Evolution Geodynamique Et Tectonique De La Ceinture De Roches Vertes Paleoproterozoique De Sefwi Craton Ouest Africain Ghana

Author : Dominique Chardon
Publisher :
ISBN 13 : 9782905532756
Total Pages : 288 pages
Book Rating : 4.5/5 (327 download)

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Download or read book ARCHAEAN CONTINENTAL DEFORMATIONS: NATURAL EXAMPLES AND THERMOMECHANICAL MODELING written by Dominique Chardon and published by . This book was released on 1997 with total page 288 pages. Available in PDF, EPUB and Kindle. Book excerpt: ln this thesis, we present the results of structural analysis in different segments of Archaean continental crust (>2.5 Ga) in which strain patterns are compatible with the development of crustaI-scale gravity instabilities. We also present analog experiments simulating the development of these gravit y instabilities in order to constrain their thermomechanical significance within the Archaean continental lithosphere. Structural results.A structural study of several greenstone belts from the Dharwar Supergroup (Dharwar craton, India) leads to a progressive deformation model for the initiation of gravity instabilities between the basin and its basement located in the upper structural level of the crust. The basal unconformity of the Dharwar Supergroup acted as a décollement zone. The kinematic criteria within fuis décollement are all compatible with a displacement of the supracrustal rocks toward the internaI parts of the greenstone belts. The décollement-related structures are progressively refolded above the zone of maximum subsidence. ln some of the greenstone belts, the gravity strain pattern interfers with the regional strain field, which is characterized by predominant E-W shortening and N-S trending strike-slip shear zones. ln the Dharwar intracratonic basin, the sagging episode was contemporaneous with the last tectonometamorphic event recognized within the craton, c.a. 2.5 Ga ago. This episode is characterized by the formation of granulites at depth as weIl as by large-scale migmatization associated with the emplacement of a large amount of juvenile materia1. We consider that the high mantle heat flux and partial melting led to the softening of the continental crust and fuis allowed the development of the gravit y instabilities. Within the western Reguibat rise (Mauritania), structural analysis reveals the occurrence of dome-and-basin structures. The strain patterns associated with these structures are interpreted as the result of interference between the diapiric rise of granitoid domes within the greenstone sequences and regional E-W shortening. At the scale of the southern part of the Archaean Reguibat rise, the strain trajectories have the same overall trends in both the high-grade and medium-grade terrains. No major tectonic break has been identified between these metamorphic domains. This implies that the formation of diapiric domes was penecontemporaneous with granulite metamorphism and regional deformation within the lower crust, which occurred c.a.2.7 Ga ago. Experimental results. We used temperature-dependent viscosity material (Gum Rosin) and an new experimental apparatus to simulate unstable granite/greenstone layers. A vertical temperature gradient was maintained, and the deformation was driven by gravit y alone. For a fixed temperature at the bottom of the model, the relative thicknesses of the different layers were changed from one experiment to another. lnsome experiments, a brittle material (sand) was introduced as a superficial layer. The experiments show that : (1) the triggering of gravity instabilities is possible only if the materials at the unstable boundary (cover/basement contact) are ductile and (2) the growth rate of gravity instabilities is an exponential fonction of the temperature at the unstable interface and the temperature gradient at the top of the uns table layer. These results suggest that this type of gravity instability could not develop within a modern continentallithosphere