PLIOCENE-PLEISTOCENE ASYMMETRIC UNROOFING OF THE IRIAN FOLD BELT, IRIAN-JAYA, INDONESIA - APATITE FISSION-TRACK THERMOCHRONOLOGY

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 oro gen was generated since the middle Miocene by the collisional underthr usting of the northern passive margin of the Australian continent into a north-dipping subduction zone beneath the Melanesian island are. So uth of Puncak Jaya, the highest peak in New Guinea, Pliocene-Pleistoce ne uplift and unroofing have exposed Tertiary carbonates and Mesozoic elastic rocks of a rift and passive-margin sequence above unmetamorpho sed Paleozoic strata and greenschist-facies slates probably of Precamb rian age. Eight magmatic apatite samples from Pliocene intrusions of i ntermediate composition in the Gunung Bijih (Ertsberg) mining district at the crest of the Central Range in Trian Jaya, Indonesia, yield poo led fission-track ages ranging between 3.7 +/- 0.9 and 2.0 +/- 0.3 Ma (+/-1 sigma), Long etchable mean track lengths (>14 mu m) and narrow t rack length distributions indicate rapid cooling, as expected for shal low-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 t he range since the Pliocene. Seven detrital apatite samples from the T riassic-Jurassic Tipuma and the Carboniferous-Permian Aiduna Formation s 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 (+/-1 sigma). Etchable mean track lengths of approximate to 9-12 mu m and wide track length distributions indic ate slower cooling compared to the Pliocene intrusions. Complete reset ting of provenance fission-track ages in detrital apatite requires bur ial deeper than approximate to 4 km, Unroofing rates are approximate t o 1.7 km/m.y. on the southern flank of the range but <0.7 km/m.y, at t he crest. These fission-track data combined with local and regional ge ologic relationships indicate that unroofing of the Irian fold belt oc curred at least two and a half times faster, and perhaps more than fiv e times faster, on the lower slope than at the crest. This difference is probably due to regional weather patterns that result in up to appr oximate to 11 m/yr of orographically induced precipitation on the sout hern slope of the Irian Central Range but only approximate to 3 m/yr n ear the highest elevations.