Pliocene-Pleistocene asymmetric unroofing of the Irian fold belt, Irian Jaya, Indonesia: Apatite fission-track thermochronology

Abstract

The Central Range of New Guinea is an orogenic belt up to 100 km wide and more than 1300 km long with numerous peaks over 3 km high. The orogen was generated since the middle Miocene by the collisional underthrusting of the northern passive margin of the Australian continent into a north-dipping subduction zone beneath the Melanesian island arc. South of Puncak Jaya, the highest peak in New Guinea, Pliocene–Pleistocene uplift and unroofing have exposed Tertiary carbonates and Mesozoic clastic rocks of a rift and passive-margin sequence above unmetamorphosed Paleozoic strata and greenschist-facies slates probably of Precambrian age. Eight magmatic apatite samples from Pliocene intrusions of intermediate composition in the Gunung Bijih (Ertsberg) mining district at the crest of the Central Range in Irian Jaya, Indonesia, yield pooled fission-track ages ranging between 3.7 ± 0.9 and 2.0 ± 0.3 Ma (±1σ). Long etchable mean track lengths (>14 µm) and narrow track length distributions indicate rapid cooling, as expected for shallow-level intrusions. These track-length data and the observation that the Grasberg pluton was emplaced into its own volcanic cover indicate that <2 km of material has been eroded from the top of this part of the range since the Pliocene. Seven detrital apatite samples from the Triassic-Jurassic Tipuma and the Carboniferous–Permian Aiduna Formations and apatite from two igneous dikes exposed halfway down the southern slope of the range yield pooled fission-track ages ranging between 2.7 ± 0.7 and 2.0 ± 0.5 Ma (±1σ). Etchable mean track lengths of ≈9–12 µm and wide track length distributions indicate slower cooling compared to the Pliocene intrusions. Complete resetting of provenance fission-track ages in detrital apatite requires burial deeper than ≈4 km. Unroofing rates are ≈1.7 km/m.y. on the southern flank of the range but >0.7 km/m.y. at the crest. These fission-track data combined with local and regional geologic relationships indicate that unroofing of the Irian fold belt occurred at least two and a half times faster, and perhaps more than five times faster, on the lower slope than at the crest. This difference is probably due to regional weather patterns that result in up to ≈ 11 m/yr of orographically induced precipitation on the southern slope of the Irian Central Range but only ≈3 m/yr near the highest elevations.