Abstract

Large explosive eruptions are generally rare, random events in the history of any particular volcano, volcanic area, or worldwide. In the Taupo Volcanic Zone, New Zealand, temporal clustering of eruptions occurs on a <1 yr to ∼300 k.y. basis, which implies that some controls lead to nonrandom eruption timing. We describe two closely paired large Taupo Volcanic Zone eruptions dated at ca. 240 ka that terminated a large-scale cluster of 7 caldera-forming and >15 smaller eruptions over a total ∼100 k.y. period. After a precursor eruption from a nearby source (and a break of years to decades), these paired eruptions in turn generated a wet ash-fall deposit and a dry pumice-fall deposit; the Mamaku ignimbrite (>145 km3 magma); a fine-grained vitric ash-fall deposit; then the Ohakuri ignimbrite (>100 km3 magma). Rotorua and Ohakuri, spaced ∼30 km apart, are the inferred collapse calderas associated with the Mamaku and Ohakuri ignimbrites, respectively. The early wet and dry fall deposits came from southerly sources, close to or within the subsequent Ohakuri caldera, while the fine-grained vitric ash is inferred to represent a co-ignimbrite ash from the Mamaku ignimbrite. At its southwest margin, the Mamaku ignimbrite overlies, but is also intercalated within and then overlain by, the pumice fall deposit, demonstrating that at least two widely spaced vents were active simultaneously for part of the eruption sequence. The post-Mamaku vitric ash-fall deposit underwent only trivial reworking prior to emplacement of the Ohakuri ignimbrite. This and other field evidence imply continuity, or time gaps of only days to months, in the whole paired sequence. Syneruptive volcanotectonic faulting may have permitted accumulation of >400 m of nonwelded Ohakuri ignimbrite through graben subsidence. Posteruptive faulting within years to decades of the eruption produced an ∼300 m extra-caldera offset of the Mamaku ignimbrite and collateral subsidence of a >40 km2 area immediately south of Rotorua caldera. Temporal linkages between ignimbrite eruptions and graben subsidence, the NNE-SSW alignment of associated faulting between the Rotorua and Ohakuri calderas, and the eruption-related subsidence indicate a tectonic control on volcanism associated with Taupo Volcanic Zone rifting processes. Statistical forecasts of the frequency of large-volume explosive events based on averages may be inaccurate because of tectonic triggering effects.

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