bluuebunny/crossref_metadata_2025 huggingface dataset:
Intensity and direction of magnetization in oceanic basalts
Authors: LOWRIE WILLIAM
Published: December 2007

The magnetic properties of oceanic basalt samples from 55 DSDP sites (301 samples) and 112 dredge haul sites (311 samples) show wide variations, indicative of heterogeneity in the basaltic layer. Remanent intensity and
Königsberger
ratio are log-normally distributed and range over three orders of magnitude for each collection. For DSDP samples between-site variation is greater than within-site variation, suggesting horizontal variation of magnetization within the source layer of magnetic anomalies. Crustal magnetization estimates from inversion of short-wavelength oceanic magnetic anomalies may therefore be in error. By using arithmetic means rather than the geometric means appropriate for log-normal distributions, estimates of the magnetization of Layer 2 of around 4 × 10
−3
Gauss are obtained for the Indian, Pacific and north Atlantic oceans and Caribbean Sea, while lower averages are found in the south Atlantic and Antarctic oceans. Using this as a representative value for abyssal areas, the thickness of the magnetic source layer is greater than commonly assumed, and probably more than a kilometre. This interpretation is supported by DSDP data from some individual sites, notably the deep holes of Leg 37.

The DSDP median destructive fields average only 123 Oe. Although many sites have much higher stabilities than this, there is evidence for unstable NRM at many sites where samples exhibit a strong tendency to acquire viscous remanence components. The low coercivity secondary magnetizations confuse identification of the stable direction during magnetic cleaning. Acquisition of high coercivity secondary remanence (CRM) as a result of maghemitization, which is extensive in the DSDP collection, may also have altered the original NRM direction.

Remanent inclination data from four Tertiary DSDP sites (ages 38–40 Ma) have been combined to give a tentative Eocene VGP for east Antarctica at 68°S 64°E. It has not been possible to compare DSDP data in this way at other sites. Palaeolatitudes deduced from DSDP basalt remanent inclinations show large discrepancies from values expected from plate reconstructions at many sites. Incompletely removed secondary magnetizations, tectonic effects and palaeo-reconstruction errors, alone or in combinations, may be responsible for these discrepancies. A major source of deviation from the expected direction may be the rapid cooling of the basalt which does not allow adequate averaging of secular variation.
Palaeomagnetic data and radiometric ages from the Cretaceous of West Kalimantan (Borneo), and their significance in interpreting regional structure
Authors: HAILE N. S. , McELHINNY M. W. , McDOUGALL I.
Published: December 2007

Potassium-argon ages of biotite and hornblende from sixteen samples of mainly granitic plutonic rocks from West Kalimantan indicate a complex history of magmatism and cooling from the Middle Jurassic to late Cretaceous, with most ages in the range 75 to 112 Ma. Samples from the Schwaner Zone in the northern part of the area give measured ages ranging from 76 to 116 Ma. In the southern region most samples are in the range 79 to 86 Ma, but two are older (127 Ma, early Cretaceous, and 153 Ma, Middle Jurassic). In both areas, concordant ages from biotite and hornblende indicate a well-defined event at about 80 to 85 Ma, which has also been recognized to the north in Sarawak, on islands of the South China Sea, in the Malay Peninsula, south Sumatra, and in the Java Sea.

Palaeomagnetic measurements on thirty-nine samples of dyke rocks, lavas, tuffs, granites, hornfels and metatuff, all believed to be late Cretaceous, yield a mean palaeomagnetic pole at 21°E 41°N, and a palaeolatitude for West Kalimantan of 0°. The palaeomagnetic pole does not differ significantly from the Cretaceous pole estimated for the Malay Peninsula. It is concluded that since the Middle Cretaceous West Kalimantan and the Malay Peninsula have behaved as a unit, have remained in much their present latitude, but have rotated anticlockwise about 50 .
Natural faulting and the mechanics of brittle shear failure
Authors: MURRELL S. A. F.
Published: December 2007

The theory of faulting developed since E. M. Anderson’s (1951) work, which is based on the concept of dislocations in an elastic continuum, is systematically discussed. Recent work enables the problem to be properly formulated in three dimensions and leads to the confirmation of Mohr’s hypothesis, on which Anderson’s theory was based, that faulting is independent of the intermediate principal stress. Estimates of the stress-differences existing in the crust and mantle (ranging up to several hundred MPa––several kb) emphasize the significance of conditions which enable the high friction existing on deep faults to be circumvented so that faulting can take place under small stress-differences. The Hubbert-Rubey (1959) hypothesis of lithostatic pore fluid pressures remains the most attractive mechanism for overcoming the problem of high friction.
Mylonitization in the Lethakane shear zone, eastern Botswana
Authors: WAKEFIELD JOHN
Published: December 2007

The Lethakane shear zone is a transcurrent ductile shear zone with a dextral displacement of
c.
50 km which developed under low amphibolite-to greenschist-facies conditions in the Limpopo orogenic belt of eastern Botswana. Three distinct minor structures occur within the shear zone: mylonitic foliation (Sa), strain-slip or fracture cleavage (Sb) and minor shear zones (Sc). The non-penetrative structures Sb and Sc have transcurrent movement senses and occur in conjugate sets orientated sub-parallel and sub-perpendicular to the shear zone margins. The penetrative fabric Sa is restricted to quartz-bearing rocks and has the same sigmoidal relationship to minor shear zones as that described for schistosity in ductile shear zones by Ramsay & Graham (1970). This relationship confirms the view, first proposed by
Johnson (1967)
, that mylonitic foliation defines the XY plane of the finite strain ellipsoid and is thus analogous to schistosity. The restriction of Sa to quartz-bearing rocks indicates that, under low amphibolite-facies conditions, the contrast between the ductile behaviour of quartz and the brittle behaviour of feldspars is the essential factor controlling deformation processes in the Lethakane shear zone.
Aspects of gravity tectonics and the development of listric faults
Authors: PRICE N. J.
Published: December 2007

Since the classic work by Hubbert & Rubey it has become generally accepted that thin but extensive sheets, or blocks of rock may glide down a gentle slope only when the fluid pressure beneath the block is abnormally high. It is pointed out that the initiation of movement of the block can be explained if it is assumed that the fluid pressure, below the block, locally and for short periods exceeds the gravitational load. Furthermore, it is argued that when the glide block encounters resistance to forward motion, fluid pressures which also exceed the gravitational load, act within the glide block and are instrumental in the initiation and development of listric faults.
Compositional and density stratification in oceanic lithosphere-causes and consequences
Authors: OXBURGH E. R. , PARMENTIER E. M.
Published: December 2007

Partial melting processes at mid-ocean ridges generate oceanic lithosphere which is chemically and mineralogically zoned. Basaltic oceanic crust overlies a 20–30 km thick zone of
depleted
upper mantle. This zone has undergone partial melting and is largely free of the high density phase, garnet, has a higher MgO/FeO ratio, and in consequence has a lower density (
Δ p
= 0 06 gm/cm
3
), than the undepleted mantle of the lower lithosphere. The lithosphere does not become gravitationally unstable upon the asthenosphere for 40–50 Ma, when increased density resulting from thermal contraction has offset the compositional buoyancy of the depleted zone and crust. During subduction the basaltic crust inverts to eclogite and the net compositional buoyancy of the lithosphere is eliminated. However, as the subducted lithosphere is heated it becomes less rigid and density differences both between different parts of the descending lithosphere and the surrounding mantle become important. The dense eclogite layer sinks through the underlying depleted zone at a rate determined by the temperature-dependent rheology. With further heating the depleted zone becomes less dense than the overlying undepleted mantle and will diapirically rise some 300-400 km behind the trench depending upon the angle and rate of subduction and the age of the subducted plate. Such diapirs are able to intitiate behind-arc spreading. In a continental setting the diapirs could both heat the lithosphere and produce exceptional elevation.
A new geological model to explain the gravity gradient across Exmoor, north Devon
Authors: BROOKS M. , BAYERLY M. , LLEWELLYN D. J.
Published: December 2007

Recent long seismic lines in South Wales and the Bristol Channel indicate a structural culmination under the southern part of the Bristol Channel, where a layer with a seismic velocity of 6.1 km/s approaches to about 2 km of the surface. It is shown that the gravity field across Exmoor and the Bristol Channel can be explained by a simple geological model in which a thick sequence of relatively low density Lower Palaeozoic or late Precambrian rocks occupies the core of this culmination. The model casts further doubt on the existence of a major thrust under Exmoor.
The Moni Mélange, Cyprus: an olistostrome formed at a destructive plate margin
Authors: ROBERTSON A. H. F.
Published: December 2007

Detailed mapping has shown that the Moni Mélange of southern Cyprus consists of two separate allochthonous components: a variety of Triassic to Cretaceous sedimentary and volcanic rocks representative of an original continental margin sequence, and substantial sheets of serpentinite derived from Upper Cretaceous Troodos-type oceanic crust. All these allochthonous rocks were emplaced by sliding into an
in situ
host matrix of Upper Cretaceous bentonitic clays and radiolarian siltstones of deep water hemipelagic facies. The postulated geotectonic setting involves a phase of late Cretaceous northward subduction of Troodos-type oceanic crust beneath a continental margin now located in southern Turkey. Subduction culminated in the Maastrichtian in a major collision of oceanic crust with the adjacent continental margin. This triggered off major oceanward gravity sliding of continental margin rocks. The collision was also associated with deep tectonic slicing of Troodos oceanic crust with the result that sheets of partly serpentinized ultramafic rocks were emplaced by sliding into the host sediments of the mélange from the opposite direction to the other olistoliths. Soon afterwards there was a brief period of submarine erosion, then local tectonic movements ceased, suggesting an end to subduction in the area.
Structure and mineralization in the Cligga granite stock, Cornwall
Authors: MOORE J. McM. , JACKSON N.
Published: December 2007

In the Cligga stock there are two groups of fractures, produced by separate episodes of brittle deformation which affected the granite and adjacent slates. Generalized models constructed by interpretation of fault and fracture traces on geological cross-sections provide a coherent explanation of the formation mechanisms for the two groups of structure: A—curved primary flat-lying joints, granite-veins and porphyrydyke fissures, early normal faults in slates, marginal thrusts and reverse faults, and B—conjugate normal faults and extensional fissures. The fractures were produced by stresses exerted from within the underlying St Agnes–Cligga cupola which forms part of the buried batholith ridge. The stress system was modified during the first deformation by local conditions within the stock.

Structures of one or both generations controlled the widespread greisen (phyllic) wallrock alteration, hydrothermal mineralization and kaolinization (argillic alteration). Time/space models illustrate relationships during the major episodes of ore mineral deposition which produced telescoped metal zonation in the belts of W/Sn/Cu/Fe veins which traverse the intrusion. The relationships between structure and mineralization at Cligga and in other small granites, indicate that undiscovered stocks in SW England may contain potentially economic W/Sn-sheeted vein or stockwork ore bodies.
Water/rock interactions and the origin of H
2
O in granitic batholiths
Authors: TAYLOR H. P.
Published: December 2007

D\H and
18
O\
16
O data have now been obtained on a wide variety of granitic batholiths of various ages. The primary δD values of the biotites and hornblendes are remarkably constant at about –50 to –85, identical to the values in regional metamorphic rocks, marine sediments and greeenstones, and most weathering products in temperate climates. Therefore the primary H
2
O in these igneous rocks is probably not ‘juvenile’, but is ultimately derived by dehydration and/or partial melting of the lower crust or subducated lithosphere. Most granitic rocks have δ
18
O = + 7.0 to +10.0, probably indicating significant involvement of high -
18
O metasedimentary or altered volcanic rocks in the melting process; such an origin is required for many other granodiorites and tonaloites that have δ
18
O = + 10 to +13. Gigantic meteoric-hydrothermal convective circulation systems were established in the epizonal portions of
all
batholiths, locally producing very low δ
18
O values (particularly in feldspars) during subsolidus exchange. Some granitic plutons in such environments also were emplaced as low-
18
O magmas probably formed by melting or assimilation of hydrothermally altered roofrocks. However, the water/rockratios were typically low enough that over wide areas the only evidence for meteoric H
2
O exchange in the batholiths is given by low D/H ratios (δD as low as –180); for example, because of latitudinal isotopic variations in meteoric waters, as one moves from through the Cordilleran batholiths of western North America an increasingly higher proportion of the granitic rocks haves δD values lower than –120. The lowering of δD values commonly correlates
The Mesozoic development of aseismic continental margins
Authors: KENT P. E.
Published: December 2007

Mesozoic basin development with in the North-west European Continental Shelf took place in belts peripheral to old Palaeozoic massifs. The area was dominated by vertical tectonic movements from the end of the Hercynian orogeny, and basin development along characterized by intermittent marginal faulting and graben formation dated as Permian, Triassic, and Lower, Middle and Upper Jurassic.

On a world scale the Mesozoic basin development and the subsequent late-Mesozoic separation of the elements of the North Atlantic and Gondwanaland was the final episode of a long-continued subsidence, peripheral to ancient blocks defined during Palaeozoic times. Marine transgression on or close to the later sutures is recognized in the Lower Palaeozoic in eastern America, West Africa, South Africa and Western Australia. Faulted basin development along what were to become the world's Atlantic-type ('aseismic') margins became widespread by the Permian, and continued intermittently until the Middle Cretaceous.

Widely around the world the long-established Mesozoic regime of synsedimentary graben formation and intermittent major faulting ended with an Aptian transgression, and was replaced by a regime of simple miogeosynclinal subsidence with prograding sedimentation which has continued to the present day. In Greenland and the Grand Banks of Canada, in Gabon and Australia, the history is strikingly similar to that of Northwest Europe, on what are now open ocean coasts. Thick earlier sediments are related to fault-controlled subsidences and rifts, a regime which was replaced worldwide within a very short period, by prograding basinal sedimentation in which faulting played a minor or very local role, and in which beds of Middle Cretaceous or somewhat earlier date transgress the planed-off structures of the previous regime.

Although this relationship is not universal, there is a broad uniformity in the dating of the fundamental change in the style of aseismic marginal basins despite the different dates ascribed to opening of adjoining oceans. This requires a world-wide mechanism, not yet understood.
Precambrian gneisses in the Dalradian sequence of northeast Scotland
Authors: STURT B. A. , RAMSAY D. M. , PRINGLE I. R. , TEGGIN D. E.
Published: December 2007

I
n this note
we draw attention to data which may require major revision of Dalradian stratigraphy and structure in northeast Scotland. The Central Highland sequence of the Dalradian and the Keith Division (
Read 1955
) of Banffshire are separated from the metasediments of the Banff Division by gneisses. These are referred to as the Cowhythe Gneiss (
Read 1923
), Queens Hill Gneiss (Harris & Pitcher 1975), Donside and Ellon Gneiss (
Read 1955
) and Inzie Head Gneiss (
Read & Farquhar 1956
) after the main areas of outcrop. The reader is referred to
Read (1955
, pl. 1) where the general distribution of the gneisses is shown. Their relationships to the Central Highland succession are often complicated by the large granitic intrusions of the post-tectonic Younger Granite suite. The generally accepted model for the region interprets the gneisses as migmatites developed from the Ben Lui and Ben Lawers schists (e.g.
Read 1955
, Harris & Pitcher 1975). Further they are considered to represent the migmatitic core of the Tay Nappe (e.g.
Ashworth 1975
).
Read (1955)
originally considered that the rocks of his Banff Division lay on a lag-surface (the Boyne Line) above the gneisses. The great recumbent fold of which the Banff Division formed the upper limb was called the Banff Nappe, and was considered to be a continuation of the Tay Nappe (
Read 1955
).

Recent investigations by the authors reveal a sharp tectonic break between the Dalradian rocks of the Central Highlands/Keith Division and the gneisses. This break is marked by discordance and a major
Pressure solution and Coble creep in rocks and minerals: a review
Authors: McCLAY K. R.
Published: December 2007

A brief review of research on pressure solution is given. The mechanisms of pressure solution and Coble creep are discussed. Deformation by diffusive mass transfer processes is generally accompanied by grain boundary sliding and this will have important effects on the textures and microstructures produced during deformation. Experiments using relaxation testing seem promising in allowing access to the slow strain rates necessary to observe pressure solution phenomena. The thermodynamics of non-hydrostatically stressed solids is discussed and an analysis of possible diffusion paths in rocks is presented. Evaluation of theoretical rate equations for pressure solution and Coble creep indicates that pressure solution in fine-grained quartz and calcite rocks may give rise to geological strain rates at temperatures from 200–350°C. Coble creep is expected to give rise to geological strain rates in fine-grained galena at low temperatures and also in fine-grained calcite rocks at temperatures around 350°C. The chemistry of natural rock pressure solution systems is expected to have significant effects and needs further detailed study. Further research is needed to elucidate the nature and role of grain boundaries during diffusive mass transfer. Pressure solution phenomena are important in the compaction behaviour of some petroleum reservoir rocks and perhaps in some faults where sliding is accommodated by pressure solution.
The relative importance of petrogenetic variables in magma genesis at accreting plate margins: a preliminary investigation
Authors: PEARGE J. A. , FLOWER M. F. J.
Published: December 2007

The relative role of petrogenetic variables at accreting plate margins varies according to the tectonic setting of the spreading centre in question. Spreading rate, heat flow, proximity to fracture zones and proximity to areas of 'anomalous' mantle are particularly important factors. This can be illustrated with reference to three well-studied areas: the FAMOUS segment of the Mid Atlantic Ridge; the Troodos ophiolite complexes and the active rift centres on Iceland. Geochemical modelling based on some critical major and trace element ratios reveals that crystal accumulation and fractional crystallization dominate variation both of the Troodos lavas (perhaps because a high degree of partial melting yielded a homogeneous primary magma) and of lavas from fast-spreading ridges (perhaps because of buffering in a large axial magma chamber). Source heterogeneities dominate variation of the Iceland lavas (perhaps because of 'anomalous' mantle beneath the centre of Iceland). The FAMOUS lavas exhibit a combination of primary magma and fractional crystallization variation.
Allochthonous origin of the Jotunheim Massif in southern Norway: a reconnaissance study along its northwestern margin
Authors: ROBERTS J. L.
Published: December 2007

The Jotunheim Massif of Precambrian granu-lite-facies rocks lies on the southeastern side of the Norwegian Caledonides. Although the geological evidence suggests that it is a far-travelled nappe derived from an internal zone to the northwest, a gravity anomaly of +58 mgals indicates that it is underlain by dense rocks to a depth of 10–15 km, which might form a local root-zone (

Smithson
et al.
1974

). The structural evidence described from the thrust-zone forming the northwestern margin of the massif indicates that the early movements on this boundary were directed towards the southeast. Since southeasterly movements also occurred on the low-angle overthrusts which form its southeastern margin, the Jotunheim Massif is clearly allochthonous, derived by overthrusting from the northwest.
Relationship between diagenesis, porosity reduction, and oil emplacement in late Carboniferous sandstone reservoirs, Bothamsall Oilfield, E Midlands
Authors: Hawkins P. J.
Published: December 2007

In the Bothamsall Oilfield, sandstone reservoirs at the base of the Lower Coal Measures comprise distributary-channel and barrier-bar facies. Diagenetic modifications were achieved principally by chemical processes, with mixed-layer illite/montmorillonite clay, ferroandolomite and ferroancalcite widespread in the barrier-bar facies; quartz, ferroandolomite, and kaolinite important in the channel facies; and quartz, ferroandolomite and siderite prevalent in the overbank facies. Porosity retention is greatest in coarse-grained sandstone located in the axial region of channel facies, and least in fine-grained sandstone where quartz cementation predominates. Early oil emplacement in coarse-grained sandstone of the channel facies arrested quartz diagenesis and inhibited further diagenetic reactions, whereas in barren water-filled zones diagenesis proceeded unimpeded.
The petrography and reservoir properties of some triassic sandstones of the Northern Irish Sea Basin
Authors: Colter V. S. , Ebbern J.
Published: December 2007

Triassic sandstones of the Northern Irish Sea have been correlated on wire-line logs with the succession at the outcrop in Cheshire (Colter & Barr 1975). Recent work on well sections confirms the lithological similarities of these units with the supposed onshore equivalents.

Porosity and permeability of the sandstones are related in the first place to original grain size, particularly argillaceous content. Superimposed on this primary variation are the effects of various diagenetic phenomena: (1) Tangential illite coatings; (2) Platy illite normal to grain surfaces; (3) Calcite cement; (4) Compaction and pressure solution; (5) Quartz overgrowths. Porosities and permeabilities measured in core plugs have been related to primary textures and to the occurrence of these diagenetic changes.
Diagenesis of part of the Brent Sand Formation (Middle Jurassic) of the northern North Sea Basin
Authors: Blanche J. B. , Whitaker J. H. Mc D.
Published: December 2007

Sandstone cores from the Brent Sand Formation were studied by petrographic and reflected light microscopy, cathodoluminescence, scanning electron microscopy and X-ray diffraction. Mineral abundances were obtained by point counting, and porosities and permeabilities by petrophysical methods. The diagenetic changes found by these techniques include (1) early cementation by poikilotopic calcite or, more commonly, pressure solution of quartz with deformation of micas and the development of quartz euhedral overgrowths; (2) partial to complete alteration of feldspars and micas resulting in authigenic growth of euhedral kaolinite and recrystallization of fibrous illite: these clay minerals form bridges across gaps and obstruct pore throats; (3) the occasional growth of small rhombohedra of calcite, dolomite or siderite or more extensive poikilotopic calcite cement with replacement of earlier-formed minerals; and (4) the development of authigenic pyrite and siderite when organic matter is present. These diagenetic changes, which result in loss of porosity and drastic reduction of permeability, are related to original lithology (Grain size and shape, mineralogy and primary porosity) and to subsequent burial history, in which the observed changes are brought about by rise of temperature and increase in pH of circulating pore solutions.
The anatomy of a batholith
Authors: Pitcher W. S.
Published: December 2007

The Mesozoic-Cenozoic Coastal Batholith of Peru is a multiple intrusion of gabbro, tonalite and granite occupying the core of the Western Cordillera over a length of 1600 km. Its structure and composition are described within the context of an intracratonic Andean zone in which vertical movements were dominant. The emplacement was controlled by growth-fractures on all scales and the magmas were channelled to high levels in the crust along a single mega-lineament, to be intruded finally as hundreds of separate plutons. Discussions follow on the space problem, on the assembly (episodic or continuous) of this immense body during 70 Ma, on the extent to which the magmas of the associated volcanic piles were vented to the surface via the subvolcanic ring-complexes, and on the physical nature of the magmas. The rocks of the batholith can be assigned to distinct plutonic units, consanguineous sequences of which form super-units. The outcrop of these along the batholith reveals a compositional segmentation which may correspond with structural and metallogenic segmentation of the Andes as a whole. Further, the super-units represent temporally distinct rhythms of magma generation and differentiation, one following the other, and with increasing overall acidity and decreasing volume. The way in which each separate melt was produced, particularly the triggering role of the gabbros, is discussed in the light of a rather 'uncomfortable' model of subduction beneath progressively thickening continental crust.
The ammonite faunas of the English Portland Beds and the zones of the Portlandian Stage
Authors: Wimbledon W. A. , Cope J. C. W.
Published: December 2007

On the basis of new and large collections of ammonites, a succession of ammonite faunas is described from the Portland Beds of southern England and a revised scheme of Standard Zones (=chronozones) proposed.

The faunas of the Portland Freestone of Dorset are shown to consist largely of hitherto undescribed species;
Titanites anguiformis
sp. nov. is described and figured therefrom. Correla­tions are proposed with other areas where the Portland Beds occur, and the scheme is integrated with the zonal scheme proposed by
Casey (1974)
for the younger Portlandian faunas.
Glaucolithites glaucolithus
Buckman and
Titanites giganteus
(Sowerby) are redefined with the aid of new material.

'Portlandian' is preferred as stage name for the terminal Jurassic stage.
Gneisses in diatremes, Scottish Midland Valley: petrology and tectonic implications
Authors: Graham A. M. , Upton B. G. J.
Published: December 2007

Gneissic clasts occur in Carboniferous volcanic vents in the Scottish Midland Valley. Samples from two widely separated areas are closely similar in petrography and are interpreted as fragments of the same gneiss complex. They exhibit mineral assemblages and textures typical of granulite-facies metamorphism. Mineral chemistry is used to suggest that pressure and temperature of metamorphism exceeded 11 kbar and 850°C in part of the complex. The gneisses are geochemically similar to Lewisian granulites in their depletion in K, Rb, and Y. Seismic evidence indicates derivation from a basement now lying 7 to 8 km down which is unconformably overlain by Ordovician and later rocks. This basement complex may have been an area of positive relief in late Precambrian and early Palaeozoic times, and its existence provides an important constraint on plate tectonic models of the Lower Palaeozoic.
The correction of stratigraphic sections for tectonic finite strain in the Bygdin area, Norway
Authors: Hossack J. R.
Published: December 2007

The deformed sediments beneath the Jotun Nappe were folded and then shortened vertically to produce the main cleavage. All the measurements from the Bygdin Conglomerate (
Hossack 1968a
) have been recalculated using harmonic means (
Lisle 1977
) to get a better estimate of the conglomerate strain. The symmetry of this strain varies locally, but the regional strain can be estimated by integrating the extensions along finite strain trajectories. The minimum value of the integrated conglomerate strain consisted of 65 per cent vertical shortening, a transverse NW elongation of 160 per cent, and a NE elongation of 11 per cent along the Caledonian strike. The vertical shortening is the most constant strain, whereas the NW elongation varies between 390 per cent and 115 per cent. The early folding is estimated to have consisted of 26 per cent shortening along the bedding with 35 per cent extension normal to the bedding. The folding and the integrated strains have been combined in the simplest manner and it is estimated that the present stratigraphic thicknesses have to be multiplied by 2.15 to calculate their original thicknesses. The positions of the nappes before thrusting have also been estimated using the folding and the integrated strains. The Jotun Nappe has a minimum displacement of 290 km and the lower nappes have been thrust for distance between 100 km and 270 km.
Deformation of soft sediments
Authors: Fitches W. R. , Maltman A. J.
Published: December 2007

In recent years there has been increasing awareness of the variety of structures formed by the deformation of unlithified sediment and of the difficulty in distinguishing these from structures produced in rock. Confusion of the two types clearly has serious implications, for example, in orogenic strain sequences where it is necessary to separate pretectonic slump structures from the results of later tectonism. Unfortunately, the features of soft sediment tend to fall within the realm of sedimentology whilst those of deformed rocks are the concern of structural geology so that the region where depositional, compactional, diagenetic and tectonic processes merge tends to be viewed from di­vergent positions and perhaps receives relatively little attention.

The meeting held at Aberystwyth provided a
forum
for workers of differing background to assess recent progress in clarifying this difficult area. About a hundred participants heard a day of talks which made it clear that a wide diversity of phenomena on a variety of scales falls within the conference topic.

The first part of the meeting was concerned with the structures and processes which are largely confined to soft sediments, beginning with large scale features such as olistostromes and moving on to the smaller structures such as convolute laminations. The later part continued through the compaction-lithification sequence, and the effects of tectonic strains became increasingly emphasised.

Discussion following the talks returned repeatedly to two points. Firstly, there was the problem of
defining terms
and especially how to define the onset of lithification. The second recurrent topic was the apparent paucity of reliable
criteria
for distinguishing between penecontemporaneous and post-lithification structures.
Construction and computation of three-dimensional progressive deformations
Authors: Flinn Derek
Published: December 2007

The deformation plot is shown to be ideal for the display of ellipsoid shape, as is its logarithmic transformation for the display of deformation paths of coaxial deformations. Other plots which have been proposed can be derived from the deformation plot by various geometric transformations and by the superposition of other coordinate axes. These variants of the deformation plot are less convenient to use. The deformation plot may also be used for the construction of deformation paths. This use depends on the definition of a number of different types of deformation ellipsoid. In particular, in deformations involving volume change, the shape change arising from the volume change needs to be represented by an ellipsoid. Under these conditions coaxial deformation paths of any imaginable complexity can be easily constructed on the deformation plot. For non-coaxial deformation, matrix representation of the deformation ellipsoids allows deformation paths to be computed, but graphic representation of these results requires both the deformation plot and a stereonet.
Radiometric dates from NW European glauconites and the Palaeogene time-scale
Authors: Odin G. S. , Curry D. , Hunziker J. C.
Published: December 2007

Glauconite is abundant in many Palaeogene rocks and its contained potassium can be used for radiometric dating. The methodology of this dating is described and sources of error and uncertainty are examined. Age determinations are presented on thirty-seven rocks mostly from NW Europe and these are discussed in relation to previous dating work. Ages are assigned to the boundaries of the Palaeocene, Eocene and Oligocene at 65, 52, 33 and 23 Ma respectively.

Des progrés rècents dans la connaissance de la genése et des propriétés géochimiques des glauconies ont permis de préciser la confiance qu'on pouvait attribuer à ce type de chronomètre. L'abondance des gisements glauconieux dans le Paléogène d'Europe permet d'effectuer le choix préalable nécessaire à l'obtention des granules verts les plus probablement representatifs de l'âge de la formation. Les méthodes de sélection sont décrites et discutées. Les âges obtenus sur une quarantaine d'échantillons sont donnés et discutes en relation avec les résultats obtenus par ailleurs. L'ensemble des résultats présèntes permet de préciser ou d'établir l'âge numérique probable des limites du Paléocène, de l'Eocène et de l'Oligocène; 65—52—33—23 · 10
6
ans. En outre on précise l'extension de l'Eocène moyen entre 45 et 39 · 10
6
ans. La position des échantillons par rapport aux biozonations est précisée et des éléments de discussions apportés au sujet des 'datations numériques' basées sur l'extrapolation des données radiométriques en fonction de biozones ou des épaisseurs de sediments.
Transformation- and reaction-enhanced ductility in rocks
Authors: White S. H. , Knipe R. J.
Published: December 2007

Enhanced ductility is expected if deformation occurs during phase changes in minerals or during metamorphic reactions. Transformation-enhanced ductility largely arises from stresses generated by the transformation and may be an important consideration during mantle deformation. Reaction-enhanced ductility is a more complex process reflecting softening due to fluid release, increased diffusivities and the formation of small, strain-free grains. It is suggested that deformation should preferentially occur during the development of new minerals and may be inhibited by subsequent grain growth.
Granite beneath Market Weighton, east Yorkshire
Authors: Bott M. H. P. , Robinson J. , Kohnstamm M. A.
Published: December 2007

A NE-SW elongated negative gravity anomaly of 40 x 20 km
2
areal extent and –15 mgal amplitude occurs near Market Weighton. Thick sediments and salt can both be ruled out as main source of the anomaly using the known geology and depth to magnetic basement. Interpretation in terms of an underlying granite batholith is supported by the pattern of second derivatives and by comparison of gravity and magnetic anomalies. The postulated Market Weighton granite intrudes the pre-Carboniferous basement and extends to more than 8 km depth. The granite penetrates the northern flank of a belt of magnetic basement rocks that give rise to a positive magnetic anomaly extending from west of the Pennine Askrigg Block to the Wash. Local magnetic highs to northwest and south of the granite are attributed to strongly magnetized basement rocks abutting against more weakly magnetized granite, analogous to the southwestern margin of the Wensleydale granite. An apparently locally thinned Carboniferous succession in the Market Weighton region can be attributed to the tectonic influence of the buoyant granite during the Carboniferous. Although the granite is not located centrally beneath the Jurassic and Lower Cretaceous stable block, nevertheless it provides a plausible mechanism for the stability, in association with basement faulting and tilting.
A gravity survey across the Gardar Igneous Province, SW Greenland
Authors: Blundell D. J.
Published: December 2007

The Gardar Igneous Province is believed to involve the remains of Proterozoic rifting which evolved through repeated phases of activity around 1300 Ma, 1250 Ma and 1170 Ma ago. A gravity survey across part of the province has established the presence of a gravity 'high' of some 300 g u amplitude which is 30 km wide and elongated ENE–WSW, centred on Tunugdliarfik and axially along one of the main zones of Gardar intrusives. This gravity anomaly is similar to that of other ancient rift systems and appears to be a residual of the axial anomaly to be found in modern active rifts. It is interpreted in terms of a basic mass underlying a late Gardar rift, some 50 km long and 25 km wide, intruding the crust. Evidence suggests the Gardar rifting cannot be explained in terms of a major crustal separation as an extension of the mid-continental rift and Keweenawan systems of North America. It is more consistent with crustal stretching of no more than 10 km and possibly diapiric mechanisms.

The coastal gravity gradient appears to be due to a change in crustal thickness coincident with the coastline, some 25 km beyond the present ice cap limit. Although Greenland may be in overall isostatic balance, it seems that the coastal zone remains overcompensated.
A use of network diagrams in depicting stratigraphic time-correlation
Authors: Smith D. G. , Fewtrell M. D.
Published: December 2007

Time-correlation among palaeontological samples or other items of geological information located in measured sections can be depicted graphically as a network, in which the samples, represented by points, are linked by arrows, each of which represents a decision as to which of the samples linked is the older and which the younger. The idea of a correlation network stems naturally from the concept of correlation as a process of ordering, not equating, items of geological information in time. Examples of correlation networks are presented for a palynological reconnaissance of Triassic-Jurassic sections in eastern Svalbard, and for correlation of Lower Carboniferous sections in North Yorkshire based on calcareous microfossils.
Vertical and lateral building of river sandstone bodies, Ebro Basin, Spain
Authors: Friend P. F. , Slater M. J. , Williams R. C.
Published: December 2007

Six examples of river sandstone bodies are described from well exposed sections of Oligocene and Miocene age in the Ebro Basin, northern Spain. Three of these are of ribbon form, varying in cross-section from 10 m wide × 2 m high to 20 m wide × 9 m high. In plan these ribbons vary from straight to highly sinuous. The sandstone bodies, in two cases, are multi-storey, clearly demonstrating a series of vertical building episodes. The fourth sandstone body may have a ribbon or sheet geometry, and is of special interest because it demonstrates lateral building by a laterally migrating channel, as well as some vertical building. The fifth and sixth sandstone bodies are of sheet geometry, and demonstrate lateral migration of channels and depositional surfaces.

Factors that appear to be most important in determining whether a river migrates laterally or remains stable laterally are: (1) river mean flow strength, (2) bank strength, (3) flood periodicity and duration, (4) vertical movement of the alluvial area.
Variscan metamorphism and K-Ar dates in the Variscan fold belt of S Brittany and NW Spain
Authors: Ries Alison C.
Published: December 2007

The metamorphism in Lower Palaeozoic rocks in the Variscan fold belt of S Brittany and NW Spain is not evenly distributed but forms HT, LP thermal domes with migmatites and granites, separated by rocks with lower grades of metamorphism. 41 mica dates, ranging from 332 to 262 Ma, obtained from granites, decrease in age westwards along the fold belt through S Brittany into NW Spain. These dates, interpreted as reflecting diachronous cooling of the Variscan fold belt, are correlated with age and intensity of the Variscan metamorphism: the oldest dates correspond to possibly earlier and lower grades of metamorphism in S Brittany while the youngest dates coincide with a later thermal dome in SW Spain. This age pattern supports a simple anticlockwise rotation of the Iberian Peninsula relative to France to form the Bay of Biscay.
Early Cambrian transgression
Authors: Matthews S. C. , Cowie J. W.
Published: December 2007

There is widespread evidence of a major transgression near the base of the Cambrian. It is difficult to establish time-controls, and therefore difficult to produce worthwhile estimates of rate of transgression. Neither the palaeontological nor the radiometric information is sufficiently exact.

Alternative means of comparing the Cambrian transgression with later events of the same kind are sought; the possibility that it is due to post-glacial (i.e. post-Varangian) rise of sea level is examined and rejected; the separation in time seems too large. The preferred explanation is that the Cambrian transgression had tectonic causes and it is argued that among the major transgressions of later geological time, it is the Cretaceous case which most closely resembles the Cambrian.
Bituminous shales and oolitic ironstones as indicators of transgressions and regressions
Authors: Hallam A. , Bradshaw M. J.
Published: December 2007

Bituminous shales can often be demonstrated to be relatively deep water deposits and tend to occur near the base of transgressive sequences, implying rather rapid rises of sea level, but they probably formed at a variety of depths controlled primarily by regional topographic restrictions on bottom water circulation. Oolitic ironstones, in contrast, frequently mark the late stages of regional regressions, with terrigenous input to the depositional area diminishing in many places as the sea shallowed. Many such regional changes in water depth can be related to eustatic rises and falls of sea level, not only for the NW European Jurassic, from which most examples are taken, but for rocks ranging widely through the Phanerozoic. The evidence from bituminous shales and ironstones supports a Jurassic eustatic model characterised by moderately rapid rises and falls of sea level, separated by longer phases of stillstand.
Causes of world-wide changes in sea level
Authors: Donovan D. T. , Jones E. J. W.
Published: December 2007

Possible factors responsible for world-wide sea level changes are reviewed. The major causes are variations in the volume of land ice and changes in oceanic ridge systems, with sediment accumulation in the oceans and desiccation of isolated basins producing second-order effects. Alterations in effective water volume (by ice sheet growth or desiccation) are much faster than changes in ocean basin capacity, but the latter are considered likely to be the cause of long-term trends, which are ultimately related to the history of mantle convection.
Structure, metamorphism an geochronology of the Arequipa Massif of coastal Peru
Authors: Shackleton R. M. , Ries A. C. , Coward M. P. , Cobbold P. R.
Published: December 2007

The Arequipa Massif, between the Andes and the Pacific, is an extensive pre-Devonian metamorphic complex. The sequence of deformations, metamorphisms and magmatism in this complex has been established. Mollendo, Atico and Marcona events are distinguished by structural and metamorphic methods and dated by Rb-Sr whole-rock isochrons, at about 1918, 440 and 392 Ma respectively. The Mollendo event led to partial melting, followed by granulite-facies metamorphism, in sediments buried to about 30 km. Further NW, sillimanite-bearing migmatites and staurolite-andalusite schists are thought to represent the same event. The tectonic trend is uncertain but the structures and metamorphism suggest a collision orogeny which probably pre-dated the Pacific Ocean.

The early Caledonian Atico and Marcona events are associated with coast-parallel batholiths, amphibolite- to greenschist-facies metamorphism and penetrative deformations. The Atico and Marcona events are separated by the deposition of the Marcona Formation, which is therefore thought to be Lower Palaeozoic (between about 440 and 392 Ma). The early Caledonian deformations are attributed to a subduction zone near the present Pacific margin. There is no penetrative Hercynian or Andean deformation in the Arequipa Massif.

Palaeomagnetic study of Jurassic andesites and dykes suggests that there has been no latitudinal motion of the Arequipa Massif relative to the Brazilian shield during the evolution of the Andes.
A Rb-Sr whole rock isochron for the Stockdale Rhyolite of the English Lake District and a revised mid-Palaeozoic time-scale
Authors: Gale N. H. , Beckinsale R. D. , Wadge A. J.
Published: December 2007

The extrusion of the Stockdale Rhyolite, an upper Ordovician lava lying within Zone 2 of the Cautleyan Stage of the Ashgill, is dated by the Rb-Sr whole rock isochron method at 421 ± 3 Ma. This datum is more precise than any published Lower Palaeozoic age. Samples of the rhyolite from within the thermal aureole of the Shap Granite show Rb-Sr systematics apparently unaffected by baking. A revised mid-Palaeozoic time-scale is presented.
The geology of the Kish Bank Basin
Authors: Dobson M. R. , Whittington R. J.
Published: December 2007

A gravity low immediately E of Dublin is interpreted as an Upper Palaeozoic to Cenozoic basin on the basis of seismic reflection profiles and direct sampling evidence. Maximum thickness of the sediment fill may exceed 4 km. It is considered to consist predominantlyof Carboniferous, undivided Permo-Triassic, and remnant proved Liassic. Thin Tertiary units, which unconformably overlie the main fill, are linked to sediments of similar character found in the Central Irish Sea Basin. Thick Quaternary deposits complete the sedimentary sequence. The basin is structurally a half graben located between two narrow, fault-bounded basement ridges, the Mid Irish Sea Uplift to the S and the Lambay Ridge to the N. The whole structure is bisected by a major NNW-SSE trending fault zone which with the margin faults has created two trapdoor-like structures. It is thought that the principal phases of movement which were instrumental in preserving the sediments occurred during the Triassic and late Jurassic period.
A Lower Eocene ash sequence in SE England
Authors: Knox R. W. O'B. , Ellison R. A.
Published: December 2007

Sections of basal London Clay in the Stour estuary area have revealed a sequence of more than 40 argillized ash layers, which are coeval with those of the Danish Mo-clay and the North Sea 'Ash Marker'. As with the 'Ash Marker', it is uncertain to what extent Hebridean volcanoes or the postulated Skagerrak volcano have contributed to the ash deposits.