We present quantitative modeling results of the dynamic interplay of
passive extension and active convective thinning of the mantle
lithosphere beneath intracontinental rift zones investigating the
relative importance of thermal buoyancy forces associated with
asthenospheric doming and far-field intraplate stresses on the style of
rifting. To this aim we employ a twodimensional numerical code based on
a finite element method formulation for nonlinear temperature dependent
viscoelastoplastic theology. Brittle behavior is modeled using
Mohr-Coulomb plasticity The models support a scenario in which passive
stretching leads to an unstable lithospheric configuration. Thermal
buoyancy related to this asthenospheric doming subsequently drives
active upwelling in a lithosphere scale convection cell. In the late
synrift to early postrift the lithospheric horizontal stresses caused by
the active asthenospheric upwelling may start to compete with the
far-field intraplate stresses. At this stage the domal forces may
dominate and even drive the system causing a change from passive to
active rifting mode. If this transition occurs, the model predicts (1)
drastic increase of subcrustal thinning beneath the rift zone, (2) lower
crustal flow towards the rift flanks, (3) middle crustal flow towards
the rift center, (4) the coeval occurrence of tensional stresses within
and compressive stresses around the upwelling region, and (5) possible
surface uplift. Late postrift thermal cooling removes the thermal
buoyancy forces. At this stage the far-field forces dominate the stress
state again and the lithosphere becomes more sensitive to small changes
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 lithosphere, the discrepancy between fault-related
extension and crustal thinning, late tend of synrift to early postrift)
mantle related volcanism, surface domal uplift succeeding rifting, and
rift hanks uplift associated with extension of a weak lithosphere.