In the easternmost part of the Manus basin, arc volcanism and back-arc rifting are occurring in a basement of Eocene to Oligocene arc crust immediately north of New Britain ( southeast rifts, Fig.5C). A 60-km-wide extensional zone has (level- oped in the old arc crust and is likely the precursor of a back- arc spreading center, although the absence of magnetic lineations indicates that true ocean-floor spreading is not yet occurring (Martinez and Taylor, 1996). A series of en echelon volcanic ridges, 20 to 30 km long,