MODELING THE PLATE-DRIVING MECHANISM

This paper demonstrates how sub-lithospheric loading gives rise to plate interior stresses and plate boundary forces. The stressing of the lithosphere resulting from plume heads in the upper mantle (hot spots), from low density asthenospheric upwelling beneath ocean ridges and from dense subducting slabs has been modelled by finite element analysis. It is first demonstrated how a sub-lithospheric load, exemplified by a sub-continental plume head, produces local deviatoric stress in the strong upper lithosphere within a plate interior region. It is shown that a 500 km wide region of anomalous density averaging -8 kg m-3 between 100 km and 400 km depths gives rise to a deviatoric tension of 75 MPa in a 20 km thick strong layer forming the upper lithosphere above the deep buoyant load. The large stresses in the strong layer are caused by the shear drag and excess pressure exerted by the buoyant load. When such a stress system is cut across by a zone or plane of weakness, then plate boundary forces and distant plate interior stresses are produced- A weak zone beneath the crest is incorporated in a model of a normal ocean ridge, and this yields a ridge push force of 2.5 x 10(12) N m-1 referenced to old ocean floor and deviatoric compression of about 40 MPa in old ocean floor. A model of a ridge underlain by an anomalously low density upper mantle (plume head) yields a much larger ridge push force, and large deviatoric compressions of nearly 100 MPa extend into the bordering continents. Subduction pull (slab pull and trench suction) of about -4.5 x 10(12) N m-1 is modelled for a 300 km deep slab separated from the overriding plate by a weak fault, and collision pull is shown to result from downbulging lower lithosphere beneath collision mountain ranges.