Preliminary results of thermal conductivity and elastic wave velocity measurements of various rock samples collected from outcrops in hanging wall of the Alpine Fault

Abstract:

The Alpine Fault is a mature active fault zone likely to rupture in the near future and DFDP aims to measure physical and chemical conditions within the fault. DFDP-2B borehole was drilled into hanging wall of the Alpine Fault. Downhole temperature measurements carried out in DFDP-2B borehole showed that the geothermal gradient in the hanging wall of the fault is very high, likely reaching to 130-150 °C/km (Sutherland et al., 2015 AGU Fall Meeting). To explain this abnormal feature, the determination of thermal properties of all the rock types in the hanging wall of the Alpine Fault is essential.

To measure thermal properties and elastic wave velocities, we collected six typical rock block samples from outcrops in Stony creek and Gaunt creek. These include ultramylonite, mylonite, muscovite schist, garnet amphibolite, protomylonite and schist, which are representative of the hanging wall of the Alpine Fault. Their wet bulk densities are 2.7 – 2.8 g/cm3, and porosities are 1.4 – 3.0%. We prepared a pair of 4 cm cube specimens of each rock type with one flat plane parallel to the foliation.

First, we measured thermal conductivity by the transient plane heat source (hot disc) method in a bulk mode, i.e. to deal with the rock as an isotropic material. However, several samples have clearly visible foliation and are likely to be anisotropic. Thus, the data measured in bulk mode provided an average value of the rocks in the range of approximately 2.4 – 3.2 W/mK. The next step will be to measure thermal conductivity in an anisotropic mode.

We also measured P wave velocity (Vp) using the same samples, but in two directions, i.e. parallel and perpendicular to the foliation, respectively. Our preliminary results suggested that Vp is anisotropic in all the six rocks. Generally, Vp parallel to foliation is higher than that in the perpendicular direction. Vp in the parallel direction ranged in 5.5 – 6.0 km/s, whereas in the perpendicular direction it was 4.4 – 5.5 km/s.

We thank the PIs and onsite staffs of the DFDP-2 project for their helps to collecting rock samples, and the financial support by JSPS (Japan-New Zealand Joint Research Program).