bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of local porosity changes in limestone samples under triaxial stress field by using X-ray computed tomography
Authors: Karacan C. O. , Grader A. S. , Halleck P. M.
Published: April 2008

The character of reservoir rocks is uncertain and variable at the depths where they are subjected to different tectonic forces and pressure changes due to drilling, well stimulation and production operations. The stress- and time-dependent deformation of the porous structure is expected to change the behaviour of the most important properties of the rock, such as porosity and permeability, which in turn changes the reservoir production predictions.

In this study, we demonstrate the use of X-ray computed tomography (CT) to investigate the porosity and permeability changes of Cordova Limestone samples during deformation in a triaxial cell. The experiments were performed in a specially designed X-ray transparent triaxial test cell, which enables applying stress as well as making flow measurements. Because the presence and value of confining pressure changes the deformation behaviour of the rock, different constant confining stresses were applied to the samples to change the deformation regime. As the axial load was increased, samples were scanned at different locations to determine the stress-dependent local changes in porosity. Absolute permeability during the deformation was also measured.

Results show that the stress condition applied to the porous medium changes the rock and fluid transport properties, compared to measurements taken without stressing the sample. X-ray CT enabled the local porosity changes to be quantified, to locate compaction bands and places where shear location occurs, and to evaluate how the inner structure of the rock changes during different modes of deformation.
Monitoring void ratio redistribution during continuous undrained triaxial compression by X-ray computed tomography
Authors: Thomson P. R. , Wong R. C. K.
Published: April 2008

It is usually implied that void ratio of a soil specimen is homogeneous and unchanging during an undrained triaxial test. In this study, X-ray computed tomography was used to measure void ratio redistribution during undrained triaxial compression of a cohesionless soil sample. A specially designed triaxial apparatus was used to scan the same sample at different axial strain levels, while the axial loads and confining pressures were maintained. Significant variation in sample void ratio was observed at the end of consolidation and sample uniformity was found to increase with increasing axial strain. The process of void ratio redistribution can be effectively illustrated in void ratio-effective stress space.
The use of X-ray computed tomography in the investigation of the settlement behaviour of compacted mudrock
Authors: O’Neill M. A. , Goodwin A. K. , Anderson W. F.
Published: April 2008

The material used in the restoration of opencast coal mines is made up predominantly of mudstone particles ranging in size from less than 60 μm to more than 60 mm nominal diameter. Post-compaction settlements are mainly sub-divided into short-term ‘collapse’ and long term ‘creep’ components, both of which can be very large and often significantly reduce the development potential of restored opencast sites. As the mechanics of these movements are poorly understood at present, X-ray computed tomography (CT) was used to investigate the fundamental nature of particulate interactions within specimens of backfill undergoing long-term creep settlement.

The preliminary results from a series of CT analyses of a number of large scale specimens are presented. Different CT procedures were used and an indication for best practice for scanning of large samples has been found. Qualitative interpretation of the images obtained to date is presented in terms of fill structure, particulate changes during settlement and effects of arching. Indications are that particle breakage is a less significant mechanism than previously thought, and that local collapse of voids, particle sliding and particle rotation are the dominant mechanisms. Quantitative data to be extracted from the images may lead to a probabilistic approach to the prediction of settlement. This possibility is noted, as is the potential for the wider use of CT in geotechnical engineering.
The Vocontian clastic dykes and sills: a geometric model
Authors: Parize O. , Friès G.
Published: April 2008

Distal Aptian-Albian deep water channelled massive sands of the Vocontian Basin (SE France) are often associated with sand injections. The Bevons and Rosans areas in the Vocontian domain present probably the most spectacular outcrops showing complex networks of clastic sills and dykes injected into a thick marly/limy succession. Most injections are found in the channel banks, fed laterally from sandy channels. The sills are up to 10 metres thick in the vicinity of the connection with the channel feeder; they thin out and die into marls 2 or 3 kilometres away from it. Most dykes are injected from the sills rather from the channel itself: a few small dykes can be found under the channel fill. They are most abundant within a few hundred metres of the channel. Today, injections extending downwards from sills have up to 275 metres vertical extent, whereas injections extending upwards from sills never reach the contemporaneous palaeo-sea floor. Ptygmatic folding of the dykes by mechanical compaction indicates the amount of local post-injection compaction of shale and clearly shows that sand injection occurred prior to burial. Outcrop mapping shows that channel bank fracturing is contemporaneous with channel infilling. This is evidence of early syn-depositional injection of the sandy material. Vocontian clastic injections provide good geometrical analogues to deep offshore clastic injectite networks and the opportunity to better understand genetic processes.
Deformation structures in Plio- and Pleistocene sediments (NW Bohemia, Central Europe)
Authors: Bankwitz P. , Bankwitz E. , Bräuer K. , Kämpf H. , Störr M.
Published: April 2008

The small intracratonic Cheb (Eger) Basin in NW Bohemia (Central Europe) is characterized by swarm earthquakes, many mineral springs and mofettes with upper mantle CO
2
degassing and by neotectonic graben and basin structures. Especially in non-lithified Upper Pliocene clay formations of the basin, a variety of deformation patterns is exposed. They include non-tectonic and tectonic activity and comprise faulting and folding from µm- to km-scale. Previously unrecognized N-S- and ENE-striking faults are sites of mantle degassing and seismic activities. Confined-layer deformation and liquefaction structures hint to palaeoseismic events and gas escape activity. Cleavage-like arranged clay mineral plates represent the microfabric of clay within fault zones. For the first time the degassing channels of Upper Mantle fluids/gases through the Pliocene clay sediments can be documented: µm-scale micro-tubes were produced by the opening of Riedel shear planes induced by fault movements.
Gas and fluid injection triggering shallow mud mobilization in the Hordaland Group, North Sea
Authors: Løseth Helge , Wensaas Lars , Arntsen Børge , Hovland Martin
Published: April 2008

During a regional seismic interpretation study of leakage anomalies in the northern North Sea, mounds and zones with a highly chaotic seismic reflection pattern in the Tertiary Hordaland Group were repeatedly observed located above gas chimneys in the Cretaceous succession. The chaotic seismic reflection pattern was interpreted as mobilized sediments. These mud diapirs are large and massive, the largest being 100 km long and 40 km wide. Vertical injections of gas, oil and formation water are interpreted to have triggered the diapirs.

On the eastern side of the Viking Graben, another much smaller type of mud diapir was observed. These near-circular mud diapirs are typically 1–3 km in diameter in the horizontal plane. Limited fluid injection from intra-Hordaland Group sands, through sand injection zones, into the upper Hordaland Group is interpreted to have triggered the near-circular diapirs.

This observed ‘external’ type of mobilization was generated at shallow burial (<1000 m) and should be discriminated from the more common ‘internal’ type of mud diapirism that is generated in deep basins (>3000 m). The suggested model has implications for the understanding of the palaeofluid system, sand distribution, stratigraphic prediction within the chaotic zone, seismic imaging, and seismic interpretation of the hydrocarbon ‘plumbing’ system.
Sinuous pockmark belt as indicator of a shallow buried turbiditic channel on the lower slope of the Congo basin, West African margin
Authors: Gay A. , Lopez M. , Cochonat P. , Sultan N. , Cauquil E. , Brigaud F.
Published: April 2008

Pockmarks on the slope of the Lower Congo Basin are distributed along a meandering band on seafloor coincident with a shallow buried palaeochannel imaged from the 3D-seismic database. Each pockmark originates systematically at the channel-levee interface and the seafloor expression of the palaeochannel’s sinuosity is mimicked by the sinuous trend of pockmarks. 3D-seismic on the slope, calibrated by biostratigraphic data from cores of the Leg ODP 175, indicate a seaward decrease of the sedimentation rate. We suggest that this condition induces a differential loading of the hemipelagic cover over the palaeochannel and propose a model for episodic dewatering of fluids trapped in the buried turbiditic channel. The consequence is a fluid flow caused by a longitudinal pressure gradient along the buried channel. A hydromechanical model proposed for the formation of shallow pockmarks indicates that the sedimentation rate cannot generate the overpressure required for pockmark formation on the seafloor. Therefore, it is suggested that hydrocarbon migration from deeper overpressured reservoirs is added to the pore fluid pressure in the shallow subsurface sediments. Horizontal drainage by the turbiditic palaeochannel and vertical migration along many vertical conduits (seismic chimneys) probably initiated at shallow subbottom depth. It is concluded that these shallow processes have important implications for fluid migration from deeply buried hydrocarbon reservoirs.
Polygonal fault systems on the mid-Norwegian margin: a long-term source for fluid flow
Authors: Berndt Christian , Bünz Stefan , Mienert Jürgen
Published: April 2008

2D and 3D seismic data from the mid-Norwegian margin show that polygonal fault systems are widespread within the fine-grained, Micene sediments of the Kai Formation that overlie the Mesozoic/Early Cenozoic rift basins. Outcropping polygonal faults show that de-watering and development of polygonal faults commenced shortly after burial. On the other hand, the polygonal fault system’s stratigraphic setting, upward decreasing fault throw and the association with fluid flow features that are attributed to de-watering of the polygonal fault systems shows that polygonal faulting and fluid expulsion is an ongoing process since Miocene times.
Soft-sediment deformation in Lake Superior: evidence for an immature Polygonal Fault System?
Authors: Wattrus N. J. , Rausch D. E. , Cartwright J.
Published: April 2008

New multi-beam sonar and seismic data collected in Lake Superior document the wide-spread development of lake-floor rings in fine-grained lake-floor sediments. The multi-beam images reveal that the rings develop as connected clusters and that individual rings have an irregular polygonal appearance. High-resolution seismic data collected with a 28 kHz echo sounder reveal extensive fracturing and faulting in the glacio-lacustrine sediments below the lake-floor. Displacement on the faults is typically normal with throws of less than 50 cm. Three styles of faulting are recognized: (a) monoclinal flexure; (b) graben-like; and (c) conjugate. Zones of acoustic blanking below the faults may be associated with de-watering and mobilization of the sediments. Lateral thickness variation in some horizons suggests that fault and fracture development is linked with lateral movement of sediment. Piston cores collected near lake-floor rings show well-developed fractures and micro-faults, suggesting that fracturing and faulting occurs on a wide range of scales. The seismic and lithological characteristics of the glacio-lacustrine section are similar to those of sediments in which Polygonal Fault Systems (PFS) have been described. This suggests that the rings in Lake Superior may be the surface expression of PFS in the near-surface sediments.
Outcrop examples of mudstone intrusions from the Jerudong anticline, Brunei Darussalam and inferences for hydrocarbon reservoirs
Authors: Morley C.K.
Published: April 2008

Mudstone intrusions in the Jerudong area represent natural hydraulic fractures developed above an inferred mobile mudstone diapir sourced from the Middle Miocene Setap Formation. Intrusions are manifest as dykes, sills and laccoliths. Intrusion geometries are strongly influenced by pre-existing weaknesses, in particular normal faults. Local inflation of country rock shale units also occurs. Dyke terminations include curved traces, splays and en echelon faults. The wall rocks tend to be smooth, or have numerous small, sub-horizontal ledges (mini jogs). Faults and bedding surfaces cause larger jogs in the dyke trend. Mudstone sills commonly change thickness at normal faults, and inflate the hanging wall so that the normal fault sense of motion is inverted, thrusting and fault bend folding are the result. An exposed mudstone laccolith passes laterally via v-shaped bedparallel intrusions into sandstones. The roof of the laccolith is arched upwards and near the intrusion, the roof is broken up by small and large intrusions. Existing models for diapir rise require the brittle shear strength of the roof sequence to be overcome. The Jerudong outcrops demonstrate diapir rise by hydraulic fracturing and stoping which limits the roof sequence strength to the minimum horizontal stress plus the tensile strength of the country rock or normal fault zones in the roof sequence. The crosscutting mudstone dykes, sills, mudstone intruded fault zones and associated cataclastic deformation form planar permeability barriers that can highly compartmentalize the reservoir rock and significantly reduce the area of reservoir rock that can be effectively produced of hydrocarbons. Intruded mudstones could easily be mistaken for stratified shales when interpreting and correlating well logs.
Remobilization of sand from consolidated sandstones: evidence from mixed bitumen-sand intrusions
Authors: Parnell John , Kelly Jason
Published: April 2008

Some sand in intrusive bodies has an origin in the disaggregation of consolidated sandstones. Evidence from the study of mixed bitumen-sand intrusions includes progressive spalling of individual grains from cemented sandstones and individual sand grains with adhering cements. Both physical forces related to rapid fluid movement and chemical dissolution may contribute to the disaggregation process. Rapid fluid flow due to hydrofracturing of an overpressured sequence is responsible for several examples of sand-bearing bitumen veins. As overpressuring is a widespread phenomenon, disaggregation of lithified sandstones may be more prevalent than hitherto appreciated. Sand may be entrapped in bitumen within vein systems cutting kilometres of stratigraphy. Fluid inclusion data from an example at Bentheim, Germany, shows that cementation of sandstone occurred at high temperatures before sand disaggregation and mobilization.
A Late Triassic pterosaur from the Northern Calcareous Alps (Tyrol, Austria)
Authors: Wellnhofer Peter
Published: April 2008

Disarticulated skeletal remains of an eudimorphodontid pterosaur from the Late Triassic of the Karwendel Mountains in Tyrol, Austria, are described and figured. It is the second record of Triassic pterosaurs from the Northern Calcareous Alps, after previous discoveries in the Southern Calcareous Alps of northern Italy. The fossil material — isolated jaws, bones and fragmentary skeletal parts of one individual — was collected from the Norian Seefeld Beds (also called ‘Bitumenmergel’ or ‘Seefelder Fischschiefer’) near Seefeld in Tyrol. The fossiliferous formation of largely bituminous and calcareous layers originated on top of an extended marine carbonate platform, an equivalent to the nearly contemporaneous limestones and dolomites in the Southern Calcareous Alps, as exposed in Lombardy and Friuli in Italy, that has produced pterosaur fossils since 1973. Based on the biostratigraphic significance of conodont index fossils, the Seefeld Beds can be dated as Late Norian, most likely Sevatian. Sufficiently well-preserved skeletal elements include a jugal, isolated teeth, both mandibular rami with a dentition of uni-, tri- and pentacuspid teeth, cervical, dorsal and caudal vertebrae, ribs, sternum, scapulocoracoids, humerus, first wing phalanx, pelvis and tibia/fibula. There are morphological characters that support a subadult stage of the individual with an estimated wing span of 70–80 cm. The dentition is comparable to
Eudimorphodon ranzii
Zambelli from the Norian Calcare di Zorzino of Cene near Bergamo, northern Italy. However, some skeletal proportions and osteological features are distinctive from this taxon, as well as from a second species,
Eudimorphodon rosenfeldi
Dalla Vecchia from the Norian Dolomia di Forni of Friuli, northern Italy. The meaning of these differences in the Seefeld specimen, in particular the relatively long tibia and short wing phalanx 1, is discussed. The caudal zygapophyses and haemal arches are not elongated into the rod-like bony extensions significant for other known rhamphorhynchoid pterosaurs. It appears, however, that all speciments of
Eudimorphodon
lack elongated caudal zygapophyses. This might be evaluated as a primitive trait for these basal pterosaurs. The typical dentition permits the assignment of the Seefeld pterosaur to the genus
Eudimorphodon
. The fragmentary state of the skeleton and somewhat different skeletal proportions allow the author only to refer the specimen to
Eudimorphodon
cf.
ranzii
Zambelli 1973.
A new azhdarchid pterosaur from the Late Cretaceous phosphates of Morocco
Authors: Suberbiola Xabier Pereda , Bardet Nathalie , Jouve Stéphane , Iarochène Mohamed , Bouya Baâdi , Amaghzaz Mbarek
Published: April 2008

A large azhdarchid pterosaur is described from the Late Maastrichtian phosphatic deposits of the Oulad Abdoun Basin, near Khouribga (central Morocco). The material consists of five closely associated cervical vertebrae of a single individual. The mid-series neck vertebrae closely resemble those of azhdarchids
Quetzalcoatlus
and
Azhdarcho
in that they are elongate, with vestigial neural spines, prezygapophysial tubercles, a pair of ventral sulci near the prezygapophyses, and without pneumatic foramina on the lateral surfaces of the centra. The Moroccan pterosaur is referred to a new genus and species of Azhdarchidae:
Phosphatodraco mauritanicus
gen. et sp.nov. It is mainly characterized by a very long cervical vertebra eight, bearing a prominent neural spine located very posteriorly. Based on comparisons with azhdarchid vertebrae, the estimated wing span of
Phosphatodraco
is close to 5 m. This discovery provides the first occurrence of Late Cretaceous azhdarchids in northern Africa.
Phosphatodraco
is one of the few azhdarchids known from a relatively complete neck and one of the latest-known pterosaurs, approximately contemporaneous with
Quetzalcoatlus
.
Giant azhdarchid pterosaurs from the terminal Cretaceous of Transylvania (western Romania)
Authors: Buffetaut Eric , Grigorescu Dan , Csiki Zoltan
Published: April 2008

Pterosaur remains from the Late Cretaceous of the Haţeg Basin of western Romania were reported by Nopcsa as early as 1899. Recent discoveries from the Late Maastrichtian Densuş-Ciula Formation include the giant azhdarchid
Hatzegopteryx thambema
, the holotype of which, consisting of skull elements and a humerus from the VĂlioara locality, is described in detail. A very large femur from the same formation at Tuştea is also described. The systematic position of
Hatzegopteryx
is discussed. The wing span of
H. thambema
is estimated to be close to that of
Quetzalcoatlus northropi
(≥12 m), but its skull is especially robust and may have been remarkably long (≥ 2.5 m). The skull bones of
H. thambema
consist of a very thin outer cortex enclosing an inner meshwork of extremely thin trabeculae surrounding very numerous small alveoli, an unusual structure reminiscent of expanded polystyrene. This peculiar structure, combining strength with lightness, can probably be considered as an adaptation to flight in a very large animal, through reduction of skull weight.
The detailed anatomy of
Rhamphorhynchus
: axial pneumaticity and its implications
Authors: Bonde Niels , Christiansen Per
Published: April 2008

An acid- and transfer-prepared, juvenile
Rhamphorhynchus muensteri
, despite some fragmentation, is in an excellent state of three-dimensional preservation, exposing exquisite anatomical details hitherto unknown in other pterosaurs. Here we describe the axial pneumatizations of the cervical and anterior dorsal vertebrae and the sternum. The interior of the cervical centra is subdivided into a pair of large camerae, presumably by air sacs entering by large pleurocoels in the sides of the centra. This so-called ‘camerate’ type of pneumatization is hitherto unknown in pterosaurs. Another excavation enters from the ventral side into the base of the neural arch and stretches between the pre- and postzygapophyses. This type of cavity also penetrates from the ventral side into the base of the first few transverse processes of the dorsal vertebrae, although these lack central pleurocoels. The cristospine also has a complex pneumatic foramen.

Skeletal pneumaticity is most probably a result of a highly derived pulmonary system, as in extant birds. Morphologically similar pneumatic features are present in most saurischian dinosaurs and it is possible that they are the result of convergence. Because basal members of the various groups, including Triassic pterosaurs, appear to lack skeletal pneumaticity, convergence seems likely, although the stem-ornithodiran parsimoniously possessed a more bird-like than ‘reptile’-like pulmonary system, albeit non-invasive. This points to possible tachymetabolism in these forms, which is in accord with the distribution of other factors such as integumentary structures and bone histology. It is concluded that evolution of this suite of advanced features, surprisingly, was among the earliest events in the ornithodiran lineage soon after it split off from its crocodilian sister-group.
Pterosaur tracks from the latest Campanian Cerro del Pueblo Formation of southeastern Coahuila, Mexico
Authors: Rodriguez-de la Rosa Ruben A.
Published: April 2008

A new vertebrate tracksite in southeastern Coahuila, northern México, known as the El Pelillal tracksite (Latest Campanian, Cerro del Pueblo Formation) has yielded an important ichnofauna that includes the tracks of turtles, crocodilians, a small theropod dinosaur, a mammal-like organism and the tracks of pterosaurs. The pterosaurian manus impressions share an extraordinary similarity, in size and morphology, with the Jurassic ichnogenus
Pteraichnus
Stokes 1957 and are referred to
Pteraichnus
sp.; however, the pes impressions differ in that they are rather very elongated. According to the geological features and ichnofauna the El Pelillal tracksite represents a shallow, freshwater or lacustrine deposit, which agrees with our knowledge about the habitat preference of Cretaceous pterosaurs. This newly discovered tracksite in southeastern Coahuila, Mexico, offers great potential for palaeoichnological research and is thus becoming important in the understanding of the palaeoecosystems during the Late Cretaceous in southern North America.
The John Quekett sections and the earliest pterosaur histological studies
Authors: Steel Lorna
Published: April 2008

In 1855, John Quekett, assistant curator to Richard Owen of the Hunterian Museum in London, published a catalogue of the museum’s thin-section specimens. Among them were 15 sections of pterosaur bone. These sections were cut from eight bones found in the Lower Chalk of Kent, the Wealden of Sussex and the Stonesfield Slate of Oxfordshire, and are believed to be the first pterosaur bone thin sections to have been produced and described. Six of these thin sections survived the bombing of London during the Second World War, although the bones from which they were cut were destroyed. The sections were re-examined in early 2001 and photographed in plane and crossed polarized light for the first time. The histology of one thin section is inconsistent with a pterosaurian origin. This case highlights the value of bone palaeohistology as an aid in confirming the identity of incomplete remains.
Histovariability in bones of two pterodactyloid pterosaurs from the Santana Formation, Araripe Basin, Brazil: preliminary results
Authors: Sayão Juliana M.
Published: April 2008

Thin sections of pterosaur bones have not been extensively studied so far. Until now mainly isolated bones were the subject of this type of analysis. Here we present preliminary results of the histological analysis of two pterodactyloid pterosaurs from the Romualdo and Crato Members of the Santana Formation. The first specimen (Museu Nacional, MN 4809-V) comprises both wings (from humerus to the third phalanx of the fourth digit) and one hindlimb (tibia, fibula and pes). The second (MN 6527-V) consists of a partial skeleton with the incomplete left wing (humerus, radius, ulna, carpals, pteroid, wing metacarpal, two phalanges of the fourth digit) and fragments of one hindlimb (complete tibia and proximal articulation of the femur). Thin sections of the diaphyseal portion of each bone were ground in transverse, longitudinal and oblique orientation and the histological structures were compared. The following features can vary in the first observed specimen (MN 4809-V): presence of primary and secondary tissues in the cortex; absence or presence and position of lines of arrested growth; resorption of primary structures. These variations in different bones of the same individual represent differential growth rates. The second specimen (MN 6527-V) is well vascularized and has fibro-lamellar bone indicative of rapid growth in the thin section of the radius, ulna and first phalanx of the fourth digit. This last feature is very similar to the condition observed in most living birds.
Ophiolite pulses, mantle plumes and orogeny
Authors: Dilek Yildirim
Published: April 2008

Ophiolites show a wide range of internal structure, pseudostratigraphy and chemical fingerprints suggesting various tectonic settings of their origin. In general, they are characterized as mafic-ultramafic assemblages and associated sedimentary and metamorphic rock units that formed during different stages of the Wilson cycle evolution of ancient oceans, and that were subsequently incorporated into continental margins through collisional and/or accretionary orogenic events. Distributions of ophiolites with certain age groups in different orogenic belts define distinct ophiolite pulses, times of enhanced ophiolite genesis and emplacement, in Earth history. These pulses coincide with the timing of major collisional events during the assembly of supercontinents (i.e. Rodinia, Gondwana and Pangaea), dismantling of these supercontinents, and increased mantle plume activities that formed widespread large igneous provinces (LIPs). Suprasubduction zone ophiolites in orogenic belts signify oceanic crust generation in subduction rollback cycles during the closing stages of basins prior to terminal continental collisions. Both collision-driven assembly of supercontinents and deep penetration of subducted slabs into the lower mantle may produce plumes that in turn facilitate continental rifting, sea-floor spreading and oceanic plateau generation, all of which seem to have contributed to ophiolite genesis. Accelerated LIP formation and seafloor spreading that are associated with superplume events are likely to have caused widespread collisions and tectonic accretion of ophiolites at global scales. Together, these spatial and temporal relations suggest close links between ophiolite pulses, mantle plumes and orogenic events in Earth history.
Melt migration in ophiolitic peridotites: the message from Alpine-Apennine peridotites and implications for embryonic ocean basins
Authors: Müntener Othmar , Piccardo Giovanni B.
Published: April 2008

Results of a field study as well as petrological and geochemical data demonstrate that substantial portions of the lithospheric mantle, exhumed during opening of the Jurassic Piedmont Ligurian ocean, were infiltrated by and reacted with migrating melts. Intergranular flow of ascending liquids produced by the underlying hot asthenosphere dissolved clinopyroxene ± spinel and precipitated orthopyroxene + plagioclase ± olivine, forming orthopyroxene + plagioclase-rich perioditite. Migrating liquids became progressively saturated in clinopyroxene, and then precipitated microgranular aggregates of clinopyroxene-bearing gabbronorite. Later, diffuse porous melt flow was replaced by focused porous flow, producing a system of discordant dunite bodies. Upon cooling, liquids migrating in dunite channels became progressively saturated in clinopyroxene and plagioclase, forming interstitial clinopyroxene at olivine triple points followed by clinopyroxene ± plagioclase megacrysts and gabbro veinlets within the dunite, and gabbro dykelets within plagioclase peridotites. Subsequent cooling during continued exhumation was accompanied by intrusion of kilometre-scale gabbroic dykes evolving from troctolite to Mg-Al and Fe-Ti gabbros. Migrating liquids, which infiltrated peridotite and formed gabbroic rocks, span a wide range of compositions from silica-rich single melt fractions to T- and N-MORB (mid-ocean ridge basalt), characteristic of the melting column beneath midocean ridges. Explanations for the progressive evolution of an igneous system from diffuse to focused porous flow and finally dyking include the competing effects of heating of the lithospheric mantle by ascending magmas from the underlying hot asthenosphere and conductive cooling by exhumation. Whether or not rift-related melt infiltration and heating is recorded by exhumed subcontinental lithospheric mantle along ocean-continent transitions and/or oceanic lithospheric mantle along slow-spreading ridges depends on the relative position to the underlying upwelling asthenosphere.
Geochemical and geochronological constraints on the origin and emplacement of the Yarlung Zangbo ophiolites, Southern Tibet
Authors: Malpas John , Zhou Mei-Fu , Robinson Paul T. , Reynolds Peter H.
Published: April 2008

The Indus-Yarlung Zangbo suture zone in southern Tibet marks the Eocene collision of the Indian continent and the Lhasa Block of Eurasia. It is characterized, particularly in its central portion, by an east-west belt of ophiolitic and related oceanic volcanic and sedimentary rocks that form a number of structurally juxtaposed geological terranes. Although tectonically disrupted in many places, almost complete ophiolite sequences exist at Luobusa and Zedong in the east and near Xigaze in the west. In Luobusa, the ophiolite sequence is thrust over the Tertiary molasse deposits of the Luobusa Formation or onto plutonic rocks of the Gangdese batholith. A mantle sequence dominates the ophiolite massif and consists chiefly of harzburgite and clinopyroxene-bearing harzburgite with abundant podiform chromitites enveloped by dunite. The Luobusa ophiolite formed the basement to an intra-oceanic volcanic arc, the Zedong terrane, which developed between the Mid-Jurassic and Mid-Cretaceous. Farther to the west, complete ophiolite sequences exist at Dazhuqu and near Xigaze. These ophiolites have suprasubduction zone geochemical signatures but there is no apparent development of a volcanic arc. Sensitive high-resolution ion microprobe U-Pb zircon analyses yield an age of 126 Ma for the crystallization of a quartz diorite from the Dazhuqu massif. Amphibolites that occur as large blocks in mélanges at the base of the ophiolites are considered to be remnants of dynamothermal metamorphic soles produced early in the ophiolite obduction process. Ar/Ar geochronology on amphibole and biotite separates from these rocks yields ages of 80 and 90 Ma, respectively, for this event, which is considered to have occurred as the Indian continental margin entered the intra-oceanic subduction zone. Continued northward subduction of the remaining portion of the Neo-Tethyan ocean floor beneath the southern margin of Eurasia produced the Gangdese continental arc on the southern margin of the Lhasa Block and led to the final closure of the ocean with the collision of India and Eurasia in the Eocene.
Ophiolites and global geochemical cycles: implications for the isotopic evolution of seawater
Authors: Gregory R. T.
Published: April 2008

Isotopic profiles through ophiolite complexes provide the necessary link between the study of global geochemical cycles and plate tectonics. The hydrothermal circulation that occurs beneath the sea floor is the primary mechanism for exchange between the mantle of the Earth and the hydrosphere. The subduction of the hydrothermally altered crust and overlying sediments is the primary mechanism for crustal recycling. Oxygen and strontium isotopes of seawater track the competition between continental weathering and mid-ocean ridge hydrothermal exchange to control the composition of the oceans. Information derived from ophiolite studies on the elemental fluxes and the depth of seawater penetration into the oceanic crust provides constraints on the important rate constants associated with these competing processes. The same tectonic rates that account for the Sr isotope evolution of seawater indicate that the oxygen isotopic composition of the ocean is constrained to vary within narrow limits (per mil level). Isotopic analysis of dredge samples and ophiolite complexes demonstrates that seawater-ocean crust interactions result in a oxygen isotopic zonation of the oceanic crust with masses (concentration times volume) centred on the initial isotopic composition of the crust. This requires that the oxygen isotopic composition of the ocean resides at near steady-state conditions over Earth history. The inferences from ophiolite complexes contrast strongly with the results of measurements on carbonates from epicontinental seaways, particularly for the Palaeozoic. Ophiolites and greenstone belts track exchange processes between the ocean and the igneous crust whereas most carbonate measurements track the surface ocean on continental shelves. For oxygen isotopes, the mass of epicontinental seaways and the rates of meteoric water input suggest a resolution to the controversy that accounts for both data sets.
Hydrothermal circulation and metamorphism in crustal gabbroic rocks of the Bay of Islands ophiolite complex, Newfoundland, Canada: evidence from mineral and oxygen isotope geochemistry
Authors: Giguère E. , Hébert R. , Beaudoin G. , Bédard J. H. , Berclaz A.
Published: April 2008

The gabbroic crust of the Ordovician Bay of Islands ophiolite complex formed in an island-arc setting near the North American continental margin. Detailed structural studies on the North Arm Mountain massif provide us with a scheme of syn-oceanic deformation events recorded in the crust. During a first transtensional stage, which generated gabbroic rocks, sheeted dykes and lavas, the temperature of formation of amphiboles in the gabbroic unit fell with time in three steps from 880–745 °C, to 790–680 °C and to 550–500 °C. The Ti, Na and Al
IV
contents of amphiboles decreased, whereas the Si activity of the fluid increased with time. The first amphibole to form has typical mid-ocean ridge basalt δ
18
O
VSMOW
indicating equilibration with a magmatic fluid or evolved seawater at low fluid/rock ratio. Lower δ
18
O values for some amphiboles (0–2.5‰) indicate the circulation of large volumes of seawater. The lowest δ
18
O values are found in the inner part of the shear zones, which channelled deep infiltration of seawater into the gabbroic unit. During brittle deformation, infiltration of low-temperature seawater produced prehnite, carbonate and quartz veins, and plagioclase with high δ
18
O. This study documents that the hydration of ophiolitic crust in the Bay of Islands ophiolitic complex occurred mainly along pre-obduction oceanic structures in an intraoceanic setting.
Ophiolites as faithful records of the oxygen isotope ratio of ancient seawater: the Solund-Stavfjord Ophiolite Complex as a Late Ordovician example
Authors: Muehlenbachs Karlis , Furnes Harald , Fonneland Hege C. , Hellevang Bjarte
Published: April 2008

Fragments of the Ordovician sea floor preserved in the Solund-Stavfjord Ophiolite Complex in Western Norway serve as proxies for the δ
18
O of Ordovician seawater. The pillow basalt sections at Oldra and Strand are both enriched in
18
O, recording their alteration by seawater at low temperature on the sea floor. In contrast, the sheeted dykes and gabbros generally are depleted of
18
O, reflecting the modal proportion of secondary, low-
18
O chlorite and epidote formed from seawater at high temperature. These isotopic contrasts simply reflect the high water to rock ratio of sea-floor alteration and the temperature dependence of the
18
O partitioning between minerals and water. Superposition of high-δ
18
O pillows on low-δ
18
O dykes and gabbros is a necessary consequence of alteration at both low and high temperatures by a fluid near 0‰ and is easily recognized in well-preserved ophiolites. Also, the δ
18
O of seawater can be independently calculated from
18
O fractionations among secondary minerals. Older, dismembered and highly metamorphosed segments of the oceanic crust may still retain the original seawater imprint because their subsequent obduction and metamorphism was relatively closed to external fluids. Suites of diamond-bearing nodules from kimberlites still have contrasting high- and low-δ
18
O eclogites, proving that even subduction into the mantle is not sufficient to erase the seawater fingerprint. Inspection of the sea-floor, ophiolite and eclogite data reveals no secular trend in δ
18
O, indicating that the δ
18
O of seawater has not changed with geological age. Because the δ
18
O of seawater itself is fixed by sea-floor-seawater exchange, the constancy of δ
18
O of seawater implies that the scale and style of sea-floor-seawater interactions has not changed over time.
What constitutes ‘emplacement’ of an ophiolite?: Mechanisms and relationship to subduction initiation and formation of metamorphic soles
Authors: Wakabayashi John , Dilek Yildirim
Published: April 2008

Ophiolites have long been recognized as on-land fragments of fossil oceanic lithosphere, which becomes an ophiolite when incorporated into continental margins through a complex process known as ‘emplacement’. A fundamental problem of ophiolite emplacement is how dense oceanic crust becomes emplaced over less dense material(s) of continental margins or subduction-accretion systems. Subduction of less dense material beneath a future ophiolite is necessary to overcome the adverse density contrast. The relationship of subduction to ophiolite emplacement is a critical link between ophiolites and their role in the development of orogenic belts. Although ophiolite emplacement mechanisms are clearly varied, most existing models and definitions of emplacement concern a specific type of ophiolite (i.e. Oman or Troodos) and do not apply to many of the world’s ophiolites. We have defined four prototype ophiolites based on different emplacement mechanisms: (1) ‘Tethyan’ ophiolites, emplaced over passive continental margins or microcontinents as a result of collisional events; (2) ‘Cordilleran’ ophiolites progressively emplaced over subduction complexes through accretionary processes; (3) ‘ridge-trench intersection’ (RTI) ophiolites emplaced through complex processes resulting from the interaction between a spreading ridge and a subduction zone; (4) the unique Macquarie Island ophiolite, which has been subaerially exposed as a result of a change in plate boundary configuration along a mid-ocean ridge system. Protracted evolutionary history of some ocean basins, and variation along the strike of subduction zones may result in more complicated scenarios in ophiolite emplacement mechanisms. No single definition of emplacement is free of drawbacks; however, we can consider the inception of subduction, thrusting over a continental margin or subduction complex, and subaerial exposure as critical individual stages in ophiolite emplacement.
Forearc ophiolites: a view from the western Pacific
Authors: Milsom John
Published: April 2008

Many of the world’s largest ophiolite masses are now being interpreted as remnants of oceanic forearcs stranded on continental margins in the course of arc-continent collision. This interpretation implies a former association between the ophiolite and a volcanic arc at a distance of 100–200 km. The New Caledonia region of the SW Pacific is one of the few areas where there is good evidence for the presence of such an arc. On New Caledonia itself, the Grand Massif du Sud ophiolite has been thrust over the Norfolk Ridge continental fragment, while the coral islands of the Loyalty Group,
c.
100 km to the NE, cap large sub-sea edifices with volcanic morphology.

New Caledonia can be correlated geologically with New Guinea, despite the considerable width of open ocean that now separates the two. In central New Guinea, where ophiolites emplaced along the northern flank of the main mountain spine lie some 100 km south of exposures of arc-volcanic basement in the north coast ranges, the similarities include the relationship between New Caledonia and the Loyalty Islands. There is, however, no obvious comparable relationship in the case of the largest of the New Guinea ophiolites, the Papuan Ultramafic Belt of the eastern peninsula, which is backed to the north and east by the oceanic Solomon Sea. An associated volcanic arc can be recognized in this area only by assuming a complicated history of collision, post-collision arc-forearc separation and sea-floor spreading, followed by renewed contraction and a very recent and continuing second collision. The case for such a sequence of events can be made on a number of grounds. If the processes responsible are general in their nature, they could explain the apparent absence of arc-volcanic belts in association with many other supposedly forearc ophiolites.
Mineral chemistry of ultramafic massifs in the Southern Uralides orogenic belt (Russia) and the petrogenesis of the Lower Palaeozoic ophiolites of the Uralian Ocean
Authors: Spadea P. , Zanetti A. , Vannucci R.
Published: April 2008

Ophiolites of the southern Uralides arc-continent collisional orogen include fertile mantle lherzolites and minor harzburgites in the Nurali and Mindyak massifs located along the Main Uralian Fault suture of the East European craton margin and the Magnitogorsk island arc. We present the first
in situ
analyses of pyroxene from Nurali spinel ± plagioclase-bearing lherzolites and harzburgites and Mindyak spinel lherzolites and harzburgites. Based on the trace element signatures of pyroxene, the Nurali peridotites are divided into: Group 1, consisting of plagioclase-bearing spinel lherzolites with moderately to extremely light rare earth element (LREE)-depleted clinopyroxenes, consistent with ⩽8% fractional melting followed by impregnation by incremental to mid-ocean ridge basalt (MORB)-like melts; Group 2, formed by a spinel peridotite with strongly LREE- to middle REE (MREE)-depleted to enriched clinopyroxenes that testify to re-equilibration with large volumes of melt of tholeiitic affinity; Group 3, consisting of amphibole-bearing spinel harzburgites that underwent pervasive percolation of alkali-enriched melts or fluids. Clinopyroxenes from the Mindyak peridotites are strongly depleted and re-equilibrated by reactive porous flow of infiltrating tholeiitic melts. Two alternative petrogenetic models are proposed. In Model 1 the peridotites derive from oceanic lithosphere generated by mid-ocean ridge processes and affected by refertilization via melt percolation. In Model 2 the peridotites were subcontinental lithospheric mantle that experienced deep-seated magmatism and sub-solidus re-equilibration prior to the opening of the Uralian Ocean, and interacted with melts generated in the asthenospheric mantle by extension-related decompression partial melting during the opening of the Uralian Ocean. In both models the final pre-orogenic events are related to the subduction of the Uralian oceanic lithosphere and to mantle wedge processes, notably intrusion of gabbro-diorite at
c.
400 Ma into the Moho sections.
A general model of arc-continent collision and subduction polarity reversal from Taiwan and the Irish Caledonides
Authors: Clift Peter D. , Schouten Hans , Draut Amy E.
Published: April 2008

The collision of the Luzon Arc with southern China represents the best example of arc-continent collision in the modern oceans, and compares closely with the Early Ordovician accretion of the Lough Nafooey arc of Connemara, Ireland, to the passive margin of Laurentia. We propose a general model for steady-state arc-continent collision in which arc crust is progressively added to a passive margin during a process of compression, metamorphism and magmatism lasting 3–10 Ma at any one location on the margin. Depending on the obliquity of the angle of collision, the timing of active collision may be diachronous and long-lived along the margin. Magmatism accompanying accretion can be more enriched in incompatible trace elements than average continental crust, contrasting with more depleted magmatism prior to collision. Accretion of a mixture of depleted and enriched arc lithologies to the continental margin allows the continental crust to grow through time by arc-passive margin collision events. During the collision the upper and middle arc crust are detached from the depleted ultramafic lower crust, which is subducted along with the mantle lithosphere on which the arc was founded. Rapid (2–3 Ma) exhumation and gravitational collapse of the collisional orogen forms the Okinawa and South Mayo Troughs in Taiwan and western Ireland, respectively. These basins are filled by detritus eroded from the adjacent collision zone. During subsequent subduction polarity reversal, continuous tearing and retreat of the oceanic lithosphere along the former continent-ocean transition provides space for the new subducting oceanic plate to descend without need for breaking of the original slab.