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Mechanical properties and processes of deformation in shallow sedimentary rocks from subduction zones: an experimental study

Abstract : To better constrain the mechanical behavior of sediments accreted to accretionary prism, we conducted triaxial mechanical tests on natural samples from the Miura-Boso paleoaccretionary prism (Japan) in drained conditions with confining pressures up to 200 MPa as well as post-experiments P-wave velocity (Vp) measurements. During experiments, deformation is principally non-coaxial and accommodated by two successive modes of deformation, both associated with strain-hardening and velocitystrengthening behavior: (1) compaction-assisted shearing, distributed in a several mm-wide shear zone and (2) faulting, localized within a few tens of μm-wide, dilatant fault zone. Deformation is also associated with (1) a decrease in Young’s modulus all over the tests, (2) anomalously low Vp in the deformed samples compared to their porosity and (3) an increase in sensitivity of Vp to effective pressure. We interpret this evolution of the poroelastic properties of the material as reflecting the progressive breakage of intergrain cement and the formation of microcracks along with macroscopic deformation. When applied to natural conditions, these results suggest that the deformation style (localized vs distributed) of shallow (z < a few km) sediments is mainly controlled by the variations in stress/strain rate during the seismic cycle and is therefore independent of the porosity of sediments. Finally, we show that the effect of strain, through cement breakage and microcracks formation, may lower Vp for effective pressure up to 40 MPa. As a consequence, the low Vp anomalies observed in Nankai accretionary prisms by seismic imaging between 2 and 4 km depth [Kitajima and Saffer, 2012] could reflect sediment deformation rather than porosity anomalies.
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Leslie Gadenne, Hugues Raimbourg, Rémi Champallier, Yuzuru Yamamoto. Mechanical properties and processes of deformation in shallow sedimentary rocks from subduction zones: an experimental study. Geochemistry, Geophysics, Geosystems, AGU and the Geochemical Society, 2014, 15 (12), pp.5001-5014. ⟨10.1002/2014GC005580⟩. ⟨insu-01097223⟩

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