Large-scale rift-related faulting linked to a caldera-forming eruption: A case study from Taupō, New Zealand

Phases of accelerated normal faulting in the Taupō Volcanic Zone have been demonstrated to be triggered by rhyolite eruptions, yet little is known about how the Taupō Fault Belt responds in the aftermath of caldera-forming events, particularly the 232 CE Taupō eruption. To address this issue, we conducted paleoseismic trenching coupled with remote and field analyses of the Whakaipō Fault (north Taupō) and the displaced post-232 CE paleoshorelines intersected by this structure. The throw profiles along the Whakaipō Fault reveal increasing throw in proximity to Lake Taupō, highlighting the importance of Taupō volcano (in particular the 232 CE caldera margin) for localising fault strain. Paleoseismic trenching exposed a ~50º dipping un-degraded paleoscarp draped by fall deposits of the 232 CE eruption, implying that fault slip occurred in the days to months preceding the eruption. Analysis of fault and paleoshoreline displacements at Whakaipō Bay on the northern shoreline of Lake Taupō suggest that two main phases of slip on the Whakaipō Fault occurred: (1) an “aftermath” phase, occurring over a ~10-20-year period after the 232 CE eruption, during which 5-10 m of throw was accrued locally on the fault; and (2) a subsequent “longer-term” phase through to the present day, during which 2.8 ± 0.3 m of fault throw has accrued. Faulting during the aftermath phase is estimated to account for ~75% of the total extension accommodated locally on the Whakaipō Fault since 232 CE, and demonstrates that exceptionally large (>5 m) normal fault displacements may accrue along the Taupō Fault Belt in association with caldera-forming eruptions.