Thermal and hydraulic characterization of the hanging-wall of the central Alpine Fault
Authors:   , Janků-Čápová Lucie 
Published: July 2022

<p><b>In an active orogen bounded by a plate-boundary fault, heat is transported not only by conduction through rock, but also by advection by exhumation of rock, and advection by fluids flowing through fractures. In order to understand the fault behaviour, it is important to identify fluid pathways and fluxes, and assess the thermal structure near the fault. This thesis investigates heat transfer in the hanging-wall of the Alpine Fault in the Whataroa Valley, South Island, New Zealand. We combine in-situ wireline, fibre-optic and hydraulic observations from the DFDP-2B borehole with laboratory measurements of representative rock samples to address heat transfer on scales from metres to hundreds of metres.</b></p>
<p>Radiogenic heat productivity estimated from geochemical composition of cuttings (1.8±0.4×10⁻⁶ Wm⁻³) and a gamma log in the DFDP-2B borehole (2.1±0.1×10⁻⁶ Wm⁻³) is in the range of the greywacke protolith of the Alpine Schist.</p>
<p>Bulk thermal conductivity (2.8±0.6 Wm⁻¹ K⁻¹) and diffusivity (1.8±0.2×10⁻⁶ m² s⁻¹) were measured with the hot disk method on saturated rock samples from outcrops and the Amethyst Tunnel. Bulk thermal conductivity and diffusivity estimated from mineralogical composition of cuttings from DFDP-2B are 3.3±0.2Wm⁻¹ K⁻¹ and 1.6±0.1×10⁻⁶ m² s⁻¹, respectively. Macroscopic structures such as folds and quartz veins intersecting the foliation at different angles reduce the microscopic effect of foliation on centimetre scale, which leads to measured anisotropy coefficients close to 1.</p>
<p>Extreme lateral thermal gradients induced near the borehole wall by mud circulation allowed us to identify nearly two hundred anomalies in sixteen temperature logs taken at different stages of drilling. We interpret them based on their evolution in time. The short-lived are attributed to fractures healed with minerals with thermal diffusivity contrast of ±0.2×10⁻⁶ m² s⁻¹ or larger. Those persistent for weeks are interpreted as fractured zones with fluid fluxes of 10⁻⁷ to 10⁻⁶ ms⁻¹.</p>
<p>Temperatures measured with distributed temperature sensing (DTS) technology during the year after drilling distinguish six zones with distinct geothermal gradient. Conductive heat flux estimated from the equilibrated temperature gradients is high and extremely variable (between 90±5 and 460±20 mWm⁻²), reflecting different dominant mechanisms of heat transfer in these zones. As thermal conductivity of the bedrock is relatively uniform, we interpret the zones with low thermal gradient as zones in which heat transfer occurs predominantly by fluid advection. We interpret the zone with low gradient (27±1°C km⁻¹) near the base of the hole (beneath 690 m true vertical depth) to be a fractured aquifer associated with the damage zone of the Alpine Fault.</p>
<p>Findings of this study may have broader implications for assessing or monitoring earthquake hazard in the South Island, as well as for potential harnessing of this unconventional geothermal resource. This thesis contributed extensive wireline and fibre-optic temperature measurements, and complemented a previously scarce dataset of thermal properties and radiogenic heat productivity of major lithologies of the central Southern Alps. A novel technique for identification, quantification and interpretation of wireline temperature data that we developed may be applicable in industry, because it can resolve even minor fluid fluxes. Finally, this thesis places new constraints on the thermal and hydrological structure of the hanging-wall of the Alpine Fault, which is late in its seismic cycle.</p>