Structural architecture and active deformation of the Nankai Accretionary Prism, Japan: Submersible survey results from the Tenryu Submarine Canyon

  • Kawamura K
  • Ogawa Y
  • Anma R
 et al. 
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Two research cruises that deployed sub- mersible surveys were undertaken along the Tenryu Submarine Canyon to directly ob- serve the structural architecture of the east- ern Nankai Accretionary Prism off the coast of southern Japan. The surveys have dem- onstrated that the accreted sediments are strongly deformed turbidite sequences that occur in repeated thrust-anticline struc- tures. From the leading edge of the prism near the trench toward the arc, the follow- ing deformation zones have been identi- fi ed within the prism: Frontal Thrust zone, Prism Toe zone, Imbricate Thrust zone, and Tokai Thrust zone (or out-of-sequence thrust or OOST zone). The Frontal Thrust zone is characterized by debris deposits within the hanging wall that have an age of 0–0.43 Ma, as determined from radiolarian biostratig- raphy. The Prism Toe zone is characterized by unconsolidated turbidite sequences that are 1.98–3.4 Ma; these sequences are cut by normal and thrust faults. The Imbricate Thrust zone includes consolidated muddy layers and unconsolidated sandy layers that contain numerous fracture cleavages. The OOST zone consists of highly deformed con- solidated sediments, ranging in age from 0.18 to 1.03 Ma. From the Prism Toe zone to the Imbricate Thrust zone, the uniaxial compres- sive strength increases gradually from 0.5– 3.0 to 1.0–6.0 MPa, while the anisotropy of magnetic susceptibility changes from oblate to prolate shapes, and porosity decreases from 40%–50% to 30%–50%. These data indicate that the eastern Nankai Accretion- ary Prism appears to have been deformed toward the Imbricate Thrust zone just south of the OOST. Stable isotope analyses of cal- cite veins and calcite cement of the sandstone samples from the Tokai Thrust zone have shown that fl uid temperatures for calcite precipitation were 24–63 °C in the OOST zone. The occurrence of highly deformed and consolidated rocks within the Nankai Accre- tionary Prism likely resulted from tectonic transportation of deeply buried rocks along major out-of-sequence thrust faults, such as the Tokai OOST. We infer therefore that out-of-sequence thrust faults play a major role in transporting deeply buried, deformed rocks in accretionary prisms to the shallower depths and even to the seafl oor during ongo- ing subduction.

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