bluuebunny/crossref_metadata_2025 huggingface dataset:
Felsic volcanism in the Kermadec arc, SW Pacific: crustal recycling in an oceanic setting
Authors: Smith Ian E.M. , Worthington Timothy J. , Stewart Robert B. , Price Richard C. , Gamble John A.
Published: April 2008

Large-scale felsic volcanic systems are a common, but not ubiquitous, feature of volcanic arc systems in continental settings. However, in oceanic volcanic arcs the erupted materials are dominated by basalts and basaltic andesites, whereas intermediate compositions are rare and dacites and rhyolites relatively uncommon. The Kermadec arc is an intraoceanic convergent system in the SW Pacific. Volcanoes occur as a continuous arc that is mainly submarine. Despite its simple tectonic setting, felsic magmatism is widespread. In the Kermadec Islands, Macauley Volcano is a basaltic volcano that produced a large felsic eruption about 6000 years ago. A comparable pattern of magmatic evolution is seen on adjacent Raoul Volcano, where basaltic activity built the main edifice of the volcano and where activity during the last 3000 years has been characterized by felsic eruptions of varying size. Elsewhere in the Kermadec arc and in its northward extension, the Tonga arc, felsic eruptions are recorded from 11 of the 30 volcanoes for which petrographic information is available, and in many cases these are the most recent eruptions. Felsic europtions are a widespread recent feature of the arc, and the scale and extent of this magmatism appears to be unusual for a tectonically simple oceanic subduction system. One explanation of the origin of the felsic magmatism is prolonged fractional crystallization from a parental basalt composition, but modelling of the chemical compositions of the felsic rocks does not support this. A second explanation, albeit apparently at odds with the oceanic setting, is crustal anatexis. An important feature of the felsic eruptives from the Kermadec arc is that each tephra sequence or occurrence has a unique chemical composition, although all show the same generalized characteristics. We suggest that this feature supports a model of crustal anatexis rather than fractionation of a range of parental magmas. We also suggest that in the thermal evolution of an oceanic arc system the processes of underplating, together with the continuous magmatic (and thermal) flux, can generate a crustal thickness in which dehydration melting of underplated arc material generates felsic magmas. Further, this condition can represent a unique ‘adolescent’ stage in a developing oceanic arc, as once the felsic melts are extracted the lower crust becomes an infertile residue.
Chemically rich and diverse submarine hydrothermal plumes of the southern Kermadec volcanic arc (New Zealand)
Authors: Massoth Gary J. , De Ronde Cornel E.J. , Lupton John E. , Feely Richard A. , Baker Edward T. , Lebon Geoffrey T. , Maenner Stacy M.
Published: April 2008

The New Zealand American PLUme Mapping Expedition (NZAPLUME) provided the first systematic survey of chemical emissions along a submarine volcanic frontal arc. Chemical plumes emanated from seven of 13 volcanoes that line a 260 km-long section of the southern Kermadec arc northeast of New Zealand. Hydrothermal plumes ranged in depth from <200 to 1500 m and are generally more shallow than plumes over mid-ocean ridges (MORs). The chemical signatures of plumes along the southern Kermadec arc are unusually diverse and have concentration anomalies for CO
2
, H
2
S and Fe that can exceed those for MOR settings by 5–10 times, or more. Projected end-member fluid concentrations of carbon and sulphur gases at some volcanoes require a magmatic vapour source, while unusually high Fe concentrations and Fe/Mn values are consistent with venting an iron-rich magmatic brine. Thus, vent-fluid emissions on the Kermadec arc volcanoes often appear as hybrid mixtures of hydrothermally
evolved
sea water influenced by water-rock reaction with compositionally diverse arc lavas, and
exsolved
magmatic fluid present as gaseous (CO
2
and SO
2
+H
2
S) and liquid (Fe-rich brines) components. While rock-buffered fluids in arc settings are expected to vary compositionally from one another and from MOR fluids, it is the magmatic components that clearly differentiate arc emissions as being super-enriched in sulphur gases and ionic metals. These first systematic observations of spatially frequent and chemically robust fluid emissions from southern Kermadec arc forecast arcs as being a potentially important source of chemicals to the oceans.
Submarine hydrothermal venting on the southern Kermadec volcanic arc front (offshore New Zealand): location and extent of particle plume signatures
Authors: Baker E. T. , Feely R. A. , De Ronde C. E. J. , Massoth G. J. , Wright I. C.
Published: April 2008

Hydrothermal activity on submarine volcanic arcs in the western Pacific Ocean is known but mostly unexplored. In March 1999, the New Zealand American PLUme Mapping Expedition (NZAPLUME) cruise conducted the first systematic exploration of hydrothermal venting along a sizeable section of an intra-oceanic arc, visiting 13 volcanoes along 260 km of the southern Kermadec arc, just northeast of New Zealand. Conclusive evidence of hydrothermal plumes exists for seven of the 13 volcanoes; at two other volcanoes plume indications were weak and uncertain. The hydrothermal origin of the particle plumes was confirmed by positive anomalies in the ratios of sulphur, iron and copper to titanium relative to non-plume particles, in mass concentrations similar to particles collected from hydrothermal plumes over mid-ocean ridges. The spatial density of active sites along the southern Kermadec arc is at least 2.7 per 100 km (2.7/100 km), probably not significantly different from the weakly constrained value of
c.
1/100 km on slow- and intermediate-rate mid-ocean ridges. An analysis of the number of hydrothermal fields produced for the magma delivery rate in each of these environments suggests that the southern Kermadec arc presently has relatively abundant hydrothermal activity. While this result cannot yet be generalized to other Pacific arcs, submarine volcanoes may contribute significantly to the global hydrothermal budget.
Geodynamic setting of Izu-Bonin-Mariana boninites
Authors: Deschamps Anne , Lallemand Serge
Published: April 2008

The Izu-Bonin-Mariana (IBM) forearc is characterized by the occurrence of boninite-like lavas. The study of the Cenozoic setting of the genesis of these boninitic lavas in light of modern geodynamic contexts in the Tonga and Fiji regions lead us to define three tectonic settings that favour the formation of boninites in back-arc basins in addition to previous settings that involve the presence of a mantle plume: (1) propagation at low angle between a spreading centre and the associated volcanic arc; (2) intersection at a high angle of an active spreading centre and a transform fault at the termination of an active volcanic arc; and (3) intersection at a right angle between an active spreading centre and a newly created subduction zone. A geodynamic model of the Philippine Sea Plate shows that boninites in the Bonin Islands are related to the second mechanism mentioned above, whereas Mariana forearc boninites are relevant to the third mechanism. In the early Eocene, the transform plate boundary bounding the eastern margin of the Philippine Sea Plate at the location of the present-day Mariana arc evolved into a subduction zone that trends perpendicular to the active spreading centre of the West Philippine Basin, somewhere around 43–47 Ma. The presence of a mantle plume in the vicinity of the subduction zone bounding the northern IBM arc explains boninites that erupted in its northern part, but only in early Eocene time.
Mantle genesis and crustal evolution of primitive calc-alkaline basaltic magmas from the Lesser Antilles arc
Authors: Pichavant M. , Macdonald R.
Published: April 2008

Most eruptive rocks in the Lesser Antilles arc are compositionally evolved. However, lavas with primitive characteristics do occur including, in the central part of the arc, a suite of rocks from Soufriere, St Vincent, and the Ilet à Ramiers basalt from Martinique. High-pressure experiments performed on a Soufriere basalt point to a spinel lherzolite source. Glass inclusion data and phase equilibria analysis suggest extraction of the Soufriere melt under relatively dry conditions (
c.
2 wt% H
2
O in melt). Using estimates of the H
2
O content of mantle sources fluxed by an hydrous slab-derived component, H
2
O concentrations as high as 5 wt% are considered possible for primary mantle melts in the Lesser Antilles arc. Experiments at low pressures (4 kbar) simulate the evolution of primitive melts within the arc crust. For elevated melt H
2
O concentrations (6–8.5 wt%), derivative liquids ranging from low-MgO basalt to basaltic andesite are generated at 1050–1100°C. Their crystallization at 950–1000°C yield andesitic liquids similar to those erupting at active volcanic centres such as Mt Pelée, Martinique, and Soufriere Hills, Montserrat. Therefore, experimental data support the derivation of Lesser Antilles arc eruptives by different degrees of fractionation from primary mantle melts.
Structure and tectonic evolution of the South Sandwich arc
Authors: Larter Robert D. , Vanneste Lieve E. , Morris Peter , Smythe David K.
Published: April 2008

Detailed analysis of marine magnetic profiles from the western part of the East Scotia Sea confirms continuous, organized back-arc spreading since at least 15 Ma ago. In the eastern part of the East Scotia Sea, the South Sandwich arc lies on crust that formed at the back-arc spreading centre since 10 Ma ago, so older back-arc crust forms the basement of the present inner forearc. Interpretations of two multichannel seismic reflection profiles reveal the main structural components of the arc at shallow depth, including evidence of trench-normal extension in the mid-forearc, and other features consistent with ongoing subduction erosion. The seismic profile interpretations have been used to constrain simple two-dimensional gravity models. The models were designed to provide constraints on the maximum possible thickness of the arc crust, and it is concluded that this is 20 and 19.2 km on the northern and southern lines, respectively. On the northern line the models indicate that the forearc crust cannot be much thicker than normal oceanic crust. Even with such thin crust, however, the magmatic growth rate implied by the cross-section of the arc crust is within the range recently estimated for two other arcs that have been built over a much longer interval.
Back-arc spreading and mantle flow in the East Scotia Sea
Authors: Livermore Roy
Published: April 2008

The East Scotia Ridge exhibits systematic variations in axial morphology and basalt geochemistry. Central segments have morphology typical of intermediate-rate spreading centres and erupt mainly normal mid-ocean ridge basalt (N-MORB). Segments near the ridge ends exhibit anomalous, inflated, axial morphology and erupt more evolved basalts, influenced by the Bouvet plume in the north. As the end segments lie closer to the volcanic arc, these variations could be caused by coupled flow within the mantle wedge, as inferred from similar studies in the Lau Basin. Three of the four zones of crustal accretion defined from the Lau Basin may be identified in the East Scotia Sea, although there is no counterpart to a zone of diminished magma supply observed at the East Lau Spreading Centre. Superimposed on the pattern of plate-driven flow is a ridge-parallel flow related to inflow of Atlantic mantle into the East Scotia Sea back-arc region at both ends of the South Sandwich slab. The inflow causes enhanced magmatism and propagation of the end segments towards the middle of the back-arc region, and may be related to trench-parallel flow beneath the rapidly retreating slab. Alternatively, it may be driven by buoyancy flux from Atlantic hot spots. There is no evidence that retreat was ever driven by escape flow of Pacific mantle.
Rare earth elements in zircon: a review of applications and case studies from the Outer Hebridean Lewisian Complex, NW Scotland
Authors: Whitehouse Martin J.
Published: April 2008

Rare earth element (REE) analysis of zircon coupled with high spatial resolution U-Pb geochronology and imaging is emerging as a useful petrogenetic tool capable of providing a link between dated zircon growth and contemporaneous magmatic conditions or metamorphic reactions. Notable amongst recent studies has been the link established between zircon growth in equilibrium with garnet and characteristic heavy REE depletion in zircon which has enabled the dating of specific pressure-temperature (P-T) evolution. Links to other REE-bearing minerals have yet to be established but have similar potential for dating P-T pathways. A brief review of recent applications in both igneous and metamorphic zircons is presented here to illustrate the range of investigations in this rapidly developing area. The method is then applied to two case studies from the late-Archaean Lewisian Composite Terrane of the Outer Hebrides, NW Scotland. In both cases, these new zircon REE analyses reveal previously unknown events in the early history of these samples, as well as clarifying the relationship of different zircon growth phases in relation to the geochronology.
Oxygen isotope equilibrium between ultrahigh-pressure metamorphic minerals and its constraints on Sm-Nd and Rb-Sr chronometers
Authors: Zheng Young-Fei , Zhao Zi-Fu , Li Shu-Guang , Gong Bing
Published: April 2008

In the Sm-Nd and Rb-Sr isotopic geochronology of metamorphic rocks, an important question is whether radiometric systems of mineral isochrons have achieved isotopic equilibrium during a given metamorphic event and preserved the equilibrium afterwards. An analogue to mineral chronometry is O isotope geothermometry. Because the rates of Sm-Nd, Sr and O diffusion in metamorphic minerals are comparable in many cases, the state of O isotope equilibrium between metamorphic minerals can provide a test for the validity of mineral Sm-Nd and Rb-Sr chronometers. In order to illustrate this applicability, O isotope geothermometry was carried out for Sm-Nd and Rb-Sr isochron minerals from ultrahigh-pressure (UHP) eclogites and gneisses at Shuanghe in the Dabie terrane of east-central China. Although the Sm-Nd isochrons give consistent Triassic ages of 213 to 238 Ma for UHP metamorphism, the Rb-Sr isochrons give Jurassic ages of 171 to 174 Ma for the same samples. O isotope geothermometry of the gneiss, eclogite and amphibolite minerals yields two sets of temperatures of 600 to 720 °C and 420 to 550 °C, respectively, corresponding to cessation of isotopic exchange by diffusion at about 225 ± 5 Ma during high pressure eclogite-facies recrystallization and at about 175 ± 5 Ma during amphibolite-facies retrogression. The preservation of Triassic Sm-Nd isochron ages, but the occurrence of Jurassic Rb-Sr isochron ages and the regular O isotope temperatures for the same samples, suggest that rates of Sr and O diffusion in such hydroxyl-bearing minerals as biotite and hornblende are faster than rates of Nd diffusion in garnet and Sr diffusion in phengite on the scale of a hand-specimen during the amphibolite-facies retrogression. While the mineral with slow diffusivity has exerted the primary control on the homogenization rate of initial isotope ratios among isochron minerals during retrograde metamorphism, the mineral with high parent/daughter ratio has exerted the principal control on the initiation of the mineral isochron clock in response to retrogression. Valid mineral isochrons can be expected to date the timing of metamorphic resetting only if the mineral with high parent/daughter ratio has a fast rate of radiogenic isotope diffusion during the metamorphic resetting.
Natural constraints on metamorphic reaction rates
Authors: Baxter Ethan F.
Published: April 2008

Quantitative constraints on the rates at which metamorphic reactions proceed in nature are now available from several sources. Most common are predictions made on the basis of laboratory kinetic data. However, the applicability of such laboratory-based predictions has long been questioned and many observations in the field now suggest much slower rates. Here, published quantitative field-based constraints on high temperature (>400 °C) reaction rates are assembled from a variety of sources. Reaction rates attending regional metamorphism are four to seven orders of magnitude slower than most laboratory-based predictions. A general rate law for
regional
metamorphism has been derived which best describes these field-based data:

log
10

(

R
net

)
≅
0.0029
T
−
9.6
±
1

where
R
net
is the net reaction rate (g/cm
2
/a) and
T
is temperature (°C). At the same time, natural reaction rates attending contact metamorphism differ from laboratory-based predictions by less than two orders of magnitude, and are in close agreement at higher temperatures. Thus, while existing laboratory-based kinetic data may be judiciously applied to some contact metamorphic systems, laboratory-based kinetic predictions clearly misrepresent regional metamorphism. To explain this kinetic discrepancy, regional metamorphic reaction rates may be limited by slow intergranular transport due to comparatively limited (or transient) availability of aqueous fluid in the intergranular medium. The general field-based rate law may be applied to regional metamorphic, and other environments (i.e. ultrahigh pressure or ultrahigh temperature metamorphism), if similar system characteristics (mainly, low aqueous fluid content) can be inferred.
Prograde and retrograde reactions, garnet zoning patterns, and accessory phase behaviour in SW Finland migmatites, with implications for geochronology
Authors: Nyström Annika I. , Kriegsman Leo M.
Published: April 2008

Rates of melt production and segregation in migmatites can be assessed using geochronology of accessory phases. Here we report on the distribution and growth patterns of accessory phases and their coupling with major phase microtextures and chemical zoning patterns in garnet. We propose that migmatization in SW Finland involved partial melting and subsequent moderate retrograde re-equilibration. The latter process has three major effects: (i) it partially obscures geochemical signatures formed during earlier equilibrium or disequilibrium melting; (ii) it excludes the possibility of very fast (< 100 years per batch) melt segregation rates in migmatites where restite-melt back reaction has operated; (iii) trace element distributions between leucosome and host rock cannot be used to infer melt segregation rates.

Garnet MnO patterns show flat cores, suggesting high-grade equilibration, and sharply increasing concentrations at the rims, attributed to garnet-melt back reaction. Trace element patterns (Zr, Y and heavy rare earth elements) also document retrogression at the rims, but in addition preserve earlier growth zoning related to incongruent melting. We report retrograde zircon and xenotime growth associated with garnet resorption, here related to restite-melt back reaction in leucosome, melanosome and mesosome. Hence, geochronological studies of migmatite terranes should take into account that the youngest zircon material is not restricted to leucosome, but can also form overgrowths on corroded grains in melanosome and mesosome.
Remembrance of things past: a hike along the Grès d’Annot research trail
Authors: Stanley Jean-Daniel
Published: April 2008

This brief
aperçu
recalls some keynote studies on the Grés d’Annot in the French Maritime Alps that paved the way for my involvement with these rocks in the late 1950s. At that time, the topic of gravitative flows was gaining momentum, and various theories on underwater avalanches, including turbidity currents capable of carrying coarse sand to the deep sea, had already been proposed. However, these transport processes still needed considerable clarification. Discoveries, including the identification of the sandstones as turbidites by Faure-Muret and others in 1956, were not conceived in a vacuum, but rested on many years of previous research by numerous geologists. Some of the key earlier findings are cited here. My interests then, and for several decades afterward, were focused on the broader context of sedimentary petrology, downslope-to-deep marine deposition, palaeogeography and regional structural geology pertaining to these spectacular Tertiary sequences. Now that more than 40 years have passed, I look back and can better appreciate the role of others and their influence serving as a base for my modest effort. The ‘cutting of teeth’ experiences on the Grés d’Annot were truly fortunate, not only for my early formation as a sedimentologist, but also as I have continued to pursue related and progressively more diverse activities.
The structural setting and palaeogeographical evolution of the Grès d’Annot Basin
Authors: Apps Gillian , Peel Frank , Elliott Trevor
Published: April 2008

A new kinematic model is derived for part of Haute Provence, which recognizes that Alpine deformation, involving both the Mesozoic and sub-Mesozoic section, has been active since the onset of Tertiary deposition. This model places the Grès d’Annot basin in an overall compressional setting, dominated by SW-directed Alpine thrusting, during deposition of the Tertiary basin fill sequence. Evidence is presented that one block of Palaeozoic material (Barrot) was strongly uplifted and deeply eroded prior to the onset of Tertiary deposition in the area. Local thin-skinned extension and strike-slip are observed, but these are secondary components, which are interpreted to be part of the overall thrust system. The region was affected by three different orogenic events. Pyreneo-Provençale thrusting established a structural fabric that influenced later, syndepositional thrust geometries. The Late Eocene to Early Oligocene Grès d’Annot Basin was a structured foreland basin created by the loading effects of the Alpine orogeny. The supply of siliciclastic sediment to the basin was controlled by tectonic events far to the south, in a subduction-related orogeny that extended from East Iberia to SE France. SW-directed Alpine compression within the basin produced significant basin-floor topography, which developed immediately prior to, and during, the Nummulitic transgression. The evolving palaeo-relief of the basin is recorded in thickness and facies variations of the older Tertiary lithostratigraphical units (Poudingues d’Argens, Calcaires Nummulitiques and Marnes Bleues), and by the onlap geometry of the Grès d’Annot turbidites. This allows us to define a set of palaeogeographical maps of the basin throughout its evolution, linked to a set of sequentially restored structural sections. This basin floor topography was complex, because Alpine thrusting overlapped the older east-west striking structures, and it created a set of partially isolated sub-basins. Major early compressional structures partitioned the basin, exerting strong control on all the Tertiary depositional systems, including the Grès d’Annot turbidite systems.
The southern Grès d’Annot outcrops (French Alps): an attempt at regional correlation
Authors: Du Fornel E. , Joseph P. , Desaubliaux G. , Eschard R. , Guillocheau F. , Lerat O. , Muller C. , Ravenne C. , Sztràkos K.
Published: April 2008

During Eocene-Oligocene times, the Grès d’Annot turbidite system (French Alps) was deposited in several tectonically controlled sub-basins, which were mainly fed from a southern major sediment source: the Corsica-Sardinia Massif. In order to establish regional correlations in the southern part of the basin, four kilometre-scale outcrop areas were studied in detail. From south to north these are: the St Antonin, Annot, Grand Coyer and Chalufy areas. The results are: (1) an updated chronostratigraphic framework, (2) a major SE-NW correlation panel, approximately 400m thick and 50 km long, parallel to palaeocurrent directions, within which all stratigraphic units are defined in terms of sedimentology and micropalaeontology and (3) some correlation panels at outcrop scale (around 5 km long and several hundred metres thick), within which all stratigraphic units are defined as before, but with the addition of a direct visual control on correlations, which enables the reconstruction of higher resolution geometry. Seven time-equivalent stratigraphic packages have been correlated from upstream to downstream, making use of micropalaeontologic constraints, and their geometric and facies evolution have been reconstructed through times. This evolution may be related to different stages in the basin deformation, induced by the east to west development of the Alpine foreland basin.
Genetic units/parasequences of the Annot turbidite system, SE France
Authors: Guillocheau François , Quéméner Jean-Marie , Robin Cécile , Joseph Philippe , Broucke Olivier
Published: April 2008

High-frequency cycles (genetic units or parasequences), with a mean duration of 20000 years, have been identified in the Grè d’Annot turbidite system of the southern Alpine foreland basin (Late Eocene/Early Oligocene). They filled a narrow (2–8 km wide) synsedimentary syncline that formed a confined basin fed upward by a fan-delta located on a narrow shelf. These cycles are 10m thick, and each consists of a basal heterolithic component and an upper sandy homolithic unit, traceable over at least 10 km. They result from the superposition of (1) a progradational phase with an accretionary system (low-angle accretionary bedsets) and a feeder system (by-pass to high preservation oblique laminasets) and (2) an aggradational phase with a ‘spread’ system overlain by a condensed interval. The progradational phase shows very low-angle clinoforms (low-angle accretionary sets, 1–2 m high, a few tens to a few hundreds of metres long) fed by erosional structures (channels or large scours, 0.2–2 m deep, a few metres to a few tens of metres wide). The aggradational phase (subplanar laminasets, ‘scour and fill’ structures) forms a ramp, and drapes the underlying sediments. These progradational geometries may be explained by the narrow sub-basin morphology and by the transition from channelled flow (updip narrow part) to unconfined flow (downdip wider part). These genetic units directly record the variations in sedimentation rate of the feeding fan-delta on the shelf, which are controlled by the 20 000 year cycles of sea-level change.
Deep-water sedimentation on an evolving fault-block: the Braux and St Benoit outcrops of the Grès d’Annot
Authors: Tomasso Mark , Sinclair Hugh D.
Published: April 2008

The record of sediment fill contained in sub-basins of the western outcrops of the Grès d’Annot indicates that, throughout the period of deposition of the sandstones, there was very little synsedimentary tectonic activity. However, evidence of syntectonic sedimentation is preserved in the basal portion of the fill of the Annot sub-basin. The lowermost portions of the turbidite infill of the Annot sub-basin crop out around Braux and St Benoit. These outcrops are separated by the St Benoit Fault, which is thought to have developed as a result of sinistral strike-slip movement on the main Rouaine Fault to the south. The St Benoit Fault has a normal offset of up to 400 m to the east. Detailed mapping within the Calcaires Nummulitiques, Marnes Bleues and Grès d’Annot, has led to the recognition of several features that indicate a syntectonic sedimentary evolution for the outcrops. These record a three-phase progressive evolution of the St Benoit Fault through the upper Calcaires Nummulitiques to the lowermost Grès d’Annot. As part of this, there are implications for deposition from turbidity currents on encountering topographic barriers. To our knowledge, this area contains one of the few outcrop examples of half-graben formation within the confined basins of the Grès d’Annot.
The Marnes Brunes Inférieures in the Grand Coyer remnant: characteristics, structure and relationship to the Grès d’Annot
Authors: Stanbrook David A. , Clark Julian D.
Published: April 2008

Part of the Alpine foreland basin, the Grand Coyer remnant lies in a NW-SE-trending syncline and contains turbidity current deposits derived from the south. Palaeotopography of the Marnes Bleues surface is complex but there is generally a NE-dipping confining slope in the west of the area. Palaeocurrent data suggest that aside from localized alteration of flow paths, the mean direction of flow was parallel to this slope. Three sections (Vallon de Fouès & Carton, Laupon and Sommet de la Mole) are used to illustrate the nature of the Marnes Brunes Inférieures—a transitional facies between the Marnes Bleues and the Grès d’Annot. The Marnes Brunes Inférieures is typically thin-bedded, sharp-based and sharp-topped, non-graded, predominantly very fine-grained sands, with planar and ripple lamination, interbedded with thin marls and shale. The finer grained Marnes Brunes Inférieures in the Grand Coyer area is interpreted as a distal and lateral equivalent of the coarser Grès d’Annot, which is generally thought to represent deposition along the main axis of flow. Examples are provided of how these characteristics change away from the onlap slope. Evidence is presented to illustrate the connection between syndepositional structural alteration and the common slumping observed within the Marnes Brunes Inférieures.
Three-dimensional full wave seismic modelling versus one-dimensional convolution: the seismic appearance of the Grès d’Annot turbidite system
Authors: Bourgeois Aline , Joseph Philippe , Lecomte Jean Claude
Published: April 2008

The increasing use of three-dimensional seismic data for reservoir characterization justifies the need for an accurate and fast seismic modelling tool. Synthetic seismic data sets, where the geological model is controlled at each location, provide crucial information for the understanding of the seismic response in connection with various geological features. The method used here involves a wave equation approximation based on ray tracing and Born formalism in the context of a one-dimensional background, thereby providing economic computation of a very useful subset of geological models. The software enables a high level of flexibility to be applied to a range of realistic experiments. In particular the software provides pre-stack multi-component seismic response of a three-dimensional heterogeneous reservoir, including both reflected and converted waves, at the level of the target. In order to analyse and reconstruct the architecture of deep-water gravity deposits, we present a realistic synthetic seismic experiment of one Grès d’Annot outcrop. A geological interpretation of the outcrop has been based on photo panoramas calibrated with vertical sedimentological sections. The internal distribution of facies has been interpolated using geostatistical simulation, with one of the stochastic realizations of the reservoir architecture serving as the three-dimensional cube of lithofacies corresponding to the reservoir model. These lithofacies were assigned elastic parameters of an analogue Brazilian field. The reservoir model is then buried within a smooth background model, and a seismic model generated. The purpose of generating three-dimensional seismic models is to analyse the seismic signature of the different sedimentary architectures, in order to provide guidelines for the seismic interpretation of comparable subsurface reservoirs. Compared with convolution results, the computed synthetic data are more realistic and closer to the actual seismic data. This is mainly due to the fact that the method takes into account the lateral heterogeneity included in the vicinity of each reflecting point.
Silled sub-basins to connected tortuous corridors: sediment distribution systems on topographically complex sub-aqueous slopes
Authors: Smith Ru
Published: April 2008

Two end-member classes of sediment distribution systems on topographically complex slopes are distinguished here: (a)
cascades of silled sub-basins
, and (b)
connected tortuous corridors
. In the first scenario a process of filling and spilling of successive silled sub-basins down a slope occurs. For each sub-basin a sill tends to hinder further downslope flow of at least the basal sandy portions of sediment gravity flows until deposition reduces the relief sufficiently to allow spill down-slope. Spill is associated with incision in the sill. In the
connected tortuous corridors
scenario, flows avoid bathymetric obstacles, but follow a (laterally confined) continuous tortuous path down the slope. Without complete three-dimensional imaging of slope architecture it can be possible to incorrectly infer from two-dimensional profiles a
cascade of silled sub-basins
model. Thus flow paths in adjacent apparent subbasins can be connected out of the plane of section. Convergent thinning and convergent baselap stratal patterns occur in both scenarios, but only in the silled sub-basin case do such patterns occur against closing frontal slopes. For a given complex slope morphology, dominant controls on fill patterns and reservoir architecture are (a) the history of sediment supply character, and (b) rates of structure growth relative to rates of smoothing of topography by erosional and depositional processes. Two particularly important aspects of sediment supply are (i) flow volumes relative to scales of receiving spaces, and (ii) flow properties (in particular, transported grain size distribution, flow thickness and flow concentration), these controlling depositional gradients and the equilibrium profiles to which slopes tend to grade.
Factors influencing the deposit geometry of experimental turbidity currents: implications for sand-body architecture in confined basins
Authors: Al Ja’Aidi Omar S. , McCaffrey William D. , Kneller Benjamin C.
Published: April 2008

Two sets of scaled laboratory experiments were performed to examine the effect of flow volume, flow density and grain-size distribution on the transport efficiency of turbidity currents. The experiments employed two sediment analogues (ballotini and silica flour) intended to model medium- to coarse-grained sand and mud respectively. In the first set of experiments each parameter was varied to examine its effect upon deposit geometry. Increases in the initial flow density, volume and proportion of fines had the effect of increasing the amount of sediment that was transferred to the floor of the experimental tank by the turbidity currents. Increase of each of these parameters has a characteristic effect on the three-dimensional geometry of the deposit: the deposits of large-volume flows are elongate, and those of fines-rich flows are broad. Increase of flow density increases the initial potential energy of the flow, thus increasing the runout distance; increase of the initial density beyond a sediment concentration of 13% by mass results, however, in a reverse of the geometrical trend of deposit elongation, possibly because of turbulence suppression at high densities. Increase of flow volume also increases the initial potential energy, and reduces the rate of velocity decrease due to gravitational spreading. Increase in the proportion of fines leads to maintenance of negative buoyancy, as the fine fraction remains suspended until the flow has virtually come to rest; it also decreases the settling velocity of the coarser fraction and thus delays its sedimentation. The second set of experiments was performed to investigate the influence of flow efficiency on the interaction of turbidity currents with topography. A single arcuate obstacle was placed in the path of the flows. In successive experiments flow efficiency was increased by progressively increasing the proportion of fines (silica flour). Both the proportion of sediment reaching the obstructing topography and the proportion of it able to surmount the topography increased as flow efficiency increased. Thus flow efficiency may determine whether or not an enclosed basin hosts deposits whose geometry has been affected by the confinement, and may also determine the relative effectiveness of the topography in confining inbound turbidity currents, and thus trapping their sediment load.
Impact of syndepositional faulting on gravity current behaviour and deep-water stratigraphy: Tabernas-Sorbas Basin, SE Spain
Authors: Hodgson David M. , Haughton Peter D. W.
Published: April 2008

Seabed faulting can have a significant impact on the routeing and behaviour of gravity currents depositing sand on deepwater basin floors. The Neogene El Cautivo Fault in the Tabernas-Sorbas Basin, SE Spain, is a rare example of a fault that demonstrably propagated through to the seabed during turbidite deposition, allowing the interplay between deepwater sedimentation and tectonics to be explored. The fault is associated with a wide (up to 350 m) gouge zone that varies significantly in thickness along its length, reflecting upward expansion towards the original seabed and progressive burial as fault activity ceased. Kinematic and stratigraphic evidence indicate that the fault was a dextral oblique strike-slip fault that accommodated an area of deeper ponded bathymetry (a ‘mini-basin’) and accelerated subsidence on its southern flank. Active faulting controlled the routeing of turbidity currents (revealed by changing provenance across the structure), rates of seabed deformation (resulting in differential subsidence and ‘growth’ of the stratigraphy), and the behaviour of the ponded currents (producing distinctive bipartite beds when deposition was in localized ponded depressions). The seabed expression of the fault varied from a forced fold, which warped the surface causing local wedging and onlap in the vicinity of the structure, and an unstable scarp that locally collapsed. The fault gouge fabrics and vein arrays are consistent with faulting of soft, water-rich sediment close to the seabed.
Down-channel variations in stratal patterns within a conglomeratic, deepwater fan feeder system (Miocene, Adana Basin, Southern Turkey)
Authors: Satur N. , Cronin B. , Hurst A. , Kelling G. , Gürbüz K.
Published: April 2008

The proximal, channelized section of a deepwater fan in the Adana Basin, southern Turkey, provides an opportunity to study down-channel changes in stratal patterns. This is a multisourced, bypass fan with at least four feeder channels. The downdip changes within one of these channels are described along a 10 km transect. Down-channel changes in grading and organization of clasts are observed for non-grading disorganized facies in the most proximal locations to more organized, inverse-normal and normal graded conglomerates midway down the transect. Bedding style changes from scoured and more sheet-like beds updip into a pattern of small channels and bars midway along the transect, and into thick-bedded and structureless conglomerates in the more distal locations within the channel. These changes in stratal patterns coincide with observed changes in depositional gradient of the channel. The gradient changes are interpreted to be a contributing factor in controlling hydrodynamics within the gravity flows and thus the depositional stratal patterns.
Sand-rich turbidite system of the Late Oligocene Northern Apennines foredeep: physical stratigraphy and architecture of the ‘Macigno costiero’ (coastal Tuscany, Italy)
Authors: Cornamusini Gianluca
Published: April 2008

The ‘Macigno costiero’ turbidite system characterized the oldest foredeep clastic wedge of the Northern Apennines during the Late Oligocene collisional phase. The cropping-out thickness is about 500 m. The features of the ‘Macigno costiero’ indicate a sand-rich, low-efficiency turbidite system. The system developed within a partially confined basin, which was part of a complex foredeep system. The stacking pattern of the turbidite system was determined through the analysis of facies and physical stratigraphy. It consists of a succession organized in sedimentary units, which are characterized by particular associations of facies linked to distinct depositional environments. Several architectural elements are seen: (1) unchannelized and channelized lobes; (2) distributary channels with channel-fill, overbank and channel-margin deposits; (3) main channel with channel-fill, channel-margin and interchannel deposits. Five turbidite stages were identified. From the bottom up they consist of four lobe stages and one proximal channel stage. The lobe stages are characterized by thickening-coarsening upward trends, from distal lobes to proximal lobes up to the channel-lobe transition zone. The uppermost, fifth stage is linked to a main channel complex with stacked channel-fill, channel-margin and interchannel deposits. This final stage also marks the maximum progradation of the system up to its closure due to the synsedimentary overthrusting of the orogenic wedge.
Diachronous Variscan late-orogenic collapse as a response to multiple detachments: a view from the internides in France to the foreland in the Irish Sea
Authors: Praeg Daniel
Published: April 2008

Models of the collapse of orogenic belts imply diachronous tectonism in which crustal uplift and extension may be compensated by peripheral compression. This first-order prediction is tested against published data on Varsican late-orogenic extensional and compressive structures along a 1500 km transect from the Variscan central internides in France to the foreland in the Irish Sea area. The collapse of the orogen is shown to have expanded northward over time, via three main stages: (i)
collapse of the central internides
(late Visean-mid-Westphalian,
c.
335–310 Ma) — crustal thinning took place by NW-SE extension within a relatively narrow (< 500 km) central axis, accompanied to the north by passive infill of basins spanning a broad seaway inherited from extension during closure of the Rheic Ocean; (ii)
reorientation and expansion of collapse
(mid-Westphalian-late Stephanian,
c.
310–300 Ma) — in the mid-Westphalian, a 90° rotation to NE-SW extension in the central internides was accompanied by changes across the northern internides (episodic basin formation and deformation), the externides (onset of thrust propagation) and the foreland (Westphalian C inversion), while from the early Stephanian, basins began to form in the central internides and expanded to the externides, coeval with final nappe emplacement along the orogenic front; and (iii)
collapse of the foreland
(late Stephanian-Early Permian,
c.
300–290 Ma) — km-scale uplift and erosion of the foreland took place, prior to widespread basin formation in the Early Permian (<290 ± 5 Ma). These three stages are argued to support a model of Variscan late-orogenic collapse in response to three successive detachments of negatively buoyant lithospheric material: of a collisionally thickened orogenic root, and of two (Rheic) oceanic slabs subducted, previously, southward (beneath the orogen) and northward (beneath the foreland). Multiple detachments are a predictable consequence of ocean closure and continental collision, so that episodic collapse may be a common process in the rise and fall of orogenic belts and the tectonic evolution of their forelands.
New constraints on the timing of late Carboniferous-early Permian volcanism in the central North Sea
Authors: Heeremans Michel , Timmerman Martin J. , Kirstein Linda A. , Faleide Jan Inge
Published: April 2008

The Permo-Carboniferous evolution of the central North Sea is characterized by three main geological events: (1) the development of the West European Carboniferous Basin; (2) a period of basaltic volcanism during the Lower Rotliegend (latest Carboniferous-early Permian); and (3) the development of the Northern and Southern Permian Basins in late Permian times. The timing of the late Carboniferous-Permian basaltic volcanism in the North Sea is poorly constrained, as is the timing of extensional tectonic activity following the main phase of inversion during the Westphalian, due to the diachronous propagation of the Variscan deformation front. Results of high precision Ar-Ar dating on basalt samples taken from a core from exploration well 39/2-4 (Amerada Hess) in the UK sector of the central North Sea suggests that basaltic volcanism was active in the late Carboniferous, at
c.
299 Ma. The presence of volcanics below the dated horizon suggests that the onset of Permo-Carboniferous volcanism in the central North Sea commenced earlier, probably at
c.
310 Ma (Westphalian C). This is contemporaneous with other observations of tholeiitic volcanism in other parts of NW Europe, including the Oslo Graben, the NE German Basin, southern Sweden and Scotland. Interpretations of available seismic data show that main extensional faulting occurred after the volcanic activity, but the exact age of the fault activity is difficult to constrain with the data available.
Carboniferous and Permian magmatism in Scotland
Authors: Upton B. G. J. , Stephenson D. , Smedley P. M. , Wallis S. M. , Fitton J. G.
Published: April 2008

Extensional tectonics to the north of the Variscan Front during the Early Carboniferous generated fault-controlled basins across the British Isles, with accompanying basaltic magmatism. In Scotland Dinantian magmatism was dominantly mildly alkaline-transitional in composition. Tournaisian activity was followed by widespread Visean eruptions largely concentrated within the Scottish Midland Valley where the lava successions, dominantly of basaltic-hawaiitic composition, attained thicknesses of up to 1000 m. Changing stress fields in the late Visean coincided with a change in the nature of the igneous activity; subsequently, wholly basic magmatism persisted into the Silesian. As sedimentary basin fills increased, sill intrusion tended to dominate over lava extrusion. In the Late Carboniferous (Stephanian) a major melting episode, producing large volumes of tholeiitic magma, gave rise to a major dyke swarm and sills across northern England and Scotland. Alkali basaltic magmatism persisted into the Permian, possibly until as late as 250 Ma in Orkney. Geochemical data suggest that the Carboniferous-Permian magmas were dominantly of asthenospheric origin, derived from variable degrees of partial melting of a heterogeneous mantle source; varying degrees of interaction with the lithosphere are indicated. Peridotite, pyroxenite and granulite-facies basic meta-igneous rocks entrained as xenoliths within the most primitive magmas provide evidence for metasomatism of the lithospheric mantle and high-pressure crystal fractionation.
40
Ar/
39
Ar geochronology of Carboniferous-Permian volcanism in the Midland Valley, Scotland
Authors: Monaghan A. A. , Pringle M. S.
Published: April 2008

Twenty-one new
40
Ar/
39
Ar step-heating experiments on mineral separates from intrusive and extrusive Carboniferous and Permian igneous rocks in the Midland Valley of Scotland yielded 17 concordant experiments with a relative age precision better than 1% (2σ). These ages resolve inconsistencies between existing K-Ar dates on the same samples and their stratigraphical constraints correlated to recently published timescales. The precise
40
Ar/
39
Ar dates are stratigraphically constrained to stage level and can contribute to Carboniferous timescale tie points at the Tournaisian-Visean boundary, within the Visean and at the Carboniferous-Permian boundary. Situated in the extending Variscan foreland, two distinct phases of extension-related transitional-alkaline volcanism have been resolved in the Dinantian: the Garleton Hills Volcanic Formation in the eastern Midland Valley near the Tournaisian-Visean boundary, 342.1 ± 1.3 and 342.4 ± 1.1 Ma; and the Clyde Plateau Volcanic Formation in the western Midland Valley during the mid-Visean, 335 ± 2–329.2 ± 1.4 Ma. Alkaline basic sills near Edinburgh, previously thought to be Namurian, appear to be coeval with the Clyde Plateau Volcanic Formation at 331.8 ± 1.3–329.3 ± 1.5 Ma. The new ages allow correlation between these short-lived Dinantian magmatic pulses and extensional and magmatic phases in the Northumberland-Solway and Tweed basins to the south. After late Westphalian, end-Variscan, compression and a regionally important tholeiitic intrusive phase at
c.
301–295 Ma, alkaline magmatism related to post-Variscan extension occurred in the central and western Midland Valley during the latest Carboniferous or Permian from 298.3 ± 1.3 to 292.1 ± 1.1 Ma. This correlates well with post-Varsican extension and magmatism observed across the NW European foreland from 300 to 280 Ma.