Plate margin processes and 'paired' metamorphic belts in Japan: Comment on 'Thermal effects of ridge subduction and its implications for the origin of granitic batholith and paired metamorphic belts' by H. Iwamori

The traditional view of 'paired' metamorphic belts forming adjacent to each other in a common arc-trench subduction system is challenged, and it is suggested that an alternative view based on terrane tectonics is more consistent with the geological evidence. For the low-P/T ratio Ryoke metamorphic belt of southwest Japan, an origin by ridge subduction is consistent with the geological evidence and results of numerical modeling by Iwamori. However, the parallel high-P/T ratio Sanbagawa metamorphic belt most likely was translated northeast after the Ryoke ridge subduction event and was juxtaposed against the low-P/T ratio metamorphic belt during the Late Cretaceous, causing progressive southwest to northeast exhumation and diachronous cooling of the low-P/T ratio metamorphic rocks. Similarly, accretion by underplating in the Shimanto belt likely caused exhumation and cooling of the high-P/T ratio metamorphic rocks of the Sanbagawa and Northern Chichibu belts concurrently with exhumation and cooling of the Ryoke belt. As a result, the prograde metamorphic evolution of the Ryoke and Sanbagawa belts cannot be modeled as a 'pair' that formed in situ; the application of results of numerical models to the origin of plate margin metamorphic belts must be based on an understanding of the geology. In the model presented by Iwamori, the low-P/T ratio metamorphism and high-P/T ratio metamorphism are ascribed to the same ridge subduction event in a common trench system. Although such a scenario may produce lower-P to higher-P metamorphism in the inboard direction, it cannot generate a high-P/T ratio belt outboard of a low-P/T ratio belt. (C) 2002 Elsevier Science B.V. All rights reserved.