DECIPHERING THE 4-D EVOLUTION OF CENOZOIC (U)HP TERRANES

Convergent plate boundary zones, where (U)HP terranes form, do not remain stationary, but evolve in both space and time, leading to considerable along strike variation in the geologic record. Exhumation mechanisms that invoke the motion of the upper plate away from the trench, and the retreat of the subduction hinge can be used to explain how buoyant (U)HP rocks travel back to the surface following subduction to >100 km depths, especially in cases where the stratigraphic record indicates erosion played a minor role. Both result in the removal of a tectonic lid, and lead to divergence within an overall convergent margin. The Late Miocene to Recent (U)HP terrane of eastern Papua New Guinea, (PNG) is a modern analogue for the evolution of (U)HP terranes. It formed within the larger obliquely convergent Australian-Pacific plate boundary zone, when a continental ribbon of Cretaceous volcaniclastic sediments and basalts was subducted northwards beneath oceanic lithosphere of the Solomon Sea (Woodlark microplate). Crustal subduction caused offscraping, subcretion/underplating, and formation of an accretionary wedge with rocks following many different P-T-t-D paths within the subduction channel. The north dipping subduction zone was eventually choked, and the counterclockwise rotation of the Woodlark microplate caused the reactivation of the former subduction thrust, leading to removal of the ophiolitic upper plate. As extension continued, rifting propagated from east to west, which triggered buoyant rise of lower density (U)HP rocks, while the upward flow of mantle asthenosphere impinged on the accretionary wedge, leading to volcanism and partial melting of the (U)HP terrane. Similarities between the (U)HP terrane of eastern PNG and those documented in the Eocene (U)HP rocks of the Western Alps include the time scales of (U)HP metamorphism and subsequent exhumation (<8 Ma), rates (>cm/yr), and mechanisms (divergence within an obliquely convergent plate boundary zone). Perhaps the greatest challenge for assessing (U)HP terrane evolution, especially in Mesozoic and older (U)HP terranes, is seeing through the effects of subsequent thermal overprinting, which drives recrystallization, resets thermochronometers, and generally destroys evidence for crustal subduction, (U)HP metamorphism and exhumation.