. This stress field is oriented at ~30° to the convergence vector (DeMets et al., 1990).
Sediments recovered at Site 1253 show a variable degree of drilling disturbance; however, some sections both above and between the igneous units such as Cores 205-1253A-4R and 11R are indurated enough to allow observation of the original geometry of sedimentary layers and deformation structures.
The sediments are characterized by tilted bedding with 30° average dips (Figs. F47, F48). This is consistent with results from Leg 170 at Sites 1039 and 1040 (Kimura, Silver, Blum, et al., 1997), where continuous coring of the sedimentary section showed that this tilting starts at ~352 mbsf at Hole 1039B (Section 170-1039B-38X-6). At Hole 1040C the bedding in the lower part of the underthrust section, starting from 640 mbsf (Core 170-1040C-51R), shows a transition from horizontal to 20° average dips (Kimura, Silver, Blum, et al., 1997). At Site 1253, paleomagnetic reorientations indicate westward dip directions clustered at 221° (Fig. F49), also consistent with results from Sites 1039 and 1040.
Small normal faults perpendicular to bedding and with millimeter-scale offset are common throughout the recovered sediments (Fig. F50A). Structures recording fluid-related processes are also commonly observed, including incipient stylolites, fluid escape structures, and fluidized sediment injections (3 to 10 cm long). These have orientations both parallel and horizontal to bedding. Reverse faults with slickenlines are present, but less common, and their orientation is compatible with subhorizontal to shallowly dipping shortening. These steeply dipping structures may be reactivated normal faults formed during compaction, or they may be reverse faults formed as such. If they are not reactivated, their orientation may be used to infer stress orientation at the time of formation. Figure F51 shows a stereographic plot of a conjugate system of these reverse faults in interval 205-1253A-2R-3, 128-135 cm (Fig. F51B), in which the principal stress orientation reconstruction indicates a north-south (183°), horizontal orientation for . This stress field is oriented at ~30° to the convergence vector (DeMets et al., 1990).
The close spatial association of reworked pelagic sediment in the lower part of Subunit U3C (Kimura, Silver, Blum, et al., 1997), westward-tilted bedding, and the gabbro sill (Subunit 4A) suggest that the lowermost part of the sedimentary section may have been deformed during the emplacement of sills by reactivation of inherited structures. Bedding perpendicular to vertical flattening and horizontal extension indicate progressive subvertical compression and may be related primarily to compaction processes. At Site 1253, the sediments do not show a clear signal that the subhorizontal shortening is related to incipient subduction.
The igneous units were carefully analyzed for fractures because of their relevance to identifying intervals for the CORK-II experiment and to provide data directly comparable with the FMS data. The only geometrically reliable sediment/igneous contact was recovered in interval 205-1253A-27R-1, 1-6 cm, and it dips 72°. This piece was recovered at the top of the core and may not have been recovered in place. The only other contacts recovered are in core pieces too small to have preserved the true orientation. Internal magmatic contacts were common, especially in the upper part of the deeper igneous unit (Fig. F52). These magmatic contacts have been recognized mainly on the basis of mesoscopic observations of grain size and may represent either chilled margins or cumulate textures (see "Petrology"). Reoriented magmatic contacts dip ~15°-20° to the northeast, mostly opposite to that of the sedimentary bedding (Fig. F53A).
The igneous units are commonly cut by veins (Fig. F52), either filled with glassy to mineral-rich groundmass (here called magmatic veins) or postemplacement alteration minerals (zeolite, clay, or calcite). The paleomagnetic reorientation of these two vein types reveals that they have the same general trend (Fig. F53B, F53C), poles oriented 60° to the north-northwest and 75° to the east-northeast. The good geometric agreement between the two vein systems suggests that the mineralized veins are a further evolution of the magmatic veins as products of alteration. Dilational joints are also frequent (Fig. F52); these are usually filled with a film of green minerals (zeolite?) and are rarely present as open fractures. Reoriented joint orientations for all joints (Fig. F53D) are similar to those determined in intervals where rocks show high magnetic intensity to allow confident reorientation (Fig. F53E). The joints show no strong preferential orientation.
The presence of joints can significantly contribute to the bulk porosity and permeability of the igneous units; their intensity downsection (Fig. F52), shows that the number of fractures increases with depth. Furthermore, joint distributions suggest that they are homogeneously distributed from Cores 205-1253A-30R through 36R, whereas in Cores 40R through 43R, comparable to the above interval in terms of recovery, fracture intensity is not constant. Other brittle deformation features include shear zones, represented by en echelon Riedel shears, usually showing reverse displacement. These structures are common in the lower part of the deeper igneous unit (Core 205-1253A-36R and below). In one case, measured conjugate fractures in interval 205-1253A-40R-1, 98-112 cm (Fig. F54), allow reconstruction of the principal stress orientation, with 1 oriented 246°/60°, at an angle of 40° to 50° to the convergence vector (Fig. F55). After reorientation, the plotting of the brittle shear zone's boundaries (Fig. F56) shows no clear indications of preferred orientation.
