COLLISIONAL DELAMINATION IN NEW GUINEA: THE GEOTECTONICS OF SUBDUCTING SLAB BREAKOFF

The formation of the island of New Guinea has long been cited as the product of a Cenozoic arc-continent collision. Geologic studies in the Central Ranges of the Irian Jaya Province of Indonesia (near Puncak Jaya, 4884 m) has revealed field and timing relationships which combined with mechanical considerations leads to a refined model for the tectono-magmatic effects of collisional orogenesis.

In west New Guinea, shelf limestone (New Guinea Limestone Group) sedimentation on top of Australian continental basement occurred as recently as 15 Ma. At the top of the ranges (Puncak Jaya), these and underlying strata (Kembelangan Group) are deformed into km-scale open folds with minor reverse faulting. Exposures along the access road to the Ertsberg (Gunung Bijih) Mining District reveal an upright, north-dipping section, at least as old as Devonian, that overlies a basement of greenschist facies slates and phyllites of probable Precambrian depositional age. Regional mapping indicates that the ~ 9 km thick road section forms the northern limb of the 30+ km wide Mapenduma anticline. Apatite fission track thermochronology indicates the mid-slope region has been unroofed at rates of 1-2 km/m.y. since the end of the Pliocene. Extrapolation of this result to the base of the slope indicates unroofing of the Mapenduma anticline began at ~8 Ma. Petroleum exploration drill holes near the southern slope indicate siliciclastic sedimentation began at ~12 Ma and abruptly changed to conglomeratic molasse deposition between 6 to 4 Ma. In the Ertsberg District, intermediate intrusions (4.4-2.6 Ma) cross-cut the axis of a km-scale fold and were emplaced in a left-lateral strike-slip fault system subparallel to the regional structural grain created by folded strata. The little studied northern slope of the Central Ranges is a belt of metamorphosed Australian passive margin strata and the Irian Ophiolite Belt.

The generation of the Puncak Jaya sector of the Central Ranges is explainable by a plate tectonic process we term collisional delamination. Northwards dipping subduction of the oceanic end of the Australian Plate began prior to 20 Ma at a Mariana-type subduction zone. Sediment accretion began once Australian margin strata entered the subduction zone. The top of an accretionary prism broke the surface at ~12 Ma. Jamming of the subduction zone by the underthrusting of Australian continental crust-capped plate occurred at ~6 Ma and was immediately preceded by the detachment and southward displacement (~10-20 km) of the large basement block forming the Mapenduma anticline. The subducted oceanic end of the Australian Plate did not stop and dangle, but rather broke off (see Sacks and Secor, 1990). The tectonic aftermath of subterranean plate rifting between 6-3 Ma included magmatism generated by adiabatic decompression melting of the lower continental lithosphere ± upwelling asthenosphere, and a rapid isostatically-driven vertical uplift of 1-2 km of the collision-generated fold belt.