The rheology of the metamorphosed oceanic crust may be a critical control on megathrust strength and deformation style. However, little is known about the strength and deformation style of metamorphosed basalt. Exhumed megathrust shear zones exposed on Kyushu, SW Japan, contain hydrous metabasalts deformed at temperatures between ~300° and ~500°C, spanning the inferred temperature-controlled seismic-aseismic transition. Field and microstructural observations of these shear zones, combined with quartz grain-size piezometry, indicate that metabasalts creep at shear stresses <100 MPa at ~370°C and at shear stresses <30 MPa at ~500°C. These values are much lower than those suggested by viscous flow laws for basalt. The implication is that relatively weak, hydrous, metamorphosed oceanic crust can creep at low viscosities over a wide shear zone and have a critical influence on plate interface strength and deformation style around the seismic-aseismic transition.

Brittle fracturing occurred locally within viscously deforming hydrated oceanic crust and subducted sediments.• Fracturing is localised at viscosity contrasts and P -T conditions of metamorphic 10 dehydration reactions. 11• Rocks that deformed at P -T conditions away from dehydration reactions record 12 dominantly viscous behaviour.

Subduction plate-boundary megathrusts produce the largest earthquakes on Earth. The up-dip limit of such earthquake ruptures may be close to the trench, as a result of dynamic weakening (Noda & Lapusta, 2013; Ujiie & Kimura, 2014), whereas the downdip limit of the seismogenic megathrust is inferred to occur where temperature exceeds ∼350°C or at the intersection with the serpentinized mantle wedge, whichever is shallower (Hyndman et al., 1997). The proposed thermal control on the downdip limit of megathrust earthquake nucleation has been thought to represent the onset of effective crystal plasticity in quartz (Hyndman et al., 1997) or pressure solution

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Thermomechanical models of subduction zones require substantial decoupling along the plate boundary in order to produce natural subduction zone geometries (Gerya et al., 2008) and fit heat flow observations (e.g., Abers et al., 2006;Wada et al., 2008). Antigorite serpentine forms in hydrous ultramafic rocks at temperatures ∼300°C-650°C and pressures up to ∼6 GPa (Evans, 2004;Ulmer & Trommsdorff, 1995), and is inferred to occur in the mantle wedge (Reynard, 2013). Serpentinites are often described as having better-developed ductile deformation structures in comparison to adjacent rocks, suggesting mechanical weakness (Hoogerduijn Strating & Vissers, 1991;Tarling et al., 2019). Accordingly, it has been inferred that serpentinite contributes to decoupling and aseismic behavior (e.g.,