bluuebunny/crossref_metadata_2025_split huggingface dataset:
Fracture zones in the North Atlantic: morphology and a model
Authors: COLLETTE B. J. 
Published: December 2007

A morphology is presented for the typical cross-section of an inactive fracture zone in the Atlantic. Inactive fracture zones typically consist of an asymmetric valley to the young side of the fracture zone plane and a high wall or scarp to the old side. Large-offset fracture zones may be accompanied by a marginal valley on the other side of the high wall. This topography is superimposed on the depth-age step due to lithospheric cooling.
A model is developed which accounts for this morphology. The model relates the inactive fracture zone morphology to the topography found at present-day intersections of the spreading axis with transform faults. The asymmetry of the median valley near fracture zones plays an important role in explaining the typical fracture zone morphology. The existence of a median valley is related to the viscous delay of the upwelling mantle material at the spreading axis. Its asymmetry near fracture zones can be accounted for by modelling the viscous drag exerted by the lithosphere on the asthenosphere. If the viscosity is low, as under Reykjanes Ridge and the East Pacific Rise, no median valley develops and a different morphology may be expected. The occurrence of marginal valleys is interpreted as the result of lithospheric warping when the graben walls in the transform domain, which are caused by tension due to horizontal thermal contraction. are excessively high.
Deep-tow seismic profiles from the Vema transform and ridge–transform intersection
Authors: BOWEN A. N. , WHITE R. S. 
Published: December 2007

A recently developed near-bottom reflection profiling system has been used to obtain high resolution seismic sections across the Vema transform fault. At the western ridge–transform intersection deep-towed 3.5 kHz echo sounder data show evidence of fresh lava flows at the seabed within the nodal deep.
Over one kilometre of sediment fills the bottom of the ridge–transform intersection deep. Many strong, discontinuous reflectors within the sedimentary section caused by lava layers indicate that the intersection deep has been the site of sustained igneous activity. On the eastern side of the deep is a large, uplifted fault block created as the newly formed crust moved out of the spreading centre. Dipping strata within the fault block document back-tilting as the new crust is extended by normal faulting. Profiles across the active transform fault reveal that the zone of deformation throughout the one kilometre thick sediment column is narrow, suggesting that the location of the fault has remained stable throughout at least the last million years.
Geology, geochronology and geodynamic implications of the Cenozoic magmatic province in W and SE Ethiopia
Authors: Berhe S. M. , Desta B. , Nicoletti M. , Teferra M. 
Published: December 2007

New K-Ar dates are presented for areas in W and SE Ethiopia. In the west, the dates distinguish the Geba Basalts of 40 to 32 Ma (previously determined by Merla
et al.
1979) from the Welega Shield Volcanics which are shown to range from 11.2 ± 2.2 to 7.8 ± 1.6 Ma. In SE Ethiopia, the Lower Stratoid flood basalts range from 30 ± 4.5 to 23.5 ± 4.5 Ma and are unconformably overlain by the Reira-Sanete shield volcanics which range from c. 15 to
c.
2 Ma. The unconformity is marked by a palaeosol as are several of the intervals between the major volcanic stages of Ethiopia.

Using new field observations, together with previously published results and unpublished data from the Ethiopian Institute of Geological Surveys, it is suggested that the Ethiopian Flood Basalts were erupted in three major stages. Stage 1, which is mainly older than 40 Ma is separated from Stage 2, 34 to 30 Ma for NW Ethiopia and 40 to 30 Ma for SW Ethiopia, by erosional unconformities. Stage 3 spans 30 to 26 Ma in NW Ethiopia, and 30 to 21 Ma in SW Ethiopia and both are marked by the incoming of silicic volcanism. In W Ethiopia, Stages 1 and 3 are not developed, whilst in SE Ethiopia the Tertiary volcanism commences with Stage 3 flood basalts. The overlying shield volcanics; (25 to 13 Ma in NW Ethiopia and 15 to 7 Ma in W Ethiopia) represent a localized terminal episode built on the Plateau and are considered a fourth stage.
The earliest volcanism is restricted to two areas: in SW Ethiopia, where the Akobo Basalts give ages as old as 49.4 Ma (Davidson & Rex 1980), and in NW Ethiopia where the Ashange Basalts underlie the Aiba Basalts which are dated at 34 to 30 Ma. The Ashange volcanism may have begun in the Palaeocene—Eocene but radiometric dates are ambiguous and inconsistent. These two volcanic centres show lithological and geochemical differences, and can be related to two separate rift zones north of Lake Turkana and further north in the Tana Graben. Volcanism appears to have spread by the expansion of these areas, and to have migrated eastwards with time towards the Main Ethiopian Rift which begun to form about 14 Ma ago. There is a complex pattern of domal uplifts within these two volcanic centres with volcanism either preceding doming, or the two being contemporaneous.
A sequential back-arc and foreland basin thrust duplex model for the Southern Uplands of Scotland
Authors: STONE P. , FLOYD J. D. , BARNES R. P. , LINTERN B. C. 
Published: December 2007

Thrust imbrication of Ordovician and Silurian submarine fan sequences overlapping pelagic deposits in the Southern Uplands has been interpreted in terms of an accretionary prism formed above a NW-directed subduction zone. Structural features invoked to support accretion are not definitive and could be explained in terms of a thin-skinned thrusting model. New palaeocurrent and compositional evidence from Llandeilo to mid-Llandovery age turbidites in the northern part of the Southern Uplands proves interdigitation of sediments with strongly contrasting petrography. Turbidites derived from the south contain significant quantities of fresh andesitic detritus whereas those from the north form more mature quartz-rich formations. This implies a back-arc situation; the turbidites being deposited in a basin with a relatively mature continental landmass to the north and a rifted continental fragment containing an active volcanic arc to the south. Oblique collision of the opposing continental margins of the Iapetus Ocean during the Llandovery caused the cessation of subduction. Underthrusting of the southern margin initiated a SE-propagating thrust stack which deformed the back-arc basin sequence and may eventually have ramped over the eroded and faulted remains of the volcanic arc.
A southward-migrating foreland basin formed ahead of the rising thrust stack and is now represented by the late Llandovery Hawick Group and Wenlock sequences. Mid- to end-Silurian sinistral strike-slip resulted from oblique collision and produced a transpressional regime during which reactivation of deep-seated structures allowed the intrusion of lamprophyre dykes and granites.
Fluid inclusion studies
Authors: FOSTER R. P. 
Published: December 2007

A thematic session on fluid inclusions was held on 15–16 December 1986, at the University of Southampton as part of the annual meeting of the Mineral Deposits Studies Group and was supported by the Applied Mineralogy Group of the Mineralogical Society. Four of the papers are published in this volume (referees: D. H. M. Alderton, A. V. Bromley, R. P. Foster, A. P. Gize, R. A. Ixer, N. J. Jackson, D. A. C. Manning and J. J. Wilkinson), and abstracts are presented for five other contributions.
Providing as always an annual forum for the reporting and discussion of mineral deposits research, the 1986 meeting focused in part on fluid inclusion studies in metallogenesis. Opening the meeting, keynote speaker Roedder (Roedder and Howard) referred to a recent study of the Taolin Zn–Pb–fluorite deposit (People's Republic of China) in which homogenization temperatures of 120–200°C and salinity determinations of 0–14 wt% NaCl equiv. were more indicative of low-temperature epithermal mineralization than a Mississippi Valley-type deposit. Some of the data, particularly the homogenization data, differed significantly from the results of an earlier study and were used to emphasize the problems that can be encountered in fluid-inclusion studies. Particular reference was made to the stretching or decrepitation of earlier, lower-Th inclusions by later hotter fluids.later hotter fluids.

In the more typical MW-type environment of the Illinois Fluorite District, petroleum-rich and aqueous inclusions were the subject of investigation by
Gize and Richardson
who observed that the petroleum phase was a strongly
Basin formation at the end of a strike-slip fault: the Cerdanya Basin (eastern Pyrenees)
Authors: CABRERA L. , ROCA E. , SANTANACH P. 
Published: December 2007

The Cerdanya basin was formed during dextral slip along the NE–SW La Tet fault (eastern Pyrenees) which formed a horsetail structure at its SW end. The resulting basin is clearly asymmetrical, formed by several basement blocks tilted towards the La Tet fault and limited by E–W faults. These faults absorbed the movement along the major strike-slip fault. The early basin evolution (Middle–Late Miocene) resulted from strike-slip, while the later evolution (Early Pliocene) involved a generally extensional regime.
A variety of depositional environments developed in the basin during its early evolution: spatially restricted and widespread alluvial fan, distributary fluvial, marginal and open lacustrine. The structural features of the basin that developed during this evolutionary stage differ from those described from most other basins related to strike-slip faulting. Despite this, the structural and sedimentary features of the Cerdanya basin fit the general model of strike-slip related basins.
Dips of normal faults in British Coal Measures and other sedimentary sequences
Authors: WALSH J. J. , WATTERSON J. 
Published: December 2007

Fault dip measurements (n = 3291) from 122 faults in British Coalfields are presented. Two classes of normal fault are recognized: (a) gently dipping (<55°) synsedimentary faults of gravity driven or, less commonly, tectonic origin and (b) steeply dipping (>55°) post-coalification tectonic faults formed at depths of about 2.5 km which comprise 90% of the faults analysed. Dip data from the steeply dipping faults are variable (standard deviation = 9°), both on single faults and between faults, and 70° is a better approximation of fault dip than the commonly accepted value of 60°. Data on tectonic normal faults, with comparable depths of formation (<3–4 km), from other sedimentary sequences show that dips of greater than 60° are quite usual. Systematic changes of dip on single fault surfaces have predictable effects on the curvature of fault traces.
Using the modified Griffith fracture criterion the dips of faults at varying depths (up to 4 km) are calculated for both unsaturated (dry) and saturated rocks. The dips of normal faults are expected to vary from 90° at the surface to 60° at 3–4 km and this variation is consistent with the limited field data available. The coalfield data are consistent with fault formation at standard hydrostatic pore fluid pressures at depths of about 2.5 km. Below depths of a few kilometres normal faults are expected to flatten out gradually to a dip of 45°, but such flattening is not necessarily an indication of an underlying detachment.
Structural controls on Upper Palaeozoic sedimentation in south-east Wales
Authors: WILSON D. , DAVIES J. R. , SMITH M. , WATERS R. A. 
Published: December 2007

Marked thickness changes in early Dinantian (Courceyan-Chadian) sediments occur across the Cardiff-Cowbridge Anticline, a major east-west Variscan fold in the Vale of Glamorgan. They resulted from differential subsidence over an active concealed basement fault zone, the Vale of Glamorgan Axis, which is coincident with the hinge of the Anticline; the latter formed in response to inversion on this basement fault. A Caledonian ancestry for the Vale of Glamorgan Axis is indicated by pronounced Upper Old Red Sandstone overstep across it.
Across the Severn Estuary, between South Wales and the Bristol-Mendips region, there is a significant offset of isopachs and facies belts in both the Courceyan-Chadian and Arundian sequences. It is suggested that these result from intra-Carboniferous dextral strike-slip along a major fault zone underlying the Severn Estuary (the Severn Estuary Fault Zone), and that the Vale of Glamorgan Axis was a synthetic structure, developed in conjunction with this strike-slip movement. By inference, the Severn Estuary Fault Zone shares a common history with the Vale of Glamorgan Axis, extending back to the Devonian and possibly earlier.
Late Dinantian (Holkerian) facies distribution reflects uplift and emergence on the north-south Usk Anticline and Malvern Lineament. Continued uplift is recorded in Silesian sequences in south-east Wales and the Bristol-Mendips area. The orientation of structures bordering the Severn Estuary is consistent with their development in a right-lateral strike-slip zone.
Samarium-neodymium isotope stratigraphy of the Lunde and Statfjord Formations of Snorre Oil Field, northern North Sea
Authors: MEARNS E. W. , KNARUD R. , RÆSTAD N. , STANLEY K. O. , STOCKBRIDGE C. P. 
Published: December 2007

Two sequences of fluviatile sediments from Snorre Oil Field are investigated by the Sm-Nd isotopic method. The results illustrate distinctly different source areas for the (Upper Triassic) Upper Member of the Lunde Formation and for the (Lower Jurassic) Statfjord Formation. The Lunde Formation has Sm-Nd provenance ages mainly in the range 1500 to 1700 Ma, which is not diagnostic of a specific source area. The Statfjord Formation has provenance ages in the range 1700 to 2150 Ma. This is old within the context of source areas in the hinterland of the northern North Sea and provenance partly from the Archaean Lewisian shield to the northwest is suggested.
Intensive sampling through the Statfjord Formation in one well illustrates that the provenance ages evolve systematically with time. The pattern provides a detailed record of how source areas evolved during deposition of these strata. The possibility is recognized of correlating changes or trends in provenance between wells.
The data also suggest that Sm/Nd ratios and isotopic compositions of these rocks are not significantly fractionated between fine- and coarse-grained sediments by mechanical or diagenetic processes.
Sedimentary dynamics of the Kimmeridge Clay: tempests and earthquakes
Authors: WIGNALL PAUL B. 
Published: December 2007

Storms are suggested to be an important influence upon the formation of epicontinental black shales, controlling both their distribution and facies types. A variety of storm-produced event beds are described from the Kimmeridge Clay of Dorset. These include horizons of graded rip-up clasts, silt laminae, thin graded mud horizons and shell pavements. Biostratinomy and population dynamics indicate that the pavements represent brief benthic colonization events in a predominantly anaerobic-dysaerobic environment. Differential near-surface lithification has also affected the sediments, leading to the formation of flow phenomena. These include loop structures, microfolding and brecciation. Earthquake-induced fissuring also occurs at a number of horizons.
The Kimmeridge Clay shows well-developed cyclically alternating organic-rich shales and mudstones on a decimetre scale. A feedback mechanism of storm-induced benthic oxygenation and temperature-stratified inhibition of storm mixing may account for the abrupt nature of the cyclicity. Subsidence and sedimentation rate were probably important factors controlling the thickness and organic-richness of the sediments whilst the influence of storms was important in controlling the distribution and facies types of these important source rocks. The model proposed for the Kimmeridge Clay may be of general application to many epicontinental black shales in the geological record.
Palaeomagnetic evidence for rotations and deformation in the Nogueras Zone, Central Southern Pyrenees, Spain
Authors: BATES MARTIN P. 
Published: December 2007

A multicomponent palaeomagnetic remanence from the Lower Triassic Buntsandstein redbeds of the Central Southern Pyrenees has been interpreted as being composed of three components; a low blocking temperature recent viscous magnetization (L), an intermediate blocking temperature Eocene remagnetization (I), and a high blocking temperature Lower Triassic remanence (H).
These palaeomagnetic results can be used in conjunction with structural evidence to identify poles of rotation during the development of the Nogueras Zone antiformal stack. Poles from individual sites reflect local deformation and folding, including vertical rotation due to shearing between adjacent thrust sheets. The I component was sometimes acquired syn-tectonically during thrust motion, and can be used to define intermediate stages of structural development.
Results from sites located on a single thrust sheet (‘No. 7’) indicate that it rotated by 45° clockwise during emplacement. This is reflected in an easterly increase in displacement estimated from the construction of balanced cross-sections. Thrust sheets appear to have been pinned close to the present day location of Pont de Suert during the later stages of structural development. The arcuate path of thrust sheet motions, and the potential movements in and out of cross-section, can lead to over- and under-estimates of minimum displacement when using line-length cross-section balancing techniques.
The San Nicolás batholith of coastal Peru: early Palaeozoic continental arc or continental rift magmatism?
Authors: MUKASA S. B. , HENRY D. J. 
Published: December 2007

Major and trace element, U-Pb zircon geochronology and Pb-isotopic data are presented on plutons from the San Nicolás batholith of central and southern Peru.
The major oxides of 9 samples indicate a high-K calc-alkaline trend, and trace element discriminant diagrams reveal affinities to volcanic arc or late- to post-collision calc-alkaline intrusions. The apparent contradiction implied by the Rb-Hf-Ta triangular diagram (within-plate) strongly suggests loss of Rb, an interpretation supported by the extensive deuteric alteration and sericitization of feldspar. Ta/Zr, Ta/Hf and Ta/Nb ratios are considerably higher than those of volcanic arc intrusions, and favour the late or post-collision calc-alkaline classification.
U-Pb zircon geochronology shows that the San Nicolás batholith was emplaced in two short-lived magmatic events at approximately 425 Ma and 394-388 Ma ago. The presence of an inherited component of radiogenic Pb of Precambrian age in the zircons suggests xenocrystic cores, which were most likely scavenged from Precambrian basement rocks of the Arequipa massif.

Time-adjusted Pb isotopic compositions of feldspars have the following ranges:
206
Pb/
204
Pb = 17.798-18.272,
207
Pb/
204
Pb = 15.571-15.626, and
208
Pb/
204
Pb = 38.182-38.291, indicating that Precambrian basement was incorporated into the magmas of the batholith. Other components are believed to have originated in an ‘enriched’ upper mantle or lower crustal wedge pervasively metasomatized by fluids extracted largely from subducted sediments.

U-Pb zircon age distribution, trace element characteristics and the linear nature of the plutons firmly establish the San Nicolás batholith as the plutonic substructure of an early Palaeozoic continental arc, which may extend northwards in Peru and southwards
Basin evolution and fault reactivation in the western Cameros Basin, Northern Spain
Authors: PLATT NIGEL H. 
Published: December 2007

Mesozoic-Cenozoic sedimentation in the Cameros Basin area strongly reflects tectonic factors. The tectonic history is dominated by two principal extensional phases (Triassic, Late Jurassic-Early Cretaceous) and strong Pyrenean compression (Oligocene). Late Variscan NW-SE and NE-SW fault sets were repeatedly reactivated as dip-slip and wrench faults respectively throughout extension and compression. E-W Variscan thrust planes showed relatively minor reactivation in this area.
Mesozoic fault control on sedimentation resulted in significant lateral variations in stratigraphic and sedimentological development. Variation in deformation style and thrust architecture reflects the differing responses of the laterally heterogeneous sequence to Oligocene compression.
Geochemistry and origin of the Archaean Beit Bridge complex, Limpopo Belt, South Africa
Authors: BORYTA MARK , CONDIE KENT C. 
Published: December 2007

The Beit Bridge complex (BBC) in the Central Zone of the Limpopo Belt in Southern Africa includes metasedimentary rocks of Archaean age (chiefly metapelites, quartzites and carbonates), amphibolites (chiefly mafic dykes and sills) and granitoids which comprise ≥50% of the complex. The amphibolites are tholeiites with relatively unfractionated REE patterns and pronounced subduction-zone geochemical signatures in terms of relatively immobile incompatible elements. Tonalites and granites, which also exhibit such a signature, can be formed by batch melting of a mafic source with residual garnet or amphibole. Most granites have negative Eu anomalies and appear to represent residual liquids.
BBC metapelites have negative Eu anomalies and other immobile element distributions similar to Phanerozoic shales. One group of metapelites that shows significant depletion in Ca, Na and Sr appears to have been derived from intensely weathered sources. The negative Eu anomalies in BBC metapelites suggest intracrustal melting of their sources and thus the presence of evolved continental crust in this part of Southern Africa by about 3000 Ma. BBC quartzites are quartz-rich sandstones that are depleted in HFS elements, Ba and Sr, relative to Phanerozoic counterparts; a feature that probably reflects less recycling during the early Archaean. BBC banded iron formation (BIF) is similar to other Archaean BIF and shows large positive Eu anomalies. BBC carbonates are dolostones depleted in REE, HFS elements, U, Th, Ba, Sr, Ni, Sc, V and Y and enriched in Fe, Mn, Na, Rb and Cs relative to Phanerozoic marine carbonates.
Sequence stratigraphy of the Palaeozoic Welsh Basin
Authors: WOODCOCK N. H. 
Published: December 2007

The timing and extent of unconformities in the Welsh Basin are investigated using ‘rock preservation curves’ derived from outcrop stratigraphic logs. Four basin-wide unconformities occur, focussed in late Precambrian, late Tremadoc, Pusgillian (early Ashgill) and mid-Devonian times. These bound three megasequences, equating with newly defined lithostratigraphic units, the Dyfed, Gwynedd and Powys Supergroups. Less extensive unconformities bound 18 component sequences.
The majority of the sequence boundaries reflect a component of tectonic or volcanotectonic activity rather than a pure eustatic sea-level change. The megasequence boundaries are attributed to late Precambrian to early Cambrian onset of rifting to form a passive margin, Tremadoc onset of subduction with intra-arc then back-arc extension, late Caradoc end to subduction, and late Early Devonian collisional deformation. The megasequences and controlling events can be tentatively matched with other basins on the Avalonian margin.
More generally, this study shows that sequence analysis is feasible in onshore basins lacking well and seismic data, and that the global eustatic interpretation of sequence stratigraphy is only partially applicable to active margin basins.