This paper provides new geochemical and isotopic data on the evolution of the western foreland to the Nubian shield of north-east Africa. There is abundant evidence for early to middle Proterozoic crust west of the River Nile, but this was severely affected by the Pan-African ( 500–900 Ma) orogenic cycle. The results are reported of Rb-Sr whole rock and zircon evaporation geochronological studies and whole rock Sm-Nd and feldspar Pb isotopic analyses for four rock units around Wadi Halfa in northernmost Sudan. These results indicate the presence of heterogeneous pre-Pan-African crustal components, preserved in mylonitic gneisses and in conglomerates that unconformably overlie the gneisses. Several episodes of crust formation, inferred from zircon ages, are preserved in the gneisses : 2.6, 2.4, 2.0, 1.7, 1.2 and 0.72 Ga. Nd model ages for the same units are invariably older than the zircon ages, yet still record a predominantly late Archaean and Palaeoproterozoic history, with depleted mantle model ages between 1.3 and 2.8 Ga. The earliest recorded Pan-African magmatic event is about 720 Ma and dates the beginning of collisional deformation. A younger Pan-African volcanic sequence ( 650 Ma) has isotopic compositions of Sr and Nd compatible with derivation from late Prote rozoic asthenospheric mantle. A 530 Ma anorogenic A-type granite also has isotopic compositions suggesting derivation from a primitive source. The inferred tectonic evolution began with rifting to form an oceanic re-entrant. This was followed by subduction leading to collision at about 700 Ma, accompanied by post-orogenic rifting at about 650 Ma. 相似文献
In this paper the author first reviews 6 major advances in the study of Precambrian geologyand tectonics of South China during the last decade, and then proposes the following new modelof Late Precambrian scissors-type opening-closing tectonic evolution in the region. (1) Besidesthe Middle-Late Proterozoic low-grade metamorphic basement, there existed a LateArchaean-Early Proterozoic medium- to high-garde metamorphic basement in South China;both of them formed the united Yangtze-Cathaysia (craton) at the end of the Early Proterozoic,with the Jiangxi-Anhui-Zhejiang palaeogulf extending into the inland and the Jiangxi-Anhui-Jiangsu Peninsula lying on the northern side of the palaeogulf.(2) From the Middle Proterozoic,the present Guangxi-Guizhou-Hunan area in the western-central oldland was subjected toextensional rifting and became a rift trough, thus resulting in counterclockwise rotation of the"Cathaysian block" and clockwise rotation of the "Yangtze block"; moreover, the southwardcompression of the Dabie-Jiaonan block which split from the southern margin of the NorthChina plate and was accreted to the northern margin of the Jiangxi-Anhui-Jiangsu Peninsula ledto the scissors-type closing of the Jiangxi-Anhui-Zhejiang palaeogulf during the Middle-earlyLate Proterozoic, which further influenced the Early Palaeozoic tectonic evolution of the region. 相似文献
We describe a waveform modelling technique and demonstrate its application to determine the crust- and upper-mantle velocity structure beneath Africa. Our technique uses a parallelized reflectivity method to compute synthetic seismograms and fits the observed waveforms by a global optimization technique based on a Very Fast Simulated Annealing (VFSA). We match the S , Sp, SsPmP and shear-coupled PL phases in seismograms of deep (200–800 km), moderate-to-large magnitude (5.5–7.0) earthquakes recorded teleseismically at permanent broad-band seismic stations in Africa. Using our technique we produce P - and S -wave velocity models of crust and upper mantle beneath Africa. Additionally, our use of the shear-coupled PL phase, wherever observed, improves the constraints for lower crust- and upper-mantle velocity structure beneath the corresponding seismic stations. Our technique retains the advantages of receiver function methods, uses a different part of the seismogram, is sensitive to both P - and S -wave velocities directly, and obtains helpful constraints in model parameters in the vicinity of the Moho. The resulting range of crustal thicknesses beneath Africa (21–46 km) indicates that the crust is thicker in south Africa, thinner in east Africa and intermediate in north and west Africa. Crustal P - (4.7–8 km s−1) and S -wave velocities (2.5–4.7 km s−1) obtained in this study show that in some parts of the models, these are slower in east Africa and faster in north, west and south Africa. Anomalous crustal low-velocity zones are also observed in the models for seismic stations in the cratonic regions of north, west and south Africa. Overall, the results of our study are consistent with earlier models and regional tectonics of Africa. 相似文献
The method of sequence stratigraphy requires the application of the same workflow and principles irrespective of the age of strata under analysis. In that respect, its application to Precambrian successions is similar to the approach used for Phanerozoic case studies. Differences, however, are recorded in terms of the preservation potential and the amount of data available for analysis; the rates and intensities of the allogenic controls on sedimentation; the environmental conditions and related physical processes; and the evolution of competing groups of organisms and associated biogenic processes. The combined effect of these contrasting aspects accounts for differences in the architecture of depositional sequences, particularly with respect to the relative contributions of various systems tracts to the makeup of a sequence.
The application of sequence stratigraphy to Precambrian basins has considerably enlarged the perspective on the fundamental principles governing the processes of sedimentary basin formation and the mechanisms controlling stratigraphic cyclicity in the rock record. These first-order principles are perhaps the most important contribution of Precambrian research to sequence stratigraphy. At the broader scale of Earth's geological history, the tectonic regimes governing the formation and evolution of sedimentary basins are shown to have been much more erratic in terms of nature and rates than originally inferred solely from the study of the Phanerozoic record. This provides important clues with respect to the criteria that should be involved in the hierarchy system of classification of stratigraphic sequences and bounding surfaces. 相似文献