Hydrothermal Fluid Flow in a Structurally-controlled Basin, Ngakuru Graben, Taupo Rift, New Zealand

Abstract

The Ngakuru Graben lies in the northern Taupo Rift, New Zealand and hosts a number of major geothermal areas on its western and eastern margins – including Te Kopia, Waikite, Orakeikorako, Atiamuri and Horohoro. As well as these active geothermal systems, areas of extinct hydrothermal activity (i.e. silica sinter, hydrothermal eruption deposits and hydrothermally-altered tephras and lake sediments) are also found at the margins of the graben, including its northern and southern terminations. The graben contains four major active faults (or fault systems) – the Ngakuru, Maleme, Whirinaki and Paeroa. These not only define the graben structurally but play host to its hydrological system which, because of the deep interconnectedness of the faults, can behave in a complex way when there are significant, sudden changes in the fault permeabilities. Such changes are expected to have implications for the location and longevity of past geothermal activity in the graben, as interpreted from fossil sinter formations and other manifestations. A numerical model of this system has been developed using the fluid flow code TOUGH2. The model represents the major geological features of the area and a recently inferred deep heat source to the east of the Paeroa fault. We explore models where there are major perturbations to the permeabilities of the faults. An increase in fault permeability is assumed to be associated with a fault rupture, while a permeability reduction to ‘background’ values can occur as a result of (unmodelled) chemical or geological processes. The key result from the modelling is that the flows in an inter-connected system of faults are complicated, and fluid pathways can be altered by changes in the fault permeabilities. More specifically, despite significant changes in the flows immediately following a rupture, even large increases in fault permeabilities do not lead to major changes such as the cutoff or reversal of flow in the faults on ~100 year timescales. Conversely, we identify a model where a reduction in fault permeability leads to drastic changes in the flows, and these have important consequences for the fossil sinter deposits found at various locations within the Ngakuru graben