Previous   |    Next

doi:10.2204/iodp.proc.314315316.136.2009

Structural geology

The two holes drilled at Site C0008 sampled the same nondeformed to weakly deformed sediments as found in the footwall of the major splay faults imaged by seismic reflection and sampled at Site C0004. Structural data measured on cores are given as a supplementary material (see C0008_STRUCT_DATA.XLS in folder 316_STRUCTURE in “Supplementary material”). Where possible, planar structures were corrected to true geographic coordinates using paleomagnetic data (see “Structural geology” in the “Expedition 316 methods” chapter). The distribution of planar structures and lithologic divisions with depth are shown in Figure F10.

The main structural features encountered in Holes C0008A and C0008C are subhorizontal bedding and normal faults. Deformation bands and sediment-filled veins were not observed at Site C0008.

Bedding and fissility

At Site C0008, bedding surfaces seldom dip >30° (Fig. F10). Figure F11 shows the distribution of poles to bedding surfaces after correction based on paleomagnetic data. The dip directions do not show any preferred orientation.

Fissility is poorly developed at Site C0008. It can be observed in Hole C0008A at depths >263 m CSF and is not observed in Hole C0008C. Given the small number of measurements, fissility data are not included in the data for Figure F11.

Faults

Cores from Site C0008 are moderately faulted. The distribution and dip of faults with depth are given on Figure F10. In Hole C0008A, they are distributed throughout, whereas in Hole C0008C, they are found between 35 and 175 m CSF. Where determinable, the sense of slip is always normal (Fig. F12). In most cases, the amount of slip does not exceed 5 cm.

At the shallowest levels, normal faults do not consist of sharp slip surfaces but rather of shear zones that range from <1 mm to ~2 cm thick. Where the shear zone is thick enough to allow internal observation, ductile deformation structures such as heterogeneous stretching of layers testify to the unconsolidated state of the sediments at the time of faulting. Striations are usually weakly developed.

At deeper levels where sediments are more consolidated, faults appear as dark seams that do not exceed 1 mm in thickness. Such structures, which were also observed at Site C0007 (see “Structural geology” in the “Expedition 316 Site C0007” chapter), are called healed faults. Like faults in unconsolidated sediments, most healed faults dip ~60° or steeper (Fig. F13A) and commonly show normal offsets of 5 cm or less. Some healed faults are horizontal or nearly horizontal and parallel to bedding (Fig. F13B). In such cases, the sense and amount of slip cannot be determined. After correction based on shipboard paleomagnetic data, fault surfaces from either hole of Site C0008 do not reveal any preferred orientation (Fig. F14).

A high concentration of normal faults is found in Hole C0008C between 35 and 80 m CSF. It is interesting to note that this concentration of normal faults is located near a discontinuity in bedding dip apparent in seismic reflection profile Inline 2675 (see Fig. F3 in the “Expedition 316 summary” chapter). X-ray CT scan image analysis suggests the existence of a 5 cm thick gently dipping shear zone in Section 316-C0008C-5H-6, 93 cm, which could correspond to this discontinuity.

Discussion and summary

After reorientation based on shipboard paleomagnetic measurements, normal faults from Site C0008 do not indicate any clear direction of extension. They rather show scattered directions suggesting vertical compaction of the sedimentary pile.

The location of Hole C0008C was chosen in order to crosscut a seismic imaged discontinuity between two packages of sediments having slightly different bedding orientation. The discontinuity could represent a low angle normal fault or the basal detachment fault of a landslide. No major shear zone could be observed in cores at the corresponding depth.

Top of page   |    Previous   |    Next