HR: 0830h
AN: V31C-0954    [PDF]
TI: Back-Arc Extension in Continental Lithosphere: Reflections From a Buoyant Mantle and Thin Crust.
AU: * Stratford, W
EM: stratfwand@student.vuw.ac.nz
AF: Victoria University of Wellington, P.O Box 600 Wellington, wellington, 6001 New Zealand
AU: Stern, T
EM: Tim.Stern@vuw.ac.nz
AF: Victoria University of Wellington, P.O Box 600 Wellington, wellington, 6001 New Zealand
AU: Pulford, A
EM: anna_pulford@yahoo.com.au
AF: Victoria University of Wellington, P.O Box 600 Wellington, wellington, 6001 New Zealand
AU: Pulford, A
EM: anna_pulford@yahoo.com.au
AF: Geosceince Australia, GPO Box 378 CANBERRA ACT Australia, Canberra, 2601 Australia
AB: Back-arc extension within the continental lithosphere of New Zealand has produced a zone of thin crust and mantle of unusual geophysical properties. The Central volcanic region (CVR), central North Island, New Zealand is the apparent extension of the Havre Trough back-arc basin, which has opened since 5ma. The area has $\sim$ 12 times the continental heat flow and numerous geothermal systems and volcanic complexes. Seismic data shot in 2001 as part of the NIGHT (North Island GeopHysical Transect) project show low crust and upper mantle velocities and two unusual reflectors beneath the volcanic area. The first is interpreted as a crust-mantle transition zone or "Moho" reflector that shallows to $\sim$ 16 km beneath the area of highest extension and volcanic activity. The second is a strong reflection at 35 km depth, which for the CVR is in the mantle. The three defining attributes of this reflector are: it has an apparent lateral extent equivalent to the surface expression of the CVR, it is of low frequency and it is of high amplitude compared to the crust-mantle reflection and Pg arrivals. AVO analysis of the mantle reflection indicates it is likely to be produced by a layer of negative impedance contrast equivalent to at least 80% drop in S-wave velocity. Laboratory studies on naturally organised, realistically orientated melt inclusions indicates such an S-wave drop could be produced by 11 - 15% partial melt. We therefore interpret the reflector to be a body of melt pooled at a viscosity boundary in the mantle. Despite the thin crust of the CVR, the topography of the area defines a broad dome with average elevation $\sim$ 300m above sea level. The low Pn wave speed measured in the area and the presence of melt in the mantle beneath the region of thinnest crust and highest extension indicates a possible contribution from decompression melting and the support for the thin crust being supplied by a mantle of increased buoyancy.
DE: 1208 Crustal movements--intraplate (8110)
DE: 1236 Rheology of the lithosphere and mantle (8160)
DE: 8109 Continental tectonics--extensional (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8145 Physics of magma and magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting