bluuebunny/crossref_metadata_2025 huggingface dataset:
An improved regional structural model of the Upper Carboniferous of the Cleaver Bank High based on 3D seismic interpretation
Authors: Schroot B. M. , De Haan H. B.
Published: April 2008

The use of 4500 km
2
of amalgamated 3D seismic surveys allowed for an improved intra-Carboniferous seismic interpretation of the Dutch Cleaver Bank High, which is part of the Southern North Sea Carboniferous Basin. The observations of faults that were active during the Late Carboniferous are reviewed in the context of what is described in literature about the regional structural framework of the basin. The high quality seismic data show at least three distinct fault trends, namely east-west, NE-SW and NW-SE, active before Rotliegend times. All of these trends are inherited from older existing zones of weakness, and furthermore, all three trends have been reactivated again during the Mesozoic or Cenozoic to some extent. The interpretation of major controlling east-west shear zones that have been reactivated in different senses throughout geological history is the result of careful examination of the data. The dominance of such systems is not obvious on simple fault maps. Their role in the regional plate tectonics fits the model of escape tectonics of the North Sea-Baltic plate. The distinction of different styles of Late Carboniferous normal fault systems results in a better understanding of the different tectonic phases during the period from Westphalian D to Autunian.
New aspects of tectonic stress inversion with reference to the TENSOR program
Authors: Delvaux D. , Sperner B.
Published: April 2008

Analysis of tectonic stress from the inversion of fault kinematic and earthquake focal mechanism data is routinely done using a wide variety of direct inversion, iterative and grid search methods. This paper discusses important aspects and new developments of the stress inversion methodology as the critical evaluation and interpretation of the results. The problems of data selection and separation into subsets, choice of optimization function, and the use of non-fault structural elements in stress inversion (tension, shear and compression fractures) are examined. The classical Right Dihedron method is developed in order to estimate the stress ratio
R
, widen its applicability to compression and tension fractures, and provide a compatibility test for data selection and separation. A new Rotational Optimization procedure for interactive kinematic data separation of fault-slip and focal mechanism data and progressive stress tensor optimization is presented. The quality assessment procedure defined for the World Stress Map project is extended in order to take into account the diversity of orientations of structural data used in the inversion. The range of stress regimes is expressed by a stress regime index
R’
, useful for regional comparisons and mapping. All these aspects have been implemented in a computer program TENSOR, which is introduced briefly. The procedures for determination of stress tensor using these new aspects are described using natural sets of fault-slip and focal mechanism data from the Baikal Rift Zone.
Oil and gas production from basement reservoirs: examples from Indonesia, USA and Venezuela
Authors: Koning Tako
Published: April 2008

Basement rocks are important oil and gas reservoirs in various areas of the world. Such reservoirs include fractured or weathered granites, quartzites, or metamorphics. In South America, basement reservoirs occur in Venezuela and Brazil. In North Africa, basement oil and gas production occurs in Morocco, Libya, Algeria and Egypt. Significant basement reservoirs occur in the West Siberia basin as well as in China. In the USA, basement-derived oil production occurs in a number of areas, including California (Wilmington and Edison fields), Kansas (El Dorado and Orth fields) and Texas (Apco field). In Southeast Asia, basement reservoirs are the main contributor of oil production in Vietnam. In Indonesia, to date oil and gas production from basement rocks has been minimal. However, the recent large gas discovery in pre-Tertiary fractured granites in southern Sumatra has led to a focusing of exploration in Indonesia for basement reservoirs.
A review of the occurrence and origin of abiogenic hydrocarbons in igneous rocks
Authors: Potter J. , Konnerup-Madsen J.
Published: April 2008

Reports on the presence of hydrocarbons in igneous rocks have been on the increase and generating greater interest in the scientific community over the last 20 years. Most of the occurrences are due to the incorporation of organic material into the magmatic systems. However, reports on the presence of hydrocarbons formed by abiogenic processes have also increased in recent years, suggesting that these hydrocarbons may not be as rare as previously thought and may have implications for natural gas resources in the future. This paper reviews these occurences and the models proposed for the generation of these hydrocarbons, in particular the nature of the hydrocarbon-bearing fluids in the alkaline complexes Khibina, Lovozero and Ilímaussaq. The origin of these hydrocarbons remains controversial, whether they are (1) derived directly from the mantle, (2) formed during late crystallization stages by respeciation of a C-O-H fluid below 500°C, or (3) formed during postmagmatic alteration processes involving Fisher-Tropsch type reactions catalysed in the presence of Fe-oxides and silicates. The reports suggest that a direct mantle origin for the hydrocarbon fluid is unlikely. A model involving near-solidus reequilibration of a C-O-H fluid to a CH
4
-rich composition is possible, although only for extreme melt compositions that have large crystallization temperature ranges (i.e. hyperagpaitic melts). The Fischer-Tropsch synthesis of hydrocarbons in igneous rocks seems to be a more applicable model for a wide variety of igneous rocks.
Fracture-dominated flow in the Borrowdale Volcanic Group at Sellafield, NW England: the identification of potential flowing features, development of conceptual models and derivation of effective parameters
Authors: Degnan P. J. , Littleboy A. K. , Michie U. McL. , Jackson C. P. , Watson S. P.
Published: April 2008

Between 1989 and 1997 United Kingdom Nirex Limited (Nirex) studied in detail the geology and hydrogeology of a rock volume near Sellafield in Cumbria, NW England. The aim of the study was to determine the suitability, or otherwise, of the site as the location for a deep repository for intermediate-level and certain low-level radioactive wastes. An important factor in determining site suitability was the nature of groundwater flow in the potential repository host rock, the Borrowdale Volcanic Group. In the host rock, interpretation of borehole core, wireline logs and hydrogeological pumping test data indicated that groundwater flow was predominantly through a limited subset of discontinuities, mainly fractures, parts of which form networks of connected channels. Within this overall understanding of the nature of groundwater flow, there is a wide range of possible geometrical descriptions for the flow channels. Determination of one or more appropriate conceptualizations of the flow system must be soundly based on site characterization data as a prerequisite for any numerical modelling study. In the first part of this paper, details of the site characterization studies that were used to identify the location, orientation and mineralogical characteristics of discontinuities, and in particular the set of discontinuities referred to as Potential Flowing Features (PFFs), are provided. These features have either demonstrable present-day open porosity, or display evidence of geologically recent groundwater flow as part of the evolution of the current groundwater system. It is inferred that the PFFs observed in boreholes correspond to flowing features and that the borehole data can be used to infer the distribution and characteristics of flowing features that are present in the unobserved rock mass. On this basis, knowledge of the distribution, orientation and permeability associated with the PFFs provided the framework for developing conceptual models for groundwater flow. Numerical models were constructed to represent the flow. It is not computationally practicable to undertake regional scale groundwater flow and transport calculations in which small-scale variability is explicitly represented. Therefore upscaled effective parameters need to be derived as a precursor to running large-scale numerical model simulations. A summary of the Nirex upscaling procedure applied to the flowing feature network in the Borrowdale Volcanic Group is provided.
Preliminary microfocus X-ray computed tomography survey of echinoid fossil microstructure
Authors: Stock S. R. , Veis A.
Published: April 2008

Microfocus X-ray computed tomography (μCT), a high resolution variant of medical CT, was used to non-invasively examine Jurassic echinoid fossils with spine, demipyramid and test plate fragments serving to assess the extent to which the microstructure remained unaffected by diagenesis. The sizes of the calcite-crystal stereom remains dictated the resolution that could be obtained. The smaller diameter spines were imaged with 9 × 9 ×
c.
25 μm voxels; the larger demipyramids and test plate fragments were imaged with 13 × 13 μm and 17 × 17 μm voxels, respectively, in the plane of reconstruction and with a proportionally larger slice thickness. The stereom structure was not seen in the μCT slices of the fossils. This is not surprising because, for the ossicles studied, the stereom pore dimensions are, for the most part, smaller than the resolution of the μCT system. Tide marks and other low absorption features were found in the spines and appear to be related to diagenetic changes. What appear to be cleavage cracks were observed in the demipyramid and some of the pores for tube feet could be seen in the test fragment.
The genesis of polygonal fault systems: a review
Authors: Cartwright Joe , James David , Bolton Al
Published: April 2008

Polygonal fault systems are widely developed in fine-grained sedimentary successions and have been recognized in over 50 basins worldwide. They are normal faults with modest throw values (typically 10–100 m), organized with a characteristic plan form pattern that is crudely polygonal, but with considerable variation in specific planform patterns. They have been attributed to four genetic mechanisms: gravity collapse, density inversion, syneresis and compactional loading. Their strain characteristics allow them to be distinguished from tectonic normal faults. The strengths and weaknesses of the four genetic mechanisms are considered in the light of these strain characteristics. It is argued that syneresis offers the likeliest mode of genesis and best explains the local and global features of these extraordinary structures. The detailed physical mechanism driving syneresis remains poorly understood.
Arc-trench rollback and forearc accretion: 2. A model template for ophiolites in Albania, Cyprus, and Oman
Authors: Dilek Yildirim , Flower Martin F. J.
Published: April 2008

Ophiolite assemblages record structural, magmatic, and metamorphic processes that preceded their entrapment in orogenic belts by continental plate collisions. Ophiolite genetic models appealing to ‘oceanic’ or ‘suprasubduction’ provenance are still unable to reconcile several basic problems, including: (1) the association of boninites with oceanic ridge-type structural settings; (2) the diachronous ‘patch-like’ distribution of ophiolites in orogenic belts; (3) disparate ages between and within their mantle and crustal sections; (4) the lack of evidence for ‘obduction’ at modern passive margins. In contrast, the proposal that ophiolite genesis is exclusive to intra-oceanic forearc settings is compelling, given their uniquely shared structural, lithological, and stratigraphic attributes. Forearcs are interpreted to record discrete stages of subduction ‘rollback’ cycles, examples of which begin with subduction nucleation and the formation of boninitic ‘proto-arcs’, followed by arc splitting and concomitant retreat of the evolving arc-forearc complex. Forearc assemblages are likely to resist subduction to become entrapped in orogens, in contrast to denser, recently formed back-arc basin lithosphere, which is reconsumed by subduction following collision of the retreating forearc. As a model for Neo-Tethyan ophiolite genesis, this is predicated on the notion that rollback cycles are driven by ductile asthenosphere mobilized prior to and during collisions of Gondwana fragments with accreting Eurasia. It is also consistent with the apparent correlation of ophiolite ages with collisional events and their conjugate plate kinematic adjustments. Here, we use the slab rollback model as a template for interpreting the structural, magmatic, and metamorphic characteristics of well-studied Tethyan ophiolites, in Albania (Mirdita), Cyprus (Troodos), and Oman (Semail).
Stratigraphic and sedimentological constraints on the age and tectonic evolution of the Neotethyan ophiolites along the Yarlung Tsangpo suture zone, Tibet
Authors: Aitchison Jonathan C. , Davis Aileen M. , Abrajevitch Alexandra V. , Ali Jason R. , Badengzhu  , Liu Jianbing , Luo Hui , McDermid Isabella R. C. , Ziabrev Sergey V.
Published: April 2008

Ophiolitic rocks distributed along the Yarlung Tsangpo suture zone in southern Tibet are the few remaining fragmentary remnants of many thousands of kilometres of the ocean space that formerly existed between India and Eurasia. Portions of mid-Jurassic and mid-Cretaceous intra-oceanic island arcs can be recognized amongst those rocks that have been studied in detail. Complete suprasubduction zone ophiolite successions are preserved in the Dazhuqu terrane, which crops out both east and west of Xigaze. Radiolarians in inter-pillow cherts and immediately overlying sedimentary rocks indicate a Barremian ophiolite generation event. Palaeomagnetic data show that this ophiolite formed at equatorial latitudes south of the Lhasa terrane before its south-directed emplacement onto the northern margin of India. Highly refractory ultramafic rocks in the Luobusa ophiolite appear to be of Mid-Jurassic age and are potentially related to intra-oceanic island arc remnants in the nearby Zedong terrane. Ophiolitic massifs along the suture in western Tibet are thrust southwards onto northern India and record Late Jurassic ocean-floor development. Miocene north-directed back-thrusting associated with India-Asia collision has further complicated interpretation of regional geology. The ophiolitic rocks of the Yarlung Tsangpo suture zone provide evidence for the former existence of multiple oceanic island arc segments within Neotethys and suggest that consumption of the oceanic space between India and Asia was more complicated than has been predicted by existing models.
Geodynamic patterns of ophiolites and marginal basins in the Indonesian and New Guinea regions
Authors: Harris Ron
Published: April 2008

Analysis of spatial, temporal, geological and geochemical patterns of ophiolites in the Indonesian and New Guinea region indicates a strong correlation with marginal basin development and closure. The spatial distribution of ophiolites is mostly linked with marginal basin producing zones of oblique convergence and collision. Strain partitioning in these zones creates a series of ephemeral plate boundaries between several independently moving lithospheric blocks. Repeated disruption of the diffuse boundaries between the blocks by changes in plate motion and collision-induced mantle extrusion creates space that is rapidly filled by new ocean basins in the upper plate of subduction zones. Suprasubduction zone (SSZ) spreading of these basins is enhanced by episodic extrusion of asthenosphere escaping collisional suture zones. Various closure events and global plate motion changes are reflected in the temporal distribution of marginal basin and ophiolite ages. Most ophiolite slabs in the Indonesian and New Guinea region represent fragments of oceanic lithosphere with a subduction zone component, as indicated by the common refractory petrochemistry of the mantle sequence and occurrence of boninite. Age and compositional heterogeneity may indicate that some ophiolite bodies are composite terranes. Collisions with buoyant lithosphere transform parts of these ocean basins into ophiolites. The connection between ophiolites and marginal basins is strongest where parts of actively spreading SSZ basins are partially represented as ophiolites in collision zones.
Controls on back-arc crustal accretion: insights from the Lau, Manus and Mariana basins
Authors: Martinez Fernando , Taylor Brian
Published: April 2008

Together, the Lau, Manus and Mariana basins encompass a broad range of conditions of back-arc basin development. Marine surveys have determined the tectonic setting and reconnaissance-scale geophysical and geochemical properties of the extension axes in these basins. We review these data to examine crustal accretion characteristics in the back-arc setting. In each basin magmatism is enhanced in spreading centres near the arc volcanic front, but decreases becoming ‘deficient’ in axes further from the arc. In the Lau and Manus basins the axes extend far behind the arc and develop typical mid-ocean ridge (MOR)-like characteristics. We propose that these variations are controlled by the subducting slab, which shapes composition and flow within the mantle wedge: water released from the slab lowers the mantle solidus and increases in concentration toward the volcanic arc. Spreading centres near the arc advect the hydrated mantle material, enhancing melt production. Slab-induced flow carries melt-depleted mantle material back beneath the basin where it mixes with ambient mantle. Spreading centres further from the arc advect this mantle mixture and produce thinner than normal crust. Far from the arc, spreading centres advect essentially mid-ocean ridge basalt (MORB)-source mantle and their characteristics are MOR-like. Thus, in addition to spreading-rate effects on mantle melting that predominate at MORs, in back-arc basins the subducting slab introduces additional systematic effects.
The subduction factory: its role in the evolution of the Earth’s crust and mantle
Authors: Tatsumi Yoshiyuki , Kogiso Tetsu
Published: April 2008

Subduction zones are major sites of magmatism on the Earth. Dehydration processes and associated element transport, which take place in both the subducting lithosphere and the down-dragged hydrated peridotite layer at the base of the mantle wedge, are largely responsible for the following characteristics common to most subduction zones: (1) the presence of dual volcanic chains within a single volcanic arc; (2) the negative correlation between the volcanic arc width and the subduction angle; (3) selective enrichment of particular incompatible trace elements; and (4) systematic across-arc variations in incompatible trace element concentrations. The occurrence of two types of andesites, calcalkalic and tholeiitic, typifies magmatism in subduction zones. Examination of geochemical characteristics of those andesites in the NE Japan arc and bulk continental crust reveals marked compositional similarity between calc-alkalic andesites and continental crust. One of the principal mechanisms of generation of calc-alkalic andesites, at least those on the NE Japan arc, is the mixing of two magmas, having basaltic and felsic compositions and being derived from partial melting of the mantle and the overriding basaltic crust, respectively. It may be thus suggested that this process would also have contributed greatly to continental crust formation. If this is the case, then the melting residue after extraction of felsic melts should be removed and delaminated from the initial crust into the mantle in order to form ‘andesitic’ crust compositions. These processes cause accumulation in the deep mantle of residual materials, such as delaminated crust materials and dehydrated, compositionally modified subducted oceanic crusts and sediments. Geochemical modelling suggests that such residual components have evolved to form enriched mantle reservoirs.
The Bouma Sequence (1962) and the resurgence of geological interest in the French Maritime Alps (1980s): the influence of the Grès d’Annot in developing ideas of turbidite systems
Authors: Bouma Arnold H. , Ravenne Christian
Published: April 2008

The two authors explain how and why the Grès d’Annot successions were chosen for their influential studies in the context of the 1960s and the 1980s. Arnold Bouma explains the origin of the Bouma Sequence in the 1960s, while Christian Ravenne focuses on the significance of the area as analogues of deep-sea fans and seismic stratigraphy in the 1980s. Ravenne recalls the main results obtained at that time: palaeogeographical maps, interpretative and synthetic sections, the spectacular onlap relationships at Chalufy, the strong interaction between seismic interpretation and field data, and the importance of large failures/collapses on the continental slope for the initiation of density surges.
Confined turbidite systems
Authors: Lomas Simon A. , Joseph Philippe
Published: April 2008

Deep-water sedimentation is currently a major focus of both academic research and industrial interest. Recent studies have emphasized the fundamental influence of seafloor topography on the growth and morphology of submarine ‘fans’: in many turbidite systems and turbidite hydrocarbon reservoirs, depositional system development has been moderately to strongly confined by pre-existing bounding slopes. This publication examines aspects of sediment dispersal and accumulation in deepwater systems where basin-floor topography has profoundly affected deposition, and the associated controls on hydrocarbon reservoir architecture and heterogeneity. The papers herein offer a global perspective which is wide-ranging in terms of both approach and location, including contrasting case studies of outcrop, subsurface, modern and experimental systems.
Timing, geodynamic setting and character of Permo-Carboniferous magmatism in the foreland of the Variscan Orogen, NW Europe
Authors: Timmerman Martin Jan
Published: April 2008

In the early Carboniferous, final subduction of the Rhenohercynian Ocean, accretion of a magmatic arc and docking of microcontinents caused fault reactivation, extension and fault-controlled basin formation in the foreland of the Variscan Orogen. Lithospheric stretching resulted in generally mildly alkaline basaltic volcanism that peaked in the Visean. In the internal Variscides, rapid uplift and granitoid plutonism shortly followed collision and was probably due to slab detachment(s) or removal of orogenic root material. A regional-scale, E-W-oriented stress field was superimposed on a collapsing orogen and its foreland from the Westphalian onwards. In the Stephanian-Early Permian, a combination of outward-propagating collapse, mantle or slab detachment and the regional stress field resulted in widespread formation of fault-controlled basins and extensive magmatism dated at 290-305 Ma. In the foreland, large amounts of felsic volcanic rocks erupted in northern Germany, accompanied by mafic-felsic volcanics and intrusions in the Oslo Rift, and dolerite sills and dyke swarms in Britain and Sweden. In the internal Variscides, mafic rocks are rare and felsic-intermediate compositions predominate. Their apparent subduction-related signature may have been inherited from metasomatized mantle sources or caused by extensive assimilation of continental crust.
New40Ar/39Ar ages and geochemistry of late Carboniferous-early Permian lamprophyres and related volcanic rocks in the Saxothuringian Zone of the Variscan Orogen (Germany)
Authors: von Seckendorff V. , Timmerman M. J. , Kramer W. , Wrobel P.
Published: April 2008

40Ar/39Ar step-heating dating of mineral separates from a series of lamprophyre dykes in the Saxothuringian Zone of the Variscan Orogen yielded Viséan-Namurian (334–323 Ma) and Stephanian-early Permian (297–295 Ma) crystallization ages indicating magma generation over a period of 30 Ma. In many cases, dyke emplacement was controlled by faults. Many are composite or show evidence for mingling of primitive and evolved magmas, and, to a certain degree, contamination with crustal melts. The high MgO (6–7 wt%), Ni (75–270 ppm) and Cr (140–1250 ppm) contents and mafic phenocryst assemblage are evidence for derivation from a mantle source. Kersantites and minettes have similar incompatible trace-element and rare earth element (REE) patterns (light REE (LREE)- and medium REE (MREE)-enriched and heavy REE (HREE)-depleted) and high, but varying Th, Zr and Hf contents. Positive Ni v. Mg# (FeO=FeOtot) correlations suggest early fractionation of olivine, and the general absence of negative Eu anomalies makes feldspar fractionation improbable. For the lamprophyres of the Spessart, the variations of Ba, Rb and TiO2indicate phlogopite fractionation. Negative Ta, Nb and Ti anomalies are common, and may be an artefact of the high large ion lithophile element (LILE) and REE contents, but are more likely to reflect derivation from a mantle source that was metasomatized during a previous (Devonian?) subduction event. The generation of the parent melts was possibly triggered by partial melting of metasomatized mantle due to lithosphere detachment, removal and replacement of metasomatized lithospheric mantle by upwelling hot asthenospheric mantle. Compared to the spessartites, the minettes and kersantites appear to have originated by partial melting of deeper-mantle sources. Lithospheric mantle detachment may have caused post-collisional Namurian uplift and cooling of the crust, and facilitated emplacement of lamprophyre dykes along fault zones at high crustal levels.
Ductile shearing, hydrous fluid channelling and high-pressure metamorphism along the basement-cover contact on Sikinos, Cyclades, Greece
Authors: Gupta S. , Bickle M. J.
Published: April 2008

On the island of Sikinos in the Cyclades a schistose carapace separates a marble-blueschist cover sequence from underlying basement rocks. The basement ‘core’, comprising metapelitic gneisses, biotite-bearing granodiorites and aplites, becomes increasingly strained towards the carapace with progressive obliteration of earlier structures and intensification of a mylonitic foliation that becomes pervasive within the carapace. Granodiorites in the ‘core’ can be traced into microcline schists within the carapace, whereas metapelitic gneisses are converted to garnet-mica schists. The carapace is therefore a simple shear zone comprising basement rocks mylonitized during overthrusting of the cover.

Biotite clusters in granodiorites of the basement ‘core’ are partially altered to phengite, whereas plagioclase shows incipient sericitization. In more strained rocks these hydration reactions show enhanced progress, until biotite and plagioclase are finally eliminated in the carapace. Rare glaucophane and chloritoid inclusions within garnets of metapelitic gneisses adjacent to the carapace are also attributed to hydration reactions. The association of higher strain with increased hydration in the basement suggests localization of fluids in the strained carapace zone, with limited percolation into underlying rocks. The restricted availability of water outside the carapace may be responsible for preservation of pre-Alpine assemblages in large parts of the Cycladic basement.
Shear zone folds: records of flow perturbation or structural inheritance?
Authors: Alsop G. I. , Holdsworth R. E.
Published: April 2008

Deformation within shear zones can be both temporally and spatially variable, resulting in multiple generations of folds which display a range of scales and overprinting relationships in mylonitic rocks associated with high strain zones. Despite such complexities, two main fold associations are broadly recognized in many shear zone settings: early tight to isoclinal sheath folds, often with mylonitic limbs that are post-dated by one or more local generations of synshearing folds which are preserved within, or root downwards into mylonitic high strain zones. These latter structures locally fold the mylonitic foliation and lineation whilst displaying geometric characteristics that are kinematically compatible with the movement regime of the major shear zone. Using examples related to ductile thrusting in Moine metasediments of north Scotland, we show that both types of fold display predictable geometric patterns on fabric topology plots. Fold axes and axial surfaces display consistent changes in asymmetry and sense of obliquity relative to local, transport-parallel mineral lineations that can be used to map out a series of culminations and depression zones. The sheath folds preserve more acute, but almost identical geometric patterns compared to the later synshearing folds, with culmination and depression zones often coinciding in location and scale. Detailed analysis also demonstrates that the distribution of finite strain is systematically linked to the architecture of all folds and that clear and predictable relationships exist between the fabric topologies of both the sheath folds and synshearing folds. These consistent topological relationships could be explained in terms of a
fold evolution model
, where sheath folds represent a more highly deformed and evolved variety of synshearing folds originally generated during perturbations in ductile flow. However, an alternative
fold inheritance model
predicts that the gross structural architecture generated during sheath folding may subsequently control the geometry and govern the orientation of the synshearing folds. Both models may be widely applicable in a broad range of shear zone environments.
Crenulation-slip development in a Caledonian shear zone in NW Ireland: evidence for a multi-stage movement history
Authors: Chew D. M. , Daly J. S. , Flowerdew M. J. , Kennedy M. J. , Page L. M.
Published: April 2008

In Scotland and Ireland, a Laurentian passive margin sequence, the Dalradian Supergroup, was deformed during the
c.
470–460 Ma Grampian orogeny, resulting in the formation of crustal-scale recumbent nappes. In Ireland, this passive margin sequence is in general bounded to the SE by the Fair Head-Clew Bay Line (FHCBL), a segment of a major lineament within the Caledonides. Adjacent to the FHCBL, Dalradian metasediments in two separate inliers have undergone post-Grampian strike-slip movement, with the initially flat-lying Grampian nappe fabric acting as a décollement-like slip surface in both cases. As the orientation of these foliation slip surfaces was oblique to the local shear plane in both inliers, displacement along these pre-existing foliation surfaces was also accompanied by crenulation slip. However, the crenulation-slip morphologies produced imply the opposite sense of movement in the two inliers.
40
Ar-
39
Ar dating of muscovite defining the crenulation-slip surfaces indicates that post-Grampian dextral displacement took place along the FHCBL at 448 ± 3 Ma. A subsequent phase of sinistral movement along the FHCBL took place at
c.
400 Ma, based on previously published Rb-Sr muscovite ages for synkinematic pegmatites. The kinematic information obtained from crenulationslip morphologies combined with geochronology can thus be used to constrain the reactivation history of a major crustal-scale shear zone.
200 years of British hydrogeology — an introduction and overview
Authors: Mather J. D.
Published: April 2008

In the early years of the 19th century William Smith and his pupil John Farey began to apply stratigraphic principles to the sinking of water wells. Between 1840 and 1870 engineers, such as Robert Stephenson, and geologists, such as James Clutterbuck, Joseph Prestwich and David Ansted began to make systematic observations. After 1870 Geological Survey officers, particularly William Whitaker, Joseph Lucas and Charles de Rance, became involved in groundwater work. Lucas introduced the term hydrogeology and produced the first British hydrogeological maps. The first half of the 20th century was a period of missed opportunities with the significant advances in hydrogeology taking place in mainland Europe and North America. The Water Act of 1945 marked the start of a new era in which the Geological Survey and, after 1965, the Water Resources Board led the way. Hydrogeology is now a mainstream branch of geology in Britain and interest in the subject is such that the Hydrogeological Group of the Geological Society has a membership of around 1050.
The contribution of geologists to the development of emergency groundwater supplies by the British army
Authors: Rose Edward P. F.
Published: April 2008

During the 19th Century, the British military pioneered geological mapping and teaching, and the operational use of Norton tube wells. In the First World War, the British army appointed its first military hydrogeologist to serve as such, to develop water-supply maps for Belgium and northern France and guide deployment of Royal Engineer units drilling boreholes into the Cretaceous Chalk of the Somme region and Tertiary sands beneath the Flanders plain. Similar well-boring units were also deployed with geological guidance in the northeastern Mediterranean region. All military geologists were demobilized after hostilities ceased, but wartime experience was quickly drawn together in the first Royal Engineer textbook on water supply. During the Second World War, several British military well-drilling units were raised and deployed, notably to East Africa and North Africa as well as northern France, normally with military geological and sometimes (in Africa) with military geophysical technical direction. A reduced well-drilling capability has since been retained by the British army, through the Cold War to the present day, supported by a small group of reserve army geologists to contribute basic hydrogeological expertise to the armed forces for peace-time projects and war-related operations.
Tectonic evolution of Palaeozoic terranes in West Junggar, Xinjiang, NW China
Authors: Buckman Solomon , Aitchison Jonathan C.
Published: April 2008

Nine separate Cambrian to Carboniferous terranes are recognized in West Junggar, northwest China. They were amalgamated as part of the Central Asian Orogenic Belt which records accretion of continental, island-arc and oceanic terranes to Archaean-Proterozoic continental nuclei. Tangbale, Kekesayi, Ebinur and Mayila terranes (Cambrian-Silurian) evolved in intra-oceanic settings and docked, along a series of north-dipping subduction zones, on to the Laba terrane to their south. This southern continent was contiguous with lithosphere of the Kulumudi Ocean to the north. Devonian subduction on the northern edge of this ocean resulted in formation of a continental arc (Toli terrane) and accretionary complex (Kulumudi terrane). The Karamay terrane formed as an accretionary complex during the Carboniferous. The ophiolitic Sartuohai terrane was emplaced as mélange between Kulumudi and Karamay terranes during the Late Carboniferous. Subduction migrated southward, continuing beneath these terranes, resulting in the intrusion of I-type granites into the Toli, Kulumudi, Sartuohai and Karamay terranes. These granites are closely associated with epithermal and porphyry-style gold mineralization. Composite terranes either side of the Kulumudi Ocean collided in the Late Carboniferous, marking the final consolidation of Central Asia. Collision was accompanied by anorogenic granite and diabase dyke intrusion, followed by widespread latest Carboniferous to Permian extension, and subsequently the formation of the Junggar Basin. West Junggar has been further disrupted by Cenozoic strike-slip faulting along Junggar and Dalabute faults.
Some ichnological concepts, methodologies, applications and frontiers
Authors: McIlroy Duncan
Published: April 2008

Ichnology straddles the boundary between palaeontology and sedimentology, and is becoming an increasingly important tool in both fields. For the palaeontologist, trace fossils allow insight into behaviour and biomechanics of animals that would otherwise be the subject of conjecture. For the sedimentologist, trace fossils have a marked impact on the interpretation of sedimentary rocks in that they destroy primary sedimentary structures, but can also reveal subtle palaeoenvironmental information beyond the resolution attainable by analysis of primary physical sedimentary structures. This contribution aims to review the major developments in the field of ichnology, and to highlight some of the tools and approaches currently used by ichnologists. A personal ethos for the study of trace fossils in core is outlined as a model ichnological protocol, and some of the frontiers of the science as a whole are briefly discussed.
Climatic control of trace fossil distribution in the marine realm
Authors: Goldring Roland , Cadée Gerhard C. , D’Alessandro Assunta , De Gibert Jordi M. , Jenkins Richard , Pollard John E.
Published: April 2008

Modern coastal and shoreface faunas exhibit strong latitude (climate) controlled distributions. In contrast, most ichnotaxa are long-ranging, and ichnofacies are widely distributed geographically. This is readily explained by the dominantly warmer and more equable climates of much of the past, as well as the diversity of the producers of most ichnotaxa. Nevertheless, in the Pleistocene, and in the Eocene, cool-water ichnofabrics can be recognized. The latitudinal distributions of thalassinidean crustaceans and infaunal spatangoid echinoids are examined because of their propensity to form distinctive and often abundant trace fossils. Three climatic zones are tentatively recognized from modern shore and shoreface sediments, and which are considered to extend back to the Mesozoic:
tropical and subtropical
with pellet-lined burrows (
Ophiomorpha
), echinoid burrows and other traces;
temperate
with echinoid burrows and mud-lined or non-lined thalassinidean burrows (
Thalassinoides
), but without
Ophiomorpha
; and
arctic
(cold waters) with only a molluscan and annelid trace fossil association. Examples demonstrating this climatic trend are drawn from the Cenozoic and Pleistocene.
Differentiation of estuarine and offshore marine deposits using integrated ichnology and sedimentology: Permian Pebbley Beach Formation, Sydney Basin, Australia
Authors: Bann Kerrie L. , Fielding Christopher R. , MacEachern James A. , Tye Stuart C.
Published: April 2008

This study integrates ichnology and sedimentology to refine the palaeoenvironmental and sequence stratigraphic interpretations of the Early Permian Pebbley Beach Formation, in the southern Sydney Basin, Australia. This succession has been interpreted previously to reflect entirely inner to outer shelf and slope environments of deposition. Detailed analysis of the formation reveals ichnological and sedimentological characteristics that contradict a fully marine interpretation. Instead, the interval reflects the vertical superposition and lateral juxtaposition of brackish-water and fully marine units. Marine facies comprise: (1) thoroughly bioturbated muddy siltstone (lower offshore); (2) thoroughly bioturbated sandy siltstone (upper offshore); (3) interbedded bioturbated sandy siltstone and laminated sandstone (delta-influenced offshore transition); (4) thoroughly bioturbated muddy sandstone (distal lower shoreface); (5) interbedded laminated sandstone, bioturbated muddy sandstone and dark claystone (delta-influenced lower shoreface); and (6) bioturbated, laterally variable sandstones (transgressive sand sheets). Estuarine facies comprise: (1) channelized heterolithic sandstone-mudstone (active estuarine channels); (2) sheet-like heterolithic sandstone-mudstone (active estuarine basins); and (3) laminated mudstone (abandoned estuarine channels and basins). The interpreted fully marine deposits contain ichnological suites that exhibit moderate to intense bioturbation, high diversities (31 ichnospecies belonging to 20 ichnogenera), uniform distributions of ichnogenera, and significant numbers of structures reflecting specialized feeding/grazing behaviours. In marked contrast, interpreted estuarine (brackish-water) deposits contain impoverished ichnological suites (9 ichnogenera), show variable but significantly reduced degrees of bioturbation intensity, pronounced variability in ichnogenera distributions and the predominance of a few, simple forms representing simple feeding strategies of resilient trophic generalists. The new analysis allows the recognition of a series of highly top-truncated and condensed sequences (cycles of relative sea-level fall and physical rise), which can be physically correlated over several kilometres. Sequence boundaries typically cut down through shoreface sandstones to directly overlie offshore facies, leading to an interface with little apparent lithological contrast. In the absence of laterally continuous exposure, these surfaces may be recognized by careful ichnofacies evaluation. Thus the re-evaluation presented herein has facilitated a more realistic sequence stratigraphic analysis of the Pebbley Beach Formation.
Pin-pricking the elephant: evidence on the origin of the Ontong Java Plateau from Pb-Sr-Hf-Nd isotopic characteristics of ODP Leg 192 basalts
Authors: Tejada M. L. G. , Mahoney J. J. , Castillo P. R. , Ingle S. P. , Sheth H. C. , Weis D.
Published: April 2008

Age-corrected Pb, Sr and Nd isotope ratios for early Aptian basalt from four widely separated sites on the Ontong Java Plateau that were sampled during Ocean Drilling Program Leg 192 cluster within the small range reported for three earlier drill sites, for outcrops in the Solomon Islands, and for the Nauru and East Mariana basins. Hf isotope ratios also display only a small spread of values. A vitric tuff with ε
Nd
(
t
) = +4.5 that lies immediately above basement at Site 1183 represents the only probable example from Leg 192 of the Singgalo magma type, flows of which comprise the upper 46–750 m of sections in the Solomon Islands and at Leg 130 Site 807 on the northern flank of the plateau. All of the Leg 192 lavas, including the high-MgO (8–10 wt%) Kroenke-type basalts found at Sites 1185 and 1187, have ε
Nd
(
t
) between +5.8 and +6.5. They are isotopically indistinguishable from the abundant Kwaimbaita basalt type in the Solomon Islands, and at previous plateau, Nauru Basin and East Mariana Basin drill sites. The little-fractionated Kroenke-type flows thus indicate that the uniform isotopic signature of the more evolved Kwaimbaita-type basalt (with 5–8 wt% MgO) is not simply a result of homogenization of isotopically variable magmas in extensive magma chambers, but instead must reflect the signature of an inherently rather homogeneous (relative to the scale of melting) mantle source. In the context of a plume-head model, the Kwaimbaita-type magmas previously have been inferred to represent mantle derived largely from the plume source region. Our isotopic modelling suggests that such mantle could correspond to originally primitive mantle that experienced a rather minor fractionation event (e.g. a small amount of partial melting) approximately 3 Ga or earlier, and subsequently evolved in nearly closed-system fashion until being tapped by plateau magmatism in the early Aptian. These results are consistent with current models of a compositionally distinct lower mantle and a plume-head origin for the plateau. However, several other key aspects of the plateau are not easily explained by the plume-head model. The plateau also poses significant challenges for asteroid impact, Icelandic-type and plate separation (perisphere) models. At present, no simple model appears to account satisfactorily for all of the observed first-order features of the Ontong Java Plateau.
Volatiles in submarine basaltic glasses from the Ontong Java Plateau (ODP Leg 192): implications for magmatic processes and source region compositions
Authors: Roberge Julie , White Rosalind V. , Wallace Paul J.
Published: April 2008

Submarine basaltic glasses from five widely separated sites on the Ontong Java Plateau (OJP) were analysed for major and volatile elements (H
2
O, CO
2
, S, Cl). At four of the sites (1183, 1185, 1186, 1187) the glass is from pillow basalt rims, whereas at Site 1184 the glass occurs as non-vesicular glass shards in volcaniclastic rocks. Glassy pillow rims from Site 1187 and the upper group of flows at Site 1185 have 8.3–9.3 wt% MgO compared with values of 7.2–8.0 wt% MgO for glasses from Sites 1183, 1184 1186, and the lower group of flows at Site 1185. Low-MgO glasses have slightly higher H
2
O contents (average 0.22 wt% H
2
O) than high-MgO glasses (average 0.19 wt%), with the exception of Site 1184, where low-MgO glasses have lower H
2
O (average 0.16 wt%). Average S concentrations are 910 ± 60 ppm for the high-MgO glasses v. 1030 ± 60 ppm for the low-MgO glasses. When compared with mid-ocean ridge basalt (MORB), the OJP glasses have lower S at comparable FeO
T
. This suggests that OJP basaltic magmas were not saturated with immiscible sulphide liquid during crystallization, but small decreases in S/K
2
O and S/TiO
2
with decreasing MgO require some sulphide fractionation. Measurements of the wavelength of the S
K
α peak in the glasses indicate low oxygen fugacities comparable to MORB values. Chlorine contents of the glasses are very high compared with MORB, and Cl/K ratios for all glasses are relatively high (>0.7). This ratio is sensitive to assimilation of hydrothermally altered material, so the high values indicate assimilation during shallow-level crystallization of OJP magmas. Ratios of H
2
O to Ce, which have similar incompatibility to each other, are higher than most depleted and enriched MORB. However, these high H
2
O/Ce values are probably also caused by the same assimilation process that results in high Cl. The water content of the high MgO-magmas before contamination is estimated to be approximately 0.07 wt% H
2
O, corresponding to H
2
O/Ce of 135 for OJP basalts, a value at the low end of the range for Pacific MORB. There is no evidence for high H
2
O contents that would have significantly increased extents of mantle melting beneath the OJP, and the estimated H
2
O content of the OJP mantle source region (170 ± 30 ppm H
2
O) is similar to that of the depleted MORB source (140 ± 40 ppm H
2
O). Instead, large extents of melting beneath the OJP must have been caused by a relatively high mantle potential temperature, consistent with upwelling of a hot mantle plume.
Particle size analysis by laser diffraction
Authors: Blott Simon J. , Croft Debra J. , Pye Kenneth , Saye Samantha E. , Wilson Helen E.
Published: April 2008

Particle size distribution is a fundamental property of any sediment or soil, and particle size determination can provide important clues to sediment provenance. For forensic work, the particle size distribution of sometimes very small samples requires precise determination using a rapid and reliable method with a high resolution. A protocol has been developed using a Coulter™ LS230 laser granulometer, which can analyse particles in the size range 0.04 μm–2000μm. The technique is essentially non-destructive, permitting the recovery of critical samples, and has been demonstrated to have high precision for a range of soils, sediments and powders of interest in forensic investigations.