No evidence for brittle tectonics is recorded in Hole 1109C; however, two types of deformation are locally observed within both unconsolidated and indurated sediments. The three structural domains, which can be distinguished from top to bottom and appear to be controlled by differing lithologic competency, follow.
Domain I is represented by unlithified and undeformed sediments of lithostratigraphic Subunit IA identified from Cores 180-1109A-1H through 180-1109C-4H (0-35.9 mbsf) (see "Subunit IA").
Domain II comprises weakly indurated, massive greenish clay-rich silts of lithostratigraphic Subunit IB (see "Subunit IB"). In Sections 4H-3 through 9H-7 (29.4-83.6 mbsf), these sediments are deformed into a series of fold structures that are clearly outlined by alternating, variously colored bands. The color variation is probably related to diagenetic processes and does not always indicate primary lithologic changes. However, some of the bands are also marked by changes in grain size, and these are likely to represent deformed bedding.
The geometry of folds can be inferred either by direct observation of core faces or by reconstruction from successively opposing bedding dips within single coherent core intervals.
Both the wavelength and amplitude of folds are typically tens of centimeters in size, when measured along two successive hinge zones and following classical fold criteria (Ramsay and Huber, 1987), as observed in intervals 180-1109C-5H-2, 110-150 cm; 5H-4, 75-120 cm; and 6H-6, 50-90 cm.
The profiles of folds range from large open structures, generally characterized by a flat-lying axial surface (intervals 180-1109C-5H-2, 120-150 cm; 5H-6, 80-90 cm; and 9H-5, 15-35 cm) to tight hinge zones, usually exhibiting moderate to steeply dipping axial planes, in addition to a well-marked fold asymmetry (Sections 5H-3, 5H-4, and 6H-6). Within the entire folded sequence, the sediments generally dip between 30º and 60º (Figs. F50, F51). However, a pronounced shallowing of the bedding attitude is observed in the lowermost part of the domain (Core 9H; 73.9 mbsf), in association with a progressive decrease in fold frequency downward from Cores 5H to 9H. The bedding dip values measured in Domain II are consistent with moderately plunging fold axes.
Most of the geometrical features of the 47-m-thick (36-83 mbsf) folded sequence described above, such as the weak induration of the sediments, or the moderate plunging attitude of the fold axes, are typical of soft-sediment deformation commonly generated either within unstable overpressured sedimentary sequences or over active seismogenic submarine areas. Folding, as well as fracturing, could also be generated as core-induced deformations. However, such an explanation is unlikely for the folds in Hole 1109C, mainly because the wavelength of the folds, as measured in a horizontal plane perpendicular to the core axis, is considerably greater than the diameter of the drill bit, and, therefore, the folded interval zone must be of substantial width.
Domain III extends from Core 10H (83.4 mbsf) through Core 41X (376.6 mbsf) at the base of Hole 1109C. It comprises well-bedded silt/claystones and minor sandstone alternations, forming lithostratigraphic Units II, III, IV, and V (see "Lithostratigraphy"). These sediments are horizontal and undisturbed, without evidence of tectonic deformation (Figs. F50, F51).
However, more indurated and dominantly silty sequences occurring from Core 20X (179.3 mbsf) downward are deformed by two types of structures that are confidently attributed to coring disturbance.
The first type of such deformation occurs in the intervals of Cores 20X to 22X (179.3-198.5 mbsf) and 26X to 31X (237.0-294.8 mbsf). It comprises a dense network of open conjugate fractures, dipping at about 45º with respect to the vertical axis of the core. Because most of these fractures typically do not show evidence of displacement and fade away laterally to a tip zone that does not reach the core liner, they are likely to have originated from brittle failure, under vertical maximum stress, and parallel to the drilling axis. However, a few individual fractures cutting the core face at higher angles (~60º) and with a more planar and longer extent (e.g., intervals 180-1109C-28X-2, 100-110 cm, and 29X-3, 120-130 cm) may be in situ fracture/fault surfaces, as confirmed locally by the observation of steeply plunging striations. These structures may account for the inclined features identified in the on board FMS images along the two intervals 2466-2476 meters below rig floor (mbrf) (255-265 mbsf) and 2483-2484 mbrf (272-273 mbsf).
The second type of coring-induced deformation is observed in regular decimeter-sized silt/clay alternations in the interval from Cores 37X to 39X (343.1-362.5 mbsf). Within this interval, clay interbeds systematically form thin depressed zones that separate more resistant dm-thick intervening silt layers, resulting in a core-scale boudinlike pattern that indirectly outlines the general bedding of the cored section. This "pinch and swell" geometry can be assigned to rotation of more competent silty material along clay-rich interlayers that have acted as horizontal layers of decoupling during the drilling process.
The structural zonation established in Hole 1109D is dominated by four main domains whose sharp boundaries seem to be closely controlled by lithology. This is supported by good correlation between the lithologic and tectonic logs (Fig. F52).
The section cored is undeformed over most of its length, except along three significant extensional fault zones confined to the top (FZ1), and to the lowermost part (FZ2 and FZ3) of Hole 1109D.
Domain IV occupies the largest part of Hole 1109D, extending ~324 m from Core 1R (352.8 mbsf) to 35R (676.6 mbsf). It mostly corresponds to the sandstones of lithostratigraphic Units V and VI. The consistently flat-lying attitude of bedding has an average dip of 0º-10º (Figs. F52, F53).
The only evidence of brittle deformation consists of discrete fault planes that are moderately to steeply inclined and cut Sections 2R-1 and 2R-2 (360.4 mbsf). In Figure F52, the latter are referred to as Fault Zone 1 (FZ1). The observed slickenside lineations typify pure dip-slip displacements that are related to extensional strain, according to the offsets identified on core faces (intervals 180-1109D-2R-1, 120-130 cm, and 2R-2, 30-35 cm; Fig. F54). In this fault zone, some of the normal faults are arranged as conjugate structures, dipping at 55º, and consistent with a vertical maximum stress (interval 180-1109D-2R-2, 25-65 cm; Fig. F54).
Exceptionally, one reverse fault is documented in Section 2R-1, 115-120 cm, from its steplike morphology, but the subsidiary fault array developed in its footwall still shows extensional kinematics (Fig. F55). Furthermore, one of the steep fractures present in the footwall of this fault zone is seen to be filled with brown sandy material that passes downward into a 1-cm-thick sandy layer, hence suggesting that some of the brittle faults of FZ1 may have nucleated along earlier open fractures initiated during the deposition of lithostratigraphic Unit V.
Domain V represents a deformed zone, ~25-30 m thick, from 671.6-690.5 mbsf (Sections 35R-5 to 37R-3; Fig. F52). It comprises an intermediate fault zone (Subdomain Vb) bounded on both sides by drilling-induced disturbed zones (Subdomains Va and Vc).
The top of Subdomain Va is located a few meters below the contact between the sandstones of lithostratigraphic Unit VI and the silty claystones of lithostratigraphic Unit VII. Its base is at the bottom of Section 35R-8 (676.3 mbsf). Its main structural features consist of a network of vertical fractures, 10-40 cm long, that penetrate only half of the thickness of the cores, but which cause the dislocation of the core pieces into elongated fragments.
Subdomain Vb represents an extensional fault zone (FZ2), 7-8 m thick, extending throughout Core 36R (676.4 mbsf). Its structural fabric is dominated by a system of discrete and planar fault planes limiting coherent core pieces, 10-100 cm long. Their average dip is moderate and clusters at ~45º (Fig. F56). The entire slickenside population indicates pure dip-slip movements and the indicators of sense of displacement, such as stretched shells (interval 180-1109D-36R-2, 135-140 cm) or steplike morphologies (interval 180-1109D-36R-3, 103-108 cm), consistently indicate extensional faulting.
Subdomain Vc is confined to Core 37R (686.0 mbsf). It is composed of individual pieces of silt/sandstones, a few centimeters long, that probably result from the dislocation of a dominantly sandy interval by drilling-induced rotations along bedding planes, similar to Subdomain IIIb of Hole 1109C discussed in "Structural Domain III."
Domain VI includes the lower part of lithostratigraphic Unit VIII down to Unit X (Cores 38R to 45R; 695.6-767.0 mbsf). The almost horizontally layered silt/clay sediments that compose its upper part (Fig. F52), as well as the individual clasts and pebbles involved in its poorly recovered basal part (see "Lithostratigraphy"), show no evidence of brittle deformation.
Domain VII is represented by a fractured dolerite, ~50 m thick, occurring between Section 45R-3 (766.2 mbsf) and the base of Hole 1109D (802.0 mbsf; Fig. F52). The dolerite is crosscut by a moderately dense network of filled veins and fractures whose mutual spatial and time relationships can be used to constrain the deformational history experienced by the intrusion since its emplacement.
The vein network is infilled with various secondary minerals that were identified by use of hand lenses, microscopic observations, and XRD analyses. They include, in decreasing order of occurrence, greigite (iron sulfide), natrolite mixed with smectite/chlorite, cristobalite mixed with smectite/chlorite, and carbonates. These infilling minerals occur, for the most part, separately and within veins showing specific morphologies. The most common mineral, greigite, is present as a fine-grained dark green material, filling a dense system of thin (1 mm) small veins that propagate through the core at shallow angles (Fig. F57A, F57B). These veins commonly show overlapping tip zones (e.g., intervals 180-1109D-47R-1, 85-90 cm, and 47R-2, 55-60 cm; Fig. F57A). Less numerous, dominantly natrolite- and cristobalite-filled veins form wide (>1 cm), linear fissures 10-100 cm long, which are systematically vertical in position (Sections 48R-1 [782.5 mbsf] and 49R-3 [789.8 mbsf]). Cristobalite-filled veins post date natrolite-filled ones (interval 180-1109D-49R-3, 0-25 cm). Locally, some of the master vertical veins connect via shorter and flat-lying cristobalite/natrolite-filled veins to form extensive small-scale relay ramps, and at some localities, minor steeply dipping greigite-filled veins are laterally offset by shallow cristobalite veins.
The composite vein pattern crosscutting the dolerite intrusion in Hole 1109D can be regarded as extensional fissures that were probably formed during a postmagmatic alteration process, possibly of hydrothermal origin. Given the consistent orientation of the resulting filled fissure network, with two main sets of fissures dipping at right angles, the vein pattern is likely to have originated under horizontal tensile stress perpendicular to the wall of the steep master veins. Such a kinematic arrangement is in agreement with the existence of horizontal-shearing features commonly observed parallel to the subsidiary shallow veinlet system (e.g., intervals 180-1109D-49R-1, 35-40 cm, and 53R-3, 52-57 cm; Fig. F57B). A large number of both steep- and shallow-dipping veins are highly striated as the result of subsequent reactivation as fault planes. Additional (newly formed?) fault surfaces also exist, usually coated by carbonates and more typically by millimeter-sized cubes of pyrite.
The histogram shown in Figure F58A reveals that the fault dip dataset comprises three main peaks around 0º, 20º-30º, and 60º. The frequency of the shallowest fault population increases markedly toward the base of the drilled section, within Cores 49R through 51R. The majority of the observed slickensides indicate dip-slip displacement (Fig. F58B). Because clear crosscutting relationships exist between kinematically contrasting faults, it is possible to reconstruct a two-dimensional map view of the overall fault pattern, as shown on the diagram in Figure F59C. Where the structures are observed in larger coherent pieces of core, it is possible to infer the direction of extension with respect to the core reference frame (Fig. F59).
To reorient core structures to true coordinates, an attempt has been made to correlate them with FMS images. Special attention was paid to the two more fractured zones (FZ2 and FZ3) because they contain steeply inclined structures that are more easily identified. The upper limit of FZ2 strictly coincides with a pronounced change in the FMS recording at 2888.5 mbrf (678 mbsf) (see "Log Unit L9," "Log Unit L10," and "Log Unit L11"). There we observe a sharp boundary between a relatively poorly layered domain that passes downward into a regularly banded zone about 5 m thick. Within the first meter, these highly conductive layers are closely spaced (interval = <20 cm), and then their frequency diminishes rapidly and disappears below 2893 mbrf (683 mbsf). The structural meaning of the horizontal features seen on FMS images is unclear; no bedding has been observed throughout the corresponding sedimentary core section (Core 36R [676.4 mbsf]). In addition, none of the steep fractures described on cores throughout FZ2 is expressed in the FMS images.
We also note that the structural boundary mentioned above corresponds to an abrupt variation of some physical properties measurements, such as the total natural gamma ray and the porosity, which both increase dramatically close to 675 mbsf, probably in relation to an increase in the clay content (see "Physical Properties"). That variation suggests a control of the grain size of the sediments on the locus of brittle strain.
The FZ3 is expressed on the FMS images with no specific signature, except in the logged interval 2990.5-2997 mbrf (780-787 mbsf), where it is finely banded with mainly horizontal and closely spaced (<10 cm) conductive horizons. Some of them might be assigned to the greigite-filled vein network, which is particularly well developed in the upper part of the intrusion (Sections 47R-1 and 47R-2; see Fig. F60). A few FMS features with a significant inclination occur in the logged interval 2993-2996 mbrf (783-786 mbsf), but we observe neither the vertical vein-filled system nor the steep fault planes (further coated with pyrite) identified on cores. At this preliminary stage of the work, no clear relationships exist between the FMS images and the core measurements.
Relatively little evidence of tectonic deformation is recorded in the sequence drilled at Site 1109 (Fig. F61). This is consistent with the almost continuous and mostly unfaulted character of the sedimentary package imaged by seismic data (see "Vertical Seismic Profile and Depth Conversion"). The most recent deformation is documented in Hole 1109C by a 20-m-thick folded zone involving weakly indurated sediments of Pleistocene age. Given the general rift tectonic setting of the Woodlark Basin, these soft-sediment structures might result from gravity-driven features generated in response to the instability of the basin floor during a syn- or postdepositional faulting event. However, their in situ origin as a megaslide of unconsolidated and overpressured sediments cannot be ruled out.
Brittle faulting and fracturing are restricted to the lower part of the cored section along two main deformed zones located at 678-685 mbsf (FZ2) and 773-802 mbsf (FZ3) (Fig. F61). Their locations, close to major lithologic boundaries between lithostratigraphic Units VII/VIII and X/IX, respectively, suggest that variation in lithologic competency was a significant factor in controlling strain. The fracture and fault patterns of FZ1 and FZ2 are quite similar, both resulting from one phase of extension, which is therefore assumed to be post-middle Pliocene in age. However, one should note that the frequency of brittle fractures increases significantly in the dolerite, but that is probably enhanced by the early disruption of the intrusion during a late magmatic hydrothermal process.