bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

Summary
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

Summary
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

Summary
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

Summary

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

Summary

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

Summary

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

Summary
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

Summary
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

Summary
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

Summary
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

Summary

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

Summary
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

Summary
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

Summary
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.