HR: 09:30h
AN: V41A-07    [PDF]
TI: Crustal Structure and Rock Uplift due to Back-Arc Extension.
AU: * Stern, T
EM: Tim.Stern@vuw.ac.nz
AF: Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand, Wellington, 6001 New Zealand
AU: Stratford, W
EM: stratfwand@student.vuw.ac.nz
AF: Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand, Wellington, 6001 New Zealand
AU: Salmon, M
EM: salmonmich@student.vuw.ac.nz
AF: Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand, Wellington, 6001 New Zealand
AU: Pulford, A
EM: anna_pulford@yahoo.com.au
AF: Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand, Wellington, 6001 New Zealand
AU: Pulford, A
EM: anna_pulford@yahoo.com.au
AF: Geoscience Australia, GPO Box 378 CANBERRA ACT, Canberra, 2601 Australia
AB: We use crustal structure and exhumation data to estimate the density and partial melt variation in the mantle wedge of a back-arc extension system. A program of explosion seismology (NIGHT) in the central North Island (CNI) of New Zealand has unearthed an intriguing, isostatic contradiction: an association of profoundly thin continental crust with recent rock uplift. We explore properties of the upper mantle that might lead to sufficient buoyancy to compensate the thin crust and raise the solid surface of the CNI into a lithospheric dome. Andesite and rhyolite volcanism have been present in the CNI for the past 4-5 my. Mudstone porosity analyses to the west and south of the volcanic region shows that over 2 km of exhumation has occurred in CNI since 4 Ma. This combined with a coeval surface uplift of $\sim$ 500 m, indicates a maximum 2.5 km of rock uplift. Marine terraces 100-150 km from the centre of maximum exhumation show uplift rates of $\sim$ 0.2-0.3 mm/y consistent with about 1 km of exhumation if averaged over 4 my, which compares well with the mudstone porosity results. The magnitude and wavelength ($\sim$ 400 km) of the uplift is similar to that observed in continental areas associated with mantle upwelling (e.g. Yellowstone). Explosion seismic studies show an arched Moho, crust that has been thinned to nearly half its original thickness and Pn wave-speeds 8% lower than normal. Seismic attenuation (Q) studies show strong attenuation throughout the mantle wedge below CNI. Isostatic modelling is used to explore processes consistent with these observations. The observed rock uplift pattern may be explained by the replacement of about 70 km of mantle lid with asthenosphere of density contrast $\sim$ 100 kg/m$^{3}$ with respect to normal mantle. Such a density contrast cannot be explained by temperature change alone, and a partial melt of up to 5% is implied. Strong seismic reflections from within a confined zone of the mantle wedge were recorded. These reflections from $\sim$ 35 km deep show strong amplitudes in an incident angle range consistent with the AVO response of a body with a high degree of fluid or melt.
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