The origin of backarc basins has been a topic of much interest and some controversy to marine geologists, geophysicists and petrologists for several decades. This interest stems in part because of their location close to island arcs where different magma types are erupted synchronously with formation of the backarc basin crust. The thermal and stress regimes of backarc basins are of interest because they represent extensional tectonics, steep thermal gradients and mantle upwelling and melting in regions which are dominated overall by compression and subduction of old and cold sediments and crustal rocks to considerable depth within the mantle. The present award will support a geologic and geochemical study of the formation of the Lau Basin which lies to the west of the Tonga Trench and Tonga Ridge in the southwest Pacific. The Lau Basin began rifting within the last few million years and has shown a progressive change in basalt chemistry from a dominance of island arc related basalts to more normal backarc basin basalts. Such a change implies an evolution in the mantle source region from which the basalts are erupted. The field program will examine both the transition regions where mixed basalts have been erupted and the spreading ridge where normal basin crust is presently being emplaced. Subsequent laboratory analyses of recovered samples should fully define the backarc melting regime and allow a comparison of the Lau Basin with the more fully studied Marianas back arc basin. Data from the field program will also aid in defining potential sites for ocean drilling.
