bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural controls on Palaeozoic basin evolution and inversion in southwest Wales
Authors: POWELL CERI M.
Published: December 2007

The tectonic evolution of SW Dyfed is presented in terms of a model of basin evolution and subsequent destruction by reactivation of the basin bounding faults in contractional mode. Movements along long-lived Palaeozoic extensional faults were reversed during Variscan compression, with the pre-existing faults accommodating the contraction in addition to the development of a thin-skinned thrust and fold system. Basin analysis of the pre-Variscan extensional system uses sedimentological and stratigraphical evidence to constrain the timing of movements of the basin bounding faults, thus allowing division of the stratigraphy into depositional megasequences related to these extensional movements. Field examples on all scales illustrate active extension during the Palaeozoic. Structural cross sections through SW Dyfed demonstrate the interrelationship of the extensional and contractional deformations, with palinspastic restorations of the cross sections illustrating the complexity of the pre-Variscan template.
The significance of garnet in the Permo-Carboniferous volcanic rocks of the Pyrenees
Authors: GILBERT J. S. , ROGERS N. W.
Published: December 2007

Garnet occurs within rhyolitic to dacitic lavas, ignimbrites and volcaniclastic sediments of the Pyrenean calc-alkaline Permo-Carboniferous volcanic rocks. On the basis of texture and major and trace element composition, the garnet is interpreted as having a phenocrystal as opposed to a xenocrystal or restite origin.

The Pyrenean silicic volcanics are strongly peraluminous, consistent with derivation as a partial melt from a sedimentary source. They were erupted contemporaneously with the Hercynian thermal anomaly, which produced regional granulite facies metamorphism and anatectic granites. The ‘Late Hercynian granodiorites’ were also intruded at about this time. Geochemical data suggest that the petrogenesis of the volcanic rocks differed from that of the granodiorites.

REE garnet/matrix partition coefficients for garnets from a welded rhyolitic ignimbrite are similar to those reported for other phenocrystal garnets in rhyolites. REE abundances imply that the garnets crystallized from a HREE depleted melt and this is considered to reflect the effects of garnet in the restite. Experimental constraints suggest that the garnet crystallized at pressures >4–5kb, implying that the rhyolites originated from > 14–17km depth and were erupted rapidly from these depths in order for the garnet to survive. These figures place constraints on the thermal regime prevailing in the late stages of the Hercynian orogeny.
Structure of some Lower Old Red Sandstone conglomerates, Kincardineshire, Scotland: deposition from late-orogenic antecedent streams?
Authors: HAUGHTON P. D. W.
Published: December 2007

A thick succession of early Lower Old Red Sandstone (ORS) conglomerates and subordinate sandstones are preserved in a small fault-bounded basin in the NE Midland Valley of Scotland. The conglomerates associated with the northern margin of this basin are distinguished by a complex provenance which involved both reworking of existing gravels and addition of contemporary volcanic and plutonic detritus. This may explain their characteristic bimodal texture. Deposition was in large, probably permanent, low sinuosity channels with evidence for both periods of uniform high stage flow and repeated flow stage fluctuations. Variable palaeoflows in the basal part of the succession suggest deposition on wet-type alluvial fans supplied by feeder channels with a high sediment and water discharge. The thickness of the succession, the poorly developed cyclothems (despite active syndepositional faulting), evidence for rapid aggradation, and the inferred scale of channel bars and coarsening-upward units produced by bar migration suggest the basin may have been externally drained. Transverse antecedent streams are invoked to account for the structure of alluvium. During the early Devonian, central Scotland lay close to the boundary between regions suffering Scandian orthogonal shortening and Acadian strike-slip shear. The tectonic controls which governed the preservation of early Lower ORS sediments may have been largely uncoupled from those determining the supply and dispersal of the sediment.
Distal alluvial fan sediments from the Upper Jurassic of Portugal: controls on their cyclicity and channel formation
Authors: HILL GRAHAM
Published: December 2007

The Lusitanian Basin of west-central Portugai is an Iberian Atlantic margin basin. During a late Jurassic rifting event, a series of alluvial fans distributed coarse, basement-derived clastics, southeastward from the western margin of the basin. The distal part of this sequence, which is about 140 m thick, consists of heterolithic, massive sandstone and massive mudrock facies, arranged in crude fining-upwards cycles 1–15 m thick. These form discrete packets of sediment in a strongly aggrada-tional sequence and are interpreted as the products of sheetflooding as the active area of sedimenta­tion on the fan surface switched. Channelized flow was not necessarily an early feature of cycle development. Considerable fluctuations in discharge occurred and were probably related to climatic factors. Large quantities of tractional and suspended sediment were transported at high stages, and were rapidly deposited as flows waned. Caliche soil profiles indicate a seasonal semi–and climate. However, other features such as widespread soft-sediment deformation, the lack of desiccation structures and the absence of evaporites suggests that areas of active sedimentation often remained wet. Three fining-upward megacycles are distinguished. These indicate rapid advance of fan toes triggered by tectonic activity followed by gradual retreat.
Vitrinite reflectance data from the Kinsale Harbour–Old Head of Kinsale area, southern Ireland, and its bearing on the interpretation of the Munster Basin
Authors: CLAYTON GEOFFREY
Published: December 2007

Vitrinite reflectance of the Upper Devonian–Carboniferous succession in the Kinsale Harbour–Old Head of Kinsale area ranges from 4.06–5.21% (

m

) and 6.21–7.82% (
max
) which is indicative of meta-anthracite coal rank. No correlation is apparent between vitrinite reflectance and stratigraphic position through
c.
2 km of section, suggesting that near-maximum temperatures were maintained until after Variscan folding. Low reflectance values of 0.62% and 0.30–0.35% (

m

) at the local base of the Jurassic succession in the North Celtic Sea Basin (Well 48/30-1) and onshore at Cloyne respectively, indicate a major discontinuity in the maturation profile in the region. Comparison with high reflectance gradients from Upper Palaeozoic successions in the Halen and Turnhout boreholes in Belgium and the Münsterland No. 1 borehole in West Germany suggests that a thick (5–7 km) cover of pre-Permo-Tnassic rocks must once have been present in the Munster Basin to account for the high maturation levels observed. Whether the cover was emplaced by Silesian sedimentation or by late Palaeozoic thrusting is unclear.
Application of geochemistry to the stratigraphic correlation of Appin and Argyll Group carbonate rocks from the Dalradian of northeast Scotland
Authors: THOMAS C. W.
Published: December 2007

240 samples from Appin and Argyll Group carbonate rock units of the Dalradian of northeast Scotland have been analysed for 10 major oxides and 14 to 19 trace elements in order to classify and correlate them using geochemistry. The carbonate rocks are chiefly limestones, with some dolostones. Rocks with 0.38 > MgO/CaO > 0.03 are classed as dolomitic limestones; those with MgO/CaO > 0.38 are classed as dolostones. The data have approximately lognormal distributions at a high level of significance except for CaO, MgO and Sr. CaO and Sr have negatively skewed distributions. Median values for groups of sample analyses plotted on normalized multi-element variation diagrams and a nonlinear mapping (NLM) algorithm have been used to classify groups of samples. In the Appin Group, multi-element diagram patterns are distinctive for each unit, allowing correlations to be made. Though distinctive compositions do occur locally in Argyll Group carbonate rocks, correlation on a regional scale is not possible. The plot of NLM coordinates for suites of samples is consistent with the results from the spider diagrams; suites of Appin Group samples cluster closely whereas there is wide spread in samples from the Argyll Group. In several areas, most notably at Sandend Bay, the data have allowed reinterpretation of the stratigraphy. The data are consistent with other geological data in suggesting that crustal conditions were relatively stable during the deposition of the Appin Group, but not during the deposition of the Argyll Group.
Structural controls on metamorphism and syn-tectonic magmatism: the Portuguese Hercynian collision belt
Authors: REAVY R. JOHN
Published: December 2007

Upper Palaeozoic collisional movements along the Ibero-Armorican arc were accommodated on oblique strike-slip ductile shear zone systems. In north-central Portugal, these shear zones are coincident with long linear belts of high temperature, low pressure metamorphism and S-type granitic plutons. A model is presented in which crustal thickening subsequent to collision was followed by anatexis at depths of 12–15 km; this was achieved by a combination of (i) increased heat flow, (ii) lowering of melting temperatures caused by fluxing of hydrous fluids, and (iii) a heterogeneous assemblage of greywackes and pelites which encouraged local high ductility contrasts. Shear zones are proposed as the most important influence in controlling the site of development of such belts; anatexis of metasedimentary crust and emplacement of the resulting S-type plutons within these zones took place without an appeal to crustal rifting or an asthenospheric heat source.
Isotopic and petrographic studies of uraniferous hydrocarbons from around the Irish Sea Basin
Authors: EAKIN PAUL A.
Published: December 2007

Stable carbon isotopic ratios of a suite of uraniferous hydrocarbons (Lower Jurassic) from the northeast Wales ore field and related deposits have been determined. The isotopic compositions (–11.41 to –29.36% PDB) become increasingly heavy with increasing uranium content (0.001–35.78wt%). Fractionation is thought to occur by repeated preferred bond breaking between
12
C–
12
C atoms with subsequent volatile production and loss, removing
12
C atoms. The extent of fractionation may also be controlled by the amount of stable aromatic material in the organic matter. Petrographic features of these uraniferous hydrocarbons, and others from occurrences in South Africa and Scandinavia, suggest reaction of radiation with organic matter. Alpha particles may be responsible for the development of high reflectance haloes around uraninite grains in uraniferous hydrocarbons from the Witwatersrand region. Bitumen reflectance falls in a non-linear trend from 3.75 to 1.75% (in oil) in a 20 μ
m
radius round the grains. During irradiation, bitumen anisotropy (measured as bireflectance) is developed in response to increasing aromaticity and the development of a short range ordered structure. Bireflectance measurements vary from 0.047%–0.173% in samples from around the Irish Sea Basin, and 0.001%–0.294% in a suite of samples from Scandinavia. The degree of anisotropy is dependent on the bitumen composition, and varies between sample suites of differing geological histories.
Genetic studies of red bed mineralization in the Triassic of the Cheshire Basin, northwest England
Authors: NAYLOR H. , TURNER P. , VAUGHAN D. J. , BOYCE A. J. , FALLICK A. E.
Published: December 2007

The Triassic sediments of the Cheshire Basin are host to a number of small mineral deposits characteristic of red beds. The mineralization comprises Cu and Pb with minor Zn, Ag, CO, V, Ni, As, Sb and Mn which, together with widespread barite mineralization, is closely associated with basin margin faults. The Wilmslow Sandstone Formation is cemented by authigenic quartz overgrowths enclosing iron sulphides, which predate the main sulphide cements of tennantite, bornite and sulpharsenides. Studies on late calcite-hosted fluid inclusions show homogenization temperatures ranging between 60–70
o
C (regarded as the minimum temperature of mineralization) and salinities of 9–22 wt% equivalent NaCl (mean = 17wt%), indicating that the mineralizing fluids were warm and saline. Consideration of the basin history suggests that fluid temperatures in the Permo-Triassic sequence may have reached 150 °C at the time of mineralization. New data significantly constrain the genetic hypothesis for the deposits, refuting a magmatic origin. A basin brine expulsion model is favoured. The sulphur isotope data from the barite mineralization (mean δ
34
S = +17‰) suggest that the bulk of the barite sulphur was derived directly from the Upper Triassic evaporites (mean δ
34
S = +19‰), whilst the distribution of δ
34
S values for sulphides (–1.8‰ to +16.2‰) is consistent with ultimate derivation of sulphur from the evaporites by closed system reduction of a sulphate-bearing brine. It seems likely that the reducing agent was organic material (methane) derived from the underlying Coal Measures, but it is uncertain at present whether the principal mechanism was thermochemical or bacterial reduction. The pattern of sulphide δ
34
S favours the former process.
Short Paper: Arc reversal in the Scottish Southern Uplands?
Authors: MITCHELL A. H. G.
Published: December 2007

Geological data permit interpretation of the Northern Belt of the Southern Uplands as an Ordovician forearc basin in a northwest-facing arc. This collided with Laurentia in the latest Ordovician, reversed, and collided with Cadomia in the early Devonian.

Since the earliest plate tectonic model for the British Caledonides (
Dewey 1969
) and the interpretation of the Southern Uplands as ocean floor and trench sediments imbricated at a southwest-facing arc (
Mitchell 1974
), the concept of northwestward subduction beneath the Northern Belt has been more or less undisputed (Mitchell & McKerrow 1975;

Leggett
et al.
1979

;
Leggett 1987
;
McKerrow 1988
; McKerrow & Soper 1989). However, the more recent geological, as opposed to palaeontological, data (
McKerrow 1987
, and references therein) are more compatible with interpretation of the Northern Belt as a forearc basin formed above southeastward-subducting lithosphere.

In the proposed model (Fig. l), by the Llanvirn or Llandeilo, subduction of an oceanic basin, Iapetus 1, had begun beneath an are system to the southeast (Fig. 1A). There was no major accretionary wedge and southeast of the forearc ridge, in the Northern Belt forearc basin, diachronous deposition of Moffat Shale was followed by greywacke sedimentation and local eruption of basaltic lavas. Until the Caradoc the associated ‘Cockburnland’ volcanic arc lay immediately to the south of the Northern Belt (
Morris 1987
;

Stone
et al.
1987

; Hutton & Murphy 1987) and possibly included the Lake District on the northern margin of Cadomia. The northwest flank of the volcanic arc supplied andesitic detritus to the forearc basin (Morris
Short Paper: Palynological evidence from the Porthleven area, south Cornwall: implications for Devonian stratigraphy and Hercynian structural evolution
Authors: WILKINSON J. J. , KNIGHT R. R. W.
Published: December 2007

Detailed structural and lithological mapping combined with a paly-nological study of Devonian sediments in the Porthleven area, have revealed that previous structural and stratigraphic interpretations are seriously in error. Palynomorphs recovered from sediments classically assigned to the Gramscatho Group and generally regarded as pre-Famennian indicate a late Famennian age. A reinterpretation of the section is proposed which emphasizes the importance of imbricate thrusting and local facies variations.

Interpretations of the Devonian geology of south Cornwall have been hindered by the lack of palaeontological control on the ages of the main lithostratigraphic units. This paper details new palynomorph assemblages which necessitate a reinterpretation of the structure and stratigraphy of the south Cornish nappe and thrust terrain (Rattey & Sanderson 1984; Barnes & Andrews 1986; Holder & Leveridge 1986).

Palynology.
A suite of 14 samples was collected from the parautochthonous Mylor Slate Formation and allochthonous Portscatho Formation (Holder & Leveridge 1986), on the west coast of the Lizard Peninsula (Fig. 1). Palynomorphs were recovered from three samples representative of the sequence exposed immediately southeast of Loe Bar [SW646237-SW647234]. Within this section, traditionally referred to the Gramscatho Group, sediments of both Mylor and Gramscatho affinity are intercalated. The finer grained (Mylor-type) lithologies were preferentially sampled and yielded comparatively diverse, rich and remarkably well-preserved (although opaque) palynomorph assemblages. Yields range from 22–146 specimens per gramme of sediment. The kerogens were oxidized using potassium chlorate and fuming nitric acid, and in transmitted light were found to be composed almost exclusively of miospores, with occasional acritarchs
Short Paper: The palynology and correlation of the Walton Crag (Read Crag Formation, Pliocene)
Authors: HUNT C. O.
Published: December 2007

The basal sands of the Red Crag Formation at Walton-on-the-Naze, Essex, contain a pollen assemblage dominated by
Pinus
and with pollen of Taxodiaceae and thermophilous trees. This is comparable with assemblages from the Dutch Reuverian ‘
C’
(Late Pliocene). It is not closely comparable with spectra from other sections previously correlated with the Red Crag Formation at Walton, which arguably have Praetiglian affinities. Dinoflagellate cysts typical of warm-temperate water are also present, and compare well with assemblages from the Red Crag Formation in the southern North Sea. The ‘Pre-Ludhamian’ and ‘Waltonian’ are therefore separate stages assignable to the Pleistocene and Pliocene, respectively.

The base of the Walton Crag, the lowest division of the Red Crag Formation, at Walton-on-the-Naze, was long considered to be the local base for the Pleistocene in the British Isles, largely because of the presence of supposedly characteristic Pleistocene mammal fossils such as
Mammuthus meridionalis, Equus
sp. and
Hipparion
sp. (
Boswell 1952
;

Mitchell
et al.
1973

; Funnell & West 1977;

Harland
et al.
1982

). Other opinion (
Zagwijn 1974
; Funnell
in

Curry
et al.
1978

;

Cameron
et al.
1984

) would attribute the Red Crag Formation to the Late Pliocene on the grounds of mollusc and foraminifer correlation with Belgian and Dutch Pliocene sequences.

The foraminifers from the Red Crag Formation at Walton-on-the-Naze are closely comparable with those from the Crag in the lower part of the Stradbroke Borehole (

Beck
et al.
1972

; Funnell & West 1977). Pollen assemblages from the same unit in the Stradbroke Borehole were dominated by
Short Paper: A standardized model for Tethyan Tertiary carbonate ramps
Authors: BUXTON M. W. N. , PEDLEY H. M.
Published: December 2007

Low latitude carbonates in massive and clinoform bedded sheets, termed ramps, occur throughout the Tertiary Tethyan realm, and are characterized by recurring biofacies usually including large foraminifera, rhodolithic algae, coralgal patch-reef and gastropod dominated sequences. All can be interpreted broadly within a standardized Tertiary carbonate ramp model. Biota can be used to define precise palaeobathymetric zones which have a predictive utility in palaeoenvironmental modelling, especially where data are limited.

The carbonate ramp model of earlier authors, has been elaborated by
Read (1982)
and ramp formation occurs mostly in passive margin and extensional tectonic settings. Ramps are characterized by slope gradients of less than 1 degree and, once established, are conservative to change. Much use has been made of the ramp model for Palaeozoic and Mesozoic lithofacies modelling. In contrast, published work on Cenozoic ramps is limited, however, Tethyan ramps are particularly widespread and extend tens to hundreds of kilometres along strike and occupy tens of kilometres of slope. Many studies have dealt with Cenozoic ramp associations and sequences (e.g.
Pedley 1983
; Reiss & Hottinger 1984;
Purser 1973
) and have demonstrated the diversity of biotal and sediment detail, but without ascribing them to a ramp model.

Biota were quick to adapt to the Tertiary environments left vacant after the K–T boundary extinction events. New genera of Foraminifera, Mollusca, Bryozoa and Echinoder-mata rapidly replaced the extinct forms in bursts of innovative opportunism. Codiacean algae, Rhodophyta and marine angiosperms were equally adaptive colonizers of the carbonate platforms. Novel biofacies associations were derived from
Thermal and tectonic history of the East Midlands shelf (onshore UK) and surrounding regions assessed by apatite fission track analysis
Authors: GREEN P. F.
Published: December 2007

Apatite fission track analysis has been used to study the thermal and tectonic history of the East Midlands Shelf and surrounding areas of onshore UK. Apatites from outcrops of Palaeozoic crystalline basement give fission track ages between 300 ± 29 and 420 ± 31 Ma, which are the oldest yet reported in the UK region and reflect the relative stability of this cratonic region. Mean track lengths in these samples are between 12 and 13
μm
. Namurian and Triassic sandstones from outcrop give apatite fission track ages between
c
. 100 and
c
. 390 Ma, with mean track lengths generally between 10 and 12.5
μm
. Distributions of single grain ages and track lengths suggest that all samples have been annealed (implying exposure to higher temperatures) since deposition, with maximum palaeotemperatures reached some time in the 50–80 Ma range prior to the onset of cooling.

Samples from five exploration wells show consistent patterns, in which apatite fission track ages decrease sharply from around or just less than the stratigraphic age at near surface, to values in the 50–80 Ma range at depths of
c
. 1–2 km (present temperatures of
c
. 40–70 °C), showing that the samples studied have all been exposed to higher temperatures. Single grain ages and track length data suggest that maximum palaeotemperatures were attained at or before
c
. 60 Ma. Subsequent cooling was protracted and the data are compatible with two phases, in the early and mid-Tertiary. Palaeotemperature analysis suggests that palaeo-geothermal gradients were similar to present values, and that elevated palaeotemperatures were due to greater depth of burial, with cooling due to subsequent uplift and erosion. Estimates of uplift and erosion vary from
c
. 1–1.5 km on the east coast (around the Humber estuary) to
c
. 2.2 km in the East Midlands and southern Pennines.

Compilation of palaeotemperature estimates from all samples analysed from the UK onshore region shows that Mesozoic burial and Tertiary uplift has affected a wide area, and that uplift was due to broad-scale, regional warping, with recognized ‘inversion axes’ being areas where uplift and erosion were particularly pronounced or where younger sediments and/or inverted basinal structures allow the identification of uplift and erosion. The recognition of Late Cretaceous/early Tertiary unconformities across wide areas of northwest Europe, both onshore and offshore, suggests that the regional uplift and erosion may have been far more widespread than previously recognized. This has important implications for hydrocarbon exploration in Northwest Europe.
The palaeomagnetism of a Precambrian metamorphic boundary in the Loch Torridon area, NW Scotland
Authors: POPPLETON T. J. , PIPER J. D. A.
Published: December 2007

A palaeomagnetic conglomerate test of Lewisian amphibolite clasts in basal Torridonian Group sediments is used to demonstrate that the magnetic record of the Lewisian complex predates Torridonian sedimentation; it appears to have survived with minimal overprinting since post-Laxfordian uplift and cooling. A traverse across a Precambrian tectono-metamorphic boundary on the north side of Loch Torridon identifies progressive replacement of a relict remanence as the zone reworked during the Laxfordian events is entered. The relict remanence (E steep positive) is predominant in the Scourian Terrane and probably dates from the Laxfordian LM1 metamorphism. It survives locally only in dyke interiors in the zone profoundly affected by LMl and LM2. A single coherent NW shallow remanence is identified on the south side of Loch Torridon and is linked to pervasive effects of the LM3 episode here. The degree of magnetic overprinting does not appear to be related to the extent of Laxfordian fabric development, but it is closely linked to dyke contacts probably because these zones permitted a more ready access of metamorphic fluids.
Short Paper: Kinematics of the Alpine arc
Authors: FRY NORMAN
Published: December 2007

Is there a continuous sweep in direction of Alpine orogenic displacements around the Alpine arc, or does the overlap of a few distinct directions give this impression? The thrust evolution of the SW Alps, which supports the latter view, does not accord with the hypothesis of indenter-induced body forces. Two displacement realms, one with directions determined by intraplate configuration and another with directions related simply to the body forces, maycoexist within the orogen, but this is not yet established.

One of the longest-standing disagreements in Alpine geology concerns the nature of the radial pattern of orogenic displacements outwards from the Alpine arc. In one view, there is a continuum of displacement directions sweeping from approximately southward in the Maritime Alps, through west and northwest to approximately northward in Austria. In the other view, there is no continuum, but a small number of distinct directions of orogenic displacements, each dominating a particular sector, which relay one to another around the Alpine arc. Recent papers mark a significant development in both sides of this argument, which is given additional importance by the attempt of Platt et al. (1989
a
) to use displacement data to constrain plate motions. This paper is intended as a contribution to the discussion and suggests one possible resolution of the argument.

Sophisticated syntheses of Alpine structure have for a long time accepted the possibility that both of the above views have some validity, and
Goguel (1963)
is a good starting point for discussion. His displacements of crystalline basement of
Short Paper: Accretion and uplift in the Shimanto Belt, SW Japan
Authors: AGAR S. M. , CLIFF R. A. , DUDDY I. R. , REX D. C.
Published: December 2007

K-Ar cleavage dating and fission track cooling ages have been combined with existing stratigraphic data to constrain the evolution of the Shimanto Belt, a Cretaceous-Neogene accretionary complex. Cleavage ages record two major phases of accretion during the late Cretaceous and late Eocene-early Oligocene. The minimum time periods between sedimentation and deformation are constrained to 9,
5
, 9 and 0Ma intervals within four tectonic units containing youngest sediments of Maastrichtian, late Eocene, late Eocene and early Oligocene ages respectively. One phase of middle to late Miocene uplift is indicated for the whole study area by the fission track cooling ages. Frontal underplating combined with out-of-sequence thrusting are suggested as mechanisms for this regional uplift.

The Cretaceous-Neogene Shimanto Belt (Fig. la) forms the outermost, onshore exposure of accreted material in SW Japan. The boundaries of constituent accreted units of the Shimanto Belt are defined by a detailed stratigraphy (Taira
et al.
1980Taira
et al.
1988) providing an opportunity to test the application of cleavage dating and fission track studies to accretionary complex evolution. The Butsuzo Tectonic Line (Fig. la) juxtaposes the Shimanto Belt to the Jurassic Sanbosan Belt in the north. Younger accreted material extends offshore to the south as far as the present-day boundary of the Eurasian Plate, the Nankai Trough (Fig.la). Continued accretion seaward of the Shimanto Belt has back-rotated older accreted units to their present-day, steeply dipping orientations.

The type section of the Shimanto Belt (Taira
et al.
1980) lies in the eastern Hata Peninsula (Fig. 1b) which contains the study
Short Paper: A note on Rb-Sr whole-rock ages from cleaved mudrocks in the Welsh Basin
Authors: EVANS J. A.
Published: December 2007

Two cleaved, epizone-grade Palaeozoic mudrock suites from Snowdonia yield a mean Rb-Sr whole-rock age of 409 ± 11 Ma. This is consistent with resetting of the Rb-Sr whole-rock isotope systems of the mudrocks during cleavage development in end-Silurian/Early Devonian (Acadian) deformation. In contrast, cleaved Palaeozoic anchizone mudrocks from north and central Wales give a Silurian mean age of 430 ± 9 Ma. This dates either a pre-tectonic isotopic event preserved through Acadian anchizone metamorphism, or mid-Silurian deformation and metamorphism at anchizone grade unaffected by the Acadian event.

The precise timing of the main Caledonian deformation in Wales has never been established firmly. Powell & Phillips (1985) suggest that it occurred in north and south Wales in latest Silurian time, whereas Ashgill and end-Llandovery tectonic events were dominant in central Wales (see also
George 1963
). However,
Woodcock (1984
, 1987)
and Soper
et al.
1987 have argued in favour of a single tectonic event of early to middle Devonian age. This chronological study was undertaken to address this controversy.

The only previous attempt to define precisely the timing of cleavage development in Wales was the K-Ar dating of cleavage-parallel chlorite from a volcanic greywacke from north Wales (

Fitch
et al.
1969

). When recalculated using modern decay constants, these data give an age of 399 ± 10 Ma.
Bath (1974)
demonstrated the feasibility of obtaining Rb-Sr ages from cleaved mudrocks of the Welsh Caledonides, and this work produced ‘Caledonian’ ages of 409 ± 20Ma and 393 ± 22Ma from north Wales and ‘late
Short Paper: Textures of forsterite–calcite marbles from the Beinn an Dubhaich aureole, Skye, and implications for the structure of metamorphic porosity
Authors: HOLNESS MARIAN B. , BICKLE M. J. , HARTE B.
Published: December 2007

In the aureole of the Beinn an Dubhaich granite, forsterite grew with dihedral angles of less than 110° against calcite, but was progressively and variably annealed towards the equilibrium dihedral angle of 165°. New experimental estimates of calcite–calcite–fluid dihedral angles are presented, ranging from 50–90°. The early low angles may be due to rapid olivine growth into a fluid-filled porosity. The preservation of a range of textural states in the Beinn an Dubhaich aureole has the potential to provide information on the kinetics of grain boundary readjustments in metamorphic rocks, which control pore geometry and permeability.
Short Paper: Evidence for a ‘Grenville’ event in the Lewisian of the northern Outer Hebrides
Authors: CLIFF R. A. , REX D. C.
Published: December 2007

Northern Britain lay between two areas of major tectonic activity in the late Proterozoic but evidence for geological events of this age within Britain are sparse. Twenty five new Rb–Sr biotite ages from a 50 km traverse through the northern Outer Hebrides show that a block, bounded in the south by the Langavat Shear Zone, was generally uniformly affected by an event close to 1.1 Ga. Consideration of the age pattern on either side of the Langavat Belt suggests that there were kilometre-scale displacements on this shear zone at that time, involving uplift and erosion of the northern Outer Hebrides as a whole. It is thus possible that the northern Outer Hebrides acted as a source area for both Laxfordian and ‘Grenville’ detritus found in the Torridonian.
A dinoflagellate cyst record for the last 0.7 Ma from the Rockall Plateau, northeast Atlantic Ocean
Authors: HARLAND R. E. X.
Published: December 2007

Organic-walled dinoflagellate cyst analysis of samples previously used to establish a high resolution oxygen and carbon isotope stratigraphy for the last 0.7 Ma in the northeastern Atlantic is detailed. Data from Hole 552A, Leg 81 of the Deep Sea Drilling Project, drilled on the western flank of the Rockall Plateau reveal peaks of dinoflagellate cyst recovery relatable with isotope stages 1, 11, 13 and 17. Cysts most prersistently present include the oceanic
Impagidinium
species,
Nematosphaeropsb labyrinthea
(Ostenfeld) Reid and the ubiquitous
Operculodinium centrocarpum
(Deflandre & Cookson) Wall. The results offer a correlation between the oceanic oxygen isotope stratigraphy and the dinoflagellate cyst record. The potential for further correlation with the extensive Quaternary marine sequences of the continental shelf around the British Isles is explored. The dinoflagellate cyst signal is an important factor in understanding oceanographic changes both at the surface and at depth.
Tobago, West Indies, a fragment of a Mesozoic oceanic island arc: petrochemical evidence
Authors: FROST CAROL D. , SNOKE ARTHUR W.
Published: December 2007

The pre-Cenozoic rocks of Tobago, West Indies, represent an exceptional cross-section through a portion of a composite Mesozoic arc system, other parts of which may be found in the Villa de Cura klippe, the Aruba-Blanquilla island chain, and perhaps in the Western Cordillera of Colombia and Ecuador. The geochemical data from Tobago presented in this paper provide a basis from which a regional view of the petrologic character of the accreted Mesozoic arc of the Southern Caribbean Mountain system can be deduced.

The Mesozoic rocks of Tobago can be divided into four main groups: the North Coast Schist, the ultramafic to mafic plutonic complex, the Tobago Volcanic Group, and late mafic dykes. Rocks of the North Coast Schist served as wallrock for the plutonic–volcanic–dyke complex, which evolved during the mid-Cretaceous. Geochemical and Nd and Sr isotopic data presented demonstrate the oceanic island arc character of all the pre–Cenozoic rocks of the island. All suites span the calc-alkaline and tholeiitic regions on discrimination diagrams. Rare Earth element (REE) patterns for the North Coast Schist, Tobago Volcanic Group and plutonic suite are essentially identical and are moderately LREE-enriched (La/Yb = 1.1–2.7). The dykes exhibit the most unevolved REE patterns, and are interpreted as most closely representative of parental liquid compositions. Initial ɛ
Nd
(+6.6 to +9.4) and
87
Sr/
86
Sr (0.70303–0.70364) for all samples except North Coast Schist and Tobago Volcanic Group argillites are similar in value and overall range to other oceanic island arcs. Limited isotopic data from argillite samples from Tobago suggest an important change in the continental-derived sediment contribution during Mesozoic arc growth. Nd isotopic data from North Coast Schist siliceous argillite indicate virtually no continental detritus in this Neocomian sediment, while Nd isotopic data from the Tobago Volcanic Group argillite suggest a definite continental contribution during Albian arc growth.
Seismic velocities from the Kohistan Volcanic Arc, northern Pakistan
Authors: CHROSTON P. N. , SIMMONS G.
Published: December 2007

The rocks bounded by the Main Mantle Thrust and the Northern Suture in northern Pakistan constitute an exhumed section through a Cretaceous volcanic arc. Samples have been collected from all the principal lithological groups of the arc, and P and S-wave seismic velocities have been measured in the laboratory with the prime objective of comparing the velocities with those determined by seismic refraction experiments on modern volcanic arcs. Velocities were measured at up to 0.7 GPa and consideration has been given to the effect of confining pressure, pore pressure and temperature and to the deformation and metamorphism involved in the Himalayan collision.

The reconstructed velocity section through the arc shows a distinct ‘upper crust’ comprising granitic-dioritic intrusions, metasediments and volcanics with a P-wave velocity of 6.2–6.4 km S
−1
depending on the parameters used. Beneath, the ‘lower crust’ of amphibolitcs and pyroxene granulite has a velocity mainly about 6.4–6.7 kms
−1
, though the garnet granulitcs extend to 7.8 km S
−1
. The more mature arc with a higher proportion of granitic rocks would show a slightly lower upper crustal velocity than the younger arc. Velocity inversions might be expected with a thermal gradient of as little as 10–15
°
C km
−1
, depending on the pore pressure. In general, the proposed velocity structure is comparable with that of volcanic arcs and with many other sections of the continental crust.
Tectonic evolution of the Northumberland Basin; the effects of renewed extension upon an inverted extensional basin
Authors: COLLIER R. E. LL.
Published: December 2007

The Northumberland sedimentary basin has a history of repeated extensional and contractional tectonic phases and a study of the deformation interference pattern and sedimentary record allows the identification of the tectonic state through time. Initial rifting is represented by a thick Dinantian sedimentary fill, but Carboniferous extension is thought to have ceased by the early Namurian.

In order to explain the development of certain facies patterns during the Westphalian thermal subsidence phase of the evolution of the Northumberland Basin, some syn-depositional extensional fault movements are required. At other times in the Westphalian, compaction-induced subsidence was the dominant control on facies distribution. A single phase of E–W to SE–NW Variscan compression commenced during or after late Westphalian times and ceased prior to the intrusion of the Whin sills and dykes. Subsequent Permo-Mesozoic extension, with a right-lateral shear component, re-activated pre-existing normal fault lines. These later structures may be interpreted as being listric in form, shallowing onto decollement surfaces within the Carboniferous basin fill and overlie the buried faulted margin at the southern side of the basin.
Tectonic fabric of Sierra Leone, West Africa: implications for Mesozoic continental breakup
Authors: VENKATAKRISHNAN RAMESH , CULVER STEPHEN J.
Published: December 2007

A lineament map interpreted from Landsat images has been integrated with available geological and geophysical data from both offshore and onshore regions of Sierra Leone. The lineament patterns are related to intraplate and plate marginal reaction of pre-existing structures during Mesozoic rifting events that resulted in strong tectonic controls on magmatism.

Of four main lineament trends, the NNW-SSE to N–S, NNE–SSW, and ENE–WSW trends are directly relatable to Archaean fabric in the Leo Uplift. A NW-SE trend reflects coast-parallel late Mesozoic dykes that follow the Rokelide Pan–African fabric (reactivated Archaean NNW and N–S trending structures).

NW–SE trending faults defining offshore basins are segmented and offset by ENE–WSW trending continental extensions of Ocean fracture zones. Both the Guinea and Sierra Leone Fracture Zones have nucleated on ENE–WSW trending sinistral Archaean shear zones in the Leo Uplift.

The four lineament trends focused Mesozoic magmatic events through protracted reactivation, Spatial and geometric relationships between the magmatic provinces and tectonic fabric indicate that intraplate deformation occurs far inland during rifting events. The angular relationships between the Permo–Triassic NE trending Guinea Belt, the Jurassic–Triassic NW–SE trending coast-parallel dykes, and the ENE–WSW trending Ocean fracture zones centred on the eariy Jurassic Freetown basic igneous complex, suggest that the Sierra Leone–Liberia continental margin evolved as an obliquely-sheared, rift–rift–transform passive margin during Mesozoic continental breakup.
Origin and tectonic setting of stratabound tungsten mineralization within the Hindu Kush of Pakistan
Authors: LEAKE R. C. , FLETCHER C. J. N. , HASLAM H. W. , KHAN B. , SHAKIRULLAH
Published: December 2007

Stratabound scheelite mineralization has been discovered in the Hindu Kush range of northern Pakistan as a result of a geochemical drainage survey. The mineralization occurs predominantly in clinozoisite-bearing calcsilicate quartzite beds within a sequence of mica schist and subordinate graphitic phyllite, mica quartzite, tourmalinite and feldspathic gneiss, but not in adjacent calcite marble. The sequence is intruded by a small leucogranite stock emplaced after the climax of Barrovian-type metamorphism during the later of two deformation phases which are related to continent-arc collisions in the Cretaceous and Eocene. In the calcsilicate quartzites, scheelite crystal-lized simultaneously with clinozoisite prior to the leucogranite emplacement and is clearly of metamorphic origin. Tungsten-rich brines, generated from hot springs in an extensional tectonic environment, are thought to have been concentrated in the relatively porous precursor of the calcsilicate quartzite during diagenesis. Subsequent metamorphism recrystallized the tungsten as scheelite along with metamorphic silicate minerals and caused limited mobility of the element into veins.
Geochronology of the late Precambrian Hamisana shear zone, Red Sea Hills, Sudan and Egypt
Authors: STERN R. J. , KRÖNER A. , MANTON W. I. , REISCHMANN T. , MANSOUR M. , HUSSEIN I. M.
Published: December 2007

The Hamisana shear zone (HSZ) is a broad zone of deformation, approximately 50 km wide and at least 300 km long, making it one of the largest basement structures in NE Africa. It has been interpreted as a Precambrian suture, as a zone of strike-slip displacement, or as a zone of crustal shortening. The results of new Rb-Sr and U-Pb zircon geochronological studies indicate that the northern HSZ was thermally active during the Pan-African event until
c
. 550 Mg ago; initiation of the structure may have begun 40–110 Ma earlier. All units have low initial
87
Sr/
86
Sr ratios, indicating juvenile derivation. The timing of activity in the HSZ is 50–150 Ma younger than collisional suturing and terrane assembly in the Arabian-Nubian Shield but is similar to the 655–540 Ma Najd fault system of Egypt and Arabia. Deformation and metamorphism along the HSZ clearly post-date terrane accretion and probably are closely related to the Najd tectonic cycle. The most important deformation of the HSZ is unrelated to suturing, and at least one late Precambrian suture must extend west from Arabia into the interior of N. Africa.
Tethyan Working Group 89
Authors: JONES G. , CLIFT P. D.
Published: December 2007

Report of a meeting of the Tethyan Working Group, held at the University of Edinburgh, on 26–27 April 1989. The conveners were P. D. Clift, G. Jones, J. E. Dixon and A. H. F. Robertson.

This was the latest in a long established series of Tethyan Working Group meetings, the purpose of which is to provide a forum for the presentation of recent research work on the Tethyan realm. It has become apparent that only by the use of an integrated variety of geological techniques, can major advances be made in our understanding of this complex orogenic belt. The meeting reflected this approach by tackling various aspects of Tethyan tectonostratigraphic development. The Working Group was attended by about 40 geologists and geophysicists, with several from collaborating continental European laboratories.

The first talk on the Greek area was given by
A. H. F. Robertson
(Edinburgh University), who documented a two stage ophiolite emplacement history on the island of Ewia, which occupies a critical tectonic position in central E. Greece. Using a multidisciplinary approach he discussed the probable root zone of a Jurassic, supra-subduction zone ophiolite and presented new evidence for the formation of late Cretaceous oceanic crust in the area.
P. D. Clift
(Edinburgh University) continued this theme, with a description of the various tectonic units within the Argolis area of southern Greece. In the light of field geological and palaeomagnetic work he demonstrated a two stage ocean basin closure history in this area and summarized evidence of spreading in a
Thrust-related metamorphism in basement windows of the central Scandinavian Caledonides
Authors: LINDQVIST J. E.
Published: December 2007

The late Caledonian metamorphic evolution of the basement beneath the Caledonian allochthons of the central Scandes has been determined by studying mafic schists and phyllonites of the crystalline basement, and the overlying Vendian and/or Cambrian sedimentary rocks. The studied rocks occur along the Caledonian front and in tectonic windows within the Scandinavian Caledonides.

A gradual westwards increase in metamorphic grade is recorded, which varies from upper diagenetic conditions in the autochthonous rocks along the eastern thrust front of the Caledonides to middle amphibolite facies in the western parts of the Nasafjäll and Tømerås windows. Locally, strain-related deviations from this pattern occur. Further west the grade of Caledonian metamorphism of the basement increases and reaches the eclogite and granulite facies.

The Scandian metamorphic conditions in the basement rocks studied prevailed in the mid- to late Silurian, at pressures of 8 ± 2 kbar and temperatures of 550 ± 50°C in the Tømmerås window, and 7 ± 2 kbar at 550 ± 50 °C in the Nasafjäll window. Uplift and cooling followed thereafter, continuing into the Devonian. The final retrograde metamorphism was characterized by temperatures of 450 ± 50°C at pressures of about 3 kbar in the Nasafjäll and Tømmerås windows.

The occurence of syn-schistosity amphibolite-facies assemblages implies heating of the basement rocks during nappe emplacement. Available time constraints suggest that this was a relatively rapid process. In late Silurian to early Devonian, the prograde phase was followed by uplift and erosion. The metamorphic evolution also demonstrates that early Devonian thrust movements were associated with a decrease in the thickness of the nappe pile.