bluuebunny/crossref_metadata_2025 huggingface dataset:
Variation in marine total organic carbon through the type Kimmeridge Clay Formation (Late Jurassic), Dorset, UK
Authors: Tyson Richard V.
Published: December 2007

Bulk geochemical data from the NERC Rapid Global Geological Events project are used to appraise the pattern of variation in mean organic content through the
cymodoce
to
rotunda
zones of the type Kimmeridge Clay Formation in southern Dorset. There is a symmetrical stratigraphic pattern in mean and modal total organic carbon (TOC) values, which increase from 1–2% at the top and base to a peak of 8–9% in the middle of the formation. To remove distortion caused by the strong positive skewness of the TOC values, the mean TOC per zone was recalculated for the first mode of the TOC distribution only, and combined pragmatically with pyrolysis estimates of the reactive (marine) TOC fraction to derive marine TOC values; these also vary symmetrically from
c
. 0.5% to peak values of
c
. 5.0%. The mean TOC is negatively correlated with sedimentation rate, indicating that dilution is a significant controlling variable. Mean palaeoproductivity was assessed using modern marine sediment relationships between carbon burial efficiency, sedimentation rate (derived from RGGE cyclostratigraphic analyses), the likely dissolved oxygen range, and water depth. Best estimates of palaeoproductivity range from 40 to 150 g C m
−2
a
−1
(low to moderate by comparison with present-day shelves) and are positively but nonlinearly correlated with mean
δ
13
C
TOC
values.
Earth geography from 400 to 250 Ma: a palaeomagnetic, faunal and facies review
Authors: Torsvik Trond H. , Cocks L. Robin M.
Published: December 2007

Palaeomagnetic and faunal data have been re-evaluated on a global basis for the period from 400 Ma (early mid-Devonian) to 250 Ma (latest Permian). The boundaries of the major terranes are considered and defined. Six new palaeogeographical maps at 30 Ma intervals, which ensure kinematic continuity, are presented for this period. The palaeomagnetic data are very useful for positioning terranes for the present-day North Atlantic area, of variable value for China and Tarim, and for much of the large superterrane of Gondwana (being notably poor during the Early Devonian and the Early Carboniferous), and are sparse or non-existent for much of the rest of Asia. The relative positions of Laurussia and Gondwana at the end of the Palaeozoic when they united to form Pangaea are discussed, and it is concluded that the most convincing reconstruction (Pangaea A) is obtained by assuming an octupole contribution of 10–15% in combination with the main dipole of the Earth's magnetic field. As well as faunal and palaeomagnetic data, the disposition of the major sediment types, including coal deposits, evaporites and glacial deposits, has also been considered, especially in the late Carboniferous and Permian.
Geology and models of salt extrusion at Qum Kuh, central Iran
Authors: TALBOT CHRISTOPHERJ. , AFTABI PEDRAM
Published: December 2007

Profiles through the summit of a small nearly axisymmetric extrusion of Oligocene and Miocene salt, and simple analogue models of it, simulate the profiles of piles of ductile nappes extruded from convergent orogens. The salt extrudes from a reactive diapir along a major strike-slip fault at about 82 mm a
−1
and rises 315 m above the central plateau of Iran. The salt has the distinctive smooth profile of a viscous fountain in which an asymmetric apron of allochthonous salt gravity-spreads over its surroundings from a summit dome. Curtain folds developed in the source layer extrude from the diapir and are refolded by major recumbent folds with circumferential axes that simulate nappes. Minor flow folds with circumferential axes refold major folds in the top 10-50 m of surficial salt. Master joints >100 m long indicate brittle failure of dilated salt by regional stress fields. Tuned to the dimensions of Qum Kuh, analytical and analogue models of viscous extrusions constrain the dynamic salt budget and a time of extrusion of at least 42 000 years. New analogue models suggest that the number, amplitude and spacing of major recumbent folds within the extruded salt (and ductile nappe piles) record the number, amount and relative timing of fluctuations in the driving forces.
Recrystallization or cooling ages:
in situ
UV-laser
40
Ar/
39
Ar geochronology of muscovite in mylonitic rocks
Authors: Mulch Andreas , Cosca Michael A.
Published: December 2007

The intra-grain
40
Ar/
39
Ar age distributions of muscovite from greenschist-facies mylonite zones are shown to be sensors of metamorphic history. Textures, compositions and
in situ
UV-laser ablation
40
Ar/
39
Ar data from two extensional shear zones demonstrate that cooling ages can be distinguished from neo- or recrystallization ages. Deformed muscovite porphyroclasts and recrystallized shear band muscovite from the Pogallo Shear Zone (Ivrea Zone, southern Alps) reveal variable intra-grain
40
Ar/
39
Ar ages with internal age variations of more than 65 Ma. The wide range of
40
Ar/
39
Ar ages within compositionally homogeneous grains is consistent with diffusion-dominated argon loss controlled by observable intra-grain microstructures. In contrast, intra-grain
40
Ar/
39
Ar ages of muscovite fish from Proterozoic mylonite of the Porsgrunn–Kristiansand Shear Zone (Southern Norway) display only minor age dispersion, and age variations are correlated with changes in muscovite composition. The sympathetic age–composition correlations in different muscovite microstructures directly relate to sequential neo- and recrystallization of muscovite and ultimately provide insight into the protracted recrystallization history during extensional deformation along the Porsgrunn–Kristiansand Shear Zone.
Magnetic fabric of granitoids from Southern Corsica and Northern Sardinia and implications for Late Hercynian tectonic setting
Authors: Gattacceca J. , Orsini J.-B. , Bellot J.-P. , Henry B. , Rochette P. , Rossi P. , Cherchi G.
Published: December 2007

Anisotropy of magnetic susceptibility (AMS) data (180 sites) from Hercynian granitoids from Southern Corsica (France) and Northern Sardinia (Italy) provide a high-density net of structural information for this part of the European Hercynian belt. Two main groups of granitoids, defined according to petrography and relative chronology, provide contrasting AMS results. For both groups, shallowly dipping magnetic foliations and lineations suggest that these granitoids were emplaced as large-scale shallowly dipping sheets or open funnel-shaped structures at a high structural level. In non-leucocratic granitoids (305–300 Ma), foliation and lineation trajectory maps reveal antiforms and synforms with a N130 axial trace. Intersection of the foliation planes indicates a N120 low-plunging ‘fold’ axis, revealing a N120 maximum stretching direction. Slightly younger leucocratic granitoids (295–285 Ma), bounded by late Hercynian faults, display N50 lineations parallel to the elongated shapes of these intrusions. This structural pattern is undisturbed by the faults, and suggests an intrusion in decakilometre-scale tensional fractures indicating a N140 instantaneous stretching direction compatible with the stretching direction evidenced in non-leucocratic granitoids. A younger
c
. N20-oriented Permian dyke swarm confirms the permanence of this stretching direction over at least 35 Ma and indicates a coaxial strain regime. To integrate these new data within a broader context in the Hercynian belt, Corsica has to be back-rotated clockwise by 70° to its Permian position, which indicates a north to N30 stretching direction for the 305–285 Ma interval, compatible with the post-collisional strain and stress fields of the French Massif Central at about the same period.
K-bentonites in the Argentine Precordillera contemporaneous with rhyolite volcanism in the Famatinian Arc
Authors: Fanning C.M. , Pankhurst R.J. , Rapela C.W. , Baldo E.G. , Casquet C. , Galindo C.
Published: December 2007

New U–Pb radiometric dates for K-bentonite horizons within the Lower Cambrian to Middle Ordovician platform carbonates from the Precordillera terrane of NW Argentina provide further constraints on models for the allochthonous or parautochthonous accretion of this terrane. Two K-bentonite layers from the Talacasto section yield indistinguishable sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon dates of 469.5 ± 3.2 Ma and 470.1 ± 3.3 Ma respectively. These are within uncertainty of the U–Pb SHRIMP zircon date of 468.3 ± 3.4 Ma for a porphyritic rhyolite from the Famatinian magmatic arc, Sierra de las Planchadas, near Rio Chaschuil. Geochemical and isotope data also demonstrate the similarity of the K-bentonite and Chaschuil rhyolite parent magmas. Thus, it is highly probable that the Famatinian arc volcanoes provided the ash for the K-bentonite horizons, suggesting proximity to the Precordillera terrane during the deposition of the Lower Cambrian to Middle Ordovician platform carbonates. This implication supports a mid-Ordovician collision model, but could also be compatible with a parautochthonous model for docking of the Precordillera terrane, by movement along the Pacific margin of Gondwana, rather than across the Iapetus Ocean.
Late Permian rifting of the South China Craton caused by the Emeishan mantle plume?
Authors: Song Xie-Yan , Zhou Mei-Fu , Cao Zhi-Min , Robinson Paul T.
Published: December 2007

Stratigraphic relationships and bulk-rock geochemical data indicate that Upper Permian metabasalts in the Songpan–Ganzi and Yidun terranes, on the eastern margin of the Tibetan Plateau, are part of the Emeishan large igneous province, which is believed to have formed from the Emeishan mantle plume. Eruption of the Emeishan basalts at 260 Ma was coincident with rifting of the western margin of the South China Craton to form the Songpan–Ganzi ocean basin. The spatial and temporal coincidence between basalt eruption and continental rifting, as well as regional doming prior to eruption, suggest that continental break-up was a response to the Late Permian Emeishan plume. The Songpan–Ganzi ocean basin was rapidly filled with Triassic flysch deposits, then deformed and uplifted during Mesozoic collision between the North China and South China Blocks and the Tertiary collision of India and Eurasia.
The North Sea Silverpit Crater: impact structure or pull-apart basin?
Authors: Smith Kevin
Published: December 2007

The Silverpit Crater consists of a set of concentric faults up to 20 km in diameter in the southern North Sea. Although formerly described as the first impact structure to be recognized in the UK, its spatial association with local tectonic and stratigraphic features suggests that it is unlikely to have been caused by a random extra-terrestrial event. This paper proposes a terrestrial origin for the Silverpit Crater as a Palaeogene pull-apart basin linked to strike-slip faulting in the Carboniferous basement. Faulting and thinning of Permian evaporites underlying the crater acted as a buffer to basement extension and caused both a flexural collapse of the overlying Triassic strata and circumferential listric faulting in the Cretaceous succession. Reactive diapirism in Lower Jurassic shales may have contributed to the formation of a central uplift at base Cretaceous level. The paper compares the tectonic setting of two contemporaneous strike-slip-related structures in southern England (Bovey Basin in Devon and the Compton Valence structure in Dorset) with that of the Silverpit Crater, and concludes by briefly considering whether a pull-apart model can help to explain the formation of Upheaval Dome, a similar putative impact structure in the Canyonlands National Park of Utah.
Formation of bifurcating chromitite layers of the UG1 in the Bushveld Igneous Complex, an analogy with sand volcanoes
Authors: Nex P.A.M.
Published: December 2007

Chromitite layers occur frequently within the Critical Zone of the Bushveld Complex. In particular, Dwars River in the eastern Bushveld is well known for the intimate association of chromitite and anorthosite layers. An explanation has been deduced for bifurcations of chromitite layers and other features frequently found in exposures of the UG1 and its footwall. It is proposed that the unconsolidated footwall anorthosite experienced liquefaction possibly caused by major magma influx and associated seismicity. This resulted in structures analogous to those seen in sediments that undergo liquefaction, including sand volcanoes or boils. Bifurcations of chromitite layers occur where there is ‘background sedimentation’ of chromite at the same time as periodic extrusion of plagioclase plus magma slurry at the magma–cumulate-pile interface. These extrusions form ‘waterlilies’ of anorthosite that build up on top of each other to form domes. Simultaneously, chromite accumulates and forms multiple layers, which vary in thickness from those formed by a single layer of chromite grains to those over 0.5 m thick. Subsequent post-depositional structures suggest continued liquefaction in the footwall rocks, mobility of residual anorthositic magma and brittle deformation of chromitite layers.
Basin thermal history favours an accretionary origin for the Southern Uplands terrane, Scottish Caledonides
Authors: Stone P. , Merriman R.J.
Published: December 2007

There is general agreement that the Siluro-Ordovician, Southern Uplands suspect terrane is an imbricate thrust belt formed adjacent to the Laurentian continental margin during closure of the Iapetus Ocean. However, there are conflicting models for its tectonosedimentary evolution, especially for the initial depositional environment. Was it a convergent forearc trench with subsequent incorporation of strata into a suprasubduction accretionary prism, or was it a back-arc or continental margin extensional basin subsequently caught up in a foreland fold and thrust belt? To address this issue the different thermal histories of the contrasting convergent and extensional sedimentary basin models have been investigated. Palaeogeothermal conditions in the Southern Uplands terrane, assessed from regional metamorphic patterns, clay mineral assemblages and K-white mica composition, rule out extension and support deposition in a low heat-flow, plate-convergent setting. There is no evidence for a change from extensional to convergent basin regimes from north to south, as required in some models. Instead, throughout the terrane, there is a close temporal and spatial relationship between deposition, burial metamorphism and tectonism that is elsewhere typical of accretion.
The structure of the south Urals foreland fold and thrust belt at the transition to the Precaspian Basin
Authors: Brown D. , Alvarez-Marron J. , Perez-Estaun A. , Gorozhanina Y. , Puchkov V.
Published: December 2007

The structural architecture of the south Urals foreland fold and thrust belt at the transition into the Precaspian Basin is that of a wedge-shaped, west-vergent thrust stack with a significant amount of basement involvement. The surface geology of this part of the thrust belt is dominated by the south Urals accretionary complex, whose internal structure was primarily developed during Late Devonian to Early Carboniferous arc–continent collision and which was subsequently incorporated into the thrust belt. A number of thrusts, identified in the field and in seismic profiles, breach the accretionary complex, suggesting that they post-date its emplacement and may be related to the development of the foreland fold and thrust belt. A basal detachment can be confidently defined only beneath the frontal part of the thrust belt, making it difficult to construct balanced and restored cross-sections and therefore to calculate the amount of shortening. Nevertheless, the amount of shortening appears to be of the order of only a few tens of kilometres. There is a major change in the structural style along the frontal part of the thrust belt that may be related to basement topography and/or basement response to the deformation across a basement fault oriented at a high angle to the structural grain of the thrust belt.
U–Pb zircon (SHRIMP) ages for the Lebombo rhyolites, South Africa: refining the duration of Karoo volcanism
Authors: RILEY T.R. , MILLAR I.L. , WATKEYS M.K. , CURTIS M.L. , LEAT P.T. , KLAUSEN M.B. , FANNING C.M.
Published: December 2007

U–Pb SHRIMP ages are reported for three rhyolite flows from the Lebombo rift region of the Karoo volcanic province. Two flows are interbedded with the Sabie River Basalt Formation and a third sample is from the overlying rhyolitic Jozini Formation. The interbedded rhyolites yield ages of 182.0 ± 2.1 and 179.9 ± 1.8 Ma, whilst the overlying Jozini Formation rhyolite yields an age of 182.1 ± 2.9 Ma. Combined with existing
40
Ar/
39
Ar geochronology, the new SHRIMP data fine-tunes the chronology of the Karoo volcanic province and indicates the 12 km succession of volcanic rocks in the Lebombo rift were erupted in 1–2 million years and lends considerable support to the links between the Pleinsbachian–Toarcian extinction event and the global environmental impact of Karoo volcanism.
Palaeokarst and its implication for the extinction event at the Frasnian–Famennian boundary (Guilin, South China)
Authors: CHEN DAIZHAO , TUCKER MAURICEE.
Published: December 2007

A prominent palaeokarst within cyclic peritidal strata reported here at the Frasnian–Famennian boundary in Guilin, southern China, displays karren (scalloped surfaces), dissolution pits and pipes. The last contain residual soil (terra rossa), and are then filled with marine sediment of the overlying strata. The features of the palaeokarst, depth of pipes and amount of missing strata indicate a significant and long-lasting (>
c
. 50 ka) fall of sea level at the end of the Frasnian. The coincidence of the significant sea-level fall with the first stage of the Frasnian–Famennian mass extinction (which mostly affected shallow-marine and reef-dwelling fauna), and of the subsequent rapid sea-level rise with the second stage of the extinction (which mostly affected deeper-water organisms), reinforces the importance of sea-level change in this mass extinction.
Organic maturation in the French Subalpine Chains: regional differences in burial history and the size of tectonic loads
Authors: MOSS S.
Published: December 2007

The French Subalpine Chains of southeastern France are the foreland fold/thrust belt of the Western Alps, formed in the mid- to late Tertiary as a result of the continental collision between the European and Italian-North African plates. With the change from passive margin to destructive margin, certain areas of the Subalpine Chains experienced enhanced temperatures through being buried to greater depths. Burial of these areas has been investigated using vitrinite reflectance
(R
o
).

Levels of organic maturity of Mesozoic and Tertiary horizons have been established using vitrinite reflectance
(R
o
)
and spore fluorescence. In the southwestern part of the chain, values of
R
o
for Mesozoic units from the Vercors and Chartreuse massifs are generally low, with all Cretaceous units having values <0.5%
R
o
. Older Jurassic horizons have higher values, up to 1.25% for the mid-Jurassic (Oxfordian). When compared to values from the same horizons, and from younger Tertiary units (published data) from the northeastern part of the chain, it is clear that this part of the belt has encountered higher temperatures, produced by greater depth of burial. In the Haut-Giffre and Aravis massifs Mesozoic units have
R
o
values as high as 2.7% and, in terms of hydrocarbon generation, can be considered mature or overmature.

Two causes of greater depth of burial of the northeastern part of the belt are indicated by field studies and by burial history modelling of specific localities of the Subalpine Chains: first, the earlier initiation of the Tertiary foredeep in the northeast compared to the southwest; and second, differences in the size of tectonic loads emplaced onto the Subalpine Chains during the final stages of Alpine compression, with the greater structural deformation of the Subalpine Chains occurring in the northeast.
Measuring lithospheric stretching on Tethyan passive margins
Authors: WOOLER D. A. , SMITH A. G. , WHITE N.
Published: December 2007

Preliminary estimates of lithospheric stretching have been made by examining 63 stratigraphic sections, from the northern and southern Tethyan margins (southeast France to Bulgaria). The study was centred on the Italian Southern Alps (the Dolomites), a portion of the southern margin within the external zones of the Alpine orogen. Sections were decompacted and backstripped using the standard procedure, allowing for errors in palaeowater-depth estimates. Theoretical subsidence curves based on the finite-duration stretching model were then fitted to the resultant water-loaded subsidence curves in an attempt to clarify the pre-compressional tectonic history of the region. According to our results, stretching started in the early to mid-Triassic and lasted 30–40 Ma. This conclusion is broadly in agreement with some previously published estimates. The stretching factor, β, is generally small (1.1–1.3), especially in southeast France and northern Italy. Larger values of β occur in the Balkans (
c.
1.5). To our knowledge, the finite-duration stretching model has not yet been applied to water-loaded stratigraphic sections from the Tethyan belt. Theory and observation appear to agree well.
Tectonic evolution of the Cuvette Centrale, Zaire
Authors: DALY M. C. , LAWRENCE S. R. , DIEMU-TSHIBAND K. , MATOUANA B.
Published: December 2007

The Cuvette Centrale (Central Basin) of Zaire is an infra-Cambrian to Recent basin with up to 9 km of sedimentary section. Basin subsidence can be largely explained by a postulated Late Proterozoic failed rift and subsequent thermal relaxation. Seismic reflection profiles and well data reveal a Palaeozoic development of the basin punctuated by two regional deformational episodes; one in the Early Palaeozoic (Late Cambrian) and a second in the Late Palaeozoic (Late Permian-Triassic). Both episodes result from NE-SW contraction, with the Late Palaeozoic episode being largely a reactivation of the Early Palaeozoic structures. The timing of the two deformations correlates with collisional events at continental margins some distance from the Congo Basin. In the Early Palaeozoic the deformation appears to be related to the late stages of the West Congolian fold and thrust belt and coincides regionally with the Pan-African event
sensu stricto.
The Late Palaeozoic deformation coincides with collisional tectonics along the southern margin of Gondwana and the generation of the Cape and Sierra Ventana Foldbelts. It is postulated that in both the Early and Late Palaeozoic, Africa and Gondwana experienced widespread intracontinental deformation generated by distant collisional processes much as Central Asia is experiencing today.
Origin and deformation of the Neogene–Recent Maghrebian foredeep at the Gela Nappe, SE Sicily
Authors: BUTLER R. W. H. , GRASSO M. , LA MANNA F.
Published: December 2007

SE Sicily contains one of the few exposures of orogenic foreland to the Maghrebian-Apennine orogenic system in the central Mediterranean. Here the African foreland is represented by the Hyblean plateau, adjacent to the frontal thrust structures of the Maghrebian chain. These structures include the disrupted foredeeps to earlier parts of the thrust system. One of these, the Gela nappe, is a major structure composed of late Oligocene to early Pliocene sediments emplaced onto Pliocene–Pleistocene rocks of the buried foreland. This paper presents a correlation of late Miocene stratigraphic units between the foreland and the foredeep basin. Abrupt lithological and thickness variations of these units suggest that the foredeep basin was limited towards the foreland by normal faults. In the hinterland these foredeep sediments lap back onto previously emplaced thrust sheets, including the far-travelled units of ‘Numidian flysch’. The precursor Oligocene–Serravallian foredeep may also have been controlled by normal faulting. Compressional tectonics are reflected in folding and thrusting which post-dates deposition of lower Pliocene chalks (Trubi Formation), although minor folding and erosion preceded this unit. Larger thrusts repeat the basin stratigraphy and carry it out onto the adjacent foreland. A minimum of 8 km shortening since Messinian times can be demonstrated although a more realistic figure is
c.
25 km, at a time-averaged rate off. 0.5cm/a
−1
. The stratigraphic relationships on the foredeep sedimentary sequences may provide analogues for offshore areas of the thrust belt.
Two decades of research on the Proterozoic-Phanerozoic transition: 1972-1991
Authors: COWIE JOHN W.
Published: December 2007

The most radical global change, involving the biology, chemistry and physics of life, occurred near and at the stratigraphical level of the boundary between the Neoproterozoic System (Precambrian) and the Cambrian System. This stratigraphical level is held by consensus to be the Proterozoic Eonothem-Phanerozoic Eonothem Boundary, and is regarded as being of fundamental importance. Research has involved not only archaeocyathans, shelly small problematica, earliest trilobites, stromatolites and algae, trace fossils, soft-bodied metazoans, microphytolites and acritarchs, but also physical and chemical aspects: radio-isotope dating, carbon, strontium and oxygen isotope stratigraphy, polar wandering relating to palaeogeography and magnetostratigraphy. A selected bibliography (1985-1990) is appended to enable further reading of the extensive research contributions.
Some problems concerning the Precambrian-Cambrian transition and the Cambrian faunal radiation
Authors: ROZANOV A. YU.
Published: December 2007

In Siberia, the so-called Khailakh and Manykay Stages of ‘pre-Tommotian’ age are inferred to be partly coeval with the type Tommotian, and to be based on a misunderstanding about the control of facies on faunal distributions. Evolutionary patterns are being studied through this interval using a variety of approaches; synoptic reviews are presented on diversity changes, on changes in skeletal composition and on archaeocyathan morphology. The latter group appear to have originated on the Siberian Platform just prior to the Tommotian and migrated globally during the Atdabanian and Botomian to form secondary centres of diversification. Olenellid trilobites may have dispersed in a similar pattern but other groups (such as
Volborthella
and relatives) appear to have had a separate, siliciclastic centre of origin. The decline in diversity of small shelly fossils after the Tommotian, and the coeval expansion in archaeocyathans and trilobites, may be related to basinal changes connected with the break up of Palaeopangea.
Patterns of phytoplankton radiation across the Precambrian-Cambrian boundary
Authors: VIDAL G. , MOCZYD£OWSKA M.
Published: December 2007

Late Proterozoic and early Phanerozoic phytoplankton comprise a wide array of microfossils of possibly bacterial and algal affinity. Their radiation patterns around the Proterozoic-Cambrian boundary interval are becoming increasingly well-known for numerous sequences in the East European and Siberian Platforms, China, Baltoscandia and North America. Transgressive sequences in the East European Platform and northern Baltoscandia comprise alternating sandstones, mudstones and claystones. Their sedimentology is well documented and microfossils (acritarchs), ‘chitinous’ fossils (sabelliditids), shelly fossils and trace fossils are well represented. Rocks immediately overlying the transitional beds have yielded lower Cambrian trilobite faunas that are utilized for regional correlations. Continuous successions of planktic microorganisms, consisting of prokaryotic photoautotrophic cyanobacteria and algal protists, are known from rocks comprising the transitional Vendian-Cambrian sequence. In the East European Platform the rock sequence attributed to the lower Cambrian
Platysolenites antiquissimus
Zone has yielded acritarchs of 'Cambrian type', abruptly replacing a morphologically simple ‘Proterozoic type’ microbiota recorded in beds of the upper Proterozoic
Sabellidites-Vendotaenia
Zone. The radiation of novel forms of phytoplankton appears to have been synchronous over a wide part of the late Proterozoicarly Cambrian shelf, and is parallelled by the nearly contemporaneous emergence of Cambrian shelly fossils and trace fossils. Along with new acritarch taxa, the diagnostic acritarch assemblage of the
Platysolenites antiquissimus
Zone consists of widely distributed species, occurring also in the lowermost Cambrian deposits of the Siberian Platform, Baltoscandia, Greenland, North America, Svalbard and Scotland. It is suggested that along with shelly faunas, trace fossils and geochemical data, the general radiation patterns of phytoplankton shed light on the events shaping the important biotic events around the Precambrian-Cambrian boundary. It is also believed that a better understanding of the regulatory mechanisms affecting the base of trophic chains may in turn be of significant importance in defining the Precambrian-Cambrian boundary.
Magmatism and the causes of continental break-up
Authors: LEAT PHILIP
Published: December 2007

A two-day meeting of the Tectonic Studies Group was held in London at the Geological Society on 3-4 October 1991, to consider relationships between continental extension, super continent break-up, magmatism and mantle plumes. The approach was multidisciplinary, with contributions from geophysists, geochemists and tectonic geologists. There were 28 talks and 11 poster presentations, with about 150 people attending.

The conference was opened by convenor
B. C. Storey
(British Antarctic Survey, Cambridge) followed by a session on relevant geophysical and geochemical principles.
D. L. Anderson
(California Institute of Technology) gave a stimulating keynote lecture on mantle reservoirs and seismic tomographic evidence for mantle circulation. He suggested that the source of all 'enriched' basalts such as ocean island basalt (OIB) and continental flood basalt was in a shallow mantle layer relative to the source of depleted mid-ocean ridge basalt (MORB). He presented new, high resolution global seismic tomography data, and argued from these that deep-rooted hot mantle up-wellings underlie mid-ocean ridges and back arc basins, but that mantle plumes, as narrow jets underlying hot spots cannot be oberved by seismic means, nor are required by geochemical data.
M. A. Menzies
(Royal Holloway and Bedford New College) reviewed the penological, age and thickness variations in continental lithospheric mantle, and its suitability as a mantle source for continental basalts. He described reaction zones and metasomatic fronts generated by intrusion of magmas into mantle rock, and argued that such processes generate a range of mantle compositions like those of source regions of continental flood basalts.
Petrology of a magma chamber: the Plutonic Complex of Guernsey (Channel Islands, UK)
Authors: de BREMOND d’ARS J. , MARTIN H. , AUVRAY B. , LECUYER C.
Published: December 2007

The Northern Plutonic Complex of Guernsey is a calc-alkaline intrusion emplaced within a metamorphic complex composed mainly of the Icart Gneiss and the Perelle Gneiss. The Northern Plutonic Complex is made up of four distinct bodies: the St Peter Port Gabbro, the Bordeaux Diorite Group, the L’Ancresse Granodiorite and the Cobo Granite, which are roughly contemporaneous, emplaced at the end of the Cadomian orogeny, at around 550–560 Ma. The St Peter Port Gabbro and the Bordeaux Diorite Group are relicts of a magma chamber, characterized by layered rocks along the margins, and heterogeneous rock associations in the core of the chamber resulting from stirring by convection. Oxygen isotopic data are consistent with melting of the country-rock (possibly the Perelle Gneiss) in response to the intrusion of the gabbro-diorite magma chamber. The resulting acidic melts mixed with crystallizing gabbro-diorite magma to yield the Cobo Granite and the L’Ancresse Granodiorite. This mechanism of acid-magma genesis can probably be extended to other late Cadomian magmatic events in the Northern Armorican Massif, suggesting that basic plutonism in this area may be more important than previously considered.
Late Proterozoic evolution of the northern part of the Hamisana zone, northeast Sudan: constraints on Pan-African accretionary tectonics
Authors: MILLER M. M. , DIXON T. H.
Published: December 2007

New structural data from the Red Sea Hills, northeast Sudan, add constraints to accretionary tectonic processes that acted in the Arabian-Nubian shield during the late Proterozoic. Geometric constraints provided by field data are used together with Landsat Thematic Mapper imagery to interpret the regional distribution and structural relations of spectrally and lithologically distinctive rock assemblages. In contrast to interpretations reported elsewhere, these studies indicate that the Hamisana zone, a prominent structural feature in the northern Red Sea Hills, post-dated terrane accretion. It is characterized by east-west crustal shortening and by steep folds and thrust faults. Calc-alkaline magmatism was contemporaneous with and outlasted main phase deformation, thus deformation occurred in an intra-arc setting. Thrust faults were reactivated after east-west contraction, compromising their usefulness in delineating 'sutures' or as piercing points across older faults.

Pan-African accretionary processes may have been analogous to Phanerozoic ophiolite and island arc accretion in the western North American Cordillera, where penetrative deformation occurred in response to periodic intra-plate shortening events rather than an ultimate collision of unrelated crustal fragments. Such an analogy implies a mechanism for crustal thickening by structural as well as igneous processes during evolution of an accretionary belt in a long-lived convergent margin setting, consistent with the long-standing observation that Pan-African magmatism became more evolved with time. This view emphasizes that an interplay of deformation and magmatism resulted in protracted accretion and crustal thickening, in contrast to models where crustal shortening results directly from collision of a volcanic arc, microcontinent, or other buoyant features.
Effects of layering and anisotropy on fault geometry
Authors: PEACOCK D. C.P. , SANDERSON D. J.
Published: December 2007

The geometry and orientation of faults are examined using several field examples of small-scale (< lm displacement) fault systems. For isotropic rocks under triaxial compression, faults normally develop in conjugate sets about the maximum compressive stress (σ
1
), with dihedral angles usually of about 50°, as predicted by the Coulomb theory of failure. In layered rocks, the geometry of faults varies with the orientation of layering with respect to the stress field. Where σ
1
, is approximately normal to layering or anisotropy, conjugate faults also develop symmetrically about σ
1
. Where rocks have interbedded layers with different mechanical behaviours, however, faults tend to initiate orthogonal to the more brittle layers (i.e. originate as extension fractures sub-parallel to σ
1
)
,
but oblique to the less brittle layers. As the fault steps through the layering, pull-aparts are developed which may reduce the dihedral angle. Where σ
1
is oblique (
c
. 25–75°) to anisotropy, one set of faults is developed at a high angle to layering, with another at a low angle, usually showing a ramp-flat geometry. Large dihedral angles (up to 90°) may result and σ
1
, does not bisect this angle.

Where σ
1
is approximately parallel to layering, two cases can be recognized. Where σ
3
is approximately normal to layering, faults with layer-parallel flats and contractional ramps develop. Where σ
2
is approximately normal to layering, conjugate faults develop which are symmetrical about σ
1
, the geometry resembling that in isotropic rocks. These observations are in agreement with rock deformation experiments which show the strong effect of anisotropy on fault orientation, but the observations incorporate the effects of layering of materials with different deformation characteristics.
Sinistral transpression and the Silurian closure of Iapetus
Authors: SOPER N. J. , STRACHAN R. A. , HOLDSWORTH R. E. , GAYER R. A. , GREILING R. O.
Published: December 2007

In recent years conflicting models have been proposed for the late Caledonian closure of the Iapetus ocean between Laurentia, Baltica and the Avalonian terranes. Recently published structural and stratigraphic evidence from Britain, Scandinavia, East Greenland and Newfoundland is reviewed and shows that Western Avalonia, Eastern Avalonia and Baltica all docked sinistrally against Laurentia in the Silurian.

Western Avalonia collided sinistrally against the previously accreted Gander and Dunnage arc terranes on the Appalachian margin of Laurentia in mid-Silurian time and then shifted dextrally during the Acadian orogeny in the Devonian. Oblique collision of Baltica with the Greenland margin induced southeasterly crustal stacking in the Scandian orogen and sinistral transpression in the NE Greenland Caledonides, and was followed by more nearly orthogonal convergence. Eastern Avalonia underwent anticlockwise with the Scottish ‘corner’ of Laurentia, rotating into a re-entrant between Laurentia and Baltica.

Some implications of this Silurian closure model are that convergence in the Tornquist zone was modest at that time, and that the Acadian (Devonian) deformation in the northern Appalahians and Britain had a subsequent external cause, most likely the impingement of Armorica and Iberia due to the northward drift of Gondwana.
Palaeomagnetic data from the Borrowdale Volcanic Group: volcano-tectonics and Late Ordovician palaeolatitudes
Authors: CHANNELL J. E. T. , McCABE C.
Published: December 2007

Nineteen palaeomagnetic sampling sites from the Borrowdale Volcanic Group between Borrowdale and Langdale (English Lake District) yield high unblocking temperature magnetization components which postdate the structural tilt, and therefore postdate the formation of the broad block-faulted syncline which characterizes this part of the central Lake District. The mean magnetization direction before tilt correction (Dec: 347.2°, Inc:–61.9°, α
95
: 6.9°) corresponds to a palaeolatitude of 43°S (+ 5°/–7°). The magnetization is associated with pervasive Late Ordovician hydrothermal alteration, and is certainly pre-Devonian based on comparison with available palaeomagnetic data from southern Britain. This implies that the block-faulted synclinal structure of the Borrowdale/Langdale region is pre-Acadian in age, consistent with recent interpretations of Late Ordovician volcano-tectonics based on mapping and volcanological studies.
Deep structure of southern Ireland: a new geological synthesis using BIRPS deep reflection profiling
Authors: FORD M. , KLEMPERER S. L. , RYAN P. D.
Published: December 2007

The BIRPS deep seismic reflection data of WIRE (West of IREland) 1/1B from Galway Bay to the Celtic Sea are integrated with geological and other geophysical data to produce a synthesis of the fundamental tectonic character of the crust in and around southern Ireland. The Iapetus Suture forms a zone of north-dipping reflectors which projects up between the Shannon estuary and the Dingle peninsula and is overlain by a late Caledonian reflector with a shallow, south dip. Across the Iapetus Suture zone the crust thickens southward and the lower crust becomes less reflective. It is proposed that the Lower Palaeozoic volcano-sedimentary rocks which form the Leinster massif extend southwestward beneath the Variscan province and at least 30 km offshore to the WIRE lines. We argue that the weak south-dipping reflectors in the upper and middle crust south of the Iapetus Suture represent an early-Caledonian fault system linked to a mid-crustal detachment which formed during the south-dipping subduction event recorded onshore by the Ordovician volcano-sedimentary successions. These faults were selectively reactivated causing the predominant ENE trend of younger regional tectonic features in and around southern Ireland. The three onshore Upper Palaeozoic basins close east of the WIRE lines.

We point out that the ‘Variscan Front’ is only a map line across the British Isles with no consistent tectonic character. Thus the south-dipping reflector below the north margin of the North Celtic Sea Basin on SWAT 4, generally referred to as the ‘Variscan Front’, is correlated by us with the northern boundary of the Rosslare complex, thus making it the northern tectonic boundary of the Caledonian basement block known as the Rosslare-Monian Terrane. Furthermore the north-dipping South Irish Sea Lineament on WINCH 4 can be correlated with the Menai Straits Lineament and could thus be the southern boundary of this terrane. The south-dipping reflector on SWAT 2, also previously called the ‘Variscan Front’ is correlated with the Johnston-Benton fault system in southwest Wales.
Seismic reflection images of intraplate deformation, central Indian Ocean, and their tectonic significance
Authors: BULL J. M. , SCRUTTON R. A.
Published: December 2007

Multichannel seismic reflection profiles collected from the intraplate deformation area in the Central Indian Ocean Basin are used to describe brittle structures produced under compressive stress. Reflectors within oceanic basement are divided into four types: north-dipping and south-dipping reverse faults that are interpreted as reactivations of structures formed at the spreading-centre as outward and inward dipping faults respectively; lower-angle, north-dipping reflectors that probably represent new faults or faults just initiating; and sub-horizontal reflectors within the uppermost crust that are interpreted as hydrothermal alteration fronts. Within the sedimentary cover upwards fault propagation shows the faults steepening from
c
. 40° just above basement to near vertical and is preceded by sediment folding. A fractal analysis of faulting suggests two fault populations possibly reflecting different criteria for brittle failure. Measurements of north-south crustal shortening indicate a shortening rate of 2.5 (± 0.9) mm a
−1
, which is at the lower end of predictions from plate motions, but significant enough to recognize this area as a diffuse plate boundary. The formation of long-wavelength basement undulations and the reactivation of fracture zones and ridge-parallel fault fabrics are linked in a unified tectonic model driven by the high level of intraplate compressive stress in the area. There is little evidence from the seismic profiles for intraplate deformation starting before the widespread unconformity dated as 7 Ma.
Sedimentary basin destruction inferred from the evolution of drainage systems in the Betic Cordillera, southern Spain
Authors: VISERAS C. , FERNANDEZ J.
Published: December 2007

The Guadix continental basin developed during the uppermost Miocene, the Pliocene and the Pleistocene. One axial fluvial system and two transverse systems are interpreted in its alluvial infill. In addition there are areas of lacustrine sedimentation, permanent in the eastern sector and ephemeral in the western part of the basin. From a sedimentological point of view, the Holocene drainage network shows significant similarities with that which occupied the ancient basin, although there are some differences in location. These provide information about the processes involved in the transformation of the sedimentary basin into an erosional basin with external drainage. Most of the data seem to indicate that the capture of drainage from the Guadix Basin by the Guadalquivir River was accelerated principally by tectonic processes related to a recent uplift of a sector of the Betic Cordillera, together with the disappearance of subsident points of the Basin. The high rate of Holocene erosion is partially conditioned by the type of sediment deposited in the basin (a low degree of lithification, textural heterogeneity, etc.) in comparison with the basement, and mainly by the position of the ancient basin in relation to the new relative base level, located almost 500 m lower than the position actually occupied by the Pleistocene relative base level of the basin.
EDITORIAL
Authors: Le Bas M. J.
Published: December 2007

To celebrate the 150th volume of the
Journal of the Geological Society,
several distinguished authors have been invited to present their personal views on significant topics that had been brought to the fore in earlier publications of the Geological Society. These include the early seminal works of Lyell, Murchison, Sedgwick and Sorby to later ones by Howell Williams, O.T. Jones, H.H. Read and even several by still active protagonists.

Two or three such contributions will appear in each issue of volume 150 of the
Journal.
In this issue, Martin Rudwick looks at the history of the
Journal,
Brian Windley applies Lyell’s Principle of Uniformitarianism to solving the Earth’s history during the Archaean and subsequently, and Stuart McKerrow considers the works of Murchison and Sedgwick and others, and sees how they created a foundation from which modern global stratigraphy has emerged. The contributions make evident the role held by the
Journal
and other publications of the Geological Society, at the forefront of geological thinking right across the many specializations that now divide the science.

Volume 150 in 1993 is not quite the sesquicentennial issue, because in 1977 two volumes were published in order to clear the back-log of good papers awaiting publication. The same feeling that too much good science was not receiving the wide appreciation deserved, was one factor that made Lyell, Murchison, Warburton, Horner and De La Beche seek an improved medium of publication for the Geological Society in the 1840s. As Rudwick points out in the first of
Historical origins of the Geological Society’s
Journal
Authors: RUDWICK MARTIN J. S.
Published: December 2007

The Geological Society’s
Transactions,
its earliest periodical (from 1811), published the full texts of a few selected papers, with fine illustrations, but generally long after they had been read at one of the meetings. Conversely, the
Proceedings
(from 1826) recorded all the papers soon after they had been delivered, but only in abstract and without illustrations. The launching of the
Quarterly Journal
1845 was an attempt to combine the advantages and eliminate the disadvantages of the earlier periodicals. After a shaky start, it proved highly successful through the rest of the nineteenth century and much of the twentieth, and was the direct forerunner of today’s
Journal.
Uniformitarianism today: plate tectonics is the key to the past
Authors: WINDLEY BRIAN F.
Published: December 2007

James Hutton published the first two volumes of
The Theory of the Earth
in 1795 and the third volume was published posthumously by the Geological Society in 1899. Charles Lyell in his four addresses (1836, 1837, 1850, 1851) to the Society put the uniformitarian paradigm of Hutton (the present is the key to the past) into the perspective of his era. Uniformitarianism today can be expressed in the view that plate tectonics is the key to the past. This paper summarizes key data and ideas which confirm that the plate tectonic paradigm can be applied convincingly back to the beginning of the geological record. In spite of the fact that heat production was greater in the early Precambrian than now, tectonophysical and geochemical processes that produced oceanic and continental rocks since the early Archaean have not been fundamentally different from those that operate today.