THE LOYALTY-NEW-HEBRIDES-ARC COLLISION - EFFECTS ON THE LOYALTY-RIDGE-AND-BASIN-SYSTEM, SOUTHWEST PACIFIC (FIRST RESULTS OF THE ZONECO PROGRAM)

The ZoNeCo 1 and 2 cruises of Ifremer's Research Vessel L'Atalante, co llected new swath bathymetry and geophysical data over the southern an d northern segments of the basins and ridges forming the Loyalty syste m. Between the two surveyed areas, previous studies found evidence for the resistance of the Loyalty Ridge to subduction beneath the New Heb rides trench near 22 degrees S-169 degrees E. On the subducted plate, except for seismicity related to the down-bending of the Australian pl ate, recorded shallow seismicity is sparse within the Loyalty system ( Ridge and Basin) where reliable focal mechanism solutions are almost a bsent. Swath bathymetry, seismic reflection and magnetic data acquired during the ZoNeCo 1 and 2 cruises revealed a transverse asymmetric mo rphology in the Loyalty system, and an along-strike horst and graben s tructure on the discontinuous Loyalty Ridge. South of 23 degrees 50' S and at 20 degrees S, the two WSW-ENE-trending fault systems, respecti vely, sinistral and dextral, that crosscut the southern and northern s egments of the Loyalty system, are interpreted as due to the early eff ects of collision with the New Hebrides Are. 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 reacti vated through flexure of the Australian lithospheric plate at the subd uction zone. Overall then, the morphology, structure and evolution of the southern and northern segments of the Loyalty system probably resu lt from the combined effects of the Australian plate lithospheric bulg e, the active Loyally-New Hebrides collision and the overthrust of the New Caledonian ophiolite.