ADVECTION OF PLUMES IN MANTLE FLOW - IMPLICATIONS FOR HOTSPOT MOTION,MANTLE VISCOSITY AND PLUME DISTRIBUTION

Because of their slow relative motion, hotspots, mainly in the Pacific , are often used as a reference frame for defining plate motions. A co herent motion of all Pacific hotspots relative to the deep mantle may, however, bias the hotspot reference frame. Numerical results on the a dvection of plumes, which are thought to cause the hotspots on the Ear th's surface, in a large-scale mantle flow field are therefore present ed. Bringing the results into agreement with observations also leads t o conclusions regarding the viscosity structure of the Earth's mantle, as well as the sources and distribution of plumes. The abrupt change in direction of the Hawaiian-Emperor chain implies an upper-mantle vis cosity under the Pacific of similar to 1.5 x 10(20) Pa s or less. Slow relative motion of hotspots requires high lower-mantle viscosity, unl ess hotspots are located at large-scale stationary upwellings that are currently unresolved by seismic tomography. For our preferred model, we obtain coherent motion of Pacific hotspots in a reference frame of no net rotation, as well as coherent motion relative to African hotspo ts, caused by return flow antiparallel to plate motion. Advection and regional differences in life expectancy can to a large part explain th e distribution of plumes in relation to ridges, subduction zones (pres ent and past) and seismic anomalies. Plume conduits are substantially tilted in the lower mantle. The surface motion of hotspots is often sm aller than the advection rate of plume conduits in the lowermost mantl e.