New Zealand Journal of Geology and Geophysics abstracts

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New eruptive vents for the Whakamaru Ignimbrite (Taupo Volcanic Zone) identified from magnetic fabric study

GEOFFROY LAMARCHE*
PAUL C. FROGGATT

Research School of Earth Sciences
Victoria University of Wellington
P.O. Box 600
Wellington, New Zealand

*Present address: ORSTOM, Laboratoire de Geodynamique, B.P. 48,06230 Villefranche sur Mer, France.

Abstract The location of possible source vents for the Whakamaru Ignimbrite was investigated using flow directions inferred from anisotropy of magnetic susceptibility (AMS). If we assume that the magnetic fabric was formed in response to processes in the formative pyroclastic flow, the magnetic lineation is related to the direction to the source vent. Lineation vectors mostly appear to dip towards the source, suggesting a flow-related imbrication of the fabric. AMS has been determined at 22 sites around Lake Taupo, and show an average bulk magnetic susceptibility (MS) of 8.04 × 10^-3 SI. The degree of anisotropy (P) ranges from 1.014 to 1.032. The magnetic susceptibility is due to titanomagnetite. Measurements showing strongly dipping magnetic foliations presumably caused by underlying topography and local deflections of the flow have been discarded. The magnetic lineation is dipping towards the source in 14 sites and 180° from the source in four sites. Four source vents for the Whakamaru Ignimbrite are interpreted along an 80 km north-south trend on the eastern shore of Lake Taupo. Our interpretation corroborates the results of previous studies which have identified vents near Wairakei and northern Lake Taupo. This study shows that there are other possible source vents south of the lake and north of Wairakei.

Keywords anisotropy; magnetic susceptibility; magnetic fabric; ignimbrite; source direction; New Zealand

Received 4 September 1992; published 25 June 1993
New Zealand Journal of Geology & Geophysics, 1991, Vol. 36: 213—222
0028Ð8306/06/3602—0213 ©The Royal Society of New Zealand 1991

PDF file of entire paper: medium quality (881K); (scanned from paper original: notes about this process). Digitisation of this article from the printed journal was kindly facilitated by the Geological Society of New Zealand

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