Present-day mantle flow and its interaction with plate tectonics inferred from observational evidence

The current inference of mantle flow is largely based on seismic tomography and anisotropic analysis. The available results suggest that the two boundary driving forces for mantle flow on a global scale are the cooling on the top and the heating on the bottom of the mantle. The density and viscosity of the mantle control its flow rates. The mantle descending entrained by subducting slabs at trenches and the mantle rising within plumes under hotspots as well as mid-ocean ridges are the expression of vertical mantle flow. Under certain conditions, the global plate motions revealed by GPS and other measurements reflect the horizontal flow in the uppermost mantle. Mantle plumes marked by hotspots on the surface of the earth can have varied depths of origins. Rising flow within plumes can deflect by several thousand kilometers and turn to lateral flow at depths 200～350 km. Around some subduction slabs there are complicated patterns of mantle flow, indicating the influence of local tectonic settings. Below continents, two mantle seismic anisotropic layers are often observed, of which the deeper one may represent mantle flow that is associated with continental roots. In various tectonic settings, the interaction between mantle flow and plate tectonics appears in different forms, either mantle flow drives plates or producing a small drag force