The 722-m-thick section cored below 205 mbsf at Site 1118 is composed of a 664-m-thick sedimentary succession, Pleistocene-early Pliocene in age (see "Biostratigraphy") that overlies a dolerite at 869 mbsf described in "Igneous and Metamorphic Petrology" and subsidiary sedimentary rocks (see "Lithostratigraphy").
The sedimentary section is composed of seven lithostratigraphic units described in "Lithostratigraphy". It is subdivided into three structural domains. Domain I is characterized by undeformed zones or zones with soft-sediment disturbances only. Domain II extends throughout the lower part of the sequence and exhibits brittle fault zones. The dolerite at the bottom of the hole comprises Domain III and is sheared, brecciated, and veined. In the following, the depth of the sections refers to their top.
Domain I constitutes most of Hole 1118A (~90% of the total thickness). It includes undeformed zones (Subdomain Ia) and two types of soft-sediment deformation zones (Subdomains Ib and Ic) that alternate along the section. The deformed zones are between 272.1 and 583.7 mbsf (Core 180-1118A-8R to Section 40R-3; lithostratigraphic Units I, II, and III).
Subdomain Ia comprises five undeformed zones where bedding dip is consistently <10º (Figs. F42, F43). Minor normal faulting is rarely observed in intervals 180-1118A-30R-4, 116-120 cm (488.76 mbsf), and 33R-2, 9-15 cm (514.27 mbsf; Fig. F42).
Subdomain Ib shows soft-sediment deformations that are fully described in "Soft-Sediment Deformation." They are principally developed within two relatively narrow zones at 320.3-335.2 mbsf (Sections 180-1118A-13R-1, to 14R-4), and 396.8-406.3 mbsf (Core 180-1118A-21R).
Deformation consists of dm-sized slump folds commonly found throughout 50-cm-long deformed intervals (intervals 180-1118A-14R-1, 55-90 cm; and 14R-2, 0-40 and 80-115 cm; and 14R-3, 5-70 cm). Individual folds are usually truncated both on top and bottom by flat-lying sliding planes with sense of motion compatible with the asymmetry of the folds (intervals 180-1118A-14R-2, 94-96 cm; 14R-3, 10-28 cm; and 21R-1, 47-48 cm). Slump folds show nearly horizontal axes (intervals 180-1118A-14R-2, 0-12 and 108-115 cm). The close association of folding and minor faulting is well documented at interval 180-1118A-14R-1, 60-70 cm, where steep and closely spaced reverse microfaults dissect a slightly overturned fold limb, both fault and fold displaying similar verging. This local compressional faulting indicates that sediments were already sufficiently lithified to deform in a relatively brittle manner, yet still plastic enough to also record folding. Slump fold limbs are also locally intensely disrupted by low-angle (20º-40º) extensional faulting (interval 180-1118A-14R-2, 110-115 cm).
The structures of Subdomain Ic also result from soft-sediment deformations, but differ from the structures described above with respect to their scale, their vertical distribution, and their frequency. Nearly each core of Subdomain Ic contains one thin discrete deformed zone a few centimeters thick separating much thicker intervals with no deformation and horizontal bedding (Table T6). Deformation consists of centimeter-scale slump folding and normal microfaulting (Fig. F44) that are assumed to have occurred at an early stage shortly after deposition of the sediments. The more demonstrative examples are vertical, locally reverse, faults that flatten downward into horizontal clay-rich intervals (interval 180-1118A-25R-3, 50-55 cm), and normal faults filled with disturbed sediments (interval 180-1118A-25R-2, 50-62 cm). The microscale deformations regularly distributed throughout Subdomain Ic may reflect instability caused by rift-related earthquakes.
The lower part of the sedimentary sequence comprises three fault zones (Subdomains IIa, IIb, and IIc).
The first evidence for brittle faulting is found in Subdomain IIa at 583.7-608.4 mbsf (Sections 180-1118A-40R-4, to 42R-8), where slickensided fault planes dip generally at moderate to low angles (<60º) and show dip-slip motion (Fig. F45A). The fragmentation of clayey material regarded as scaly fabrics is related to either shallow-angle faulting (intervals 180-1118A-42R-2, 66-70 cm; 42R-3, 3-7 cm; and 42R-8, 7-11 cm), or more rarely to nearly vertical dip-slip faulting (interval 180-1118A-41R-4, 120-145 cm).
Evidence for multistaged brittle deformation is provided in interval 180-1118A-40R-4, 50-98 cm, where a well-laminated upward-fining sequence is cut by two steep (70º) normal faults facing to the east (core reference frame) with flat-lying beds in the hanging wall. They are labeled Fe in Figure F46A. On the contrary, the footwall is much more intricate and has shallow (25º-30º), west-dipping normal faults (Fw) whose footwall has bedding steepening progressively upward. These two opposite-facing extensional fault patterns may involve early shallow structures (Fw) with progressively rotated hanging walls as faulting propagated forward (Fig. F46B). As a response to the possible locking of this earlier shallow fault system, steeper faults (Fe) might have developed later with opposite vergence, probably facilitated by the favorable dipping attitude of the previously rotated strata.
In addition to this two-staged brittle faulting, earlier soft-sediment deformation is also documented in interval 180-1118A-40R-4, 50-98 cm. It is expressed in the dominantly clayey hanging-wall sequences of the fault (Fe) by a nearly vertical fault cutting through horizontal sandy layers with a normal offset (~1 cm) that fades away rapidly upward and downward. This fault is, therefore, likely to form an early compaction-induced structure, and finally, the structure observed in Core 180-1118A-41R is assumed to have recorded at least three stages of extensional synsedimentary faulting.
Subdomain IIb represents a narrow normal fault zone at 714.4-715.8 mbsf (Section 180-1118A-54R-1) that includes steep (70º-85º) conjugate fault surfaces with dip-slip slickensides.
Faulting is well developed in Subdomain IIc at 784.5-813 mbsf (Sections 180-1118A-61R-3 through 64R-2), where steep conjugate fault planes dipping 50º-80º show exclusively dip-slip sense of motion (Fig. F45B). Curved secondary faults are locally observed in the hanging wall of the faults, as in intervals 180-1118A-63R-1, 40-50 and 100-110 cm.
The dolerite was first encountered within Section 180-1118A-70R-3 (871 mbsf) below a conglomeratic unit composed of a carbonate matrix containing well-rounded basaltic clasts and fragments from a lagoonal environment (refer to "Lithostratigraphic Unit VII"). Two subunits are defined within the dolerite (Fig. F47).
An unbrecciated dolerite (Subdomain IIIa) exists within interval 180-1118A-70R-3, 82-140 cm (872.07-872.65 mbsf), and Sections 74R-1 and 2 (897.5-900.4 mbsf), 75R-3 (908.6-909.7 mbsf), and 76R-1 (917.0-918.3 mbsf). In this part of the sequence the rocks are mainly undeformed and a granular magmatic texture is preserved (see "Igneous and Metamorphic Petrology"). In some samples conjugate fractures or veins filled with calcite and zeolite (Fig. F48) crosscut the dolerite. These fractures and veins are oriented north-south and dip 35º-70º to the east or to the west (core reference frame). No motion is visible along the planes.
A massively brecciated dolerite (Subdomain IIIb) is found in Sections 180-1118A-71R-1 to 73R-2 (878.4-889.4 mbsf), and 74R-3 to 75R-2 (900.4-908.6 mbsf). These two zones show a progressive increase in brecciation from the fractured dolerite to the brecciated dolerite. Perpendicular veinlets invade the rock from main veins fragmenting the dolerite as fluid circulation proceeded (Fig. F49). Figure F50 shows a network of veins in the dolerite. Veining resulted in the fragmentation of the dolerite into angular domains, which are visible on the lower right part of the sample where an angular dolerite fragment in dark brown matrix has been separated from the massive dolerite. The ultimate stage of the brecciation shows a dark brown clay-sized, iron oxide-rich matrix containing angular dolerite clasts (Fig. F51). In some samples the breccia consists of individual pebbles without matrix (Fig. F52).
In the upper part of Subdomain IIIb (Sections 180-1118A-71R-1 and 72R-1 and interval 180-1118A-73R-1, 0-15 cm) the pebbles of the breccia differ from the pebbles of Sections 74R-3 to 75R-2 because they show fault planes bearing slickensides. These structures represent a deformation stage developed in the upper part of the brecciated Domain III. Thus, it is difficult to determine if the shearing occurs prior, after, or contemporaneous to the brecciation of the dolerite.
The brecciated zones localized fluid circulation, which was responsible for the alteration (weathering) of the rocks in subaerial conditions. This breccia could have been reworked later on because some fragments of the Subdomain IIIb are well rounded (see "Lithostratigraphy").