Fig. 5

a Map of the Japanese subduction systems. Black contours show depth to the top of the subducting slab (from Hayes et al. 2018) for the Japan, Nankai, Kyushu, and Ryukyu segments at 10 km intervals (50 km intervals are in bold). Red triangles show locations of arc volcanoes. Orange lines are plate boundaries from Bird (2003). Age of oceanic lithosphere is from Müller et al. (2008). b 3D model showing subduction of the Philippine Sea Plate (PHS) and Pacific slab below the Kanto region (modified from Wada and He 2017). MDD = maximum decoupling depth (denoted as \(d_c\) in this paper). c Heat flow comparison between observations (Tanaka et al. 2004) and model predictions (also modified from Wada and He 2017). d Predicted “blueschist-out” boundary below Tohoku (modified from Morishige 2022) assuming this occurs, as in van Keken et al. (2012), at T=617–52P (in \(^\circ\)C with P in GPa). Compare with Fig. 2b. e as frame d but now for the serpentinite-out boundary using, as in Faccenda et al. (2012), T=740−1.8\(P-\)3.9\(P^2\) at \(P\ge\)2.1 GPa and T=478\(+\)180\(P-\)31\(P^2\) at \(P\le\)2.1 GPa