Basement Geology and Structure of TVZ Geothermal Fields, New Zealand

Abstract

The basement beneath the volcanic and sedimentary deposits in geothermal fields in the Taupo Volcanic Zone is a potential, but poorly understood resource for future deep production. A basement exploration drilling programme carried out by ECNZ (now Contact Energy Ltd) at Ohaaki showed that geywacke-dominated Torlesse Terrane is permeated by hot fluids to depths of 2000 m below sea level, but has poor permeability even where cut by major faults. A rhyolite dyke drilled 575 m below the basement surface may have intruded a fault plane and sealed permeability at that point. Greywacke-dominated basement probably does not extend more than 10 km beyond the eastern margin of the TVZ at Ohaaki. Hot plutons (granite and minor diorite), and metamorphosed Torlesse Terrane intruded by dyke complexes may comprise the basement further west in the Whakamaru caldera area.