A geophysical reconnaissance survey of Great Barrier Island, North Island, New Zealand

Abstract

Results of a gravity and an airborne magnetic survey of Great Barrier Island are described. The interpretation of residual Bouguer gravity anomalies shows that a sequence of Miocene andesites up to 2 km thick (Coromandel Group volcanics) has been deposited in the elongated, northwest-trending Great Barrier Depression which extends offshore to the west and northwest of the island. Late Miocene rhyolites (Whitianga Group volcanics) erupted from the Mt Hobson caldera, a 2 km deep depression within the Miocene volcanics, and at Rakitu Island and Okupu.

Contemporaneous geothermal activity produced widespread alteration of near-surface volcanics, mineral deposition, and partial demagnetisation which is reflected in the low-amplitude pattern of magnetic anomalies over the central part of the island.

Keywords:

• gravity anomalies
• magnetic anomalies
• Miocene
• volcanic rocks
• Great Barrier Depression
• Mt Hobson caldera
• geothermal systems
• Great Barrier Island