HR: 0800h
AN: T41C-1212    [Abstracts]
TI: Metamorphic Tectonites and Differential Exhumation Reveal 3D Nature of Extension and Lower Crustal Flow in the Active Woodlark Rift, Papua New Guinea
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
AU: Monteleone, B D
EM: bmontele@mailbox.syr.edu
AF: Dept. of Earth Sciences, Syracuse University, Syracuse, NY 13244-1070 United States
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
AB: The D'Entrecasteaux Islands metamorphic core complexes (MCCs) occur in the Woodlark rift, a continental region where $\sim$200 km of extension since $\sim$6 Ma has been focused into a relatively small number of normal faults, some dipping at $<$30$\deg$. Flow of a low viscosity lower crust on a time scale of $<$4 m.y. is evidenced by the narrow width of the rift zone, and the apparent large magnitude of extension (Beta $\sim$5) , as contrasted by observed relief on the Moho ($<=$10 km). Gneisses in the footwalls of MCCs, including retrogressed eclogites, have been exhumed from depths of $\sim$30 km since $\sim$3 Ma. Our structural and thermochronometric data covers parts of the D'Entrecasteaux MCCs, including Normanby Island. These are mantled by serpentinized ultramafic rocks of the Papuan ophiolite. Remnants of this upper plate are preserved along the margins of the MCCs. In underlying mylonites, exhumation-related fabrics, microstructures and quartz lattice preferred orientations reveal a regional pattern of lineations and top-north shear inconsistent with diapir tectonics. The gently dipping mylonites were later warped by uplift of the $\sim$30 km-wide domes, perhaps in response to magmatic underplating during intrusion of dolerite dikes and granodiorites at $\sim$2 Ma. Southward across Fergusson Island, muscovite Ar/Ar ages, increase by $\sim$2 m.y. along the detachment, suggesting footwall exhumation by top-to-the-north slip at $>$12 mm/yr. To the east, an MCC on eastern Normanby Island has top-north footwall mylonites that dip gently SW and that were exhumed during the Pliocene as part of a northward progression of normal faulting that did not arrive at the offshore Moresby Seamount until $\sim$1.2 Ma (ODP Leg 180 site), relationships that suggest a rolling-hinge style uplift. Importantly, its detachment exposes no rocks deeper than blueschist-facies. Ductile deformation fabrics in the MCCs reveal patterns of lower crustal motion that can be evaluated against seafloor spreading-derived plate motions. Shear fabrics in MCCs closest to the Woodlark spreading ridges, including Normanby and Misima Islands are parallel to the NNE direction of 0.5-3.6 Ma Solomon Sea-Australia spreading. Farther west, lineations in the lower plates of the D'Entrecasteaux MCCs locally deflect $\sim$40-50$\deg$ clockwise from this direction. This obliquity is interpreted to reflect inhomogeneous lower crustal extension to the west of the Woodlark spreading ridges. A rift corridor extending $\sim$100 km to the north of Goodenough and Fergusson Islands is defined by active normal faulting and subsidence of the Trobriand margin. We infer that a previously subducted, locally eclogite-bearing, slab of thinned Australian lower crust to the north of the islands is being pulled out from beneath this zone as it is being sinistrally sheared along its eastern edge. Today the rift zone steps $\sim$70 km south towards the Papuan Peninsula to define a right-step, an asymmetry that is enhanced by seafloor spreading east of $\sim$151.4$\deg$ E. Published focal mechanisms suggest that N-S sinistral shear along the northern Woodlark rift is continuing. In central Normanby Island, $\sim$2 Ma andesites may have erupted along a transverse fault bounding the deeply exhumed D'Entrecasteaux corridor. Its offshore extension trends N along a major embayment in the Woodlark Rise.
DE: 8000 STRUCTURAL GEOLOGY (New field, replaces single entry 8165)
DE: 8109 Continental tectonics--extensional (0905)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting