ABSTRACT

The Finisterre Mountains of northern Papua New Guinea are the site of an active, southeast-propagating collision between the Finisterre terrane and the Australian continental margin. The clastic sedimentary sequence on the southern flank of the range records the history of the collision. The age of initial collision can be deduced from the second of two provenance shifts that occur in the sequence, as revealed by a study of 69 dated sandstones. The first shift, at about 16-18 Ma, was from volcanolithic sediments to mixed-provenance sediments rich in quartz and metasedimentary lithic fragments. These latter sediments were probably derived from the orogenic belt active along the Australian continental margin at that time. At 3.0-3.7 Ma the volcanic source was rejuvenated. Initial collis on at this time uplifted the Finisterre terrane, causing the provenance shift.

Here we break the stratigraphic sequence into three units: the Sarawaget beds, the Erap Structural Complex, and the Leron Formation. The Sarawaget beds are Middle Eocene to Early Miocene volcanolithic sandstones that were deposited as clastic aprons un the flanks of the Finisterre volcanic are. Volcanic activity ceased in the Early Miocene. The Erap Complex is divided into two clastic subunits, the Sukurum and Nariawang units, and one basaltic subunit, the Gorambampan unit. The Sukurum unit consists of turbidites deposited south of the extinct Finisterre Arc from the latest Early Miocene through the Early Pliocene. Much of the sediment in the unit was derived from low- to medium-grade metasedimentary units on the active northern margin of the Australian continent. Renewed subduction i the Late Miocene or Pliocene occurred along the New Britain Trench, just south of the remnant Finisterre Arc, closing an eastward-widening small ocean basin. The Sukurum unit probably represents an accretionary prism that formed at this subduction zone. Scattered seamounts that were scraped off the downgoing plate are preserved as the Gorambampan unit. Initial collision of the Finisterre terrane with the continental margin at 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 incorporation of the Nariawang unit into the accretionary wedge. Rapid uplift raised the terrane foreland above sea level sometime after 1.3 Mo. The fluvial, lacustrine and marginal-marine deposits of the Leron Formation accumulated in a terrane foreland basin that developed south of the collision zone. Further propagation of the thrust front toward the foreland incorporated the Leron Formation into the fold-and-thrust belt. Rapid uplift of the Finisterre Range and voluminous volcanolithic sedimentation continue to the present.

In the Finisterre Range, arc-continent collision is marked by a provenance shift from metasedimentary and quartzose detritus to volcanic detritus. In another modern arc-continent collision on Taiwan, collision has caused a shift from volcanic to metasedimentary detritus. We attribute this discrepancy to the presence of the Finisterre terrane, an extinct arc, in the forearc region of 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 zones shows that local factors can profoundly influence the provenance histories of collision zones. No one model of sediment provenance evolution can characterize all collision zones, whether modern or ncient.