bluuebunny/crossref_metadata_2025_split huggingface dataset:
Contrasting styles of fluvial-aeolian interaction at a downwind erg margin: Jurassic Kayenta-Navajo transition, northeastern Arizona, USA
Authors: Herries Robert D. 
Published: April 2008

Abstract
Fluvial-aeolian interactions result from both short-term interplay between an ergmargin and adjacent fluvial system, and much longer-term expansion and contraction of the erg in response to climatic, tectonic or eustatic effects.
Near Moenkopi, in northeastern Arizona, a zone of intertonguing characterizes the transition between the fluvial Kayenta Formation and the overlying aeolian Navajo Sandstone. Three fluvial to aeolian drying-up cycles form the largest scale interaction and represent the repeated advance of the Navajo erg across the Kayenta alluvial plain, probably in response to an increasingly arid climate. During the aeolian phase of each cycle, small-scale interactions resulted from fore-erg deposition on the alluvial plain, and fluviallyfed interdunal flooding, which both occurred during periods of erg activity.
The top of each cycle marked a return to wetter climatic conditions which terminated erg activity, forming a stabilization surface. As a consequence, prograding fluvial systems were able to transgress the denuded surface, covering the area formerly occupied by the erg to form the base of the next drying-up cycle.

From a hydrocarbon reservoir viewpoint, the two types of interaction have contrasting geometries. Fluvial deposits of the large-scale cycles have regionally extensive sheet-like geometries and are likely to form widespread barriers to vertical flow. Small-scale interdunal flooding is less significant and forms ribbon-like baffles to flow which extend roughly parallel to the strike of the aeolian cross-strata. The key to differentiating each type of interaction lies in determining whether the fluvial syste coexisted with the
active
erg, or prograded as a result of erg
stabilization
.
Palaeomagnetic study and magnetostratigraphy of the Triassic Skagerrak Formation, Crawford Field, UK North Sea
Authors: Rey D. , Turner P. , Yaliz A. 
Published: April 2008

Abstract
A detailed palaeomagnetic study has been made of the Skagerrak Formation (Triassic) in two wells from the Crawford Field in the northern North Sea. In 9/28a-9 (unorientated) a well-defined magnetostratigraphy is present comprising up to four reversed and three normal zones. There are secondary magnetizations (Tertiary 8 Recent in age) particularly in areas which are oil stained. In 9/28a-13 (orientated), three normal and four reverse magnetozones can be isolated. Despite the existence of orientation data it is more difficult to establish a reliable Triassic magnetostratigraphy. This is partly due to the fact that the sandier sequence in this location is impregnated with oil and associated diagenetic effects have resulted in pervasive magnetic overprints which are difficult to separate from the original Triassic magnetization. The effects of oil emplacement included widespread dissolution and the precipitation of a suite of secondary iron minerals including pyrite and possible pyrrhotite or magnetite. These later phases carry a secondary component of magnetization which overprints the original Triassic characteristic remanence. Directions of secondary magnetization are consistent with oil emplacement in Tertiary times.
In attempting to correlate the two wells a number of factors have to be taken into account, in particular variations in sedimentation rate and the lack of precision in the published Triassic polarity time scale. The results of this study strongly suggest that the cored sequences in the two wells are not time equivalent. However, the sequence in 9/28a-9 compares most closely with the Upper Triassic polarity time scale.
The Sapat mafic-ultramafic complex, Kohistan arc, North Pakistan
Authors: Jan M. Qasim , Khan M. Asif , Qazi M. Sufyan 
Published: April 2008

Abstract

The Sapat layered complex, occupying the tectonic base of the Cretaceous Kohistan island arc, occurs in the immediate hanging wall of the Indus suture zone north of the Kaghan valley. Previously undescribed, much of the complex consists of gabbroic rocks, but in the basal part there are ultramafic cumulates (15 × < 2 km) in direct tectonic contact with the Indian plate. In the central part, the complex can be divided into five zones. From the base these are: (1) homogeneous serpentinized dunites (350 m thick), (2) dunites with thin chromite layers (100 m), (3) thinly layered dunites-pyroxenites (30 m), (4) layered dunites-pyroxenites-gabbros (170 m) with chromite layers in the lower part, and (5) gabbros. The gabbroic rocks are amphibolitized and consist of a variety of lithologies ranging from troctolites to anorthosites, with local pyroxenite (± olivine) layers and horizons of olivine-rich ultramafics. These rocks are locally cut by pyroxene-rich pegmatites containing up to 45 cm long pyroxene crystals. Preliminary microprobe studies on the ultramafic rocks show that the clinopyroxene is Mg-Ca-rich diopside, and olivine has a forsterite content of 92 to 78, but in rare cases > 95. The chromite displays high Cr No. (75–67) and low Fe
3+
/R
3+
(<10). These data, especially the chromite analyses, are akin to those of ultramafic rocks of island arcs, including the Jijal complex, 80 km to the west. We propose that the Sapat complex formed the basal part of the Kohistan terrain and is related to arc magmatism. It has been metamorphosed under epidote amphibolite facies conditions.
Compression- and transpression-related deformation in the Kohat Plateau, NW Pakistan
Authors: Pivnik David A. , Sercombe William J. 
Published: April 2008

Abstract
Detailed surface mapping and recently acquired subsurface data from the Kohat Plateau of northwestern Pakistan show complex structural styles created by the overprinting by transpression-related structures of pre-existing compression-related structures. The Kohat Plateau is roughly divided into two structurally distinct regions; a northwest region (NW) and a southeast region (SE). Surface structures in the Plateau include doublyoverturned folds (both limbs overturned) which incorporate a lower Eocene to Pleistocene stratigraphic succession. Intensely folded with the stratigraphy in the NW region is a relict thrust belt, representing an earlier compressional episode of deformation. In the SE region many folds (related to wrench faulting at depth) have had either their southern or northern limbs faulted out by high-angle reverse faults which flatten near the surface along a Lower Eocene shale and evaporite horizon. In the NW region, lower Eocene evaporites and limestones are replaced by thick shales which served as a fault-flat forming horizon. This is believed to have hindered the surfaceward propagation of faults, leaving detachment folds exposed at the surface. Steeply dipping strata at depths of over 4 km, steeply dipping faults that become shallow near the surface forming positive flower structures, large vertical displacements across faults, and left-stepping folds suggest transpressional rather than purely compressional deformation. Regions adjacent to the Kohat Plateau also contain transpression-related structures, rather than thin-skinned imbricate thrusts. In view of the position of the Kohat Plateau near a major syntaxial bend in the Himalayan orogen, both north-south and east-west convergence played a dominant role in creating structural styles, and the combination of these two convergence directions influenced the development during the Plio-Pleistocene of transpression-related structures.
Evolution of the Himalaya since Miocene time: isotopic and sedimentological evidence from the Bengal Fan
Authors: France-Lanord Christian , Derry Louis , Michard Annie 
Published: April 2008

AbstractWe report Sr, Nd, O, and H isotopic data and clay mineral abundances for turbidite sediments recovered in ODP Leg 116 cores from the Bengal Fan at 1°S. The samples studied cover the period betweenc.17 Ma and the present. We also present new and compiled data on the isotopic compositions of potential source regions for the Bengal Fan sediments. ɛNd(0) values in the Bengal Fan sediments (all samples) define a narrow range about − 16.0.87Sr/86Sr values (all samples) are also in a narrow range near 0.741. δ18O values in quartz separates define a narrow range at +12.8±0.5‰. Coarse biotite-chlorite separates give δ18O = 3.6−5.6‰. Combined δ18O values of quartz and biotites indicate a metamorphic source. Clay mineral abundances define two clay facies: an illite-chlorite-rich assemblage (IC) and a smectite-kaolinite-rich assemblage (SK). δ18O in the IC clay fractions is 11.5–15‰, while SK clays are 18.2–22.6‰. The narrow range of isotopic values throughout the deposition history implies that the source of the Bengal Fan sediments has not changed since the early Miocene, despite changes in sedimentation rate, sedimentary facies, tectonic history and climactic regime. The difference between δ18O in the IC and SK clay fractions represents different alteration histories of the same source material. The SK clays appear to have been altered at low T in the Indo-Gangetic Plain, while the IC clays and coarse fractions preserve metamorphic signatures. The narrow range of the Sr values, despite wide variation in Rb/Sr ratio, also argues for a source that underwent isotopic homogenization shortly before erosion and deposition of the sediment. The source that meets these criteria is the High Himalayan Crystalline series (HHC) or a close analogue, although subordinate contributions (probably <20%) from the Lesser Himalaya (LH) and Tibetan Sedimentary Series (TSS) are possible.A model in which the HHC are exposed to erosion since the early Miocene on the south flank of the orogen by thrusting along the MCT, while the TSS is simultaneously removed by northward-directed normal faulting satisfies the constraints above. The results of this study require that the Himalaya have been a significant topographic feature since at least the early Miocene. Independent evidence supports this contention. Variations in the sedimentation style in the Bengal Fan since that time appear to represent a combination of factors, including tectonic activity and the coupled effects of climate and sea-level changes.
Downstream variation in bed material characteristics: a turbulence-controlled form-process feedback mechanism
Authors: Clifford N. J. , Hardisty J. , French J. R. , Hart S. 
Published: April 2008

Abstract
Downstream changes in bed material characteristics represent an element of the sediment transport process which is of immediate geomorphological and economic significance. Particle size trends represent the joint effect of abrasion and sorting, often complicated by in situ weathering and variable sediment supply. In principle, the combined effects of abrasion and shorting can be explained if attention is paid to the role and characteristics of near-bed turbulence, via the mechanisms of ‘abrasion in place’ and the size-dependent particle acceptance or rejection involved in Bluck’s ‘turbulence template’. Field data describing spatially- and stage-dependent differences in turbulence characteristics by which the significance of such processes can be quantified are, however, lacking.
In this paper, high-frequency three-dimensional velocity data obtained with an ultrasonic current meter in a braided section of Langden Brook, Lancashire, are used to demonstrate significant variations in turbulence parameters at the bar-pool scale. These are related to grain abrasion, sorting and the arrangement of grains in surface microtopographic bedforms using a force-balance model which incorporates turbulent velocity components in two dimensions. Following Bluck, it is suggested that bar forms act as, and result from, sediment trapping phenomena, whose fundamental controls lie in the characteristics of turbulence at the bar surface. Variations in turbulence provide a physical rationale for differing values of sorting/abrasion coefficients used in descriptive models of longitudinal size contrasts. In contrast to single-thread channels, however, spatially- and stage-dependent variation in turbulence characteristics is more marked in braided environments. Consequently, greater research effort is required to fully incorporate this into a spatially-realized form-process feedback mechanism applicable at the reach-scale and beyond.
Planform pattern and channel evolution of the Brahmaputra River, Bangladesh
Authors: Thorne Colin R. , Russell Andrew P. G. , Alam Muhammad K. 
Published: April 2008

Abstract
The Brahmaputra is one of the world’s greatest rivers, ranking fifth in terms of discharge and eleventh in terms of drainage area. It also has a very high sediment discharge, ranking third in the world. The river is braided with meta-stable islands and nodal reaches, mobile sand bars, shifting anabranches and severe bank erosion. The dominant discharge is about 38 000 cumecs, which is a high in-bank flow. Islands have top elevations that are adjusted to bankfull discharge, with a spacing scaled on the width of the primary channel. Bar top elevations are adjusted to the dominant discharge and bars are scaled on the width of the major anabranches. The dominant bedforms are large dunes scaled on the third order sub-channels of the major anabranches. Sustained right bank erosion has occurred during the last 35 years, with average rates of about 90 m per year. The left bank shows both erosion and accretion that together produce relatively low rates of net movement. Erosion is faster in island reaches and slower in nodal reaches. Recognition of the geomorphic controls of bank erosion allows its spatial distribution to be related to the development of the braided pattern of the river and a tentative hypothesis is proposed to explain recent trends in both bankline movement and the evolution of the channel planform pattern.
Provenance of braided alluvial deposits of the Thari Formation, Rhodes, SE Aegean: evidence for major erosion of an ophiolite-bearing thrust sheet
Authors: Edgington Lisa , Harbury Neil 
Published: April 2008

Abstract
The Middle to Upper Oligocene Thari Formation sediments in central and southern Rhodes, Greece were deposited adjacent to an active thrust front. They consist of clast-supported massive to horizontally stratified conglomerates, massive matrix-supported conglomerates interbedded with ophiolitic sandstone, and silty mudstone forming lenticular, discontinuous beds with common erosional contacts. Debris flow, stream channel, sheet flood deposits and the high gravel to sand ratios (90: 10), suggest deposition by processes within a gravel-dominated braided alluvial system. Facies relationships demonstrate a clear proximal (NW) to distal (SE) variation across an alluvial fan. Provenance relationships based on 2400 clast counts show that the main constituents of the conglomerates in the north west area are rounded to well rounded pebbles and cobbles derived from an ophiolite sequence (80%) and include harzburgite, dunite, gabbro, serpentinite, and dolerite whilst the other major components, pelagic limestone and chert (20%), have an increasing importance to the south west of the area. These relationships suggest that the Thari Formation formed part of a flexural foreland basin-fill that developed adjacent to a deformed ophiolite stack eroded after early Paleocene to mid-Oligocene convergent tectonics.
Tectonic significance of the Hellenic-Dinaric ophiolites
Authors: Smith Alan G. 
Published: April 2008

Abstract
Isotopic and biostratigraphic data show the Hellenic ophiolites (those SE of the Scutari-Pec transform zone in Albania) are of Middle to Late Jurassic age. They are of two contrasting types. The first type of ophiolite is complete, usually harzburgitic, forms allochthonous sheets, commonly develops metamorphic soles, generally includes significant chrome deposits and shows supra-subduction zone (SSZ) trace-element characteristics with an evolution from MORB to IAT and occasionally boninites. The second type is characterized by incomplete ophiolites which lack chrome and mantle tectonite rocks. In contrast to the first type, these are slightly younger, are exposed in situ, show contact metamorphism against the country rock, are similar to MORB and show little influence of a subduction zone.
The harzburgitic ophiolites from a western and an eastern outcrop belt. The relationship between the two is unclear, principally because of large, probably strike-slip, offsets in the Vardar zone, best documented in former Yugoslavia.
The western Hellenic ophiolites (Pindos, Vourinos, Othris, Evvia ( = Euboea) and Argolis) all formed above a SW-dipping subduction zone in early Middle Jurassic time. They are continuous with a similar belt in the Dinarides of Yugoslavia whose SW margin gradually becomes lherzolitic as it is traced northwards. It is totally lherzolitic at its termination in NW Yugoslavia. This gradual change in ultramafic character is speculatively linked to the changes in spreading rates in a wedge-shaped marginal basin.
Emplacement took place in later Middle to Late Jurassic time probably along a low-angle detachment zone that was close to the lithosphere/asthenosphere boundary shortly after spreading ceased. The SW half of the basin was pushed over the NE half, which was subducted to the SW but has left very little trace of its former existence. In the Hellenides the ophiolites were emplaced onto the Pelagonian continent and in the Dinarides onto the Drina-Ivanjica continent.
Precise analogues of the complete ophiolites appear to be absent in the present-day major ocean basins — because of their SSZ character — or any island arcs that border the present-day oceans — because their marginal stratigraphy shows no trace of arc sediments or volcanics.
The incomplete ophiolites, which appear to be restricted to the Axios zone in Greece, seem to be the expression of transient motion on short extensional segments of a plate margin whose geometry resembled the early stages of the opening of the Gulf of California.
Tectonics of the Mozambique Belt in East Africa
Authors: Shackleton Robert M. 
Published: April 2008

Abstract
The Late Proterozoic Mozambique Belt in East Africa is reviewed, using new geochemical and other data. This review leads to the following tectonic interpretation.
A first collision, between 750 and 800 Ma ago, after the obduction of the Baragoi and associated ophiolites of north-central Kenya, produced intense regional recumbent fabrics and structures, a trans-Mozambique Belt stretching lineation, crustal thickening and metamorphism reaching granulite-facies in the lower crust.
Uplift and erosion were followed by closure of a second oceanic area, obduction of the West Pokot ophiolite and collision along the West Pokot suture, possibly about 580 Ma ago. This second collision produced strong within-plate deformation (Baragoian-Barsaloian) especially in N-S dextral shear zones over 100 km east of the West Pokot suture. The shear zones are associated with late Barsaloian within-plate granites.
It is inferred that eastward subduction led to the successive accretion of plates to an eastern foreland, perhaps represented by Malagasy, culminating in the collision of east and west Gondwana.
The isotope and trace element geochemistry of basalts from the volcanic islands of the southern Red Sea
Authors: Rogers N. W. 
Published: April 2008

Abstract
Alkali basalts from the Hanish-Zukur and Zubair island groups in the southern Red Sea have been analysed for major, trace elements and Sr, Nd and Pb isotopes. They have close to primary compositions with high Mg# (60–70) and Ni and Cr concentrations up to 298 ppm and 597 ppm respectively. Many have only experienced olivine fractionation and this can be readily corrected. Compositional variations in the corrected analyses are interpreted to result from variations in the degree of melting, and a simple inversion of the corrected incompatible element abundances reveals a source composition intermediate between that of N-MORB and primitive mantle. This is suggested to result from the interaction of depleted asthenospheric mantle with less depleted material derived from the nearby Afar plume.

The overall geochemical characteristics of the basalts are comparable with ocean island basalts erupted through thin (young) lithosphere. SiO
2
varies from 44.0–47.9%, Nb/Y from 0.8–1.4 and Ce/Y averages 1.64, close to values from Ascension Island.
143
Nd/
144
Nd ratios show minimal variation between 0.51301–0.51309 whereas
87
Sr/
86
Sr ranges between 0.70323–0.70396, and the analyses define a horizontal trend extending to the right of the mantle array. Pb isotopes also show marked variation,
206
Pb/
204
Pb values varying from 18.675–19.077, but the analyses remain within the ranges defined by MORB and are generally comparable with samples from the Red Sea axial trough and the Gulf of Aden.

Comparison with basalts from Afar and the Main Ethiopian Rift (MER), together with those from other parts of the Red Sea and Gulf of Aden reveal systematic shifts in isotope ratios with degree of extension. Along the constructive plate margins and including the island basalts, isotope results are dominantly asthenospheric, showing variations from N-MORB towards a possible plume end-member. In Afar, where β factors are lower (<2.5), isotope ratios show a greater deviation from oceanic basalts and this is most marked in the MER basalts where β factors are lowest (< 1.2).
These data are tentatively related to a model of passive extension over a mantle plume, driven by plate motions, with the possibility of active, plume-driven extension being restricted to the early stages of Ethiopian flood basalt volcanism and the subsequent development of the MER during the Miocene.
Advances in analytical technology and its influence on the development of modern inorganic geochemistry: a historical perspective
Authors: Potts Philip J. , Hawkesworth Chris J. , van Calsteren Peter , Wright Ian P. 
Published: April 2008

Abstract
Since the 1960s, a range of new instrumental analytical techniques have become widely available in geochemical laboratories, replacing traditional wet chemical procedures that were in common use throughout the first half of the twentieth century. These new techniques have resulted not only in an enormous advance in analytical productivity, but also in the sensitivity with which a wide range of trace elements and isotope ratios may now be determined on a routine basis. It is argued that there is a symbiosis between analytical technology and geochemical application which has resulted in geochemistry currently being regarded as one of the dominant strands of modern geological sciences. The present paper reviews the development of this analytical revolution, evaluates the impact that successive techniques have had on geochemical thinking and looks to the future to suggest areas that may result in advances over the next decade.