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Regardless of the origin of the forces driving extension, and regardless on the presence of not of voluminous volcanism, the geometry continental rifts have been classified into three categories (Buck, 1991). In the narrow rift mode, exemplified by the East African System, extensional deformation localizes along typically 100 to 150 km wide, continuous or segmented, rifts. In the wide rift mode, such as the Basin and Range Province, extensional deformation is more diffuse spreading over an horizontal length-scale several times larger than the thickness of the lithosphere. In contrast to narrow rifts, wide rifts display relatively small gradient in crustal thickness. The Aegean domain exemplifies the core complex mode of continental extension. As with the wide rifts, core complexes display little variation in crustal thicknesses, however, they differ from the wide-rift mode in that deeper crustal levels are exhumed to the surface forming a domical metamorphic core wrapped by a low-angle normal fault connected laterally to a crustal-scale detachment fault. The décollement/detachment system is overlaid by low-grade rocks and detrital sediments.
The consensus is not yet reached on the significance of the three different modes of continental extension. Some authors see in the core complex mode a mere variant of wide rift mode. For some, the three modes may represent various stages of long-lasting continental extension. Rheology is commonly accepted as a key parameter controlling the rifting mode. Wide rift mode is thought to be promoted by high temperature and low strain rate, whereas lower temperature and high strain rate are thought to favor narrow rifts. The depth of the brittle-ductile interface in the crust, the coupling between brittle and ductile layers, as well as the number of mechanically contrasting layers in the lithosphere, are of primary importance in controlling the style of continental extension. Rifting modes are also discussed in terms of competition between gravitational forces and tectonic driving forces. There is a lot of overlap between the various interpretations of the three modes of continental extension and overall they all point toward the conclusion that, continental geotherm, rheology, and strain rate conspire to dictate the rift mode. |
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