The Upper Cretaceous extrusive ophiolite unit of Western Anatolia, having unusual stratigraphic and structureal setting contrasts with ophiolites but is comparable with modern oceanic crust. The lavas of the extrusive ophiolite unit are dominantly alkali olivine basalt, basanite and subordinately transitional to olivine tholeiitic basalt, all of which are uniformly undersaturated. The compositional range and chemical variations available from major oxide data are consistent with an extensional back-arc environment where magma either vented to the surface from multiple centres or through multiple conduits. This model contrasts to interard basins with axial spreading, and can also be inferred on geological grounds.