The ∼10 ka multiple vent pyroclastic eruption sequence at Tongariro Volcanic Centre, Taupo Volcanic Zone, New Zealand:: Part 1. Eruptive processes during regional extension

A sequence of six major (each ∼1 km3) and several minor pyroclastic eruption episodes occurred in the Tongariro Volcanic Centre (TgVC) during a period of uniquely intense activity at ∼10 ka (14C years B.P.). This andesitic/dacitic sequence, named Pahoka–Mangamate (PM), had an apparent duration of ∼200 to 400 years. Rhyolite tephras erupted from Taupo are interbedded within the PM deposits. Our mapping of PM ejecta distribution has shown that multiple vents between Tongariro and Ruapehu volcanoes were active during each of the ∼10 ka episodes. The PM vents are located on a 20-km long NNE-trending linear vent zone, lying within a graben structure defined by regional fault zones. Some of these faults appear to have ruptured during the ∼10 ka eruptive sequence. The pattern of fault and vent zones suggests that an episode of accelerated extension (rifting) occurred across the TgVC at 10 ka, marked by normal displacements of the graben margin faults, and by dike intrusion beneath the vent zone on the graben axis. Juvenile ejecta from each of the PM eruptions is chemically and petrographically diverse, indicating that a number of separate magma bodies were tapped during the ∼10 ka eruption sequence (see companion paper). Despite this compositional diversity, each of the PM pyroclastic deposits is characterised by poorly vesicular juvenile lapilli with fracture-bounded surfaces, indicating interaction at depth between rapidly rising magmas and abundant groundwater. Widespread (plinian) dispersal of the PM tephras demonstrates that relatively high eruption columns were produced, despite this water-cooling of the erupting magmas. The PM eruptions and associated faulting were triggered by rifting processes. The coincident eruption of voluminous rhyolite tephras from a linear multiple vent zone in the Taupo Volcanic Centre, 50 km to NE of Tongariro, demonstrates that the 10 ka extensional episode was of regional extent.