bluuebunny/crossref_metadata_2025 huggingface dataset:
Tectonic and volcanic events at the Jan Mayen Ridge microcontinent
Authors: Gudlaugsson S. T. , Gunnarsson K. , Sand M. , Skogseid J.
Published: April 2008

Two main tectonic phases were responsible for the formation of the Jan Mayen Ridge microcontinent: (1) the opening of the Norway Basin in late Palaeocene/early Eocene times, and (2) subsequent rifting within the Greenland margin by which complete separation was achieved in early Miocene times. During the first phase the eastern ridge flank developed as a volcanic passive margin. The initial break-up was associated with flexuring and the formation of sequences of eastward-dipping basalt flows, which are considered equivalent to similar features beneath the Vøring and Faeroe-Shetland marginal highs off Norway. Rifting along the Greenland margin during the second phase was accompanied by uplift, listric normal faulting and the formation of large extensional fault blocks. To the W and S of the ridge a flat volcanic marker of probable earliest Miocene age covers the subsided rift and masks the ocean-continent transition. It was formed by a volcanic event of large magnitude, either as submarine lava flows or as a sill complex.
Early Tertiary volcanism at the western Barents Sea margin
Authors: Faleide J. I. , Myhre A. M. , Eldholm O.
Published: April 2008

The western Barents Sea margin is composed of two regional shear segments linked by a NE-trending rifted margin SW of Bjørnøya. The primary marginal structures developed in response to the Cainozoic opening of the Norwegian-Greenland Sea. A marginal high characterizes the rifted margin segment. It is suggested that the high forms an edifice of subaerial extrusives emplaced during the initial opening of the southern Greenland Sea in the early Eocene. The extrusives also cover the adjacent continental crust and the volcanic event may have caused the deposition of tuffs in the basinal province of the southwestern Barents Sea. Renewed local volcanic activity in the early Oligocene is related to the major reorganization in plate motion at this time.
Nd, Sr and Pb isotope geochemistry of the Lower Lavas, E Greenland Tertiary Igneous Province
Authors: Holm P. M.
Published: April 2008

New Sr, Nd and Pb isotopic and trace-element data are presented for the Lower Lavas and some Plateau Basalts. Ranges of isotopic compositions are large among the lower and middle Vandfaldsdalen Formation lavas (87Sr/86Sr=0.7033–0.7073;143Nd/144Nd=0.5124–0.5130;206Pb/204Pb=15.1–17.5;207Pb/204Pb=14.7–15.6;208Pb/204Pb=35.0–39.1) and show a decrease in range of values with stratigraphic height. The youngest flows among the Lower Lavas have compositions similar to the Plateau Basalts of the late Skrænterne and Igtertiva Formations as well as of Nansen Fjord and Wiedemann Fjord, all of which are near to Icelandic composition (87Sr/86Sr=0.7031–0.7036;143Nd/144Nd=0.5129–0.5131;206Pb/204Pb=17.1–18.4;207Pb/204Pb=15.3–15.4;208Pb/204Pb=37.6–38.0). The lavas constitute two groups, one of which (group 2) is restricted to the lower parts of the succession. The source for this group can be modelled by mixing between an enriched old lithospheric mantle and an ocean island basalt (OIB)-type mantle with a relatively small component of depleted asthenosphere. The other group (group 1) is the most common and can be modelled by mixing between a typical Icelandic source component and a second, less enriched, lithospheric mantle component. Contamination from continental crust was of minor overall importance.

The geochemical evolution in basic E Greenland Tertiary lavas is believed to reflect the interaction between the asthenospheric mantle, 3 Ga old continental lithospheric mantle and the mantle jet which is presently positioned under Iceland. With time the most enriched lithospheric source quickly decayed while the less enriched lithospheric source was important during most of Vandfaldsdalen Formation times. The OIB-type source prevailed throughout lava eruption with the asthenospheric contribution larger in the Miki Formation and the Plateau Basalts.
A review of silicic pyroclastic rocks of the British Tertiary Volcanic Province
Authors: Bell B. R. , Emeleus C. H.
Published: April 2008

Silicic airfall and ashflow tuffs constitute a significant component of the extrusive products of the British Tertiary Volcanic Province. These deposits typically formed early in the evolution of individual volcanic centres as proximal facies accumulations intercalated with polylithic pyroclastic breccias, and occur throughout the evolution of the Province. Rhyolites and, less commonly, trachytes occur as lavas in close spatial and temporal association and, together with the silicic pyroclastic rocks, are interpreted as the extrusive equivalents of the felsic and felsic-mafic (mixed-magma) subvolcanic intrusions which are important members of the central intrusive complexes of the Province. The field relations of the silicic tuffs suggest that the associated pyroclastic breccias are also often extrusive and together they form crater infills.
The origins of lacustrine oil source rocks: evidence from the lakes of tropical Africa
Authors: Talbot M. R.
Published: April 2008

New data from core samples are presented on organic matter accumulation during the last 10 000–15 000 years in six tropical African lakes. Pyrolysis and maceral studies indicate that in most of these lakes the preserved organic matter is a mixture of phytoplankton and higher plant remains showing varying degrees of bacterial degradation. Organic assemblages dominated by phytoplankton material have not been encountered. Sediments with the richest oil potential accumulate in meromictic or monomictic lakes during periods when the climate is humid and surface winds are relatively slack. The evidence from tropical Africa does not support current theories that shallow saline lakes may be especially favourable sites for the accumulation of oil prone sediment.
Microbial and biogeochemical processes in Big Soda Lake, Nevada
Authors: Oremland R. S. , Cloern J. E. , Smith R. L. , Culbertson C. W. , Zehr J. , Miller L. , Cole B. , Harvey R. , Sofer Z. , Iversen N. , Klug M. , Des Marais D. J. , Rau G.
Published: April 2008

Meromictic, alkaline lakes represent modern-day analogues of lacustrine source rock depositional environments. In order to further our understanding of how these lakes function in terms of limnological and biogeochemical processes, we have conducted an interdisciplinary study of Big Soda Lake. Annual mixolimnion productivity (ca. 500 g m
−2
) is dominated by a winter diatom bloom (600f annual) caused by upward transport of ammonia to the epilimnion. The remainder of productivity is attributable to chemoautotrophs (30%) and photosynthetic bacteria (10%) present at the oxic-anoxic interface from May to November. Studies of bacterial heterotrophy and particulate fluxes in the water column indicate that about 900f annual productivity is remineralized in the mixolimnion, primarily by fermentative bacteria. However, high rates of sulphate reduction (9–29 mmol m
−2
yr
−1
) occur in the monimolimnion waters, which could remineralize most (if not all) of the primary productivity. This discrepancy has not as yet been fully explained. Low rates of methanogenesis also occur in the monimolimnion waters and sediments. Most of the methane is consumed by anaerobic methane oxidation occurring in the monimolimnion water column. Other bacterial processes occurring in the lake are also discussed. Preliminary studies have been made on the organic geochemistry of the monimolimnion sediments. Carbon-14-dating indicates a lower depositional rate prior to meromixis and a downcore enrichment in
13
C of organic carbon and chlorophyll derivatives. Hydrous pyrolysis experiments indicate that the sediment organic matter is almost entirely derived from the water column with little or no contribution from terrestrial sources. The significance of the organics released by hydrous pyrolysis is discussed.
Palaeo-environment information from deep water siderite (Lake of Laach, West Germany)
Authors: Bahrig B.
Published: April 2008

A comparative study from modern and ancient sediments from the Lake of Laach was performed by means of chemical and stable isotope analyses. In the modern environment siderite is precipitated around CO
2
-spring vents. In contrast, siderite from the Pleistocene/Holocene boundary occurs as small layers in a varved sequence. The stable isotope data provide strong evidence for an analogous formation of both types of siderite. The different CO
2
sources of the siderites allow reconstruction of a distinct rise of productivity at the Pleistocene/Holocene transition. It is proposed that siderite may be developed as a useful tool for tracing both palaeo-environmental and early diagenetic processes.
Spores and pollen in oils as indicators of lacustrine source rocks
Authors: De-xin Jiang
Published: April 2008

More than 200 species of fossil spores and pollen extracted from 68 crude oil samples collected from the lacustrine sedimentary basins of China, including the Talimu Basin, the Zhungeer Basin, the Jiuquan Basin, the Chaidamu Basin, the Eerduosi Basin, the Xialiaohe Depression, etc., have been investigated. The spores and pollen found in the crude oil are interpreted as a kind of ‘trace element’ indicating the pathway of the petroleum migration from the source rock to the reservoir. The principle of why some of the spores and pollen found in the crude oil can act as a kind of indicator of source rock, and the methods used to find and recognize this kind of indicator, are explained in this paper. Based on the investigations of the spores and pollen from the crude oils, the spore and pollen indicators of lacustrine petroleum source rocks of the continental sedimentary basins are proposed.

The spores and pollen acting as the indicators of the lacustrine petroleum source rocks usually indicate a tropical or subtropical wet depositional climate. It is therefore, concluded that a wet and hot climate is favourable for the deposition of lacustrine petroleum source rocks.
Lacustrine petroleum source rocks in the Dinantian Oil Shale Group, Scotland: a review
Authors: Parnell J.
Published: April 2008

The Lower Carboniferous Oil Shale Group includes lacustrine rocks with petroleum source-rock potential. Existing data is reviewed, and new pyrolysis data is presented which shows that the oil shales
sensu stricto
are excellent source rocks with up to 30% TOC and yields of up to 180 mg/g. Other shales and dark limestones in the group also have some source-rock potential. The oil shales contain mixtures of laminar algal matter (lamalginite) and discrete algal bodies (telalginite) which represent planktonic and benthonic algal deposits. The algal bodies are
Botryococcus
-type and referable to
Pila
. There is a general correlation between % alginite and hydrocarbon yield.
Lacustrine sequences in an early Mesozoic rift basin: Culpeper Basin, Virginia, USA
Authors: Gore P. J. W.
Published: April 2008

Lacustrine sequences in the Culpeper Basin, Virginia, USA were deposited in a half-graben dominated by fluvial sheetflood deposition. During periods of increased precipitation and runoff, lakes formed within the basin. Numerous lacustrine sequences, ranging from less than 1 m to more than 35 m thick, document the expansion and contraction of perennial freshwater and saline lakes controlled by Milankovitch-type cyclic climatic change. Many of the lacustrine sequences contain black, laminated shale, deposited in deep, anoxic water in stratified lakes. Most of the lakes were hydrologically closed, as indicated by the presence of evaporite crystal moulds and pseudomorphs in associated mudflat deposits.
Organic geochemical characteristics of major types of terrestrial petroleum source rocks in China
Authors: Jiamo Fu , Guoying Sheng , Dehan Liu
Published: April 2008

Terrestrial source rocks, deposited under non-marine conditions in continental environments, have been divided into five major groups: Group A, sedimentary formations of big lake basins in the interior of the Chinese plate; Group B, lacustrine clastic formations in fault-bounded basins; Group C, salt lake evaporite-clastic formations in fault-bounded basins; Group D, lagoonal-lacustrine volcano-clastic formations of intermontane basins; and Group E, paralic coal-bearing formations of platform areas. The abundance, type and evolutionary characteristics of the organic matter, as well as the biomarker characteristics of various groups of source rocks are discussed briefly. There are several problems relating to evaluation of hydrocarbon prospects in terrestrial sedimentary basins, including the origin of Type I kerogens and their hydrocarbon potentials, immature oils, and coal-bearing formations as potential source rocks.
Geochemical characteristics of crude oils and source beds in different continental facies of four oil-bearing basins, China
Authors: Tieguan Wang , Pu Fan , Swain F. M.
Published: April 2008

In order to correlate Chinese continental crude oils with their sources and, thus, the different depositional facies of the sources, the geochemical characteristics of crude oils and of extracts from related source rocks in four different types of continental facies were studied. The four types of facies were those of (1) inland and paralic freshwater to brackish lake basins, (2) saline lake basins, (3) inland swampy lake and swamp basins, and (4) terrestrial and marine intertonguing facies basins. Oils from these facies show both similarities and differences. Some specific biomarkers, such as the stereoisomers of onocerane, serratane and long chain acyclic polymethylalkanes which could be useful indicators of a palaeoenvironment, are also discussed. All the components were identified using mass spectrometry and a few retention index data: therefore, the identification of some compounds must be tentative.
The lacustrine Condor oil shale sequence
Authors: Hutton A. C.
Published: April 2008

Organic matter in sedimentary rocks is derived from numerous types of organisms that lived in many different environments. Australian Tertiary oil shales, for example the Condor deposit, are termed lamosites and were formed in a lacustrine environment with the bulk of the organic matter derived from freshwater planktonic algae such asPediastrum. The Condor oil shale deposit, located near Proserpine, Queensland, contains four oil shale units and a brown coal unit. The oil shales contain abundant lamalginite (or lamellar alginite), derived from planktonic freshwater algae, with a small quantity of vitrinite derived from terrestrial plants and other minor constituents. The lower oil shale unit contains a dark brown oil shale whereas the upper three oil shale units contain predominantly brown oil shale. The coal unit contains liptinite-rich cannel coals. Reflectance data for vitrinite indicate that the lower oil shale unit and the underlying coal unit are within the oil generation window.
Sand turbidites and organic-rich diatomaceous muds from Lake Malawi, Central Africa
Authors: Crossley R. , Owen B.
Published: April 2008

Seven cores taken from 350 m water depth are described, near the foot of a rift valley boundary fault, in the anoxic hypolimnion of Lake Malawi. The sediments average 29and turbidites, 18% largely undisturbed ‘pelagic’ mud and 53% ‘pelagic’ mud disturbed by slumping and bottom currents. Diatom-rich laminae can be correlated between cores and comparison of the lithostratigraphy with limnological records permits age estimation. Sand turbidite bed thickness/maximum grain size ratios are also examined. Blooms of phytoplankton production appear to have made a major contribution to the organic-carbon content in the undisturbed ‘pelagic’ muds.
Deltaic systems and their contribution to an understanding of basin-fill successions
Authors: Elliott T.
Published: April 2008

Deltaic systems are prominent features of many clastic basin-fill successions throughout the geological record and are also important as sites of fossil fuel reserves. The process-based classification of deltaic systems into fluvial-, wave- and tide-dominated types adopted in recent years remains valid, but requires additional information on, for example, the sediment load of the system, its position in the basin and the extent of synsedimentary deformation in order to be a meaningful characterization of a delta. The range of delta types is further extended by research into ancient deltaic systems which can result in the definition of a non-actualistic delta which differs from modern systems. This is an important trend which should develop in the future as well-constrained ancient systems are characterized to their full extent. A further avenue of future research lies in examining the nature of links of deltaic systems to shelf, slope and submarine-fan systems, with particular reference to depositional analysis of seismic stratigraphic information.
Fjord sedimentation as an analogue for small hydrocarbon-bearing fan deltas
Authors: Syvitski J. P. M. , Farrow G. E.
Published: April 2008

Fjord basins have sediment accumulating at rates between 0.1 and more than 1000 mm a
−1
with sea-level fluctuations up to 50 mm a
−1
, lithologies varying from poorly sorted conglomerates to unimodal sands and muds, carbon contents from less than 1.0% to more than 10% for methane-producing sediments found in some anoxic basins, and variably consolidated and sensitive sediment. Four major mechanisms affect the rate and style of basin infilling: (i) hemipelagic sedimentation of particles carried seaward by river plumes; (ii) delta-front progradation, particularly as affected by bed-load deposition at river mouths; (iii) proximal slope bypassing, primarily by turbidity currents and cohesionless debris flows; (iv) down-slope diffusive processes, mainly creep and slides, that work to smear previously deposited sediment into deep water.

Fjords, with their rugged topography and dynamic history of basin infilling, are prone to slope failures. Release mechanisms include sediment loading, ground motion from earthquakes, giant waves, sea-level fluctuations and changes in the mass physical properties of basin sediments. Slope failures occur on a semicontinuous basis and with displaced volumes of 10
3
–10
6
m
3
, or with much less frequency (1000 year intervals) but affecting larger volumes of sediment (10
6
–10
9
m
3
). Although slope failures are common to the delta front, they may occur anywhere along the prograding prodelta surface, along the basin margins and over basement highs.

Sediment accumulations in fjords have many similarities to small hydrocarbon-bearing basins, such as those of the North Sea, that once received submarine fans, often experienced anoxic conditions and contained high rates of sedimentation. Fjords typically contain more sand and conglomerate than other commonly employed modern analogue environments such as the Indus, Mississippi or Amazon fans.

Two stratigraphic plays are suggested on the basis of fjord-infilling processes. The first involves the semicontinuous failure of foreset sands where, through progradation, gravity flow sands form an areally extensive reservoir (up to 10 km wide, 100 m thick and 100 km long) sandwiched between basinal muds and prodelta muds. The second play involves the catastrophic failure of a major portion of the prodelta where reservoir-quality sands (from either the basin-head delta or marginal delta-beach complexes) infill a slide-formed megachannel (up to 500 m wide, 50 m thick and tens of kilometres long). These megachannel sands would be underlain by faulted and disturbed mud blocks, and capped by conformable hemipelagic muds.
Diagenesis of Upper Carboniferous sandstones: southern North Sea Basin
Authors: Cowan G.
Published: April 2008

Upper Carboniferous sediments from the central part of the southern North Sea Basin were deposited on a broad fluvio-deltaic plain. Potential reservoir rocks occur within distributary channel facies sandstones, but porosity and permeability have been reduced by authigenic mineral growth and improved by at least two separate phases of porosity enhancement: firstly by the infiltration of meteoric waters during post-Carboniferous uplift, and secondly by the action of acidic fluids generated during burial. The depth of invasion of meteoric fluids is highly variable, ranging from a few metres to hundreds of metres, and the best porosity is preserved in sediments which have undergone both phases of porosity enhancement. Virtually all the observed porosity in Carboniferous sandstones can be interpreted as having a secondary origin. Integrating the deduced diagenetic history with a burial and temperature history for specific wells allows the timing of diagenetic events to be estimated.
Detailed sedimentology and gamma-ray log characteristics of a Namurian deltaic succession I: Sedimentology and facies analysis
Authors: Bristow C. S. , Myers K. J.
Published: April 2008

The aim of this study was to investigate the relationship between sedimentary facies and the gamma-ray log characteristics of the Upper Namurian Rough Rock exposed in a road section at Elland, Yorkshire. The overall succession coarsens upwards and is divided into five facies which record a prograding deltaic system. Coarse sands of a major braided fluvial distributary (Facies 1) rest upon channelled coarse sands of minor distributaries or crevasse channels (Facies 2) which overlie mouth-bar deposits (Facies 3) and more distal deltaic deposits (Facies 4). Abandonment deposits (Facies 5) occur above and between the channellized sands and the mouth-bar sands. Thus the delta front featured a series of small distributaries characterized by periodic switching in discharge and sediment transport. The shoal-water delta may have been similar to the Lafourche Delta on the Mississippi.
Arcuate and bird’s foot deltas in the late Pleistocene Palaeo-Tokyo Bay
Authors: Okazaki H. , Masuda F.
Published: April 2008

Two types of prograding deltaic sequences are recognized in the upper part of the late Pleistocene Shimosa Group, which crops out in southern Ibaraki Prefecture and northern Chiba Prefecture, eastern Kanto, Japan. The older sequence is interpreted as a wave- and tide-dominated arcuate delta and the younger sequence as a fluvial-dominated bird’s foot delta. The older sequence is characterized by thick molluscan shell beds (
c.
10 m) in the middle part. In contrast, the younger sequence is characterized by the rhythmic alternation of fine-grained sand and mud with ripples in the lower part and coarse-grained sandy deposits with trough cross-stratification in the upper part. Both sequences show distinct coarsening-upward sequences. The difference between these two types of delta was caused by the water depth in the delta front during the lowering of the sea level from the interglacial to the glacial period during the last stage of Palaeo-Tokyo Bay.
A fluvial-dominated lacustrine delta in a volcanic province, W Greenland
Authors: Pedersen G. K.
Published: April 2008

A 50 m thick sequence of dark grey to black non-marine shales are exposed in a 12 km section along the NE coast of Disko in W Greenland. The shales are sandwiched between sandy fluvial deposits, and the sedimentological logs demonstrate a marked lateral continuity of facies within the shale unit. The shale is fissile and micaceous, and contains locally comminuted plant debris. The average total organic carbon (TOC) content is 6%, the organic matter corresponds to type III kerogen and the dominant clay mineral is kaolinite. The shale unit constitutes one coarsening-upward sequence interpreted as lacustrine prodelta mud, with thin sheets of sand deposited from density underflows which pass transitionally up into cross-laminated mouth-bar sand finally overlain by cross-bedded sand deposited in fluvial channels. This sequence represents progradation of a fluvial-dominated delta into a lake partially bounded by subaqueous hyaloclastic breccias.
Review and computer modelling of the Brent Group stratigraphy
Authors: Helland-Hansen W. , Steel R. , Nakayama K. , Kendall C. G. St. C.
Published: April 2008

The depositional history of the Brent Delta system can be analysed on three different levels: (i) the individual formations and their facies distribution, used for the interpretation of the depositional processes; (ii) the vertical succession of formations in each well, indicating the temporal changes in environment; (iii) the two- or three-dimensional array of sequences illustrating both the temporal and spatial variations within the delta system. The Brent Group is essentially one regressive-transgressive megacycle. The regressive part comprises the Rannoch and Etive Formations (delta front) and the lower Ness Formation (delta plain), and the transgressive system contains the Tarbert Formation (delta front) and the (upper) Ness Formation (delta plain). With the aid of a computer program, the succession geometry can be analysed in terms of the relative importance of sediment supply, eustasy and tectonics. The change from regressive to transgressive conditions was probably initiated by an increase in subsidence rates, whereas regressive pulses in the overall transgressive Tarbert Formation can be explained in part by eustatic sea-level falls.
Magmatic processes at and near constructive plate margins as deduced from the Troodos (Cyprus) and Semail Nappe (N Oman) ophiolites
Authors: Gass I. G.
Published: April 2008

The Troodos and Semail Nappe ophiolites, both produced during Upper Cretaceous (Cenomanian) times at Neotethyan supra-subduction zone constructive margins, are sufficiently comparable to allow a generalized model of constructive-margin magmatic processes to be presented. Such processes can be traced from (i) the partial melting of lherzolitic mantle within rising asthenospheric diapirs and the formation and intramantle fractionation of picritic magmas through (ii) the feeding of these melt batches into high-level, open-system crustal magma chambers to produce phase-layered mafic and ultramafic cumulate assemblages to (iii) the upward injection of melts to form sheeted dyke complexes and eruptive sequences. Evidence suggests that there is a series of mantle diapirs, each limited in time and space, beneath a constructive margin, each feeding its own magma chamber, sheeted dyke complex and volcanic sequence. Thus the oceanic lithosphere is seemingly the product of numerous diapiric ‘point’ sources embellished by a variety of off-axis magmatic products. The spreading axes are interrupted by transform fault zones which, during extensional phases, are the sites for magmatic products that are geochemically different from those produced at the axes.
Upper mantle segregation processes: evidence from alpine-type peridotites
Authors: Spray John G.
Published: April 2008

Studies of alpine-type peridotites indicate that the upper mantle is pervaded by various segregations whose origin cannot be exclusively attributed to the effects of crustal emplacement. The segregations typically occur as veins and layers (less than 1 m thick). They are common in the lherzolitic varieties of alpine-type peridotite but are of more restricted occurrence in true ophiolites. The segregations are represented by predominantly metamorphic mineral assemblages which span the garnet, spinel and plagioclase peridotite stability fields. Low-pressure segregations may remain igneous. Many segregations are varieties of aluminous clinopyroxenite with or without garnet, spinel or plagioclase. Segregations possess variable bulk chemistries and mineralogies so a number of different origins for them must be considered. The available evidence indicates that they are (i) metamorphosed trapped partial melts, melt residua or conduit platings, (ii) recycled and streaked-out ocean crust (via subduction zones), (iii) solid-state metamorphic differentiates or (iv) products of mantle metasomatism. The formation of segregations renders the mantle heterogeneous on a macroscopic scale, but this is combated in time by diffusive processes and segregation recycling at spreading centres, both of which contribute to its rehomogenization.
Pressure of origin of primary mid-ocean ridge basalts
Authors: Elthon Don
Published: April 2008

High-pressure phase equilibrium studies of mid-ocean ridge basalts (MORBs) have shown that some samples are in equilibrium with olivine + orthopyroxene (OPX) + clinopyroxene ± plagioclase at about 10 kbar, whereas others are not in equilibrium with OPX at this pressure. Samples that are OPX-saturated at about 10 kbar are characterized by higher SiO
2
(>49.7 wt%) than most (about >75%) MORB glasses with more than 9.5 wt% MgO, suggesting that only about 25% of these glasses could be derived from primary magmas generated at about 10 kbars. Orthopyroxenes in abyssal peridotites are characterized by Al
2
O
3
contents that range from 2.0 to 6.0 wt% (probably about 2.5–6.5 wt% before subsolidus equilibration), but orthopyroxenes in peridotite partial-melting experiments and liquidus orthopyroxenes for MORBs at about 10 kbar have only 3.5–4.3 wt% Al
2
O
3
. Liquidus orthopyroxenes for high-MgO basalts at 25 kbar and 20 kbar have about 6.6 wt% and 5.5 wt% Al
2
O
3
respectively. It appears that the least depleted (Al
2
O
3
-rich end) of abyssal peridotite orthopyroxenes indicate the dominance of melting and primary MORB genesis at pressures substantially greater than 10 kbar, and probably at 20–25 kbar.
Geochemical features of intraplate oceanic plateau basalts
Authors: Floyd P. A.
Published: April 2008

Coeval with the Cretaceous thermal uplift of a large region of the ‘old Pacific’ oceanic lithosphere was the widespread development of two expressions of intraplate volcanism: edifice building (ocean islands, seamounts) and submarine sheet-flows (oceanic plateaux). The geochemical features of the development of oceanic plateaux are reviewed using the Nauru Basin complex as a model. Ocean plateau basalts (OPBs) are mildly altered, poorly phyric,
hy
- or
qz
-normative tholeiites, rapidly extruded submarine sheet-flow lavas with a relatively uniform chemistry. Although their eruptive setting is apparently intraplate they have a chondrite-normalized incompatible-element-depleted pattern similar to normal-type mid-ocean ridge basalt (MORB) formed at ocean spreading centres. They are chemically distinguished from normal-type MORB by being enriched in selected incompatible elements and can be matched by various enriched or ‘transitional’ MORB types. Isotopically they have similarities to some types of ocean island basalt (OIB), such as Hawaii. They were probably derived by high degrees of melting from an upper mantle MORB-type source with an enriched OIB-type component; melts being mixtures derived from the two mantle components. In terms of crustal development they are characteristic of anomalously thickened oceanic crust and were formed in an off-axis (or near-axis) eruptive setting shortly after the development of axis-generated oceanic basement over which they flowed. OPB appears to be restricted to the widespread Cretaceous magmatic event.
A combined chemical and Pb-Sr-Nd isotope study of the Azores and Cape Verde hot-spots: the geodynamic implications
Authors: Davies G. R. , Cliff R. A. , Norry M. J. , Gerlach D. C.
Published: April 2008

The mafic lavas of the Azores and Cape Verde Islands are of a highly varied composition, reflecting a complex history of magma genesis and a variety of source compositions. The lavas of the Cape Verde Islands are characteristically highly silica undersaturated, with alkali-rich ankaramites, larnite-normative melilitites and carbonatites. In contrast, the lavas of the Azores vary from strongly nepheline- to hypersthene-normative types. Isotopic ratios and trace elements also show considerable variation, consistent with derivation from multicomponent mantle sources. Three distinct groupings of lava compositions can be seen in Pb-Sr-Nd isotope space, and each is characterized by its own trace-element enrichment. (i) The majority of the lavas from the Azores and northern Cape Verdes Islands have isotope and trace-element systematics similar to basalts recovered from elevated segments of the mid-Atlantic Ridge (
eg
during Deep Sea Drilling Project Leg 82) (Ba/La <12,
206
Pb/
204
Pb=19.3–20.0). (ii) The islands of Sao Miguel and Faial in the Azores are characterized by radiogenic strontium and lead isotope ratios and marked fractionation of the most incompatible trace elements (
eg
87
Sr/
86
Sr=0.70522,
206
Pb/
204
Pb=19.88,
207
Pb/
204
Pb=15.75, Ba/Nb=12, La/Nb=0.84). (iii) In contrast, the southern Cape Verde Islands have relatively unradiogenic Pb-Sr-Nd isotope ratios (
eg
87
Sr/
86
Sr=0.70393,
206
Pb/
204
Pb=18.74,
207
Pb/
204
Pb=15.53, Ba/La=22, Ba/Nb=15, La/Nb=0.67).

Mixing relationships between isotope and trace-element ratios indicate the involvement of at least four, and possibly five, chemically distinct source regions in the petrogenesis of mafic lavas. It is shown that these different source regions represent combinations of the depleted upper mantle (mid-ocean ridge basalt source), recycled oceanic lithosphere and two components of the subcontinental lithospheric mantle. No direct unequivocal evidence is found for a fifth component, namely undepleted primitive mantle reservoir, although helium isotope data suggest influxes of material and heat from the lower mantle. Similarly, it is suggested that continental crust has no direct contribution to the petrogenesis of these ocean island basalts and its role in the mantle is limited to the production/modification of the subcontinental lithosphere above subduction zones.
Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes
Authors: Sun S.-s. , McDonough W. F.
Published: April 2008

Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs.

In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (⩽1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (⩽2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (
eg
St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by the recycling of the enriched oceanic lithosphere back into the mantle.
Analogue models of inversion tectonics
Authors: McClay K. R.
Published: April 2008

Inversion of dip-slip fault systems by horizontal recompression has been modelled using sand, sand-mica and sand-clay analogues. Three extension-inversion systems have been tested: uniform basal detachment, simple listric detachment and ramp/flat detachments. Extension above a uniform basal detachment produces a domino or conjugate array of near planar faults. Upon inversion only faults at angles < 60° to the layering and suitably oriented with respect to the compression direction undergo reactivation. With increased contractional strains all faults are rotated to high angles and reactivation ceases. Shortcut faults may develop in the footwall of steeply dipping faults. Extension above a simple listric detachment fault produces a characteristic rollover anticline and associated crestal collapse graben structure. Upon inversion asymmetric uplift is associated with the major detachment. For models with no competency contrast the rollover anticline is accentuated. The crestal collapse graben faults are reactivated and the graben tightens producing a ‘flower-like’ fault system. Reactivation of synthetic faults ceases when they become rotated into angles too steep for contractional deformation. Extension of ramp/flat detachment systems produces characteristic rollover anticlines and crestal collapse graben systems above each concave-up segment of the listric fault system. It also produces a hanging wall syncline associated with the convex-up segment of the detachment. Inversion results in asymmetric uplift at the emergence of the reactivated detachment and above the convex-up portion of the detachment surface. The crestal collapse graben faults become reactivated and rotated into steep angles resulting in flower-like geometries. The rollover anticline is accentuated resulting in nappe-like structures. The results of these analogue studies of inversion structures indicate that considerable further research is needed to understand the geometries and kinematics of inversion fault systems.
Basin inversion in and around the British Isles
Authors: Roberts D. G.
Published: April 2008

The development of basins of Mesozoic and Tertiary age both onshore and offshore the British Isles is largely a consequence of episodic rifting that led to the progressive northward opening of the North Atlantic. In these basins, extension was controlled by the structure of underlying Palaeozoic terrains and succeeded by passive thermal subsidence.

In this context, however, the British Isles represent an anomaly both in terms of their present elevation and in the widespread exposure of Pre-Cambrian to Tertiary rocks. The outcrop geology of the onshore and offshore basins is an obvious demonstration of uplift although the timing and origin and magnitude of the uplift have remained uncertain. Evidence from onshore and offshore the British Isles is used to document the influence of compressional inversion of Tertiary age in shaping the present day outcrop geology.
Inversion, reactivated faults and related structures: seismic examples from the southern North Sea
Authors: Badley M. E. , Price J. D. , Backshall L. C.
Published: April 2008

This study examines the geometry and timing of inversion features in offshore Quadrants 53/54 of the UKCS. The area studied lies south of the Zechstein salt basin and is dominated by two major and temporally-persistent structural elements, the Wales-Brabant Massif (in the SW of the area) and the South Hewett Fault (in the centre of the area). The Wales-Brabant Massif is a stable basement block, flanked to the north by an inverted Triassic basin. This Triassic basin in turn forms the footwall block to the South Hewett Fault. NE, and locally SW, of the South Hewett Fault lies a series of Jurassic tilted fault blocks symmetrically disposed about a central ‘keystone’. These central blocks contain no synextension sediment fill and are interpreted as outer-arc extensional structures parasitic on a large wavelength uplift produced by an underlying thermal anomaly. All major structural features lying to the NE of the Wales-Brabant Massif were inverted during the late Cretaceous. This inversion produced forced folds in the cover rocks above deeper basement faults, and harpoon-shaped structures at the sites of earlier rotational half-grabens. This account considers the effects of both this late Cretaceous inversion and a succeeding period of inversion during the Miocene on earlier extensional structures.
Basin inversion in the Osen-Røa thrust sheet, Southern Norway
Authors: Morley C. K.
Published: April 2008

The Osen-Røa thrust sheet is the most external tectonic unit in the southern Norwegian Caledonides. This extensive thrust sheet may have originally covered over 100 000km
2
. It is largely composed of Late Precambrian and Lower Palaeozoic sedimentary rocks. Thickness variations in the Late Precambrian sedimentary package can be attributed to the passage from a rift basin about 3 km thick (the Hedmark Basin) into rift margin sediments a few hundred metres thick (Kumpulainen & Nystuen 1985). Sedimentary facies distribution in the Hedmark Basin indicates that frontal and lateral ramps in the sole thrust followed the rift margins. As the basin was thrust onto a footwall flat, inversion of the basin occurred and produced a large culmination. The short wavelength of the basin (approximately 70 km) indicates, that little downwarping occurred beneath the displaced Hedmark basin, if an elastic lithosphere is assumed. Hence, the culmination probably remained a positive feature and would have bowed up the overlying thrust sheets.
Tertiary kinematics in the Lepontine dome
Authors: Merle O. , Cobbold P. R. , Schmid S.
Published: April 2008

In the Pennine zone of the central (Lepontine) Alps, the mineral-zone boundaries of Tertiary metamorphism and the regional attitudes of foliation and thrust sheets define a dome. We investigate the Tertiary kinematic history of the dome using structural, metamorphic and geochronological data. Thus we distinguish two main ductile deformations, that occurred under differing metamorphic conditions. The first high-temperature deformation (HTD) developed under amphibolite facies conditions. It is responsible for (1) a radiating pattern of stretching lineation trends at the scale of the Lepontine area, and (2) structures indicating overthrusting towards the European foreland in the lineation direction (i.e. N and NW). The HTD event is attributable to NW-SE Tertiary convergence between Europe and the Adriatic sub-plate. The second retrograde deformation (RD) is responsible for differential uplift of the Pennine zone, with formation of an earlier subdome to the E (Ticino subdome, 25-20 Ma) and a later one to the W (Simplon subdome, 15-10 Ma).

In the Ticino subdome, kinematic indicators reveal a horizontal shearing top W. Some major structures such as the Maggia steep zone and the vertical duplication of the Antigorio basement nappe (Bosco-Gurin area) may be related to this westward phase. Dextral transcurrent shearing along the southern steep belt and backthrusting of the central Alps over the southern Alps along the N dipping Insubric mylonite belt are probably coeval with the retrograde deformation recorded in the Ticino subdome. In the Simplon subdome, kinematic indicators reveal a radial motion parallel to the lineation, which plunges towards the SSW in the Verampio window and towards the WSW at the Simplon Line.

Serial uplift of Ticino and Simplon subdomes is attributed to continuing dextral transpression at the boundary between Europe and the Adriatic sub-plate.

Finally, the back-folding event located at the northern limit of the Pennine zone postdates both HTD and RD deformational events described above. It is part of a later pop-up responsible for uplifting the external crystalline massifs.
Thrusting and backthrusting in the Briançonnais domain of the western Alps
Authors: Platt J. P. , Cunningham P. C. , Weston P. , Lister G. S. , Peel F. , Baudin T. , Dondey H.
Published: April 2008

The Briançonnais domain comprises the deformed remnants of a semi-independent microcontinental strip that lay close to the European margin of the Mesozoic Tethys. A Triassic carbonate platform of south-Alpine type was broken up by extension in Early to Middle Jurassic time, producing a syn-rift sequence characterized by small basins with marginal breccia wedges, internal unconformities, and local terrestrial sediments. The pre-Triassic basement was probably reduced to about half its original thickness during this extensional phase. Late Jurassic and Cretaceous subsidence was accompanied by pelagic sedimentation. Flysch sedimentation began shortly before the onset of compressional tectonics in late Palaeocene or Eocene time.

Underthrusting and accretion of the Briançonnais domain to the developing Alpine thrust wedge caused an early phase of thrusting, producing antiformal stacks of kilometrescale thrust sheets in parts of the external Briançonnais. The internal margin of the domain was underplated at depths of up to 40 km, and was affected by high-P low-T metamorphism and regional ductile deformation. Stretching lineations and slip-vectors related to this stage trend NW in the northern French Alps, but swing to WSW in the S.

Backthrusting in Oligocene time was probably associated with the start of collision with the European margin proper. Families of back thrusts and associated recumbent folds cut, deform or reactivate older structures. The members of each family appear to have been activated in sequence towards the internal margin of the Alps, forming large-scale duplexes, the floor and roof thrusts of which are in part reactivated early thrusts.

The trend of stretching lineations and slip-vectors in the Briançonnais domain vary in both space (NW in the N, SW in the S), and time (later trends lie consistently anticlockwise of early ones). These trends and their changes are incompatible with calculated Africa/Europe plate motions for the Tertiary. Independent NW to WNW motion of Adria relative to Europe is probably required. Anticlockwise rotation and bending of the internal Alps during collision could explain the radial pattern of the early vectors. The radial pattern of the later backthrusting matches the radial pattern of coeval forethrusting in the external zones, however; this is therefore unlikely to be a result of later rotations, and may reflect the dominance of gravitational body forces during the later stages of mountain-building.
The role of the Periadriatic Line in the tectonic evolution of the Alps
Authors: Schmid S. M. , Aebli H. R. , Heller F. , Zingg A.
Published: April 2008

The Periadriatic Line and related lineaments formed as a result of post-collisional deformations which severely modified the Alpine chain. This post-late Oligocene deformation is the result of dextral transpression between the Adriatic sub-plate and the European foreland. Indentation of the western edge of the southern Alps caused uplift, related to backthrusting and associated deformations of the Lepontine region combined with E-directed escape of the central Alps. In the eastern Alps the response to dextral transpression is mainly by lateral escape along conjugate strike slip zones with minor or no vertical movements. Older deformations along this essentially late Alpine lineament can still be inferred locally and include: extension and transfer faulting in the late Palaeozoic to early Mesozoic, Cretaceous deformations, and Tertiary phases of compression (Eocene) and possibly extension (Oligocene). The geometry of crustal thinning associated with the formation of the passive continental margin of the southern Alps (associated with initial uplift of the Ivrea zone) has a profound influence on strain localization and the kinematics of movements along and north of the present day Periadriatic Line.