Diapiric exhumation of Earth's youngest (UHP) eclogites in the gneiss domes of the D'Entrecasteaux Islands, Papua New Guinea

The Woodlark Rift in Papua New Guinea hosts the world's youngest (2–8 Ma) eclogite-facies rocks and extensional deformation has played a key role in exhuming these (U)HP rocks at rates of > 20 mm/yr. During the Eocene Papuan arc-continent collision Australian Plate-derived continental rocks were subducted to (U)HP depths. There they remained for up to 30 m.y. until the Pliocene when asthenospheric circulation ahead of the west-propagating Woodlark spreading ridge introduced heat and fluids. This caused rocks to break away from the paleosubduction channel, recrystallize in the eclogite facies, and rise as Rayleigh–Taylor instabilities. The diapirs ascended adiabatically undergoing partial melting, which lowered their viscosity and increased buoyancy. (U)HP crust ponded near the Moho at ~ 2–4 Ma, thickening the crust to ~ 40 km (11 kb). Domal uplifts emerged above sea level, and these are still underlain by an unusually thick crust (> 26 km) for a rift that has stretched by factor of ~ 3 since 6 Ma. After ponding, they acquired a flat-lying foliation during amphibolite-facies retrogression. Vertical shortening accompanied the gravitationally driven outflow of ponded lower crust. The weak material was extended parallel to the rift margin, thinning ductilely by < 1/3. The flow was dominated by pure shear (Wk ~ 0.2), and was mechanically decoupled from – and orthogonal to – plate motion in the rift. Top-E shear fabrics suggest that this flow was westward, perhaps driven by isostatic stresses towards a strongly thinned rift corridor ahead of the Woodlark spreading ridge. At < 2 Ma, the gneisses were upwardly juxtaposed against an ophiolitic upper plate to form nearly symmetric gneiss domes that cooled at > 100 °C per m.y. and were mechanically incorporated into the rift's upper crust. Final exposure was by normal faulting and minor erosion. Such exhumation may also apply to other (U)HP terranes where less evidence for Moho ponding is preserved.

► Rocks stay cool in dead subduction zone for > 20 m.y., then UHP-crystallized as result of heating.
► Partially molten diapirs rise at > 2 cm/yr to form World's youngest (U)HP terrane.
► Diapirs ponded at Moho, then flowed outward under gravity, parallel to rift margin.
► Ductile flow was pure-shear dominant; thinning caused 10 km of exhumation.
► Rocks emplaced into upper crust as gneiss domes; exposed by normal faulting.