The dynamics of rifting: deformation of the lithosphere by active and passive driving forces

The thermal and mechanical behaviour of the lithosphere during rifting is investigated using a two layer model for its rheology and different driving forces for the rift process. The uppermost crustal layer is assumed to deform by brittle failure, while the lower lithosphere, including the lower crust, is assumed to behave as a Newtonian viscous fluid. Results of numerical modelling of the deformation, flow and heat transport in the viscous layer are presented, in which viscosity is a strong function of depth (and temperature). Deformation of the upper brittle layer is complex and difficult to quantify. Thus deformation of this upper layer was approximated by a simple kinematic model in which the layer is thinned by an amount proportional to the amount of extension it experiences. Two fundamentally different driving forces were investigated. They correspond to active rifting, in which the lithosphere is thinned by an upwelling limb of a convection cell or mantle plume, and to passive rifting, or stretching of the lithosphere. The results show that for zones of stretching which are wider than the thickness of the viscous layer, thinning at all levels in the lithosphere can be described by a single parameter, β, as in earlier stretching models. This is not always true when the rift zone is narrow, or near the edges of the rift zone. Initial subsidence due to isostatic and thermal changes during stretching slows as stretching progresses, giving an exponential character to the rift stage subsidence. Active rifting is associated with thermal uplift. In this case lithospheric deformation is confined to a narrow zone near the base of the lithosphere, and thinning of the crust does not occur until the base of the lithosphere approaches that level. The rate of lithospheric thinning and width of the rift zone depend on the convection velocities in the underlying asthenosphere and on the viscosity-depth gradient near the lithosphere-asthenosphere boundary. Also contributing to the elevation changes during rifting are dynamic stresses which result from the flow. These dynamic affects may significantly modify the uplift or subsidence of the rift zone. Another important process which results from viscous flow is the generation of secondary convection in the upwelling asthenosphere. Ths will further thin the lithosphere and can cause depth-dependent stretching, in which the lower lithosphere is thinned more than the upper lithosphere. Both of these properties of the flow; dynamic stresses and secondary convection, critically depend on absolute values of viscosity and so are not well constrained at present. The model results presented here exhibit the degree of complexity which results from viscous flow in the lower lithosphere and which is matched by the wide variety of observations on rifting.