Abstract:
The Woodlark-Australia plate boundary is the only actively extending region within a
complex zone of microplates between the Australian and Pacific plates in SE Papua
New Guinea. Along the eastern part of this plate boundary, sea floor spreading in the
Woodlark Basin has been occurring since ~6 Ma and has propagated westward into the
extending continental crust of the Woodlark rift where it is now stalled at 151.4° E.
Continental extension rates of ~23 mm/yr (Wallace et al., 2004) west of the spreading
tip are among the fastest in the world. The rift to the west of the current spreading tip is
a known area of active low-angle normal faulting (e.g., the Moresby fault) and
metamorphic core complex (MCC) formation in the D’Entrecasteaux Islands. Misima
Island on the southern margin of the oceanic Woodlark Basin is an older MCC situated
on the Pocklington Rise, ~75 km to the south of the Woodlark spreading centre and 100
km east of its current tip. In this study geological observations, structural field data, aerial photographs,
metamorphic petrology, microstructures, and quartz CPOs, have been used to document
the tectonic evolution of Misima Island including the formation of a MCC on the island.
Metamorphic basement rocks on Misima Island contain ductile fabrics that I have
interpreted to have evolved during a progression from contractional (D₁ and D₂) to
extensional (D₃) deformation. In the Miocene deep-seated (lower crustal) collision
related fabrics in the continental rocks of the lower plate of the Weipoou detachment
fault underwent extensional shearing and a strain-related rotation and intensification in a
major (~2.2 km thick) ductile shear zone. This shearing was also expressed by
imposition of a new NE dipping mylonitic foliation (S₃ₐ) and NE trending stretching
lineation (L₃ₐ) onto these gneisses. The Weipoou shear zone was active at temperatures
of ~600º C at depths of 12-20 km and was supplanted in time by the brittle detachment
fault that currently bounds its upper surface. The sense of shear in the Weipoou shear
zone was top-to-the-NE and the finite strain path was strongly non-coaxial and 3D
including an imprint of constrictional deformation. As rocks from the Weipoou shear
zone were translated up the Weipoou detachment fault they were brittlely overprinted by
faulting, fracturing and brecciation. The upper amphibolite-facies ductile fabrics in the
Weipoou shear zone are well preserved on the surface today and did not experience a significant greenschist-facies deformational overprint. This relationship may reflect
their exhumation from the lower crust by dominantly brittle processes, involving
localised slip on a detachment fault that penetrated deeply into the middle-lower crust.
Deep embrittlement and localisation of slip on the fault could have been due to a
weakening effect of high strain-rates on a thick (>3-10 m thick) zone of fault gouge
material or due to prolonged maintenance of high pore-fluid pressures or high strain
rates to cause a deepening in the position of the brittle ductile transition.
