bluuebunny/crossref_metadata_2025_split huggingface dataset:
Leucocratic rocks from the Bela ophiolite, Khuzdar District, Pakistan
Authors: Ahmed Zulfiqar 
Published: April 2008

Abstract
This is the first detailed report of the well-developed leucocratic rocks, dominantly plagiogranites, from the Bela ophiolite, the most complete and largest ophiolitic outcrop of Pakistan. A wide range of leucocratic rock types occur within the ophiolite including leucogabbro, gabbroic pegmatite, diorite, quartz diorite, trondhjemite, epidote trondhjemite, granite, keratophyre and acidic pegmatite. Two suites of acidic leucocratic rocks are recognized: a very low potash and high soda suite containing magma-fractionated plagiogranite characteristic of most ophiolites, is accompanied by an anatectic suite with higher potash content. Each suite possesses distinct trace element and rare earth contents, together with distinctly different Sr and Nd systematics. These differences indicate differences in tectonic setting for each leucocratic suite. U-Pb isotopic measurements on zircons separated from the acidic rocks yield a crystallization age of 65 ± 1 Ma for both the rock suites. On the basis of geochemical data a suprasubduction zone origin dominated by a marginal basin is proposed for the Bela ophiolite. The plagiogranitic rock suite could have been derived from basic magma by fractional crystallization. The potash-richer granitic rocks may have been generated by crustal anatexis forming a melt that locally invaded the ophiolite.
Chemistry of chromite and associated phases from the Shangla ultramafic body in the Indus suture zone of Pakistan
Authors: Arif Mohammad , Jan M. Qasim 
Published: April 2008

AbstractThe partly to wholly serpentinized ultramafic rocks of the Shangla area constitute the northern part of the Mingora ophiolitic melange which lies along the Main Mantle Thrust (MMT) zone between the Kohistan island arc and the Indo-Pakistan plate. These rocks contain disseminated grains and massive bodies of segregated chromite. Field, textural, and chemical characteristics of the segregated chromite closely resemble those of podiform chromite deposits. The chemical composition of the chromite varies considerably from sample to sample with modal proportion, from grain to grain in the domain of a thin section, and from core to margin within individual grains. In general, the segregated chromites are richer in Cr and Mg, but poorer in Fe, Al and Mn than the accessory chromites. The inter- and intragranular chemical variations are much greater and more common in the case of accessory chromite due probably to a greater degree of subsolidus re-equilibration with the associated abundant silicate matrix. The intragranular variation, probably a result of alteration, mostly involves enrichment in Fe, Mn (and Ti), and depletion in Al, Mg and Cr, leading to the development of ferritchromit and, rarely, magnetite along fractures and margins of individual grains.The studied rocks from the Shangla area display cumulus textures but contain abnormally magnesian olivine (Fo96.0–98.5). The 100Cr/(Cr + Al) ratios of their chromite suggest a complex origin. These rocks may have formed under conditions transitional between those of arc and oceanic settings. The Shangla rocks underwent a low- to ?medium-grade metamorphism as indicated by the development of talc and antigorite. This prograde metamorphic process probably also accounted for the extreme chemical zoning of the Shangla chromite.
A section through the Indian Plate, Kaghan Valley, NW Himalaya, Pakistan
Authors: Greco Antonio , Spencer David A. 
Published: April 2008

Abstract
A map is presented which summarizes the work of the ETH Himalayan Group in the Kaghan Valley and adjacent areas of the NW Himalaya in Pakistan. The stratigraphical, structural and metamorphic features of the lithologies belonging to a virtually complete section through the Indian plate are described. Evidence is presented which shows a stratigraphic range in ages from the Precambrian Salkhala Formation to the Eocene Murree Formation which have been metamorphosed at various metamorphic grades ranging from very low grades/unmetamorphosed to eclogite facies. An inverse metamorphism, associated with the Main Central Thrust is also described. A large-scale tectonic subdivision of the area into the classical Sub-, Lesser and Higher Himalayan elements can be correlated with the Kashmir Himalaya to the east. A tectonic model is presented which suggests that an anticlockwise rotation of the transport direction caused the refolding and stacking of the nappe piles in the Paleogene.
The tectonic evolution of the Northwestern Himalaya in eastern Ladakh and Lahul, India
Authors: Steck A. , Spring L. , Vannay J.-C. , Masson H. , Bucher H. , Stutz E. , Marchant R. , Tieche J.-C. 
Published: April 2008

Abstract
Geological studies along a transect across the Himalaya in eastern Ladakh and Lahul provide new insights into the Tertiary structural evolution of this region. The initiation of the Nyimaling-Tsarap Nappe is related to an early phase of underthrusting of India below Asia. In Lahul, an opposite vergent intra-continental underthrusting develops immediately after continental collision (NE-vergent Tandi Syncline and Shikar Beh Nappe). This NE-vergent nappe stack is responsible for the amphibolite-facies regional metamorphism of the lower Chandra Valley. The subsequent phase corresponds to the main thrusting of the SW-vergent Nyimaling-Tsarap Nappe, developed by ductile shear (87 km Eocene shortening). This nappe pile is responsible for the regional metamorphism of SE Zanskar (kyanite-staurolite near Sarchu). The root zone and the frontal part of the Nyimaling-Tsarap Nappe are subsequently overprinted by two NW-SE-orientated dextral transpressional shear zones. To the south of the investigated area, the Main Central Thrust has been developed as a shear zone in the regional metamorphic ductile crustal rocks below the older nappes to the north. In the Sarchu and Nyimaling regions, the following tectonic phase corresponds to NE-vergent ‘backfolding’ (Miocene). Normal faults in the Sarchu area record a late extension of approximately 14 km.
Field relations, petrogenesis and emplacement of the Bhagirathi leucogranite, Garhwal Himalaya
Authors: Searle M. P. , Metcalfe R. P. , Rex A. J. , Norry M. J. 
Published: April 2008

Abstract
The Bhagirathi leucogranite forms a series of low-angle en echelon, lensoidal intrusions at the top of the High Himalayan slab in the central Himalaya of Garhwal, northern India. The leucogranite comprises the assemblage: K-feldspar + quartz + plagioclase + tourmaline + muscovite ± biotite ± garnet. Compared to other High Himalayan leucogranites it is particularly rich in tourmaline. The granite is generally compositionally homogeneous although it is magmatically banded in both the upper and lower portions.
The Bhagirathi leucogranite is situated structurally above the kyanite and sillimanite gneisses of the Vaikrita Group which, in turn, overlie the north-dipping Main Central Thrust zone of inverted metamorphic isograds. A pegmatite — aplite leucogranite sill and dyke swarm is present around the margins of the leucogranite. Vaikrita Group gneisses below the leucogranite contain a pronounced northeasterly component to generally randomly orientated mineral stretching lineations. This reflects localized reorientation of early, coaxially constrained, mineral growth by later, non-coaxial deformation. Various shear criteria in the gneisses immediately below the granite document the existence of a major zone of ductile NNE-SSW-directed extension across a northeastward dipping, low-angle normal fault zone. The top of the 1500–1800 m thick leucogranite sheet exposed on the peaks of Bhagirathi, Shivling and Thalay Sagar are intrusive into Martoli Formation metasediments of the Tethyan sequence which locally contain andalusite and staurolite. The roof complex shows numerous stoped blocks, xenoliths and elongate rafts of host rock in the top 100 m together with an extensive zone of layer-parallel leucogranite veining.

The northward-dipping Bhagirathi leucogranite was intruded syn-tectonically during the ductile to brittle transition and has been deformed into linked, en echelon bodies within the extensional shear zone at the interface between the Vaikrita Group gneisses and the Tethyan sedimentary cover. The long axes of the leucogranite lenses lie parallel to the
y-z
plane of the finite strain ellipsoid for this extensional duplex. During extension, sub-orthogonal dilatational forces exceeded sub-horizontal shear stresses thus facilitating the repeated emplacement of sheeted granite melt, a process analogous to low-angle tension gash development. The final emplacement level in the crust must have been ultimately controlled by the density contrast between melt and country rocks, the thermal blanketing of the Tethyan sedimentary cover, and the extensional stress field along the top of the High Himalayan slab.
Extensional tectonics in the Himalayan orogen, Zanskar, NW India
Authors: Patel R. C. , Singh Sandeep , Asokan A. , Manickavasagam R. M. , Jain A. K. 
Published: April 2008

Abstract
Large-scale nappes of the deformed Higher Himalayan Crystallines (HHC) were thrust southward over the Lesser Himalayan Proterozoic foreland during the Late Cenozoic. Critical evaluation of shear fabrics, reveals that the HHC underwent an earlier phase of ductile shearing with top-to-southwest overthrust-type sense of movement. On which was superposed layer-parallel NE-trending extension. As a consequence, the Zanskar Shear Zone (ZSZ) along the northern boundary of the HHC is re-interpreted as a complex ductile shear zone with an initial top-to-southwest sense of thrusting followed by the later superposition of top to northeast-directed extension. This period of extension is recognized across the whole of the HHC and is the result of the uplift and exhumation of the HHC by southwestward directed thrusting and ramping along numerous thrusts including the Main Central Thrust (MCT).
Annapurna detachment fault in the Greater Himalaya of central Nepal
Authors: Brown Richard L. , Nazarchuk Jeffrey H. 
Published: April 2008

Abstract
Displacement on the Annapurna detachment fault (ADF) has exhumed the Greater Himalayan metamorphic sequence in central Nepal. The fault is most likely a continuation of the South Tibetan detachment system recognized elsewhere in the Himalayan orogen.
The characteristics of detachment faulting and the structural and metamorphic histories of the hanging wall and footwall in the Thakkhola region are described. Two stages of normal-sense shearing on the ADF are recognized: the first stage occurred at mid-crustal depth; the most recent detachment faulting on the system was brittle and may have been accompanied by orogen-parallel extension.
First-phase isoclinal folds in the Tibetan sedimentary sequence above the ADF reflect a southwesterly sense of shear and are refolded by spectacular northeasterly verging second-phase folds that have affected a crustal thickness of over 6 km. Third-phase southwesterly directed thrust faults and folds have disrupted the earlier structures but predate the most recent period of normal-sense shearing on the ADF.
The kinematic significance of the northeasterly verging second-phase folds is not clear; these structures may have formed during early motion on the ADF but, alternatively, may reflect older events associated with compression and crustal thickening of the orogen.
Pressure, temperature and time constraints on metamorphism across the Main Central Thrust zone and High Himalayan Slab in the Garhwal Himalaya
Authors: Metcalfe R. P. 
Published: April 2008

Abstract

Following the early Eocene collision of India and Asia, continental subduction occurred on the northward-dipping Main Central Thrust (MCT). In western Garhwal, N. India, upper amphibolite-facies gneisses on the High Himalayan Slab are thrust southwards over unmetamorphosed to greenschist facies quartzites, carbonates and metabasics of the Lesser Himalaya. In the Bhagirathi valley, the MCT forms a
c.
10 km thick shear zone composed of mylonitic augen gneiss, amphibolite and metasediments. Metamorphic grade increases both northwards and with structural height. The MCT zone is bounded to the north by the Vaikrita (roof) Thrust and to the south by the Munsiari (floor) Thrust. The Vaikrita Thrust is a diffuse high-temperature shear zone, whereas the Munsiari Thrust is a relatively discrete fault formed under brittle-ductile conditions. North of the MCT zone, at the top of the High Himalayan Slab a northward-dipping extensional shear zone, the Jhala normal fault, was responsible for the downthrow of the Tethyan sediments to the north with respect to the uplifting High Himalayan Slab gneisses to the south.

Thermobarometic transects reveal a sudden increase in both pressure and temperature across the Vaikrita Thrust from south to north but with subsequent decreases accompanying structural height in the High Himalayan Slab. Temperatures increase going up-structural section from about 500° C to 770° C across the MCT zone, but then decrease again to the north varying between about 550 and 640° C. Similarly, pressures increase sharply up-structural section across the MCT zone from 6 to 12 kbar, then decrease towards the top of the slab to between 7 and 8.9 kbar. The inverted
P-T
gradient across the MCT zone changes to approximately isothermal and isobaric conditions in the top 9 km (horizontal distance) of the High Himalayan slab.

Cooling rates for the upper MCT zone determined from
40
Ar/
39
Ar (hornblende) and K-Ar (muscovite and biotite) cooling ages suggest a return to erosion-controlled denudation following extension at the top of the High Himalayan Slab. Additional K-Ar (muscovite) cooling ages from a transect through the MCT zone and High Himalayan Slab are progressively younger towards the south, reflecting the southward propagation of the deformation sequence with time. Hornblende
40
Ar/
39
Ar cooling ages from the MCT zone suggest that structurally lower rocks have not been heated above
c
. 500° C since the Precambrian, whilst a 19.8 ± 2.6 Ma hornblende age from the MCT zone dates the latest high-temperature shearing at higher structural levels in the MCT zone and places a minimum age constraint on Himalayan metamorphism in the Garhwal sector of the Himalaya.
Deformation and metamorphism within the Main Central Thrust zone, Arun Tectonic Window, eastern Nepal
Authors: Meier K. , Hiltner E. 
Published: April 2008

Abstract
The Lesser Himalayan Arun Tectonic Window comprises the Upper Precambrian, polymetamorphosed metasediments of the Nawakot Complex. These are tectonically overlain by Precambrian gneisses of the Higher Himalayan Kathmandu Complex. The main phase metamorphism within the Higher Himalayan rocks predated thrusting along the Main Central Thrust. By contrast, the main metamorphism (M2) within the Nawakot Complex commenced synchronously with it being underthrust during D2 along the Main Central Thrust Zone beneath the amphibolite facies rocks of the Kathmandu Complex. During D2 rocks on both sides of the Main Central Thrust Zone underwent intensive mylonitization within a several hundred meter wide zone of non-coaxial shearing. Although the stratigraphic order within the Nawakot Complex is the right way up, rocks within the Arun Tectonic Window show a concentric inverted metamorphic zoning, ranging from lower greenschist facies in the core of the window, up to lower amphibolite facies at the outer edge of the window. This ‘inverted metamorphism’ (M2) of the Nawakot Complex originated as a result of syn- to post-D2 downward heat conduction from the ‘hot’ hanging wall of the Main Central Thrust onto the ‘cold’ footwall. This heat flow also caused the strong retrograde overprinting at the base of the Kathmandu Complex. After a static period, a late- to post-M2 NNE-shortening (D3) commenced along the Main Boundary Thrust. The Arun Tectonic Window is the result of doming of the Main Central Thrust during D4 followed by erosional exhumation.
Evolution of foreland structures: an example from the Sulaiman thrust lobe of Pakistan, southwest of the Himalayas
Authors: Jadoon I. A. K. , Lawrence R. D. , Lillie R. J. 
Published: April 2008

Abstract
Three cross-sections based on integrated surface geology, borehole and seismic reflection data have been used to interpret the style and evolution of the evolving Sulaiman foreland. A 10 km thick stratigraphic section outcrops at the deformation front. Available evidence suggests detachment of nearly all the stratigraphic sections from the crystalline basement. The decollement is believed to be located along the brittle-ductile boundary at the base of the wedge in the Sulaiman lobe of gentler topography. Coherent stratigraphy and structural uplift of 4–8 km is interpreted to be a result of a passive-roof duplex style of deformation. The duplex sequence consists of competent Jurassic and older strata. It has a floor thrust (decollement) in Palaeozoic rocks and a roof thrust (passive-roof backthrust) in thick Cretaceous shales. The roof sequence preserved over several horses along length of the section shows hindward translation.

The structural variation in cross-sections has led to the modification of an earlier interpretation that folding precedes duplexing. The present chronology of structures is the: (1) development of an incipient (
c.
3.5 km half-wavelength) Uch fault-propagation fold at the tip of the decollement; (2) formation of a low amplitude, broad (
c.
20 km half-wavelength) concentric detachment fold (Sul); (3) increase in amplitude of a detachment fold (Loti/Zin) to a critical level for development of ramp and duplex structures.
Braided-channel pattern and palaeohydrology using an index of total sinuosity
Authors: Robertson-Rintoul M. S. E. , Richards K. S. 
Published: April 2008

Abstract

Channel patterns are commonly represented as discrete types (e.g. straight, meandering, braided) separated by threshold values of slope and discharge. However, for gravel-bed river patterns, an index of total sinuosity (∑
P
) can be defined which varies continuously with both stream power per unit channel-belt length (represented as the product of mean annual flood and valley gradient) and median bed material diameter. An empirical relationship between ∑
P
and these controlling variables can be used to provide palaeo-hydrological reconstructions for braided stream traces on terrace fragments, as an alternative to methods based on bed material sizes and criteria for the threshold of grain motion.
Anabranch confluence kinetics and sedimentation processes in gravel-braided streams
Authors: Ashmore Peter 
Published: April 2008

Abstract
Based on observations of channel changes and bar sedimentation in small-scale hydraulic models, it is argued that confluence zones play the key role in sedimentation patterns and processes in gravel-bed braided streams. The control is exerted because confluences are nodes in the channel network and therefore sedimentation and channel pattern downstream of a confluence are mediated by the response of the confluence to changes in discharge, bedload, and planform of the upstream confluent channels. Examples of confluence kinetics and sedimentation occurring over time-scales equivalent to days or weeks of competent flows in the prototype are described for the first time. Renewed braiding is directly associated with flow expansion downstream of confluences and triggered by migration of unit bars or bedload sheets through the confluence. Lateral translation of confluences may lead to the accretion of point bar complexes, while downstream translation often causes erosion or complete removal of existing bars. The effects of a change in position or orientation of one of the upstream channels are similar to those of confluence translation. In addition to the changes caused by progressive shifting of the confluence zone, a second group of adjustments involving changes in the discharge or bedload transport rate in the confluent channels is also common. These include: rotation of the scour axis redirecting flow and sediment movement downstream of the confluence and leading to downstream channel avulsion in some cases; a change in the total discharge of the confluence causing expansion or contraction of the scour zone; and the virtual abandonment of one of the confluent channels resulting in obliteration of the scour hole. Observing a confluence for several hours in the model leaves the visual impression of sediment being distributed downstream of the confluence rather like water spurting from a hose as it plays about. This descriptive synthesis of confluence kinetics provides the basis for analysis and quantitative modelling of confluence dynamics, which requires the acquisition of field data, as well as further laboratory experiments.
Radar facies: recognition of facies patterns and heterogeneities within Pleistocene Rhine gravels, NE Switzerland
Authors: Huggenberger Peter 
Published: April 2008

Abstract
Pleistocene braided-river deposits in river valleys constitute a large fraction of natural groundwater reservoirs in Switzerland. A key for estimating the residence times of water and for determining the extent of macrodispersion, which describes large-scale mixing processes in the aquifers, is a knowledge of the distribution of hydraulic conductivities. In many contamination problems, sedimentological information is sparse and drill-core descriptions and pumping-tests only give a limited picture of the geometry of inhomogeneities. Ground-probing radar (GPR) offers the potential to resolve sedimentary structures and lithofacies in gravel deposits. For example, the geometry of characteristic sedimentary structures and the textures of late Pleistocene Rhine gravel are portrayed on GPR reflection images.
Because of the ability of the GPR method to detect changes in water content, the reflection image can be related to small changes in the degree of sediment saturation, which may also reflect a change in sediment composition. This allows a distinction between several characteristic lithofacies that are typical of late Pleistocene braided-river depositional systems. The main limitation of the GPR method is the rapid attenuation of electromagnetic waves in the ground, especially in clay-covered regions.
Avulsive channel systems: characteristics and examples
Authors: Richards Keith , Chandra Shobhit , Friend Peter 
Published: April 2008

AbstractFluvial processes in large subsiding sedimentary basins (e.g. tectonic foredeeps) must be adjusted in the long term to distribute sedimentation areally, although in the short term deposition is localized along the river courses. This means that the major long-term characteristic of the river pattern is that it is avulsion-dominated. Individual channels in an avulsive system display one of the conventional pattern types (e.g. meandering, braiding), but these patterns may reflect the behavioural relationships between the ‘dominant’ and ‘secondary’ channels in the system. These relationships change through time as aggradation along the dominant channel encourages avulsion into the course of former secondary channels, and the channels exchange roles. Thus, channel system behaviour is not explained by the variation of stream power used to analyse the behaviour of individual (meandering or braided) rivers, and palaeohydrological reconstructions for individual channels within such a system are of limited significance. Examples of avulsive systems are noted, including the northern Gangetic Plain.