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 evidence 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 compared to the East Pacific Ridge. The Lau basin displays an intermediate stage with a complex system of accretionary ridges. This difference in the evolution stage is translated in the nature and geochemistry of the rocks.

Rock samples have been dredged clming S0-35 and SEAPSO JV cruises (1984 and 1985) from cliJTemnl spreading ridges in lhe Lau Basin, soulh-weslern Pacific. They were analyzed for major and lrace elemenls and isotopic ralios (Sr, Nd). Nmlhem Lau Basin samples show N-MORB type characlerislics, bolh in major andlrace elemenls, whereas soulhern Lau Basin ones are differenlialed and show significanl Ta-Nb negalive anomalies in lheir extended REE pallerns, which is characlerislic of subcluclion zones.lsolopic ralios (Sr, Nd) are quile helerogeneous and inlermediale bel ween MORB and island arc values. The norlhern Lau Basin Ridge has geochemical characlerislics suggeslive ofmedio-oceanic lype spreading, whemas lhe soulhem LauBasin Ridge (Valu Pa Ridge) wilh ils geochemical characleristics influenced by subcluclecl slab material, could eilher be a nascenl island arc or corresponcllo an inilial slage of back-arc opening. A comparison of lhe LauBasin resulls vvilh lhose from the North Fiji Basin andlhe Okinawa Trough shows geochemicallrencls thalmflecllhe evolution from a rifling slags (island arc charac:Lerislics) loa malum slage (MORB characteristics) of back-arc basins.

The aim of the Japanese-French Kaiyo 87 cruise was the study of the spreading axis in the North Fiji Basin (SW Pacific). A Seabeam and geophysical survey allowed us to define the detailed structure of the active NS spreading axis between 16° and 22° S and its relationships with the left lateral motion of the North Fiji Fracture Zone. Between 21° S and 18°10′ S, the spreading axis trends NS. From 18°10 S to 16°40 S the orientation of the spreading axis changes from NS to 015°. North of 16°40′ S the spreading axis trends 160°. These two 015° and 160° branches converge with the left lateral North Fiji fracture zone around 16°40′ S to define an RRFZ triple junction. Water sampling, dredging and photo TV deep towing give new information concerning the hydrothermal activity along the spreading axis. The discovery of hydrothermal deposits associated with living communities confirms this activity.