[1] We developed a theoretical method for predicting effective stress and pore pressure based on rock physics model. We applied the method to reveal the pore pressure distribution within the Nankai accretionary prism off southwestern Japan and to investigate variations in pore pressure associated with evolution of the plate boundary décollement. From the crack aspect ratio spectrum estimated from laboratory and well-log data, we calculated a theoretical relationship between acoustic velocity and mean effective stress by using differential effective medium theory. By iteratively fitting the theoretically calculated velocity to the seismic velocities derived from 3D tomographic inversion, we estimated in situ mean effective stress within the accretionary prism. Pore pressure is then the difference between the effective stress and the confining stress. When we calculated pore pressure, we considered compressive state of stress in the accretionary prism. Our results confirm that pore fluid pressure is high within the subducting sedimentary sequence below the décollement; we determined a normalized pore pressure ratio (l*) of 0.4-0.7. Abnormal pore pressures develop in the underthrust sequence as a result of the increase in overburden load because of the thickened overlying prism and a low permeability barrier across the décollement. Overpressuring within the accreted sequence is initiated at the deformation front and proceeds landward. The increase in horizontal compaction within the accreted sequence may raise pore pressures within the accreted sequence, and the pore pressure (mean effective stress) contrast at the décollement becomes smaller landward of the deformation front.Citation: Tsuji, T., H. Tokuyama, P. Costa Pisani, and G. Moore (2008), Effective stress and pore pressure in the Nankai accretionary prism off the Muroto Peninsula, southwestern Japan,

[1] A three-dimensional prestack depth-migrated seismic reflection data volume acquired off Shikoku Island, Japan covers the seaward portion of the Nankai Trough accretionary prism. We calculate and interpret total horizontal shortening lengths along three cross-sectional profiles through the volume, incorporating a technique addressing the significant amount of water volume sediments lose during accretion, constrained by porosity values derived from seismic interval velocities. The results reveal a total horizontal shortening of ∼40% within sediments of the first three thrust sheets in the wedge. This indicates that structural restorations applied to water-saturated young sediments, or other domains displaying large tectonicinduced porosity changes (e.g., accretionary prisms, subaerial and submarine fold and thrust belts), must account for the substantial amount of distributed compactive strain that affects the sediment during the initial stages of accretion. Our analysis of the porosity reduction also allows an estimate of dewatering rates across the outer accretionary wedge. We find that porosity loss and associated dewatering decrease with distance landward from the trench and correspond to a progressively decreasing contribution of diffuse compactive strain to the total shortening. We compute a dewatering rate of 10.5 km 3 /Ma (per km along strike) over the outer ∼7 km of the accretionary wedge. This relatively high rate of dewatering when compared to other well-studied subduction systems probably reflects the large thickness of accreted sediment and high sediment permeability that allows efficient consolidation. These results highlight the importance of considering distributed compactive strain in structural restorations for any setting where deformation occurs in sediments.

[1] Target-oriented 3-D prestack depth migration (PSDM), tied to known depths of key reflections at three ODP drill sites in the Nankai Trough, provides an improved representation of the subsurface depths and P-wave velocities around the drill holes. The resulting velocity volume is representative of the ''in situ'' velocities and allows us to extend the 1-D logging and core velocity data from the drill sites to the entire seismic transect. The PSDM velocity gradients closely match those at the drill sites, although the absolute values of the velocity curves are somewhat different. The average PSDM vertical velocity gradient above the décollement decreases from 0.99 s À1 seaward of the trench axis to 0.87 s À1 landward in the proto-thrust and frontal thrust zone, with a velocity inversion of about 50-150 m/s at the décollement. Below the décollement, the vertical gradient in the underthrusting section decreases from 0.97 s À1 in the basin to 0.72 s À1 beneath the frontal thrust. The lateral velocity gradient in the underthrusting section between the basin and the deformation front is 0.02 s À1, corresponding to $10% thinning in the direction of subduction.

Upper oceanic crust within the Cocos Plate offshore northwestern Costa Rica is dominated by sill intrusions of various ages, recognized regionally as smooth, high amplitude seismic reflections at the base of the sedimentary sequence and locally by dolerites and microgabbros recovered by drilling. Earlier interpretation of seafloor magnetic anomalies documented a set of spreading ridge jumps at 19.5 and 14.5 Ma, continuing to the present. In addition to these events, we present evidence for a widespread tectonic event in the period 8–10 Ma displayed in reflection seismic data recorded during the Ticoflux 1 and 2 experiments and dated by seismic correlation to ODP Site 1039. The 8–10 Ma events may have been triggered by collision of the Cocos Ridge at this time and facilitated by widespread sill intrusion associated with the Galapagos Hot Spot.