Sites 1040 and 1043 penetrated the prism, décollement, and underthrust unit, recovering 371.2 and 150.57 m of core, respectively (Fig. F3). The recovered material is similar at both sites, comprising claystone and silty claystone (Kimura, Silver, Blum, et al., 1997). Deformation within the prism is indicated by a wide range of structures, distinguished both by their morphology and relative distribution throughout the prism. Inclined bedding, microfaults with small (0.5 to 1 cm) offsets, and fracture networks are widely distributed throughout the prism. Fissility, closely spaced fracture networks, brittle-ductile stratal disruption, deformation bands, and incipient scaly fabric tend to be concentrated within certain horizons (terminology used is consistent with definitions within Maltman, 1998). Fluid conduits inferred from geochemical anomalies that may indicate shear zones (Kimura, Silver, Blum, et al., 1997) and structural discontinuities identified from paleontological age reversals are not always coincidental with mesoscopic features. The lower boundary of the prism (i.e., the top of the décollement zone) is arbitrarily placed at the first occurrence of fractures with polished surfaces, typical of incipient scaly fabric.
Scanning electronic microscopic (SEM) and optical observations on undeformed sections within the prism show that the uppermost 80 m of the section consists of high-porosity sediment with only a weak preferred orientation of clay minerals. Below this depth, clay minerals show increasing levels of particle alignment. Given that depth is the only variable in these relatively homogeneous sediments (only thin lenses of more silt-rich horizons were encountered), clay orientation has arisen primarily in response to gravity induced compaction. Core-based physical properties and logging while drilling (LWD) porosity and bulk density measurements show a gradual decrease in porosity from ~65% to 55% and are in good agreement with microscopic observations. Similar porosity/depth trends have been observed at other convergent margins, such as Cascadia (Westbrook, Carson, Musgrave, et al., 1994).
Deformed horizons are characterized by three principal modes of deformation: kink bands, deformation bands, and small, discrete faults. Kink bands sharply deflect the primary sedimentary fabric. Deformation bands is a general term used to describe planar zones with an appreciable thickness where clay minerals are approximately parallel to the edges of the deformed zone. Although appearing brittle at the macroscopic scale, they maintain a distinctly ductile appearance at the electron-microscopic scale. Faults have a similar appearance to deformation bands but tend to be more localized and brittle, with well defined offsets even at the hand-specimen scale.
The relative timing of each type of deformation structure can be ascertained by crosscutting relationships. Kink bands are the earliest structures formed but are neither abundant nor well developed. They are indiscernible at the macroscopic scale and microscopically occur only at depths greater than 150 meters below seafloor (mbsf). In addition, they tend to localize at high angles (between 40° and 70°) (Fig. F4) to regions where a strong compaction fabric has developed. External kink-band measurements are relatively high (40° to 55°), with internal angles between 40° and 50°. The kink band morphologies at this margin are similar to those observed in the Nankai prism (Suppe, 1985; Byrne et al., 1993) and are indicative of progressive porosity loss through consolidation. The fact that they do not form until at least 150 mbsf suggests that overburden pressure must exceed a critical value before they can nucleate and grow.
Deformation bands displace kink bands and thus were formed at a later stage. They represent the most common features observed in the entire prism, particularly at Site 1040. The length and width of deformation bands vary but do not exceed 5 cm and 2 mm, respectively. Internal structures of the deformation bands are ubiquitously characterized by clay mineral alignment parallel to the shear-zone wall. The overall geometry of the deformation bands varies from numerous networks of bands to singular, thick shear zones (Fig. F5). Where deformation bands form parallel to bedding, they tend to be thicker (0.1 mm) and longer (1 cm) than where they crosscut obliquely to bedding fabric. crosscutting relationships within individual deformation bands suggests that increased levels of strain occur through progressive thickening of individual zones. Below 200 mbsf, zones of deformation bands are so numerous that the sediment has two clay mineral orientations, one due to the compaction fabric and the other due to the shear zones. The intense grain alignment has reduced the porosity relative to surrounding, undeformed sediment, suggesting that the overall mode of deformation was compactional in nature. Reverse kink bands and oblique deformation bands may result from rotation of the principal stress component from vertical at the reference site to subhorizontal within the prism.
In contrast to the deformation bands, faults are narrow, discrete discontinuities that are dark red in color (Fig. F6). They crosscut both kink bands and deformation bands and are dominantly 0.01 mm wide and <1 cm in length. Faults are commonly oriented at a high angle to the preferred clay mineral orientation and show a wide spectrum of dihedral angles. SEM observations show that the fault planes are frequently polished and lineated. X-ray energy dispersive spectroscopy analyses on the fault surface do not record any chemical differences between the fault and surrounding sediments that can explain the difference in color, although the technique is not sensitive to any changes in the oxidation state of elements (e.g., Fe). Two shear zones at Site 1040 did contain authigenic barite crystals that, like Barbados (Labaume et al., 1997) are thought to be indicative of fluid flow.
Structural features observed within extracted cores were found to coincide well with geochemical, physical properties, and micropaleontological data, constraining the décollement within a well-defined zone of 38.6 m thickness at Site 1040 and 9 m at Site 1043. Mesoscopic observations reveal a heterogeneous distribution of brittle and ductile deformation features, which gradate from a more brittle upper part to a ductile lower part (Fig. F7). At Site 1040, the brittle domain is 24.2 m thick and occupies 63% of the entire décollement thickness.
In the brittle domain, fracture networks disseminate the cores into lenticular, blocky fragments on the millimeter to centimeter scale. Anastomosing, discontinuous, and interpenetrative fractures represent the most disrupted sediments. Some fracture surfaces are polished, and a few, thin horizons of incipient scaly fabric have been recovered. Several millimeter-scale veinlets filled with calcite and rhodocrosite also were recovered.
The ductile domain consists of plastic, silty clay that has suffered intense drilling disturbance, which precludes structural interpretation.
SEM observations from within the brittle domain of the décollement indicate a high level of heterogeneity in the grain size of the sediments, in the deformation style and the clay mineral fabric (Fig. F8). Siltier zones are arranged in laminae or lenses surrounded by clayey material that have experienced sporadic incipient cementation and smectite authigenesis. More clay-rich domains are characterized by poorly oriented smectite with incipient carbonate cementation interspersed with areas of well-developed preferred orientation.
Although the brittle part of the décollement maintains a similar appearance with increasing depth, there is an increase in intensity of fractured sediment toward the lower part of the décollement. Deformation is constrained to more clay-rich horizons as irregular anastomosing bands, whereas adjacent areas remain undeformed, indicating extreme strain localization. Incipient scaly fabrics are formed in more clay rich lithologies that are slightly cemented and are characterized by lineated surfaces with some or no polishing. Small aggregates of rhodocrosite crystals also are present in carbonate-poor regions, testifying to fluid flow.
Given the minor amounts of accretion, the underthrust section at both Sites 1040 and 1043 represents the same stratigraphic sequence examined at Site 1039. Clayey diatomites (Unit U1, mid- to late Pleistocene in age) overlie silty claystones (Unit U2, late Miocene to mid-Pleistocene in age), which in turn overlie nannofossil-rich chalks (Unit U3, middle to late Miocene in age). The base of the sequence is marked by a pyroxene gabbro, which contains a K/Ar datum of approximately 15 m.y. (J. Griffin, pers. comm., 1998).
Underthrusting results in a progressive increase in overburden pressure as the overlying wedge thickens. The response of each unit to burial can therefore be quantified by comparing the respective physical properties at each site. For instance, at Site 1040, Unit U1 records a volume loss of 34.73%, Unit U2 has suffered 14.66% volume loss, and Unit U3, 58.92% (Fig. F3) (Kimura, Silver, Blum, et al., 1997). The deformation style shows an abrupt change between the bottom of the décollement and the upper part of the underthrust unit. The plastically deformed lower section of the décollement is replaced by largely undeformed to occasionally brittle deformed sediments in which primary sedimentary fabrics such as burrows are well preserved. The faults are morphologically different from within the prism, displaying discrete, narrow displacements with no ductile component. Where offsets can be discerned, they commonly show a reverse sense of displacement and may form from partitioning of the compressive stress component prevalent in the prism and décollement into the upper part of the underthrust section. With increasing depth, the number of faults decreases and shows geometries similar to those faults observed at the reference site. If the dominant stress orientation at the reference site is vertical, then extensional structures seen both at Site 1039 and within underthrust sediments attest to early differential compaction and may indicate structures arising from the initial flexure of the oceanic crust as it enters the subduction zone. Furthermore, the décollement represents a major structural discontinuity that divides dominantly contractional deformation features above in the prism (implying near horizontal principal stress) to vertical stress within the underlying underthrust units. Such efficient decoupling has been noted at other margins from palaeomagnetic measurements (Housen et al., 1996). Deformation becomes more intense in the vicinity of the gabbro intrusion encountered at the base of Site 1040, where layer-parallel extension structures such as pinch and swell and boudinage occur.
Compaction within the upper part of Unit U1 is largely taken up by draping of clay minerals around more rigid silty grains. Porosity loss occurs primarily by reorientation and more intensely aligned grains than seen in equivalent horizons at the reference site. Occasional layers show anastomosing and discrete deformation bands that contain silt grains oriented parallel to the band edges. The lower part of Unit U1 contains undeformed pyrite-filled fractures and framboids.
Unit U2 is markedly less deformed than unit U1 but contains sporadic well developed faults. U2 faults can reach several centimeters in length and are detectable mesoscopically. They are characterized by strong clay orientation and lineated surfaces. The intensity of deformation fabrics increases with depth, where occasional fracturing of microfossils can be observed. The majority of Unit U3 is undeformed, in contrast to the vicinity of the gabbroic intrusion, where porosity is markedly lower and all mineral grains are intensely aligned.
Quartz and silica recrystallization occurs sporadically throughout the underthrust section (Fig. F9) but shows no prevalence for particular horizons. At Site 1039, recrystallization occurs at 375.94 mbsf, in contrast to Site 1040, where no recrystallization was observed above 411.7 mbsf. Silty grains are commonly surrounded by calcite, which can form a patchy, incipient cementation.