Journal of African Earth Sciences (1983)最新文献

Geophysical studies have revealed extensive rift systems throughout Central Africa, among them the White Nile Rift runnng SE-NW across central Sudan. Few earthquakes have been detected teleseismically in areas of Sudan away from the East African or Red Sea Rift systems. Relocations presented here confirm three earthquakes to have occurred at shallow depth at the NW extreme of the White Nile Rift, around Jebel Dumbeir, Kordofan province. Ambiguity in a teleseismic P-wave first-motion solution for the 9 October 1966 0648GMT event is resolved by use of the P/pP/sP relative amplitude method. The fault plane is near-vertical, striking to 005–025° with a downthrow to the SE and a minor sinistral strike-slip component. This is consistent with ground deformation and location of an aftershock. Its downthrow direction appears to reflect subsidence of the White Nile Rift and central Sudan. Its strike is common with present-day activity in the Ethiopian Rift, suggesting that a near ESE-WSW stress regime persists across central and southern Sudan.

The electrical anisotropy of the Basement Complex area of the University of Ife has been established using apparent resistivity maps.

The electrical anisotropy and hence inhomogeneity in the area is primarily due to foliation; structural trends of concealed basement rocks deduced from resistivity maps correlate significantly with strikes of foliation/geologic boundary.

It follows therefore that strikes of foliation of concealed solid rocks, in which the predominant structural feature is foliation or where the fracture/joint directions are generally in line with strikes of foliation, can be reasonably determined from surface geophysical measurements.

The Eocene Formi Formation in southeastern Nigeria consists, in its type area, of the following four lithologic units, in ascending order: (1) silty to fine calcareous sandstone; (2) grey to dark shale with interacaltions of siltstone; (3) silty to fine argillaceous sandstone; and (4) fine to coarse pebbly sandstone. A palaeoecological study of the fauna of these units is presented. The most significant fossils are the foraminifera comprising chiefly of calcareous benthonic forms. The dominant families are Miliolidae and Nonionidae. Arenaceous forms are absent, while planktonic forms, which are rare, are represented by the generaGlobigerina and Globorotalia. The ostracod assemblage is mainly dominated by the shallow water genera, Togoina, Buntonia, Loxoconcha, Paracypris, Bythocypis, Costa, Basslerites and Cytherella. Based on an integrated analysis of lithologic and faunal data, the following depositional environments are suggested for the Ameki Formation, in ascending order: subtidal and intertidal zones of the shelf environment; barrier ridge, outer lagoon, inner lagoon and beach ridge of the near-shore environment. The water temperature of the Ameki sea was partly influenced by the Bengeula Current.

Petrological, geochemical and K/Ar radiometrical whole rock age data of volcanic rocks from SW Egypt and N Sudan in the area of Gebel Kamil-Lake Nasser-Laqia Arbain-Nile are presented. These rocks are (i) trachytic-phonolitic rocks and (ii) alkali olivine basalts occurring mostly as volcanic plugs. Their petrographic and their major and trace element patterns are typical for continental within-plate volcanism. Two groups of ages were determined: Triassic (240, 233 Ma) with predominantly silicic, differentiated magmas, and Tertiary (60-20 Ma) with predominantly basaltic magmas. Major element abundances of the basalts show a regional trend from west to east from low to high fractionation irrespective of the age of the rock. Their occurrence can be correlated with two W-E uplift zones which are important for the structural development of the large sedimentary basins of the Libyan desert.

The mafic to ultramafic rocks of the Barberton greenstone belt, South Africa, form a pseudostratigraphy comparable to that of Phanerozoic ophiolites. This Archaean complex, referred to here as the Jamestown Ophiolite Complex, consists of a high temperature tectono-metamorphic peridotite overlain by an intrusive extrusive igneous section, which in turn is capped by a chert-shale sequence. There is a complete range from komatiitic to tholeiitic compositions within single intrusive units. Crustal contamination and magma mixing is evident from field and geochemical data.

Pillow structures, ⁴⁰Ar/³⁹Ar ages and oxygen isotope analysis suggest that hydrothermal interaction with the Archaean ocean severely hydrated and chemically altered the entire simatic section during its formation. As a consequence, only a ‘ghost’ igneous geochemistry is preserved. This regional open-system alteration may have increased the MgO content of the igneous rocks by as much as 13%, and the most primitive liquids, from which the extrusive sequence evolved, were ‘picritic’ in character. Rocks with a komatiitic chemistry were derived during crystal accumulation from picritic-crystal mushes (predominantly olivine-clinopyroxene) and/or by metasomatism during one or more subsequent episodes of hydration-dehydration.

In contrast to Phanerozoic ophiolites, the Jamestown complex is relatively thin (≦3 km), which implies that locally at least the ca 3.5 Ga oceanic crust was also thin. This is consistent with the regionally extensive metasomatic alteration, and is compatible with theoretical and experimental models predicting higher Archaean heat transfer from the mantle concentrated within Archaean oceans.

Two sets of data are used to estimate heat flow density in Morocco: 36 determinations were made in shallow boreholes (100–500 m) in equilibrium conditions and 44 determinations in oil exploration boreholes (1000–5000 m).

These two sets of data are apparently consistent and are used to define the gross feature of heat flow in northern Morocco: Anti-Atlas, located at the West African craton boundary, is affected by a low heat flow (40 ± 5 m Wm⁻²) close to the average value obtained in this craton (37 ±8 m Wm⁻²); Mesetas, formed by a paleozoic and mesozoic basement, are characterized by a “normal” heat flow density (around 60 m Wm⁻²); northeastern part of Morocco (Eastern Rif, Middle Atlas and Horst chain) is affected by a higher heat flow density ranging from 80 m Wm⁻² to 110 m Wm⁻². It is suggested that this anomaly is directly connected with the recent quaternary volcanic activity.

A preliminary account of Albian and Cenomanian ostracods from Tocqueville (Rass el Oued), North Algeria, agrees well with the known faunal homogeneity between North Africa and the Middle East and, consequently, with the distinct heterogeneity between the latter two regions compared with southern Europe, especially southern France and Spain. The Albian and Cenomanian ammonite essemblages comprise species of wide geographical distribution, with a certain concentration to the central reaches of the Tethys.

Radar imageries revealed three major geomorphological units and a series of long, short and arcuate lineaments. The different lineament systems seem to delineate blocks containing folded structures. The metamorphosed terrain is easily recognisable as a highly deformed area. Including the metamorphosed rocks, three lithologic units have been recognised and demarcated in the area; consequently a radar-geological map at 1:50,000 scale of area 20,000 km² based on interpretation of radar imagery and a brief period of field work has been produced from this study.

The radar imagery studies have disclosed regional lineaments of tectonic origin, and indicate that an important tectonic episode occurred in the Oban massif and post the Precambrian period.

Integration of petrographic and stable isotopic analyses of dolomite from the reefal facies of the Lower Cretaceous Shuaiba Formation of Abu Dhabi, indicates two types of dolomites: host (very fine to medium crystalline dolomite formed in a meteoric-marine mixing zone) and baroque (white, coarsely crystalline dolomite). The baroque dolomite is characterized by coarsely crystalline mosaics of anhedral to subhedral crystals with undulose extinction, curved crystal faces and cleavage planes, and abundant inclusions. Baroque dolomite from this formation was precipitated in veins and narrow solution channels and is commonly composed of coarse bladed crystals, sometimes fan-shaped toward the center, precipitated as void filling cements that partially occlude porosity. Analysis of the formation of water chemistry and stable isotope for the dolomite of the Shuaiba Formation reveals that it probably formed by mixing seawater or meteoric water in the subsurface, with interstitial brines derived from halite-bearing evaporites. Also, oxygen isotope analyses (δ ¹⁸O PDB values range from −4.8 to −9.6% and δ ¹³C PDB values range from 2.4 to 4.9%) suggest that baroque dolomite formed at temperatures ranging from 67 to 112°C.

From the N.S. linear structure of the Itombwe syncline arise some problems in central Africa. Some authors considered it as a Kibarian structure and others as an isolated fold belt of Pan-African orogeny. This ambiguity is the result, in its greatest part, of the scarceness of field work and the lack of dating on the rocks of this syncline. A study on the central area showed the existence of two lithostructural units. The western area is correlated with a Kibarian structure while the eastern area had some similarities with the tectonic Pan-African troughts from NE of Zaïre and west of Uganda. The existence of an area folded during the Pan-African orogeny and in the same environment as the great African lakes area must be considered.