African, southern Indian and South American cratons were not part of the Rodinia supercontinent: evidence from field relationships and geochronology   总被引：2，自引：0，他引：2  

Alfred Krner  Umberto Cordani 《Tectonophysics》2003,375(1-4):325

We discuss the question whether the late Mesoproterozoic and early Neoproterozoic rocks of eastern, central and southern Africa, Madagascar, southern India, Sri Lanka and South America have played any role in the formation and dispersal of the supercontinent Rodinia, believed to have existed between about 1000 and 750 Ma ago. First, there is little evidence for the production of significant volumes of ˜1.4–1.0 Ga (Kibaran or Grenvillian age) continental crust in the Mozambique belt (MB) of East Africa, except, perhaps, in parts of northern Mozambique. This is also valid for most terranes related to West Gondwana, which are made up of basement rocks older than Mesoproterozoic, reworked in the Brasiliano/Pan-African orogenic cycle. This crust cannot be conclusively related to either magmatic accretion processes on the active margin of Rodinia or continental collision leading to amalgamation of the supercontinent. So far, no 1.4–1.0 Ga rocks have been identified in Madagascar. Secondly, there is no conclusive evidence for a ˜1.0 Ga high-grade metamorphic event in the MB, although such metamorphism has been recorded in the presumed continuation of the MB in East Antarctica. In South America, even the Sunsas mobile belt, which is correlated with the Grenville belt of North America, does not include high-grade metamorphic rocks. All terranes with Mesoproterozoic ages seem to have evolved within extensional, aulacogen-type structures, and their compressional deformation, where observed, is normally much younger and is related to amalgamation of Gondwana. This is also valid for the Trans-Saharan and West Congo belts of West Africa.Third, there is also no evidence for post-1000 Ma sedimentary sequences that were deposited on the passive margin(s) of Rodinia. In contrast, the MB of East Africa and Madagascar is characterized by extensive structural reworking and metamorphic overprinting of Archaean rocks, particularly in Tanzania and Madagascar, and these rocks either constitute marginal parts of cratonic domains or represent crustal blocks (terranes or microcontinents?) of unknown derivation. This is also the case for most terranes included in the Borborema/Trans-Saharan belt of northeastern Brazil and west-central Africa, as well as those of the Central Goíás Massif in central Brazil and the Mantiqueira province of eastern and southeastern Brazil.Furthermore, there is evidence for extensive granitoid magmatism in the period ˜840 to <600 Ma whose predominant calc-alkaline chemistry suggests subduction-related active margin processes during the assembly of the supercontinent Gondwana. The location of the main Neoproterozoic magmatic arcs suggests that a large oceanic domain separated the core of Rodinia, namely Laurentia plus Amazonia, Baltica and West Africa, from several continental masses and fragments now in the southern hemisphere, such as the São Francisco/Congo, Kalahari and Rio de La Plata cratons, as well as the Borborema/Trans-Saharan, Central Goiás Massif and Paraná blocks. Moreover, many extensional tectonic events detected in the southern hemisphere continental masses, but also many radiometric ages of granitois that are already associated with the process of amalgamation of Gondwana, are comprised within the 800–1000 age interval. This seems incompatible with current views on the time of disintegration of Rodinia, assumed to have occurred at around 750 Ma.  相似文献   

Paleomagnetic evidence for post-Cretaceous internal deformation of the Chuan Dian Fragment in the Yangtze block: a consequence of indentation of India into Asia   总被引：3，自引：0，他引：3  

Shinya Yoshioka  Yu Yan Liu  Ken Sato  Hiroo Inokuchi  Li Su  Haider Zaman  Yo-ichiro Otofuji   《Tectonophysics》2003,376(1-2):61-74

Paleomagnetic samples of Paleocene–Eocene red sandstones were collected at 36 sites from the Jiangdihe-4 and Zhaojiadian formations around the Yongren (26.1°N, 101.7°E) and Dayao areas (25.7°N, 101.3°E). These areas are located in the Chuxiong basin of the Chuan Dian Fragment, southwestern part of the Yangtze block. After stepwise thermal demagnetization, a high-temperature component with unblocking temperature of about 680 °C is isolated from 26 sites. The primary nature of this magnetization is ascertained through positive fold and reversal tests at 95% confidence level. The tilt-corrected mean paleomagnetic directions for the Yongren and Dayao areas are D=17.2°, I=26.6° with α₉₅=5.8° and D=16.5°, I=31.1° with α₉₅=4.8, respectively. Easterly deflected declinations from this study are consistent with those reported from other areas of the Chuxiong basin, indicating its wide presence in the Cretaceous–Eocene formations of the said basin. Comparison with declination values expected from the Cretaceous–Eocene APWP of Eurasia indicates that the magnitude of clockwise rotation systematically increases toward the southeast within the Chuxiong basin as well as in the Chuan Dian Fragment. This trend of the differential tectonic rotation in the Chuan Dian Fragment is consistent with curvature of the Xianshuihe–Xiojiang fault system. Deformation of the Chuxiong basin can fairly be associated with the formation of eastward bulge in the southern part of the Chuan Dian fragment. During southward displacement, the Chuan Dian Fragment was probably subjected to tectonic stresses as a result interaction with the Yangtze and Indochina blocks, which resulted into east–west extension and north–south shortening.  相似文献   

Skills development in tourism: South Africa's tourism-led development strategy     

Lucy Kaplan 《GeoJournal》2004,60(3):217-227

Skills development for tourism has attracted only a limited scholarship in the developing world. In this paper the role of skills development in advancing South Africa's tourism-led development strategy is investigated. Skills development has a central role to play in ensuring the effective and sustainable transformation and development of the tourism industry in developing countries and, in the case of South Africa, in ensuring that poorer South Africans begin to benefit from this industry. It is argued that the current lack of an integrated and co-ordinated approach to tourism skills development seriously limits the potential of skills development to impact positively on tourism transformation and development in South Africa. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

Geographical Research about African Americans: A Survey of Journals, 1911–1995     

Owen J. Dwyer 《The Professional geographer》1997,49(4):441-451

This paper surveys research about African Americans in six geography journals between 1911 and 1995. 176 journal articles about African Americans were identified. A chronological analysis of a subset of these articles shows that it was not until the 1960s that African Americans were studied to any appreciable degree by geographers. Since that time, geographers have continued this research, but unevenly; annual output varies widely. Popular topics have included considerations of spatial distributions, social problems, political participation, the development of black residence areas, and historical and cultural studies of African Americans. Recent research in this area has problematized the concept of “race” and investigated the mutual constitution of racial identity and place.  相似文献   

Proximity to Job Opportunities and African American Male Unemployment: A Test of the Spatial Mismatch Hypothesis in Indianapolis   总被引：1，自引：0，他引：1  

Thomas J. Cooke 《The Professional geographer》1993,45(4):407-415

Despite the lack of substantive research, the spatial mismatch hypothesis remains a pervasive explanation for the high rate of African American male unemployment. This research tests the hypothesis that census tract African American male unemployment rates are negatively related to the number of local job opportunities. This hypothesis is tested for Marion County, Indiana, the central county of the Indianapolis MSA. Weighted partial correlation coefficients of the logit of African American male census tract unemployment rates and indices of the availability of local job opportunities in manufacturing, services, and retail trade are statistically insignificant. The findings reject the spatial mismatch hypothesis in the specific context of Indianapolis in 1990.  相似文献   

A numerical study of the seasonal variations for tracer redistribution by clouds over West Africa     

M. Renard  N. Chaumerliac  S. Cautenet  N. Audiffren  E. C. Nickerson 《Journal of Atmospheric Chemistry》1995,20(3):237-258

The sensitivity of the horizontal and vertical transport of an inert tracer to seasonal variability of the complex West Africa circulations is explored by means of a 2D mesoscale meteorological model including explicit microphysics and describing the cloud dynamics of these circulations. The seasonal variations of the location and spatial extent of the cloudy masses associated with the Inter Tropical Convergence Zone (ITCZ) are reproduced in a meridional cross section over West Africa. The redistribution of the inert tracer is shown to be due to the northward migration of the ITCZ from January to July and also to the enhancement of the convective activity of the ITCZ in July. The redistribution of carbon monoxide during the dry and wet seasons is discussed, highlighting the importance of the biomass burning during the dry season as a source of trace gas enrichment in the upper levels of the tropical troposphere over West Africa.  相似文献   

Emissions of Polycyclic aromatic hydrocarbons by savanna fires   总被引：2，自引：0，他引：2  

Pierre Masclet  Hélène Cachier  Catherine Liousse  Henri Wortham 《Journal of Atmospheric Chemistry》1995,22(1-2):41-54

Although Polycyclic aromatic hydrocarbons (PAH) are known as anthropogenic compounds arising from the combustion or the pyrolysis of fossil fuels, they may be also emitted by the combustion of vegetation. A field study was carried out in January 1991 at Lamto (Ivory Coast) as part of the FOS DECAFE experiment (Fire Of Savanna). Some ground samplings were devoted to the qualitative and quantitative characterization of atmospheric emissions by savanna fires during prescribed burns and under background conditions. Specific collections for gaseous and particulate PAHs have shown that the African practice of burning the savanna biomass during the winter months is an important source of PAHs. These compounds are emitted mainly in gaseous form but a significant fraction, essentially heavy PAHs, is associated with fine carbonaceous particles and can therefore represent a hazard for human health, since some of these compounds are mutagenic and carcinogenic. Twelve compounds were identified during the fire episodes and in the atmospheric background. The total concentration in the fires is of the order of 10 ng m^–3 for the gas phase and from 0.1 to 1 ng m^–3 in the aerosols. In the atmospheric background the mean concentrations are regular, 0.15 ng m^–3 and 2 pg m^–3, respectively. These concentrations are comparable with what is observed in European rural zones. The particulate emissions of PAHs by the savanna fires are distinguished by the abundance of some compounds which can be considered as tracers, although they are also slightly emitted by fossil fuel sources. These compounds are essentially pyrene, chrysene and coronene. In the gas phase, although no individual PAH may be considered as specific of the biomass combustion emissions, the relative abundances of the main PAHs are characteristic of the biomass burning. The concentrations of pyrene and fluorene are always predominant; these compounds could be considered as characteristic emission products of smoldering and flaming episodes, respectively. In the background the PAH composition shows that in a tropical region the air consists of a mixture coming from the various sources, but the biomass combustion is by far the most important source.The fluxes of total PAH emitted by savanna biomass burning in Africa were estimated to be of the order of 17 and 600 ton yr^–1, respectively, for the particulate PAHs and the gaseous PAHs, respectively.  相似文献   

The nature and location of the suture zone in the Rokelide orogen, Sierra Leone: Geochemical evidence     

Jennifer Lytwyn  Kevin Burke  Stephen Culver 《Journal of African Earth Sciences》2006,46(5):439-454

The boundaries of the West African Craton mark the location of a continuous suture zone that records Neoproterozoic to Early Cambrian oceanic closure. The western part of the circum-West African suture zone extends through the line of outcrop of the Mauritanide, Bassaride and Rokelide mountain belts. Our geochemical analyses are consistent with the idea that igneous and metamorphic rocks of the Rokelide and Southern Mauritanide mountain belts of West Africa occupy a suture zone that records the closing of a Neoproterozoic to Early Cambrian ocean basin during the Pan-African orogeny and final assembly of Gondwana. The closing of that basin was marked by the collision between Archean rocks of the Leo massif of the West African Craton and reactivated Archean and Paleoproterozoic rocks that now outcrop nearer to the coast of Africa in Sierra Leone and Liberia. Within the Rokelides, the geochemistry of the Kasewe Hills volcanic rocks and Marampa amphibolite indicate that remnants of an arc system are caught up in the suture zone. The geochemistry of Guingan schists that outcrop along strike of the Rokelides is compatible with the idea that the metamorphosed equivalents of the Marampa and Kasewe Hills arc volcanic rocks extend through the Bassarides and into the Southern Mauritanides.  相似文献   

Distributed Nubia–Somalia relative motion and dike intrusion in the Main Ethiopian Rift     

R. Bendick  S. McClusky  R. Bilham  L. Asfaw  S. Klemperer 《Geophysical Journal International》2006,165(1):303-310

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The Olduvai buffalo Pelorovis and the origin of Bos     

Bienvenido Martínez-Navarro  Juan Antonio Pérez-Claros 《Quaternary Research》2007,68(2):220-226

The origin of the genus Bos is a debated issue. From ∼ 0.5 Ma until historic times, the genus is well known in the Eurasian large mammal assemblages, where it is represented by Bos primigenius. This species has a highly derived cranial anatomy that shows important morphological differences from other Plio-Pleistocene Eurasian genera of the tribe Bovini such as Leptobos, Bison, Proamphibos-Hemibos, and Bubalus. The oldest clear evidence of Bos is the skull fragment ASB-198-1 from the middle Pleistocene (∼ 0.6-0.8 Ma) site of Asbole (Lower Awash Valley, Ethiopia). The first appearance of Bos in Europe is at the site of Venosa-Notarchirico, Italy (∼ 0.5-0.6 Ma). Although the origin of Bos has traditionally been connected with Leptobos and Bison, after a detailed anatomical and morphometric study we propose here a different origin, connecting the middle Pleistocene Eurasian forms of B. primigenius with the African Late Pliocene and early Pleistocene large size member of the tribe Bovini Pelorovis sensu stricto. The dispersal of the Bos lineage in Western Europe during middle Pleistocene times seems to coincide with the arrival of the Acheulean tool technology in this continent.  相似文献