bluuebunny/crossref_metadata_2025 huggingface dataset:
Examples of Multi-attribute, Neural Network-Based Seismic Object Detection
Authors: De Groot P. , Ligtenberg H. , Oldenziel T. , Connolly D. , Meldahl P.
Published: April 2008

Certain seismic objects, like faults and gas chimneys, are often difficult to delineate using conventional attribute analysis. Many attributes contain useful information about the target object but each new attribute provides a new and different view of the data. The challenge is to find the optimal attribute for a specific interpretation. In this paper the optimal attribute is found with a pattern recognition approach based on multi-dimensional/multi-attributes and neural network modelling. Multi-dimensional attributes, as opposed to point attributes, can provide the spatial information on the seismic objects. The role of the neural network is to classify the input attributes into two or more output classes. Neural networks are trained on seismic attributes extracted at representative example locations that are manually picked by a seismic interpreter. This approach is a form of supervised learning in which the network learns to recognize certain seismic responses associated with the identified target objects. Application of the trained network yields an 'object probability' cube for the target object. Essentially, the neural network can target any seismic or geological feature requiring detailed analysis. In this paper the method is described and examples are shown of gas chimneys, faults, salt domes and 4D anomalies. Some interpretation aspects are discussed.
The Neoproterozoic Timanide Orogen of eastern Baltica: introduction
Authors: Gee D. G. , Pease V.
Published: April 2008

This volume was conceived during EUROPROBE’s investigations into the dynamic evolution of the Palaeozoic Uralide Orogen and relationships northwards into the Eurasian high Arctic. During these European Science Foundation studies, the preservation of Neoproterozoic deformation over large regions of northern Europe became increasingly apparent. This mainly Vendian tectonic event is referred to as the Timanian Orogeny and became the focus of many recent and on-going investigations. Much progress has been made in understanding Timanian Orogeny and a Memoir synthesizing our current knowledge is not only timely, but also relevant to Neoproterozoic global tectonic reconstructions.

The type area for the Timanide Orogen is located in the Timan Range of northwestern Russia, which separates the East European Craton from the Pechora Basin and Polar Urals. The orogen extends over a distance of at least 3000 km, from the southern Ural Mountains of Kazakhstan to the Varanger Peninsula of northernmost Norway, flanking the eastern margin of the older craton (Fig. 1). From the Timan Range, it reaches northeastwards below the thick Phanerozoic successions of the Pechora Basin and Barents Shelf (O’Leary
et al.
2004), and reappears in the Polar Ural Mountains and northwards through Pai Khoi to Novaya Zemlya. Timanian orogeny thus influenced a vast region of northwestern Russia. The Phanerozoic cover, Arctic shelf areas and, further east, Uralian deformation, obscure the importance of this orogenic event for the geodynamic evolution of Europe.

The Timanide Orogen has been referred to by various other names, most frequently as the ‘Baikalides’. The term ‘Baikalian Orogeny’
Structural and tectonic development of the Timanide orogen
Authors: Roberts David , Olovyanishnikov Vesvolod
Published: April 2008

The northeastern margin of the East European Craton developed passively in an extensional regime from late Mesoproterozoic through to the later stages of Neoproterozoic time. Along the exposed parts of the Timan-Varanger Belt, a major fault zone separates pericratonic (platformal) and basinal domains. Successions of the basinal domain can be traced beneath the Pechora Basin, via drillcore and geophysical data, to where intra-oceanic subduction systems with island arcs are inferred to have existed in the later stages of the Late Riphean. In terminal Riphean to Vendian time, inferred subduction polarity reversal resulted in a progressive telescoping, dissection and accretion of these diverse magmatosedimentary assemblages against the northeastern margin of the craton, culminating in Mid to Late Vendian, Timanian orogenesis. The Timan Range exposes SW-verging upright folds with anchizone to lower greenschist-facies cleavages. Higher-grade rocks in the Kanin-North Timan area occur in anticlinal cores and thrust slices. Isotopic dating constraints suggest that peak Timanian metamorphism occurred during the time interval 600–550 Ma.
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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.
Sedimentation in the southern Cape Verde Basin: seismic and sediment facies
Authors: Auffret G. A. , Le Suave R. , Kerbrat R. , Sichler B. , Roy S. , Laj C. , Muller C.
Published: April 2008

A detailed seismic and sedimentological study of a small area at the lower continental rise to abyssal plain transition off NW Africa has revealed a complex pattern of Plio-Quaternary sedimentation. The area is one of extremely low relief. However, the sediment facies distribution mirrors the slight morphological expression of basement structures and downslope shallow channels. Fine-grained hemipelagic sediments are dominant throughout, although in some cases the influence of low-density turbidity currents and of bottom currents is observed. Sandy turbidites, up to 2 metres thick in the channels, were emplaced during periods of lowered sea-level.
North Atlantic contourites: an overview
Authors: Stow D. A. V. , Holbrook J. A.
Published: April 2008

We present a brief overview of known bottom circulation and Tertiary to Recent sediment drifts in the North Atlantic Ocean, and outline the distinctive features of contourite sediments recovered from these drifts. There is a gradation of structural and textural characteristics between the finer-grained clayey and muddy contourites and the coarser-grained silty and fine-sandy contourites. There is also a gradation between the three compositional types identified: biogenic, mixed biogenic/terrigenous, and volcanogenic. Other features such as microfabric, or facies sequences are less well documented but potentially diagnostic.
Facies and sequence analysis of Nova Scotian Slope muds: turbidite vs ‘hemipelagic’ deposition
Authors: Hill P. R.
Published: April 2008

Five fine-grained mud facies were identified in piston-cores obtained from the Nova Scotian Slope. The sedimentary structures of the muds indicated deposition under varying bottom-current conditions and rates of sediment supply. Two facies associations were recognized using Markov sequence analysis: (a) turbidite and (b) non-turbidite associations. The problems of recognizing turbidite, contourite and hemipelagic deposits in fine-grained sequences are discussed in the context of low-concentration, low-velocity flows. The dynamics of unconfined flows indicates that turbidity currents become unstable and are deflected by Coriolis to near contour-parallel flow. Dilution and deceleration of the flow may produce a transition to hemipelagic-type deposition. Transitions of this type within individual flows make recognition of discrete turbidite, contourite or hemipelagic processes very difficult in parts of the geological record.
Facies, facies-associations and sediment transport/deposition processes in a late Precambrian upper basin-slope/pro-delta,, Finnmark, N. Norway
Authors: Pickering K. T.
Published: April 2008

The lower Naeringselva Member of the 2.5 to 3.5 km thick Båsnaering Formation varies from 60 to 540 m thick, and is part of the 9 km thick late Precambrian Barents Sea Group cropping out on Varanger Peninsula, north-east Finnmark, N. Norway. It is underlain by a minimum 3.2 km of submarine fan deposits and passes up into delta front and fluvio-deltaic deposits of the Båsnaering Formation. Within the lower Naeringselva Member (LNM) four main facies are recognized: parallel-laminated mudstone; current-rippled siltstone; structureless mudstone, and wave-rippled fine-grained sandstone. Soft sediment deformation was abundant as slide, slump and
in situ
disturbed layers up to tens of metres thick. Erosional-depositional discordances are evident between packets of beds. Palaeocurrent data, together with a limited number of slide fold readings, suggest a west to south-westerly source area for the submarine fan, upper basin-slope/pro-delta and delta deposits. The stratigraphic position and sedimentology of the LNM suggests that it forms part of an ancient upper basin-slope/pro-delta, with fan deposits on the lower basin-slope. Apart from sporadic storm events visibly affecting the slope, it was below wave-base. Modern analogues for the LNM are the upper part of continental slopes separating fan from delta deposits, as in the Mississippi Delta-Slope-Fan system.
Models for the deposition of Mesozoic-Cenozoic fine-grained organic-carbon-rich sediment in the deep sea
Authors: Arthur M. A. , Dean W. E. , Stow D. A. V.
Published: April 2008

The widespread occurrence of organic-carbon-rich strata (‘black shales’) in certain portions of Jurassic, Cretaceous and Cenozoic sequences has been well-documented from Deep Sea Drilling Project sites in the Atlantic and Pacific Oceans and from sequences, now exposed on land, originally deposited in the Tethyan ocean. These ancient black shales usually have been explained by analogy with examples of modern deep-sea sediments in which organic matter locally is preserved by (1) increasing the supply of organic matter, (2) increasing the rate of sedimentation, and/or (3) decreasing the oxygen content of the bottom water. However, detailed examination of many black shales reveals characteristics that cannot be explained by simple local models, including: their approximate coincidence in time globally; their occurrence in a variety of different environments, including open oxygenated oceans, restricted basins, deep and shallow water; their interbedding with organic-carbonpoor strata which often dominate a so-called black shale sequence; their deposition by pelagic, hemipelagic, turbiditic and other processes; and the variations in type and amount of organic matter that occur even within the same sequence.

A more complex model for the origin of black shales therefore appears most appropriate, in which the cyclic preservation of organic matter depends on the interplay of the three main variables, namely supply of organic matter, sedimentation rate, and deep-water oxygenation, each of which varies independently to some extent. The variation and relative importance of these parameters in individual basins and widespread black shale deposition in general are linked globally and temporally by changes in global sea-level, climate and related changes in oceanic circulation. An important and often overlooked factor for the supply of organic matter to deep-basin sediments is the frequency and magnitude of redepositional processes. The interplay of these variables is discussed in relation to the middle Cretaceous and Cenozoic organic-carbon-rich strata, in particular, which show marked differences in the relative importance of the different variables.
Plasticity and compaction characteristics of the Quaternary sediments penetrated on the Guatemalan Transect—DSDP Leg 67
Authors: Faas R. W.
Published: April 2008

The plasticity characteristics of the Quaternary sediments of the Guatemalan continental margin were determined from five sites drilled on Leg 67 of the Deep Sea Drilling Project. Sixty-four samples were analysed from various marine environments, including the Cocos oceanic plate, Middle America Trench, and the trench lower slope to mid-slope of the Guatemalan continental slope.

Quaternary sediments on the Guatemalan continental slope are generally mixed siliceous and calcareous hemipelagic silty-clays and clayey-silts, frequently interbedded with volcanic ash. In the Middle America Trench, similar sediments alternate with sandy turbidites. Siliceous hemipelagic silty-clays and clayey-silts form the uppermost 170 metres of the Cocos Plate.

The sediments are generally classified as ‘organic clays of medium to high plasticity, containing micaceous sands and silts,’ with 14% classed as ‘inorganic clays of medium to high plasticity.’

Plasticity (measured by the Plasticity Index,

I
P

) decreases downslope to the trench, then increases sharply in the Cocos Plate sediments. Organic matter is greatest highest on the slope (Site 496) and decreases seaward, averaging 1.14% at Site 495 on the Cocos Plate.

Sediment compaction varies, according to position along the traverse and to the dominant sediment type. Greatest compaction (44.5%) occurs at Site 494 at the base of the lower trench slope. Least compaction (18.6%) is found at Site 495 on the Cocos Plate. Compaction of the trench and mid-slope sediments ranges between 31–38.5%.

High sedimentation rates in Quaternary sediments are due, in part, to sediment gravity-flows which depend upon rheological properties, i.e. sediment plasticity. Mudflows and cohesive debris-flows appear to be significant downslope transport mechanisms in these highly plastic sediments.
Subaqueous pyroclastic flows: their development and their deposits
Authors: Yamada Eizo
Published: April 2008

Subaqueous pyroclastic flows are generated by eruptions in shallow water or close to a shore. Their flow mechanism is similar to that of their subaerial counterparts, except that they incorporate steam and not air. Near the source area, their deposits are non-sorted and resemble subaerial flow deposits. In some cases evidence of high temperature is recognized. Within a short distance (less than a few km) they are sorted and five subdivisions can be determined. In upward succession these are: (i) massive graded pumice tuff; (ii) parallel-laminated pumice tuff; (iii) parallel-laminated sandy tuff; (iv) parallel-laminated fine tuff; and (v) massive very fine tuff. With increasing distance from the source, and laterally from the main direction of the flow, the lower divisions gradually die out. Deposition of the upper three divisions is greatly influenced by sedimentary processes. The thickness and constitution of each division vary according to the volume and nature of the associated eruption.
Taupo-Rotorua Depression: an ensialic marginal basin of North Island, New Zealand
Authors: Cole J. W.
Published: April 2008

New Zealand lies on the boundary between the Pacific and Indian plates. In the vicinity of North Island, oceanic crust of the Pacific plate is subducted obliquely beneath continental crust of the Indian plate to form the Taupo-Hikurangi arc-trench system.

The volcanic component of this system is the Taupo Volcanic Zone which comprises a young (<50,000 yr) andesite-dacite arc, to the west of which is the Taupo-Rotorua Depression, a basin filled with approximately 2 km of low-density volcanics. Most of these volcanics are rhyolitic (total volume > 10,000 km
3
) and have been erupted in the last 0.6 Ma from the Rotorua, Okataina, Maroa and Taupo Volcanic Centres. Approximately 2 km
3
of high-alumina basalt has also been erupted from the Depression.

Normal faults are common in the Taupo-Rotorua Depression and some have been active in historic time when open fissures formed. No strike-slip movement has been recorded. This evidence, together with the presence of basalt dykes, suggests the area is an extensional ensialic marginal basin. A change in orientation of the faults, from 040° at Taupo to 090° at the N end of the Okataina Volcanic Centre, is considered to be a result of dextral strike-slip movement within the North Island Shear Belt superimposed on the extensional tectonics of the marginal basin. The resultant oblique extension would also account for the
en échelon
basalt dykes in the 1886 Tarawera rift.

Crustal thickness in the marginal basin is estimated to be 15 km compared to 30–35 km outside the basin. This may account for the high heat flow recorded (average may be up to 1575 mW m
−2
) and for partial melting of the base of the crust to produce the voluminous rhyolitic volcanics. The high-alumina basalts are considered to be derived from the upper mantle. They form dykes in the crust which in places reach the surface to form a series of vents aligned parallel to the regional fault pattern. The basalts are considered to be intruded and extruded mainly as a response to the extensional tectonics.
Late Cenozoic volcanism and extension in Eastern Papua
Authors: Smith I. E. , Milsom J. S.
Published: April 2008

The sea floor around eastern New Guinea is divided into a number of deep basins, separated by submarine ridges which are capped in places by islands of metamorphic, volcanic or coralline rock. The area has been subject to phases of extension since at least the Palaeocene, when the Coral Sea basin was formed. There have also been major compressive events, including the thrust emplacement of an ophiolite, the Papuan Ultramafic Belt, on to the Papuan Peninsula in the Oligocene. This peninsula, on the E of New Guinea, has been volcanically active from the Middle Miocene to the present day, and there have been eruptions on many of the surrounding islands and in the marine basins. Extension-related volcanic rocks include low-K tholeiites dredged from the floor of the Woodlark Basin and a peralkaline rhyolite association on islands near the E end of the peninsula. Volcanic rock types usually regarded as indicators of subduction, including andesites and high-K trachybasalts (shoshonites), are common.

During the Neogene, two phases of extension with associated igneous activity can be interpreted; the first in the Middle and Late Miocene, and the second from the Middle Pliocene to the present day. Only in the first phase was the formation of marginal basins related to subduction. The later phase is seen as part of the response of the complexly fragmented Melanesian area to changes in relative motions of the surrounding major plates. Sea-floor spreading is currently occurring in the Woodlark Basin, and the post-Miocene calc-alkaline and shoshonitic rocks of the Papuan Peninsula and offshore islands reflect reactivation of subduction-modified mantle under this tensional regime, and not renewed subduction.
Heat flow and magmatism in the NW Pacific back-arc basins
Authors: Sychev P. M. , Sharaskin A. Y.
Published: April 2008

High heat flow is a characteristic feature of a number of back-arc basins of the NW Pacific Ocean. Temperature calculations related to the geological and geophysical data reveal that the sources of the high heat flow are located at shallow depths in the upper mantle and probably are zones of partial melting. From the location of these zones, the character and composition of magmas, and the evolution of the basins, it is proposed that injection of magmas, by a magmafracting (magmarupture) mechanism, is the main deep process within the back-arc basins. It is assumed that ‘primitive’ ultrabasic magma rises from great depths in the mantle along vertical or sub-vertical channels and is then distributed nearly horizontally close to the lower boundary of the crust, under the back-arc basins. The initial cause of the magmatic process is apparently a stepped gravity differentiation, beginning perhaps on the core-mantle boundary, and not the supposed subduction of oceanic lithosphere.
Crustal extension in the Southern Andes (45–46°S)
Authors: Bartholomew D. S. , Tarney J.
Published: April 2008

As a result of back-arc extension along the Andean margin of southern Chile, marginal basins formed during both Mesozoic and Tertiary times. Crustal extension was limited but was accompanied by tholeiitic basalt or bimodal (acid-basic) volcanism rather than the calc-alkaline magmatism normally characteristic of the Andean margin.

The basement in southern Chile is composed of highly deformed metasediments, part of a major accretionary wedge built out from the Pacific margin in late Palaeozoic to early Mesozoic times in front of the late Palaeozoic magmatic arc now situated in Argentina. In the mid-Jurassic the locus of calc-alkaline magmatism moved W to near the present continental margin, and a narrow marine basin developed behind the magmatic arc from the late Jurassic to the early Cretaceous. The formation of the basin was marked by extensive silicic volcanism, probably a product of melting of the metasedimentary basement, followed by bimodal acid-basic volcanism in the early to mid-Cretaceous and the emplacement of a major batholith. Extension was less than in the contemporaneous ‘rocas verdes’ back-arc basin in southernmost Chile in which new mafic crust was formed. The basin was closed and uplifted during compression in the mid-Cretaceous. However, in the late Cretaceous a new basin, floored by mafic crust, opened within the calc-alkaline arc. Toleiitic volcanism and plutonism continued in this basin during the Tertiary while alkaline volcanism occurred in the back-arc regions. Compression in the Miocene, accompanied by further calc-alkaline plutonism, closed the basin and resulted in deep levels of the Patagonian batholith being juxtaposed with the volcanic sequences of the basin.
Processes of formation and filling of a Mesozoic back-arc basin on the island of South Georgia
Authors: Storey B. C. , Macdonald D. I. M.
Published: April 2008

South Georgia contains a diverse suite of rocks representing different stages in the development of a back-arc basin on the Gondwana margin during the Mesozoic. Basin formation initially involved intrusion of large gabbroic plutons into deformed paragneisses and calc-alkaline rocks of a magmatic arc, and then extension of this crust by dyke emplacement. Partial melting of the metasediments formed a migmatitic aureole around the basic rocks. Rifting of this thinned continental crust led to the emplacement of mafic crust composed of lavas, dykes and plutonic gabbros with oceanic crustal characteristics. On an upper slope marginal to the arc, the mafic crust is overlain by a thinly bedded sequence of tuffs and mudstones deposited by turbidity currents. Overlying andesitic volcaniclastic rudites, deposited by debris flows, represent a major channel-fill sequence. The basin was filled by thick sequences of both arc-derived volcaniclastic sandstones and silicic detritus with a continental provenance. The volcaniclastic sediments form a fault-controlled aggrading system, deposited by high- and low-density turbidity currents in an elongate basin.
Ordovician marginal basin development in the central Norwegian Caledonides
Authors: Roberts D. , Grenne T. , Ryan P. D.
Published: April 2008

In the Caledonides of central Norway, volcano-sedimentary successions, of the Lower and Upper Hovin and Horg Groups (Ordovician to ?Lower Silurian), unconformably overlie a variable substrate which includes fragmented oceanic crust and ensimatic immature arc assemblages initially deformed and metamorphosed in earliest Ordovician times. In western areas, remnants of a mid-Ordovician, evolved island-arc complex comprise a volcanic-plutonic suite of clear calc-alkaline affinity. To the E, in the Trondheim district, continental margin-type basic to andesitic volcanics, and sediments of comparable age, denote accumulation in a fault-dissected, back-arc marginal basin characterized by marked facies variations and bipolar sediment dispersal patterns. Sediment composition reflects the nature of the contemporaneous volcanism, although an input of metamorphic detritus from a continental margin source can also be determined. Thick, massive to pillowed tholeiitic greenstones, mainly of OFB chemistry, occur in several areas and some contain thick sheeted-dyke units, gabbro complexes and subordinate plagiogranites. These ophiolitic assemblages appear to occur at two stratigraphic levels within the Lower Hovin Group, reflecting separate phases of crustal thinning and oceanic lithosphere accretion. Mega-breccias and an olistostrome may relate to abortive ophiolite displacement or intra-basinal movements in late Arenig times. During the Caradoc and Ashgill, and possibly in earliest Silurian times, continental margin-type acidic volcanism developed widely, particularly in the S, while in central and northern parts of the basin, turbidites and resedimented conglomerates, derived from the basin margins, show SW transport in the deeper-water axial trough. Although there is strong evidence that the marginal basin and arc lay at the edge of the continental plate (Baltoscandia) on the SE side of the Iapetus, with SE subduction, this is not supported by the faunas in the lowest parts of the Ordovician successions, which are largely of North American affinity. A compromise model involving an elongate microcontinental plate within the Iapetus is invoked.