Structural evolution of an inner accretionary wedge and forearc basin initiation, Nankai margin, Japan

Core recovered during Integrated Ocean Drilling Program (IODP) Expedition 319 from below the Kumano forearc basin of Japanʹs Nankai margin provides some of the only in situ samples from an inner accretionary wedge, and sheds light on the tectonic history of a seismically hazardous region. The 84 m of core comprises Miocene-age well-bedded muds, silts, and volcaniclastic sediments. Beds increase in dip with depth, and are cut by (i) soft-sediment deformation bands (“vein structures”), (ii) ∼1-cm thick shear zones within ∼10-cm thick regions of high shear strain, and (iii) <1-mm thick slickensided faults which are the youngest structures in the core and highly localized. Microstructural analyses of the shear zones suggest that they formed via multiple increments of shear localization and a mixed granular and cataclastic flow. Kinematic analysis of slip indicators in shear zones further reveals that they formed via north–south shortening. In contrast, the faults cut the shear zones with mixed slip kinematics, and accommodated northwest–southeast shortening, roughly parallel to the modern shortening direction. The entire section was also rotated ∼15° counterclockwise about a roughly vertical axis. Therefore the principle strain axes and stratigraphic section rotated during or postdating development of the major sub-basin (∼5.6–3.8 Ma) unconformity, a time that generally coincides with a change in the Philippine Sea plate convergence direction. Forearc basin development therefore postdates a protracted geologic evolution of shear-zone development, tectonic rotations, and inner-wedge development, the last of which coincides with a rheological evolution toward localized frictional faulting.