Volcanology and ignimbrite stratigraphy along the paeora fault, taupo volcanic zone

Abstract

Normal movement on the Paeroa Fault has exposed a 500 m thick sequence of crystal-rich ignimbrites, which span an age range of about 570-370 ka. The ignimbrites are collectively known as the Paeroa Range Group, and comprise (from oldest to youngest) the Te Kopia, Te Weta and Paeroa ignimbrites. The Paeroa Range Group ignimbrites are petrographically and geochemically similar and are interpreted as the eruptive products of a single magma chamber. Most of the ignimbrites were emplaced by low and moderate energy pyroclastic flows. Lithic-rich, co-ignimbrite breccias record more violent activity and are considered to correlate to episodes of caldera collapse. A well bedded, accretionary lapilli-rich unit represents a brief phase of phreatomagmatic activity. The distribution of ignimbrite facies along the fault and the occurrence of very proximal facies in some of the ignimbrites, indicate that the ignimbrites were erupted from at least two source calderas which were located nearby. The Te Kopia ignimbrites were erupted from a source caldera at the northern end of the study area, while the Te Weta and Paeroa ignimbrites were erupted from a southern source. No topographic evidence of the source calderas for either the Te Kopia or Te Weta ignimbrites exists today due to burial by younger units. A possible source caldera for the Paeroa Ignimbrites is located to the the east of the Paeroa Fault, and has been partially buried beneath the younger Waiora Formation. Following eruption of the Paeroa Ignimbrites and subsequent collapse of the caldera, uplift of the study area occurred along west to northwest-striking normal faults. Younger formations erupted from sources elsewhere in the Taupo Volcanic Zone were not deposited within the centrally uplifted part of the study area and appear to have been channelled around the topographic highs. A later phase of faulting along northeast-striking normal faults commenced at about 150 ka and has continued up to the present day.