Regional variations in cleavage and fold development in North Wales

The arcuate pattern of the main Caledonian cleavage and associated fold axial plane traces in North Wales is due partly to NW-SE compression with tectonic transport to the southeast against the concealed crop of the Tan y grisiau Microgranite. Low-angle cleavage close to the microgranite is shown to be a local variant of the regional cleavage formed during the main deformation and not an earlier phase as previously supposed. Transcurrent movements along several major fault systems are also related to compression around the microgranite and the Harlech Dome block.     

Tokaido — megalopolis of Japan

The Tokaido east coast road has been the main road of Japan since Mediaeval times, and the journey from Tokyo at one end to Kyoto or Osaka at the other, which used to take a fortnight, can now be completed in about three hours by bullet train, and an even faster linear-motor car is likely to be in operation in the near future.Already during the 18. cent., Edo (Tokyo) was the largest city in the world, with a population over a million, and the rapid urbanization of Japan's population since Meiji times, and particularly during the post-WW II period, has been quite unprecendented. In 1950, the median size of place was 13,000 and by 1975 it was 140,000. About 60 million lived in the Tokaido zone.The Kanto, Nobi and Osaka plains, adjacent to the good harbours of Tokyo, Ise and Osaka bays, enjoying the relatively mild climate of the Pacific coast, and being within 600 km of each other, have been the focii of urban and industrial development in Japan. The emergence of Tokaido megalopolis was boosted by capital investment in this zone, and was contingent upon the industriousness and high level of education of the people.The concept of megalopolis in Japan is popularly associated with rapid urbanization, poly-nuclear and linear form, and concentration of population, capital and information, all of which elements are typified by the Tokaido zone. The linear megalopolis pattern has been postulated as a more efficient growth form for high-dense society than the radial metropolitan pattern. It has even been suggested that megalopolis is a concept perceived by the intellect, its physical structure determined by information networks, metropolis being perceived by the eye and its physical structure being determined by transport and energy networks.Quite irrespective of the concept of megalopolis, there can be no denying that Japan's society is a high-dense society. In 1975, 57% of the population lived in Densely Inhabited Districts (DIDs) at minimum densities of 40 persons per hectare, and these DIDs covered only 2.2% of the land area of Japan. The current trend is for more and more people to live in DIDs, but for overall DID densities to decrease. During the past 25 years, there has been a huge influx of population into the Tokaido zone, and while until 1960 the greatest increases were in the three main metropolitan centres, as these became saturated, rapid urbanization spread into the neighbouring prefectures. Since the mid-sixties, the central metropolitan wards have begun to lose residents, but the daytime population has continued to increase, giving rise to increasingly complex commuting patterns. To give an example, the commuting field of Yokohama includes almost all the prefectures of Tokaido megalopolis.Like the image of megalopolis itself, life in Tokaido megalopolis has its good and bad aspects. Although per capita space in dwellings is increasing somewhat, housing is extremely expensive and people commute long distances. Incomes are high but environmental problems persist. There is a U-turn phenomenon, but metropolitan suburbs remain a popular choice of residence.Central management functions and knowledge and information oriented occupations are predominantly concentrated in Tokyo and Osaka, the two main nodes of Tokaido megalopolis. In the intermediate cities, new employment opportunities are stimulated by the expansion of second-level managerial functions. The transport and communications networks of Tokaido are becoming congested as mobility and information flow increase.Planning in the eighties will be affected by the switch from industries dependent on raw materials to knowledge intensive industries; from investment in production to investment in public facilities and pollution control. Within Tokaido megalopolis, there is room for local governments to expand efforts to improve the existing situation, and at its fringes to avert some of the less desirable consequences of rapid urbanization.     

The petrology and petrogenesis of the Tertiary anorogenic mafic lavas of Southern and Central Queensland,Australia — Possible implications for crustal thickening

The Miocene-Oligocene volcanism of this region is part of the larger Tertiary volcanic province found throughout E. Australia. Within the S.E. Queensland region, the volcanism is strongly bimodal, and has emanated from six major centres, and many additional smaller centres. The mafic lavas (volumetrically dominant) range continuously from ne-normative through to Q-normative and are predominantly andesine-normative; Mg/Mg+Fe (atomic ratios range from 30–60; K₂O ranges from 0.42–2.93%, and TiO₂ from 0.81–3.6%.Phenocryst contents are low (averaging 6.7 vol.%), and comprise olivine (Fa_18–75; Cr-spinel inclusions occur locally in Mg-rich phenocrysts), plagioclase (An_25–68), and less commonly augite, which is relatively aluminous in lavas of the Springsure volcanic centre. Very rare aluminous bronzite occurs in certain Q-normative lavas. Groundmass minerals comprise augite, olivine (Fa_33–77), feldspar (ranging from labradorite through to anorthoclase and sanidine), Fe-Ti oxides, and apatite. Within many of the Q-normative lavas, extensive development of subcalcic and pigeonitic pyroxenes occurs, and also relatively rarely orthopyroxene. Mineralogically, the ne- and ol-normative lavas, and some of the Q-normative lavas are indistinguishable, and in view of the gradations in chemistry, the term hawaiite has been extended to cover these lavas. The term tholeiitic andesite is used to describe the Q-normative lavas containing Ca-poor pyroxenes as groundmass phases.Megacrysts of aluminous augite, aluminous bronzite, olivine, ilmenite, and spinel sporadically occur within the lavas, and their compositions clearly indicate that they are not derived from the Upper Mantle. Rare lherzolite xenoliths are also found.The petrogenesis of these mafic lavas is approached by application of the thermodynamic equilibration technique of Carmichael et al. (1977), utilizing three parental mineral assemblages that could have been in equilibrium with the magmas at P and T. The models are: (a) standard upper mantle mineralogy; (b) an Fe-enriched upper mantle model (Wilkinson and Binns 1977); (c) lower crust mineralogy, based on analysed megacryst compositions. The calculations suggest that these mafic magmas were not in equilibrium with either mantle model prior to eruption, but show much closer approaches to equilibrium with the lower crust model. Calculated equilibration temperatures and pressures (for the lower crust model) range from 995°–l,391° C (average 1,192), and 7.2–16.3 kb (average 12.4). These results are interpreted in terms of a model of intrusion and magma fractionation within the crust-mantle interface region, with consequent crustal underplating and thickening beneath the Tertiary volcanic regions. Some support for the latter is provided by regional isostatic gravity anomalies and physiographic considerations.     

Antithethic and homothetic faults

Antithetic faults are dip slip faults that displace rocks so as to prevent or reduce structural relief, typically producing tilted fault blocks. They may or may not occur in conjugate systems with their opposites, homothetic faults, which serve to increase structural relief. Several examples serve to illustrate the concept and to correct current misunderstandings.

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Zusammenfassung Die Arbeit soll die Begriffe antithetisch und homothetisch klären helfen. Antithetische Verschiebungen wirken dem tektonischen Relief entgegen, homothetische Verwerfungen unterstützen es. Die Begriffe werden an einigen Beispielen erläutert.

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Résumé Ce travail constitue une tentative d'élucider les termes »antithétique« et »homothétique«. Le rejet des failles antithétiques s'oppose à la création d'un relief tectonique. Le rejet des failles homothétiques augmente ce relief. Quelques exemples sont discutés.

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Geochemistry and petrogenesis of late proterozoic volcanic rocks from north-western Africa

The Upper Proterozoic volcanism of northwestern Africa is characterized by the predominance of calc-alkaline rocks. Volcanics with tholeiitic affinities and alkali basalts are rare. The geochemistry and the relative proportions of calc-alkaline rocktypes in the Silet zone (Algeria) and the Ouarzazate formation (Morocco) are similar to those of recent island arc suites where basalts are most abundant while in the Tassendjanet and Gara Akofo zones (Algeria) they resemble contintal margin volcanic suites with a predominance of andesites. The volcanic rocks have undergone low-grade metamorphism which strongly affected alkali and alkali-earth elements and also to a smaller degree, the less mobile elements such as REE, Zr, Hf, Nb, and P. The geochemistry of the calc-alkaline rocks point to a complex origin involving low-pressure fractional crystallization, crustal contamination and derivation from a source already enriched in LILE.     

The Tugtutoq Younger Giant Dyke Complex, South Greenland: Fractional Crystallization of Transitional Olivine Basalt Magma

The Younger Giant Dyke Complex consists of a set of massivecoalescing dykes of Proterozoic age (c. 1170 Ma), resultingfrom intrusion of a suite of transitional olivine basaltic/hawaiiticmagmas in a continental rift setting. The suite, compositionallyrelated by low pressure (< 10 kb) olivine-plagioclase fractionation,is believed to have had a deeper level evolution dominated bypyroxene and possibly garnet fractionation. Slow cooling insitu of the interior parts of the dyke complex produced cumuliticsuites. Those exposed range from gabbroic to syenitic; residualbodies of riebeckite granite and, very subordinate, feldspathoidalsyenite were also generated. The basic magmas had notably lowfO₂ values, leading to delayed magnetite and clinopyroxene precipitation,relatively iron-rich differentiates and some residual liquidsof pantelleritic composition. The basic magmas had high F/Clvalues and are inferred to have had low H₂O contents. They werealso characterized by relatively high K/Rb and low ⁸⁷Sr/⁸⁶Srvalues; these characteristics imply a mantle source with highF/Cl but depleted in Rb relative to K and Sr. Basaltic magmasresponsible for (a) the preceding Older Giant Dyke Complex and(b) a suite of anorthositic xenoliths within the Younger GiantDyke Complex, are inferred to have been derived from separateprimary magma batches independent of those that yielded theYounger Giant Dyke Complex. The giant dykes are the highest-levelrepresentatives of a larger basic complex responsible for theextensive linear gravity ‘high’ in the Tugtutôq-Narssaqarea.