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Th Lundqvist - One of the best experts on this subject based on the ideXlab platform.

Karin Hogdahl - One of the best experts on this subject based on the ideXlab platform.

Alastair H F Robertson - One of the best experts on this subject based on the ideXlab platform.

  • evidence from late cretaceous paleogene volcanic rocks of the kyrenia range northern cyprus for the northern active continental margin of the southern neotethys
    Lithos, 2021
    Co-Authors: Guohui Chen, Alastair H F Robertson
    Abstract:

    Abstract Late Cretaceous felsic and latest Cretaceous-Paleogene basaltic volcanic rocks are exposed throughout the Kyrenia Range, N Cyprus. Field mapping of the key, well-exposed western range indicates that the felsic volcanics mainly crop out in the southerly, frontal part of the range, separated from the latest Cretaceous-Paleogene basaltic volcanics farther north by a thrust. New U-Pb zircon dating of the felsic volcanics indicates a primary age of c. 74.0 Ma (Late Campanian). The felsic volcanics are characterised by evolved compositions that were probably generated in an extensional subduction-related setting. Their relatively low Nb and Y but high Rb concentrations, together with characteristic Th/Ta ratios (6–20), suggest a mature continental arc setting. The latest Cretaceous-Paleogene basaltic volcanics have mainly within-plate chemical characteristics (in the east), although some have a subordinate subduction influence; e.g. negative Nb (in the west). Sr-Nd-Hf isotopic signatures (i.e. high positive eNd(t) and eHf(t) values) suggest derivation of the basalts from several OIB-like mantle sources, with probable involvement of a crustal (recycled) component (i.e. elevated Nb/Y ratios). Comparisons with SE Turkey, where Late Cretaceous arc-related granitic rocks are widely developed, suggest that the Kyrenia Range igneous rocks may have originated to the south of the Alanya metamorphic massif (N of Cyprus) and correlative continental units in SE Turkey. The latest Cretaceous-Paleogene basaltic volcanics in N Cyprus and SE Turkey (Maden Complex) are interpreted to represent incipient Marginal Basin formation, possibly in an oblique-convergent setting, prior to Miocene suturing with Arabia. In the light of alternatives, we infer genesis of the N Cyprus Late Cretaceous and Paleogene volcanic rocks related to stages in the development of the northerly active continental margin of the Southern Neotethys.

  • shyok suture zone n pakistan late mesozoic tertiary evolution of a critical suture separating the oceanic ladakh arc from the asian continental margin
    Journal of Asian Earth Sciences, 2002
    Co-Authors: Alastair H F Robertson, Alan S Collins
    Abstract:

    Abstract The Shyok Suture Zone (Northern Suture) of North Pakistan is an important Cretaceous-Tertiary suture separating the Asian continent (Karakoram) from the Cretaceous Kohistan–Ladakh oceanic arc to the south. In previously published interpretations, the Shyok Suture Zone marks either the site of subduction of a wide Tethyan ocean, or represents an Early Cretaceous intra-continental Marginal Basin along the southern margin of Asia. To shed light on alternative hypotheses, a sedimentological, structural and igneous geochemical study was made of a well-exposed traverse in North Pakistan, in the Skardu area (Baltistan). To the south of the Shyok Suture Zone in this area is the Ladakh Arc and its Late Cretaceous, mainly volcanogenic, sedimentary cover (Burje-La Formation). The Shyok Suture Zone extends northwards (ca. 30 km) to the late Tertiary Main Karakoram Thrust that transported Asian, mainly high-grade metamorphic rocks southwards over the suture zone. The Shyok Suture Zone is dominated by four contrasting units separated by thrusts, as follows: (1). The lowermost, Askore amphibolite, is mainly amphibolite facies meta-basites and turbiditic meta-sediments interpreted as early Marginal Basin rift products, or trapped Tethyan oceanic crust, metamorphosed during later arc rifting. (2). The overlying Pakora Formation is a very thick (ca. 7 km in outcrop) succession of greenschist facies volcaniclastic sandstones, redeposited limestones and subordinate basaltic–andesitic extrusives and flow breccias of at least partly Early Cretaceous age. The Pakora Formation lacks terrigenous continental detritus and is interpreted as a proximal base-of-slope apron related to rifting of the oceanic Ladakh Arc; (3). The Tectonic Melange ( Reconnaissance further east (north of the Shyok River) indicates the presence of redeposited volcaniclastic sediments and thick acid tuffs, derived from nearby volcanic centres, presumed to lie within the Ladakh Arc. In addition, comparison with Lower Cretaceous clastic sediments (Maium Unit) within the Northern Suture Zone, west of the Nanga Parbat syntaxis (Hunza River) reveals notable differences, including the presence of terrigenous quartz-rich conglomerates, serpentinite debris-flow deposits and a contrasting structural history. The Shyok Suture Zone in the Skardu area is interpreted to preserve the remnants of a rifted oceanic back-arc Basin and components of the Asian continental margin. In the west (Hunza River), a mixed volcanogenic and terrigenous succession (Maium Unit) is interpreted to record syn-deformational infilling of a remnant back-arc Basin/foreland Basin prior to suturing of the Kohistan Arc with Asia (75–90 Ma).

  • age and significance of radiolarian sediments within basic extrusives of the Marginal Basin guevgueli ophiolite northern greece
    Geological Magazine, 1996
    Co-Authors: Taniel Danelian, Alastair H F Robertson, Sarantis Dimitriadis
    Abstract:

    Well-preserved Radiolaria have been discovered in calcareous silt turbidites and mudstones intercalated with basic extrusives of the Guevgueli Ophiolite, northern Greece. The mudstones contain terrigenous silt, probably derived from adjacent continental basement of the Serbo-Macedonian and/or Paikon units. Volcanic quartz and rare volcanic glass were probably derived from an active continental margin arc (Paikon volcanic arc) to the west. The radiolarian sediments were deposited within fault-controlled hollows in the ophiolitic extrusives, and then covered by massive and pillowed extrusives. The radiolarian assemblage is indicative of an early Late Jurassic (Oxfordian) age, which therefore dates the genesis of the Guevgueli Ophiolite. Our data are consistent with the age of the intrusive Late Jurassic Fanos Granite, believed to be contemporaneous with the Guevgueli Ophiolite. In general, the Guevgueli and related ophiolites of northern Greece are thought to have formed within a transtensional intra-continental Marginal Basin, generated in response to oblique eastward subduction of older Tethyan oceanic crust (Almopias ocean).

  • late palaeozoic Marginal Basin and subduction accretion the palaeotethyan kure complex central pontides northern turkey
    Journal of the Geological Society, 1994
    Co-Authors: Timur Ustaomer, Alastair H F Robertson
    Abstract:

    The Kure Complex, located in the Central Pontides of northern Turkey, is a c . 20 km thick pile of thrust-imbricated deep-sea sediments, intercalated with a dismembered ophiolite. The ophiolitic rocks include serpentinized harzburgite, massive gabbro (cumulate and isotropic), sheeted dykes and basic volcanic rocks. The extrusive rocks are mainly pillow lavas, sheet flows and lava breccias. Geochemical evidence of immobile major- and trace-elements and pyroxene chemistry indicate mid-ocean ridge (MOR) and volcanic arc basalt (VAB) compositions, and suggest that the Kure Ophiolite was generated above a subduction zone. Chrome spine1 analysis also supports this interpretation. In some thrust sheets the extrusives are depositionally overlain by hemi-pelagic shales, passing up into terrigenous turbidites and shales, which are up to several hundred metres thick in intact successions. Cyprus-type massive sulphides, interpreted as precipitates from black smokers, are located along the lava-sediment contact. Individual, intact sediment units, up to several kilometres thick within the Kure Complex are separated by narrow (up to 10 m wide) zones of intense shearing, layer-parallel extension and melange formation. Asymmetrical folds, reverse faults, thrusts and duplex structures throughout the Kure Complex indicate mainly northwards emplacement. The Kure Complex is thought to have opened in the late Palaeozoic, as a Marginal Basin above a northward-dipping Palaeotethyan subduction zone, located along the southern margin of Eurasia. The Kure Marginal Basin later closed, in response to inferred southward subduction, leading to development of an accretionary prism, prior to late Jurassic time. The Kure Complex is unlikely to represent the main Palaeotethyan suture, as has been suggested in some earlier tectonic reconstructions. Regional comparisons with Dobrogea, Crimea and Caucasus suggest the Kure Basin opened, then closed, along a south Eurasia active margin.

  • a late palaeozoic early mesozoic Marginal Basin along the actives sourthern continental margin of eurasia evidence from the central pontides turkey and adjacent regions
    Geological Journal, 1993
    Co-Authors: Timur Ustaomer, Alastair H F Robertson
    Abstract:

    Remnants of two ‘Palaeotethyan’ oceanic Basins are exposed in the Central Pontides of northern Turkey, separated by a continental sliver and an oceanic arc. The southern Basin corresponds to the main Tethys (‘Palaeotethys’), which partially closed in Early Mesozoic time following northward subduction under the southern, active continental margin of Eurasia. The northern Basin (Kure Complex) opened above the ‘Palaeotethyan’ subduction zone as a Marginal Basin, following rifting of a continental fragment (Istanbul fragment) from Eurasia. Marginal Basin opening apparently dates from the Late Palaeozoic in the east (Kure Basin) and from the Triassic in the west (Kocaeli Basin). Basin closure was achieved by southward subduction-accretion, in pre-Late Jurassic times, leaving ‘Neotethys’ open to the south. Counterparts of the Kure Complex are found in the adjacent Crimea (Taurian Series), Istranca (Zabernevo Complex), Dobrogea (Nalbant flysch) and Caucasus (pre-Late Jurassic Southern Slope Basin) regions. Basin opening was accompanied by oceanic crust genesis, at least in the Pontides and Caucasus. Closure before Mid-Jurassic time was achieved by subduction-accretion processes, whereby oceanic crust and deep-sea sediments (including sulphides) were detached and structurally assembled, while oceanic basement was subducted. Marginal Basin opening and closure is seen as one in a series of events along a long-lived, active south Eurasian continental margin.

Bruno Quirino Araujo - One of the best experts on this subject based on the ideXlab platform.

  • biomarkers of photosynthetic sulfur bacteria in lower cretaceous crude oils east brazilian Marginal Basin
    Organic Geochemistry, 2020
    Co-Authors: Bruno Quirino Araujo, Vinicius B Pereira, Francisco Radler De Aquino Neto, Debora A Azevedo
    Abstract:

    Abstract Diagenetic aromatic carotenoids and degradation products, which are derived from the carotenoids of photosynthetic sulfur bacteria, were characterized in the aromatic fraction of Brazilian Cretaceous crude oils (NE Brazil, Sergipe-Alagoas Basin) by high-temperature comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (HT-GC × GC–TOFMS). Unresolved complex mixtures (UCM), coeluting compounds, and limited mass spectrometric identification were noted by traditional one-dimensional (1D) GC–MS screening of aromatic compounds. Group-type separation on a two-dimensional chromatographic plane, first- and second-dimension retention times (1tR and 2tR, respectively), and mass spectra enabled the assignment of alkylated mono-, di-, tri-, and tetra-aryl hydrocarbons with confidence. A series of diagenetic C40 carotenoids and degraded products, which are likely indicators for green-brown sulfur bacteria, were detected. This finding corroborates previous results, and isorenieratane was relatively more abundant than 2,3,6-trimethylaryl isoprenoids. Moreover, atropisomeric biphenyls (m/z 237), which coeluted using 1D GC–MS analyses, were resolved by GC × GC and identified by mass spectrometry. The identification of minor abundances of chlorobactane, diagenetic biphenyl chlorobactane, and related compounds, for the first time in these Brazilian Cretaceous oils, may indicate a contribution of green sulfur bacteria (Chlorobiaceae) and photic zone euxinia in shallow waters.

  • an assay on alkyl aromatic hydrocarbons unexpected group type separation of diaromatic hydrocarbons in cretaceous crude oils from brazilian Marginal Basin
    Energy & Fuels, 2019
    Co-Authors: Arkellau K. S. Moura, Danilo O. Ribeiro, Bruno Quirino Araujo, Vinicius B Pereira, Debora A Azevedo, Iolanda Souza Do Carmo, Antonia Maria Das Gracas Lopes Cito
    Abstract:

    Numerous aromatic biomarkers such as aromatic carotenoids and aryl isoprenoids have been reported in Brazilian Cretaceous oils. The n-alkyl aromatics such as n-alkyl benzenes (ϕ-Cn), toluenes (ϕT-Cn), xylenes, and mesitylenes have also been detected in geological samples and can be valuable tools in geochemical and environmental investigations. In this work, comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC×GC–TOFMS) and GC–tandem MS (GC–MS/MS) were applied to identify the alkyl aromatic series in two crude oils from the Sergipe-Alagoas Basin, Brazil. Geochemical characteristics indicate a different depositional paleoenvironment for each sample, marine and lacustrine end-members. Aromatic compounds were identified using full mass spectrum information, diagnostic ions (i.e., m/z 92 for alkylbenzenes and m/z 106 for alkyltoluenes), selected reaction monitoring transitions, and coinjection with an authentic C21 n-pentadecylbenzene standard. The Sergipe-Alagoas o...

  • uncommon steranes in brazilian Marginal crude oils dinoflagellate molecular fossils in the sergipe alagoas Basin brazil
    Organic Geochemistry, 2016
    Co-Authors: Bruno Quirino Araujo, Debora A Azevedo
    Abstract:

    Comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GC×GC–TOFMS) was used to identify unusual steranes in branched-cyclic hydrocarbon fractions of crude oils from Sergipe-Alagoas Basin, Brazil and infer the geochemical settings of their Cretaceous source rocks. Hopane/sterane and sterane ratios, and the presence of gammacerane and β-carotane in the oil samples indicates a saline source rock depositional environment and predominantly marine organic matter (OM) input. Steroidal hydrocarbons were represented by alkyl steranes and several sterane series, including abundant short-chain steranes (C21 and C22), regular steranes (C27–C29), and diasteranes (C21–C23, C27–C29). The ring-A methyl sterane series contained 3β-methyl steranes (C22 and C23, C28–C30), abundant 3β-methyldiginane (C22), and 3β-methyldiacholestanes (C28), including 4-methyl steranes with abundant C30 4α,23,24-trimethylcholestanes in the branched-cyclic fraction of these crude oils. The C29 sterane isomers, 23,24-dimethylcholestane and 24-ethylcholestanes, were resolved by GC×GC–TOFMS, and a C29 dinosterane derivative was identified for the first time in Brazilian crude oils. The 23,24-dimethyl and 4,23,24-trimethyl steroids can be related to dinoflagellate OM and abundant dinosteranes suggests dinoflagellate blooms during source rock deposition in this Brazilian Marginal Basin. Additionally, the detection of C28–C32 3β-alkyl cholestanes, C29–C33 3β-alkyl 24-methylcholestanes, and C30–C34 3β-alkyl 24-ethylcholestane with ethyl to pentyl groups at C-3 resulted in a complex sterane fingerprint, as expected for petroleum generated from source rocks deposited under predominantly marine saline conditions with high microalgal input. These biomarker results provide additional insight into the character and geologic processes responsible for the source rocks of crude oils in the Brazilian Marginal Basins. This information will be a valuable exploration tool that can be used to constrain the distribution of different oil types in these Basins.

R Roye W Rutland - One of the best experts on this subject based on the ideXlab platform.

  • a regional 1 92 ga tectonothermal episode in ostrobothnia finland implications for models of svecofennian accretion
    Precambrian Research, 2008
    Co-Authors: Ian S Williams, R Roye W Rutland, Jukka Kousa
    Abstract:

    Abstract The mostly metasedimentary Svecofennian Western Pohjanmaa belt in Ostrobothnia, Finland, can be divided into two stratigraphic groups separated by a major unconformity that reflects deformation following regional metamorphism. The western Lappfors group, interpreted as a Svionian basement complex, has strong W-trending folding and aeromagnetic signatures that contrast with the overlying eastern Evijarvi group, interpreted as lower Bothnian, which has more open N-trending folding and magnetic patterns. Several lines of evidence date the unconformity at ∼1.92 Ga. Detrital zircons from two samples of Lappfors group metasediment, and a sample of the basal Nivala gneisses in the Eastern Pohjanmaa belt, have 1.92–1.91 Ga post-depositional low-Th/U metamorphic overgrowths. The maximum deposition age of the Lappfors sedimentary protoliths, based on detrital zircon ages, is between ∼1.99 and ∼1.95 Ga. Three samples of Bothnian sediments lack pervasive ∼1.91 Ga overgrowths, instead having a variety of detrital zircons as young as ∼1.95–1.91 Ga, reflecting recycling of the underlying basement complex. The maximum deposition age of the Bothnian sedimentary protoliths is inferred to be ∼1.91 Ga. The Niska granitoid, which intrudes the Evijarvi group and is deformed only by the younger tectonic episode affecting that sequence, has a zircon age of 1896 ± 6 Ma. That episode, which established the present relationships between basement and cover, is dated by ∼1.88 Ga metamorphic zircon overgrowths in both the Svionian and Bothnian samples, and by 1878 ± 4 Ma metamorphic monazite from a metasediment from the Savo belt, east of the Nivala district. The post-1.91 Ga volcanic sequences of the Svecofennian Province are unlikely to represent arc accretion. The Svionian metamorphic sequences are probably the remnants of a widespread Marginal Basin that formed between ∼1.97 and 1.92 Ga, then was accreted to the craton during an Early Svecofennian (∼1.92–1.91 Ga) orogenic phase, forming the basement on which the Bothnian volcano-sedimentary sequences were subsequently deposited.

  • successive 1 94ga plutonism and 1 92ga deformation and metamorphism south of the skellefte district northern sweden substantiation of the Marginal Basin accretion hypothesis of svecofennian evolution
    Precambrian Research, 2006
    Co-Authors: Torbjorn Skiold, R Roye W Rutland
    Abstract:

    Abstract In contrast to the prevailing arc-accretion hypothesis, it has recently been argued that the most extensive deformation and metamorphism (D1) in the Skellefte region of north-central Sweden took place in older metasedimentary sequences before the eruption of the ∼1.90–1.87 Ga volcanic sequences. The new data presented here provide a basis for a refined hypothesis involving accretion of a pre-1.92 Ga Marginal Basin during an Early Svecofennian orogenic episode between ∼1.92–1.91 Ga. To the south of the Skellefte district, in a part of the metasedimentary Robertsfors Group dominated by NE (D1) structural trends, 40 SIMS analyses of zircon from a metasediment (Robertsfors-1) have been made. Fourteen analyses of overgrowths and structureless zircon grains provide strong evidence for a post-depositional metamorphic event dated to 1916 ± 5 Ma, which we correlate with the observed D1 metamorphism and migmatisation. This age is indistinguishable from that of a corresponding event in the Vammala Migmatite Belt (VMB) in Finland. This is the most important crustal thickening episode in the evolution of the Svecofennian province, here distinguished as the early Svecofennian orogenic episode. Nine Archaean and 15 older Palaeoproterozoic (OP) detrital zircon grains also yielded data comparable to that from the VMB. The age of 1980 ± 6 Ma obtained from the total OP population possibly indicates derivation from a single magmatic source area, with similarly aged metamorphic overgrowths on some of the grains. Two sampled localities to the NE show D1 schistosity overprinted by D2 shears and granitic veining. Sample Ronnskar-3, from a relatively homogeneous granitoid, and sample Ronnskar-2, from a garnetiferous felsic layer in well-banded layered gneisses, both contain igneous zircon populations. These have been affected by D1 and D2 events but the SIMS data allow the age of magmatism in both rocks to be placed between 1940 and 1950 Ma, thus reinforcing the links to the Knaften/Storuman areas to the WNW. In the granitoid, six analyses from highly luminescent rims, yielding an imprecise age of 1856 ± 25 Ma, are consistent with the earlier more precise 207Pb/206Pb dating of the D2 event at 1858 ± 2 Ma. The D2 event is also recorded by ID-TIMS analyses of monazite from five widely spaced localities in the Skellefte structural zone with ages ranging from 1849 ± 3 to 1866 ± 2 Ma. The D2 deformation has similar characteristics in the Skellefte/Robertsfors and Tampere/Vammala areas. We suggest that it can be allocated to a second and Middle Svecofennian orogenic episode between 1.88 and 1.85 Ga, affecting a wide area. It has been demonstrated that the protoliths of the Robertsfors Group were deposited before ∼1.95 Ga; the group was intruded by granitoids between ∼1.94 and 1.95 Ga; and it was strongly deformed and metamorphosed between ∼1.92 and 1.91 Ga. Proximity to the post-1.90 Ga Skellefte Group during the deposition of the latter is indicated for both Robertsfors and Knaften/Storuman areas by the presence of ∼1.90 Ga granitoids and ∼1.88 Ga conglomerates like those in the Skellefte district. The regional evidence for the continuity of metamorphic complexes over wide areas, and the similarity in pre- and post-depositional histories of the Robertsfors Group and the Vammala Migmatite Belt 300 km apart, support the hypothesis that accretion of a large (Svionian) Marginal Basin at ∼1.92–1.91 Ga provided the basement for the subsequent volcanism.