bluuebunny/crossref_metadata_2025 huggingface dataset:
Geochemical characterization of the organic matter from some recent sediments by a pyrolysis technique
Authors: Vandenbroucke M. , Behar F.
Published: April 2008

A comparative study of humic acids and kerogens from recent sediments was performed using geochemical analyses of hydrocarbons released by pyrolysis of these fractions. Lacustrine samples came from the Eastern African Rift (Lake Bogoria, Lake Tanganyika) and Europe (Lake Leman), marine samples from upwelling zones (Mauritania, Arabia), and coastal land-derived sediments from Kerguelen and Svalbard islands and from a deltaic facies in Indonesia. The deposited aquatic organic matter consisted of clearly identifiable algae (diatomae, coccolithophoridae). C
14+
saturated and unsaturated hydrocarbons generated by programmed temperature pyrolysis under an inert gas flow (argon) were recovered, hydrogenated and analysed by GC. The iso- + cyclo-alkane fraction was separated, then analysed by GC-MS. Pyrolysates of humic acids, kerogens and fulvic acids reveal diverse compositions important for comparative studies. Hydrogenated hydrocarbon distributions of kerogens and humic acids can be distinguished by diminished amounts of C
14+
hydrocarbons in the latter, whereas fulvic acids generate unique hydrocarbon distributions atypical of both kerogens and humic acids. Major iso- + cyclo-alkanes in the kerogens, the main fraction of the organic matter, are isoprenoids (pristane and phytane), steranes (α cholestane) and triterpanes (C
27
and C
29
hopanes and moretanes), whatever the origin of the kerogen, whether lacustrine or marine.
Application of biological markers in the recognition of palaeohypersaline environments
Authors: ten Haven H. L. , de Leeuw J. W. , Sinninghe Damsté J. S. , Schenck P. A. , Palmer S. E. , Zumberge J. E.
Published: April 2008

In this study the saturated and aromatic hydrocarbon fractions of a marl sample from a Messinian (late Micoene) evaporitic basin located in the northern Apennines, and four oils, Rozel Point oil (Utah, USA; Miocene) and three seep oils from Sicily (Messinian), have been studied by GC with simultaneous FID and FPD detection and by GC-MS. All samples show characteristics which might be linked to hypersaline conditions prevailing during the time of deposition. Some of these characteristics are: a very low pristane/phytane ratio (<0.1), a relatively high abundance of docosane (C
22
) and gammacerane and a series of extended hopanes and/or hop-17(21)-enes maximizing at C
35
. The aromatic hydrocarbon fraction of all samples is dominated by organic sulphur compounds of which 2,3-dimethyl-5-(2,6,10-trimethylundecyl) thiophene is the most abundant compound. The suggestion of Meissner
et al.
(1984), that the source rock of Rozel Point oil was deposited under hypersaline conditions in a playa-like system, is supported by the organic geochemical characteristics of this oil.
Possible relationships of stratigraphy and clay mineralogy to source rock potential in lacustrine sequences
Authors: Yuretich R. F.
Published: April 2008

Petroleum accumulations in lacustrine environments often possess characteristics different from those in marine settings. In general, lacustrine petroleum fields are smaller, the oil has often not migrated far from the source, and oil shales are common. Some of these differences can be attributed to the relative youthfulness of lacustrine basins and to differences in the type of organic matter present. However, additional influences may arise from the stratigraphic framework and clay composition of the source rocks. The scale of the environment may be a major controlling factor. Gradations between large lakes and marginal marine basins exist, but lake basins will more often be areally small, relatively shallow and subject to frequent lateral migrations in depositional environments. These characteristics make the resulting sedimentary sequences much more heterogeneous, both in stratigraphic relationships and in sediment composition. Stratigraphy will ultimately influence the migration and accumulation of the oil; the sediment composition—particularly clay mineralogy—will affect the generating potential of the source rock. Shifting sediment source areas coupled with lake level fluctuations and early diagenetic reactions may produce thin mudstones containing very heterogeneous clay mineral assemblages, which, accordingly, may not undergo significant transformations during burial diagenesis.
Cenozoic lacustrine basins of South-east Asia, their tectonic setting, depositional environment and hydrocarbon potential
Authors: Gibling M. R.
Published: April 2008

Cenozoic strike-slip tectonism, which caused local crustal extension in South-east Asia, generated many short-lived but deeply subsiding basins. Under tropical conditions and assisted by intensive karstic weathering, the basins formed ideal sites for lakes during early basinal history in Oligocene to Miocene times. In several basins in northern Thailand, a shallow-lacustrine assemblage of mudstone and coal contains thin, alginite-rich oil shales of moderate grade which formed under oxic or anoxic conditions during the initial flooding of the lake, which was accompanied by high organic productivity. In the Mae Sot Basin, a carbonate-rich lacustrine assemblage contains thick, alginite-rich oil shales of high grade which formed under anoxic conditions in deeper, probably stratified lakes where the organic material was well preserved. The kerogen in both assemblages is Type I, hydrogen-rich. Under conditions of rapid sedimentation and abnormally high heat flow associated with active tectonism, the shallow-lacustrine oil shales have generated hydrocarbons which are exploited in several basins.
Anoxic-oxic cyclical lacustrine sedimentation in the Miocene Rubielos de Mora Basin, Spain
Authors: Anadón P. , Cabrera L. , Julià R.
Published: April 2008

The Rubielos de Mora Basin is a Miocene half-graben (3 × 10 km) with a sedimentary fill of over 600 m of alluvial and lacustrine sequences. Three main units make up this basin infill: lower (alluvial) and middle and upper lacustrine units. The upper lacustrine unit is mainly formed in the western part of the basin, by cyclical sequences. These sequences consist of organic-poor, non-laminated mudstones and marls cyclically alternating with thinly laminated facies such as sandy mudstones, bioclastic laminae, oil shales, rhythmites (carbonate-clay, varve-like couplets) and marls. Low-Mg calcite and non-stoichiometric (Mg-poor) dolomite are the dominant carbonate minerals in the non-laminated facies, whereas variable amounts of low-Mg calcite, high-Mg calcite, aragonite and non-stoichiometric dolomite have been recorded in most of the laminated facies. Low-Mg calcite is the main carbonate mineral in sandy mudstones and bioclastic laminae which occur at the lower part of laminated intervals in the cycles. Analysis of oil shales reveals a major macrophyte contribution of organic matter, displaying an early diagenetic evolutionary stage. Cyclical sequences record alternating oxic-anoxic bottom conditions in marginal zones of a meromictic lake, due to cyclical changes in lake water volume. This feature is recorded in the cyclical sequences of mineralogical changes and facies transitions. Strong tectonic subsidence (not balanced by sedimentation and taking place in an active rift setting) and palaeoclimate were the most striking features which could favour the establishment of a meromictic lake during this stage of the basin history.
Profiles of fan deltas and water depth in the receiving basin
Authors: Ito M.
Published: April 2008

Profiles of prograding fan deltas are largely controlled by the water depth in the receiving basin. The initial slope of the fan-delta front with respect to that of the fan-delta plain, which is sometimes detectable in stratigraphic packages, decreases with the increase in water depth at the toe of fan-delta front deposits. The resultant relationship provides important evidence and information for restoring the depositional architecture of ancient fan deltas that may reflect the marginal geometry of sedimentary basins. Relatively steep gradients in fan-delta accretion surfaces may lead to an overestimation of the
vertical
stratigraphic thickness, particularly if it is calculated using a simple cumulative bed thickness. Furthermore, fan-delta successions which appear to be anomalously thick may not be due to progressive or incremental basin subsidence, but rather to overestimated stratigraphic thickness because the fan-delta clinoform dips were ignored. A part of the thick fan-delta deposits, therefore, can best be interpreted in terms of lateral accretion of fan-delta front deposits on the dipping clinoform surfaces, responding to the shallowing in the receiving basin.
Detailed sedimentology and gamma-ray log characteristics of a Namurian deltaic succession II: Gamma-ray logging
Authors: Myers K. J. , Bristow C. S.
Published: April 2008

A portable gamma-ray spectrometer was use to measure the total radioactivity, potassium content (% K), uranium content (ppm U) and thorium content (ppm Th) in the Upper Namurian Rough Rock at an outcrop in a road section at Elland, Yorkshire. The total gamma radiation generally decreases upwards in the succession, reflecting an upward increase in grain size. Gamma-ray peaks within the succession are due partly to facies/grain-size changes and partly to concentrations of thorium-bearing monazite in mouth-bar sandstones and basal channel lags. Failure to identify the true nature of these peaks could lead to erroneous correlations in the subsurface. The Th/K ratio decreases upwards in association with the increase in grain size and a decrease in the quartz/feldspar ratio. The Th/K ratio and Th-K cross-plots are effective in distinguishing some of the deltaic facies.
Nile Delta: a review of depositional environments and geological history
Authors: Sestini G.
Published: April 2008

The distribution of the Holocene-Recent sedimentary environments of the Nile Delta is related to secular variations of the semi-arid climate and of the Nile River sediment input and flux, and to coastal subsidence and high wave energy, together with an eastward littoral drift. The delta started to form in the Late Pliocene, with its main development taking place in the Pleistocene, associated with progradation of large volumes of coarse coastal delta sands and the offshore accumulation of turbidities (Nile Cone). In the Oligocene-Early Miocene, an ancestral Nile discharged to the NW. Fluvio-deltaic deposits appeared in the present delta area in the Late Miocene, where they built a narrow prism close to an active faulted flexure, which in the subsurface now separates the South Delta Block from the deep North Delta Basin. The stratigraphy, structure and depocentre development of the region are functions of the interaction between the E-W- to WSW-ENE-trending Mediterranean margin structures, active since the end of the Cretaceous, and the NW-, NNW- and NNE-trending Red Sea-Gulf of Suez structural fabric associated with arching and rifting from the Oligocene to Pleistocene. The main phases of tectonic activity and of clastic deposition, which also correlate with lowered sea levels, were in Late Oligocene-Early Miocene, late Middle Miocene, Tortonian-Messinian, Middle Pliocene and Early-Middle Pleistocene. Petroleum exploration in the Nile Delta has produced modest results to date with a few medium-sized gas fields and minor oil discoveries.
The sedimentology of a Middle Jurassic lagoonal delta system: Elgol Formation (Great Estuarine Group), NW Scotland
Authors: Harris J. P.
Published: April 2008

Deposition of N-S prograding lagoonal deltas took place in two separate extensional basins (Sea of the Hebrides and Inner Hebrides Basins) of different size and hence wave energy. Salinity of water in the basins varied spatially and temporally from marine to fresh-brackish. The reduced salinities were controlled by diminished connections with the open sea together with copious run-off from a hilly hinterland with a subtropical seasonal climate. Sections in N Trotternish, S Trotternish, Raasay (Sea of the Hebrides Basin) and Strathaird (Inner Hebrides Basin) represent three different styles of delta sedimentation corresponding to different salinities and basinal energy regimes (waves and tides). In N Trotternish, marine macrofaunas and microfloras occur in a wave-tide interaction delta system in which tidal currents controlled distributary-channel and mouthbar hydrodynamics and high wave energies generated beach-ridge sediments at the delta shoreline. In S Trotternish and Raasay, probable brackish-marine salinities and a deeper basin are reflected in buoyant mouth-bar hydrodynamics. In Strathaird (Inner Hebrides Basin) probable fresh-brackish salinities correspond to a fluvial-dominated delta recording friction-dominated mouth-bar hydrodynamics. This lobate delta is characterized by low angle offshore inclined bar-front sandstones. In the Inner Hebrides Basin, the Elgol Formation delta reaches a depositional limit between Strathaird and Eigg. The N-S transition from marine to fresh-brackish salinities records the gradual establishment of a non-marine depositional system (which is maintained throughout the rest of the Great Estuarine Group) during progradation of the Elgol Formation delta.
Facies associations and sand-body geometries in the Ness Formation of the Brent Group, Brent Field
Authors: Livera S. E.
Published: April 2008

The Brent Group in the Brent Field contains over 1000 × 10
6
bbl of recoverable oil in a simple gently dipping half-graben structure. It is 243 m (800 ft) thick and represents a sequence amplified by synsedimentary fault-block subsidence close to the axis of the Viking Graben. On a regional scale the depositional model is of a major shoreface and coastal sequence, the Rannoch and Etive Formations, underlying the partly time-equivalent shallow lagoonal to emergent delta-plain deposits of the Ness Formation. However, more localized and detailed sedimentary modelling is required to predict reservoir geometries and production behaviour, particularly in the Ness Formation.

The Ness Formation is the largest reservoir-bearing unit in the Brent Field, being up to 160 m (525 ft) thick. Five main facies associations form reservoir-quality rock: varied channel sandstones, fluvial sheet sandstones, major mouth bars, lagoonal shoals (which are usually associated with, and capped by, thin beaches) and lagoonal sheet sandstones. These associations make up 50%–60% of the sand-rich sequence and are embedded in the non-reservoir facies associations: lagoonal mudstones, emergent flood-plain mudstones and allochthonous and autochthonous coals. The coals are often field wide in extent. Within the amplified Ness Formation the most important environments were the lagoonal deltas which were wave and fluvial dominated with minor tidal influence. As a result of this importance the sand bodies produced by these deltas are subjected to a more detailed description in this paper. Wave energies were sufficient to redistribute large amounts of sediment and indicate significant fetch in the brackish lagoons diluted by major freshwater input.

Sand-body geometries are very variable, reflecting both localized controls on sediment distribution, particularly immediately above the compacting shoreface pile, and more regional effects including coarse-sediment supply, basinal processes and fault-block subsidence. The dominant controls on sedimentation resulted in regular drowning events on the delta plains and had a significant impact by producing a strongly layered reservoir.
Coal reviewed: depositional controls, modern analogues and ancient climates
Authors: Haszeldine R. S.
Published: April 2008

Ancient economic coals usually contain more than 90% organic matter and less than 1% sulphur. Sedimentological reconstructions show the interbedded clastics to be fluvial-plain, deltaic, barrier coastline, estuarine or alluvial-fan deposits. However, bases of coal beds unconformably transgress these various facies, and coals only rarely pass laterally into clastic facies. Thus coal-forming peats
post-dated
the underlying clastic sedimentation and most coals did not form in deltas. Local coal thicknesses can be controlled by compaction of underlying sediment, syndepositional faulting, or basement structure.

Coals with more than 1%–2% sulphur are usually overlain within 10 m by marine sediments which provided SO
4
2−
by diffusion. However, most coals contain less sulphur and must be freshwater in origin. Lithotype and spore profiles vertically through coals indicate a characteristic change of peat floras. This resulted from equisites-dominated seat earths, followed by diverse tree-sized vegetation which formed bright coal and lastly by low diversity stunted vegetation which formed dull coal. These features compare well with peats formed in modern ombrogenous blanket or raised bogs. The ancient mires formed in rainwaters (low sulphur), excluded clastic sediments (low ash), had a lateral floral zonation, usually post-dated clastic deposition and occurred inland from the shoreline.

Modern ombrogenous mires are abundant, especially in humid temperate climates. These peats are forming today during a period of long-term climatic cooling in areas with a frequent reliable rainfall. Ancient coals may also have formed during long-term climatic cooling epochs when volcanic CO
2
was being removed from the atmosphere. This could be regarded as a Gaian biosphere maintaining Earth temperatures similar to those of the present day.
Deltaic coals: an ecological and palaeobotanical perspective
Authors: Scott A. C.
Published: April 2008

Coals occur widely in deltaic sequences but are rarely of economic quality, being thin, split and with high ash content. Coals with brackish-water influence or overlain by marine rocks tend to have high sulphur values. Not all coals in deltaic sediments formed in swamps or marshes. Peats may form under a wide variety of conditions, and ecological and hydrological interpretations of mire types are essential and should be integrated with facies studies of accompanying clastic sediments. Botanical, physical and chemical data are needed for the interpretation of the coals.

Considerable time may elapse between delta abandonment and the onset of peat formation. Rheotrophic or flow-fed peats may form initially on abandoned delta lobes, but if peat accumulation continues under high rainfall conditions ombrotrophic low ash peats may form as raised bogs. In this case vertical and lateral zoning of the peat (coal) may occur. In any interpretation of the coals based on analogies of modern peats care must be exercised in using ecological data from Recent plant communities, as peat-forming vegetation shows significant changes from the Devonian to Recent.
Fan-delta-lacustrine sedimentation and coal development in the Tertiary Ombilin Basin, W Sumatra, Indonesia
Authors: Whateley M. K. G. , Jordan G. R.
Published: April 2008

The Ombilin Basin is a Tertiary intermontane basin situated in the Barisan mountain range of W Sumatra. It covers an area of some 1500 km
2
and is known for the coal resources which have been mined in the Sawahlunto area since 1891. Seismic and borehole data indicate that the basin has been filled with approximately 4600m of middle Eocene to early Miocene sediments. The coal developed early in the basin’s history (middle Eocene). During this study the fan-delta sediments below the oldest seam, the C seam, the seam itself and the sediments immediately overlying the C seam were investigated along the western margin of the Ombilin Basin in the Sawahlunto area.

The oblique convergence of the Indian Ocean plate weith the SE Asia plate created a stress field of the type that can produce a primary dextral wrench fault which in this case is named the Great Sumatran Fault Zone. The Ombilin Basin is a graben-like ‘pull-apart’ basin resulting from localized extension failure and related deformation. The failure and deformation are a function of irregular strike-slip movement along the Great Sumatran Fault Zone during the early Tertiary. Tertiary sedimentation began in the Eocene following graben formation. High basement relief adjacent to the newly formed basin resulted in debris flows and fan-delta formation around the margin of the central lake. Concurrent with the deposition of coarse-grained fan-delta sediments around the basin margins, fine-grained lake sediments accumulated in the basin centre.

Reduction in relief difference between the source area and the basin was associated with an expansion of the lacustrine environment. Shallowing of the lake and a humid tropical climate allowed plant colonization to take place. Peat accumulation was influenced by the underlying fan-delta lobe geometry. In interlobe areas thick sequences of low ash peat accumulated, while central lobe areas through the same interval were sites of continued sediment influx and relatively slow peat accumulation. In these areas high ash coal or carbonaceous shale are laterally equivalent to the low ash coal. A detailed investigation of the character of the coal in the C seam indicates that, while the lateral dispersion of clastic material may play some part in the variation in the ash content of the coal from one place to another, the most significant variable is the rate of accumulation of peat.
Asthenospheric control on magmatism in the ocean basins
Authors: White Robert S.
Published: April 2008

Asthenospheric processes exert a dominant control over the volume and distribution of magmatism in oceanic basins. In particular, the temperature of the asthenosphere controls the amount of melt and hence the volume of igneous crust produced at rifted continental margins and oceanic spreading centres. Changes of as little as 100–150°C in the asthenosphere temperature cause massive differences in the thickness of the igneous crust.

Where the asthenosphere wells up passively during rifting, such as at continental margins and oceanic spreading centres, partial melt is produced by adiabatic pressure release. Continental rifts near hot-spots are often emergent above sea level, with considerable extrusive and intrusive magmatism, whilst those away from hot-spots subside gently without volcanism. Oceanic spreading centres are fed by sublithosphere feeders that generate spreading cells typically 40–80 km long which operate independently of one another. At the intervening fracture zones, where the magma supply from the adjacent feeder is least, the crust thins, sometimes to less than 1 km.

Hot-spots are fed by mantle plumes. The partial melt and resultant surface volcanism comes from the rising plume which is only 100–200 km wide. The broad surrounding topographic swells, 1000–2000 km across, are dynamically supported by the mushrooming head of abnormally hot mantle beneath the lithosphere.
Strontium, neodymium and lead isotope characteristics of the Granadilla Pumice, Tenerife: a study of the causes of strontium isotope disequilibrium in felsic pyroclastic deposits
Authors: Palacz Z. A. , Wolff J. A.
Published: April 2008

The Granadilla Pumice deposit, the product of a single explosive eruption of phonolitic magma, exhibits stratigraphically systematic internal compositional variations interpreted as the result of fractionation of the observed phenocryst assemblage in the pre-eruptive magma body. Whole-rock
87
Sr/
86
Sr data exhibit a consistent profile, with the most differentiated compositions having the most radiogenic strontium. The resultant
87
Sr/
86
Sr versus 1/Sr mixing line has a positive slope, suggesting the addition of a relatively radiogenic strontium component to the existing strontium concentration profile. There is no significant systematic variation, however, in neodymium or lead isotopes. Separated alkali feldspar phenocrysts have
87
Sr/
86
Sr values of 0.7031–0.7032, identical with Tenerife mafic magmas; the strontium isotopic variation is restricted to the complementary glasses. The more radiogenic component in the glasses is only partially removed by acid leaching and has a strontium isotope ratio of about 0.707, deduced from analysis of the leachant. Strong acid leaching, accompanied by up to 25% sample dissolution, results in greater, but not complete, loss of the more radiogenic component, with a good correlation between pre- and post-leaching glass isotopic compositions. These relationships cannot be easily explained by either post-eruptive alteration or contamination of the magma chamber with high-
87
Sr/
86
Sr country rock. The data are instead consistent with a model of interaction between vesiculating magma and hydrothermal fluids with high
87
Sr/
86
Sr, immediately before and/or during explosive eruption, involving limited diffusion of strontium from the fluid into bubble walls before quenching of the magma to form glassy pumice. This process may be common during large explosive eruptions, and we suggest that whole-rock strontium isotope variations found in strontium-poor felsic pyroclastic deposits may not reflect purely magmatic processes, even though they closely mimic plausible magmatic variations.
Axial volcanism on the East Pacific Rise, 10–12°N
Authors: Thompson G. , Bryan W. B. , Humphris S. E.
Published: April 2008

The East Pacific Rise axis, 10–12°N, consists of three discrete topographic or tectonic units each of which has erupted compositionally distinct magmas. The two major units have an axial bathymetric high, are bounded by transform or overlapping spreading centres and are shown to be discrete volcanoes. One of the units shows systematic variation in magma composition along the rift consistent with injection of lava down-rift and concomitant low-pressure fractional crystallization with distance from the topographic high. Compositional variation within and between magmatic units is largely controlled by fractional crystallization at low and intermediate pressures. The eastern and western limbs of a large overlapping spreading centre at about 11°45′N have compositionally distinct lavas, apparently from separate magma batches. Axial eruptions in the vicinity of a large transform fault (Clipperton Fracture Zone) show a large variation in composition and at least one independent magma injection. Highly fractionated lavas with compositions up to 59% silica are included in these eruptions. One of the axial tectonic units has, in addition to normal tholeiitic eruptions, some lavas highly enriched in incompatible elements such as potassium, titanium, strontium, barium, zirconium and lanthanum. These lavas are also isotopically enriched with respect to
87
Sr/
86
Sr, suggesting that the mantle beneath some portions of the axis is compositionally heterogeneous. The presence of compositionally distinct magma batches, often in close proximity, suggests that magma chambers underlying the East Pacific Rise axis are small and probably intermittent.
Characterization of the St Helena magma source
Authors: Chaffey D. J. , Cliff R. A. , Wilson B. M.
Published: April 2008

Combined Sr-Nd-Pb isotope and trace-element data for St Helena are interpreted in terms of changing thermal and chemical fluxes impinging on and interacting with the base of the lithosphere over a period of 6 Ma. The data reveal the existence of two geochemically distinct components in the St Helena source region: (i) a HIMU (high U/Pb) component which has extremely radiogenic lead isotopes (
206
Pb/
204
Pb >20.8) with
87
Sr/
86
Sr and
143
Nd/
144
Nd ratios displaced below the mantle array; (ii) a component with less radiogenic lead and strontium isotopic compositions and more radiogenic neodymium compositions. Coupled trace-element and isotope variations are evident during the activity of each volcano. During shield development an increase in incompatible-trace-element enrichment occurs. This is coupled to a decrease in
143
Nd/
144
Nd, whilst strontium and lead isotope ratios become progressively more radiogenic with time. The time-dependent variations are thought to be consistent with high-level processes occurring at the base of, or within, the lithosphere. A decrease in the signature of the depleted component with time is shown to be the result of a decreasing thermal flux acting on the base of the lithosphere. As a consequence the degree of partial melting of the depleted component (which may reside in the lithosphere or asthenosphere) is reduced, increasing the signature of the HIMU component in the erupted magmas.
Hygromagmaphile element distributions in oceanic basalts as fingerprints of partial melting and mantle heterogeneities: a specific approach and proposal of an identification and modelling method
Authors: Joron J.-L. , Treuil M.
Published: April 2008

On the basis of the properties of hygromagmaphile (HYG) elements we propose a specific method of identifying and modelling the evolution of mantle geochemical properties using simple mass balance equations and taking into account a large volume of data obtained for basaltic series belonging to characteristic oceanic domains. This method uses both binary and ternary HYG element ratio diagrams to distinguish between the effects of partial melting and mantle heterogeneity. It is assumed that batch partial melting is the simpler and more suitable process of basaltic magma generation, which produces linear correlations on binary HYG element ratio diagrams. Melting of a homogeneous source is projected on ternary HYG element ratio diagrams as a single point approximating the composition of the mantle source.

We show that basalt HYG element ratios are closely dependent on four main factors, namely the nature of melting (batch partial melting), the degree of partial melting, the composition of the mantle and the mineralogy of the mantle.

Partial-melting trends and complex mantle heterogeneities are clearly indicated by binary diagrams, and a simple and regular suboceanic mantle HYG element array is obtained in ternary diagrams. Taking into account the close relation with isotopic trends, the HYG element array is considered to be the fingerprint of ancient mantle differentiation. It can be reproduced by a simple fractional crystallization process, and mostly excludes a large-scale and intensive chemical mixing process. These results are in good agreement with a marble cake mantle model, limiting convection effects to a mechanical stirring of the upper mantle, but they do not necessitate any recycling of crustal material into the suboceanic mantle.
Evaluation of rare gas data in relation to oceanic magmas
Authors: Fisher D. E.
Published: April 2008

The isotopic compositions of the rare gases measured in oceanic rock samples present stimulating but incomplete information concerning the time and extent of magma degassing processes. Isotopic abundances of gases trapped from suboceanic mantle source regions are related to the depleted/undepleted mantle concepts, while atmospheric or crustal components form an important part of the data set. A reasonable story can be presented for the rare gas component from normal mid-ocean ridge basalt mantle source regions, but oceanic islands and island arc basalts result from more complex sources; the rare gas evidence is not yet sufficiently free of ambiguities and experimental uncertainties to provide a firm constraint to various models.
Geometry and kinematics of inversion tectonics
Authors: Williams G. D. , Powell C. M. , Cooper M. A.
Published: April 2008

Positive inversion tectonics involves the reversal of extensional fault movement during contractional tectonics. Basin stratigraphy developed before, during and after extensional fault movements may be described as pre-, syn- and postrift sequences. Growth fault activity may be graphically displayed using thickness changes in stratigraphic intervals from fault footwall to hanging wall. Alternatively, it may be recorded using a hanging wall displacement/distance plot. Contractional reactivation of extensional faults puts progressively older synrift markers into net contraction. The point of change from net extension to net contraction is the null point. Its position in the synrift stratigraphy may be used to quantify the inversion ratio, which is defined as the ratio of contractional to extensional movement. Negative inversion is the reactivation in extension of a significant portion of an existing contractional system. Stratigraphic separation diagrams constructed from geological maps may be used to define the null point of individual faults and to quantify their inversion ratio.
Some geometrical characteristics of inversion
Authors: Hayward A. B. , Graham R. H.
Published: April 2008

The inversion of extensional fault systems results in the reversal of slip on the faults and expulsion of the synrift fill. During inversion the beds in the cover sequence shorten before the net extension at the basement level has been cancelled. Shortening of the sedimentary cover generates folding and backthrusting in the still downthrown hanging wall block. Intracratonic inverted basins in different parts of the Alpine Foreland show similar structural geometries with the major extensional faults which controlled basin development reactivated during subsequent compression. We use examples from the Western Approaches and offshore Holland (Broad Fourteens Basin) to illustrate the structural styles developed during inversion. The fundamental control on compressional structural geometry exerted by pre-existing extensional structures is also visible in more complexly deformed orogenic belts, like the Western Alps and the Pyrenees. In these areas inversion also occurs, but more commonly extensional faults which may not have inverted act as an indirect control on the location of ramps, and/or thrust orientation. Seismic data are normally required to establish these effects with certainty. However, as the body of knowledge builds up, it is possible to recognize certain geometrical characteristics which suggest the control of extensional faults in thrust belts. These include footwall shortcuts, out of sequence structures and arcuate thrust-fold traces.
The inverted margin of the French Alps and foreland basin inversion
Authors: de Graciansky P. C. , Dardeau G. , Lemoine M. , Tricart P.
Published: April 2008

The whole of the Western Alpine realm can be considered to result from the inversion of the European margin of Ligurian Tethys as a consequence of the collision between Apulia and Europe. The margin was developed due to Tethyan rifting during the Liassic and Middle Jurassic and regional thermal subsidence as the Ligurian ocean was spreading in the Late Jurassic and Early Cretaceous. Coevally, the margin underwent additional stretching in response to North Atlantic rifting. Inversion was initiated by Late Cretaceous times when the Ligurian oceanic crust began to be subducted, with localized obduction, beneath the previously passive margin. The most recent inversion of the Western Alps occurred in the external Molasse basin of Manosque-Digne-Valensole during the Late Miocene and Pliocene. Regions of mild inversion allow useful comparisons to be made with more strongly inverted basins. The geometric characteristics of the inverted basins depend on the lithology, geometry of the extensional structures, orientation of extensional faults with respect to the compressive Alpine stresses and the amount of compression. Comparisons of the external and interal zones suggest that the ductilely deformed thrust sheets may have originated from inverted half-grabens.
Notes and cross-sections illustrating inversion tectonics in the Wessex Basin
Authors: Simpson I. R. , Gravestock M. , Ham D. , Leach H. , Thompson S. D.
Published: April 2008

Four cross-sections based on seismic and well data illustrate the Tertiary inversion tectonics in the Wessex Basin. Inversion is concentrated in Early Cretaceous depocentres with Permian and Triassic depocentres undergoing limited uplift. In the Portland—South Wight Basin inversion formed a northward facing monocline with a major reverse fault in the S. In contrast, in the Weald Basin a broad arch of 50 km wavelength was formed. Both basins show an overall uplift of 1.5 km. These variations in inversion style are probably a function of the geometry of the basin-controlling faults and underlying detachments.
The kinematics of inversion in the Danish Central Graben
Authors: Cartwright J. A.
Published: April 2008

The Danish Central Graben is a complex mosaic of half-grabens and structural highs that formed during Permo-Triassic and Upper Jurassic rift phases at the confluence of the NW-SE and N-S trending elements of the Central North Sea Rift. The extensional structures forming the margins to the rift were reactivated in a reverse sense in the Late Cretaceous as part of the regional inversion of basins in the southern North Sea. Deposition proceeded throughout the deformation, and thickness variation in the deep marine Chalk succession is directly attributable to compressional deformation of the basin floor. The total shortening across the rift amounts to only a few percent, and the structural expression of the reactivated margins is typically as low amplitude folds with minor associated reverse faulting. Compressional reactivation was restricted to basement faults, and the shallow inclination of the margin faults may have been an important factor in their subsequent reactivation. Zechstein evaporites and overpressured Jurassic shales influenced the development of some of the compressional structures. The orientation of the growth fold axes is consistent with a net convergence in an approximately NE-SW direction. The fold axes are arranged in a segmented pattern which can be related to the underlying basement fabric. This segmentation not only resulted in variations in the amounts and style of shortening along the length of the rift, but seems to have influenced the timing of the deformation. Seismic-stratigraphic analysis of the Upper Cretaceous sequence indicates that the compressional deformation was markedly diachronous. Abrupt changes in the deformational chronology are found to occur across the transverse segment boundaries.
The Permian to Cretaceous structural evolution of the Western Approaches Basin (Melville sub-basin), UK
Authors: Chapman T. J.
Published: April 2008

The tectonic-sedimentary evolution of the western part of the Western Approaches Basin (Melville sub-basin) was initiated during early Permian times when synrift clastic sedimentation and volcanism commenced within an ENE trending half-graben. This sequence thickened to the south and onlapped a high to the north. During the Permian an early NNW-SSE basin extension was accommodated on an array of ENE-WSW trending normal faults linked by NNW-SSE orientated transfer faults. This was followed by a period of ENE-WSW extension along the basin axis when the normal and transfer faults of the previous faulting episode swapped roles. This was possibly related to early rifting in the Bay of Biscay. Both sets of faults appear to be controlled to some extent by Variscan thrusts and strike-slip faults within the basement. During the Upper Triassic the ridge in the north subsided allowing a thick sequence of salt to be deposited where there had previously been a topographic high. During Jurassic times sedimentation extended across the Permian-Triassic half-graben into the French sector. This was followed by an episode of rifting and wrench deformation (and inversion?) during which the Permian to Jurassic basin was domed and uplifted. The Cretaceous to Tertiary sedimentation which followed probably resulted from postrift thermal subsidence related to cooling of the continental platform adjacent to the opening Atlantic Ocean. Steep normal and/or strike—slip faults data from the earliest part of this postrifting stage.
Devonian basins of Northern Scotland: extension and inversion related to Late Caledonian — Variscan tectonics
Authors: Coward M. P. , Enfield M. A. , Fischer M. W.
Published: April 2008

The Devonian sedimentary basins of Northern Scotland illustrate the different processes of inversion along reactivated frontal and lateral thrust ramps, and the importance of strike-slip fault movements to late and post-Caledonian tectonics. The basins of the West Orkneys and West Shetland, west of the Great Glen Fault developed by collapse of thickened Caledonian crust during the Devonian. The basins extended in a NW-SE direction on dominantly SE-dipping faults, which curve and become parallel to the earlier Caledonian fabric at depth. The Caledonian structures developed a crustal anisotropy which was subsequently reworked during the later Palaeozoic and Mesozoic by extensional and compressional movements. Hence in the West Orkney and Minches basins, Caledonian structures influenced the kinematics of Early to Middle Old Red Sandstone extension, later (Devonian and Permo-Carboniferous) compression, Mesozoic extension and later Mesozoic-Tertiary compression. During basin extension, minor inversion of early listric normal faults occurred within the basin sediments, driven by gravitational gliding down tilted fault blocks. SE of the Great Glen Fault in eastern Shetland and possibly also the Inner Moray Firth, basins were affected by NE-SW extension during Middle to Upper Devonian times. This is possibly a result of sinistral strike-slip displacements on the Great Glen-Walls Boundary Fault system. This strike-slip episode may have been responsible for inversion of the earlier and more regionally widespread Lower and Middle Devonian basins formed by NW-SE extension. Localized zones of folding in Caithness and the Orkney Islands and wide zones of more intense folding in western Shetland are thought to be related to this deformation. Late Palaeozoic Variscan dextral strike-slip displacements juxtaposed basins along the Great Glen-Walls Boundary fault system which have distinct stratigraphies and structural evolutions and formed separately to the east and west of these faults. This strike-slip deformation inverted basins both to the east and west of the Great Glen Fault. In the East Shetland basins it produced local zones of overturned beds above oblique and frontal ramps. Further subsequent dextral movements in the Mesozoic produced pull-apart basins in the Moray Firth and East Orkneys.
A new tectonic model for the Helvetic nappes
Authors: Dietrich D. , Casey M.
Published: April 2008

A tectonic model must be based on geometrical information of both large and small scales. A new model of the Helvetic nappes of western Switzerland has been developed using constraints from:

(1)

The geometric shape of the nappes. Accurate profiles of the Morcles, Diabelerets and Wildhorn nappes show that all three nappes are wedge-shaped and that the axial planes of the major nappe internal folds converge towards the root zone.

(2)

The strain state of the nappes. All the existing pressure shadow fibre data of the region are reviewed and reveal the existence of very steep gradients of increasing strain towards the root zone of each individual nappe.

(3)

The metamorphism of the nappes. A compilation of illite crystallinity data indicates the existence of a vertical gradient of increasing metamorphism, but as well of a horizontal gradient of metamorphism increasing towards the root zone.

A geometrical and finite element compatibility model is presented which best accounts for these data. This model shows the following geometrical features:

(1)

Combinations of simple shear and pure shear deformation to account for the horizontal strain- and metamorphic gradients.

(2)

The heterogeneous simple shear part of the deformation is constant in magnitude from the root zone to the external part of the nappes.

(3)

The pure shear part of the deformation decreases towards the external part of the nappes.

(4)

Varying amounts of overthrust displacement are taken up by distributed ductile shear in the internal part of the nappes and by thrust faulting in the external part.

This model has kinematic implications. We envisage tectonic extrusion from the root zone to be responsible for the pure shear component of the deformation. The nappes formed sequentially being extruded between the underthrusting basement and the backfolded Gotthard massif.
Strain-displacement compatibility and restoration of the Chaînes Subalpines of the western Alps
Authors: Gratier J.-P. , Ménard G. , Arpin R.
Published: April 2008

Finite strain values and trajectories of the sedimentary cover of the French Chaînes Subalpines (Chartreuse, Vercors, Diois, Baronnies) with respect to the stable platform west of these regions were deduced from the strain measurements of a folded and faulted competent layer (Tithonian). The removal of this finite deformation was obtained by a finite element analysis (84 elements of initial size of about 15×15 km) and the finite displacement field of this cover was calculated. Assuming a stable margin (one fixed element, and one line with a constant direction during the progressive deformation) the displacement of the eastern, southern and western limits of the Chaînes Subalpines were estimated. A displacement towards the west, associated with a counterclockwise rotation (15–20°) was found for the Miocene—Present basement thrust of the Belledonne massif. The presence of local heterogeneities (such as Jurassic normal faults) deflect the general E-W displacement along dextral NE-SW and N-S strike-slip faults and induce a displacement of the Chaînes Subalpines towards the southwest. The comparison of the restored depth of the Tithonian with its estimated initial depth reveals an excess of elevation of the Chaînes Subalpines with a maximum value (1000–1500 m) for the Diois—Vercors which might be associated with an external basement thrust under the Chaînes Subalpines.
Southwestward thrusting and tectonics of the western Alps
Authors: Fry N.
Published: April 2008

The gross geometry of the western Alps is the result of three processes operating since Eocene times: thrusting to WNW; thrusting to SW; arcuate lithospheric flexure due to loading. In the
external zones
, taken to include the SubBriançonnais domain, these processes are more substantial than earlier phases of Alpine tectonics. In the
internal zones
, these processes rework substantial earlier Alpine structures. A major feature of Alpine thrusting is the preservation of basement topography, which influences the geometry of the thrust system. This is demonstrated by the Digne thrust system W from Remollon and is implicated in both formation and emplacement sequence of the Lower Embrunais-Ubaye nappes, in the Embrunais-Ubaye arc, in the localization of WNW thrust outcrops S of the Pelvoux Massif and perhaps in the southerly derivation of Gres d’Annot sediments. A major (>100 km) phase of SW thrusting affects the Embrunais-Ubaye terrain (continuity of external and SubBriançonnais) before the development of the lower Embrunais-Ubaye nappes and the arcuate deformation of their external floor. All these Embrunais-Ubaye processes are consistent with maintenance of a single 230° transport direction, and with thrusts arising from depth, causing real crustal shortening. Later events with southwestward transport are the emplacement of the combined Parpaillon-Briançonnais thrust stack, the Argentera Massif ‘pop-up’, and the Digne thrust system. Despite circumstantial evidence, there is no substantial contribution to the regional tectonics of the western Alps from strike-slip tectonics, inversion tectonics, gravity tectonics, rotation during thrusting, radial thrusting or arc bending. The dominant features are the maintenance, spanning hundreds of kilometres and tens of millions of years, of the two principal directions of thrust transport (230°; 290°) and the regional continuity of displacements which this implies.