Two Small Crustal Plates in the South Pacific near Easter Island

Abstract

THE evolution of the South Pacific during the Cainozoic involved several patterns of crustal plates in relative motion for various periods of time¹. The present East Pacific Ridge, which now outlines the Pacific-Nasca Plate boundary, has been the only major spreading centre north of 36° S for the past 9 m.y. Before then, a fossil ridge, continuous with the still active Chile Ridge^2,3 was a locus of spreading in what is now the middle of the Nasca Plate; between 9 and 20 m.y. ago, both the East Pacific and the Fossil Ridges were active, suggesting that the Antarctic Plate, which now occupies the area south of the Chile Fracture Zone between the Pacific-Antarctic and Chile Ridges, extended almost to the equator only 10 m.y. ago. The apparent ease with which the northern boundary of the Antarctic Plate has moved during the past 20 m.y. suggests the triple junction between the Antarctic, Pacific and Nasca Plates has been unstable.