NEW GUINEA OROGEN: MODEL FOR THE 200 MILLION YEAR OLDER ZAGROS OROGEN OF IRAN

The Alpine-Himalayan orogenic system, which includes the Zagros Orogen of Iran, extends another 10,000 km southeast to New Guinea. Although having started almost 200 million years later, the New Guinea and Zagros Orogens incorporate similar tectonic elements, including re-attached continental blocks, magmatic arcs, deformation belts, foreland basins, and ophiolites. The orogens are similar because they were constructed by similar events.

The Zagros Orogen bounds the NW-SE suture between the Eurasian and Arabian plates. The Zagros orogenic cycle began with microcontinents (Sanandaj-Sirjan, central Iran, Helmand, other terranes) rifting from the African-Arabian plate margin in the Permian forming the Neotethys Ocean. The terranes moved northward on subducting Paleotethys seafloor toward Eurasia until docking in the Middle to Late Triassic. Northward subduction then jumped outboard, and a Jurassic-Cretaceous magmatic arc, possibly beginning in the Late Triassic, was generated at the Eurasian plate margin. Convergence was also responsible for formation of Mesozoic back-arc basins and for Jurassic and Cretaceous-Paleocene deformation. In contrast, the Arabian continent margin on the other side of Neotethys subsided passively until the Late Cretaceous when the margin became convergent and seafloor was obducted southwestward over the continent edge. Continental collision of Arabian and Eurasia and closure of Neotethys likely occurred during the Eocene to possibly Oligocene. The Neyriz-Kermanshah ophiolite trend marks the suture. Continued convergence formed the Neogene Zagros imbricate zone and fold-thrust belt and was responsible for major Pliocene-Quaternary strike-slip faulting.

Papua New Guinea from SW to NE consists of the Fly Platform, which is an extension of the Australian continent, the Papuan fold-and-thrust belt, the New Guinea Orogen, and several mostly volcanic islands. The New Guinea Orogen in PNG began with separation of microcontinents (Eastern and Papuan Plateaus, Owen Stanley and other terranes) from the Australian continent in the Paleocene, although rifting may have started in the Late Cretaceous. Paleocene-Middle Eocene oceanic crust of the Coral Sea Basin formed during separation. Another ocean apparently opened between detached microcontinents to the north as indicated by inferred Paleocene to Eocene ophiolites (Kami, Uyaknji, Marum terranes) between later re-sutured continental blocks. Mantle and oceanic crust of the Papuan Ultramafic Belt was obducted southward over the Owen Stanley terrane approximately concurrently with Paleocene seafloor spreading creating the Eastern Papuan composite terrane (EPTC).

The Australian plate began moving rapidly northward in the Eocene, and northward subduction began concurrently under the southern edge of the EPTC and an Eocene magmatic arc was generated above. Cretaceous strata and Paleocene-Eocene chert, shale, and deep-water limestone along the southwestern EPTC edge are strongly deformed and likely represent an accretionary wedge associated with the subduction. Continental terranes including EPTC accreted to the north-moving Australian continent starting in the Oligocene. Oligocene accretion is indicated by Oligocene siliciclastics overlapping sutures and by Late Oligocene-Early Miocene radiometric dating of metamorphism and minor magmatism in the orogen. The former subduction zone between the EPTC and the Eastern and Papuan Plateaus is represented by the Moresby Trough. Depression of the continent edge and formation of a foreland basin on the Australian continent edge followed docking with continued compression. The foreland basin includes the Aure Trough above the EPTC suture.

A new south-dipping subduction zone formed outboard of the accreted microcontinents in the Miocene. Melting related to this subduction generated the dominantly Middle Miocene Maramuni volcanic arc. The modern Trobriand Trough is a remnant of the former subduction zone. The Finisterre terrane-Bismarck arc, in turn, collided probably in the Late Miocene-Early Pliocene due to still-ongoing northward subduction at the New Britain Trench. Docking and continued convergence caused faulting and uplift in the orogen and formation of the Late Miocene-Quaternary Papuan fold and thrust belt and a second foreland basin. In addition, extension in the Woodlark Basin since 6 Ma likely is the result of by slab pull of the Solomon Sea plate under New Britain. The Australian plate is currently moving northward and the Caroline and Pacific plates are moving to the northwest. In time, if the motions continue (100 million years?), the Australian plate will collide with Eurasia possibly incorporating additional magmatic arcs and ophiolite slivers, and forming a continent-continent collision zone similar to the Zagros Orogen.