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