The 158.9-m-thick section cored at Site 1116 is subdivided into four structural domains (Fig. F22). The top and bottom of the section are undeformed (Domain I) and little deformed (Domain IV), respectively. In between, Domain II shows soft-sediment deformation whereas Domain III is affected by brittle faulting. However, the boundaries between the three upper structural domains are not precisely defined because of low recovery.
Bedding dips range from 0º to 50º throughout Hole 1116A with an average at 20º (Figs. F22, F23). High values, mostly related to soft-sediment deformations, are scattered throughout the section, except within Domain IV at the bottom of Hole, where they do not exceed 30º.
Domain I extends from the seafloor to 24.2 mbsf (Cores 180-1116A-1R to 3R), and involves sandstones, siltstones, and claystones of all but Core 4R of Lithostratigraphic Unit I (see "Lithostratigraphic Unit I").
Its overall structure is poorly imaged because of very low recovery in Cores 180-1116A-1R (0-6.6 mbsf) and 3R (16.2-24.2 mbsf) where only pebbles were recovered. The nearly continuous section lying from 6.6 to 7.9 mbsf (Section 180-1116A-2R-1) exhibits beds dipping from 0º to 10º (Fig. F22). Rare evidence for brittle deformation is present, consisting mainly of mm-sized veins filled with either calcite (interval 180-1116A-1R-CC, 55-60 cm) or quartz (interval 180-1116A-2R-1, 13-15 cm).
Domain II includes the interval from 24.2 to 101.0 mbsf (Cores 180-1116A-4R to 12R), and is composed of the lowermost core of lithostratigraphic Unit I, all of Unit II, and the upper half of Unit III (see "Lithostratigraphy"). The few (7) measurements of bedding dips show relatively high values, between 5º and 50º (Fig. F22), that are confidently assigned to soft-sediment deformation. Additional soft-sediment deformation is observed at intervals 180-1116A-4R-1, 90-130 cm, and 9R-1, 115-120 cm.
The most significant structure (interval 180-1116A-4R-1, 90-130 cm; Fig. F24) consists of a steeply dipping (50º), finely laminated silt/clay lying along the normal limb of an isoclinal and overturned fold whose hinge zone occurs at 100-110 cm. The asymmetry of the subsidiary S-shaped folds developed along the normal flank of the structure typifies a west-verging structure with respect to the core reference frame. The fold limb is disrupted by two sets of low-angle extensional microfaults with opposite senses of displacement, involving either thicker sandy layers or more silty material. The base of the fold is sharply truncated at 113 cm by a shallow-dipping plane, which in turn lies on top of a 10-cm-thick horizontal recumbent folded zone, probably detached from underlying and undeformed sandstones along a second flat-lying sliding surface.
The association of such folding and low-angle extensional faulting is typical of gravity-driven structures that may have preferentially developed within clay-rich intervals detached from bounding sandy levels along shallow-dipping shear planes. These deformations are likely to have taken place shortly after deposition of the synrift sediments during compaction, so that dominantly-clayey sequences could still be deformed by folding, because more competent material accommodated strain by faulting.
Evidence for later faulting is observed in interval 180-1116A-4R-2, 5-7 cm, where a flat-lying and dominantly sandy succession is affected by a small-scale asymmetric graben-like structure bounded by steeply dipping (60º-70º) normal microfaults (Fig. F25). The steep attitude of the faults may indicate that they developed through relatively indurated sandy material, probably at a later stage than the low-angle structures described above.
At 62.6-63.6 mbsf (interval 180-1116A-9R-1, 0-100 cm) and 72.2-73.3 mbsf (interval 180-1116A-10R-1, 0-110 cm), fine-grained siltstones and claystones are intensely broken into mm- to cm-sized angular fragments with polished surfaces. Whether the origin of this scaly-like fabric is coring induced or tectonic is questionable because only one slickensided surface has been observed in interval 180-1116A-10R-1, 95-97 cm. Core 180-1116A-10R is, therefore, placed into Domain II, and the top of the underlying fault zone (Domain III) is located below the poorly recovered Cores 11R and 12R (e.g., at 101.0 mbsf).
Domain III corresponds to a fault zone lying from 101.0 to 130.0 mbsf (Sections 180-1116A-13R-1 to 15R-2). Its upper limit is clearly marked on Figure F22 by an abrupt increase of fault frequency. On the histogram of Figure F26A, the fault dip population displays a bimodal distribution with two peaks at 35º-45º and 75º-90º.
Steep faults are mainly confined to the upper part of Domain III (Sections 180-1116A-13R-1 and 2; 101.0-102.7 mbsf), and mostly show strike-slip displacements. On the other hand, dip-slip faults preferentially occur in the lower part of the fault zone, in Sections 180-1116A-14R-1 (110.6-111.9 mbsf) and 15R-1 and 2 (120.3-122.7 mbsf). The fault dip vs. slickenside plunge diagram (Fig. F26B) shows that dip-slip and strike-slip faults are predominant with respect to oblique-slip faults. At interval 180-1116A-15R-2, 90-106 cm, normal offsets a few mm long are well documented along a steep and sigmoidal fault. Calcite-filled veins occur in an extensional relay zone along the curved fault (Fig. F27). In intervals 180-1116A-14R-1, 0-10 cm, and 85-95 cm, scaly-type fabrics associated with steep dip-slip normal faults likely result from fragmentation of fine-grained silts during brittle extensional faulting.
Domain IV lies from Section 180-1116A-16R-1 to the base of the hole (130.0-158.9 mbsf). Its upper boundary corresponds to a rapid decrease in fault frequency (Fig. F22). However, a few dip-slip normal faults and strike-slip faults occur throughout Domain IV, especially in intervals 180-1116A-17R-1, 65-75 cm (dip-slip faults), and 140-145 cm (strike-slip faults). In interval 180-1116A-17R-1, 100-115 cm, nearly vertical fault planes, ~10 cm long, are typically sigmoidal and show both apparent normal and reverse offsets a few mm long. These faults are likely to have been generated as early compaction-related features, as also evidenced in interval 180-1116A-17R-2, 15-25 cm (141.35-141.45 mbsf), where a similar vertical fault fades away downward. Arrays of mm-sized calcite-filled veins are present in the uppermost part of Domain IV in intervals 180-1116A-16R-1, 3-6 and 100-110 cm, and 16R-2, 0-7 and 22-30 cm. Domain IV generally contains long unbroken pieces of siltstones a few dm long, which are locally disrupted by sharp coring-induced (?) conjugate fractures (intervals 180-1116A-16R-5, 40-50 cm; 18R-2, 5-35 and 85-115 cm). Throughout Domain IV, bedding dip distribution is similar to that for the complete Hole 1116A with a well-marked peak at 15º, but with fewer high values (Fig. F28).