Transition from passive to active rifting: Relative importance of asthenospheric doming and passive extension of the lithosphere

We present quantitative modeling results of the dynamic interplay of passiv e extension and active convective thinning of the mantle lithosphere beneat h intracontinental rift zones investigating the relative importance of ther mal buoyancy forces associated with asthenospheric doming and far-field int raplate stresses on the style of rifting. To this aim we employ a twodimens ional numerical code based on a finite element method formulation for nonli near temperature dependent viscoelastoplastic theology. Brittle behavior is modeled using Mohr-Coulomb plasticity The models support a scenario in whi ch passive stretching leads to an unstable lithospheric configuration. Ther mal buoyancy related to this asthenospheric doming subsequently drives acti ve upwelling in a lithosphere scale convection cell. In the late synrift to early postrift the lithospheric horizontal stresses caused by the active a sthenospheric upwelling may start to compete with the far-field intraplate stresses. At this stage the domal forces may dominate and even drive the sy stem causing a change from passive to active rifting mode. If this transiti on occurs, the model predicts (1) drastic increase of subcrustal thinning b eneath the rift zone, (2) lower crustal flow towards the rift flanks, (3) m iddle crustal flow towards the rift center, (4) the coeval occurrence of te nsional stresses within and compressive stresses around the upwelling regio n, and (5) possible surface uplift. Late postrift thermal cooling removes t he thermal buoyancy forces. At this stage the far-field forces dominate the stress state again and the lithosphere becomes more sensitive to small cha nges in the intraplate stresses. The model results may explain several key observations that are characteristic of a large number of intracontinental rift basins. These features include differential thinning of extending lith osphere, the discrepancy between fault-related extension and crustal thinni ng, late tend of synrift to early postrift) mantle related volcanism, surfa ce domal uplift succeeding rifting, and rift hanks uplift associated with e xtension of a weak lithosphere.