INTERMEDIATE-WAVELENGTH (400-600 KM), SOUTH-PACIFIC GEOIDAL UNDULATIONS - THEIR RELATIONSHIP TO LINEAR VOLCANIC CHAINS

Intermediate-wavelength (400-600 km) geoidal undulations observed in SEASAT data from the South Pacific appear to be: (1) much more pronounced and extensive than the short-wavelength lineations of Haxby and Weissel (which start, and are confined to within 1500 km of the EPR); (2) copolar, aligned roughly parallel to the absolute motion of the Pacific plate; (3) continuous across transform faults or fracture zones; and (4) surmounted at their crests, in places, by linear volcanic seamount chains formed since 43 Ma. Whereas the short-wavelength lineations could be caused by shallow gravitational instabilities forming in the uppermost mantle just below the base of the lithosphere, we think that the intermediate-wavelength lineations result from dynamic, elastic deformation of the lithosphere caused by well-developed, secondary convection rolls extending much deeper into the upper mantle. Because individual hotspot volcanic chains invariably coincide with crestal highs of discrete positive geoid lineations, the location of the hotspots is probably controlled by the rolls.