Abstract
The Pacific Plate is being consumed under the Lau-Tonga Ridge down a subduction zone dipping at 45°. The trench inner wall has ultramafic rocks exposed, possibly within the accretionary wedge or prism. The fore-arc basin has a pre-Late Eocene basement overlain by 3?5 km of Late Eocene to Recent sediments. Plio-Pleistocene volcanics occur at the rear of the present fore-arc zone. The Lau basin was formed by rifting apart of the former Lau-Tonga Ridge between the fore-arc and the volcanic arc in the Early Pliocene. The Lau Ridge (the old volcanic arc) is capped by a line of volcanic piles certainly as old as Miocene but probably extending back to the Late Eocene. The size of the accumulated pile increases towards the north. The western flank of the Lau Ridge was built by a volcanic accumulation of over 3 km thickness on oceanic crust of the South Fiji Basin formed during the Oligocene. This sea floor postdates the inception of arc volcanism. In the discussion it is suggested that (a) the region has had high heat flow since the volcanism started and most of the magma has come from the mantle wedge overlying the downgoing slab, (b) the volcanism was initiated in the Tonga region over a steeply dipping Benioff Zone, (c) the locus of volcanism moved rearwards during the Late Eocene as the dip of the Benioff Zone became gentler, (d) the crust of the region reached its present thickness by (i) addition of sediment to the Tonga Ridge after the first volcanic phase and volcanic activity on the Lau Ridge and (ii) intrusions into the deeper crust, (e) the crustal thickening by both processes increased to the north as subduction rates increased away from the Pacific-India plate poles of relative rotation.
Published Version
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