Strength and deformation behavior of the Shimanto accretionary complex across the Nobeoka thrust

<jats:title>Abstract</jats:title><jats:p>A rapid reduction in sediment porosity from 60 to 70 % at seafloor to less than 10 % at several kilometers depth can play an important role in deformation and seismicity in the shallow portion of subduction zones. We conducted deformation experiments on rocks from an ancient accretionary complex, the <jats:styled-content style="fixed-case">S</jats:styled-content>himanto <jats:styled-content style="fixed-case">B</jats:styled-content>elt, across the <jats:styled-content style="fixed-case">N</jats:styled-content>obeoka <jats:styled-content style="fixed-case">T</jats:styled-content>hrust to understand the deformation behaviors of rocks along plate boundary faults at seismogenic depth. Our experimental results for phyllites in the hanging wall and shale‐tuff mélanges in the footwall of the <jats:styled-content style="fixed-case">N</jats:styled-content>obeoka <jats:styled-content style="fixed-case">T</jats:styled-content>hrust indicate that the <jats:styled-content style="fixed-case">S</jats:styled-content>himanto <jats:styled-content style="fixed-case">B</jats:styled-content>elt rocks fail brittlely accompanied by a stress drop at effective pressures < 80 <jats:styled-content style="fixed-case">MP</jats:styled-content>a, whereas they exhibit strain hardening at higher effective pressures. The transition from brittle to ductile behavior in the shale–tuff mélanges lies on the same trend in effective stress–porosity space as that for clay‐rich and tuffaceous sediments subducting into the modern <jats:styled-content style="fixed-case">N</jats:styled-content>ankai subduction zone. Both the absolute yield strength and the effective pressure at the brittle–ductile transition for the phyllosilicate‐rich materials are much lower than for sandstones. These results suggest that as the clay‐rich or tuffaceous sediments subduct and their porosities are reduced, their deformation behavior gradually transitions from ductile to brittle and their yield strength increases. Our results also suggest that samples of the ancient <jats:styled-content style="fixed-case">S</jats:styled-content>himanto accretionary prism can serve as an analog for underthrust rocks at seismogenic depth in the modern <jats:styled-content style="fixed-case">N</jats:styled-content>ankai <jats:styled-content style="fixed-case">T</jats:styled-content>rough.</jats:p>