bluuebunny/crossref_metadata_2025_split huggingface dataset:
Early Ordovician volcanism in Dyfed, SW Wales
Authors: Bevins R. E. , Roach R. A. 
Published: May 2008

Summary
A review of early Ordovician volcanism in SW Dyfed, Wales is presented in the light of recent work. There were a number of major and minor volcanic episodes within the area during Arenig and Llanvirn times. For the most part, submarine volcanism occurred within a restricted basin, in which black muds were deposited and to which coarse terrigenous material had very limited access. Basic and intermediate magmas were generally erupted as flows. Acidic magmas were also erupted quietly, to produce thick, frequently autobrecciated flows and domes. Volcaniclastic rocks resulted by deposition from sediment gravity flows originating on unstable flanks of lava domes. Evidence for occasional episodes of explosive activity is provided by sediment gravity flows carrying a high proportion of pyroclastic debris. Subaerial or shallow water conditions are suggested only for the oldest volcanic rocks on Ramsey Island. Numerous basic and less frequent intermediate and acid high-level intrusive sheets have a close temporal, spatial and genetic relationship with the volcanic rocks.
The Capel Curig Volcanic Formation, Snowdonia, North Wales; variations in ash-flow tuffs related to emplacement environment
Authors: Howells M. F. , Leveridge B. E. , Addison R. , Evans C. D. R. , Nutt M. J. C. 
Published: May 2008

Summary
A detailed correlation of the tuffs of the Capel Curig Volcanic Formation is presented. The depositional environment of the sediments associated with the tuffs varied from subaerial to submarine. Variation in the ash-flow tuffs are related to this changing environment. Subaerially emplaced tuffs are characterised by even bases, distinctive recrystallisation textures, the absence of reworked tops and sparse siliceous nodules. Tuffs emplaced in a subaqueous environment show irregular undersurfaces with welding to the lower contact, reworked tops and common siliceous nodules.
The age of mineralisation at Parys Mountain, Anglesey
Authors: Nutt M. J. C. , Ineson P. R. , Mitchell J. G. 
Published: May 2008

Summary
Borehole exploration of the mineralisation in the Ordovician and Silurian rocks of Parys Mountain, Anglesey has produced two hypotheses to explain the emplacement of the ore deposits. Mineralisation was either syngenetic, associated with volcanicity in the Ordovician, or epigenetic, occurring during or after the Caledonian orogeny. An outline of the geological evidence is given together with the results of K-Ar isotope age determinations on thirty-two core samples. Two isotopic events have emerged from the K-Ar determinations, 394±9 Ma and 353±7 Ma; these ages are discussed in the light of the evidence available.
The petrogenesis of Old Red Sandstone volcanic rocks of the western Ochils, Stirlingshire
Authors: French W. J. , Hassan M. D. , Westcott J. 
Published: May 2008

Summary
Lavas and intrusions ranging from basalt to felsite have been examined. These form a high-K, calc-alkaline series of rocks characterised by acid groundmasses and zoned plagioclase phenocrysts which sometimes have rims of sanidine. The intrusions are generally more acid than the lavas and the differentiation trend seems to relate to subtraction of olivine and feldspar. Formation of the secondary magmas may have been due to removal of olivine, clinopyroxene and garnet from a crystal-melt system generated some 250 km below the contemporary surface. The rocks match other Old Red Sandstone volcanic provinces and are chemically analogous with the Caledonian plutonic series.
Geochemical and geophysical constraints on the origin and evolution of Caledonian granites
Authors: Brown G. C. 
Published: May 2008

Summary

Most British Caledonian granites, which were emplaced during the period 600–390 Ma, are characterised by temporal progression towards low K/Rb ratios, low total Sr, low initial
87
Sr/
86
Sr ratios and high U contents coupled with increasingly large negative Bouguer gravity anomalies and sometimes large magnetic anomalies. It is proposed that some early granites from N Scotland represent low mobility, migmatitic low volume melts resulting from overthrusting which reached a climax in mid-Ordovician times. Early granites from S Britain and late granites from both the north and south provinces have geochemical characteristics of mantle-derived parental magmas capable of metamorphosing the lower crust to a high grade and ascending to become voluminous high level and possibly sub-volcanic intrusions. Late Caledonian granite emplacement ignores the major lateral break in crustal continuity near the Southern Uplands fault that is identified on seismic, geochemical, aeromagnetic and geological evidence. Seemingly, these granites were produced after Silurian continental plate closure. Yet their characteristics have little in common with the modern Alpine-Himalayan plutonic rocks and a source within sub-continental lithosphere under tension is postulated.
Petrogenetic significance of Rb-Sr and U-Pb isotopic systems in the 400 Ma old British Isles granitoids and their hosts
Authors: Halliday A. N. , Aftalion M. , van Breemen O. , Jocelyn J. 
Published: May 2008

Summary

Rb-Sr isotopic data for 17 Caledonian granitic plutons in Scotland and northern England define low
87
Sr/
86
Sr
i
(<0.709) for all granitoids emplaced between ∼435 and 390 Ma with the exception of Moy and Aberdeen (which may be older). There is no apparent correlation of
87
Sr/
86
Sr
i
with distance from the proposed Iapetus suture near the Southern Uplands. Zircons from the Moinian and Dalradian rocks and Southern Uplands Ordovician greywacke roughly define U-Pb discordia lines with upper concordia intersections of 1200–1800 Ma and the position along the discordia related to the age of the sedimentary unit. Zircons from the Strontian tonalite and granodiorite and the Loch Doon granite contain little if any Pb isotopic memory, whereas the Strontian central granite contains a marked inherited zircon component. We conclude that, in strong contrast to the
c.
550 Ma and
c.
460 Ma granitoids, the
c.
400 Ma magmas were largely hybrids derived from the mantle and, or, ‘new’ lower crust with varying degrees of incorporation of upper crustal-type material.
Caledonian granites in relation to regional geochemistry in northern Scotland
Authors: Johnstone G. S. , Plant J. , Watson J. V. 
Published: May 2008

Summary
Regional geochemical maps based on stream sediment sampling provide means of comparing the relative abundances of trace elements in granitic bodies with those characteristic of the enclosing Moine and Dalradian country rocks. Migmatites, vein complexes and pre-Devonian forceful intrusions in northern Scotland differ little from their country rocks on these maps (apart from higher levels of Sr): this observation supports the widely held view that they are derived mainly from remelting of Proterozoic crustal materials. The latest mainly Devonian granites and alkaline complexes tend to show levels of U, Li, Be and Rb well above the background values of the province and from this and other evidence it is suggested that their parent magmas were derived at least in part from material underplated on the crust, and perhaps metasomatically enriched in incompatible elements through mantle processes connected with subduction. The magmas were released during phases of deep fracturing. There are indications of compositional variations related to distance from the Caledonian suture.
The Carn Chuinneag granite and its aureole
Authors: Wilson D. , Shepherd J. 
Published: May 2008

Summary

The Carn Chuinneag granite comprises a suite of acid and subsidiary basic igneous rocks, intruded into the Moine of Easter Ross. Structural evidence indicates that the granite may have been sheet-like or laccolithic in form, and now occupies the core of a major D
3
synform. Hornfelsing of pelitic rocks in the aureole has preserved a variety of sedimentary structures. The former presence of andalusite and cordierite within the hornfels indicates that the granite was emplaced at a high structural level; their alteration to aggregates containing kyanite, may have resulted from burial of the area under the superincumbent load of the Ross-shire nappe during the second episode of deformation.
The Assynt alkaline suite
Authors: Parsons Ian 
Published: May 2008

Summary
The alkaline intrusives in Assynt overlap the thrust movements and can be used to put limits on their magnitude and timing. The Ailsh syenite is the earliest igneous body within the thrust belt; the Borralan complex is later and overlapped major movements on the Assynt thrust. Large scale displacements on the Sole and Assynt thrusts ended before movement ceased on the Moine thrust. Pyroxene variation in the Ailsh complex confirms that the leucosyenites form a series fractionated before emplacement, but the pyroxene range is unusual for an igneous series and consistent with a metasomatic origin for pyroxene-rich members.
Caledonian minor intrusions of the N Highlands of Scotland
Authors: Smith D. I. 
Published: May 2008

Summary

Three major suites of minor intrusions cut the Caledonian rocks north of the Great Glen fault. (1)
The amphibolite suite
was intruded between D
1
and D
2
deformations as a swarm of dolerites (locally gabbroic) trending sub-parallel to the fold belt. (2)
The microdiorite suite
, consisting of a wide variety of rock types, was emplaced during the closing phases of Caledonian tectonism principally in the southern part of the area. Early members were locally involved in the final phase of folding and metamorphism. The majority, however, occur as sheared sheets occupying easterly dipping planes which transgress all the Caledonian fold structures. These sheared intrusions exhibit a variety of internal fabrics, the geometry of which indicates that they were recrystallised under an ESE, subhorizontal maximum stress. Over most of the N Highlands, the recrystallised mineral assemblages belong to the greenschist facies metamorphism, but in the south an area of amphibolite facies has been defined. This is believed to reflect a residual Caledonian temperature gradient. (3)
The minette suite
, centred on the Ratagain granite is a post tectonic dyke swarm which marks the end of igneous activity in the N Highlands Caledonides. Members of the swarm are widely dispersed throughout the Moine and are also present in the foreland rocks west of the Moine thrust. Variations in K-Ar ages from these dykes may relate to differential uplift or to the continuing presence of a local Caledonian thermal high.
Plutonic rocks in the Irish Caledonides
Authors: Kennan Padhraig S. 
Published: May 2008

Summary
Major occurrences of plutonic rocks are briefly reviewed. The important granites are described with special reference to the features that makes each of them unique. The association of basic intrusives with all of the major granites is noted. Some of the basic intrusions may have had, prior to deformation, a lopolithic or funnel-shaped form. Most, however, occur as dykes or sills or as large numbers of small appinitic bodies associated, in a special way, with certain diapiric granites. Possible controls on the siting and mechanism of intrusion of the granites are briefly examined.
The Newry igneous complex, County Down
Authors: Meighan I. G. , Neeson J. C. 
Published: May 2008

Summary

New field and petrographic observations indicate that the ultramafic rocks and closely associated meladiorites at the northeast end of the complex are principally clinopyroxene-biotite-apatite (±plagioclase) cumulates from intermediate magmas. The meladiorites grade into more evolved oversaturated dioritic-monzonitic rocks, many of which may also be accumulative, and consequently the main intermediate and ultramafic body is thought to be part of a layered intrusion. New geochemical data for major and trace elements (including R.E.E.) and initial
87
Sr/
86
Sr ratios show compositional similarities between some of the intermediate rocks and the younger granodiorites. The granodiorites are areally most abundant and probably constitute three contiguous plutons formed mainly by crystal fractionation at depth in intermediate magmas of Upper Mantle origin.
The age of the Saltees granite in the Rosslare complex
Authors: Max M. D. , Ploquin A. , Sonet J. 
Published: May 2008

Summary
A 10 point whole-rock Rb-Sr isochron of 436 ± 7 Ma from this granite probably indicates emplacement age. The granite is thoroughly deformed and its foliation passes into the Rosslare complex to form a composite fabric. A suite of metadolerites, which were intruded before the Saltees granite had fully crystallised, were metamorphosed in the low amphibolite facies even well away from the granite. This Caledonian tectono-thermal event was associated with the emplacement of the granite and the metadolerites and overprints at least two major thermal events seen in the Rosslare complex gneisses.
The Haweswater complex
Authors: Nutt M. J. C. 
Published: May 2008

Summary
The largest basic intrusive complex of the Lake District cuts Borrowdale Volcanic Group rocks around Haweswater. It is composed mainly of gabbros, dolerites, andesites and microdiorites. These have been altered in places by hydrothermal and tectonic activity as well as by later igneous intrusions. Petrology and geochemistry have established a consanguineous relationship between the Haweswater complex and the coeval Borrowdale volcanic rocks. It is suggested that the complex was intruded into an active continental margin island arc.
What is a Thrust? What is a Nappe?
Authors: Dennis J. G. , Price R. A. , Sales J. K. , Hatcher R. , Bally A. W. , Perry W. J. , Laubscher H. P. , Williams R. E. , Elliott D. , Norris D. K. , Hutton D. W. , Emmett T. , McClay K. R. 
Published: May 2008

Summary
The definitions of thrusts and nappes are discussed and the criteria used to define these structures are analysed. A ‘thrust fault’ is defined as a map-scale contraction fault. A ‘thrust nappe’ is an allochthonous tectonic sheet which has moved along a thrust fault. Suggested definitions are also given for, a fold nappe; extension fault; contraction fault; listric normal fault; reverse listric fault; klippe; tectonic slide; ramps and flats; duplex structure; blind thrusts.
Thoughts on the tectonics of folded belts
Authors: Bally A. W. 
Published: May 2008

Summary
Balanced cross sections and their palinspastic reconstruction in structurally simple external zones of folded belts suggest that a limited amount of continental lithosphere was subducted (Ampferer or A-subduction). This process appears to be associated with preceding or synchronous basement remobilization. Therefore, the rigid or ductile nature and the timing of basement deformation remain as some of the most important orogenic problems. Decoupling and ductility contrasts within the lower continental crust and within the overlying sedimentary sequences are responsible for varying structural styles in mountain ranges.
Gravity gliding as an important factor for mountain building is examined in some detail. Soft sediment gravity tectonics on passive continental margins are dominated by listric normal growth faults. This style contrasts with observed styles of deformation in folded belts. Gravity tectonics induced by stretching of the underlying basement area is commonly observed during the rifting phase of passive continental margins and in episutural basins associated with orogenic systems.
Pore pressure, discontinuities, isostasy and overthrusts
Authors: Gretener P. E. 
Published: May 2008

Summary

Thrusting represents the shifting of large loads. This has the following consequences: (a) in the overridden sequence high pore pressures develop initially due to rapid loading and later due to aquathermal pressuring caused by slower thermal adjustment. This leads to the activation of deeper and more outward emerging thrust planes: (b) the isostatic response to the loading in front and the simultaneous unloading in the back produces the
wandering bulge
.

These two processes tend to make thrust faulting a self-perpetuating process once it has been initiated.
Gravitational gliding in deltas
Authors: Mandl G. , Crans W. 
Published: May 2008

Summary
Many cases of faulting in prograding delta slopes can be genetically attributed to submarine gravity gliding (Terzaghi 1956) on slope-parallel slip planes. Essential in this ‘thin-skinned’ model of gravitational faulting is the overpressuring of the pore fluid, which not only permits the formation of very gently dipping slip planes, but also determines the listric shapes of normal and thrust faults at the head and toe of the gliding path.
The development of the gliding process and the associated primary and secondary structures (antithetic faults, ‘roll-over’ anticlines) are described for an infinitely long slope and for somewhat idealized pore-pressure profiles, a main parameter being the distance of the overpressure top from the (rising) sediment surface.
Realistic deviations from the idealizing assumptions are discussed in qualitative terms.
The gliding process is ‘slow’ and controlled by sediment compaction in the slope-parallel direction. Continued sedimentation gives rise to changes in effective stresses and these, in turn, cause active faults to adjust their shapes. Examples of this adjustment are discussed.
Thrust sheet deformation at a ramp: summary and extensions of an earlier model
Authors: Wiltschko D. V. 
Published: May 2008

Summary
A previous model for the equilibrium of a linearly viscous thrust sheet at a ramp shows that fault-zone drag is the most important energy sink, though bending can be an important resisting force and gravity can be a moderate driving force. The model was developed for the Pine Mountain block of the southern Appalachians, though it is general and may be adapted to other geometries.
Increasing the viscosities, ramp dip and thickness of the sheet each impedes forward motion whereas increasing the dip of the erosional surface and fraction of incompetent material and including the effect of an overriding thrust sheet aid movement. Doubling the viscosities doubles the bending resistance. Doubling the block thickness increases bending resistance as well, but since the force required to overcome this added resistance acts over twice the surface area, the net traction is reduced. Doubling the ramp dip greatly increases bending resistance, but if the length of the ramp is adjusted to maintain the same stratigraphic climb, this increase is lessened. The dip of the erosional surface is linearly related to the traction due to gravity in the ‘glacier analogy’ model. However, gravity cannot overcome all other sources of resistance for a sheet of the dimensions of the Pine Mountain block unless the surface slopes at 16° in the direction of transport. Increasing the fraction of incompetent material decreases bending stresses and therefore reduces the net traction necessary for movement. Finally, in the case of an overriding thrust sheet, if the upper fault zone is equal in shear strength to the lower fault zones, the resistance to movement of the lower one is cancelled; only bending resists motion. Successively overriding thrust sheets is a documented kinematic scheme in major thrust belts and the resulting applied shear stresses may be one mechanism by which stress and displacement are transferred from higher to lower sheets.
Deformation and secondary faulting near the leading edge of a thrust fault
Authors: Rodgers D. A. , Rizer W. D. 
Published: May 2008

Summary
Elastic dislocation models are used to calculate the displacements of the ground surface produced by a thrust fault climbing section. The models show that the surface uplift is asymmetrical with the steepest slope above and ahead of the fault. The gradient of the surface uplift becomes steeper and the magnitude of the uplift increases as: (1) the fault dip increases; (2) the fault becomes shallower; and (3) the shear modulus of the material above the fault decreases relative to the shear modulus of the material containing the fault. Elastic dislocation, photoelastic, and clay models are used to predict the orientation of secondary faults which might be produced by a thrust fault climbing section. In general, the secondary faults strike parallel to the master fault, but the dislocation models show that there are regions in both the hanging and foot walls where the secondary fault strikes are not parallel to the master fault. The models suggest that a thrust fault in a homogeneous medium is likely to extend itself with about the same dip, while a thrust fault moving from a stronger material into a weaker material is likely to flatten and become layer-parallel. High-angle antithetic faults or back thrusts in the hanging wall are associated with a thrust fault propagating through a homogeneous medium, but layer-parallel antithetic faults are more likely than high-angle antithetic faults in the hanging-wall when a thrust fault is propagating from a stronger to a weaker medium.
Mechanical model of thrust sheet gliding and imbrication
Authors: Mandl G. , Shippam G. K. 
Published: May 2008

Summary
A competent thrust sheet being pushed over incompetent substrata is idealized as an elastic block overriding a stack of perfectly plastic substrata. The stress fields induced in both the plastic substrata and the thrust sheet have been analysed for various thrust sheet dimensions and boundary conditions.
In the case of the plastic substrata the stress analysis accounts for the formation and location of a basal slip plane, its stepping-up to a higher gliding horizon, and for secondary faulting.
For the competent thrust sheets, finite-element analyses have located a well-defined spot where a push force of sufficient magnitude is likely to induce fracturing and, consequently, may control the formation of an imbrication thrust, assuming that faulting is not preceded by compressional folding. Some further conclusions are drawn with respect to the development and spacing of subsequent imbrication thrusts.
The rock mechanics of thrust and nappe formation
Authors: Murrell S. A. F. 
Published: May 2008

Summary
Three models of thrust formation have been proposed: sliding opposed by friction, movement by flow in a thin layer, and spreading by more general flow. The field evidence suggests that the shear stresses involved must be low. Experimental and theoretical rock mechanics provide criteria which may be able to distinguish the applicability of the three models, with the constraint of low shear stresses, and the present paper investigates this possibility.
Subduction and coeval thrust belts, with particular reference to North America
Authors: Smith A. G. 
Published: May 2008

Summary
Although thrust belts at present-day subduction zones are sub-parallel to such zones, several ancient thrust belts dip in the opposite direction to the inferred dip of the contemporaneous subduction zone. Gravity spreading, gravity sliding or compressional stresses transmitted from the subduction zone are unlikely causes of such thrust belts. The driving force is attributed instead to the hydrostatic head of a fluid-like rock welt. If the welt is hot—as in a granitic/metamorphic belt—foreland thrusts may form.
The emplacement stresses are similar in magnitude to those of gravity spreading, but the thrust wedge need not be weak. The essential requirements of a thrust belt created by a fluid welt are a downhill surface slope in the direction of thrust transport and weak decollement horizon(s).
In the Mesozoic foreland thrust belts of western North America, the volume of rock pushed onto the foreland is comparable to the volume of the batholiths in the orogenic core and to the volume of new material added by subduction to the overriding American Plate.
After collision with another continent or an island arc the recognition of thrust belts emplaced by fluid welts is more difficult, particularly when ophiolites are present.