STYLES OF CONTINENTAL RIFTING - CRUST-MANTLE DETACHMENT AND MANTLE PLUMES

Observations made in different continental rift systems (European, Red Sea-Gulf of Aden, and East African Rift Systems) were investigated in terms of the influence of different parameters on the style of riftin g. Apart from the lithospheric thermal regime at the time of rift init iation, the process of rifting seems to be mainly controlled by the fa r-field stress regime and the presence or absence of a mantle plume. I n a hot lithosphere the low viscosity of the lower crust enables the u pper crust to be detached from the mantle and be deformed independentl y under far-field stresses. Therefore, in western Europe the main rift s could open obliquely to the direction of mantle movement in crustal levels without appreciable extension in the lithospheric mantle. In co ntrast, the colder lithosphere of Arabia did not allow detachment of c rust and mantle. Therefore, despite being in a similar tectonic situat ion as in western Europe, i.e. rifting in front of an orogen, the whol e lithosphere deformed congruently lift opening occurred parallel to m antle movement, i.e. parallel to the direction of extensional stress i n the lithospheric mantle induced by the pull of the subducting slab a t the orogenic front. The forces needed to extend the whole relatively cool Arabian lithosphere could, however, not be produced by slab pull alone. Additional forces and weakening of the lithosphere were produc ed by the Afar mantle plume. Mantle plumes are generally not able to b reak very thick cratonic lithosphere but they deflect sidewards when h itting this kind of lithosphere. Warmer (but still relatively cool) li thosphere like in the surroundings of the East African Tanzania craton or in Arabia can, by the buoyancy of a plume, be bent strongly enough to break. As a consequence, long linear rift structures develop with generally high shoulders. The presence of a plume explains thus the po sition of the East African and Red Sea-Gulf of Aden rifts. Under far-f ield compression, rifts will open only a small amount, whereas under f ar-field extension continental break-up may occur. A plume hitting a h ot lithosphere may penetrate it without producing long linear rifts. I nstead, crustal deformation will be distributed in parallel basins ove r a wide area with only minor amounts of rift shoulder uplift as has h appened in northern Kenya and the French Massif Central.