STRATIGRAPHIC CONSTRAINTS ON THE DEVELOPMENT AND TIMING OF ARC-CONTINENT COLLISION IN NORTHERN PAPUA-NEW-GUINEA

The Finisterre Mountains of northern Papua New Guinea are the site of an active, southeast-propagating collision between the Finisterre terr ane and the Australian continental margin. The clastic sedimentary seq uence on the southern flank of the range records the history of the co llision. The age of initial collision can be deduced from the second o f two provenance shifts that occur in the sequence, as revealed by a s tudy of 69 dated sandstones. The first shift, at about 16-18 Ma, was f rom volcanolithic sediments to mixed-provenance sediments rich in quar tz and metasedimentary lithic fragments. These latter sediments were p robably derived from the orogenic belt active along the Australian con tinental margin at that time. At 3.0-3.7 Ma the volcanic source was re juvenated. Initial collision at this time uplifted the Finisterre terr ane, causing the provenance shift. Here we break the stratigraphic seq uence into three units: the Sarawaget beds, the Erap Structural Comple x, and the Leron Formation. The Sarawaget beds are Middle Eocene to Ea rly Miocene volcanolithic sandstones that were deposited as clastic ap rons on the flanks of the Finisterre volcanic arc. Volcanic activity c eased in the Early Miocene. The Erap Complex is divided into two clast ic subunits, the Sukurum and Nariawang units, and one basaltic subunit , the Gorambampan unit. The Sukurum unit consists of turbidites deposi ted south of the extinct Finisterre Arc from the latest Early Miocene through the Early Pliocene. Much of the sediment in the unit was deriv ed from low- to medium-grade metasedimentary units on the active north ern margin of the Australian continent. Renewed subduction in the Late Miocene or Pliocene occurred along the New Britain Trench, just south of the remnant Finisterre Arc, closing an eastward-widening small oce an basin. The Sukurum unit probably represents an accretionary prism t hat formed at this subduction zone. Scattered seamounts that were scra ped off the downgoing plate are preserved as the Gorambampan unit. Ini tial collision of the Finisterre terrane with the continental margin a t 3.0-3.7 Ma uplifted the Paleogene volcanic rocks, which were rapidly eroded and deposited as the volcanolithic turbidites of the Nariawang unit. Southeastward propagation of the collision tip caused incorpora tion of the Nariawang unit into the accretionary wedge. Rapid uplift r aised the terrane foreland above sea level sometime after 1.3 Ma. The fluvial, lacustrine, and marginal-marine deposits of the Leron Formati on accumulated in a terrane foreland basin that developed south of the collision zone. Further propagation of the thrust front toward the fo reland incorporated the Leron Formation into the fold-and-thrust belt. Rapid uplift of the Finisterre Range and voluminous volcanolithic sed imentation continue to the present. In the Finisterre Range, arc-conti nent collision is marked by a provenance shift from metasedimentary an d quartzose detritus to volcanic detritus. In another modern arc-conti nent collision on Taiwan, collision has caused a shift from volcanic t o metasedimentary detritus. We attribute this discrepancy to the prese nce of the Finisterre terrane, an extinct arc, in the forearc region o f the Finisterre arc-continent collision and the absence of a similar terrane in the forearc area on Taiwan. The very different evolution of sandstone provenance in the active Taiwan and Finisterre collision zo nes shows that local factors can profoundly influence the provenance h istories of collision zones. No one model of sediment provenance evolu tion can characterize all collision zones, whether modern or ancient.