standard two-dimensional modelling techniques. The southern margin of the Arc, the Main Mantle Thrust (MMT), dips to the north at approximately 45° and gradually flattens out at a depth of 7–9 km. The northern margin of the Arc, the Main Karkoram Thrust (MKT), also dips towards the north, but at a shallower initial angle (15°). From the models, the Arc terrane now appears to be around 7–9 km thick with the thicker sections occurring closer to the southern margin. The proposed model, in particular the angle of the MMT and the MKT, may have been significantly affected by the recent and rapid uplift that is occurring along the Nanga Parbat-Haramosh Massif.

Collision zone between the Kohistan arc and the Asian plate in NW Pakistan

ABSTRACTThis paper describes the suture zone between the Asian plate and the accreted Kohistan island arc in the Chitral district of NW Pakistan.The southern part of the Asian plate consists of two tectonic units separated by the N-dipping Reshun fault. The northwestern unit comprises Devonian carbonates and quartzites overlain by Devonian to Permian shales and slates with some limestones (Lun shales). Its structure is complex with S-verging thrusts and isoclinal folds. Along the Reshun fault, the relatively undeformed Reshun Formation may represent molasse. The central unit includes N-dipping Upper Palaeozoic slates and quartzites (Darkot Group), probably faulted against an antiformal tract of slates, schists derived from a volcanic assemblage and Cretaceous limestones (Chitral slate, Koghozi greenschist, Krinj and Gahiret limestones). Asian plate sediments are intruded by granitic and granodioritic plutons, variably deformed and locally porphyritic.The Northern suture melange of volcanic, sedimentary and serpentinite blocks in a slate matrix separates the Asian plate from the southeastern unit, the Kohistan arc. This comprises Cretaceous volcanic rocks with some sediments (Shamran Volcanic Group, Drosh, Purit and Gawuch Formations) intruded by aphyric diorites, tonalites and granites. These intermediate plutonic rocks pass southwards into a mafic layered complex and amphibolites representing deep levels of the arc. The volcanic rocks and sediments dip to the N and have a horizontal lineation. The structural history of southern Asia and Kohistan is consistent with an originally curved Northern suture: motion of the arc was initially to the NE relative to Asia and subsequently to the NW.