Collisional delamination in New Guinea: The geotectonics of subducting slab breakoff
Mark Cloos, Benyamin Sapiie, Andrew Quarles van Ufford, Richard J. Weiland, Paul Q. Warren, Timothy P. McMahon, 2005. "Collisional delamination in New Guinea: The geotectonics of subducting slab breakoff", Collisional Delamination in New Guinea: The Geotectonics of Subducting Slab Breakoff, Mark Cloos, Benyamin Sapiie, Andrew Quarles van Ufford, Richard J. Weiland, Paul Q. Warren, Timothy P. McMahon
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The spine of the island of New Guinea, the Central Range, is a 1300 km long by 100 to 150 km wide mountain belt with numerous peaks over 3 km elevation. This mountain belt began to form when the Australian passive margin entered a north-dipping subduction zone in the Middle Miocene. Regional relationships and fieldwork near the Ertsberg (Gunung Bijih) mining district in the western Central Range are the basis for making a detailed reconstruction of the events leading up to, and during, collisional orogenesis.
Most of New Guinea can be divided into five lithotectonic belts. From north to south, these are an accreted arc terrane, an upturned forearc basement, an underlying metamorphic belt, a highlands fold-and-thrust belt, and a foreland basin. In western New Guinea, the accreted arc/forearc is the northwestern end of the Melanesian Arc Terrane that was the product of two phases of subduction volcanism since the Eocene. This terrane is largely buried under debris shed northwards from the Central Range. The crystalline leading edge of the accreted arc/forearc terrane, the Irian Ophiolite Belt of Jurassic age, is upturned forearc basement forming the north flank of the Central Range. The ophiolite is underlain by the Ruffaer Metamorphic Belt, which comprises continental rise and slope sediments and probably some trench axis deposits that underwent subduction deformation and metamorphism since the Early Miocene. The metamorphic belt grades into the highlands fold-and-thrust belt, which contains carbonate shelf strata at least as young as 15 Ma. Kilometer-scale, angular to rounded folds are the dominant structures. Regional sedimentologic relationships indicate the highlands area has constituted a 500+ km long landmass since ca. 12 Ma. The southern flank of the western Central Range is a giant 300 km long by 30 km wide basement block that has been thrust southwards since 8 Ma, forming the Mapenduma anticline. Minor, but widely distributed, magmatism occurred along the spine of the western highlands from ca. 7.5 to 2.5 Ma. There is abundant evidence for minor left-lateral strike-slip faulting subparallel to the upturned bedding that was concurrent with igneous activity at 4–3 Ma.
These relationships, combined with consideration of the mechanical properties of the crust and lithospheric mantle, are the basis for the construction of a series of lithospheric-scale cross sections illustrating the process of collisional delamination. Subduction tectonism and metamorphism began at ca. 30 Ma. Underthrusting of Australian continental margin sediments was well under way by ca. 15 Ma, when small isolated islands emerged. Bulldozing of the sediment cover formed an elongate landmass by ca. 12 Ma, and siliciclastic sediment was shed southwards, overwhelming carbonate shelf sedimentation. Collisional orogenesis due to the jamming of the subduction zone and initiation of thick-skinned crust-involved deformation began at ca. 8 Ma. Magma generation due to asthenospheric upwelling and decompression of stretched lithospheric mantle occurred from ca. 7.5 to 3 Ma. Contractional deformation in the western highlands ended at ca. 4 Ma, when this region became a site of minor northwest-striking, left-lateral strike-slip faulting. Since ca. 2 Ma, offset has been localized along the Yapen fault zone near the north coast of the island.
Collisional delamination involved the decapitation of the crust, continued sinking of the subducted lithosphere, and the upwelling of asthenosphere into the rupture as fast as it separated. This ensuing adiabatic decompression melting manifested itself as a short-lived magmatic event and up to 2.5 km of vertical uplift, both centered on the spine of the collision-generated orogenic belt. Collisional orogenesis is still under way beneath the eastern Central Range, with delamination-generated magmatism in its waning stages. Starting at ca. 8 Ma, the tear rupturing the subducting Australian lithosphere propagated from west to east at a rate of ∼150 km/m.y.