bluuebunny/crossref_metadata_2025 huggingface dataset:
Microfacies associations in Asbian carbonates: an example from the Urswick Limestone Formation of the southern Lake District, northern England
Authors: Horbury A. D. , Adams A. E.
Published: April 2008

Semi-quantitative analysis of allochems from the Urswick Limestone Formation (Asbian) of the southern Lake District area of northern England has revealed a distinctive cyclicity of the microfacies. Cycle-top grainstone microfacies contain an algal flora comprising
Koninckopora, Anatolipora
and
Polymorphocodium
, with
Girvanella
filmaents and
Ortonella
lumps. Other allochems include intraclasts, large peloids and thick-shelled bivalves and gastropods. The middles of cycles are mostly packstones and micro-grainstones and contain allochems dominated by small peloids and the algae
Kamaeena, Kamaenella
and
Epistacheoides
, with the microproblematicum
Ungdarella
and relatively high abundances of micritic-walled formainifera such as endothyrids. Cycle bases contain a diverse algal assemblage including
Coelosporella
and
Stacheoides
, with other allochems represented by trilobites, ostracodes,
Saccamminopsis
, foraminifera such as Archaediscidae, the base late Asbian guide
Howchinia
, the base early Asbian guides
Gigasbia gigas
and
Vissariotaxis
, bivalves, small gastropods, bryozoans, sponge spicules and bored grains. Other allochems are found throughout most cycles, decreasing only in the very shallowest (intertidal) facies, or have an irregular distribution, and include brachiopod debris, crinoid ossicles and coral fragments.

There are significant variations in allochem distribution according to palaeogeography. Close to the shelf margin there are higher abundances in the cycle top grainstones of the algae
Koninckopora
and
Anatolipora
, and also of the calcified filaments
Girvanella
and
Ortonella
, with thick-shelled gastropods, intraclasts and coarse peloids. At cycle bases, echinoderm arm plates and bryozoans are particularly abundant in packstone-wackestone textures. Platform interior facies are differentiated into a diverse open-marine type, with a high total abundance of bioclasts in the cycle base pack-wackestones including trilobites,
Coelosporella, Stacheoides, Kamaena
and bored grains, grainstones are dominated by small peloids,
Kamaenella
and
Ungdarella
), and a more restricted cycle type, in which total bioclast abundances are low.
Mid-Dinantian brachiopod biofacies from western Ireland
Authors: Harper David A. T. , Jeffrey Anna L.
Published: April 2008

New silicified brachiopod faunas from lower and middle Viséan horizons have been retrieved from the vicinity of Loughs Carra, Corrib and Mask in west Connaught. In addition, carbonate shell beds dominated by
Linoprotonia
are recorded from mid County Galway. At present over 40 species of brachiopod are known, from current sampling programmes and the literature, variably associated with bryozoan, coral, echinoderm, mollusc and trilobite faunas; microvertebrates such as sharks teeth are locally common. Four main assemblages are recognized; in ascending order, Arundian strata at Ardnasillagh, Lough Carra, Kibeg Wood and Ballintober in a northern belt are variably dominated by
Rhipidomella, Schizophoria, Leptagonia, Rugosochonetes, Krotovia, Echinoconchus, Tylothyris, Punctospirifer
and
Spiriferellina
. In addition to genera recorded from Arundian strata elsewhere,
Streptorhynchus, Plicochonetes, Dictyoclostus, Pleuropugnoides
and ‘
Spirifer
’ also occur in the diverse Kiltiernan fauna assembled last century. In a southern belt, possible Holkerian horizons at Dunsandle are less diverse, with faunas dominated by
Composita
, whereas limestones of Holkerian-early Asbian age at Kiltullagh Bridge have quite different faunas with
Schizophoria, Brochocarina, Minythyra
and
Cleiothyridina
; these faunas are more similar to the diverse Asbian assemblages described from County Fermanagh in the north of Ireland. In a central belt, in the Bunoghanaun area, Holkerian to early Asbian horizons are dominated by relatively thick shell beds in pure limestones with mainly opportunist large linoproductids; this fauna is not silicified.
Volcano instability development: a planetary perspective
Authors: Head James W.
Published: April 2008

Analysis of the types and causes of volcano instability using the record seen on other planets, where volcanoes form in a wide variety of environments and are often better preserved, provides a perspective on the processes involved and deposits produced in edifices on Earth. The records on these bodies illustrate the potential significance of neutral buoyancy zones and rheological boundaries in relation to initial magma ascent and storage, related eruption styles, and subsequent edifice development and behaviour. On the Moon no Hawaii-like shield volcanoes are observed; the thick low-density anorthositic crust provided a density barrier to mantle plumes and the low frequency of dyke-emplacement events precluded development of shallow reservoirs and large shields. On Venus, the very high atmospheric pressure reduces volatile exsolution and fragmentation, inhibiting the formation of neutral buoyancy zones (NBZ) and shallow magma reservoirs near or below mean planetary radius; eruptions are predicted to be characterized by relatively high total volumes and effusion rates, and poorly developed edifices. Decrease in atmospheric pressure with elevation favours the development of NBZs and reservoirs at higher elevations. These and other factors result in low and broad volcanic edifices, with reservoirs predominantly in the substrate. Smaller loads, thicker lithospheres, and dry substrates combine to minimize large-scale vertical edifice growth and instability development. Exceptions include a wide array of collapse runout features associated with steep-sided domes. On Mars, low gravity and low surface atmospheric pressure cause density profile differences and flow length differences and massive volcanoes have been emplaced over long periods on a stable lithospheric plate and a volatile-containing upper crust. They exhibit a wide range of rift zone development, internal deformation related to lithospheric loading and flexure, flank and slope failure, and summit failure and caldera development. These planetary examples permit one to begin to isolate various factors in the development of volcano instabilities, including edifice geometry (height/width) and size, relation to lithospheric thickness as a function of time, potential role of neutral buoyancy zones and magma reservoir size and development, and the presence of fluids in the substrate. The extensive and well-preserved flank-failure deposits on Mars provide a significant areal perspective and analogue for the interpretation of less-well preserved deposits on Earth.
Slope stability and eruption prediction on the eastern flank of Mount Etna
Authors: Murray J. B. , Voight B.
Published: April 2008

Recent work applying knowledge of the failure of materials to volcanic eruptions in Kamchatka, Alaska and mainland USA, has shown that inverse rate analysis of seismic data can accurately predict eruption times up to 2 or 3 weeks in advance. We find evidence that application of the same principles to ground deformation data on Mt Etna’s eastern flank prior to the eruptions of 1985 and 1989 provides good eruption time predictions more than 3 months in advance. The same data also point to an eruption triggering mechanism involving an interplay between slope creep and magma pressure that has not been described before.
A simple model for the genesis of large gravitational landslide hazards in the Canary Islands
Authors: Carracedo J. C.
Published: April 2008

Natural hazards in the Canarian Archipelago are mainly in relation to volcanism (at least five islands with Holocene eruptive activity, 17 eruptions in the last 500 years) and massive gravitational landslides related to edifice overgrowth and dyke intrusion. In the volcanically active islands, the emission vents tend to group in clearly aligned clusters, evolving to form steep ridges that behave as true polygenetic active volcanoes with clear rift affinities, and constitute by far the most probable location of any future volcanic eruption in the archipelago. Tunnels excavated for water mining show a narrow band of tightly packed parallel dykes running through the centre of the rifts. The rift zones play a key role in the growth and shape of the island edifices and in the generation of massive landslides. Cumulative gravitational stresses related to the growth of the volcanic edifices and more ephemeral mechanisms associated with intense eruptive phases, such as dyke wedging, increase of slope angles and strong local seismicity associated with magma movement can finally exceed the trigger-threshold of gravitational slides. This mechanism may be the explanation for the numerous horseshoe-type valleys and calderas developed in the Canary Islands. The geometry of these rift zones is frequently three-branched at 120°C (Mercedes-Benz star configuration), suggesting a least-effort fracturing by magma-induced upwelling. The rift zones play a major role in the distribution of geological hazards: eruptive vents and the failure planes of gravitational landslides are located preferentially along these volcanic lineaments.
Volcanological and structural evolution of Roccamonfina volcano (Italy): origin of the summit caldera
Authors: De Rita D. , Giordano G.
Published: April 2008

Roccamonfina volcano (Roman Magmatic Province) sits at the margin of the NE-trending Garigliano graben. The most important episodes in the volcano’s history (630–50 ka
bp
) have been controlled by tectonic activity associated with the graben’s master-faults. Roccamonfina volcano comprises two main parts: a stratovolcano developed inside the graben, and a complex of centres developed on the south-eastern horst. The summit of the stratovolcano is truncated by a horse-shoe shaped caldera (dimensions 6.5 km by 5.5 km) with the longest axis trending NW. The caldera opens towards the SE along NE-trending faults, which belong to the same system as the graben faults. Stratigraphical evidence indicates that caldera collapse was not caused by explosive eruptive events emplacing ignimbrites, or by sector collapse preceding ignimbrite eruptions. Geomorphological and structural observations, together with geophysical evidence, suggest that collapse of the volcano summit occurred as a mechanical re-adjustment to the high rate of the Garigliano graben extension during a climax of the regional tectonism at around 400 ka
bp
. The present elliptical shape of the collapsed area is due to the superposition of a linear NE-trending graben structure in the east, and a sector collapse in the west.
Structural evolution of the Bracciano volcano-tectonic depression, Sabatini Volcanic District, Italy
Authors: De Rita D. , Di Filippo M. , Rosa C.
Published: April 2008

The Latian volcanoes of Central Italy form several K-rich volcanic districts, some of which are characterized by large volcano-tectonic depressions filled with lakes. The development of these large depressions (>8 km in diameter) occurred from 0.4–0.3 Ma
bp
, during a climax of regional extension. Analysis of geophysical and deep drilling data and detailed field analysis, allows reconstruction of the evolution of the Bracciano volcanotectonic depression in the Sabatini Volcanic District, northwest of Rome. This depression developed during the Upper Pleistocene inside a NE-trending half-graben structure. The evolution of the half-graben has been driven by NE-trending regional faults which were reactivated during volcanism. Magma rose along these faults and came to rest at 4–7 km depth, probably as a laccolithic body, and was then erupted with emplacement of large volume ignimbrites. A lava plateau recognized by drilling data in the southern and western sectors of the depression is interpreted as being associated with NE-striking swarms of feeder dykes. The final downsagging was caused by NE and NW-trending regional faults after 0.17 Ma. The eastern depression morphology was modified by late Pleistocene N-trending strike-slip faults which were accompanied by intense hydromagmatic activity. Hydromagmatic craters are distinguished in the eastern margin of the area and in the southeastern part of Lake Bracciano in high-frequency seismic sections.
Modified volcanic domes and associated debris aprons on Venus
Authors: Bulmer M. H. , Guest J. E.
Published: April 2008

The Magellan SAR images show that volcanic domes occur on the surface of Venus. Edifices with scalloped margins are similar to volcanic domes and fall within a spectrum ranging from unmodified to remnant forms. Over 320 domes have been located of which more than 80% have modified morphologies. Broadly, the modified domes can be described by five sub-categories which are related to three unmodified dome sub-categories. Many modified domes have deposits associated with them that possess characteristics indicative of their having been formed from mass movements. Evidence for slope failure is not seen on volcanic shields on Venus due to them having shallow flanks. Several steep-sided cones show some evidence of slope failure but on a much smaller scale than failures associated with domes. Slope failures on venusian domes appear to have been triggered by non-explosive and explosive events. Collapse on the edges of domes, either as a result of explosive events of oversteepening, is common on Earth but the collapses on Venus are on a scale that is more in common with major sector collapses of volcanoes on Earth. Four morphological sub-groups of debris deposit have been recognized. The deposits in groups one and two are analogous to terrestrial volcanic debris avalanche deposits and the deposits in group three are analogous to pyroclastic flows on Earth. The characteristics of the fourth group of deposits are similar to those resulting from deep-seated slides on Earth.
High-grade metamorphism of 2.45–2.4 Ga age in mafic intrusions of the Belomorian Belt in the northeastern Baltic Shield
Authors: Bogdanova S. V.
Published: April 2008

In the Late Archaean Belomorian Belt of the Baltic Shield, shearing and high-grade metamorphism occurred in association with bimodal gabbroic-granitic igneous activity between 2.45 and 2.40 Ga. That period is generally regarded as one of anorogenic rifting and extension. In the studied case of the Tolstik intrusion, the time of metamorphism is constrained by the 2434 ± 7 Ma age of a metamorphosed mafic intrusion and the 2405 ± 20 Ma age of a subsequently intruded potassic granite. Several generations of mafic dykes allow a detailed study of the metamorphic evolution during that time span.

The conditions of post-intrusion peak metamorphism in the mafic intrusion were 700–800°C and 11–12 kb which was followed first by near-isothermal decompression and, after the loss of
c.
5 km crustal thickness, by near-isobaric cooling to
c.
600° C at 8–9 kb.

These results suggest metamorphism possibly triggered by igneous intrusions into previously thickened crust, but the P/T-ratios of metamorphism were moderately high. Later followed crustal extension concomitantly with continuous igneous input and the P/T- ratios decreased. Finally, a post-extensional period of cooling ensued as igneous activity ceased.
The Palaeoproterozoic Southeastern Churchill Province of Labrador-Quebec, Canada: orogenic development as a consequence of oblique collision and indentation
Authors: Wardle R. J. , Van Kranendonk M. J.
Published: April 2008

Palaeoproterozoic orogenic development in the northeastern Canadian Shield was controlled by the successive, oblique collision of the Archaean Nain (North Atlantic) and Superior cratons with a southwards projecting promontory of the Archaean Rae Province (part of the northern Churchill Province hinterland). By this process the Rae Province became sutured to the Nain craton by the Torngat Orogen and to the Superior craton by the New Quebec Orogen, resulting in the formation of a 400 km wide orogenic belt known as the Southeastern Churchill Province (SECP). Initial rifting at 2.45–2.1 Ga, and early (arc?) magmatism and deformation at 2.3–1.9 Ga, were restricted to the Rae Province. They were followed by arc magmatism in the Torngat Orogen at
c.
1.91–1.88 Ga and Rae/Nain collision 1.87–1.86 Ga, which resulted in the formation of an orogen with east- and west-verging structures. Arc magmatism in the New Quebec Orogen commenced at
c.
1.845 Ga and was succeeded by Rae/Superior collision and widespread deformation across the SECP at
c.
1.83 Ga. Deformation at this time was dominated by west-vergent thrusting in the New Quebec Orogen and Rae Province, and by renewed east-vergent thrusting in Torngat Orogen. Deformation was then progressively transferred to major sinistral (1.845–1.82 Ga in the eastern SECP) and dextral (1.83–1.80 Ga? in the western SECP) shear systems that accommodated continued northwards motion of the Nain and Superior cratons relative to the Rae Province. Juvenile crust expelled by thrusting effectively doubled crustal thickness in parts of the Rae Province and the northwestern edge of the Nain craton. Late stage development (1.8–1.71 Ga), was restricted to the margins of the SECP, where deformation was associated with renewed outward-directed overthrusting and transcurrent shear in conjunction with uplift of the orogenic core.
The Mesoproterozoic cratonization of Baltica — new age constraints from SW Sweden
Authors: Connelly J. N. , Åhäll K-I.
Published: April 2008

Comparable early Mesoproterozoic tectonic histories resulted in the formation of 1.71–1.62 Ga Labradorian and 1.75–1.55 Ga Gothian crust in Laurentia and Baltica respectively, along a postulated contiguous margin. New U-Pb geochronology in southwest Sweden has constrained the timing of three consecutive stages of the Gothian Orogeny. Calc-alkaline magmatism, presumably related to subduction, occurred as late as 1587 ± 3 Ma, indicating convergence and consumption of oceanic crust until this time. These magmas intruded terranes of previously accreted arcs, thus providing a minimum age for their juxtaposition and accretion. The age of the latest stage of Gothian migmatization is constrained between 1555 ± 2 and 1553 ± 2 Ma, synchronous with the transition from ductile to brittle conditions at the presently exposed erosional level. This transition permitted the emplacement of a suite of mafic-felsic 1555–53 Ma intrusions during an extensional regime that may have coincided with the post-deformational collapse of the Gothian Orogen. These age refinements in southwest Sweden indicate that the final stage of the Gothian Orogeny in Baltica occurred considerably later than that of the Labradorian Orogeny in Laurentia. The preservation of Gothian-aged titanite from southwest Sweden indicates that post-Gothian metamorphism, including that of Sveconorwegian-Grenvillian age, was not penetrative in this region.
Iodine in the environment and endemic goitre in Sri Lanka
Authors: Dissanayake C. B. , Chandrajith R. L. R.
Published: April 2008

The prevalence of endemic goitre is extremely high in certain parts of Sri Lanka where rates as high as 44% have been observed. With nearly 10 million people at risk the aetiology of endemic goitre in Sri Lanka needs to be clearly ascertained. The endemic goitre belt of Sri Lanka coincides with the wet climatic zone, indicating an apparent relationship of iodine geochemistry with climatic factors. However, the fact that there is a correlation of only −0.64 between the iodine content and the prevalence of goitre suggests the existence of other factors such as humic substances in the soil, clay minerals and soil pH that exert an influence on the bioavailability of iodine.

Iodization of salt used in food is the most convenient method of goitre prevention.
The protective role of trace elements in preventing aflatoxin induced damage: a review
Authors: Nair M. G. , Maxwell S. M. , Brabin B. J.
Published: April 2008

Recent evidence has shown that there are important interactions in experimental models and human beings between trace element status and aflatoxin B
1
(AFB
1
) exposure. The hypothesis is presented that these interactions operate through altered host antioxidant status leading to excess free radical production resulting in cell damage and clinical disease. Geographical differences in the geochemical environment could have a major influence on the pattern of disease. Appropriate nutritional intervention studies are required to determine potential benefits of trace element supplementation in aflatoxin exposed populations.
A Kimmeridgian time-slice through the Humber Group of the central North Sea: a test of sequence stratigraphic methods
Authors: Veldkamp J. J. , Gaillard M. G. , Jonkers H. A. , Levell B. K. , Gaillard M. G. , Jonkers H. A.
Published: April 2008

Reservoir quality of the Fulmar Formation in the UK Central Graben depends on the original distribution of facies as well as subsequent modification by partly facies-dependant diagenesis. Numerous sequence stratigraphical analyses of the Humber Group have been published recently, each of which has elements which are convincing geologically. Nevertheless, it is our experience that facies and reservoir prediction within the Humber Group remain difficult.

Sequence stratigraphical interpretations in the Group are clearly non-unique due to major uncertainties in the data.

(1)

Biostratigraphical uncertainties include species concepts, recognition of agediagnostic taxa in thermally altered floras, validity of repeated micropaleontological events and equivocal paleoenvironmental information.

(2)

Sedimentological uncertainties are due to the gross facies similarity of the Fulmar Formation over a large geographic area and throughout the Upper Jurassic, which make the detection of potential maximum flooding surfaces and especially sequence boundaries difficult even in many cored wells. Furthermore strong syndepositional subsidence variations due to both salt movement and tectonics, lead to aliased sampling of the subsurface in wells, which are mostly situated on highs.

(3)

Seismic correlation difficulties are due to persistent seismic imaging problems below the Late Cimmerian (‘X’) Unconformity.

(4)

The depositional and sequence stratigraphical models for Humber Group deposition need to be set in the context of a transgressively skewed, back-stepping system with strong local subsidence control. The overall transgression is thought to be due to the combination of sea-level rise, widening and deepening of the basin due to tectonics, and reduced sediment supply through time. In this situation existing models have to be applied with even more care than normal if they are not to be misleading (e.g. concepts of forced regressions).

These difficulties are clarified in the context of a Kimmeridgian time slice through the Humber Group. Multiple options exist in well log correlations. As a result palaeogeographic reconstructions of the time-slice have an error margin of ±15 km, which has serious implications on the validity of reservoir quality predictions.
The application of sequence stratigraphy to the understanding of Late Jurassic turbidite plays in the Central North Sea, UKCS
Authors: Carruthers Alan , Mckie Tom , Price John , Dyer Robin , Williams Gwydion , Watson Paul
Published: April 2008

The identification and correlation of genetic sequences has significantly enhanced our understanding of the Late Jurassic stratigraphy in the Central North Sea. The integration of genetic sequences with an appreciation of the basinal mudrock stratigraphy, and the identification of depositional sequence boundaries provides a powerful tool to understand and potentially predict the distribution of subtle stratigraphic turbidite plays.

The largely syn-rift Late Oxfordian-Kimmeridgian was deposited during a low frequency, second-order sea-level rise, culminating in the
eudoxus
condensed section. Deposition was dominated by shallow marine shelfal sandstones and offshore mudrocks. Sandy turbidites are rare. Whilst the requisite depositional slopes and deep water areas are likely to have existed, the effects of high frequency relative sea-level falls are likely to have been suppressed. As the second-order sea-level rise accelerated towards the
eudoxus
condensed section, the supply of sand into the basin was reduced such that the potential for turbidite deposits was minimal.

The largely post-rift Kimmeridgian-Volgian section is dominated by basinal mudrocks. However, regionally correlatable ‘hot’ and ‘cool’ units provide information on sediment flux into the basin which may be linked to relative sea-level changes on the basin margin, with cool units interpreted to be the product of enhanced depositional rates during lowstand. Turbidites commonly occur within the ‘cool’ mudrock units. A Middle Volgian ‘cool’ unit is likely to represent a second-order sea-level fall, within which are composite turbidite packages which are considered to be the product of higher frequency lowstands (lowstand sequence set).
Image analysis and microscopic investigation of annually laminated lake sediments from Fayetteville Green Lake (NY, USA) Lake C2 (NWT, Canada) and Holzmaar (Germany): a comparison
Authors: Zolitschka Bernd
Published: April 2008

Counts and measurements of annually laminated sediments are often time consuming and subjective. An automated and computerized method to accelerate the generation of varve data and to make these data more comprehensible is required. The approach taken here is to digitize thin sections of varved lacustrine sediments of different composition and mean lamination thickness. Density variations within an area scan of the digitized image are then measured. The data obtained provide peaks and troughs of light transmission corresponding to the pale and dark alternating laminae. After comparing these results with microscopic varve measurements it is clear that light transmission data alone do not allow the counting of varves. The composite nature of laminations causes high-frequency noise which is hard to distinguish from thin varves without additional microscopic information.
Composition and origins of laminae in late Quaternary and Holocene sediments from the Santa Barbara Basin
Authors: Bull David , Kemp Alan E. S.
Published: April 2008

Late Quaternary and Holocene sediments from the Santa Barbara Basin have been analyzed using backscattered electron imaging (BSEI) techniques to identify the origins of laminae and document intra- and inter-annual ocean/climate variability over the last glacial cycle. Laminae observed in X-ray radiography and BSEI are entirely defined by variations in the relative abundance and grain size of biogenic and terrigenous components. There is no evidence to suggest that bacterial mat mediation may have played a role in lamina formation. The simplest lamination type comprises silt-rich, silt-poor couplets which originate from seasonal variation in the supply of silt to the basin. The second lamination type contains an additional lamina of diatom ooze to form terrigenous sediment-diatomaceous triplets. The thicker diatom ooze laminae commonly exhibit a vertical succession of diatom floras which represents intra-annual variation in productivity. The variation in thickness and assemblage of diatom ooze laminae is probably a function of the amount of diatom flux, which is controlled by upwelling-driven primary productivity in the basin. Laminated sediments deposited during cooler periods generally contain thicker diatom ooze laminae, consistent with more vigorous regional circulation and increased upwelling.
The Southeast Indian Ridge between 127° and 132°40′E: contrasts in segmentation characteristics and implications for crustal accretion
Authors: Sempéré Jean-Christophe , West Brian P. , Géli Louis
Published: April 2008

The Australian-Antarctic Discordance is an anomalously deep section of the Southeast Indian Ridge which overlies a colder than normal region of the upper mantle. The Southeast Indian Ridge exhibits large contrasts in its geophysical and geochemical characteristics across the eastern boundary of the Discordance. We present new geophysical data collected along the Southeast Indian Ridge between 127°00′E and 132°40′E which define the segmentation characteristics of this portion of the Indo-Australian-Antarctic plate boundary. The Southeast Indian Ridge within our survey area can be broken into three first-order segments bounded by one transform fault and two propagating rifts. The transform fault, located at the west end of the study area, forms the eastern boundary of the Australian-Antarctic Discordance.

The morphology of the spreading axis is that of an axial high akin to that found along the East Pacific Rise. The non-transform discontinuities found along the axis of accretion are also similar to those encountered at fast-spreading centres. These characteristics are in marked contrast with those of the spreading axis within the Australian-Antarctic Discordance, which resembles a slow-spreading centre despite its intermediate spreading rate (
c.
74 mm a
−1
). We suggest that the geophysical and geochemical transitions that occur along the Southeast Indian Ridge near the eastern boundary of the Australian-Antarctic Discordance may be ascribed to varying mantle temperature and melt production rate along the spreading centre at constant spreading rate.

The two propagating rifts, located near 127°45′E and 131°E, are actively migrating toward the Discordance down the regional, along-axis gradient in bathymetry at the rates of 43 and 46 mm a
−1
, respectively. Propagating rifts play a dominant role in the reorganization of the spreading centre east of the Australian-Antarctic Discordance. Satellite gravity data reveal a number of V-shaped, west-pointing structures on the flanks of the Southeast Indian Ridge. These structures appear to have been spawned near the George V ridge-transform intersection at 139°E. Four of these features display a clear association with active propagating rifts. Some of the remaining features may be due to the migration of melting anomalies entrained by asthenospheric flow along the axis of the Southeast Indian Ridge.
Segmentation of the Mid-Atlantic Ridge south of the Azores, based on acoustic classification of TOBI data
Authors: Blondel Philippe
Published: April 2008

The European MARFLUX/ATJ project has investigated volcanic, tectonic and hydrothermal processes along 500 km of the Mid-Atlantic Ridge near the Azores. During cruises CD89-CD90, multibeam bathymetry and high-resolution TOBI sidescan sonar data were acquired and added to the existing Geographical Information System. From the Azores Triple Junction, between 35° and 38°N, the ridge runs 227°, oblique to the 100° spreading direction. It is partitioned into several short ridge segments dominated by tectonism rather than volcanism, and is characterised by abundant hydrothermal activity. Textural analysis quantifies the second-order statistics of TOBI imagery, enabling the detection of details invisible to the human eye. The results of this analysis are used to quantify the relationship between tectonism and volcanism along-axis, and suggest declining neovolcanic activity away from the Triple Junction. They show that the discontinuities are heavily sedimented, whereas the segments are mostly volcanic and the segment ends are more densely faulted. Textural analysis quantifies the differences in style and activity of the different spreading segments, and the evolution of the discontinuities.
Initiation and evolution of boundary-wall faults along the Mid-Atlantic Ridge, 25–29°N
Authors: McAllister Eddie , Cann Johnson R.
Published: April 2008

The major faults that bound the median valley on slow spreading ridges form a staircase that migrates upwards and outwards from the valley floor like a slow escalator. Each boundary wall fault is generated at the margin of the median valley floor at about 2 km from the spreading axis. Boundary wall faults that appear to be single scarps on the relatively low resolution Seabeam multibeam echosounder maps are shown by high resolution TOBI sidescan sonar images in some cases to be single faults, but in others to be complex anastomosing fault zones. The major faults in such zones are often scalloped in plan view. There is no systematic relation between fault zone geometry and segment type.

We conclude that during the growth of boundary wall faults the strain is first accommodated over a wide area, in which an array of small faults dissects the volcanic surface. However, with time, and with continued extension, the strain becomes organized into increasingly narrower zones of deformation until a major boundary wall fault develops. Faults grow by propagation and linkage with other nearby faults. The early stages of linkage include tilted ramps, but at later stages brittle faults cut across the ramps, and eventually become part of a scalloped fault trace. In some places, linkage is through zones of small-scale diffuse faulting, rather than through a single fault. Fault capture and linkage can occur over lateral distances (perpendicular to the mean azimuth of the faults) of up to 1.5 km.

This deformation takes place within a fault growth window fixed relative to the spreading axis, through which the lithosphere passes as seafloor spreading continues. Most of the growth of each boundary fault takes place before the next fault is initiated. We conclude that the location and size of the narrow fault growth window is controlled by the changing mechanical properties of the ocean lithosphere as it spreads away from the axis. Its inner boundary represents the position at which a normal fault reaches the sea floor after originating at the base of the lithosphere at the spreading axis. Its outer boundary is interpreted as the point at which the strength of the boundary wall fault, increasing as the lithosphere ages and thickens, becomes greater than the strength of the lithosphere at the spreading axis, when a new fault is generated. The process of fault generation at mid-ocean ridges has relevance to fault generation in other extensional environments.
Hydrothermal activity and ridge segmentation on the Mid-Atlantic Ridge: a tale of two hot-spots?
Authors: German C. R. , Parson L. M. , Murton B. J. , Needham H. D.
Published: April 2008

In the past five years we have completed systematic seabed and water column surveys of two key sections of the Reykjanes Ridge and the Mid-Atlantic Ridge, between 36° and 38° North, close to the Iceland and Azores hot-spots, respectively. These surveys have provided a data set which provides a unique opportunity to address the relationship between the incidence and location of hydrothermal venting relative to volcanism/tectonism and ridge segmentation. Along the Reykjanes Ridge, running SW from Iceland, deep-tow (TOBI) sidescan sonar and swath bathymetry data have indicated a pattern dominated by volcanism in the form of axial volcanic ridges with only minor tectonic fracturing. A water column survey along this section of ridge crest, using conventional profiling techniques, indicated no evidence for high-temperature hydrothermal activity except at 63°N where the Reykjanes Ridge intercepts the bathymetric platform (≤500m water depth) which surrounds Iceland. Along the Mid-Atlantic Ridge between 36–38°N, running SW from the Azores archipelago, TOBI and swath bathymetry data have imaged a ridge with spreading rate and obliquity of spreading motion similar to the Reykjanes Ridge. But this ridge-section is dominated by tectonism rather than volcanism, the axis being partitioned into a series of short (20–55 km length) segments of spreading axis, each systematically displaced from its neighbours by right-lateral non-transform offsets of 15–50 km. Hydrothermal activity (as detected by a combination of deep-tow instruments and conventional profiling) is much more common along this latter section of spreading centre: evidence for discrete sources of hydrothermal discharge was obtained on fourteen different occasions, in seven separate segments of ridge crest. Sources of hydrothermal activity, including the major Rainbow hydrothermal field, were observed to coincide consistently with ridge-offset intersections suggesting a strong link between tectonic segmentation and the occurrence of high temperature venting.
Regional metamorphism due to anorogenic intracratonic magmatism
Authors: Gibson Roger L. , Stevens Gary
Published: April 2008

The Vredefort Dome, the central uplifted portion of the
c.
300 km diameter, 2.02 Ga Vredefort Impact Structure, exposes a
c.
20 km deep profile through the crust of the Kaapvaal Craton of southern Africa which displays evidence of greenschist- to granulite-facies low-pressure metamorphism. Reaction textures in the medium- to high-grade metamorphic rocks indicate an anticlockwise isobaric cooling
P-T
evolution, and peak metamorphic temperatures consistent with a mid- to upper-crustal geotherm of 40–50°C km
−1
. The metamorphism is attributed to craton-wide intraplating of mantle-derived magmas into the deep crust during the 2.05–2.06 Ga Bushveld Event. Felsic volcanic rocks and high-level intrusions in the Bushveld Complex represent anatectic derivatives from the deeper, high-grade parts of the metamorphic terrane. The intracratonic setting, the absence of evidence of concomitant orogenic deformation, and the preservation of the pre-existing diamondiferous lithospheric root beneath the craton, suggest that the thermal event was triggered by the transient positioning of the Kaapvaal Craton over a mantle plume.
Disequilibrium during metamorphism: the role of nucleation kinetics
Authors: Rubie David C.
Published: April 2008

The sluggishness of nucleation, one of the most poorly understood and least-discussed processes involved in mineral reactions, is an important cause of disequilibrium during metamorphism. Both field-based and experimental petrological studies show that metastable phases can persist far from equilibrium on long time scales because the stable phases are unable to nucleate. This behaviour applies not only to solid-solid reactions but also to some dehydration and melting reactions. Although the degree of overstepping required for nucleation during polymorphic phase transformations is relatively small (2–4 kbar on an experimental time scale), nucleation can be much more sluggish for reactions involving two or more product phases. When reactions occur at conditions of large overstepping due to delayed nucleation, reaction rates can be very fast because of the effect of the large free energy change on growth kinetics. Under these circumstances, new metastable phases may form instead of the stable mineral assemblage, as predicted by the Ostwald step rule. Such metastable phases can persist during metamorphism, even during high-strain ductile deformation and fluid influx, but without any obvious texture evidence to indicate that they are metastable.
Conduit flow and fragmentation
Authors: Mader H. M.
Published: April 2008

This chapter provides a critical overview of syn-eruptive processes between magma chamber and vent in magmatic, sustained, Plinian-type eruptions. Phreatomagmatic effects are largely neglected. The main sources of information derive from textural studies of ejecta, theoretical models and physical experiments of eruption dynamics. Textural studies commonly find bimodal vesicle populations, indicative of several discrete nucleation events. The degree of disorder increases with explosivity. Dynamical parameters, such as nucleation density and rate and bubble growth rate, can be inferred from studies of the moments of bubble size distributions. Vesicularity in pumices is observed to vary significantly both within and between deposits, which suggests that the vesicularity at fragmentation is affected by the flow dynamics. Vesicularity variations correlate most closely with changes in magmatic composition and viscosity, but not with discharge rate. Conduit flow models can be broadly grouped into first- and second-generation models; the former generally impose a lithostatic pressure gradient and a constant Newtonian viscosity, whereas the latter include equations for the rheological changes that take place during vesiculation and solve for the pressure. Second-generation models derive highly non-lithostatic pressure gradients with the result that most of the vesiculation occurs at a high rate over a short distance just prior to fragmentation. The mechanisms of brittle and ductile fragmentation have been investigated in separate studies in non-vesiculating magmas, but which mechanism operates in explosive eruptions is not known. Dynamical laboratory experiments provide observations of the physical processes operating in conduit flows. Gas-expansion experiments have shown that it is possible to generate violent explosions by unloading in cool magmatic materials. Expanding dusty flows are found to be stable only if the bulk density increases with height. Exsolution experiments have demonstrated that acceleration precedes fragmentation and that gas evolution is enhanced by advection and bubble deformation. Deformed vesicles similar to those found in ‘woody’ pumice have been generated in an analogue system that has similar rheology to that found in vesiculating magmas. Large-scale exsolution experiments suggest that explosive volcanic eruptions are inherently heterogeneous: the fluctuations in discharge rate and discrete pulses and shocks commonly observed are a consequence of the large physical scale of volcanic systems. The effect of the magma chamber and conduit geometry has also been investigated. Eruption of material from a spherical flask up a narrow cylindrical tube generates quasi-steady flow conditions after an initial transient during which the discharge rate grows, as frequently observed in volcanic eruptions. The fragmentation surface does not propagate down into the magma chamber.
Palaeosurfaces and associated saprolites in southern Sweden
Authors: Lidmar-Bergström Karna , Olsson Siv , Olvmo Mats
Published: April 2008

Saprolite remnants from different palaeosurfaces in southern Sweden have been analysed by XRD and SEM analyses. They represent two clearly different types. The first is a clay- and silt-rich saprolite with a kaolinite-dominated clay mineral association representative of mature saprolites and with chemically altered quartz grains. This saprolite type is associated with sub-Cambrian, sub-Jurassic and sub-Cretaceous denudation surfaces. The second type is gravelly and in a youthful stage of alteration with mainly vermiculitic clay minerals in the fine fractions. Microtextures developed on quartz grains indicate a mainly mechanical breakdown. Compared to overlying till beds this saprolite is in a more advanced stage of chemical alteration. It is not associated with any specific denudation surface. Its characteristics and thickness indicate a Plio-Pleistocene age.

The saprolites represent deep weathering of the bedrock surface at different times. The weathering resulted in thin kaolinitc saprolites during the Late Proterozoic, thick kaolinitic saprolites from the latest Triassic through the Jurassic and Cretaceous, and medium thick immature saprolites from the Pliocene and onwards. The depth of the deep weathering has been decisive for the shape of the present relief and thus etch processes have been of fundamental importance in shaping the relief, even in a formerly glaciated area.
Preservation of old palaeosurfaces in glaciated areas: examples from the French western Alps
Authors: Battiau-Queney Yvonne
Published: April 2008

Remnants of deeply-weathered rock have been recorded in many regions which were ice-covered during the Pleistocene. They generally belong to shields and Palaeozoic massifs. This paper presents an example from the French Alps which testifies to the good preservation of very old (> 200 Ma) landforms in a high relief area situated on the western side of the ‘Grandes Rousses’ which was entirely covered by ice 30 000 years ago. Sedimentary structures, such as ripples on sandstone bedding planes and other erosional near-shore features, dating back to the Late Triassic before the development of the north, passive, margin of the Tethys ocean, are found in this high relief area. These are remarkably well preserved and occur in close proximity to typical glacial landforms. More generally, widespread elements of a pre-Triassic erosional surface are still recognized in the present landscape. Evidently, ice was unable to obliterate these 200 Ma old features in a high mountain area which was strongly glaciated 30 000 years ago.
Rock weathering in blockfields: some preliminary data from mountain plateaus in North Norway
Authors: Whalley W. Brian , Rea Brice R. , Rainey Michelle M. , McAlister John J.
Published: April 2008

The formation of blockfields is a process usually attributed to weathering. In mountain areas this is generally assumed to be mechanical weathering (frost shattering). Evidence from two high plateaus [900 and 1350 m above sea level (a.s.l.)] in North Norway (
c.
70°N) suggests that chemical action is at least as important as mechanical activity in blockfield formation. The bedrock in both areas consists of complex banded gabbros. Blockfields circumscribe ice masses and are generally > 1 m thick. They contain high percentages of material in the silt and clay sized fractions, including a variety of clay minerals: gibbsite, chlorite, vermiculite and kaolinite, as well as magnetite/maghemite. The blockfield thickness and presence of these weathering products suggests both a considerable (pre-Pleistocene) length of time required for development as well as warmer conditions than are found now (mean annual air temperature
c.
0°C) or in the period since deglaciation. It is suggested that these blockfields represent a preglacial palaeosurface which formed initially under warmer conditions and has survived, largely intact, beneath all the Pleistocene ice sheets.
Environmental changes during the Tertiary: the example of palaeoweathering residues in central Spain
Authors: Borger Harald
Published: April 2008

In central Spain three main weathering phases are found which can be put in a chronological sequence with tectonic and sedimentation phases. Weathering intensity, as recorded by micromorphological analysis, has plainly decreased during the course of the Tertiary reflecting the prevailing environmental conditions.

During the Palaeogene intensive chemical activities produced, on the mineral grains of the deeply decomposed rocks, features typical of humid tropical climate.

In the Miocene the Palaeogene regolith of the southern Tajo Basin was truncated and partly incorporated into sediments which comprise reworked weathering residua of this basin and of the adjoining Montes de Toledo. After sedimentation another weathering phase started. During this phase the feldspars were heavily corroded, but in contrast to the Palaeogene residues, the quartz shows hardly any features caused by etching.

In the Pliocene the rañas were deposited during a second main transportation phase. Particularly in the SE of the Montes de Toledo, these sediments were subjected to intense weathering, but only where conditions were favourable. Due to sufficient soil humidity in intramontane basins, high temperatures, and presence of iron, quartz corrosion was locally possible once more during Pliocene times. The rañas of the Tajo Basin, however, were substantially less attacked.