INTRAPLATE EN ECHELON VOLCANIC RIDGES IN THE SOUTH-PACIFIC WEST OF THE EASTER MICROPLATE

Satellite altimetric data for the south-central Pacific ocean show lin ear geoid anomalies extending from Pitcairn island (130 degrees W-24 d egrees S) to the western border of the Easter microplate (118 degrees W-26 degrees S). Previously, these anomalies have been interpreted as a submarine volcanic chain more than 1300 km long, inclusive of Hender son island, Ducie island and Crough seamount, and constructed on young (< 30 Ma) oceanic lithosphere. Recent bathymetric data and side-scan sonar images (GLORIA) from the eastern part of this volcanic chain sug gest that the anomalies correspond to a succession of elongate ridges arranged in a right-stepping en echelon pattern along a fracture zone. At least five volcanic ridges were recognized during the SO80a cruise (1992) between 123 degrees W and 118 degrees W at 25 degrees S, trend ing 275 degrees-280 degrees (parallel to plate motion). The ridges ran ge from 70 to 200 km long, 20 to 35 km wide and 1100 to 2900 m high. T heir volumes, similar to 600 to 1500 km(3), are equivalent to those of large intraplate submarine volcanoes from the Polynesian hotspots. Se diment thickness and the degree of alteration of samples increase prog ressively from east to west, suggesting that the chain grew from a vol canic centre, located near 118 degrees W-25 degrees S, close to the We st Rift that forms the western border of the Easter microplate. The ri dges are built along a fracture zone of the Pacific plate, trending N1 00 degrees-110 degrees, which shifts the magnetic anomalies more than 300 km to the left. The origin of this fracture zone is not well asses sed but it strikes in the continuity of the present-day Orongo transfo rm fault forming the southwestern boundary between the Easter and Paci fic plates, east of 114 degrees 50'W. The crests of the volcanic ridge s are oriented N055 degrees-075 degrees, at angles of 30 degrees-50 de grees with respect to the fracture zone. The arrangement of both featu res could be compared to that of right-stepping opening cracks (volcan ic ridges) along a sinistral shear zone (transform fault). An alternat ive model is based on the orientation of the abyssal hills which is al most perpendicular to that of the ridges, especially in the eastern pa rt of the volcanic alignment. The origin of the en echelon ridges coul d also be related to tension and fracturing in the youngest lithospher e along leaky transform faults.