bluuebunny/crossref_metadata_2025_split huggingface dataset:
U-Pb zircon ages from the Bindal Batholith, and the tectonic history of the Helgeland Nappe Complex, Scandinavian Caledonides
Authors: NORDGULEN Ø. , BICKFORD M. E. , NISSEN A. L. , WORTMAN G. L. 
Published: December 2007

The Helgeland Nappe Complex forms part of the Uppermost Allochthon in the central Scandinavian Caledonides. It consists of metasupracrustal rocks of assumed Precambrian age which are juxtaposed with metasupracrustal rocks present as non-conformable cover sequences to subjacent fragments of Early Ordovician ophiolitic rocks. All of these rock units have been intruded by the Bindal Batholith, which ranges in composition from mafic gabbro to leucogranite. In this paper, U-Pb zircon ages from five plutons in the southern part of the batholith are presented. These are a porphyritic granite of the Andalshatten pluton (447 ± 7 Ma), a tonalite from the Kråkfjellet pluton (443 ± 7 Ma), a granodiorite west of Gåsvassfjellet (437 ± 4 Ma), the Kalvvatnet monzonite (435 ± 10 Ma), and a monzodiorite near Tosbotn (430 ± 7 Ma). The dates are essentially concordant and are considered to reflect the crystallization ages of the intrusions.
Based on regional correlations and the age of the oldest of the dated plutons, it is concluded that internal accretion and polyphase tectonometamorphism of the Helgeland Nappe Complex took place in Ordovician times. The final Silurian (Scandian) thrusting of the nappe stack across the continent of Baltica appears to post-date emplacement of the Bindal Batholith.
Palaeosols as indicators of geomorphic stability in two Old Red Sandstone alluvial suites, South Wales
Authors: MARRIOTT S. B. , WRIGHT V. P. 
Published: December 2007

Palaeosols (fossil soils) are abundant in the alluvial late Silurian–early Devonian Moor Cliffs Formation and Rat Island Mudstone (Freshwater West Formation) of south Dyfed, South Wales. The palaeosols occur in thick mudstones deposited on floodplains under a seasonal, semi-arid climate, and represent Vertisol-type soils. Simple, single profiles are rare and most of the mudstone intervals exhibit complex overprinted profiles. However, a major difference exists between the palaeo-Vertisols in each of the two units studied. The Moor Cliffs Formation contains thick floodplain intervals with evidence of frequent erosion and reactivation of earlier soil horizons. The associated coarser channel deposits mainly represent ephemeral streams. The alluvial system was unstable, flashy and prone to extensive stripping of stored floodplain sediment. Arroyos (unstable ephemeral stream channels) probably developed during this interval. Evidence for any marine influence on soil development is missing and previous speculations that the formation may represent deposition in an upper-intertidal or supratidal system are not supported.
The Rat Island Mudstone represents a complete contrast in that there was little or no stripping of floodplain sediments. The channel deposits are typically of the fining-upwards type representing more stable, low sinuosity channels and with ephemeral-stream floods.
By integrating the palaeosols, a more detailed picture of the geomorphic history of these units can be formulated. Comparison with contemporary drainage basins indicates that a change in climate was a possible cause for the differences between the two units, less frequent and more severe storms occurring during the deposition of the Rat Island Mudstone.
Petrographic analyses of organo-mineral relationships: depositional conditions of the Oxford Clay Formation (Jurassic), UK
Authors: BELIN S. , KENIG F. 
Published: December 2007

Five samples representing the range of variability of palynofacies and organic geochemical parameters of the organic-rich facies of the Peterborough Member (formerly Lower Oxford Clay) and one sample from the Stewartby Member (Middle Oxford Clay) were studied using optical microscopy (transmitted light and fluorescence) and scanning electron microscopy (backscattered electron mode).
The organic matter assemblage of the Oxford Clay Formation is dominated by amorphous organic matter of marine origin (75 to 95%). Whatever their organic content, the sediments display a highly microbioturbated texture, with edge-to-face disposition of clay minerals and numerous microburrows, signifying that the sea floor was never completely anoxic. The presence of abundant organic matter in the sediment has enhanced diagenetic transformations, and in particular is related to the formation of pyrite, calcite and authigenic quartz. The nature of diagenetic changes, particularly those involving silica species, provides information on the interstitial environment. The sediment displays, at the micrometric scale, numerous microenvironments with varying reducing conditions, resulting in different diagenetic products. Pyrite displays two habits, framboidal and euhedral. Euhedral pyrite, which characterizes more anoxic interstitial environments than framboids, is found in the most organic-rich sediments. The abundance of large coccolith-rich faecal pellets in the most organic-rich sediments is correlated with chemical evidence of intense trophic activity.
A lithofacies study of the Peterborough Member, Oxford Clay Formation (Jurassic), UK: an example of sediment bypass in a mudstone succession
Authors: MACQUAKER J. H. S. 
Published: December 2007

Thirteen samples from the Peterborough Member (Oxford Clay Formation, Middle Jurassic), from Quest Pit, Stewartby, central England, were examined in detail using a combination of optical microscopy and backscattered electron imagery. The majority of the samples were clay-rich mudstones, two were silt-rich mudstones and one was a muddy sandstone. The dominant components of these samples are fine-grained clays (illite, mixed layer illite/smectite, kaolinite) and amorphous organic matter; minor components include authigenic pyrite (framboidal and euhedral), silt-sized quartz grains, calcareous nannoplankton (mainly disarticulated coccolith plates) and a variety of early carbonate cements. Some of the mudstones were found also to contain abundant foraminifera. Temporal trends between samples in grain size, foraminiferal abundance and total organic carbon contents were observed.
These mudstones were deposited in a shallow epeiric sea, where productivity was high and the bottom waters were oxic. Relative sea-level change, coupled to a lack of available accommodation space within the basin at low sea-level stands is invoked to explain the observed mudstone stacking patterns.
A late Caledonian melange in Ireland: implications for tectonic models
Authors: WILLIAMS D. M. , HARKIN J. , ARMSTRONG H. A. , HIGGS K. T. 
Published: December 2007

The Clew Bay area in western Ireland contains the remnants of a Caledonian terrane which separates the Dalradian of North Mayo from the Ordovician of South Mayo to the south. Rocks of the area have been variously interpreted as representing part of the Dalradian succession, as a Cambrian to Ordovician rifted margin to subduction related basin, and as a shear carpet derived from an accretionary wedge overridden by an ophiolite in the early Ordovician. The lithologies present include discontinuous outcrops of ultrabasic and basic rocks, together with quartzose and semipelitic schists on the south shore of Clew Bay. These are in contact to the north with a sedimentary and volcanic sequence exposed both on the mainland and on Clare Island for which a detailed stratigraphy had previously been established. We reinterpret this latter sequence as a melange containing blocks up to 500 m long of sandstone, conglomerate, chert and volcanics.
Although microfossil evidence had previously shown that a chert block on Clare Island was of Late Llanvirn age, new fossil control shows that this date must be extended upwards. Microfossils extracted from a variety of lithologies within the melange show that the cherts range from Middle Ordovician to at least Caradoc. Further, spores extracted from the melange matrix show that the formation of the melange took place in the Silurian, probably in the Wenlock or later. These data indicate that the melange is unrelated to any early Ordovician obduction event and in fact represents the effects of a significant tectonic episode in the late Silurian of the British and Irish Caledonides.
Vitrinite reflectivity and the structure and burial history of the Old Red Sandstone of the Midland Valley of Scotland
Authors: MARSHALL J.E.A. , HAUGHTON P.D.W. , HILLIER S. J. 
Published: December 2007

Vitrinite reflectivities from Lower Old Red Sandstone (ORS) sediments of the Strathmore region in the Midland Valley of Scotland give values of 1.2% for the Stonehaven Group (Kincardineshire), 0.9% for the Arbuthnott Group (Angus) and 1.3% for the Strathmore Group (Perthshire). These values are corroborated by clay mineral diagenesis studies. The Stonehaven Group reflectivity values are considerably lower than expected under the 9 km thickness of sediments aggregated for the Lower Old Red Sandstone of Kincardineshire and demonstrate a maximum burial of some 3 to 5 km. These results together with detailed vitrinite reflectivity measurements in the Dundee Formation, consideration of post-Devonian burial history and kinetic burial modelling, show that the present level of Lower ORS thermal maturation results from Late Carboniferous maximum burial. This is confirmed by comparison with a vitrinite reflectivity gradient from the Strathclyde Group of Fife.
To reconcile the vitrinite reflectivity data with the stratigraphy and structure of the Lower Old Red Sandstone, burial models invoking structural repetition, depocentre migration and structurally assembled stratigraphy are considered. It is concluded that the Lower Old Red Sandstone is a structurally assembled series of basins with the Cowie Formation of the Stonehaven Group being part of a separate depositional succession and structurally juxtaposed into its present position as part of an apparently contiguous sequence.
Structural evolution of the Gander-Avalon terrane boundary: a reactivated transpression zone in the NE Newfoundland Appalachians
Authors: HOLDSWORTH R. E. 
Published: December 2007

In the Appalachian Orogen of NE Newfoundland, the Gander and Avalon zones are separated by the 1–2 km wide, dextral, Dover Fault Shear Zone. This structure overprints sinistral transpressive deformations that affect broad regions either side of the Gander-Avalon boundary and is post-dated by ?Alleghenian (or younger) brittle dextral faults that are not thought to record large regional displacements. In ?Lower Palaeozoic sedimentary rocks of the Gander Lake Subzone, a steeply dipping sinistral shear zone (up to 20 km wide) was the focus of syn-tectonic sillimanite grade metamorphism, migmatization and granitic plutonism. In lower greenschist facies late Precambrian sedimentary and volcanic rocks of the Avalon Zone, early sinistral displacements were focused into a high strain zone (
c
. 2km wide) along the boundary. Although developed at different metamorphic grades, these early sinistral transpressive shear zones can be correlated and are thought to record the main phase of sinistral-oblique accretion and regional orogenesis during the mid-Silurian (
c.
415-435 Ma). The switch to dextral displacements forming the Dover Fault Shear Zone appears to have initiated during later stages of syntectonic plutonism in the Gander Lake Subzone, suggesting that it may also be Silurian in age.

No evidence links the Gander and Avalon zones prior to sinistral transpression. Sinistral fabrics overprint flat-lying structures in the Gander Lake Subzone that are thought to be related to mid-Ordovician accretion of the superjacent Exploits (Dunnage) arc-ophiolite terrane to the west. The Gander and Dunnage allochthons were probably accreted to the Laurentian margin prior to the sinistral-oblique collision of Western Avalonia during the Silurian and they overlie the suture that separates Laurentia from the Central Lower Crustal Block of Avalonian affinities. The Gander-Avalon boundary in NE Newfoundland would then form the steeply-dipping surface expression of a collision-linked sinistral shear zone that allowed lateral decoupling of the Avalon and Central Lower Crustal Blocks during or following oblique collision.
Along the margin of the Gander Lake Subzone, the shear zone-hosted belt of sheeted granites and melting increasingly became the focus of deformation during the later stages of sinistral transpression. This suggests significant rheological softening of the crust that would account for the ability of transpressive terrane boundaries to accommodate large displacements. Furthermore, the estab-lishment of a thermal anomaly would have a long-lived weakening effect that could explain why such terrane boundaries are prone to reactivation.
Fractography in geology
Authors: AMEEN M. S. 
Published: December 2007

The first thematic meeting on fractography in geology, held on 13 September at Burlington House, London, was jointly sponsored by the Petroleum Group and the Tectonic Studies Group of the Geological Society. It was attended by 50 participants from Australia, Canada, France, Germany, Holland, Norway, Sweden, United Kingdom and the United States of America, a gathering which included a wide spectrum of specialists from the oil, environmental, engineering and mining industries and from universities and colleges. Three posters and 14 papers were presented, including two keynote addresses.
A one-day field trip led by Ameen, the convenor, preceded the conference during the course of which the fracture characterization of the Upper Chalk in the Thanet monocline, Kent, was discussed.

The purpose of the conference was to discuss those aspects and applications of fractography which have evolved into an invaluable discipline within the vast field of fracture characterization of rocks. Fractography can be defined as the science which deals with the diagnosis, description, analysis and interpretation of fracture surface morphology or topography, and links them to the causes and mechanisms of the fractures. The term fractography was first used by the metallurgist Car1 A. Zapffe (Zapffe & Clogg 1944), and introduced to geology by Byron Kulander in the 1970s (

Kulander
et al.
1979

).

An introduction by the convenor outlined several factors which prompted the meeting.

The discipline of fractography in geology is new, even though
Woodworth (1896)
was the first to describe joint surface morphology from field observations. It is hardly mentioned
Geochemistry of metamorphosed mafic rocks from Saih Hatat: pre-obduction history of NE Oman
Authors: EL-SHAZLY A. K. , WORTHING M. A. , JAYAWARDANE J. , VARNE R. 
Published: December 2007

Field and petrological data, combined with major, trace and rare earth element analysis of mafic rocks that structurally underlie the Semail ophiolite in Saih Hatat, have led to their subdivision into five groups: (a) crossite-bearing mafic schists that represent metamorphosed within-plate alkali basalts, erupted in a Proterozoic continental rift or ocean basin setting; (b) greenschists, blueschists, garnet-blueschists, clinopyroxene-rich layers and eclogites, representing metamorphosed Permian basaltic flows and tuffs with enriched-MORB affinities; (c) crossite-bearing metavolcanics, representing alkali basalts, trachyandesites and dacites erupted on oceanic islands in a small ocean basin or through rifted continental crust during the late Permian; (d) lawsonite- and pumpellyite-bearing metavolcanic blocks incorporated in a Cretaceous tectonic melange, and originally erupted as alkali basalts on oceanic islands in the Hawasina basin; and (e) unmetamorphosed tholeiites formed at a spreading centre in the Hawasina basin and preserved in the Hawasina thrust sheets.
A reconstruction of the tectonic history of Saih Hatat based on these data suggests that continental rifting took place in the Permian, developing two distinct basins separated by rifted continental material. Some of the basaltic blocks that formed in the outboard Hawasina basin were tectonically dislocated and incorporated into a tectonic melange formed in a foredeep during the early stages of obduction. Emplacement of the Semail ophiolite onto the continental margin resulted in high-pressure, low-temperature metamorphism of this melange and the underlying units.
The magmatic, metamorphic and tectonic evolution of the Northern Marginal Zone of the Limpopo Belt in Zimbabwe
Authors: ROLLINSON H. , BLENKINSOP T. 
Published: December 2007

The Northern Marginal Zone of the Limpopo Belt in southern Africa comprises a Plutonic Assemblage of granitoids including a distinctive suite of porphyroclastic granites. and a much less abundant Supracrustal Assemblage of metabasites and iron formations. These rocks are at granulite facies above a normal thickness of continental crust. Most of the Plutonic Assemblage are intrusive rocks that crystallized from dry melts from 2800 to 2600 Ma, with a relatively simple thermal history. They may have been derived from partial melting of a mafic source. Some supracrustal rocks have experienced two thermal events at granulite facies.
A reverse-sense shear zone forms the boundary of the Northern Marginal Zone with the Zimbabwe craton. The southern boundary is the Triangle shear zone, which is proven as a continuous structure along a much greater strike length than previously documented. A widespread sub-vertical foliation in the Northern Marginal Zone and the reverse shear zone formed during progressive NNW-SSE shortening. Crustal thickening occurred both magmatically and tectonically in the late Archaean, and was accompanied by synchronous uplift. Protracted magmatism provides a mechanism to incorporate supracrustal rocks into the lower crust, and can explain the occurrence of more than a single thermal event.
Westward continuation of the craton–Limpopo Belt tectonic break in Zimbabwe and new age constraints on the timing of the thrusting
Authors: MKWELI S. , KAMBER B. , BERGER M. 
Published: December 2007

The Umlali Thrust Zone has been mapped between the granite–greenstone rocks of the Zimbabwe craton and the granulite-facies rocks of the Limpopo Belt. A tectonic break had not been previously recognized in the southwest because of the intrusion of syn- to post-kinematic porphyritic granites. These intrusions locally obliterate evidence for the tectonic break, although in some localities the associated mylonites are preserved as xenoliths within the intrusive porphyritic granite bodies.
The porphyritic granite bodies were emplaced during and after the main regional fabric forming event. A U–Pb zircon age of 2627 ± 7 Ma obtained from a late-kinematic microgranite is interpreted to be the minimum time of thrusting. A whole-rock Rb–Sr age of 2583 ± 52 Ma obtained from plutonic rocks and retrogressed zones within the North Marginal Zone falls within error of the zircon age, suggesting widespread magmatic and hydrothermal activity coeval with the thrusting.
Plagiogranite from the Andaman ophiolite belt, Bay of Bengal, India
Authors: JAFRI S. H. , CHARAN S. N. , GOVIL P. K. 
Published: December 2007

Plagiogranites occur on the eastern margin of the southern part of South Andaman in the outer sedimentary arc of the Andaman-Nicobar islands, a part of the Sunda-Burmese double-chain arc system, in the Bay of Bengal. They intrude gabbros that are associated with pillow basalt, East Coast volcanic rocks (basalt, basaltic andesite to acid differentiates), radiolarian cherts, conglomerate and grit.
The plagiogranite, characterized by the occurrence of interstitial vermicular and micrographic intergrowths of quartz and plagioclase, shows some geochemical similarities with ophiolitic plagiogranites, but Rb, Yb, Nb, Ta and Y abundances in the Andaman plagiogranite are characteristic of volcanic-arc granites rather than the ocean-ridge granites. It is suggested that the Andaman plagiogranite has been generated by low-pressure crystal fractionation of a low-K tholeiitic magma derived from a mantle source in a supra-subduction zone tectonic setting. Further, it is suggested that this plagiogranite intruded the gabbro of the Andaman ophiolite probably during Mid-Eocene times, as the Andaman ophiolitic rocks (Late Cretaceous to Early Eocene) are believed to have been obducted onto the leading edge of the Eurasian continent during the Mid-Eocene to Late Oligocene subduction event, prior to the currently active Andaman-Java subduction system, that was initiated during the Late Miocene.
Palaeocene uplift and Eocene subsidence in the northern North Sea Basin from 2D forward and reverse stratigraphic modelling
Authors: NADIN P. A. , KUSZNIR N. J. 
Published: December 2007

Structural and stratigraphic basin modelling has been used to examine quantitatively the Cretaceous and Tertiary post-rift subsidence history of the northern North Sea Basin following Late Jurassic extension. 2D forward and reverse, syn-rift and post-rift modelling has been used to determine the magnitude and timing of departures from the McKenzie post-rift thermal subsidence trend for regional stratigraphic profiles in the Outer Moray Firth, the South Viking Graben and the North Viking Graben. Observed stratigraphy has been reverse post-rift modelled using flexural backstripping combined with decompaction and reverse thermal subsidence calculations, and forward modelled through the syn-rift and post-rift period using the flexural cantilever model of continental rift basin formation. Long-term global eustasy has been included in the analysis. Only reliable palaeobathymetric markers, such as erosion surfaces and coals, have been used to constrain subsidence history.
Both forward and reverse 2D modelling show that a Palaeocene regional uplift followed by a rapid Eocene subsidence must be superimposed on McKenzie post-rift Cretaceous and Tertiary basement subsidence in order to generate the stratigraphy observed. The magnitude of Palaeocene uplift deduced from modelling was of the order of 375 m in the Outer Moray Firth (central North Sea) increasing to 525 m in the North Viking Graben (northern North Sea). Rapid Early Eocene subsidence was of the order of 160–310 m. The most likely mechanism for regional Palaeocene uplift and rapid Eocene subsidence in the northern North Sea was the Palaeocene development of the Iceland plume situated at a distance of 700–900 km from the study area in the Early Tertiary. Forward and reverse modelling predict axial bathymetries in the graben of up to 1300 m for the Early Cretaceous and 900 m for the Upper Cretaceous. The large palaeobathymetries at the end of the Cretaceous provided much of the accommodation space for Tertiary sedimentation.