The Lau Basin, which lies between the Tonga and Lau Ridges, is characterized by an absence of sediment, high but variable heat flow, and a confused magnetic anomaly pattern. A ridge 300 km long and 40 km wide runs northwest-southeast through the western part of the basin. This ridge is composed of fresh tholeiitic basalt and is associated with a linear band of shallow-focus earthquakes ( < 70 km). Chemical analysis of unaltered glassy and aphanitic phenocryst-free pillow fragments indicates that the parent liquids may have been derived from mantle depths of about 50 km.

The geophysical and geological data are interpreted within the framework of the theory of plate tectonics. It is suggested that the ridge and related earthquake epicenters mark the boundary between the India plate and a much smaller plate, the Tonga plate, which lies between the India plate and the Tonga Trench. The boundary is a transform fault marking the direction of motion of the two plates. A revised crustal consumption rate of 11 cm/yr for the Tonga Trench is required if the Tonga-India plate separation is added to the India-Pacific plate convergence. Continued NW-SE dilation and basalt intrusion during the past 10 m.y. has separated the Tonga and Lau Ridges and can explain the fresh basalt, the absence of sediment, and the high heat flow in the Lau Basin. This basin, although lying below the ridges on either side, has an average elevation more than 3,000 m above that of the deep Pacific. Intrusion of hot material at the center of the basin can account for this increase in elevation. The classic concept of the “andesite line,” separating oceanic and continental crust, clearly has no meaning as an indication of crustal type, as oceanic crust can be generated behind island arcs.

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