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Chad has an area of about 1.2 million km2, is located in the centreof the African continent and is not well explored. Results of importance to the local economic geology have been acquired recently, mainly during mineral exploration:
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i) all geological formations within Chad territory were reworked/influenced by the Pan-African Orogeny (by the end of the Proterozoic) terminating the crustal evolution of the area with most of the Chad granitoids being formed during this event; 2.
ii) The Precambrian formations are of Proterozoic age and contain volcanosedimentary series with considerable mineral potential (Au,…); 3.
iii) the vast Chad Basin (extending into neighbouring countries) has a complex structure and includes several sub-basins and troughs, whose development started during the break-up of Gondwana; they have been filled by up to 10, 000 m of sediments and petroleum and gas occur within these structures; 4.
iv) well preserved fossil remnants of an Australopithecus have been recently found in Chad; and 5.
v) large reserves of oil and vaste resources of a variety of minerals (Au, ornamental stones, marbles, diatomites, etc.) have been found.
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W.M. Edmunds 《Journal of African Earth Sciences》1996,22(4):385-389
The scope for using hydrogeochemical techniques in water quality studies in Africa is reviewed as a background to a set of thematic papers. Water quality problems are emerging as a key issue in Africa either:
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i) in view of the pressures of man-made pollution on finite resources; or 2.
ii) the existence of regions with naturally induced geological problems, for example fluoride endemic areas.
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Analysis of mineral assemblages and illite crystallinity of the Arganan Triassic formations show that:
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i) mixed-layers are present across almost all of the section; and 2.
ii) values of illite crystallinity are quite dispersed and lie mostly in the diagenetic zone, with no gradient related to burial.
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A sampling of Mesozoic and Tertiary basalts in Lebanon yielded the following information:
These results confirm and amplify earlier work by Van Dongen et al., and can be interpreted as indicating a net anticlockwise rotation of Lebanon relative to the African tectonic plate amounting to about 70° during the Late Jurassic-Pliocene interval. This could have resulted from differential movement between the African and European plates as they made way for the growing Atlantic Ocean. 相似文献
Age | D | I | α95 | Pole position | dp | dm |
Upper Jurassic | 95 | +21 | 10.6 | 114E 2N | 5.9 | 11.2 |
66W 2S | ||||||
Lower Cretaceous | 122 | +2 | 9.0 | 105E 25S | 4.5 | 9.0 |
75W 25N | ||||||
Upper Pliocene | 2 | +46 | 7.7 | 169E 88N | 6.3 | 9.8 |
11W 88S |
Full-size table
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Occurrence of small (3 ML < 4) earthquakes on two 10-km segments of the Calaveras fault between Calaveras and Anderson reservoirs follows a simple linear pattern of elastic strain accumulation and release. The centers of these independent patches of earthquake activity are 20 km apart. Each region is characterized by a constant rate of seismic slip as computed from earthquake magnitudes, and is assumed to be an isolated locked patch on a creeping fault surface. By calculating seismic slip rates and the amount of seismic slip since the time of the last significant (M 3) earthquake, it is possible to estimate the most likely date of the next (M - 3) event on each patch. The larger the last significant event, the longer the time until the next one. The recurrence time also appears to be increased according to the moment of smaller (2 < ML < 3) events in the interim. The anticipated times of future larger events on each patch, on the basis of preliminary location data through May 1977 and estimates of interim activity, are tabulated below with standard errors. The occurrence time for the southern zone is based on eight recurrent events since 1969, the northern zone on only three. The 95% confidence limits can be estimated as twice the standard error of the projected least-squares line. Events of M 3 should not occur in the specified zones at times outside these limits. The central region between the two zones was the locus of two events (M = 3.6, 3.3) on July 3, 1977. These events occurred prior to a window based on the three point, post-1969 slip-time line for the central region.
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Latitude | Longitude | Depth | Mag. | Target date | Standard error (days) |
37°17′± 2′N | 121°39′±2′W | 5.0 ±2 km | 3.0–4.0 | 7-22-77 | 22.3 |
37°26′± 2′N | 121°47′±2′W | 6.0 ± 2 km | 3.0–4.0 | 9-02-77 | 8.0 |
Full-size table
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A study of the synoptic situation which produced the catastrophic floods of November 1988 in Catalonia (in the northeast of the Iberian Peninsula) is presented. Analyses of the vertical structure, potential instability, precipitable water, and instability index are made through the radiosounding data from Palma, Majorca. It is found that the 1988 situation is included in type I intense convective events in Catalonia (classification obtained from all the events since 1950, (Llasat, 1989)). It was characterized by:
相似文献
(a) | -pattern in the middle and high troposphere, the ridge axis east of Catalonia. |
(b) | High pressure over Europe. |
(c) | South-easterly winds in the lower troposphere with warm and moist humid air advection and south-westerlies aloft over Catalonia. |
(d) | Strong instability (convective and latent). |
(e) | Penetration of Atlantic air. |
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Ligang Zhang 《中国地球化学学报》1988,7(2):109-119
Based on the oxygen isotopic compositions of 133 wolframite samples and 110 quartz samples collected from 30 tungsten ore
deposits in south China, in conjunction withδD values and other data, these deposits can be divided into four types.
Based on theδ
18O values of the coexisting quartz and wolframite and temperature data, two calibration equilibrium curves have been constructed,
and the corresponding equations have been obtained:
(1) | Reequilibrated magmatic water-hydrothermal tungsten ore deposits. Theδ 18O values of wolframite and quartz samples from this type of tungsten ore deposits are about +5–+12‰, respectively. The calculatedδ 18O values of ore fluids in equilibrium with quartz are about +6.5‰, and theδ values of fluid inclusions in quartz range from −40 to −70‰ |
(2) | Meteoric water-hydrothermal tungsten ore deposits. Theδ 18O values of wolframite in this type of tungsten deposits are around −1‰ |
(3) | Stratiform tungsten ore deposits. In these deposits, theδ 18O values of quartz and wolframite are about +17 and +3‰, respectively. It is considered that these stratiform tungsten ore deposits are genetically related to submarine hot-spring activities. |
(4) | Complex mixed-hydrothermal tungsten ore deposits. These tungsten ore deposits are characterized by multi-staged mineralization. Theδ 18O values of early wolframite are around +5‰, but of later wolframite are lower than +4‰, indicating that the early wolframite was precipitated from reequilibrated magmatic water-hydrothermal solutions and the late one from the mixture of hydrothermal solutions with meteoric waters or mainly from meteoric waters. |
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Experimental evidence of decompression melting during exhumation of subducted continental crust 总被引:16,自引:2,他引:14
E. Auzanneau D. Vielzeuf M. W. Schmidt 《Contributions to Mineralogy and Petrology》2006,152(2):125-148
Experiments have been carried out on a metagreywacke at 800, 850 and 900°C, in the pressure range 0.5–5 GPa to locate the solidus and the eclogite/amphibolite facies transition in felsic rocks, identify the nature of the reactions responsible for major mineralogical changes, and determine the proportions of phases as a function of pressure. The mineral assemblage phengite + clinopyroxene + garnet + quartz/coesite is stable above 2.3 GPa while biotite + plagioclase + garnet + quartz is stable below 2 GPa. The model reaction for the eclogite/amphibolite facies transition in metagreywackes is:
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