IMPLICATIONS OF A HIGH HEAT-FLOW ANOMALY AROUND NEW-PLYMOUTH, NORTH-ISLAND, NEW-ZEALAND

A high heat flow anomaly exists over the northern Taranaki peninsula, North Island, New Zealand, with the heat flow rising to a maximum of 73 mW/m(2) in oil exploration wells drilled in New Plymouth. This compares to an average heat flow of close to 60, mW/m(2) for the whole of the Taranaki Basin. The spatial association between the heat flow anomaly and the Quaternary volcanics on the peninsula suggests a causal relationship. Conductive thermal modelling shows that either magmatic intrusions into the crust, or magmatic underplating at the base of the crust, are capable of explaining the increased surface heat flow anomaly. If mid-crustal intrusions are the main cause of the anomaly, continual intrusions over the last 0.2-0.5 m.y., which amount to a cumulative thickness of c. 500 m, may have occurred. If magmatic underplating is the cause of the anomaly, intrusions over the last 2-4 m.y. totalling c. 5 km in thickness are required. The resulting rise in temperature within the overlying sediments has caused upper Eocene coal measures to begin expelling oil. The high heat flow may be the main source of the oil in shallow reservoirs in the Kaimiro-Ngatoro structures and at New Plymouth. Deep-penetrating seismic reflection surveys may be able to detect the presence of past intrusions within or below the crust. Microearthquake surveys at the time of future eruptions from Mt Taranaki may resolve the extent of crustal underplating and/or crustal intrusion of magma.