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Double trouble: Paired ignimbrite eruptions and collateral subsidence in the Taupo Volcanic Zone, New Zealand

D.M. Gravley^,1, C.J.N. Wilson¹, G.S. Leonard² and J.W. Cole³

¹ Geology Department, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand
² GNS Science, P.O. Box 30368, Lower Hutt 5040, New Zealand
³ Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand

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.

Key Words: caldera collapse • collateral subsidence • ignimbrite eruption • Taupo Volcanic Zone • Mamaku ignimbrite • Ohakuri ignimbrite

This article has been cited by other articles:

				C. J. N. Wilson and B. L. A. Charlier Rapid Rates of Magma Generation at Contemporaneous Magma Systems, Taupo Volcano, New Zealand: Insights from U-Th Model-age Spectra in Zircons J. Petrology, May 13, 2009; (2009) egp023v1. [Abstract] [Full Text] [PDF]

				P. W. Lipman and W. C. McIntosh Eruptive and noneruptive calderas, northeastern San Juan Mountains, Colorado: Where did the ignimbrites come from? Geological Society of America Bulletin, July 1, 2008; 120(7-8): 771 - 795. [Abstract] [Full Text] [PDF]