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The Sanbagawa belt is a “coherent” oceanic subduction-type metamorphic region representing a rock package predominantly derived from oceanic crust and accreted at depths of 20–80 km (300–700 °C). The thermal structure and lithological layers are complexly deformed but semi-continuous, in contrast to more commonly reported subduction-related domains dominated by mélange. The coeval Shimanto accretionary complex records accretion at depths 15 km and the rocks are primarily terrigenous sediments. The Sanbagawa belt has a greater proportion of mafic rocks than the Shimanto complex, implying progressive peeling-off of oceanic plate stratigraphy with more basaltic oceanic crust slices accreted at deeper levels. Tectonic exhumation can be explained by three separate phases dominated by buoyancy-driven upflow, ductile thinning, and normal faulting.
Endo et al. (Mon,) studied this question.