Abstract

The Bewani-Torricelli-Prince Alexander Mountains, along the northern margin of Papua New Guinea, probably formed as a tholeiitic island arc in the Late Eocene-Early Oligocene. The arc is interpreted to have accreted to the margin by the Late Oligocene, but may have formed on a ribbon of extended continental crust along the New Guinea margin. Inferred roll-back of the subducting slab beneath New Guinea in the Early Miocene placed the margin into extension, creating starved graben in northern New Guinea and causing regional subsidence. Near the graben, metamorphic core complexes resulted as the upper crust was pulled off the lower crust along low angle detachments such that lower crustal rocks cooled rapidly from temperatures >500°C. The two inferred core complexes that have been dated show rapid cooling from 27-23 Ma and 20-18 Ma. Continued subduction beneath New Guinea resulted in formation of the Maramuni arc in the Middle Miocene and the end of extension. In the Late Miocene, collision of the Melanesian arc caused regional uplift of all basement of northern Papua New Guinea, mainly from 8-5 Ma, causing at least 3–4 km of denudation. The compressional deformation propagated south causing uplift, denudation and cooling in the Papuan Fold Belt at c. 4 Ma, but is continuing at the present.