A variety of deformation structures is present at Site 1174, summarized in relation to lithostratigraphy in Figure F11. All the numerical data are given in Table T7. The chief features at the site are (1) deformation bands developed in the upper part of the hole, (2) zones of fractured and, in places, distinctly steepened bedding within a zone of protothrusting, (3) the basal décollement, and (4) the relatively little-deformed underthrust section. The distribution with depth of the main core-scale structures is portrayed in Figure F12.
All the cores retrieved from Hole 1174A, which reached 67.61 mbsf, show approximately horizontal bedding with a distinct fissility in places and a complete absence of deformation.
Horizontal bedding with fissile intervals also typifies Hole 1174B down to 464 mbsf, although there is evidence of some slump folding with associated steepened bedding. However, within this largely horizontally bedded interval, we define a structural domain in which deformation bands are concentrated.
The shallowest example of a deformation band that we recognized, albeit weakly developed, is at 199 mbsf, indicating that tectonic deformation is acting on sediments that have undergone surprisingly little burial and lithification. The main development of deformation bands arises between 218 and 306 mbsf. The structures appear to be best developed in silty claystones that lack bioturbation. They are manifest in the core face as roughly planar dark zones between ~1 and 8 mm across and are commonly in paired, oppositely inclined arrays. Bands can abruptly decrease in width (Fig. F13A) and bifurcate into twin strands that coalesce a few millimeters along the band (Fig. F13B). Such observations are in line with earlier descriptions of deformation bands in the Nankai prism (e.g., Maltman et al., 1993), where at the microscopic scale, the bands have various kinklike and shear-zonelike aspects. On occasion, it was possible with the Site 1174 cores to detect a sense of displacement along the bands; in all the observed cases, this was a reverse sense.
Figure F14 shows the orientations of all the deformation bands we observed. Despite the diversity of orientations of the structures in the cores, paleomagnetic correction (see "Paleomagnetism" in the "Explanatory Notes" chapter) revealed a concentration into two distinct sets, both with a strike of 033°—roughly that expected if the bands are due to prism contraction. The lesser dihedral angle between the two sets is 67°, the bisector of which is inclined 10° from vertical toward the northwest. The few cases where sense of movement was discernible from the bands are consistent with the principal shortening being shallowly inclined and bisecting the obtuse angle between the sets, which is congruent with typical kink-band geometry. A few millimeter-wide planar zones, which we classified as deformation bands, were observed at much deeper levels: 510 and 590 mbsf. Both cases, unusually, are developed in and confined to thin bands of volcanic ash.
The concentration of the majority of the deformation bands between ~200 mbsf and a sheared interval at 306 mbsf is striking. This latter horizon is a fault zone a mere 35 cm thick consisting of foliated breccia with fragments as small as a few millimeters in length and a distinctly inclined fabric (Fig. F15). It therefore contrasts markedly with the little disturbed sediments above and particularly with the virtually undeformed material below. Indications of reverse movement such as small parasiticlike folds imply that the structure may be a thrust. Paleomagnetic reorientation of the fold axial planes (Fig. F16A; great circle labeled 1) and the inclined fractures (Fig. F16 great circle labeled 2.) suggest that the fault is a southeast-dipping feature, possibly a backthrust within the prism. The deformation bands may therefore record pervasive strain in the hanging wall of this fault. An alternative explanation for the localization of deformation bands involves some lithologic influence on their development. At the present site their lower limit approximately coincides with the transition from the trench-wedge to outer trench-wedge facies; at Site 808 of Leg 131, where deformation bands were observed at deeper levels, their lower boundary corresponds to the top of the Shikoku Basin sediments.
The shear zone mentioned above is the shallowest of a number that appear irregularly throughout the section above the basal décollement. Their distribution with depth is depicted in Figure F12. For 157 m below the one already described, there is very little deformation and bedding dips rarely depart from horizontal, although core recovery was relatively poor. Abrupt, therefore, is the 1.5-m-thick zone of fine rubble at 463.0 mbsf, which may or may not be of natural origin. The underlying 2.6-m-interval was unrecovered. Between 467.10 and 469.95 mbsf is an interval of fractured and steepened bedding (Fig. F17), in places vertical, followed by a 6.55-m unrecovered interval and then virtually undeformed subhorizontal beds. A 30-cm-thick zone of shearing at 504 mbsf (Fig. F16C) ends this zone of deformation as the underlying cores, to depths well over 600 mbsf, show approximately horizontal beds with little deformation.
The significance of the above observations, and indeed one underlying reason for drilling Site 1174, is that seismic interpretations show a prism protothrust at this location. In the preliminary seismic depth section, this protothrust is at a depth of ~550 mbsf, yet we saw no deformation in the cores from around that depth. Good core recovery constrains any thrust located at such depths in unrecovered material to have a thickness <5 m. On the basis of the core data, therefore, a protothrust of significant thickness at this site must be represented by the structures between ~463 and 500 mbsf mentioned above. The beds could be steepened by fault-related folding similar to that inferred for the frontal thrust at Sites 573 (Shipboard Scientific Party, 1986) and 808 (Taira, Hill, Firth, et al., 1991), but details at this site remain unclear. The vertical thickness of the fault zone is a maximum of 41 m if all rubble and unrecovered intervals are included, in comparison to the 30 m reported from Site 808.
Further intervals of fractures and brecciation are present between 688 and 807 mbsf, the latter being the top of the décollement (Fig. F12). The appearance of these intervals of broken core is highly variable, and it is possible that those lacking fabric have been artificially induced or that naturally fractured intervals have been enhanced by the coring process. Those zones with a clear alignment of clasts or some regularity of fractures are thought to be natural, but even here their appearance varies. Nowhere are they thick, ranging between 10 and 90 cm. Some cores contain more than one of these zones, separated by intact material, but they are typically many meters apart. Preliminary analysis suggests that some of the fracture fabrics dip toward the southeast; the significance of this remains unclear. The intensified fracturing between 699 and 720 mbsf, sharply bounded at its base and including a northwest-dipping fracture fabric, is shown on Figure F12 as a fault zone. It may correspond to a protothrust apparent at these depths on the seismic section.
Small faults are present throughout Site 1174, but they become noticeably more common between ~495 and 611 mbsf. The faults appear as dark seams no more than a millimeter or two across, commonly braided and distinctly curviplanar to irregular (Fig. F18). Offset markers are infrequent but, where observed, in almost all cases show a normal sense of displacement, typically between a millimeter and a centimeter. The faults are vertical to steeply inclined; any breakage along them reveals slickensides and, commonly, down-dip slickenlines. These structures are more commonly developed than at Site 1173 and less widespread than similar structures at Site 808 and so could be recording either increasing tectonic strain or greater burial. Their apparently random strike after paleomagnetic reorientation (Fig. F19) indicates a lack of tectonic influence. In line with previous interpretations, the faults probably reflect the effects of burial compactional strains on these clayey silt lithologies at this degree of lithification.
The deepest of the brecciated intervals reported above is found at 784 mbsf and is underlain by generally intact sediment with moderate dips as deep as 807 mbsf. Here a distinctive set of planar, inclined fractures divides the cores into trapezoidal blocks (Fig. F20A) that decrease downward in size eventually to be replaced by brecciated material (Fig. F20B). The overall orientation of the fractures is shown in Figures F21A and F21B.
We take the onset of these distinctive, inclined fractures to mark the top of the basal décollement at 807.6 mbsf. The striking feature of the décollement zone is the heterogeneity of the deformation, which in a general way increases downward in intensity. Bedding is irregularly steepened in the zone (Fig. F21C). The topmost 3 m (disregarding unrecovered core) consists largely of the trapezoidal blocks mentioned above, mostly 5 cm and more in length, but the size of the blocks tends to diminish in the underlying 7 m of core until intervals with fragments on the scale of millimeters are common. Six meters or more of unrecovered core separates these foliated breccias from 7 m of much more uniformly comminuted material, with only rare clasts exceeding 1 cm in length and a foliated aspect being variably developed (Fig. F22). The base of this zone contains a few larger blocks but also a 20-cm interval of very finely pulverized material. Just 60 cm below this, a further interval of finely broken sediment is underlain by intact sediment; this contact, at 840.20 mbsf, clearly marks the décollement base. The décollement zone therefore has a vertical thickness at this site of 32.6 m. Because the core containing the base of the structure (Core 190-1174B-73R) was almost completely recovered, any allowance for unseen material would increase this figure by <1 m.
The brecciated aspect of the décollement zone is similar to that reported from Site 808 and contrasts with, for example, that of the northern Barbados prism with its scaly clay and S-C fabrics (Maltman et al., 1997). Nevertheless, there are some differences between the décollement cores from Sites 1174 and 808, such as a 13-m greater thickness at Site 1174. Also, the greater proportion of relatively intact intervals suggests a somewhat more heterogeneous deformation. The meters-long sections of comminuted material reported here were not seen at Site 808, although with the less successful recovery there it is possible such material exists but was simply not retrieved.
The underthrust sediments show little deformation overall, as at Site 808. Between 950 and 1000 mbsf, bedding dips >20° were recorded, and at ~1020 mbsf, a 40-cm-thick zone of shallowly inclined slickensided fractures and nearby bedding dips up to 54° testify to some localized tectonic deformation in these downgoing sediments (Fig. F12). The rest of the section, however, is characterized by subhorizontal bedding. Rare, weakly developed healed normal faults and dewatering features (e.g., Fig. F23) probably record early compaction processes.
Measurements from Site 1174 are presented in Figure F24. Figure F25 provides a further illustration of the fine resolution that is made available by the instrument. It is important to reemphasize, however, that the shipboard results summarized here are semiquantitative measurements based on nitrogen flow in water-saturated sediments.
The sediments in Hole 1174A, reaching only 68 mbsf, were soft and delicate, but the determinations were sufficient to document the contrast in values between the interturbidite silty clays, which measured <10-16 m2, the sandy intervals, which exceeded 10-13 m2, and the thick layers of black sand, which approached 10-11 m2. In Hole 1174B, the sands of the axial-trench turbidites were generally too soft to allow reliable measurement, but two attempts gave measurements >10-13 m2, whereas the silty clays yielded values within an order of magnitude above and below 10-16 m2.
The lower part of the trench sediments, lacking the thick sand intervals, generally showed values ~10-16 m2 or slightly greater, but measurements well over 10-15 m2 were given by siltier horizons and some ash bands. Thicker ash bands show some variation in detail, illustrated in Figure F25. Also yielding values an order of magnitude greater than the normal silty clays were the bioturbated intervals that involve ash fragments.
Below 480 mbsf, in the Shikoku Basin facies, the cores consistently gave measurements that deviate little from 10-16 m2 (apart from a few aberrant values near 600 mbsf that are not understood). This consistency includes the horizons of ash at these depths, which presumably give values less than shallower equivalents because of their greater degree of alteration. It also includes the décollement zone, that is, it applies to those fragments >~3 cm across that were large enough to allow measurement. Such clasts may not, of course, represent the permeability of the intervening, more highly sheared material. The underthrust sediments show some variation in appearance, such as degree of bioturbation and diagenetic alteration to carbonate, but all these materials gave measurements that deviated little from 10-16 m2. In the lowest recovered sediments, values of 10-17 m2 and less were obtained.