From intracratonic extension to mature spreading in back arc basins: Examples from the Okinawa, Lau, North Fiji basins

Three cruises (POP 1, SEAPSO 3 and SEAPSO 4) carried out during the "Tour du Monde" of the R.V. Jean Charcot allow us to obtain new structural and geochemical data for the Okinawa, North Fiji and Lau basins. Taken together, the structural and geochemical data illustrate a difference in evolution stages of these sites. The less evolved stage is represented by the Okinawa basin, where accretion is evidenced only by volcanic intrusion into a thinned continental crust. The most evolved stage is illustrated in the North Fiji basin, where the present-day accretion can be directly compm·ed to the East Pacific Ridge. The Lau basin displays an intem1ediate stage with a complex system of accretionary ridges. This difference in the evolution stage is translated in the nature and geochemistry of the rocks : in the Okinawa trough they are basalts, andesites and dacites enriched in hydromagmaphile m1d rare earth elements. In the Lau basin they are typical N. Morb basalts in the central pmt of the basin and inteimediate composition between Morb and m1desites on Valu Fa ridge. Lastly, the North Fiji basin axis provides only typical Morb basalts.