bluuebunny/crossref_metadata_2025 huggingface dataset:
The complex nature of a Jurassic multistorey, alluvial sandstone body, Whitby, North Yorkshire
Authors: Alexander J. , Gawthorpe R. L.
Published: April 2008

The complex alluvial sandstone body of the Saltwick Formation (Aalenian) exposed in the town of Whitby, North Yorkshire is 700–1000 m wide, up to
c.
25 m thick and extensive in the palaeoslope direction (N-S). Exposed sections are approximately parallel and perpendicular to the palaeochannel orientation. The sandstone body is made up of a large number of channel storeys stacked in a complex vertical and lateral pattern and it is composed predominantly of fine- to medium-grained sandstone (> 85% by volume) with subordinate amounts of mudstone and siltstone. The sedimentary facies architecture indicates that channels repeatedly reoccupied the same site and that periods of erosion and incision alternated with periods of rapid bed aggradation. The sandstone facies patterns are related to migration of channel bars, dunes and ripples. Fine-grained sediment facies which occur between some of the storeys are laterally discontinuous because of their position within abandoned channels and their partial erosion by later channels.

Within the sandstone, permeability ranges from 0.8 to 682.1 mD with a geometric mean of 83.5 mD. Permeability heterogeneity is apparent from laminae to sandstone body scale. An attempt was made to characterize permeability on the basis of sedimentary facies. However, there is considerable scatter in the data and individual facies show a varying degree of heterogeneity of permeability. Two major sandstone storeys which appear very similar in origin and lithological character were found to have systematically different permeability distributions (geometric mean permeabilities of 36.0 and 108.5 mD). Facies variations can only partly explain this. There is a difference in the mean permeability of the same facies in the two sandstone storeys (e.g. geometric mean permeability of cross bedded sandstones in the two units are 63 mD and 100 mD respectively).
Production evidence for geological heterogeneities in the Anschutz Ranch East Field, western USA
Authors: Lewis Helen , Couples Gary D.
Published: April 2008

The Anschutz Ranch East Field is a large asymmetric anticlinal trap in the Wyoming-Idaho-Utah thrustbelt of the western USA. The aeolian Jurassic Nugget Sandstone reservoir unit is a layered sequence of dunes and stratigraphically flat interdunes. This interpretation is supported by patterns of reservoir performance in the gently-dipping, nearly planar backlimb of the fold where there is little structural deformation. Structural features which developed during emplacement of the Anschutz Ranch East truncation anticline are partially dependent on the distribution of lithofacies, and the changes in rock properties which result from the deformation are overprinted onto the pre-existing rock properties. An extensive coring programme demonstrates that most of the strain in the Nugget Sandstone is in the crest and forelimb of the fold. Throughout the structure, the deformation is primarily of the compactive (sealing) type, leading to lowered porosities and permeabilities, and thus both to degraded reservoir quality and to flow barriers. Structurally-created low-permeability zones are identified or interpreted at a range of scales, and production histories provide sufficient evidence to establish the role of the larger features as flow partitions.
Compartmentalization of an aeolian sandstone by structural heterogeneities: Permo-Triassic Hopeman Sandstone, Moray Firth, Scotland
Authors: Edwards H. Ewart , Becker Andrew D. , Howell John A.
Published: April 2008

Many excellent aeolian sandstones suffer a decline of reservoir quality associated with structural compartmentalization by tectonic features. An onshore analogue, the Permo-Triassic Hopeman Sandstone, which crops out along the Southern side of the Moray Firth, Scotland, has been studied due to the difficulty in quantifying tectonic heterogeneities in the sub-surface. A hierarchy of structural flow barriers, with impermeable fault rock, has been observed. These vary from solitary deformation bands, via compound zones of bands to major slip surfaces/faults. Episodic faulting leads to the growth from bands to larger fault zones. Increased cementation of the sandstones is seen adjacent to these tight fault zones, and a zone of enhanced cementation, spatially governed by the main fault in the region, has been mapped. Conjugate extensional systems of deformation bands decrease in density away from major faults. Porosity reduction is related to proximity to faults, with porosity increasing abruptly away from the tectonic barrier; the deformation producing low-quality reservoir and flow barriers. Tectonic features have, intermittently, acted as both fluid conduits and barriers: enhanced cementation of sedimentary features adjacent to ‘tight’ faults shows migration of diagenetic brines along fractures. The relatively well sorted aeolian Hopeman Sandstones display better developed flow barriers than the poorly-sorted, mixed facies fluviatile Burghead Beds. Interaction of fluid-flow, diagenetic cementation and structural deformation produce flow barriers that compartmentalize the sandstones on all scales.
Recent gravity measurements in the Karakoram
Authors: Caporali Alessandro
Published: April 2008

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

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

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

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

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

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

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

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

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

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

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

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.
Cause and effect among thrust and normal faulting, anatectic melting and exhumation in the Himalaya
Authors: England Philip , Molnar Peter
Published: April 2008

The transition from compression to extension in a regime of overall convergence in a narrow mountain belt requires a change with time in at least one of the following: horizontal normal stress applied to the belt by its surroundings; the distribution of shear traction on the base of the belt; the constitutive relation between stress and strain rate of material in the belt itself; the surface height of the belt. The following processes could have caused one or other of these changes in the Himalaya in Miocene time and thus could have triggered the onset of extension by normal faulting on planes parallel to thrust faults that were active at the same time. The shear stress on the thrust fault at the base of the Himalaya could have decreased, for instance because of melting on the fault plane. Alternatively, the creation of a new deeper thrust fault, and hence a new locus of convergence, could have initiated extensional faulting if the distribution of shear stress on the new thrust fault were unable to support the Himalaya above it. The Himalaya could have been weakened by an increase in pore fluid pressure, for instance owing to metamorphic dehydration reactions within the wedge. The Himalaya could have been elevated, for instance following the detachment of a tongue of mantle lithosphere from beneath it, or by the underthrusting of a lens of material beneath its base.

The apparent simultaneity of thrust and normal faulting on nearly parallel planes, of anatexis and of exhumation in the Himalaya in Miocene time can be explained if normal faulting was responsible for most of the exhumation and, through decompression melting, for anatexis. Existing data suggest that the most probable cause for Miocene normal faulting was either uplift of the Himalaya or the cessation of slip on the Main Central Thrust at the base of the orogenic wedge and the creation of a new, deeper thrust fault and a new base to the deforming Himalaya.
The emplacement of the Manaslu granite of Central Nepal: field and magnetic susceptibility constraints
Authors: Guillot S. , Pêcher A. , Rochette P. , Le Fort P.
Published: April 2008

This paper investigates the relationship between the emplacement of the Manaslu leucogranite and the regional deformation. Structural data are derived from field measurements and from the anisotropy of magnetic susceptibility of oriented samples. The geometrical relationship between the internal structure of the granite, north vergent folds affecting the Higher Himalayan Sedimentary Series and the E-W dextral shear-zone, separating the Higher Himalayan Crystalline Series and its sedimentary cover, indicate that in the sedimentary series, the collapse structures and E-W dextral shearing are two distinct structural features. Emplacement of the granite post-dates the collapse structures but is related to the E-W dextral shearing. A model for the Manaslu granite emplacement is proposed which requires development of extensional fracturing at 45° to the shear direction, followed by intrusion of granitic magma in successive pulses which results in lateral expansion of the pluton towards the NE.
Numerical modelling of the thermal history of the NW Himalayas, Kullu Valley, India
Authors: Grasemann Bernhard
Published: April 2008

This paper demonstrates two-dimensional time-dependent finite-difference modelling of Himalayan metamorphism illustrated by the Kullu-S Lahul area (Himachal Pradesh) after the collision between India and Asia and the over-thrusting of the Higher Himalayas over the Lesser Himalayas.

It is shown that a one-dimensional model with crustal thickening by a single thrust cannot satisfy the observations present in the section along the Kullu valley. A two-dimensional solution demonstrates that during thrust sheet emplacement at geologically realistic strain rates of several centimetres a year, a temperature inversion does not occur, in contrast to the inversion assumed by the one dimensional instantaneous thrust models. Therefore the variability of two other model parameters were explicitly considered. (i) There is a significant conductivity contrast between the different types of rocks. As conductivity layers function as a barrier to the transport of heat, the conductivity limit should be variable for different regions in the grid. (ii) Rocks in the Higher Himalayas record a phase of metamorphism that preceded the movements on the Main Central Thrust. This observation, together with the knowledge that rocks with different radiogenic heat production and conductivity are not uniformly distributed in the crust, emphasizes that there were considerable horizontal temperature variations across the whole region. For that reason the initial temperature profile, too, has to be adapted for a realistic model.
A study of illite crystallinity and fluid inclusions in the Kathmandu Klippe and the Main Central Thrust zone, Nepal
Authors: Morrison C. W. K. , Oliver G. J. H.
Published: April 2008

The Kathmandu Klippe is an outlier of the Tibetan crustal slab resting on the Main Central Thrust (MCT); it contains a 20 km thickness of Precambrian and Lower Palaeozoic metasediments and Ordovician granites folded into a large syncline. The regional metamorphism predates granite emplacement and, on the basis of illite crystallinity and index mineralogy, is lowest grade in the core of the syncline (epizone-chlorite zone) and highest on the north and south limbs (garnet zone). The distribution of metamorphic zones is ‘normal’ for a burial sequence. The klippe rests on the Benighat Slates of the Midland Formations of the Indian crustal slab. Fluid inclusion studies of the syn-MCT quartz veins in the Benighat Slates show H
2
O, CO
2
, CH
4
, Na
+
, K
+
, Cl′ fluids which were hotter than the country rocks. The MCT contact at Dolalghat does not show the effects of ‘reverse’ metamorphism expected from the Le Fort model of heat transfer from the hot klippe several kilometres down into the Midland Formations. A study of index minerals, illite crystallinity and illite b
0
indicates that the grade of regional metamorphism within the anticlinal Midland Formations varies between epizone-chlorite and garnet zone with the highest grades occurring in the core of the structure. Further to the N around Barahbise, the contact along the MCT between the Midland Formations and the Tibetan Slab is a 2 km thick zone of highly sheared Benighat Slates. The grade of metamorphism in the Benighat Slates is chlorite-epizone grade i.e. lower than either of the sequences above (kyanite zone) or below (garnet zone). The inference drawn from these relationships is that although the inverse metamorphic zonation in the base of the Tibetan Slab might be explained by cooling against the Indian Slab during the Tertiary Himalayan collision, metamorphism in the Indian plate was largely pre-collision.
The Main Boundary Thrust and propagation of deformation into the foreland fold-and-thrust belt in northern Pakistan near the Indus River
Authors: McDougall James W. , Hussain Ahmad , Yeats Robert S.
Published: April 2008

The Main Boundary Thrust in northwestern Pakistan is a floor thrust along which a thrust system incorporating Precambrian and Phanerozoic rocks of the Kala Chitta and Attock-Cherat Ranges was emplaced over Cenozoic strata of the northern Kohat and Potwar Plateaus. The MBT and successive thrusts toward the foreland are interpreted as low angle to flat decollement thrusts at 8–10 km depths that bound thrust sheets with large lateral dimensions. Admissible cross sections indicate over 60 km of shortening along the largely pre-Palaeocene Khairabad, Cherat, and Hissartang thrusts within the MBT allochthon. In the toe of the MBT allochthon, contraction is manifest at the surface as tight folds in largely Mesozoic and lower Tertiary strata, decoupled in the upper third of the allochthon. Displacement of the MBT is estimated at over 40 km and south of the MBT, shortening is interpreted along blind thrusts accompanied by shallow (0–3 km deep) backthrusting. The MBT allochthon was deformed prior to its low angle displacement over the northern Kohat and Potwar Plateaus, and folded and eroded during or after its emplacement. The MBT thrust front may participate in its own later deformation, but not as an emergent low angle thrust through the Quaternary.
The interaction between channel geometry, water flow, sediment transport and deposition in braided rivers
Authors: Bridge John S.
Published: April 2008

Models of braided-river deposition must be detailed, fully 3D, and preferably quantitative to be of use in understanding and predicting the nature of ancient deposits. In order to construct and validate adequate predictive models it is necessary to have information on: (1) variation and interaction of channel geometry, water flow and sediment transport in time and space in modern channel belts, as these control erosion and deposition, the formation and migration of channels and bars, and channel abandonment and filling; (2) 3D variation of bed geometry, texture, sedimentary structures and paleocurrents throughout modern channel-belt deposits, including the age and spatial arrangement of preserved parts of bars and channel fills; (3) long-term (more than hundreds of years) trends in channel and floodplain geometry, flow and sedimentary processes in order to understand channel-belt movements such as avulsions, and the spatial arrangement of channel-belt deposits relative to overbank deposits. Such information is rare because: (1) it is difficult to study modern braided-river geometry, flow and sedimentary processes throughout a range of the all-important high discharges; (2) detailed reconstructions of braided channel and bar geometry and movement are only available for the past half-century and cannot readily be linked to causative mechanisms; (3) 3D documentation of modern deposits below the water table (especially large scale features like lateral-accretion bedding) requires extensive coring and dating of the deposits, and geophysical profiling. As a result of this lack of information, and because of the quality of analysis and presentation of the information available, existing braided-river facies models are virtually useless as interpretive and predictive tools. The nature of the information available is critically reviewed. Using information from recent detailed field and laboratory studies of the geometry, flow and sedimentary processes in braided rivers of simple geometry, in single river bends, in channel confluences, and using some theoretical reasoning, it has been possible to construct fully 3D qualitative and quantitative models of braided river deposits. These models can be used to provide sophisticated quantitative interpretations of palaeochannel geometry, hydraulics and migration, as illustrated by comparison with some particularly well described examples of ancient braided river deposits.
Understanding braiding processes in gravel-bed rivers: progress and unsolved problems
Authors: Ferguson R. I.
Published: April 2008

This paper is a review, from a geomorphologist’s point of view, of recent progress in understanding the interrelationships between channel form, flow, bed texture, and bedload transport in braided gravel-bed rivers. The fundamental geomorphological units in such rivers are not bars alone but bar-pool units, usually associated with confluences and bifurcations. Braiding can develop not only by midstream deposition but by bar dissection and partial avulsion; the hydraulic conditions favouring different mechanisms deserve study. Braiding is highly dynamic, with rapid feedback between channel configuration, flow, and sediment transport. These interactions can profitably be observed in Froudescaled flume models, and can to some extent be quantified by intensive field measurements, but numerical modelling is potentially the best approach. Sediment is sorted both laterally and longitudinally within pool-bar units, but some of the possible mechanisms have not been adequately studied. The downstream fining which occurs along entire braidplains can be explained by selective transport but associated changes in bar types may not follow any single sequence.
Braiding and meandering parameters
Authors: Friend P. F. , Sinha R.
Published: April 2008

Modifications to standard definitions of braiding and meandering are proposed to indicate the morphology of every river channel reach quantitatively, whether it has single channel or multiple channels. Sinuosity (
P
) is defined as,
P
=
L
cmax
/
L
R
, where
L
cmax
is the length of the midline of the channel (in single-channel rivers), or the widest channel (in multi-channel rivers), and
L
R
is the overall length of the reach. Braiding is a measure of channel multiplicity and a new term ‘braid-channel ratio’ (
B
) has been defined as,
B
=
L
ctot
/
L
cmax
, where
L
ctot
is the total of the mid-channel lengths of all the channels in a reach. Another expression for the braid-channel ratio which gives the same numerical result is,
B
=
P
ctot
/
P
, where the total sinuosity,
P
ctot
=
L
ctot
/
L
R
, and
P
has been defined above. Scatter plots on braid-channel ratio/sinuosity axes show a negative correlation between these parameters, as would be expected from the above relationships. Single-channel rivers (
B
= 1) have relatively higher sinuosities and the upper limit of these is the point at which cut-off becomes highly probable. For multi-channel rivers (
B
> 1), sinuosity remains low, reflecting the limiting effect of braid bars on the development of fully developed spiral secondary flow. Data analyses showing that increases in channel slope and bankfull discharge are associated with changes from meandering to braided morphology are diverting attention from the importance of the increased availability of bed-load-grade sediment as a control.
Braided stream and flood-plain deposition in a rapidly aggrading basin: the Escanilla formation, Spanish Pyrenees
Authors: Bentham Peter A. , Talling Peter J. , Burbank Douglas W.
Published: April 2008

Models of braided stream deposition have largely been developed from studies of regionally degrading and laterally confined alluvial environments. Glacial outwash streams, in particular, have supplied important and widely cited descriptions of intra-channel processes. These fluvial systems are typically confined within quite narrow valleys. It is felt that such systems have low long-term preservation potential and are unlikely to be present in the geologic record in large quantities. Therefore, the study of these modern laterally confined degradational systems may not provide holistic analogs of the larger-scale alluvial architecture developed in braided river environments in the ancient. The Escanilla Formation of the Spanish Pyrenees provides a well-exposed example of an Eocene fluvial system flowing axially within the Pyrenean foreland basin. Sedimentologic study shows coarse channelized deposits of braided character wholly enclosed within large amounts of fine-grained overbank mudstones and siltstones (>40% by volume), with both being deposited coevally across the Escanilla floodplain.

A new depositional model is proposed that combines facets of existing models derived from other fluvio-morphologic systems. This consists of a laterally confined channel belt, internally preserving a braided stream character, capable of rapid vertical aggradation on short geological time-scales (about a thousand years). Avulsion processes are used to explain finer sediment deposition in interfluve settings, as well as the large-scale architectural geometries within the lower Escanilla Formation. This new model illustrates that discrete channel belt avulsion, and the preservation of thick sequences of overbank material are not exclusively characteristics of higher sinuosity fluvial systems.
Variability of late Holocene braiding in Britain
Authors: Passmore David G. , Macklin Mark G. , Brewer Paul A. , Lewin John , Rumsby Barbara T. , Newson Malcolm D.
Published: April 2008

Braided channel patterns are relatively rarely found in contemporary British rivers, although site studies have been undertaken for reaches in catchments ranging in size from 2.6 km
2
to 2850 km
2
in area. Systematic study of two major British rivers, the Tyne (2927 km
2
) and the Upper Severn (1000 km
2
), reveal braiding at several sites, albeit at a limited scale. An examination of historical maps and air photographs shows, however, that braiding processes have been more widely active in the eighteenth and nineteenth centuries. Episodic braiding activity is discussed for selected sites. Floodplain stratigraphy shows significant channel aggradation and transformation to braided river planforms also occurred in the late Roman period and in the thirteenth and fourteenth centuries. Possible explanations are suggested in terms of flood frequency and magnitude variations (reflecting climatic fluctuations and land-use change), alterations in sediment supply (including the impact of historic mining activities), and channelization control.
A meso-scale field investigation of channel change and floodplain characteristics in an upland braided gravel-bed river, New Zealand
Authors: Warburton J. , Davies T. R. H. , Mandl M. G.
Published: April 2008

Detailed data from active gravel-bed braided rivers over time periods greater than a decade is relatively sparse. The aim of this study is to establish a field site for examining channel change over an extended period (> 10 years) and determining the main mechanisms which produce the braidplain morphology. The prototype is the Ashley River, Lees Valley, New Zealand, which is 2.5 km in length, has an average braidplain width of 400 m and slope of 0.012 m m
−1
. The flow regime is dominated by low flows (average flow 3.15 m
3
s
−1
) with 2–3 winter flood events in excess of 25 m
3
s
−1
per year. Planform information on this reach dates back to the 1860s with ten sets of maps/air photographs up to the present. Since 1989 the sedimentary and morphological characteristics of the braidplain have been carefully surveyed. There is a downstream decrease in sediment size which is accompanied by a reduction in bank height at the margin of the floodplain and a decrease in cross-section bed amplitude. Certain elements in the floodplain (vegetated bars, channels with coarse lag sediments and banks with armoured bank toes) persist while other features such as unconsolidated gravel banks and fine-gravel sheets do not. Channels are migrating towards the northern edge of the braidplain with old bars becoming vegetated on the southern margin. Channel patterns change most dramatically when the flood flows reoccupy old channel segments. Observations of channel change from flood to flood indicate that, although some channel segments switch dramatically, other segments are left intact and largely unaltered. Movement of small sediment lobes affected the flow at node-points within the channel (e.g. areas of flow divergence and convergence) causes avulsion. The dominant mechanisms of channel change are avulsion and bank notching/scalloping.