| Title | Modelling of Flow Paths in a Structurally-Controlled Basin, Ngakuru Graben, Taupo Volcnic Zone, New Zealand |
|---|---|
| Authors | W.M. Kissling, A.J. Rae, P. Villamor and S.E. Ellis |
| Year | 2013 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Ngakuru Graben, geothermal systems, silica sinter, hydrothermal alteration, fluid flow, modelling, TOUGH2 |
| Abstract | The Ngakuru Graben is bounded by the eastern Paeroa and western Ngakuru Faults. The structural fabric within the graben is characterised by northeast-trending normal faults, with opposing northwest- and southeast-facing faults defining the central rift axis. The geology of the graben is dominated by ignimbrite sequences that are overlain by lake sediments and Quaternary surface alluvium. Surface expressions of geothermal activity occur on the graben margins, especially along the eastern and western boundaries, at Te Kopia, Waikite, Orakeikorako, Atiamuri, Ngapouri and Horohoro. Similarly, areas of extinct hydrothermal activity (i.e., silica sinter and hydrothermal eruption deposits, and hydrothermally-altered lake sediments) are also located marginal to the graben. Dating of organic material (14C) in fossil sinter from various locations shows that hydrothermal activity has been ongoing in the area for at least the last 34 ± 0.5 ka. The temporal and spatial characteristics of geothermal activity within the Ngakuru Graben raise important questions regarding the controls of fluid flow paths in a structurally controlled basin. What are the important parameters that influence permeability? Why is hydrothermal activity restricted to the margins of the graben, and why has activity at many of these geothermal areas been extinguished? A TOUGH2 model of this system has been developed to address these questions. The simplified model represents a northwest-southeast section across the graben and contains four geological units - shallow and deep volcanics within the graben, a low permeability unit exterior to it and a high permeability unit associated with faulting at the graben margins. We show that both upflows of warm water and downflows of cool surface waters can occur within these faults, and that these flows and the regional-scale flows within the graben can be controlled by small changes in the fault permeabilities. We discuss the dynamics of these upflows and how they might relate to the areas of hydrothermal activity observed at the basin margins. |