bluuebunny/crossref_metadata_2025_split huggingface dataset:
Plio-Quaternary volcanism in Ecuador
Authors: Barberi F. , Coltelli M. , Ferrara G. , Innocenti F. , Navarro J. M. , Santacroce R. 
Published: May 2009

AbstractExtensive sampling, major element chemistry on over 300 samples and K-Ar radiometric dating have been carried out on the Ecuadorian Upper Tertiary–Quaternary volcanoes. The results show important space–time variations of the volcanic activity, between Late Miocene time and the present. In Late Miocene time a continuous volcanic belt, located approximately along the present volcanic front (VF), affected the whole country from the Cuenca basin to the south, up to Colombia to the north. Major changes occurred at about 5 Ma: volcanic activity stopped south of the Guayaquil fault belt and never resumed; to the north the active volcanic axis shifted eastward to the Cordillera Real (CR) area with a simultaneous relative decrease in intensity. Since Early Quaternary time the volcanic belt widened westward to the Western Cordillera where the volcanism resumed at about 1.5–1.0 Ma, giving rise to the very wide active volcanic zone of Ecuador.The Plio-Quaternary products show significant longitudinal and latitudinal chemical and mineralogical changes. Volcanics of the VF and Interandean Depression contain amphibole and define a calc-alkaline trend with a K2O content lower than that of the CR products, which are characterized by a mostly anhydrous phenocryst assemblage. In both areas andesites dominate, but extreme compositions (basaltic andesites and rhyolites) are more diffuse in the CR than the VF. No significant transverse zoning has been detected in the northern region (north of the Chota-Mira transverse tectonic line). The observed temporal and spatial variations are interpreted as a result of the subduction of the Carnegie Ridge anomalous oceanic crust, underthrusting of which began approximately 6 Ma ago.
Deformation of Anhydrite–gypsum rocks in central Spitsbergen
Authors: Harland W. B. , Mann A. , Townsend C. 
Published: May 2009

AbstractEarly Permian anhydrite–gypsum strata in central Spitsbergen show deformation of sulphate nodules in dolostone (chicken-wire structure) to varying degrees from imbricate to extreme flattening and elongation. These shear zones often occur between undeformed layers of apparently similar strata.The shear zones indicate a minimum displacement of between 0.5 and 1.5 km to the ENE of Zechstein and later strata over the Nordfjorden Block. Resultant folding and faulting over the Billefjorden Fault Zone represent an eastward manifestation of the Eocene West Spitsbergen Orogeny.
Volcanism on Gough Island: a revised stratigraphy
Authors: Maund J. G. , Rex D. C. , Le Roex A. P. , Reid D. L. 
Published: May 2009

AbstractRecent field work on Gough Island combined with K–Ar dating of the lavas requires revision of the age and volcanic stratigraphy. Four main periods of volcanic activity on the island are recognized. These comprise the eruption of the Older Basalt Group which ranges in age from 2.5 to 0.52 Ma, the intrusion of aegerine-augite trachyte plugs (0.8−0.47 Ma), voluminous trachyte extrusion (0.30–0.12 Ma) and finally to eruption of the Edinburgh Basalt (0.20–0.13 Ma).Within the Older Basalt Group three phases of activity can be recognized; the earliest involving the eruption of pillow basalts and hyaloclastites when the island emerged from below sea level. This was followed by subaerial as-type lava flows and also dyke intrusion (phase two) which probably contributed to forming a large shield-type volcanic island, which in turn supported the eruption and deposition of flat-lying flows on an angular unconformity (phase three). Intrusion of aegirine-augite trachyte plugs occurred concurrently with the latter stages of Older Basalt eruption. After a period of considerable erosion the voluminous trachyte lavas and pyroclastics were erupted. The Edinburgh Basalt, erupted in the vicinity of Edinburgh Peak, represents the youngest volcanic activity on the island.
The nodular granite of Castanheira, north central Portugal: origin of the nodules and evidence for diapiric mobilization of granite
Authors: Reavy R. J. , Hutton D. H. W. , Finch A. A. 
Published: May 2009

AbstractThe Castanheira pluton in north-central Portugal is a small (1000 m × 600 m) granite body of Hercynian age which contains a remarkable abundance of granite-cored, biotite-rimmed nodules. The nodules are interpreted as representing original bubbles in the uppermost volatile-rich zone of a granitic pluton. Strong depletion in K and Rb in the host granite around the nodules suggests that the biotite is magmatic in origin. The nodules may have formed by reaction between chloroferrate(II) complexes in the vapour phase and silicate melt, possibly followed by condensation of the vapour phase to a small granitic core. Motion of the vapour bubble stabilized a gradient in chemical potential with respect to the host granite, giving rise to the nodules. Chemical, petrological and structural data suggest that the pluton was part of a larger granite body, which was forcefully emplaced during synchronous transcurrent shearing. The inferred presence of volatiles, in addition to the pervasive tourmalinization of the roof zone, suggest that the magma was halogen-rich; this may imply that the magma had low viscosity.
Accretion tectonics in northern Eritrea revealed by remotely sensed imagery
Authors: Drury S. A. , Berhe S. M. 
Published: May 2009

AbstractNew details from remotely sensed images of the structure and disposition of broad lithological variations in the Pan-African of northern Eritrea are discussed in the context of accretionary tectonics. The recognition of major north-south structural discontinuities allows the area to be divided into three discrete terranes with apparently different histories of deformation and metamorphism, magmagenesis and sedimentation. The central Hagar Terrane is dominated by large ultramafic masses with a volcano-sedimentary layered sequence, and shows the effects of major sinistral transpression and lateral expulsion. It is bounded to the west by a major fault, the Barka suture, and abuts the older Barka Terrane that comprises metasediments with evidence for polyphase ductile deformation and pre-kinematic dyke emplacement. The Hagar Terrane is thrust against the eastern Nacfa Terrane, which is dominated by low-grade calc-alkaline metavolcanics and immature volcanoclastic sediments intruded by syn-kinematic plutons. These units are pre-dated by an earlier high-grade basement and post-dated by high-level unmetamorphosed silicic volcanics and redbed sediments. The complex is suggested to have been assembled by oblique accretion from the southeast after arc volcanism in the Nacfa Terrane and back-arc extension in the Hagar Terrane ended with the cease of subduction.
An Acadian mountain front in the English Lake District: the Westmorland Monocline
Authors: Kneller B. C. , Bell A. M. 
Published: May 2009

AbstractThe structure of the southern and central English Lake District is that of a southeast-facing monocline, named here the Westmorland Monocline. This 10 km wide zone of highly cleaved, southeast-dipping rocks separates gently dipping, poorly cleaved Borrowdale Volcanic Group to the north from extensively folded but regionally subhorizontal Windermere Group (foreland basin) rocks to the south. The monocline formed early in the local Acadian deformation sequence, and accommodates at least 8 km of uplift. It coincides with the steep concealed margin of the Lake District batholith. A major northwest-dipping shear zone is revealed in the deepest levels now exposed within the monocline, in the Skiddaw Group rocks of the Black Combe inlier.The monocline has the characteristics of a mountain front, providing significant tectonic elevation across a foreland-dipping panel of rocks, with no hinterland-dipping thrust visible at the surface. We interpret the uplift as the consequence of a southeast-vergent thrust with a gently northwest-dipping ramp beneath the central Lake District, continuing southeastwards as a flat detachment beneath the Windermere Group. A displacement up the ramp of about 20 km is accommodated by backthrusting within the monocline and by shortening within the Windermere Group of the hangingwall southeast of the monocline. The tip lies beyond the limit of the Lower Palaeozoic inlier, beneath Carboniferous cover.
A modern analogue for the Lower Ordovician Obolus conglomerate of Estonia
Authors: Hiller Norton 
Published: May 2009

AbstractPhosphate-bearing rocks of the lower Ordovician Kallavere Formation, northern Estonia, contain diverse fragments and, more rarely, complete shells of the phosphatic inarticulate brachiopods Schmidtites and Ungula. In places the concentration of brachiopod debris in sandstones is so dense that economically exploitable seams of phosphorite are formed. A directly analogous situation occurs along the coast of Namibia today. In places the extant phosphatic inarticulate brachiopod Discinisca is washed up on the beach in such large numbers that its shells dominate the littoral sediment. The distribution range of this species suggests that it is a product of the Benguela upwelling ecosystem, and the inference is drawn that the Estonian deposits are the products of a similar palaeo-upwelling system.
Basement fractures in North Wales: their recognition and control on Caledonian deformation
Authors: Wilkinson I. , Smith M. 
Published: May 2009

AbstractA detailed survey of the patterns of regional deformation in North Wales has revealed the presence of several discrete, linear zones of anomalous deformation. These zones are distinguished by atypical finite strain values, intense tight to isoclinal folding, non-axial planar cleavages and high metamorphic gradients. A history of prolonged tectonic activity along these zones is indicated by their influence on sedimentation and volcanism. It is proposed that these zones reflect the deformation of the Lower Palaeozoic cover above pre-existing basement fractures. These basement fractures form a framework which divides North Wales into a series of blocks and sub-blocks. The end-Caledonian deformation is explained by a transpression model dominated by SE-directed shear with minor components of strike-slip localized in these zones. The application of this model to other parts of the Welsh Basin is briefly considered.
Carbon- and oxygen-isotope stratigraphy of the English Chalk and Italian Scaglia and its palaeoclimatic significance
Authors: Jenkyns H. C. , Gale A. S. , Corfield R. M. 
Published: May 2009

AbstractA detailed carbon- and oxygen-isotope stratigraphy has been generated from Upper Cretaceous coastal Chalk sections in southern England (East Kent; Culver Cliff, Isle of Wight; Eastbourne and Seaford Head, Sussex; Norfolk Coast) and the British Geological Survey (BGS) Trunch borehole, Norfolk. Data are also presented from a section through the Scaglia facies exposed near Gubbio, Italian Apennines. Wherever possible the sampling interval has been one metre or less. Both the Chalk and Scaglia carbon-isotopic curves show minor positive excursions in the mid-Cenomanian, mid- and high Turonian, basal Coniacian and highest Santonian–lowest Campanian; there is a negative excursion high in the Campanian in Chalk sections that span that interval. The well-documented Cenomanian–Turonian boundary ‘spike’ is also well displayed, as is a broad positive excursion centred on the upper Coniacian. A number of these positive excursions correlate with records of organic-carbon-rich deposition in the Atlantic Ocean and elsewhere. The remarkable similarity in the carbon-isotope curves from England and Italy enables cross-referencing of macrofossil and microfossil zones and pinpoints considerable discrepancy in the relative positions of the Turonian, Coniacian and Santonian stages.The oxygen-isotope values of the various Chalk sections, although showing different absolute values that are presumably diagenesis-dependent, show nonetheless a consistent trend. The East Kent section, which is very poorly lithified, indicates a warming up to the Cenomanian–Turonian boundary interval, then cooling thereafter. Regional organic-carbon burial, documented for this period, is credited with causing drawdown of CO2and initiating climatic deterioration (inverse greenhouse effect). Data from other parts of the world are consistent with the hypothesis that the Cenomanian–Turonian temperature optimum was a global phenomenon and that this interval represents a major turning point in the climatic history of the earth.
A new Arenig (Ordovician) graptolite fauna from the Kerman District, east-central Iran
Authors: Rickards R. B. , Hamedi M. A. , Wright A. J. 
Published: May 2009

AbstractA single slab of silicified silty and cross-bedded mudstone from the Katkoyeh Formation, east-central Iran yieldsYutagraptuscf.mantuanusRiva which indicates a late Arenig age. The slab also bearsDictyonema ghodsiaesp. nov., regarded as the only unequivocally planktonic dendroid from post-Tremadoc strata. The sicula is surrounded by at least twelve vesicular bodies which constitute a raft of floats. These are the only certain floats yet identified in the Order Dendroidea, and one of very few cases known in the Class Graptolithina.
Plio-Pleistocene radiolarian biostratigraphy and palaeoceanography of the mid-latitude North Atlantic (DSDP Site 609)
Authors: Haslett Simon K. 
Published: May 2009

AbstractRadiolaria were examined throughout the Plio-Pleistocene of Deep Sea Drilling Project (DSDP) Site 609. Eight radiolarian datum-levels (first and last appearances) were identified, some for the first time in the North Atlantic. The recognition of these datums allows correlation between the Atlantic, Pacific and Indian oceans, through a previously published zonal scheme (Johnson et al. 1989). Zones NR1 to NR11 were recognized, although some zones had to be combined (NR1–2 and NR8–10) due to the absence of some stratigraphically important taxa. The relative abundance distribution of the radiolarian palaeoceanographical proxy Didymocyrtis tetrathalamus indicated three cool phases (0/0.56–0.75 Ma, 1.2–1.33/1.69–1.86 Ma, and 2.14–2.32/3.73– > 4.1 Ma) interrupted by two relatively warm episodes (0.56–0.75/1.2–1.33 Ma and 1.69–1.86/2.14–2.35 Ma). These fluctuations in sea-surface temperature (SST) correspond with palaeoclimatic events indicated by other proxies (e.g. Foraminifera), such as the onset of Northern Hemisphere glaciation. This study illustrates the usefulness of radiolaria in North Atlantic stratigraphical and palaeoceanographical analysis.
Relationships between distributed and localized shear in the tectonic evolution of a Caledonian fold and thrust zone, northwest Ireland
Authors: Alsop G. I. 
Published: May 2009

AbstractBroad zones of distributed shear operating through mid-crustal regions of orogenic belts may incorporate narrow horizons of intense localized deformation culminating in discrete, large magnitude displacements. The relative importance and relationship between distributed and localized shear are influenced by a variety of factors including lithological variation, pre-existing structural anisotropy, strain rate and migration of fluids. Rigorous structural analysis of lower amphibolite facies Dalradian metasediments in northwestern Ireland demonstrates that an early (D1) discrete ductile detachment was subsequently reactivated during distributed non-coaxial D2 deformation operating in a broad zone through the structural pile. Regional shear was directed towards the southeast and resulted in the generation and translation of kilometre-scale, isoclinal, recumbent sheath folds which close and face towards the transport direction. The D1 detachment is clearly folded around the hinges of these major folds, whilst on fold limbs it was reactivated and acted as a local décollement within the zone of distributed shear. Shear criteria along the detachment indicate a southeast-directed translation of the major folds, in sympathy with regional shear. A broad zone of D3 translation operating through the nappe pile resulted in coaxial refolding of large scale F2 folds by the D3 Ballybofey Nappe producing a complex fold interference pattern. Non-coaxial D3 deformation resulted in continued reactivation of local decollements, together with the initiation of east-southeast directed oblique thrusts and partial dismemberment of D2 folds. Detailed structural investigation allows concepts of distributed and localized shear to be evaluated and models of crustal deformation to be assessed.
Geology and geochemistry of bitumen vein deposits at Ghost City, Junggar Basin, northwest China
Authors: Parnell John , Ansong Geng , Jiamo Fu , Guoying Sheng 
Published: May 2009

AbstractVeins of solid bitumen occur in Cretaceous sandstones at the northwest margin of the Junggar Basin, China. The bitumen has a low aromaticity and a composition comparable to gilsonite. The bitumen contains abundant steranes and terpanes, and β-carotane, although most n- and i- alkanes have been removed, which is characteristic of the local crude oil. The sterane and triterpane maturity parameters show that the bitumen, local crude oil, and source rocks are all mature. Bitumen–wallrock relationships suggest that the host sandstone was not completely consolidated at the time of emplacement of the bitumen veins, although bitumen emplacement was a relatively late diagenetic event. The burial history for the northwest Junggar Basin shows that hydrocarbon generation from the assumed upper Permian source rocks commenced in late Triassic/early Jurassic times and suggests that rapid hydrocarbon generation may have resulted in overpressure contributing to the bitumen emplacement.
Thermal maturation of the Lower Palaeozoic strata of northern Greenland from conodont colour alteration index (CAI) data: implications for burial history and hydrocarbon exploration
Authors: Armstrong H. A. , Smith M. P. , Aldridge R. J. , Tull S. J. 
Published: May 2009

AbstractConodont colour alteration data for the Lower Palaeozoic strata of the North Greenland carbonate platform indicate a pattern of increased thermal maturity northwards within the Franklinian Basin. There is little variation in values through the Canadian–Llandovery (Lower Ordovician–Lower Silurian) interval at any given locality. A simplified thermal model for the platform suggests that the predominant control of conodont colour alteration and thermal maturation was maximum depth of burial, which occurred during the mid- to late Silurian. A preliminary integrated scheme for conodont and organic thermal maturity indicators can be compiled from the data now available from North Greenland.
Mineralization, antiforms and crustal extension in andesitic arcs
Authors: Mitchell A. H. G. , Carlile J. C. 
Published: May 2009

AbstractThe distribution and stratigraphic position of porphyry copper and epithermal gold deposits in andesitic arcs of the western Pacific and eastern Europe suggest that porphyry copper and epithermal vein deposits of adularia–sericite type develop successively under different stress regimes in an evolving arc, rather than being genetically related as commonly supposed. Absence of coeval high-level stocks in the root zones of many adularia-sericite deposits suggests that circulation of the dominantly meteoric hydrothermal fluids is not driven by shallow intrusions. The location of several world-class deposits on basement geanticlines, and on more localized antiforms of which at least one has been interpreted as a metamorphic core complex, implies that elevation of the arc, emplacement of magmatic sills at depth and adularia–sericite type gold mineralization are genetically related to subduction-induced crustal extension. Ascent of deep hydrothermal fluids, predominantly meteoric but with a metamorphic or magmatic component, may be controlled by regional low-angle structures at depth, analogous to those inferred for some mesothermal gold deposits. Mineralization at shallow (epithermal) depths in high-angle structures largely reflects the high geothermal gradient and mixing of deep fluid with cool meteoric water in or at the base of the permeable volcanic cover. Andesitic magmatism may resume following porphyry copper mineralization, adularia–sericite epithermal gold mineralization, or continued extension to form a ‘back arc’ spreading system, depending on the relative plate motion.
Neoproterozoic sedimentation on the northeast margin of Laurentia and the opening of Iapetus
Authors: Soper N. J. 
Published: May 2009

AbstractVery thick shallow water sedimentary sequences were deposited in Neoproterozoic time along the future margins of Laurentia. On the eastern margin these include the Eleonore Bay and Hecla Hoek sequences of Greenland and Svalbard; these are described and their geotectonic context briefly reviewed. They present both geotectonic and geodynamic problems: why did subsidence continue for some 300 Ma prior to the opening of Iapetus, and how could 15–20 km of sediment be accommodated in an ensialic environment?Prolonged slow stretching appears to have affected the eastern margin of Laurentia while the western (Cordilleran) margin progressed through the rift-drift transition as the Pacific opened. It is proposed that expansion of the Pacific was associated with both the convergence of Proto-Gondwanan continental terranes during the Pan-African orogeny (the extended SWEAT hypothesis) and also the maintenance of very slow extension rates on the future Iapetus margin. The strain-hardening effect of slow stretching may have been inhibited by a continuous basin-fill of juvenile heat-producing Grenville detritus. The onset of subduction in the Pacific freed up this margin; major rifting took place between East Greenland and possibly the Tornquist margin of Baltica in Vendian time, followed by the opening of northern Iapetus.
Chemostratigraphy of predominantly siliciclastic Neoproterozoic successions: a case study of the Pocatello Formation and Lower Brigham Group, Idaho, USA
Authors: Smith Loren H. , Kaufman Alan J. , Knoll Andrew H. , Link Paul Karl 
Published: May 2009

AbstractIsotopic chemostratigraphy has proven successful in the correlation of carbonate-rich Neoproterozoic successions. In successions dominated by siliciclastic rocks, chemostratigraphy can be problematic, but if thin carbonates punctuate siliciclastic strata, useful isotopic data may be obtained. The upper Pocatello Formation and lower Brigham Group of southeastern Idaho provide an opportunity to assess the potential and limitations of isotopic chemostratigraphy in overwhelmingly siliciclastic successions. The 5000 m thick succession consists predominantly of siliciclastic lithologies, with only three intervals that contain thin intercalated carbonates. Its depositional age is only broadly constrained by existing biostratigraphic, sequence stratigraphic and geochronometric data. The lowermost carbonates include a cap dolomite atop diamictites and volcanic rocks of the Pocatello Formation. The δ13C values of these carbonates are distinctly negative ( −5 to −3), similar to carbonates that overlie Neoproterozoic glaciogenic rocks worldwide. Stratigraphically higher carbonates record a major positive δ13C excursion to values as high as +8.8 within the carbonate member of the Caddy Canyon Quartzite. The magnitude of this excursion is consistent with post-Sturtian secular variation recorded elsewhere in the North American Cordillera, Australia, Svalbard, Brazil and Namibia, and exceeds the magnitude of any post-Varanger δ13C excursion documented to date. In most samples, Sr-isotopic abundances have been altered by diagenesis and greenschist facies metamorphism, but a least-altered value of approximately 0.7076 supports a post-Sturtian and pre-Marinoan/Varanger age for upper Pocatello and lower Brigham rocks that lie above the Pocatello diamictite. Thus, even though available chemostratigraphic data are limited, they corroborate correlations of Pocatello Formation diamictites and overlying units with Sturtian glaciogenic rocks and immediately post-Sturtian successions in western North America and elsewhere.