Brian D. Monteleone

Pliocene eclogite exhumation at plate tectonic rates in eastern Papua New Guinea

As lithospheric plates are subducted, rocks are metamorphosed under high-pressure and ultrahigh-pressure conditions to produce eclogites and eclogite facies metamorphic rocks. Because chemical equilibrium is rarely fully achieved, eclogites may preserve in their distinctive mineral assemblages and textures a record of the pressures, temperatures and deformation the rock was subjected to during subduction and subsequent exhumation. Radioactive parent–daughter isotopic variations within minerals reveal the timing of these events. Here we present in situ zircon U/Pb ion microprobe data that dates the timing of eclogite facies metamorphism in eastern Papua New Guinea at 4.3 ± 0.4u2009Myr ago, making this the youngest documented eclogite exposed at the Earths surface. Eclogite exhumation from depths of ∼75u2009km was extremely rapid and occurred at plate tectonic rates (cmu2009yr-1). The eclogite was exhumed within a portion of the obliquely convergent Australian–Pacific plate boundary zone, in an extending region located west of the Woodlark basin sea floor spreading centre. Such rapid exhumation (> 1u2009cmu2009yr-1) of high-pressure and, we infer, ultrahigh-pressure rocks is facilitated by extension within transient plate boundary zones associated with rapid oblique plate convergence.

Late Miocene coesite-eclogite exhumed in the Woodlark Rift

Late Miocene-Pliocene eclogites were exhumed in the Woodlark Rift of eastern Papua New Guinea, an actively extending region west of the Woodlark Basin seafl oor spreading center. We report the discovery of coesite in late Miocene eclogite from the lower plate of one of the DEntrecasteaux Islands metamorphic core complexes within the Woodlark Rift. Zircon crys- tallization temperatures (650-675 °C) and 238 U/ 206 Pb age (ca. 8 Ma), and rutile thermometry (695-743 °C) combined with garnet-pyroxene thermometry (600-760 °C) and garnet-pyroxene- phengite barometry (18-27 kbar), indicate that the coesite-eclogite was exhumed from mantle depths (≥90 km) to the Earths surface at plate tectonic rates (cm yr -1 ). This late Miocene coesite- eclogite is the youngest exhumed ultrahigh-pressure (UHP) rock on Earth, and its preservation ahead of the westward-propagating seafl oor spreading center forces reevaluation of models for UHP exhumation, as well as the geologic and tectonic evolution of the Woodlark Rift.

Continental rifting and metamorphic core complex formation ahead of the Woodlark spreading ridge, D'Entrecasteaux Islands, Papua New Guinea

[1]xa0We evaluate the role of a metamorphic core complex (MCC) on Normanby Island in the Woodlark rift. Located 1 km thickness of blueschist-derived mylonites formed in a midcrustal shear zone during the Pliocene at ∼400–500°C. This top-to-the-north zone appears to have reactivated the gently dipping base of the Papuan ophiolite (Papuan Ultramafic Body, PUB), and its continued activity appears to control the north dipping asymmetry of active half grabens to the north of the MCC and rapid subsidence of the Woodlark Rise. Mylonites in the MCCs lower plate have been exhumed along a detachment as a result of >50 km of slip at rates of >12 mm/yr. The inactive, back-tilted detachment preserves fault surface megamullions and mylonitic lineations parallel to the Plio-Pleistocene plate motion. A second SE vergent detachment has been established on the opposite flank of this rolling-hinge style MCC, probably since 0.8) at depth, and provide a sufficient mechanism for activating low-angle normal faults in the rift. MCC inception was not localized to the tip of the Woodlark MOR. Instead, extreme crustal thinning near the MCC preconditioned later continental breakup. The lower crust appears to be weak, thickening beneath unloaded footwalls to uplift MCCs above sea level, and flowing laterally to even out regional crustal thickness contrasts on a 1–6 m.y. timescale. Deep-seated transforms separate rheologically distinct domains in which extension has been localized along the weak PUB to cause MCC formation, vs. those in which slip is distributed across an imbricate zone of more uniform strength normal faults. The Trobriand fault connects in the eastern Woodlark rift to the Owen Stanley fault in the Papuan Ranges, which is probably moving at nearly the full plate velocity.