Thermal consequences of the formation of a slab window beneath the Mid‐Cretaceous southwest Japan arc: A 2‐D numerical analysis

Abstract

Abstract The development of voluminous granitic magmatism and widespread high‐grade metamorphism in Mid‐Cretaceous southwest Japan have been explained by the subduction of a spreading ridge (Kula–Pacific or Farallon–Izanagi plate boundaries) beneath the Eurasian continent and the formation of a slab window. In the present study, the thermal consequences of the formation of a slab window beneath a continental margin are evaluated through a 2‐D numerical simulation. The model results are evaluated by comparison with the Mid‐Cretaceous geology of southwest Japan. Of particular interest are the absence of an amphibolite‐ to granulite‐facies metamorphic belt near the Wadati–Benioff plane, and significant melting of the lower crustal‐mafic rocks sufficient to form a large amount of granitic magma. Because none of the model results simultaneously satisfied these two geological interpretations, it is suggested that subduction of plate boundaries in Mid‐Cretaceous southwest Japan was not associated with the opening of a slab window. According to previous studies, and the results of the present study, two different tectonic scenarios could reasonably explain the geological interpretations for Mid‐Cretaceous southwest Japan: (i) The spreading ridge did not subduct beneath the Eurasian continent, but was located off the continental margin, implying the continuous subduction of very young oceanic lithosphere; (ii) ridge subduction beneath the continental margin occurred after active spreading had ceased. Consequently, in both tectonic scenarios, the subduction of plate boundaries at the Mid‐Cretaceous southwest Japan was not associated with a slab window, but very young (hot) oceanic lithosphere.