ON THE COMPLEX RELATIONSHIP BETWEEN PLATE TECTONICS, LITHOSPHERIC AND SURFACE PROCESSES REVEALED IN METAMORPHIC CORE COMPLEXES OF THE WOODLARK RIFT, EASTERN PAPUA NEW GUINEA

Timescales and rates of HP-UHP exhumation in the Woodlark Rift provide new insight into how rapidly orogens at active plate boundaries can evolve. Subducted crust is tracked from mantle depths, where mixing with the mantle has occurred, through the brittle-ductile transition to the surface, where the landscape reflects continued rock uplift and ongoing mantle influence. Subduction of the Woodlark microplate at the New Britain trench is driving sea-floor spreading in the Woodlark Basin and extension in the Woodlark Rift to the west. There, metamorphic core complexes (MCCs), active since the Pliocene, form the D'Entrecasteaux Islands. MCC lower plates expose previously subducted sediments and basalts that have been variably deformed, metamorphosed, intruded by magmas, and partially melted during exhumation. Some of the mafic eclogites display Hf and Nd isotopic compositions consistent with contributions from a juvenile source (i.e. ε Hf of +11.3 to +11.9 and ε Nd of + 5.2 to +6.2), interpreted to reflect crystallization, at HP-UHP conditions, of basaltic melt derived from juvenile mantle. Quaternary andesites and rhyolites erupted through the core complexes have similar Hf and Nd isotopic compositions. The MCCs form a near-linear belt of increasing topography to the west, reaching progressively higher elevations (>2500 m in the D'Entrecasteaux Islands and >3500 m in the Suckling-Dayman massif). Low-angle normal faults bound the MCC's and define dip-slopes, beneath which mylonites and ductile shear zone rocks are exposed. Stream profile analysis of MCC footwalls reveal dynamic topography, with prominent knickpoints inferred to result from an increase in rock uplift following a shift in Australian-Woodlark relative plate motions at ~0.52 Ma. Crust beneath the western D'Entrecasteaux Islands is relatively thin (26-29 km) with high topography supported by buoyant mantle. Details of the depth-time relationships, and mantle influence on the role of magmatism and migmatization in the Woodlark Rift are only beginning to be revealed. However, results indicate that since ~8 Ma juvenile mantle has contributed basaltic melts during the subduction-to-rifting plate boundary transition, MCC evolution, and dynamically supports the high topography in the D'Entrecasteaux Islands.