Extension with lateral material accommodation - 'Active' vs. 'passive' rifting

In many rifted areas of the world, differential thinning is observed between upper and lower crust or between crust and lithospheric mantle. A kinematic model based on a finite element algorithm is presented which allows thermal modelling of differential stretching. In order to avoid space problems, the velocity field is defined under the assumption that the total amount of extension is the same for every lithospheric layer. Several combinations of far-field velocity and local extensional velocities in the rift area have been used to simulate basin-formation under different conditions of active (far field velocity smaller than extensional velocities) and passive rifting. Consequences on elevation, surface heat flow and lithospheric yield strength are investigated. Far-field velocities lower than local extensional velocities produce narrow basins bordered by high shoulders, whereas a wide basin without shoulders is obtained when the far-field velocity is similar to the maximum extensional velocity. A detailed study is presented for the first velocity distribution. When the detachment is located in the lower crust, a thickening of the crust occurs below the rift shoulders, which continue to rise during the post-rift phase. Lithospheric strength below the shoulders increases strongly during rifting due to cooling of the lithospheric caused by the down-welling of material in this area. In contrast, post-rift thermal relaxation leads to a rapid decrease of strength resulting eventually in a minimum located at the rift shoulders.