bluuebunny/crossref_metadata_2025_split huggingface dataset:
The causes of the high-grade metamorphism of the Scourie complex, NW Scotland
Authors: Barnicoat A. C. 
Published: April 2008

Summary

A number of previous studies of the possible causes of Archaean high-grade metamorphism have suggested that the heat source was large bodies of (intermediate) magma emplaced within pre-existing crust. Geochronological data for the Lewisian indicate that there was probably a gap of
c.
200 Ma between the cessation of significant igneous activity and the peak of high-grade metamorphism. This is sufficiently long to allow conductive dissipation of magmatic heat and to render a model of the metamorphism relying upon heat transported in by intrusions unsatisfactory. The quantity of deep-derived CO
2
that would be required to make it the primary cause of the metamorphism is larger than petrological considerations allow. Alternative models, based on enhanced basal heat-flow in a static crust or heat derived from the decay of radionuclides resident within the crust coupled with a more normal mantle heat-flow and tectonic crustal-thickening have been erected, and reveal that the
P-T
conditions and age data recorded by the Scourian granulites can be satisfied by such models.

Thermal models of this nature, and a consideration of the available petrological and geochronological data, suggest a very slow cooling rate which would allow considerable intercrystalline diffusion to occur. This could give rise to the observed zoning profiles and other forms of re-equilibration found within the granulite-facies rocks of the Lewisian complex.
Petrology of Scourian supracrustal rocks and orthogneisses from Stoer, NW Scotland: implications for the geological evolution of the Lewisian complex
Authors: Cartwright I. , Barnicoat A. C. 
Published: April 2008

Summary

The Lewisian complex at Stoer comprises a diverse assemblage of paragneisses together with several generations of acid to basic orthogneiss. All lithologies are early-Scourian in age and underwent granulite-facies metamorphism in the Badcallian (
c.
2.7 Ga) event. Petrogenetic modelling coupled with geothermometry yields a
P-T
estimate for the peak of metamorphism of 920–990°C and >11 kb, as well as estimates of conditions encountered during Inverian (
c.
2.6–2.3 Ga) and Laxfordian (<1.7 Ga) retrogression. Certain lithologies, especially those of supracrustal origin, show evidence of partial melting. It is suggested that anatexis, controlled by the internal buffering of metamorphic fluids, may have been an important process in the evolution of the complex.
Early Proterozoic structure and kinematic evolution of the southern mainland Lewisian
Authors: Park R. G. , Crane A. , Niamatullah M. 
Published: April 2008

Summary

The southern mainland region of the Lewisian is a belt of intense Early Proterozoic deformation with a minimum width of 55 km, in which all the main structures trend NW-SE. Archaean structures are dominant in the central block to the northeast, and in a small enclave (the Ruadh Mheallan block) between Gairloch and Loch Torridon. Major structures within the belt include upright NW-SE folds (the Carnmore antiform, the Letterewe synform, and the Tollie and Torridon antiforms) and the Gairloch and Diabaig shear zones. The structural history is divided into six phases: (1) Inverian (2600–2400 Ma)—the whole belt represents a major steep Inverian shear zone. Structures in areas little affected by subsequent deformation indicate a N-up overthrust movement with a small dextral component (dextral transpressional), (2) Emplacement of dykes and Loch Maree Group (2400–1900 Ma)—evidence of dextral shear during dyke emplacement indicates a change to a dextral transtensional regime, (3) Laxfordian D
1
and, (4) D
2
deformation (1900–1800 Ma)—oblique dip-slip shears and asymmetric shear folds suggest a continuation of the previous dextral transtensional movements on inclined lateral ramps, connected with major gently-inclined shear zone ‘flats’, (5) Laxfordian D
3
deformation (
c.
1600 Ma)—upright folding of previously sub-horizontal fabrics and a reversal of movement to N-up on the Gairloch shear zone indicate a change to a dextral transpressional regime, which is accompanied by retrogressive metamorphism, (6) Laxfordian D
4
deformation (
c.
1400 or
c.
1000 Ma)—steeply plunging large and small-scale asymmetric folds and crush belts indicate a change to a sinistral strike-slip regime. We suggest that these phases of movement represent major changes in relative plate movement direction probably separated by relatively long periods of comparative stability.
Physical properties of Lewisian rocks: implications for deep crustal structure
Authors: Hall J. 
Published: April 2008

Summary
Data on the densities, seismic velocities, magnetization, and permeability of Lewisian rocks are summarized. These properties have been used to estimate the shallow-depth extension of exposed boundaries within the Lewisian, and between the Lewisian and other rocks; and also to predict downward changes in porosity and electrical conductivity. Geophysical surveys of the deep crust can be interpreted in terms of subsurface distributions of Lewisian, or Lewisian-like, rocks.

Among the more significant conclusions relevant to the interpretation of surface structure of the Lewisian is that the Kylesku Group rocks of the central belt in Sutherland may be isolated from deeper crustal rocks of similar composition which are characterized by
P
-wave velocities of 6.4 km s
−1
and over. Such rocks may form the mid crust below NW Scotland, shallowing north of the mainland possibly with a sharp discontinuity upwards into more acid basement rocks. West of the mainland direct evidence of the 6.4 km s
−1
rocks is missing; they appear to be absent from the upper crust and buried rather deeply, perhaps with a transitional top, in the middle or lower crust.

Granulite-facies rocks occurring in the upper crust west of the mainland are likely to be more acid than average Kylesku Group rocks. The layered basic rocks of the central belt provide an analogue of the deepest crust in NW Britain.
Permeability/porosity studies allow estimates of electrical conductivity to be made in an attempt to explain the high conductivity of the lower crust in terms of trapped saline fluids. Porosities in the deep crust may be high enough to account for deep crustal seismic reflections in areas where porous layers are sub-horizontal.
Deep seismic reflection profiling of the Lewisian foreland
Authors: Smythe D. K. 
Published: April 2008

Summary
A number of marine deep seismic reflection profiles cross the Hebridean shelf, the Lewisian foreland to the Caledonian orogen. They include the BIRPS MOIST, WINCH and DRUM profiles, several recent commercial deep lines, and many old and new conventional commercial exploration profiles. Basement, comprising the upper crust, is largely devoid of coherent seismic reflections. In contrast, the mid-crust contains many reflectors which may be relics of early Palaeozoic, Caledonian (or earlier Grenvillian) eastward-dipping thrust zones, which pass into an acoustically strongly layered lower crust.
The Outer Isles Thrust is mapped from the surface to the mid-crust, and tied into its land outcrop on north Lewis. Reactivation of this thrust as a normal fault caused the formation of the Sea of the Hebrides, Minch and North Lewis Basins. The Moho is defined by a strong band of reflections at a rather uniform 27 km depth. The eastward-dipping Flannan Thrust can be mapped into the upper mantle from about 15 to 45 km depth.
The Moine Thrust, which carries rocks of the orogen over Lewisian foreland, dips at 20–25° to the east on MOIST, and is either the westernmost of a series of easterly-dipping reflections (thrusts) which flatten or terminate at 17–20 km depth, or a more easterly thrust which structurally overlies these easterly-dipping reflectors. In neither case are the easterly-dipping reflectors themselves likely to be simply ‘Caledonised’ Lewisian foreland.
Mineralogy, petrology and geochemistry of the Scourie dykes: petrogenesis and crystallization processes in dykes intruded at depth
Authors: Tarney J. , Weaver B. L. 
Published: April 2008

Summary
The mineral chemistry, petrology and geochemistry of representative members of the Scourie dyke swarm are described, and discussed in relation to crystallization processes in deep-seated dykes and to their petrogenesis. Four types of dykes can be defined on petrological and geochemical grounds: bronzite-picrites, norites, olivine-gabbros and quartz dolerites. The latter constitute by far the most abundant dyke-type. In the central Scourian granulite zone in particular the dykes were emplaced at depth into hot country rocks; this enables the through-flow behaviour of different magma types in dyke conduits to be compared. Whereas the dolerites and norites have chilled margins, the bronzite-picrites and olivine-gabbros have coarse grained orthopyroxene-rich and augite-rich margins respectively. This probably reflects turbulent-flow conditions in the less viscous, more ultramafic magmas which permitted rapid crystallization of pyroxenes on the dyke walls during intrusion. The picrite dykes, and to a lesser extent the olivine-gabbro dykes, display additional across-dyke modal and textural variations. These are not accompanied by significant variations in mineral composition, and can largely be attributed to crystal settling in variably inclined dyke sheets.
All dykes typically show a ‘continental’ trace element signature (enrichment in light rare-earth elements and large-ion-lithophile elements), but this must have been inherited from the sub-continental lithosphere because contamination by Lewisian gneisses would not generate the observed trace element characteristics. Both trace element and mineral chemical data indicate that the dykes were derived from at least two distinct mantle sources, that supplying the picrite and norite dykes being more refractory with respect to major elements, but showing a greater relative enrichment in light REE and LIL elements.
The emplacement of the Scourie dyke swarm represents considerable crustal extension, but also poses a thermal problem in the generation of high temperature picritic magmas. It is suggested that the dykes and their compositional characteristics reflect the processes of growth and evolution of the sub-continental lithosphere, and that the extensive retrogression of the granulites, which occurred penecontemporaneously with dyke intrusion, may also be linked with these processes.
The East Greenland Nagssugtoqidian mobile belt compared with the Lewisian complex
Authors: Myers J. S. 
Published: April 2008

Summary
The rocks, structures and geological history of the Lewisian complex are similar to those of the Nagssugtoqidian mobile belt in East Greenland. Both regions comprise a heterogeneous, mainly tonalitic, gneiss complex which formed 2900–2700 Ma. Both gneiss complexes were intruded by basic dykes and intensely deformed and metamorphosed during two major episodes. The first episode, called Inverian in Scotland, is broadly equivalent to the Nag. 1 episode in Greenland dominated by major dextral strike-slip shear zones about 2600–2300 Ma. The second episode, the Laxfordian of Scotland, is broadly equivalent to the Nag. 2 episode of Greenland associated with the southward translation of sheets of rock along northward-dipping thrusts and shear zones about 1900–1700 Ma. Both gneiss complexes were raised to a high crustal level by 1600–1400 Ma and extensively eroded.
High-grade terrains in and around the Yilgarn Block of Western Australia
Authors: Myers J. S. 
Published: April 2008

Summary
Two kinds of high-grade terrain occur in the western part of the Archaean Yilgarn Block. One consists of repeatedly deformed and metamorphosed granites (3650 and 3500 Ma), layered basic rocks (3750 Ma), and siliceous metasedimentary rocks, which were metamorphosed to granulite facies at 3300 Ma. The other consists of much less deformed granites and some basic rocks which form deeper, granulite-facies equivalents of the adjacent high-level 3000–2600 Ma granite-greenstone terrain.
The Yilgarn Block is bounded by Proterozoic belts of intense deformation and high-grade metamorphism. These belts were active between 2000 and 1600 Ma and comprise both reworked Archaean and early Proterozoic rocks. Two belts were mobile again and metamorphosed to granulite or amphibolite facies between 1300 and 1000 Ma, and one of these belts was also intruded by plutonic rocks, deformed and metamorphosed to granulite facies at about 650 Ma.
The Archaean geological evolution of Enderby Land, Antarctica
Authors: Harley S. L. , Black L. P. 
Published: April 2008

Summary

The Napier Complex of Enderby Land, Antarctica, is a major terrain exhibiting Archaean granulite-facies metamorphism at extreme
PT
conditions. Combined Rb-Sr, Sm-Nd and U-Pb zircon isotope systematics define an age of 3070 Ma for initial granulite-facies metamorphism and intense D
1
deformation. Orthogeneissic sheets intruded into the tectonically thickened crust at times up to 3070 Ma resulted in the attainment of peak regional metamorphic conditions of 950°C and 7–10 kb subsequent to D
1
. These high-grade conditions prevailed through a second, less intense, deformation (D
2
) after which the Napier Complex became a stable craton for some 500 Ma. The presently exposed granulites record a near-isobaric cooling path over this time interval prior to reactivation at 700–650°C and 5–8 kb in a deformation event (D
3
) at
c.
2450 Ma. The D
3
event resulted in open dome-and-basin folding of the pre-existing recumbent- and isoclinally-folded pile. Protolith ages for some granulite-facies gneisses, obtained from conventional and ion-microprobe investigation of zircons and deduced from Sm-Nd modelling, are in the range 3750–3950 Ma, some 700–900 Ma prior to the first recognized metamorphism.
Development of extension and mixed-mode sedimentary basins
Authors: Gibbs A. 
Published: April 2008

SummaryAnalysis of onshore and offshore basins using a variety of techniques including deep seismic reflection profiles has shown that basins develop above linked fault systems. These fault systems comprise both steep and gently dipping faults which can have either dip- or strike-slip displacements. Whilst linked fault systems are common to all basins they are developed to different degrees and hence produce a variety of structural expressions within, and between the basins. This allows basins to be classified either in terms of their dominant structural elements or seen as a mechanical and geological continuum.As the basins grow, changes in crustal thickness in the upper brittle crust can take place, on staircase arrays of low-angle listric faults, on steeper planar (domino) faults or commonly on a combination of both linked by crustally conservative strike-slip systems.The development of the controlling basement elements affects the distribution and pattern of internal deformation in the evolving sedimentary fill resulting in a characteristic basin architecture. This resulting architecture has profound implications for models of the stratigraphy and sedimentology of basins, and provides an insight into the occurrence of certain facies and stratigraphic changes.A general model for the evolution of such basin architectures is presented in this paper including both the basement structural elements and the overlying carapace of sediments with both stratigraphic and structural components.
Physical models of extensional tectonics at various scales
Authors: Vendeville B. , Cobbold P. R. , Davy P. , Choukroune P. , Brun J. P. 
Published: April 2008

Summary
In a preliminary series of experiments, using physical models mechanical processes of extensional tectonics have been investigated at various scales. By a suitable choice of model materials, experiments were performed at low cost in a natural gravity field. Upper layers of the lithosphere were modelled using sand; lower layers, using silicone putties of two different densities; the mantle asthenosphere was modelled using honey. The models deformed under their own weight or under absolute horizontal tension. Rates of extension were controlled using a stepper motor. Surface deformation and faulting were monitored using 35 mm time-lapse photography. Lower lithosphere topography was photographed through the transparent asthenosphere. Fault patterns in models with lithosphere only, were observed by serial sectioning. Otherwise, the brittle-ductile interface was observed after suctioning off the sand.
Simple experiments with uniformly extended sand layers only show that; (i) spacing of normal faults is a measure of the layer thickness; (ii) the length of fault trace increases with the amount of downthrow; and (iii) faults tend to form domino domains. Some experiments with a brittle layer on a ductile substrate show a mechanism of passive rifting where; (i) major faults occur in conjugate pairs, defining rift valleys; (ii) minor faults localize additional extension in rift-valley floors; and (iii) isostatic uplift of the viscous substrate causes uplift and tilting of rift rims. In freely floating continents, gravitational spreading leads to: (i) highly localized extension and thinning at continental margins and (ii) internal rifting.
Analogue models of extensional fault geometries
Authors: McClay K. R. , Ellis P. G. 
Published: April 2008

Summary
The progressive development of extensional fault geometries in a sedimentary cover sequence above a rigid or deforming basement has been experimentally investigated using analogue models. Quartz sand (700 μm) was used as a modelling material. The experiments were recorded using time-delay 16 mm cine photography and 35 mm photography. Final models were impregnated with resin and serially sectioned to investigate the three-dimensional fault geometry. Four series of experiments were carried out: (i) extension above a linear basement dislocation; (ii) extension above a uniformly stretching basement; (iii) extension controlled by a planar fault; and (iv) extension controlled by a listric fault.
Extension above a linear basement dislocation produced a single graben structure in which initial high-angle bounding faults were cut by later listric faults. Fault nucleation occurred into the hanging wall of the graben and only minor rotation occurred in the fault blocks within the graben. Extension above a uniformly extending basement produced a variety of fault structures. Heterogeneous nucleation of faults occurred with initial planar geometries giving way to more listric faults. Significant fault-block rotation was observed. In some instances rotation of pre-existing fault planes produced a negative (convex upward) listric-fault geometry. Extension controlled by a planar fault produced a single graben structure in which new faults developed into the graben. Little rotation of fault blocks occurred. Extension controlled by a predetermined listric fault showed the progressive development of a rollover anticline. The crest of the anticline collapsed producing a second-order crestal collapse graben. The nucleation of new faults was consistently in the hanging wall above the major detachment faults.
In all of the experiments carried out to date, once a major fault had developed the second-order fault nucleation was consistently in the hanging wall fault block. Footwall collapse and hence fault migration into the footwall was not significant. Listric faults produced considerable rotation of hanging wall blocks. In some instances heterogeneous rotation of preexisting faults generated negative listric-fault shapes. In rollover structures collapse of the crestal region of the fold produced a second-order graben structure.
Brittle modes of foreland extension
Authors: Hancock P. L. , Bevan T. G. 
Published: April 2008

Summary
Complex systems of mesoscopic brittle fractures striking transversley to orogenic margins are characteristic upper-crustal structures in forelands (and hinterlands) that have experienced extension as a consequence of continental collision. They are especially well developed in foreland peripheries and some forelands lacking major grabens and strike-slip faults, the best-known expressions of horizontal extension. Despite the contribution of the mesofractures to foreland strain being less than that achieved by macrofault zones they are important structures because: (i) they are widespread and pervasive at outcrop scale, (ii) they influence the location of some major topographic lineaments and (iii) they permit regionally significant extension directions to be determined where macrostructures are absent.
The principal styles of mesoscopic brittle structures are planar normal faults, vertical extension joints and veins, and steeply inclined, conjugate hybrid and shear joints. Commonly, the strike of fracture sets is uniform throughout large areas and although it is generally normal to a deformation front, this simple relationship is not present everywhere, particularly distant from some markedly arcuate fronts.
Two examples of parts of forelands containing brittle extensional structures approximately coeval with late-Alpide events are the European platform in S England and N France, and the Arabian platform neighbouring the Gulf coast. In both regions, the fractures are superimposed obliquely on older Tertiary structures and comprise dominant sets of extension fractures accompanied locally by normal faults and hybrid joints. The dominance of extension and hybrid fractures among the failure modes indicates that differential stresses remained low throughout large areas of the forelands and that the effective minimum stress was tensile.
Tectonic heredity and the layered lower crust in the Basin and Range Province, western United States
Authors: Allmendinger Richard W. , Oliver Jack , Hauge Thomas A. , Hauser Ernest C. , Potter Christopher J. 
Published: April 2008

SummaryCOCORP deep seismic reflection data were collected in a transect at 40°N latitude across the entire Basin and Range extensional province and its boundaries, the Sierra Nevada Mountains and the Colorado Plateau. In general, the data display little of the province-wide symmetry of the modern Basin and Range, and instead can be more directly related to the pre-17 Ma asymmetric orogens of the western United States, particularly in the eastern part (112–115°W) of the region. No one model of intracontinental extension is applicable to the entire province: it may be that pre-existing structure of the continental crust predisposes it to a particular mode of extension unless thermally activated processes render the crust mechanically isotropic. Such processes (e.g. enhanced ductility, magmatism, etc.) may be responsible for the development of a highly layered lower crust and pronounced reflection Moho; features not commonly seen in reflection profiles from cratonic regions.