Paleoshorelines as Deformation Markers for Magmatic Related Faulting at Whakaipō Bay, Lake Taupō

Abstract

The Taupō Volcanic Zone (TVZ) is a volcanic arc associated with the continental intra-arc rift, the Taupō Rift, located in the North Island of New Zealand. The Central TVZ is characterised by voluminous rhyolitic volcanism that has produced large, caldera-forming eruptions. This study investigates the impact that the caldera-forming Taupō 232 ± 10 CE eruption had on faulting, and thus rifting, in nearby regions of the Taupō Rift. After the 232 CE eruption Lake Taupō’s water level rose then fell, leaving behind paleoshoreline terraces that can be mapped in the present day to constrain the nature of deformation around the lake over the last 1800 years. At Whakaipō Bay on the north shore of Lake Taupō and surrounding bays to the northeast, normal faults and paleoshorelines were mapped using a LiDAR digital elevation model (DEM) and field techniques. Two faults were the main focus in this study, the West Whakaipō Fault and Whakaipō Fault. These faults hold significance as they are proximal to the Taupō caldera and displace the 232 CE Taupō ignimbrite and associated paleoshoreline deposits, which act as critical time markers that provide constraints on the timing and rates of fault displacements. Based on the vertical displacements obtained on the Taupō ignimbrite and paleoshoreline deposits, and an examination of paleoshoreline morphologies around the observable faults, this study reveals the history of faulting at Whakaipō Bay for the ~20 year period immediately after the 232 CE eruption, and also up to the present day. Since the Taupō 232 CE eruption, it is estimated that the Whakaipō Fault has accrued a total vertical displacement of 12.8 ± 0.44 m; however, ~75% of this displacement accumulated in the 20 year time period immediately after the 232 CE eruption. The close temporal association between large-scale faulting and the 232 CE eruption highlights the significant affect that large-scale volcanic eruptions can have on surrounding extensional fault systems, suggesting that silicic volcanism may trigger continental rifting events in the Taupō Volcanic Zone.