New Zealand Journal of Geology and Geophysics abstracts

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Geophysical characterisation of large silicic volcanic structures in the Coromandel Peninsula, New Zealand

BARBARA MALENGREAU^1,2
DAVID SKINNER^3,4
CHRIS BROMLEY⁵
PHILIPPA BLACK¹

¹Geology Department
The University of Auckland
P.O. Box 92 019
Auckland, New Zealand

²Present address: PYROS SARL, 27 Route du Cendre, F-63800 Cournon d'Auvergne, France.

³Institute of Geological & Nuclear Sciences
P.O. Box 30 368
Lower Hutt, New Zealand

⁴GeoGnostics Consultancy
New Zealand

⁵Wairakei Research Centre
Private Bag 2000
Taupo 2730, New Zealand

Abstract  Previous stratigraphic and tectonic studies of Coromandel Peninsula, New Zealand, have defined regionally significant structures, including calderas, of Miocene age. A new gravity map of the area reveals significant details not seen in the earlier geophysical surveys. Results from a new low-level aeromagnetic survey and from two new magnetotelluric resistivity soundings help interpret the volcanic structures. 2-D/2.5-D models are used to visualise the internal structure.

Basement greywacke and andesite signatures are observed in the northern and western parts of Coromandel Peninsula, and in the areas of the Aldermen Islands and Mercury Islands, offshore to the east. Rhyolite and ignimbrite signatures define four main volcanic structures: (1) the Whitianga Caldera over the Whitianga Volcanic Centre, with an estimated volume of low-density material of 179 km³, a diameter of 15 km, and a depth of 1-1.5 km; (2) the Kapowai Caldera, previously defined by geological evidence, with a volume of remaining material of 256 km³ and a depth of 1.1-1.4 km; (3) a north-south-trending area south of the Kapowai Caldera which could be a trough filled with low-density silicic rocks and includes the previously suggested but undocumented Wharekawa Caldera; and (4) a trough filled with low-density silicic rocks offshore of the east coast of Coromandel Peninsula.

Keywords  Coromandel Peninsula; New Zealand; silicic volcanic structures; calderas; gravity surveys; low-level aeromagnetic survey; magnetotelluric resistivity soundings; hydrothermal alteration

New Zealand Journal of Geology & Geophysics, 2000, Vol. 43: 171-186

0028-8306/00/4302-0171 $7.00/0 (c) The Royal Society of New Zealand 2000

PDF file of entire paper: medium quality (4599K); (scanned from paper original: notes about this process)

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