Plate movements, ductile deformation and geochronology of the Sanbagawa belt, SW Japan: tectonic significance of 89–88 Ma Lu–Hf eclogite ages

Abstract

AbstractPrograde P–T paths and thermal modelling suggest metamorphism in the Sanbagawa belt represents unusually warm conditions for subduction‐type metamorphic belts, and these likely reflect conditions of a convergent margin a few million years before the arrival of an active spreading ridge. Radiometric age data and kinematic indicators of ductile deformation suggest the Sanbagawa belt formed in a Cretaceous convergent margin associated with a plate movement vector that had a large sinistral oblique component with respect to the belt, the East Asian margin. Plate reconstructions for the Cretaceous to Tertiary for this region show that the only plausible plate compatible with such motion at this time is the Izanagi plate. These reconstructions also show that progressively younger sections of the Izanagi plate were subducted beneath eastern Asia, i.e. a spreading ridge approached, until 85–83 Ma when the Izanagi Plate ceased to exist as an independent plate. The major reorganization of plates and associated movements around this time is likely to be the age of major interaction between the ridge and convergent margin. The ridge‐approach model for the Sanbagawa metamorphism, therefore, predicts that peak metamorphism is a few million years older than this age range. New Lu–Hf dating of eclogite in the Sanbagawa belt gives ages of 89–88 Ma, in excellent agreement with the prediction. Combining this estimate for the peak age of metamorphism with published P–T‐t results implies vertical exhumation rates of greater than 2.5 cm yr−1. This high rate of exhumation can explain the lack of a significant thermal overprint in the Sanbagawa belt during subduction of the ridge.