Active transpression along the fast Pacific-Australian transform boundary revealed by swath mapping around the Futuna-Alofi Islands.

The Futuna-Alofi Ridge is one segment of the active transform boundary betw een the Australian and the Pacific plates located in a poorly known region of the Melanesian Borderland. The bathymetry and the structure of the ridge have been revealed during a short survey with the R.V. L'Atalante in Octob er 1999 using the EM-12 multibeam echo-sounder system. We show that the rid ge is located within a domain showing a typical oceanic fabric and could re present a piece of uplifted oceanic crust such as a wall of a previous frac ture zone. The ridge is connected to active transcurrent faults and suffers current compression along its western side and transpression along its eas tern-southeastern side. It must be considered as a compressional relay zone along a splay of the major Pacific-Australian transform boundary. (C) 2000 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.