Continent-arc collision and reversal of arc polarity: New interpretations from a critical area

Northern New Guinea has been regarded as a region where the polarity of an island arc reversed following collision with the Australian continent in the Tertiary. However, the evidence for this reversal is not compelling. Because present-day volcanism off the north coast of mainland Papua New Guinea is associated with a steeply northward-dipping Benioff zone (almost vertical) and late Cainozoic volcanoes in the central highlands to the south cannot be related to any present-day Benioff zone, a more acceptable interpretation is that, following collision, the northward-dipping slab beneath the arc became suspended nearly vertically. The active marginal basin lying to the north of the arc is unlikely to be subducted southwards beneath the mainland, because the lithosphere beneath marginal basins appears to be neither thick nor cold enough for the initiation of subduction. Polarity reversal, therefore, may not be the inevitable consequence of continent-arc collisions. Instead, the downgoing slab may steepen, equilibrate with the surrounding mantle and lose its identity. Continuing convergence may be taken up at other plate boundaries and the accreted arc may never again become active.