Stratigraphy, paleomagnetism, geochronology and structure of silicic volcanic rocks, Waiotapu/Paeroa range area, New Zealand

Abstract

The altered ignimbrite sequence underlying the Waiotapu Geothermal Field has long been correlated with unaltered ignimbrite sequences exposed in the Ngapouri and Paeroa fault-scarps to the west. Recent age-dating (fission-track and K-Ar) and magnetic polarity studies have indicated this correlation is invalid. The Paeroa Scarp section comprises three major ignimbrites (Paeroa, Te Weta and Te Kopia) erupted between 0.34 and 0.38 Ma ago from local caldera sources. The Caldera Boundary Fault (CBF) marking the eastern boundary of the Paeroa Caldera crosses the northern Paeroa Range between the Waiotapu and Waikite thermal areas. East of the CBF, the Ngapouri Scarp section comprises the Waiotapu Ignimbrite (0.58 ± 0.03 Ma) unconformably overlying a reversely magnetised sequence of late Matuyama age ignimbrite (Akatarewa A) and epiclastic tuff (Unit X), intruded by rhyolite (Ngapouri Rhyolite). Episodic caldera collapse has spalled exotic blocks of Waiotapu Ignimbrite and Ngapouri Rhyolite into the caldera, where they form a discontinuous layer between two sheets of Paeroa Ignimbrite. In the Waiotapu Geothermal Field - separated from the Ngapouri Ridge by a younger rhyolite dome - the Waiotapu Ignimbrite is underlain by Akatarewa ignimbrites (A,B) separated by epiclastic tuffs (Unit Y) overlying a truncated andesite cone (Ngakoro Andesite) and an older unnamed Ignimbrite C; all are of late Matuyama age, possibly extending back to the Jaramillo sub-chron. Younger sheets of Rangitaiki (0.35 ± 0.03 Ma), Matahina (0.28 ± 0.04 Ma), Ohakuri (0.27 ± 0.03 Ma) and Kaingaroa (0.22 ± 0.04 Ma) ignimbrite were emplaced across the area, the latter from the nearby Reporoa Basin (caldera). The progressive uplift of the Paeroa and Ngapouri tilted fault-blocks and further subsidence of the Reporoa Basin allowed the accumulation of thick lake beds (Huka Group) which capped geothermal reservoirs, originally established and later rejuvenated by episodic caldera formation. Present-day upflow zones are largely concentrated by active faults and fault intersections breaching the cap-rocks. Hydrothermal eruptions from self-sealed shallow reservoirs throughout the Waiotapu-Waikite area were commonly initiated by volcanic and/or tectonic events on major faults cutting and linking major calderas.