bluuebunny/crossref_metadata_2025_split huggingface dataset:
Recent gravity measurements in the Karakoram
Authors: Caporali Alessandro 
Published: April 2008

Abstract

Two new gravimetric profiles have been obtained in the Karakoram as part of the activities of the Ev-K2-CNR Project. One profile, established in 1988, starts from Bazar Dara (China), north of the Aghil range, and covers the portion of the Shaksgam valley between the confluence of the Urdok-Gasherbrum glaciers to the SE and, along the Shaksgam river, Tek ri-Sughet Jangal to the NW. The second profile, carried out in 1990, starts from Skardu and ends in Gilgit via the Biafo Hispar glaciers. During both expeditions a number of astrogeodetic stations were established for the measurement of the deflection of the vertical, using the satellites of the Global Positioning System (GPS) to determine geodetic reference coordinates. Two astronomic stations were established in 1988 and fourteen in 1990. The new data, when added to those collected by Marussi and co-workers from 1954 to 1979, improve the coverage north of the Main Karakoram Thrust, where the Bouguer anomalies attain their largest negative values. For all available data Bouguer and Airy isostatic corrections have been computed using the ETOPO5 height grid. A mean crust density of 2.85 g cm
−3
, a density contrast of 0.3 g cm
−3
and a normal thickness
T
= 30 km have been assumed, so that the surface of isostatic compensation locally best fits the Moho. Two profiles have been selected, one continuing northward of the Nanga Parbat — Haramosh massif the deep seismic sounding profiles done in the 1970s. The other is such that it crosses the Indus Suture Zone, the Main Karakoram Thrust and the Karakoram Fault nearly at right angles. In both profiles, from Pakistan to China, the horizontal gradients of the Bouguer anomalies are in the interval −1.5 to −2 mgal km
−1
, comparable to gradients found in the region between the Indo-Gangetic Plains and the Himalaya. A marked steepening of the Moho is implied here with increased detail. Isostatic anomalies underneath the Karakoram are found to be more negative than those computed in the earlier work of Marussi, who made different assumptions of density and density contrast. Clearly the new data do not uniquely constrain the local structure of the lithosphere but may be of help in better understanding non-negligible deviations from isostatic equilibrium.
Structural evolution of the North Karakoram cover, North Pakistan
Authors: Zanchi Andrea 
Published: April 2008

Abstract
The oldest deformational structures recognized in the Karakoram sedimentary cover north of the Karakoram Axial Batholith (KAB) consist of an E-W deformed belt composed of Permian-early Cretaceous sediments. This belt is overlain with a stratigraphical unconformity by the Cretaceous Tupop conglomerates and subsequent Late Cretaceous marine sediments. In the Chapursan Valley a complex and polyphase antiformal stack consisting of a Permian to Cretaceous succession was recognized. Post-Cretaceous northward and northeastward directions of transport are shown by slip vectors along thrust planes and by fold axes in the Chapursan Valley, whereas NW-SE-trending folds due to northeastward transport were observed in the Shimshal Valley. The north-vergent structure is crossed by successive north-dipping thrusts along the northern side of the Chapursan Valley and along the contact between the sedimentary cover and the northern continuation of the KAB.
The last tectonic phase recognized in the Chapursan Valley caused the activation of an important set of E-W-orientated sinistral strike-slip faults. In the Shimshal Valley, large E-W sinistral and NW-SE dextral faults are present as well, and may be interpreted as a consequence of a NW-SE-directed simple-shear due to deformation along major NW-SE dextral strike-slip faults.
The sedimentary cover of the Karakoram was deformed under anchimetamorphic conditions in the Chapursan Valley, whereas sericite to muscovite low-grade metapelites with occurrence of chloritoid, epidote and biotite are present in the lower Shimshal Valley. Near the Shimshal Pass, andalusite, garnet, muscovite and biotite were found in phyllites at the contact aureole of an important belt of large previously unknown granitic bodies.
Tectonics of the Himalayan collision between the Indian Plate and the Afghan Block: a synthesis
Authors: Treloar Peter J. , Izatt Christopher N. 
Published: April 2008

Abstract
A synthesis of the available tectonic, structural, magmatic and geochronologic data from Afghanistan and western Pakistan suggests that collision between the Indian Plate and the Afghan Block (a collage of blocks assembled during the Cimmeride orogeny), took place during the Pliocene. Prior to this, the thinned western margin of the Indian Plate, together with the transitional to oceanic crust which separated the Indian Plate from the Afghan Block, was overlain by a wedge of Tertiary clastic sediments deposited in a large flexural basin that included what is now the Katawaz basin as well as the Sulaiman foredeep. At least three phases of collision can thus be recognised in the northwest Himalaya: the first, at about 100 Ma involved the suturing of the Kohistan-Ladakh-East Nuristan island arc to southern Asia; the second, at about 55 Ma involved India-Asia collision and the third, during the Pliocene, involved India-Afghanistan collision. During the Eocene and Miocene, the Afghan block was displaced southwards in response to Indian indentation into Asia. Simultaneous counterclockwise rotation of the Afghan Block resulted in intracontinental extension causing alkaline magmatism and right lateral displacements along the Herat Fault. As the Indian Plate moved northward with respect to Afghanistan, some of the wedge of clastic sediments deposited on the Indian transitional crust were accreted to the leading edge of the Afghan block and displaced sinistrally with respect to the rest of the sediments in the wedge. These displaced sediments now form the Makran sequence. Collisional structures on both sides of the India-Afghan collision zone in the Helmand block and the Sulaiman-Kirthar Ranges probably represent the inversion of continental margin basins rather than thrust belts with large shortening amounts. After initial inversion, the dominant feature of this highly oblique collision has been transpressional tectonics. Prior to its final docking with the Afghan Block, the Kabul block was a continental slice, completely surrounded by oceanic crust, and located northwest of continental India. Palaeocene collision between India and the Kabul block could have led to thickening of the Indian Plate prior to initiation of India-Asia collision.
Evolution of the lower arc crust in Kohistan, N. Pakistan: temporal arc magmatism through early, mature and intra-arc rift stages
Authors: Khan M. Asif , Jan M. Qasim , Weaver B. L. 
Published: April 2008

Abstract
The middle to lower island-arc crust in southern Kohistan represents a 40 Ma life-span of subduction-related magmatism in an intraoceanic setting. The Kamila amphibolite belt comprises two varieties of metavolcanic amphibolites. One enriched in high-field strength (HFS) and heavy rare-earth (HRE) elements is transitional in character between N- and E-MORB with a minor subduction-related component, and represents the earliest arc basement in Kohistan. The other variety of the metavolcanic amphibolites, together with deformed and amphibolitised intrusive basic plutons, has a transitional tholeiitic to calc-alkaline nature marked by depleted HFS and HRE elements and a distinct negative anomaly for Nb, suggesting emplacement in the early to mature stages of arc growth. The subsequent magmatism in southern Kohistan took place in two stages. The extensive calc-alkaline gabbronorites of the Chilas complex were derived from partial melting of a mantle diapir emplaced into a mature arc or, more probably, during the initial stages of sub-arc splitting. This intra-arc rifting, at its advanced stages, generated tholeiitic picrite to high-Mg basalts which crystallized the ultramafic-mafic-anorthosite (UMA) association and basic dykes of the Chilas complex. The Late Cretaceous accretion of the Kohistan island arc with the Karakoram plate in the north ceased the intraoceanic history of magmatism in southern Kohistan.
A palaeogeographic reconstruction of the Dir Group: evidence for magmatic arc migration within Kohistan, N. Pakistan
Authors: Sullivan M. A. , Windley B. F. , Saunders A. D. , Haynes J. R. , Rex D. C. 
Published: April 2008

Abstract
The Late Palaeocene Dir Group records the resumption of volcanic activity along the southern margin of Eurasia after a prolonged period of uplift and erosion. The group forms an integral part of the Kohistan Batholith and is readily divisible into two distinctly contrasting volcanic successions.

The Baraul Banda Slate Formation comprises 2700 m of fore-arc sandstones and siltstones. The basin formed during the collapse of the Kohistan continental margin and was filled initially by subaerial debris and mass-flow deposits. Subsidence was rapid such that >900f the sedimentary record consists of thin-bedded sheet turbidites. These maintain a remarkable lateral continuity and indicate deposition in a restricted deep-water environment. Rare interbedded limestones have yielded a Thanetian (60.2–54.9 Ma) marine fauna of
Miscellanea miscella
and
Actinosiphon tibeticus
.

In direct contrast the Utror Volcanic Formation comprises 3000 m of volcaniclastic sedimentary material, lava flows and ignimbrites. Eruptive style was predominantly explosive with fragmental material dominating the volcanic record. Lavas range in composition from 53% to 79% SiO
2
with rhyolite the predominant rock type. A basaltic andesite has an
40
Ar-
39
Ar age of 55 ± 2 Ma. Although the volcanic stratigraphy is complex preliminary investigations suggest accumulation in a predominantly subaerial ring-plain or flanking facies distal to the main focus of volcanic activity. Sedimentary thickness and the abundance of silicic lavas and pyroclastic flows imply extensional tectonic control even though the arc developed at a convergent margin.

Juxtaposition of the two facies along the Dir Thrust excludes any record of shore-line or coastal processes from the volcanic stratigraphy. The inferred original geographical separation of the two environments coupled with their relationships to isotopically dated plutonic suites reveals that between Late Cretaceous and Early Eocene times the locus of magmatic activity within the Kohistan arc migrated progressively northward.

The Shamran volcanic rocks although originally correlated with the island-arc Chalt Volcanic Group are now identified on the basis of a 58 ± 1 Ma (
40
Ar-
39
Ar) age as a northern continuation of the Late Palaeocene volcanic arc.
High-pressure metamorphism in the High Himalayan Crystallines of the Stak valley, northeastern Nanga Parbat-Haramosh syntaxis, Pakistan Himalaya
Authors: Pognante Ugo , Benna Piera , Fort Patrick Le 
Published: April 2008

Abstract
The High Himalayan Crystallines (HHC) from the little-known NE termination of the Nanga Parbat-Haramosh syntaxis (Stak and upper Turmik valleys, Pakistan) consist of kyanite-bearing gneiss with minor garnet-granulite and garnet-amphibolite. The HHC underwent a Himalayan metamorphism with a peak at high pressure (8–13 kbar) and high temperature (650–700° C). During exhumation the HHC rocks followed a rapid exhumation path at high temperature with little or no medium to low pressure re-equilibration. These lines of evidence, combined with geochronological and petrological data from Eocene eclogites recently found in the Kaghan nappe, indicate that, after subduction and high to very high pressure metamorphism, part of the HHC from northern Pakistan underwent very rapid cooling and exhumation. By contrast, exhumation along paths of increasing temperature are recorded by the HHC from regions located east of Pakistan (e.g. Nepal). These differences along strike in the HHC suggest that, during the Eocene collision between India and Eurasia, subduction and exhumation occurred at higher rates in northwestern than in central-eastern Himalaya.
Early Permian granitic dykes of alkaline affinity in the Indian High Himalaya of Upper Lahul and SE Zanskar: geochemical characterization and geotectonic implications
Authors: Spring L. , Bussy F. , Vannay J.-C. , Huon S. , Cosca M. A. 
Published: April 2008

Abstract

In the Tethyan sedimentary series and the High-Himalayan ‘crystalline’ units of NW India, two generations of granite have previously been described. These rocks include large Cambro-Ordovician batholiths and small Tertiary bodies. A third type, the Yunam granite, has been discovered in the SE Zanskar — Upper Lahul region. It crops out as discordant dykes in the Ordovician Thaple Fm. Radiometric (U-Pb) measurements on zircons give an Early Permian crystallization age of 284 ± 1 Ma. Tertiary metamorphic cooling ages of 33.5, 20.8 ± 0.4, 22.8 ± 0.5 and 19.3 ± 0.2 Ma have been obtained by
40
Ar/
39
Ar and K-Ar methods on amphiboles, muscovites and biotites of the granite and associated rocks. In contrast to the crustal affinity of the Cambro-Ordovician and Tertiary Himalayan granites, the Yunam granite is associated with an anorogenic, possibly thinned lithosphere context. This magmatic event is most probably related to the Permo-Mesozoic rifting of Neo-Tethys.
Sedimentology and tectonic implications of the Lamayuru Complex: deep-water facies of the Indian passive margin, Indus Suture Zone, Ladakh Himalaya
Authors: Robertson Alastair H. F. , Degnan Paul J. 
Published: April 2008

Abstract
The Lamayuru Complex, exposed in the Indus Suture Zone of the Ladakh Himalaya, comprises remnants of the Triassic to Upper Cretaceous, north-facing Indian passive margin. This margin experienced pulses of extension and/or transtension in the Late Permian, early Mid-Jurassic, Late Jurassic and Early Cretaceous. Two areas studied preserve mainly relatively intact slope, to base-of-slope settings.
Initial sea-floor spreading in the Late Permian is reflected in early deposition of base-of-slope turbidites in the Lamayuru Complex, and exotic limestones and volcanics in associated melange units. Quartzose turbidites, calciturbidites and shales then accumulated in slope, to base-of-slope settings during the Triassic and Early Jurassic, related to subsidence of the adjacent Zanskar shelf after spreading began. The carbonate platform edge underwent drastic extensional collapse in the early Mid-Jurassic (Aalenian), giving rise to limestone ‘exotics’ in the west and carbonate-filled submarine channels in the east. Extension related alkaline volcanism, dated as Mid-Late Jurassic by radiolarians, exploited the inferred continent-boundary area in the west. Mixed quartzose clastic, volcaniclastic, radiolarian and pelagic carbonates accumulated in the Cretaceous, but remain poorly dated. Radiolarian cherts reflect siliceous plankton productivity along the passive margin. During the latest Cretaceous, ophiolitic melange was accreted at a trench within the Tethys ocean to the north, but there is, as yet, no definite evidence of emplacement onto the Zanskar shelf until formation of an early Eocene foredeep, followed by emplacement of the Spontang ophiolite and related melange.
Metamorphic zonation, migmatization and leucogranites along the Everest transect of Eastern Nepal and Tibet: record of an exhumation history
Authors: Pognante Ugo , Benna Piera 
Published: April 2008

Abstract

A polyphase evolution and a thermal and mineral zonation characterize metamorphism of the High Himalayan Crystallines (HHC) in the Everest transect. After an early Barrovian-type metamorphism (M1:
T
= 550–680° C;
P
= 8–10 kbar) caused by subduction and crustal thickening during India-Eurasia collision, exhumation of the HHC occurred at rates which allowed the isotherms to rise and the thickened crust to suffer partial melting. In the upper structural levels, the thermal conductivity contrast between crystalline and upper sedimentary rocks together with the infiltration of metamorphic fluids released during exhumation, favoured migmatization and formation of leucogranite magmas through partial melting of muscovite-bearing metasediments chiefly at medium (M2:
T
= 650–750° C;
P
= 4–7 kbar) but also at low-pressure (M3:
T
= 600–700° C;
P
= 2–4 kbar). The leucogranite melts segregated as dykes and kilometre-sized bodies into extensional shear zones situated at the uppermost levels of the HHC and at the base of the overlying Tibetan Series. During exhumation, a metamorphic zonation was imposed on the HHC in response to the complex interplay between rise of the isotherms in the thickened crust, emplacement of leucogranites in the uppermost levels of the HHC and conductive cooling near the ‘cold’ Lesser Himalaya and Tibetan Series coupled with thrust imbrication.
The metamorphism of the Tibetan Series from the Manang area, Marsyandi Valley, Central Nepal
Authors: Schneider Christian , Masch Ludwig 
Published: April 2008

Abstract

In the Manang area a distinctive metamorphic zonation with grade increasing downward to deeper structural levels of the Tibetan Series can be observed. The first appearance of prograde biotite/phlogopite introducing the metamorphic zonation can be located in the Devonian Tilicho Pass Formation. The Silurian Dark Band Formation contains prograde titanite. Tremolite is not observed to the west of the Mutsog region (Nilgiri Limestone). In the lower parts of the Nilgiri Limestone, tremolite appears together with K-feldspar. Diopside only occurs in the lowest parts of the Nilgiri Limestone. Maximum temperatures for the prograde metamorphism, derived from carbonate solvus thermometry, are in the range of 510–530°C for the middle Nilgiri Limestone and 370–390°C for the Lower Carboniferous. The fluid composition during the prograde metamorphism at the base of the Tibetan Series was CO
2
rich (
X

CO
2

between 0.4 and 0.7). A strong retrogressive phase with a fluid composition rich in H
2
O is documented by secondary growth of sphene, clinozoisite and amphibole. This retrograde phase is rare in formations above the Nilgiri Limestone and might be contemporaneous with detachment at the base of the Tibetan Series. The prograde mineral phases are related to a first deformation phase. A second deformation phase, most probably related to detachment movements between the Tibetan Series and the High Himalayan Crystalline, resulted in a partly developed crenulation cleavage unrelated to any metamorphic change. The formation of the dominating E-W-orientated syncline took place in a third deformation phase.
Structural evolution of the High Himalayan Gneiss sequence, Langtang Valley, Nepal
Authors: Reddy Steven M. , Searle Michael P. , Massey John A. 
Published: April 2008

Abstract
Within the High Himalayan Crystalline Sequence (HHCS) of the Langtang Valley of central Nepal, four different lithotectonic units have been identified in the field. The structurally lowest unit, immediately in the hanging wall of the Main Central Thrust (MCT) records an inverted metamorphic sequence with kyanite-grade rocks overlain by rocks containing sillimanite. Pressure-temperature-deformation histories of the different units since Himalayan collision 50 Ma ago, in combination with geochemical data, indicates that the ‘apparent’ metamorphic inversion is a result of the tectonic juxtaposition along high-temperature shear zones that accommodated top-to-southwest thrusting. It is also argued that the contacts between different sillimanite-grade units are also tectonic in origin and that the HHCS in the Langtang Valley does not represent a structurally undisturbed section through the crust. Internally the different units contain minor extensional and thrust faults which at the same structural level developed at similar metamorphic conditions. Deformation along the high temperature shear zones of the HHCS are coeval with the earliest of these minor fault zones at the lowest structural levels. With time, this deformation migrated upwards and minor faulting within the upper levels of the HHCS took place at chlorite-grade. These minor fault zones are interpreted to be the result of dynamic spreading of the developing orogenic wedge associated with changes in the geometry of this wedge due to underplating and accretion of material to the base of the metamorphic pile.
The role of fluids in the formation of High Himalayan leucogranites
Authors: Harris Nigel , Massey John , Inger Simon 
Published: April 2008

Abstract
During fractional melting of the crust, the availability of fluids is a critical variable that influences the melt fraction obtained, the capacity of the melt to migrate from its source and the relationship between melting and uplift. For the High Himalayan leucogranites, isotope systematics and metamorphic phase equilibria have identified micaceous metasediments as the most likely source, and the incongruent melting of muscovite as the appropriate melting reaction. High-field strength trace elements are controlled by dissolution kinetics and incomplete crystal/liquid segregation of accessory phases during crustal anatexis. In contrast, Rb and Sr are controlled by major phases contributing to the melting reaction. Hence the Rb/Sr ratios in primary melts can be correlated with the availability of fluids during partial melting.
Geochemical modelling of melts resulting from anatexis of a wide range of metasedimentary compositions (plagioclase 5–30%, muscovite 5–30%, biotite 5–20%, quartz 30–85%) indicates that Rb/Sr ratios will vary between 4 and 10 during equilibrium melting and vapour-absent conditions, but are less than 3.5 during vapour-present melting. In either case, Rb/Sr ratios in melts are reduced under disequilibrium conditions. In general Rb/Sr ratios of Himalayan leucogranites vary between 3 and 6. Such high ratios could not have formed under vapour-present conditions unless feldspar fractionation has increased the Rb/Sr ratios of primary melts. Trace-element systematics and petrographic evidence do not support significant fractionation of feldspar. Hence for most, if not all, Himalayan granites, vapour-absent conditions prevailed during fractional melting, irrespective of either source composition or the kinetics of the melting reaction. This is consistent with the observation that the melts have migrated from their source, and allows decompression melting to play a role in their genesis.