HR: 17:00h
AN: V14A-05    [Abstracts]
TI: Lower Crustal Earthquakes Near a Continental Rift - a Soggy Jelly Sandwich Works in New Zealand
AU: * Reyners, M
EM: m.reyners@gns.cri.nz
AF: GNS Science, P.O. Box 30368, Lower Hutt, 6315 New Zealand
AU: Eberhart-Phillips, D
EM: d.eberhart@gns.cri.nz
AF: GNS Science, Private Bag 1930, Dunedin, 9020 New Zealand
AU: Stuart, G
EM: graham@earth.leeds.ac.uk
AF: School of Earth and Environment, University of Leeds, Leeds, LS2 9JT United Kingdom
AB: Subduction of the Pacific plate beneath the North Island of New Zealand is accompanied by active back-arc spreading within continental crust in the Taupo Volcanic Zone (TVZ). This spreading terminates in the central North Island at the latitude of the andesitic Mt Ruapehu. Southwest of this termination, there is an unusual concentration of lower crustal seismicity in the overlying plate, often referred to as the Taranaki-Ruapehu line. A recent dense deployment of seismographs in the central North Island has provided insight into the genesis of this seismicity. A tomographic inversion for 3-D Vp and Vp/Vs reveals that southwest of the TVZ, the crust thickens by about 10 km. Furthermore, the pattern of seismic velocity heterogeneity in the mantle wedge suggests that this thicker crust has choked off mantle return flow, leading to a termination of the volcanic front. When combined with shear-wave splitting results, the seismic tomography results suggest a pattern of mantle flow normal to the strike of the subducted plate beneath the TVZ, but along strike to the southwest. We have used the seismic tomography results to relocate 1618 earthquakes in the crust of the overlying plate recorded during the dense seismograph deployment. In the TVZ, seismicity is largely concentrated in the top 10 km of the crust, in a region of relatively low Vp/Vs. Both the seismicity and low Vp/Vs are characteristics of hydrostatic fluid transport in the extensional regime. Beneath Mt Ruapehu this upper crustal seismicity deepens, forming a continuous c. 10 km-thick band which dips to the southwest, before becoming horizontal again at the base of the thicker crust. The region of low Vp/Vs shows the same dipping trend as the seismicity. This continuous linking of upper and lower crustal seismicity suggests a common genesis for both - fluids in the crust. The presence of fluids is also suggested by the reflective nature of the lower crust to the southwest. At the base of the crust beneath Mt Ruapehu, there is a very strong gradient to high Vp/Vs in the uppermost mantle, which can be related to an extensive region of partial melt. The fluids in the lower crust may thus result from water released from the top of a solidifying magma body in the uppermost mantle. This wholesale weakening of the crust by water at the southwestern termination of the TVZ is a transient effect. However, it has probably been operating for millions of years, providing a ready explanation for the southward propagation of the TVZ with time.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8109 Continental tectonics: extensional (0905)
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8180 Tomography (6982, 7270)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005