Inhomogeneous deformation of metamorphic tectonites of contrasting lithologies: Strain analysis of metapelite and metachert from the Ryoke metamorphic belt, SW Japan

摘要

To clarify the deformation history of rocks formed within an arc-trench system, we studied the strain geometry and strain path of upper greenschist to lower amphibolite facies metamorphic rocks of contrasting lithologies (i.e., metapelite and metachert) from a metamorphosed accretionary complex (the Cretaceous low-pressure/high-temperature Ryoke metamorphic belt, SW Japan). Strain analysis focused on deformed radiolarian fossils in the metachert and pebbles in the metapelite. The metachert records flattening strain, whereas the samples of metapelite record flattening (non-folded samples) and constrictional strains (folded samples). The samples that plot in the flattening field may record strain related mainly to the schistosity-forming deformation, whereas the samples that plot in the constriction field may record the total tectonic strain of both the schistosity-forming deformation and late-stage upright folding. The fold structures observed in the layered metachert are distinct from those in the metapelite; it is likely that they mainly formed during accretion. The preservation of primary fold structures in the metachert may reflect lower strain than that in the metapelites for the period during and after the main metamorphism event. The present results demonstrate that the geometries and strain paths of different lithologies differ over a small area; consequently, in using strain analysis to constrain tectonic models, strain analysis must be undertaken in conjunction with a clear distinction of the deformation phases recorded in different lithologies.