Heat flow in an active plate margin: New Zealand's crustal thermal regime from borehole temperatures and numerical modelling

Abstract

Geothermal heat provides a low-carbon energy source, which can be used directly or to produce electricity. It is an important part of New Zealand's energy mix, but to date geothermal use in New Zealand has been focused in known hot regions, particularly the Taupō Volcanic Zone (TVZ), above the subducting Pacific Plate. This study examines the complexity of New Zealand's heat flow and crustal temperature distribution, using borehole temperature measurements and conductive heat flow modelling, with a focus outside the TVZ. The modelling includes the transient heat flow effects of exhumation, basin subsidence and changes in crustal thickness, allowing gaps between sparse direct subsurface temperature measurements to be filled. Variations in New Zealand's heat flow largely reflect its position on a major plate boundary. The forearc region of the Hikurangi Subduction Margin has broadly low heat flow (30–50 mWm⁻²), associated with the down-going Pacific Plate, consistent with evidence from geophysical models and fluid chemistry. Heat flow is elevated in the Coromandel Volcanic Zone (100–140 mWm⁻²), north of the TVZ, which may be associated with volcanism from 18 to 4 Ma. High heat flow in Northland (100–220 mWm⁻²) is associated with more recent igneous activity. High heat flow along the Alpine Fault collisional plate boundary (150–250 mWm⁻²) is largely a result of rock advection (exhumation), which has been occurring at up to 6 mm yr⁻¹ over the last 2 Ma. These results demonstrate the importance of including transient processes in modelling heat flow in active areas.

Plain language summary

Understanding the temperatures in the upper 5–10 km of the Earth is important for many reasons. These include a growing drive to expand the use of geothermal energy, or heat energy in the earth, as a low-carbon way of producing electricity or direct use of heat for processes that would otherwise use electricity or fossil fuels. Knowing the temperature distribution is also important in understanding processes such as earthquake rupture. Subsurface temperatures have been measured in boreholes in some locations, but there are also large areas without any measurements. This paper uses our knowledge of geological processes occurring in New Zealand to predict underground temperatures between measurement locations. The underground temperatures of the Earth generally increase with depth, but the rate of increase varies greatly in different places. This variation is largely due to New Zealand's location on the boundary between two tectonic plates, the Pacific and Australian Plate. In the North Island, the Pacific Plate is subducting beneath the Australian Plate. This causes low crustal temperatures along the southeastern margin of the North Island but high temperatures in the central North Island (Taupō Volcanic Zone). In the South Island, the Pacific and Australian Plates are colliding, and uplift and erosion of the Southern Alps causes high temperatures at shallow depths along this mountain range.