bluuebunny/crossref_metadata_2025 huggingface dataset:
Eruptions from zoned magma chambers
Authors: Blake S.
Published: December 2007

SUMMARY: An appreciation of the positions within a magma chamber from which magma will be erupted can be gained from an understanding of the pattern of magma removal. Upwardly converging streamlines followed by a fluid escaping from a flat-topped reservoir into a cylindrical conduit cause magma from widely separated horizontal and vertical locations in a pre-eruption chamber to be simultaneously erupted. The volume erupted (
V
) is related to the maximum depth tapped (
R
) by the equation
V
= (2π/9)
R
3
. Large ash-flow sheets (100 km
3
) may therefore sample magma up to 5 km below the chamber roof.

Analysis of the flow field suggests that about one half of a given sample of erupted magma represents material from the deepest part of the chamber being tapped at the time of eruption. Many petrographic features of chemically zoned ignimbrites can best be explained by the operation of such a flow-field during eruption, where a series of sub-spherical sampling shells intersect the horizontal layers in the magma chamber to produce heterogeneous ejecta. Mixed magmas erupted late in some zoned ash-flow sequences can originate by tapping across the boundary between silicic and underplating basaltic magmas which coexist in the source chamber rather than by convective mixing prior to eruption. Flow banding in rhyolites can result from the same fluid dynamic processes.
The geology and tectonic setting of the Velasco Alkaline Province, eastern Bolivia
Authors: Fletcher C. J. N. , Litherland M.
Published: December 2007

The Velasco Alkaline Province of Mesozoic age is situated near to the Bolivian-Brazilian border, and is enclosed in Precambrian gneisses of the Brazilian Shield. It consists of an early volcanic suite, and a series of 14 interfering circular, ring and elliptical plutons, and is associated with a silicified carbonatite complex. The combined length of the province is nearly 80 km and it outcrops over an area of 440 km
2
. The plutonic rock types range from foyaite to aegirine granite but each individual pluton may be classified as either over- or under-saturated. The province was built up by a series of plutonic events which for the most part were aligned along a NE-trending lineament. Normal faulting was active during the development of the province.

Although the Velasco Alkaline Province lies over 1500 km from the eastern continental margin it is deduced that it defines a plume-generated triple point initiated at the same time as the break-up of the South American-Africa plate.
The significance of isotopic dates from the English Lake District for the Ordovician–Saurian time-scale
Authors: Rundle C. C.
Published: December 2007

The Lower Palaeozoic time-scale is discussed in the light of a new date for the Threlkeld Microgranite and a reassessment of other published dates from the Lake District which suggest ages of
c.
410 Ma for the base of the Devonian;
c.
421 Ma for the Ashgill;
c.
429 Ma for the late Caradoc;
c.
439 Ma for the early Llandeilo, and
c.
468 Ma for the early Llanvirn.
A ground water resources databank in the Kingdom of Saudi Arabia
Authors: Marsh J. M. , Sagaby I. , Sooley R. R.
Published: December 2007

In rapidly developing countries, one of the chief impediments to efficient assessment and planning of geologic resources is the frequent lack of awareness of existing data and difficulty of access to available information. Such problems can be reduced by employing a computerized data storage and retrieval system.

Such a system is being implemented by the Water Resources Development Department and Ministry of Agriculture and Water, Saudi Arabia, to store data of water resources and hydrogeology. The lack of an adequate system of data storage has impeded rationalization and adjudication of existing data; to enable the introduction of sophisticated data processing techniques, all available data are being input to a multiple database. So far only the well-site data file has been implemented and details of 23,000 wells have been entered. A further 6 files are planned to cover all aspects of water resources and the hydrochemistry and surface water data files are now in preparation.

The data are managed by SYSTEM 2000 software loaded on an IBM 370/158 computer housed in the National Computer Centre.
The Eurasian-Arabian and African continental margin from Iran to Greece
Authors: Scott Barry
Published: December 2007

The continental margin between Eurasia and Arabia can be traced as a continuous thrust zone for
c
. 2000 km from the Gulf of Iran westwards through Iran and Turkey towards the Mediterranean Sea. In this sector it forms a coherent geological whole with increasing structural complexity from S to N across the junction. This clarity is terminated 125 km NE of the Gulf of Iskenderun, on the Mediterranean Sea, by the N–S trending Levant Fault System (LFS). W of the LFS, for 1400km to the W of Greece, the boundary significantly changes to curved micro-plates, which are not directly observable as they are beneath the sea. Their location is in doubt for
c.
650 km from the LFS westwards to the eastern margin of the Aegean plate, at the Anaximander Mountains. Over this sector, several major tectonic units occur which obscure and complicate such a margin. To the W the Aegean plate continues for 750 km with the characteristics of an island arc.
Phanerozoic granulites
Authors: Windley B. F.
Published: December 2007

Granulites can form at every stage of the Wilson cycle: during plate accretion (Mid-Atlantic Ridge), in active plate margins of island-arc and Andean type (Ivrea Zone, main complex in Kohistan in the Karakorums), in the roots of Andean-type batholiths (Coast Range Complex, British Columbia; Santa Lucia Range of California), during the closure of a marginal basin (Aracena belt, Spain), during Himalayan-type collision (Jijal Complex in the Karakorums of N Pakistan and at I’Agly in the Pyrenees) and during post-tectonic intrusion at depth(Sä0 Paulo, Brazil). Himalayan-type orogenic belts may, in principle, contain any of such granulites. The majority of granulites form along active plate margins in the roots of magmatic arcs, which are the main sites of crustal growth in the Phanerozoic. Layered stratiform complexes, formed largely by cumulate processes, are common in Phanerozoic granulites (Kohistan, Ivrea, Calabria in Italy, Cabo Ortegal in NW Spain, Brittany and N Pyrenees). Their granulite-grade parageneses may have formed during cooling from an igneous temperature, or during superimposed metamorphism. Variations in the types of granulites may help in understanding how different types of lower continental crust are constructed.
Magma and sediment-I Emplacement mechanism of late Carboniferous tholeiite sills in northern Britain
Authors: Francis E. H.
Published: December 2007

Like other great tholeiite sheets of the world, the Midland Valley Sill and Whin Sill were emplaced in thick accumulations of sediments which, at the time of intrusion, were not much older nor had been orogenically deformed. However, the Carboniferous sediments of northern Britain had been deposited under the influence of continuous differential subsidence, so that they were already disposed in syn-sedimentary basins and swells when intrusion began. By drawing palinspastic contours of the sills relative to a late Westphalian marine band (approximating to the surface at the time of emplacement) it is established that the dolerites mainly follow bedding planes, dipping at angles of as much as 5
°
down to the bottoms of the basins. Isopachs of the sills indicate a further relationship, for the intrusions are thickest at the bottoms of the basins.

These facts are not explained by any intrusion mechanism previously postulated, though there seems to be broad agreement as to the processes which have given rise to thinning and
en echelon
lensing at leading edges as well as to step-and-stair transgression of sediment bedding. It is proposed that emplacement must have been controlled in part by gravitational flow down dip from feeder dykes which extended to between 0.5 and 1.0 km below the contemporary surface. Thereafter hydrostatic equilibrium was attained first by accumulation at the bottom of the basins and then by further advance up-dip under pressure of head. The sheets are so similar in aspect to flood basalts that it must be assumed that the sediments offered only low frictional resistance to splitting and flow.
Gravity and magnetic studies in the Channel Islands
Authors: Briden J. C. , Clark R. A. , Fairhead J. D.
Published: December 2007

The gravity and total magnetic field over Jersey, Guernsey, Herm and Jethou, and the gravity field over Alderney and Sark, have been surveyed in detail at 470 gravity stations and 847 magnetic locations as the basis of Bouguer and magnetic anomaly maps, which can be integrated with the marine gravity and aeromagnetic maps of the region. There is a striking correlation in location and relative amplitude of gravity and magnetic anomalies on the islands. The St Peter Port Gabbro of Guernsey can be modelled as a laccolith no more than ∼0.8 km thick and extending several km NE subsurface beyond its present outcrop limit. The aeromagnetic and gravity high over SE Jersey can be modelled as a circular laccolithic lens
c
. 12 km in diameter and
c
. 2 km thick with about 1 km of cover; thus the dioritic and gabbroic outcrop in that area does not reflect a great volume of such material at <l km depth. Smaller gravity and magnetic highs are centred over Alderney and Sark and may be due to similar but smaller buried intrusions, and other small anomalies near Le Doit in SW Guernsey and in the sea ∼10km S of Guernsey may also have the same origin. There are positive magnetic anomalies E of Sark and N of Guernsey which have zero or negative gravity expression. The granites of western Jersey have low gravity and magnetic expression; modelling indicates that the NW and SE granites are linked at shallow depths and also underlie central and much of eastern Jersey; the existence of negative magnetic anomalies to the NE and S of Jersey suggest that the granites may also extend offshore in those directions subsurface. The Pentevrian outcrop area of Guernsey is characterized by intermediate values of the Bouguer anomaly and positive magnetic anomaly fields, and the geophysical maps suggest that Guernsey lies at the N end of a promontory of Pentevrian outcrop beneath shallow sedimentary cover.
Genesis of stromatactis cavities between submarine crusts in Palaeozoic carbonate mud buildups
Authors: Bathurst R. G. C.
Published: December 2007

It is concluded that the sedimentary structure ‘stromatactis’ is a calcite cement-fill of a swarm of labyrinthine cavities developed in marine carbonate muds of Palaeozoic bioherms, commonly with geopetal sediment. Field and petrographic evidence is examined from the Ordovician of Dalarna, Sweden, the Devonian of the Canning Basin, Australia, and of Belgium and the Carboniferous of Lancashire, U.K. In all four occurrences the host microspar encloses masses of radiaxial fibrous calcite with sub-horizontal floors. Following other workers, radiaxial fibrous calcite is regarded as an altered marine cement, a conclusion supported by the presence of overlying geopetal sediments with marine fossils, widespread evidence of early submarine cementation in Palaeozoic mud mounds, isotopic analyses, and the presence of microdolomites. The stromatactis masses are separated from each other by laminae of host biomicrospar, which show discontinuities resembling fractures which are healed with the early marine cement, or filled with marine internal sediments. The fractures are therefore of submarine origin. The evidence points to episodic submarine lithification of the carbonate muds, yielding successive crusts between which cavities were eroded and then filled. The crusts were broken and rotated during bioturbation, differential compaction or shearing during burial.
Sedimentary basin in-fill in the northernmost Natal Valley, hiatus development and Agulhas Current palaeo-oceanography
Authors: Martin A. K. , Goodlad S. W. , Salmon D. A.
Published: December 2007

Sedimentation over large areas of a marginal plateau, the northernmost Natal Valley (NNV), is influenced by the tropical/sub-tropical Agulhas Current. Through mobilization of detritus and concentration of sedimentation into localized depo-centres, current action provides a mechanism for hiatus development both in on-shelf and off-shelf areas. Post-Gondwana break-up sedimentary basin in-fill is interrupted by several regional hiatuses. These correlate with coastal regressions, global sea-level low-stands, proposed phases of invigorated oceanic circulation and with global phases of slow deep-sea sedimentation. An assessment of these mechanisms indicates detritus supply related to sea-level changes cannot explain correlation of on-shelf and off-shelf hiatuses. Current action may have been a contributory factor in development of a Cenomanian/Turonian hiatus and facies change and a Cretaceous/Tertiary boundary hiatus.

An Early Oligocene hiatus clearly marks a change in the Agulhas Current regime. Current-moulded features, asymmetric sediment distributions and re-working attest to enhanced current action. Establishment of a cold bottom layer in the ocean at this time stimulated Indian Ocean surface/near-surface circulation. New circulation patterns were established after a Miocene/Pliocene boundary hiatus. Since then the main flow-paths have been fairly stable, although local depo-centre shifts late in the Pliocene–Recent sequence suggest eastward movement of a counter-current and cyclonic eddy off Maputo.
Petrochemistry and origin of sub-ophiolitic metamorphic and related rocks in the Oman Mountains
Authors: Searle M. P. , Malpas J.
Published: December 2007

The Semail ophiolite complex of the northern Oman Mountains was obducted during Cenomanian to Campanian (Upper Cretaceous) times onto the Arabian continental margin. During this time span, schistose metamorphic rocks of amphibolite and greenschist facies exhibiting polyphase deformation were formed and accreted below the ophiolite slice. Whole-rock and mineral analyses confirm that the amphibolites were originally tholeiitic basalts comparable to present day occurrences at mid-ocean ridges. A comparison of trace element geochemistries, however, suggests that the protoliths were not equivalent to the Haybi alkaline and transitional tholeiites found in the underlying structural slice, or to the Semail ophiolite volcanic rocks which lie structurally higher in the sequence. It is therefore suggested that the amphibolites may have been derived from Jurassic or Lower Cretaceous oceanic basalts not recognized elsewhere in the Oman Mountains.

Structurally beneath the amphibolites within the metamorphic sheet, greenschist facies metasedimentary rocks, including quartzites and marbles, are interbanded with metabasite horizons containing actinolite and, less commonly, blue crossitic amphiboles.

It is suggested that the sedimentary and volcanic protoliths were metamorphosed in a subduction zone environment during initial ophiolite displacement, and were accreted to the base of the ophiolite during progressive underthrusting.
The geochemistry and origins of the Precambrian rocks of the Rosslare Complex, SE Ireland
Authors: Winchester J. A. , Max M. D.
Published: December 2007

Geochemical studies of gneisses from the Rosslare Complex have indicated that the grey gneisses comprise a suite of sediments consisting mainly of greywackes before metamorphism. These were associated with tholeiitic gabbros, which are now represented by the Greenore diorite, and early granodiorites during migmatization. After this early high amphibolite-facies metamorphic event, the St Helen's gabbro was intruded. Subsequently, tholeiitic older basic dykes were also emplaced. A low amphibolite-facies metamorphic event, accompanied by localized alkali metasomatism, modified the chemistry and mineralogy of these basic intrusive rocks. Finally, tholeiitic younger basic dykes, which have subsequently been altered or mildly metamorphosed, were emplaced before the complete crystallization of the Ordovician Saltees granite.
The ophiolitic Bath-Dunrobin Formation, Jamaica: significance for Cretaceous plate margin evolution in the north-western Caribbean
Authors: Wadge G. , Jackson T. A. , Isaacs M. C. , Smith T. E.
Published: December 2007

Two neighbouring areas of tholeiitic basalts, dolerites, isotropic gabbros and tonalites in eastern Jamaica are recognized as the upper section (~2.5 km thick) of an ophiolite. Limestones associated with this Bath-Dunrobin Formation contain a fauna of Upper Campanian and Lower Maestrichtian age. The basalts are overlain by a thick sequence of Maestrichtian volcaniclastic rocks and are faulted against a blueschist terrain. Since its emplacement the ophiolite has been subjected to uplift and major left-lateral strike-slip tectonism beginning in the Eocene. Recognition of this subduction zone complex and regional considerations of island arc polarity necessitates that ocean crust must have been consumed by a southward-dipping subduction zone. The overlying late Cretaceous/Palaeogene arc stretched from Guatemala through Jamaica to Oriente Province, Cuba. This arc separated the Yucatan Basin to the N, where active spreading accompanied subduction, from the Colombian Basin and the southern Nicaraguan Rise to the SE, where tholeiitic magmatism (B") penetrated older ocean crust. The Bath-Dunrobin Formation probably represents ocean crust from the Yucatan Basin obducted during the Maestrichtian. The major ophiolite terrains of Guatemala and Oriente, Cuba are by
inference
also derived from a proto-Yucatan Basin.
The composition and emplacement of basaltic magmas produced during the development of continental-margin basins: the Gulf of California, Mexico
Authors: Saunders A. D. , Fornari D. J. , Morrison M. Ann
Published: December 2007

The Gulf of California is a region of active ensialic basin development and transform faulting Legs 64 and 65 of the Deep Sea Drilling Project have demonstrated the diversity of basement stratigraphy: drilling at Sites 474, 482, 483 and 485 at the mouth of the Gulf recovered massive basaltic flows and occasional pillow lavas and sills, whereas drilling in the central part of the Gulf, in the Guaymas Basin, recovered a sequence of sediments and massive doleritic sills. Very high sedimentation rates (over 1000 m/Ma) are characteristic of the Guaymas Basin and prevent extrusion of the basaltic magma onto the seafloor, thus resulting in at least partial intercalation of oceanic layers 1 and 2. Such a process of ocean crust formation could be locally important during the early stages of ocean and marginal basin development, when sedimentation rates may be very high

Over 200 samples of basalts and dolerites from the Gulf of California have been analysed by X-ray fluorescence and neutron activation techniques. Basalts from the mouth of the Gulf are tholeiitic and contain very low abundances of the more-hygromagmatophile (more-HYG) elements (K, Rb, Ba, Th, Ta, Nb and light REE) relative to the HYG elements (Y, Zr, Ti and heavy REE), thus resembling depleted N-type MORB from elsewhere along the E Pacific Rise (EPR). Dolerites from the Guaymas Basin also have a tholeiitic chemistry and some EPR-basalt characteristics (e.g. La
N
,/Sm
N
, < l), but they have higher La/Yb, Sr/Zr, Zr/Ti and Th/Hf ratios than the basalts from the Gulf mouth region. It is suggested that these differences are due in part to variations in mantle source composition, the mantle underlying the central and northern parts of the Gulf containing a minor sub-continental (residual calc-alkaline) component.
Upper Palaeozoic basins of the British Isles—Caledonide inheritance versus Hercynian plate margin processes
Authors: Leeder M. R.
Published: December 2007

Hypotheses to explain the origin and location of Upper Palaeozoic sedimentary basins are presented. Group A basins were those Old Red Sandstone basins whose location and subsidence were controlled by Caledonide features, including active and remnant fore-arc basins (Midland Valley) and major transcurrent movements (Orcadian basin). Newer Granite plutonic magmas in the Southern Uplands subduction complex post-dated subduction and resulted from post-collision cratonization processes as yet little understood. Group B basins were those whose initial positions in space were controlled by palaeo-plate factors, in particular the location of granitoid plutons, but whose subsidence was controlled by active Hercynian (neoplate) processes. Some Group A basins were regenerated by this process (Midland Valley). A new hypothesis is presented to account for the origin of a Carboniferous rift-to-sag trend similar to that proven in modern continental margin basins such as the North Sea. The hypothesis is an adaptation of McKenzie's lithospheric stretching model, some of whose major features are well shown in the Carboniferous history of the north and central British Isles. Group B processes were controlled by Ligurian/Bretonic plate consumption in the S Brittany–Massif Central–Vosges areas. Group C basins in the Rheno-Hercynian zone also owe their location and origin to back-arc rifting and continental crustal ‘oceanization’ processes caused by Liguro-Bretonic subduction.
A comparison of K-Ar and Rb-Sr ages of rapidly cooled igneous rocks: two points in the Palaeozoic time scale re-evaluated
Authors: Williams I. S. , Tetley N. W. , Compston W. , McDougall I.
Published: December 2007

K-Ar and Rb-Sr ages were measured on rapidly cooled igneous rocks from Kinnekulle, Sweden (Ordovician bentonite), the Cerberean Cauldron, Australia (late Devonian felsic volcanics), Mount Dromedary, Australia (Cretaceous monzonite) and Cooma, Australia (Jurassic syenite). No systematic bias was detected between the ages determined by the two techniques, calculated using the constants recommended by the I.U.G.S. Subcommission on Geochronology. The K-Ar and Rb-Sr ages of the Jurassic syenite also were indistinguishable from its zircon U-Pb age. The Kinnekulle A
1
bentonite gives a minimum age of 446 ± 3 Ma for the early Caradocian
Diplograptus molestus
Zone. The Cerberean Cauldron volcanics give a minimum age of 367 ± 2 Ma for the early Frasnian. These data are consistent with a small upwards revision of the ages in Gale
et al.
's 1979 time scale as proposed by Gale
et al
. (1980).
Rb-Sr geochronology of Graham Land, Antarctica
Authors: Pankhurst R. J.
Published: December 2007

Thirty-five new Rb-Sr whole-rock and mineral isochrons are reported for igneous rocks from Graham Land. Magmatism was essentially continuous but peaks of activity occurred in early Jurassic, early Cretaceous, late Cretaceous and Eocene times. The oldest dated granite is Triassic (209 ± 3 Ma). Volcanic rocks were erupted throughout this history and some previously supposed Upper Jurassic volcanic rocks must be mid-Cretaceous or younger. A change in magma type, from uniformly acid to predominantly basic or intermediate, combined with westward migration of the arc, may be partly responsible for previously reported transverse geochemical variations. There is a marked trend of decreasing initial
87
Sr/
86
Sr ratio of the plutonic rocks with time, from c. 0.707 to 0.704, although volcanic rocks tend to have higher values. This trend is ascribed to the waning influence of an, as yet unidentified, pre-magmatic crustal basement, due to increasing depth of magma generation. Close parallels to the observed age pattern imply continuity of the Andean belt through southern South America into Graham Land back to early Mesozoic times and are not compatible with reconstructions of Gondwana in which the Antarctic Peninsula is located W of South America.
Models for the deposition and simple shear deformation of a turbidite sequence in the South Georgia portion of the southern Andes back-arc basin
Authors: Tanner P. W. G. , Macdonald D. I. M.
Published: December 2007

The Cumberland Bay Formation on South Georgia (South Atlantic) is an Upper Jurassic–Lower Cretaceous andesitic turbidite sequence >8 km thick, derived from a contemporary volcanic are and deposited in part of the southern Andes back-arc basin. 4 main lithofacies are recognized which show coarsening and thickening upward cycles. A Lower Cretaceous palinspastic reconstruction shows that lobes of sand-rich material were banked against the (?) fault-bounded SW margin of the basin, adjacent to the volcanic arc. The basin had a northerly-directed palaeoslope and evidence for both axial and lateral palaeocurrents is preserved. Sedimentation of the formation cannot be explained by the classical fan model but resembles that described from linear, tectonically-controlled troughs (
cf
. Grenada Basin, Lesser Antilles). Folding resulted from mid-Cretaceous closure of the basin and increases in intensity to the NE: open, upright folds pass into trains of inclined chevron folds with progressive decrease in interlimb angle, axial plane dip and wavelength. The turbidites were affected by up to 55% shortening and had a minimum original cross-strike width of about 60 km. The geometry of the structures is consistent with deformation above a gently inclined simple shear zone resulting from limited underthrusting or subduction of the basin floor beneath the arc terrane. Extension of both sedimentary and tectonic models to the southern Andes is discussed.
A comparison of the ammonite faunas of the Antarctic Peninsula and Magallanes Basin
Authors: Thomson M. R. A.
Published: December 2007

Ammonite-bearing Jurassic and Cretaceous sedimentary successions are well developed in the Antarctic Peninsula and the Magallanes Basin of Patagonia. Faunas of middle Jurassic-late Cretaceous age are present in Antarctica but those of Patagonia range no earlier than late Jurassic. Although the late Jurassic perisphinctid-dominated faunas of the Antarctic Peninsula show wide-ranging Gondwana affinities, it is not yet possible to effect a close comparison with faunas of similar age in Patagonia because of the latter’s poor preservation and our scant knowledge of them. In both regions the Neocomian is not well represented in the ammonite record, although uninterrupted sedimentary successions appear to be present. Lack of correspondence between the Aptian and Albian faunas of Alexander I. and Patagonia may be due to major differences in palaeogeographical setting. Cenomanian-Coniacian ammonite faunas are known only from Patagonia, although bivalve faunas indicate that rocks of this age are present in Antarctica. Kossmaticeratid faunas mark the late Cretaceous in both regions. In Antarctica these have been classified as Campanian, whereas in Patagonia it is generally accepted, perhaps incorrectly, that these also range into the Maestrichtian.
Nature and distribution of metalliferous mineralization in the northern Antarctic Peninsula
Authors: Hawkes Donald D.
Published: December 2007

Known mineralization in the N Antarctic Peninsula, although sparse, is comparable to that of the Andean metalliferous province. It includes porphyry-style Cu (MO and Fe) mineralization, polymetallic (Cu, Pb. Zn and Ag) veins, and a volcanogenic Fe deposit of Laco Sur type. Extensive areas of pyrite-rich hydrothermally altered rocks represent exposures of the high-level parts of porphyry copper systems. The known mineralization can be related to magmatism associated with subduction of the Pacific plate beneath the Antarctic Peninsula in the ?late Cretaceous and Cenozoic. A longitudinal distribution of metallogenic belts can be demonstrated; these are terminated by transverse tectonic boundaries which may reflect segmentation of the underlying subducted ocean plate. There is evidence that mineralization is concentrated along these tectonic boundaries.
Diversity in Precambrian microbial communities
Authors: Baur M. E.
Published: December 2007

A key problem for the interpretation of the evolution of microbial ecosystems and physiologies in the Precambrian is that of the time of onset of an oxygenic atmosphere. This onset constrains the time of appearance of O
2
-producing photosynthesis as well as of the various structural and enzymatic protective devices enabling cells to function aerobically. It is suggested that evidence bearing on this matter can be derived by examining the distribution of carbon isotope values in samples obtained from rock formations of a given period. Prior to the appearance of free O
2
, in large, stable quantities, microbial diversity could have been greater than it was subsequently and this might be reflected in a wider variability in the carbon isotopic composition of organic carbon in sediments from the preoxygenic Earth. Examination of the available isotope data offers support for this picture. Whatever the ultimate cause, microbial variability appears to have diminished markedly by about 1.5 Ga and to have remained relatively low thereafter
Osmotic regulation and the evolution of brackish- and fresh-water faunas
Authors: Croghan P. C.
Published: December 2007

Present-day brackish-water areas contain a variety of animal species with differing abilities to adapt to dilute media and many studies on the mechanisms of adaptation to dilute media have been made using these species. However, many of these areas are unstable in composition, small in volume and geologically transitory and thus hardly present a likely avenue for large-scale evolutionary processes. Many of these areas are relatively poor in numbers of species and this can be interpreted as indicating that speciation has not proceeded to equilibrium. Large-scale and geologically long-lived brackish-water areas, resulting from continental drift, such as the Sarmatic Sea, may have provided the main types of situation in which the evolution of brackish- and fresh-water faunas has occurred and from where species have become dispersed into present-day localized niches.
Pan-African structures, ophiolites and mélange in the Eastem Desert of Egypt: a traverse at 26°N
Authors: Ries A. C. , Shackleton R. M. , Graham R. H. , Fitches W. R.
Published: December 2007

A detailed structural traverse across the basement rocks of the Eastern Desert of Egypt shows that they consist, apart from intrusions, of four broadly recumbent tectonic units. The lowest, of arkosic metasediments of continental shelf facies, is exposed in a dome. This unit is overlain by an allochthonous ophiolitic mélange containing complete and dismembered ophiolitic masses in a matrix of deep-oceanic graphitic pelites and turbidites. A near horizontal, schistose to mylonitic fabric, most intense near and below the base of the mélange, indicates extensive lateral tectonic transport of the ophiolitic mélange over the shelf metasediments, but the mélange originated as an olistostrome before being tectonically transported. The mélange extends over an area of at least 10 000 km
2
. It is locally overlain by calc-alkaline volcanics, in which deformation is less intense but increases, and schistosity flattens downwards, indicating some translation over the mélange. Unconformable on the mélange and calc-alkaline volcanics is a molasse-facies series, itself also locally strongly deformed. Late tectonic granites preceded, and locally post-date, the molasse-facies sediments. Still later, diapiric peralkaline riebeckite granites locally up-domed the recumbent structures. The tectonic evolution is related to a late Proterozoic subduction zone to the SE of the region.
The relationship between Caledonian nappe tectonics and Silurian turbidite deposition in North Greenland
Authors: Hurst J. M. , McKerrow W. S. , Soper N. J. , Surlyk F.
Published: December 2007

The relationship between the northern part of the Caledonian fold belt of East Greenland and the North Greenland fold belt is explored within the context of Silurian sedimentation and tectonics. A model is developed relating the Caledonian nappes of eastern North Greenland to major tectonic and sedimentological features of the Silurian of the remainder of North Greenland. The abrupt change from a thin succession of predominantly slowly deposited muds to a thick succession of rapidly deposited sandstone turbidites at the Ordovician/Silurian boundary, in the deep-water basin N of the carbonate platform in North Greenland, is directly related to uplift and westerly directed nappe emplacement in the eastern Caledonides. It is suggested that uplift to the E of North Greenland, which commenced in the late Ordovician or early Silurian, provided the main source of the Silurian turbidites of North Greenland. This is corroborated by the dominant westerly transport direction of the Silurian turbidites. Nappe emplacement during the latest Llandovery (Silurian), and foundering of the whole of the carbonate platform of eastern North Greenland E of the Victoria Fjord Arch (approximately 60 000 km
2
), is marked by the replacement of shallow water carbonates by basin turbidites. These events were directly related to isostatic readjustments as the nappes advanced onto the eastern fringe of the platform.
Variations in fluvial style on the Eocene–Oligocene alluvial fan of the Scala Dei Group, SE Ebro Basin, Spain
Authors: Allen P. A. , Cabrera L. , Colombo F. , Matter A.
Published: December 2007

The Scala Dei Group represents one of several coarse clastic wedges along the eastern margin of the Tertiary Ebro Basin. This group, of Eocene–Oligocene age, is partly concealed by faulted and locally overthrust older sediments, but a segment of Oligocene age is spectacularly exposed for about 15 km along the newly constructed A2 motorway.

Near the edge of the basin multistorey conglomerate channel-bodies are composed of large scale foresets up to 3 m high, representing side bars or possibly point bars. Smaller (up to 40 cm) normally graded beds with imbricated clasts represent diagonal or transverse bars, and horizontally-stratified sheets may be longitudinal bars. Flow was highly variable. The multistorey conglomerate bodies are restricted to several crudely radiating axes representing major feeder systems and give rise to a ‘goose-foot’ depositional palaeomorphology.

Towards the basin centre and in quiet zones between the main feeder axes single-storey channel-bodies of conglomerate and sandstone are common. Some plug-like conglomerate and sandstone channel-bodies are ribbons which are essentially straight in plan, and other, generally asymmetrical, sandstone channel-bodies preserve the point bars of channel bends where limited lateral migration took place.

Many channel-bodies possess extremely well-developed wings representing levees and locally amalgamated crevasse splay sequences. Conspicuous crevasse channels cut through the levees. The wings and crevasse channel-bodies contain carbonate-rich palaeosols.

This sequence is interpreted as an essentially distributive but ephemeral fluvial system.
The base of a sheeted dyke complex, Oman ophiolite: implications for magma chambers at oceanic spreading axes
Authors: Rothery D. A.
Published: December 2007

The sheeted dyke complex in the Maydan syncline area of the Oman ophiolite was formed during a single sea-floor spreading episode. In some areas, individual dykes from this sheeted dyke complex have been seen to root in the high-level gabbro, and others are truncated by high-level gabbro, diorite and trondhjemite. However, elsewhere dykes penetrate laterally into the layered part of the plutonic sequence. This situation suggests that active magma chambers and zones of dyke injection were discontinuous along the spreading axis at any one time.
The character and evolution of hydrothermal fluids associated with the kaolinized St. Austell granite, SW England
Authors: Alderton D. H. M. , Rankin A. H.
Published: December 2007

A complex assemblage of fluid inclusions is present in quartz from the St. Austell granite. Homogenization temperatures range from below 100°C to above 570°C, and salinities vary from <l to 42 wt % NaCl equivalents. On a regional scale the fluid inclusion populations show marked variations in their abundance and distribution of types. These variations can be correlated with the petrology of the host rock. Low-temperature, low-salinity fluids are most abundant in extensively-kaolinized granite. These represent the latest phase of fluid activity in the St. Austell granite and were probably responsible for the formation of the extensive ‘china clay’ deposits in the region, at temperatures ranging from about 150°C to below 70°C. The most saline fluids (with halite as a daughter mineral) are restricted to the lithionite and fluorite granites. They could either represent the earliest fluids associated with these granites or the concentrated residue formed by boiling of an earlier, more dilute fluid. In the biotite granite, the earliest fluids have salinities of about 10–20 wt % NaCl equivalents, and it can be demonstrated that these fluids boiled at temperatures around 520°C. Abundant gas-rich inclusions in all rock types show further that episodic boiling did occur during the evolution of the hydrothermal phase associated with all these granites.
Isotope techniques for tracing migration of gases in sedimentary basins
Authors: Schoell Martin
Published: December 2007

The isotopic composition of methane is primarily dependent on the stage of maturation of organic matter present in the hydrocarbon source rocks and is not changed during migration. The isotopic composition of
in situ
methane can be estimated from the maturity of the source organic matter. Migrated methane from deeper, more mature sources is recognized by its relative
13
C enrichment compared to indigenous methane. Examples are presented in this paper to evaluate the migration pattern of gases in sedimentary basins.
Hydrocarbon generation and migration from Jurassic source rocks in the E Shetland Basin and Viking Graben of the northern North Sea
Authors: Goff J. C.
Published: December 2007

In the E Shetland Basin oil generation began 65 Ma ago; peak oil generation maturity occurs today at 3250 m (0.7% R
0
) and was first reached 40–50 Ma ago; the oil generation threshold is at 2500 m. Highest oil saturations in the Kimmeridge Clay occur at 0.8% R
0
; oil expulsion efficiencies are >20–30%. Oil phase migration has probably occurred through oil wet kerogen laminae, and through interconnected large pores aided by low oil/water interfacial tensions. Oil migrated along strong lateral fluid pressure gradients, from overpressured source rocks in half grabens to Jurassic reservoirs in tilted fault blocks.

In the Viking Graben the Kimmeridge Clay is at oil floor maturity below 4500 m; oil and peak oil generation began 7C–80 and 55–65 Ma ago respectively; 40 Ma ago the Kimmeridge Clay passed through peak generation, and gas generation by cracking of oil had begun. Peak dry gas generation from Brent coals occurs today below 5000 m, and began 40 Ma ago. The Frigg Field gas, probably generated from late Jurassic source rocks, migrated through microfractures in overpressured mudstones below 3500 m; above 3500 m methane probably migrated in aqueous solution and was exsolved in the early Tertiary aquifer.
Petroleum geochemistry in the Neogene sedimentary basin of Japan, with special reference to primary migration
Authors: Taguchi K.
Published: December 2007

This paper presents geochemical and some geological features of the presumed source beds of the Neogene Tertiary in Japan. Distribution patterns of total organic carbon, bitumen and hydrocarbon contents in the presumed source beds, variation of bitumen and hydrocarbon ratios, vitrinite reflectance with burial depth, the time-temperature relationship to the threshold of intense oil generation, negative evidence for primary migration in water solution and the depth-porosity curves of the Japanese oil-producing sedimentary basins were examined. The results show that primary migration, by oil phase transport of immature or mild mature hydrocarbons derived from primary bitumen takes place at a relatively shallow depth,
c.
800–1300 m in Japanese Neogene oil fields.
Sm-Nd garnet age for the Ordovician Borrowdale Volcanic Group, English Lake District
Authors: Thirlwall M. F. , Fitton J. G.
Published: December 2007

Garnets separated from andesites and dacites of the calc-alkaline Borrowdale Volcanic Group are highly enriched in heavy rare earth elements (HREE) relative to their host rocks (D
Gt
Yb
from 9 in andesite to 37 in dacite). Sm/Nd ratios and D
Gt
LREE
show a wide range, probably resulting from small quantities of an REE-rich impurity in the separates. The four garnet-host rock pairs analysed yield Sm-Nd ages identical within analytical error of a weighted mean of 457 ± 4 Ma. The Borrowdale. Group overlies upper Llanvirn sediments and is unconformably overlain by mid-Caradoc. The age of 457 Ma is considered to be the best estimate available for the age of the Llandeilo, as the samples analysed are from the lower part of the Group. The age suggests that a Lower Palaeozoic timescale intermediate between those of

Gale
et al
. (1979)

and

McKerrow
et al
. (1980)

might be the most appropriate.
Regional metamorphism of the Vassalboro Formation, south-central Maine, USA: a case study of the role of fluid in metamorphic petrogenesis
Authors: Ferry J. M.
Published: December 2007

Progressively metamorphosed carbonate rocks of the Vassalboro Formation serve a′ natural laboratory for the investigation of fluid/rock interactions during metamorphism. An integrated study of their whole-rock chemistry, mineralogy, mineral chemistry and mineral abundances has led to the following results. (a) Prograde mineral reactions involved hydrolysis as well as dehydration and decarbonation. The hydrolysis reactions have resulted in an almost quantitative extraction of K and Na from high-grade metacarbonates. (b) Prograde mineral reactions buffered the X

CO
2

of coexisting metamorphic fluid during almost all of the rocks’ metamorphic history. (c) The carbonate rocks were infiltrated by large volumes of H
2
O-rich fluid while the buffering reactions progressed during the metamorphic event. Some high-grade metacarbonates were infiltrated by
at least
3 rock volumes of fluid. Combined buffering and infiltration appears to be a general metamorphic phenomenon in carbonate-bearing rocks to depths of ∼30 km in the crust. (d) There is an excellent positive correlation between the heat budget of individual metacarbonate rock samples and calculated fluid/rock ratios. Convective heat transfer by metamorphic fluids is therefore likely to be an important item in the heat budget of metamorphic terrains.

Consideration of these results leads to a model of regional metamorphism which involves infiltration of rock by large volumes of acid, H
2
O-rich fluid (large-scale acid infiltration metasomatism?). In this model, the infiltrating fluid exerts enormous control over the mineralogical evolution of the carbonate rocks during the metamorphic event: it drives mineral reactions, it changes whole-rock chemistry and it supplies heat.
Petrogenesis of Dalradian albite porphyroblast schists
Authors: Watkins K. P.
Published: December 2007

A linear belt of schists within which abundant albite porphyroblasts are developed outcrops over approximately 2000 km
2
of the SW Scottish and NE Irish Dalradian. Other features which characterize these rocks are chloritization, the common occurrence of magnetite and development of quartz segregations. The albite porphyroblast schists occur in the biotite and garnet Barrovian metamorphic zones and occupy the crests of regional F3 antiforms.

Textural, mineralogical and petrological data indicate that retrogression of original (Barrovian) metamorphic assemblages and concomitant hydrogen metasomatism occurred. As a consequence of these reactions pH gradients were induced between quartz-albite and mica-garnet-rich bands in the rocks which resulted in redistribution and recrystallization of albite as porphyroblasts. No input of sodium on a regional scale is necessary to explain porphyroblast growth.

An influx of water was responsible for these processes. It is likely that the water was a product of Barrovian regional metamorphism and accumulated in the crests of F3 antiforms as a result of restricted fluid flow.
Oxygen-isotope equilibration and permeability enhancement during regional metamorphism
Authors: Rumble Douglas , Spear F. S.
Published: December 2007

The degree to which oxygen-isotope equilibrium has been attained between the minerals of different, contiguous rock layers during metamorphism is correlated with evidence of enhanced permeability. Quartz separated from various rock layers of naturally occurring 'diffusion couples' has uniform values of δ
l8
O where rocks have either (1) experienced intense devolatilization reactions, or (2) been severely fractured. Quartz from interlayered rock types lacking evidence of enhanced permeability differs by 1–2%
o
over distances as small as 1 cm. These results suggest that intergranular diffusion through static pore fluid is not an efficacious process for exchanging oxygen isotopes between different rock types. Infiltrating pore fluid, however, can serve as a medium of isotope exchange over distances greater than bedding thickness where rocks are sufficiently permeable for fluid flow to occur
Quartz veins and devolatilization during metamorphism
Authors: Yardley Bruce W. D.
Published: December 2007

Fluid inclusion studies indicate that quartz veins represent pathways of rapid fluid movement in the Connemara Schists, where fluids have passed through contrasting lithologies without re-equilibrating. This suggests that fluid flow over distances of tens of metres must have occurred within a matter of hours. It is further suggested that since fluid generation is slow, fluid pressure and permeability may fluctuate during prograde metamorphism, with periodic episodes of hydraulic fracturing and fluid loss after slow increases in fluid pressure. Veining does not always occur during metamorphism and, where veins are absent, fluid loss may have been achieved through enhanced permeability or fine scale pervasive fracturing. Hydraulic fracturing is more likely if deviatoric stresses are large, and an association of veining with belts of more intense deformation is noted.
Mechanisms of crustal deformation
Authors: Turcotte D. L.
Published: December 2007

The energy source for crustal deformation is isotopic heating and secular cooling of the mantle. In a true solid, heat would be lost to the surface by conduction; however, solid-state creep processes allow the Earth’s solid mantle to exhibit a fluid behaviour. Thus, thermal convection can convert heating into directed motion. Variations in temperature lead to variations in densitythrough thermal expansion and contraction. Although the resultant body forces are vertical, horizontal variations in temperature lead to horizontal body forces. At an ocean ridge the variations in the thickness of the lithosphere lead to the elevation of the ridge. The result is a substantial horizontal ridge push force.This force, along with the trench pull force on the descending lithosphere is the primary force driving plate tectonics. Lithospheric thinning must occur beneath continental rifts and plateau uplifts. I suggest that lithospheric thinning is caused by the diapiric penetration of hot asthenospheric rock to the crust/mantle boundary. This penetration may also cause lithospheric stoping and plateau uplifts. Lithospheric thinning leads to large tensional stresses above the zone of thinning. These forces are generally much larger than the compressional forces generated by crustal thinning. The forces generated by lithospheric thinning are quite large and may be responsible for the propagation of continental rifts. In island arcs and Andean type orogenic belts both lithospheric thinning and crustal thickening result in large tensional forces. Probably the most complex zone of crustal deformation is the continental collision. Variations in types of continental collisions are discussed.