High permeability and low temperature correlates with proximity to brittle failure within mountains at an active tectonic boundary, Manapouri tunnel, Fiordland, New Zealand

- Despite >1 km overlying relief, the Manapouri tunnel is close to isothermal.
- Topography plays an important role controlling permeability and fluid flow.
- Permeability correlates with proximity to brittle failure.

Temperature and fluid flow data from a 10 km-long tunnel at the active plate boundary in New Zealand constrain the shallow (<3 km) permeability structure beneath mountains of fractured crystalline rock. The tunnel is at sea level and almost isothermal (10.6±2.5 °C), despite >1 km of topographic variation along its length. Permeability is sufficiently high for advective heat transport to be significant and 3D fluid flow models show permeability cannot be described as a simple function of depth. Permeability must be higher within mountains than beneath valleys. 3D stress models show that proximity to brittle failure increases beneath mountains and that a permeability structure correlated with this effect can explain temperature observations. Topography influences brittle failure and hence permeability, but contributes <30% of the stress load in Fiordland. Far-field tectonic stresses and transient phenomena such as earthquake shaking provide the main loads, but have regional rather than local extent, when averaged over geological time. The depth affected by topography is of similar magnitude to topographic amplitude. The heterogeneous permeability structure in active tectonic settings like New Zealand results in little or no thermal signal at depth associated with topography, with significant implications for modelling thermal structure and thermochronology data in past and present orogens. Our formulation can be applied in regions with topography and plate-boundary stress forcing, as well as less-active regions, and provides an improved conceptual basis for understanding fracturing processes and predicting the spatial distribution of rock mass physical properties such as permeability.