| Abstract |
The majority of thermal waters in the South Island discharge along the Alpine Fault. The rest occur in coastal Canterbury, Taieri Basin, West Coast and Southland. Surface discharge (maximum of 66oC measured in this survey) and inferred subsurface temperatures (200 + 10oC) are higher in the Alpine Fault than in the rest of South Island. About 99% of the annual thermal water flow and surface heat output are released from the Alpine Fault springs at ~7 x 108 L/a and ~113 TJ, respectively. Along the Alpine Fault Zone thermal mineral waters are derived from heated meteoric waters gaining solutes from interaction with fault comminuted rock, contributions from the subducted slabs in the northeast and southwest of the South Island and probable metamorphic fluids at depth. These are diluted by cold meteoric waters. Serpentinisation reactions in Fiordland result to unusually high pH and very low total dissolved solid contents in mineral waters. Anomalous heat along the Alpine Fault Zone is caused by rapid uplift. Formation waters occur in the West Coast and cold seawater mixes with heated meteoric waters in coastal Canterbury. In the South Island the relative permeability of faults or fractures, topographic gradient, elevation and uplift rates affect fluid flow, heat transfer and solute transport. Relatively higher permeability in the Marlborough Fault System compared to other parts of the Alpine Fault allows inflow of cold meteoric water from the surface but also rapid ascent of hot solutions from depth. In contrast the recirculation of solutions in the No. Alpine Fault may be more circuitous, the fault zone relatively less permeable or the fluid source deeper. Fault-comminuted rock along the Alpine Fault enhances and hastens kinetic rates of reaction with aqueous solutions resulting to high HCO3/Cl ratios with low Mg/Ca and K/Na ratios. Where fluid flow is restricted, rates of reaction between fault-comminuted rock and solutions may be protracted, giving rise to oversaturated silica solutions at depth. Where high permeability faults allow large throughput of cold meteoric waters, silica concentrations in surface discharges decrease. |