HR: 17:00h
AN: T52F-05    [PDF]
TI: Making Room for the Pacific: Southward Migration of Pacific Subduction Beneath New Zealand and Tectonic Consequences of Delaminating the Australian Lithosphere
AU: Kamp, P J
EM: p.kamp@waikato.ac.nz
AF: Universty of Waikato, Dept Earth Sciences, Hamilton, 2001 New Zealand
AU: * Furlong, K P
EM: kevin@geodyn.psu.edu
AF: Penn State University, Geodynamics Research Group, University Park, PA 16802 United States
AB: Over the past 20 Ma, as a consequence of the substantial obliquity to plate motions, the southwestern edge of the subducting Pacific slab has migrated southward sweeping beneath the North Island of New Zealand to its present position beneath the northern South Island. In many locations where a well-defined slab edge migrates, such as northern California where the Gorda slab migrates with triple junction motion, the slab edge is on the trailing edge of the plate leaving a slab window in its wake. In contrast, in the New Zealand case, the slab edge is the leading edge of the plate and thus it is moving into a region with a pre-existing lithospheric structure that needs to be modified and/or removed to allow the slab to migrate. Present day lithospheric structure in the vicinity of the slab edge and the record of vertical motions preserved in the geologic record provide the key to unraveling the tectonic response of New Zealand to this slab migration. Associated with the slab migration is the development of a series of ephemeral basins that trail the slab edge. These range from the ~ 20 Ma Waitemata Basin in the Auckland region to the present day Wanganui Basin, whose active depocenter lies between the North and South Islands of New Zealand. These basins are characterized by rapid subsidence/deposition following passage of the slab edge, followed by more gradual uplift and exhumation. An analysis of seismicity in the vicinity of the present slab edge identifies a near vertical region of deep seismicity, spatially distinct from the slab, which we interpret to be a delaminated sliver of the mantle lithosphere of the Australian plate. The delaminated sliver appears to initiate in the region where the edge of the slab interacts at shallow levels (30-80 km), in a style akin to a chisel, with the Australian lithosphere. The tectonic consequences of this active delamination and subsequent sinking of the detached mantle lithosphere include localized uplift in the vicinity of the slab edge, rapid subsidence and basin formation, followed by more gradual inversion of the basins. The vertical motions recorded in the history of the basins are inferred to be driven by viscous coupling between the sinking sliver and the base of crust. The spatial and temporal scales of evolution of these basins provide constraints on the extent of this viscous coupling. The transition from a transform to subduction boundary in the northern South Island of New Zealand represents an ideal location to study the processes by which lithosphere adapts to abrupt changes in plate interactions.
DE: 7230 Seismicity and seismotectonics
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8155 Plate motions--general
DE: 8159 Rheology--crust and lithosphere
SC: Tectonophysics [T]
MN: 2003 Fall Meeting