Thermomechanical models of active rifting

Geological and geophysical evidence (in particular thermal and gravity anomalies) suggests the presence of hot mantle material below the zones of continental rifting. Using a thermomechanical numerical model we quantify the lithospheric thinning caused by such a deep thermal anomaly. The mantle is assumed to be an incompressible non‐Newtonian fluid with temperature‐ and pressure‐dependent viscosity. The index power law is 3 in the mantle and 7 in the crust. This yields a viscosity minimum at the base of the lithosphere and therefore favors rapid convective thinning. The corresponding mass displacements disturb the lithosphere. The stresses at the bottom of the crust are used to quantify the vertical movements and the tectonic regime within the continental crust. The heat flow anomalies are also computed. The present work indicates that these deep geodynamic processes can induce well‐localized extensive tectonic stresses and the thinning of the lithosphere on a short time scale. This analysis also suggests that the subsidence in the rift valley is mainly due to crustal thinning while the uplift of the shoulders of the rift is a consequence of the advection of hot material from the asthenospheric channel. A regional extensional stress enhances the convective phenomena and favors the crustal thinning but is not necessary. A decrease in this far‐field force drastically increases the uplift of the shoulders and reduces the width of the rift valley. This case may be representative of the African rift.