HR: 17:30h
AN: T24B-07    [Abstracts]
TI: Evolution of subduction and Back-arc Extension at a Continental Margin
AU: Stratford, W R
EM: stratfwand@student.vuw.ac.nz
AF: Institute of Geophysics, Victoria University of Wellington, Wellington, 6005 New Zealand
AU: Salmon, M L
EM: salmonmich@student.vuw.ac.nz
AF: Institute of Geophysics, Victoria University of Wellington, Wellington, 6005 New Zealand
AU: * Stern, T A
EM: tim.stern@vuw.ac.nz
AF: Institute of Geophysics, Victoria University of Wellington, Wellington, 6005 New Zealand
AB: In central North Island, New Zealand, there exists one of the more complete geological and geophysical records of subduction initiation and evolution. Geological data provide us with evidence of how subduction initiated in late Oligocene times, while geophysical data provide an image of how back-arc spreading and an active mantle wedge have developed beneath central North Island in the past 5 my. Principal information on subduction initation processes come from oil-industry bore hole data located $>$ 500 km from the trench where subduction initiated. These data show a rapid and coeval platform subsidence across a wide area ($> $200 km) of western New Zealand. This is interpreted as being due to a broad subduction-induced, hydrodynamic flow in the mantle. Nearly 20 my of compression and foreland basin development in western North Island followed the platform subsidence. At around 5 Ma a rapid switch in tectonics took place that was marked by the development of lithospheric, dome-like uplift centred on the central North Island. This uplift signal (2500 m of rock uplift or 1200 m of tectonic uplift) is principally derived from mudstone porosities that are used as a proxy for exhumation. From about 4 Ma to the present andesitic and then rhyolitic volcanism developed across the domal uplift, accompanied by extension and tectonic rotation of central North Island. Recent geophysical experiments in the central North Island have begun to define the structure and conditions within the subjacent mantle wedge. Pn wave speeds are exceedingly low (7.4-7.8 km/s) and the attenuation (Q-1) is high. Explosion seismology results show the pre-existing greywacke-schist crust ($œ$ 6 km/s) has been stretched to at least half its original thickness (i.e. 30 to 15 km). Beneath 15 km we see, from wide-angle reflection analysis, a reflective sequence down to about 18 km where velocities increase rapidly from 6.7 to 7 km/s. At 20 $\pm$ 2 km P-wave speeds of $\sim$ 7.4 km/s are measured and these are seen from passive seismic studies to extend down through the mantle wedge. We interpret the rocks from 15 to 20 km as new lower crust and from below 20 km as an anomalous upper mantle. On the basis of our seismic velocity model and the rock uplift data constraints are placed on the buoyancy and degree of partial melt in the mantle wedge. Our best estimate is that the mere replacement of say 70 km of the mantle lid by asthenosphere (density contrast $\sim$ - 40 kg/m3) is insufficient to explain the rock uplift. Instead we require a density contrast more like -70 to -100 kg/m3, which implies an important negative buoyancy contribution from fluids and/or melts in the mantle wedge.
DE: 8115 Core processes (1507)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 1734 Seismology
DE: 1744 Tectonophysics
DE: 0935 Seismic methods (3025)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting