It has been still unknown that the timing of the exhumation of the metamorphic belts shows intermittent and sometimes periodic along the subduction plate boundary, though plate subduction is still continuous. There seem two basic mechanisms for this: one is the mechanism depending on the tectonic framework of the subduction zone, and second is the mechanism controlled by the changing the parameters of constituent materials such as the accretion of masses and viscosity depending on water content. There are several models of metamorphic exhumation from the subduction boundary: one is the viscous drag exhumation model, and the other is the friction drag exhumation model in the along the subduction boundary or in the wedge mantle (Toriumi 2022). In both models, the driving force of the exhumation or extrusion should be the gravitational one yielded by density difference and total mass of the extrusion body against the surrounding upper mantle. The net force acting on the metamorphic mass as, dF = Fb - 2kt S = S h drm g (w/ h - 2 k m dr/drm) , in which t is shear stress, dr is density difference, w is mass aperture, h is depth, g is gravitational acceralation, and k is extensional coefficient. In the condition of extensioal stress (k<0.5), the critical depth of massive extrusion attains 100 km.Reference,Toriumi, M., 2022, Geochemical Mechanics and Deep Neural Network Modeling, Springer.
