Kohistan Arc Research Articles

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.

Towards resolving the metamorphic enigma of the Indian Plate in the NW Himalaya of Pakistan

Abstract The Pakistan part of the Himalaya has major differences in tectonic evolution compared with the main Himalayan range to the east of the Nanga Parbat syntaxis. There is no equivalent of the Tethyan Himalaya sedimentary sequence south of the Indus–Tsangpo suture zone, no equivalent of the Main Central Thrust, and no Miocene metamorphism and leucogranite emplacement. The Kohistan Arc was thrust southward onto the leading edge of continental India. All rocks exposed to the south of the arc in the footwall of the Main Mantle Thrust preserve metamorphic histories. However, these do not all record Cenozoic metamorphism. Basement rocks record Paleo-Proterozoic metamorphism with no Cenozoic heating; Neo-Proterozoic through Cambrian sediments record Ordovician ages for peak kyanite and sillimanite grade metamorphism, although Ar–Ar data indicate a Cenozoic thermal imprint which did not reset the peak metamorphic assemblages. The only rocks that clearly record Cenozoic metamorphism are Upper Paleozoic through Mesozoic cover sediments. Thermobarometric data suggest burial of these rocks along a clockwise pressure–temperature path to pressure–temperature conditions of c. 10–11 kbar and c. 700°C. Resolving this enigma is challenging but implies downward heating into the Indian plate, coupled with later development of unconformity parallel shear zones that detach Upper Paleozoic–Cenozoic cover rocks from Neoproterozoic to Paleozoic basement rocks and also detach those rocks from the Paleoproterozoic basement.

Timing of subduction initiation, arc formation, ophiolite obduction and India–Asia collision in the Himalaya

Abstract Reconstruction of the Western Himalaya requires three subduction systems operating beneath the Spong arc, Dras–Kohistan arc and the Asian continent during the Late Cretaceous–Paleocene. The timing of the closure of the Neo-Tethys Ocean along the Indus Suture Zone (ISZ) in Ladakh and south Tibet has been proposed to be as old as c. 65 Ma and as young as c. 37 Ma. The definition of the India–Asia collision can span >15 myr from the first touching of Indian continental crust with Asian crust to the final marine sedimentation between the two plates. There is good geological evidence for a Late Cretaceous–Early Paleocene phase of folding, thrusting and crustal thickening of Indian Plate shelf carbonates associated with obduction of ophiolites. There is no geological evidence of any oceanic ‘Greater Indian Basin’ separating the northern Tethyan and Greater Himalaya from India. There is clear evidence to support final ending of marine sedimentation along the ISZ at 50 Ma (planktonic foraminifera zone P7–P8). There is no evidence for diachroneity of collision along the Pakistan–Ladakh–South Tibet Himalaya. The timing of ultrahigh-pressure metamorphism cannot be used to constrain India–Asia collision, and the timing of high-grade kyanite- and sillimanite-grade metamorphism along the Greater Himalaya can only give a minimum age of collision.

Structural and metamorphic evolution of the Karakoram and Pamir following India–Kohistan–Asia collision

Abstract Following the c. 50 Ma India–Kohistan arc–Asia collision, crustal thickening uplifted the Himalaya (Indian Plate), and the Karakoram, Pamir and Tibetan Plateau (Asian Plate). Whereas surface geology of Tibet shows limited Cenozoic metamorphism and deformation, and only localized crustal melting, the Karakoram–Pamir show regional sillimanite- and kyanite-grade metamorphism, and crustal melting resulting in major granitic intrusions (Baltoro granites). U/Th–Pb dating shows that metamorphism along the Hunza Karakoram peaked at c. 83–62 and 44 Ma with intrusion of the Hunza dykes at 52–50 Ma and 35 ± 1.0 Ma, and along the Baltoro Karakoram peaked at c. 28–22 Ma, but continued until 5.4–3.5 Ma (Dassu dome). Widespread crustal melting along the Baltoro Batholith spanned 26.4–13 Ma. A series of thrust sheets and gneiss domes (metamorphic core complexes) record crustal thickening and regional metamorphism in the central and south Pamir from 37 to 20 Ma. At 20 Ma, break-off of the Indian slab caused large-scale exhumation of amphibolite-facies crust from depths of 30–55 km, and caused crustal thickening to jump to the fold-and-thrust belt at the northern edge of the Pamir. Crustal thickening, high-grade metamorphism and melting are certainly continuing at depth today in the India–Asia collision zone.

The mantle source of island arc magmatism during early subduction: Evidence from Hf isotopes in rutile from the Jijal Complex (Kohistan arc, Pakistan)

The Cretaceous–Paleogene Kohistan arc complex, northern Pakistan, is renowned as one of the most complete sections through a preserved paleo-island arc. The Jijal Complex represents a fragment of the plutonic roots of the Kohistan arc, formed during its early intraoceanic history. We present the first Hf isotope determinations for the Jijal Complex, made on rutile from garnet gabbros. These lithologies are zircon-free, but contain rutile that formed as an early phase. Recent developments in analytical capabilities coupled with a careful analytical and data reduction protocol allow the accurate determination of Hf isotope composition for rutile with <30 ppm Hf for the first time. Rutile from the analysed samples contains 5–35 ppm Hf, with sample averages of 13–17 ppm. Rutile from five samples from the Jijal Complex mafic section, sampling ~2 km of former crustal thickness, gave indistinguishable Hf isotope compositions with εHf(i) ranging from 11.4 ± 3.2 to 20.1 ± 5.7. These values are within error of or only slightly more enriched than modern depleted mantle. The analysed samples record variable degrees of interaction with late-stage melt segregations, which produced symplectitic overprints on the main mineral assemblage as well as pegmatitic segregations of hydrous minerals. The indistinguishable εHf(i) across this range of lithologies demonstrates the robust preservation of the Hf isotope composition of rutile. The Hf isotope data, combined with previously published Nd isotope data for the Jijal Complex garnet gabbros, favour derivation from an inherently enriched, Indian Ocean type mantle. This implies a smaller contribution from subducted sediments than if the source was a normal (Pacific-type) depleted mantle. The Jijal Complex thus had only a limited recycled continental crustal component in its source, and represents a largely juvenile addition of new continental crust during the early phases of intraoceanic magmatism. The ability to determine the Hf isotope composition of rutile with low Hf contents is an important development for zircon-free mafic lithologies. This study highlights the potential of Hf isotope analysis of rutile to characterise the most juvenile deep arc crust cumulates worldwide.

Arc crust formation and differentiation constrained by experimental petrology

We review igneous experimental data designed to understand fractionation processes in the roots of magmatic arcs, focusing on fractional crystallization experiments in dry and hydrous basalts to andesites to simulate the liquid and cumulate lines of descent of primary mantle derived magmas. An evaluation of the major element composition indicates that the cumulate line of descent (CLD) of hydrous systems is fundamentally different from dry systems. Cumulates derived from hydrous experiments display elevated Al2O3 and CaO contents at low SiO2, producing voluminous andesitic to rhyolitic liquids that closely overlap with compositions of natural systems. The mineralogical and chemical composition of cumulates converges for very different hydrous primary mantle magmas, indicating that fundamental phase equilibria under the conditions prevailing in the roots of magmatic arcs exert a strong control on the compositions of derivative andesitic to rhyolitic liquids. Melting experiments on amphibolite or basaltic to andesitic compositions equally produce granitic – rhyolitic liquids of similar composition yet their restites do not present the same variability of the CLD of hydrous fractional crystallization experiments. The comparison of our experimentally produced cumulates with natural rock suites from the Jurassic Talkeetna arc (South Central Alaska) and the Cretaceous Kohistan arc (Pakistan) show that fractionation experiments closely reproduce the natural rock compositions and in particular the cumulate sequences exposed in the roots of island arcs. Both dry and hydrous fractionation series are preserved yet only the hydrous fractionation series produces voluminous ultramafic cumulates (50–70%) that are negatively buoyant with respect to mantle peridotite. Delamination of dense ultramafic roots can explain the andesitic composition of continental crust. Our fractionation experiments show that generation of granites is a natural consequence of fractional crystallization of primary basaltic magmas. Discrepancies of liquid compositions produced in the lower crust and the arc volcanic record can readily be explained by further differentiation, partial assimilation and mingling and mixing processes during ascent and decompression in transcrustal magmatic systems. We therefore favor the interpretation that Earth-like continental crust is primarily produced by fractional crystallization in the lower crust and upper mantle, and that magma mixing, assimilation, metamorphism and crustal melting are important secondary processes that refine and amplify the difference and stratification between mafic lower crust and a granitic upper crust.

Seismic anisotropy from compositional banding in granulites from the deep magmatic arc of Fiordland, New Zealand

We present calculated seismic velocities and anisotropies of mafic granulites and eclogites from the Cretaceous deep lower crust (∼40–65 km) of Fiordland, New Zealand. Both rock types show a distinct foliation defined by cm-scale compositional banding. Seismic properties are estimated using the Asymptotic Expansion Homogenisation – Finite Element (AEH-FE) method that, unlike the commonly used Voigt–Reuss–Hill homogenisation, incorporates the phase boundary network into calculations. The predicted mean P- and S-wave velocities are consistent with previously published data for similar lithologies from other locations (e.g., Kohistan Arc), although we find higher than expected anisotropies (AVP ∼ 5.0–8.0%, AVS ∼ 3.0–6.5%) and substantial S-wave splitting along foliation planes in granulites. This seismic signature of granulites results from a density and elasticity contrast between cm-scale pyroxene ± garnet stringers and plagioclase matrix rather than from crystallographic orientations alone. Banded eclogites do not show elevated anisotropies as the contrast in density and elastic constants of garnet and pyroxene is too small. The origin of compositional banding in Fiordland granulites is primarily magmatic and structures described here are expected to be typical for the base of present day magmatic arcs. Hence, we identify a new potential source of anisotropy within this geotectonic setting.

Metamorphic P–T conditions and CO2 influx history of medium-grade metapelites from Karakorum, Trans-Himalaya, India

The medium grade metapelites of Pangong-Tso area in the trans-Himalayan region underwent sillimanite-grade metamorphism initiated during the Cretaceous, associated with the collision of the Kohistan arc and the Indian plate with Asia. This paper present results from a petrological and fluid inclusion study to understand the metamorphic P–T conditions and fluid history of these rocks. The calculated phase equilibria in the Na2O–CaO–K2O–FeO–MgO–MnO–Al2O3–SiO2–H2O–TiO2 (NCKFMMnASHT) system suggest P–T conditions of 8kbar and 650°C for the peak metamorphic event. Primary fluid inclusions occur in staurolite and garnet, whereas quartz carries mostly secondary fluid inclusions. The trapped fluids in primary inclusions show initial melting temperatures in the range of −56.9 to −56.6°C, suggesting nearly pure CO2 composition. The secondary fluids are of mixed carbonic-aqueous nature. The re-equilibrated inclusions show annular morphology as well as necking phenomena. The CO2 isochores for the primary inclusions indicate pressures of 6.1–6.7kbar, suggesting that the CO2-rich fluids were trapped during post-peak exhumation of the rocks, or that synmetamorphic carbonic fluids underwent density reversal during isothermal decompression. The secondary CO2–H2O fluids must have been trapped during the late exhumation stage, as their isochores define further lower pressures of 4.8kbar. The morphology of re-equilibrated fluid inclusions and the rapid decrease in pressure are consistent with a near-isothermal decompression trajectory following the peak metamorphism. The carbonic fluids were probably derived locally from decarbonation reactions of the associated carbonate rocks during metamorphism or from a deep-seated reservoir through Karakorum fault.

Role of Arc Processes in the Formation of Continental Crust

We review data and recent research on arc composition, focusing on the relatively complete arc crustal sections in the Jurassic Talkeetna arc (south central Alaska) and the Cretaceous Kohistan arc (northwest Pakistan), together with seismic data on the lower crust and uppermost mantle. Whereas primitive arc lavas are dominantly basaltic, the Kohistan crust is clearly andesitic and the Talkeetna crust could be andesitic. The andesitic compositions of the two arc sections are within the range of estimates for the major element composition of continental crust. Calculated seismic sections for Kohistan and Talkeetna provide a close match for the thicker parts of the active Izu arc, suggesting that it, too, could have an andesitic bulk composition. Because andesitic crust is buoyant with respect to the underlying mantle, much of this material represents a net addition to continental crust. Production of bulk crust from a parental melt in equilibrium with mantle olivine or pyroxene requires processing of igneous crust, probably via density instabilities. Delamination of dense cumulates from the base of arc crust, foundering into less dense, underlying mantle peridotite, is likely, as supported by geochemical evidence from Talkeetna and Kohistan. Relamination of buoyant, subducting material—during sediment subduction, subduction erosion, arc-arc collision, and continental collision—is also likely.

Titanite evidence for Triassic thickened lower crust along southeastern margin of North China Craton

Abstract Titanite U–Pb isotopic and major and trace element compositions of one mafic garnet granulite from a rare suite of lower crustal xenoliths (e.g., eclogite, garnet pyroxenite, and mafic garnet granulite) hosted in Early Cretaceous dioritic porphyries in the Xu–Huai area along the southeastern margin of the North China Craton (NCC) were analyzed by laser ablation ICP-MS. Titanite occurs as granular grains or coronary rims on rutile. The coronary titanite is clearly a secondary product of rutile decomposition. The granular titanite exhibits zonation in U–Pb age and chemical composition. Petrographic and geochemical evidence suggests that the zonation was formed by thermal diffusion and later fluid-assisted recrystallization. Occurrences of granular titanite between garnet grains point to a pressure of > 10 kbar, while inclusions of rutile inside granular titanite rims imply that the pressure might have reached 15 kbar. Granular titanite cores give U–Pb ages of 237–241 Ma and Zr-temperatures of 794–831 °C at 10 kbar and 850–892 °C at 15 kbar, indicating high-pressure granulite-facies metamorphism. Together with previous P–T estimates of coeval eclogite-facies xenoliths, a geotherm of above 60 mW m− 2 is implied. The geotherm plots below the temperature field of amphibole dehydration melting, consistent with presence of abundant amphibole. This geotherm is similar to that of the Kohistan arc, which has preserved a 12-km-thick dense lower crust, but significantly cooler than the geotherm of the Talkeetna arc, where most of the dense lower crust has been foundered. Our results provide new evidence for Triassic thickened dense lower crust along the southeastern margin of the NCC. By comparison with the Kohistan and Talkeetna arc crusts, we suggest that this dense lower crust was not hot enough to be foundered in the Triassic. Foundering must have occurred in the Jurassic–Cretaceous in order to explain the present-day seismic velocity structure characterized by a sharp Moho, overall slow velocities in the lower crust, and a thin crustal thickness in the Xu–Huai area and other parts of the eastern NCC. We suggest that the Jurassic–Cretaceous foundering was related to the Pacific subduction. On one hand, the Jurassic subduction may have further thickened the southeastern margin of the NCC prior to Cretaceous extension, leading to greater instability of the lower crust. On the other hand, the subduction-related magma provided heat and water that weakened the lower crust, resulting in the final foundering. The large contrast in mineralogy between the Xu–Huai eclogite-facies xenoliths and nearby Nushan garnet-free granulite xenoliths entrained by Quaternary basalts indicates > 20 km removal of the lower crust along the southeastern margin of the NCC.

Arc crustal differentiation mechanisms

The detailed vertical compositional and thermal structure of the entire Kohistan arc section is constructed using ∼63P/T constraints and 209 plutonic whole rock analyses. The aim is to better understand the nature of the chemical differentiation in the plutonic arc crust at different crustal levels.Results indicate that a distinct difference in character exists between the upper and lower plutonic arc crust. Plutonic rocks in the lower crust (⩾25–30km) have whole-rock chemical characteristics that indicate mineral accumulation and residual melt loss. In these cumulate rocks there is a systematic chemical stratification of decreasing incompatible trace elements with depth (e.g. Rb, K2O). In contrast, the plutonic rocks of the upper crust represent mainly frozen liquids and show no systematic relationship between chemical composition and intrusion depth. These results clearly indicate that magmatic differentiation in the Kohistan arc dominantly occurred in the deeper arc crust (⩾30km) consistent with the lower crustal ‘hot zone model’. To understand the role of density barriers for melt stagnation in the crust, and thereby for the observed chemical stratification, the detailed density structure of the plutonic arc crust is compared to melt densities calculated for plutonic rocks with near liquid compositions. Results indicate that melts are consistently less dense than rocks, and there is no evidence for a “neutral buoyancy line” controlling melt emplacement. It is speculated that melt stagnation in the lower arc crust and subsequent chemical differentiation of a felsic upper and mafic lower continental crust is dominantly controlled by temperature and to a lesser extent to density or rheological barriers.

Seismic hazard assessment of Kashmir and Kangra valley region, Western Himalaya, India

A complete earthquake catalogue of the Western Himalaya (latitudes 30°N–36°N and longitudes 72°E–78°E) for the period of 1501–2010 has been compiled with earthquake magnitude computed in moment magnitude (Mw) scale. Pre- and early twentieth century records of earthquake damage have been documented from rare and out of print publications. Seismotectonics and seismic hazard for Kohistan arc, Kashmir–Hazara Syntaxis, Nanga-Parbat (Western Syntaxis), Karakoram and Himachal Himalaya are discussed with special reference to 1905 Kangra and 2005 Muzaffarabad earthquakes. Analyses of spatio-temporal variation in b-value from the region indicate significant precursor prior to the 2005 Muzaffarabad earthquake; progressive rise of background b-value observed and the main shock locates close to relative high b-value domains. Regions surrounding the location of the 1905 Kangra earthquake also display such high b-value for the period of 2005–2010 that calls for closer scrutiny. Temporal analysis of b-value from the epicentral block of Muzaffarabad earthquake clearly showed a high–low b-value couplet of 1.45–0.72, which may be treated as a typical precursor before an imminent large earthquake. Gumbel extreme value statistics indicate probability of occurrence of an event of Mw > 7.0 within 50 years in the region.

TTG-type plutonic rocks formed in a modern arc batholith by hydrous fractionation in the lower arc crust

We present the geochemistry and intrusion pressures of granitoids from the Kohistan batholith, which represents, together with the intruded volcanic and sedimentary units, the middle and upper arc crust of the Kohistan paleo-island arc. Based on Al-in-hornblende barometry, the batholith records intrusion pressures from ~0.2 GPa in the north (where the volcano-sedimentary cover is intruded) to max. ~0.9 GPa in the southeast. The Al-in-hornblende barometry demonstrates that the Kohistan batholith represents a complete cross section across an arc batholith, reaching from the top at ~8–9 km depth (north) to its bottom at 25–35 km (south-central to southeast). Despite the complete outcropping and accessibility of the entire batholith, there is no observable compositional stratification across the batholith. The geochemical characteristics of the granitoids define three groups. Group 1 is characterized by strongly enriched incompatible elements and unfractionated middle rare earth elements (MREE)/heavy rare earth element patterns (HREE); Group 2 has enriched incompatible element concentrations similar to Group 1 but strongly fractionated MREE/HREE. Group 3 is characterized by only a limited incompatible element enrichment and unfractionated MREE/HREE. The origin of the different groups can be modeled through a relatively hydrous (Group 1 and 2) and of a less hydrous (Group 3) fractional crystallization line from a primitive basaltic parent at different pressures. Appropriate mafic/ultramafic cumulates that explain the chemical characteristics of each group are preserved at the base of the arc. The Kohistan batholith strengthens the conclusion that hydrous fractionation is the most important mechanism to form volumetrically significant amounts of granitoids in arcs. The Kohistan Group 2 granitoids have essentially identical trace element characteristics as Archean tonalite–trondhjemite–granodiorite (TTG) suites. Based on these observations, it is most likely that similar to the Group 2 rocks in the Kohistan arc, TTG gneisses were to a large part formed by hydrous high-pressure differentiation of primitive arc magmas in subduction zones.

High-Pressure - Low-temperature evolution in the Indus-Tsangpo Suture along the Kohistan Arc (Kaghan Valley, NE Pakistan)

North of Naran in the Kaghan Valley (NE Pakistan), the metabasites of the melange units lying below the Kohistan Arc, contain glaucophane. Typically they reflect blueschist-metamorphic conditions (0.7 GPa, 400°C). Associated graphite-rich metapelites with quartz veins underwent upper greenschist to amphibolite conditions. Near Naran we observed in quartz grains of type 3 veins first minute relics of Fe-Mg carpholite indicating earlier blueschist metamorphic conditions. P-T estimates indicate 1.2-1.6 GPa at 380-410 °C, pressure and temperature values typical for blueschist metamorphic conditions. Changes in mineral assemblages and abundant sudoite component in associated chlorite point to a pressure drop after peak I conditions. We assign the observed changes to peak I conditions occurring during a Cretaceous subduction event. Temperatures estimated with Raman graphite-thermometry clearly indicate a significant subsequent rise of post- peak I temperatures up to 500°C. This is compatible with the amphibolite peak II assigned to the Tertiary continental collision that leads to subduction of the Indian Plate and ultra-highpressure metamorphism. During subduction the blueschist metamorphic metapelites underwent dehydration, which caused alteration in the overlying lithospheric mantle. In a hydrated lithospheric mantle density is significantly reduced which enhanced subduction of continental crust in the Higher Himalaya. This P-T evolution is typical for a collision orogen with a high plateau but remarkably contrasting findings from Eastern Anatolia, where plateau building is in statu nascendi (e.g., Oberhansli et al., 2010).

Seismic properties of the Kohistan oceanic arc root: Insights from laboratory measurements and thermodynamic modeling

AbstractP‐wave velocities (Vp) have been measured in the laboratory and calculated using thermodynamic modeling for seven representative rock samples from the lower crust to mantle section of the Kohistan paleo‐island arc. Lower crustal rocks comprise plagioclase‐rich gabbro, garnet‐bearing gabbro, and hornblendite; mantle rocks comprise garnetite, pyroxenite, websterite, and dunite. Measurements were performed at confining pressures up to 0.5 GPa and temperatures up to 1200°C. Vp were also calculated using rock major element chemistry with the Perple_X software package. Calculated Vp match closely the laboratory measurements. At depths representative for the arc root, Vp of upper mantle rocks vary from 7.7–8.1 km/s, whereas the lower crustal rocks have velocities between 6.9–7.5 km/s. P‐wave anisotropy is small, with exceptions of sheared gabbros. Measured and calculated seismic properties are consistent with, and complement a growing database of published seismic properties from the Kohistan arc. In the light of such data, we discuss seismic imaging of present‐day island arcs. Intermediate Vp (7.4–7.7 km/s) in arc roots can be explained by pyroxenites and garnet‐bearing mafic rocks. Strong seismic reflectors may be related to garnetites (8.0–8.2 km/s).

The composition of the foundered complement to the continental crust and a re-evaluation of fluxes in arcs

Most primitive arc melts are basaltic in composition, yet the bulk continental crust, thought to be generated in arcs, is andesitic. In order to produce an andesitic crust from primitive arc basalts, rocks complementary to the andesitic crust have to be fractionated and subsequently removed, most likely through density sorting in the lower arc crust. The Kohistan Arc in northern Pakistan offers a unique opportunity to constrain the composition and volume of material fluxes involved in this process. In a lower crustal section >10km cumulates (dunites, wehrlites, websterites, clinopyroxene-bearing garnetites and hornblendites, and garnet-gabbros) are exposed that are 0.1–0.3g/cm3 denser than the underlying mantle. The cumulates combine with the andesitic bulk Kohistan Arc crust to reproduce the major and trace element composition of primitive basaltic arc melts. Our petrochemical analysis suggests that fractionation and subsequent foundering of wehrlites+ultramafic hornblende–garnet–clinopyroxene cumulates+garnet-gabbros is a viable mechanism for producing andesitic crust from a calc-alkaline/tholeiitic primitive high-Mg basalt. The mass of the foundered material is approximately twice that of the arc crust generated. For an overall andesitic arc composition, we estimate a magma flux into the arc (11–15km3/yr) about three times the rate of arc crust production itself. Foundering fluxes of cumulates (6.4–8.1km3/yr) are one third to half those of the globally subducted oceanic crust (~19km3/yr). Hence, the delaminate forms a volumetrically significant, albeit refractory and depleted geochemical reservoir in the mantle. Owing to its low U/Pb and high Lu/Hf the foundered material evolves with time to a reservoir characterized by unradiogenic Pb and highly radiogenic Hf isotopes, unlike any of the common mantle endmembers defined by OIB chemistry. The unradiogenic Pb of the foundered arc cumulates could counterbalance the radiogenic Pb composition of the depleted mantle. The predicted highly radiogenic Hf (at rather unradiogenic Nd) of the foundered material can explain the εHf–εNd systematics observed in some abyssal peridotites and mantle xenoliths.

Age, Nd–Hf isotopes, and geochemistry of the Vijayan Complex of eastern and southern Sri Lanka: A Grenville-age magmatic arc of unknown derivation

The ca. 1.0–1.1 Ga Vijayan Complex (VC) of eastern and southeastern Sri Lanka is one of three high-grade metamorphic terranes making up the basement of the island and is in tectonic contact with the adjacent, older Highland Complex. It consists predominantly of granitoid gneisses ranging in composition from diorite to leucogranite, with a distinct calc-alkaline geochemical signature, and is interpreted as a magmatic arc. Strong ductile deformation has obliterated almost all original intrusive relationships. High-grade metamorphism during the Pan-African event at ca. 610–520Ma has produced widespread granulite-facies assemblages that are now largely retrogressed and were affected by extensive late metamorphic K-metasomatism. In southern Sri Lanka the Vijayan gneisses are tectonically interlayered with rocks of the Highland Complex in a so-called mixed zone. We report zircon ages, whole-rock Nd and Hf-in-zircon isotopic systematics and geochemical data for a large selection of Vijayan gneissses in order to better characterize this complex and its tectonic setting. The zircon ages are predominantly in the range 1100–1000Ma with a few early Neoproterozoic intrusions, and identify the VC as a Grenville-age magmatic arc. Many zircons experienced minor to significant lead-loss at about 580Ma. The Nd and Hf isotopic data confirm a generally primitive origin for most Vijayan gneisses, but significant variations in both isotopic systems argue for source heterogeneities and the possible involvement of minor amounts of older continental material in their genesis. Immobile trace and rare earth element distributions favour an origin of the gneiss protoliths through melting at relatively shallow depth, and the chemical variation from diorite and tonalite via granodiorite to granite in the VC can be explained by increasing fractionation of plagioclase and biotite, accompanied by amphibole and accessory phases. The VC may be comparable, in some respect, to the composition and evolution of the Kohistan Arc in Pakistan. The unique composition, age and isotopic characteristics of the VC find no convincing counterpart in other fragments of the Gondwana supercontinent, and its origin therefore remains exotic and enigmatic.

Long wavelength satellite gravity and geoid anomalies over Himalaya, and Tibet: Lithospheric structures and seismotectonics of deep focus earthquakes of Hindu Kush – Pamir and Burmese arc

Free air anomaly map of Himalaya, Tibet and adjoining regions derived from global models including satellite observations reflect most of the tectonic elements of this region. Fold and thrust belts and suture zones are characterized by small wave length free air high anomalies representing shallow high density mafic/ultramafic rocks including ophiolitic mélange. On the other hand, medium to large wavelength negative gravity anomalies which are related to crustal thickening due to isostatic compensation are better visualized in the Bouguer anomaly map. This emphasizes the importance of both free air and Bouguer anomaly maps of complex orogenic belts.Spectral analysis of the Bouguer and the geoid anomaly maps and their low pass filtered maps and modeling along a N–S profile (90°E) constrained from various seismic studies suggest low density rocks (3250kg/m3) in the lithospheric mantle between depths of ∼130–320km under Tibet. The low pass filtered geoid anomaly shows a prominent geoid low in the same depth range oriented NE–SW, in the direction of motion of the Indian plate that is near perpendicular to the prominent tectonics features. It coincides with the NE–SW oriented seismic anisotropy in the central part of the Himalayan collision zone. Low density rocks in the lithospheric mantle suggest under thrusted Indian and Asian lithospheres in this section that constitutes the thick lithosphere under Tibet. The Moho in general lies at a depth of 50–75km under Tibet with deepest being under South Tibet. The Low density rocks in the upper mantle make this region buoyant that explains large scale uplifts.The prominent negative free air, Bouguer and geoid anomalies are observed over the Hindu Kush – Pamir Seismic Zone between the Kohistan arc towards the south and Pamir towards the north suggesting predominance of low density rocks approximately at a depth of ∼120km in the lithospheric mantle. Gravity anomalies and its modeling along a profile (73°E) constrained from the hypocenters of deep focus earthquakes in this region and results from receiver function analysis in adjoining regions suggest following characteristics of the subduction model in this section: (i) the subducting plates from two sides, viz. the Indian (Kohistan arc) and the Asian plates show relatively high angle subductions compared to the east of it. (ii) The subducting plates are affected by the strike slip Chamman – Panjshir faults towards the west and Karakoram strike slip fault towards the east. (iii) The subducting plates from the two sides interact with each other in the lithospheric mantle. (iv) The subducted rocks in this section are generally comprised of high density mafic/ultramafic rocks of the Kohistan arc and Tien Shan. Combination of these factors would induce fast subduction and slab break off that usually facilitate deep focus earthquakes, as observed in case of Mariana type subductions.Similar is the case along the Burmese arc that is the only other section characterized by deep focus earthquakes in the long Himalayan collision zone (∼5000km). Modeling of the Bouguer anomaly along 23.5°N in this case also shows relatively high angle subduction of the Indian plate which is affected by the Sagaing strike slip fault towards the east that would facilitate slab break off and generation of deep focus earthquakes as described above. Gravity modeling across Tibet, Hindu Kush and Burmese arc suggest the presence of high density rocks (∼3100kg/m3) in the lower crust and low density rocks (∼3250kg/m3) in the upper mantle that may be attributed to partial eclogitization and serpentinization of rocks, respectively in these sections that are typical of several orogenic belts. Such conditions are expected to facilitate delamination and detachment.

The formation and bulk composition of modern juvenile continental crust: The Kohistan arc

The intraoceanic Kohistan arc, northern Pakistan, exposes a complete crustal section composed of infracrustal basal cumulates formed at ≤55km depth, a broadly basaltic/gabbroic lower crust, a 26km thick calc-alkaline batholith, and 4km of a volcanoclastic/sedimentary sequence. The bulk composition of the Kohistan arc crust is approximated by estimating the relative volumes of exposed rocks through detailed field observations, in particular along a representative km-wide transect across the arc, through geobarometric constrains to determine the unit thicknesses, and through satellite images to estimate their lateral extent. We separated the arc into three major units: lower, mid-, and mid- to upper crust, which contain a total of 17 subunits whose average compositions were derived from employing a total of 594 whole rock analyses. The volume-integrated compositions of each unit yield the bulk composition of the arc crust. While the details of the resulting bulk composition depend slightly on the method of integration, all models yield an andesitic bulk supra Moho composition, with an average SiO2 of 56.6–59.3wt.% and XMg of 0.51–0.55. The Kohistan arc composition is similar to global continental crust estimates, suggesting that modern style arc activity is the dominant process that formed the (preserved) continental crust. A slight deficit in high field strength and incompatible elements in the Kohistan arc with respect to the global continental crust can be mitigated by adding 6–8wt.% of (basaltic) intraplate type magmas. Our results document that infra arc processes, even in a purely oceanic environment, result in an overall andesitic crust composition in mature arcs, contrary to the widely accepted view that oceanic arcs are basaltic. Bulk crust differentiation from a basaltic parent occurs through foundering of ultramafic cumulates. Our results imply that secondary reworking processes such as continental collision are of secondary importance to explain the major element chemistry of the bulk continental crust composition.

New paleomagnetic results from the northern margin of the Kohistan Island Arc

Different phases of remanent magnetizations have been identified in the Cretaceous–Tertiary rocks collected from the northern margin of the Kohistan Island Arc, northern Pakistan. Among them, a magnetite-related component is recognized as the most useful one because of its relatively stable behaviour and widespread presence in the volcanics and red beds. Calculation of mean direction for this component reveal two visible groupings in terms of paleomagnetic directions (Yasin + Baris Group: D = 341.6o, I = 10.6o, α 95 = 23.3 o, k = 11.7, N = 5; Sandhi Group: D = 28.4o, I = −27.4o, α 95 = 32.5 o, k = 8.96, N = 4). The fold tests applied to both these groups are inconclusive, indicating a syn-folding to post-folding origin for this component. A set of inclinations from this study provide new insight into post-collision northward displacement of the Kohistan Arc with respect to its surrounding terranes. Reliability of the paleomagnetic declinations from this study is not fully guaranteed, but when compared with previously reported paleomagnetic directions, a systematic increase in counter-clockwise deflections towards west has been detected along this paleo-island arc. This trend of declinations is well matched with the extrusion model of Asia, where counter-clockwise rotation has been suggested for the tectonic terranes around Western Himalaya. Another important observation is a divergence in paleomagnetic declinations across the rivers, which may indicate the presence of faulted zones under the cover of flowing water. This aspect can be compared to recent geological interpretations that Kohistan may not have acted as a rigid block following its collision with India but may have been deformed through localized shears and faults.