EXHUMATION OF THE YOUNGEST HP ROCKS, AT PLATE TECTONIC RATES, DURING PLIO-PLEISTOCENE CONTINENTAL EXTENSION IN SE PAPUA NEW GUINEA

The transition from continental rifting to seafloor spreading (sfs) occurs in the Western Woodlark Basin (WWB) of SE Papua New Guinea (PNG), the most rapidly extending active continental rift system on Earth. West of the sfs rift tip eclogites, formed during prior subduction, are found as relics in the lower plates of active metamorphic core complexes (mccs) on Goodenough and Fergusson Islands. In situ ionprobe analysis of zircons in high pressure (HP) mineral assemblages document Latest Miocene (7-8 My) and Early Pliocene (4 My) eclogite facies metamorphism prior to mcc formation. Assuming exhumation along shallow (23-35°) extensional shear zones, average eclogite exhumation rates (15-41 mm/yr) determined from P-T-t-D data are comparable to sfs rates in the WWB.

Recent fieldwork in the eastern Prevost Range of Normanby Island, ~50 km SW of the rift tip, has documented the presence of retrogressed blueschists in the deepest structural levels exposed on the domal, northeastern part of the island. The main foliation in the range is interpreted as an exhumation-related lower plate fabric. This fabric dips 10-20° SSW and has a strong SSW stretching direction and shear fabrics indicative of top-to-the-NE shear, parallel to sea floor spreading in the adjacent WWB, suggesting that extension in the continental and oceanic crust are kinematically linked. Active NNE striking normal faults and/or transfer faults post-date the main foliation and tilt the range westward.

The discovery of retrogressed blueschists on Normanby Island, together with 1) eclogite relics preserved in the lower plates of mccs on Fergusson and Goodenough Islands and 2) blueschists previously reported in the Dayman Dome on the Papuan Peninsula, lead us to propose that a regionally extensive subduction complex was exhumed during continental rifting associated with the reorganization of the Australian-Solomon Sea/Woodlark plate boundary zone. The plate tectonic rates at which these HP rocks were exhumed indicate that the transition from convergent to extensional tectonism and accompanying decompression reactions can occur on timescales < 1 Ma. Ongoing P-T-t-D studies will determine whether the blueschists were exhumed during convergence, or during subsequent continental extension associated with the propagation of the WWB spreading center.