1) that formed the normal fault system was subvertical. This suggests either extension or compaction-related vertical shortening. This observation is also valid for other observed conjugate fault systems in Units 2 through 4.
A variety of deformation structures, including folds, faults, and shear zones in sediments, were observed at Site 1276. Apart from compaction-related deformation, which can be observed throughout the cored sequence, deformation structures are rare but nonetheless occur in all sedimentary units. The measured apparent dip (i.e., on the face of the split core) of sedimentary bedding shows gentle dips of as much as 25° from 800 to 853 mbsf (Section 210-1276A-7R-4). Lower dips (<10°) are observed downhole to 1060 mbsf (Section 210-1276A-29R-1), and dips of as much as 15° occur from 1060 mbsf to the bottom of the hole (Fig. F133). Changes in measured apparent dip of bedding generally match the lithostratigraphic unit boundaries (see "Lithostratigraphy").
We here describe some of the deformation structures observed in the different lithologic units (see "Lithostratigraphy"). Describing deformation, particularly where strain is low, depends on the presence of strain markers such as laminations, lithologic boundaries, or bioturbation. The common occurrence of massive, homogeneous claystone and mudstone makes it difficult to determine the strain distribution in the sediments at Site 1276. Discrepancies between the observed distribution of deformation structures and the measured apparent dip of sedimentary bedding (Fig. F133) suggest that some of the deformation may have been distributed in the more ductile claystones and mudstones. The low finite strains encountered at Site 1276 also make it difficult to distinguish between compaction-related deformation (mainly pure shear dominated) and deformation related to gravitational instabilities or tectonic processes (mainly simple shear dominated).
Compaction-related structures are observed throughout Site 1276. They are responsible for a flattening of burrow structures (i.e., a shortening parallel to their subvertical axis). Other compaction-related structures, such as cleavage or foliation, are not observed, except for zones of high shear deformation, which were observed mainly in Unit 1. Stylolites indicating the presence of pressure solution are observed, but they occur only in carbonate-rich lithologies (e.g., interval 210-1276A-25R-4, 47-62 cm). Veins were observed only in intervals 210-1276A-19R-2, 102-105 cm, and 87R-6, 6-55 cm (see "Igneous and Metamorphic Petrology" and Fig. F121).
Another commonly observed structure that may be misinterpreted as deformation structure is convolute lamination, which occurs mainly in the silt fraction of graded turbidites in Unit 5 as discussed in "Lithostratigraphy". Despite these limitations, the observed structures allow us to draw some conclusions on the postdepositional history of the sediments drilled at Site 1276.
For further description of core deformation, see also "Lithostratigraphy".
Deformation structures in Unit 1 are represented by rare faults, folds, and zones of increased shear strain marked by incipient formation of a foliation. These structures are observed in Cores 210-1276A-1R to 3R and Section 3R-4 but are rare or absent in the remainder of Unit 1. Faults are mainly normal faults, but conjugate systems and reverse faults also occur locally. The displacement along these faults is small, commonly <1 cm.
Folds observed in Unit 1 are on a centimeter scale, commonly observed in sediments that are highly strained. The axial planes of the folds are subhorizontal. All the observed deformation structures affect sediments that are already bioturbated. In interval 210-1276A-2R-3, 34-36 cm, a localized deformation zone is observed in which sedimentary structures are rotated into the inferred direction of the shear transport. Evidence for ductile plastic deformation is observed in claystones in Core 210-1276A-7R.
The observations made in Unit 1 show that deformation may have occurred after consolidation of the sediments but probably not at a very deep level. Although deformation structures are observed only down to Section 210-1276A-3R-4 in Unit 1, sedimentary bedding shows high apparent dips down to Core 210-1276A-8R at the base of Unit 1 (Fig. F133). Thus, tilting of sedimentary bedding may also be a consequence of distributed ductile deformation accommodated in mudstones and claystones, which is not documented by small-scale deformation structures. The scarcity of both contractional structures (e.g., reverse faults and folds with inclined fold axial planes) and conjugate normal faults suggests that the rocks cored in Unit 1 were deformed on a slope. In contrast, contractional structures might be expected in a base-of-slope setting oceanward of Site 1276.
Deformation structures are rare in Unit 2 and are observed mainly in Cores 210-1276A-14R and 15R. The measured apparent dip of the sedimentary bedding varies between 0° and 5° (Fig. F133). Deformation structures include faults and folds. The folds are commonly on a centimeter scale, and fold axial planes are commonly subhorizontal and parallel to bedding. In interval 210-1276A-14R-2, 8-32 cm, a well-developed chevron fold is observed (Fig. F134). The presence of such ductile folds indicates that deformation occurred after deposition and compaction but before lithification of the sediments.
In Unit 2, faults are observed only in Core 210-1276A-15R; all are normal faults. These faults show apparent displacements of as much as 1 cm. A well-developed conjugate normal fault system is also observed in interval 210-1276A-15R-2, 71-74 cm, at the base of Unit 2 (Fig. F135). The conjugate fault system shows a classic graben structure bounded by symmetric faults, indicating that the orientation of the maximum principal stress (1) that formed the normal fault system was subvertical. This suggests either extension or compaction-related vertical shortening. This observation is also valid for other observed conjugate fault systems in Units 2 through 4.
The scarcity of deformation structures as well as the general subhorizontal dip of sediment bedding in Unit 2 suggests that the unit was only very weakly affected by postdepositional deformation. Thus, Units 1 and 2 experienced different postdepositional deformation histories, with Unit 1 showing much more evidence of disturbance after deposition.
In Unit 3, deformation structures are very similar to those in Unit 2 but are more abundant. The most common structures are normal faults, which are observed in 4 out of 10 cores. Folds are rare in Unit 3. In interval 210-1276A-17R-5, 83-88 cm, a folded burrow is observed (Fig. F136). The folds have subhorizontal fold axial planes, and the fold limbs are commonly extended, indicating subvertical compaction of the sediments. Further evidence for compaction is indicated by the occurrence of stylolites in interval 210-1276A-21R-4, 125-132 cm (Fig. F137). These structures are observed only in the sandy fraction of calcareous turbidite units.
Relatively scarce deformation structures and the low apparent dip values of the sedimentary bedding (<5°) (Fig. F133) indicate that Unit 3 is only weakly deformed, similar to Unit 2. This suggests a similar postdepositional history for the two units.
Deformation structures are observed in the majority of cores in Unit 4. The most common structures are normal faults and ductile shear zones in mudstones. A good example of a ductile shear zone is observed in interval 210-1276A-27R-3, 34-40 cm (Fig. F138). The shear component is indicated by the presence of asymmetric folds and elongated burrows that are aligned at a slightly oblique angle to the shear zone boundaries.
Unit 4 is more deformed than the overlying Units 2 and 3, and the apparent dip of the sedimentary bedding increases at the base of the unit (Fig. F133). Unlike Unit 1, Unit 4 shows a direct relationship between the dip of sedimentary bedding and an increased occurrence of deformation structures in the cores (Fig. F133). Thus, the dip and the deformation structure probably developed at the same time, most likely in poorly consolidated sediments.
In Unit 5, deformation structures are observed in Cores 210-1276A-30R through 32R, 41R through 45R, 61R, 65R, 66R, 69R, 71R, 76R, 79R, 80R, 85R, 86R, 90R, and 94R (Fig. F133). The majority of these deformation structures are normal faults. The faults range from microscale normal faults with displacements of a few millimeters to larger faults with displacements >5 cm. Displacements >1 cm are restricted to the lowermost part of Unit 5 in Sections 210-1276A-80R-4, 69 cm, 90R-3, 37 cm, 94R-5, 22 cm, 94R-6, 39 cm, and 94R-6, 56 cm. Normal faults with slickensides are recorded in mudstones and claystones in Cores 210-1276A-80R, 86R, and 90R. Some faults also show a strike-slip component.
Reverse faults are observed in Sections 210-1276A-32R-1, 32R-3, 94R-4, and 94R-5. Figure F139 illustrates a reverse fault in interval 210-1276A-94R-5, 20-27 cm. Other structures occurring in Unit 5 include subvertical faults and microduplex structures in Section 210-1276A-79R-4, ductile shear zones in Section 76R-1, and styolites in Section 92R-3.
Postdepositional folds are observed only in Cores 210-1276A-76R, 79R, and 94R. Folds are distinguished from convolute-bedding folding by the deformation of sedimentary structures such as bioturbated layers, showing that the folding took place some time after deposition. Most of the structural folds recorded in Unit 5 are shear folds (e.g., Sections 210-1276A-76R-1 and 76R-2).
Dips of apparent bedding increase slightly toward the bottom of Unit 5 (Fig. F133). This may represent deviation of the hole. Hole deviation was measured with the Tensor tool during retrieval of Core 210-1276A-92R (1662.1 mbsf), and the tool indicated that the hole deviated 7.4° from vertical. On the other hand, intervals of high apparent dip (as much as 10°) correspond to sections with increased frequency of normal faults as well as the presence of a few reverse faults. Other deformation structures are more common, and displacement (accommodated strain) is more important in the lowermost part of the hole at Site 1276. Thus, the increase in apparent dip may be a real feature.
In summary, the various (nondepositional) structures recovered at Site 1276 can be attributed to postdepositional gravity instability and sedimentary compaction on the Newfoundland rifted margin. In some cases the instability occurred very soon after sediment deposition and apparently continued until the sediments became relatively well lithified.
