The 802.5-m-long middle Miocene to Pleistocene sedimentary succession cored at Site 1115 is relatively undeformed. The only evidence of deformation is in the lowermost part of the section where three discrete fault zones alternate with undeformed zones. Undeformed zones, referred to Domain I, extend from the seafloor to 551.8 mbsf (Subdomain Ia), from 566.1 to 603.85 mbsf (Subdomain Ib) and from 689.8 to 724.9 mbsf (Subdomain Ic). Domain II consists of three fault zones that form Subdomains IIa (551.8 to 566.7 mbsf), IIb 609.4 to 696.5 mbsf) and IIc (725.4 to 802.5 mbsf). They are labeled FZ1, FZ2, and FZ3 in Figure F30. In general, bedding dips are very shallow, with a mode between 0º and 5º. The few steep dips (50º and 60º) shown in Figures F30 and F31A are inferred to be mostly related to soft-sediment deformation. The fault network identified at Site 1115 displays a relatively large range of dips, as illustrated on the histogram of Figure F31B, which shows three maxima at 45º-50º, 70º-75º, and 85º-90º, respectively.
Subdomain Ia extends from the seafloor to 551.8 mbsf and is composed of unconsolidated and weakly indurated late Miocene to Pleistocene oozes and silty clays of lithostratigraphic Units I to VII (see "Lithostratigraphy"). Bedding is generally horizontal (Fig. F30), but some gentle dips up to 10º are locally observed. At interval 180-1115B-17H-3, 121-125 cm (153.91-153.95 mbsf), strata are exceptionally inclined at 15º, probably as the result of slope instability. Additional evidence is suggested by a minor high angle (75º-80º) normal fault filled with greenish clayey material, and inferred to be an early compaction-induced feature (Section 180-1115B-18H-2 [160.7-162.2 mbsf]).
In Core 180-1115B-29X (264.3-273.9 mbsf), the silty material becomes more lithified, and exhibits drilling-induced brittle deformation especially in Cores 180-1115B-29X to 180-1115C-9R (264.3 to 369.5 mbsf). Two types of coring-induced deformation are observed. Steep individual fractures ~10 cm long, cut through the central part of cores. In addition boudinlike structures are formed by regularly spaced (5-10 cm) silty pieces separated by more clay-rich horizons along which rotation has preferentially occurred during coring. This coring-induced fabric is in the interval from Section 180-1115C-1R-2 to Core 11R (284.6-388.5 mbsf), and is particularly well developed in Section 8R-2 (351.4 to 352.9 mbsf).
Subdomains Ib and Ic are located within the middle Miocene forearc sequences forming lithostratigraphic Units X and XII, respectively, although the former straddles the major unconformity identified in Section 180-1115C-31R-1 at 571.87 mbsf (see "Lithostratigraphy").
Subdomain Ib extends from 566.01 to 603.85 mbsf (Sections 180-1115C-30R-5 to 34R-3), and is characterized by almost horizontal bedding, dipping from 0º to 7º-8º (Fig. F30). The highest dips (9º-20º) measured in Section 180-1115C-34R-3 correspond to slump structures marked by thin layers of medium- to coarse-grained sandstones injected into underlying clays (see "Lithostratigraphy"). Evidence for fracturing is documented in intervals 180-1115C-31R-1, 90-95 cm; 31R-3, 8-10 cm; and 31R-CC, 0-5 cm, by very few mm-thick calcite-filled veins lying in either horizontal or vertical positions. In addition, Section 180-1115C-31R-4 is cut throughout most of its length (interval 180-1115C-31R-4, 15-95 cm) by a vertical, 1-mm-thick quartz-filled vein.
This subdomain is present from 689.8 to 724.9 mbsf (Sections 180-1115C-43R-3 to 46R-7). Bedding is nearly horizontal everywhere with only a few beds dipping between 0º and 12º (Fig. F30). Weak brittle deformation is expressed by either minor steeply dipping normal faults or coring-induced structures (boudinlike features in Section 180-1115C-43R-3 [689.8 to 691.0 mbsf]).
Subdomain IIa represents a discrete fault zone (FZ1 in Fig. F30), extending from Sections 180-1115C-29R-3 to 30R-4 (554.3-565.7 mbsf) and corresponds to lithostratigraphic Unit VIII (see "Lithostratigraphic Unit VIII"). Ten minor faults, including three shallow dip-slip slickensided fault surfaces, exhibit dips ~20º-40º. Figure F30 shows that bedding remains nearly horizontal throughout FZ1, with no marked change with respect to the two bounding undeformed zones. The few moderate bedding dips (20º) measured in interval 180-1115C-30R-3, 10-40 cm, are bounded on top and bottom by horizontal strata, and they are, therefore, assigned to local soft-sediment deformation.
This domain (FZ2) constitutes the most significant fault zone at Site 1115 (Fig. F30) and extends from 609.4 to 696.5 mbsf (Cores 180-1115C-35R to 43R). Within the fault zone, bedding is still nearly horizontal (Figs. F30, F32A). A few steep beds, dipping at 50º-60º, are present in interval 180-1115C-43R-2, 45-100 cm, and because their overlying and underlying strata are horizontal, they may represent intraformational slump features. Soft-sediment deformation is also observed in interval 180-1115C-40R-4, 90-100 cm.
The fault system of Domain IIb displays almost the same dips as the total fault population, with a mode of 40º-60º with another peak at 90º (Fig. F32B). Some shallow-dipping faults are occasionally associated with scaly fabrics in fine-grained silts, as in interval 180-1115C-41R-3, 135-140 cm. Most of the steepest fractures are usually filled with fine-grained material similar to the clay-rich host rock in interval 180-1115C-43R-2, 50-90 cm. They are, therefore, inferred to be early brittle structures formed during sediment compaction. The histogram of Figure F32C illustrates the composite nature of the Domain IIb fault network, including dip-slip, oblique-slip, and strike-slip faults. Dip-slip faults are dominant, with either extensional or reverse sense of motion. In intervals 180-1115C-40R-4, 40-50 cm, and 42R-3, 90-130 cm, the reverse displacements systematically take place along almost vertical and sigmoidal surfaces, in close association with normal faults. Since steeply dipping faults are likely to be early structures, generated during compaction, the reverse motions are not interpreted as true compressional structures, but are the result of slightly varying dip angle of the nearly vertical curved normal fault.
Subdomain IIc forms the third fault zone within Cores 180-1115C-47R to 54R (725.4-802.45 mbsf). Bedding dips are similar to those of the above-mentioned structural domains, with a mode between 0º and 10º, and maximum values between 15º and 28º (Figs. F30, F33). Faults are generally steep, dipping on the average of 70º-90º, and they include both strike-slip and pure dip-slip structures. Most of the dip-slip faults are extensional; however, reverse structures are present in intervals 180-1115C-47R-6, 95-103 cm, and 48R-2, 55-60 cm, with offsets a few millimeters long. The reverse motions can be explained in the same way as those discussed above.
The comparative analysis of the histograms of Figure F34 clearly shows that there is no major contrast in bedding dips on both sides of the intra-Miocene unconformity, which is in accord with the local strata pattern imaged by reflection seismic (see "Vertical Seismic Profile and Depth Conversion").