NEOGENE TECTONIC RECONSTRUCTION OF THE ADELBERT-FINISTERRE-NEW BRITAIN COLLISION, NORTHERN PAPUA-NEW-GUINEA

The Finisterre terrane is presently colliding with the Australian continent in northern Papua New Guinea. The terrane is exposed in two distinct blocks - the Adelbert block and the Finisterre block. A elastic sedimentary sequence in the suture zone provides constraints on the age of initial collision for each block and on paleotectonic reconstructions of New Guinea. Provenance shifts within the Finisterre block sedimentary section date the collision of that block at 3.0-3.7Ma. Analysis of the Adelbert block sedimentary section has not revealed similar provenance shifts. This absence of temporal trends in source area may be caused by oblique collision of the Adelbert block with an allochthonous terrane composed of oceanic crust. These factors would likely lead to a ''soft'' collision involving little uplift and hence little disruption of the pre-collisional sedimentation patterns. In contrast, the Finisterre block collided orthogonally with terranes composed of continental crust. This ''hard'' collision has led to rapid uplift and significant modification of the pre-collisional depositional system. A deep water basin existed between the Adelbert block and the continent in the Late Pliocene. Deep marine sediments deposited in this basin were subsequently overthrust by older lithologies of the Adelbert block. When combined with geochemical, seismic and plate kinematic data published by other workers, these data suggest collision of the eastern portion of the Adelbert block in the Middle to Late Pliocene. The western portion of the Adelbert block probably collided in the latest Miocene. Many tectonic reconstructions of northern Papua New Guinea have favored collision of the Finisterre terrane over a doubly-subducting Solomon Sea Plate in an extension of the modern tectonic configuration of the Solomon Sea. The presence of continentally-derived sediment in the Finisterre accretionary wedge casts doubt on this scenario. The trench of a southward-dipping subduction zone would be likely to block continentally-derived sediment from reaching the Finisterre accretionary wedge. The Maramuni Are, the igneous association usually attributed to the hypothesized southward-dipping subduction zone, appears to have erupted on allochthonous terranes rather than on autochthonous crust. These observations suggest that collision of the Adelbert block and most of the Finisterre block occurred above a single, northward-dipping subduction zone. The double subduction present in the Solomon Sea probably never extended more than 200 km west of its present location.