A structural review of Wairakei-Tauhara Geothermal Field

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McNamara, D.; Massiot, C.; Lewis, B. 2013 A structural review of Wairakei-Tauhara Geothermal Field. Lower Hutt, N.Z.: GNS Science. GNS Science report 2013/03 20 p.

Abstract: Understanding of the structural framework of the Taupo Volcanic Zone (TVZ), Taupo Rift, and the Wairakei-Tauhara Geothermal Field has evolved over time. Initial geophysical surveys of the TVZ determined it to be a NE-SW trending zone of extension accommodated by complex normal faulting. Stress orientation studies confirm that TVZ faulting is almost purely normal dip-slip, with small, local strike-slip components. Many studies have integrated geophysical, field mapping, and remote sensing data of the TVZ and Taupo Rift to show subordinate NW-SE, N-S, and E-W structural fabrics in addition to the dominant NE-SW regional trend. Recent work has shown that the TVZ and Taupo Rift have an overall segmented structure, though the nature of the links between these segments is unresolved. The Wairakei-Tauhara Geothermal Field is the southernmost commercially developed geothermal field in the TVZ. It occupies a NE-SW striking graben centred under the west of the field. Early work established the main structure of the field divided into the central, up-faulted, NE-SW trending Wairakei Block, which is down-faulted into the Te Mihi Basin to the WNW and the Taupo-Reporoa Basin to the SE. Historic and recent field mapping and aerial photographic interpretation show most active faults strike NE-SW and only a minor set striking NW-SE, N-S and E-W. Whether these faults represent reactivation of deep inherited structures or secondary faults formed in the same tectonic regime as the primary NE-SW trend is a subject of debate. Extrapolation of field mapping from outside the geothermal fields, and geophysical and remote sensing studies have also suggested the presence of regional, basement NW-SE trending faults in the region of the Wairakei-Tauhara Geothermal Field. However, direct evidence of faults and fractures in borehole image logs and fracture analysis of relevant surface outcrops suggest that faults and fractures with trends other than NE-SW are rare. Given that the geothermal activity and distribution of the TVZ and Taupo Rift is thought to be dominantly controlled by regional structures and that structural permeability in the Wairakei-Tauhara Field has been proven by recent correlation of PTS logs with borehole image data, further understanding of the structural framework of the Wairakei-Tauhara Geothermal system will be vital for the continued successful development of this field and for establishing the larger role of faulting and fracturing in geothermal expression in the TVZ. (auth)