DEFORMATION BANDS FROM THE TOE OF THE NANKAI ACCRETIONARY PRISM

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%. 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 σ1 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. -from Authors