Sources of solutes and heat in low-enthalpy mineral waters and their relation to tectonic setting, New Zealand

Abstract

Approximately 90% of the low-enthalpy spring systems in New Zealand are distributed along the present-day convergence of the Pacific and Australian plates, which varies from subduction to oblique plate collision. Forearc subducted waters rising in the Hikurangi Accretionary Prism consist of pore waters containing labile organic material, water of hydration from clays, and seawater. These subducted waters are traceable in the North Island Axial Ranges and the northern Marlborough Fault System in the South Island albeit modified by temperature, mixing with surface and heated meteoric waters and protracted interaction with the rock. The signature of subducted marine sediments in the Hikurangi Trench is retained in the chemical and isotopic compositions of the Taupo Volcanic Zone magmatic arc-type waters. The concentrations of B, Ba, Br, Ca, Cl, I, K, Li, Mg, Na, Rb and Sr—solutes found in high concentrations in forearc subducted waters—increase from northeast to southwest towards the incipient subduction zone in the Puysegur Trench southwest of South Island. Most thermal mineral waters in the South Island are expelled along the Alpine Fault Zone where plate collision results to rapid uplift of the Southern Alps, elevated geothermal gradients and fault-shearing of rocks. In the Alpine Fault Zone, variations in temperatures (175°± 30 °C) and mineral water compositions are affected by elevation, hydrological gradient, intensity of tectonic activity, permeability, fluid volume and rate of throughput, and rates of uplift. As the main trace of the Alpine Fault Zone is approached, concentrations of Cl and other solutes generally increase suggesting increasing contributions from ascending Cl-rich “metamorphic waters”. Outside the active convergent zone, low-enthalpy springs discharge diagenetically-modified ancient forearc waters; Na–Cl sedimentary formation waters and heated seawater; Na–HCO 3–Cl waters; and Na–HCO 3 deep-circulating heated meteoric waters. The total surface heat flow in the North Island low-enthalpy mineral waters is more than 10× higher than in the South Island, with projected subsurface temperatures as high as 250 °C. This is because of the widespread occurrence of shallow magmatic and mantle sources, associated with Pliocene to Holocene volcanism and back-arc continental rifting. The sources of heat and solutes in low-enthalpy mineral waters, outside the Taupo Volcanic Zone and Ngawha, are decoupled except for rapidly ascending hot pore waters in the Hikurangi Accretionary Prism and low-temperature serpentinization Ca–Cl waters in Fiordland.