RIFT INITIATION BY LITHOSPHERIC RUPTURE

Field evidence from lherzolite massifs related to asthenosphere upwelling below rifts and recent tomographic imaging of the East African Rift suggests that the upwelling occurred within a narrow and steep conduit, implying a lithospheric rupture. This evidence prompts a two-stage model of rift development. The first stage is characterized by a lithospheric rupture creating, in the upper crust, narrow troughs, with expansion being mainly caused by intrusion of mantle wedges at depth and expansion of basalt dykes above. The second stage is characterized by homogeneous stretching of the lithosphere. This stage is prepared by thermal relaxation following, 10 Ma later, the fracturing stage and hot injections. The thermomechanical behaviour of a stable continental lithosphere subjected to stretching suggests that the lithosphere has been previously weakened (a small thermal plume or presence of a mechanically weak zone). Rifting of a structurally and thermally homogeneous lithosphere seems to require both plume heating and tensional forces, thus limiting the meaning of the concept of 'active' and 'passive' rifting.