The How and Why of Hydrothermal Circulation on the Alpine Fault, New Zealand

Drilling of the DFDP-2B borehole into the Alpine Fault hanging wall in the Whataroa Valley, South Island, New Zealand, has revealed an unusually high geothermal gradient and pore fluids in excess of hydrostatic pressure. A previous thermal model [Sutherland et al. 2017] considered bulk convection of groundwater within the hanging wall and tectonic uplift of hanging wall rocks to account for the high fluid temperatures. However, this model did not consider other regimes or mechanisms, such as concentrated fluid flow within a high permeability damage zone, or periodic shear heating due to regular M_w ≥ 8 earthquakes (seismic cycle 300 years; Cochran et al. [2017]).

We have developed a series of alternative Alpine Fault models using a geothermal reservoir simulator AUTOUGH2 modified to approximate uplift and shear heating effects. Our models incorporate fluid flow driven by local topography, and explore the possible combinations of uplift, fault plane and fault block flow, and shear heating, that are consistent with drilled temperatures.

While we have been able to replicate the findings of Sutherland et al. [2017] using a similar model setup, we also detail several alternative configurations that produce similar or better borehole temperature matches. These include flow systems hosted exclusively within the fault plane, those developed within a permeable extended-damage zone proximate to the fault, and including both shallow (4 km) and deep limits on circulation (8 km). Using the calibration at Whataroa, we have extended our flow models to approximate the different possible flow patterns along the 500 km strike of the central Alpine Fault. This enables us to estimate the geothermal potential of fault-influenced hydrothermal flow in the South Island, and suggest potential locations of other hot upwellings.