Deformation mechanisms and fluid conditions of mélange shear zones associated with seamount subduction

Lithologic heterogeneity and the presence of fluids have been linked to seamount subduction and collocated with slow earthquakes. However, the deformation mechanisms and fluid conditions associated with seamount subduction remain poorly understood. The exhumed Chichibu accretionary complex on Amami-Oshima Island preserves m & eacute;lange shear zones composed of mudstone-dominated m & eacute;lange and basalt-limestone m & eacute;lange deformed under sub-greenschist facies metamorphism. The mudstone-dominated m & eacute;lange contains sandstone, siliceous mudstone, and basalt lenses in an illitic matrix. The basalt-limestone m & eacute;lange contains micritic limestone and basalt lenses in a chloritic matrix derived from the mixing of limestone and basalt at the foot of a seamount. The basalt-limestone m & eacute;lange overlies the mudstone-dominated m & eacute;lange, possibly representing a submarine landslide from the seamount onto trench-fill terrigenous sediments. The asymmetric S-C fabrics in both m & eacute;langes show top-to-SE shear consistent with megathrust-related shear. Quartz-filled shear and extension veins in the mudstone-dominated m & eacute;lange indicate brittle failure at near-lithostatic fluid pressure and low differential stress. Microstructural observations show that deformation in the mudstone-dominated m & eacute;lange was accommodated by dislocation creep of quartz and combined quartz pressure solution with frictional sliding of illite, whereas the basalt-limestone m & eacute;lange was accommodated by frictional sliding of chlorite and dislocation creep of coarse-grained calcite, with possible pressure solution creep and diffusion creep of fine-grained calcite. The m & eacute;lange shear zones formed in association with seamount subduction record temporal changes in deformation mechanisms, fluid pressure, and stress state during megathrust shear with brittle failure under elevated fluid pressure, potentially linking tremor generation near subducting seamounts.