Is Microplate Rotation Driving HP–UHP Exhumation In Eastern Papua New Guinea?

The discovery of Late Miocene coesite-eclogite in Papua New Guinea (PNG) provides an unprecedented opportunity to examine mechanisms for exhumation from high–ultrahigh-pressure (HP–UHP) conditions to the surface in an active tectonic setting. Rapid oblique convergence between the Australian and Pacific plates has resulted in the formation of several microplates. Along the Papuan Peninsula, a NW–SE transition from a convergent to a divergent plate boundary is associated with counter-clockwise rotation of the Woodlark microplate relative to the Australian plate. HP–UHP rocks are found in the lower plates of metamorphic core complexes distributed along the divergent segment of the plate boundary, the Woodlark Basin, where a seafloor spreading system is propagating westward into a zone of continental extension. Exhumation of the HP–UHP rocks was facilitated by top-to-the-N or NE, km-scale ductile shear zones that reactivated a subduction thrust. Stretching lineations in metamorphic tectonites on Fergusson, Normanby, and Misima Islands trend parallel to Pliocene plate motion vectors of the Woodlark microplate. On the northern dip-slope of the Dayman Dome lineations trend approximately parallel to the present-day Woodlark–Australia plate motion vector. While exhumation from HP–UHP conditions to the surface in PNG can be attributed to two apparently distinct phases, non-coaxial normal-sense reactivation of the subduction thrust followed by coaxial rifting of the exhumed slab, the entire process represents a continuum in an evolving tectonic system. This neotectonic example of HP–UHP exhumation provides the basis for a 3-D model in which the magnitude of exhumation is a function of the relative rotation between upper and lower plates, the distance from the pole of rotation, and the geometry of the subduction zone. The model provides a mechanism for rapid exhumation and predicts spatial and temporal patterns of exhumation to assess its applicability to HP–UHP terranes worldwide.