Abstract
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.
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