bluuebunny/crossref_metadata_2025 huggingface dataset:
Geochemistry and volcanology of the Tertiary basalts of the Giant's Causeway area, Northern Ireland
Authors: LYLE PAUL. , PRESTON JOHN.
Published: December 2007

The columnar jointed basalts of the Giant's Causeway in Northern Ireland are a localized group of flows, the Causeway Tholeiite Member within the Tertiary Antrim Lava Group. Major and trace element analyses, including rare-earth element (REE) data, are presented which show a range of compositions from hy-normative tholeiites with mid-ocean ridge basalt (MORB) characteristics, to qz-normative examples with SiO
2
> 54% and K
2
O > 1.0%. Within the Causeway tholeiites are three well defined chemical sub-groups which can be correlated stratigraphically across the area. Zr/Nb data suggest a single mantle source for the lavas with the range of composition the result of variable degrees of partial melting of a progressively depleted mantle, followed by fractional crystallization of the magmas at higher crustal levels. Mass balance calculations are used to model the fractionation processes involved. The lavas of the Causeway result from tectonically controlled decompression melting with magma equilibration in ephemeral chambers at high crustal levels followed by eruption as sheet flows. Disruption of the contemporaneous drainage system and the subsequent flooding of hot flow surfaces modified the cooling regime and formed the characteristic multi-tiered flows with colonnade and entablature development.
Ichnofabrics containing
Ophiomorpha:
significance in shallow-water facies interpretation
Authors: POLLARD J. E. , GOLDRING R. , BUCK S. G.
Published: December 2007

In an attempt to interpret
Ophiomorpha
ichnofabrics observed in core, three ichnofabrics are described from outcrops where
O. nodosa
is a conspicuous element. These ichnofabrics enable sandy shoreline sedimentary environments to be characterized and differentiated: (1) shoreface with mottled-
Ophiomorpha—Planolites
ichnofabric generally without primary lamination; (2) offshore tidal shelf sand wave facies with
Macaronichnus-Ophiomorpha
ichnofabric associated with primary, mainly cross-laminated or cross-bedded sands; (3) estuarine facies with
Ophiomorpha
ichnofabric associated with primary lamination and, commonly, heterolithic sands and mudstones. Distinctions between the ichnofabrics are attributed to differences in primary stratification, the total ichnocoenoses, morphological features (such as burrow attitude, shaft restriction, pellet wall lining), to the nature of the substrate and, particularly, to the time available for colonization (larval settlement or relocation) and burrow construction, referred to here as the
colonization window
The analysis is applied to an interval of core (Upper Jurassic, Central Graben, North Sea) and a sequence in Eocene sediments in southern England.
Environmental oxygen fluctuations and the adaptive response of marine benthic organisms
Authors: OSCHMANN W.
Published: December 2007

Oxygen depletion is widespread in modern and ancient benthic environments. Most modern examples exhibit a significant cyclic fluctuation in O
2
availability, ranging from tidal to seasonal cycles, and longer term periods (e.g. El Niño). In contrast, only long term changes have been generally recognized in ancient environments. Benthic organisms adapt along two pathways: (1) to low O
2
-values and free H
2
S (including chemosymbiosis); (2) synchronization of individual life cycles to the O
2
-fluctuation cycle by living as planktonic larvae in oxygenated surface waters, using oxygenated periods in bottom waters during the adult stage.

Examples of both adaptive pathways can be recognized in the benthic faunas of fossil oxygen-controlled environments, thus recording short and longer term fluctuations.
Charles Lapworth and the biostratigraphic paradigm
Authors: FORTEY RICHARD A.
Published: December 2007

Lapworth’s paper on ‘The Moffat Series’ (1878) provided a model for deciphering the ‘interminable greywackes’ of the Southern Uplands, and one which lasted for a century. The same paper established graptolites in a dominant position in Lower Palaeozoic biostratigraphy. The changes in the biostratigraphic paradigm are discussed with reference to Lapworth’s contribution; issues implicit in his 1878 paper are still contentious. Graptolites have exemplified the conflict that can arise between the use of fossils as stratigraphic ciphers, on the one hand, or as complex organisms to be interpreted biologically on the other. They have been subjected to the vicissitudes of stratigraphic fashion. It is shown that Lapworth’s biostratigraphy has been enduring in contrast to his structural or palaeogeographic interpretations. However, the subsequent separation of litho- and chrono- from biostratigraphy, while conceptually necessary, has encouraged an idealistic pursuit of the perfect stratigraphic section for the purpose of defining stratigraphic boundaries. This has not always been constructive, not least because such boundaries often coincide with events which militate against the preservation of ideal sections. But Lapworth’s close integration of biostratigraphic range with observations on lithology, ‘barren beds’ and fossil preservation may have a new lease of life in the context of event stratigraphy.
U-Pb ages from SW Poland: evidence for a Caledonian suture zone between Baltica and Gondwana
Authors: OLIVER G. J. H. , CORFU F. , KROGH T. E.
Published: December 2007

Faunal and palaeomagnetic evidence suggests the existence of a
c.
3000 km-wide Tornquist Sea between Gondwana and Baltica in Early Ordovician times, which narrowed to <1000 km by the Late Ordovician. The inferred suture zone between sequences with ‘Baltic’ faunas in Poland and the ‘Gondwana’ faunas in Czechoslovakia is characterized by a collage of six terranes which have distinct Cambrian to Carboniferous histories. They have geochemical characteristics and sedimentary associations of volcanic rocks in marginal basins, island arcs, ophiolites and volcanic arc granites, give Tremadoc and Arenig protolith U–Pb zircon ages, and have been regionally metamorphosed. A syn-metamorphic granite is dated as pre-Lower Ashgill (461
−50
–2
Ma). An unmetamorphosed ophiolite gives a 420
−20
–2
Ma age, whilst in two other terranes amphibolite-facies island-arc lavas have fossiliferous Lower Ludlow (424–415 Ma) sequences: all are unconformably overlain by unmetamorphosed Upper Devonian conglomerates. A sinistral transpressive regime is observed in regionally extensive mylonite zones. Titanite and zircon ages (338
−2
–3
and 339 ± 4 Ma) record Variscan magmatism. The data suggest considerable narrowing of the Tornquist Sea during the Ordovician, continuing ocean floor and island arc activity in the Silurian, and final sinistral transpressive closure by the Mid-Devonian.
Volcanic plumbing systems
Authors: McGUIRE W. J.
Published: December 2007

Report of a meeting held by the Volcanic Studies Group at the Open University on 7 December 1991.

This research seminar, targeted principally at postgraduates and senior undergraduates, was convened with the object of reviewing current ideas on the functioning and form of volcanic and sub-volcanic plumbing systems. The seminar was topical, with the recent activity at Mount Pinatubo not only serving to keep volcanoes and volcanology in the public eye, but also providing a timely reminder, in human terms, of the importance of understanding the mechanisms which operate within and beneath volcanoes. Increased knowledge in this area can lead only to the more accurate prediction of volcanic events, and consequently to the more effective execution of mitigation policies and reduced risk at the world’s 700 or so currently active volcanoes.

The first two speakers concerned themselves primarily with the application of physical parameters measured at the surface.
R. S. J. Sparks
(University of Bristol) presented a simple physical model designed to interpret observations of lava eruptions in terms of changing eruption conditions. The model assumes that magma overpressure is proportional to erupted volume which is in turn directly related to the effusion rate. These relationships were used, together with field observations, to monitor factors controlling variations in lava effusion rate. Application of the model to three well documented eruptions demonstrated that variations in magma flow rate during the course of an eruption may be explained by changes in magma overpressure, modified at times by additional factors including magma solidification
Geochemical differences between the Cadomian granites of Mancellia and the St Malo migmatites, Armorican Massif, France
Authors: POWER G. M.
Published: December 2007

Data sets used to suggest geochemical similarity between the St Malo migmatites and Mancellian granites (Brown & D’Lemos 1991) are reinterpreted. The samples have complex distributions with outliers. Three non-parametric tests show significant statistical differences between the samples in A1
2
O
3
, CaO, K
2
O, Cr, Ni, Sr, Y and Zr. If outliers are culled from the samples the differences become more extreme. The differences are also apparent on both principal component and multidimensional scaling plots. Geotectonic models based on a similar petrogenesis for the two rock units (D’Lemos
et al.
1992) must be able to account for these differences.
The Nadi Basin: Neogene strike-slip faulting and sedimentation in a fragmented arc, western Viti Levu, Fiji
Authors: HATHWAY B.
Published: December 2007

The Nadi Basin in western Viti Levu is one of a number of sedimentary basins controlled by E–W to NE–SW faults that developed on the Fiji Platform in the Late Miocene.

The initial basin fill, belonging to the Upper Miocene Nadi Sedimentary Group, consists of mainly basement-derived breccias and sandstones, deposited at bathyal depths by sediment gravity flows on a basin floor dissected by braided E–W faults. The main source area lay to the south, across the actively down-faulting basin margin.

In the latest Miocene, a change in provenance took place as debris spread southward from a newly developed E–W chain of shoshonitic volcanoes (Koroimavua Volcanic Group) some 12 km north of the southern basin margin. Continued normal displacement along the southern margin allowed the deposition of a thick sequence of shoshonitic sandstones at bathyal depths on the downthrown side.

In the earliest Pliocene, the onset of basin inversion was marked by shallowing to neritic depths and the onlap of sandstones and shoshonitic conglomerates southward onto pre-Nadi Sedimentary Group basement. As inversion proceeded the basin fill was folded and southward-directed reverse displacement took place along the southern basin margin.

Evidence from the Nadi and other Late Miocene basins indicates that they developed as a result of broadly E–W-directed-sinistral strike-slip, probably related to a shearing collision of the Melanesian Border Plateau with the northern margin of the Fiji Platform during the breakup of the formerly continuous Outer Melanesian arc.

It is suggested that prior to the establishment of the Fiji fracture zone at 7-6 Ma, much of the left-lateral displacement currently taking place along that plate boundary was accommodated by strike-slip deformation within the Fiji Platform.
Magnetic susceptibility of the Chinese loess–palaeosol sequence: environmental change and pedogenesis
Authors: XIUMING LIU , SHAW JOHN , TUNGSHENG LIU , HELLER FRIEDRICH
Published: December 2007

Pronounced differences in concentration, alignment and grain-size distribution of the magnetic minerals have been identified between loess and palaeosol samples from the section at Xifeng (Central Chinese loess plateau). Superparamagnetic magnetite formed in situ plays an important role in controlling magnetic low field susceptibility. The variable brown-red colouration of the loess-palaeosol sequence is due to a significant hematite component produced by pedogenesis. The magnetic fabric of the palaeosols is weaker than that of the loesses. This is taken as evidence that the formation of ultrafine ferromagnetic minerals by pedogenesis is the most likely explanation for the observed susceptibility enhancement in palaeosols (4 to 10 times higher than in loesses). It is argued that variations of frequency dependent susceptibility are sensitive recorders of second order palaeoclimatic variations during glacial periods, whereas bulk susceptibility seems to be an appropriate measure of palaeoclimate change during interglacial periods.
Mesozoic cover over northern England: interpretation of apatite fission track data
Authors: HOLLIDAY D.W.
Published: December 2007

Whether a significant thickness of Mesozoic cover rocks once rested upon the Palaeozoic rocks, which form the structural highs of the English Lake District and the Northern Pennines, is a subject of long debate which has been revived recently by apatite fission track analysis (AFTA) studies. Evidence from adjacent basins suggests that 1200–1750 m of Mesozoic cover was probably formerly present on the highs, prior to Cenozoic erosion. Higher estimates (
c
. 3000 m) of cover thickness, based on such fission track-derived palaeotemperatures, result from underestimation of early Cenozoic surface temperatures and from not allowing for the relatively low thermal conductivity of the eroded cover rocks. For similar reasons, apatite fission track estimates of eroded cover rocks elsewhere in Britain may also prove to be overestimates.
Correlating the Eocene Oligocene mammalian biochronological scale from SW Europe with the marine magnetic anomaly sequence
Authors: LÉVÊQUE F.
Published: December 2007

Magnetostratigraphic studies have been undertaken on sedimentary continental formations from Western France and Spain which have yielded Palaeogene mammals. The results obtained provide new constraints on the chronological distribution of the mammalian reference levels. A magnetozone sequence associated with the reference levels is defined. The correlation of this sequence with the geomagnetic polarity time scale is proposed. This correlation links the late Mid-Eocene to Oligocene mammalian biochronological scale from Western Europe to other stratigraphic scales.

A numerical calibration of the Palaeogene polarity sequence is made, taking into account recent radiometric dating. The equations to convert the ages proposed by several authors are given.
Provenance of turbiditic cover to the Caledonian Solund–Stavfjord ophiolite from U-Pb single zircon dating
Authors: PEDERSEN R. B. , DUNNING G. R.
Published: December 2007

U-Pb dating has been carried out on single zircon and titanite grains in greywacke intercalated with lavas of the late Ordovician (443 ±3 Ma) Solund-Stavfjord ophiolite. The ages range from 2495 Ma to 462 Ma indicating that detritus was provided by a Precambrian continental region as well as an Ordovician Caledonian terrane. The Caledonian component of the detritus was probably derived from the early Ordovician ophiolitic terrane of west Norway, and the data therefore link two generations of Caledonian ophiolites. The mapped relationships of the ophiolitic terranes suggest that the late Ordovician–early Silurian evolution of the Caledonides was dominated by rifting and the formation of marginal basin(s).
Gondwanaland significance of Lower Palaeozoic deformation in central India and SW Western Australia
Authors: HARRIS L. B. , BEESON J.
Published: December 2007

The Stirling Range Formation of Western Australia and the Vindhyan Supergroup of central India have undergone two identical Lower Palaeozoic deformation events. Folding related to dextral reactivation of basement structures suggests a possible continuation of the Indian Narmada-Son-Brahmaputra lineament into SW Western Australia. Subsequent open folding during NE-SW crustal shortening in both terranes is possibly related to Gondwanaland assembly. Whilst the Shillong Group in NE India appears similar to the Stirling Range Formation and Vindhyan Supergroup, further geochronology is required to substantiate a common tectonic history.
Early Acadian sinistral shear in north-central Maine, USA
Authors: HIBBARD J. , HALL S.
Published: December 2007

A previously unrecognized sinistral shear event recently has been identified in Silurian–Devonian rocks of the Millimagassett Lake region, north-central Maine, USA. This event predates the ‘classic’ Early-Mid-Devonian Acadian orogeny of the region, yet affects rocks as young as Early Devonian. The Millimagassett Lake sinistral shear zone places significant time constraints on an early phase of Acadian tectonics that may well extend throughout the northern and Newfoundland Appalachians.
Contrasting metamorphic
P–T
–time paths of the Kohistan granulites and tectonics of the western Himalayas
Authors: YAMAMOTO HIROSHI
Published: December 2007

In the Swat-Indus Kohistan area, lower crustal rocks of the Kohistan arc including amphibolites, granulites and ultramafic rocks are widely exposed. Detailed petrological-structural studies reveal that the Chilas-Jijal metamorphic complex comprises essentially intermediate- to high-pressure granulites with a granulite/amphibolite transition controlled by extensive retrograde metamorphism and ductile deformation. Contrasting patterns of compositional zoning in garnet and orthopyroxene suggest complicated
P–T
paths in the granulites. Geothermobarometry based on garnet-bearing granulites identify two distinct
P–T–time
paths: a high-pressure anticlockwise path of the Jijal complex and a low-pressure retrograde path of the Chilas complex. These
P–T
–time paths represent the evolution of middle to lower crustal rocks of the Kohistan arc. The anticlockwise
P–T
-time path combined with published isotopic ages reveals that: (i) the high pressure granulitefacies metamorphism is related to the Kohistan-Asia collision and (ii) the crust of the Kohistan arc had been thickened at least 55 km before the India-Asia collision.
Modelling the plate-driving mechanism
Authors: BOTT M. H. P.
Published: December 2007

This paper demonstrates how sub-lithospheric loading gives rise to plate interior stresses and plate boundary forces. The stressing of the lithosphere resulting from plume heads in the upper mantle (hot spots), from low density asthenospheric upwelling beneath ocean ridges and from dense subducting slabs has been modelled by finite element analysis. It is first demonstrated how a sub-lithospheric load, exemplified by a sub-continental plume head, produces local deviatoric stress in the strong upper lithosphere within a plate interior region. It is shown that a 500 km wide region of anomalous density averaging −8 kg m
−3
between 100 km and 400 km depths gives rise to a deviatoric tension of 75 MPa in a 20 km thick strong layer forming the upper lithosphere above the deep buoyant load. The large stresses in the strong layer are caused by the shear drag and excess pressure exerted by the buoyant load. When such a stress system is cut across by a zone or plane of weakness, then plate boundary forces and distant plate interior stresses are produced. A weak zone beneath the crest is incorporated in a model of a normal ocean ridge, and this yields a ridge push force of 2.5 × 10
12
Nm
−1
referenced to old ocean floor and deviatoric compression of about 40 MPa in old ocean floor. A model of a ridge underlain by an anomalously low density upper mantle (plume head) yields a much larger ridge push force, and large deviatoric compressions of nearly 100 MPa extend into the bordering continents. Subduction pull (slab pull and trench suction) of about −4.5 × 10
12
Nm
−1
is modelled for a 300 km deep slab separated from the overriding plate by a weak fault, and collision pull is shown to result from downbulging lower lithosphere beneath collision mountain ranges.
Pattern of mantle convection and Pangaea break-up, as revealed by the evolution of the African plate
Authors: PAVONI N.
Published: December 2007

A tensional stress regime has governed the tectonic evolution of most of the African continent. The long history of crustal extension is documented by the widespread occurrence of continued intraplate rifting and volcanism since Triassic times and by the large-scale plate movements following the break-up of Pangaea. A remarkable result of the present investigation is the recognition that the extension displays a continent-wide and plate-wide radial pattern, centred in equatorial Africa near 10°E/0°N, the African Centre A. The radial pattern is evident from the diverging block movements, documented by the intra-continental rifting in Africa, by the break-up of Pangaea and the subsequent, diverging drift of North America and the Gondwana fragments away from Africa, entailing a radial growth of the African plate. All these phenomena are manifestations of what may be referred to as the ‘African lithospheric divergence’.

The African plate is surrounded, to 85%, by active oceanic ridges. Ridge-push forces exerted on the African plate have to be balanced by some other forces in order to explain the tensional stress regime which, even at present, dominates large parts of the African continent . Shear traction exerted by the convecting asthenosphere on the base of the African lithosphere offers an explanation for the observed phenomena. It is proposed that relatively warm and less dense mantle material rises from the deep mantle below the African plate. In the upper mantle and at the base of the African lithosphere, the ascending material diverges and flows radially away from the centre of ascent. Upper mantle flow below the sea-floor spreading axes is unidirectional and horizontal and is directed away from the African centre A.

It is proposed that the ascending flow beneath Africa forms part of a very large, bicellular circulation system in the Earth's mantle. The origin of geotectonic cycles is probably interrelated with the onset, build-up, main phase and decay of such large-scale convection systems.
The development of two oceanic microplates: Easter and Juan Fernandez microplates, East Pacific Rise
Authors: SEARLE R. C. , BIRD R. T. , RUSBY R. I. , NAAR D. F.
Published: December 2007

In this paper we synthesize and discuss the results of recent detailed geophysical surveys and reconstructions of two active mid-ocean ridge microplates on the East Pacific Rise. We quantify their development in terms of the principal dimensions of the microplates and their boundaries, and the temporal changes in these dimensions. The results are discussed in the framework of the ‘roller-bearing’ microplate model of Schouten and others, which postulates that microplates are driven by shear couples imposed on them by the motions of their bounding major plates. We test the specific predictions of this model for the geometry of the ‘pseudofaults’ (traces of the propagating tips of the microplates’ spreading boundaries) and the histories of rotation of the microplates. Both microplates are currently rotating at the rates predicted by the model on the assumption of complete coupling (no slip or deformation) between the microplates and the major plates at instantaneous points of contact near the propagating tips of the spreading boundaries. Juan Fernandez microplate was also rotating at the predicted rate just before 2 Ma when it was driven by the Nazca–Pacific couple rather than the Nazca–Antarctic couple that currently applies. However, earlier rotations were slower than predicted, implying some degree of decoupling or deformation at earlier times. The microplates appear to have grown from moderate-sized (
c
. 100 km), probably left-stepping offsets of the East Pacific Rise since about 5 Ma. The nature of these offsets is currently uncertain, but they may have been propagating rifts, which changed from shearing by ‘bookshelf faulting’ to rigid block (microplate) rotation on reaching a critical size. Once rigid block rotation began, the microplates grew by seafloor spreading, accompanied by propagation of their bounding spreading centres. The effects of the neighbouring Easter mantle plume and Pacific–Nazca–Antarctic triple junction can be discerned in the bathymetry and ridge propagation rates, but do not appear to have had a major influence on the microplates’ development.
Geochemical signatures of oceanic and continental basalts: a key to mantle dynamics?
Authors: WILSON M.
Published: December 2007

The distinctive trace element and Sr-Nd-Pb isotopic compositions of oceanic island basalts (OIB), believed to be related to the upwelling of deep mantle plumes from the core-mantle boundary, provide important constraints for models of mantle dynamics. Whilst the present-day plume flux is small compared to the plate-scale flow, during the Early Cretaceous there appears to have been a major phase of superplume activity which changed, rather drastically, the pattern of convection within the upper mantle, may have modulated the reversal frequency of the Earth’s magnetic field, and considerably accelerated plate motions. Major foci of upwelling occurred in the Pacific and Indian Oceans and beneath Gondwana, along the site of the future South Atlantic rift. During such cycles of superplume activity outward flow from the axis of the plumes may provide an important driving force for plate tectonics.

Early Cretaceous continental and oceanic flood basalts, possibly related to the initial impact of the superplume heads on the base of the lithosphere, have distinctive Nd-Pb isotopic signatures which suggest that the source of the super-plumes (? D” layer at the base of the lower mantle) was ancient (
c
. 2 Ga) recycled oceanic crust plus subducted pelagic sediment. In addition the Pacific super-plume may also have entrained lower mantle material with distinctive Nd-Pb isotopic characteristics. Within the Atlantic domain, the distinctive isotopic fingerprint of two plumes, St Helena and Tristan da Cunha, whose compositions are close to those of the HIMU and EM I end-members of the OIB spectrum respectively, can be recognized in the magmatic rocks generated throughout the 130 Ma history of opening of the South Atlantic Ocean.
Seismic reflections from within the Lake District batholith, Cumbria, northern England
Authors: EVANS D. J. , ROWLEY W. J. , CHADWICK R. A. , MILLWARD D.
Published: December 2007

Seismic reflection data from the Wasdale area, Cumbria, reveal reflector-poor and reflector-rich areas within components of the Lake District batholith. A representative panel of seismic data crossing the western boundary of the intrusion has been modelled with synthetic seismograms. The preferred model indicates that the Ennerdale Granophyre is a sheet-like body some 1100 m thick. Beneath this, the batholith comprises multiple poorly-reflective laccolithic granitic intrusions, interfingered with moderately reflective wedges of country rock, to give the steep western margin of the granite complex a 'cedar-tree' profile. Within the main body of the batholith, reflector-rich zones may arise from either areas of included country rock, basic sills or zones of mafic material within granite.
Proposed methods for correcting external detector fission track counts for tracks lost during etching
Authors: MILLER J. A. , HORSFALL J. A. C. , PETFORD N. , TIZARD R. H.
Published: December 2007

The ways in which minerals and mica detectors respond to chemical etching are described. While tracks with angles of inclination less than a critical value will be lost from minerals, a steady state is reached. With continued etching all tracks in an external detector can be destroyed. Methods are proposed and numerical expressions presented by which the number of tracks lost during etching external mica detectors can be determined. An example of the process applied to automatically counted tracks in an external mica detector is given.
Structure of the late Proterozoic Nakasib suture, Sudan
Authors: ABDELSALAM M. G. , STERN R. J.
Published: December 2007

The Nakasib suture is a prominent structural belt in the central Red Sea Hills, Sudan. It is one of the ophiolite-decorated sutures along which the late Proterozoic (Pan-African) island arc/back arc terranes and continental micro-plates of the Arabian–Nubian Shield were welded together. It juxtaposes the 900–800 Ma Haya terrane in the south with the 830–700 Ma Gebeit terrane to the north. New structural data from the Nakasib suture show that the suture had evolved through three phases of deformation which gave rise to a fold and thrust belt. During the early two phases of deformation, SE-verging tight folds and thrusts were developed. The third phase of deformation refolded the earlier structures into NE-trending, upright, and horizontal antiforms and synforms and culminated in the development of NW-verging thrusts. The structural data are used, together with previous stratigraphic and geochemical data, to suggest a tectonic model for the evolution of the Nakasib suture. The suture followed a Wilson Cycle. This started with rifting of the Haya terrane, extrusion of rift volcanics and deposition of passive margin sediments, and development of a NW-dipping subduction zone over which the arc volcanics of the Gebeit terrane erupted. The closing of the oceanic basin gave rise to the Nakasib suture with the present structural configuration.

The structural styles of other sutures of the Arabian-Nubian Shield are examined in light of structural data from the Nakasib suture. These sutures show a common feature of steepening of the early subhorizontal ophiolite-obduction structures during a late event of upright folding which may be related to collision between terranes.
Stratigraphy and regional significance of the Upper Jurassic-Lower Cretaceous Byers Group, Livingston Island, Antarctica
Authors: CRAME J. A. , PIRRIE D. , CRAMPTON J. S. , DUANE A. M.
Published: December 2007

The Byers Group, exposed on Byers Peninsula, western Livingston Island, Antarctica, comprises a mudstone dominated sequence at least 1 km thick which accumulated in a marginal fore-arc environment. The basal, 105 m thick Anchorage Formation consists of radiolarian mudstones and tuff-rich interbeds of Kimmeridgian-Tithonian age; it correlates with Upper Jurassic organic-rich mudstone units throughout the proto-South Atlantic region. The succeeding 244 m thick Devils Point Formation marks the first major pulse of coarse volcaniclastic material into the basin. It is in turn followed by the extensive President Beaches Formation, comprising several hundred metres of finely laminated mudstones with at least two major sandstone intercalations. Molluscan and dinoflagellate cyst taxa indicate a Berriasian age and comparatively nearshore depositional environment for this unit. An unconformity of late Berriasian or early Valanginian age separates the three lowest formations from the Chester Cone Formation. The fine-grained Sealer Hill Member at the base of the latter is dated as Valanginian, and grades up into several hundred metres of pebbly sandstones and pebble-granule conglomerates. These mark the second major volcaniclastic pulse and may be of Hauterivian or even younger age. Definition of this major new group will facilitate more precise Upper Jurassic-Lower Cretaceous stratigraphical correlations within the southern South America-Scotia arc-Antarctic Peninsula region. It will also aid our understanding of the critical palaeogeographical transition in the northern Antarctic Peninsula from anoxic basin to active magmatic arc.
A depositional model for the Mid-Westphalian A to late Westphalian B Coal Measures of South Wales
Authors: HARTLEY A. J.
Published: December 2007

Mid-Westphalian A to late Westphalian B sediments of the South Wales Coalfield comprise mainly lacustrine, raised mire and flood deposits developed in an upper coastal plain environment. Occasional high sinuosity, fine-grained channel systems drained the subdued topography of the Wales-Brabant Massif to the north and east of the basin. Low sinuosity channels with lithic, coarse-grained fills drained a rising source to the south. Direct marine influence was restricted to a single marine band.

Facies distributions were controlled by (1) local scale (10
2
-10
3
m
2
) autocyclic sedimentary processes, (2) regional scale (10 km
2
) synsedimentary fault activity and differential subsidence and 3) basin-wide scale (10
2
km
2
) response to changes in relative sea-level. Base-level changes are recorded by basin-wide bivalve horizons (up-dip or landward extension of marine bands) developed in lacustrine mudstones above basin-wide coal seams. Peat accumulation was terminated by a water table rise related to base-level rise. Following flooding, re-establishment of the clastic supply resulted in the gradual infilling of the basin-wide lake to produce coarsening- upwards cyclothems or parasequences. A eustatic origin for parasequence development is supported by the development of identical facies associations and parasequences in coeval sediments from the Pennine and Midland Valley Coalfields.
Transfer zones in extensional basins: their structural style and influence on drainage development and stratigraphy
Authors: GAWTHORPE R. L. , HURST J. M.
Published: December 2007

Transfer zones form important structural elements in extensional basins, accommodating displacement changes between individual fault and basin segments. Transfer zone geometry is related to the extension direction and the displacement, dip polarity, overlap and overstep of fault/basin segments adjacent to the zone. Topographic changes associated with transfer zones have a direct influence on drainage basin evolution, sediment transport and stratigraphy.

Two main categories of transfer zone can be identified: (i)
interbasin transfer zones,
linking individual half graben, and (ii)
intrabasin transfer zones,
linking individual fault segments within a half graben. Interbasin transfer zones range from interbasin ridges to broad faulted highs and major relay ramps. They have a marked influence on basin stratigraphy and drainage evolution, often separating half graben with distinct stratigraphies and acting as conduits through which major axial depositional systems enter the rift zone. Intrabasin transfer zones range from relay ramps separating adjacent en echelon normal faults to discrete fault jogs. Intrabasin transfer zones commonly act as a conduit for local sediment transport, but have minimal effect on basin-scale stratigraphy. Transfer zones also affect early post-rift sedimentation and are important elements in controlling fluid migration in the subsurface.
Current research in the British Tertiary Igneous Province
Authors: KERR A. C.
Published: December 2007

A one-day meeting on current and recent research in the British Tertiary Igneous Province, convened by Andrew Kerr, was held at the Department of Geological Sciences, University of Durham, on 1 May 1993. The meeting attracted over 50 participants, from a wide range of disciplines, including geochemistry, volcanology, petrology and structural geology. There were nine speakers and six poster presentations.

After a brief welcome, by
R. N. Thompson
on behalf of the Durham Department,
S. A. Gibson
(Durham University) reviewed her work on the Little Minch Sill Complex, Skye. Field and petrographic evidence were used to show that the internal stratigraphy of the sills could be explained by multiple intrusion. Previous workers had proposed that the sill complex were fed by the same magmatic conduits as the Skye lavas. Geochemical data, however, prove that this can not be the case since the sills had been contaminated with amphibolite facies upper crust, in contrast to the predominantly lower crustal contamination of the lavas.
G. R. Kitchener
(Glasgow University) spoke on oxygen isotope exchange during hydrothermal alteration around the Cullin intrusion, Skye. Focusing on one lava flow, close to the margin of the intrusion, he showed how δ
18
O varied with height throughout the flow, and cast some doubt on the interpretations of δ
18
O values by previous workers on the Skye lavas. Analysis of secondary mineral separates revealed that the chlorite and the zeolites have different δ
18
O, which suggests that the hydrothermal alteration was caused by many different pulses of fluid with variable
Silurian basin slope sedimentation and mass movement in the Wigmore Rolls area, central Welsh Borderland
Authors: WHITAKER J. H. McD.
Published: December 2007

In the Wigmore Rolls area of the central Welsh Borderland, westward-directed mass movements (sliding and slumping) on the Silurian Welsh basin slope removed at least 275 m of the Wenlock and Ludlow (Homerian to upper Ludfordian) succession of mudstones and calcilutites. The resulting slide scar was more than 2.5 km wide. Lower Leintwardine Formation (lower Ludfordian) flaggy siltstones rest with marked unconformity in this erosional hollow. Continuing instability is shown by a basal coarse boulder bed up to 10 m thick, channellized in places, that resulted from a debris flow coming to rest downslope, and by widespread slumping during the accumulation of the Lower Leintwardine Formation siltstones. Instability at this time outside the Wigmore Rolls area is confirmed by the formation of submarine canyon heads, channelling and hardgrounds. Comparable mass movements, leaving slide scars and channels, on modern basin slopes are becoming better known, but ancient examples such as those described here are rare. The slide scar and channels were filled by the end of Early Leintwardine times, and sandier siltstones of the Upper Leintwardine, Lower Whitcliffe and Upper Whitcliffe formations prograded uniformly over the whole area in shallower water. During the Acadian orogeny, the area was folded and faulted into a southwesterly continuation of the Ludlow anticline.
Allochthonous oceanic basalts within the Mesozoic accretionary complex of Alexander Island, Antarctica: remnants of proto-Pacific oceanic crust
Authors: DOUBLEDAY P. A. , LEAT P. T. , ALABASTER T. , NELL P. A. R. , TRANTER T. H.
Published: December 2007

The Mesozoic LeMay Group accretionary complex of Alexander Island, Antarctica, contains thrust-bound slices of accreted ocean floor, ocean islands and seamounts. They represent fragments of proto-Pacific oceanic crust, of which only a tiny remnant (the Phoenix plate) remains off northern Antarctic Peninsula. They therefore provide an excellent opportunity to sample the ancient oceanic crust that formerly occupied the southern Pacific Ocean.

All the basalts experienced sea-floor and subduction/accretion metamorphism ranging from zeolite to transitional blueschist facies. On the basis of rare–arth and other immobile trace element characteristics, the basalts are divided into depleted MORB, N-MORB, E-MORB, and tholeiitic and alkaline OIB.

Oceanic basalts occur within two rock associations on Alexander Island, basalt-volcaniclastite-chert and basalt-volcaniclastite-tuff. The basalt-volcaniclastite-chert rock association is dominated by pillow lavas which have light REE-depleted N-MORB geochemical characteristics, and is interpreted as representing ocean floor formed at spreading centres. Locally, sills of tholeiitic OIB intrude the sequence. The basalt-volcaniclastite-tuff rock association exposed in the Lully Foothills was formed in shallow water during the Early Jurassic. It is geochemically varied, consisting of basalts with N-MORB, E-MORB and tholeiitic OIB characteristics. The association is interpreted to have been formed on a large seamount or ocean island.
40
Ar-
39
Ar ages for dykes from the Falkland Islands with implications for the break-up of southern Gondwanaland
Authors: MUSSETT A. E. , TAYLOR G. K.
Published: December 2007

40
Ar-
39
incremental-heating spectra analyses were made of five microgabbro dykes from the Falkland Islands, two of which yielded an acceptable plateau, and a third a close maximum estimate. All three indicate an age of dyke intrusion of about 190 Ma, supporting a single, published K-Ar date. This provides further support for the geological continuity of the Falkland Islands and the eastern Karoo Basin where dyke swarms of similar petrological affinities were intruded at about the same time. The age constrains the previously published palaeoreconstruction of the Falkland Islands, based on palaeomagnetic data, which places the Falklands immediately to the east of southern Africa, to be valid until at least the late Early Jurassic.
U-Pb zircon and Sm-Nd dating of Moldanubian H
P
/H
T
granulites from South ohemia, Czech Republic
Authors: WENDT J. I. , KRÖNER A. , FIALA J. , TODT W.
Published: December 2007

Conventional multigrain U-Pb systematics of zircon and single-grain evaporation data of zircons in combination with Nd model ages are presented for pyroxene-free, felsic, garnetiferous granulites and orthopyroxene-bearing granulites of felsic, mafic and intermediate composition from the Blansky Lés mountains granulite complex, S Bohemia. Zircons from mafic and intermediate granulites yield Concordia intercept ages between 346 ± 5 and 351 ± 6 Ma as well as a single-grain evaporation age of 346 ± 12 Ma. These ages define the time of Variscan high-grade metamorphism in S Bohemia and are supported by an internal Sm-Nd garnet-zircon-plagioclase-whole rock isochron age of 343 ± 21 Ma. Euhedral zircons from felsic granulites of probable granitic derivation yield U-Pb ages of 366 ± 5 Ma and 373 ± 12 Ma interpreted as intrusion ages of the granitic precursor. An inherited Proterozoic crustal component is documented by poorly defined Concordia upper intercept ages around 1700 Ma and Nd-model ages between 1.3 and 2.1 Ga.
Geochemical evidence for progressive, rift-related early Palaeozoic volcanism in the western Sudetes
Authors: FURNES H. , KRYZA R. , MUSZYNSKI A. , PIN C. , GARMANN L. B.
Published: December 2007

The volcanigenic rocks of the Kaczawa Mts (western Sudetes, Poland), in the eastern Variscides, show changing geological and geochemical evolution during early Palaeozoic time. The lower part of the succession (Cambrian (?)–Ordovician) has three components. 1. Shallow marine to subaerial metabasalts, associated with limestones and volcaniclastics. The lavas are dominantly of a transitional tholeiitic–alkaline type and their trace element patterns typically represent a rift-related environment. They pass laterally (and upwards ?) into more depleted basalts resembling enriched MORB, with Nd-isotopic characteristics indicating contamination by continental crust. 2. Interlayered rhyodacitic lavas and volcaniclastics which show negative ɛ
Nd
values, suggesting formation of the original magma by crustal melting. 3. An overlying alkaline bimodal suite of lavas and volcaniclastic rocks, as well as alkaline metabasites of shallow-intrusive character. The geochemistry of the latter resembles oceanic island volcanics, but they may well have been emplaced in the same evolving rift environment.

The upper part of the Kaczawa succession, Ordovician-Silurian (?) in age, comprises a thick and monotonous sequence of deep-marine pillowed and massive metabasalts, associated with black shales and cherts. These lavas exhibit MORB trace element characteristics, with minor evidence of crustal contamination. During this stage of rifting, true oceanic crust probably formed. It is thus suggested that the studied part of the Kaczawa succession developed in a progressively evolving rift, initially within an ensialic environment, and finally reaching the stage of a basin underlain by oceanic-type crust. Together with similar Cambrian-Ordovician volcanic-sedimentary associations, widely distributed in western Europe, from Portugal, through France and Germany, they represent a record of the Early Palaeozoic rifting in the northern periphery of Gondwana.
Isotopic biogeochemistry of the Oxford Clay Formation (Jurassic), UK
Authors: KENIG F. , HAYES J. M. , POPP B. N. , SUMMONS R. E.
Published: December 2007

A total of 165 samples was obtained from the Oxford Clay Formation at seven different sites. Nearly all were from the Peterborough Member (Lower Oxford Clay), but seven were from the Stewartby and Weymouth Members (Middle and Upper Oxford Clay respectively). Five samples from the underlying Kellaways Formation were also examined. Stratigraphic relationships were estimated on the basis of ammonite subzones and results from all locations can be placed along a single stratigraphic scale. The following were determined for all samples: abundance and isotopic composition of organic carbon, abundances of carbonate carbon and total sulphur, and the Rock-Eval pyrolysis parameters hydrogen index, oxygen index and T
max
. For a subset of eight samples selected to be representative of geochemical and apparent palaeoenvironmental variations, soluble organic compounds were extracted and the isotopic composition of pristane, phytane, and long-chain
n
-alkanes determined by isotope-ratio-monitoring gas chromatograph mass spectrometry.

Concentrations of organic carbon in samples from the Peterborough Member ranged from 0.5 to 16.6 % and δ values of total organic carbon (TOC) ranged from –27.7 to –23.1‰ v. PDB. Shales dominated by epifaunal bivalve assemblages have high concentrations of TOC and values of H index approaching 800, indicating preservation of hydrogen-rich organic material. Conversely, shell beds and calcareous and silty clay beds have lower abundances of TOC and values of H index dropping below 100, indicating extensive oxidation of the organic matter.

Isotopic composition of pristane and phytane in the Peterborough and Stewartby Members average –31.7‰, those in the Weymouth Member average –29.8‰. Values of δ for long-chain
n
-alkanes average –28‰Together these results indicate δ values for primary inputs as follows: terrestrial vascular plants, –23.5‰; Peterborough Member algae, –28.2; Stewartby Member algae, –29.1‰; Weymouth Member algae, –26.6%. Comparison of primary δ values to those of TOC indicates that in some cases secondary processes enriched TOC relative to primary inputs by as much as 4‰ Palaeontological evidence in these same beds indicates development of extensive food-webs and supports attribution of this isotopic enrichment to heterotrophic reworking.
The trophic structure of the biota of the Peterborough Member, Oxford Clay Formation (Jurassic), UK
Authors: MARTILL D. M. , TAYLOR M. A. , DUFF K. L. , RIDING J.B. , BOWN P. R.
Published: December 2007

The Peterborough Member of the Oxford Clay Formation is organic-rich and contains an abundance of well-preserved vertebrate and invertebrate fossils. A high nutrient input supported a diverse biota. Phytoplankton was exceptionally abundant in the surface water, and formed the basis for an intricate food web in both surface and bottom waters. Top predators include some of the largest known Mesozoic marine reptiles. A giant teleost fish was analogous to modern filter feeding whales and sharks. Benthic faunas depended on organic matter sinking from surface waters, and two parallel food webs may have existed. Trophic partitioning allowed the higher level predators to become diverse, especially the plesiosaurs. Productivity was high in the surface waters, probably high in the mid-water column, and high on the sea floor at times, although benthic diversity may have been reduced due to substrate consistency and/or dysoxia. Bacterial activity within the sediment was also intense. The contribution of organic material to the sea floor was high, but heterotrophic reworking probably reduced the abundance considerably during early diagenesis. A decrease in organic carbon content in the Upper Callovian and Lower Oxfordian parts of the Oxford Clay Formation parallels a decreasing abundance of vertebrate fossils.