Our team are exploring more fractures in Aotearoa’s ancient basement rocks to better understand how to unlock supercritical resources. This research builds on our prior work mapping fractures in Marlborough and exploring deep basement rock from the surface. Understanding the fluid flow beneath the Taupō Volcanic Zone is one key element to support the New Zealand Government’s move to advance superhot exploration.

The paper, recently published in the New Zealand Journal of Geology and Geophysics, brings together fieldwork and analysis by geologists Cécile Massiot, Sarah Milicich, Siru Jylhänkangas, Nick Mortimer, Regine Morgenstern and Isabelle Chambefort.

The story in the stone

Deep beneath the Taupō Volcanic Zone (TVZ), geothermal fluids move through New Zealand’s basement rocks. What makes flow possible are fractures that are pathways for hot fluids. Through time, fractures can fill with mineral deposits like quartz and become veins that we can observe today as records of past fluid flow.

However, basement rocks are very complex. Greywacke rocks have a long history starting at the bottom of the oceans with layers of sediments deposited on top of each other. Then rocks were buried, faulted, brought back up to the surface, and in some cases experienced hot geothermal fluids. All these steps caused deformation and cracking. This makes it difficult to predict if there are open fractures in deep superhot systems, how many there would be, where they would be and how connected the fluid pathways are.

Understanding how and where these fractures form and how they connect is vital for predicting where geothermal systems at 4–6 km depth will allow enough fluid to circulate and generate large amounts of energy.

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From rock face to reservoir model

In this new study, the team examined fracture and vein patterns in greywacke at three different field sites across the motu: Awakeri Quarry in the Bay of Plenty where rocks have not been to very great depths before, Whakatāne Heads which is part of a major old fault zone, and Rarangi in Marlborough where rocks became schists at very great depth. In addition, they reviewed prior studies around the motu that represent different context of burial, and prior faulting and/or hot fluid circulations.

Using a combination of field mapping, drone imagery photogrammetry and 3D visualisations, the team found what they were looking for – lots of veins. Even better, they found they are fairly well connected to each other. In the new outcrops, they even found some long veins that connect sweet spots of high vein densities.  

This quantitative analysis is the foundation to model realistic fracture networks and help mapping fluid circulations in geothermal reservoirs, as developed by Kissling and Massiot (2023)

Want to see these fracture networks for yourself? Check out our interactive 3D visualisations for Awakeri Quarry and Rarangi Schist Outcrop.

What does this mean for geothermal energy?

With this new knowledge, our team have proposed a way to select some parameters that matter at different scales. They have determined that the complexity of the old sedimentary layering does not seem to be important when looking for “where to drill”, but it is important when interpreting data collected in boreholes.  

For modern geothermal development, particularly on superchot resources, this work helps answer key questions about the viability of these deep systems:

• How permeable is basement greywacke at depth?
  The answer depends not just on how many fractures exist, but how well they’re connected across scales.

• Can fracture patterns be predicted from the surface?
  Yes, to a degree. Mapping at outcrop scale gives insights into deeper, less accessible systems.

• Do sedimentary layers matter?
  Not as much as previously thought, once buried, it’s the tectonic history and stress regime that dominate. But the layers are important to understand small-scale patterns imaged in boreholes.

his study is more than just a geological deep-dive. It’s a roadmap for how we can use the Earth’s natural plumbing system to power our homes, businesses, and communities.

“These ancient veins are more than just geological patterns. They’re clues to how we can sustainably harness energy for generations to come.” says lead author Dr. Cécile Massiot.

Thanks to J-swap for access to the Awakeri Quarry. This study is part of GNS Science’s Geothermal the Next Generation research programme funded by the New Zealand Ministry of Business, Innovation and Employment Endeavour Research; and Nick Mortimer’s James Cook Research Fellowship awarded by the Royal Society Te Apārangi.

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