bluuebunny/crossref_metadata_2025 huggingface dataset:
Proterozoic anorogenic magmatism and its orogenic connections
Authors: WINDLEY BRIAN F.
Published: December 2007

There is a major belt of Proterozoic, so-called anorogenic, igneous rocks that extends from the Baltic Shield through southern Greenland and eastern Canada to southern USA. All rocks are situated within a zone of 1.9-1.6 Ga arc-accretionary orogens that consist of juvenile mantle-derived material especially accretionary prism sediments that provided the fertile source for abundant granitic crustal melts. There were two main periods of formation of the anorogenic rocks. 1.76-1.55 Ga rocks are mainly rapakivi granites and rhyolites that formed about 60-200 Ma after the last compressional events in the Svecofennian, Ketilidian and Penokean orogens, the time-lag being caused by the slow thermal relaxation of thickened lower crust that had commonly been depleted by the extraction of earlier granitic crustal melts. 1.45-1.4 Ga anorogenic rocks are mainly rhyolites and peraluminous granites in central/southern USA and anorthosites in eastern Canada that formed in the continental margin of the Grenvillian ocean, the subduction of which led to the Grenville orogen. These anorogenic igneous rocks formed in extensional regimes during the formation and breakup of a 1.5 Ga supercontinent.
Early Caledonian tectonothermal evolution in outboard terranes, central Scandinavian Caledonides: new constraints from U-Pb zircon dates
Authors: STEPHENS M. B. , KULLERUD K. , CLAESSON S.
Published: December 2007

Early Caledonian deformation in an outboard terrane (Storfjället) in the central Scandinavian Caledonides is constrained to the time-period Arenig to Caradoc on thebasis of U-Pb zircon age determinations. A trondhjemite clast in a conglomerate belonging to a supracrustal sequence that was affected by the early deformational event yields an age of 489
+10
–5
Ma. A granite intruded after this event defines an age of 445
+24
–6
Ma. The clast is similar in age to a trondhjemite intrusion in a contiguous terrane (Gjersvik) dated at 483
+5
–3
Ma and the granite age is in agreement with a Rb–Sr whole-rock isochron of 438 ± 6 Ma determined in a previous study. The three new ages provide further evidence for the separation of igneous events in outboard terranes in the Scandinavian Caledonides into older (Tremadoc to Arenig) and younger (Late Ordovician to Early Silurian) episodes. More critically, they argue against simple correlation of early Caledonian deformational events in outboard terranes with the Late Cambrian to Early Ordovician tectonometamorphic history along the margin of the continent Baltica (the so-called Finnmarkian orogenic phase). This age disparity provides support for significant spatial separation of the Storfjället terrane from Baltica during the early–mid Ordovician and an influence of the Taconic orogenic event on this outboard terrane is proposed.
Concretions of the Valtos Sandstone Formation of Skye: geochemical indicators of palaeo-hydrology
Authors: WILKINSON MARK.
Published: December 2007

The calcite concretions of the Jurassic Valtos Sandstone Formation record the origin of the porewaters within which they grew, and the approximate flow rates. The concretions grew within meteoric porewaters, part of a Middle or Upper Jurassic meteoric water lens extending into the Sea of the Hebrides Basin from the then Hebridean landmass. Modal porefluid flow rates were low, resulting in an approximately spherical concretion geometry. The concretions are zoned due to episodic recharge of the aquifer (which is tentatively related to seismic pumping). The sandstone was open with respect to Mg, Fe, Mn and Sr, but closed with respect to Ca. The sandbody was rock-dominated with respect to Ca, Sr, C and Mn, which were derived from within the sandbody by internal redistribution; but water-dominated for Fe, Mg and O.

The Valtos concretions grew by the diagenetic redistribution of aragonitic shells, and comprise only a single phase of ferroan calcite (FeCO
3
=0.89−2.7 mole %), with a small range of carbon compositions (δ
13
C = +0.25 to -0.97‰PDB). The cements have a large range of oxygen isotopic compositions (δ
18
O = −8.3 to −18.2‰ PDB) interpreted to represent changes in porewater composition, not formation temperature. Cement crystal size increases radially, reflecting decreasing degrees of supersaturation during growth. The cements have only a small range of 'bulk' Fe and Mg concentrations (as determined by ICPS), but a large range of concentrations determined by microprobe. This pattern is explained by disequilibrium trace element partitioning associated with sector zoning within the cement.
Geochemical and mineralogical variations in the upper Mercia Mudstone Group (Late Triassic), southwest Britain: correlation of outcrop sequences with borehole geophysical logs
Authors: LESLIE A. B. , SPIRO B. , TUCKER M. E.
Published: December 2007

The Mercia Mudstone Group comprises up to 1200m of predominantly red mudstones and siltstones laid down in rift-related basins during a period of regional subsidence. The mudstones are commonly dolomitic and contain horizons of sulphate and halide salts. Up to 150m of undifferentiated red mudstones of Norian and Rhaetian age (Late Triassic) were examined in coastal outcrops in Devon and Somerset, southwest Britain, and mineralogical and stable isotopic studies were carried out in order to identify any subtle changes in the succession. Gamma-ray measurements were taken at outcrop to provide correlation with published borehole logs in which the Mercia Mudstone Group has been subdivided on the basis of gamma-ray and sonic response. One of the borehole subdivisions was identified at outcrop within a sequence of otherwise undifferentiated red mudstones and was related to a transition in clay mineral assemblage and stable isotopic composition over 20 to 30 m of section. The red mudstones below this transition have a higher proportion of magnesian clay minerals and enriched carbonate oxygen isotope compositions, indicating deposition from Mg
2+
-rich marine-derived waters. Clay mineral assemblages in the succeeding red mudstones are dominated by illite, and oxygen isotopic compositions are relatively depleted, indicating a greater influence of K
+
-rich continental-derived waters.
Late Eocene–Recent volcanism and faulting in the southern main Ethiopian rift
Authors: EBINGER C. J. , YEMANE T. , WOLDEGABRIEL G. , ARONSON J. L. , WALTER R. C.
Published: December 2007

Few constraints on the timing, amount and distribution of lithospheric extension associated with flood-basalt magmatism were available from the southern Main Ethiopian rift system, where the base of the Cenozoic volcanic succession is exposed by faulting. New structural observations, together with K–Ar and
40
Ar/
39
Ar geochronology data from a transect of the Chamo basin–Amaro horst–Galana basin, show that basins are bounded by faults with steep dips at the surface, and the stratal dips of Eocene–Recent volcanic and sedimentary units are generally less than 20°. Little or no extension accompanied the extrusion of a 0.5 to 1 km thick sequence of transitional tholeiitic flood basalts between 45 and 35 Ma. Stratigraphical correlations with basins to the north and southwest suggest that felsic eruption(s) at
c.
37 Ma blanketed much of the southern Ethiopian plateau region with a felsic tuff unit. A second, less widespread, episode of alkali basalt and trachyte volcanism occurred between 18 and 11 Ma, and Recent alkali basalt volcanism occurs within the Chamo basin. The attitude, distribution, and diversity of Neo–gene–Recent volcanic and sedimentary strata within the Chamo and Galana basins indicate that crustal extension, basin subsidence, and rift flank uplift began during or after the second flood-basalt phase. Based on cross-sectional reconstruction to the top of the Oligocene tuff, we estimate a minimum of 12 km crustal extension (β ≈ 1.12), and infer that maximum extension across the southern Ethiopian rift is less than 25 km. Extension is primarily accommodated by slip along the border faults bounding the asymmetric basins, with small amounts of extension occurring within the hanging walls. Crude estimates of original basalt layer thickness prior to erosion in the Amaro region suggest that roughly comparable volumes of basaltic material erupted during the two episodes of flood-basalt magmatism (45–35 Ma and 18–11 Ma). The small amounts of lithospheric extension and the large volumes of magma estimated in this study of the southern Main Ethiopian rift suggest a very hot plume and/or efficient thinning of the mantle lithosphere from below by mantle plume processes during the two discrete episodes of flood-basalt volcanism.
The structural evolution and mechanics of volcanoes and subvolcanic intrusions
Authors: DAY S. J.
Published: December 2007

A joint meeting of the Volcanic Studies and Tectonic Studies Groups, held at the University of Liverpool, 13 May 1992. Convenor: S. J. Day.

This meeting, the first joint meeting of the Volcanic and Tectonic Studies Groups for some time, was called to consider and promote the application of techniques and constraints from structural geology and geophysics to the work of volcanologists and igneous petrologists on both ancient and modern volcanoes, and to illustrate to structural geologists some of the problems and procedures involved in work on volcanic systems.

The meeting was opened by
B. P. Kokelaar
(Liverpool University), who illustrated the range of structural problems confronting the volcanological and petrological communities through a discussion of the possible ways in which structural lineaments may influence magmatism. This led into the first session of the meeting, which considered problems with the classical models, developed by Anderson, Mogi, Roberts, Phillips and others (see review in Chevallier, L. & Verwoerd, W. J. 1988. A numerical model for the mechanical behaviour of intraplate volcanoes. Journal of Geophysical Research,
93
, 4182–4198), of the relationship between deformation around large intrusions and the stresses they impose on the rocks around them.
R. W. England
(BIRPS, Cambridge) & S. J. Day (Liverpool University) used examples from the British Tertiary Province to illustrate the complexities of the finite strain produced during emplacement of large volumes of magma close to the free surface, as contrasted to the simple stress trajectories of the classical models, which do not allow for
P–T–t
evolution of orogenic belts and the causes of regional metamorphism
Authors: BROWN MICHAEL
Published: December 2007

Barrow (1893)
introduced three important ideas that furthered understanding of metamorphic processes: (i) the use of critical index minerals in argillaceous rocks to define metamorphic zones and elucidate spatial features of regional metamorphism; (ii) the concept of progressive metamorphism; and (iii) the concept of magmatic advection of heat as a possible cause of regional metamorphism. This article expands upon these themes by reviewing our understanding of the dynamic evolution of orogenic belts as interpreted from the
P–T–t
paths of metamorphic rocks, and by considering the likely causes of the different kinds of regional metamorphism that we observe within orogenic belts.

Understanding metamorphic rocks allows the distinction of two fundamentally different types of orogenic belt defined by relative timing of maximum
T
and maximum
P.
Orogenic belts characterized by clockwise
P–T
paths achieved maximum
P
before maximum
T,
the metamorphic peak normally post-dated early deformation within the belt and additional heating above the ‘normal’ conductive flux has been related to the amount of overthickening. By contrast, orogenic belts characterized by counterclockwise
P–T
paths achieved maximum
T
before maximum
P,
the metamorphic peak normally pre-dated or was synchronous with early deformation within the belt and additional heating above the ‘normal’ conductive flux has been related to the emplacement of plutons. Techniques used to constrain portions of
P–T–t
paths include: the use of mineral inclusion suites in porphyroblasts and reaction textures; thermobarometry; the use of fluid inclusions; thermodynamic approaches such as the Gibbs method; radiogenic isotope dating; fission track studies; and numerical modelling. We can utilize specific mineral parageneses in suitable rocks to determine individual
P–T–t
paths, and a set of
P–T–t
paths from one orogenic belt allows us to interpret the spatial variation in dynamic evolution of the metamorphism. Recent advances are reviewed with reference to collision metamorphism, high-temperature–low-pressure metamorphism, granulite metamorphism, and subduction zone metamorphism, and some important directions for future work are indicated.
A major Oligo-Miocene detachment in southern Rhodope controlling north Aegean extension
Authors: SOKOUTIS D. , BRUN J. P. , VAN DEN DRIESSCHE J. , PAVLIDES S.
Published: December 2007

Plutonic and metamorphic rocks of the southwest part of the Rhodope massif in Greece correspond to ductile lower crust exhumed and deformed along a major detachment during post-thickening extensional tectonics. Extension started during the Oligocene and is responsible for the development of Miocene–Quaternary sedimentary basins. Both brittle and ductile deformations result from gravity collapse of previously thickened lithosphere, as proposed for others large extended terranes. This interpretation disagrees with the previous models which attributed Tertiary ductile deformation to Alpine thrusting and brittle extensional deformation to back arc tectonics above a subduction zone.
Fracture patterns in clasts of diamictites (? tillites)
Authors: HARKER R. I.
Published: December 2007

Fracture patterns in some of the clasts of the Gowganda Formation, Ontario, are inconsistent with aqueous deposition and suggest that locally the diamictites are tillites.

Similar fracture patterns have been found in the clasts of modern tills in the Cascade Mountains. Rare examples were also seen on the Garvellachs, Scotland.

Laboratory experiments indicate that such patterns can be developed by the impaction of one clast against another while surrounded by a compliant matrix. The stresses which induced cracking were of the same order of magnitude as those which can be expected to occur in a sub-glacial environment.
Palaeoecology and sedimentology across a Jurassic fault scarp, NE Scotland
Authors: WIGNALL P. B. , PICKERING K. T.
Published: December 2007

A detailed palaeoecological and sedimentological analysis of the Kimmeridgian coastal outcrops around Helmsdale (northeast Scotland) has been related, for the first time, to a detailed biostratigraphical scheme allowing the depositional history of this important area (adjacent to some North Sea oilfields) to be elucidated. A broad, shallow marine, sub-tropical, shelf was disrupted by the onset of normal faulting along the Helmsdale fault in earliest Kimmeridgian times. Initially, water depths remained shallow on the hanging wall, reflected by a normal benthic fauna, but a rapidly steepening southeast-directed palaeoslope produced frequent sediment sliding. Shoreface sands developed immediately to the west of the fault and were transferred to the hanging wall via a fault-transfer zone where they now constitute the Allt na Cuile Formation. Continued rapid subsidence during the remainder of the Kimmeridgian and the Portlandian produced deep, poorly oxygenated to anoxic benthic conditions on the hanging wall in which a combination of rock slide/debris flow/turbidity current deposition alternated with hemipelagite deposition to produce the Helmsdale Boulder Bed Formation. The Eathie locality (Cromarty), previously regarded as a deeper, more distal setting than Helmsdale, contains a slightly more diverse benthic fauna implying shallower water. The deepest waters were immediately adjacent to the fault scarp as predicted by the standard half-graben depositional model.
The structure of the western part of the Southern Uplands of Scotland
Authors: NEEDHAM D.T.
Published: December 2007

The structures of two areas, one in the Northern Belt and one in the Central Belt of the Southern Uplands are described and compared. The main structural histories are the same, both show a main deformation phase related to SE-directed thrusting with associated fold and cleavage formation. Both areas exhibit late, minor NW-directed thrusting with related refolding and crenulation cleavage development in the case of the Central Belt. Both areas also show evidence of sinistral strike-slip or transpressive deformation which reactivates steepened thrust faults. When these deformation histories are reviewed in the context of geophysical, radiometric and biostratigraphical data from the region a forearc accretionary setting for the whole of the Southern Uplands appears the most likely. The strike-slip deformation has internally disrupted an accretionary prism but has not juxtaposed originally unrelated terranes. The effects of the late Silurian closure of Iapetus and subsequent events are also represented in the Southern Uplands.
Age and tectonothermal record of Laurentian basement, Caledonides of NE Greenland
Authors: TUCKER R. D. , DALLMEYER R. D. , STRACHAN R. A.
Published: December 2007

Isotope mineral ages provide constraints on the age and tectonothermal record of Laurentian basement in Dronning Louise Land, NE Greenland Caledonides. West of the Caledonian thrust zone, U-Pb dates for detrital zircon grains within late Proterozoic–early Palaeozoic foreland cover sequences define an age range of detritus between 3001 Ma and 1700 Ma. Detrital muscovites record40Ar/39Ar mineral cooling ages ofc.1700–1600 Ma. A reworked granitic orthogneiss east of the Caledonian thrust zone yields a U-Pb zircon age of 1909+6−5Ma which is interpreted as dating emplacement of the protolith. A lower concordia intercept of 422+28−25Ma dates Caledonian reworking. The data indicate that the Pre-cambrian basement in NE Greenland forms part of an early Proterozoic(c.1900–1700 Ma) mobile belt comprising, in part, reworked Archaean rocks. In contrast to central E Greenland, there is no isotopic evidence for regional tectonothermal activity of late Proterozoic age(c.1000 Ma).
Sinistral strike-slip and transpressional tectonics in an ancient oceanic setting: the Mamonia Complex, southwest Cyprus
Authors: SWARBRICK R. E.
Published: December 2007

Structural contacts between the Triassic to Cretaceous Mamonia Complex and the Upper Cretaceous Troodos Complex have been examined in southwest Cyprus. The contacts are invariably near vertical in the south of the region, and moderate to high-angle in the north. Field evidence suggests that these two contrasting oceanic basement types were juxtaposed by strike-slip tectonics. During the late Cretaceous, a sinistral strike-slip fault zone was active in the southern area, whilst in the northern area a central block was uplifted at a restraining bend. This model can explain the presence of interleaved tectonic slivers of both Mamonia and Troodos basement and earlier formed metamorphic rocks. It also explains the presence of diapirs of sepentinized harzburgite and dunite along the fault contacts, which originated as deep oceanic fractures. Finally it provides a satisfactory explanation for the origin and distribution of submarine debris flows, only found in the northern area, derived from erosion of a transpressional ‘push-up’ structure. Constraints on the timing of strike-slip faulting suggest that this episode of tectonism in southwest Cyprus is coeval with the 90° anti-clockwise rotation of parts of Cyprus, indicated by palaeomagnetic data. A model is proposed which reconciles anti-clockwise rotation of the Troodos Complex with sinistral strike-slip displacement along its contact with the Mamonia Complex. This model differs significantly from several recently published models which invoke dextral strike-slip along the same contacts associated with anti-clockwise rotation of the Troodos microplate.
Tectonic evolution of the Nakasib suture, Red Sea Hills, Sudan: evidence for a late Precambrian Wilson Cycle
Authors: ABDELSALAM M. G. , STERN R. J.
Published: December 2007

The Nakasib suture is a late Proterozoic (Pan-African) ophiolite-decorated structural belt in the central Red Sea Hills of the Sudan. It represents one of the sutures along which the island arc/back-arc terranes and continental microplates of the Arabian-Nubian Shield are welded together. The Nakasib suture separates the 900–850 Ma old Haya terrane in the south from the 830–720 Ma old Gebeit terrane to the north. Five thrust-bounded stratigraphical groups are identified across the suture. These are, from south to north, Arbaat volcanic group (c. 730 Ma rift-related volcanic rocks), Salatib group (early passive margin volcanic and sedimentary rocks), Meritri group (the lower part of which contains alluvial and submarine fans of an incipient ocean basin, while its upper part can be tentatively equated with a platform of clastic sedimentary and carbonate intercalations which mark the stage of mature ocean basin opening), Nakasib ophiolite, and Shalhout group (arc-related volcanic and sedimentary rocks). These are intruded by syn-tectonic and post-tectonic plutonic rocks.

The tectonic setting of the above lithological groups suggests that the early stage of the Nakasib suture development was marked by rifting of older crust, probably the Haya terrane, which culminated in development of a passive margin on the south margin of an oceanic basin prior to collapse into an ophiolite-decorated suture. This indicates that the Nakasib suture evolved through a complete Wilson cycle orogeny from rifting, through passive margin formation, to closure of the basin and suturing.
U-Pb zircon age for the Lagoa augen gneiss, Morais complex, Portugal: tectonic implications
Authors: DALLMEYER R. D. , TUCKER R. D.
Published: December 2007

The Morais complex is a structural unit within a succession of allochthonous, exotic crystalline Variscan nappes exposed in northwestern Iberia. U-Pb analyses of tips broken from needle-like zircon in mylonitized Lagoa augen gneiss indicate a crystallization age of 496
+3
−2
Ma. Four other zircon fractions from the Lagoa augen gneiss suggest inherited zircon components with upper-intercept ages ranging between
c.
2242 and 1941 Ma. The Cambrian to earliest Ordovician crystallization age is incompatible with previous interpretations of the Lagoa augen gneiss as Iberian Precambrian basement unconformably overlain by early Palaeozoic sedimentary rocks (Lagoa mica schist). The U-Pb results indicate that the Lagoa augen gneiss is coeval with, or perhaps younger than, the host sedimentary succession (the Lagoa mica schist). A 496 Ma crystallization age for the Lagoa augen gneiss is consistent with its derivation from an Avalonian-Cadomian source, at present situated west or northwest of the central Iberian autochthon.
Early Cretaceous environments
Authors: TAYLOR K. G. , RUFFELL A. H.
Published: December 2007

A one day meeting of the British Sedimentological Research Group (BSRG), convened by K. G. Taylor and A. H. Ruffell, was held at the Postgraduate Research Institute for Sedimentology, University of Reading on 23 June 1992. The meeting set out to consider what is known about palaeoenvironments of the NW European early Cretaceous and to present current areas of new research that may significantly expand our understanding of this topic. Over 50 people attended this multidisciplinary meeting, coming from the fields of sedimentology, geochemistry, palaeontology and tectonics. There were 13 speakers and five poster presentations. Financial assistance from the BSRG to speakers from overseas is gratefully acknowledged. The Institute is also thanked for the use of their facilities.

After a brief welcome by the conveners,
D. J. Batten
(Aberystwyth University) gave a talk reviewing his work on the palynology of NW European Wealden sediments. In this he stressed how palynological data, integrated with other available evidence, can contribute significantly to interpretations of early Cretaceous environments and should be utilized more by sedimentologists. In particular, the total recoverable organic matter in a sample should be studied, not just the identification of miospores and dinoflagellate cysts.
J. Eyers
(Open University) described how detailed sedimentology, grain-size analysis and gamma-ray logging have been used to correlate the Lower Cretaceous of the Bedfordshire Province. Using isopachyte maps, she was able to show that basement structures, reactivated as a result of North Sea rifting, were a major control on sedimentation. She also tentatively compared a combined
Dinantian (Lower Carboniferous) biostratigraphy and chronostratigraphy in the British Isles
Authors: RILEY N. J.
Published: December 2007

Vaughan’s (1905)
zonation of the Carboniferous Limestone in the Bristol district was a pioneer biostratigraphical study, which because of its meticulous execution can be reinterpreted in a modern context. The replacement of his scheme with a chronostratigraphical one by

George
et al.
1976

had a similar revolutionary affect on British Dinantian stratigraphy. However, it is now time to revise the British Dinantian stages so that they more closely correspond to biostratigraphical events. Dinantian biostratigraphy still requires considerable refinement, but it has now achieved a diversity of techniques and resolution far beyond that which was available at the time of these earlier proposals. It is the most pragmatic and closest approximation to widespread chronostratigraphical correlation available. This paper discusses these and related issues and presents a review and correlation of current biozonations.
Bedload-transported pedogenic mud aggregates in the Lower Old Red Sandstone in southwest Wales
Authors: ÉKES CSABA
Published: December 2007

Siltstones which exhibit large scale cross stratification and three dimensional bedforms in the Ridgeway Conglomerate Formation are interpreted as partly representing bedload transported pedogenic mud aggregates. The siltstones contain mixed-layer swelling clays, abundant desiccation cracks, calcrete nodules and pseudoanticlinal slip planes which suggest the presence of vertic palaeosols and a hot, seasonal climate. Compaction obliterated much of the original mud aggregate texture which is retained only where quartz grains surround individual aggregates.
Sediment dynamics of the Severn Estuary and inner Bristol Channel
Authors: McLAREN P. , COLLINS M.B. , GAO S. , POWYS R.I.L.
Published: December 2007

Net sediment transport pathways have been determined in many environments by studying grain size trends. This approach is extended here to an inner shelf environment, with improved statistical techniques.

Patterns of net surficial sediment transport are proposed, indicating that the area is dominated by up-estuary transport. Exceptions exist in the form of radial mud transport at the river mouths and (clockwise) sand circulation around linear sandbanks.

Six different sedimentary environments are defined on the basis of the shape of the ‘transfer function’ and its relationship to the grain size distributions. Erosion takes place along the axis of the estuary. Dynamic equilibrium is reached around the linear sandbanks, whilst accretion occurs at the head of the estuary and where there are riverine inputs.

The results are compared with numerical model outputs, representing water movement and sediment transport. Over the seaward part of the study area, there is agreement between the two approaches; contradictions increase, however, towards the upper reaches of the estuary. The discrepancies can be attributed to: (1) inherent differences between patterns of water and sediment movement; (2) difficulties in modelling intertidal boundary conditions; (3) inadequate representation of river influences in the models; and (4) time-scale differences, associated with sediment transport processes.
Early Precambrian crustal development: changing styles of mafic magmatism
Authors: HALL R. P. , HUGHES D. J.
Published: December 2007

The work of Sutton & Watson (1951) was one of the cornerstones in the interpretation of the evolution of early Precambrian (Archaean and early Proterozoic) gneiss complexes and in the documentation of mafic dyke swarms that intruded recently accreted Archaean cratons. Since then, the concept of geological terranes and terrane accretion has been extended back to the Archaean, and different events can be distinguished by increasingly precise isotopic dating techniques. There is growing evidence that, with certain clear provisos, the principle of uniformitarianism can be applied as equally to the early Precambrian as it can through the rest of the geological record. In terms of mafic magmatism, there are similarities between some ancient and modern suites, both intrusive and extrusive, but there are also some major differences, such as the presence of komatiites in the Archaean and the formation of large-scale layered intrusions and extensive dyke swarms at the end of the late Archaean and in the early Proterozoic. The mantle has evolved continuously with the progressive extraction of mafic magmas but, allowing for this, continental tholeiites appear to have changed little with time. Ancient mid-ocean ridge basalts are elusive because of their poor preservation record. The geochemistry of mafic volcanics occurring in many greenstone belts commonly suggests a continental contamination component and that these volcanic rocks are most closely akin to those from present-day arc systems. The relative abundance of early Precambrian intrusive mafic suites parallels the crustal growth curve, reflecting the global crustal accretion–differentiation superevent towards the close of the Archaean. Bimodal tholeiitic–noritic mafic magmatism associated with many of these intrusive suites stems from this rapid turnover in the mantle–mafic crust–continental crust system. This, in turn, gives a possible link between the respective evolutions of Archaean to Recent komatiitic, noritic and boninitic suites.
Multivariate techniques for biostratigraphical correlation
Authors: KOVACH W. L.
Published: December 2007

Multivariate techniques summarize large sets of data and present their major trends in a graphically simplified manner. This paper discusses ways in which these techniques can be used to correlate biostratigraphical data. Stratigraphically constrained cluster analyses, which highlight distinct zones, may be performed on a number of sections. These can be plotted together to provide a concise summary that allows sequences to be compared. Ordinations, such as principal components and detrended correspondence analyses, can be performed on several sections simultaneously to extract a new set of axes that represent the combined trends of various taxa. The first few major axes can be plotted against depth as ‘biostratigraphical logs’; the rest can be ignored as insignificant variation or noise. Sections can then be correlated based on these logs, either manually or using procedures such as sequence slotting. These techniques have been tested using artificial data from
Edwards (1984)
, consisting of taxa of known spatial and temporal ranges. Correlations between sections were found to match closely the true ages. Inconsistencies in the data, such as time-transgressive taxa, were isolated on one axis, allowing them to be removed or studied further.
Proximal deposits at a fault-controlled basin margin, Upper Miocene, SE Spain
Authors: LONERGAN L. , SCHREIBER B. C.
Published: December 2007

Upper Miocene sediments of a proximal basin-margin facies association were deposited on the eastern edge of the Sierra Espuña mountains of the Betic Cordillera, SE Spain. These are interpreted as having been deposited along an active, basin-bounding fault. The sequence is composed of mixed marine carbonate and clastic sediments with conglomerates and calcarenites containing coralgal detritus passing distally into finer-grained sediments. Laterally along the faulted margin there are marked variations in facies and clast composition over very short distances. Clastic conglomerates interfinger both laterally and upwards with algal and coral reefs. Sedimentary facies analysis and provenance data are used to infer that mass flows coming off the steep basin margin fed talus cones. Laterally to the talus cones a fan delta coexisted with active algal and coral reefs.
Structure of Blackdown pericline, Mendip Hills, from gravity and seismic data
Authors: ATES A. , KEAREY P.
Published: December 2007

Blackdown pericline is the western pericline of the Mendip Hills, which were formed during the Variscan Orogeny at the end of the Carboniferous. Unlike the more easterly periclines, Blackdown pericline has no associated magnetic anomaly, indicating the absence of the Silurian volcanic rocks responsible for the anomaly in eastern Mendip. The gravity field of Blackdown pericline cannot be adequately modelled by the known geology of the Mendip Hills and requires the presence of an anomalously dense body at shallow depth. The geometry of the body is inferred from a seismic reflection profile and is interpreted as a wedge of Precambrian basement rocks. Its high density and negligible susceptibility may indicate an amphibolitic composition. Three models involving Variscan thrusting and post-Variscan listric normal faulting are presented in explanation of the presence of the wedge. The anomalous body is shown to be laterally continuous to west and east. A linear negative gravity anomaly to the north of Blackdown pericline is interpreted to be caused by a small occurrence of Coal Measures, which have not before been described from this area.
Neotectonics: current themes and future prospects
Authors: STEWART I. S.
Published: December 2007

This was a two-day conference entitled ‘Neotectonics: recent advances’ convened by L. Owen, I. Stewart, C. Vita-Finzi & N-A. Mörner at Burlington House on 16–17 June 1992. The meeting, held under the auspices of the Quaternary Research Association, the Tectonic Studies Group and the INQUA association, was designed as a forum for discussion on the current state of play of an emerging discipline and the future directions to which the subject might look.

With seven years having passed since Hancock & Williams (1986) convened a meeting on the burgeoning theme of ‘Neotectonics’ at Burlington House, it appeared timely to assess the degree to which the subject had subsequently progressed and to promote new developments taking place within it. In contrast to the initial meeting, which was organized on behalf of the Tectonic Studies Group and attended largely by UK-based structural geologists and seismologists, the recent meeting attracted a large international response from researchers in a wide range of fields, reflecting more fully the multi-disciplinary nature of the subject.

An important facet of the meeting was to encourage international collaboration between neotectonic communities, and to this end the conference hosted business meetings for the INQUA Neotectonics Commission, a subcommission on the Formalisation of Neotectonic Maps and the International Lithosphere Programme’s ‘Palaeo-seimicity of the Holocene’. It was particularly gratifying that, of over one hundred papers submitted for oral and poster presentation (Mörner
et al.
1992), the vast majority of these were from overseas scientists. In all, the meeting itself was attended
Structure and origin of limestones
Authors: SELLWOOD B. W.
Published: December 2007

Sorby’s Presidential Address of 1897 on the structure and origin of limestones was essentially an interim report of research in progress. His petrographic approach to limestones, stemming from three decades of research, laid the foundations for a wide range of research lines, some of which were not fully exploited for almost a century, such as fluid inclusion studies in diagenesis. The significance of many of his discoveries (e.g. that some Jurassic ooids and Palaeozoic corals were originally calcitic) had geochemical implications that have only recently achieved research prominence (e.g. CO
2
and the ‘greenhouse’ Earth). In addition, Sorby’s legacy was the example of his peerless approach to research, applying ruthless empiricism to the problem at hand. In the case of limestones, this involved an application of meticulous descriptive petrography and innovative experimentation. His approach to understand more fully the complex problems posed by carbonates remains unsurpassed, involving a thorough integration of detailed observation, imaginative thinking and judicious use of analytical techniques.
Deflection versus reflection of sediment gravity flows in the late Llandovery Rhuddnant Grits turbidite system, Welsh Basin
Authors: CLAYTON C. J.
Published: December 2007

The Rhuddnant Grits turbidite system was deposited in an elongate, fault-bounded trough, an apparently ideal locality for the reflection of sediment gravity flows as solitons. However, 280 palaeocurrent readings from turbidite sandstones do not show the expected high angle discrepancies between palaeocurrent directions from sole structures and those from cross-lamination. Palaeocurrents orthogonal to the bounding lineament are not generally observed. Instead there is a gradual, progressive, anticlockwise deflection of the currents with distance from source. This may be explained by banking of the currents against an intrabasinal slope, by the Coriolis effect, or by a combination of the two processes.
The polarity of the Silurian magnetic field: indications from a global data compilation
Authors: TRENCH A. , McKERROW W. S. , TORSVIK T. H. , LI Z. X. , McCRACKEN S. R.
Published: December 2007

Silurian palaeomagnetic data from all continents are compiled in an attempt to define the polarity history of the Earth’s magnetic field during the period. Data sets from Llandovery, Ludlow, and Pridoli strata often display evidence of field reversals (i.e. the presence of both normal and reverse polarity magnetization directions) despite the fact that the palaeomagnetic sample collections may cover only a limited stratigraphic range in each case. Palaeomagnetic data from rocks of Wenlock age are exclusively of normal magnetic polarity. A comparison of the available Silurian data with published Ordovician palaeomagnetic results suggests that the frequency of magnetic field reversals increased from Ordovician to Silurian times. Palaeomagnetic data sets are not yet available from a continuous Silurian stratigraphic sequence of any great duration, so the Silurian magnetostratigraphy is still at a much more preliminary stage than that for the Ordovician. Apart from the Wenlock, all Silurian series have some intervals with frequent (<2 Ma) polarity reversals. Constant polarity (for durations in excess of 2 Ma) in the Ordovician and Silurian are only apparent during the Arenig (reverse), the early Caradoc (normal), the late Caradoc (reverse), the Ashgill (normal) and the Wenlock (normal). Data are still particularly sparse in Tremadoc and Llandovery rocks.
Late Proterozoic to Early Palaeozoic tectonism in Dronning Maud Land, Antarctica: supercontinental fragmentation and amalgamation
Authors: MOYES A. B. , BARTON J. M. , GROENEWALD P. B.
Published: December 2007

The Mid-Proterozoic-Early Palaeozoic tectonic development of western Dronning Maud Land is examined in the light of recent proposals that suggest geological continuity between Laurentia and Gondwana during the Middle Proterozoic (the SWEAT hypothesis). Gondwana amalgamation from the remaining constituents followed fragmentation and rifting of Laurentia from this supercontinent, culminating in a Late Cambrian orogeny at
c
. 500 Ma. The Mid-Proterozoic-Early Palaeozoic tectonic development of western Dronning Maud Land was significantly different to that observed in the Transantarctic Mountains, and the
c.
500 Ma event in particular is characterized by: (1) a regional isotopic resetting event at
c.
470 Ma, (2) scarce coeval magmatism, comprising only one major intrusion of alkaline affinity, and volumetrically small ‘S-type’ granitic sheets derived from local anatexis of gneisses, (3) structural deformation considerably less intense than during the preceding Kibaran (1100 Ma) orogeny, comprising relatively open to gentle folding, (4) reactivation of thrusting initiated during the Kibaran orogeny, with associated metasomatism. It is concluded that the effects of the Late Cambrian (
c
. 500 Ma) event are consistent with crustal thickening through large-scale underplating, rather than subduction-related processes as seen in the Ross orogeny of the Transantarctic Mountains. It is also concluded that Gondwana amalgamation occurred along suture lines not currently exposed in outcrop in Dronning Maud Land. These data neither contradict nor support the SWEAT hypothesis as currently argued.
The Permian to Jurassic stratigraphy and structural evolution of the central Cheshire Basin
Authors: EVANS D. J. , REES J. G. , HOLLOWAY S.
Published: December 2007

New seismic reflection data reveal details of the Permo-Triassic basin fill in the central Cheshire Basin. The seismic stratigraphy indicates that the Permian and Triassic succession is far thicker than previously estimated. Seismic data and backstripping of the preserved Permian to early Jurassic sedimentary succession suggest that at least two phases of fault-controlled subsidence occurred, in early Permian and early Triassic times. An intra-Triassic unconformity is seen on the seismic data near the top of the Sherwood Sandstone Group. This may equate with the Hardegsen Unconformity. It is possible that this unconformity was generated by syn-extensional regional uplift accompanying lithospheric thinning. The gross morphology and location of the faults on the eastern margin of the Cheshire Basin are interpreted to be, at least locally, controlled by extensional re-activation of a westwards-dipping reverse fault, possibly an extension of the Pontesford–Linley lineament.
Incremental caldera collapse of Suswa volcano, Gregory Rift Valley, Kenya
Authors: SKILLING I. P.
Published: December 2007

Suswa volcano, located at 1°10′S, 36°20′E, is Quaternary in age (<0.4 Ma), dominantly trachytic-phonolitic in composition, and has two calderas. Regional extension was a fundamental control on caldera collapse, providing pathways for the siting, drainage and recharge of magma chambers. Caldera I collapse was associated with magmatic overpressure from volatile exsolution, magma-water interaction, influx of denser magma and magma drainage at depth. Trachybasalt ash, trachyte globular-ash ignimbrites, trachyte pumice lapilli air-fall tuffs and carbonate-trachyte ignimbrites characterize the initial subsidence. Air-fall tuffs, erupted during caldera collapse at Longonot, are interbedded, suggesting a regional collapse event. Incremental, but dominantly Valles-type, collapse continued with the eruption of trachyte agglutinate flows from concentric ring-fractures outside the caldera ring-fault (Ring-Feeder Zone) and trachyte pumice lapilli air-fall tuffs from west caldera I.

Following caldera I collapse, phonolite lava flows were erupted from the caldera floor. Centrally-erupted phonolite lava flows led to the construction of Ol Doinyo Onyoke lava cone. A pit-crater on the cone was a precursor to the collapse of caldera II, both of which were generated entirely by magma withdrawal. Regional decompression caused ring-fault bounded, block-resurgence of the caldera floor
Welded breccias in andesite lavas
Authors: SPARKS R. S. J. , STASIUK M. V. , GARDEWEG M. , SWANSON D. A.
Published: December 2007

Flow breccias, formed at the margins of blocky andesite lavas, can be reheated, welded and sometimes deformed to form rocks reminiscent of welded pyroclastic rocks. Reheating occurs due to advection of heat from the flow to the basal breccias. Surface breccias also infill extensional crevasses which are later closed and compressed. Strongly deformed welded breccia bands form within the flow levees. The angular breccia clasts can be deformed into fiamme.
Cenozoic intra-plate volcanism related to extensional tectonics at Calatrava, central Iberia
Authors: LÓPEZ-RUIZ J. , CEBRIÁ J.M. , DOBLAS M. , OYARZUN R. , HOYOS M. , MARTÍN C.
Published: December 2007

The Calatrava Volcanic Province is characterized by an intra-plate alkaline undersaturated magmatic association of melilitites, nephelinites, alkali olivine basalts and leucitites that were extruded in the central Iberian Peninsula (Spain) during Late Miocene–Quaternary time. This volcanism can be related to the following two-stage scenario: (1) an initial mantle-diapir stage, during which magma generation would have occurred at the lithosphere/asthenosphere boundary triggering minor accumulation processes at the crust/mantle interface, and consequent weakening/extension of the upper crust; and, (2) a final indentation stage related to disruption effects caused by the tectonic welding of the Prebetic Arc onto the Iberian foreland.
Plate-moving mechanisms: constraints and controversies
Authors: WILSON M.
Published: December 2007

The nature of the driving forces for plate tectonics has been a subject for intense, often heated debate, for more than a quarter of a century. On 15 April 1992 Peter Ziegler delivered the annual William Smith lecture to the Geological Society on the subject of Plate-Moving Mechanisms, followed by a series of lectures on related themes. This thematic set of papers, resulting from the meeting, covers some of the major issues discussed.

For more than a quarter of a century Earth Scientists have debated the nature of the forces which drive the lithospheric plates. One of the major problems has been whether mantle convection, including the activity of mantle plumes, dominates or whether boundary and plate forces, such as slab pull and ridge push, exert the most important controls on plate motions. Clearly the relative importance of the different forces involved may vary, both in space and time, and they may either reinforce or counterbalance each other depending upon the tectonic setting.

On 15 April 1992 Peter Ziegler delivered the annual William Smith Lecture to the Geological Society on the subject of
Plate-Moving Mechanisms.
This was followed by a further eight presentations on related themes by an international group geoscientists from Europe and North America. Of the nine papers presented at the meeting, six are included in this thematic set

(Ziegler, Bott, Wilson, Searle
et al.,
Doglioni, Pavoni).

The other speakers were Randall M. Richardson, University of Arizona, Tuscon, USA, John F. Dewey, Oxford and Sierd Cloetingh, Free University,
Vertebrate fissure faunas with special reference to Bristol Channel Mesozoic faunas
Authors: SAVAGE ROBERT J. G.
Published: December 2007

Throughout the Mesozoic and Cenozoic, tectonically activated joint fissures and karstic cave systems have acted as reservoirs for terrestrial vertebrate remains. They yield concentrations of microfaunal elements rarely if ever preserved in other situations. In the 1850s, Charles Moore realized their potential and exploited them to discover the earliest known mammals in late Triassic fissure infillings in the Carboniferous Limestone of Somerset. The history of that discovery and its associated problems of interpretation are recorded. His work greatly influenced succeeding generations of palaeontologists and, as techniques developed, fossil fissure discoveries have made a major impact on our understanding of vertebrate evolution.