Geology of the subduction thrust : insights from exhumed shear zones on Kyushu, SW Japan

Along plate interfaces in subduction zones, displacement occurs by a spectrum of slip styles, including ∼steady creep, slow-slip events, tectonic tremor, and earthquakes. However, the factors which determine where, and how, different slip styles occur, are not clear. In Japan, on Kyushu, three exposures of plate boundary shear zones, exhumed from different metamorphic conditions along a similar paleo-margin, within the temperature range 300-550 ◦C, represent a natural laboratory to understand processes which influence slip style along the subduction plate interface. In exposures exhumed from conditions corresponding to localised metamorphic dehydration reactions, foliations and quartz veins are mutually overprinting, indicating episodic brittle behaviour within an otherwise viscous regime. Local fluid pressurisation, and amplified shear stresses along lithological contacts, are inferred to drive embrittlement. A quartz recrystallised grain size piezometer is applied to viscously deformed quartz veins, and used to constrain the level of differential stress within hydrated metabasalt. Results suggest that hydrated oceanic crust is far weaker than dry crust, and weak enough to control plate interface rheology. Microstructural observations of antigorite serpentinite deformed at ∼ 500 ◦C suggest that foliation development results in a reduction in viscosity. Geometric relationships within the shear zone deformed at ∼ 500 ◦C suggest that serpentinite may be more viscous than hydrated oceanic crust and subducted sediment, and therefore plate interface strain may be concentrated in the subducting plate, along the slab-mantle boundary. Outcrop and microstructural observations suggest that, within the plate boundary shear zone, subducted sediments are slightly weaker than hydrated oceanic crust, and antigorite serpentinite is stronger than both subducted sediments and hydrated oceanic crust. Overall, brittle instabilities occur in all of the deformed lithologies due to a combination of metamorphic dehydration reactions locally reducing effective stresses, and local shear stress amplifications along lithological contacts.