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Showing papers in "Journal of the Geological Society in 1977"


Journal ArticleDOI
TL;DR: In this paper, physical factors likely to affect the genesis of the various fault rocks are examined in relation to the energy budget of fault zones, the main velocity modes of faulting and the type of fault, whether thrust, wrench, or normal.
Abstract: Physical factors likely to affect the genesis of the various fault rocks—frictional properties, temperature, effective stress normal to the fault and differential stress—are examined in relation to the energy budget of fault zones, the main velocity modes of faulting and the type of faulting, whether thrust, wrench, or normal. In a conceptual model of a major fault zone cutting crystalline quartzo-feldspathic crust, a zone of elastico-frictional (EF) behaviour generating random-fabric fault rocks (gouge—breccia—cataclasite series—pseudotachylyte) overlies a region where quasi-plastic (QP) processes of rock deformation operate in ductile shear zones with the production of mylonite series rocks possessing strong tectonite fabrics. In some cases, fault rocks developed by transient seismic faulting can be distinguished from those generated by slow aseismic shear. Random-fabric fault rocks may form as a result of seismic faulting within the ductile shear zones from time to time, but tend to be obliterated by continued shearing. Resistance to shear within the fault zone reaches a peak value (greatest for thrusts and least for normal faults) around the EF/QP transition level, which for normal geothermal gradients and an adequate supply of water, occurs at depths of 10–15 km.

1,948 citations


Journal ArticleDOI
TL;DR: In this paper, the carbon and oxygen isotopic composition of carbonate sediments, limestones and calcite cements are compiled and reviewed, and an elementary exposition of the factors that control them is given.
Abstract: Data on the carbon and oxygen isotopic composition of the carbonate sediments, limestones and calcite cements are compiled and reviewed, and an elementary exposition of the factors that control them is given. Typical compositions and diagenetic trends are displayed on δO 18 –δC 13 scatter diagrams, and an attempt is made at an isotopic categorization of environments of carbonate lithification. Many limestones pass through several diagenetic environments, which are recorded isotopically by cement generations of distinct isotopic composition. Most limestones contain essentially 9marine9 carbon, but extreme compositions can result from organic reactions, especially those involving methane; some of these may be important in petroleum exploration, but their volumetric abundance is probably quite small.

642 citations


Journal ArticleDOI
TL;DR: In this paper, a wide variety of granitic batholiths of various ages have been obtained and the primary δD values of the biotites and hornblendes are remarkably constant at about −50 to −85, identical to the values in regional metamorphic rocks, marine sediments and greeenstones.
Abstract: D\H and 18 O\ 16 O data have now been obtained on a wide variety of granitic batholiths of various ages. The primary δD values of the biotites and hornblendes are remarkably constant at about –50 to –85, identical to the values in regional metamorphic rocks, marine sediments and greeenstones, and most weathering products in temperate climates. Therefore the primary H 2 O in these igneous rocks is probably not ‘juvenile’, but is ultimately derived by dehydration and/or partial melting of the lower crust or subducated lithosphere. Most granitic rocks have δ 18 O = + 7.0 to +10.0, probably indicating significant involvement of high - 18 O metasedimentary or altered volcanic rocks in the melting process; such an origin is required for many other granodiorites and tonaloites that have δ 18 O = + 10 to +13. Gigantic meteoric-hydrothermal convective circulation systems were established in the epizonal portions of all batholiths, locally producing very low δ 18 O values (particularly in feldspars) during subsolidus exchange. Some granitic plutons in such environments also were emplaced as low- 18 O magmas probably formed by melting or assimilation of hydrothermally altered roofrocks. However, the water/rockratios were typically low enough that over wide areas the only evidence for meteoric H 2 O exchange in the batholiths is given by low D/H ratios (δD as low as –180); for example, because of latitudinal isotopic variations in meteoric waters, as one moves from through the Cordilleran batholiths of western North America an increasingly higher proportion of the granitic rocks haves δD values lower than –120. The lowering of δD values commonly correlates

598 citations


Journal ArticleDOI
TL;DR: The mineral facies of rocks reaching the surface do not reflect any one geotherm through the pile but lie on a locus of P-T conditions, the metamorphic geotherms which will generally be concave towards the temperature axis as mentioned in this paper.
Abstract: Metamorphism of tectonically thickened continental crust or subducted sediment wedges is likely to take place in a thermal regime where temperature increases by conductive relaxation whilst concurrently pressure decreases by erosion of the pile. The mineral facies of rocks reaching the surface do not reflect any one geotherm through the pile but lie on a locus of P–T conditions, the metamorphic geotherm, which will generally be concave towards the temperature axis. Maximum pressures on the metamorphic geotherm are significantly less than maximum pressures experienced by rocks during the early stages of recrystallization. The metamorphic geotherm is polychrome, points at lower temperatures reflecting conditions earlier in the development than those at higher temperature; crustal melts are developed after low-medium temperature metamorphism and the amount of such melts could be significant. Blueschists develop on the low temperature end of the metamorphic geotherm and are succeeded in exposure at the surface by greenschist- or amphibolite-facies rocks; the time-scale for this process is consistent with the virtual absence of Precambrian blueschists. Crust thickened by addition of hot magma is likely to yield a metamorphic geotherm convex towards the temperature axis. Recognition of differently curving metamorphic geotherms can be used to assess the part played by magmatic activity in older metamorphic terrains.

544 citations


Journal ArticleDOI
TL;DR: Basaltic oceanic crust overlies a 20-30 km thick zone of depleted upper mantle, which is largely free of the high density phase, garnet, and has a higher MgO/FeO ratio, and in consequence has a lower density ( Δ p = 0 06 gm/cm 3 ), than the undepleted mantle of the lower lithosphere.
Abstract: Partial melting processes at mid-ocean ridges generate oceanic lithosphere which is chemically and mineralogically zoned. Basaltic oceanic crust overlies a 20–30 km thick zone of depleted upper mantle. This zone has undergone partial melting and is largely free of the high density phase, garnet, has a higher MgO/FeO ratio, and in consequence has a lower density ( Δ p = 0 06 gm/cm 3 ), than the undepleted mantle of the lower lithosphere. The lithosphere does not become gravitationally unstable upon the asthenosphere for 40–50 Ma, when increased density resulting from thermal contraction has offset the compositional buoyancy of the depleted zone and crust. During subduction the basaltic crust inverts to eclogite and the net compositional buoyancy of the lithosphere is eliminated. However, as the subducted lithosphere is heated it becomes less rigid and density differences both between different parts of the descending lithosphere and the surrounding mantle become important. The dense eclogite layer sinks through the underlying depleted zone at a rate determined by the temperature-dependent rheology. With further heating the depleted zone becomes less dense than the overlying undepleted mantle and will diapirically rise some 300-400 km behind the trench depending upon the angle and rate of subduction and the age of the subducted plate. Such diapirs are able to intitiate behind-arc spreading. In a continental setting the diapirs could both heat the lithosphere and produce exceptional elevation.

302 citations


Journal ArticleDOI
TL;DR: Precambrian granulites are representative of the lower continental crust and are refractory and low in heat-producing elements, being 9depleted9 in K, Rb, Cs, U and Th as mentioned in this paper.
Abstract: Precambrian granulites are representative of the lower continental crust. They are, relative to upper crustal gneisses, refractory and low in heat-producing elements, being 9depleted9 in K, Rb, Cs, U and Th; they have normal or even higher than normal abundances of Zr, Sr, and Ba, highly fractionated REE patterns and high Ba/Rb, Ba/Sr, Ce/Yb, K/Cs and K/Rb ratios. Having low Rb/Sr, U/Pb and Sm/Nd ratios they are likely to be unradiogenic with respect to Sr, Pb and Nd isotopes. They do not seem to have marked positive Eu anomalies which might compensate for the significant negative Eu anomalies observed in upper crustal granitic rocks and sediments. The granulites formed largely under an intermediate goethermal gradient with P of 7–12 kbar and T of 700–1000°C. The continental crust completed a substantial part of its growth by 2500 Ma ago. We suggest that growth took place at Cordillerantype continental margins with underthrusting of oceanic crust, generation and underplating of extensive calc-alkaline tonalitic-granodioritic material containing early remnants of hornblende gabbro/calcic anorthosite complexes, and that it was associated with widespread nappe stacking and imbricate interthrusting. This crustal generation process culminated in the granulite metamorphism deep in the tectonically and magmatically thickened continents.

201 citations


Journal ArticleDOI
TL;DR: In this article, the authors measured oxygen and hydrogen isotope compositions on 63 whole-rock samples of both fresh and altered granitoids and xenoliths and found that the average δD values (and water contents) are −62 ± 4 (1.10%) and −77 ± 12 (0.73%) for both igneous and sedimentary granitoid types, respectively.
Abstract: Two contrasting types of Palaeozoic granitoids are of widespread occurrence in SE Australia and can generally be distinguished by their chemistry, mineralogy, field relations and initial strontium isotope ratios. Chappell & White (1974) have proposed that the granitoids are derived by partial melting of two different types of source materials: (1) igneous or ‘I-type,’ and (2) sedimentary or ‘S-type.’ Oxygen and hydrogen isotope compositions were measured on 63 whole-rock samples of both fresh and altered granitoids and xenoliths. Analyses were also made of representative mineral separates from these rocks. For ‘S-type’ plutons, average δ 18 O values range from 9.9 to10.5, whereas for the ‘I-types’ the range is 7.9 to 9.4 Xenoliths are about 1 per mil depleted in 18 O and generally enriched in D relative to the hose rocks. The average δD values (and water contents) are –62 ± 4 (1.10%) and –77 ± 12 (0.73%) for ‘S’ and ‘I’ granitoids, respectively. Individual δD values range from –50 to –102 and correlate well with water contents: the more water-rich the granitoid the greater the deuterium content. Per mil 18 O fractionations between quartz and biotite are rather large ranging from 5.0 to 6.9 (independent of granitoid type), with a mean of 6.0. Thus typical isotope temperatures of about 520°C are inferred and retrograde effects are indicated.

194 citations


Journal ArticleDOI
TL;DR: In this article, field and petrographic evidence indicate an intra-oceanic island arc environment for the mainly volcanoclastic sediments and geochemical data suggest that magmatic products were generated at or above easterly inclined Benioff zones.
Abstract: The crystalline basement of NE Africa and Arabia has all the geophysical and geological characteristics of continental crust. But, field and petrographic evidence indicate an intraoceanic island arc environment for the mainly volcanoclastic sediments and geochemical data suggest that magmatic products were generated at or above easterly inclined Benioff zones. Some of the mafic-ultramafic masses that lie in NE zones across the region have been identified as ophiolites and the zones are thought to mark the approximate margin of individual arc systems; four or five arc systems are identified. The episodic evolution of these Proterozoic island arcs took c. 600 Ma (1100–500 Ma) before the continental character of the area was attained. As calc-alkaline igneous rocks and intervening ophiolite bodies occur elsewhere in northern Africa, the Pan African island arc model may have wider application.

172 citations


Journal ArticleDOI
TL;DR: A brief review of the literature on pressure solution is given in this article, where the mechanisms of pressure solution and Coble creep are discussed and the thermodynamics of non-hydrostatically stressed solids are discussed.
Abstract: A brief review of research on pressure solution is given. The mechanisms of pressure solution and Coble creep are discussed. Deformation by diffusive mass transfer processes is generally accompanied by grain boundary sliding and this will have important effects on the textures and microstructures produced during deformation. Experiments using relaxation testing seem promising in allowing access to the slow strain rates necessary to observe pressure solution phenomena. The thermodynamics of non-hydrostatically stressed solids is discussed and an analysis of possible diffusion paths in rocks is presented. Evaluation of theoretical rate equations for pressure solution and Coble creep indicates that pressure solution in fine-grained quartz and calcite rocks may give rise to geological strain rates at temperatures from 200–350°C. Coble creep is expected to give rise to geological strain rates in fine-grained galena at low temperatures and also in fine-grained calcite rocks at temperatures around 350°C. The chemistry of natural rock pressure solution systems is expected to have significant effects and needs further detailed study. Further research is needed to elucidate the nature and role of grain boundaries during diffusive mass transfer. Pressure solution phenomena are important in the compaction behaviour of some petroleum reservoir rocks and perhaps in some faults where sliding is accommodated by pressure solution.

144 citations


Journal ArticleDOI
TL;DR: In this paper, it was suggested that these differences are not a result of simple partial melting, fractional crystallization, zone refining, residual minerals or isotopic disequilibrium between minerals and melt during melting, but have existed for some 1500 Ma or more.
Abstract: The suboceanic mantle has on the basis of incompatible element and isotope ratios two chemically distinct units; the mantle source for ocean island basalts and the mantle source for ocean ridge basalts. Ocean island alkali basalts have an enriched light rare earth element (REE) source with K/Rb = 400, K/Ba = 28, Zr/Nb = 6.6. Ocean ridge tholeiites have a depleted light REE source, with K/Rb = 1060, K/Ba = no, and Zr/Nb = 37. 87 Sr/ 86 Sr and 206 Pb/ 204 Pb ratios are generally lower in the ocean ridge basalts than in the basalts from ocean islands. It is suggested that these differences are not a result of simple partial melting, fractional crystallization, zone refining, residual minerals or isotopic disequilibrium between minerals and melt during melting. Instead, the differences between the sources are real, and have existed for some 1500 Ma or more. Nephelinite, alkali basalt, and tholeiites from ocean islands have similar isotopic and incompatible element ratios. The nephelinites and alkali basalts are, however, progressively much more enriched in the light REE than are the tholeiites, yet the light REE in nephelinites and alkali basalts appear to be incompatible. This may be explained by the occurrence of small (average 4 cm thick), closely spaced pyroxenite veins (within tens of cm) in the mantle source for ocean island basalts. The occurrence of a network of veins which melt preferentially may lead to melts which have apparent sources with a much greater enrichment in incompatible elements than the total mantle source and may also give the appearance of a source whose ratio for slightly compatible elements changes as a function of extent of melting.

140 citations


Journal ArticleDOI
TL;DR: In this article, the morphology and structure of a large submarine slump (Agulhas Slump) on the sheared continental margin off SE Africa are described from bathymetric and continuous seismic reflection records, which is a composite feature consisting of proximal and distal allochthonous sediment masses separated by a large glide plane scar.
Abstract: The morphology and structure of a large submarine slump (Agulhas Slump) on the sheared continental margin off SE Africa are described from bathymetric and continuous seismic reflection records. It is a composite feature consisting of proximal and distal allochthonous sediment masses separated by a large glide plane scar. The locations of the various structural elements of the slump are related to underlying features: in the head region these are controlled by large-scale Cretaceous and Palaeogene depositional features, and in the toe region by older basement ridges. In the western part of the slump, the basement ridges have dammed the slump over continental crust, whereas in the east, allochthonous material has spread into the oceanic Transkei Basin. Characteristics related to a structural setting on a sheared continental margin are emphasized and discussed. The Agulhas Slump is probably the largest slumped mass so far recognized from modem oceans (750 km long, 106 km wide, with a volume of over 20,000 km3) and is post-Pliocene in age. A seismic triggering mechanism is tentatively proposed: the slump lies on two major fault zones whose extensions are known to be seismically active (the Cape Fold Belt and the Agulhas marginal fracture zone).

Journal ArticleDOI
TL;DR: The application of stable isotope fractionation to geothermometric problems is introduced through considerations of statistical thermodynamics first developed by H. C. Urey as discussed by the authors, and a set of thermometric equation is derived for the most common mineral constituents of rocks.
Abstract: The application of stable isotope fractionation to geothermometric problems is introduced through considerations of statistical thermodynamics first developed by H. C. Urey. The available calibrations of isotope fractionation between species of geochemical interest are reviewed. These calibrations are discussed in the light of simple theoretical considerations, and a new set of thermometric equation is derived for the most common mineral constituents of rocks. Examples of application are given for igneous and metamorphic rocks. The significance of ‘isotopic’ temperatures is discussed in terms of environmental and kinetic considerations. As an example of possible future developments we derive the relationship between observed isotope temperatures and the speed of ascent of a rock system during a postorogenic isostatic readjustment.

Journal ArticleDOI
TL;DR: Palaeomagnetic measurements on thirtynine samples of dyke rocks, lavas, tuffs, granites, hornfels and metatuff, all believed to be late Cretaceous, yield a mean palaeomalagnetic pole at 21°E 41°N, and a palaeolatitude for West Kalimantan of 0° as discussed by the authors.
Abstract: Potassium-argon ages of biotite and hornblende from sixteen samples of mainly granitic plutonic rocks from West Kalimantan indicate a complex history of magmatism and cooling from the Middle Jurassic to late Cretaceous, with most ages in the range 75 to 112 Ma. Samples from the Schwaner Zone in the northern part of the area give measured ages ranging from 76 to 116 Ma. In the southern region most samples are in the range 79 to 86 Ma, but two are older (127 Ma, early Cretaceous, and 153 Ma, Middle Jurassic). In both areas, concordant ages from biotite and hornblende indicate a well-defined event at about 80 to 85 Ma, which has also been recognized to the north in Sarawak, on islands of the South China Sea, in the Malay Peninsula, south Sumatra, and in the Java Sea. Palaeomagnetic measurements on thirty-nine samples of dyke rocks, lavas, tuffs, granites, hornfels and metatuff, all believed to be late Cretaceous, yield a mean palaeomagnetic pole at 21°E 41°N, and a palaeolatitude for West Kalimantan of 0°. The palaeomagnetic pole does not differ significantly from the Cretaceous pole estimated for the Malay Peninsula. It is concluded that since the Middle Cretaceous West Kalimantan and the Malay Peninsula have behaved as a unit, have remained in much their present latitude, but have rotated anticlockwise about 50 .

Journal ArticleDOI
TL;DR: In this paper, it was shown that the presence of an unstable bulk density gradient dating from the time of deposition is not necessary for the manifestation in sediments of this type of Rayleigh-Taylor instability.
Abstract: Gravitationally unstable vertical gradients of bulk density can arise during the re-sedimentation of water-logged normally graded sand beds following liquidization, even though the density gradient imposed on the beds by the depositional process was originally stable. Convolute lamination, found chiefly in turbidites, is therefore capable of being formed under a wider range of circumstances than was formerly appreciated, and the presence of an unstable bulk density gradient dating from the time of deposition is not necessary for the manifestation in sediments of this type of Rayleigh–Taylor instability. Although the analysis promoting these conclusions cannot at present be tested experimentally because of practical difficulties, a laboratory study of the analogous sudden heating of a layer of viscous liquid from below affords some evidence in its support.

Journal ArticleDOI
TL;DR: In this paper, D/H and 18 O/16 O ratios were determined on minerals and whole-rock samples of fresh granites, granites altered during greisenization and kaolinization of the Hercynian Cornubian batholith, detrital kaolinitic sediments of the Bovey Formation, and modern meteoric waters.
Abstract: D/H and 18 O/ 16 O ratios were determined on minerals and whole-rock samples of fresh granites, granites altered during greisenization and kaolinization of the Hercynian Cornubian batholith, detrital kaolinitic sediments of the Bovey Formation, and modern meteoric waters. The granites are high −18 O rocks with δ 18 O = 10.8 to 13.2‰ due to melting, assimilation and/or exchange with argillaceous-rich metasediments at depth. Vein-controlled greisening was dominated by meteoric-hydrothermal fluids, Kaolinites from the major china clay deposits are of weathering origin. They are isotopically consistent with having formed in a tropical to warm temperature climate during the Cretaceous-Tertiary. Although intense supergene kaolinization was probably localized by earlier post-magmatic processes, there is no evidence that china clay kaolinites originally formed during hydrothermal activity and subsequently underwent post-formational isotopic exchange.

Journal ArticleDOI
TL;DR: In this paper, upper crustal data obtained during the LISPB seismic experiment reveals the velocity structure of the pre-Caledonian basement in northern Britain, with a relatively high seismic velocity (> 6.4 km/s) extending from the Caledonian foreland into the Midland Valley and probably terminates at the Southern Uplands fault.
Abstract: Interpretation of upper crustal data obtained during the LISPB seismic experiment reveals the velocity structure of the pre-Caledonian basement in northern Britain. Lewisian-like basement with a relatively high seismic velocity (> 6.4 km/s) extends from the Caledonian foreland into the Midland Valley and probably terminates at the Southern Uplands fault. To the south, beneath northern England, the basement has a lower velocity (

Journal ArticleDOI
TL;DR: The Barham Arctic Structure Soil and the associated Barham Loess were formed during the early Anglian Glacial as discussed by the authors, which is the basis of a revised Middle Pleistocene stratigraphy which comprises a rubified sol lessive formed in warm temperate conditions, which was altered to an arctic structure soil by periglacial processes.
Abstract: A conspicuous horizon of rubification, clay and iron enrichment, involutions and ice-wedge casts is attributed to two stages of soil formation. It comprises a rubified sol lessive formed in warm temperate conditions, which was altered to an arctic structure soil by periglacial processes. These palaeosols indicate that the sand and gravel deposits beneath the Lowestoft Till comprise two units, and provide the basis of a revised Middle Pleistocene stratigraphy. The Kesgrave Sands and Gravels were formed during the Beestonian in a periglacial environment by a major river which drained towards the north-east. Subsequently the Valley Farm Rubified Sol Lessive was formed during the Cromerian Interglacial, and the Barham Arctic Structure Soil and the associated Barham Loess were formed during the early Anglian Glacial. Both soils and the wind-blown sediments were formed on low relief terrace topography and are the remnants of a formerly extensive land surface. This feature was subsequently trimmed by meltwater rivers, then overlain by the Barham (outwash) Sands and Gravels and the Lowestoft Till which were deposited during the Anglian Glaciation.

Journal ArticleDOI
TL;DR: The Scourie gneisses equilibrated their strontium isotopes to upper mantle ratios 2.8 Ga (1 Ga = 10 9 years) ago at the base of a c. 50 km thick crust as mentioned in this paper.
Abstract: General considerations of the mineral assemblages likely to be developed at the crust-mantle boundary, the manner in which such rocks are brought to the surface and the criteria for identifying them are followed by a specific model of events in the Scourie gneiss complex. The Scourie gneisses equilibrated their strontium isotopes to upper mantle ratios 2.8 Ga (1 Ga = 10 9 years) ago at the base of a c. 50 km thick crust. Conditions of 15 kbar, 1250°C with low water fugacity, imply thick crust (c. 50 kin) very high crustal geothermal gradient (c. 25°C/km over 50 km) and probable residual character of the entire complex due to loss of granitic fractions. Recovery history was marked by (i) rapid reduction in pressure to c. 11 kbar at high temperature (consistent with near adiabatic uplift) followed by (ii) rapid fall in temperature to c. 450°C at pressures of 11–7 kbar (consistent with rapid cooling of the outer parts of the earth in the late Archaean) prior to dyke intrusions at 2.2–2.4 Ga. Subsequent thermal history until surface exposure 10 Ga ago is poorly marked by mineralogical events. Closure of different mineral systems to chemical change at several stages allows the thermal history of excavation to be unravelled.

Journal ArticleDOI
TL;DR: A 14 point Rb/Sr whole rock isochron obtained from the Morar Pelite in the Northern Highlands suggests an age of 1024± 96 Ma for the main metamorphism.
Abstract: A 14 point Rb/Sr whole rock isochron obtained from the Morar Pelite in the Morar area of the Northern Highlands suggests an age of 1024± 96 Ma for the main metamorphism of the Morar Moines. Within error this age overlaps that recently given for the Ardgour Gneiss and thus confirms the existence of Grenvillian elements within the Northern Highland Block. It suggests that the tectono-metamorphic history of the southwestern Moines is more complex than hitherto thought.

Journal ArticleDOI
TL;DR: A series of model experiments which were designed to simulate certain aspects of the development of large-scale folds in bedded sedimentary rocks are described and illustrated in this paper, where the models used were multilayers made up of thin sheets of gelatine.
Abstract: A series of model experiments which were designed to simulate certain aspects of the development of large-scale folds in bedded sedimentary rocks are described and illustrated. The models used were multilayers made up of thin sheets of gelatine. Two types of experiment were carried out. In the first a section of the base of a horizontally layered model was slowly raised to simulate uplift and gravity gliding. In the second a horizontally layered model was laterally compressed by a slow moving piston. The initial development of lateral compressive stress in both types of model was achieved by the propagation of a stress front along the length of a model. In a number of experiments this stress front played an important role in localizing the initiation of folds. The existence and rate of propagation of the stress front were related to the presence of shear stresses along the base of a model. The development of folds in the majority of experiments resulted in well defined anticlines separated by broad, poorly defined synclinal zones. Individual anticlines were initiated at different times during the course of an experiment and this resulted either in the serial development of a train of folds or in the development of isolated anticlines. Asymmetric anticlines were, during the initial stages of amplification, upright symmetrical structures. Asymmetry tended to develop late in the amplification history of such folds. In three dimensions, folds tend to be non-cylindrical and adjacent structures form an array of en echelon periclines.

Journal ArticleDOI
TL;DR: In this article, it is suggested that the geometry and products of ductile and brittle displacements can be examined in eroded Precambrian faults, and the Ikertoq shear belt in western Greenland is cited as an example of how, due to uplift and erosion, all the deformation products which occur through the vertical extent of fault systems and the geometry of deformation processes associated with the formation of these products, can be observed at the present erosion level.
Abstract: Deformation in the brittle (seismic) mode is characteristic of the upper parts of fault systems, while at lower levels displacements are predominantly ductile (aseismic). It is suggested that the geometry and products of ductile and brittle displacements can be examined in eroded Precambrian faults. The Ikertoq shear belt in western Greenland is cited as an example of how, due to uplift and erosion, all the deformation products which occur through the vertical extent of fault systems, and the geometry of deformation processes associated with the formation of these products, can be observed at the present erosion level in a Precambrian shear belt. It is argued that this circumstance allows the use of Precambrian faults to infer the various deformation processes occurring in modern, highly active, fault systems.

Journal ArticleDOI
TL;DR: In this paper, it is concluded that the structure and thickness of the continental slab modulates the escape of heat and volatiles from the deep mantle, and the lithosphere thus also controls the character of the magmatism.
Abstract: The proposition that intra-continental magmatism is caused by plumes which have fixed sources below the lithosphere is critically examined and found to be false. Repetition of alkaline/carbonatite activity at continental rift intersections, over periods of large-scale plate movement, demands that the structural fabric of the lithosphere is determining the site of the magmatism. It is concluded that the structure and thickness of the continental slab modulate the escape of heat and volatiles from the deep mantle, and the lithosphere thus also controls the character of the magmatism.

Journal ArticleDOI
TL;DR: A series of transcurrent faults with northerly trend is well exposed in the cliffs of Shetland as mentioned in this paper, including the Walls Boundary Fault and the Nesting Fault.
Abstract: A series of transcurrent faults with northerly trend is well exposed in the cliffs of Shetland. The Walls Boundary Fault which cuts through the middle of Shetland has a continuously changing trend so that it forms a very flattened S. Dextral movement along the fault seems to have created further dextral transcurrent faults, including the Nesting Fault, across the eastern concavity in the trend. These main faults lie within broad zones of cataclasis, subsidiary faulting and local folding. The offsets on the subsidiary faults are very much less than on the main faults, and the crushed rocks have isotropic fabrics. The zones probably arose during faulting from varying local stresses caused by the interaction of the unevennesses on the two sides of the non-planar main faults. Gouge in the Walls Boundary Fault contains analcite. Gouge-like laumontite in subsidiary shears was probably formed by mechanochemical reactions. The occurrence of these minerals and of blastomylonitic scapolite veins near the Walls Boundary Fault may indicate up to three phases of movement taking place between Cretaceous and Devonian times under different depths of overburden. Slices of secondarily cataclastic mylonite occurring along the Walls Boundary Fault are probably relics of the Great Glen Fault now partially cut out by the nearly coincident Walls Boundary Fault.

Journal ArticleDOI
TL;DR: In this article, the Moni Melange of southern Cyprus consists of two separate allochthonous components: a variety of Triassic to Cretaceous sedimentary and volcanic rocks representative of an original continental margin sequence, and substantial sheets of serpentinite derived from Upper Cret Jurassic Troodos-type oceanic crust.
Abstract: Detailed mapping has shown that the Moni Melange of southern Cyprus consists of two separate allochthonous components: a variety of Triassic to Cretaceous sedimentary and volcanic rocks representative of an original continental margin sequence, and substantial sheets of serpentinite derived from Upper Cretaceous Troodos-type oceanic crust. All these allochthonous rocks were emplaced by sliding into an in situ host matrix of Upper Cretaceous bentonitic clays and radiolarian siltstones of deep water hemipelagic facies. The postulated geotectonic setting involves a phase of late Cretaceous northward subduction of Troodos-type oceanic crust beneath a continental margin now located in southern Turkey. Subduction culminated in the Maastrichtian in a major collision of oceanic crust with the adjacent continental margin. This triggered off major oceanward gravity sliding of continental margin rocks. The collision was also associated with deep tectonic slicing of Troodos oceanic crust with the result that sheets of partly serpentinized ultramafic rocks were emplaced by sliding into the host sediments of the melange from the opposite direction to the other olistoliths. Soon afterwards there was a brief period of submarine erosion, then local tectonic movements ceased, suggesting an end to subduction in the area.

Journal ArticleDOI
TL;DR: The magnetic properties of oceanic basalt samples from 55 DSDP sites (301 samples) and 112 dredge haul sites (311 samples) show wide variations, indicative of heterogeneity in the basaltic layer as mentioned in this paper.
Abstract: The magnetic properties of oceanic basalt samples from 55 DSDP sites (301 samples) and 112 dredge haul sites (311 samples) show wide variations, indicative of heterogeneity in the basaltic layer. Remanent intensity and Konigsberger ratio are log-normally distributed and range over three orders of magnitude for each collection. For DSDP samples between-site variation is greater than within-site variation, suggesting horizontal variation of magnetization within the source layer of magnetic anomalies. Crustal magnetization estimates from inversion of short-wavelength oceanic magnetic anomalies may therefore be in error. By using arithmetic means rather than the geometric means appropriate for log-normal distributions, estimates of the magnetization of Layer 2 of around 4 × 10 −3 Gauss are obtained for the Indian, Pacific and north Atlantic oceans and Caribbean Sea, while lower averages are found in the south Atlantic and Antarctic oceans. Using this as a representative value for abyssal areas, the thickness of the magnetic source layer is greater than commonly assumed, and probably more than a kilometre. This interpretation is supported by DSDP data from some individual sites, notably the deep holes of Leg 37. The DSDP median destructive fields average only 123 Oe. Although many sites have much higher stabilities than this, there is evidence for unstable NRM at many sites where samples exhibit a strong tendency to acquire viscous remanence components. The low coercivity secondary magnetizations confuse identification of the stable direction during magnetic cleaning. Acquisition of high coercivity secondary remanence (CRM) as a result of maghemitization, which is extensive in the DSDP collection, may also have altered the original NRM direction. Remanent inclination data from four Tertiary DSDP sites (ages 38–40 Ma) have been combined to give a tentative Eocene VGP for east Antarctica at 68°S 64°E. It has not been possible to compare DSDP data in this way at other sites. Palaeolatitudes deduced from DSDP basalt remanent inclinations show large discrepancies from values expected from plate reconstructions at many sites. Incompletely removed secondary magnetizations, tectonic effects and palaeo-reconstruction errors, alone or in combinations, may be responsible for these discrepancies. A major source of deviation from the expected direction may be the rapid cooling of the basalt which does not allow adequate averaging of secular variation.

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TL;DR: In this paper, the upper pillow lavas of the Troodos Igneous Complex are directly overlain by up to 75°m of volcaniclastic sediments, ranging in age from Lower Campanian to Mid-Maastriehtian.
Abstract: In SW Cyprus the upper pillow lavas of the Troodos Igneous Complex are directly overlain by up to 75° m of volcaniclastic sediments, ranging in age from Lower Campanian to Mid-Maastriehtian. The basal sediments are pre-dominantly non-calcareous bentonitic clays and radiolarian mudstones representative of relatively slow deposition, probably below the carbonate compensation depth. In contrast, stratigraphically higher calcareous bentonitic clays, volcaniclastic siltstones and volcaniclastic sandstones show evidence of much more rapid deposition by a combination of mass flow, traction and minor turbidity currents. The basal volcaniclastic sandstones were partly derived by submarine erosion of the subjacent Troodos lavas. In contrast, the overlying sequences contain abundant fine-grained pumice, basic to acid volcanics, and also terrigenous material derived predominantly from outside the present area of southern Cyprus. The hetero-geneous provenance of the Kannaviou sediments can be explained by postulating late Cretaceous subduction of Troodos-type oceanic-crust with genesis of a related volcanic arc and eventual tectonic juxtaposition of the Troodos Massif with a Mesozoic continental margin.

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TL;DR: In this article, it was pointed out that the initiation of movement of the block can be explained if it is assumed that the fluid pressure, below the block, locally and for short periods exceeds the gravitational load.
Abstract: Since the classic work by Hubbert & Rubey it has become generally accepted that thin but extensive sheets, or blocks of rock may glide down a gentle slope only when the fluid pressure beneath the block is abnormally high. It is pointed out that the initiation of movement of the block can be explained if it is assumed that the fluid pressure, below the block, locally and for short periods exceeds the gravitational load. Furthermore, it is argued that when the glide block encounters resistance to forward motion, fluid pressures which also exceed the gravitational load, act within the glide block and are instrumental in the initiation and development of listric faults.

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TL;DR: In this article, a wide range of values in sediments can be attributed to bacterial reduction of sea-water sulphate under various conditions, and in detail relate to the conditions of petrogenesis.
Abstract: Variation in sulphur isotope ratios is caused by equilibrium fractionation and kinetic fractionation. Two groups of samples show a narrow range of 34 S/ 32 S: sulphur from sea-water sulphate, and from meteorites and basic rocks; the former show an enrichment, δ 34 S, +20% 0 with respect to the latter. They are identifiable source materials from which the complete range of values is derived by fractionation processes. Experimental and theoretical work shows the magnitude of these effects and allows interpretation of data from natural samples in terms of isotopic temperatures or source of the sulphur or both. Data mainly from rocks not associated with ore deposits is reviewed. The wide range of values in sediments can be attributed to bacterial reduction of sea-water sulphate under various conditions. Basalts give ratios near to the meteorite value, 0% 0 , and in detail relate to the conditions of petrogenesis. Some granites are enriched up to +30% 0 , probably from late-stage differentiation, but samples from the SE Autralian, New England batholith have ratios which fall into two groups: those within a small range from −3.6 to +5.0% 0 and those which are generally higher or lower than that range. These respectively relate to an igneous or sedimentary source for the rocks as distinguished by other geological criteria.

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TL;DR: In this article, the authors compare the San Andreas fault system and the Alpine fault system within the reference frame of their regional slip vectors, and argue that a genetic relationship exists between the development of these complexities.
Abstract: When the San Andreas fault system of California and the Alpine fault system of New Zealand are compared within the reference frame of their regional slip vectors, their major tectonic elements are seen to occupy similar positions. This argues that a genetic relationship exists between the development of these complexities. Furthermore, three distinctive modes of slip, or ‘seismic style’, can be recognized for different segments of these major transform fault systems. These are: (1) slip during great (M ≥ 8) earthquakes separated by long periods of quiescence; (2) slip during more frequent large (6.5 ≤ M ≤ 7.5) earthquakes, and (3) aseismic slip (creep). These different seismic styles occur in distinctive tectonic settings and it is argued that they represent fundamental variations in the mechanical properties of the faults. I suggest that style 1 occurs along sections of the fault orientated such that a component of tectonic convergence occurs, leading to a high effective normal stress and hence high frictional strength. Style 2 occurs along sections of faults which strike close to the regional slip vector, and hence have a lower normal stress. Aseismic slip occurs only along sections of faults in central California where anomalously high pore pressures reduce the effective normal stress to a very low value and the fault is entirely within the stable sliding frictional field.

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TL;DR: In the Andes, major lineaments controlled sedimentation, vulcanicity, batholithic intrusions and mineralisation throughout the Mesozoic and Cenozoic.
Abstract: In the Andes, major lineaments controlled sedimentation, vulcanicity, batholithic intrusions and mineralisation throughout the Mesozoic and Cenozoic. These lineaments represent major fault lines in an ancient crystalline basement. The magmas of the Coastal Batholith were emplaced along one such lineament and the emplacement of the individual plutons was closely controlled by transcurrent faults and by smaller scale joint patterns. Such structures were continuously exploited throughout the long emplacement history of the batholith. Exceptionally, plutons were forcefully emplaced along early lineaments and lifted roof blocks along shear zones coincident with pre-existing faults. More generally subsidence of large fracture-bound crustal blocks provided the space for the emplacement of magma in bell-jar and cauldron-subsidence structures while stoping along the regular, closely spaced joints resulted in a rectilinear pattern of contacts at outcrop scale.