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

Previous stratigraphic and tectonic studies of Coromandel Peninsula, New Ze aland, have defined regionally significant structures, including calderas, of Miocene age. A new gravity map of the area reveals significant details n ot seen in the earlier geophysical surveys. Results from a new low-level ae romagnetic survey and from two new magnetotelluric resistivity soundings he lp interpret the volcanic structures. 2-D/2.5-D models are used to visualis e 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 Is lands and Mercury Islands, offshore to the east. Rhyolite and ignimbrite si gnatures define four main volcanic structures: (1) the Whitianga Caldera ov er the Whitianga Volcanic Centre, with an estimated volume of low-density m aterial of 179 km(3), 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 km(3) and a depth of 1.1-1.4 km; (3) a north-s outh-trending area south of the Kapowai Caldera which could be a trough fil led with low-density silicic rocks and includes the previously suggested bu t undocumented Wharekawa Caldera; and (4) a trough filled with low-density silicic rocks offshore of the east coast of Coromandel Peninsula.