HR: 0800h
AN: T31A-1275    [Abstracts]
TI: Mantle Deformation and Seismic Anisotropy due to Oblique Collision, South Island, New Zealand
AU: * Bourguignon, S
EM: bourgusand@student.vuw.ac.nz
AF: Institue of Geophysics, Victoria University of Wellington, Wellington, 6005 New Zealand
AU: Savage, M
EM: marttha.savage@vuw.ac.nz
AF: Institue of Geophysics, Victoria University of Wellington, Wellington, 6005 New Zealand
AU: Stern, T
EM: tim.stern@vuw.ac.nz
AF: Institue of Geophysics, Victoria University of Wellington, Wellington, 6005 New Zealand
AU: Baldock, G
EM: stru_uth@hotmail.com
AF: Institue of Geophysics, Victoria University of Wellington, Wellington, 6005 New Zealand
AB: An important question in geodynamics is how does continental mantle lithosphere shorten in transpressional zones? We address this by measuring properties of the upper mantle beneath central South Island, New Zealand. The obliquely-convergent, Australian-Pacific plate boundary passes through the South Island and effectively links two subduction zones of opposite polarities by the Alpine Fault. Crustal deformation is now well documented across this continental transform, with oblique thrusts occurring at the Alpine Fault and distributed deformation to the east of the fault. About 90 km of shortening has occurred across central South Island in the past 7 myr. Deformation in the mantle, however, remains controversial and surface observations are equally explained by a variety of models. We use aftershocks of the 2003 Fiordland EQ to determine Pn beneath the root of the Southern Alps. High Pn speeds of 8.6 $\pm$ 0.1 km/s, as well as thickening of the crustal root from 45 $\pm$5 km in central (Mt Cook) to 49 $\pm$ 6 km in southern (Queenstown-Wanaka) South Island are the main results of our experiment. Comparison with parallel and crossing lines both on- and off-shore suggest similar Pn speeds on the Pacific and the Australian plate boundaries, but high values of Pn anisotropy of 11.5 $\pm$ 2 %\ on the Australian side than the 7.7 $\pm$ 2.7 %\on the Pacific side. We interpret the anisotropy as being due to finite strain in the mantle lid. Two further Pn-anisotropy measurements off-shore of 0 $\pm$ 2.5 %\ and a 6.5 $\pm$ 3 %\, define a E-W boundary to uppermost mantle deformation east of South Island. Furthermore, gravity modelling of the thick low-density crustal root shows, that the Southern Alps are not sufficiently high to compensate for it, requiring a region of positive density contrast in the mantle, and probable widening towards the South. We interpret this region as cold, thickened lithospheric mantle. Concomitant crustal root thickening, widening of Pn anisotropy and the mantle positive density contrast, suggest material accumulation, e.g. extrusion towards the southeast of South Island, and favour the thesis of thickening of the entire lithosphere.
DE: 5475 Tectonics (8149)
DE: 3010 Gravity
DE: 1734 Seismology
DE: 0935 Seismic methods (3025)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting