On the effects of continental collisions on mantle flow

Both the thermal properties and the accretionary history of the continental cratons and fragments which assemble to form a supercontinent may govern the growing continent's stability. The influence of continental collisions on deep mantle flow is considered here by simulating continental aggregation in numerical mantle convection models incorporating distinct continental and oceanic plates. The results presented were obtained using a two-dimensional Cartesian geometry numerical mantle convection model with reflecting side walls. The continental plates included in the models differ from oceanic plates by their enforced buoyancy. Plate velocities adjust to the mantle flow as each model evolves so that plate motion neither drives nor impedes flow in the modelled mantle. In addition, migrating sites of oceanic plate subduction and oceanic plate creation are included in the models which respond to changes in plate velocity. A systematic survey investigates the effects of continental collisions on mantle flow. The influences of model Rayleigh number, continental width, model aspect ratio, continental accretionary history and internal heating of the mantle are each considered. The results of this study indicate that following a continental collision, a reorganization of mantle flow may result in the development of deviatoric stresses at the base of the continental lithosphere which are able to exert a net force capable of initiating continental breakup. The factors favouring eventual continental breakup are Rayleigh number dependent and therefore have different implications for whole mantle and upper mantle convection. An upper mantle convection scenario requires the presence of active mantle upwellings in continental breakup models. In contrast, supercontinents that form over a largely internally heated whole mantle (in which active upwellings are intrinsically absent) are less stable than similar continents which form over an entirely basally heated mantle. In the former case forces associated with subduction were found to dominate the stress fields within the supercontinents.