Thermal properties of orogenic flysch and schist, Rakaia Terrane, Torlesse Composite Terrane, New Zealand

ABSTRACT Voluminous, deformed, and variably metamorphosed argillite and greywacke sediments (flysch) are common orogenic rock types. Knowledge of their physical properties is required to make tectonic and geothermal models of modern and ancient plate boundaries. Portable X‐ray fluorescence data from 89 representative samples of such a unit in New Zealand were used to construct an argillite‐greywacke index (AGI). The porosity (1.40 ± 0.71%), density (2715 ± 196 kg m –3 ), volumetric heat capacity (2.17 ± 0.23 MJ K −1 m 3 ) and specific heat capacity (803 ± 110 J K –1 kg –1 ) show no significant differences between lithologies. Thermal conductivity and diffusivity varies by rock type: argillite, 2.40 ± 0.74 W m –1 K –1 , 1.09 ± 0.30 mm 2 s −1 ; greywacke, 3.06 ± 0.50 W m –1 K –1 , 1.43 ± 0.26 mm 2 s ‐1 ; semischist perpendicular to foliation, 2.47 ± 0.40 W m –1 K –1 , 1.20 ± 0.29 mm 2 s ‐1 ; semischist parallel to foliation, 4.02 ± 0.63 W m –1 K –1 , 1.74 ± 0.37 mm 2 s –1 ; schist perpendicular to foliation 2.33 ± 0.72 W m –1 K –1 , 1.11 ± 0.41 mm 2 s ‐1 ; and schist parallel to foliation, 3.96 ± 0.84 W m –1 K –1 , 1.90 ± 0.35 mm 2 s ‐1 . Thermal conductivity is correlated with grainsize, textural zone, and metamorphic grade; and is higher in rock with a biogenic component. Measurements parallel and perpendicular to foliation demonstrate thermal anisotropy factor increases with metamorphic grade: 1.15 ± 0.14 for greywacke; 1.39 ± 0.34 for argillite; 1.68 ± 0.35 for semischist; and 1.82 ± 0.50 for schist. Quantity, composition, and alignment of phyllosilicates is more important than compositional layering (bedding or segregations) as a cause of thermal anisotropy.