Pakistan Himalaya Research Articles

Ductile extrusion in continental collision zones: ambiguities in the definition of channel flow and its identification in ancient orogens

Abstract Field characteristics of crustal extrusion zones include: high-grade metamorphism flanked by lower-grade rocks; broadly coeval flanking shear zones with opposing senses of shear; early ductile fabrics successively overprinted by semi-brittle and brittle structures; and localization of strain to give a more extensive deformation history within the extrusion zone relative to the flanking regions. Crustal extrusion, involving a combination of pure and simple shear, is a regular consequence of bulk orogenic thickening and contraction during continental collision. Extrusion can occur in response to different tectonic settings, and need not necessarily imply a driving force linked to mid-crustal channel flow. In most situations, field criteria alone are unlikely to be sufficient to determine the driving causes of extrusion. This is illustrated with examples from the Nanga Parbat-Haramosh Massif in the Pakistan Himalaya, and the Wing Pond Shear Zone in Newfoundland.

Plate velocity exhumation of ultrahigh-pressure eclogites in the Pakistan Himalaya

U-Pb ages of zircon and allanite from the coesite-bearing ultrahigh-pressure (UHP) units in the Kaghan Valley, northern Pakistan, demonstrate that peak UHP metamorphism along the northern margin of the Indian plate occurred at 46.4 ± 0.1 Ma at peak pressure-temperature conditions of >27.5 kbar (>100 km) and 720–770 °C. Much lower pressure retrogressive growth of titanite took place between 46.4 and ca. 44 Ma, indicating that the eclogites were exhumed to 35 km depth at or before 44 Ma, implying very rapid exhumation rates within the mantle of ∼30–80 mm/yr or more, comparable to rapid plate velocities. Once entrained in the base of the crust, the eclogites followed a slower cooling history from 45 Ma, similar to the amphibolite facies gneisses of the Pakistan Himalaya.

Documentation of traditional knowledge about wild medicinal plants of moist temperate Himalayas in Pakistan

The moist temperate Himalayas, as one of the major ecological zone of Pakistan, deserve specific attention to the conservation of environment and the sustainable development of natural resources. During the last hundred years, the area has been subjected to major structural changes leading to a decrease of about fifty per cent of the potential forest area. The decrease in forest cover, combined with major changes in community structure has been responsible for the decline of indigenous medicinal plants resources and their traditional knowledge also. Ayubia National Park has been identified as one of the priority area to be focused for medicinal plant conservation. The study was aimed to analyze traditional knowledge including local names, general distribution, flowering period, part used, medicinal and other uses, market values and taxonomic diversity of the medicinal plant of the area The field surveys were conducted by adopting predefined questionnaires through guided and transect walks. The market oriented indigenous species have been subjected through IUCN criterion for evaluation of their conservation status. Traditional knowledge about 117 indigenous medicinal plants (including 8 cultivated ones) have been collected from 140 informants. Women followed by children have been identified as the principle gatherers of medicinal plants. About 44 species were found to be market oriented. According to this criterion, eleven species including two trees (Juglans regia, Taxus wallichiana), one shrub (Berberis lycium) and eight herbaceous species (Asparagus adscendens, Atropa acuminata, Colchicum luteum, Dioscorea deltoidea, Podophyllum hexandrum, Rheum australe, Saussurea costus and Valeriana jatamansi) have been found as endangered. It has been concluded that Traditional knowledge in Moist Temperate Himalayas of Pakistan is under threat of being lost. Availability of cultivated land is quite less, the establishment of botanical gardens, home gardens or kitchen gardens may be the best ex situ conservation strategy, which can be adopted for sustainable utilization of medicinal plants. While clearly defined land tenure system and community participation in park management will be the best in situ conservation measure for adoptation.

Exhumation of early Tertiary, coesite-bearing eclogites from the Pakistan Himalaya

Early Tertiary, coesite-bearing eclogites crop out in the internal zones of the Indian Plate in the Kaghan Valley of North Pakistan. Field and geochronological data suggest that UHP metamorphism, at c . 46 Ma, accompanied subduction of the leading edge of continental India beneath Kohistan. A new U–Pb rutile age of 44.1 ± 1.1 Ma can be interpreted as either a growth or a cooling age, given the uncertainty of the closure temperature of rutile for Pb, but clearly documents a very short-lived metamorphic peak, with cooling occurring between 44 and 40 Ma. Eclogitic assemblages are preserved in Permo-Triassic mafic rocks in both cover and basement sequences, with the best-preserved eclogites hosted in the anhydrous basement. The eclogite-facies rocks lie on the hanging wall of a major south-directed ductile thrust zone, which placed basement rocks that contain coesite eclogites southward onto lower-pressure cover metasediments. Shear criteria show that those sequences that contain eclogite-facies rocks are flanked above by a zone of pervasive north-directed extensional shearing. Mineral assemblages contained within both the thrust and extensional shear zones are consistent with them both having operated synchronously during the amphibolite- to greenschist-facies transition. Amphibole Ar–Ar cooling history data suggest that this was at c . 40–42 Ma. As in the Alps this implies exhumation from the metamorphic peak to greenschist-facies conditions within a few million years. This short time scale has important implications, as it is likely that both the thrust and extensional shear zones represent the late stage of a deformation continuum that commenced at UHP and brought the UHP rocks back to ‘mid’-crustal levels. This late Eocene extensional shearing, and associated exhumation of HP or UHP rocks, has not previously been recorded in the Pakistan Himalaya. The thrusts and shears were later folded by major east-trending folds, themselves cut by Miocene-aged, north-vergent extensional faults that currently separate the Indian Plate from the structurally overlying Kohistan island arc.

Early-Middle Miocene paleodrainage and tectonics in the Pakistan Himalaya

The 18–14 Ma Kamlial Formation Himalayan foreland basin sedimentary rocks in the Chinji Village region, Potwar Plateau, Pakistan, are characterized by: (1) lithofacies indicative of deposition by a large river; (2) a dominant magmatic arc provenance completely unlike the ‘recycled orogen’ foreland basin deposits stratigraphically below, above, or coeval with these rocks; and (3) subordinate contribution from a rapidly exhuming source, interpreted as either the Nanga Parbat Haramosh Massif or the southern margin of the Asian crust. The start of Kamlial Formation deposition at this locality at 18 Ma marks a major break with the older Murree Formation rocks, which were deposited by rivers draining predominantly the Himalayan thrust stack south of the arc. We interpret this change as the result of diversion of the paleo-Indus River to its present position, which crosses the Kohistan arc and Himalayas and debouches into the foreland. If the rapidly exhuming subordinate source region were the Nanga Parbat Haramosh Massif, then initiation of its uplift would have resulted in significant arc detritus to the basin as the overlying arc carapace was exhumed. As the carapace was progressively breached, arc material would have become a less substantial component of detritus to the basin, consistent with the reported petrography of the overlying Siwalik deposits.

Textural, chemical and isotopic insights into the nature and behaviour of metamorphic monazite

Monazite is a mineral of choice for dating metamorphism in amphibolite- and granulite-grade metapelites. However, there exist a number of difficulties that complicate the interpretation of monazite geochronological data and prevent its application to many geological problems. The two main obstacles addressed in this contribution are firstly, the minor but significant (e.g. 1–30 Ma) dispersal in duplicate isotope dilution thermal ionisation mass spectrometry (ID-TIMS) U–Pb age data commonly recorded from a single rock, and secondly, the difficulty of attaching monazite age data to pressure and temperature information. Through a multidisciplinary approach utilising TIMS and laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) isotope data, quantitative and qualitative EMP chemical analyses of monazite, and textural studies, we assess the significance of Pb loss, older components, and continuous and episodic monazite growth in the generation of dispersed age data. Three samples from the Canadian Cordillera and one sample from the Himalaya of Pakistan are examined. Each sample exhibits an age dispersion of between 1 and 12 Ma for single crystal and multi-grain TIMS U–Pb monazite age determinations. Consideration of the closure temperature for Pb diffusion in monazite and the metamorphic temperatures experienced by these samples suggests diffusive Pb loss did not play a significant part in generating this age dispersal. The LA-MC-ICPMS study indicates that an older component (<100 Ma older than the TIMS ages) contributed to the age dispersal in three of the four samples. In all the samples however, chemical analyses identified that the majority of monazites examined exhibited significant intra-crystalline zoning in Y content. The LA-MC-ICPMS analysis of one sample that was constrained to zones of distinct Y content indicates that these zones are of distinct age. We suggest that monazite grown before the appearance of garnet and during garnet breakdown is relatively rich in Y, whereas monazite grown after garnet is relatively poor in Y. A combination of these chemical data with textural observations suggests that once monazite had entered the mineral assemblage it grew or recrystallised episodically throughout the prograde and retrograde paths of the metamorphic event. This behaviour contributes to, and in one of the samples controls, the observed age dispersal. This recognition allows the generation of pressure–temperature–time points by combining textural and chemical information of monazite with in situ age determinations, and pressure–temperature information from garnet. Thus, the episodic growth of compositionally distinct monazite throughout a metamorphic event provides the geochronologist with a very valuable chronological tool.

Mechanical links between erosion and metamorphism in Nanga Parbat, Pakistan Himalaya

BACKGROUND--The increases in airspace size within the human lung associated with microscopic emphysema can be assessed by measuring the airspace wall surface area per unit volume of lung tissue (AWUV). In a previous study the limits of normality of AWUV with age were estimated in lifelong non-smokers by the 95% prediction limits of the regression line for these variables. The aims of this study were to study the incidence of microscopically assessed emphysema in a group of smokers and to examine the influence of smoking habit on the susceptibility to and severity of microscopically assessed emphysema. METHODS--AWUV was measured on tissue sections from 125 lung specimens obtained from tobacco smokers (mean age 61.1 (range 33-85) years) with the fast interval processor, a rapid automatic scanning device. The mean AWUV value was calculated for each specimen and this figure was plotted against the age of the subject. The limits of normal AWUV were plotted, and AWUV values below these limits were taken as indicative of microscopically assessed emphysema. Details of the number of cigarettes smoked each day were obtained for 97 of the smokers. These subjects were grouped according to smoking habit: group 1, 1-19 cigarettes/day; group 2, 20-29 cigarettes/day; group 3, at least 30 cigarettes/day. The AWUV results from each of these groups were then assessed. RESULTS--Mean AWUV decreased with age in this group of smokers, but only 26% had microscopically assessed emphysema, indicating that within the group there were two subgroups of smokers with differing susceptibility to microscopically assessed emphysema. There were no sex differences in the incidence of microscopically assessed emphysema, nor were the incidence and severity increased with increased daily cigarette consumption. CONCLUSIONS--Susceptibility to microscopically assessed emphysema was found to be similar in male and female smokers. Daily cigarette consumption did not appear to be the primary factor influencing the susceptibility to or severity of microscopically assessed emphysema. Susceptibility differences within the smoking population should be taken into consideration in studies of the pathogenesis of emphysema.

Overview of hydrothermal activity associated with active orogenesis and metamorphism: Nanga Parbat, Pakistan Himalaya

The Nanga Parbat Massif is located in northwestern Pakistan and occurs in the interior of the western syntaxis of the Himalaya. Recent studies have shown that the massif exposes an active metamorphic anomaly developed in Indian- Plate rocks exhumed from beneath and surrounded by rocks of the Kohistan/Ladakh island-arc. Geochemical and geophysical data from Nanga Parbat show that an active hydrothermal system exists within the massif. In the upper 5 to 6 kilometers of the crust, above a shallow brittle/ductile transition constrained by microseismic data, there is a shallow hydrothermal system with meteoric fluids channelized along active fault zones. In contrast, below the brittle/ductile transition fluids are metamorphic in origin and restricted to unconnected packets along grain boundaries. The shallow system is stratified with water-rich fluid inclusions near the surface grading to vapor-rich inclusions at the brittle/ductile transition, as evidenced from fluid inclusion studies. These fluids are meteoric in origin and enter the massif along shear and fracture zones and exit the massif along the bordering fault zones. Evidence for these channelized meteoric fluids comes from stable isotopic values of minerals and rocks collected from fault zones that show that these rocks have interacted with meteoric fluids, as well as magnetotelluric (MT) data that show that the fault zones are electrically conductive and embedded in very resistive rocks. The deeper portions of the hydrothermal system consist of unconnected metamor- phic fluids. Stable isotopic and fluid inclusion data show that fluids were present during the recent metamorphism but the rocks did not extensively interact with these fluids. Existence of an unconnected fluid system is supported by the MT studies showing that below the brittle/ductile transition the rocks are relatively resistive (values ranging from 1000-10,000 ohm-m), ruling out an interconnected fluid network. Seismic and MT data also show no evidence of a large magma body at depth. We suggest that the type of hydrothermal system observed at Nanga Parbat can be an important feature of late-stage orogenic evolution, especially near the tectonically complex edges of orogens. The tectonism in these areas exposes deep, relatively dry rocks that have previously lost their volatile content during the earliest stages of tectonic thickening and metamorphism. These relatively dry rocks are rapidly uplifted and exposed within the massifs where they interact with surface waters at high temperatures and shallow depths.

Vertical stretching and crustal thickening at Nanga Parbat, Pakistan Himalaya: A model for distributed continental deformation during mountain building

The localization of strain in the continental crust during compressional tectonics is examined using the active structures at the Nanga Parbat massif, an exhumed tract of Indian continental crust in the Pakistan Himalaya. This large‐scale (∼40 km wavelength) structure is considered to involve the whole crust. Thrusting at the modern surface places gneisses of the Indian continental crust onto Holocene deposits. At the Raikhot transect, the thrust zone carries a relatively narrow (2 km wide) shear zone within which minor structures are asymmetric and the deformation apparently noncoaxial. However, modeling of foliation and augen preferred orientation/ellipticity suggests that the bulk deformation is a combination of relatively small simple shear strains (γ = 1) with larger stretching strains. Heterogeneous stretching within the shear zone was accommodated by localized shearing on metabasic layers so that strain is partitioned. Outside this shear zone on the transect there is penetrative deformation throughout the Nanga Parbat massif. This broadly distributed deformation shows no asymmetry or evidence of rotation. Rather this deformation is better described as near pure‐shear subvertical stretching. Augen ellipticities suggest subvertical stretches of greater than 200%. Consideration of plausible changes in crustal thickness during the amplification of the Nanga Parbat structure suggests the magnitude of vertical stretch decays with depth. Presumably these strains in the deep crust are more distributed but weaker than in the exposed middle crustal sections, assuming conservation of horizontal shortening displacement with depth. These studies suggest that penetrative vertical stretching through dominantly pure shear deformation is an effective mechanism for thickening the continental crust and that models which assume that simple shear zones penetrate the whole crust need not be of ubiquitous applicability.

First evidence for high-grade, Himalayan-age synconvergent extension recognised within the western syntaxis—Nanga Parbat, Pakistan

We present evidence for horizons of medium to high grade (400–600°C) deformation accompanying normal sense displacement in high-strain Himalayan gneisses and schists from the southeastern Nanga Parbat massif, the western syntaxis in the Pakistan Himalaya. In their present orientation, the broadly N–S trending, steeply-dipping gneisses show microstructural and outcrop scale evidence for dextral and sinistral shear in discrete layers. We interpret these as horizons of thrust and normal motion within the footwall of the original Main Mantle Thrust, once Neogene antiformal folding is removed. This is the first report of significant normal motion in the Main Mantle Thrust footwall in the Nanga Parbat syntaxis and may indicate that synconvergent extension in the Himalaya extended to the western syntaxis. This episode of extension could correspond to a period of similar normal motion in the central Himalaya, or represent a separate event earlier in the orogeny.

Book Reviews: The Past in the Present: Horizons of Remembering in the Pakistan Himalaya and Perspectives on History and Change in the Karakorum, Hindu-Kush, and Himalaya

Book Reviews: The Past in the Present: Horizons of Remembering in the Pakistan Himalaya and Perspectives on History and Change in the Karakorum, Hindu-Kush, and Himalaya

Timing of normal faulting along the Indus Suture in Pakistan Himalaya and a case of major 231Pa/235U initial disequilibrium in zircon

We report age data by TIMS U–Pb, LA-ICP-MS and 39Ar–40Ar techniques for main magmatic events in the Lower Swat region of Pakistan, in order to constrain the tectonic evolution of the northwestern Himalaya. The pre-Himalayan history of the Indian continent is documented by single-zircon U–Pb results from the peraluminous Choga granite gneiss, which yielded a 468±5 Ma lower concordia intercept interpreted to approximate the time of magmatic emplacement. The presence of a well-defined 870±7 Ma inherited component (upper intercept) suggests a plutonic or volcanic protolith residing at unexposed levels of the Indian crust. Zircon data for the Swat granite gneiss from the northern part of the Loe Sar dome give an emplacement age of 267+6/−3 Ma, which is at variance with earlier correlations favoring an early Paleozoic origin. Subsequent to metamorphic overprint by the Himalayan orogeny, the Swat granite was intruded by late kinematic alkali-granite dykes. Single zircons from one of these dykes show reproducible 206Pb/238U data giving a precise mean age of 29.26±0.12 Ma, whereas the 207Pb/235U ages scatter between 34 and 81 Ma, pointing to huge and variable enrichment in 207Pb. The unsupported 207Pb can be explained by incorporation of 231Pa, an intermediate long-lived daughter nuclide in the 235U decay chain, in excess of the secular equilibrium ratio. LA-ICP-MS measurements confirm the presence of unsupported 207Pb, but do not show any correlation between the latter and other selected trace element concentrations in these zircons. A concordant Ar–Ar muscovite age of 28.4±1.1 Ma obtained for the same dyke postdates regional mica ‘cooling’ ages and indicates a lack of younger regional events capable of resetting the K–Ar system in muscovite. Because the dyke is pre- to synkinematic relative to normal faulting and related north-vergent folds, its emplacement age provides a maximum age for this event or reflects already ongoing local extension. A lower limit of 15 Ma has previously been established by apatite fission track analysis. The 29–15 Ma age bracket for normal faulting is coeval with extension along the South Tibetan Detachment System (STDS) on the north side of the High Himalaya. This suggests that the Indus Suture in Pakistan has acted as a western continuation of the STDS and that related faulting was roughly contemporaneous over most of the Himalaya.

Seismic characterization of an active metamorphic massif, Nanga Parbat, Pakistan Himalaya

Research Article| July 01, 2001 Seismic characterization of an active metamorphic massif, Nanga Parbat, Pakistan Himalaya Anne Meltzer; Anne Meltzer 1Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA Search for other works by this author on: GSW Google Scholar Golam Sarker; Golam Sarker 1Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA Search for other works by this author on: GSW Google Scholar Bruce Beaudoin; Bruce Beaudoin 1Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA Search for other works by this author on: GSW Google Scholar Leonardo Seeber; Leonardo Seeber 2Lamont-Doherty Earth Observatory, Palisades, New York 10964, USA Search for other works by this author on: GSW Google Scholar John Armbruster John Armbruster 2Lamont-Doherty Earth Observatory, Palisades, New York 10964, USA Search for other works by this author on: GSW Google Scholar Geology (2001) 29 (7): 651–654. https://doi.org/10.1130/0091-7613(2001)029<0651:SCOAAM>2.0.CO;2 Article history received: 28 Aug 2000 rev-recd: 02 Mar 2001 accepted: 13 Mar 2001 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Anne Meltzer, Golam Sarker, Bruce Beaudoin, Leonardo Seeber, John Armbruster; Seismic characterization of an active metamorphic massif, Nanga Parbat, Pakistan Himalaya. Geology 2001;; 29 (7): 651–654. doi: https://doi.org/10.1130/0091-7613(2001)029<0651:SCOAAM>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Earthquakes recorded by a dense seismic array at Nanga Parbat, Pakistan, provide new insight into synorogenic metamorphism and mass flow during mountain building. Microseismicity beneath the massif drops off sharply with depth and defines a shallow transition between brittle failure and ductile flow. The base of seismicity bows upward, mapping a thermal boundary with 3 km of structural relief over a lateral distance of 12 km. Anomalously low seismic velocities are observed at the core of the massif and extend to depth through the crust. The main locus of seismicity and low velocities correlates with a region of high topography, rapid exhumation, high geothermal gradients, young metamorphic and igneous ages, and crustal fluid flow. We suggest a genetic link between these phenomena in which hot rocks, rapidly advected from depth, are pervasively modified at relatively shallow levels in the crust. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Late Cretaceous blueschist metamorphism in the Indus Suture Zone, Shangla region, Pakistan Himalaya

Rb–Sr and 39Ar– 40Ar phengitic muscovite dating of the transitional blueschist–greenschist facies rocks from the Shangla region of Pakistan Himalaya resulted in a concordant ca. 80 Ma age interpreted as a time of metamorphic peak. 39Ar– 40Ar age spectra of sodic amphiboles were unsuitable for precise age determination. Mass-balance calculations show that the K concentration in glaucophane/crossite is ca. 25 ppm, and the K–Ar budget is dominated by inclusions of phengitic muscovite. E–W-oriented (parallel to Kohistan arc) stretching lineation defined by preferred orientation of amphiboles was accompanied by synkinematic growth of riebeckite rims on the edges of microboudins. Evolution of the pressure conditions based on the Al 2O 3 content in sodic amphibole shows that this deformational stage was accompanied by a decrease in pressure from ca. 700 to ca. 400 MPa. This, together with our dating results, strongly suggests ≥10 km late Cretaceous exhumation of the Shangla blueschists. The exhumation may have been associated with the increase of arc parallel displacement rate due to changes of arc obliquity. This led to thinning caused by arc parallel extension and to exhumation of the Shangla blueschists. A lack of strong Eocene metamorphic overprint implies that after the peak metamorphism, blueschists were accreted to the Kohistan arc, which helped to prevent burial and metamorphism during continental collision. The final emplacement of the Shangla blueschists most likely occurred during the India–Kohistan arc collision and was accompanied by the weak, locally developed greenschist facies overprint.

Isotopic constraints on the evolution of metamorphic conditions in the Jijal-Patan complex and the Kamila Belt of the Kohistan arc, Pakistan Himalaya

Abstract Pressure-temperature data and Sm-Nd and Rb-Sr garnet ages are presented for retrogressed granulitic rocks of the Jijal-Patan complex and the Kamila Amphibolite Belt. Despite the retrogression and hydration, the two samples contain garnet and hydrous minerals that yield pressures and temperatures similar to previous estimates for pristine granulite facies rocks from the same area. The Sm-Nd and Rb-Sr garnet ages are concordant at 95–100 Ma for the two samples and for both isotopic systems. These ages are interpreted as dating cooling through 700–800 °C following magmatic crystallization and granulite facies metamorphism. In the case of the garnet amphibolites from the Kamila Belt, the garnets retain prograde major element zonation. In addition, the closure temperature for the Rb-Sr system is very close to the recorded temperature. For these reasons, the age of 100 Ma must represent a time close to that when the pressure and temperature preserved in the mineralogy and its chemistry was recorded. The isotopic equilibrium between garnet and paragonite at 90–100 Ma suggests that the regional hydration event that affected the lower crust of the Kohistan arc also occurred at this time. Cooling rates calculated from the Rb-Sr and Sm-Nd ages for the partially retrogressed granulite give a minimum of 3–6°C Ma −1 and imply a different tectonic mechanism for the exhumation of the lower crust than is typical for granulites. This process may be related to early regional decompression following the collision of the Kohistan arc with Eurasia.

Structural evolution of the western margin of the Nanga Parbat massif, Pakistan Himalaya: insights from the Raikhot-Liachar area

Abstract There are several competing interpretations of the structure of the margins of the Nanga Parbat massif: that the massif is bounded by the original suture between the Indian continent and the Kohistan-Ladakh island arc—the Main Mantle Thrust; that the massif is entirely bounded by neotectonic faults; that it is bounded by a combination of early and late faults and shear zones. If the marginal structures of the massif are to be related to local and regional geo-tectonic evolution then their correct characterization is critical. The Raikhot Bridge area on the western margin of the massif is useful in this regard, as it provides accessible and near-continuous outcrops. This contact, sometimes called the Raikhot Fault, is composite. Sheared metagabbros of the Kohistan arc are juxtaposed tectonically against metasediments and orthogneisses of the Nanga Parbat massif along an early ductile shear contact, developed under amphibolite facies conditions. In this regard it may be a preserved segment of the Main Mantle Thrust. However, this ductile shear zone has been strongly modified, flattened and rotated, and is cut by younger shears and faults. The original kinematics of the shear zone have been largely overprinted by these subsequent deformations. The younger structures include NE-SW striking, dextral strike-slip faults and a major top-to-NW thrust and shear zone. A sequence of metamorphism, deformation and igneous emplacement may be used to study the history of structural evolution within the massif. The use of a single name (e.g. Raikhot Fault) for the present-day map contact between the Nanga Parbat massif and neighbouring Kohistan is misleading. The early contact (termed here the Phuparash Shear Zone, possibly the northeastern continuity of the Main Mantle Thrust) is modified by the Buldar Fault Zone (dextral strike-slip) and the Liachar Thrust Zone (top-to NW carriage of the Nanga Parbat massif across the Phuparash Shear Zone and onto Kohistan). The activity of the Buldar Fault and Liachar Thrust Zone continued during exhumation of the massif, through amphibolite facies to the Earth’s surface. The interaction between these structures is at present unknown. However, establishing these and similar interactions within the Nanga Parbat area remain central to establishing the role of regional NE-SW dextral transpression in the modern structure of the massif.

Geological structure of the southern part of the Nanga Parbat massif, Pakistan Himalaya, and its tectonic implications

Abstract The Nanga Parbat massif lies in the core of the major north-south trending, broadly upright antiform that marks the NW syntaxis of the Himalayan arc. However, this antiformal structure is not evident in the trend of foliation and banding within the central and southern parts of the massif. Reconnaissance field studies in this region (Astor, Rama and Rupal areas) have delineated an important shear zone with top-to-the-south overthrust kinematics. This Rupal Shear Zone carries the migmatitic core of the massif onto nonmigmatitic metasediments locally termed the Tarshing Group. The shear zone traces north into a broad high strain zone of steep foliation with gently plunging mineral elongation lineations with no consistent sense of shear. A tentative model is proposed whereby top-to-the-south overshear in the Rupal area passes northwards into a steep belt of apparently constrictional N-S elongation. This type of large-scale transpression may record the early growth of the syntaxis. However, relating these structures to Himalayan orogenesis and the amplification of the NW syntaxis is problematic. The Nanga Parbat massif displays a long and complex history of polyphase deformation, metamorphism and magmatism, as might be expected of a terrane derived from the basement of the Indian sub-continent. Although at least the later part of the constrictional steep belt developed with syn-kinematic leucogranite intrusions (&lt; 10 Ma), the old age limit on the Rupal Shear Zone remains unconstrained.

Tectonics of the SW margin of the Nanga Parbat-Haramosh massif

Abstract We present an analysis of the tectonic evolution of the southwestern portions of the Nanga Parbat massif, Pakistan Himalaya, based upon detailed mapping and structural analyses from the Bunar, Biji, Diamir, Airl, Niat and SW Rupal valleys. Mainly metasedimentary cover rocks of the Indian plate are divided into upper and lower cover. There is a marked structural thinning of the cover in the main Bunar valley from south to north, and this is attributed to a major frontal ramp in the original Main Mantle Thrust (MMT). A hitherto unmapped shear zone, the Diamir Shear Zone, is identified, that is associated with a syn-kinematically intruded belt of granitic rocks, the Jalhari Granite. The shear zone is a several kilometre thick, generally W-vergent, ductile to brittle shear zone that is associated with local overturning of the entire MMT section, typified by the Gashit Fold. 40 Ar/ 39 Ar cooling ages from across the area indicate a steep cooling age gradient across the Diamir Shear Zone from &gt;40 to &lt;5 Ma. The Diamir Shear Zone is mechanically linked to part of the Raikhot Fault System and, together, they are seen to be a crustal-scale reverse fault that has allowed relative uplift and overthrusting of the core of Nanga Parbat.

Glaucophane and barroisite eclogites from the Upper Kaghan nappe: implications for the metamorphic history of the NW Himalaya

Abstract This paper presents the results of a petrographical and mineral chemical study of glaucophane- and barroisite-bearing eclogites from the Upper Kaghan nappe in the Higher Himalayan Crystallines of the Pakistan Himalaya, and discusses the implications of the P-T path recorded in such rocks for the tectonic and metamorphic history of the NW Himalaya. The eclogites described here come from a previously undescribed outcrop at Gittidas, but belong to the same suite as the garnet-omphacite-phengite-quartz-rutile eclogites previously described elsewhere in the Upper Kaghan nappe. The metamorphic peak assemblage is garnet-omphacite-rutile-quartz in glaucophane eclogite and garnet-omphacite-zoisite-rutile±kyanite±phengite±ankerite in barroisite eclogite. Most samples contain a significant amount of amphibole, white mica and quartz. White mica may be present either as part of the peak assemblage (phengite) or as a retrogressive phase after kyanite (paragonite). Amphibole is later than the metamorphic peak assemblage and is barroisite in most samples, but in relatively Fe-rich eclogite it is glaucophane with significant Na in the A site, Ca in the M4 site and tetrahedral A1. Garnet displays strong prograde zoning in the barroisite eclogites, with Mg increasing and Fe decreasing from core to rim. The iron-rich core is crowded with mineral inclusions of the peak assemblage, but inclusions of earlier paragonite, green and blue-green amphibole were also found. Peak metamorphic conditions in the barroisite eclogites have been estimated at T = 610 ± 30 °C and P = 24 ± 2 kbar from Fe/Mg partition in garnet-omphacite pairs, and from the garnet-omphacite-phengite barometer. These values are close to the equilibration conditions estimated for the eclogites of the North Himalayan Tso-Morari Dome. Strong similarities between metamorphic evolution of the Upper Kaghan nappe and metamorphic evolution of the eclogite-bearing units of the Neelum valley just to the east of the Kaghan valley indicate that the eclogite occurrences of the Kaghan-Neelum area define an eclogite-bearing terrane of regional extent, which was subjected to high pressure metamorphism in middle Eocene times. A comparison of the metamorphic evolution recorded in the eclogites of the NW Himalaya with that of the granulitized eclogites recently discovered in the E Himalaya suggests the possibility of a Himalaya-wide eclogitic metamorphism of pre-Miocene age. Therefore, the main difference between the Higher Himalayan Crystalline nappes of the NW Himalaya and those of the E Himalaya appears to lie less in the early part of their metamorphic evolution than in the different P-T paths they followed during exhumation.

Mazeno Pass Pluton and Jutial Pluton, Pakistan Himalaya: age and implications for entrapment mechanisms of two granites in the Himalaya

Zircon and monazite U-(Th)-Pb ion microprobe analysis were performed on the Mazeno Pass pluton and the Jutial pluton, two leucogranite bodies within the Nanga Parbat-Haramosh massif (NPHM), Pakistan Himalaya. Zircon rim ages and monazite ages indicate the Mazeno Pass pluton in southwest NPHM intruded at 1.40 ± 0.05 Ma; the Jutial pluton, to the north, similarly yields concordant zircon and monazite ages suggesting crystallization at 9.45 ± 0.06 Ma. The Jutial pluton was subsequently intruded by leucogranite dikes at 5.3 Ma, as revealed by monazite ages. Concordancy of U-Pb and Th-Pb accessory mineral ages demonstrates the robustness of the technique on young rocks. Both plutons, some of the youngest in the Himalaya, have a general association with nearby shear zones that we interpret to have played an integral role in granite evolution and emplacement setting (`deformation enhanced ascent'). Together with new field observations, these results provide an insight on the spatial and temporal relationship between plutonism and deformation relating to the development of the massif.