The evolution of past and present magmatic systems in the north Taupo area: Implications for modern hazards

Summary

Lava dome eruptions have been more frequent in the north Taupō area than previously thought and their vents have shifted spatially through time over a widespread 80 x 40 km area. New 40Ar-39Ar geochronology data show the vents become younger from the Northwest Dome Complex (NWDC, north of the Waikato River) to the current Lake Taupō configuration, which aligns with current understanding. The 40Ar-39Ar dates show that lava dome eruptions have been a continuous feature of the volcanism over ~400,000 years. There were quieter periods with few eruptions, and “busier” periods of more frequent eruptions. Whether a caldera volcano is better characterized by frequent small volume eruptions interspersed by large ignimbrite forming eruptions is yet to be decided.

Lavas along the western border of the study (Western Dome Belt, WDB), previously thought to pre-date the Whakamaru supereruption and resulting caldera collapse, are now recognised to in part predate these events by up to 100,000 years. This result suggests that the magma systems involved in WDB eruptions were not completely destroyed by the Whakamaru events. The magma system that fed the WDB lavas is separate to those that supplied the Whakamaru eruption. The 40Ar-39Ar age data support Brown’s (1994) hypothesis that a series of magma chambers or an irregular chamber would let separate magma batches at varying stages of evolution co-exist and erupt. This hypothesis would account for the presence of the highly evolved NWDC lavas to the north of the inferred Whakamaru caldera rim (which are dated as the oldest). A revision to the Whakamaru caldera structure would further help us understand how modern hazards are affected by rift faulting.

The whole rock geochemistry shows a clear distinction between magma types from different eruptions. The magma types change in crystal content and Sr values further south towards Lake Taupō. There is a steady progression in magma types, however this is not reflected in a sequential manner in the 25,000 – 350,000 years ago time period as the most evolved magma type (NWDC) is the oldest. The central Maroa lava domes and NWDC lava domes are both derived from the same magma source, whilst the WDB lavas are a mixture of at least three magma types from two magma systems. 87Sr/86Sr isotope ratios reveal the presence of two large-scale deep crustal magma systems controlling the isotopic characteristics of most of the eruptives studied. These magma systems co-existed up to ~50,000 years ago and appear to lead into the two modern systems under the area but with a shift in vent areas and an intense focusing of activity into an area beneath the modern Lake Taupō. Two magmatic systems are currently viable for unrest or eruption: the highly productive system submerged beneath Lake Taupō, active in its current configuration since ~25,000 years ago and a less frequently active system beneath the Wairakei-Tauhara area that has been active since ~45,000 years ago.

 

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