Figure F15. Crust develops that is “continental” in velocity structure and seismically similar beneath both the volcanic front and rear arc but is heterogeneous in chemical composition, schematically shown in blue and red. Red shows crust and mantle that are rich in fluid-mobile, recycled slab components but also strongly depleted in mantle-derived fluid-immobile elements. Blue shows those areas where the diminished slab signature and lower degrees of mantle melting create crust that is more typical of the present-day overall continent composition. The “from the beginning” hypothesis stipulates that this heterogeneity established from the Eocene arc inception through to the Neogene. The “from the middle” hypothesis stipulates that this heterogeneity only developed after the cessation of the Shikoku Basin (i.e., only in the Neogene).
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