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

Abstract

We evaluate the role of a metamorphic core complex (MCC) on Normanby Island in the Woodlark rift. Located <30 km from an active mid‐ocean ridge (MOR), a >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 MCC's 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 <0.5 Ma. Centimeters per year slip rates on these two faults can account for most of the Pleistocene plate motion in this eastern sector of the Woodlark rift, and confirm the important role of MCCs in exhuming very young HP rocks in this rift. Paleopiezometry of mylonites using recrystallized quartz grain size indicates flow stresses of ∼30 MPa before the rocks were overprinted by extension fractures. These results imply high pore fluid pressures (λ > 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.