Figure 2. Geologic map of the southwest Japan forearc region and the Leg 190 Muroto and Ashizuri drilling transects. The Shimanto accretionary prism provides a landward analog of the Nankai accretionary prism. Note the widespread 17- to 12-Ma igneous activity, probably due to the initial subduction of the young Shikoku Basin oceanic lithosphere. Previous ODP/DSDP drill sites are shown by open circles. The dashed line shows the location of the cross section shown in Figure 3.
Figure 3. Crustal cross section of the Nankai Trough forearc (modified after Kodaira et al., 2000). Crustal structure, crustal velocities, and subducting plate earthquakes are shown. Note that the updip limit of 1946 Nankaido earthquake rupture zone possibly reaches to the Nankai Trough accretionary prism. MTL = median tectonic line.
Figure 4. Paleogeographic reconstruction of the Shimanto Belt and Nankai forearc evolution. After Taira et al. (1989). Arrows show the direction of convergence.
Figure 5. Historical recurrence time interval of class M8 earthquakes along the Nankai Trough. Zones A to D represent rupture area segments.
Figure 6. ODP Leg 190 (solid circles) and previous ODP/DSDP drill sites (solid squares) in the Nankai Trough. The shaded outline shows the 3-D seismic survey of Bangs et al. (1999) and Moore et al. (1999). Contour interval = 100 m.
Figure 7. Schematic interpretation of seismic line 141-2D in the Muroto Transect showing tectonic domains and location of Leg 190 drill sites.
Figure 8. A. Correlation of facies units, magnetic susceptibility, and major time boundaries within stratigraphic successions of the reference and prism toe sites at the Muroto and Ashizuri Transects at Nankai margin. Time boundaries are in red (solid line). Facies boundaries are in blue (Muroto Transect) and purple (Ashizuri Transect)(patterned lines). Data for DSDP Site 297 are from Shipboard Scientific Party (1973). Data for DSDP Site 582 are from Shipboard Scientific Party (1986). Data for ODP Site 808 are from Shipboard Scientific Party (1991). Note that the effects of facies imbrication along the frontal thrust of Site 808 have been removed and that the position of the Pliocene/Miocene boundary has been shifted in response to reinterpretation of paleomagnetic data. B. Correlation of facies units, magnetic susceptibility, and major time boundaries within stratigraphic successions cored at upslope sites of the Muroto Transect, Nankai margin. Time boundaries are in red (solid line). Facies boundaries are in blue (patterned line).
Figure 9. Seismic reflection profile through the Muroto Transect reference (Site 1173) and prism toe sites (Sites 1174 and 808). Correlation of sedimentary facies to the seismic data is shown on the right. Seismic data are from the 3-D seismic survey of Bangs et al. (1999) and Moore et al. (1999). Xline identifies the crossing line number in the 3-D seismic volume.
Figure 10. Seismic reflection profile (NT-2) through Ashizuri Transect reference site (Site 1177), trench site (Site 582), and prism toe site (Site 583).
Figure 11. Seismic reflection profile through the Muroto Transect slope Sites 1175, 1176, and 1178. Seismic data are from the 3-D seismic survey of Bangs et al. (1999) and Moore et al. (1999). Xline identifies the crossing line number in the 3-D seismic volume.
Figure 12. Summary of results at Site 1173.
Figure 13. Photograph of volcanic ash beds interbedded with silty clay from Unit II (interval 190-1173A-13H-6, 100124 cm).
Figure 14. Photograph of bioturbated silty claystone and interbedded siliceous claystone from Unit III (interval 190-1173A-57X-4, 6671 cm).
Figure 15. Summary of results at Site 1174.
Figure 16. Photograph of graded interval of medium- to fine-grained sand with some mudchips in the upper part of Subunit IIA (intervals 190-1174A 8H, 040 and 4080 cm).
Figure 17. Photographs showing deformation bands. A. Note the varying width and the tendency of the bands to bifurcate (interval 190-1174B-17R 2, 103112 cm). B. Note the variation in the width of the more shallowly inclined set (interval 190-1174B-15R-2, 1924 cm).
Figure 18. Stereographic equal-area lower hemisphere projections of deformation bands, illustrating the effectiveness of paleomagnetic reorientation. A. Deformation bands in the core liner reference frame, before paleomagnetic reorientation. B. Data in A after paleomagnetic correction to real geographic coordinates (excluding some planes for which the paleomagnetic information was not available). Note the concentration into two oppositely dipping sets. C. Poles to the planes shown in B. D. Average of the two sets of deformation bands showing the dihedral angle and the inclination of the acute bisectrix from vertical.
Figure 19. A. Photograph of the upper part of décollement zone, showing breakage into angular blocks along inclined fractures (interval 190-1174B 71R-2, 4879 cm). B. Photograph of the lower part of décollement zone, showing comminution of sediments (interval 190-1174B-73R-1, 96118 cm).
Figure 20. Details of fracturing across the décollement zone. The density of fracturing is expressed by the nature and size of the brecciated fragments. Most of the fracture surfaces are slickensided and slickenlined. Note the trend of increasing fracturing downward through the zone, peaking a few meters above a sharply defined base.
Figure 21. Summary of results at Site 1175.
Figure 22. Photograph showing chaotic bedding of Unit I (interval 190 1175A-9H-1, 65110 cm).
Figure 23. Photograph showing pebbly mudstone of Unit III (interval 190 1175A-37X-4, 70110 cm).
Figure 24. Summary of results at Site 1176.
Figure 25. Photograph of pebbles and gravel of quartz and lithic clasts in muddy matrix from Unit III (interval 190-1176A-47X-CC, 028 cm).
Figure 26. Summary of results at Site 1177.
Figure 27. Photograph of a wood-rich sandy turbidite interbedded with silty claystone from Unit III (interval 190-1177A-43R-3, 025 cm).
Figure 28. Photograph of laminated and bioturbated volcanic ash of Unit IV (interval 190-1177A-49R-4, 77101 cm).
Figure 29. Photograph showing contact between green basal mudstone (Unit IV) and basalt (Unit V) (interval 190-1177A-56R-3, 012 cm).
Figure 30. Summary of results at Site 1178.
Figure 31. Bedding-oblique foliation typical of Domain III (interval 190 1178B-9R-5, 527 cm). Steeply dipping bedding can be observed over the intervals 510 and 2227 cm, bounding an interval with lower angle foliation.
Figure 32. Incipient scaly and foliated clays with fine black seams crosscutting the fracture fabric (interval 190-1178B-29R-3, 3553 cm).
Figure 33. Incipient web-like structure in fine sand (interval 190-1178B 27R-3, 2741 cm).
Figure 34. Bio- and magnetostratigraphic correlation between Leg 190 reference and prism toe sites of the Ashizuri and Muroto Transects.
Figure 35. Age-depth plots based on biostratigraphic (solid squares) and paleomagnetic (open circles) data for prism toe sites of the Ashizuri and Muroto Transects.
Figure 36. Comparison of the structurally identified décollement interval and its physical properties and pore-water geochemistry across the Leg 131/190 transect. The décollement interval at Site 1174 has been projected to the reference Site 1173 based on correlation of patterns in magnetic susceptibility data (see Fig. 8A).
Figure 37. Structural and stratigraphic interpretation of seismic profile across the Ashizuri Transect based on drilling sites.
Figure 38. Chloride concentrations in interstitial water samples from the Muroto Transect reference (Site 1173) and prism toe sites (Sites 1174 and 808).
Figure 39. Porosities and velocities across the Muroto Transect (Sites 1173, 1174, and 808) and Ashizuri reference site (Site 1177). Lithologic units and major structural features are shown. The décollement location is shown by gray shading where it was observed and by a dashed line at the stratigraphically equivalent depth at the reference sites.
Figure 40. Biostratigraphic and paleomagnetic ages as a function of solid thickness (thickness of the sediment after vertical compaction to 0% porosity) for Sites 808, 1174, 1173, and 1177. Lithostratigraphic units and their correlation between holes are indicated in blue (solid and dashed). The décollement interval is indicated at Sites 808 and 1174 (green/shaded). The red lines (patterned) mark the 4- to 7-Ma age interval. This interval appears thicker at the décollement Sites 808 and 1174 than at the reference Sites 1173 and 1177.
Figure 41. Multiple sources (biogenic, thermogenic, and catagenic) and production mechanisms for the hydrocarbons encountered during Leg 190 were identified by plotting the methane (C1), Bernard ratios (C1/[C2+C3]), and sulfate profiles for the individual sites drilled during Leg 190. Note that depth scale is different for each plot. Vertical lines represent zones for thermogenic, mixed, and bacterial regions for the hydrocabon profiles.
Figure 42. Biogeochemical profiles in sediments from Nankai Trough sites (Leg 190). A. Total bacterial populations at Sites 1173, 1174, and 1177. The small dashed curve represents a general regression line of bacterial numbers vs. depth in deep-sea sediments (Parkes et al., 1994), with 95% upper and lower prediction limits shown by large dashed curves. BD. Sulfate, methane, and total organic carbon depth profiles for Sites 1173, 1174, and 1177, respectively. E. C1/C2 ratios for Sites 1173, 1174, and 1177.
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