Deformation bands from the toe of the Nankai Accretionary Prism

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Deformation in the toe of the Nankai accretionary prism of SW Japan occurs not only by displacement on seismically defined thrust faults but also by diffuse thickening, shortening, and porosity loss. Sediments recovered from this setting are only very slightly tilted, and the only structures noted were subtle deformation bands, which are semiplanar zones of brittle‐ductile shear associated with subhorizontal shortening. Most bands dip between 25° and 60°, both in and opposite to the direction of bedding dip, and show no obvious trends with depth or distance from the trench. There is no change of mineralogy within deformation bands and only a very slight reduction of grain size, but porosity is reduced by 5%. Bands with dips between 25° and 40° are often marked by fractures, the surfaces of which show reverse slip with slickenlines and downdip steps. These bands crosscut and are younger than the steeper bands. All deformation bands consist of subbands that dip 10°–15° less steeply than the main band and within which the clay fabric is rotated to near vertical. The subbands are probably Riedel shears, oriented at angles to 01 in the bands appropriate to a brittle Coulomb response. A few deformation bands show subsidary sets of many short bands, at angles of ∼85° to the main bands. These may be R′ shears. The steeper deformation bands, especially the conjugate sets that dip 50°–60°, are interpreted to have been passively rotated toward the maximum finite extension direction (ε1) by ductile flow. The strain necessary to rotate these bands would require an ellipticity, R = (1+ε1)/(1+ε3), of ∼2.5. Based on seismic profiles and porosity data, R is only 1.5, demonstrating that if ductile flow were responsible for band rotation, such flow is not representative of the bulk of the toe. Seismic profiles across the proto thrust zone show 45° dipping “seismic discontinuities,” which appear to be panels of slightly steeper bedding dips (10°–15°) and perhaps greater deformation. Ductile flow is to be expected in the compactively deforming sediments of the Nankai toe, but the coexistence with brittle to brittle‐ductile shears is not easily explained with experimental soil mechanics.