Members of our GNG team recently co-authored a paper in the Journal of Geophysical Research : Solid Earth. This study determines the thermal structure of Zealandia using magnetic data to map the depth to the temperature at which magnetisation vanishes, also known as the depth to base of magnetic sources (DBMS). This temperature is called the Curie point temperature and is typically about 580°C, although it can vary depending on the chemical make up of the rocks.

To model the DBMS, we used a method that analysed the frequency content of an updated compilation of magnetic data over central Zealandia, including on shore New Zealand. By modelling 1000’s of magnetic spectra from 150 km wide grids of magnetic data spaced every 10 km across NZ, we could model the top and thickness of the magnetic crust, from which we can derive the depth to the base by simple addition.

Our results show that the shallowest depth at which magnetisation vanishes in the Taupo Volcanic Zone is at around 9-11 km. The deepest area at which magnetisation vanishes (28-30 km) is in the central South Island. We compare our model with other crustal markers, such as the depth at which most earthquakes stop, and the thickness of the crust. There is a general correlation of the depth to base of magnetisation with crustal thickness, although the relationship is complicated and sometime the depth to base of magnetisation may be below the crust, in the upper mantle.

The depth at which the DBMS occurs is also important as this isotherm is a control on crustal heatflow and hence geothermal resource distribution within Zealandia. Other uses of this model are for understanding the depth to which rocks become ductile (about 400°C) rather than brittle. Earthquakes typically do not occur below the brittle-ductile transition and this has important implications for understanding earthquake hazard in New Zealand, as shallow earthquakes may cause more damage than deeper. We find that, in most cases, the brittle ductile transition zone is shallower than the DBMS, as expected, although the relationship is complex and varied, and highlights the complex tectonic and crustal makeup of Zealandia across the plate boundary that separates the Pacific and Australian plates.

Finally, we used our DBMS model to create a new heatflow model of onshore New Zealand, calibrated by heatflow measurements made in previously in boreholes. Heatflow models are important for a range of users, including geothermal. Areas of high heatflow are regions that can be used for a range of geothermal uses, including power generation or direct use. The combination of DBMS and heatflow modelling allows continent to regional scale assessment of heatflow and can be used to guide future more detailed exploration into geothermal resources, including supercritical.

Citation:

Miller, C.A., Kirkby, A., Mortimer, N., Aden, F., Barretto, J.,Black, J., Chambefort, I., Stagpoole, V. (2023) The thermal structure of central Zealandia continent as determined from the depth to base of magnetic sources. Journal of Geophysical Research: Solid Earth.