New Perspectives on the Volcano-Tectonic Evolution of the Wairakei-Tauhara Geothermal System, Taupo Volcanic Zone, New Zealand, from U-Pb Dating of Zircons

Abstract:

Wairakei-Tauhara (WK-TH) is one of the largest geothermal systems in the actively rifting Taupo Volcanic Zone. SIMS U-Pb dating of magmatic zircons (using SHRIMP-RG) from hydrothermally altered rhyolitic lavas and ignimbrites has provided new ages which constrain the major episodes of locally sourced volcanism and basin subsidence that were major influences on structural evolution and hydrology of the system. The WK-TH system is located in the southern Taupo-Reporoa Basin (TRB) across a horst-graben structure defined by predominantly NE-SW striking faults of the Taupo Rift. The geothermal system is hosted in late Quaternary volcanic and sedimentary strata and underlying Jurassic-Cretaceous greywacke basement. When combined with a robust stratigraphic architecture refined using data from >200 geothermal wells up to 3 km deep, the dating reveals that the majority of all known WK-TH rhyolite lavas (each ~0.1 km³ to ~7 km³) were extruded in a relatively short time interval from ~ 310 ka to ~120 ka and occur in a spatial cluster in the uppermost ~1.5 km of strata beneath the north-western (Wairakei) area. Stratigraphy and dating revealed at least one local rhyolite extrusion occurred at ~ 940 ka, and also an apparent >500 ka lapse in volcanic activity or local deposition after that event. The ages of strata from the Tauhara (south-eastern) area make it clear that a substantial volume of ignimbrite was emplaced into then eroded from the southern TRB between ~0.35 and 0.31 Ma, and then after ~0.31 Ma, replaced with as much as 2.5 km thickness of tuffs and sediments. Correlation by age of the locally named Wairakei Ignimbrite with the regional ~349 ka Whakamaru group of ignimbrites reinforces the value of this unit as a key stratigraphic marker and constrains the timing of major fault formation/reactivation, potentially with implications for understanding the structure and subsidence of Whakamaru caldera and evolution of the Taupo-Reporoa Basin.