West of the Tonga-Kermadec trench is a series of ridges and basins including, from east to west, the Tonga-Kermadec ridge (frontal arc), the Lau-Havre trough (interarc basin) and the Lau-Colville ridge (third arc). The frontal and third arcs have asymmetric cross sections with scarps on the flanks facing the interarc basin and substantial accumulations of sediment on the outer ridge flanks. Geology of exposed islands indicates that the Tonga-Kermadec ridge has been a frontal arc since the early Tertiary; the third arc is probably at least as old. A thin cover of rapidly deposited sediment suggests that the morphology of the interarc basin, which consists of linear ridges and troughs with approximate relief of 1000 meters, is no older than latest Tertiary. The interarc basin is underlain by a crust of oceanic character and is the locus of high heat flow. The regional morphology and distribution of sediment must be ascribed to tectonic activity, involving either creation of new oceanic crust or modification of older oceanic crust. An origin of the interarc basin by extensional rifting within an older frontal arc best satisfies the available data and suggests that the extension is related to intensification of island arc tectonism at the close of the Tertiary. Additional studies indicate that the South Fiji basin and at least several other marginal basins on the concave sides of western Pacific trenches have had a similar mode of origin at an earlier time. The result of such activity would be the migration of the trench-frontal arc complexes away from the Asian continent, with creation of new basins with oceanic crust on the convex sides of the frontal arcs.