Volcanic and tectonic perspectives on the age and evolution of the Wairakei-Tauhara geothermal system

This thesis presents a revision and update of the geology of the Wairakei-Tauhara (WKTH) geothermal system. The research is focussed around the use of U-Pb dating of zircons in key stratigraphic units, coupled with presentation of a robust stratigraphic architecture to establish the timing of major volcanic and volcano-tectonic events, including those that help to constrain the broader scale evolution of the Whakamaru Caldera and Taupo-Reporoa Basin (TRB) and those that initiated or rejuvenated the geothermal system. The present day Wairakei-Tauhara geothermal system is much younger than previously estimated and is now proposed to be a The earliest TVZ volcanism is represented locally by silicic pyroclastic units that were emplaced prior to 1.8 Ma on a basement now at >3 km depth (beyond the limits of drilled wells). At the Wairakei and Tauhara fields, these pyroclastics are intercalated with greywacke sediment, demonstrating that the area was within a long-lived basin westwards of the North Island axial ranges. At ~1.8 Ma, the earliest dated volcanism at Wairakei began with eruption of andesitic lavas at the eastern margin of TVZ. These lavas were later buried by sediments and rhyolitic pyroclastics, including the locally named Stockyard ignimbrite that is correlated by its U-Pb zircon age spectrum with 1.0 Ma Raepahu Formation ignimbrites erupted from the Mangakino Volcanic Centre. Magmatic activity in the following ~0.3- 0.4 Ma is represented locally by only two WK-TH volcanic episodes (at 0.95 Ma and 0.73 Ma). Wairakei well stratigraphy also highlights a major hiatus in local volcanism and deposition until the ~0.35 Ma Whakamaru eruptions. These and associated structural collapse were the events that may have initiated frequent volcanic and volcanotectonic episodes in the Wairakei-Tauhara area. Syn-eruptive piecemeal collapse of the caldera is inferred to have created the fault pathways for small batches of crystal-poor rhyolite, probably derived from a Whakamaru-like magma system, to intrude and erupt in much the same locations in the WK-TH area. This post-Whakamaru period (0.31 Ma until 0.22 Ma), overlaps with the age span of the majority of surficial rhyolitic volcanism in the Maroa Volcanic Centre, north and northwest of the WK-TH area. The Taupo-Reporoa Basin evolved in major volcano-tectonic episodes beginning around 0.35 Ma. In the south, displacement of Whakamaru Group ignimbrite across an elongate, central graben-like structure between the Wairakei and Tauhara fields is explained by 0.9-1.7 km subsidence of large NE-SW trending fault blocks, during piecemeal collapse of the Whakamaru Caldera. Further local collapse is proposed, which was nearly contemporaneous with 0.31 Ma eruptions that formed a ~10 x 12 km block faulted, newly recognised Waiora caldera and generated >30 km³ of Waiora Formation ignimbrites that syn-eruptively filled the new caldera. Rhyolite lava eruptions also starting at ~0.31 Ma on the eastern margin of the Tauhara Field and in the Wairakei Field, were the first of many spatially overlapping lavas and intercalated local pyroclastic units collectively mapped as the Waiora Formation. U-Pb and U-Th dating of zircons, zircon growth textures revealed by cathodoluminescence, and whole rock REE geochemistry are integrated with petrography to outline overlapping compositions of the lavas, several of which are also analytically indistinguishable in age. Correlations between Waiora Formation rhyolitic lavas and ignimbrites are equivocal with the available data, but the geochemistry suggests that early Waiora Formation ignimbrites may have been derived from a Whakamaru-like magma source that was distinct from contemporaneous Maroa rhyolite lavas. Infilling and subsidence of the Waiora caldera continued at varied rates until ~0.22 Ma. After 0.22 Ma there was a hiatus in volcanism in the local WK-TH area, during which lacustrine sediments accumulated in the local part of Lake Huka at ~1 mm/y. These fine grained Huka Falls Formation deposits became the impermeable cap rock of the modern WK-TH geothermal system. Volcanism did not wholly cease in the local area, or elsewhere in the TRB, however. There were shallow intrusions and subaerial dome extrusions of rhyolite at ~0.14-0.12 Ma, and at ~0.1 Ma there was the subaqueous eruption of middle Huka Falls Formation rhyolite. The new U-Pb ages reflect magma systems active in the local area at least at 1.8 Ma, 0.95 Ma, and 0.73 Ma, but there is no evidence of any associated geothermal system during one of these episodes. The Whakamaru eruptions and collapse episodes may have initiated the first geothermal system in the WK-TH area. The overlapping ages of Waiora Formation rhyolites, and spatial clustering beneath the Wairakei Field, show that underlying magmatism might have supported a geothermal system in more or less the same location at numerous times during the past ~0.3 Ma. Previously, the geothermal system was inferred to have been active since ~0.5 Ma, but in its modern state, the system is now interpreted to have begun at ~60 ka and has been sustained since then by the presence of an underlying NE dome magma system.