HR: 1340h
AN: T43B-1409    [Abstracts]
TI: Asymmetric Spreading in a Continental Back-Arc.
AU: * Stratford, W
EM: Wanda.Stratford@vuw.ac.nz
AF: Victoria University of Wellington, P.O Box 600, Wellington, 6001 New Zealand
AU: Stern, T
EM: Tim.Stern@vuw.ac.nz
AF: Victoria University of Wellington, P.O Box 600, Wellington, 6001 New Zealand
AB: Where back-arc spreading propagates into continental lithosphere we get a rare opportunity to study continental extension at rapid rates. The wedge-shaped Central Volcanic Region (CVR) of North Island, New Zealand is the onshore extension of the oceanic Lau-Havre trough back-arc system. Long-range refraction and reflection data recorded in the continental back-arc provides evidence for the attenuation of the original ~ 30 km thick crust to ~ 15 km or β = 2. This extension factor will be a minimum as an unknown amount of new crust has been added by magmatic intrusion. Beneath the extended and intruded original crust, a ~ 5 km thick layer of new crust, of seismic P-wave velocity 6.7-7.2 km/s, is interpreted as magmatic underplating. From the top of the underplated layer P-wave velocities increase with depth and the transition zone from lower crust to upper mantle is not distinct. However, low Pn wave-speeds of 7.4 ± 0.2 km/s are reached at a depth of 20 ± 2 km. At about 8% below those of normal mantle, the Pn wave-speeds imply the presence of a hot, buoyant mantle with a small percent of partial melt. Continental break-up is fundamentally controlled by how extension is accommodated in the lithosphere. In a back-arc spreading regime, the mode of extension, whether it be pure-shear, simple-shear or a combination of both, is less obvious. Tectonic forces controlling extension in the CVR have rotated and translated, producing a shifting extensional locus with time. Apparent strain rates in the volcanic region of ~4.0 x 10^-14 s^-1 are ~ twice that of most continental rifts. Offshore seismic data shows the CVR as a whole has extended by similar amounts but there is a distinct younging of fault motion to the east. This younging direction is consistent with a fanlike opening of the back-arc. Two general end-members of opening have been ascribed to the CVR. A rifting model where the original crust is still present, but thinned by normal faulting, and a spreading model where some of the extension is accommodated by magmatic intrusion. Surface heat flux can help differentiate between extension models. In the CVR, heat flux is concentrated in the east in a region of geothermal activity where the equivalent heat flow is ~ 12 times the continental average. The present day heat flux can be explained by complete replacement of the original crust beneath the geothermal systems by magmatic intrusions.
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 7205 Continental crust (1219)
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8109 Continental tectonics: extensional (0905)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005