HR: 0800h
AN: T41C-1213    [Abstracts]
TI: High Ductile Shear Strains and Strongly Localised Brittle Slip Associated With a Low-Angle Detachment Fault on Misima Island, Woodlark Rift, Papua New Guinea
AU: * Peters, K J
EM: peterskati@student.vuw.ac.nz
AF: School of Earth Sciences, Victoria University P.O. Box 600, Wellington, 6000 New Zealand
AU: Little, T A
EM: timothy.little@vuw.ac.nz
AF: School of Earth Sciences, Victoria University P.O. Box 600, Wellington, 6000 New Zealand
AU: Baldwin, S L
EM: sbaldwin@syr.edu
AF: Dept. of Earth Sciences, Syracuse University, Syracuse, NY 13244-1070 United States
AU: Fitzgerald, P G
EM: pgfitzge@syr.edu
AF: Dept. of Earth Sciences, Syracuse University, Syracuse, NY 13244-1070 United States
AB: The Woodlark rift system forms the boundary between the Solomon Sea Plate and Australian Plate. Misima Island is a metamorphic core complex (MCC), on the Pocklington Rise, $\sim$75 km to the south of the Woodlark spreading centre and $\sim$100 km east of the western tip of sea-floor spreading. Structural data and microstructural analysis reveal that Misima Island is different from other MCCs, including the D'Entrecasteaux Islands MCCs that lie west of the propagating ridge tip. Geological fieldwork on Misima Island has confirmed the presence of a low angle detachment fault that dips $\sim$25$\deg$ to the NE. Amphibolite-facies felsic and mafic gneisses, occur in the lower plate, whereas unmetamorphosed Quaternary-Tertiary volcanic and sedimentary rocks, and low-grade phyllites occur in the upper plate. A flight of 6 coral terraces on the southern coast of the island have been uplifted to a maximum elevation of $\sim$300 m on the lower plate. The eastern side of the island has been uplifted to a lesser extent resulting in $\sim$40 m high coral cliffs at the coast. The juxtaposition of amphibolite-facies rocks in the lower plate against phyllites and unmetamorphosed conglomerates in the upper plate implies that the magnitude of oblique-slip on the fault is on the order of tens of kilometres. Exhumation related ductile deformation has resulted in NNE trending stretching lineations that occur within a $\sim$2 km thick shear zone immediately beneath the detachment. The shear strain gradient increases upwards towards the detachment fault, where there are abundant $<$1 m thick shear zones. The measured NNE slip direction (slickenlines, lineations) is parallel to the Australia-Solomon Sea plate vector between 500 ka and 3.5 Ma, calculated from seafloor spreading data (Taylor et al., 1999). Pre-existing E-W trending stretching lineations show a pattern of progressive rotation towards the detachment fault. We suggest these lineations have been passively rotated by ductile shear and have numerically modelled this process as a function of oblique-slip simple shear parallel to the fault. The results of this model fit the observed lineation pattern implying that ductile deformation in the lower plate did occur chiefly by simple shear. The model suggests that shear strains as high as 10 to 20 are obtained immediately below the detachment in this shear zone and that the zone accommodates a ductile displacement of $>$20 km. Tectonites from the lower plate show little evidence for overprinting of amphibolite-facies deformation fabrics by greenschist-facies mylonitic textures as is commonly seen in other MCCs, including the nearby D'Entrecasteaux Islands MCCs. This implies strongly localised slip on the brittle detachment fault during exhumation of these rocks through the middle parts of the crust. Factors that might contribute to such a situation include; 1) A high geothermal gradient to markedly reduce the envelope of greenschist-facies metamorphic conditions at higher structural levels; 2) A possible weakening effect of high strain-rates on fault gouge material; 3) Deep embrittlement of the crust through prolonged maintenance of high pore-fluid pressures or high strain rates. Preliminary $^{40}$Ar/$^{39}$Ar results, from lower-plate tectonites on Misima Island, yield mica cooling ages of $\sim$8 Ma (Baldwin et al., 2000). Thermochronologic analysis (in progress) will help to determine the relationship between the processes of MCC formation on Misima Island and the sea-floor spreading history of the Woodlark rift.
DE: 8000 STRUCTURAL GEOLOGY (New field, replaces single entry 8165)
DE: 8109 Continental tectonics--extensional (0905)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting