MANTLE PLUMES AND CONTINENTAL TECTONICS

It is now recognised that plumes originate from a thermal boundary layer deep in the mantle. A new plume consists of a large head with a diameter of approximately 800 to 1200 km and it is followed by a smaller tail which has a diameter of 100 to 200 km. At the top of its ascent the head flattens to form a disk 2000 to 2500 km in diameter that may melt to produce a continental flood basalt or oceanic plateau. The tail melts to produce chains of volcanic activity such as the Hawaiian-Emperor Chain. The ascent of a new plume leads to surface uplift of 500 to 1000 m over a length scale of similar to 2000 km. This places the lithosphere under tension but the horizontal forces produced do not result in runaway extension unless the continental lithosphere is already anomalously hot and therefore weak. If the lithosphere is cold and strong, slow extension leading to a new spreading ridge may postdate plume arrival by 10 to 40 m.y.; the time difference being the time taken to conduct heat from the plume to the overlying lithosphere. Plume heads may control the timing and position of new ocean ridges but are not the fundamental force that drives plate tectonics. The heat from plume tails may weaken the overlying plate and pin the position of spreading. The ascent of a mantle plume can also have an important influence on subduction. Large oceanic plateaus, such as the Ontong-Java Plateau, ark more buoyant than normal oceanic crust because they have a thicker basaltic layer and because they are underlain by a 200 km thick layer of hot material from a mantle plume head. As a consequence oceanic plateaus are more difficult to subduct than normal oceanic crust. This can lead to jamming of plates at subduction zones, resulting in a ''flip'' in the sense of subduction as appears to have happened in the Caribbean region or to obduction of the plateau. Finally, when a plume head is emplaced beneath continental crust, heat released from the plume may lead to melting of the overlying crust. The granites associated with many greenstone belts and intercontinental magmatic provinces such as the Paleozoic Lachlan Fold Belt of Eastern Australia and the Middle Proterozoic ''anorogenic'' granites of the central and southwestern United States may have formed in this way.