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• Mafic schists from subducted oceanic crust deform by semi-brittle processes at the conditions of experiments. • Strength of the maciffic schists increases with increasing degree of prograde metamorphism. • All sample types exhibit a high stress exponent (n > 15) and strain rate strengthening (i.e., positive a–b values). • Our experimental results are consistent with that the general absence of crustal seismicity in hot subduction zones. • The semi-brittle deformation could control the slab-mantle decoupling and the exhumation process of these rocks. We conducted triaxial deformation experiments using a Griggs-type solid pressure-medium apparatus on two mafic schists that experienced different peak metamorphic conditions from the Sambagawa metamorphic belt; a greenschist and an epidote-amphibole schist. At a confining pressure of 1 GPa, 400 °C and strain rate of 10–5 1/s, differential stresses for all mafic schists were higher than 1 GPa. The greenschist samples were weaker than epidote-amphibolite samples under all experimental conditions. Microstructures of recovered samples of greenschist indicate that deformation of mafic schist is accommodated by semi-brittle deformation. Samples of the epidote-amphibole schist deformed at 500 °C show shear localization along a single fault oriented parallel to the foliation, while mechanical data shows no rapid stress drop. On the surface of the fault, we observe the formation of slickensides with little gouge. All sample types exhibit a high stress exponent (n > 15) and strain rate strengthening (i.e., positive a–b values); frictional behavior that inhibits earthquake nucleation. Differential stress increased with increasing confining pressure, while the friction coefficient decreased with increasing confining pressure and temperature. Greenschist and epidote-amphibolite facies metamorphism typically occurs at hot subduction zones such as Cascadia and southwest Japan. The lack of seismicity within the subducting oceanic crust at hot subduction zones could be explained by semi-brittle deformation, or frictional sliding with a positive (a – b) during deformation of these types of metamorphic rocks. Weak and aseismic fault zones in subducting slabs might promote slab-mantle decoupling at the corner of the mantle wedge, and detachment and underplating of oceanic crust from the subducting slab to forearc crust. [ABSTRACT FROM AUTHOR]

Abstract: We measured gas permeability of Neogene sedimentary rocks from Wakkanai, Koetoi and Yuchi Formations in the Horonobe area in northern Hokkaido, Japan. Measurements were done at room temperature and during pressure cycling to simulate underground conditions. Fresh intact samples from surface outcrops yielded a similar range of permeabilities as did intact samples from drill cores, within lithological variations. Comparison of laboratory and in-situ measurements has revealed that the permeability of Wakkanai Formation (middle to late Miocene, siliceous mudstone) from in-situ measurements is bounded by the permeability of brecciated specimen on the upper side and by the permeability of intact specimens on the lower side. Fragments in fault breccia are smaller than sample size and laboratory data should be applicable to natural fault rocks. Permeability of fractured specimens falls in between those upper and lower bounds, and complex scale issues will not be important in determining upper and lower bounds of permeability. The hydraulic conductivity values from in situ tests for Koetoi Formation (late Miocene to Pliocene, diatomaceous mudstone) are similar to those of the gas permeability of intact specimens. Effects of faults and fractures on permeability seems to be minor for this formation, and this was confirmed by permeability measurements with fractured samples. The hydraulic conductivity values from in situ tests for Yuchi Formation are lower than estimates from laboratory tests by within an order of magnitude. Thus permeability measurements in laboratory on fresh surface samples (intact, fractured and brecciated), combined with geological survey on the development of faults and fractures, leads to quantitative estimates, even if not complete, of underground permeability structures. [Copyright &y& Elsevier]