Abstract:
Cores retrieved from Sites C001 and C0002 during Integrated Ocean Drilling Program (IODP) Expedition 315 offshore SW Japan provide an excellent opportunity to examine deformation processes occurring across the Nankai Trough. Historically, this region is well known for its great (magnitude >8) earthquakes. On-board core logging and analysis of X-ray computed tomography scans revealed numerous core-scale deformation structures. These structures include faults, vein structures, kind bands, deformation bands, brecciated horizons, shear zones and rare folds. Here we document the first order geometries and textural characteristics specifically of faults and vein structures. This is done principally through mm-scale microscope observation and petrographic analysis. At the hand-sample-scale the faults and vein structures are mm-scale thick structures that appear visually as darker anastomosing features against the lighter colored wallrock. In thin section these structures show some sign of grain orientations oblique to bedding. The faults and vein structures have diffuse tips and terminations, respectively. The faults typically occur as isolated structures whereas the vein structures typically occur in groups of three or four. At Site C0001 these structures occur in zones with abundant faults and shear zones as noted during on-board core logging, and mostly occur below an m-scale thick zone of breccia encountered at approximately 220 meters below the sea floor. A single sample from C0002 displays particularly well-preserved cross-cutting relations between several faults. Preliminary analyses suggest that that the steeper dipping (>60 degrees ) faults are older than the shallower dipping ( approximately 45 degrees ) faults. These findings are being examined in the context of fault kinematic data obtained from core observations during the expedition in hopes of shedding light on the sequence of faulting within the accretionary wedge. Understanding how these deformation structures might fit into the earthquake cycle remains an important question.