Fig.1

a Tectonic setting, showing individual arc segments and subduction zones of the Japanese island arc system (Kimura et al. 2014). Notice the distinct trench morphology between the NE Japan arc and the SW Japan arc. Seismic tomography shows the deep slab of the Pacific plate (in green) and the Philippine Sea Plate (red – seismic, white – aseismic) from Zhao et al. (2012). b: Basement geology map of the Japanese arc system (Taira 2001). Several tectonic units have been summarized for simplicity and intrusive rocks have been excluded. A progressive younging of individual units from an inner zone (adjacent to the Japan Sea) to the outer zone (Nakai trough) is particularly well developed in SW Japan. Basement units in the back-arc region include new continental crust as well as thinned and rifted older cratonic crust. c: names of distinct geographical and geological zones within SW Japan. The westernmost region of Honshū, is also known as the Chūgoku region.