Part 4. Plate boundary evolution in the Solomons region: The Solomons as a non-arc

Abstract

The Solomon Islands have been described as a fractured island arc (Coleman, 1970) and, more recently, as an arc which has undergone polarity reversal (Karig and Mammerickx 1972) probably within the last 10 m.y.. Both situations are envisaged as in keeping with the current widely accepted model of arc development as a consequence of the subduction process (Karig, 1974; Coleman, 1975). This in turn implies that during the Palaeogene the bulk of the Solomons was built as a result of subduction, south-dipping, bordering the northeastern flank. The evidence for such subduction is indirect and ambiguous, especially if the relationship of the Solomons to the Ontong Java Plateau (crust about 40 km) is considered (for localities and relevant illustrations see Hackman, 1973). Supporting features include a belt of high heat flow to the south of and parallel to the Solomons (Halunen and von Herzen, 1974); the suggestion of hanging, remnant slabs of lithosphere, an inference derived from the pattern of hypocentres south-east of Bougainville (Denham, this volume); a set of troughs (up to 6000 m) on the north-east flank, interpreted as a possible old trench; and the supposed obduction of the Ontong Java Plateau over the Solomon block (Kroenke, 1972). Against the notion of a NE-facing arc are the pattern and distribution of rock types and the apparent absence of discernible morphologic arc elements, on land or submarine. The presence of basal hi-T amphibolites and amphibolite schists on the northeastern side but of lower grade metamorphics on the other side does not support a north-east polarity, nor does the comparative dearth of calc alkaline rocks. The supposed old trench is actually on the Ontong Java Plateau. It might just as well be the expression of a fracture zone. As regards another supposed arc feature, the partial double chain character of the group is more apparent than real and does not in itself prove the existence of an inter-arc basin: the double chain can be explained as the effect of Neogene displacement of a single chain (Bougainville?Choiseul?Santa Isabel?Gaudalcanal?San Cristobal). The gravity picture, in which the higher positive values tend to lie along the southwestern side, is ambiguous. If it could be proved that the outer Pacific edge (e.g. Malaita, Ulawa) was a tectonic flake from the Ontong Java Plateau then this would be compelling evidence that the Plateau had indeed choked off an active subduction zone. But the evidence in support of this can also be used to support the idea of a left-lateral shearing collision between the Plateau and the Solomon chain. That this outer Pacific edge is indeed an uplifted part of the Ontong Java Plateau is accepted. Finally, there is no evidence of a tholeiite-calcalkaline-high K progression. The case for viewing the Solomons as a NE-facing arc during the Palaeogene is not proven.