Chronology of Sanbagawa metamorphism

By collating age data based on the fossil age of the protoliths, radiometric dating of the metamorphic minerals, and sedimentary records of erosion at the earth's surface, the history of the Sanbagawa metamorphism can be summarized as follows. (1) The pre‐metamorphic sedimentary rocks (Carboniferous‐Jurassic + Early Cretaceous?) became mixed and formed a thickened packet in the vicinity of an ancient trench through a variety of subduction‐related tectono‐sedimentary processes, probably in Early Cretaceous time ( c. , 130‐120 Ma). (2) The subducted protoliths underwent progressive metamorphism reaching a maximum depth of c. 30 km in late Early Cretaceous time ( c. 116 ± 10 Ma). (3) The high‐ P/T metamorphic rocks began to rise toward the surface (during the interval 110‐50 Ma) with minimum estimates for the average cooling rate around 9‐12°C/Ma and an average uplift rate around 0.4‐0.5 mm/year. (4) Finally, at some stage after reaching the erosional surface, the high‐ P/T metamorphic rocks were covered unconformably by the middle Eocene ( c. 50‐42 Ma) Kuma Group. On the basis of the present chronological summary of the Sanbagawa metamorphism, the areal extent of the Sanbagawa metamorphism is also discussed with respect to the weakly metamorphosed subduction‐accretion complex of the next tectonic belt to the south, the Northern Chichibu belt.