bluuebunny/crossref_metadata_2025 huggingface dataset:
Interpretation of the gravity field of the Lac Fournier & Romaine River anorthosite massifs, eastern Grenville Province: significance to the origin of anorthosite
Authors: Kearey P. , Thomas M. D.
Published: December 2007

A large positive gravity anomaly (amplitude >600 g.u.) coincides with the E part of the Lac Fournier Massif, an anorthosite complex in the Quebec part of the eastern Grenville Province. A more mafic phase of anorthosite than found elsewhere in the complex is present in the anomalous region, and density sampling indicates that the anomaly is probably caused by a large subsurface body of gabbro. The residual gravity anomaly indicates that the body’s anomalous mass is at least 2.6 × 10
15
kg, corresponding to a minimum gabbro volume of 1.0 × 10
4
km
3
. Negative residual anomalies flanking the positive appear to be caused by bodies of relatively low density anorthosite.

The preferred three-dimensional interpretation of the causative body of the positive anomaly is of an uppermost flat inverted cone extending to a depth of
c
. 5 km with two roots extending to
c
. 12 km. A model with outward sloping contacts can also simulate the observed anomalies, but is considered unlikely on petrogenetic grounds and by comparison with other basic intrusions whose geometry is known. Bodies of anorthosite flank the inferred gabbro body and extend to depths of
c
. 5 km.

The gabbro represents the lowest level of a differentiated magma body initially emplaced by forcible intrusion. During differentiation mafic minerals sank in the magma chamber while plagioclase feldspar separated at a higher level, perhaps by flotation. The present flanking locations of anorthosite may have been accomplished by buoyant ascent and lateral spreading.

The interpreted gabbroic body would have significant gravitational effects even at great depth. This places constraints on the origin of anorthosites in the western Grenville Province as no such positive gravity anomalies have been observed over these complexes.
The recognition and division of the Tournaisian Series in Britain
Authors: Ramsbottom W. H. C. , Mitchell M.
Published: December 2007

A recent re-definition of the base of the Tournaisian at the base of the Carboniferous and a revised correlation of the base of the Chadian Stage with the base of the Viséan allows the re-adoption in Britain of the Series name Tournaisian, which was not used in the Geological Society's Special Report on the Dinantian (George
et al.
1976). In addition, the two Belgian Tournaisian stages Hastarian and Ivorian can also be recognized in Britain and their use is recommended. The regional stage name Courceyan becomes redundant. 4 new biozonal names are proposed.
Metamorphism and orogeny in the Irish Dalradian
Authors: Yardley B. W. D.
Published: December 2007

The Irish Dalradian was locally heated to garnet zone temperatures prior to the development of major isoclinal fold structures, possibly at moderately high pressures. The peak of Barrovian metamorphism was reached after the major structures had formed. In Connemara, metamorphism may have been initially of Buchan type in the S, where synorogenic intrusions occur. Connemara suffered subsequent development of major northward facing nappe structures and the high grade rocks were thrust over low grade metasediments in the S, possibly indicating continent-continent collision. This deformation was accompanied by uplift, and low pressure assemblages were superimposed on earlier Barrovian metamorphism in the N. Thermal aureoles around post-tectonic granites show that most Irish Dalradian rocks underwent uplift and erosion after folding, and before granite emplacment. The highest metamorphic grades are now exposed in Connemara, Tyrone and in a region from Belmullet to Castlederg that is centred around windows of probable basement.
Volcanic rocks beneath the Semail Ophiolite nappe in the northern Oman mountains and their significance in the Mesozoic evolution of Tethys
Authors: Searle M. P. , Lippard S. J. , Smewing J. D. , Rex D. C.
Published: December 2007

Volcanic rocks, locally up to 700 m thick, occur within a complex imbricate zone beneath the Upper Cretaceous Semail Ophiolite nappe in the northern Oman mountains. Although the rocks were considerably disrupted during late Cretaceous thrusting and nappe emplacement, intact sequences occur and show that there is a lower unit of alkaline lavas and pyroclastics, including ankaramites, alkali basalts and trachytes, and an upper one of predominantly tholeiitic pillow lavas. The latter contain interbedded cherts and limestones and well as large blocks of Permian and Triassic ‘exotic’ limestones. In places the volcanics are intruded by minor intrusives of alkaline peridotite and gabbro.

Geochemical studies, particularly of the ‘immobile’ elements, show that the lower volcanics and the intrusives are strongly alkaline with high Ti, P, Zr and Nb contents and steep LREE enriched rare earth patterns. They are typical ‘within-plate’ alkaline magmas characteristic of continental rift zones and some ocean islands. Within the tholeiitic basalt lava pile two types are recognized: a predominant relatively trace element enriched ‘transitional’ type which probably formed in a within-plate oceanic setting, and a ‘depleted’ type with the geochemical characteristics of some island arc tholeiites.

K-Ar ages of biotite separates from the alkaline lavas give Triassic ages (230-200 Ma) while one of the alkali periodotite sills gives an Upper Cretaceous age (92 Ma). The bulk of the tholeiites are late Triassic as they are interbedded with or enclose the Upper Triassic ‘exotics’. The Triassic volcanics are interpreted as having formed during continental rifting and then as marginal ocean islands during the early stages of ocean basin development. However, preliminary radiolarian ages from interbedded cherts in the uppermost lavas are Cretaceous. The presence of these island-arc type tholeiites is taken as evidence that subduction off the Oman continental margin began prior to ophiolite emplacement in the late Cretaceous.
Magma accumulation in the ascending mantle
Authors: Maaløe S.
Published: December 2007

SUMMARY: The most important factors which control the accumulation of primary magma in the mantle are:

(1) The degree of partial melting required for permeability, which is suggested as10–20%.

(2) The creep properties of the partially molten mantle, the type of creep probably being Nabarro-Herring creep.

(3) The type of flow of the ascending mantle material; the flow may be parallel or divergent dependent on the depth.

(4) The depth of extension of the feeder dykes depending on the tensional stress distribution with depth and the rheological properties of the mantle. It is shown that the partially molten mantle becomes stratified into layers of magma and residuum, the layers of residuum being one or several metres thick. The stratified mantle constitutes the primary source of magma. A dispersed source system is formed in ascending convection currents, while a large magma chamber is formed in ascending plumes.

The small diffusion rates imply that the melting is of a fractional type rather than batch melting.
A seismic refraction investigation of the Charnian basement and granitic intrusions flanking Charnwood Forest
Authors: Whitcombe D. N. , Maguire P. K. H.
Published: December 2007

Results from two seismic refraction profiles radiating NE and SW from the Precambrian Charnian block of Charnwood Forest in the central Midlands of England are described and interpreted using surface geology, borehole, aeromagnetic and gravity data, and previous seismic research.Charnian rocks probably extend beneath Palaeozoic and Mesozoic cover along the length of the profiles.However, to the NE of Charnwood the Charnian basement is intruded by a NE extension of the Mountsorrel granodiorite complex, while to the W it is intruded by a possible NW extension of the S Leicestershire and/or S Charnwood diorites.
Thermal and kinematic control on ocean-ridge magma fractionation: contrasts between Atlantic and Pacific spreading axes
Authors: Flower M. F. J.
Published: December 2007

Differences in the lithology and chemistry of mid-ocean ridge basalts generated at Pacific and Atlantic spreading axes reflect a simple correlation to the spreading rate. On the basis of experimental data and thermal models, these differences are attributed to variation in the nature of magma supply and fractionation processes as a function of spreading rate, rather than to processess in the upper mantle. Thermal and kinematic models for ocean ridges predict stable steady-state magma reservoirs at faster-spreading axes (>5 cm/yr) and more restricted, transient systems at slower-spreading (<5 cm/yr) axes. The chemical and lithological character of basalts confirms this pattern and suggests that slow-spreading axes(e.g. the Mid-Atlantic Ridge) are characterized by polybaric magma fractionation in complex reservoir systems, reflected by the widespread accumulation of calcic plagioclase in low viscosity melt, phenocryst reaction morphologies and pyroxene-dominated fractionation ‘extracts’. Fast-spreading axes (e.g. East Pacific Rise) show low-pressure basaltic fractionation trends to iron-rich compositions with little evidence for plagioclase accumulation or crystal-liquid reaction, in comparatively stable shallow-level chambers. Spreading rates of 5–6 cm/yr in the Gulf of California may be reflected by ‘intermediate’ fractionation regimes influenced by some magma crystallization at depth with incipient accumulation of plagioclase followed by further fractionation at low pressure. The major effect of spreading rate differences appears to be on the lithological and chemical patterns of basalts induced after partial melting, with minimal influence on the composition of primary melts.
A revised sea-level curve for the early Jurassic
Authors: Hallam A.
Published: December 2007

A comparison is undertaken of three different methods of eustatic sea-level analysis: (1) shallowing and deepening/transgressive events inferred from the facies sequence, (2)changes through time in the areal spread of epicontinental seas,(3) seismic stratigraphy. The results of these methods are compared, mainly with reference to the Lower Jurassic. New data are brought forward from several continents in a stage-by-stage study and it is concluded that theLias was characterized by interrupted sea-level rise with no major regressions.Both the transgressions and regressions were relatively rapid,with the major transgressions associated with widespread anoxic bottom waters.Estimates of the net amount of sea-level rise through early Jurassic time range from ∼75 m to ∼110 m.
The geology of the Dibba zone, northern Oman mountains; a preliminary study
Authors: Lippard S. J. , Smewing J. D. , Rothery D. A. , Browning P.
Published: December 2007

The Dibba zone is a narrow, structurally complex, NE-SW trending part of the Oman mountains thrust front. It comprises a series of imbricated and folded thrust sheets of shelf-edge and slope facies Mesozoic carbonates, basinal facies sediments and a sedimentary mélange, overlain by a thrust sheet of sub-ophiolitic greenschist and amphibolite facies metamorphic rocks and the basal part of the Upper Cretaceous Semail ophiolite nappe. Thrusting during the Late Cretaceous was directed towards the W and NW across the former continental margin. It is postulated that during the Mesozoic the area lay on a SE-facing, probably transform fault controlled, section of the NE Arabian continental margin. There is no evidence for major post-early Mesozoic high-angle faulting along the Dibba zone, as has previously been postulated in tectonic syntheses of the area.
Geophysical evidence concerning the southern boundary of the London Platform beneath the Hog’s Back, Surrey
Authors: Smalley S. , Westbrook G. K.
Published: December 2007

The results of a gravity survey have been interpreted with seismic reflection and borehole data to show that a major change in the structure of the upper crust underlies the Hog’s Back W of Guildford, Surrey. The pre-Mesozoic basement beneath the thin Mesozoic succession N of the Hog’s Back is of a lower density than that underlying the thick Mesozoic succession to the S. This lower density basement was probably the principal cause of the relative stability of the London Platform during the Mesozoic era. The sourthern boundary of the London Platform may have been initiated by Variscan thrusting with subsequent reversal of displacement, or may be inherited from an older crustal lineament.
Plate tectonics and the evolution of the British Isles
Authors: Dewey John F.
Published: December 2007

This paper is an essay on the rationalism and difficulties of the geological corollaries of plate tectonics, with illustrations particularly from the geological history of the British Isles. There is special emphasis on the mechanics of continental lithosphere in extension, when basins are formed and in convergence whereby continental lithosphere thinned in extension is shortened and restacked to form orogenic belts. An upper lithosphere stress-concentrating elastic lid suffers extremely complex inhomogeneous deformations above a simpler, viscoelastic lower lithosphere whose deformation drives strain and displacement in the elastic lid. The very nature of plate tectonics as relative motion among a multi-plate mosaic of plates provides a rational semi-quantitative framework for large-scale geological complexity in space and time. The Caledonian and Hercynian history of the British Isles may be explained semi-quantitatively in terms of plate boundary evolution but it is not possible to define unequivocal solutions. The rationalism of plate tectonics is exhibited on a smaller, more direct field-scale by many of the complexities of ophiolite complexes, such as Ballantrae (Scotland), that can be explained as the effects of the evolution of ridge/ridge transforms and by diachronous deformation and sedimentation in subduction accretion prisms such as the Southern Uplands. Deformational and sedimentary patterns in NW Europe from Triassic times to the present day resulted from a complex interaction of extensional events related to diachronous Atlantic opening and compressional events related to sequential Alpine shortening phases. Those widespread events are typical of intracontinental plate boundary systems and create great difficulties for the geologist wishing to decipher relative plate motion from geological sequences, because huge areas, much larger than the British Isles, must be analysed and synthesized before much sense can be extracted. Further difficulties arise from the palinspastic and preservation problems of convergent boundaries; orogenic shortening telescopes and partially subducts facies belts. Most orogenic belts were assembled as a collage of displaced and rotated terrain elements that were subsequently flattened, elongated and straightened to an apparent but deceiving simplicity. Therefore, while plate tectonics provides a rational geological framework against which, for example, British basin evolution can be tested and explained, we cannot yet use the geological effects of plate boundary evolution to reconstruct relative plate motion quantitatively.
The plate tectonic setting of NW Britain and Ireland in late Cambrian and early Ordovician times
Authors: Yardley B. W. D. , Vine F. J. , Baldwin C. T.
Published: December 2007

Deposition of Dalradian sediments in a stretching ensialic basin peripheral to Iapetus continued into Cambrian times. Late in the history of Dalradian deposition a southern continental landmass was removed by strike-slip motions and a subduction zone then developed beneath the Dalradian sediments. Deformation and moderately high pressure metamorphism of Dalradian rocks occurred and was followed by thermal relaxation and development of a volcanic arc with magmatic centres in Connemara by Tremadoc times and in NE Scotland by the Arenig. The S Mayo Trough was a back-arc basin that opened shortly after the initiation of the arc, the oldest volcanic rocks seen there being the extrusive equivalents of the basic intrusions of Connemara. Dalradian rocks outcropping in Ordovician times over the Highlands and probably extending southwards are a possible source for high pressure metamorphic clasts at Ballantrae, while clasts of arc volcanics may have come from N of the Highland Boundary Fault. Plate models that involve a separate arc in the Midland Valley are rejected because the tectonic elements are impossibly small by comparison with modern convergent plate margins. The arc rooted in Dalradian rocks ceased to be active at the end of the Llanvirn as a result of a further change in plate motions, and there are considerable spatial problems in locating an arc coupled to the Southern Uplands accretionary prism after Llanvirn times.
Strike-slip orogens—an explanation for the Hercynides
Authors: Badham J. P. N.
Published: December 2007

The nature of oblique plate interactions is not yet well understood. The oblique interaction of Pacific and N American plates has resulted in large scale migration, rotation and accretion of microplates. Intervening basins are ephemeral, individually filled and deformed. Oblique motion results in zones of both compression and extension. The latter may be sufficient to generate new oceanic crust and the former to obduct it. Both subduction and collision are restricted in time and space. The resulting orogen is extraordinarily complex in detail. The far more complicated Hercynides are the result of dextral interaction between continental plates—Europe and Africa. The lateral variability of the orogen, the major and long-lived strike-slip faults, the bimodal volcanism and related ore deposits of the European trough, the Lizard ophiolite and its mélanges, the NW Spanish ophiolites and the Iberian Pyrite Belt are all explained using an oblique collision model. In particular the generation, deformation and obduction of ophiolites in oblique-slip mobile zones is discussed.
The evolution of the Hercynides of central SW England
Authors: Isaac K. P. , Turner P. J. , Stewart I. J.
Published: December 2007

Towards the end of Lower Carboniferous times, sedimentation, accompanied by widespread igneous activity, proceeded in a tectonically active, unstable and constricted basinal zone. Viséan flysch and olistostromic sequences were generated in front of an advancing deformation front which caused erosion of submarine rises and heralded the beginning of the Hercynian orogeny in this region.

Between Dartmoor and Bodmin Moor, Upper Devonian basin and rise facies sediments were thrust northwards over both Upper Devonian and Lower Carboniferous northern equivalents which were themselves incorporated onto the allochthon. The tectonic deformation history began in the late Viséan and early Namurian with the gravity sliding (Dl) of the bulk of the Lower Carboniferous northwards to form the Greystone Nappe. Subsequently the main thrusting event (D2) from late Lower Namurian times onwards led to the successive overthrusting and emplacement of the Heathfield, Petherwin and Tredorn Nappes. Towards the end of the Namurian and in the Westphalian the intrusion of the granite batholith caused the fold belt to tighten, which resulted in the formation of numerous low angle normal faults (D3) offsetting earlier thrusts to the N. Overall the regime is one of very thin-skinned thrust and nappe tectonics with the allochthon probably not exceeding 1 km in thickness.
An interpretation of the Variscan structures in SW England
Authors: Shackleton R. M. , Ries A. C. , Coward M. P.
Published: December 2007

The Variscan structures in SW England, apart from the high-level structures in the Culm synclinorium of N Devon, are recumbent with an overall gentle southward dip of cleavages, thrusts and recumbent folds. Translation was towards the NNW. The structures are interpreted on a thin-skinned model, with a basal décollement dipping gently S, related to a S-dipping subduction zone S of the Lizard. The granites are thought to have been generated to the S of their present position and injected northwards above the décollement zone.
The late Palaeozoic to early Mesozoic evolution of southern South America: a plate tectonic interpretation
Authors: Forsythe R.
Published: December 2007

The limits of 3 distinct upper Palaeozoic to lower Mesozoic tectono-stratigraphic zones of the ancestral Pacific margin of Gondwana in southern South America are documented. These are: a fore-arc region consisting in part of ‘oceanic’ components; a magmatic arc that is coincident with Carboniferous to Triassic continental sedimentation; and a back-arc region comprising the upper Palaeozoic epicratonic sequences of the ‘Samfrau Geosyncline’. These zones, traceable 2300 km from 29–56
°
S (Cape Horn), document the semi-continuous subduction of the ancestral Pacific floor from the Middle Devonian to the Triassic. Between 29
°
S and 37
°
S the palaeo-subduction zone was roughly coincident with the present Pacific coast line. Between 37
°
S and Cape Horn, the subduction zone migrated progressively westward as an ‘Alaskan’ style accretionary prism grew to a width of 250 km. While in the northern sector, Jurassic are intrusions are coincident with the upper Palaeozoic are intrusions, in the S they intruded the upper Palaeozoic to lower Mesozoic accretionary prism 200 km W of their Palaeozoic equivalents. No post middle Palaeozoic ‘sutures’ have been identified in southern South America. Hence, there is no evidence of late Palaeozoic, Mesozoic, or Cenozoic accretion of discrete microplates.
Radiometric evidence for the age of the subduction complex in the South Orkney and South Shetland Islands, West Antarctica
Authors: Tanner P. W. G. , Pankhurst R. J. , Hyden G.
Published: December 2007

New K-Ar and Rb-Sr data are presented for the age of part of the supposed late Palaeozoic/early Mesozoic metamorphic complex in West Antarctica (here renamed the ‘Scotia metamorphic complex’). These results, together with a review of other radiometric and palaeontological evidence, fail to confirm a pre-Mesozoic age for the subduction complex in this region although rocks belonging to the albite-epidote-amphibolite facies could be as old as Permian. Blueschists and greenschists from the South Shetland Is appear to be significantly younger and were probably formed, in part at least, during Cretaceous subduction.
Composition and origin of garnet from the Antarctic Peninsula Volcanic Group of Trinity Peninsula
Authors: Hamer R. D. , Moyes A. B.
Published: December 2007

Garnet is widely found as a minor constituent in rocks of the Antarctic Peninsula Volcanic Group (APVG) of Trinity Peninsula. It occurs as conspicuous megacrysts, or in xenoliths within volcanic rocks of andesitic-rhyolitic composition and as detrital grains in the associated terrestrial sediments. It is also found as an accessory mineral in many plutonic rocks from the E coast of the Antarctic Peninsula. Evidence is presented to show that the garnet can be divided petrographically and chemically into two main groups: Type A: almandine-rich primary igneous garnet, and Type B: less almandine- and more pyrope-rich garnet as xenocrysts or included in xenoliths within the volcanic rocks.

Comparison with published experimental data on garnet occurrence in acidic igneous rocks suggests that high almandine-low spessartine garnet in volcanic rocks is a remnant phase of high pressure crystallization from magma at pressures of >7 kbar. Garnet with a higher pyrope content is regarded as xenocrystal in origin, having been derived from garnet-bearing country rocks at depth, either as accidental inclusions or through direct partial melting (restite) of the lower crust, and implies that a considerable thickness (>25 km) of crustal material was in existence before the generation of the Mesozoic magmatic arc. The origin of these calc-alkaline magmas may therefore be due, at least in part, to partial melting of pre-existing sialic crustal material.
The gravity anomaly of the St. George's Channel Basin, southern Irish Sea—a possible explanation in terms of salt migration
Authors: Dimitropoulos K. , Donato J. A.
Published: December 2007

Gravity profiles across the St. George's Channel Basin show a marked discrepancy between the observed anomalies and the anomalies calculated using a model based upon an interpretation of seismic data. This difference may be explained in terms of salt migration away from the centre towards the flanks of the basin. Evidence for such a migration path is provided by the seismic data. An excess of salt on the SE side of the basin has intruded into a major fault giving rise to a salt wall. The gravity models suggest that the basin reaches a thickness of 10 km near its centre but that Permian and Carboniferous sediments are possibly thin or absent.
Coralline algal reef frameworks
Authors: Bosence D. W. J.
Published: December 2007

Published and unpublished accounts of coralline algal reef frameworks are reviewed. The descriptions are divided into frameworks constructed from crustose coralline algae and those constructed from branching corallines. Crustose frameworks are briefly described and illustrated from the Miocene of Malta, Recent ‘coralligène’ from the Mediterranean, Eocene reefs from Spain, Recent algal reefs, of St Croix, U.S. Virgin Is. and algal reefs from Bermuda. Branching frameworks are briefly described and illustrated from the Recent maerl of the NE Atlantic, Recent mud mounds from Florida, Recent algal reefs from St Croix and Recent algal ridges on Pacific reefs. Crustose and branching frameworks show an increase in strength of construction, early submarine cements and macro-borers and a decrease in erodibility from low to high energy environments. The occurrence of genera in coralline frameworks is primarily controlled by their geographic and water-depth ranges. The construction of coralline frameworks is seen as the result of competition for living space on shallow marine hard substrates.
Some potential applications of pyrolysis to basin studies
Authors: Horsfield B. , Dembicki H. , Ho T. T. Y.
Published: December 2007

A variety of pyrolysis systems can be used to obtain source rock evaluation and other data. Systems with bulk flow capabilities are best used to screen samples for subsequent detailed analysis, but data must be interpreted in the light of possible mineral-pyrolysate interactions. Non-indigenous hydrocarbons can be detected by the same techniques, but pyrolysis-FID/-gc or alternatively pyrolysis-gc plus pyrolysis-bulk flow are required to identify and differentiate migrated hydrocarbons and refined petroleum products. Screening can be carried out in the laboratory or at the well location. In the latter case, data can be used for on-site decision making, as well as providing the groundwork for subsequent geochemical studies. Pyrolysis-FID/-gc provides the most comprehensive means for screening in both laboratory and well-site situations. For detailed geochemical analysis, pyrolysis-gc is the most versatile technique, providing both quantitative and qualitative ways to identify kerogen type, classify bitumens, and recognize contamination.
Zabargad (St. John’s) Island: an uplifted fragment of sub-Red Sea lithosphere
Authors: Bonatti E. , Clocchiatti R. , Colantoni P. , Gelmini R. , Marinelli G. , Ottonello G. , Santacroce R. , Taviani M. , Abdel-Meguid A. A. , Assaf H. S. , El Tahir M. A.
Published: December 2007

Zabargad is a small island located in the Red Sea about 50 km W of its axis, between 23º and 24ºN. The island is not volcanic but probably represents an uplifted block of upper mantle and crustal rocks, and yields information on the nature of the underlying lithosphere.

Peridotites are the main rock type, consisting of exceptionally fresh spinel lherzolites, which equilibrated last at a depth >30 km in the mantle. Other ultramafic facies, some representing rocks which mixed with a basaltic melt fraction, and some which underwent metasomatic exchange at depth, are also present. The peridotite bodies are in tectonic contact with a metamorphic formation consisting of gneissic and amphibolitic rocks similar to those outcropping in the Pan African metamorphic basement of eastern Egypt, Peridotites and metamorphic rocks are both crossed by several generations of basaltic dykes. The sedimentary Zabargad Formation, of probable Cretaceous or Palaeocene age, consists of alternations of silicified limestones, sandstones, black shales and phosphoritic lenses, and is overlain by evaporites, probably of Miocene age, and by reef limestone units, one probably of early Pleistocene age, the other probably deposited during the last interglacial. Various formations are affected by localized metasomatic mineralizations (recrystallized large, gem-quality olivine, cancrinite, scapolite. garnierite). The uplift of upper mantle ultramafic bodies with fragments of crust, which created the island, might have occurred in connection with the early stages of development of the rift which preceded the formation of the Red Sea. An alternative explanation, that the Zabargad peridotites are remnants of a Pan-African Palaeozoic ophiolitic complex, is possible but less likely.
The structural evolution of the Tibchi ring-complex: a case study for the Nigerian Younger Granite Province
Authors: Ike E. C.
Published: December 2007

In the Tibchi Complex, an initial gas build-up in the ascending felsic magma was most probably accompanied by updoming of the overlying basement rocks. Paroxysmal volcanic eruptions followed and culminated in a central shield volcano whose conduit was eventually sealed. Magmatic activity was renewed, with consequent increase in magma pressure but the volcano's load effect hindered a new central updoming or reopening of the already welded volcanic neck. Instead there resulted a peripheral stress concentration around the volcano to form a master cone-fracture that was apparently complete as a ring-fracture and which in attitude was inward-dipping at depth, steepening towards the ground surface. Fluidization along the ring-fracture brought about surface cauldron subsidence of the early volcanic edifice, and intra-caldera extrusion of crystal-rich ignimbrites. Fluidization waned as the magma became depleted in gas; the feeders to the crystal-rich ignimbrites were frozen in the ring-fracture as quartz porphyries. Gas-poor magma corresponding to the present granite porphyry ring-dyke displaced the frozen quartz porphyries by ring-fracture stoping and marked the transition to the plutonic phase of intrusion.
The thrust and shear zones of the Moine thrust zone and the NW Scottish Caledonides
Authors: Coward M. P.
Published: December 2007

The Caledonian thrust belts of NW Scotland exhibit many features typical of foreland fold-and-thrust belts. The major structures propagated westwards towards the foreland, the thin carapace of Cambro-Ordovician sediments being accreted in imbricate and duplex zones beneath the major thrust sheets. On a broad scale, the rules of thrust geometry are obeyed but many local anomalies are found, particularly in the Assynt region.

In Assynt, several large fault-bounded gravity-spreading structures, surge zones, have been identified. Extensional faults have been traced into strike-slip and then contractional faults. There are zones of extensional and contractional imbricate faults and extensional and contractional ductile flow. Similar structures have been found along the length of the Moine thrust belt, suggesting that gravity spreading was an important mechanism during the later stages of movement.

Within the main Caledonian fold belt there are several major tectonic slides, which are recognized by a local increase in intensity of deformation. The majority are NW-verging and were originally gently dipping. In the Highlands SE of the Great Glen there are early major SE-verging recumbent folds associated with considerable crustal thickening reflected in high pressure (~12kbar) metamorphism. Many of the major slides may have formed by spreading away from this thickened crust. The Caledonian deformation and metamorphism occurred later in the Moines and Moine thrust zone than in the Dalradian and Grampian Moines to the SE of the Great Glen. This may reflect different levels of deformation; alternatively, it may argue for a large strike-slip displacement on the Great Glen fault, juxtaposing crustal blocks which had experienced different ages of Caledonian deformation and metamorphism.

The late movement on the Moine thrust involved spreading from an area of thickened Caledonian crust formed either by the stair-step trajectory of the basal thrust to the Moines and, or, by thickening due to intrusion of large masses of late Caledonian granites
The Loch Quoich Line—a limit of early Palaeozoic crustal reworking in the Moine of the Northern Highlands of Scotland
Authors: Roberts A. M. , Harris A. L.
Published: December 2007

The Loch Quoich Line, a major structural line at least 60 km in length, within the Moine of the Northern Highlands of Scotland, has hitherto been variously interpreted as a major tectonic dislocation. the root zone of a nappe, and an unconformity. The work reported here was carried out at the type locality of the line, and was directed at resolving these ambiguities. The lithostratigraphic succession established by sedimentary structures shows that the rocks of the Glenfinnan Division are older than, but are stratigraphically continuous into, those of the Loch Eil Division. Rocks of the Loch Eil Division crop out extensively to the W of the Loch Quoich Line, and are not restricted in outcrop to the area E of the line, as was previously thought. The early (D
1
–D
2
) structures were initially sub-horizontal. Steeply inclined, NNE-trending D
3
structures are weakly developed E of the axial trace of a major fold, the Beinn Beag Synform. In this area the metasediments remain essentially in their post-D
2
, gently inclined attitude. W of the axial trace of the synform, the earlier folds and fabrics were severely reworked into steep attitudes during D
3
, producing NNE-trending zones of 'platy' rocks, which mark regions of severe strain on D
3
fold limbs. Earlier (D
1
–D
2
) structures are clearly preserved in D
3
hinge zones, which mark zones of low D
3
strain. It is concluded that the Loch Quoich Line coincides with the axial trace of the Beinn Beag Synform, and that it is the eastern limit of severe tectonic reworking (probably Caledonian) over-printing early (probably Precambrian) structures. Recognition of structural and stratigraphic continuity between the Glenfinnan and Loch Eil divisions, implies that the Loch Quoich Line cannot be correlated across the Great Glen Fault with the putative unconformity separating the Grampian (cover) and Central Highland (basement) divisions, in the Moine rocks of the Central Highlands.
Isotopic evolution of the Lizard Complex
Authors: Davies G. R.
Published: December 2007

An Sm-Nd mineral isochron from an olivine gabbro yields an age of 375 ± 34 Ma (MSWD = 0.006) which represents the formation age of the Lizard Igneous Complex. Associated dolerite dyke suites have ɛ
Nd375
values between +8.9 and +11.7 and record derivation from a recently trace element-enriched, but heterogeneous, source region.

The petrology and Sm-Nd isotope systematics of the Lizard peridotites record the local infiltration of a magma. Significantly, the Sm-Nd isotope systematics of the ‘metasomatized’ peridotites are the same as calculated sources of the dolerite dyke suites: their sources are probably equivalent at depth.

Finally, the magmatic evolution of the Lizard Complex is likened to the initial development of the Red Sea, with early trace element-enriched volcanism followed by MORB-Iike activity.
Possible Lizard-derived material in the underlying Meneage Formation
Authors: Barnes R. P.
Published: December 2007

The Lizard Complex rests tectonically on the Meneage Formation within the top of the flysch dominated Gramscatho Group of sediments. The Meneage Formation is a Middle to Upper Devonian sedimentary mélange sequence with interbedded rudites. The possibility that one or both of these lithologies include Lizard-derived debris is examined in detail. The rudaceous debris is characterized by a range of meta-psammitic, pelitic and basic lithologies which cannot be matched with a Lizard source. Parts of this suite, notably granite gneiss and amphibolitic material, are found to be quite different from superficially similar, possible Lizard source rocks. A cratonic source is required. The only widespread type of mélange phacoid with a possible Lizard source is greenstone. Geochemically these have ocean floor tholeiite affinities. Gabbro and amphibolite occur within, and serpentinite near, one particularly large greenstone phacoid. However, no precise geochemical comparison can be made between greenstones and Lizard dykes. Autochthonous oceanic tholeiites in the Gramscatho Group sequence, and the absence of varied and widespread possible Lizard-derived material, suggest that the Lizard Complex is an unlikely source for the allochthonous greenstones. A separate local development of oceanic crust now no longer exposed, and autochthonous volcanics, are postulated as more likely sources.
MOIST and the continuity of crustal reflector geometry along the Caledonian-Appalachian orogen
Authors: Brewer J. A. , Smythe D. K.
Published: December 2007

New aspects of the deep structure of the Scottish Caledonides are revealed by the Moine and Outer Isles deep seismic reflection traverse (MOIST). The Caledonian foreland is underlain by an easterly-dipping, strongly reflecting surface cutting through the Moho and traceable to more than 45 km depth. Thrusts within the foreland basement and Caledonian orogen have been reactivated as normal faults bounding half-grabens filled with sediments of late Palaeozoic to Mesozoic age. The Moine Thrust, which carries rocks of the orogen over Lewisian foreland, dips at 20–25° to the E on MOIST, and is either: (a) the westernmost of a series of easterly-dipping reflections (thrusts) which flatten or terminate at 17–20 km depth, or (b) a thrust lying further E which structurally overlies these easterly-dipping reflectors. By comparison with easterly-dipping reflections on COCORP lines in the northern and southern Appalachians the latter interpretation is preferred, implying that many of the easterly-dipping reflections on MOIST correspond to off-shelf metasedimentary rocks, with slivers of basement, stacked and imbricated against the Lewisian basement edge. Regional seismic refraction and conductivity data from northern Scotland suggest that regions of the Caledonian orogen have lower mid-crustal velocity gradients and higher conductivities than the foreland, which might be explained by this interpretation of the easterly-dipping reflections.

Despite similarities in crustal reflector geometries, there are important differences between the MOIST and COCORP lines, including (1) a remarkably continuous, relatively horizontal Moho seen on MOIST data at about 25 km depth, and (2) the sedimentary basins offshore from northern Scotland which have formed by reactivation under crustal extension of the easterly-dipping thrusts. Furthermore, it is probable that the easterly-dipping reflectors in these areas of the MOIST and COCORP lines were formed at different times in the early Palaeozoic.
Early curvilinear fold structures and strain in the Moine of the Glen Garry region, Inverness-shire
Authors: Holdsworth R. E. , Roberts A. M.
Published: December 2007

Four regional deformation episodes (D1-D4) are recognized in the Moine rocks (Loch Eil and Glenfinnan divisions) of the Glen Garry area, Inverness-shire, NW Scotland. These episodes fall into two groups: early, sub-recumbent D1-D2 structures and later, upright D3-D4 structures. D2 structures occur on all scales and appear, in a regional sense, to be more important than the D1 structures. The D1-D2 structures are only preserved in their original disposition where subsequent upright D3 reworking was minimal, i.e. in the region to the E of the Loch Quoich Line and in low strain augen developed in D3 hinge zones to the W of this structure.

The F2 folds are intensely curvilinear and at many localities F2 hinges lie approximately parallel to the N-S trending extension lineation. Examination of these structures in three dimensions and application of the
Sanderson (1973)
model provides information on the nature and orientation of the regional D2 strain ellipsoid in this part of the Moine. The D2 XY plane is sub-horizontal with the X direction trending approximately N-S. This is markedly different in orientation from the D3 ellipsoid in this region, which, being of post-Glen Dessary Syenite (456 Ma) age, is demonstrably Caledonian.

The D2 linear fabrics suggest that the direction of tectonic transport was S to N or N to S, markedly different from the generally accepted WNW-directed movement attributed to Caledonian orogenesis. These findings are consistent with the proposal that the Loch Quoich Line represents an eastern limit of intense Caledonian (D3) reworking, and that the D1-D2 structures are of Precambrian age.
An investigation of the Reading-Newbury magnetic anomaly
Authors: Ellis P. F. , Kearey P.
Published: December 2007

The complex magnetic anomaly situated in the region of Reading and Newbury has been investigated using several techniques. Analysis of the power spectrum of the anomaly indicates that it results from two groups of sources: a shallow group at a maximum depth of some 1.1 km and a more extensive, deeper group at a depth in excess of 2.5 km. The anomalies due to these two sources have been separated by wavenumber filtering and continuation methods. The deeper group of sources appears to have a significant remanent magnetization and could be modelled by an approximately prismoidal body with a side length of some 20 km. The shallow group of sources appears to correlate with igneous rocks encountered in local boreholes. The magnetic properties of these rocks have been determined.
The tectonic evolution of the Pyrenees: a workshop
Authors: McCaig A. M. , Wickham S. M.
Published: December 2007

Report of a meeting of the Tectonic Studies Group held at the University of Cambridge on 21 May 1983.

About 70 geologists and geophysicists from Britain, France, Holland and Spain met to discuss the tectonic evolution of the Pyrenees and surrounding areas. Twenty short talks and 12 posters were presented and the meeting closed with a two-hour 'round table' discussion.

Hercynian geology and Alpine overprinting
Vielzeuf referred to lower crustal granulites of probable Hercynian age along the North Pyrenean Fault. Zircon age determinations for these rocks are in progress. In the Aston Massif, Vissers described the progressive folding and transposition of a steep early schistosity (suprastructure) into a relatively flat-lying attitude (infrastructure) with increasing metamorphic grade. The refolding accompanied low-pressure metamorphism and probably resulted from tectonic doming during granite emplacement. Similar low-pressure metamorphic rocks and granite petrogenesis in the Trois Seigneurs Massif were discussed in a poster by Wickham. Although comparatively few participants spoke on Hercynian orogenesis, it appears to have been characterized by unusually high heat flow and intense plutonic activity; the tectonic setting of the area at this time remains obscure.

Andrews, Austin and Colwill described high strains measured on pyrite-cube pressure-shadows in the Llavorsi syncline. These indicate up to 10 km of Hercynian S-directed thrust movement in this area. A similar reverse displacement with a dextral component on mylonites along the Merens Fault was described by McCaig and Miller.
39
Ar-
40
Ar dating clearly shows this movement to be of Alpine age, and probably <70 Ma, although excess argon
The petrology and geochemistry of Cretaceous to Recent volcanism in Colombia: the magmatic history of an accretionary plate margin
Authors: Marriner G. F. , Millward D.
Published: December 2007

The Pacific margin of Colombia records a complex history of subduction-related magmatism from at least Cretaceous times. Cretaceous lavas from the Western Cordillera and part of the Central Cordillera of the Colombian Andes exhibit some chemical characteristics typical of island-arc tholeiites, including low LIL-element abundances and flat to light depleted REE patterns, but also have chemical and geological affinities with deposits from marginal basins, and with transitional type ocean floor basalts. An appraisal of the geological and chemical evidence leads to the conclusion that these rocks were erupted in an oceanic environment in close proximity to the continent. A suite of basic rocks from the Coastal Cordillera of Upper Cretaceous to Tertiary age has been interpreted elsewhere as abyssal tholeiites.

Following accretion of the Cretaceous sequences on to cratonic S America, volcanism continued in the upper Miocene with the eruption of basalts and basaltic andesites on the eastern flanks of the Western Cordillera. This tholeiitic suite is LREE and LIL-element enriched compared with the Cretaceous lavas, and represents the trenchward portion of an ensialic island-arc sequence. Subduction-related magmatism continued from the Miocene with the formation of composite calc-alkaline strato-volcanoes, mostly along the axis of the Central Cordillera. These Miocene to Recent lavas are dominantly andesitic, with strong LIL and LREE enrichment and
87
Sr/
86
Sr ratios of 0.7042:0.7047.
Cenozoic plate kinematics, subduction and magmatism: South American Andes
Authors: Pilger Rex H.
Published: December 2007

Revised plate reconstructions and enlarged isotopic age data sets strongly support three mechanisms for controlling the configuration and associated volcanism and tectonism of the Andean subduction zone. These include age of subducted plate, convergence rate, and interaction with hotspot traces. From 50 to 25 Ma, in the Andes, the age of oceanic plate subducted progressively decreased, apparently resulting in a'gradual decrease in subduction angle and consequent eastward shift in the loci of magmatic activity. A major reorganization of plate motions at 25 Ma resulted in an increased convergence rate normal to the Chilean Andes and was accompanied by subsequent broadening of the locus of magmatic activity into adjacent portions of Bolivia and Argentina. Convergence rates changed only slightly across the Peruvian margin and no clear effect on subsequent magmatism is indicated. Interaction of the Andean subduction zone with the Juan Fernandez and Easter-Nazca hotspot traces apparently resulted in lower angle subduction and consequent gaps in magmatic activity as predicted continuations of the traces correspond with regions lacking dated volcanic rocks. The current gaps in igneous activity correspond with contemporary subduction of the two traces, and this association apparently occurred in the past as well.
Long-range sonar observations of sedimentary facies patterns in and around the Amirante Trench, western Indian Ocean
Authors: Roberts D. G. , Kidd R. B. , Masson D. G.
Published: December 2007

Long-range side-scan sonographs are used to map two distinct acoustic facies in the Amirante Trench, western Indian Ocean. A structural barrier formed by a seamount within the trench separates the two facies types. The acoustic facies developed S of the seamount is characterized by weak reflectivity returned from sediments deposited by the flow of the Deep Western Boundary Undercurrent through the channels of the Amirante Passage into the southern part of the Amirante Trench. N of the seamount the acoustic facies is characterized by high reflectivity and is confined to the trench axis: it arises from sediments episodically deposited by turbidity currents or debris flows possibly originated by slumping of reef talus from the atolls surmounting the adjacent Amirante Ridge.