HR: 1330h
AN: S22B-0450    [PDF]
TI: Attenuation and Electrical Resistivity in an Asymmetric Back Arc Extensional Environment
AU: * Salmon, M
EM: salmonmich@student.vuw.ac.nz
AF: Victoria University of Wellington, PO Box 600 Wellington New Zealand, Wellington, 6001 New Zealand
AU: Bannister, S
EM: s.bannister@gns.cri.nz
AF: Institute of Geological and Nuclear Sciences, 69 Gracefield Rd PO Box 30-368 Lower Hutt New Zealand, Lower Hutt, 6001 New Zealand
AU: Bibby, H
EM: h.bibby@gns.cri.nz
AF: Institute of Geological and Nuclear Sciences, 69 Gracefield Rd PO Box 30-368 Lower Hutt New Zealand, Lower Hutt, 6001 New Zealand
AU: Savage, M
EM: martha.savage@vuw.ac.nz
AF: Victoria University of Wellington, PO Box 600 Wellington New Zealand, Wellington, 6001 New Zealand
AU: Stern, T
EM: tim.stern@vuw.ac.nz
AF: Victoria University of Wellington, PO Box 600 Wellington New Zealand, Wellington, 6001 New Zealand
AB: We use seismic attenuation and magnetotelluric data to determine the nature and location of a boundary that marks the transition from extension to compression, North Island New Zealand. This boundary, termed the Taranaki-Ruapehu Line (TRL), has long been recognised as a fundamental boundary between lithosphere of contrasting nature. Geophysical evidence indicates that the TRL runs approximately east to west, nearly perpendicular to the present day plate boundary configuration. The area north of the boundary is associated with slightly elevated heat flow (70-80 mWm$^{-2}$), thinned crust (25 km) and crustal extension. The area south of the boundary has normal continental heat flow (60 mWm$^{-2}$), crustal thickness of 35 km and is in geodetically determined compression. Previous attenuation studies indicate that attenuation is higher to the north of the boundary. A steep gravity gradient connects the two regions. In 2002 six long period magnetotelluric soundings straddling the boundary were recorded in a line 120 km long. These data have been used for 2D inversion models of resistivity down to 45 km. Modelling indicates low resistivity ($<$ 300 ohmm) present below 25 km depth in the north that is absent in the south. This change in resistivity structure is north of most estimates for the boundary position. A new seismic network of six short period instruments was deployed along the same line to collect local earthquake data. Preliminary results from this data suggest the attenuation boundary is contiguous with the resistivity boundary. For earthquakes greater than 150 km depth path averaged P wave values of Q are as low as 200 on the extensional side of the boundary, which is comparable with Q values found below many volcanic arcs. On the compressional side of the boundary Q values are $>$1500.
DE: 0925 Magnetic and electrical methods
DE: 1645 Solid Earth
DE: 1749 Volcanology, geochemistry, and petrology
DE: 7218 Lithosphere and upper mantle
DE: 8109 Continental tectonics--extensional (0905)
SC: Seismology [S]
MN: 2003 Fall Meeting