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Forearc ophiolites: a view from the western Pacific

2003; Geological Society of London; Volume: 218; Issue: 1 Linguagem: Inglês

10.1144/gsl.sp.2003.218.01.26

2041-4927

John Milsom,

Geological and Geophysical Studies

Abstract Many of the world’s largest ophiolite masses are now being interpreted as remnants of oceanic forearcs stranded on continental margins in the course of arc-continent collision. This interpretation implies a former association between the ophiolite and a volcanic arc at a distance of 100–200 km. The New Caledonia region of the SW Pacific is one of the few areas where there is good evidence for the presence of such an arc. On New Caledonia itself, the Grand Massif du Sud ophiolite has been thrust over the Norfolk Ridge continental fragment, while the coral islands of the Loyalty Group, c. 100 km to the NE, cap large sub-sea edifices with volcanic morphology. New Caledonia can be correlated geologically with New Guinea, despite the considerable width of open ocean that now separates the two. In central New Guinea, where ophiolites emplaced along the northern flank of the main mountain spine lie some 100 km south of exposures of arc-volcanic basement in the north coast ranges, the similarities include the relationship between New Caledonia and the Loyalty Islands. There is, however, no obvious comparable relationship in the case of the largest of the New Guinea ophiolites, the Papuan Ultramafic Belt of the eastern peninsula, which is backed to the north and east by the oceanic Solomon Sea. An associated volcanic arc can be recognized in this area only by assuming a complicated history of collision, post-collision arc-forearc separation and sea-floor spreading, followed by renewed contraction and a very recent and continuing second collision. The case for such a sequence of events can be made on a number of grounds. If the processes responsible are general in their nature, they could explain the apparent absence of arc-volcanic belts in association with many other supposedly forearc ophiolites.