Thermal properties of the hanging wall of the central Alpine Fault, New Zealand

Abstract

ABSTRACT Samples of schist, protomylonite, mylonite and ultramylonite from the hanging wall of New Zealand’s Alpine Fault were measured using a transient plane source, yielding mean bulk thermal conductivity of 2.04 ± 0.33 W m–1 K–1 in a dry state and 2.78 ± 0.51 W m–1 K–1 in a wet state at room temperature. Corresponding thermal diffusivity is 1.35 ± 0.32×10–6 m2 s–1 in a dry state and 1.72 ± 0.27×10–6 m2 s–1 in a wet state. Thermal conductivity and diffusivity calculated from the mineralogical composition of cuttings from the DFDP-2B borehole in Whataroa Valley and previously published thermal properties of minerals are 3.26 ± 0.16 W m–1 K–1 and 1.56 ± 0.08×10–6 m2 s–1, respectively. We attribute the significant difference in thermal conductivity to regional mineralogical variations in composition of rock samples versus borehole cuttings. Anisotropy of thermal properties is inferred to exist on a centimetre scale, due to competing effects of aligned quartz veins, minerals, and microfractures; but our measurements do not confirm significant anisotropy. Radiogenic heat productivity calculated from geochemical data and a gamma log in DFDP-2B (1.8–2.1 μW m–3) falls within the previously reported range for greywacke and schist.