Computed Tomography (ct) Scan Image Analysis of Site 808 Cores: Structural and Physical Property Implications

X-ray computed tomography (CT) is a promising tool that yields data useful for understanding the fine-scale density structure of partly lithified and tectonically deformed sediments. We conducted 21 CT scans of ODP Leg 131 sediments, including whole-round cores and thin-section chips, obtained from the toe of the Nankai accretionary prism. The samples range from highly deformed pieces from the frontal thrust and décollement to homogeneous and essentially undeformed sediments above the frontal thrust and beneath the décollement. In the CT images, kink-like deformation bands and faults are recognized as obvious bright seams, bands, or stripes with relatively high linear attenuation coefficients. The differences in linear attenuation coefficients relative to the matrix range from 0.021 cm/g (kink-like deformation band) to 0.038 cm/g (fault). These data suggest a 0.10 g/cm to 0.18 g/cm increase in bulk density within the deformation structures, and they appear to be 13% and 33% more compacted than the nondeformed matrix, respectively. In contrast to the samples from the frontal thrust zone, CT images of the décollement sample exhibit relatively homogeneous textures. The attenuation coefficient of the sample of the décollement indicates bulk density and porosity values of 2.45 g/cm and 18%, respectively. The sample, hence, is approximately 50% more compacted than the sediment outside the décollement zone.