bluuebunny/crossref_metadata_2025_split huggingface dataset:
Late Neoproterozoic granitoid magmatism in the basement to the Pechora Basin, NW Russia: geochemical constraints indicate westward subduction beneath NE Baltica
Authors: Pease V. , Dovzhikova E. , Beliakova L. , Gee D. G. 
Published: April 2008

Abstract

A unique collection of deep (up to 4.5 km) drillcores from across the Pechora Basin of NW Russia indicate that the Precambrian basement beneath Palaeozoic sediments mainly comprise Neoproterozoic metasedimentary and magmatic complexes. These Neoproterozoic rocks are variously deformed and metamorphosed. They are intruded by granitoids of latest Neoproterozoic age (557 ± 6 Ma, 95% confidence). The granitoids are variably deformed, metamorphose their country rock, and represent syn- to post-tectonic magmatism. Major element, trace element, and Sm-Nd and Rb-Sr isotopic data indicate that these intrusions represent continental arc magmatism. Their geochemistry indicates assimilation of older crust by a magma derived from a depleted mantle source. We infer that the granitoids were generated by westward subduction of oceanic crust beneath Baltica (present day coordinates) at
c.
560 Ma.
Northeastern and northwestern margins of Baltica in Neoproterozoic time: evidence from the Timanian and Caledonian Orogens
Authors: Siedlecka Anna , Roberts D. , Nystuen J. P. , Olovyanishnikov V. G. 
Published: April 2008

Abstract
The Neoproterozoic depositional histories of the Timanian and Baltoscandian, orthogonal margins of Baltica show several important differences but also some similarities. The Timanian margin comprises mainly low-grade, terrigenous sedimentary successions with a distinctive, margin-parallel fault zone separating pericratonic and basinal domains. Magmatic rocks are comparatively rare on land, but are common in deep drillcores recovered from beneath the Pechora Basin. Conversion to an active margin occurred in latest Riphean time, ultimately leading to the accretionary and transpressional regime of the Vendian-age, Timanian Orogeny. Along the Baltoscandian margin, successions of low to high metamorphic grade are preserved in diverse Caledonian nappe complexes. Three main types of palaeobasin are distinguished, based largely on sedimentary facies and basin geometry. Magmatic rocks are more common than in the Timanides, ranging from mafic dyke swarms to the voluminous Seiland Igneous Province. This margin remained passive throughout the Neoproterozoic era. The Vendian-dated dyke swarms signify the onset of Iapetus/Ægir ocean opening at precisely the time when the orthogonal Timan margin was being deformed and telescoped during the Timanian Orogeny.
Svalbard: a fragment of the Laurentian margin
Authors: Gee D. G. , Teben’kov A. M. 
Published: April 2008

Abstract
The last decade of structural and isotope-age dating studies in Svalbard and East Greenland has provided strong support for the close correlation of these segments of the Caledonide Orogen, as had previously been inferred from stratigraphic evidence. Prior to Tertiary opening of the Norweigan-Greenland Sea, Svalbard’s Caledonian terranes were an essential part of the Laurentian margin, as witnessed not only by the Early Palaeozoic depositional environments and fauna, but also by the character of the Palaeoproterozoic basement, the Meso- to Neoproterozoic cover, the evidence of late Grenvillian tectono-thermal activity, Caledonian structural style and timing of movements, Caledonian granitic magmatism and Old Red Sandstone (ORS) deposition.
Recently published maps of East Greenland show the hinterland allochthons of central East Greenland to strike out obliquely into the continental shelf. The hypothesis promoted here requires that they continue offshore northwards, extending to the northern edge of the NE Greenland shelf and that most of the Svalbard terranes were northerly continuations of the East Greenland Caledonides. Only along the west coast of central Spitsbergen are ‘foreign’ terranes exposed that have affinity with Pearya, having been located north of the North Greenland foldbelt, apparently unrelated to Laurentia, prior to Ellesmerian Orogeny.
The unambiguous affinity of the Svalbardian and Greenlandian (Laurentian) Caledonides contrasts markedly with the Timanide evolution of northeastern Baltica. It confirms previous interpretations that an important Caledonian suture-zone transgresses northeast-wards across the Barents Sea, separating Laurentian domains in the NW from the Timanides of Baltica in the SE. The Timanides of northeastern Europe are truncated by, and terminate in the Barentsian Caledonides of the Barents Shelf.
Neoproterozoic Orogeny along the margins of Siberia
Authors: Vernikovsky V. A. , Vernikovskaya A. E. , Pease V. L. , Gee D. G. 
Published: April 2008

Abstract

The Siberian Craton is bounded by fold-and-thrust belts involving Neoproterozoic (locally Mesoproterozoic) complexes on the southern (Baikal-Vitim), western (Yenisey Ridge and Turukhansk-Igarka), northern (Taimyr), and eastern (Verkhoyansk) sides. This paper focuses on the geological structure and evolution of these formations. Previous and new geochronological data show that passive continental margins existed around most of the Siberian Craton during the early Neoproterozoic and possibly the late Mesoproterozoic. Between about 850–760 Ma, the southern, western and northern passive margins of the Siberian Craton were transformed into active margins. Middle-late Neoproterozoic island arcs and ophiolites were formed between
c.
750–650 Ma along these margins; they are inferred to have been obducted onto the Siberian continental margin at
c.
600 Ma, prior to late Vendian deposition. New ion microprobe U-Pb ages of ophiolitic rocks from Taimyr’s Central domain are presented. The Neoproterozoic record in the Cretaceous Verkhoyansk fold-and-thrust belt indicates that the eastern part of the Siberian Craton remained a passive continental margin during the Neoproterozoic and Palaeozoic. Baltica-Siberia relationships are also discussed.
Depths and temperatures of mid-ocean-ridge magma chambers and the composition of their source magmas
Authors: Fisk M. R. 
Published: April 2008

Summary
Electron-microprobe analyses of submarine basalt glass and coexisting olivine from the Galapagos Rise and the Mid-Atlantic Ridge (60–63°N) reveal that olivine phenocrysts are virtually unzoned and olivine compositions are identical to those predicted by assuming that the olivine crystallized at the liquidus temperature of the host glass. The absence of chemical zoning of olivine indicates that their host magmas must have cooled by less than 15°C and the MgO/FeO of the magma must have been nearly constant (less than 3 wt% olivine removed) during the time that olivine phenocrysts existed in the magma. Olivine phenocrysts re-equilibrate with their host magma relatively quickly and any changes in temperature or composition that occurred in the 24 h previous to eruption would be recorded in the olivine composition. The settling rates of 1-mm olivine phenocrysts are too slow compared to magma ascent rates to allow early-formed olivine to settle out. There, many magmas that erupt at mid-ocean-ridge crests must ascend from their final reservoir nearly isothermally and isochemically, or alternatively they ascend at superliquidus temperatures and cool below the liquidus just prior to eruption. In either case the magmas identify the temperature (±15°C) and the composition of the magma reservoir immediately prior to eruption. The depth of these magma reservoirs may be estimated through experimental and empirical determination of the pressure-temperature phase diagrams of oceanic basalts. For Galapagos Rise and Mid-Atlantic Ridge basalts these reservoirs are at 3–25 km beneath the ridge crust and in some cases individual reservoirs may be located within ±3 km.
Cryptic mineral-chemistry variations in a detailed traverse through the cumulate ultramafic rocks of the North Arm Mountain massif of the Bay of Islands ophiolite, Newfoundland
Authors: Elthon D. , Casey J. F. , Komor S. 
Published: April 2008

Summary

The compositions of cumulus olivine, orthopyroxene, and clinopyroxene from a detailed traverse through the upper portion of the cumulate ultramafic rocks from the North Arm Mountain massif of the Bay of Islands ophiolite complex have been determined. The compositions of olivine (Fo
91.5–83.6
), clinopyroxene (100 × Mg/(Mg + Fe) = 94.2–87.5) and orthopyroxene (100 × Mg/(Mg + Fe) = 90.8–85.1) within this traverse, when compared to the compositions of minerals that crystallize from oceanic basalts at various pressures, suggest that these cumulate ultramafic rocks crystallized from basaltic liquids at high pressures (>10 kb) in a subaxial magma conduit system. The mineral compositions, when plotted against stratigraphic height within the complex, define eleven different cryptic units over a ‘stratigraphic’ thickness of 136 m. The boundary of each of these cryptic units is marked by an abrupt discontinuity in mineral chemistry from somewhat fractionated to very primitive compositions, suggesting that batches of magma passing through the conduit system do not intermix with other batches of magma undergoing crystallization in the system.
Atlantic oceanic crust: seismic structure of a slow-spreading ridge
Authors: White R. S. 
Published: April 2008

Summary

The seismic structure of the slow-spreading Atlantic ridge is reviewed using results constrained by synthetic seismogram modelling and all available seismic determinations of the crustal structure of fracture zones. The variation of velocity with depth in normal oceanic crust is characterized by a series of velocity gradients: large (1–2 s
−1
) gradients in the upper crust and more gentle ones (∼0.1 s
−1
) in the lower crust, with a variable crust-mantle transition. Petrologic changes, modified by hydrothermal circulation, metamorphic alteration and by cracks and fractures control the seismic structure.

Superimposed on the ‘typical’ crustal structure are marked lateral variations caused by: (i) crustal formation in areas away from a mature spreading centre (e.g. aseismic ridges, the oceanic-continental transition); (ii) accretion within transform faults, generating low-velocity material of greatly attenuated or variable thickness; (iii) ageing of the young crust as it moves away from the spreading centre; (iv) localized and generally small variations in regions away from fracture zones caused by spatial and temporal changes in the igneous and tectonic activity.
The observed variations in seismic structure can be explained by a model of the spreading centre consisting of a string of independently accreting segments separated by transform faults every 50–80 km. Restricted magma supply at the ends of the segments may result in accretion of thinned crust in transforms whilst faulting lowers the seismic velocity and allows the deep penetration of water and possibly serpentinization at depth. Growth faulting within the median valley causes the isovelocity contours in the lower crust to be rather flatter than the surface expression shown by the basement relief. Episodic accretion in the spreading centre results in small-scale lateral variability in the extrusive carapace.
Variations in structure and petrology in the Coastal Complex, Newfoundland: anatomy of an oceanic fracture zone
Authors: Karson J. A. 
Published: April 2008

Summary
The Coastal Complex of western Newfoundland is perhaps the world’s best example of an ancient oceanic fracture zone preserved within an ophiolite complex. The structural style, metamorphic zonation, and magmatic history of the Coastal Complex contrasts sharply with that of the contiguous Bay of Islands Complex. The exposures of the Coastal Complex provide cross sections of the fracture zone at various vertical and lateral positions. A reconstruction of these sections reveals structural and petrological variations that are a function of depth as well as position along strike. For example, deep crustal sections show a history of strike-slip deformation that is fairly homogeneously distributed across a 4 km wide high-strain zone. High-temperature metamorphic rocks, partial melting relationships, and peridotite intrusive bodies are also present. Diabase dykes cut the entire assemblage. At shallow crustal levels, deformation is concentrated along relatively narrow, well-defined faults and shear zones that overall have an anastomosing aspect. High-grade metamorphic rocks occur only locally and have in most places suffered retrograde effects. Felsic intrusive bodies occur locally and serpentinite mélange is widespread. Volcanic rocks non-conformably overlie a tectonically brecciated horizon that transects the entire assemblage. The most striking lateral variations are the discontinuous nature of lithologic units and the apparent bifurcation of major high-strain zones along strike.
Oceanic fracture zones and their ophiolite analogues probably display a wide range of geological relationships that vary with depth, with position along strike, and in scale. Thus, the Coastal Complex should probably be viewed only as a single example of how exceedingly complex and variable the oceanic lithosphere may be near transform faults and their non-transform extensions. Clearly, similar ophiolitic assemblages should not be considered when attempting to model processes along purely extensional plate boundaries.
Mantle flow pattern at oceanic spreading centres: relation with ophiolitic and oceanic structures
Authors: Nicolas A. , Rabinowicz M. 
Published: April 2008

Summary

The rolling-mill effect concerning asthenosphere circulation beneath a fast-spreading centre (Rabinowicz
et al.
in press) is recalled and discussed in terms of its geological assessment. The model predicts a fast and narrow jet of asthenosphere which carries 70% of the total melt produced beneath the ridge. This explains both the structural data in most ophiolites and the formation of the crust within a few kilometres at fast-spreading ridges. It is proposed that the more diffuse activity at slow-spreading ridges from mid-oceans or marginal basins, along with the more diffuse magmatism recorded in a few ophiolites is related to the disappearance of this narrow stream as a consequence of an increase of viscosity due to lower temperatures beneath the ridge.
Modelling of oxygen-isotope data from the Sarmiento ophiolite complex, Chile
Authors: Elthon D. , Lawrence J. R. , Hanson R. E. , Stern C. 
Published: April 2008

Summary

The results of an oxygen-isotope investigation of the Sarmiento ophiolite complex show that the extrusive rocks are isotopically enriched in
18
O compared to fresh basalts as a consequence of low-temperature interaction with seawater. The basaltic sheeted dykes, dykes cross-cutting plutonic rocks, and the plutonic rocks themselves are depleted in
18
O compared to fresh basalts as a consequence of high-temperature interaction with seawater. K
2
O contents of pillow lavas indicate minimum water/rock ratios of 15 to 90. Within the sheeted dyke complex and the plutonic section, the inferred water/rock ratios become <0.6. Modelling of basalt-seawater interaction indicates that hydrothermal fluids in equilibrium with actinolite-facies assemblages (525–700°C) have δ
18
O=+5 to +7‰, whereas hydrothermal fluids in equilibrium with greenschist-facies assemblages (230–525°C) have δ
18
O=0 to +5‰. It appears that an inverse correlation exists between the temperature of metamorphism and the water/rock ratio. This would be anticipated from the cooling effect that circulating seawater has on the oceanic crust.
Obduction processes in ancient, modern and future ophiolites
Authors: Searle M. P. , Stevens R. K. 
Published: April 2008

Summary
Geologic and tectonic processes of ophiolite obduction deduced from structural mapping, timing of deformation and metamorphism and palinspastic reconstruction of ancient ophiolite complexes compare closely with modern processes operative in the western Pacific. Many ongoing arguments about the origin of ophiolite complexes (oceanic, island arc or marginal basin) and their emplacement (subduction-related or not) are largely semantic and depend on exactly what individuals regard as island arcs, marginal basins or subduction zones. A review of tectonic processes currently active in the western Pacific region shows a wide range in size, shape and duration of arc-basin complexes and some of these show uniformitarian models similar to those proposed for emplacement of ancient ophiolites. The Tethyan ophiolites of the Semail (Oman Mountains) and Spontang (Western Himalaya of Ladakh-Zanskar) and the Ordovician western Newfoundland ophiolites are compared with recent examples of the Mariana basin, Yap trench, Banda arc, Luzon arc and Papua New Guinea. It is concluded that they are all connected in some way with marginal basins and island arcs in varying stages of development and maturity. Emplacement of these ophiolites is related to continental underthrusting of a trapped marginal basin and accretionary prism comparable to present-day examples of the N Australian margin underthrusting Timor and the Banda Sea and the SE China margin underthrusting the Luzon arc.
The structural variety in Tethyan ophiolite terrains
Authors: Woodcock N. H. , Robertson A. H. F. 
Published: April 2008

Summary
We examine the structural arrangement of some important ophiolitic terrains from Greece, through Turkey, Cyprus and Iran, to Oman. No two areas have an identical geometry or kinematic history and gross comparisons cannot support any single model of ophiolite emplacement. However, more detailed comparisons reveal similarities between individual structural units in the different areas. These units include, for example, the ophiolite sheet itself, slivers of metamorphic rocks, imbricated platform margin sequences, deformed syn-orogenic clastics or mélange, and allochthonous supra-ophiolite sheets. Each of these units may reflect a distinct process operating during emplacement. We describe and interpret these units in the framework of two stacking geometries, one produced by dip-slip-dominated emplacement and the other by strike-slip. These geometries highlight how the presence or absence of particular units may closely constrain possible ophiolite emplacement models. This point is illustrated by the difficulties in applying the ‘emplaced forearc’ model to Oman and Cyprus.
The gravity field of the Marum ophiolite complex, Papua New Guinea
Authors: Milsom J. S. 
Published: April 2008

Summary
The ophiolitic Marum complex crops out in northern New Guinea and to the south is in fault contact with sialic rocks of the continental core of the island. It is overlain to the north by thick sediments of the Ramu basin which give rise to a major gravity low.
Gravity surveys over the complex and its margins have defined a Bouguer anomaly high offset towards the northern edge of the outcrops of basic rock. The anomaly is similar in form to, but much smaller than, the anomaly associated with the Papuan Ultramafic Belt to the east. Also, it is superimposed on a very much lower background field, so that positive Bouguer anomaly values occur only at the very peak of the high. A geological model of the area consisting of only three major units; continental crust, Neogene sediments and a heavily faulted island-arc frontal zone of mainly ultramafic rocks, is compatible with the gravity data. Other interpretations which are gravitationally acceptable are less easily accomodated to the geological constraints. A problem still exists in explaining the ‘thin-skin’ overthrust in the southeastern part of the complex.
Ophiolites of the northern Caribbean: A reappraisal of their roles in the evolution of the Caribbean plate boundary
Authors: Wadge G. , Draper G. , Lewis J. F. 
Published: April 2008

Summary
The ophiolites of the northern Caribbean represent latest Jurassic and Cretaceous oceanic crust which was obducted from the mid-Cretaceous to the Paleogene. The majority of these ophiolites are early Cretaceous in age and were emplaced during the late Cretaceous. The late Cretaceous geological history can be successfully interpreted in terms of a northward-migrating island arc which subducted Atlantic crust to the south beneath Pacific crust. This arc collided with southern Yucatan in Campanian times to form the Guatemalan ophiolites, and in Paleogene times with the Bahamas platform to form the central and western Cuban ophiolites. Both these collisions involved underthrusting by carbonate-mantled continental forelands. However, the eastern extension of this subduction zone, the North Coast belt of Hispaniola, has not been thrust over the Bahamas platform. The provenance of these arc-related ophiolites should be Atlantic ocean crust. Back-arc spreading to the south of this arc during the late Cretaceous — Paleocene may have produced the source material for the Jamaican and Oriente ophiolites and also for the Dumisseau Fm. of the Southern Peninsula of Haiti. The oldest ophiolites in central Hispaniola and Puerto Rico are possible candidates for crust of Pacific provenance, perhaps isolated during an early Cretaceous change of subduction polarity. However, our preferred model for the Cordillera Central of Hispaniola is as the base of an accretionary prism with southward-dipping subduction. All but the Cuban ophiolites have suffered varying degrees of strike-slip dismemberment during the Cenozoic.
A conspectus of Scandinavian Caledonian ophiolites
Authors: Sturt B. A. , Furnes H. , Roberts D. 
Published: April 2008

Summary
Dismembered and fragmented ophiolite assemblages constitute important elements of the metamorphic allochthon of the Scandinavian Caledonides. Ophiolites recognized to date range in age from probable Vendian to Middle Ordovician and occur at two principal tectonostratigraphic levels. Field criteria, with support from geochemical data, permit a classification of many of the ophiolite complexes into two fundamental groups. Group I is characterized by a generally well-developed pseudostratigraphy, MORB as well as IAT and WPB petrochemistry, plagiogranites at higher levels, and a conformable cap of oceanic sediments with local ocean or immature arc volcanism. The group is representative of either a major ocean or evolved marginal-basin regime. Evidence points to their obduction, and initial internal deformation, in the Finnmarkian orogenic event in pre-Middle Arenig time. Group II complexes are younger, of late Arenig to Llanvirn and possibly Llandeilo age, and show a poorer-defined pseudostratigraphy. They are laterally and vertically intercalated with either siliciclastic sediments from a contemporary magmatic arc or arcs, and are considered as having been generated in a restricted marginal-basin setting. Their prime tectonic deformation, and translation within nappes, is essentially of Silurian age. Group I ophiolites were also further dissected and deformed during this main Caledonian event.
Fine-grained sediments of the Zaire deep-sea fan, southern Atlantic Ocean
Authors: van Weering T. C. E. , van Iperen J. 
Published: April 2008

Summary

The Zaire deep-sea fan is a large mud-dominated system fed mainly by the Zaire (Congo) River. A submarine canyon deeply incised into the outer estuary funnels part of the river’s high suspended load directly out to the deep sea. Sediment is also contributed to the fan via slumping and debris-flows initiated on the upper slope. A number of 11–17 m long piston cores have been collected from this part of the SE Atlantic Ocean, and three of these are described in detail in terms of structure, texture and composition. Four sediment facies are recognized: (1) turbidite muds with mainly T
E1
, T
E2
and T
E3
structural divisions (after Piper 1978); (2) pelagic and hemipelagic biogenic muds; (3) homogeneous uniform muds that probably result from slow settling of dilute suspensions; and (4) debris-flow deposits. There are marked differences in facies distribution, turbidite thickness and organic carbon content between the upper and outer regions of the fan.