University of Southampton, United Kingdom

Core recovered during Integrated Ocean Drilling Program (IODP) Expedition 319 from below the Kumano forearc basin of Japan's 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 84m 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) nearly equal 1-cm thick shear zones within nearly equal 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 nearly equal 15 degrees counterclockwise about a roughly vertical axis. Therefore the principal strain axes and stratigraphic section rotated during or postdating development of the major sub-basin ( nearly equal 5.6-3.8Ma) 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. Abstract Copyright (2012) Elsevier, B.V.