[S-MP24] Deformed rocks, Metamorphic rocks and Tectonics

[SMP24-P19] The importance of shear heating in subduction-type metamorphism: evidence from the Sanbagawa belt, SW Japan.

*Simon Richard Wallis¹ (1.The University of Tokyo)

Keywords:Stress, Shear heating, Subduction, Sanbagawa belt

Shear heating along lithospheric shear zones can be an important source of heat energy. The presence of pseudotachylites along shear zones including at the base of landslides shows that such shear heating can raise the temperature of rocks to above the solidus. There is no good consensus about the significance of shear heating on a regional scale. The peak metamorphic conditions of subduction type metamorphic rocks typically show higher temperatures than those predicted by thermal models of subduction zones that do not include shear heating. One possible explanation for this mismatch is the influence of shear heating. Alternative explanations include the possibility that subduction type metamorphic rocks do not represent typical conditions of the subduction. This implies that metamorphic rocks only exhume at times when the subduction zone is unusually warm such as the time immediately after subduction initiation and when an active spreading ridge is close to the subduction zone and a young warm slab with an age of less than a few million years is subducting. The Sanbagawa belt is well suited to assessing these alternative hypotheses. Firstly, it has long been recognized as an example of relatively warm conditions in a subduction zone and there is abundant information on the P-T conditions. Secondly, plate reconstructions in the Pacific realm for the time of the Sanbagawa metamorphism allow the metamorphic conditions to be compared to quantitative thermal models.
Thermal models that incorporate shear heating show strongly curved P-T paths. Similar paths are shown during subduction of very young slabs. However, radiometric dating shows the slab subducting during the main phase of Sanbagawa metamorphism was around 60 million years old. Such old slabs are thermally mature and not suitable for explaining high-temperature subduction zones. The estimated shear stress from P-T paths is similar to that estimated directly from metamorphic rocks. Supporting evidence for the general importance of shear heating comes from a new analysis of heat flow data from modern NE Japan. Shear heating is mainly developed in the seismogenic zone and focused near the plate boundary. Variations in the metamorphic conditions in the Sanbagawa belt are explicable for a subduction channel with a thickness of around 1 km.