MANTLE CONVECTION AND STABILITY OF DEPLETED AND UNDEPLETED CONTINENTAL LITHOSPHERE

We address the question of how convective processes control the thickn esses of oceanic and continental lithospheres. The numerical convectio n model involves a Newtonian rheology which depends on temperature and pressure: A repeated plate tectonic cycle is modelled by imposing a t ime-dependent surface velocity. One part of the surface, representing a continent, never subducts. The asymptotic equilibrium thickness of t he lithosphere varies with the viscosity at the base of the lithospher e, but is not directly sensitive to the pressure, dependence of the vi scosity law and to the plate velocity. For small activation volumes, a nd average upper mantle viscosities deduced from postglacial rebound, the equilibrium plate thickness is more than 400 km (regimes 1 and 2). The equilibrium thickness of the oceanic lithosphere (around 100 km) implies that the viscosity in the,asthenosphere is less than 7x10(18) Pa s. Only models with strongly pressure-dependent viscosity laws (act ivation volumes greater than 9x10(-6) m(3)/mol) are able to reconcile this value with the average upper mantle viscosity (5x10(20) Pa a). Fo r these models, there are two lithospheric thicknesses such that the h eat supplied by convection at the base of the lithosphere equals the s urface conductive heat flow (regime 3). They could be that Of an aged oceanic lithosphere and that of a shield Lithospheric root; Thy indeed appear as points of preferred thickness in our numerical model. Howev er, convection triggered by the lateral density jumps at the boundarie s between the root and the thinner lithosphere slowly destabilizes the thick lithosphere: A plausible degree of chemical buoyancy in a deple ted lithospheric root does not prevent convective erosion. In our simu lations, long-term stability of a cratonic lithospheric root is best a chieved when its material is both buoyant and more viscous than the su rrounding mantle. Extensive devolatilization of the refractory rocks f orming the root is invoked to explain this viscosity increase.