Nanga Parbat Massif Research Articles

Granitoids of the western Himalaya and Karakoram as potential geothermal reservoirs – A petrological, geochemical and petrophysical study

Hot springs in various granitoid and gneissic complexes in northern Pakistan indicate elevated geothermal gradients, which warrants their evaluation for geothermal applications. Evaluation of such prospects requires extensive knowledge of rock properties and their interaction with reservoir fluids. Our contribution provides first-order information on petrophysical, geochemical, and petrographic descriptions derived from outcrop analogs. About seventy samples of (mostly) granitoids and gneisses from the Nanga Parbat Massif (NPM), the Kohistan-Ladakh Batholith (KLB), and the Karakoram Batholith (KB) were collected. Biotite, K-feldspar, and plagioclase in the granitoid and gneiss show weak to moderate alteration, which increases in shear zones, suggesting fracture-assisted fluid interaction. Geochemical results indicate that the gneisses and granites of the NPM are mostly peraluminous S-type and show enrichment in most LREEs and depletion in HREEs. In addition, depleted Sr and Ba and enriched Rb and U in granites indicate partial melting and high fractionation. On the contrary, granitoids from the KLB are of calc-alkaline I-type and characterized by the depletion of REEs and the enrichment in Ba and Sr. The granitoids of the KB, due to their different magmatic histories, are more diverse, ranging from older I-type calc-alkaline granodiorite, and younger I-type alkaline syenite and S-type calc-alkaline granite. They show an overall enrichment in HREEs along with Ba, Th, Ta, Sr, and Lu. Allanites from syenite of the KB show significant concentrations of Th and U.Petrophysical measurements reveal low matrix porosities (0.6–3.5 %), with primarily average thermal conductivities (1.48–3.37 W m−1K −1) and thermal diffusivities (0.68–1.95 ∙10–6 m2 s−1), with minor variation in specific heat capacities (744–767 J kg−1 K−1). The NPM displays higher average thermal conductivity, thermal diffusivity, and lower porosity than the KB, while the petrophysical properties of the KLB range between these two domains.The geothermal systems in the area operate due to the thickening of a granitoid-dominant crust, which is enriched with radiogenic elements. The fault zones provide channels for meteoric water to access deeply these hot and thermally conductive rocks in the subsurface. After heat exchange, the water is discharged back to the surface as hot springs. The present data set provides a better understanding of regional geothermal regimes from petrological, geochemical, and petrophysical perspectives and assists in numerical modeling for potential geothermal assessment.

Application of in-situ gamma spectrometry for radiogenic heat production estimation in the Western Himalaya, Kohistan, and Karakoram in northern Pakistan

The Himalaya, Kohistan, and Karakoram ranges comprise Proterozoic to Cenozoic crystalline complexes exposed in northern Pakistan. Numerous hot springs in the area indicate high subsurface temperatures, prompting a need to evaluate the local contribution of radiogenic heat to the general orogenic-related elevated geothermal gradients. The current study employed a portable gamma spectrometer to estimate the in-situ radiogenic heat production in the Nanga Parbat Massif, Kohistan–Ladakh batholith, and the Karakoram batholith. Heat production in the Nanga Parbat Massif is high, with a range from 0.2 to 10.8 µWm−3 and mean values of 4.6 ± 2.5 and 5.9 ± 1.9 µWm−3 for gneisses and granites, respectively. By contrast, the heat production is low in the Kohistan–Ladakh batholith, ranging from 0.1 to 3.1 µWm−3, with the highest mean of 2.0 ± 0.5 µWm−3 in granites. The Karakoram batholith shows a large variation in heat production, with values ranging from 0.4 to 20.3 µWm−3 and the highest mean of 8.4 ± 8.3 µWm−3 in granites. The in-situ radiogenic heat production values vary in different ranges and represent considerably higher values than those previously used for the thermal modeling of Himalaya. A conductive 1D thermal model suggests 93–108 °C hotter geotherms, respectively, at 10 and 20 km depths due to the thick heat-producing layer in the upper crust, resulting in a surface heat flow of 103 mWm−2. The present study provides first-order radiogenic heat production constraints for developing a thermal model for geothermal assessment.

A modern pulse of ultrafast exhumation and diachronous crustal melting in the Nanga Parbat Massif

We combine monazite petrochronology with thermal modeling to evaluate the relative roles of crustal melting, surface denudation, and tectonics in facilitating ultrafast exhumation of the Nanga Parbat Massif in the western Himalayan syntaxis. Our results reveal diachronous melting histories between samples and a pulse of ultrafast exhumation (9 to 13 mm/year) that began ~1 Ma and was preceded by several million years of slower, but still rapid, exhumation (2 to 5 mm/year). Recent studies show that an exhumation pulse of similar timing and magnitude occurred in the eastern Himalayan syntaxis. A synchronous exhumation pulse in both Himalayan syntaxes suggests that neither erosion by rivers and/or glaciers nor a pulse of crustal melting was a primary trigger for accelerated exhumation. Rather, our results, combined with those of recent studies in the eastern syntaxis, imply that larger-scale tectonic processes impose the dominant control on the current tempo of rapid exhumation in the Himalayan syntaxes.

Zircon U‐Pb Age Constraints on NW Himalayan Exhumation From the Laxmi Basin, Arabian Sea

AbstractThe Indus Fan, located in the Arabian Sea, contains the bulk of the sediment eroded from the Western Himalaya and Karakoram. Scientific drilling in the Laxmi Basin by the International Ocean Discovery Program recovered a discontinuous erosional record for the Indus River drainage dating back to at least 9.8 Ma, and with a single sample from 15.6 Ma. We dated detrital zircon grains by U‐Pb geochronology to reconstruct how erosion patterns changed through time. Long‐term increases in detrital zircon U‐Pb components of 750–1,200 and 1,500–2,300 Ma record increasing preferential erosion of the Himalaya relative to the Karakoram between 8.3–7.0 and 5.9–5.7 Ma. The average contribution of Karakoram‐derived sediment to the Indus Fan fell from 70% of the total at 8.3–7.0 Ma to 35% between 5.9 and 5.7 Ma. An increase in the contribution of 1,500–2,300 Ma zircons starting between 2.5 and 1.6 Ma indicates significant unroofing of the Inner Lesser Himalaya (ILH) by that time. The trend in zircon age spectra is consistent with bulk sediment Nd isotope data. The initial change in spatial erosion patterns at 7.0–5.9 Ma occurred during a time of drying climate in the foreland. The increase in ILH erosion postdated the onset of dry‐wet glacial‐interglacial cycles suggesting some role for climate control. However, erosion driven by rising topography in response to formation of the ILH thrust duplex, especially during the Pliocene, also played an important role, while the influence of the Nanga Parbat Massif to the total sediment flux was modest.

Recycling of subducted Indian continental crust and its significance on post-collisional ultrapotassic magmatism in southwestern Tibet

The petrogenesis of the Eocene (43–42 Ma) Nb-rich granitoid dykes from the Kohistan–Ladakh island arc provides insights into melting of the down-going Indian continental crust during the Indian-Eurasian continental collision. These Nb-rich granitoids (SiO2 = 53.8–72.3 wt%, Nb = 24.0–44.1 ppm) have high Sr/Y (41.2–76.8) and (La/Yb)N (15.6–36.8) ratios. Their geochemical and Sr–Nd–Hf isotopic compositions are distinct from those of the Kohistan–Ladakh basement (Eurasian continent), but similar to those of coevally metamorphic amphibolites (42–40 Ma) in the Nanga Parbat massif (Indian continent). This implies that the magma of the Nb-rich granitoids would be derived from partial melting of the subducted Indian continental plate. The biotites from the Nb-rich granitoids show high Mg# (up to 61) and Cr2O3 (up to 2.36 wt%) and low TiO2 (0–3.21 wt%). Some samples of the Nb-rich granitoids contain 2–3% phengites with SiO2 ranging from 48.33 to 51.74 wt% and calculated pressure of 1.6–0.6 GPa, indicating initial magma crystallization of the Nb-rich granitoids at high-pressure condition (depth > 55 km). We propose that partial melting of the subducted Indian continental crust occurred when it underthrusted into the Kohistan–Ladakh asthenosphere mantle and the resultant melts upward migrated and significantly modified the overlying lithosphere and the residual Indian continental crust sank into the deep mantle. Both the metasomatized lithospheric mantle and the residual Indian continental crust played a critical role in the formation of the Miocene ultrapotassic rocks in southwestern Tibet.

Multimineral Fingerprinting of Transhimalayan and Himalayan Sources of Indus-Derived Thal Desert Sand (Central Pakistan)

As a Quaternary repository of wind-reworked Indus River sand at the entry point in the Himalayan foreland basin, the Thal Desert in northern Pakistan stores mineralogical information useful to trace erosion patterns across the western Himalayan syntaxis and the adjacent orogenic segments that fed detritus into the Indus delta and huge deep-sea fan throughout the Neogene. Provenance analysis of Thal Desert sand was carried out by applying optical and semi-automated Raman spectroscopy on heavy-mineral suites of four eolian and 11 fluvial sand samples collected in selected tributaries draining one specific tectonic domain each in the upper Indus catchment. In each sample, the different types of amphibole, garnet, epidote and pyroxene grains—the four dominant heavy-mineral species in orogenic sediment worldwide—were characterized by SEM-EDS spectroscopy. The chemical composition of 4249 grains was thus determined. Heavy-mineral concentration, the relative proportion of heavy-mineral species, and their minerochemical fingerprints indicate that the Kohistan arc has played the principal role as a source, especially of pyroxene and epidote. Within the western Himalayan syntaxis undergoing rapid exhumation, the Southern Karakorum belt drained by the Hispar River and the Nanga Parbat massif were revealed as important sources of garnet, amphibole, and possibly epidote. Sediment supply from the Greater Himalaya, Lesser Himalaya, and Subhimalaya is dominant only for Punjab tributaries that join the Indus River downstream and do not contribute sand to the Thal Desert. The detailed compositional fingerprint of Thal Desert sand, if contrasted with that of lower course tributaries exclusively draining the Himalaya, provides a semi-actualistic key to be used, in conjunction with complementary provenance datasets and geological information, to reconstruct changes in paleodrainage and unravel the relationship between climatic and tectonic forces that controlled the erosional evolution of the western Himalayan-Karakorum orogen in space and time.

DEM Generation from Multi Satellite PlanetScope Imagery

Planet Labs have recently launched a large constellation of small satellites (3U cubesats) capable of imaging the whole Earth landmass everyday. These small satellites capture multiple images of an area on consecutive days or sometimes on the same day with a spatial resolution of 3–4 m. Planet Labs endeavors to operate the constellation in a nadir pointing mode, however, the view angle of these satellites currently varies within a few degrees from the nadir leading to varying B/H ratio for overlapping image pairs. Due to relatively small scene footprint and small off-nadir angle, the baseline to height ratio (B/H) of the overlapping PlanetScope images is often less than 1:10, which is not ideal for 3D reconstruction. Therefore, this paper explores the potential of Digital Elevation Model generation from this multi-date, multi-satellite PlanetScope imagery. The DEM generation from multiple PlanetScope images is achieved using a volumetric stereo reconstruction technique, which applies semi global matching in georeferenced object space. The results are evaluated using a LiDAR based DEM (5 m) over Mount Teide (3718 m) in Canary Islands and the ALOS (30 m) DEM on rugged terrain of the Nanga Parbat massif (8126 m) in the western Himalaya range. The proposed methodology is then applied on images from two PlanetScope satellites overpasses within a couple of minutes difference to compute the DEM of the Khurdopin glacier in the Karakoram range, known for its recent surge. The quantitative assessment of the generated elevation models is done by comparing statistics of the elevation differences between the reference LiDAR and ALOS DEM and the PlanetScope DEM. The Normalized Median of Absolute Deviation (NMAD) of the elevation differences between the computed PlanetScope DEM and LiDAR DEM is 4.1 m and the elevation differences for the ALOS DEM over stable terrain is 3.9 m. The results show that PlanetScope imagery can lead to sufficient quality DEM even with a small baseline to height ratio. Therefore, the daily PlanetScope imagery is a valuable data source and the DEM generated from this imagery can potentially be employed in numerous applications requiring multi temporal DEMs.

From crystal to crustal: petrofabric-derived seismic modelling of regional tectonics

Abstract The Nanga Parbat Massif (NPM), Pakistan Himalaya, is an exhumed tract of Indian continental crust and represents an area of active crustal thickening and exhumation. While the most effective way to study the NPM at depth is through seismic imaging, interpretation depends upon knowledge of the seismic properties of the rocks. Gneissic, ‘mylonitic’ and cataclastic rocks emplaced at the surface were sampled as proxies for lithologies and fabrics currently accommodating deformation at depth. Mineral crystallographic preferred orientations (CPO) were measured via scanning electron microscope (SEM)/electron backscatter diffraction (EBSD), from which three-dimensional (3D) elastic constants, seismic velocities and anisotropies were predicted. Micas make the main contribution to sample anisotropy. Background gneisses have highest anisotropy (up to 10.4% shear-wave splitting, AVs) compared with samples exhibiting localized deformations (e.g. ‘mylonite’, 4.7% AVs; cataclasite, 1% AVs). Thus, mylonitic shear zones may be characterized by regions of low anisotropy compared to their wall rocks. CPO-derived sample elastic constants were used to construct seismic models of NPM tectonics, through which P-, S- and converted waves were ray-traced. Foliation orientation has dramatic effects on these waves. The seismic models suggest dominantly pure-shear tectonics for the NPM involving horizontal compression and vertical stretching, modified by localized ductile and brittle (‘simple’) shear deformations.

Sediment provenance, reworking and transport processes in the Indus River by U–Pb dating of detrital zircon grains

We present new major and trace element data, together with U–Pb ages for zircon sand grains from the major tributaries of the Indus River, as well as the adjacent Ghaggar and Yamuna Rivers and from bedrocks within the Sutlej Valley, in order to constrain the origin of the sediment reaching the Arabian Sea. Zircon grains from the upper Indus are generally younger than 200 Ma and contrast with those from the eastern tributaries eroded from Himalayan sources. Grains younger than 15 Ma, which typify the Nanga Parbat Massif, comprise no more than 1–2% of the total, even in the upper Indus, showing that this terrain is not a major sediment producer, in contrast with the Namche Barwe Massif in the eastern Himalayan syntaxis. The Sutlej and Yamuna Rivers in particular are very rich in Lesser Himalayan-derived 1500–2300 Ma zircons, while the Chenab is dominated by 750–1250 Ma zircons, mostly eroded from the Greater Himalaya. The upper Indus, Chenab and Ravi yield zircon populations broadly consistent with the outcrop areas, but the Jhelum and the Sutlej contain many more 1500–2300 Ma zircons than would be predicted from the area of Lesser Himalayan rock within their drainages. A significant population of grains younger than 200 Ma in the sands of the Thar Desert indicates preferential eolian, monsoon-related transport from the Indus lower reaches, rather than reworking from the local rivers. Modelling of observed zircon ages close to the delta contrasts with modern water discharge. The delta is rich in zircons dating 1500–2300 Ma, while discharge from modern rivers carrying such grains is low. The modest size of the Sutlej, the richest source of these materials in the modern system, raises the possibility that the compositionally similar Yamuna used to flow westwards in the recent past. Our data indicate a non-steady state river with zircon transport times of 5–10 k.y. inferred from earlier zircon dating of delta sands. The modern delta zircons image an earlier, likely Early-Mid Holocene, erosional state, in which the Lesser Himalaya were more important as sediment suppliers. Early-Mid Holocene sands show much less erosion from the Karakoram–Transhimalaya compared to those deposited at the Last Glacial Maximum, or calculated from the modern discharge. We favour variations in summer monsoon intensity as the primary cause of these temporal changes.

Pleistocene melting and rapid exhumation of the Nanga Parbat massif, Pakistan: Age and P– T conditions of accessory mineral growth in migmatite and leucogranite

Rapid Pleistocene exhumation of the core of the Nanga Parbat massif (northwestern Himalayan syntaxis) is inferred by combining U-Pb dates from monazite, xenotime, and zircon from migmatitic rocks and a leucogranite dike with pressure estimates that are closely linked to dated events in the melting and crystallization history. Exhumation rates of ∼ 11–13 mm/a were calculated from (i) migmatitic rocks that were produced at ∼ 1.7 Ma and 5.0 kbar by dehydration melting of biotite on decompression and (ii) veins with garnet and cordierite that crystallized at ∼ 1.0 Ma and 3.5 kbar. Tourmaline-bearing leucogranitic dikes separated from the source and ascended to crystallize near their solidus at ∼ 0.7 Ma. Modeling of Th/U in the leucogranite magma based on Th/U and U-Pb data from monazite, xenotime, and zircon shows a decrease from 1.1 to 0.2 over a span of 0.15 Ma. The implied acceleration of exhumation at ∼ 1.7 Ma may be linked to mid-crust flow as the evolving thermal structure of the Neogene metamorphism encountered the biotite dehydration–melting reaction. The rapid exhumation may have resulted from significant lowering of the effective viscosity of mid-crustal rocks, leading to vertical channel flow into the core of the Nanga Parbat massif along bounding shear zones.

Fluctuations of Raikot Glacier during the past 70 years: a case study from the Nanga Parbat massif, northern Pakistan

AbstractThe Himalaya has some of the largest glacier concentrations outside the polar regions. Despite this, long-term measurements detecting the impact of global warming and changing precipitation patterns on glaciers are rare. The Nanga Parbat massif in northern Pakistan is an exception. The cartographer and glaciologist R. Finsterwalder investigated glacier dynamics of this mountain massif in the 1930s, and several other studies document changes since then. The aim of this study is to detect and analyse the changes of Raikot Glacier over the past seven decades. We use a multitemporal and multiscale approach, based on repeat terrestrial images, additional historical data and remotely sensed imagery (Corona, ASTER, Landsat, QuickBird). The multitemporal approach covers the period 1934–2007. While the analyses show a total glacier retreat of ~200 m in 73 years, this general trend was interrupted by a significant glacier advance between the 1950s and 1980s. Although down-wasting processes can be inferred from an increase in debris-covered area, a general trend of reduced glacier thickness does not appear significant over the whole observation period.

Land, Life, and Environmental Change in Mountains

One of the greatest challenges facing mountain scholars is to separate environmental change caused by human activities from change that would have occurred without human interference. Linking cause and effect is especially difficult in mountain regions where physical processes can operate at ferocious rates and ecosystems are sensitive to rapid degradation by climate change and resource development. In addition, highland inhabitants are more vulnerable to natural hazards and political-economic marginalization than populations elsewhere. This address focuses on the Nanga Parbat massif in the Himalaya Range of Pakistan, Garhwal Himalaya of northwest India, and Manaslu-Ganesh Himals of central Nepal. I have highlighted three special insights that geographers offer to increase understanding of human impacts on the stability of mountain landscapes. First, the mixed methods and theories we employ—quantitative and qualitative, postpositivist science and social theory, muddy-boots fieldwork linked with GIScience—together position geographers to resolve the debate over human-triggered changes in the physical landscape in mountains and explain the frequent disconnect between mountain science, policymaking, and resource management. Second, academic scholars and policymakers have come to realize that most problems require training, experience, and expertise in understanding physical and human systems. Third, modern techniques of measuring rates of geomorphic change help place the human factor in perspective and explain spatial variability of natural hazards. Forecasting environmental change remains elusive in “the perfect landscape” of mountains.

Petrology of Indus River sands: a key to interpret erosion history of the Western Himalayan Syntaxis

The Indus River has been progressively transformed in the last decades into a tightly regulated system of dams and channels, to produce food and energy for the rapidly growing population of Pakistan. Nevertheless, Indus River sands as far as the delta largely retain their distinct feldspar- and amphibole-rich composition, which is unique with respect to all other major rivers draining the Alpine–Himalayan belt except for the Brahmaputra. Both the Indus and Brahmaputra Rivers flow for half of their course along the India–Asia suture zone, and receive major contributions from both Asian active-margin batholiths and upper-amphibolite-facies domes rapidly exhumed at the Western and Eastern Himalayan syntaxes. Composition of Indus sands changes repeatedly and markedly in Ladakh and Baltistan, indicating overwhelming sediment flux from each successive tributary as the syntaxis is approached. Provenance estimates based on our integrated petrographic–mineralogical data set indicate that active-margin units (Karakorum and Transhimalayan arcs) provide ∼81% of the 250±50 10 6 t of sediments reaching the Tarbela reservoir each year. Partitioning of such flux among tributaries and among source units allows us to tentatively assess sediment yields from major subcatchments. Extreme yields and erosion rates are calculated for both the Karakorum Belt (up to 12,500±4700 t/km 2 year and 4.5±1.7 mm/year for the Braldu catchment) and Nanga Parbat Massif (8100±3500 t/km 2 year and 3.0±1.3 mm/year). These values approach denudation rates currently estimated for South Karakorum and Nanga Parbat crustal-scale antiforms, and highlight the major influence that rapid tectonic uplift and focused glacial and fluvial erosion of young metamorphic massifs around the Western Himalayan Syntaxis have on sediment budgets of the Indus system. Detailed information on bulk petrography and heavy minerals of modern Indus sands not only represents an effective independent method to constrain denudation rates obtained from temperature–time histories of exposed bedrock, but also provides an actualistic reference for collision-orogen provenance, and gives us a key to interpreting provenance and paleodrainage changes recorded by clastic wedges deposited in the Himalayan foreland basin and Arabian Sea during the Cenozoic.

Sediment flux in the modern Indus River inferred from the trace element composition of detrital amphibole grains

The drainage basin of the modern Indus River is composed of tectonic blocks that include oceanic arc units in the Indus Suture, reworked components of the Indian Plate and various parts of the former southern margin of Eurasia against which India collided. We investigate how these source units contribute to the modern bedload of the Indus River using the major and trace element composition of single detrital amphibole grains. Sediments from rivers eroding restricted drainage areas were analyzed to characterize the major potential source units. Samples taken at various localities downstream in the main Indus River allow the evolving provenance to be assessed. When coupled with existing bulk sediment Nd isotope data, a basic mass balance for the river can be constructed. The rapidly exhuming Southern Karakoram Metamorphic Complex is the dominant sediment source to the deep-sea Indus Fan. Although the Nanga Parbat Massif, located adjacent to the river's course is also rapidly exhuming, this is not an important source of sediment, probably because of its small area and the tectonic mechanism for its exhumation. The trace element composition of detrital amphibole grains can be a useful provenance indicator, especially in the identification of dominant sources in a complex mixed sediment environment. Trace element ratios Nb/Zr and Ba/Y were identified as being especially useful in discriminating detrital amphibole grains from various sources.

Geology, geochemistry and Ar–Ar geochronology of the Nangimali ruby deposit, Nanga Parbat Himalaya (Azad Kashmir, Pakistan)

The Nangimali ruby deposit in the southern part of the Nanga Parbat Himalaya, has been investigated through field work, geochemistry, stable and radiogenic isotopes. It outcrops in the Shontar valley in a large north-vergent syncline consisting of high-grade metamorphic gneisses capped by a metasedimentary series dominated by marbles and amphibolites. The ore-body is stratiform. Ruby is found within 0.1–2 cm thick shear-veinlets and gash veins cutting dolomitic marbles and carbonate-bearing bands. The marbles of the Nangimali Formation display restricted ranges in δ 18O (from 23.6 to 27.6‰ relative to SMOW) and in δ 13C (from −1.9 to 2.6‰ relative to PDB). Fluid infiltration along the shear-zone in the marble has no effect on the isotopic signatures of the carbonates. Fluids are metamorphic and CO 2 is derived from the decarbonation of marbles. Mass-balance and geochemical analyses suggest that the mobilisation by the fluids of aluminium and chromium in the marbles is sufficient to enable the formation of ruby in the shear-zone. Rubies have been indirectly dated using a stepwise 40Ar– 39Ar laser heating technique on syngenetic phlogopites. The Miocene age records a Neogene cooling in the South of the Nanga Parbat massif and a minimum formation age for ruby of 16 Ma.

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.

Crustal anatexis and its relation to the exhumation of collisional orogenic belts, with particular reference to the Himalaya

AbstractWe review the causes, mechanisms and consequences of crustal anatexis during the exhumation of metamorphic terranes, from a petrological perspective. During both prograde and retrograde metamorphism, limited influx of free hydrous fluids may result in small volumes of very hydrous melts, which cannot ascend far (if at all) before reaching their solidus. If thermal conditions for dehydration melting are attained in fertile micaceous crustal layers, much larger volumes of water-undersaturated granitic magmas may result, especially where limited external fluid influx raises water activities above those that may be buffered by dehydrating hydrous phases. Magmas have specific trace element characteristics depending on the reaction which formed them which, combined with accessory phase thermometry, may enable the (P-T) conditions of melting to be ascertained. Small volume-fraction magmas will typically remain as in situ migmatites unless their extraction is assisted by deformation. In turn, deformation will be focused in weaker partially molten zones, so that water-undersaturated magmas may often be mobilized. Once segregated, their ascent is limited by the rate of dyke propagation, and they may reach shallow levels (<2 kbar) before crystallizing. The complex interplay between deformation and melting is exemplified by the Miocene evolution of the central Himalaya, where thrust and normal faulting, melting and exhumation were all simultaneously active processes which were linked by feedback relations. In the Nanga Parbat Massif of the western Himalaya, rapid post-Miocene denudation and vigorous fluid flux enabled rocks to experience more than one episode of melting simultaneously, at different levels of the same exhuming crustal section.

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.

Tectonically driven fluid flow and gold mineralisation in active collisional orogenic belts: comparison between New Zealand and western Himalaya

Hydrothermal activity and mesothermal-styled gold mineralisation occurs near the main topographic divide of most active or young collisional mountain belts. The Southern Alps of New Zealand is used in this study as a model for the mineralising processes. The collisional tectonics results in a two-sided wedge-shaped orogen into which rock is transported horizontally. Upper crustal rocks pass through the orogen and leave the orogen by erosion, whereas lower crustal rocks are deformed into the mountain roots. High relief drives meteoric water flow to near the brittle–ductile transition. Lower to upper greenschist facies metamorphic reactions, driven by deformation at the crustal decollement and in the root, release water-rich fluids that rise through the orogen. Intimate chemical interaction between fluid and rock results in dissolution and later precipitation of gold, arsenic and sulphur. Fluid flow and mineralisation in the topographic divide region is facilitated by a network of steeply dipping faults and associated rock damage zones where oblique strike-slip faults intersect the thrust faults that strike subparallel to the main mountain range. The Nanga Parbat massif of the western Himalaya is an example of an active collisional zone which hosts hydrothermal activity but no gold mineralisation. The lack of gold mineralisation is due to the following factors: CO 2-dominated rising metamorphic fluid in dehydrated amphibolite-granulite facies metamorphic rocks does not dissolve gold and arsenic; hot (up to 400 °C) meteoric water confined to fractures in the gneiss limits dissolution of gold and arsenic; low density of hot water/dry steam, and low reduced sulphur content of fluid, restrict solubility of gold and arsenic; absence of fracture networks in the core of the massif and the small volumes of circulating fluid limit metal concentration; and lack of reactive rock compositions limits chemically mediated metal deposition.