Intraplate en echelon volcanic ridges in the South Pacific west of the Easter microplate

Abstract

Satellite altimetric data for the south-central Pacific ocean show linear geoid anomalies extending from Pitcairn island (130°W-24°S) to the western border of the Easter microplate (118°W-26°S). Previously, these anomalies have been interpreted as a submarine volcanic chain more than 1300 km long, inclusive of Henderson 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 suggest 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°W and 118°W at 25°S, trending 275°–280° (parallel to plate motion). The ridges range from 70 to 200 km long, 20 to 35 km wide and 1100 to 2900 m high. Their volumes, ∼ 600 to 1500 km 3, are equivalent to those of large intraplate submarine volcanoes from the Polynesian hotspots. Sediment thickness and the degree of alteration of samples increase progressively from east to west, suggesting that the chain grew from a volcanic centre, located near 118°W–25°S, close to the West Rift that forms the western border of the Easter microplate. The ridges are built along a fracture zone of the Pacific plate, trending N100°–110°, which shifts the magnetic anomalies more than 300 km to the left. The origin of this fracture zone is not well assessed but it strikes in the continuity of the present-day Orongo transform fault forming the southwestern boundary between the Easter and Pacific plates, east of 114°50′W. The crests of the volcanic ridges are oriented N055°–075°, at angles of 30°–50° with respect to the fracture zone. The arrangement of both features could be compared to that of right-stepping opening cracks (volcanic ridges) along a sinistral shear zone (transform fault). An alternative model is based on the orientation of the abyssal hills which is almost perpendicular to that of the ridges, especially in the eastern part of the volcanic alignment. The origin of the en echelon ridges could also be related to tension and fracturing in the youngest lithosphere along leaky transform faults.