ALBERT

Description: Complete penetration of frontal thrust and décollement of the Nankai Trough accretionary complex in Site 808 during ODP Leg 131 provided a wealth of structural observations and physical property data. In this paper possible mechanisms are discussed that could be responsible for the development of irregular downhole trends in acoustic anisotropy observed in Site 808. After various steps of data reduction and screening, a paleomagnetic reorientation procedure is applied to a selected group of physical property data sets. This facilitates the integration of the observed changes in physical properties with the geotectonic framework at the deformation front of the Nankai Trough accretionary complex. The paleomagnetic database was employed in the reconstruction of directional properties of acoustic velocities of the Lower Shikoku Basin sedimentary sequence, which is divided by a sharply defined décollement into an accreting and a subducting portion. P-wave velocity anisotropies derived from paleomagnetically oriented samples in the upper part of this 420-m-thick hemipelagic sequence show maximum values in the direction parallel and normal to the inferred vector of plate convergence (31O°-315°). No preferred orientation of P-wave velocity anisotropy is found in the subducting part of the sequence. The preferred direction of maximum anisotropy parallel to the convergence vector is also in accordance with the true direction of the observed macro- to mesoscale structural features in Site 808. Microfractures and microcracks forming as stress relief and tensile fractures in cores of semilithified sediment normal and perpendicular to the maximum horizontal stress are discussed as control mechanisms for the development of the observed anisotropy pattern.