MID-CRUSTAL EPISODIC GRANULAR VORTICES CAN BOOST ANOMALOUS HEAT-RELEASE IN THE TAUPO VOLCANIC ZONE NZ

Abstract

The Taupo Volcanic Zone (TVZ) in New Zealand is famous for its focussed heat transfer causing massive historic ultra Plinian eruptions, the latest observed in 186 AD in ancient Rome through vivid sunsets. The quantities of heat measured at the surface place the TVZ into an extreme position. Previous explanations have solved the extreme focusing of heat in the upper 5-8km of the crust through fluid convection, along channelized and anomalously high localised heat sources below. However, the question remains, where does the heat come from? Here we show that, by considering the tectonic setting of high speed extension of the crust by 2 cm/yr in a zone of 40km width, solid vortices are triggered that can explain the heat flow anomaly. We found that vortices of approximately 10 km radius with the high extension velocity directly boost the heat transfer into the upper crust. A system that has a vortex lifetime larger than 100k years increases the heat transfer by a factor of 2-5. This rationalises the observed anomalies, and casts a new light on the mechanism causing ultra-Plinian eruptions. Our results demonstrate the role of solid vortices, recently discovered in granular physics, for solving long-standing anomalies of heat transfer in shear zones. Similar observations of heat focussing have been made for the Unzen Graben in Japan and the Himalayan Geothermal belt. The model can equally explain the defocussing of heat measured across the fault following the Chi-Chi earthquake or the heat flow paradox of the San Andreas Fault.