Pakistan Himalaya Research Articles

Early Miocene anatexis identified in the western syntaxis, Pakistan Himalaya

Evidence for typical Himalayan Early to Middle Miocene anatexis has remained elusive in the Nanga Parbat massif in the western Himalaya of Pakistan; previous work has identified only young plutonism (10–1 Ma). New U–(Th)–Pb data from the Southern Chichi granite, a leucogranite in southern Nanga Parbat, reveal that crustal melting occurred during the Early Miocene. This largely undeformed, fine-grained pluton intrudes the Indian metasedimentary cover sequence adjacent to the Rupal shear, a major shear zone at Nanga Parbat. Th–Pb ion microprobe analyses of monazites from the Chichi granite yield ages between 22 Ma and 16 Ma, with the majority of analyses lying at 19–18 Ma. U/Pb zircon analyses yield ages which fall along a chord with a lower intercept age of 19 Ma. The zircons also contain an ∼1850 Ma inherited component. These data indicate that the Early Miocene anatexis that is ubiquitous in central portions of the Himalayan orogen, unreported anywhere in the NW Himalaya, also occurred in the western Himalayan syntaxis, and demonstrates that Nanga Parbat has a protracted melting history. A small granitic dike that cross-cuts the outer portion of the Rupal shear yields monazite ages between 22 Ma and 9 Ma, where the young ages correlate with high U concentrations. 40Ar– 39Ar biotite ages from adjacent gneisses indicate cooling by 10 Ma, requiring significant displacement on this portion of the Rupal shear to be older than ∼10 Ma and possibly as old as ∼20 Ma.

Isotope systematics of granites and gneisses of the Nanga Parbat Massif, Pakistan Himalaya

Isotope systematics of granites and gneisses of the Nanga Parbat Massif, Pakistan Himalaya

Incision and differential bedrock uplift along the Indus River near Nanga Parbat, Pakistan Himalaya, from 10Be and 26Al exposure age dating of bedrock straths

Well preserved, river-cut bedrock surfaces (straths) abandoned by the progressive incision of the Indus River in northern Pakistan were dated using in situ cosmogenic 10Be and 26Al exposure age dating. Measurements on young and modern straths show no significant ingrowth of 10Be and 26Al, implying that strath exposure ages closely approximate their abandonment age. Strath exposure ages range up to 65 ka and increase with height above the river. The straths can be divided into two groups: low straths dated at about 7 ka, which yield very rapid bedrock incision rates (9–12 m/ka), and high straths >7 ka in age, which yield somewhat slower incision rates (1–6 m/ka). Incision rates for both groups increase by ∼1–3 m/ka downstream toward an active reverse fault, the Raikot fault, that forms the western border of the Nanga Parbat–Haramosh massif (NPHM). The downstream increases in incision rates for the low straths mimic downstream increases in the modern gradient, suggesting that bedrock incision rates have been proportional to stream power over the last 7 ka. Gradient increases are probably the result of the creation of knickpoints by bedrock uplift along the Raikot fault. The longitudinal profile and steep gorge of the Indus River in the study area indicate that bedrock incision appears to keep pace with differential vertical motions between the NPHM and surrounding regions. Thus, the 1–3 m/ka difference in incision rates likely reflects the differential bedrock uplift rate between the NPHM and the Skardu Basin. The differential incision rate is consistent with differential exhumation rates obtained from apatite fission track ages assuming geotherms of ≤80–90°C/km in the NPHM and 35°C/km in the Skardu Basin. This relation suggests that rapid bedrock incision of at least 1–3 m/ka has persisted within the NPHM for the last 0.5 Ma. The acceleration of incision rates throughout the study area since 15 ka is likely to be a climatic signal that records an increase in the discharge and/or sediment load of the Indus River in the study area related to deglaciation of the surrounding terrane.

Geochemical and isotopic constraints on subduction polarity, magma sources, and palaeogeography of the Kohistan intra-oceanic arc, northern Pakistan Himalaya

Geochemical and isotopic data are presented for 18 representative samples from the intraoceanic phase of the Kohistan arc. A restricted range of initial 87 Sr/ 86 Sr (0.7036–0.7066) and ºNd (+2.8 to +7.4) along with measured 206 Pb/ 204 Pb (18.0–18.6) are consistent with formation of the arc complex in an intra-oceanic setting. The isotopic data demonstrate the involvement of enriched, DUPAL-type mantle, suggesting that the Kohistan arc formed at or south of the present equator. Subduction polarity inferred from geochemical and isotopic data indicate that the Chalt Volcanics and Kamila Amphibolites represent a forearc and backarc basin sequence, respectively. These inferences are most simply resolved with a tectonic model whereby the intra-oceanic Kohistan arc evolved over a south-dipping subduction zone, implying that Kohistan and India moved northwards on the same plate, although separated, during much of Cretaceous time. Collision of Kohistan with the Karakorum caused a new, north-dipping subduction zone to form on the south side of Kohistan, leading to collision with India in early Tertiary time.

Regional waveform calibration in the Pamir‐Hindu Kush region

Twelve moderate‐magnitude earthquakes (mb 4–5.5) in the Pamir‐Hindu Kush region are investigated to determine their focal mechanisms and to relocate them using their regional waveform records at two broadband arrays, the Kyrgyzstan Regional Network (KNET), and the 1992 Pakistan Himalayas seismic experiment array (PAKH) in northern Pakistan. We use the “cut‐and‐paste” source estimation technique to invert the whole broadband waveforms for mechanisms and depths, assuming a one‐dimensional velocity model developed for the adjacent Tibetan plateau. For several large events the source mechanisms obtained agree with those available from the Harvard centroid moment tensor (CMT) solutions. An advantage of using regional broadband waveforms is that focal depths can be better constrained either from amplitude ratios of Pnl to surface waves for crustal events or from time separation between the direct P and the shear‐coupled P wave (sPn + sPmP) for mantle events. All the crustal events are relocated at shallower depths compared with their International Seismological Centre bulletin or Harvard CMT depths. After the focal depths are established, the events are then relocated horizontally using their first‐arrival times. Only minor offsets in epicentral location are found for all mantle events and the bigger crustal events, while rather large offsets (up to 30 km) occur for the smaller crustal events. We also tested the performance of waveform inversion using only two broadband stations, one from the KNET array in the north of the region and one from the PAKH array in the south. We found that this geometry is adequate for determining focal depths and mechanisms of moderate size earthquakes in the Pamir‐Hindu Kush region.

Interactions between deformation, magmatism and hydrothermal activity during active crustal thickening: a field example from Nanga Parbat, Pakistan Himalayas

AbstractThe Nanga Parbat massif is a rapidly eroding, thrust-related antiform that is distinct from other regions of the Himalayan orogen in being both intruded by Late Miocene-Pliocene anatectic granites and permeated by a vigorous hydrothermal system. Exhumation is achieved by erosion during thrusting along the Liachar thrust in the apparent absence of extensional tectonics. At depths in excess of 20 km, small batches of leucogranitic melt have been generated by fluid-absent breakdown of muscovite from metapelitic lithologies. These melts ascend several kilometres prior to emplacement, aided by low geothermal gradients at depth and by interaction with meteoric water as they reach shallow levels. At intermediate depths (∼15 km) limited fluid infiltration is restricted to shear zones resulting in localised anatexis. Within the upper 8 km of crust, magmatic and meteoric fluid fluxes are channelised by active structures providing a feedback mechanism for focusing deformation. Leucogranite sheets show a range of pre-full crystallization and high-temperature crystal-plastic textures indicative of strain localisation onto these sheets and away from the country rocks. At subsolidus temperatures meteoric fluids promote strain localisation and may trigger cataclastic deformation. Since near-surface geothermal gradients are unusually steep, the macroscopic transition between distributed shearing and substantial, but localised, cataclastic deformation occurred at amphibolite-facies conditions (∼600°C). Even with the greatest topographic relief in the world, the meteoric system of Nanga Parbat is effectively restricted to the upper 8 km of the crust, strongly controlled by active structures.

A re-evaluation of the stratigraphy and evolution of the Kohistan arc sequence, Pakistan Himalaya: implications for magmatic and tectonic arc-building processes

New field mapping and structural data, combined with published geochemical data, from the Kohistan arc in the NW Himalaya, enable a re-evaluation of the arc stratigraphy. Key lithological units and their relationships are more clearly defined, permitting the construction of a revised magmatic-tectonic history for the arc. The oldest units are transitional oceanic-type basalts, which form the basement to the subduction related sequence. Arc-type gabbroic sheets and plutons intrude the oceanic basalts: together these form the Kamila Amphibolite Belt. Metasediments and basaltic lavas were deposited, within an extensional basin, onto the Kamila Amphibolite Belt basement. This sequence, exposed across the arc, forms a distinct stratigraphic unit which is formally defined here as the Jaglot Group. Sediment-charged turbidity currents transported material into the basin, whilst submarine eruptions contributed the basaltic component. This period of extension culminated in the eruption of high-Mg boninites of the Chalt Volcanic Group which overlie the rocks of the Jaglot Group. The earliest granitoids of the Kohistan Batholith predate suturing and intrude the Jaglot and Chalt sequences. At c. 100 Ma Kohistan sutured to Asia. suturing being accompanied by thickening of the arc with the development of major intra-arc shear zones and a penetrative, regionally developed steep cleavage. At c. 85 Ma intra-arc rifting permitted the emplacement into the arc of the voluminous gabbronorites of the Chilas Complex which clearly intrudes the Kamila Amphibolite Belt to the south and the Jaglot Group to the north. The Chilas Complex has been regarded as part of the pre-suturing, juvenile arc sequence. Field evidence summarized here show this to be not so. Heat advection associated with emplacement of the Complex caused amphibolite facies regional metamorphism, melting of the lower arc and plutonism. Some of the resultant granitoid plutons were unroofed and eroded during a compressional phase at between 80 and 55 Ma, before emplacement of further plutons and extrusion of basaltic through to rhyolitic volcanic rocks at between 55 and 40 Ma. At least three phases of extension and rifting, each separated by short lived phases of compression, charac­terized arc evolution. Much of the magmatism is controlled by extensional tectonics within the overriding plate of the kind commonly associated with a retreating subduction zone.

Structural evidence for back sliding of the Kohistan arc in the collisional system of northwest Pakistan

Research Article| August 01, 1996 Structural evidence for back sliding of the Kohistan arc in the collisional system of northwest Pakistan Jean-Pierre Burg; Jean-Pierre Burg 1Geologisches Institut, ETH-Zentrum, Sonneggstrasse 5, CH-8092, Zürich, Switzerland Search for other works by this author on: GSW Google Scholar M. Nawaz Chaudhry; M. Nawaz Chaudhry 2Institute of Geology, Punjab University, Quaid-e-Azam Campus, 54590 Lahore, Pakistan Search for other works by this author on: GSW Google Scholar Munir Ghazanfar; Munir Ghazanfar 2Institute of Geology, Punjab University, Quaid-e-Azam Campus, 54590 Lahore, Pakistan Search for other works by this author on: GSW Google Scholar Robert Anczkiewicz; Robert Anczkiewicz 1Geologisches Institut, ETH-Zentrum, Sonneggstrasse 5, CH-8092, Zürich, Switzerland Search for other works by this author on: GSW Google Scholar David Spencer David Spencer 1Geologisches Institut, ETH-Zentrum, Sonneggstrasse 5, CH-8092, Zürich, Switzerland Search for other works by this author on: GSW Google Scholar Author and Article Information Jean-Pierre Burg 1Geologisches Institut, ETH-Zentrum, Sonneggstrasse 5, CH-8092, Zürich, Switzerland M. Nawaz Chaudhry 2Institute of Geology, Punjab University, Quaid-e-Azam Campus, 54590 Lahore, Pakistan Munir Ghazanfar 2Institute of Geology, Punjab University, Quaid-e-Azam Campus, 54590 Lahore, Pakistan Robert Anczkiewicz 1Geologisches Institut, ETH-Zentrum, Sonneggstrasse 5, CH-8092, Zürich, Switzerland David Spencer 1Geologisches Institut, ETH-Zentrum, Sonneggstrasse 5, CH-8092, Zürich, Switzerland Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1996) 24 (8): 739–742. https://doi.org/10.1130/0091-7613(1996)024<0739:SEFBSO>2.3.CO;2 Article history 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 Jean-Pierre Burg, M. Nawaz Chaudhry, Munir Ghazanfar, Robert Anczkiewicz, David Spencer; Structural evidence for back sliding of the Kohistan arc in the collisional system of northwest Pakistan. Geology 1996;; 24 (8): 739–742. doi: https://doi.org/10.1130/0091-7613(1996)024<0739:SEFBSO>2.3.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 In the Naran region of the Pakistan Himalayas, a regionally distributed second generation of folds results from northward-directed shear deformation. These folds are collapse structures associated with back sliding of the hanging wall, namely the Kohistan paleo–island arc. They are explained by a geometrical model that combines coeval vertical and horizontal shortening. Accordingly, they are synconvergence collapse features that indicate at least 5 km of vertical shortening of the imbricate thrust slices derived from the Indian continent in the northwest Himalayas. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Structural and metamorphic evolution of the Nanga Parbat syntaxis, Pakistan Himalayas, on the Indus gorge transect: The importance of early events

AbstractField relationships are as important as radiometric age data for unravelling the history of polycyclic metamorphic regions. Field and petrographic data were used to investigate such a region, the Nanga Parbat syntaxis ‐ a major N‐trending transverse structure in the Pakistan Himalayas. Continental rocks of the Indian plate are exposed in the core of this structure due to uplift relative to the surrounding and structurally higher amphibolites of the overthrust Kohistan Island Arc. In the Indus gorge, the Indian plate gneisses are migmatitic, highly deformed and cut by Himalayan granite sheets. Field relationships show that the migmatization pre‐dates the emplacement of a suite of basic intrusive sheets (now amphibolites) which are themselves unlikely to be Himalayan; these amphibolite sheets are cut by the Himalayan granite sheets. There is therefore no genetic link between the granites and the migmatization: the latter might be Precambrian. During the pre‐Himalayan evolution, an eastern group of metapelitic migmatites underwent biotite dehydration melting at granulite facies conditions, leading to the development of assemblages with garnet, orthoclase and kyanite. Melt was not completely lost and back‐reacted during cooling to give biotite‐kyanite coronas around garnets. A western group of orthogneisses is characterized by foliated muscovite and biotite; orthoclase is never found with either garnet or aluminium silicate. In this second group, partial melting and small‐scale segregation gave rise to the migmatitic appearance, but melt was not lost and back‐reacted fully to regenerate the micas.The migmatites and basic sheets were strongly deformed at amphibolite facies during Himalayan collision, producing LS tectonites, mylonitic in places. This strong fabric was then tightly folded on large scale. The NNE trending fold is slightly oblique to the earlier N‐S stretching lineation; this has been reoriented on the steep limbs, but its plunge remains shallow. No new fabric is seen associated with this folding. On the western margin, steeply plunging lineations are associated with shear bands in gneisses, indicating relative uplift of the syntaxis. In contrast, on the steep eastern margin, relative uplift of the syntaxis appears to have been due to rotation without the development of new fabrics. There is no evidence for further melting during this Himalayan evolution, so the granite sheets cannot have been derived locally. This study highlights the importance of distinguishing pre‐Himalayan and Himalayan events in this polymetamorphic region, and the usefulness of field relationships in addition to geochronological constraints in such a task.

Where is (Is there a?) Main Central Thrust in the NW Himalayas of Pakistan?

The term "Main Central Thrust" was first introduced into Himalayan terminology in 1939. Still today, much discussion exists about whether the Main Central Thrust is actually one, two or three thrust surfaces, especially in the Central Himalaya of India and Nepal. As the Main Central Thrust is widely accepted as the intracontinental thrust that separates the Higher and Lesser Himalaya, its recognition is therefore implicit in using these unit names in the Pakistan Himalaya. The location of any "Main Central Thrust" in the Pakistan area of the Northwest Himalayan region is likely to remain controversial for some time. This is simply because the area where the Main Central Thrust was last seen, to the south of the Kashmir Basin, was some 100 km away from the next suggested location in Neelum and Kaghan Valleys. This intervening area between the last known location and any new suggested location is only scantily mapped in part. Eleven suggestions of locations of the "Main Central Thrust" have been made in Pakistan and a review of their indicative features is given for discussion. Only one position of the Main Central Thrust, at Batal in the Kaghan Valley, is generally accepted by the majority of northwest Himalayan workers (who do believe in a continuation of the MCT) as the correct location for a major tectonic contact which might correlate with the Main Central Thrust.

Active hydrothermal systems during the recent uplift of Nanga Parbat, Pakistan Himalaya

During the last 10 m.y., the Nanga Parbat Haramosh Massif in the northwestern Himalaya has been intruded by granitic magmas, has undergone high‐grade metamorphism and anatexis, and has been rapidly uplifted and denuded. As part of an ongoing project to understand the relationship between tectonism and petrologic processes, we have undertaken an isotopic study of the massif to determine the importance of hydrothermal activity during this recent metamorphism. Our studies show that both meteoric and magmatic hydrothermal systems have been active over the last 10 m.y. We suggest that the rapid uplift of the massif created a dual hydrothermal system, consisting of a near‐surface flow system dominated by meteoric water and a flow regime at deeper levels dominated by magmatic/metamorphic volatiles. Meteoric fluids derived from glaciers near the summit of Nanga Parbat were driven deep into the massif along the transpressional faults causing δ18O and δD depletions in the gneisses and marked oxygen isotopic disequilibrium between mineral pairs from the fault zones. The discharge of these meteoric fluids occurs in active hot springs that are found along the steep faults that border the massif. At deeper levels within the massif, infiltration of low δ18O magmatic fluids caused δ18O depletions in the gneisses within the migmatite zone. These low δ18O fluids were derived from the young (&lt;4 Ma), relatively low δ18O granites (∼8‰c) that are found within the core of the massif. Geochronological evidence in the form of fission track and 40Ar/39Ar cooling ages and U/Pb ages on accessory minerals from the granites and gneisses provide a constraint on the timing of fluid flow in the surface outcrops we examined. Fluid infiltration in the migmatite zone rocks located along the Tato traverse was coeval with metamorphism, granite emplacement, and rapid denudation, in the interval 0.8–3.3 Ma. Finally, we infer from the presence of active hot springs that significant flow systems continue to be active at depth within the central portion of the Nanga Parbat‐Haramosh Massif.

Relocation analysis of earthquakes near Nanga Parbat‐Haramosh Massif, northwest Himalaya, Pakistan

Locations of earthquakes that have occurred between 1970 and 1988 in the northwest Himalaya of Pakistan, as computed by the ISC, were analyzed to improve knowledge of the spatial distribution of seismicity near the Nanga Parbat‐Haramosh Massif (NPHM). A set of 15 earthquakes were relocated using the multiple‐event relocation technique of Jordan and Sverdrup [1981]. Earthquakes located using records from at least 25 to 30 stations tend to be well‐located in the ISC database: smaller error ellipses and shorter distance of relocation. These analyses indicate that earthquakes with magnitudes mb &gt; 4.0 have occurred since 1970 along the NPHM.

Characteristics and mitigation of the snow avalanche hazard in Kaghan Valley, Pakistan Himalaya

Snow avalanche hazards in mountainous areas of developing countries have received scant attention in the scientific literature. The purpose of this paper is to describe this hazard and mitigative measures in Kaghan Valley, Pakistan Himalaya, and to review alternatives for future reduction of this hazard. Snow avalanches have long posed a hazard and risk to indigenous populations of the Himalaya and Trans-Himalaya mountains. Land use intensification due to population growth, new transportation routes, military activity and tourism is raising levels of risk. The history of land use in the study area is such that investigations of avalanche hazard must rely on different theoretical bases and data than in most industrialised countries. Despite the intensive use of valley-bottom land which is affected by avalanches, a number of simple measures are currently employed by the indigenous population to mitigate the hazard. Out-migration during the winter months is the most important one. During the intensive use period of summer avalanche-transported snow provides numerous resources for the population. In Kaghan the avalanche hazard is increasing primarily as a result of poorly located new buildings and other construction projects. The large scale of avalanche activity there rules out any significant improvement or protection of the currently difficult winter access. Instead, future mitigation of the hazard should focus on protecting the small number of winter inhabitants and minimising property damage.

Direct evidence for a steep geotherm under conditions of rapid denudation, Western Himalaya, Pakistan

Recent fluid-inclusion and 40 Ar/ 39 Ar cooling-age data show that currently exposed basement rocks in the Raikhot glacier valley of the Nanga Parbat-Haramosh massif, Pakistan Himalaya, were at temperatures of 350 ± 50 °C at depths of 6 ± 2 km (hydrostatic pressure correction). These data imply the presence of a steep thermal gradient in the upper crust at 1 Ma (29-100 °C/km) and denudation rates over the past 1.0 m.y. of 3-6 mm/yr, providing independent corroboration of previous estimates of rapid denudation at Nanga Parbat (4.5 mm/yr over 3.3 m.y.). Our data provide direct documentation of near-surface compaction of isotherms under conditions of rapid denudation, a result that has long been supported by thermal modeling.

Metamorphism and Melting within the Nanga Parbat Syntaxis, Pakistan Himalaya

Metamorphism and Melting within the Nanga Parbat Syntaxis, Pakistan Himalaya

The Himalayan passive margin from Precambrian to Cretaceous times

The Himalaya passive margin should be separated into the Indian Himalaya and the Pakistan Himalaya, divided by the Nanga Parbat-Haramosh basement uplift in the northwestern syntaxes. The Indian passive margin shows a complex history involving splitting of microcontinents off the northern Gondwana margin from early Paleozoic times until the Jurassic. In contrast, the Pakistan Himalaya formed part of the stable Indian shelf until separation from Africa started in the Jurassic. Each margin has a distinct stratigraphic history until the mid-Mesozoic after which they show similar histories until the Tertiary.The Himalaya mountains in India consist of three distinct tectonic units juxtaposed during the Neogene by southward thrusting of the northern Indian Precambrian to Mesozoic passive margin. The Lesser Himalaya on the south forms a late Precambrian passive margin usually directly overlain by late Cretaceous to Tertiary clastics: its stratigraphy is closest to that of the Indian Shield. In contrast, the High Himalaya has a thin Ordovician to Carboniferous shelf sequence deposited after early Ordovician deformation and granite intrusion. Above this, thick Permian and Mesozoic shelf sequences mark the separation of continental blocks off the northern Indian margin and the opening of the Neotethys ocean. The North Himalaya formed the slope and basin of this ocean and consists of reactivated Paleozoic gneiss domes overlain by thick Mesozoic sediments which pass, in the northeastern Himalaya, into an enormously thick sedimentary sequence resting on oceanic crust.Paleomagnetic and structural evidence indicates at least 500 km of southward thrusting along the Main Central Thrust between the Lesser and High Himalaya. This thrusting, together with a further 250 km of erosion at the front of the nappes, has removed the entire inner shelf of the Himalaya passive margin and can explain the startling contrast between the High and Lesser Himalaya stratigraphic sequences. But there are still discrepancies, particularly in the eastern Himalaya, where Precambrian basement is juxtaposed with the North Himalaya Mesozoic slope. These discrepancies can be partly resolved by strike-slip movements roughly parallel to the Himalaya which have removed parts of the northern Indian passive margin. Such faults of the requisite orientation, displacement and age occur in southeast Asia where their cumulative displacements can add up to several thousand kilometres. Similar faults, of undoubted pre-Tertiary age, occur in the Pakistan Himalaya. Here, the passive margin shows a much simpler history of Jurassic to Miocene subsidence.

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 &gt; 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.

Synchronous anatexis, metamorphism, and rapid denudation at Nanga Parbat (Pakistan Himalaya)

The Nanga Parbat-Haramosh massif of the western Himalaya is a north-trending half-window of Indian crust that provides spectacular exposures of Precambrian basement gneisses that have been overprinted by Himalayan metamorphism. We report here petrologic data and U/Pb dates on zircon and monazite which document that Nanga Parbat gneisses underwent a Pliocene-Pleistocene episode of high-grade metamorphism and anatexis during an interval in which the Nanga Parbat massif was undergoing rapid denudation at mean rates of ∼5 mm/yr. We speculate that by initiating decompression melting, this denudation may be at least partly responsible for the anatexis and high-grade metamorphism.

High-pressure metamorphism in the High Himalayan Crystallines of the Stak valley, northeastern Nanga Parbat-Haramosh syntaxis, Pakistan Himalaya

Abstract The High Himalayan Crystallines (HHC) from the little-known NE termination of the Nanga Parbat-Haramosh syntaxis (Stak and upper Turmik valleys, Pakistan) consist of kyanite-bearing gneiss with minor garnet-granulite and garnet-amphibolite. The HHC underwent a Himalayan metamorphism with a peak at high pressure (8–13 kbar) and high temperature (650–700° C). During exhumation the HHC rocks followed a rapid exhumation path at high temperature with little or no medium to low pressure re-equilibration. These lines of evidence, combined with geochronological and petrological data from Eocene eclogites recently found in the Kaghan nappe, indicate that, after subduction and high to very high pressure metamorphism, part of the HHC from northern Pakistan underwent very rapid cooling and exhumation. By contrast, exhumation along paths of increasing temperature are recorded by the HHC from regions located east of Pakistan (e.g. Nepal). These differences along strike in the HHC suggest that, during the Eocene collision between India and Eurasia, subduction and exhumation occurred at higher rates in northwestern than in central-eastern Himalaya.

The role of erosion and extension in unroofing the Indian Plate thrust stack, Pakistan Himalaya

AbstractIn north Pakistan cooling history data show that metamorphism within the Indian Plate predated 40 Ma, and that the post-metamorphic thrust stack developed within the crystalline internal zones had cooled to less than 100 °C by c. 18 Ma. Much of this cooling occurred during late Oligocene to early Miocene time and can be equated to substantial unroofing of the metamorphic pile. This unroofing was by a combination of erosion, recorded in Lower Miocene molasse deposits within the foreland basins, and by large scale hinterland (northward) directed extensional normal faults developed within the upper parts of the Indian Plate and within the Kohistan–India suture zone and operative as late as 20 Ma. As up to 20 km of material was removed during exhumation, substantial uplift must have been synchronous with exhumation. Part of this may be accounted for by isostatic rebound of the thickened Indian Plate, and part by uplift in the hanging wall of major south-verging thrusts developed at the base of the crystalline pile.