The 352-m-thick section drilled in Hole 1114A penetrated 286 m of Pliocene-Pleistocene claystones, siltstones, and sandstones (see "Lithostratigraphy") that overlie a tectonic breccia and 67 m of metamorphosed basic igneous rocks (Fig. F21). The petrology of this metadolerite is described in "Igneous and Metamorphic Petrology".
This section describes the deformation of the sedimentary column, the nature of the breccia that occurs between 286 and 292 mbsf, and the structure of the underlying metamorphic basement.
Structural observations and measurements indicate that the recovered sedimentary section can be subdivided into three distinct structural domains. Domain I corresponds to undeformed sediments of lithostratigraphic Unit I (Section 180-1114A-1R-1 [0-6.6 mbsf]). Because they are exempt of tectonic deformation, they are not described in this section. Domain II represents slightly fractured and inclined siltstones and claystones of lithostratigraphic Unit II and the upper part of Unit III (Cores 180-1114A-2R to 7R [6.6-64.6 mbsf]).
Domain III extends from Cores 180-1114A-8R (64.6 mbsf) through 30R (285.8 mbsf). It includes moderately to strongly deformed sandstones, siltstones, and claystones of lithostratigraphic Units III and IV, and part of Unit V. Deformation is characterized by highly inclined bedding, faults, and scaly fabrics. The lowermost part of Domain III is obviously more deformed but poor recovery (~10%), especially from 141 to 180 mbsf, precludes a comprehensive description of the deformation in this domain. In this latter interval the structural description is based on FMS data only and can be found in "Downhole Measurements." Underlying Domain III is Domain IV (Section 180-1114A-31R-1 [285.8-287.1 mbsf]), which consists of a 1.30-m-thick (recovered thickness) tectonic breccia that separates the deformed sediments from the massive metamorphic basement rocks (Domain V).
Evidence of deformation is only occasionally observed in Domain II. It mainly exists in the inclined bedding that usually dips between 10º and 20º with a mode of 15º (Fig. F22). Because no evidence of soft-sediment deformation is observed along this section, the bedding dip is attributed to fault-induced tilting. Brittle deformation is weakly expressed by rare shallow-dipping faults that are locally coated by pyrite (e.g., interval 180-1114A-7R-2, 30-35 cm) and exhibit striations. The observed slickensides indicate dip-slip movements (e.g., interval 180-1114A-3R-2, 0-40 cm). The frequency of faults increases significantly downward from 55.24 to 56.87 mbsf (Sections 180-1114A-7R-2 and 3), and the lower limit of Domain II is defined at the bottom of Section 7R-3.
The boundary between Domains II and III is placed at the first occurrence of scaly fabrics at 64.6 mbsf (Section 180-1114A-8R-1). This limit is also marked by an increase in the frequency of the fracture network and the changes might reflect an increasing gradient of deformation downward.
Bedding measurements show a wide range of dip from horizontal to 80º, but average ~15º (Fig. F23). The higher bedding inclinations are found in the middle part of Domain III from 113 to 199.4 mbsf with values of 35º in Section 180-1114A-21R-1 (189.8-190.4 mbsf); 50º in Section 13R-1 (112.8-114.0 mbsf); and 65º-80º in Section 15R-1 (132.0-133.3 mbsf; Fig. F24). In Section 180-1114A-23R-1 (208.8-209.9 mbsf), steeply inclined strata dipping at 60º in the flank of a tight core-scale fold of probable gravitational origin indicate that part of the bedding dip may have been caused locally by early soft-sediment deformation. However, the consistent development of inclined beds throughout most of the recovered section of Domain III favors their tectonic origin, which is consistent with seismic reflection data. Very poor recovery (0.8%-3.6%) from 141.6 to 180.1 mbsf (Cores 180-1114A-16R to 19R) and from 247.3 to 266.5 mbsf (Cores 27R to 28R) makes it difficult to state whether steeply dipping beds are present along this entire section. However, FMS images confirm that within Domain III strata are inclined between 10º and 55º, as at 131 mbsf (40º) and in the interval 177-180 mbsf (25º; see "Downhole Measurements"). Faults occur throughout Domain III and their frequency is nearly constant, as shown in Figure F21. The fault population of Domain III is characterized by a large range of dips from 0º to 90º, but with an average ~30º-40º (Fig. F25). As observed between 74.2 and 95 mbsf (Sections 180-1114A-9R-1 to 11R-1) most of the shallow-dipping (0º-30º) structures occur in close association with the scaly-fabric bands described below (Fig. F26A, F26B), but are not seen in the FMS images. On the other hand, steeper faults are commonly observed in more sandy intervals, and the nature of the crosscutting relationships between shallow and steeply dipping fault sets is not clearly established from core observations.
Although the majority of the slickensided fault surfaces measured in Domain III are dip slip (66%), oblique (19%) and pure strike-slip faults (15%) also are present (Fig. F27). Oblique extensional faults are well documented in Section 180-1114A-21R-1 (189.8-190.4 mbsf) where finely laminated siltstones and claystones of lithostratigraphic Unit IIIA are offset with an apparent normal displacement of a few millimeters by an array of microfaults dipping at 65º and showing a shallow plunging slickenside lineation indicative of a lateral movement. Evidence for reverse microfaults is also documented in Section 180-1114A-15R-1 (132.0-133.3 mbsf; Fig. F28); therefore, Domain III appears to be a composite fault zone, including dip-slip (normal and reverse), oblique, and strike-slip faults.
On FMS images, most fractures dip toward the north and the north-northwest, but a few dip to the south-southeast (conjugates?) and to the northeast (see "Downhole Measurements"). The west-southwest-dipping fractures present from 280 to 295 mbsf mark the master fault (see below) that separates the sedimentary sequence from its basement. The histogram of fracture dip vs. depth deduced from FMS data also indicates that the mean inclination of the fractures steepens markedly with depth from 40º to 60º in the interval 40-120 mbsf and from 50ºto 80º in the interval 180-210 mbsf. The lower dips observed below in the interval 280-295 mbsf are related to the master fault discussed later.
Scaly fabrics are the most characteristic tectonic features of Domain III. They occur in bands of various thicknesses involving fine-grained, clay-rich material. The origin of scaly fabrics as either tectonic or core-induced features has been debated during previous ODP legs (e.g., see Shipley, Ogawa, Blum, et al., 1995). The scaly fabrics recognized in Hole 1114A are interpreted to result primarily from tectonic deformation. However, this fabric might have been later emphasized during coring and the relative contribution of the two processes is difficult to estimate. Nevertheless, the vertical distribution and variations displayed by the scaly fabrics are confidently assigned to changes in the deformation, and they are, therefore, used as qualitative strain markers. Three main types of scaly fabrics have been distinguished, and their distribution is shown in Figure F21:
At several levels, intervals up to 5-10 cm long of intact siltstones and claystones pass progressively upward and downward into zones of closely spaced joint/fracture networks in the incipient scaly fabric. Furthermore, the different types of scaly fabrics commonly show vertical gradational boundaries, for example, in Sections 180-1114A-9R-1 (74.2-75.6 mbsf) and 11R-1 (93.5-94.5 mbsf; Fig. F26). These observations allow us to assign the development of scaly fabrics to tectonic processes, probably involving local increase in strain (under the same stress conditions) throughout particular fine-grained intervals. Within each individual scaly-fabric zone no reliable marker of strain is observed. However, most of them contain discrete striated fault planes lying roughly parallel to the lithologic boundaries, as clearly evidenced in Sections 180-1114A-9R-1 and 11R-1 (Fig. F26). Assuming that both scaly fabric and sliding planes have been formed synchronously as a response to the same ambient stress, it can be argued that a significant component of layer-parallel shearing has taken place during deformation, which in turn excludes a strict relation of the initiation of scaly fabrics to flattening under vertical stress.
Evidence for the development of scaly fabrics is provided by observations from 113.4 to 114 mbsf (interval 180-1114A-13R-1, 60-120 cm), where highly fractured sandstones pass downward into silty and clayey material displaying incipient scaly fabric (Fig. F29). The sandstones form decimeter-long coherent pieces cut by steep conjugate fractures dipping between 60º and 70º. Further down, the density of fractures increases, resulting in the fragmentation of the siltstones and sandstones into decimeter- to centimeter-sized elongate angular fragments. The size of the fragments decreases downward toward the silty and clayey scaly-fabric band, which in turn is disrupted by parallel shallow-dipping fault planes.
These sliding surfaces are likely to be coeval with the above-mentioned steeper conjugate fault network present in the more competent sandy levels and their synchronous development may result from an almost vertical principal maximum stress oblique to the lithologic boundary, thereby inducing a shear component (Fig. F29). Substantial decoupling must have also occurred between the contrasting grain-sized layers involved in the deformation. Each scaly-fabric band is, therefore, likely to form a discrete shear zone and the bands' distribution determines the vertical structural zonation of Domain III.
The cumulative thickness of scaly-fabric bands measured in each core reveals that the frequency of occurrence and thickness of individual shear zones increase downward from Cores 180-1114A-8R through 30R (Fig. F21). The prominent scaly-fabric band identified in Cores 180-1114A-25R to 26R (228.0-247.3 mbsf) roughly coincides with a zone where logging caliper data show that the diameter of the hole was enlarged (see "Downhole Measurements"). The upper part of the cored section from 64.6 to 151.3 mbsf (Cores 180-1114A-8R to the top of 16R) is typically dominated by incipient and intermediate scaly fabrics, whereas intermediate and pervasive scaly fabrics are prevalent in the middle and lower parts of the section from 189.8 to 295.4 mbsf (Cores 21R to 31R). A quite similar vertical polarity is observed in the proportion of deformed and undeformed clay intervals within each core (Table T6). It clearly shows that the ratio of total clay thickness vs. scaly-fabric thickness increases from ~40% from 64.6 to 151.3 mbsf (Cores 8R to 16R, or possibly 21R, because of the recovery problem) to almost 100% in the deepest part of the section, where all of the clay-rich material is deformed.
The lowermost part of the deformed sedimentary section is very poorly recovered from 247.3 to 276.1 mbsf (Cores 180-1114A-27R to 29R), and it then passes at 276.1 mbsf (top of Core 30R) into a weakly indurated and fine-grained pale reddish clay-rich matrix containing pieces of silty claystones and subordinate sandstones. This interval is extensively deformed and exhibits pervasive scaly fabric; it is further crosscut by steeply dipping faults (60º-90º). No indication for sense of displacement is observed.
In contrast with basement rocks described below, the sedimentary section cored in Hole 1114A lacks extensive hydrothermal mineralization. Only two examples of fault surfaces coated with pyrite are present in intervals 180-1114A-7R-2, 30-35 cm (55.54 mbsf), and 11R-CC, 10-14 cm (95.17 mbsf), and very few millimeter-thick calcite-filled veinlets were observed in Section 24R-2 (218.8-219.6 mbsf), whereas quartz-filled veins are present in one sandstone pebble in Section 26R-1 (237.6-238.8 mbsf).
Sections 180-1114A-31R-1 and 31R-CC consist of a greenish tectonic breccia extending from 286.05 to 287.35 mbsf that is subdivided into two parts. The upper part (interval 180-1114A-31R-1, 25-74 cm) is composed of angular to subangular clasts of greenschist in a matrix containing quartz, plagioclase, clay, accessory minerals, opaque grains, and inorganic calcite. It also includes scattered well-rounded red, brown, and colorless grains (see "Lithostratigraphy").
The lower part (intervals 180-1114A-31R-1, 74-130 cm, and 31R-CC, 0-18 cm) is composed of a fine-grained matrix enclosing scattered angular millimeter- to centimeter-sized fragments that are similar to the underlying dolerite. The clasts inherited from the dolerite are crosscut by numerous quartz- and epidote-filled veins that do not extend through the matrix (Fig. F30). The preferred orientation of the clasts within the matrix outlines a weakly developed layering in the breccia. A second generation of calcite-filled veins crosscut the breccia; they are very often at the boundary between the clasts and the matrix (Fig. F31). These veins are sheared in a plane parallel to the layering defined by the oriented clasts (Fig. F32). Late fractures crosscut the breccia at high angles with respect to the layering (Fig. F31).
The lowermost part of the breccia is characterized by a reduction of grain size associated with massive alteration of both the matrix and the clasts. The layering defined in the upper part of breccia is still well expressed.
From these observations we infer that the formation of the tectonic breccia postdates the emplacement of quartz + epidote veins in the dolerite and is contemporaneous with the formation of calcite veins. The breccia was later disrupted by steep fractures.
The massive metadolerite was first recovered at 295 mbsf in Core 180-1114A-32R through Core 37R (bottom at 352.8 mbsf). However, FMS data indicate a sharp change of resistivity compared to the overlying sediments at 292 mbsf. The increase of resistivity related to the presence of the dolerite shows the upper limit of the dolerite, which constitutes Domain V. Although the poor and fragmented recovery does not allow a complete description of its tectonic evolution, two types of tectonic breccia are observed.
The first one (B1) consists of a penetrative breccia resulting from brittle to ductile fragmentation of the dolerite throughout the entire section with an intensity that decreases downward. This breccia consists of a centimeter-sized cataclastic zone composed of a fine-grained dark brown clay matrix containing fragments of pyroxene and plagioclase ("Lithostratigraphic Unit VI").
The second type of breccia (B2) consists of a fine-grained blue-green matrix containing fragments of the underlying dolerite. This 8.4-m-thick breccia is localized in the upper part of the section at the boundary between the overlying sediments and the dolerite; it corresponds to tectonic Domain IV described in "Domain IV" and to lithostratigraphic Unit V (see "Lithostratigraphic Unit V").
The deformation history of the dolerite can be summarized as follows:
Stage 3: In the upper part of the sequence (Sections 180-1114A-32R-1 to 34R-2), the dolerite is massively fractured. The interval between the fractures is 2-4 cm, and the angle between the fractures is at ~90º, visible in both the hand specimens (Fig. F35) and thin sections (Fig. F31). This brittle event is also observed in the sediments overlying the dolerite.
In summary, the following chronological evolution can be deduced from the observations described above (Fig. F36):
Stage 3: Continuing evolution of the fault zone, shearing of the calcite veins in the tectonic breccia, and fracturing of the whole sequence from the bottom to the top of the dolerite, including the breccia B2 and the overlying sediments.
As this tectonic evolution is associated with a retrograde metamorphic evolution under greenschist facies conditions (see "Igneous and Metamorphic Petrology"), the three tectonic stages described above are related to the unroofing of the dolerite along a 60º-65º dipping normal fault expressed by the development of the breccia.
In Hole 1114A, Pliocene-Pleistocene synrift sediments overlie metamorphic basement rocks along a major extensional fault zone dipping at 60º-65º to the south-southwest. In the metadolerite forming the footwall of the fault, extensional faulting is expressed by fracturing that increases in intensity upward close to the fault contact and, thus, results in the brecciation of the metadolerite. The brittle deformation recorded by the sedimentary sequence in the hanging wall of the fault is typical of a large-scale shear zone comprising discrete shear bands parallel to the bedding, which consistently dips 0º-40º to the north-northwest. The intensity of the shear strain decreases gradually upward throughout the sedimentary succession.
The layer-parallel shear fabrics are disrupted by a composite fault system including dip-slip normal faults as well as oblique and reverse faults. Most of the faults dip to north, north-northwest, and northwest, and their dip shallows significantly upward (Fig. F37). Two northeast-dipping faults are also imaged by FMS in the upper part of the logged section between 100 and 120 mbsf where we also note that both bedding and fault strikes are anticlockwise rotated by ~20º-30º with respect to their deeper counterparts (Fig. F37B).
The wide range of bedding dips observed from 65 to 286 mbsf, in association with the more intense network of steep faults downhole, may indicate that differential rotations of strata have occurred at depth within fault-bounded compartments. In map view and in vertical section, this steep fault network lies at relatively high angles (30º-40º) to the master fault mentioned above (Fig. F37).
Likewise, the layer-parallel shear zones present in the lower part of the sedimentary sequence are sharply cut at depth by the master fault. Such angular relationships between the most prominent structures forming the Moresby Seamount suggest a multistaged extensional fault history involving notably late oblique faulting in agreement with recent regional kinematics.