Abstract Often described as a natural laboratory, the Himalaya are probably the ideal place in which to study ongoing continent-continent collision. This volume focuses on the geology of the northwestern part of the Himalaya which provides the most complete and best-exposed transect across the range. Here, in northern Pakistan and in Ladakh in northwest India, the full profile across the south Asian continental margin, and the north Indian margin is superbly exposed in mountains reaching as high as K2 (8611 m) and Nanga Parbat (8125 m). The south Asian geology is exemplified in the Karakoram and Hindu Kush ranges along the north and northwestern frontiers of Pakistan. The unique Kohistan-Dras island-arc terrane is sandwiched within the Tethyan suture zone between India and Asia. Rocks of the northern margin of the Indian Plate are exposed in both the Zanskar and the Pakistan Himalaya. The northern sedimentary carbonate platform of the Indian Plate, magnificently exposed in the mountains of Zanskar and Ladakh, is largely missing in Pakistan where the Kohistan arc has been obducted southward onto the metamorphosed rocks of the internal crystalline zones of the Indian Plate. The Nanga Parbat syntaxis represents an orogenic bend developed within a convergent zone in the thrust belt where the south-vergent thrusts of the central and eastern Himalaya swing around through 300 degrees.

Abstract A ‘blank on the map’ only 60 years ago, the Karakoram Range has been explored and surveyed with greater difficulty than the Himalaya and Tibet due to its rugged terrain and extensive glaciation. In the past ten years we have succeeded in doubling the number of gravity stations. A substantial improvement in coverage and overall quality was obtained by concentrating on previously unsurveyed areas and by validating older data with more accurate measurements. Our data were merged with earlier data, converted to full Bouguer anomalies and gridded. The resulting Bouguer anomaly map defines very precisely the gravimetric low associated with the Nanga Parbat-Haramosh syntaxis, and the huge negative anomalies between the Karakoram Fault and the Main Karakoram Thrust. Large negative values are now visible also in the Ghujerab-Khunjerab areas. Correlation of the topography and Bouguer anomaly shows that a plate of flexural rigidity with D = 2 × 10 24 Nm fits the coherence data in the Karakoram at all but two distinct frequency ranges centred at wavelengths of 80 and 300 km. In a rheologically layered lithosphere developing a buckling instability under horizontal compression, the observed spectral features of the topography and Bouguer gravity anomalies constrain the depth of the competent layers to be in the range 13–20 km and 50–75 km respectively.

Abstract Indian plate, granulite facies, migmatitic basement gneisses exposed within the Nanga Parbat syntaxis host at least two generations of mafic sheets. In the southern part of the syntaxis, concordant sheets yield Palaeo-Proterozoic model ages of 2.2–2.6 Ga, which probably date early stages of continental growth. In the northern part of the syntaxis the sheets include a suite of discordant, silica-saturated or oversaturated sub-alkaline basalts extracted from a slightly depleted sub-continental mantle. Nd model ages and an imprecise Sm-Nd isochron yield an age of emplacement at between 1.6 and 1.8 Ga. That these dykes cross-cut granulite facies migmatitic fabrics implies that peak metamorphism in the Indian plate gneisses was, at latest, Meso-Proterozoic and not Tertiary in age. Zircon and amphibole ages published elsewhere suggest that this metamorphism was probably c. 1850 Ma in age. That the basement gneisses were refractory by the Tertiary has implications for the derivation of leucogranite sheets during the Neogene. Although the gneisses experienced a Tertiary-aged metamorphism, it was to lower temperatures than the Meso-Proterozoic metamorphism. Unless the gneisses were rehydrated during the Tertiary, the leucogranites need to have been sourced from more fertile rocks underplating the granulite facies basement complex.

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.

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 > 40 to < 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.

Abstract Neogene events in the Nanga Parbat-Haramosh massif have obscured much of its earlier evolution. However, structural mapping of the eastern margin reveals a ductile contact zone preserving many features of the original Main Mantle Thrust that emplaced the Ladakh island arc over the Indian margin in the late Cretaceous. The sequence of ductile deformation was controlled both by the contrasting rheologies of the Ladakh island arc and the Main Mantle Thrust footwall, and the changing thermal regime during subduction, collision and burial. Preliminary P–T estimates indicate conditions during southward thrusting on the Main Mantle Thrust of c. 650 °C and 9.5 kbar, with later deformation (post-dating garnet growth) in some units at c. 500 °C and 7.4 kbar. The concordant fabrics and lithological boundaries on either side of the contact are only disrupted by a NW-vergent, brittle thrust south of the village of Subsar (Indus gorge) which cross-cuts the steepened Main Mantle Thrust Zone. This thrust is related to the neotectonic Liachar Thrust on the western margin of the massif, and is an expression of the regional tectonics at the western termination of the Himalayan arc. This late thrusting followed formation of the syntaxial antiform in Neogene times.

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 non-migmatitic 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 (< 10 Ma), the old age limit on the Rupal Shear Zone remains unconstrained.

Abstract New amphibole, muscovite and biotite Ar-Ar and K-Ar data and zircon and apatite fission track data are presented from the western margin of the Nanga Parbat syntaxis as well as from the Indus and Astor valley sections which cross the syntaxis. Amphibole data date a regional cooling through 500 °C at 25 ± 5 Ma and are inconsistent with earlier suggestions that the peak of regional metamorphism was Neogene in age, although there is no doubt that some rocks were still at upper amphibolite facies temperatures as recently as 5 Ma. The data can be used to constrain structural models for syntaxial uplift. After an initial phase of crustal-scale buckling, bodily uplift of the syntaxis was along subvertical shear zones developed along its margins, although with a significantly higher time-averaged strain rate for shears developed along the western margin than along the eastern margin. The latter may be antithetic to the former. These shears were operative from 10 to < 1 Ma. In the southwestern part of the syntaxis, this subvertical uplift was superseded, since 6 Ma, by uplift along moderately SE-dipping NW-vergent shears on the hanging wall of which are located Neogene-aged migmatites.

Abstract The rapid erosional unroofing of the Nanga Parbat Himalaya in late Cenozoic time is thought to have been initiated when the Indus River, initially flowing somewhat north and well to the west of its present location, was captured and diverted south close to the massif of today by extensional structures and downfaulted topography across the Kohistan-Ladakh island arc terrane. It is hypothesized that the Nanga Parbat pop-up structure was initiated at c. 12–10 Ma, as a tectonic aneurysm caused by rapid incision by the Indus River and other surface processes. Because of this subsequent rapid unroofing of the region, however, the oldest sediments to record erosion in the immediate region of Nanga Parbat are < 200 ka old: most sediments and our cosmogenic and ISRL exposure dates are more than five times younger. Diverse field measurements of rates of local incision and areal denudation for mass movement, glacial, river and catastrophic floods for the past c. 55 ka are highly differential but internally replicative and externally consistent with research indicating long-term, severe denudation. Averaged rates of maximum incision at more than 15 points around the massif are 22 mm ± 11 mm a –1 . Late Pleistocene surface processes at Nanga Parbat were capable of erosional unroofing of the massif sufficiently vigorous to produce the pronounced relief of today.

Abstract The dynamic mountains of the western Himalaya are the result of complex interactions involving tectonic, structural, lithological, climatic and surface processes. The multi-scale dynamics of surface processes in this region are largely unknown. This paper assesses the spatial complexities of the topography at Nanga Parbat, as we seek to understand erosion dynamics, differential denudation and the geodynamics of uplift and denudation. Spatial analysis of a high resolution digital elevation model and three-dimensional terrain simulations using satellite imagery indicate that the topographic complexity of Nanga Parbat is highly scale-dependent and exhibits a hierarchical order that is reflective of erosion dynamics. Observations and analyses reinforce prior understandings of rapid rates of uplift and high rates of surficial denudation. Results indicate that climate controls the topographic complexity of the massif, although a tectonic influence is present and is largely masked by the overprinting of surface processes with time. Consequently, Nanga Parbat is seen to owe its origin to erosionally induced tectonic uplift and rapid surficial denudation. Rapid uplift altered erosion dynamics and further accelerated erosion resulting in extreme relief. Nonetheless, the differential denudation resulting from erosion dynamics does not appear to be in spatial balance with the regional scale tectonic mass flux. Systematic integration of dynamic models that account for the scale-dependencies of subsurface and surface processes are required to study the nature of this complex system and explain topographic evolution.

Abstract New Sr and Nd isotope data for basement gneisses and leucogranites are presented from two contrasting areas of the western Himalaya; the Nanga Parbat-Haramosh massif (NPHM) and Zanskar. Sr-isotope systematics of metapelites and anatectic migmatites from the Zanskar Himalaya are characterized by εe Sr of 515–930, typical of the High Himalayan Crystalline unit as exposed for more than 2000 km along strike. Moreover, Zanskar leucogranites are typical of the belt of Early Miocene granites intruding the High Himalayan Crystallines across the orogen (mean ε Sr = 834). In contrast, the NPHM leucogranites show an elevated average e Sr of 2400, and basement samples show a wide range in ε Sr from 1850 to 8150. Errorchrons for the metasedimentary gneisses indicate isotopic homogenization of the basement at c. 500 Ma for the Zanskar samples compared with c. 1800 Ma from the NPHM, confirming that the two terrains have experienced contrasting pre-Himalayan histories. Nd isotopic data from the NPHM indicate model ages from 2300 to 2800 Ma, indicating the mean crustal formation ages of the protoliths from which the sediments were derived. A compilation of published Nd data from the Himalaya indicates average protolith formation ages of 2640 ± 220 Ma for the Lesser Himalaya lithologies, compared with 1940 ± 270 Ma for the High Himalaya unit. Gneissic lithologies from Zanskar and the NPHM have previously been correlated with the High Himalayan Crystalline Series, since both display high-grade Himalayan metamorphism and are intruded by syn- to post-tectonic tourmaline-bearing leucogranites. Isotopic systematics in the Zanskar region confirm this correlation. In contrast, the NPHM basement rocks are better correlated with Lesser Himalayan lithologies, exposed south of the Main Central Thrust. We conclude that the NPHM represents either a lower structural level of the Lesser Himalaya Series, or its protolith.

Abstract The eastern and western Himalayan syntaxes are large-scale, coeval antiforms developed late in the history of India-Asia collision. We use two-dimensional finite element models of lithospheric folding to develop a mechanically plausible structural interpretation. The models mimic the coeval development of adjacent synformal basins, analogous to the Peshawar and Kashmir basins on either side of and adjacent to the western syntaxis. Pure-shear thickening and symmetric buckling accommodate shortening until, at a certain strain, an asymmetric thrust-like flow pattern occurs on a crustal to lithospheric scale. Similarities between geological data and calculated models suggest that lithospheric buckling is a basic response to large-scale continental shortening. To generalize these results, we suggest that a typical shortening history would include: (1) locking of an early thrust system in hinterland regions, followed by (2) pure shear shortening and symmetric buckling of the shortened lithosphere, and (3) loss of symmetry leading to the formation of an asymmetric fold in which a new thrust system will nucleate.

Abstract The left-lateral strike-slip Tirich Mir Fault, Chitral, NW Pakistan, is associated with a belt of peridotites, metagabbros and gneisses named the Tirich Boundary Zone (TBZ), separating the Late Palaeozoic-Mesozoic units of the East Hindu Kush from the Palaeozoic successions of the Karakoram block. These rocks were metamorphosed up to upper amphibolite facies conditions, followed by a greenschist facies overprinting, and then thrust on to very low grade metasediments; they were finally intruded at shallow levels by the mid-Cretaceous Tirich Mir pluton. Ultramafic rocks along the fault zone include well-preserved spinel lherzolites and harzburgites (Tirich Gol, Barum valley, Arkari Gol), whereas schistose serpentinites occur in the Rich Gol. Whole-rock analyses and mineral chemistry of olivine, clinopyroxene, orthopyroxene and spinel from these peridotites show a depleted signature. Microstructural and petrological features suggest a mantle origin for these ultramafic bodies, which equilibrated at temperatures ranging from 1000–1100 °C. Peridotites are faulted against partially metamorphosed igneous bodies including hornblende-gabbros, hornblende cumulates and quartz-diorites. Metamorphic rocks of the TBZ, which lay south of the ultramafic-mafic complex, include quartzites, amphibolites, garnet-sillimanite (± kyanite ± K-feldspar)-biotite gneisses and mica schists, locally displaying migmatitic textures. A sub-continental character of the peridotites indicated by low temperatures of equilibration and by the presence of a deep crustal sequence. These characters along with the absence of an ophiolitic sequence may suggest that the TBZ represents a fragmented crust-mantle boundary developed along a zone of attenuated continental crust. The TBZ is interpreted as a sheared lithospheric section of a Jurassic-Early Cretaceous orogenic complex, formed as a consequence of the accretion of the Karakoram terrane to the southern side of the Pamir belts, which were progressively accreted to the Asian margin.

Abstract The Nubra-Shyok confluence in northern Ladakh is a key area for understanding the tectonic evolution of NW Himalaya and provides the basis for linking the geology of Pakistan to that of Tibet. The geology of the confluence area has been the subject of much speculation centred mainly on the existence of ophiolites and their regional significance. These ophiolites are thought to represent the eastward extension of the Shyok Suture Zone (SSZ), which separates the Dras island arc from the southern margin of Eurasia, and which was overprinted by movement along the Khalsar Thrust (often thought to represent the eastern continuation of the Main Karakoram Thrust). The geology of the area is relatively complex and the little information available has hampered regional geological correlations. The Khalsar Thrust (KT) and the dextral Karakoram Fault (KF), two regional tectonic features of NW Himalaya, merge at the confluence defining a triple point and three blocks: the Ladakh block to the south, the Saltoro block to the northwest, and the Karakoram block to the northeast. Close to the triple point, the KF changes strike and movement direction. Movement vector analysis of the triple point indicates that the KT and the two parts of the KF could have moved contemporaneously, and allows prediction of the movement vectors across the faults. The KT and KF shear preferentially volcano-sedimentary rocks of the Shyok and Nubra formations, respectively. Contrary to previous interpretations, these sheared rocks do not represent disrupted ophiolites. Regional tectonic reconstructions, however, require suturing between the Ladakh block and Eurasia and the strike of the SSZ in Baltistan suggests that the suture zone might crop out north of the KT, either along the southern slopes of the Saltoro Range or further north along the Saltoro valley. In the few outcrops of the Saltoro block we were able to visit, we found no evidence of ophiolitic rocks. Instead we found outcrops of the calc-alkaline Tirit batholith. Although our observations do not confirm the presence of the suture-related rocks in the southern Saltoro block, this possibility cannot be ruled out. Zircons from a sample of Tirit granite (U-Pb ion-microprobe age) yielded an age centred at 68 ± 1 Ma. The similar range of modal composition and age of the Tirit and Ladakh batholiths suggest that they are part of the same magmatic event. This result and a number of other observations indicate that the post–75 Ma geology of the Ladakh and Saltoro blocks is similar. Thus, if there is a suture zone in the southern Saltoro block, suturing must have occurred before 75 Ma, as concluded by others along the same tectonic boundary to the west in Pakistan. The KF represents a much younger terrane boundary, juxtaposing rocks of the Ladakh and Saltoro blocks to those of the Karakoram terrane. Rocks related to suturing of continents were not found along the KF. Karakoram leucogranites cropping out in the southern part of Karakoram terrane yielded a U-Pb zircon age centred at 15.0 ± 0.4 Ma (2σ). Because these leucogranites were not found south of the KF, this fault must have initiated after leucogranite intrusion and must therefore be younger than 15 Ma old. At the confluence the KF cuts across the regional rock sequence than can be followed from Kohistan into Baltistan and into the confluence area. Movement on the fault displaces the sequence by approximately 150 km to southeastern Tibet where the regional rock sequence can be regained.

Abstract A geological map of the eastern Hindu Kush, northwest of Chitral, Northern Pakistan, is presented. The lithologies are placed into two main categories, divided by the Tirich Mir Fault Zone. To the northwest, the units of the eastern Hindu Kush are dominated by monotonous sequences of graphite-rich pelitic rocks. Southeast of the fault, the phyllites and diamictites are thought to be lateral equivalents of the Northern Sedimentary Belt of the Karakoram. A structural analysis of the area studied identifies a major, early deformation phase which is usually characterized by tight to isoclinal folding with a well developed axial-planar schistosity. This deformation is thought to have been related to the northward-directed subduction and accretion beneath the southern margin of Asia during the Mesozoic, and may have taken place over a considerable period of time. A major phase of crustal melting at c. 24 Ma generated migmatites and biotite + muscovite ± garnet ± tourmaline leucogranites (including dykes and the Gharam Chasma pluton). This age is comparable to that of the Baltoro pluton in the Karakoram to the east, confirming the regional importance of crustal melting along the southern margin of the Asian plate during the earliest Miocene. The crustal melting was associated with thrusting and folding of the earlier schistosity. Subhorizontal stretching lineations indicate a phase of strike-slip deformation that is thought to have been associated with anticlockwise rotation of the regional foliation strike from E to NE and N after the emplacement of the Gharam Chasma pluton at c. 24 Ma. This deformation and rotation was probably a direct result of the northward-moving Indian plate forcing Kohistan to indent into Asia, resulting in a left-lateral transpressional tectonic environment which remains today. The anomalous height of the Tirich Mir massif, relative to other peaks in the Hindu Kush and the nearby Hindu Raj, may be accounted for by the onset of this transpression. Intensely active seismicity to depths of 300 km beneath the Hindu Kush is associated with seismic shear wave velocities that are significantly faster than those beneath Tibet, where earthquake occurrence is restricted to the upper crust, and previous geophysical studies indicate elevated thermal conditions and possible crustal melts. U-Pb ages suggest that post-India-Asia collision crustal melting beneath the Hindu Kush is restricted to c. 24 Ma, whereas in the Karakoram, the record is both more voluminous and more continuous from c. 37 to c. 9 Ma. These observations reflect major differences in the thermal histories of these regions, where the relatively cooler conditions beneath the Hindu Kush are associated with continental subduction-related seismicity.

Abstract Ductile strain localization commonly forms a pattern of shear zones anastomosing around lenses of less deformed rock. Initiation and development of anastomosing shear zones are studied through description of the structures and deformation history of plutonic rocks that form the lower crust of the Kohistan arc. Structures and textures developed in these rocks result from primary magmatic to solid state regional strain, overprinted by anastomosing shear zones. The primary strain was mainly acquired during magmatic emplacement at 100–90 Ma. Strain localization took place continuously from magmatic emplacement to solid state deformation during cooling of the plutons and formed three successive sets of shear zones. Set 1 is composed of associated discrete Riedel and thrust shear zones developed above solidus conditions during southwestward thrusting. Continuous deformation from solidus to amphibolite facies conditions between 100 and 83 Ma formed the second set of shear zones. The lower amphibolite facies set 3 shear zones are differentiated by larger strains recorded in the thicker mylonitic zones and enlargement of the spacing between shear zones during cooling. The anastomosing pattern of shear zones described here probably represents arc-related deformation during subduction of the Tethys oceanic lithosphere below the Kohistan arc.

Abstract Mafic to ultramafic granulites in the northeastern part of the Jijal complex include two-pyroxene granulite, garnet-clinopyroxene granulite and garnet hornblendite. Field and textural relations indicate that two-pyroxene granulite is a relict left after formation of the garnet-clinopyroxene granulite and garnet hornblendite was an originally intrusive rock which dissected the protoliths of mafic granulites. Sm-Nd mineral isochron ages of 118 ± 12 Ma, 94.0 ± 4.7 Ma and 83 ± 10 Ma were determined for two-pyroxene granulite, garnet-clinopyroxene granulite and garnet hornblendite respectively. These ages, together with previously reported chronological data, led to the following tectonic implications: (1) crystallization of the granulite protoliths predates, or is coeval with, the tectonic accretion of the Kohistan arc to the Asian continent; (2) crustal thickening related to the accretion was probably responsible for the high-pressure granulite-facies metamorphism in the Jijal complex; (3) formation of the garnet hornblendite assemblage was probably after crystallization of garnet-clinopyroxene granulite.

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.