bluuebunny/crossref_metadata_2025 huggingface dataset:
Basement geology and tectonic evolution of Ireland as deduced from Pb isotopes
Authors: DIXON PAUL R. , LeHURAY ANNE P. , RYE DANNY M.
Published: December 2007

In Ireland, lead isotope data collected on Carboniferous-hosted Zn-Pb deposits can and has been interpreted to indicate that lead and other metals were derived from the underlying basement rocks. In order to test this hypothesis, whole rock lead isotope measurements were made on greywackes from the Longford-Down Inlier, which represent the along strike continuation of the northern part of the Caledonian basement underlying the base metal deposits. The lead isotope data define a linear array on a
207
pb/
204
pb versus
206
Pb/
204
Pb plot, with end points at (
207
Pb/
204
Pb) = 15.542, (
206
Pb/
204
Pb) = 18.456 and (
207
Pb/
204
Pb) = 15.461, (
206
Pb/
204
Pb) = 17.973. No evidence of lead isotope overprinting by the mineralizing event could be found in the samples studied. When the lead isotope data are corrected for addition of radiogenic lead since the time of base metal mineralization (c. 350 Ma ago), there is a 1:1 correlation between the lead isotopic composition of galenas in the base metal deposits and the Longford-Down grewyackes. These results support the hypothesis that lead and other metals were derived from the greywackes underlying the base metal mineralization.

On a
208
Pb/
204
Pb versus
206
Pb/
204
Pb plot, neither whole rock greywacke nor galena lead isotope data define a linear array. The
208
Pb/
204
Pb-
206
Pb/
204
Pb trend is best explained by tectonic or sedimentological mixing of three isotopically distinct lead isotope end members. These end members are: (1) old unradiogenic basement in NW Ireland (Lewisian complex precursor?), (2) Ordovician arc material from NW Ireland and (3) Ordovician arc material form SE Ireland. Isotopic modelling of these end members shows that the present-day mixing line seen in the Ordovician and Silurian sediments can be calculated back to values inferred for the Ordovician arc end members using the same values of μ as were used to back calculate the lead isotopic composition of the greywackes to the time of mineralization. This information is further confirmation of the sediment-mixing model presented.

A tectonic model developed from these and established geological data suggest a late Ordovician closure of the Iapetus Ocean, and that the Silurian greywackes are a by-product of eroded Ordovician arc material from NW and SE of the Iapetus suture.
Extensional and compressional zones in slumps and slides in the Namurian of County Clare, Ireland
Authors: MARTINSEN OLE J. , BAKKEN BENTE
Published: December 2007

Extensional and compressional zones in slumps and slides have been studied in a well-exposed Namurian (Carboniferous) base-of-slope and delta slope sedimentary succession in County Clare, Ireland. Extensional zones are in slumps dominantly characterized by single, downslope-dipping normal faults, while in slides they are characterized by normal, listric fault families. These faults sometimes show minor growth. Compressional zones in slumps are dominated by a single reverse fault with upslope-dip. They are only rarely characterized by imbricate fault fans. Compres­sional zones in slides fall into a tri-partite division of thick packages, thin packages and basal zones. Thick packages compare with tectonic imbricate zones, while thin packages are simpler and characterized by very shallow synthetic thrusts. Basal zones compare with blind imbricate fans. Lateral margins of both extensional and compressional zones in slumps and slides show transfer faults with transtensional, transpressional or purely strike-slip motion, depending on the width variability of the slide and slump scars. The similarity in structural style between soft-sediment deformational structures and deformation structures in lithified rocks shows that, on the basis of structural style alone, it is problematical to differentiate between these two groups of structures.
The contact zone of the Rhum ultrabasic intrusion: evidence of peridotite formation from magnesian magmas
Authors: GREENWOOD R. C. , DONALDSON C. H. , EMELEUS C. H.
Published: December 2007

The Eastern Layered Series of the Rhum ultrabasic complex formed in situ, and was not tectonically emplaced as previously suggested. Field relations along the contact show no evidence for the existence of a late ‘Marginal Gabbro’, instead peridotite and allivalite layers can be traced to within 2 m of the surrounding partially melted country rocks. Local preservation of a chilled margin to one peridotite layer indicates that the parental magma consisted of an olivine tholeiite liquid carrying approximately 19% (vol.) olivine crystals in suspension.
Pb-Sr-Nd isotope and chemical study of the origin of undersaturated and oversaturated shoshonitic magmas from the Borralan pluton, Assynt, NW Scotland
Authors: THIRLWALL M. F. , BURNARD P.
Published: December 2007

Pb–Sr–Nd isotope and major and trace element data are reported for a suite of samples from the Siluro–Ordovician Loch Borralan pluton, NW Scotland. Rock types ranging from pyroxenite through highly potassic melasyenites to quartz syenite were primarily generated by extensive fractional crystallization from mantle derived magmas. All rock types show pronounced Nb-depletion indicating shoshonitic, subduction-related, affinities for the magmas. Sr–Nd–Pb isotope compositions of the undersaturated rocks vary little and indicate derivation from a time-integrated mildly LREE-enriched mantle source very similar to that of the Silurian Lorne shoshonitic lavas further south. This mantle source is very unlikely to be lithosphere stabilized in Archaean times. The slightly younger quartz syenite intrusion contains 10–20% Archaean Pb and Nd indicating that its oversaturated and mildly peraluminous character is a function of assimilation of local Lewisian crust.
Eustatic and tectonic controls upon Quaternary coastal sedimentation in the Corinth Basin, Greece
Authors: COLLIER R. E. LL.
Published: December 2007

The late Quaternary sedimentology and palaeogeography of the Corinth Isthmus in central Greece is re-interpreted. A series of beach-to-shoreface sub-sequences are seen, each characteristically having an overall fining-up character. These occur within a coastal facies belt which has prograded to the west.

Computer modelling illustrates the interaction of tectonic displacements and eustatic sea-level variation as controls of facies patterns. Late Pleistocene-Holocene facies geometries are predicted on the basis of published sea-level data over that time interval. On coastlines undergoing tectonic uplift, such as the Isthmus of Corinth, depositional patterns may be characterized by transgressive wedges. These sub-sequences merge basinwards below sea-level minima. Each transgression relates to a major eustatic sea-level peak. Such first order models are modified by variations in the rate of tectonic deformation, the second control on relative sea-level position.

Modelled facies patterns are tested on the basis of new
230
Th/
234
U dates obtained from sclerac-tinian corals of the Corinth canal area. Dated sub-sequence sediments correspond in age to late Pleistocene eustatic highs and observed facies patterns correlate with the major late Pleistocene-Holocene eustatic transgressions of
c
. 100 ka periodicity. Episodes of intrabasinal normal faulting are isolated from a structural history otherwise dominated by uplift across the Isthmus. Quantification of post-depositional vertical tectonic displacements is achieved, constraining the eustatic base-level at the time of deposition of a dated horizon by reference to the global late Quaternary sea-level curve. The original depositional level is then subtracted from the present outcrop level. Using available
230
Th/
234
U dates to establish syn-depositional base-levels, minimum uplift rates of the order of 0.3 m ka
−1
are calculated across the area, averaged through the late Pleistocene and Holocene.
Late Cretaceous–early Tertiary palaeoclimates of northern high latitudes: a quantitative view
Authors: SPICER ROBERT A. , PARRISH JUDITH TOTMAN
Published: December 2007

Analyses of plant community structure, vegetational and leaf physiognomy, and growth rings and vascular systems in wood provide qualitative and quantitative data that can be combined to define non-marine palaeoclimatic parameters with better resolution than is available from other, principally sedimentological, methods. Application of these techniques to Cenomanian through Paleocene floras from high palaeolatitudes (75°-85°N) indicates a polar light regime similar to that of the present. Plant data suggests Cenomanian sea level mean annual air temperatures (MATs) of 10 °C, and MATs of 13 °C, 5°C and 6-7 °C in the Coniacian, Maastrichtian, and Paleocene respectively. Evapotranspirational stresses at sea level were low and precipitation was in most part uniform throughout the growing season in the Cenomanian, with possible seasonal drying occurring by the Maastrichtian. Maastrichtian winter freezing was likely, but periglacial conditions did not exist at sea level. Permanent ice was likely above 1700 m at 75°N in the Cenomanian, and above 1000 m at 85°N in the Maastrichtian. These near-polar data provide critical constraints on global models of Late Cretaceous to early Tertiary climates.
Equatorial aridity and climatic oscillations during the early Carboniferous, southern Britain
Authors: WRIGHT V. P.
Published: December 2007

Evidence from palaeosols, palaeokarsts and styles of meteoric diagenesis shows that the early Carboniferous climate of southern Britain was dominantly seasonally semi-arid. This semi-aridity occurred while the region was in an equatorial palaeolatitudinal position. The cause of the seasonal aridity was possibly the deflection of easterly winds into a major low pressure zone set up over Gondwana during the southern hemisphere summer.
The influence of a sudden climatic change on marine deposition in the Kimmeridgian of northwest Europe
Authors: WIGNALL PAUL B. , RUFFELL ALASTAIR H.
Published: December 2007

A sudden change from humid style to semi-arid style deposition markedly affected the accumulation of the Upper Kimmeridge Clay in southern England. Many of the changes appear to be related to a change in sedimentation rate at this time. Thus softground faunas are replaced by firmground faunas; diagenetic dolostones formed in the methanogenic zone are replaced by sulphate reduction zone carbonate nodules; and depositional gradients, recorded by lateral biofacies changes, becomes steeper. The evidence available is in accord with a decline in offshore sedimentation rates during this interval. Other changes, such as a decline in kaolinite abundance, were more directly controlled by the ‘drying-out’ of the hinterland. Similar changes, elsewhere in the marine geological record, could be used as climatic indicators.

The climatic change is part of a wider, northern hemisphere dry event which affect a broad area in the late Jurassic. The Kimmeridge Clay of southern England was one of the last depositional environments to be influenced by the climatic change at this latitude.
Distinguishing climatic and current effects in mid-Pleistocene sediments of Hatton and Gardar Drifts, NE Atlantic
Authors: HUIZHONG WANG , McCAVE I. N.
Published: December 2007

Samples taken from DSDP sites 552 and 553 on Hatton Drift and site 611 on Gardar Drift in the northeast Atlantic Ocean were examined by analyses of carbonate content, coarse fraction amount and composition, fine fraction size and composition, and oxygen isotope ratios. The age of both sections is the mid-Pleistocene, ranging from the end of the Jaramillo Normal Event to the boundary between the Matuyama and Brunhes epochs (about 0.77–0.96 Ma). Records of this time show a change from a dominance of 23 ka to longer periodicity at about 0.818 Ma. This is marked at site 552, but less so at site 611. The grain size is mainly controlled by coccoliths, foraminifera and ice-rafted material, all of which are related to glacial-interglacial changes. Zones barren of and abundant in coccoliths are sensitive indicators of climatic effect. Carbonate increase lags behind foraminiferal oxygen isotope records of surface warming in several glacial to interglacial transitions at both drifts. Different types of carbonates oozes occur on Hatton and Gardar drifts. Of several possible factors influencing these, bottom current effects seem more significant than others, deduced from size analysis of the fine fraction. Sedimentological results in this study further confirm that sediment drifts in the NE Atlantic have maintainted their basic characteristics at least since the mid-Pleistocene.
The significance of metre-scale allocycles in the quest for a fundamental stratigraphic unit
Authors: ANDERSON E. J. , GOODWIN P. W.
Published: December 2007

The major ‘transgressive’ sequence in the lower Helderberg Group (Devonian) of New York State and a ‘regressive’ sequence of similar scale in the first Courceyan mesothem (Carboniferous) of South Wales are entirely divisible into metre-scale allocycles. The boundaries of these cycles are produced by non-sedimentological processes: larger-scale stratigraphic events associated with relative sea-level fluctuations. The existence of these boundaries as basin-wide discontinuities precludes the application of Walther’s Law except within cycles. Thus palaeogeographic reconstructions based on the assumption of contemporaneity of large-scale fades tracts (formations and members) must be abandoned. Instead stratigraphic interpretation requires the recognition and application of a genetic unit of analysis consistent with the actual internal allogenic fabric of such sequences. Application of the metre-scale allocycle, the smallest allogenic unit of stratigraphic accumulation, results in fundamentally different interpretations of the Helderberg and Courceyan sequences. Rather than being continuously and unidirectionally ‘transgressive’ or ‘regressive’, both sequences were discontinuously generated by episodes of rapid deepening and gradual shallowing at more than one scale.
Recognition and documentation of depositional sequences from outcrop: an example from the Aptian and Albian on the eastern margin of the Wessex Basin
Authors: HESSELBO STEPHEN P. , COE ANGELA L. , JENKYNS HUGH C.
Published: December 2007

Stratigraphic data relating to the Aptian and Albian sediments around Folkestone in southeast England, on the eastern margin of the Wessex Basin, are plotted graphically to illustrate biostratigraphic gaps and condensed intervals. These are then used to suggest candidate sequence boundaries, notated LG1-4 and Gl-3, which are named after the Lower Greensand and Gault. The biostratigraphic subzones apparently unrepresented in the area are: LG1
obsoletus
and
bodei;
LG2
gracite
and
debile;
LG3
schrammeni;
Gl
bulliensis.
Candidate sequence boundaries G2
(cristatum
Subzone) and G3
(auritus
Subzone) are erosion surfaces correlating with probable biostratigraphic gaps elsewhere in the basin. The age of LG4 is uncertain but must lie within the
mammillatum
Superzone. All candidate sequence boundaries correspond closely to those predicted by recent global sea-level cycle charts, and additionally there is a close match between predicted maximum flooding surfaces and the occurrence of condensed intervals. Most of the candidate sequence boundaries could have been formed as a result of rapid, and at least regionally extensive, relative sea-level falls, but the sedimentological contexts of LG3 and Gl suggest that these may have been generated by transgression-related sediment condensation. The minor sequence boundary in the
loricatus
Zone
(intermedius
Subzone) illustrated in the cycle charts cannot be unambiguously recognized at outcrop in the Folkestone area. Additional sequence boundaries not shown in the cycle charts may also be present in the Wessex Basin.
Provenance of the pre-Devonian sediments of England and Wales: Sm-Nd isotopic evidence
Authors: THOROGOOD E. J.
Published: December 2007

ɛ
Nd
values and clast petrography of the Longmyndian sediments of Shropshire indicate a provenance dominated by juvenile (high ɛ
Nd
(
t
)) Uriconian volcanic sources, but with an increasing detrital contribution from older (low ɛ
Nd
(
t
)) metamorphic sources up-section. By contrast, most Lower Palaeozoic strata of England and Wales have low ɛ
Nd
(
t
)) values, typical of sediments derived from long established continental crust. The Sm–Nd isotopic evidence suggests that Avalonia received sediment from an adjacent large continent in the Cambrian to Lower Ordovician and in the Silurian. Ordovician and Silurian volcanicity supplied juvenile detritus only during and shortly after active volcanism.
The stratigraphy of a sheet of supracrustal rocks within high-grade orthogneisses and its bearing on Late Archaean structure in southern West Greenland
Authors: CHADWICK BRIAN
Published: December 2007

An extensive sheet of presumed middle Archaean supracrustal rocks within high-grade orthogneisses in inner Godthåbsfjord includes a section 3 km thick with exceptional preservation of pillow structures in low K tholeiitic amphibolites unambiguously indicating way up. The amphibolites are associated with pillow-structured and pyroclastic, komatiitic ultramafic schists, dunitic-harzburgitic sills, a stratiform gabbro, pyritous metacherts and quartzo-feldspathic gneisses and schists with field and chemical characteristics suggesting they formed as acid-intermediate tuffaceous sediments.

The sheet is tectonically overlain by older orthogneisses (Amîtsoq) with tracts and enclaves of the early Archaean Isua/Akilia supracrustal association. It is underlain by younger orthogneisses and granites with tracts of older, variably reworked Amîtsoq gneisses. Rocks of the higher level were emplaced on a tectonic slide which marks the upper boundary of the supracrustal sheet. The original lower boundary with Amîtsoq gneisses in the lower level may also have been a slide which has been obscured by emplacement of younger orthogneisses. Composite dome-in-dome structures with intervening synclinal cusps were superimposed on the sub-horizontal layer cake structure of the supracrustal sheet and the higher and lower levels of orthogneisses. Syntectonic granitic and amphibolite dykes (Kangilíngua dykes) were emplaced in parts of the synforms, whilst granite emplacement and feldspar blastesis took place in the cores of some domes. The dome and cusp system appears to be part of a regional series of late Archaean, steeply inclined to upright structures trending northeast in the Godthåbsfjord region. In inner Godthåbsfjord the system evolved in response to buoyancy forces generated by injection of basaltic magmas and partial melting of orthogneisses at depth within a regime of regional compression associated perhaps with transcurrent intraplate motions.
An oceanic model for lithological and faunal changes tested on the Silurian record
Authors: JEPPSSON LENNART
Published: December 2007

A model for oceanic cycles is presented. The cycles are expressed as changes between more humid low latitude and cooler high latitude climates (P episodes), and dryer low latitude and warmer high latitude climates (S episodes). The cyclicity may be self-regulating through changes in the CO
2
storage capacity of the deep ocean due to temperature changes and through changes in the oceanic input and output of dissolved carbonate. The model permits over 30 predictions regarding observable changes in both deep water and shelf lithologies and variations in the abundance, diversity and extinction rate of planktic and benthic faunas. Sedimentary changes involve fluctuations in the rate of clay deposition and carbonate production, the advance and retreat of carbonate deposition on the shelf edge, the timing of oolite, reef, and black shale formation, and changes between oxic and anoxic deep water. Faunal effects include both iterative evolution and intermittently widespread taxa (‘Lazarus taxa’). These predictions agree well with changes observed in well known sequence of Wenlock and Ludlow age, as well as changes seen in selected older and younger sequences.
Extinction and migration in Silurian trilobites and conodonts of northwestern Canada
Authors: CHATTERTON BRIAN D. E. , EDGECOMBE GREGORY D. , TUFFNELL PAMELA A.
Published: December 2007

Appearances and disappearances (lineage turnover) of trilobite and conodont evolutionary species’ clades in Silurian shelf marginal sections of the Mackenzie Mountains are concentrated at particular stratigraphical levels. A depth/energy curve generated for this region shows three major and several minor transgressive-regressive cycles in the Llandovery and Wenlock Series. A comparison of this curve with conodont and trilobite distributional data supports the thesis that there are cause/effect relationships between changes in sea level and migration, evolution and extinction events of these marine organisms. The rate, amount and direction of change in depth were important factors in the migration and extinction of clades. The depth/energy curve for the region fits the pattern expected for a passive downdropping shelf responding to global, eustatic sea-level changes. Emigrations and extinctions are associated with rapid transgressive episodes and concentrated at three levels: lower Llandovery, lower Wenlock, and middle or lower part of the upper Wenlock. The early Wenlock event was the most impressive. A further concentration of extinctions and/or migrations, not considered here, occurred later in the Ludlow, associated with a regional regression. Phyletic evolution is most convincingly demonstrated in fossil collections from intervals of gradual regression.

A model is proposed that predicts that a similar pattern of extinctions occurred around the Silurian world, in regions of similar palaeotectonic regime, and that global extinction patterns of clades, between episodes of mass extinction, were not random. Episodes of high extinction (turnover pulses) were caused by perturbations of the environment, foremost of which were rapid changes in the depth of the seas on the continental shelves. Because taxonomic loss during these extinction/emigration episodes was generally at low (below generic) levels, resolution of Silurian turnover patterns requires data collected at the evolutionary species level.
Post-metamorphic cooling history of the Indian Plate crystalline thrust stack, Pakistan Himalaya
Authors: TRELOAR PETER J. , REX DAVID C.
Published: December 2007

A post-metamorphic Himalayan cooling history for the crystalline internal zones of the northern Indian plate is defined by amphibole and mica K-Ar and Ar-Ar geochronology. Hornblende, muscovite and biotite cooling ages from cover sequences metamorphosed during the Himalayan orogeny are 35–40, 30 to 23, and 29 to 23 Ma respectively. The mica ages, together with those derived from zircon and apatite fission track data demonstrate a cooling rate of about 30°C/Ma during the late Oligocene to early Miocene. This rapid cooling was initiated during the post-metamorphic development of the Indian Plate south-verging crustal-scale thrust stack. Most of the cooling occurred during the stripping of some 10 ± 2 km of overburden through a combination of erosion, recorded in the Miocene molasse sediments of the foreland basin, and major crustal extension within upper levels of the Indian Plate and the Main Mantle thrust zone. Both erosion and extension were the direct consequence of the evolution of the thrust stack.
Tidal and storm influenced sedimentation from a Cambrian transgressive passive margin sequence
Authors: McKie THOMAS
Published: December 2007

The basal Cambrian clastic sediments in northwest Scotland record a transgressive sequence of barrier island to tidal shelf quartz-arenites (Eriboll Sandstone Lower Member and Pipe Rock) overlain by a storm-dominated interval of mixed clastic-carbonate sediments (Fucoid Beds). Two shallowing events interrupt this sequence: the lower near the middle of the Pipe Rock, and the upper at the top of the Fucoid Beds which led to the deposition of the quartz-arenites of the Salterella Grit. Variations in the proportion of storm deposited facies and in the thickness of tidally generated structures suggests that tidal currents progressively increased during deposition of the quartz-arenites and then decreased until deposition of the mixed clastic-carbonate deposits. These features suggest amplification of tides as the shelf approached a resonant width during transgression. Further transgression produced a wide, non-resonant shelf where tides were frictionally damped, and storm processes were dominant. Similar sequences of the same age in western Newfoundland, eastern Greenland and Spitsbergen may also have been produced by the shelf bordering the north American craton responding to tidal amplification followed by frictional damping.
A composite tectonic–eustatic origin for shelf sandstones at the Cambrian–Ordovician boundary in North Greenland
Authors: BRYANT IAN D. , SMITH M. PAUL
Published: December 2007

Shelf sandstones deposited at the Cambrian–Ordovician boundary in central and western North Greenland have a sheet-like large-scale geometry, and extend for over 300 km along strike within a shallow marine, carbonate-dominated, platform sequence. Conodont faunas from units above and below the most extensive sand sheet, the Kap Coppinger Member of the Cass Fjord Formation and the Permin Land Formation, indicate that deposition of the sands spanned the
proavus, intermedius
and
lindstromi
conodont biozones. Sand was derived by reworking of older, quartzite-dominated formations following the uplift and unroofing of a platform bulge to the east. During the Cambrian, this sediment was transported northwards from the uplifted area across an unstable platform into a deep-water trough. However, during sea-level lowstands within the Cambrian–Ordovician boundary interval this clastic material was deflected westwards and transported across the stable platform by progradation of tide- and storm-dominated shorelines. Facies sequences in the thickest part of the Permin Land Formation suggest at least three such regressive cycles combined to form a composite sand body. Each regressive cycle comprises a shallowing upward transition from tide- and storm-deposited marine sediments to clean sandstones of marine or terrestrial origin. The cycles are separated by thin but laterally extensive dolomites interpreted to represent drowning events. Thinner sequences further to the east are attributed to reduced accommodation space as a consequence of lower subsidence rates close to the uplifted source area, whilst thin sequences in the west are ascribed to the effect of increasing distance from the sediment source so that only the youngest of the three regressive cycles is present.
Petrography of thoriferous hydrocarbon nodules in sandstones, and their significance for petroleum exploration
Authors: PARNELL J. , MONSON B. , TOSSWILL R. J.
Published: December 2007

Nodules of thorium-rich hydrocarbon are recorded in sandstone sequences from several basins. The hydrocarbon nodules contain mineral inclusions, particularly of thorite or monazite. There is evidence both for accretion of hydrocarbons around detrital heavy mineral grains and for epigenetic growth by interaction of hydrocarbons with thorium-bearing fluids. Both mechanisms depend upon the precipitation of solid hydrocarbons induced by irradiation from thorium. The nodules exhibit aggressive replacement of other grains, a characteristic of radioanomalous hydrocarbons. The occurrence of the nodules is dependent upon an appropriate (plutonic) source for the thorium, but given this constraint they are important evidence for hydrocarbon migration pathways.
Fracture zones across the Cape Verde Rise, NE Atlantic
Authors: WILLIAMS C. A. , HILL I. A. , YOUNG R. , WHITE R. S.
Published: December 2007

More accurate locations have been made for 13 fracture zones across the Cape Verde Rise and four previously unknown fracture zones mapped. Evidence for their existence is from analysis of the M series of magnetic anomalies together with evidence from seismic reflection profiles where these data exist. The Cape Verde Islands themselves lie among a series of five fracture zones. The continuity of magnetic lineations and fracture zones across the region of the Cape Verde Islands indicates not only that the islands lie on oceanic crust but also that the original Mesozoic seafloor features and magnetization persist and were not obliterated by the Neogene reheating, uplift and volcanism.
Age of amphibolitic metamorphism in the ophiolitic unit of the Morais allochthon (Portugal): implications for early Hercynian orogenesis in the Iberian Massif
Authors: DALLMEYER R. DAVID , GIL IBARGUCHI JOSÉ I.
Published: December 2007

Structurally lower portions of the allochthonous Morais complex are comprised of an ophiolitic unit exposed in two contrasting structural units separated by a ductile thrust fault. The lower unit is dominated by variably foliated amphibolite whereas the upper unit is largely comprised of metaperidotite and metagabbro. Most of the ophiolitic unit has recrystallized synkinematically under amphibolite facies metamorphic conditions (
c.
500°C and 5 kbar). Amphiboles formed during this metamorphism are typically magnesio-hornblende. An amphibole concentrate from the lower structural unit records a
39
Ar/
40
Ar plateau age of 397.3 ± 4 Ma. An amphibole concentrate from the upper structural unit displays an internally discordant
39
Ar/
40
Ar age spectrum corresponding to a total gas age of 431.1 ± 27.3 Ma. Gas fractions evolved from each amphibole concentrate at intermediate and high experimental temperatures are characterized by similar apparent K/Ca ratios and yield well-defined
36
Ar/
40
Ar v.
39
Ar/
40
Ar isotope correlations which result in similar ages of 384.2 ± 5.2 and 392.4 ± 7.3 Ma. These are considered to be more reliable than the plateau ages and are interpreted as dating the last cooling through those temperatures required for intracrystalline retention of argon within constituent amphibole grains. The associated tectonothermal event represented a first stage of Hercynian orogenesis in the Iberian massif. It was probably related to oceanic closure and associated obduction of granulitic/eclogitic massifs which at present structurally overlie the ophiolitic unit.
Evidence for Silurian sinistral accretion of Avalon composite terrane in Canada
Authors: DOIG R. , NANCE R. D. , MURPHY J. B. , CASSEDAY R. P.
Published: December 2007

The Kingston dyke complex forms a regionally developed belt of sheeted bimodal dykes within the Avalon composite terrane of southern New Brunswick, Canada. Dyke orientation relative to the belt suggests their injection accompanied sinistral movement, and the belt has previously been attributed to transform activity within the late Precambrian magmatic arc regime typical of the Avalon composite terrane in the Northern Appalachians. However, rhyolite dykes of the sheeted complex yield a U-Pb age of 435.5 ± 1.5 Ma that is interpreted to date their emplacement. The complex therefore provides evidence of Early Silurian sinistral displacement in the Northern Appalachians. Such movement is suggested by palaeomagnetic data and may record the accretion of the Avalon composite terrane to cratonic North America.
The identification of granitic diapirs
Authors: ENGLAND R. W.
Published: December 2007

Diapirism is a themomechanically viable ascent mechanism for granitic magmas. However there is very little direct evidence of actual granite diapirs. Many of the characteristics we may expect to use to determine ascent mechanisms are destroyed by changes in the shapes of intrusions resulting from the interaction of buoyancy and tectonic forces during emplacement. Many features of ballooning plutons are identical to those we would expect for diapirs. Notably, patterns of strain deduced from deformed xenoliths and concentric foliations, cannot be used as unambiguous evidence for diapirism. Diapirism can only be proven beyond doubt where it is possible to demonstrate vertical uplift of country rocks in excess of the vertical thickness of the intrusion.
Stress in the Earth's crust: the link between crustal fluids and extensive-dilatancy anisotropy
Authors: LOVELL J. H. , CRAMPIN S. , SHEPHERD T. J.
Published: December 2007

Observations of seismic shear-wave splitting (birefringence) along almost all raypaths in the Earth's crust have led to the recognition that the fluid-filled cracks, microcracks and pores known to pervade most rocks in the crust have been preferentially aligned by the prevailing stress-field. This phenomenon is known as extensive-dilatancy anisotropy or EDA, and the fluid-filled microstructures as EDA-cracks because the seismic effects can be modelled by distributions of parallel cracks. The effects of these stress-aligned EDA-cracks have been observed in many different areas, and they appear ubiquitous in the upper 10 to 20 km of the crust. Many details of the crack geometry and the current stress-field can be readily determined from records of the shear-waves which have propagated through these aligned cracks. In separate studies, geochemists have confirmed the presence of microscopic fluid-filled cavities in almost all geological material. These are referred to as fluid inclusions, and provide unequivocal evidence of crustal fluid circulation, often along stress-controlled fractures. Different stress episodes can be recognized in many rock samples and studies of fluid inclusions can be used to interpret the palaeo-stress history of a rock, where the in situ morphology of these inclusions is related to the current regional stress-field. This paper discusses the relationship between the fluid inclusions of the geochemical literature and EDA-cracks, and concludes that the former may be regarded as one of the constituent inclusion types of extensive-dilatancy anisotropy.
The geochemistry of the Trotternish sills, Isle of Skye: crustal contamination in the British Tertiary Volcanic Province
Authors: GIBSON SALLY A.
Published: December 2007

Geochemical and isotopic data are presented for a suite of Palaeocene picrites, picrodolerites and crinanites from the Trotternish sills, northern Skye. The whole-rock geochemistry of the Trotternish sills is comparable with the overlying Skye Main Lava Series, although the sills are relatively Mg-rich (up to 30 wt% MgO) due to abundant cumulus olivine. Rare-earth element patterns exhibit overall LREE enrichment, relative to chondrite, and suggest an alkali basalt parental magma. Incompatible element patterns show Ba and K enrichment, relative to Nb, Th and U. Sr, Nd and Pb isotope ratios indicate that crinanites, which are the most fractionated magmas, are the most contaminated. Isotope ratios suggest up to 22% amphibolite-facies Lewisian gneiss may have been assimilated during fractionation in the upper crust, prior to multi-phase intrusion as sills. This is in contrast to the Skye Main Lava Series, which show evidence of crustal contamination by granulite-facies material in the lower crust, but is comparable with the Skye Preshal Mhor basalts and the Rhum intrusion. The Trotternish sills and the overlying plateau lavas therefore have a more complicated genetic relationship than was previously supposed.
Volcaniclastic carbonatite at Kaluwe, Zambia: age and relations to sedimentary rocks in the Zambezi rift valley
Authors: TURNER D. C. , REX D. C.
Published: December 2007

The Kaluwe complex, a stratified sheet-like body of volcaniclastic carbonatite has given K-Ar ages of 113–101 Ma. The carbonatite overlies Upper Karoo sandstone, but its cover of continental siliciclastic sediments is not Karoo as previously thought, but post-dates the carbonatite. The complex is interpreted as a relict of the distel deposits of a large, long–long lived carbonite volcano.
The Kabus ophiolitic melange, Sudan, and its bearing on the western boundary of the Nubian Shield
Authors: ABDELSALAM MOHAMED G. , DAWOUD AHMED S.
Published: December 2007

The Precambrian high-grade gneisses of a continental terrane and the late Proterozoic (Pan-African) low-grade volcanogenic oceanic assemblage of northeastern Nuba Mountains are separated by the Kabus ophiolitic melange. This zone reflects a structural style of imbricate thrusting and strata1 disruption that is attributed to the collision of the Pan-African oceanic assemblage of the Nubian Shield against the pre-Pan-African continental crust to the west. The geological, structural, geochemical and geophysical data indicate that the melange zone marks a fossil plate boundary. The tectonic units of the region demonstrate in-situ deformational patterns that argue against the detached nature that had been previously suggested for the region. Hence, the low-grade assemblage of northeastern Nuba Mountains represent the southwestern extension of the ensimatic Nubian Shield. It is suggested here that the western margin of the Shield is marked by a N-S trending suture extending from the Bayuda Desert in northern Sudan up to the western margin of the Mozambique Orogenic Belt in northwestern Kenya. This suture passes through the Kabus ophiolitic melange.
The Tavua Caldera, Fiji: a complex shoshonitic caldera formed by concurrent faulting and downsagging
Authors: SETTERFIELD T. N. , EATON P. C. , ROSE W. J. , SPARKS R. S. J.
Published: December 2007

The shoshonitic Tavua Volcano in Viti Levu, Fiji, developed from a submarine to a subaerial environment, and comprises lava flows, dominantly of absarokite composition, with debris flows and conglomerates on its flanks. The Tavua Caldera formed in the centre of the volcano. Within the caldera, avalanche deposits are overlain by the Turtle Pool Formation, which consists of lacustrine sediments, shoshonite lavas and hydrovolcanic/pyroclastic lapilli tuffs. Subsequently the Inner Caldera formed in the centre of the Tavua Caldera. This was infilled by the Morrisons Pool Formation, which comprises lavas and pyroclastic rocks of banakite composition, with minor intercalated sediment. Thus the magmas became progressively more evolved.

In addition to normal movement along regional and concentric faults, subsidence (which exceeds 3 km) was accomplished via substantial downsagging. Evidence for this is the consistent centripetal dip of caldera fill material, which locally exceeds 50° in parts of the Turtle Pool Formation. Centripetal dips are also recognized in pre-caldera rocks immediately outside the Tavua Caldera.

Subsidence was episodic, caused by periodic, non-explosive withdrawal of underlying magma to erupt on the flanks and in part within the caldera. Downsagging and fault movement were responsible for approximately equal amounts of subsidence, and their interaction produced a caldera with the shape of a stepped funnel.
Analysis and tectonic interpretation of gravity data over the Variscides of southwest Ireland
Authors: FORD MARY , BROWN COLIN , READMAN PETER
Published: December 2007

The gravity data set for southwest Ireland comprising 5500 stations with a spacing of about 1–3 km has been filtered using standard wavenumber domain techniques. A band pass filtered map, emphasizing gravity anomalies originating to a maximum depth of 3–4 km, correlates well with surface geology. It shows major faults and density contrasts among six lithostratigraphical units, the most important being between the Old Red Sandstone and underlying basement. A low pass filtered map, emphasizing gravity anomalies from a maximum depth of 10 12 km shows major negative gravity anomalies at Killarney and Fermoy and positive anomalies beneath the Shannon estuary, on the Dingle peninsula and along the south coast. These data suggest that Caledonian basement was involved in Variscan deformation and any detachment lies at mid-crustal levels.

Two north-south gravity profiles are modelled along Eastings 100 and 200, constrained by surface geology and published seismic refraction data. On Line 100 the source of the negative anomaly at Killarney is modelled as either a granite within basement or a deformed Devonian basin. The source of the Shannon positive anomaly is modelled as a large dense body within the crust whose southern limit coincides with the Iapetus Suture Zone. On Line 200, the Fermoy low is modelled as a laccolithic granite within basement. The positive anomaly along the south coast is a consequence of Mesozoic crustal thinning.
End-Silurian modifications of Ordovician terranes in western Ireland
Authors: WILLIAMS D. M. , HARPER D. A. T.
Published: December 2007

The Ordovician elements in the Caledonides of western Ireland may be divided into the Clew Bay, South Mayo, North Connemara and South Connemara sub-zones. Evidence suggests the presence of a number of proximal suspect terranes, and the remnants of one, or possibly two, arc complexes. Palaeontological and sedimentary evidence suggests an allochthonous relationship between the Silurian rocks of Croagh Patrick and North Galway. These successions may therefore not represent a single sedimentary overlap to the Ordovician suspect terranes. End-Silurian deformation was the result of transpression, which was responsible in part for the present configuration of the area’s geology. Deep seated shear zones, with fuchsite and gold mineralization were foci for lateral translation concomitant with southward directed thrusting, northward directed back thrusts and variable degrees of metamorphism being of higher grade in the north. The end-Silurian event is intermediate between the high grade metamorphism in parts of the Appalachians and the non-metamorphic Silurian sequences of Scotland.
A geological and tectonic cross-section of the Caledonides of western Ireland
Authors: RYAN P. D. , DEWEY J. F.
Published: December 2007

Geological and geophysical criteria allow division of the Caledonides of western Ireland into four principal zones. Zone 1, in the north, contains Grenvillian basement with late Proterozoic supra-crustals. Zone 2 contains the Westport Complex, the Deer Park Complex and the South Mayo Trough. Connemara and the Delany Dome Formation occupy Zone 3. Zone 4 contains the South Connemara Group and mid- to late-Ordovician and Silurian sediments and volcanic rocks. The various tectonic models proposed for these zones are discussed and a ‘minimum’ tectonic model is presented, which involves early Ordovician collision of the northern part of Zone 2 with the Laurentian margin (Zone 1) followed by subduction flip (northern part of Zone 4). The site of early collision between Zones 1 and 2 subsequently underwent orogenic collapse and strike-slip dismemberment leading to the sinistral transpressive emplacement of Zone 3. Assemblages in Zone 4 were accreted to the now south-facing arc. Northwards stress transfer consequent on suturing deformed the thinned crust and overlying Silurian sequences in Zones 2 and 3.
A foreland basin on the southern margin of Iapetus
Authors: KNELLER B. C.
Published: December 2007

The Windermere Group basin was situated on the northwestern margin of Eastern Avalonia, dose to the suture with Laurentia. Sediment accumulation rates within the basin, estimated using a graptolite time-scale, show a history of accelerating subsidence through the mid- to late Silurian, with rates exceeding 1mm per year in the Ludlow. The characteristic subsidence curve, high sedimentation rate, and lack of syndepositional extensional faulting or basic magmatism indicate a foreland basin. The basin was generated by subduction of Avalonian continental crust beneath Laureutia from the mid-Wenlock onwards, which resulted in a slowing of continental convergence. Further slowing occurred towards the end of the Silurian.
Reflection seismics for shallow geological investigations: a case study from Central England
Authors: ALI J. W. , HILL I. A.
Published: December 2007

The applicability of the seismic reflection method for exploration at shallow depths (<100 m) has been investigated at Bardon Hill Quarry in Charnwood Forest where Triassic sedimentary rocks overlie a Precambrian basement. A high-resolution system, comprising high-frequency response equipment coupled with a data- logging mimompnter, was developed. The reflection data provide a significantly more reliable model of the sub-Triassic surface than do either existing widely-spaced boreholes or seismic refraction investigations; in addition, they provide information on the internal structure of the Triassic strata. The resolution and accuracy of the reflection data demonstrate the potential of this method in engineering geophysical investigations.
Quantitative spore colour
Authors: MARSHALL J. E. A.
Published: December 2007

Spores exhibit changes in colour with increasing depth of burial; these changes have found wide application for thermal maturity determination in hydrocarbon source rocks. At present colours are determined visually. Here the application of the Commission Internationale de l’Eclairage colour system to the measurement of spores is described such that colour can be defined by the chromaticity co-ordinates (
x, y
) and the luminance (
L
) which define a colour solid. The colour changes seen in spores with increasing burial define a maturation pathway in this colour solid which shows two distinct trends. The first is a well-defined progressive colour change to a cusp at which a broad spread of colours from yellow to brown occur in a single sample. This is the interval over which hydrocarbons are generated and which is clearly defined by these colour differences. The second trend is through a series of brown colours with an increasing lack of sensitivity of the spores to colour changes. These changes are calibrated to vitrinite reflectivity. The data and conclusions are amplified and supported through changes in colour, chemistry and exine reflectivity in artificially matured
Lycopodium
spores. Comparisons are made with existing qualitative spore colour scales. The implications for maturation determination are that spore colour is a good thermal maturity index for the interval prior to oil generation and that it is suitable for quantitative modelling studies. It demonstrates the interval of oil generation at the cusp but after this point it becomes inaccurate and inferior to other methods such as vitrinite reflectivity.
Geological evidence for intra-Jurassic faulting in the Wessex Basin and its margins
Authors: JENKYNS H. C. , SENIOR J. R.
Published: December 2007

Geological observations on Jurassic outcrops close to major faults in the Wessex Basin–Mendip area reveal the local presence of ammonite- and brachiopod-bearing sediments penetrating underlying strata. Toarcian and Bajocian neptunian dykes and particularly sills are associated with the Eypemouth Fault and Bajocian sills with the Bride Fault and Mere Fault. In the Mendip area numerous neptunian dykes of Hettangian, Sinemurian, Pliensbachian and Bajocian ages, cross-cutting Carboniferous Limestone, are recorded, typically also associated with major basement faults (e.g. Cranmore and Leighton Faults). These periods of assumed sediment injection are taken as indicating times of displacement along the faults in question.

Variations in facies (Hettangian–Sinemurian, Toarcian, Bajocian, uppermost Oxfordian, Kimmeridgian) spatially linked to faults are documented from some areas, and boreholes reveal considerable fault-controlled thickness changes in Hettangian–Sinemurian, Bajocian and Kimmeridgian sediments. The timing of Jurassic faulting in the Wessex Basin–Mendip area thus polarizes into two intervals: Hettangian–Bajocian and latest Oxfordian onwards, correlating with the early rifting phases of the Central and North Atlantic respectively.