HR: 0800h
AN: T41C-1215 INVITED     [Abstracts]
TI: Spatial and Temporal Variations Along the New Zealand Plate Boundary: Decoupling, Delamination, and Localization
AU: * Furlong, K P
EM: kevin@geodyn.psu.edu
AF: Geodynamics Research Group, Dept. Geosciences Penn State University, University Park, PA 16802 United States
AU: Kamp, P J
EM: p.kamp@waikato.ac.nz
AF: Department of Earth Sciences, University of Waikato, Hamilton, 6 New Zealand
AU: Malservisi, R
EM: rmalservisi@rsmas.miami.edu
AF: RSMAS, 4600 Rickenbacker Cswy University of Miami, Miami, FL 33149 United States
AB: New Zealand sits astride the Pacific-Australia plate boundary and hosts two fundamental transitions in plate interactions. Subduction of the Pacific plate beneath the North Island along the Hikurangi margin ends and the plate motion is taken up along the Alpine Fault translational plate boundary in the Marlborough-Kaikoura regions of the South Island. This subduction-translation transition has migrated southward since 20-25 Ma. In the south, a small sliver of the Australia plate subducts beneath the Fiordland region, accommodating an $\sim$ 100 km transpressional left-step in the Australia-Pacific plate boundary south of the South Island. At the northern termination of this small subduction zone the Alpine Fault initiates. This subduction-translation-subduction plate boundary structure has undergone significant evolution over the past 15-20 million years, driven by changes in relative plate motion and significant lithospheric deformation focused in the transition zones along the plate boundary. At the northern transition, we argue that the encroaching subducted Pacific slab acts as a chisel on the lower lithosphere of the overriding Australian plate driving the active delamination of much of the mantle lithosphere. This mass removal makes the necessary space to accommodate the slab. Additionally it drives substantial vertical tectonics of the Australia plate producing rapid and localized uplift in the zone of the active delamination. Also a series of ephemeral sedimentary basins have developed and subsequently been exhumed in the wake of the advancing slab edge. At the southern transition, we argue that the localized subduction of Australia beneath Fiordland is enabled by the progressive tearing of a sliver from the Australian plate. This leaves a newly formed edge to the Australian plate that translates northward along the plate margin becoming the western side of the Alpine Fault plate boundary. It is useful to distinguish between the well-described near-surface Alpine Fault (AF) and the less well understood deeper plate boundary shear zone, which we term the Southern Alps plate boundary (SAPB). As a result of the southward migration of the Hikurangi subduction, the SAPB has been shortening in time. Concurrent with the shortening of the SAPB (since $\sim$15 Ma) plate motions between the Pacific and Australia plate have changed, driving a clockwise rotation in the azimuth of motion along the plate boundary through New Zealand. This rotation produces a mismatch between the sense of shear in the ductile lower crust/upper mantle of the plate boundary and the orientation and location of the upper crustal AF. Localization of deformation along the SAPB shear zone can lead to a significant decoupling between the crust and mantle lithosphere. Evidence from upper mantle shear-wave anisotropy (SKS splitting) and deformational modeling suggest that such a decoupling has occurred, and the resulting spatial and temporal variability in crust-mantle coupling across South Island, New Zealand may lead to variability in deformational style along the Southern Alps orogen.
DE: 8107 Continental neotectonics
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8123 Dynamics, seismotectonics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting