Tephra beds in deep-sea cores off northern New Zealand: implications for the history of Taupo Volcanic Zone, Mayor Island and White Island volcanoes

摘要

Twenty piston cores were collected from water depths of ～650-3300 m from offshore Bay of Plenty, North Island, New Zealand. They contain tephra from 11 Okataina Volcanic Centre and 4 Taupo Volcanic Centre rhyolite eruptions during the last ～50 kyr that produce a tephrostratigraphic framework across the tectonically and volcanically complex region of southern Havre Trough, Alderman Trough and the Bay of Plenty continental shelf. This allows correlation between offshore and onshore sequences up to 200 km from source volcanoes, covering much of Marine Isotope Stages 3 and 2. The framework temporally constrains poorly dated and newly recognised volcanic events. Macroscopic tephra layers from the peralkaline Mayor Island volcano are documented for the first time at pre-50, post-50, 40.5, 37.4, 22.2, and 14.2 ka, in addition to the well known 7 ka (Tuhua) event. These macroscopic layers represent some of the most explosive events from this volcano, and provide new marker horizons. They are dispersed up to 90 km north-east to east of the edifice. Minor tephra dispersal is also recorded at 17.8, 25.5, ～35 and pre-50 ka. The tephra contain high SiO_2 (73-75.5 wt.%) glass and subordinate basaltic components, and each tephra represents a distinct magma batch that can be fingerprinted. The Mayor Island tephra form two temporal trends toward less evolved magma compositions, punctuated by a large caldera-forming event at 36 ka. The historically active, andesitic White Island volcano does not have widely dispersed tephra, and the oldest primary deposit found is ～21 ka. Five pre-50 ka rhyolite eruptions from an unknown Taupo Volcanic Zone source provide evidence for explosive activity in a time interval poorly documented on-land. The cores demonstrate the patchy and uneven preservation of large magnitude tephra falls caused by local faulting, bioturbation and ponding in bathymetrically complex regions. Reworked tephra layers are common and often lack indicative lithological features. Such units could easily be misinterpreted as primary events without micro-beam geochemical analyses of glass shards.