bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental studies of collapse calderas
Authors: MARTI J. , ABLAY G. J. , REDSHAW L. T. , SPARKS R. S. J.
Published: December 2007

We present experiments to investigate the formation of calderas using inflatable balloons in a medium of fused alumina powder. This system provides a model of a magma chamber in an homogeneous elastic media which scales to volcanic systems in terms of geometry and strength of the media. Three kinds of experiment were performed: (i) the balloon was inflated and then deflated simulating pre-emption tumescence and caldera collapse: (ii) the balloon was deflated without prior inflation, simulating caldera formation without pre-eruption tumescence: (iii) three balloons were inflated and deflated in a line, simulating multiple magma chambers. The influence of magma chamber shape was investigated using different shaped balloons. Balloon inflation formed a dome with a surface pattern of shallow polygonal fractures centrally, and radial fractures peripherally. Uplift occurs along deep inward-dipping concentric reverse faults. Deflation generated depressions with an outer set of concentric ring fractures surrounding a central funnel formed by sagging along numerous minor faults, some of which formed during doming. During collapse the concentric reverse faults were converted to normal faults and the radial normally-faulted fractures were converted to reverse faults. Without prior tumescence, an outer set of near vertical ring faults surround a flat central depression due to collapse of a coherent block. An inner set of outward-dipping reverse ring faults also develop. The experimental caldera area increased with balloon size and increased as the balloon depth decreased. For a given balloon volume and depth, collapse area is significantly smaller if doming had occurred prior to collapse. Arcuate fractures can provide the pathways for magma ascent during pre-emption tumescence (Smith & Bailey 1968), but the shallow radial and polygonal fracture systems may also influence vent location. Changes in fault geometry and sense of movement (extension to compression and vice versa) occur when subsidence initiates, and can account for changes in vent location that occur in many eruptions. Multiple overlapping collapse calderas are associated with magma chamber migration, whereas nested calderas can result from the activity of a single magma chamber.
Carbonatite near the Loch Borralan intrusion, Assynt
Authors: YOUNG B. N. , PARSONS I. , THREADGOULD R.
Published: December 2007

A suite of intrusive carbonate rocks, some containing xenoliths of nepheline syenite and Durness Group dolomite, has been discovered near the shore of Loch Urigill, Assynt, Scotland,
c
. 400m outside the contact of the strongly alkaline Loch Borralan intrusion. Four varieties occur: porphyritic white sövite, phlogopite sövite, sövite breccia and foliated silicocarbonatite. Initially only the white sövite was found in situ, intrusive into Durness dolomites, but excavation has revealed all types, except the silicocarbonatite, in place. The rocks are largely Sr-rich calcite, often enclosing phlogopite poikilitically, together with fluorapatite rosettes. Fluorite, titanite, pyrrhotite, pyrite, chalcopyrite and ilmenite also occur. Diopside, potassic richterite and chondrodite are found in the silicocarbonatite and in the vicinity of xenoliths of silicate-rock and dolomite in the other carbonatite types. The carbonate rocks have the high Sr. Rb and Ba, rare earth element patterns, and carbon and oxygen isotopic ratios characteristic of carbonatites from a mantle source region and are chemically distinct from the Durness dolomites, even when the latter are sampled as xenoliths in nepheline syenite. Although the absence of Nb and the lack of significant fenitization of accompanying silicate rocks are somewhat uncharacteristic, we conclude that the rocks at Loch Urigill are true carbonatites related to the Loch Borralan complex. The strongly alkaline Loch Borralan intrusion and its associated carbonatite represent the most westerly expression of Caledonian magmatism in Britain, which supports the view that they represent the products of small-scale partial melting in old, cold lithospheric mantle on the flanks of the orogen
Geochemistry of metamorphosed mafic rocks from Saih Hatat: pre-obduction history of NE Oman
Authors: EL-SHAZLY A. K. , WORTHING M. A. , JAYAWARDANE J. , VARNE R.
Published: December 2007

Field and petrological data, combined with major, trace and rare earth element analysis of mafic rocks that structurally underlie the Semail ophiolite in Saih Hatat, have led to their subdivision into five groups: (a) crossite-bearing mafic schists that represent metamorphosed within-plate alkali basalts, erupted in a Proterozoic continental rift or ocean basin setting; (b) greenschists, blueschists, garnet-blueschists, clinopyroxene-rich layers and eclogites, representing metamorphosed Permian basaltic flows and tuffs with enriched-MORB affinities; (c) crossite-bearing metavolcanics, representing alkali basalts, trachyandesites and dacites erupted on oceanic islands in a small ocean basin or through rifted continental crust during the late Permian; (d) lawsonite- and pumpellyite-bearing metavolcanic blocks incorporated in a Cretaceous tectonic melange, and originally erupted as alkali basalts on oceanic islands in the Hawasina basin; and (e) unmetamorphosed tholeiites formed at a spreading centre in the Hawasina basin and preserved in the Hawasina thrust sheets.

A reconstruction of the tectonic history of Saih Hatat based on these data suggests that continental rifting took place in the Permian, developing two distinct basins separated by rifted continental material. Some of the basaltic blocks that formed in the outboard Hawasina basin were tectonically dislocated and incorporated into a tectonic melange formed in a foredeep during the early stages of obduction. Emplacement of the Semail ophiolite onto the continental margin resulted in high-pressure, low-temperature metamorphism of this melange and the underlying units.
History of classification of Mesozoic ammonites
Authors: DONOVAN D. T.
Published: December 2007

The ammonites were first subdivided at generic level on the basis of shell coiling, heteromorphs or ‘uncoiled’ shells being separated after 1799 from the normally coiled shells. The latter were referred to a single genus,
Ammonites,
until the late nineteenth century. After the publication of Charles Darwin's
Origin of species
in 1859, the more forward-looking workers adopted the ideal of a classification based on phylogenetic relationships. This necessitated the use of a wider range of characters. Increasing realization of the complexity of phylogenetic relationships brought a proliferation of names at lower taxonomic levels—genus and family. For a while, the belief that the growth stages of the shell provided a reliable key to ancestry dominated the work of several specialists.

In the early twentieth century, Schindewolf favoured reliance on a single character, sutural ontogeny, as the decisive criterion in determining relationships. Other workers favoured a balanced appreciation of several characters. In the mid-twentieth century some felt that the detailed lineages, of which glimpses are seen through the fossil record, might never be disentangled, and that classification at generic and specific level must often be arbitrary. Recently, more abundant material and better stratigraphical information have led to the recognition of lineages based on closely-spaced assemblages which can be used as a basis for classification. The usefulness of the present classification is briefly considered.
Characterization of cement mineralogy in agglutinated foraminifera (Protista) by Raman spectroscopy
Authors: ROBERTS S. , MURRAY J. W.
Published: December 2007

Using micro-laser Raman spectroscopy, it is shown for the first time that calc-agglutinated foraminifera use one of either of two polymorphs of CaCO
3
, namely calcite or aragonite, as the mineralized cement to bind together the detrital grains. With the exception of one optical microscopy study, previous studies have used indirect methods, such as X-ray diffraction of whole tests or energy dispersive X-ray analysis, to infer the cement composition and mineralogy. All records to date have suggested that the mineral phase is calcite. We confirm this for
Spirorutilis wrightii
and provide the first record of the use of aragonite as biomineralized cement in the agglutinated foraminiferan
Texrularia crenara.
Previously aragonite was known only from the hyaline walls of the suborder Robertinina. This discovery has potential implications for the classification of the group.
Vendian and Riphean rifting in NW Scotland
Authors: SOPER N. J. , ENGLAND R. W.
Published: December 2007

In the Neoproterozoic, NW Scotland was located in a tract of extending continental lithosphere that evolved into the eastern passive margin of Laurentia when Iapetus opened in the Vendian. On the NW Caledonian foreland, major rift events are inferred in the Riphean and Vendian, corresponding to the intra-Torridonian and sub-Cambrian unconformities respectively. In this context, current views that the Moine and Dalradian rocks of the Scottish Highlands were subjected to repeated orogeny during the same period become difficult to sustain. We propose a tectonostratigraphic history for the two areas in which Neoproterozoic orogeny plays no part.
The magmatic, metamorphic and tectonic evolution of the Northern Marginal Zone of the Limpopo Belt in Zimbabwe
Authors: ROLLINSON H. , BLENKINSOP T.
Published: December 2007

The Northern Marginal Zone of the Limpopo Belt in southern Africa comprises a Plutonic Assemblage of granitoids including a distinctive suite of porphyroclastic granites. and a much less abundant Supracrustal Assemblage of metabasites and iron formations. These rocks are at granulite facies above a normal thickness of continental crust. Most of the Plutonic Assemblage are intrusive rocks that crystallized from dry melts from 2800 to 2600 Ma, with a relatively simple thermal history. They may have been derived from partial melting of a mafic source. Some supracrustal rocks have experienced two thermal events at granulite facies.

A reverse-sense shear zone forms the boundary of the Northern Marginal Zone with the Zimbabwe craton. The southern boundary is the Triangle shear zone, which is proven as a continuous structure along a much greater strike length than previously documented. A widespread sub-vertical foliation in the Northern Marginal Zone and the reverse shear zone formed during progressive NNW-SSE shortening. Crustal thickening occurred both magmatically and tectonically in the late Archaean, and was accompanied by synchronous uplift. Protracted magmatism provides a mechanism to incorporate supracrustal rocks into the lower crust, and can explain the occurrence of more than a single thermal event.
Westward continuation of the craton–Limpopo Belt tectonic break in Zimbabwe and new age constraints on the timing of the thrusting
Authors: MKWELI S. , KAMBER B. , BERGER M.
Published: December 2007

The Umlali Thrust Zone has been mapped between the granite–greenstone rocks of the Zimbabwe craton and the granulite-facies rocks of the Limpopo Belt. A tectonic break had not been previously recognized in the southwest because of the intrusion of syn- to post-kinematic porphyritic granites. These intrusions locally obliterate evidence for the tectonic break, although in some localities the associated mylonites are preserved as xenoliths within the intrusive porphyritic granite bodies.

The porphyritic granite bodies were emplaced during and after the main regional fabric forming event. A U–Pb zircon age of 2627 ± 7 Ma obtained from a late-kinematic microgranite is interpreted to be the minimum time of thrusting. A whole-rock Rb–Sr age of 2583 ± 52 Ma obtained from plutonic rocks and retrogressed zones within the North Marginal Zone falls within error of the zircon age, suggesting widespread magmatic and hydrothermal activity coeval with the thrusting.
Age of Mesozoic igneous rocks in northwesternNamibia, and their relationship to continental breakup
Authors: MILNER S. C. , LE ROEX A. P. , O'CONNOR J. M.
Published: December 2007

Mesozoic igneous rocks of northwestern Namibia comprise continental flood basalts and quartz latites of the Paraná–Etendeka province, and a suite of central sub-volcanic intrusions, the Damaraland complexes. Conventional K–Ar ages published for these rocks range from 377–88 Ma, with the majority of samples yielding ages of 150–110 Ma. This large spread of ages is interpreted as being due to disturbance of the K–Ar system by low-temperature alteration, which is ubiquitous among the volcanic rocks. Rb–Sr and
40
Ar/
39
Ar dating methods, which are less susceptible to such disturbance, yield a narrower range of ages (132–124 Ma, with one outlier at 149 Ma). New Rb–Sr internal isochron ages of 129.8 ± 3.8 Ma, 126.8 ± 1.3 Ma and 126.1 ± 7.3 Ma have been determined for an Etendeka quartz latite, and nepheline syenite intrusions in the Messum and Okorusu complexes, respectively. In addition,
40
Ar/
39
Ar release spectrum ages of 137.0 ± 0.7 Ma, 135.0 ± 0.7 Ma and 126.9 ± 0.6 Ma have been determined for samples from the Paresis (comendite), Cape Cross (nepheline syenite) and Okorusu (nepheline syenite) complexes, respectively.

A 13 Ma period between 137 and 124 Ma is considered to be the best estimate of the timing of Mesozoic igneous activity in northwestern Namibia. The continental flood basalt volcanism was probably limited to the early part of this period and is inferred to have been relatively short-lived (1–2 Ma). Magmatic activity among the Damaraland complexes was contemporaneous with the onset of flood basalt volcanism, but continued for approximately 15 Ma and terminated after the onset of sea floor spreading in this area (130–125 Ma). Both the flood basalt volcanism and the magmatic activity of the Damaraland complexes are thought to have occurred in response to rifting across the upwelling Tristan mantle plume. The linear distribution of intrusions in Damaraland is interpreted as being due to magmatism focused along a structural discontinuity between the Pan-African Damara terrain to the south and Proterozoic cratonic basement to the north.
Biogenic structure of early sediment fabric visualized by low-temperature scanning electron microscopy
Authors: PATERSON D. M.
Published: December 2007

The accurate visualization of early biogenic sediment fabric is important in the search for modern analogues to relict fossil structure and, therefore, for the interpretation of past environmental conditions. Water may have a significant structural role within hydrated material and sample dehydration may be detrimental to the structural interpretation of natural sediments and their associated microbiota. Water is present in sediments as free water or in the form of bound or hydration water. Extracellular polymeric substances associated with microbial assemblages have a high content of hydration water and are distorted by dehydration. The use of low-temperature scanning electron microscopy for the visualization of water-saturated sediments and associated biogenic structures is examined. Low-temperature SEM allows samples to be examined without the removal of water that is normally associated with the preparatory procedures for traditional ambient SEM. Samples for low-temperature SEM are first frozen (cryofixed) and then examined, fully-hydrated and still frozen, at low temperature (cf.–180°C) on the stage of a specially adapted SEM. The process of low-temperature SEM for the examination of hydrated sediments is described and several examples of cohesive and mixed-flat sediments and their associated biogenic fabrics are given. Detail of the internal fabric and associated microbiota can be examined by freeze-fracture techniques to preserve the natural condition of the sediments.
Pelletal structures in Peruvian upwelling sediments
Authors: BRODIE I. , KEMP A. E. S.
Published: December 2007

Late Quaternary sediments from the Peru upwelling zone have been investigated using backscattered electron imagery. The dysaerobic to anaerobic conditions which persist within the strong oxygen minimum zone intersecting the shelf and upper slope sediments, have resulted in the preservation of a wide range of microscopic fine structure relating to biological activity. Two forms of pelletal structures are present: pellets and aggregate structures. (1) Pellets are oval in cross section with a mean diameter of 610μm and sharp boundaries; three common pellet types have been identified: (a) crushed diatom and clay pellets; (b) intact diatom pellets; and (c) silt pellets. (2) Aggregate structures are smaller than pellets with a mean diameter of 360 μm. They have indistinct boundaries and may be composed either of pure diatom ooze or a mix of clay, silt and diatoms. The size, shape and mode of occurrence of aggregate structures suggest that they are the product of a meiofauna, redistributing the topmost sediment during periods of very slight increases in oxygen. Most of the crushed diatom/clay pellets and intact diatom pellets are probably also the product of a surficial deposit-feeding fauna tolerant of low dissolved oxygen levels such as polychaete worms. Silt pellets may record the chamber building activities of agglutinating foraminifera.
Early carbonate diagenetic fabrics in the rhizosphere of late Pleistocene aeolian sediments
Authors: McLaren S. J.
Published: December 2007

Analysis of a range of aeolianite deposits from the Western Mediterranean (Roquetas in southern Spain and Campo de Tiro in Mallorca) and the Bahamas (Crab Cay on San Salvador Island) has shown that biota can be important in the diagenesis of deposits in the vadose zone, and this may aid in the interpretation of palaeoenvironments, particularly where pedogenic calcretes are absent. This paper looks at how the growth of roots into dune sediments and the associated profusion of micro-fauna and micro-flora can have, on a local scale, a profound affect on the early diagenetic and sediment fabric. These fabrics are affected at three levels: (1) at the site of the former root; (2) directly surrounding the root (the peritrophic zone); and (3) from the peritrophic zone to a few tens of centimetres from the former root.
Evidence for multiple high-
T
metamorphism and regional migmatization within the Cadomian belt of northern Brittany, France
Authors: HÉBERT R.
Published: December 2007

The Cadomian Orogeny is characterized by two major tec-tonometamorphic events, one that pre-dates 570 Ma and another at
c
. 540 Ma. Until recently, the majority of migmatites occurring within the Cadomian belt of northern Brittany were interpreted as having been formed during the second event. The discovery of the Jouguet migmatites, which have been hornfelsed by the St-Brieuc diorite, emplaced at 533 ± l2 Ma, suggests that a migmatitic event occurred before 540 Ma. These data demand a reinterpretation of the structural sketch of the Cadomian Orogeny.
New evidence for evaporitic Middle Devonian lacustrine sediments with hydrocarbon source potential on the East Shetland Platform, North Sea
Authors: DUNCAN W. I. , BUXTON N. W. K.
Published: December 2007

The UKCS (United Kingdom Continental Shelf) well 9/16-3 drilled on the western flank of the Beryl Embayment encountered a Middle Devonian sequence interpreted as part of the lacustrine Orcadian Basin. An anhydrite layer found within the sequence provides confirmation of the local development of hypersaline environments within the Orcadian Basin. Detailed geochemical analysis of a representative source interval equivalent to the Achanarras/Sandwick fish bed has revealed a close -affinity with contemporaneous Middle Devonian source rocks found in the Inner Moray Firth and crude oil from the Beatrice oil field.
The geometry and evolution of a transpressional strike-slip system: the Carboneras fault, SE Spain
Authors: KELLER J. V. A. , HALL S. H. , DART C. J. , McCLAY K. R.
Published: December 2007

The Carboneras fault system is a 40 km long, 1 km wide Neogene NE-SW-trending left-lateral transpressional strike-slip fault system which is part of the Trans-Alboran shear zone in SE Spain. The Carboneras fault system is an anastomosing array of sub-vertical, individual fault planes or fault zones which surround pods or lenses of less intensely strained rocks. Displacements along the individual fault planes exhibit reverse components of slips and form positive flower structures. Faults have either sharp boundaries or wider bands of gouge, typically a few metres thick and are hundreds of metres in length. Second-order fault splays are well developed and usually exhibit P-shear rather than Riedel-shear orientations. These are interpreted to be related to the transpressional displacement and may also characterize other oblique convergent zones elsewhere. The second-order faults are interpreted to have formed as shear strain increased along the first-order fault and was transferred laterally to branch segments. This process produced pods, or shear lenses, bounded by the fault segments. The length to width aspect ratios of the shear lenses were found to be scale-independent across five orders of magnitude with the most common values between 3:l and 6:l. A model is proposed for the development of the fault zone by incremental finite displacements along the segmented fault surfaces. This model is based on field evidence that displacement switched with time from one fault strand to another.
Palaeostresses associated with thrust sheet emplacement and related folding in the southern central Pyrenees, Huesca, Spain
Authors: MARTINEZ-PEÑA B. , CASAS-SAINZ A. M. , MILLAN-GARRIDO H.
Published: December 2007

Brittle mesostructures have been studied in the detached Mesozoic and Lower Tertiary cover rocks of the south-central Pyrenees (Huesca province, Spain), in order to reconstruct the state of stress within the thrust sheets during Tertiary compression. The structure of the area studied consists of NW–SE- to E–W-striking, south-verging thrust-sheets, located west of the South-Pyrenean Central Unit. The palaeostresses obtained show dominant N–S to ENE–WSW compression directions for stress tensors with horizontal σ
1
axes, and E–W to NW–SE extension directions for stress tensors with vertical
σ
l
axes. The values of the stress ellipsoid ratios for stress tensors with horizontal
σ
1
are predominantly between 0 and 0.5 (indicating uniaxial compression to wrench regime). Regarding their relationship with macro-structures, the palaeostresses show transport-parallel compression in most sites, although thrust-parallel compression is also found. There are no differences between palaeostresses found in the hanging walls and the footwalls of thrusts. Stress tensors with vertical
σ
1
do not show systematic relations with macro-structures, but transport-parallel extension and thrust-parallel extension are found in some sites. The analysis of palaeostresses in areas of fold-and-thrust belts shows a high degree of complexity and the results are not easy to interpret. The continuous variation from N–S to ENE–WSW in the orientation of the compression axes is here interpreted in two ways: (1) as the result of the clockwise rotation of the Mesozoic and Tertiary cover, which was pushed in the east by the South-Pyrenean Central Unit with a constant N to NNE regional compression, and (2) due to NE–SW compression resulting from a SW-directed local transport of minor thrusts. Palaeostress analysis can, therefore, reveal important information about thrust kinematics, at high structural levels where ductile deformation is absent.
The discovery of a new Devonian tetrapod trackway in SW Ireland
Authors: STÖSSEL I.
Published: December 2007

A tetrapod trackway has been discovered in Devonian sedimentary rocks of SW Ireland. It is the first discovery of its kind in Europe. Details of the footprints are not preserved, probably because of strong Variscan pressure solution cleavage. However, after removing the effect of Variscan deformation, the large number of footprints (more than 150) makes it possible to calculate the original dimensions of the footprint pattern: stride (approximately 34 cm) and pace (approximately 35 cm). These values are higher than those for other traces of comparable age. The total length of the tetrapod is estimated to be 1 m. Lithostratigraphical and biostratigraphical arguments indicate an age of Mid- to Late Devonian.
Early Palaeozoic rather than Neoproterozoic volcanism and rifting within the Transantarctic Mountains
Authors: MILLAR I. L. , STOREY B. C.
Published: December 2007

New U-Pb dating of zircons separated from felsic volcanic rocks of the Patuxent and Gambacorta formations from the Pensacola Mountains in the Transantarctic Mountains, Antarctica, yields earliest Ordovician ages of 500 ± 8 and 501 ± 3 Ma respectively. The dated felsic volcanic rock of the Gorecki Felsite Member of the Patuxent Formation is important tectonically, as the felsite, together with mafic volcanic rocks, were previously considered to provide key evidence of a Neoproterozoic rifting event prior to separation of Laurentia from East Antarctica. The new data do not necessarily refute this event but indicate a previously unrecognized early Ordovician period of bimodal magmatism and extension along the Transantaractic Mountains.
Reconstruction of land and freshwater palaeoenvironments near the Eocene-Oligocene boundary, southern England
Authors: HOOKER J. J. , COLLINSON M. E. , VAN BERGEN P. F. , SINGER R. L. , DE LEEUW J. W. , JONES T. P.
Published: December 2007

Mammalian assemblages in the Bembridge Limestone Formation of late Eocene age, Headon Hill, Isle of Wight, England, indicate habitats ranging from open woodland to closed forest. Distinctive 'lower' and 'upper' mammalian faunas reflect different fauna1 provinces, probably in response to climatic fluctuations that foreshadowed the terminal Eocene event. In order to improve our understanding of these patterns, we have examined a variety of other palaeoenvironmental indicators from this section. These include palynological organic matter (POM), plant macrofossils, non-mammalian faunas, organic geochemistry and stable isotopes. The evidence shows that the depositional setting was a tranquil, shallow, freshwater lake, with a brief lagoonal interval. However, evidence for habitats surrounding the lake is contradictory, emphasizing the necessity for multidisciplinary approaches to palaeoenvironmental reconstruction. The mammal faunas give unequivocal evidence for woodland or forest, yet, apart from some of the land snails, there is no other indication of the presence of trees. Furthermore, according to the mammalian evidence the lake was bordered by distinctive vegetation at different times, with closed forest/woodland during mar1 deposition and open woodland during black mud deposition, but there are no parallel fluctuations in other biotic elements.
Late Jurassic palaeogeography and anaerobic-dysaerobic sedimentation in the northern Antarctic Peninsula region
Authors: PIRRIE D. , CRAME J.A.
Published: December 2007

Late Jurassic anaerobic-dysaerobic mudstones crop out on both the Weddell Sea (back-arc) and Pacific (fore-are) margins of the northern Antarctic Peninsula. The only known occurrence on the Pacific margin of the Antarctic Peninsula is the Anchorage Formation of Livingston Island. This mudstone dominated unit comprises interbedded volcaniclastic sandstones, pyroclastic/epiclastic tuffs and radiolarian mudstones. The volcaniclastic sandstones are interpreted as representing deposition from turbidity currents. The tuffs represent sedimentation by both primary airfall processes and resedimentation by low concentration turbidity currents. The radiolarian mudstones represent suspen­sion sedimentation, and reveal an upward increase in bioturbation with a transition from anaerobic-dysaerobic conditions to dysaerobic-aerobic conditions. These facies and the observed vertical change in oxygenation conditions are similar to those seen in the Nordenskjöld Formation on the Weddell Sea margin of the Antarctic Peninsula. However, biostratigraphical investigations show that the transition from dysaerobic to aerobic conditions occurred during the late Kimmeridgian-early Tithonian in the Anchorage Formation but late Tithonian or early Berriasian in the Nordenskjold Formation. This diachroneity is related to the palaeogeographical development of the Antarctic Peninsula magmatic arc. A wide epicontinental sea and subdued arc relief in the early Kimmeridgian was followed in the Tithonian by arc uplift, increasing oxygenation in the fore-are basin, and the development of a restricted basin in the hack-arc region. In latest Tithonian-earliest Berriasian times a substantial arc had developed which supplied volcaniclastic sediment to the fore-arc basin; only then was the back-arc basin undergoing the transition from dysaerobic to aerobic conditions. Anaerobic conditions initiated by regional upwelling and expansion of the oxygen minimum zone were perpetuated in a silled basin in the back-arc area, formed by the emergent arc.
Are enigmatic erosional sandy wave-like bedforms in Jurassic Bridport Sands, Dorset, due to standing waves?
Authors: PICKERING K. T.
Published: December 2007

A field of sandwave-like bedforms produced by erosion, rather than deposition-with-bedload traction, is identified within the Lower-Middle Jurassic shallow-marine Bridport Sands of the Wessex Basin, which were previously interpreted as simple channel-like erosional scours by Davies in 1967. Here, this interpretation is refined by proposing that these sandy bedforms formed as near-symmetrical scours into horizontally-stratified and bioturbated very fine- to fine-grained sands in shallow water below standing waves, possibly internal edgewaves. These sandy bedforms, and contemporaneous erosional surface which defines the base of the Ham Hill Stone, may represent different responses to a
moorei
Subzone lowstand of sea level.
A new method for determining magnetization direction from gravity and magnetic anomalies: application to the deep structure of the Worcester Graben
Authors: ATES A. , KEAREY P.
Published: December 2007

The aeromagnetic field over the Worcester Graben, a major N-S-oriented, fault-bounded sedimentary basin, shows a large positive anomaly of 100 nT amplitude near the town of Tetbury. A new method for the determination of the magnetization direction of the causative body was employed that searches for the maximum correlation between the observed gravity anomaly and the pseudo-gravity anomaly computed for a range of orientations of the magnetization vector. The method suggests a vertical magnetization for the causative body of the Tetbury anomaly. A three-dimensional magnetic model of the body has the form of a cone, elongated N-S, whose base is at 6.0 km and upper surface at 3.1 km as indicated by power spectrum analysis of the anomaly. A two-dimensional model derived from the gravity field and constrained by the surface geology, borehole data and existing density measurements cannot simulate the observed anomaly over the centre of the graben. However, when the magnetic body is included in the gravity model, the fit is much improved. The western side of the body is probably imaged on a seismic reflection profile to the north. The body magnetic model of the body has the form of a cone, elongated N-S, whose base is at 6.0 km and upper surface at 3.1 km as indicated by power spectrum analysis of the anomaly. A two-dimensional
Plagiogranite from the Andaman ophiolite belt, Bay of Bengal, India
Authors: JAFRI S. H. , CHARAN S. N. , GOVIL P. K.
Published: December 2007

Plagiogranites occur on the eastern margin of the southern part of South Andaman in the outer sedimentary arc of the Andaman-Nicobar islands, a part of the Sunda-Burmese double-chain arc system, in the Bay of Bengal. They intrude gabbros that are associated with pillow basalt, East Coast volcanic rocks (basalt, basaltic andesite to acid differentiates), radiolarian cherts, conglomerate and grit.

The plagiogranite, characterized by the occurrence of interstitial vermicular and micrographic intergrowths of quartz and plagioclase, shows some geochemical similarities with ophiolitic plagiogranites, but Rb, Yb, Nb, Ta and Y abundances in the Andaman plagiogranite are characteristic of volcanic-arc granites rather than the ocean-ridge granites. It is suggested that the Andaman plagiogranite has been generated by low-pressure crystal fractionation of a low-K tholeiitic magma derived from a mantle source in a supra-subduction zone tectonic setting. Further, it is suggested that this plagiogranite intruded the gabbro of the Andaman ophiolite probably during Mid-Eocene times, as the Andaman ophiolitic rocks (Late Cretaceous to Early Eocene) are believed to have been obducted onto the leading edge of the Eurasian continent during the Mid-Eocene to Late Oligocene subduction event, prior to the currently active Andaman-Java subduction system, that was initiated during the Late Miocene.
Benthic foraminiferal evidence for the existence of an early Miocene oxygen-depleted oceanic water mass?
Authors: SMART C. W. , RAMSAY A. T. S.
Published: December 2007

This study presents new data derived from changes in benthic foraminiferal assemblages between 24 and 8 Ma in the Atlantic and Indian oceans which are integrated with published stable isotope and fauna1 records. We have identified an intermediate to deep water mass which was lower in oxygen and was restricted to the NW Indian, Atlantic and the Tethys (Mediterranean) oceans. Our interpretations are based on the coeval occurrences (19.5–16.5 Ma) of high abundances of smooth-walled bolivinids in these oceans. Our observations agree with the interpretations of previous workers for the occurrence of Tethys-derived intermediate and deep water in the northern Indian Ocean between 24 and 14 Ma and for Northern Component Water production in the Atlantic Ocean between 20 and 16 Ma. The water mass defined by these bolivinids formed a southward flowing Northern Component Water in the Atlantic Ocean and contributed, via Tethys, to the intermediate and deep water input into the Indian Ocean.
Cool Cretaceous climates: new data from the Albian of Western Australia
Authors: PIRRIE D. , DOYLE P. , MARSHALL J. D. , ELLIS G.
Published: December 2007

Although previous geological, palaeobotanical and geochemical data from the Albian of Australia have suggested the possibility of cool palaeoclimates, there are few well-constrained stable isotope palaeotemperatures studies from this region. In this paper we present the results of stable isotope analyses of dimitobelid belemnites from the Albian Gearle Siltstone, Carnarvon Basin, Western Australia. Oxygen isotope values for diagenetically least-altered samples give values ranging between δ
18
O −0.02 and 0.85% PDB, which equate to a mean palaeotemperature of 10.08°C (assuming δ
w
of −1.2% SMOW). This value is broadly consistent with recently published isotope data from the Albian Antarctic and with estimates for mid-Cretaceous latitudinal temperature gradients, implying that cool palaeoclimates occurred during a period of 'Greenhouse' Earth.
Palaeocene uplift and Eocene subsidence in the northern North Sea Basin from 2D forward and reverse stratigraphic modelling
Authors: NADIN P. A. , KUSZNIR N. J.
Published: December 2007

Structural and stratigraphic basin modelling has been used to examine quantitatively the Cretaceous and Tertiary post-rift subsidence history of the northern North Sea Basin following Late Jurassic extension. 2D forward and reverse, syn-rift and post-rift modelling has been used to determine the magnitude and timing of departures from the McKenzie post-rift thermal subsidence trend for regional stratigraphic profiles in the Outer Moray Firth, the South Viking Graben and the North Viking Graben. Observed stratigraphy has been reverse post-rift modelled using flexural backstripping combined with decompaction and reverse thermal subsidence calculations, and forward modelled through the syn-rift and post-rift period using the flexural cantilever model of continental rift basin formation. Long-term global eustasy has been included in the analysis. Only reliable palaeobathymetric markers, such as erosion surfaces and coals, have been used to constrain subsidence history.

Both forward and reverse 2D modelling show that a Palaeocene regional uplift followed by a rapid Eocene subsidence must be superimposed on McKenzie post-rift Cretaceous and Tertiary basement subsidence in order to generate the stratigraphy observed. The magnitude of Palaeocene uplift deduced from modelling was of the order of 375 m in the Outer Moray Firth (central North Sea) increasing to 525 m in the North Viking Graben (northern North Sea). Rapid Early Eocene subsidence was of the order of 160–310 m. The most likely mechanism for regional Palaeocene uplift and rapid Eocene subsidence in the northern North Sea was the Palaeocene development of the Iceland plume situated at a distance of 700–900 km from the study area in the Early Tertiary. Forward and reverse modelling predict axial bathymetries in the graben of up to 1300 m for the Early Cretaceous and 900 m for the Upper Cretaceous. The large palaeobathymetries at the end of the Cretaceous provided much of the accommodation space for Tertiary sedimentation.
Stratigraphic and structural development of the Eastern Pontides, Turkey
Authors: ROBINSON A. G. , BANKS C. J. , RUTHERFORD M. M. , HIRST J. P. P.
Published: December 2007

This paper describes the stratigraphic and structural development of the Eastern Pontides, North Turkey, based on section logging at eight localities, construction of three structural profiles across the mountains, and published literature. The Eastern Pontides comprise a Late Carboniferous to Miocene sequence resting on a basement of Hercynian metamorphics. Late Carboniferous to Scythian sediments are continental clastics interpreted as having been deposited in an extensional half graben. The Middle Triassic is limestone, reflecting the development of a south facing passive margin. In the Late Triassic, the ocean (Palaeotethys) began to close: flysch was deposited in most places with minor shallow water limestones in the Sinemurian above an inverted half graben. All pre-Aalenian strata were deformed during the Cimmerian orogeny. After a phase of subduction-related volcanism in the Mid-Jurassic, limestones were deposited during the Late Jurassic and early Cretaceous, mainly in shallow water. The Aptian and Albian are absent due to doming in the Western Pontides prior to the opening of the Western Black Sea, but there is a thick sequence of Upper Cretaceous arc volcanic rocks and intervening turbidites. The Upper Palaeocene is absent, possibly due to rifting in the Eastern Black Sea. Major compression affected the Pontides from the Eocene to the Pliocene associated with the closure of the Tethyan Ocean. Oligocene and younger rocks are accordingly non-marine.
A new crested maniraptoran dinosaur from the Santana Formation (Lower Cretaceous) of Brazil
Authors: MARTILL D. M. , CRUICKSHANK A. R. I. , FREY E. , SMALL P. G. , CLARKE M.
Published: December 2007

The skull of a new, and highly unusual crested dinosaur with an elongate rostrum is the first dinosaur to be named from the Santana Formation of NE Brazil.
Irritator challengeri
gen. et sp. nov was most likely a maniraptoran dinosaur, but its affinities to other maniraptorans remain to be established. Unique features for
Irritator
include the pattern of tooth replacement, a highly reduced supratemporal fenestra, extreme lateral compression of the rostrum and a saggital crest comprised of the frontal and parietal bones.
Irritator
was most probably a piscivore. A land link between South America and the dinosaurian faunal province of North America and Asia is indicated, probably via Africa.
Sulphur sources for epithermal and mesothermal veins in Cretaceous–Tertiary magmatic-arc rocks, Livingston Island, South Shetland Islands
Authors: WILLAN ROBERT C. R. , SPIRO BARUCH
Published: December 2007

Epithermal veins in early Triassic turbidites on Hurd Peninsula are isotopically homogeneous over an area of 13 km × 2 km (mean δ
34
S = +2.2‰, 1
σ
= 1.7,
n
= 65) suggesting derivation from a deep circulating, neutral-chloride hydrothermal plume containing magmatic sulphur. The sense of fractionation (pyrite = +3.5, arsenopyrite = +3.2, sphalerite = +2.9, chalcopyrite = +2.5, galena = +0.9‰) and isotopic temperatures between 250 and 345°C suggest partial isotopic equilibration. Sulphides in massive dark carbonate/peperitic dyke breccias, with abundant magnetite, hematite and titanite and traces of barite, garnet, zircon and monazite are isotopically heavy (+7 to +14.9‰,
n
= 12) due either to dyke intrusion into the hydrothermal system resulting in degassing of H
2
S, or to a second, isotopically heavy and sulphate-bearing solution in fault zones along which dykes were subsequently intruded.

Vein sulphides in nearby Cretaceous volcanic rocks are similar to the epithermal system (mean = + 1.8‰, 1
σ
= 0.9,
n
= 6, pyrite = +2.0, chalcopyrite = +1.5), suggesting sulphur-derivation by degassing of sub-volcanic magma, or remobilization of disseminated sulphides during plutonism. Sulphide in an Eocene tonalite pluton is slightly
32
S-enriched (mean = 0‰, 1
σ
= 1.9,
n
=7, molybdenite = +0.7, pyrite = +1.5, chalcopyrite = –2.6) and was exsolved directly from the cooling tonalitic magma.
Fission-track evidence for the thermotectonic evolution of a Mesozoic–Cenozoic fore-arc, Antarctica
Authors: STOREY B. C. , BROWN R. W. , CARTER A. , DOUBLEDAY P. A. , HURFORD A. J. , MACDONALD D. I. M. , NELL P. A .R.
Published: December 2007

Zircon and apatite fission-track data from the LeMay Group accretionary complex and Fossil Bluff Group fore-arc basin sequence on Alexander Island (Antarctica) record a common regional Cretaceous and Cenozoic thermal and denudational history. With the exception of zircon data from samples closest to the trench, the apatite and zircon central ages are substantially less than known and, inferred stratigraphic ages. Thermal modelling of the data indicate cooling from maximum palaeotemperatures in the range 180–350°C at
c
. 100 Ma. A younger period of accelerated cooling occurred between 40 and 35 Ma with final cooling to surface temperatures taking place at reduced rates through the Tertiary. The start of cooling was close in time to the end of deposition within the fore-arc basin and is consistent with structural evidence for Cretaceous deformation in a strike-slip setting. The accelerated, Early Tertiary cooling episode was broadly coeval with, and may have been caused by ridge–trench collisions and cessation of subduction off-shore Alexander Island.

Zircon and apatite age data, from magmatic rocks emplaced in the fore-arc region in Late Cretaceous and Tertiary times, are close to their age of crystallization. This indicates rapid cooling of both fission-track systems from temperatures >350°C to <
c
. 60°C at high crustal levels of emplacement. The formation of these magmatic rocks, together with the cooling history of the accretionary prism, are related to changes in subduction zone parameters during Cretaceous and Cenozoic times. Slowing of subduction rates and role-back of the slab prior to cessation of subduction by ridge–trench collision may have been responsible for migration of the magmatic focus into the fore-arc region, uplift of the prism and formation of the high mountain ranges, and the extensional graben that separates Alexander Island from the Antarctic Peninsula.
Avalonian magmatism and terrane linkage: new isotopic data from the Precambrian of North Wales
Authors: HORÁK J. M. , DOIG R. , EVANS J. A. , GIBBONS W.
Published: December 2007

New U–Pb isotope data from the Precambrian (Sarn Igneous Complex) of North Wales establishes that calc-alkaline plutonic rocks of the same age occur on either side of the Menai Strait fault system. Zircon fractions from the Sarn Igneous Complex gabbro give a near concordant U–Pb age of 615 ± 2 Ma which is interpreted as the first accurate age of igneous crystallization and supersedes previous dating attempts using the Rb–Sr method. The new U–Pb age is the same, within error, as recent zircon dates obtained from the Arfon Group ignimbrites along strike from the Sarn Igneous Complex, and from the Coedana Complex granite on Anglesey. These data emphasize the importance of what appears to have been
the
major phase of calc-alkaline magmatism at 630–600 Ma recognizable along the Avalonian arc in Britain, Newfoundland and elsewhere. Despite the similar ages and tectonic setting of the Coedana and Sarn plutonic rocks, however, they remain petrogenetically and geochemically distinct and are not directly linked as part of the same igneous complex. Instead the Coedana Complex granite is interpreted as having been generated in the Avalonian arc during a widespread magmatic event that is also recorded in Britain not only by the Sarn Igneous Complex and Arfon Group, but by volcanic and plutonic rocks in central England. Investigation of granite clasts in the Gwna Group melange (Monian Supergroup) on Anglesey failed to produce zircons adequate for dating purposes but revealed Rb–Sr. whole-rock geochemical and petrographic characteristics similar to the nearby Coedana Complex and suggest linkage between fault-bounded 'Monian' terranes on Anglesey. In the light of these new data rocks on either side of the Menai Strait terrane boundary are interpreted as belonging to the same arc system that was dismembered, dispersed and laterally duplicated by transcurrent faulting after late Precambrian magmatism.
Evolution of the Northern Belt, Southern Uplands: implications for the Southern Uplands controversy
Authors: ARMSTRONG H. A. , OWEN A. W. , SCRUTTON C. T. , CLARKSON E. N. K. , TAYLOR C. M.
Published: December 2007

Conodont and graptolite dating provide a high resolution biostratigraphical framework for correlation across the Southern Upland Fault. The establishment of an early Caradoc age for the red chert and volcanic sequences within the Northern Belt of the Southern Uplands confounds the accretionary prism model. and our data support the view that following the obduction of the Ballantrae ophiolite, fan complexes prograded southeastwards across a narrow, fault-bounded shelf into the deep basin of the Northern Belt. The fan complexes were initiated by uplift in the hinterland, caused by the intrusion of plutons into the Midland Valley Terrane. Sedimentation was controlled by regional extension, subsidence and eustasy through the Llanvirn and Caradoc. Basic lavas, with attenuated within-plate geochemical signatures, were erupted during the early Caradoc peak of extension and may have just post-dated the initiation of southwards subduction beneath the Lake District.