SUBDUCTION TO RIFTING EVOLUTION OF THE AUSTRALIAN-WOODLARK PLATE BOUNDARY ZONE OF EASTERN PAPUA NEW GUINEA: INSIGHTS INTO THE 4-D NATURE OF CONTINENTAL SUBDUCTION AND EXHUMATION PROCESSES

Oblique Australian–Pacific (AUS–PAC) convergence (10–11 cm/yr) in eastern Papua New Guinea led to rapidly evolving plate boundaries, and the formation and rotation of microplates. It is widely recognized that active northward subduction of the Solomon Sea lithosphere occurs at the New Britain and San Cristobal trenches, however, controversy exists as to whether southward subduction of the Solomon Sea lithosphere at the Trobriand trench is, or has been active. Within the southernmost (i.e., AUS- Woodlark) plate boundary, a rifted subduction complex transitions along strike into a seafloor spreading system.

Mafic eclogite and blueschist relicts occur in the lower plates of metamorphic core complexes west of the active Woodlark Basin seafloor spreading rift tip. These HP/UHP rocks formed when the Australian passive continental margin was subducted northwards beneath a late Paleocene–Early Eocene island arc built on oceanic lithosphere. Zircon growth (≈8-2 Ma) under eclogite facies conditions, structural and field evidence indicates both mafic eclogites and host quartzo-feldspathic gneisses were metamorphosed together. Thermochronology, thermobarometry and fabric analysis indicates exhumation occurred at plate tectonic rates (cm/yr) and was diachronous from east to west, prior to, and synchronous with seafloor spreading in the Woodlark Basin (≤6 Ma). Mylonites preserve top-to-the-north shear indicators with stretching lineations parallel to Plio-Pleistocene plate motion. Low geothermal gradients (<8°C/km), attained only in subduction zones, were required to form ≈8 Ma coesite eclogite that was subsequently exhumed from >90 km. Coesite-quartz transformation kinetics and preservation of blueschist and eclogite facies assemblages do not support back-arc basin exhumation models. Instead counter-clockwise rotation of the Woodlark plate relative to the Australian plate likely reactivated the former subduction thrust, resulting in extensional eversion of the subduction complex. The Late Miocene-present 4-D evolution of the AUS-Woodlark plate boundary provides an unprecedented opportunity to study how lithosphere evolves petrologically, rheologically, and thermally during the transition from subduction to rifting and seafloor spreading.