HR: 1340h
AN: T13B-0478    [Abstracts]
TI: Seismic Attenuation, Temperature, H₂0, Mantle Melting and Rock Uplift, Central North Island New Zealand
AU: * Salmon, M
EM: michelle.salmon@vuw.ac.nz
AF: School of Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, 6001 New Zealand
AU: Savage, M
EM: martha.savge@vuw.ac.nz
AF: School of Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, 6001 New Zealand
AU: Stern, T
EM: tim.stern@vuw.ac.nz
AF: School of Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, 6001 New Zealand
AB: Back-arc basins of the western Pacific are elevated some 1-2 km above the adjacent oceanic floor. Where oceanic back-arc basins propagate into continental lithosphere we also see an uplift signal, which can be mapped and evaluated with geological methods. Trying to understand the driving force for this uplift requires seismological methods to quantify temperatures, and therefore buoyancy, in the upper mantle. New Zealand's North Island is one such place where the back-arc basin has propagated into continental lithosphere. Geological records show that the North Island has undergone up to 2.5 km of broad wavelength rock uplift since 5 Ma. We use earthquake data to map variations in seismic attenuation (Qp^-1) beneath the North Island. Results are used to determine some constraints on the effects of temperature, water and melt on buoyancy in the mantle wedge above the subduction zone. A region of high attenuation extending to depths of ~140 km correlates, spatially, with the region of back-arc extension, volcanism and high heat flow (Central Volcanic Region or CVR). In this region the path-averaged Qp^-1 for frequencies from 1-15Hz is 4.0×10^-3±0.3×10^-3 and shows little variation with depth. West of the CVR, the north-western North Island shows a decrease in attenuation but Qp^-1 remains slightly elevated (path-averaged Qp^-1 1.4×10^-3±0.2×10^-3). Here attenuation increases with depth until it reaches similar values as the CVR mantle at approximately 80 km. We use Qp values to calculate temperatures at 30 km and 80 km depth below these two regions. Temperatures at 30 km below the CVR are elevated to just above the melting temperature (1.02 Tm) while to the west temperatures are just below the solidus (~0.95 Tm). At 80 km depth attenuation indicates temperatures for both regions are just above the solidus. To reconcile temperatures calculated from heat flow measurements in the north-western North Island with those calculated from attenuation, melting temperatures must be lowered by the presence of water. Heat flow and seismic attenuation return to normal continental values abruptly at a line that runs approximately perpendicular to the present day plate boundary termed the Taranaki-Ruapehu line. This line also marks termination of volcanism despite the continuation of subduction for another 200 km to the south.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Tectonophysics [T]
MN: Fall Meeting 2005