Dr. Wallis moved to Japan in 1988 after receiving his doctorate from Oxford University. He worked at Kyoto and Nagoya Universities before moving to his current position at the University of Tokyo in 2017. His research focuses on the deformation and mineralogy of metamorphic rocks and how this rock record can be used to address questions about what happens when two tectonic plates converge — in subduction zones and zones of continental collision. He has worked in many parts of the world including the European Alps, Japan, central America, Tibet and the Andes. Together with members of his group he has made significant contributions to developing methods to quantify rock deformation and to estimate metamorphic temperature based on Raman spectroscopy of carbonaceous material. He has combined these results with numerical modeling to understand how studies of ancient mountain belts can inform geophysical models of modern plate boundaries. Dr. Wallis was awarded the Geological Society of Japan Prize in 2017 and made a corresponding member of the Torino Academy of Sciences in 2023. He has taken on numerous leadership roles including Editor in Chief of the journal Island Arc 2004–2008 and President of the Japan Geoscience Union since 2024.
The thermal structure of subduction zones provides the fundamental framework for understanding first-order geological processes in convergent plate margins, including megathrust earthquakes, arc volcanism (including economically important mineral resources), and large-scale material transport and cycling of rocks and volatiles from the Earth's surface to its deep interior.
To estimates the thermal structure of subduction zones, geoscientists use numerical modelling complemented by studies of metamorphic rocks, seismic attenuation, and surface heat-flow measurements. These studies show subduction zones are strongly cooled by the rapid descent of cold oceanic lithosphere, which outpaces heat conduction from the warmer surrounding mantle. This cooling is reflected in the development of high-pressure/low-temperature subduction-type blueschist metamorphic belts. Active magmatism is maintained despite the overall cooling by the vertical rise of fluids from the subducting plate into hot domains of the overlying mantle where partial melting occurs.
While this general framework is well established, major uncertainties remain and even for the same subduction zone temperature estimates can vary by up to several hundreds of degrees. One source of uncertainty is the effect of shear heating. Better understanding of this heat source is needed to determine the down dip temperature limit of the seismogenic zone; it has also been invoked as a possible cause for the discrepancy between relatively warm metamorphic rocks and the cooler results for most numerical models. A second major source of uncertainty is the depth at which the subducting plate becomes viscously coupled to the overlying mantle. This coupling is commonly assumed to start at ~80 km irrespective of subduction zone conditions, but is only well constrained in a few cases and there is little information on potential variability. This talk reviews current understanding of subduction zone thermal structure and highlights geological evidence for the conditions of deep subduction, focusing on key unresolved problems.