bluuebunny/crossref_metadata_2025_split huggingface dataset:
Pattern of mantle convection and Pangaea break-up, as revealed by the evolution of the African plate
Authors: PAVONI N. 
Published: December 2007

A tensional stress regime has governed the tectonic evolution of most of the African continent. The long history of crustal extension is documented by the widespread occurrence of continued intraplate rifting and volcanism since Triassic times and by the large-scale plate movements following the break-up of Pangaea. A remarkable result of the present investigation is the recognition that the extension displays a continent-wide and plate-wide radial pattern, centred in equatorial Africa near 10°E/0°N, the African Centre A. The radial pattern is evident from the diverging block movements, documented by the intra-continental rifting in Africa, by the break-up of Pangaea and the subsequent, diverging drift of North America and the Gondwana fragments away from Africa, entailing a radial growth of the African plate. All these phenomena are manifestations of what may be referred to as the ‘African lithospheric divergence’.
The African plate is surrounded, to 85%, by active oceanic ridges. Ridge-push forces exerted on the African plate have to be balanced by some other forces in order to explain the tensional stress regime which, even at present, dominates large parts of the African continent . Shear traction exerted by the convecting asthenosphere on the base of the African lithosphere offers an explanation for the observed phenomena. It is proposed that relatively warm and less dense mantle material rises from the deep mantle below the African plate. In the upper mantle and at the base of the African lithosphere, the ascending material diverges and flows radially away from the centre of ascent. Upper mantle flow below the sea-floor spreading axes is unidirectional and horizontal and is directed away from the African centre A.
It is proposed that the ascending flow beneath Africa forms part of a very large, bicellular circulation system in the Earth's mantle. The origin of geotectonic cycles is probably interrelated with the onset, build-up, main phase and decay of such large-scale convection systems.
Structure of the late Proterozoic Nakasib suture, Sudan
Authors: ABDELSALAM M. G. , STERN R. J. 
Published: December 2007

The Nakasib suture is a prominent structural belt in the central Red Sea Hills, Sudan. It is one of the ophiolite-decorated sutures along which the late Proterozoic (Pan-African) island arc/back arc terranes and continental micro-plates of the Arabian–Nubian Shield were welded together. It juxtaposes the 900–800 Ma Haya terrane in the south with the 830–700 Ma Gebeit terrane to the north. New structural data from the Nakasib suture show that the suture had evolved through three phases of deformation which gave rise to a fold and thrust belt. During the early two phases of deformation, SE-verging tight folds and thrusts were developed. The third phase of deformation refolded the earlier structures into NE-trending, upright, and horizontal antiforms and synforms and culminated in the development of NW-verging thrusts. The structural data are used, together with previous stratigraphic and geochemical data, to suggest a tectonic model for the evolution of the Nakasib suture. The suture followed a Wilson Cycle. This started with rifting of the Haya terrane, extrusion of rift volcanics and deposition of passive margin sediments, and development of a NW-dipping subduction zone over which the arc volcanics of the Gebeit terrane erupted. The closing of the oceanic basin gave rise to the Nakasib suture with the present structural configuration.
The structural styles of other sutures of the Arabian-Nubian Shield are examined in light of structural data from the Nakasib suture. These sutures show a common feature of steepening of the early subhorizontal ophiolite-obduction structures during a late event of upright folding which may be related to collision between terranes.
Allochthonous oceanic basalts within the Mesozoic accretionary complex of Alexander Island, Antarctica: remnants of proto-Pacific oceanic crust
Authors: DOUBLEDAY P. A. , LEAT P. T. , ALABASTER T. , NELL P. A. R. , TRANTER T. H. 
Published: December 2007

The Mesozoic LeMay Group accretionary complex of Alexander Island, Antarctica, contains thrust-bound slices of accreted ocean floor, ocean islands and seamounts. They represent fragments of proto-Pacific oceanic crust, of which only a tiny remnant (the Phoenix plate) remains off northern Antarctic Peninsula. They therefore provide an excellent opportunity to sample the ancient oceanic crust that formerly occupied the southern Pacific Ocean.
All the basalts experienced sea-floor and subduction/accretion metamorphism ranging from zeolite to transitional blueschist facies. On the basis of rare–arth and other immobile trace element characteristics, the basalts are divided into depleted MORB, N-MORB, E-MORB, and tholeiitic and alkaline OIB.
Oceanic basalts occur within two rock associations on Alexander Island, basalt-volcaniclastite-chert and basalt-volcaniclastite-tuff. The basalt-volcaniclastite-chert rock association is dominated by pillow lavas which have light REE-depleted N-MORB geochemical characteristics, and is interpreted as representing ocean floor formed at spreading centres. Locally, sills of tholeiitic OIB intrude the sequence. The basalt-volcaniclastite-tuff rock association exposed in the Lully Foothills was formed in shallow water during the Early Jurassic. It is geochemically varied, consisting of basalts with N-MORB, E-MORB and tholeiitic OIB characteristics. The association is interpreted to have been formed on a large seamount or ocean island.
A pre-D2 age for the 590 Ma Ben Vuirich Granite in the Dalradian of Scotland
Authors: TANNER P. W. G. , LESLIE A. G. 
Published: December 2007

Regional mapping demonstrates that S2 is the main fabric in the Central Highland Dalradian and that it is this fabric, not S3 as previously thought, which overprints the 590 Ma Ben Vuirich granite. The granite appears to have been emplaced following a tectonic event (D1) in the Dalradian rocks, for a finely-spaced penetrative fabric seen in newly-discovered hornfelses from the granite aureole is overgrown by contact metamorphic cordierite and andalusite porphyroblasts (now both altered) which predate the D2 metamorphism.
The Ben Vuirich granite cuts the Neoproterozoic Dalradian sequence of the Central Highlands of Scotland (Fig.1,

inset), and was first recognized as an
older foliated granite
by Barrow
et al.
1905. Later workers (

Bradbury
et al.
1976

;
Bradbury 1978
) concluded that the granite was intruded between D2 and D3 in the Dalradian rocks (

Harris
et al
. 1976

) and for many years an U-Pb zircon age of 514 ±7 Ma from the intrusion (Pankhurst & Pidgeon 1976) provided a valuable datum point in the evolution of the Scottish Caledonides. However, Dalradian chronology has recently undergone a radical revision following the determination of a precise U-Pb age of 590 ± 2 Ma for the emplacement of the Ben Vuirich granite (

Rogers
et al.
1989

). In accepting the interpretation of Bradbury
et al.
1976 that the regional penetrative fabric which affects the country rocks (and is continuous with that in the granite) is of D3 age, Rogers
et al.
were also guided by the conclusions of Harte
et al.
1984 and Dempster & Harte
Geochemical evolution of an Ordovician island arc, South Mayo, Ireland
Authors: CLIFT P. D. , RYAN P. D. 
Published: December 2007

Early Ordovician volcanic rocks exposed in the South Mayo region of western Ireland document the history of a volcanic arc complex, produced following the initiation of south-dipping subduction within the Iapetus Ocean in the Late Cambrian or Early Ordovician. Lavas of the Lough Nafooey Group (Tremadoc-Arenig) show an eruption history marked by an initial period of tholeiitic basaltic volcanism, followed by a shift to increasing silica compositions of calc-alkaline affinity. Subsequent volcanism of the Tourmakeady Group (Arenig) shows a mixture of rhyolitic and andesitic compositions, which are in turn succeeded by tuffs of the Rosroe and Mweelrea Formations of the Murrisk Group (Llanvirn). These are also rhyolitic and andesitic but are generally more siliceous. The overall increase in silica contents may reflect increasing fractional crystallization due to crustal thickening.
Trace element studies of the basaltic members of the Lough Nafooey Group show a strong subduction zone influence. While absolute levels of immobile trace and rare earth elements decrease upsection, relative depletion of the most incompatible elements decreases over the same time period. These trends suggest decreasing degrees of partial melting of a progressively more depleted source mantle upsection.
The basaltic Bohaun Volcanic Formation (Tremadoc-Arenig ?) shows greater depletion in its incompatible trace elements than the Lough Nafooey Group, so indicating a more depleted mantle source. Comparison with modern arc systems suggests that the Bohaun Volcanic Formation was probably generated shortly after the initiation of subduction and was erupted in a more trenchward position than the Lough Nafooey Group (i.e. in the forearc). This confirms a south-dipping polarity to the subduction zone, and suggests an age of initiation during the Late Cambrian or earliest Ordovician.
Pelagic fluxes and sedimentation
Authors: CRONAN D. S. , WAKEFIELD S. J. 
Published: December 2007

In February 1992 there was a two day meeting of the Marine Studies Group dealing with pelagic fluxes and sedimentation. Day 1 covered several topics while Day 2 focused on BOFS (Biogeochemical Ocean Flux Study). The BOFS work is being published elsewhere and so the present thematic set and the meeting summary below concentrates on the topics covered during Day 1.
The floor of the ocean is the site of deposition for particles from any source that are transported down through the water column. The sediments record both about past activities in the overlying waters and about past inputs to those waters. Present-day oceanographic processes are analogues for the past, but the data must be interpreted correctly. To do that some idea is needed of what happens to particles on their way down, and what modifications to the record take place upon sedimentation.

Amongst the main themes of the meeting were the variability of the fluxes, and the coupling of the surface waters with the bottom sediments. Although the story that can be gained from isotopes provides extra details, there is still a great deal of information to be gleaned from detailed examination of the trace-metal content of particulates.
R. Chester
showed how the trace-metal composition of particulate material undergoes modification during its journey from the sea surface to the sea-floor. What is essentially a ‘fingerprint’ of solid-state speciation can be determined by looking at which carrier phase contains which trace element. It was shown that organically bound copper in
Subsidence analysis of Eastern Avalonian sequences: implications for Iapetus closure
Authors: KING L. M. 
Published: December 2007

Basin subsidence analysis, employing the backstripping method, indicates that fundamentally different basin-generating mechanisms controlled Silurian basin development in northern England and in Wales. Accelerating subsidence with time suggests that the dominant control on basin formation in northern England was flexure of the Eastern Avalonian lithosphere. Mid-Wales, however, displays dominantly decelerating subsidence, suggesting an extensional control. The errors involved with the backstripping method, particularly concerning Lower Palaeozoic application, have been addressed.
In conjunction with sedimentological and tectonic data, the subsidence patterns for the Windermere Supergroup indicate deposition in a flexurally-loaded foreland basin which migrated southwards with time. Similar data for the Powys Supergroup of Mid-Wales imply an extensional control on Silurian deposition, possibly in a strike-slip setting. However, later Pøídolí and early Devonian Welsh sedimentation may represent the onset of foreland basin deposition. The new data are consistent with a diachronous closure of the Iapetus ocean, related to rotational collision of Eastern Avalonia with Laurentia during the early Silurian. Early Silurian oblique collision produced transtension in Wales. Subsequent rotation of Eastern Avalonia produced more orthogonal collision. Thus, continental flexure was felt initially in northern England in late Wenlock times, followed by migration of the resultant foreland basin to the southeast into Wales in early Devonian times.
The versatility of the backstripping method is shown by this application to Lower Palaeozoic sequences.
Structural synthesis of S Staffordshire, UK: implications for the Variscan evolution of the Pennine Basin
Authors: WATERS C. N. , GLOVER B. W. , POWELL J. H. 
Published: December 2007

The presence of strata of Westphalian A to possibly Stephanian age in south Staffordshire, located at the southern margin of the Pennine Basin, provides a rare control on the timing of late Carboniferous deformation in Central England. During Westphalian A—C times, sedimentation in south Staffordshire was controlled by thermal subsidence, with a superimposed influence of synsedimentary normal and reverse faulting. Alkaline basic igneous activity occurred during Westphalian C times, synchronous with uplift of the southern margin of the Pennine Basin. Uplift culminated, during Westphalian C—D times, in a phase of Variscan inversion along basement lineaments and the development of a regional unconformity. Renewed subsidence and deposition of a relatively thick sedimentary sequence of Westphalian D to possibly Stephanian age was followed by a final phase of Variscan deformation, comprising possibly two pulses of uplift, tilting and erosion.
Observations presented for south Staffordshire, in conjunction with studies from other parts of the Pennine Basin, indicate a complex interrelationship between rifting, thermal subsidence and Variscan basin inversion. Pulses of N—S extension and E—W to NW—SE compression are interpreted as the product of regional strike-slip deformation, superimposed upon a regime of thermal subsidence. Alkaline igneous activity is considered to be the product of mantle partial melting in response to limited decompression due to transtensional displacements on crustal thickness faults.
Correlation of Upper Jurassic-Lower Cretaceous continental sequences from the southern Biscay margin, northern Spain
Authors: PLATT N. H. , PUJALTE V. 
Published: December 2007

Biostratigraphical data indicate that fresh-water carbonate deposits from the southern part of the the Cantabrian Basin, northern Spain are of earliest Cretaceous (Berriasian) age. Charophyte assemblages enable correlation with fresh-water carbonate rocks of the western Cameros Basin to the south. The charophytes also support correlation with Berriasian marginal marine carbonate sequences in the Jura Mountains, southern England and northern Germany, indicating that these continental carbonate rocks represent fresh-water equivalents of the 'Purbeckian' facies. The biostratigraphical correlation is enhanced by the parallel facies evolution observed in the Upper Jurassic-Berriasian of the Cameros and Cantabrian basins. Interpretation of these successions as non-marine depositional sequences permits member-scale correlation between the two basins. Further strong facies similarities in the overlying alluvial clastic rocks of the two areas suggest a revised correlation of the overlying Valanginian-Albian deposits.
The two basins were connected at least intermittently during the Late Jurassic-Early Cretaceous; only during periods of higher sea-level did facies differences develop as a result of greater marine influence in the north. At other times, vast alluvial, lake and marsh systems extended over the region, leading to the deposition of genetically-related alluvial and lacustrine or palustrine deposits in these linked half-graben systems.
In Cantabria and the Cameros Basin, strong lateral variations in facies, thickness and stratigraphy reflect erosion at the onset of Late Jurassic rifting, differential subsidence in the Early Cretaceous, and erosion beneath Aptian-Albian unconformities.
Improved understanding of the successions described here may assist interpretation of similar Upper Jurassic-Lower Cretaceous sequences on the northern margin of the Biscay rift, such as those in the Wessex and North Celtic Sea basins.
Trace element geochemistry of orogenic igneous rocks and crustal growth models
Authors: TARNEY J. , JONES C. E. 
Published: December 2007

Some of the more important constraints on crustal growth mechanisms are reviewed in relation to the trace element composition of orogenic igneous rocks, with specific reference to granitoids, and in relation to tectonothermal models of subduction zones. The wide range of magma types associated with subduction zones is difficult to explain by a single magma-generating mechanism. This variation can be accommodated if some lavas ( are tholeiites and boninites) are formed through wet melting of upwelling asthenosphere at the initiation of oceanic subduction, whereas others are generated from the mantle wedge during the more mature stage of ares, with either hornblende or phlogopite dominated assemblages, where induced convection in the mantle wedge enables source replenishment. Yet other products are generated through melting of the slab (±subducted sediment), but only where the subducting ocean crust is very young and/or warm.
The range and variation of trace-element patterns displayed by basic lavas is also prevalent in more siliceous compositions, implying that the mantle component in many granitoids may be much higher than is commonly assumed. As well as the traditional I-type, S-type and A-type granites with generally low-Ba and low-Sr characteristics, there is another important group of granitoids that has high-Ba and high-Sr concentrations, and that consistently has about 10 trace-element characteristics that are complementary in nature to the first groups. This type of granitoid is common in the Arehaean, rare in the Proterozoic, but is surprisingly abundant amongst post-Cretaceous to Recent orogenic volcanic and plutonic rocks. Trace element patterns may be inherited from mineralogically-controlled element fractionation occurring deep in the lithosphere; major element characteristics are more dependent on mineral-fluid control during magma generation.
Tectonomagmatic evolution of the Kenya rift valley: some geological perspectives
Authors: MACDONALD R. , WILLIAMS L. A. J. , GASS I. G. 
Published: December 2007

The Cenozoic Kenya rift valley has developed in an area of great lithospheric complexity, partly related to late Proterozoic orogenic events, which has strongly influenced the regional patterns of faulting, subsidence, uplift and magmatism. The mantle sources of the magmas are poorly understood, but seem to be heterogeneous as regards trace elements and isotopes, and to have undergone one or more depletion-enrichment events. Mafic magmas erupted in the rift zone may have both asthenospheric and lithospheric components, but the evidence is equivocal. Uncertainties about geothermal gradients and the abundances and speciation of volatiles in the melting zone make it difficult to constrain the depths and degrees of partial melting. Partial fusion may, however, have taken place over the depth range 100+ to 23 km. The largest volumes of basalt last equilibrated at pressures <l5 kb and have trace element characteristics consistent with generation in the spinel-garnet peridotite transition zone.
The thickness and internal structure of the lithospheric mantle and attempts to interpret rift evolution in terms of lithosphere-asthenosphere interactions are poorly constrained. Evidence in favour of active (plume-driven) rifting includes the long wavelength gravity and topographic anomalies, the volume of eruptive rocks, the relative timing of magmatism and rifting, the distribution of geophysical anomalies, and the pattern of the tectonomagmatic evolution of the rift. A passive role for the asthenosphere is suggested by the continent-scale correlation between episodes of rifting and magmatism and by fault geometry. Active and passive mechanisms may have acted together in rift development.
Basin structure and Tertiary magmatism on Skye, NW Scotland
Authors: BUTLER R. W. H. , HUTTON D. H. W. 
Published: December 2007

The emplacement of igneous material into upper crustal rocks of sedimentary basins is likely to be strongly controlled by the geometry of the pre-existing basin structures. These controls are investigated using examples from the Tertiary igneous complexes of Skye, part of the Sea of Hebrides basin of NW Scotland. The basin consists of an array of half-graben related to SE-dipping normal faults. These pre-volcanic, Mesozoic structures are traced near the igneous complexes using geological relationships preserved unconformably beneath the widespread basaltic lava fields. The unconformity represents a period of Cretaceous uplift and denudation of the basin and its flanks, entirely pre-dating the Tertiary volcanism of NW Scotland. This unconformity seals stratigraphically the major basin faults, preserving field relationships that permit the tracing of these faults in the country rocks to the Tertiary intrusions. The major Camasunary fault is separated from the Raasay fault via a series of minor graben, linked by a series of steep, NW–SE-trending faults that transfered Mesozoic displacements between the principal fault strands.
A broad range of igneous material of various compositions was intruded into part of the Mesozoic Sea of Hebrides basins and their flanks during Palaeocene times. Different emplacement styles and different structural controls are found. The major gabbroic centres do not appear to be controlled by upper crustal structures, having been emplaced into the footwalls of major faults. However, minor synmagmatic displacements on the basin faults may have been sufficient to generate dilatational sites in these footwall positions, thereby facilitating emplacement. In contrast, the granitic melts have been emplaced as sheets and domed into the sediments and overlying lava pile, reactivating segments of the basin fault network. Doming occurred from an array of sills, the stratigraphic levels of which can be reconstructed using structural relationships preserved in the roofs and walls of the intrusions. The sill levels and their transgressive forms are strongly related to inferred Mesozoic basin structures. The major fold structures of Tertiary age in southern Skye are interpreted as accommodating granitic emplacement rather than crustal shortening. The NW-SE Mesozoic transfer fault trend appears to have strongly influenced the segmentation of the granite domes. These interpretations are illustrated using field relationships mapped in the vicinity of the Coire Uaigneich granophyre. It is concluded that although the higher parts of the basin faults were reactivated to facilitate the doming of granitic intrusions, the deeper levels of the Mesozoic faults show no evidence of substantial reactivation.