An Integrated Approach to Correlation of Geology in Geothermal Systems: a Case Study from the Kawerau Geothermal Field, New Zealand

Abstract

The utilisation of geothermal systems benefits fromunderstanding the host rock geology, locations/controls onpermeability pathways, and the source/timing of heat inputinto the system. Using an integrated approach, i.e.,combining age dating with systematic petrographicexamination of relevant rock units, researchers can obtaingreater insights into the geological and hydrologicalstructure of a geothermal system, and hence its successfulexploration and development at reduced risk. Here wepresent a revised geological framework for the Kaweraugeothermal system, as a case study that outlines thestructural and thermal history of the system.The oldest definable event at Kawerau is faulting ofbasement greywacke along NW-SE orientated, dominantlystrike-slip structures, which progressively generated halfgrabensthat were filled with sediments (and twoignimbrites, U-PB zircon-dated at 2.38±0.05 and 2.17±0.05Ma, respectively). By the time the 1.46±0.01 Ma ignimbrite(Te Teko Formation) was deposited across the field, anylocal topographic relief was subdued. Subsequent depositionof ignimbrites occurred at about 1.0, 0.55-0.6, and 0.32 Ma,interspersed with sedimentary sequences that accumulated ataverage rates of 0.06 mm/yr. Andesite lavas from a buriedcomposite cone occur as a conformable package betweenunits dated at 1.0 and 0.6 Ma. Bodies of coherent rhyoliteoccur at multiple stratigraphic levels, with dikes and domesthat are still exposed emplaced at 0.138 ± 0.007 Ma and aseries of domes, sills and associated tuffs that wereemplaced at 0.36 ± 0.03 Ma. The andesitic Putauakicomposite cone first erupted around 8 ka, but evidence ofhydrothermal eruption breccias point to magma beingintruded to shallow depths as early as ~16 ka. Current ratesof regional tectonic subsidence (2±1 mm/yr) and thermaloutput at Kawerau are geologically recent, and areassociated with latest Quaternary rifting processes (