Deciphering the 4-D geological framework of the Taupo-Reporoa Basin: Implications for hydrothermal fluid flow and volcanism

Abstract

The Taupo-Reporoa Basin (TRB) represents the easternmost basin in the central Taupo Volcanic Zone (TVZ), North Island, New Zealand, and hosts >2000 MW of the 4200±500 MW of heat output within this arc setting. Its geographic alignment with active volcanic centers (Okataina, Taupo), geophysical interpretations, and extraordinary heat output indicate that it is magmatically active. Yet the tectonic, volcanic, and magmatic processes that influence TRB evolution, and its tectonic context relative to the North Island, are poorly understood. The first part of this thesis (chapters 2 and 3) outlines the stratigraphic architecture of the TRB through mapping of exposed Quaternary age lavas, volcaniclastic, sedimentary, and primary pyroclastic rocks. Corresponding sedimentology, physical volcanology, geochemical analyses, and 40Ar/39Ar age determinations have allowed surface outcrops to be correlated with geothermal drillhole records to at least ~3 km depth. This has resulted in the development of an age constrained 4-D basin evolutionary model. Volcanism within the TRB started >1.9 Ma with the building of large andesitic complexes that overlap spatially and temporally with rhyolitic eruptives. Deposition of these units occurred in a single basin that spanned the entire modern TVZ (120 km long by 60 km wide). Closely coincident with emplacement of the Paeroa Subgroup ignimbrites at 339±5 ka, the single central TVZ basin was separated into the two modern basins: the TRB to the east, and the Taupo Fault Belt (TFB) to the west. These two basins are geographically separated by the Paeroa Fault. This fault closely coincides with the eruptive center (Paeroa linear vent zone) of the Paeroa Subgroup ignimbrites that are exposed in the upstanding blocks on both sides of the fault, which are known as the Paeroa Block (within the TRB) and Te Weta Block (within the TFB). The second part of this thesis (chapters 4 and 5) discusses silicic magma generation rates and heat sources of the TRB and greater North Island, and controls on heat and mass transfer. Aeromagnetic data has been processed using an upward continuation filter to a maximum depth of 4800 m. Geologically constrained magnetic anomalies are used to infer magnetic lineations as basement structures. Intersecting basement structures at geothermal fields and alignment with caldera boundaries indicate that these structures strongly influence the locations and geometries of geothermal fields and volcanic vents. Corresponding heat sources and magma generation rates are interpreted as resulting from Hikurangi Plateau subduction processes.