bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural histories in the SW Southern Uplands, Scotland
Authors: KNIPE R. J. , CHAMBERLAIN M. I. , PAGE A. , NEEDHAM D. T.
Published: December 2007

The detailed structural histories in the SW part of the Southern Uplands are reviewed. The evolutionary sequences described from the southern part of the Central Belt are characteristic of a fold/thrust fault system which has been affected by strike-slip deformation. The present structure is characterized by tectonic domains usually less than 2 km across strike which show different internal deformation patterns and histories. The generalized deformation history of the area involves downslope movement, thrusting and folding followed by strike-slip deformation. The domain character of the area is considered to have developed by the juxtaposition of blocks throughout this deformation history. The concentration of anastomosing stratal disruption zones (developed by mixing of partially lithified sediment during deformation), and the geometry of folds and faults indicate the presence of an important shear zone towards the base of the Central Belt. A complex relationship between faulting, folding and cleavage development exists within this shear zone. Although cleavage development appears to post-date movement in some disruption zones, there is evidence that the disruption zone displacement was synchronous with or continued after cleavage development. These features indicate that the shear zone had a complex movement history spanning the lithification of the sediments. The structural histories described are compatible with an accretionary wedge model for the area although the nature of the basement which underlay the Southern Uplands during its evolution cannot be assessed from the outcrop-scale structures.
A cryptic Caledonian flysch terrane in Scotland
Authors: HAUGHTON P. D. W.
Published: December 2007

A substantial part of the Lower Old Red Sandstone Crawton Group in the north-east Midland Valley of Scotland has a first cycle provenance from the south and east, i.e. from a source block within the Midland Valley. From the clast assemblages in proximal conglomerates, it is clear that this provenance comprised a flysch terrane cut by granodioritic and tonalitic plutons. Angular clasts of lithic arenite and metagreywacke derived from this cryptic flysch sequence show it to be distinct from a metasedimentary sequence which flanked the Lower Old Red Sandstone basin to the north and north-east. The basin must overlie, or lie adjacent to, a boundary between two quite different sedimentary assemblages. Petrographic examination and chemical analysis of the clasts show that the southerly cryptic succession includes an important mafic/ultramanc provenance, implying that ophiolitic source rocks may have originally contributed to it. Sr and Nd isotopic compositions for the clasts establish that the Midland Valley flysch is isotopically unrelated to either Southern Uplands shales and greywackes or Dalradian metasedimentary rocks. The clasts have lower Sr contents than either suite, and Sr isotopic compositions (
87
Sr/
86
Sr in the range 0.7099-0.7176 at 450 Ma) which are more radiogenic than the Southern Uplands, but less radiogenic than the Dalradian. Crustal residence ages
(t

DM

)
have a narrow range of 1.76-1.94 Ga. The mean of 1.87 ± 0.06 Ga compares with a mean residence age of 1.49 ± 0.21 Ga for Southern Uplands greywackes and shales. Mixing calculations demonstrate that the incorporation of young ophiolite-derived basaltic detritus into the Caledonian sedimentary mass can explain properties of the Nd data previously attributed to a bimodal Proterozoic provenance. The distinct compositions of the Southern Uplands and Midland Valley flysch sediments suggest they originally tapped different parts of the Laurentian margin and have subsequently been brought together structurally as displaced fore-arc slivers. The diverse isotopic compositions of outboard flysch terranes has implications for the nature of the sedimentary component incorporated in inboard magma suites.
A basin model for the Silurian of the Midland Valley of Scotland and Ireland
Authors: WILLIAMS D. M. , HARPER D. A. T.
Published: December 2007

Identification of terranes is frequently based on the apparent disparity of source area for sediment supply, palaeontological evidence and the presence of faults whose sense and amount of displacement is sometimes conjectural. As an example of such superficially unrelated stratigraphies, the Silurian successions of the Midland Valley of Scotland and the northern part of Ireland are compared. A diachronous late Llandovery (early-middle Telychian) transgressive episode, which is earlier in the Midland Valley inliers, is common to many of the successions. The transgression resulted in the development of comparable sedimentary facies and fossil assemblages. The clast compositions of Llandovery and Wenlock conglomerates suggest erosion of a common provenance of volcanic rocks founded on a metaquartzite basement. Areas of active volcanism appear to have migrated in a north-westerly direction with time; a similar sequence is evident in Canada. It is suggested that the common features of the Scottish-Irish successions may be accommodated in a unified basin model for Silurian sedimentation and volcanism. Such a basin may have formed in an intra-arc environment and been controlled by oblique-slip fault mechanisms although evidence for the presence of a Silurian arc is equivocal.
Tectonic controls of Ordovician arc and marginal basin volcanism in Wales
Authors: KOKELAAR P.
Published: December 2007

Ensialic destructive plate margin volcanism persisted in Wales from late Tremadoc to Caradoc times. Late Tremadoc uplift and erosion in NW Wales is attributable to onset of subduction and development of a frontal ridge system (fore-arc). South-east of this, in the vicinity of the Harlech Dome and in SW Wales, more localized uplift was associated with the development of arc volcanoes at a volcanic front. In North Wales the arc volcanism occurred in a regime of E-W crustal extension and was centred on a deep crustal discontinuity. During Arenig times the arc terrane was transgressed. Subsequently, marine conditions, with local and temporary emergence, persisted through Arenig to Caradoc episodes of marginal basin volcanism. While there was widespread and persistent subsidence in Wales during development of the marginal basin, the volcanism occurred mainly along complex and relatively narrow grabens which were sites of repeated channelling of magmas and pronounced subsidence. The grabens mark the sites of deep and steep crustal discontinuities (‘fractures’), one of which controlled the earlier arc volcanism. Changing sites of volcanism in Wales can now be seen as due to transfer of active extension from one fracture to another. There is evidence that the fractures were present by Tremadoc times, and they almost certainly formed before development of the Cambrian marine rift-basin. They are most likely to have been strike-slip faults or shear zones, and their pattern in Wales indicates that they may represent part of a complicated deep-crustal strike-slip duplex system. The volcanically active grabens of the marginal basin are considered to reflect upward propagation of splay faults (flower structure) which channelled magmas through the cover during extension across the fractures. The fractures and associated grabens strongly influenced the location and orientation of zones of relatively high strain that developed during Late Caledonian compression and transpression.

In North Wales the volcanically active grabens trended
broadly
N-S, oblique to the exposed NE-SW basement tectonic grain, and dominantly E-W extension seems to have persisted throughout the Ordovician volcanism. While there is considerable uncertainty concerning Ordovìcian plate tectonic configurations and displacements, the extension is best explained by invoking a N-S convergence vector between the oceanic and over-riding microcontinental plates, with resultant sinistral shear stress in the latter and sinistral transtensional displacements across the arc and marginal basin. The model highlights the suspect nature of the relationship between the Welsh Ordovician terrane and others to the north.
Sealed microcracks in the Lewisian of NW Scotland: a record of 2 billion years of fluid circulation
Authors: HAY S. J. , HALL J. , SIMMONS G. , RUSSELL M. J.
Published: December 2007

Examination of a suite of Lewisian gneisses from NW Scotland has revealed the sequential development of microcrack networks sealed by minerals deposited from fluids passing along them. The sealants were deposited during periods of both localized and regional fluid permeation through the gneisses. The study has shown that both microcracks and shear zones may remain open as permeable pathways over several fluid permeation phases; and that an open porosity within crystalline rock may be maintained, between permeation events, by partially sealed microcracks. The separate microcrack sets, identified by their mineral sealing phases have widespread regional distributions.

Several magnetite sealing events, specific to lithologies and structures, occurred during pre-Torridonian times associated with the Scourian and Laxfordian metamorphic cycles.

Four main post-Laxfordian regional sealing events have been identified, characterized by the mineral suites calcite + K-feldspar; prehnite + albite + calcite; pumpellyite + quartz + calcite; and stilpnomelane, respectively. Of these, calcite + K-feldspar is pre-, or earliest, Torridonian. Prehnite + albite + calcite, and pumpellyite + quartz + calcite are associated with Torridonian events: a model of a downward permeating hydrothermal system involving non-marine ground water is envisaged to account for their formation at about 1000–800 Ma, before the deposition of the Applecross Formation Later stilpnomelane, along with less well developed K-feldspar; barite; and pyrite, may either be a result of this system or perhaps a later one associated with the Cambrian marine transgression of the craton at about 600 Ma.
A new palaeogeographic reconstruction for the Middle Jurassic of the northern North Sea
Authors: RICHARDS P. C. , BROWN S. , DEAN J. M. , ANDERTON R.
Published: December 2007

An estuarine sequence equivalent in age to the Brent Group deltaic deposits further north is recognized in the central Viking Graben. It indicates a persistent marine connection southwards from the Boreal Ocean during deltaic accumulation, and suggests that the Brent delta did not prograde from the south, along the graben as previously suggested. Evidence from the UK north Viking Graben indicates the Brent delta was probably derived transversely rather than axially, as were the other Jurassic clastic systems infilling the graben.
A search for ancient detrital zircons in Zimbabwean sediments
Authors: DODSON M. H. , COMPSTON W. , WILLIAMS I. S. , WILSON J. F.
Published: December 2007

86 detrital zircon grains from Archaean quartzites from the Mweza and Shurugwi greenstone belts have been dated by the ion probe SHRIMP. Nine of these give ages of 3.75–3.80 Ga, older than any Zimbabwean rocks so far satisfactorily dated, and the Tokwe-Zvishavane gneisses are a possible source for these old grains. However, none are comparable in age with the 4.2 Ga zircons discovered in Western Australia. If such old grains exist in the sample it is unlikely (P>0.05) that their abundance could exceed 3%. Ages of the younger grains are broadly consistent with the presumed age of the quartzites sampled, though some high Th/U grains in the Shurugwi sample may be anomalously young.
Middle Jurassic within-plate granites in West Antarctica and their bearing on the break-up of Gondwanaland
Authors: STOREY B. C. , HOLE M. J. , PANKHURST R. J. , MILLAR I. L. , VENNUM W.
Published: December 2007

Five post-tectonic granitic plutons isolated within the central Ellsworth-Whitmore mountains crustal block in West Antarctica form a distinctive geochemical suite. All have some characteristics of S-type granites and are atypical of active continental margins. They range in composition from a within-plate granite (WPG) end member, with the lowest
87
Sr/
86
Sr initial ratio (0.707), to granites with a much more marked crustal signature and high initial ratios (0.722). The granitic suite was emplaced over a restricted Middle Jurassic time interval at the same time as the extensive Ferrar-Karoo-Tasman mafic suite and just prior to the disintegration of the supercontinent Gondwanaland. Petrogenetic modelling suggests that the WPG end member could have been derived entirely by differentiation of the enriched mantle-derived Ferrar magma, and the end member with the highest initial ratio by partial melting of a crustal source. Low initial
143
Nd/
144
Nd ratios and Proterozoic model ages are compatible with a Precambrian crustal component but may alternatively, as in the case of Ferrar Supergroup magmas, reflect partial inheritance from enriched lithospheric mantle geochemically coupled to the lower crust since Precambrian differentiation. Data from these granites are consistent with large-scale underplating of mafic magma and crustal melting in response to a thermal disturbance in the Gondwanaland lithosphere related in some way to break-up of the supercontinent.
Periodicity in extinction and the problem of catastrophism in the history of life
Authors: SEPKOSKI J. JOHN
Published: December 2007

The hypothesis that extinction events have recurred periodically over the last quarter billion years is greatly strengthened by new data on the stratigraphic ranges of marine animal genera. In the interval from the Permian to Recent, these data encompass some 13,000 generic extinctions, providing a more sensitive indicator of species-level extinctions than previously used familial data. Extinction time series computed from the generic data display nine strong peaks that are nearly uniformly spaced at 26 Ma intervals over the last 270 Ma. Most of these peaks correspond to extinction events recognized in more detailed, if limited, biostratigraphic studies. These new data weaken or negate most arguments against periodicity, which have involved criticisms of the taxonomic data base, sampling intervals, chronometric time scales, and statistical methods used in previous analyses. The criticisms are reviewed in some detail and various new calculations and simulations, including one assessing the effects of paraphyletic taxa, are presented. Although the new data strengthen the case for periodicity, they offer little new insight into the driving mechanism behind the pattern. However, they do suggest that many of the periodic events may not have been catastrophic, occurring instead over several stratigraphic stages or substages.
A geophysical investigation of post-Alpine granites and Tertiary sedimentary basins in northern Greece
Authors: MALTEZOU F. , BROOKS M.
Published: December 2007

The geology of the Rhodope region of northern Greece consists of crystalline basement rocks of the Rhodope Massif in which Tertiary extensional basins and granitic intrusions of Oligocene age occur widely.

Geophysical modelling has highlighted the close spatial relationship of the Tertiary sedimentary basins and post-tectonic granites of closely similar age. Geochemical data for selected granites are used to support the suggestion that there is a close genetic relationship between the episode of granite intrusion and the tectonic development of extensional basins.
Palaeomagnetism of Permo-Triassic rocks in the Iberian Cordillera, Spain: acquisition of Secondary and characteristic remanence
Authors: TURNER P. , TURNER A. , RAMOS A. , SOPEÑA A.
Published: December 2007

Magnetostratigraphy is widely used as a tool for stratigraphical correlation but there are problems in its application to rocks which have suffered complex diagenesis, particularly sandy, continental red beds. In such cases the complete magnetic record should be studied in relation to its depositional, diagenetic and structural history. To this end a detailed palaeomagnetic study has been made of Permian and Triassic rocks in the northern Iberian Cordillera, Spain. Four stratigraphical units comprising the Autunian, 'Saxonian', Buntsandstein and Muschelkalk have been studied in a single section near Molina de Aragon which is well-documented sedimentologically. Autunian rocks have a characteristic remanence which is exclusively reversed and corresponds to the Kiaman Interval of Lower Permian age. A pole position (49 N, 167 W) is consistent with previous studies which indicate the rotation of Iberia in post-Triassic times relative to the stable European plate. The Saxonian facies show complex, multicomponent magnetizations; no polarity zonation can be resolved but individual components were acquired prior to the rotation of Iberia.

The Buntsandstein is remarkable in that much of it is completely remagnetized in a direction similar to the present local geomagnetic direction. This secondary remanence is carried by fine grained hematite which has been formed as a result of carbonate dissolution and other diagenetic reactions. Initially, these iron oxides were probably superparamagnetic and remanence was only blocked-in by structural inversion of the Iberian Cordillera in Tertiary times and after rotation of the Iberian plate. Some of the primary remanence is preserved in some fine grained lithologies of the Buntsandstein, indicating that normal and reversed zones of magnetization were originally present. The magnetization of the Muschelkalk carbonates is also complex; secondary components similar to those of the Buntsandstein are present but there is evidence that primary components were mainly normal during Karnian times.
Phototrophy, diazotrophy and palaeoatmospheres: biological catalysis and the H,. C, N and O cycles
Authors: RAVEN J. A. , SPRENT J. I.
Published: December 2007

Photolithotrophy (use of light as energy source, CO
2
, as carbon source) and then diazotrophy (use of N
2
, as N source), became important as prebiotically-synthesized organic compounds were consumed.
13
C/
12
C data suggest that ribulose bisphosphate carboxylase-oxygenase has dominated photolithotrophic CO
2
, fixation over the last 3.5 Ga. The long-term decline in atmospheric CO
2
, can be related to carbon isotope evidence for a CO, accumulating mechanism in aquatic plants over the last 100 Ma and of C
4
, metabolism in terrestrial plants over 10 Ma. Increasing biogenic O
2
, levels permitted, via an O
3
, UV screen, photolithotrophs to grow in high-light environments and, via O
2
, availability for respiration and biosyntheses, to produce large and (on land) homoiohydric plants. This greatly increased the productivity and diversity of photolithotrophy. Diazotrophy is less strongly coupled to homoiohydry and to photochemistry than is CO
2
, fixation. Symbiotic N
2
, fixation in land plants partially effects these two couplings, and permits novel methods of dealing with O
2
, inactivation of nitrogenase.
Some ecological implications of palaeoatmospheric variations
Authors: MOORE P. D.
Published: December 2007

The past composition of the atmosphere is gradually being elucidated with increasing precision. But advances in the understanding of the ecological and evolutionary implications of atmospheric changes are not keeping pace. A graphic model is presented here to indicate the complexity of interacting forces resulting from a change in atmospheric carbon dioxide level, and it can be seen that any resultant ecological outcome will be influenced by a series of (sometimes opposing) factors. This is particularly true of the hydrological consequences of atmospheric change and hydrology will undoubtedly prove a major influence on vegetation.

Changes in the levels of both atmospheric carbon dioxide and oxygen have had profound effects on the evolution of plant life by modifying the selective pressures favouring alternative systems of carbon assimilation. These photosynthetic strategies are also closely related to local hydrology. More ecological and physiological information is clearly needed before the biological consequences of Variations in the atmosphere of the past can be appreciated.
Short Paper: Ophiolitic detritus in the Ordovician sediments of South Mayo, Ireland
Authors: WRAFTER' JOHN P. , GRAHAM JOHN R.
Published: December 2007

Sandstones from Ordovician sediments of the South Mayo Trough, western Irish Caledonides, show marked stratigraphic variation in their major and trace element geochemistry. High Cr, Ni, and Mg concentrations of ultramafic derivation succeed high Ti and Fe concentrations of mafic provenance This sequence is interpreted as the result of the progressive unroofing of an ophiolite. A possible sedimentary linkage with ophiolite lithologies in the Clew Bay Domain to the north would indicate minimal post-Arenig transcurrent displacement. The South Mayo flysch with its ophiolitic detritus is typical of Caledonian flysch sequences which pre-date the mid-Ordovician Taconic deformation, but differs in character from those of late Ordovician age.
Swaley cross-stratification produced by unidirectional flows, Bencliff Grit (Upper Jurassic), Dorset, UK
Authors: ALLEN PHILIP A. , UNDERHILL JOHN R.
Published: December 2007

Unusual bedforms occur in the Bencliff Grit at Osmington Mills, Dorset. The geometry of cross-strata in stacked, predominantly fine-grained sandstone units is dominated by low-angle troughs with (locally) perfectly-preserved antiforms which wedge-out onto climbing low-angle erosion surfaces. Many of the antiforms (hummocks) and troughs (swales) appear to have grown at similar rates of sediment fall-out. Others show draped antiforms and thickening into swales. In both cases laminae are concordant with the lower bounding surfaces of the cross-sets, demonstrating that vertical accretion was the dominant process. This geometry together with the conspicuous absence of angle-of-repose cross-strata give the bedforms a resemblance to amalgamated hummocky cross-stratification or swaley cross-stratification. Many workers consider amalgamated hummocky cross-stratification and swaley cross-stratification to be identified with storm sedimentation on the shelf to shoreface, thereby bracketting the structures both in terms of process and environment. However, the structures in the Bencliff Grit suggest that such a precise interpretation of amalgamated hummocky cross-stratification or swaley cross-stratification is inadvisable. The geometry of the cross-stratification suggests deposition under a highly sediment charged but essentially unidirectional flow. This study therefore supports recent views that hummocky and swaley styles of cross-stratification are in reality a form of low-angle trough cross-stratification produced primarily by unidirectional flows. Clearly, since undulatory forms of stratification are substrate- and process-dependent and
not
environmentally-specific, great care must be taken in the interpretation of palaeoenvironmental settings, especially where exposures are limited, as for instance in borehole cores. It is unlikely that core data can be used to identify bedforms analogous to hummocky cross-stratification or swaley cross-stratification unless boreholes are highly deviated.
Diagenesis of the Kimmeridge Clay Formation, onshore UK
Authors: SCOTCHMAN I. C.
Published: December 2007

Study of sedimentology, carbonate geochemistry and clay mineralogy of four laterally continuous beds from the outcrop of the Kimmeridge Clay Formation indicates strong lateral variations in the depositional and early diagenetic processes, reflecting the basin and swell topography of the depositional basin.

The mudstones from the swell environment with low sedimentation and subsidence rates are shelly and carbonate–rich with thin, poorly developed oil shales. They were deposited under dysaerobic conditions with consequent poor organic-matter preservation. Early diagenetic processes were limited to the sulphate reduction and methanogenesis zones with the formation of discrete horizons of concretions. The shallow burial depths attained are reflected by the immaturity of the organic matter in the mudstones although ordered illite-smectites are present.

Sediments from other locations comprise black, organic-rich mudstones, the thick beds and good organic-matter preservation indicating deposition under high sedimentation and subsidence rates with anoxic bottom waters in a basinal environment. These indurated mudstones contain a few shell fragments and much pyrite, indicating intense reaction in the sulphate reduction zone. Carbonates were precipitated in the methanogenesis and decarboxylation zones as ferroan calcites and dolomites. The much greater burial depths are indicated by the higher maturity of the organic matter and by the ordered nature of the illite–smectites and the loss of kaolinite.
The biotic response to the late Cenomanian oceanic anoxic event; integrated evidence from Dover, SE England
Authors: LEARY P. N. , CARSON G. A. , COOPER M. K. E. , HART M. B. , HORNE D. , JARVIS I. , ROSENFELD A. , TOCHER B. A.
Published: December 2007

The late Cenomanian sea-level rise was coupled with an expansion of the oxygen-minimum zone. The position of its upper surface may be discerned from the sequential effects on the benthonic and planktonic biota.
An oxygen-minimum palaeoceanographic signal from Early Toarcian cavity fills
Authors: METTRAUX MONIQUE , WEISSERT HELMUT , HOMEWOOD PETER
Published: December 2007

Moderately deep-marine Toarcian sedimentary rocks of the Médianes Nappe (Alpine Front Ranges, Switzerland) comprise calcarenites and lime-mudstones. They were deposited under storm and fairweather conditions, in the dysaerobic zone, near the storm wave base. These remnants of the northern margin of the Tethyan Ocean show facies where primary sediment ('deep water' stromatolitic micrite) was reworked during early diagenesis. Stromatactis-like cavities were formed by carbonate dissolution near the sediment-water interface. Suboxic early diagenetic conditions are indicated by corroded pyrite and newly formed glauconitic oncoids. Carbon and oxygen stable isotope data indicate cavity stabilization and cementation above the sulphate-reducing zone. Oceanographic conditions during the time of cavity development are reconstructed from the carbon and oxygen isotope composition of cavity cements, of bulk sediment and of associated ammonites.δ
13
C values of up to +4‰ are related to the Oceanic Anoxic Event of the Early Toarcian and oxygen-isotope values from cements and ammonites define a shallow Toarcian thermocline (<100m) and surface-water temperatures up to 25°C.
Short Paper: Seismic reflection evidence for the location of the Iapetus suture west of Ireland
Authors: KLEMPERER SIMON L.
Published: December 2007

A new deep seismic profile recorded west of Ireland by the British Institutions’ Reflection Profiling Syndicate (BIRPS) images prominent north-dipping reflectors west of the Shannon estuary. Qualitatively similar north-dipping reflectors have been imaged on previous BIRPS profiles recorded across the Solway Line both east and west of Britain, and have there been interpreted as structures related to the Iapetus suture. By analogy with these other north-dipping reflectors, and because the newly discovered reflectors lie close to previously hypothesized locations of the Iapetus suture, the structures seen on the new BIRPS profile are here interpreted as representing part of the Iapetus suture zone.
Short Paper: Palaeontological constraints on the definition and development of Irish Caledonide terranes
Authors: HARPER D. A. T. , PARKES M. A.
Published: December 2007

The Irish Caledomde terranes occupied a variety of sites marginal to the cratons of Laurentia, Baltica and Gondwana. Palaeontological data provide age constraints on the timing of sedimentary linkage and the age of overlap sequences. The Northwestern terrane is characterized by the development of Toquima–Table Head faunas in the early Ordovician in contrast to the Celtic faunas of the Bellewstown and Rosslare terranes. During the middle Ordovician the Northwestern terrane and the Scottish equivalent of the Central terrane were occupied by the Scoto–Appalachian fauna. The Leinster terrane was dominated by Anglo-Welsh elements with varying proportions of the Scoto-Appalachian and Baltic faunas across the terrane. In the Iapetus suture zone in the east of Ireland the Grangegeeth terrane is characterized by a predominantly Scoto-Appalachian fauna.
Petrography, geochemistry and origin of the NW Jordan phosphorites
Authors: ABED ABDULKADER M. , AL-AGHA MOHAMMED R.
Published: December 2007

Phosphorites from NW Jordan were examined petrographically and chemically. Their mineralogy is basically simple and includes apatite (francolite), carbonate (mainly micritic calcite), some dolomite, and minor clay minerals and organic matter. These mineral phases control the chemistry. The phosphorite deposits are made up of several depositional cycles that indicate shallowing and show decreasing particle size. They are believed to have formed by authigenic precipitation followed by reworking and redeposition in the inner parts of the basin, where only minor amounts of micrite and clay were being deposited.
Lower crustal heterogeneity beneath Britain from deep seismic reflection data
Authors: CHADWICK R. A. , PHARAOH T. C. , SMITH N. J. P.
Published: December 2007

450 km of deep seismic reflection data have been acquired over the UK landmass. A qualitative assessment of lower crustal reflectivity (based on the amplitude and continuity of deep crustal events) has been made. In the cratonic area of central England the lower crust is generally poorly reflective, while in the foldbelt terrains of southern, eastern and northern England it is more strongly reflective. Beneath the Variscides of southern England an important distinction can be made between poorly reflective ‘foreland-type’ lower crust lying beneath the Variscan Front thrust and strongly reflective ‘orogenic’ lower crust farther south. A brief review is given of hypotheses for the origin of lower crustal reflectivity, and the relationship of the latter to crustal tectonic regime, both in the UK and elsewhere. It is concluded that the types of lower crustal heterogeneity required to produce the observed reflections are more easily generated in orogenic provinces (fold/thrust/metamorphic belts) than in areas which have undergone only low to moderate crustal extension (beneath typical sedimentary basins). The generation of granites by partial melting within the lower crust, and processes of metamorphic differentiation and recrystallization which accompany orogenic activity, are likely to play a significant role in the development of such heterogeneities. In addition, post-orogenic processes of magmatic underplating and low-viscosity ordering may also have played a part.
Stable isotopes in ore genetic studies
Authors: MANNING D. A. C.
Published: December 2007

A thematic session on the use of stable isotopes in the study of mineral deposits was held as part of the annual meeting of the Mineral Deposits Studies Group in Newcastle upon Tyne on 14th December 1987. Five of the papers presented at the meeting are published here, together with abstracts for six other contributions.

The thematic session consisted of 15 papers, with invited contributions from D. T. Rickard and S. M. F. Sheppard. In order of presentation, S. H. Bottrell (University of Leeds) discussed the results of a stable isotope study of the Dolgellau black shale hosted gold mineralization, and demonstrated that the ore fluids were probably derived from an external igneous source, with considerable modification of isotope ratios by interaction with the host rock. P. Eakin (Queen's University of Belfast) then examined evidence for
13
C isotopic fractionation as a consequence of radioactivity within uraniferous hydrocarbons, comparing data for samples from the North Wales orefield with others from Scandinavia and South Africa. This study showed that fractionation effects in excess of 10% may be caused by this process, requiring caution when interpreting C isotope data for organic matter from uraniferous mineral deposits. The following two contributions introduced two recently developed techniques to those within the audience who were not specialist isotope geochemists. S. P. Kelley (with A. J. Boyce, A. E. Fallick, P. J. Hamilton and R. M. McIntyre; SURRC, East Kilbride) described the use of laser microprobe extraction techniques which allow small samples to be studied without the
Short Paper: The extent of Grampian orogenic activity in the Scottish Highlands
Authors: LINDSAY N. G. , HASELOCK P. J. , HARRIS A. L.
Published: December 2007

Structures in Atholl nappe Grampian Division rocks in the Tayside Region can be traced NW into the supposedly older Central Highland Division in Speyside and Strath Nairn where they are the earliest fabrics present. Grampian Division/Central Highland Division contrasts appear to involve a gradational metamorphic change, and the existence of the Grampian slide supposed, by previous workers to separate the two units has not been confirmed. Atholl nappe fabrics have been shown to be coeval with those of Grampian orogenesis in the Tay nappe where their age is constrained by the 590 ± 2 Ma Ben Vuirich granite. Grampian orogenic structures can thus be traced, as the earliest structures present, as far as the Great Glen fault, a conclusion which may have a bearing on the significance of the fault as a terrane boundary.

Central Highlands.
The metamorphic rocks of the Scottish Central Highlands (Fig. 1) comprise units referred to the Southern Highland (youngest), Argyll, Appin and Grampian (oldest) groups. While Harris
et al.
1978 regarded the Grampian Group as the oldest Dalradian group, it is broadly equivalent to the Grampian Division (Moine) (GD) of
Piasecki (1980)
who also identified the Central Highland Division (Moine) (CHD) which occupies several hundred square kilometres of Inverness-shire east and south of Inverness and which forms three other small inliers in Speyside, not distinguished on Fig. 1. Working largely in Speyside and adjacent areas, Piasecki & van Breemen (1983) regarded the CHD as lithostratigraphically broadly equivalent to Moine rocks of the N Highlands and
Catastrophic extinctions and the inevitability of the improbable
Authors: HSÜ KENNETH J.
Published: December 2007

The Earth as we know it today has been shaped by natural processes of the past. Uniformitarianism emphasizes the cumulative effects of processes operating at uniform rates. Catastrophists single out the importance of rare and unusual events. This article proposes a scale to measure the magnitude of rare events, and cites the inverse magnitude/frequency relation of naturally occurring events to formulate the philosophical basis for actualistic catastrophism. Mass extinction at the end of the Cretaceous is held up as an example that a catastrophe such a bolide impact could influence the course of biologic evolution. Improbability of such an event is no argument against such a hypothesis, because the improbable, even those with odds of once every billion years, must have taken place a few times in the geological history. The immensity of geological time is not only important because of the immensity of the cumulative effects, but also because the long duration provides a chance for the improbable to take place
The structure and evolution of the Northumberland Trough from new seismic reflection data and its bearing on modes of continental extension
Authors: KIMBELL G. S. , CHADWICK R. A. , HOLLIDAY D. W. , WERNGREN O. C.
Published: December 2007

An 18 km long N–S seismic reflection profile has been acquired across the faulted boundary between the Alston Block and the Northumberland Trough. A NW–SE geological cross-section has been constructed across the trough by integrating this line with other data, and a Tricentrol seismic profile confirms the suitability of this model. The trough has a markedly asymmetric form, with a thickness of more than 4.2 km of Dinantian strata adjacent to its faulted southern margin. The present day surface faults (Stublick, Ninety Fathom) are related to Variscan and later inversion and transpression, and do not everywhere correspond precisely to the earlier syn-depositional normal faults. The basin is believed to have formed in response to reactivation of the Iapetus Suture in a dominantly N–S extensional stress field. Basin evolution has been analysed by fault restorations and subsidence modelling. The fault restorations indicate an upper crustal extension factor of 1.15 to 1.19. Subsidence modelling indicates a whole crustal extension factor of 1.30, with similar sub-crustal lithospheric extension. It is possible that the difference between the inferred extension factors is due to non-uniform (increasing downwards) lithospheric extension, but the uncertainties inherent in both methods are such that this cannot yet be confirmed.
Structural and sedimentological evolution of part of the west central Spanish Pyrenees in the Late Tertiary
Authors: NICHOLS GARY
Published: December 2007

During the late Oligocene and early Miocene, the frontal thrust complex of one of the south Pyrenean thrust sheets (the Guarga or Gavarnie sheet) became emergent to form part of the northern margin of the Ebro Basin. The principal structures in the frontal thrust complex show a progressive development from a simple recumbent fold in the west to an emergent thrust in the east. Southward movement at the thrust front decreased from east to west and in the west the thrust tip line is buried. These structures influenced Miocene continental sedimentation on two scales. The emergent thrust front formed an orographic barrier to rivers draining the higher Pyrenees. These rivers passed around the barrier rather than across it, and the positions of the apices of fluvial distributary systems in the Ebro Basin were thus controlled by the low points in the structure of the thrust front. On a smaller scale, sedimentary sequences and syn-depositional structures in small alluvial fan bodies at the basin margin were controlled by late thrust movements in the adjacent frontal thrust complex. Syn-depositional faults, folds and unconformities in the alluvial fan conglomerate sequences can be related to movements on individual faults within the thrust complex.
A geophysical study of the Vardar Zone ophiolite belt in Chalkidhiki, northern Greece
Authors: KIRIAKIDIS L. , BROOKS M.
Published: December 2007

The structure of the Chalkidhiki ophiolite belt, located at the border between the Vardar Zone and the Serbomacedonian Massif of the Internal Hellenides, has been investigated by gravity and magnetic methods. New density measurements and pre-existing magnetic susceptibility measurements have been used to constrain geophysical models of subsurface structure. The northern part of the ophiolite belt, which crops out locally, is interpreted as a shallow body extending to a depth of about 2.5 km and having a width of about 10–15 km. The concealed extension of the ophiolite belt to the south, under the sediments of Toroneos Gulf, is interpreted as a massive body up to 35 km wide and 8-10 km thick. The ophiolite belt represents a major allochthonous tectonic unit overlying a continental crustal basement. Its overall subsurface geometry may be explained by the early obduction of an ophiolite slab with major thickness variations along strike, modified by later Alpine thrusting.
A decade of phosphorite research and development
Authors: NOTHOLT A. J. G. , JARVIS I.
Published: December 2007

Report of the eleventh (and final) International Field Workshop and Symposium of Project 156: Phosphorites, organized by A. J. G. Notholt, National Correspondent, 1. Jarvis, and W. C. Burnett (Florida State University) and held at Hertford College, Oxford, England, 5–8 September, 1988. The meeting attracted over 70 delegates and speakers representing 14 countries. It consisted of 41 talks and 2 poster presentations covering a wide range of phosphorite research. A selection of papers will be published by the Geological Society as a Special Publication.

Project 156: Phosphorites is one of the most successful and probably the largest of long-term, interdisciplinary geological research projects supported by the International Geological Correlation Programme (IGCP). It owes its origin to the hypothesis by P. J. Cook, currently of the Bureau of Mineral Resources, Geology and Geophysics, Canberra, that a major Proterozoic-Cambrian phosphogenic province extends through the Asian and Australian region, perhaps comparable in size with the late Cretaceous-Eocene Tethyan province of northern Africa and the Middle East. In his Keynote Address, Dr Cook recalled that although Project 156 was established in 1977 as a research programme on Proterozoic-Cambrian phosphorites of Asia and Australia, such had been the ensuing international interest that three other Working Groups were formed within the Project, devoted respectively to an International Phosphate Resource Data Base; Young Phosphogenic Systems; and Cretaceous-Eocene Phosphorites. All four working groups had generated much research in the Project’s lifetime, a reflection also of the great scientific interest in phosphorites by virtue of their complex petrography and
The structure of the Bracco Ophiolite complex (Ligurian Apennines, Italy): a change from Alpine to Apennine polarity
Authors: HOOGERDUIJN STRATING E. H. , VAN WAMEL W. A.
Published: December 2007

The Bracco ophiolite complex, exposed in the area studied as the Bracco Subunit of the Melange Nappe, is tectonically intercalated between the underlying Canetolo Nappe and the overlying Lavagna Nappe. Detailed structural analysis and mapping in these units reveals a multi-phase deformation history. In the Bracco Subunit and the Lavagna Nappe, this history is characterized by a change, from SW (i.e. ‘Alpine’) directed folding of the ophiolitic basement and its sedimentary cover, to NE (i.e. ‘Apennine’) directed folds and thrusts. The Canetolo Nappe only shows Apennine vergent structures.

The Alpine vergent structures are considered to be developed during the Late Cretaceous to Palaeocene in an accretionary wedge associated with an east-dipping subduction zone. The first Apennine directed structures are low-angle extensional faults and cascading folds, indicating that the change to Apennine polarity resulted from gravitational collapse of the accretionary wedge during the Palaeocene to Eocene. Gravitational instability is considered to have resulted from a drastic decrease of subduction rate during the Palaeocene, which has led to extension and uplift of the wedge. The rapid decompression under isothermal to slightly decreasing temperature conditions of the high-pressure terrains in the Voltri Massif concurs well with this tectonic scenario.
GLORIA: the second decade
Authors: FRANCIS T. J. G.
Published: December 2007

Report of a meeting jointly sponsored by the Marine Studies Group of the Geological Society and the Challenger Society for Marine Science held at Burlington House on
1
February
1989.

T. J. G. Francis,
the convenor, opened the meeting by briefly summarizing the development of the GLORIA (Geological Long Range Inclined Asdic) long range sidescan sonar at the Institute of Oceanographic Sciences since the mid-nineteen sixties. GLORIA has established itself in the nineteen eighties as the ideal tool for surveying Exclusive Economic Zones (EEZs). GLORIA surveys were carried out for 241 days in 1988 from two different ships, 140 of which were spent surveying the United States EEZ with the U.S. Geological Survey. By the end of 1988, some 13 million km
2
of ocean floor has been surveyed, which is about 4% of the ocean deeper than 200m. The scale of USGS activity with GLORIA was equivalent to all UK deep Ocean geology and geophysics.

R. C. Searle
(IOSDL, Wormley) spoke on the contribution of GLORIA to understanding mid-oceanic ridge tectonics. After reviewing a wide range of observations from the Atlantic, Pacific and Indian Oceans, he reported the results of the recent survey of the whole of the Easter Microplate camed out from RRS Charles Darwin last November. This revealed for the first time the complex plate boundaries, spreading fabric and associated propagating rifts and transform faults of the whole of this unusual feature.

D. Twichell
(USGS, Woods Hole) spoke about the integration of digital bathymetry (SEABEAM) and sidescan (GLORIA) data with particular reference to the Gulf of Mexico. Gridding of the two data sets was done on the USGS Mini Image Processing System (MIPS). Since the sidescan backscatter
Kaluwe carbonatite, Zambia: an alternative to natrocarbonatite
Authors: NGWENYA B. T. , BAILEY D. K.
Published: December 2007

Fine grained carbonatite in the Kaluwe complex has been inferred, on textural grounds, to be calcified natrocarbonatite. Electron microprobe analysis, however, shows that the brown laths thought to be pseudo-nyerereite are dolomitic, with Sr values much higher than normal in low-temperature carbonates. This chemistry effectively rules out near-surface replacement of an original natrocarbonatite, and the compositions must closely reflect that of the magma from which they crystallized. The observed fabrics can be interpreted in terms of the system CaO–MgO–FeO–CO
2
, where CaO:MgO and cooling history have largely determined the crystallization paths.

Carbonatite pedogenesis has long been a controversial topic and there is still debate about how far observed carbonatite compositions represent the magma from which they crystallized. Lava eruptions from Oldoinyo Lengai (
Dawson 1962
) removed any doubt that carbonate melts could exist at the Earth's surface. This lava, composed of the Na-K-Ca minerals nyerereite [(Na
0.82
K
0.18
)
2
Ca(CO
3
)
2
] and gregoryite [(Na
0.78
K
0.05
)
2
(Ca
0.17
CO
3
)] with whole rock Na
2
O up to 30%, is referred to as natrocarbonatite whereas other carbonatites are low in alkalis.

This discovery led to the idea that many older extrusive carbonatites were erupted as natrocarbonatite, from which the alkalis were leached by meteoric water, and the nyerereite and gregoryite pseudomorphed by secondary calcite (

Dawson
et al.
1987

and references therein). Some workers, however, have questioned whether alkali carbonate volcanics could have retained their original structure during total replacement by ground-water calcite (
Keller 1981
; Bailey in
Turner 1988
).

One of the most recent candidates for replacement of original natrocarbonatite is the extensive
Shear zones at the Laxford front, NW Scotland and their significance in the interpretation of lower crustal structure
Authors: COWARD M. P.
Published: December 2007

The Lewisian rocks in the Scourie-Laxford region, NW Scotland, show variations in shear strains and associated structures on the southern (upper) margin of a zone of intense Laxfordian reworking. The Scourian block was thrust a large distance obliquely northwards over the northern Laxfordian gneisses as part of a Laxfordian episode of crustal thickening. This was followed by crustal extension, during which time the Scourian block moved down to the south. Granite sheets were intruded along the main shear zone in a dilational region at the base of a rotational extensional block.

The Laxfordian contractional and extensional deformation occurred under amphibolite facies conditions and analogies can be made with middle crustal structures inferred from deep seismic data. For example, structures produced during late Laxfordian crustal spreading can be compared to those seen on the BIRPS offshore Scottish data, which formed during Devonian basin development following Caledonian crustal thickening.

The Scourie–Laxford structures give an idea of fault kinematics at the brittle-ductile transition in the middle to lower crust. Steep faults pass down into more gently dipping ductile shears and then to a zone of anastomosing shears which accommodate the strains and rotations at the base of the rigid upper crust. The shears may concentrate along a more planar zone of deformation, forming a detachment or decoupling zone, below which the extension may be variable, depending on the mode of lithospheric stretching.