Thermal Properties of the Central Alpine Fault, New Zealand: Implications for Regional Heat Flow and Earthquake Processes

The Alpine Fault is New Zealand’s largest onshore source of seismic hazard, is late in its seismic cycle, and has regularly ruptured in magnitude 8 earthquakes. Temperatures within fault zones strongly influence fault behaviour before, during, and after earthquake slip, yet little is known about the thermal properties of fault rocks themselves. This study quantifies the thermal properties of the Alpine Fault zone, which consists of a ~50 m-thick sequence of fault gouge and cataclasite that grades into a ~1 km-thick mylonitic sequence derived from the protolith Alpine Schist. 130 thermal conductivity, diffusivity, and volumetric heat capacity measurements were made in situ of fault gouge, cataclasite, and the footwall Quaternary gravel using a linear heat source needle probe. 40 foliated mylonite and Alpine Schist samples were collected and measured with an electronic portable divided bar in directions parallel and perpendicular to foliation.

Thermal conductivity decreases with increasing proximity to the principal slip zone from 2.15 ± 0.36 W m-1 K-1 in cataclasite to 1.37 ± 0.19 W m-1 K-1 in fault gouge, controlled by increasing porosity and the alteration of primary minerals to phyllosilicates of lower thermal conductivities. Mylonite and Alpine Schist have anisotropic thermal conductivity with values parallel to foliation (3.46 ± 0.59 W m-1 K-1) being 1.8 times higher than perpendicular to foliation (1.93 ± 0.29 W m-1 K-1) caused by structural and crystal anisotropy associated with aligned mica in foliation planes. Thermal conductivity parallel to foliation is dominantly controlled by quartz content. Anisotropic thermal conductivity of mylonite and Alpine Schist refracts heat flow of the hanging-wall north-westwards towards the Alpine Fault because foliation dips 40–50⁰ to the southeast.

Fault gouge and cataclasite thermal properties were applied to models of critical slip distance before the onset of weakening from thermal pressurisation. Critical slip distances of <15 cm and >10 m in fault gouge and cataclasite, respectively, support the hypothesis that thermal pressurisation causes dynamic slip weakening on the principal slip surface, but not in the cataclasite, and hence could explain why rupture repeatedly occurs on the same surface.