Coring at Site 1254 targeted two different structural domains based on coring and logging while drilling at Site 1040: (1) a zone from 150 to 223 mbsf, containing fractured and locally steepened bedding within the prism, described as the prism fault zone and (2) the basal décollement zone from 300 to 368 mbsf. The deformation structures observed in these intervals are summarized, along with lithostratigraphy, in Figure F23. All structural data are given in Table T10. Core descriptions of deformation including breccia size, foliation (if present), hardness of breccia clasts, and the presence of polished surfaces are also shown.
The concentration of deformation structures at ~180 mbsf at Site 1040 (Kimura, Silver, Blum, et al., 1997) and the clear sheared nature of that horizon (Vannucchi and Tobin, 2000) mark the presence of a fault zone, associated with a strong geochemical anomaly (Kimura, Silver, Blum, et al., 1997). This interpretation was confirmed by coring in Hole 1254A from 150 to 223 mbsf (Fig. F23). Structural observations in poorly lithified material require high-quality cores. Because Cores 205-1254A-1R through 2R were heavily disturbed by drilling, structural data were collected starting at a depth of 165.10 mbsf (Core 205-1254A-3R). The entire recovered interval is characterized by deformation, consisting of stratal disruption, foliated breccia with fragments as small as a few millimeters in length, brittle shear zones, deformation bands, and inclined bedding (Fig. F24). There is no unequivocal evidence that the stratal disruption and the deformation bands are natural rather than drilling induced. The intensity of deformation, particularly brecciation and brittle shearing, increases downward, reaching a peak in the interval from 215.95 to 216.40 mbsf (interval 205-1254A-8R-2, 105-150 cm) (Fig. F23). Deformation is not continuous but is localized along preferentially sheared horizons that range in thickness from 2 m to 20 cm. The distribution of sheared and brecciated zones with depth is depicted in Figure F23. Reidel shears within a 24-cm-thick, exceptionally well preserved foliated breccia (interval 205-1254A-8R-5, 0-24 cm) indicates reverse displacement consistent with the results from Site 1040 (Vannucchi and Tobin, 2000). Paleomagnetic reorientation of the shear zone shows that it dips to the northeast (Fig. F25). This is the only shear zone among those encountered in the recovered sediments for which paleomagnetic reorientation yields a reliable geometry. Given the complexity of the prism, particularly in three dimensions, the relationship between the fault zone observed here at 210-220 mbsf and that encountered at 180-190 mbsf at Site 1040 cannot be uniquely established.
Coring of the décollement zone started after washing from 223 to 300 mbsf, and well-preserved structural features can be observed beginning with Core 205-1254A-11R, corresponding to a depth of 319.30 mbsf (Fig. F23). The cores typically exhibit significant drilling disturbance, like that encountered and described during Leg 170 as "spiraliferous" (Kimura, Silver, Blum, et al., 1997), consisting of rotation and continuous torque of clay-rich sections. Despite the drilling disturbance, some bedding plane orientations were recorded (Fig. F26). Bedding and fissility dips are varied, indicating heterogeneous deformation (Fig. F26A), but paleomagnetic reorientation of the features, where possible (Fig. F26B), shows that they consistently dip northeast or southwest, parallel to the trench strike. The entire recovered section from 319.30 to 367.96 mbsf is characterized by intense deformation. A comparison with less-deformed sediments away from the fault zone was not possible because coring was only conducted in the fault zone. There are, however, small changes with depth in the fault zone that define a detailed structural "stratigraphy" and distinguish different subdomains within it (Fig. F23). The heterogeneity of the deformation is the most striking feature (Fig. F23), and the degree of brecciation, usually associated with a strong foliation, allows division of the recovered interval into two zones. The upper zone from 319.30 to 328.90 mbsf is characterized by generally increasing brecciation with depth, with breccia fragments <0.3 cm. One exception is in the interval between 323.76 and 324.15 mbsf (interval 205-1254A-11R-4, 23-62 cm), where there are fragments up to 5 cm (Fig. F27). Foliation is common throughout Core 205-1254A-11R, resulting in a clear alignment of clasts, except for the above-mentioned zone with larger fragments that are equidimensional but internally strongly foliated.
Below 324.15 mbsf (Core 205-1254A-12R), deformation sharply decreases and consolidated and coarsely brecciated (fragments >10 cm) sand layers are common. These sandstone layers contain dipping lamination and rare web structures. This well-defined change in deformation intensity marks the top of a relatively less deformed rock volume, which can be interpreted in two ways: (1) it may mark a basal décollement zone starting at 338.5 mbsf, underlying a separate fault identified at a depth of 319.30-328.90 mbsf. This interpretation suggests a more articulated geometry for the décollement system than that described at Site 1040 (Kimura, Silver, Blum, et al., 1997; Tobin et al., 2001). (2) Alternatively, this horizon of less-deformed sand layers could reflect rheologic differences between the different lithologies within a single, thicker décollement zone—although sand-rich sections of cores have been strongly deformed deeper in the fault zone, suggesting that this interpretation is less plausible (Fig. F23).
Following interpretation 1 above, the upper boundary of the décollement zone is defined by increasing deformation intensity beginning in Core 205-1254A-13R at 338.5 mbsf. The exact location of the décollement upper boundary is difficult to place, mainly because the deformation gradually increases in intensity toward zones of concentrated shear and no sharp discontinuity is observed between Cores 205-1254A-12R and 13R.
The décollement zone itself is heterogeneous, with a general downward increase in brecciation intensity, breccia fragment aspect ratio, and hardening of the sediments. Despite the high recovery, spiraliferous drilling disturbance affects the cores, although less extensively than at Site 1040. Unlike Site 1040, a concentration of spiraliferous disturbance is not present in the lowermost part of the décollement zone. In zones of intense brecciation, breccia fragments are characterized by polished surfaces; the development of scaly fabric is probably precluded by the abundant silt and sand in the sediments. From 354.78 to 355.86 mbsf, sandstone layers are brecciated and foliated. At 360.60 mbsf, the appearance of diatoms in the sediments marks the lithologic boundary between the prism and the hemipelagic units (Fig. F5). The lithologic boundary underlies 50 cm of finely brecciated sand and 10 cm of highly sheared clay, possibly indicating a surface of ductility contrast that appears as a major structural discontinuity.
The hemipelagic sediments below the lithologic boundary are also deformed and brecciated and contain aligned clasts with a strong internal foliation (Fig. F6). Deformation decreases with depth and becomes discrete at 364.2 mbsf, localized along 3- to 8-cm-thick brittle shear zones characterized by gouges or Riedel shears (Fig. F7). These brittle shear zones consistently exhibit a normal sense of displacement and landward dips when reoriented to true geographical coordinates (Fig. F28). As defined by the criteria discussed above, the hemipelagic sediments above the zone at 364.2 mbsf are also deformed by normal faults, a few of them present as conjugate features. The change in deformation style and intensity within hemipelagic sediments at 364.2 mbsf is interpreted as the base of the décollement zone. The décollement zone at Site 1254 has a vertical thickness of 25.7 m. Because the recovered interval at Site 1254 does not contain intact or mildly deformed sediments, the geometry of any structurally higher faults that may be part of the décollement system in Hole 1254A cannot be determined. The complex geometry of the décollement system at this site contrasts with that described at Sites 1040 and 1043, where the top of the décollement coincides with an increase in brecciation, and the lithologic boundary between the prism and the hemipelagic unit is coincident with the base of the décollement.