Coring at Site 1255 began at 123 mbsf with the target to recover the décollement as recognized at Site 1043 (Kimura, Silver, Blum, et al., 1997). From the first core recovery at 132.7 to 144.08 mbsf, sediments exhibit a downward increase in deformation intensity. Deformation is characterized by brecciation and polished clast surfaces, indicating an incipient scaly fabric (Fig. F14). The top of the décollement could not be defined because of the limited cored interval and poor recovery, but its base is sharp and well defined (Fig. F15). This lower boundary also coincides with the contact between Unit T1 and Subunit U1A as described at Site 1043 (Kimura, Silver, Blum, et al., 1997). At Site 1043, spiraliferous drilling disturbance was observed; this was not observed at Site 1255, although other types of drilling disturbance affected some intervals (Fig. F14). Only one measurement of bedding dip was possible in the décollement zone (44°).
The underthrust section starts at Section 205-1255A-3R-2, 18 cm (144.08 mbsf), and is characterized by alternating turbiditic and hemipelagic clay-rich layers. The clay-rich layers exhibit some brecciation with unpolished surfaces (Fig. F14), likely related to the shear zone, whereas the sandy layers are remarkably undeformed.