Rifting History of the Woodlark Basin in the Southwest Pacific

Abstract

Marine geophysical evidence has been obtained for the rate and history of sea-floor spreading in the Woodlark Basin. The eastern part of this basin is presently separating from the Australian Plate at over 4 cm per yr in a northerly direction. The western part of the basin is not presently spreading. This spreading rift marks the southern boundary of the Solomons Plate which is bounded by subduction zones in the north and east (the New Britain and northern Solomons Trenches, respectively) and in the west by a combination strike-slip rifting (dip-slip) boundary in eastern Papua (New Guinea). A vector triangle solution near the Solomons Trench-Woodlark Rift triple point gives underthrusting of the Solomons Plate beneath the northern Solomon Trench in a northeasterly direction at about 11 cm per yr. The Woodlark Basin began opening as a sphenochasm, with a pole near the tip of eastern Papua about 20 m.y. B.P. This was caused by left-lateral shear in the region induced by a change in the relative motion pole of the Australia and Pacific Plates. The basin opened only a few degrees at this time, then stopped. Rifting in the entire basin resumed about 3 m.y. B.P., based on magnetic anomaly data. About 1 m.y. B. P., the spreading center in the western basin shifted to the Woodlark Rise.