The Loyalty—New Hebrides Arc collision: Effects on the Loyalty Ridge and basin system, Southwest Pacific (first results of the ZoNéCo programme)

Abstract

I. ' I' ' , ,. , - * ' The Southwest Pacific region in the area of study is ~ dominated by the subduction of the Australian plate beneath the New Hebrides Arc at a rate of 12 cdy, with a direction of convergence that trends WSW- ENE (Dubois et al., 1977) (Figure 1). The subduction GULWS collision between the Loyalty Ridge and the New. Hebrides Arc. West of the collision zone, three Abstract. The ZoNeCo 1 and 2 cruises of Ifremer's Research Vessel L'Afalante, collected new swath bathymetry and geophysical data over the southern and northern segments of the basins and ridges formine the Lovaltv svstem. Between the two surveyed areas, previ- - - _- OU~ studies found evidence for the resistance of the Loyalty Ridge to subduction beneath the New Hebrides trench near 22 5-169 E. On the subducted plate, except for seismicity related to the down- ,parallel geological units that trend W-SE north of 22030' s and N-S south of this latitude are present . from east to west (Figure 1): bending of the Australian plate, recorded shallow seismicity is sparse within the Loyalty system (Ridge and Basin) where reliable focal mechanism solutions are almost absent. Swath bathymetry, seismic reflection and magnetic data acquired during the ZoNeCo 1 and 2 cruises revealed a transverse asymmetric morphology in the Loyalty system, and an along-strike horst and graben structure on the discontinuous Loyalty Ridge. South of 23'50' S and at 20 S, the two WSW-ENE-trending fault systems, respectively, sinistral and dextral, that crosscut the southern and northern segments of the Loyalty system, are interpreted as due to the early effects of collision with the New Hebrides Arc. A NNW- SSE trend, evident along the whole Loyalty system and on the island of New Caledonia, is interpreted as an inherited structural trend that may have been reactivated through flexure of the Australian lithospheric plate at the subduction zone. Overall then, the morphology, structure and evolution of the southern and northern segments of the Loyalty system probably result from the combined effects of the Australian plate lithospheric bulge, the active Loyalty-New Hebrides collision and the overthrust of the New Caledonian ophiolite.