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doi:10.2204/iodp.pr.343343T.2012

Principal results/Site summaries

Although severe weather and failure of various equipment delayed operations at Site C0019, nothing occurred that would require abandonment of the primary goals of the expedition and shifting to contingency sites. Accordingly, all drilling and coring activities occurred at IODP Site C0019 (Fig. F3) for the duration of the expedition. During JFAST (Expedition 343) in April and May 2012, the LWD/MWD pilot was drilled in Hole C0019B to a TD of 850.5 mbsf (Fig. F4). In preparation for deploying the autonomous temperature measurement observatory, a wellhead and 20 inch casing were successfully set in Hole C0019D. Finally, the coring hole was successfully completed in Hole C0019E where spot cores were acquired from various depths to the TD of 844.5 mbsf. The LWD/MWD and coring holes were blind-spudded with an estimated 5 m of separation at the seafloor. During JFAST II (IODP Expedition 343T) in July 2012, the wellhead in Hole C0019D was successfully reentered to extend the borehole to a TD of 854.8 mbsf (Fig. F4). Subsequently, the 4½ inch casing containing the miniature temperature logger (MTL) autonomous observatory was installed and the observatory was completed (Table T1).

Site C0019

Geophysical logging

Hole C0019B was drilled to 850.5 mbsf, and the entire hole was successfully logged using a LWD/MWD assembly. The logging tools used during Expedition 343 include two of Schlumberger’s VISION series tools, namely geoVISION and arcVISION, in addition to their MWD TeleScope tool. Four logging units were defined on the variability of log responses (Fig. F5). In the upper 800 m of the borehole, in logging Units I and II, the overall responses of both gamma ray and resistivity increase with depth. In the gamma ray log this trend is broken by significant low values that occur at 168 and 535 mbsf. There is evidence of potential cyclicity in the resistivity log response, and the resistivity-at-the-bit (RAB) images also show multiple zones transitioning from darker, more conductive layers to lighter, more resistive bands with depth. Similarly, the abundance of borehole breakouts is variable with low occurrences below 230, 470, and 730 mbsf. The patterns described above could infer sediment packages of different lithologies or an overall homogeneous sediment unit cut by thrust faults (producing repeated sediment packages), or a combination of these. The gamma ray and resistivity log responses from the upper 800 m of Hole C0019B compare favorably to those of diatomaceous muds that were recovered and logged at neighboring ODP Sites 1150 and 1151 ~100 km away (Sacks, Suyehiro, Acton, et al., 2000) and at DSDP Sites 436 and 434 (Shipboard Scientific Party, 1980a, 1980b).

Logging Unit III (820.6–835.9 mbsf) is defined by a significant increase in gamma ray intensity relative to the units above that remains high for 16 m before dropping sharply (Fig. F5). RAB images also indicate this unit is more conductive compared to the units above and below. Such log response could indicate a clay-rich unit, possibly similar to the brown clay unit recovered from Leg 56 Site 436 (Shipboard Scientific Party, 1980b). Logging Unit IV (835.9–850 mbsf) is characterized overall by low gamma ray and high resistivity values. RAB images primarily show regions of very high resistivity in layers and patches throughout this logging unit. The resistivity and gamma response is consistent with lithologies such as chert, and RAB images may indicate some interbedding of chert and a more conductive material. Chert was also observed below the brown clay at Site 436 (Shipboard Scientific Party, 1980b).

Bedding and fracture occurrence and orientation were determined from borehole resistivity images (Fig. F6). Three structural domains may be defined on the basis of bedding dip distributions: (1) upper frontal prism (0–275 mbsf) with more gently inclined bedding, (2) lower frontal prism (276–820 mbsf) with variable and steeply dipping beds, and (3) base section (820 mbsf to base of hole) with shallow to horizontal bedding. The poles-to-bedding in the lower frontal prism are consistent with cylindrical folding with a mean axial trend of 030°, which is about 20° northeast of a line parallel to the regional slope and nearly normal to the convergence direction of 292° (DeMets et al., 2010; Argus et al., 2011). Structural Domain 1 shows sparse fractures, whereas fractures are more numerous in Domains 2 and 3. Similar to bedding, the strike of fractures displays a preferred northwest orientation. A zone of enhanced fracturing occurs around a significant conductive feature at 720 mbsf. The feature is shown as a large negative excursion in the standard logs and a dark band in the RAB images, which may reflect a zone of faulting (Fig. F5). This feature was regarded as a potential site of the Tohoku earthquake rupture and was targeted for coring; the feature is hereafter referred to as the 720 fault.

Another probable fault was identified at 820 mbsf between structural Domains 2 and 3 and between logging Units II and III, hereafter referred to as the 820 fault. The structural transition resembles that of a classic décollement, with variably dipping beds above and shallow dipping beds below that are sitting above the basaltic crust (Fig. F6), and interpreted as a brown clay layer and chert characteristic of the incoming oceanic plate of the north Pacific Ocean (Shipboard Scientific Party, 1980b).

Breakouts were identified in the image logs of Hole C0019B and are found to occur over a broad depth range in logging Units I and II and structural Domains 1 and 2 (Fig. F7). Breakout distributions display preferred orientations that vary with depth. The mean azimuth of breakouts in the deeper portion of the hole (537–820 mbsf) is 049° and the standard deviation is 23°. Thus the maximum horizontal compressive stress orientation can be interpreted as 319° ± 23° at the base of the prism. At immediate depths, the breakouts are less frequent and orientations are highly variable. In the shallower portion of the borehole, the orientation of breakout azimuth progressively rotates clockwise down to 140 mbsf and then rotates back counterclockwise, suggesting the presence of a discontinuity, perhaps a fault. This is supported by a conductive peak in the resistivity logs. Changes in bedding dips are also consistent with the presence of a discontinuity at this depth.

Downhole measurements and observatory

No observatory components were deployed during the main portion of Expedition 343; however, some downhole observations were made to measure temperature and pressure using 1–3 of the MTLs (TDR2050) placed in the inner core barrel of the RCB coring assembly. Deplugger operations typically use the inner core barrel with a special center adaptor to clear the RCB bit; we used the inner barrel to hold the MTL sensors for in situ pressure and temperature measurements. The inner core barrel containing the instruments was deployed by a free-fall release down the drill pipe in a procedure that was developed onboard during the expedition. Three different measurement runs were completed on 12 May, 16 May, and 22–24 May.

Temperature measurements made within the borehole were highly affected by drilling operations, particularly measurements recorded during pumping and circulation. Although pumping and circulation was stopped prior to making the third run, we did not expect the borehole temperature to have equilibrated by the time measurements were taken, and this was confirmed by observations that the temperature changed with time even though the pipe was fixed in position.

Pressure measurements showed significant fluctuations, reflecting changes in depth of the instrument with time. The dominant signal indicated cyclic changes in depth consistent with ship heave. However, additional pressure variations were recorded that implied depth variations of much greater magnitude than the heave. It was determined that these large variations in pressure (depth) reflected vertical oscillations that were triggered by rapid accelerations of the drill stem or core line at the ship, as occurs for example during addition or removal of pipe stands and retrieval of the inner barrel holding the MTL sensors. This effect is magnified during free hanging of the extremely long drill stem and core line that were employed for the deepwater operations of this expedition.

During Expedition 343T in July, the autonomous temperature observatory consisting of MTLs was successfully deployed in Hole C0019B (Fig. F8). The instrument string of 55 MTLs was suspended in 4½ inch pipe from a hanger at the wellhead. Three types of MTLs from two different manufacturers were used, which differed in data storage volume. Also, some of the instruments included pressure sensors for accurate estimates of water depth. The sensors are designed to monitor the changing temperature at sample intervals of 10 s to 10 min over several months until their retrieval, which is scheduled for October 2012 or February 2013. Also, two of the instruments were programmed to turn on at high sample rates during the time of the anticipated retrieval, in order to obtain a temperature profile during pullout. The MTLs are distributed over about 550 m with dense spacing of 1.5 m near the 820 fault. Because of the possibility of postseismic slip on the fault that may deform the borehole and cut the instrument string, weak links that can break at designated positions were interspersed along the instrument string to help minimize loss of MTLs.

Lithology

As a result of time constraints, a total of 21 coring runs at four depths were completed. Lithologic Unit 1 was sampled by a single spot core taken from the 176.5–186.0 mbsf interval (Fig. F9). The core consists of a medium olive-gray siliceous mudstone with secondary lithologies including isolated, few millimeter to 2 cm thick layers of yellowish gray ash and dark gray to black, millimeter-scale laminations with gradational contacts. The siliceous mudstone is dominated by abundant clay- to silt-sized siliciclastic material and includes abundant siliceous microfossils (diatoms, sponge spicules, and radiolarians) and common ash shards.

Lithologic Unit 2 was sampled by two coring runs taken from the interval 648.0–660.5 mbsf. Recovered sediment comprises bluish gray ashy mudstone and grayish brown ashy mudstone, both of which are heavily brecciated and thoroughly mixed throughout by drilling and recovery processes. Both mudstones contain dominant siliciclastic grains, common volcaniclastic grains, and biogenic fragments that are present and occasionally common.

The sediments of lithologic Unit 3 were relatively well sampled from 13 coring runs covering intervals 688.5–729.0 and 770–821.5 mbsf (Fig. F9). Unit 3 comprises four dominant lithologies, an olive-brown-gray ashy mudstone, a dark gray mudstone with black interlayers, a clay-rich mudstone, and a dark gray mudstone with abundant pyrite. These units are interbedded on a meter scale with occasional centimeter-scale interlayers and intermittent centimeter-scale clay and silt beds. All lithologies are dominated by siliciclastic material and contain trace or present siliceous microfossils and fossil fragments, with rare horizons containing up to 15% siliceous fragments. Ash is abundant in the ashy mudstone, but otherwise ash layers are rare and ash content decreases significantly downsection below the ashy mudstone. X-ray diffraction data show the overall quartz, feldspar, and clay content of Unit 3 is generally consistent throughout the section except for discrete clay and silt beds. Unit 3 has a much higher component of siliciclastic material and a lesser component of volcanic and siliceous grains than Unit 1. Unit 3 contains none of the grayish brown or bluish gray ashy mudstones of Unit 2. Unit 3 reflects a more terrigenous environment of accumulation than Units 1 and 2 and a time of lesser accumulation of siliceous microfossils and increased distance from, or lesser activity of, volcanic sources.

Unit 4 occurs in only one core taken from the interval 821.5–824.0 mbsf. Unit 4 consists of strongly deformed clays that are pervasively sheared, forming sharp strongly aligned phacoids with polished, striated surfaces defining a scaly fabric. Two main components can be recognized from visual inspection and smear slide analysis: a red-brown clay and a dark brown to black clay. The reddish brown clay is composed predominantly of clay minerals with rare coarser grains and vitric grains, whereas the dark clay is composed mainly of dark red-brown clay minerals probably mixed with Fe or Mn oxides/hydroxides. Sections 0.5–4 cm thick of reddish to dark brown and black clay alternate at the top of the core. A 13 cm section of gray mudstone is enclosed inside the clay. Downsection from the mudstone interval, dark brown to black clay is dominant. The foliation defined by the scaly fabric varies from horizontal to ~25° inclination. The upper and lower contacts of Unit 4 with Unit 3 above and Unit 5 below were not captured in the recovered core section.

Unit 5 is a yellowish to grayish brown mudstone composed of dominant siliciclastic grains, minor volcaniclastic grains, and trace siliceous microfossils (radiolarians). The siliciclastic component of the rock is predominantly clayey in the upper part of the unit (>60% clay and <40% silt) and becomes more silty in the lower part (<60% clay, >40% silt) with trace sand. Bedding is often indistinct but is loosely defined by alignment of the long axes of the elliptical lenses of all colors. The contact between Units 5 and 6 is contained in Core 343-C0019E-20R, and it appears conformable and is likely stratigraphic. The overall nature of the unit, particularly the dominance of silt in the lower portion, indicates a significant influx of siliciclastic and terrigenous material relative to the biogenic and pelagic clay deposits in Unit 6 below.

Unit 6 comprises laminar yellow-brown and dark brown clay with occasional pink, red-brown, and white laminae. It is 0.65 m thick, occurs within the lower portion of Core 343-C0019E-20R, and is in contact with Unit 5 above and Unit 7 below. The unit consists of >80% siliclastic grains, with <10% volcanic grains and trace–10% siliceous microfossils. Diatoms are not observed in this interval, but radiolarians are present. More than 75% of the material falls in the clay grain size fraction. The bottom section consists of green to light green clays and gradationally transforms into chert (Unit 7). Radiolarian molds are present in the basal section of Unit 6. Unit 6 is interpreted as pelagic clay deposited on the incoming Pacific plate.

Unit 7 consists of yellow-brown and chocolate-brown laminar chert that occurs as fragments at the base of Core 343-C0019E-20R and in Core 21R, taken from the interval 831–844.5 mbsf (TD). Similarity in color and lamination of the chert to that observed in the overlying clay, and the intermixing of clay and chert observed in Cores 20R and 21R, suggest that the underlying chert is formed by silicification of the clay. The transition from the pelagic clays in Unit 6 to the chert in Unit 7 was not sampled directly, but the presence of chert nodules and intercalated chert layers within Unit 6 suggests the transition is a diagenetic front. Unit 7 is correlated to the chert recovered in the incoming Pacific plate during Leg 56 at Site 436 (Shipboard Scientific Party, 1980b). Similar to Leg 56, the fine-scale laminations within in the chert represent the only primary sedimentary structures, implying minimal transport and reworking of the clays prior to silicification.

Site 436 is located off Northern Honshu and represents the most proximal drilling site that can be used as a reference for the section cored at Site C00019. Site 436 consists of three lithologic units: Unit 1, vitric diatomaceous silty clay and claystone; Unit 2, radiolarian diatomaceous claystone; and Unit 3, pelagic clay with chert and porcellanite. Site 436 Units 1 and 2 are lithologically similar to Site C0019 Units 1–3, which are composed of mudstone, siliceous mudstone, and locally ashy mudstone. This lithologic description is very general and does not necessarily imply a direct correlation between the units in Hole C0019E and the incoming plate strata. Site C0019 Units 4–7 correlate to Site 436 Unit 3. There are direct matches between the brown clay, the multicolored clays, and the chert between these unit correlations. For example, chert in Units 6 and 7 closely resembles descriptions of chert at Site 436 interval 378.5–397.5 mbsf, in which scarce Albian–Cenomanian radiolarians indicate a Cretaceous depositional age (Shipboard Scientific Party, 1980b). However, an equivalent of Site C0019 Unit 5 (brown mudstone) was not identified in Site 436 Unit 3. The lithology of Site C0019 Unit 5 could represent a stratigraphic variation between the two holes or could be structurally emplaced from the upper plate at Site C0019. The former interpretation is favored because of the absence of a sheared basal contact of Unit 5 at Site C0019.

Structural geology

Investigation of structures in core samples through combined X-ray computed tomography (CT) imaging and visual observation provided a wealth of data that both verify and extend the structural data derived from the LWD/MWD borehole logs. Within the intervals sampled by coring, numerous orientation measurements of bedding and fractures and orientation and kinematics of faults were obtained. Unfortunately, appropriate paleomagnetic poles to correct bedding strike to true azimuths have not yet been determined, so here we concentrate on dip magnitudes and shear sense indicators.

In core samples from lithologic Unit 1 in structural Domain 1, direct measurement of bedding indicates a magnitude of dip of 30°. Faults are diffuse, narrow (few millimeters thick) zones of deformed sediment with similar composition to the adjacent sediment. In the X-ray CT images, faults are tabular zones of material with high CT number (which appear as bight features), several millimeters thick, that truncate and offset worm burrows and bedding they intersect. Fault surfaces in the core are polished and display slickenlines defined by aligned clay particles, which are correlated to bright planes. Of 10 mapped faults, all are dip-slip. Three are normal faults, and the shear sense of the others was not determined. Fault dip averages 66° ± 9°, ranging from 55° to 81°. Measured offsets are of the order of 1 cm, but several faults had offsets greater than the diameter of the core. Both bedding and fault dip determined from the core are consistent with bedding and fracture orientations determined from image logs.

Because of the extreme brecciation of core containing lithologic Unit 2, few structural measurements were possible. A single probable bedding surface that dips ~3° is marked by a dark band that grades gradually darker toward the top of the core. Additionally, five dip-slip faults were identified. Some were recognized in CT images as bright surfaces cutting burrows and were then located in core. Of the faults, one was inferred as a normal shear sense. Fault dip is variable, ranging from ~10° to steeper than 70°.

Coring from 688.5 to 729.0 and 770 to 821.5 mbsf sampled lithologic Unit 3 and structural Domain 2 across the 720 fault to the 820 fault. Core observations confirm the variation in bedding dip magnitude determined from the image logs but define some patterns in dip magnitude in particular intervals. Between 660 and 680 mbsf, bedding dips average 37° ± 20° and are variable through the entire interval. From ~770 to 790 mbsf, steeper dips dominate, including some potentially overturned intervals with very steep dips (average 64° ± 15°). From ~800 mbsf to the base of the unit at ~820 mbsf, moderate dips dominate (average 38° ± 11°).

The most common structures observed in Unit 3 over the depth intervals 688.5 to 729.0 and 770 to 821.5 mbsf are dark seams and dark bands. Dark seams are planar to curviplanar and are <1 mm in thickness (usually hairline ~100 µm width), whereas dark bands are tabular or curviplanar to irregular with their thickness ranging from <1 mm to 2–3 mm along their length. On the CT images, most dark seams and some dark bands are marked by bright seams and bright bands, respectively, with higher CT numbers than those of the surrounding material. The different characteristics of these features were investigated both visually and in X-ray CT imaging. The truncation and offset relationships and X-ray CT brightness properties suggest dark seams are likely solution surfaces or very thin shear surfaces, whereas dark bands are usually shear surfaces but may also rarely be bedding or bedding-parallel shear surfaces. The dip angles of these features are highly variable throughout and occur over a range of dips at each depth interval. However, overall low-angle faults are more prevalent deep in the section between 800 and 820 mbsf.

A noteworthy zone of fractured and brecciated sediment occurs between 719 and 725 mbsf. Over a 0.27 m interval centered around 719.85 mbsf, beds are crosscut and offset by a 15 mm thick, anastomosing, 60° dipping fault zone. This feature correlates quite closely with interval of low resistivity identified in resistivity image logs from the neighboring Hole C0019B (i.e., the 720 fault). From 719 to 725 mbsf (below the fault), the mudstone is broken into angular fragments ranging from 1 to 10 cm diameter along sets of inclined fractures that may be parallel to, or exploit, dark seams. The fractures are commonly polished and slickenlined and sharply cut burrows and the mottled texture and compositional layering in the mudstone. Stepped slickensides and drag of bedding along the fault indicates reverse shear sense.

Another notable fault (at 697.2 mbsf) was identified in CT images as a ~2–7 mm thick bright band that dips 10° with respect to a horizontal plane. The section above the bright band shows inclined fissility in homogeneous sediment, whereas fissility is absent in the section below, which is distinctively mottled, suggesting a slip magnitude sufficient to juxtapose different sedimentary layers. The interval including the bright band was taken as a structural whole-round sample because this interval is close to the location of an H₂ anomaly at 697.9 mbsf; the sample was not further described during the shipboard operations but will be the focus of some shore-based studies.

Unit 4 is only observed within, and makes up all of, Core 343-C0019E-17R. Structures in Unit 4 were identified from observation of the whole round and X-ray CT images. Most of the core is composed of clay with a variably intense scaly fabric. There is also an interval (0.22–0.34 m from the top of the section) of relatively intact and only slightly sheared mudstone with upper and lower boundaries in contact with the sheared clay. The intense scaly fabric within the clay is characterized by polished lustrous surfaces, commonly striated, enclosing narrow, variably shaped and sized lenses of less fissile material, termed phacoids. The major and intermediate axes of the phacoids define the dominant, pervasive foliation. In any observation section, the phacoids appear bounded by surfaces with two predominant orientations, but locally one surface orientation predominates. In the uppermost 22 cm of the core, phacoids are sometimes asymmetric when viewed in a section perpendicular to the foliation and parallel to the dip direction of the foliation; the observed asymmetry indicates reverse shear sense.

In general, phacoid size increases with depth in Unit 4; however, there are several abrupt, discontinuous changes in color and phacoid size implying compositional and structural layering parallel to the foliation. The most finely foliated scaly fabric is observed between 0 and 0.20 m, where the red-brown clay forms platy phacoids with minor axes <1 mm in length. Within this interval, a visibly obvious curviplanar contact juxtaposes the predominantly red-brown clay and predominantly dark brown to black clay. The contact surface is slightly wavy at the centimeter scale with amplitudes <1 mm; the foliation on either side is truncated without deflection at the contact. These features are consistent with a meters thick shear zone hosting mesoscopic-scale surfaces of localized slip.

The interval of intact mudstone bounded by the sheared clay layers above and below displays three major sets of intersecting dark seams, some of which offset each other by a few millimeters. The major dark seams are shallow dipping subparallel to the bounding surfaces. The deformation features on the mudstone interval are similar to those observed within lithologic Unit 3 in Core 343-C0019E-15R, as well as to the fracture sets that were opened during drilling in the top of Unit 5 (Cores 18R and 19R). Deformation of the mudstone may have occurred during shearing of the entire unit. Alternatively, because similar structures are observed in Units 3 and 5, the observed deformation could predate the incorporation of the mudstone as a tectonic lens within the sheared clay.

Overall, the sheared zone of clay displaying scaly fabrics in Core 343-C0019E-17R markedly contrasts with relatively coherent and much less deformed sediments recovered from above and below Core 17R. The fact that the primary bedding has been completely destroyed by shear deformation and a penetrative scaly fabric containing localized slip surfaces developed in the layer indicates it has accommodated significant shear displacement and constitutes the core of a major fault. The total thickness of the sheared clay layer is unknown because the upper and lower contacts were not recovered and recovery of Core 17R was 38.8%. If unrecovered intervals in Cores 16R–18R are also composed of sheared clay, the maximum thickness of the sheared zone is 4.86 m. Regardless of true thickness, the structure of the layer is compatible with displacement on the order of hundreds to thousands of meters of slip. Also compatible with significant displacement is the abrupt change in lithology and bedding dip across the interval of scaly clay; the sediments in the hanging wall show moderate dip magnitudes and those in the footwall are subhorizontal. Moreover, the footwall strata are consistent with a pelagic sediment sequence deposited on the oceanic crust of the Pacific plate and thrust under the prism sediments in the hanging wall. Thus, the sheared clay interval at 821.5–822.5 mbsf is interpreted as the plate boundary décollement zone between the subducting Pacific plate and the overlying prism of accreted sediment. It is noteworthy that the scaly clay décollement zone is more similar to the décollement zones in Costa Rica (e.g., Kimura, Silver, Blum, et al., 1997; Vannucchi and Tobin, 2000) and considerably thinner than the few tens of meters thick décollement zones in Nankai and Barbados (Maltman et al., 1997; Wallace et al., 2003; Kinoshita et al., 2008).

The brown silty clayey mudstone of lithologic Unit 5 displays progressively less deformation downward to a relatively undisturbed state at the basal contact with Unit 6. The unit is relatively homogeneous and bedding orientation is not clear on the core surface, possibly due to extensive bioturbation indicated in X-ray CT images. However, the sedimentary contact at the base of this unit dips 7°, similar to the few observed stratigraphic contacts within the unit. The unit is traversed by shear fractures and locally dark bands. These brittle deformation features are observed in the cut surface as well as in CT images. Located near the top of the unit are discrete zones of particularly high fracture density, and incipient scaly fabrics may be indicated by intersecting fracture networks in zones <0.10 m thick. Shear surfaces within this unit, particularly near the base, dip between 31° and 62°, averaging about 45°.

The primary structural features in Units 6 and 7 are bedding surfaces in the clays that on average dip 6° ± 2° and three natural faults dipping 14°, 55°, and 78°. Other apparent faults probably result from drilling-induced damage, as they are bounded by significant amounts of soft, structureless, intruded clay. Overall, intact lamination in Unit 6 indicates that very little structural deformation or bioturbation affected these units of the underthrust sediments.

Biostratigraphy

Although samples were collected for analysis during the expedition, the actual analysis will be performed by a shore-based specialist. Results will be provided in the Expedition 343/343T Proceedings volume as an appendix or data report.

Geochemistry

Interstitial water geochemistry and headspace sampling from cores collected during the expedition were processed aboard the D/V Chikyu. Interstitial water samples were collected from 12 cores. Interstitial water samples showed evidence of a sulfate-bearing fluid reservoir at depth, which on the basis of the condition of the intact whole-round sections sampled, is unlikely to be from drilling-induced contamination. Chlorinity and bromide also decreased with depth, consistent with the presence of a reservoir of deeper fresher fluids.

Gas chemistry, particularly the presence of varying H₂ concentrations, may reflect mechanochemical production of H₂ during high-velocity frictional sliding and sediment disruption as would be expected for the Tohoku earthquake. Although the sampled interval is not complete and is therefore missing some key zones, the presence of a sudden sharp increase in H₂ concentration in Section 343-C0019E-5R-1 suggests the presence of a recently activated fault. Below this location, methane concentrations decrease with depth (Section 20R-2), being 2–3 orders of magnitude below the previous core; this also suggests that methane concentrations have been dissipated through subseafloor fluid flow and supports the presence of a deeper reservoir of fluids.

Microbiology

Twelve whole-round core samples were taken from various depths in Hole C0019E, and sampling procedures were completed within 60 min of the core arriving on deck. Fixed samples were stored at –80°C for later DNA/RNA analyses. Results of the contamination tests showed that perfluorocarbon (PFC), which was added to the drilling mud, was present in the exterior portions of the whole-round samples. Also, some PFC contamination was present in the interior portions of Cores 343-C0019E-5R, 7R, 8R, 15R, and 20R but not inside of Cores 4R, 12R, and 19R. Microbiological and interstitial water data will be carefully evaluated to assess the degree of possible contamination from drilling mud and seawater.

Physical properties

Bulk density of core samples determined from moisture and density (MAD) measurements and from gamma ray attenuation measured by the whole-round multisensor core logger (MSCL-W) compare favorably. The density values generally increase with depth as expected for mechanical consolidation. The discrete samples taken from the 820 fault (Unit 4) have bulk density of 1.98 g/cm³, whereas in the clay-rich zone just below the fault, bulk density values range from 1.76 to 2.03 g/cm³. Porosity determined from MAD measurements decrease with depth from 55.3%–68.7% at shallow levels to ~45%–50% in the vicinity of the 720 and 820 faults. The lowest values are found in the immediate vicinity the 720 fault zone, compatible with shear-enhanced compaction.

Overall, resistivity of discrete samples from lithologic Unit 1 and into Unit 3 increases with depth but is lowered locally, possibly from the presence of ash layers. Resistivity markedly increases from the lower part of Unit 3 into Unit 4. In Units 5 and 7 from 826 to 836 mbsf, below Unit 4, resistivity is again significantly lower. Resistivity is high within the chert of Unit 7.

Qualitatively consistent with logging data, natural gamma radiation (NGR) data from the MSCL-W measured on whole rounds indicate the NGR magnitude in Unit 4 is about twice that in units above, consistent with a significant increase in the clay fraction from Unit 3 to Unit 4. NGR decreases progressively from Unit 4 downward, indicating a gradual decrease in clay content in Units 5 and 6.

P-wave velocity measured on unconfined discrete samples ranges from 1400 to 3300 m/s, but there is a clear trend of increasing velocity with depth. There is a decrease in the velocity measured on samples from lithologic Unit 5, below the 820 fault zone. A maximum velocity of ~3272 m/s occurs in the cherts of Unit 7. P-wave velocity has an inverse relationship with porosity, with the majority of samples following an approximately linear trend except for the chert, which has a much higher velocity. On five select cubic samples, both P- and S-wave velocities were measured in multiple directions under stepwise increasing and decreasing confining pressure to determine pressure dependence and anisotropy. These measurements are useful for determining seismic properties at in situ pressures, as well as for inferring properties of similar sediments at greater depths in the wedge.

Thermal conductivity was measured on the working half of core sections at 45 discrete locations; the mean of all measurements is 1.139 W/(m·K) (standard deviation = 0.118 W/[m·K]). Significant deviation from the mean occurs in Unit 1 with a mean of 0.874 W/(m·K) (standard deviation = 0.0141 W/[m·K], n = 2), in Unit 6 with a mean value of 1.086 W/(m·K) (standard deviation = 0.0870 W/[m·K], n = 3), and in the chert of Unit 7 with a very high value of 1.622 W/(m·K) due to its high density and silica content.

Unconfined compressive strength (UCS) was determined for nine minicores from various depths. In general, UCS increases downhole, with locally low strength values exhibited by samples that may represent major faults or shear zones. In the lower portion of Unit 3 (below 800 mbsf), UCS is 6.4–7.6 MPa, but Unit 4 from the 820 fault zone immediately below shows lower strengths of 3.4 and 4.7 MPa. A chert sample from Unit 7 was the strongest tested, with a minimum UCS of 65.3 MPa.

Core-Log-Seismic integration

Logging units based on resistivity and gamma ray response, structural units based on bedding and fracture features in image logs, lithologic units determined from visual core description of the cores, and seismic units identified in seismic reflection profiles are integrated to develop a unified interpretation of the geology and geophysics of the drill site (Fig. F10). Unfortunately, core was acquired from only a portion of the entire drilled section, albeit from critically important sections, so, where possible, core samples were used to ground-truth lithologic interpretations of the logging data. Similarly, the limited suite of logging tools did not provide multiple independent measures of rock properties such as velocity, density, and porosity. Accordingly, to quantify density and seismic velocity, we determined formation factor from the resistivity logs, calculated porosity using Archie’s law, and used direct measurement of discrete samples from core to constrain Archie’s law. In addition, MSCL-W data were used as an intermediate step for correlation of discrete measurements from core and log data. Finally, to integrate core and log data with seismic data, a synthetic seismic trace was created for Hole C0019B. A wavelet was extracted from seismic Line HD33B, and the reflectivity series were calculated using density and velocity logs determined from resistivity-based porosity with some adjustment of the velocity model based on crosscorrelation of seismic data and the synthetic seismic trace.

The logging units, based on gamma ray and resistivity response, compare favorably with lithologic units identified through core analysis, particularly at the base of the borehole where contrasts in lithology and geophysical response are most dramatic (Fig. F10). The boundary between logging Units II and III, structural Domains 2 and 3, and lithologic Units 3 and 4 correlate exactly. The changes observed across this boundary, specifically the abrupt change in bedding dip seen in image logs and core, the increase in clay content of core samples and concomitant increase in gamma ray, and the presence of the fault rocks that make up lithologic Unit 4 point to this boundary as a significant fault contact. The similarity of the lithologic units seen in core below the boundary with strata deposited on the Pacific plate (Shipboard Scientific Party, 1980b) suggests this is the plate boundary interface. Furthermore, the fact that observations of both the core samples and image logs indicate that the entire sediment section above the boundary is variably, often steeply dipping and fractured is consistent with it comprising a shortened and accreted sequence of strata.

Above the plate boundary interface and within the frontal prism, the logging unit boundaries, structural domain boundaries, and lithologic boundaries do not correlate well. Boundaries demarcated on the basis of changes in stress indicated by borehole breakout patterns also do not correlate. Moreover, the entire prism appears relatively transparent in seismic profiles. This likely reflects both the presence of inclined and faulted bedding and the relatively uniform properties of the sediment. Although the spot cores taken at 176.5–186.0 and 648.0–660.5 mbsf are considered to represent different lithologic units, both of these sections, as well as core from deeper intervals in the prism (688.5–729.0 and 770–821.5 mbsf), are mudstones composed predominantly of terrigenous silt and clay with varying amounts of vitric ash and biogenic silica. The subtle cyclicity in gamma ray and resistivity log response with depth in the prism may result from stacking packages of sediment by thrust and reverse faulting. The apparent gradual decrease in porosity with depth from progressive consolidation interrupted by stepwise increases in porosity from faulting would be is consistent with such a structural interpretation. Given the variable but dominantly steep eastward dip of the sediment throughout most of the prism, even relatively small displacement on contractional faults would lead to significant repetition of strata as seen in a vertical borehole.

A correlation exists between abrupt changes or discontinuities in different signals at a couple of different locations in the prism. For example, at ~140 mbsf, an abrupt reversal in the progressive change in borehole breakout orientation correlates with a sharp change in bedding from moderate dips to very shallow dips, which likely reflects a fault contact. Another marked change in borehole breakout distribution occurs at ~550 mbsf, and this appears to correlate with a local low gamma ray response. The 720 fault identified on the basis of image logs and resistivity curves correlates with a fault zone in core samples and an abrupt change in bedding dip.

Regional seismic images display three prominent seismic units below the lower slope of the Japan Trench where Site C0019 boreholes were drilled (Fig. F10). Uppermost seismic Unit 1 is wedge-shaped with an acoustically chaotic character and without continuous reflectors and corresponds to the frontal prism. Seismic Unit 2 consists of a relatively thin section of fairly continuous subhorizontal reflectors located below Unit 1 and above Unit 3. Lowermost seismic Unit 3 is the acoustic basement likely corresponding to oceanic igneous basement. Generation of the synthetic seismic trace based on resistivity logs, discrete core sample measurements of density and seismic velocity, and correlation with the seismic profile through the drill site allows location of the plate boundary fault in the seismic profile.

The synthetic seismic trace shows five possibly significant reflectors at the seafloor and at 70, 269, 368, and 839 mbsf. The middle three reflectors on the synthetic trace, at 70, 269, and 358 mbsf, fall within chaotic seismic Unit 1. Coherent reflectors within this unit are rare and, where present, are often masked by steeply dipping noise, making confident correlation within the unit difficult. Moreover, bedding at these depths in the borehole dip moderately to steeply and are thus unlikely to be apparent in seismic sections. The lowermost strong reflector on the synthetic trace falls within bedded seismic Unit 2. After crosscorrelation-based alignment with the strong reflectors at the base of the borehole, the synthetic reflector at 839 mbsf correlates best with the first strong reflector of the bedded unit at 9994 ms two-way traveltime. In the borehole, the plate boundary décollement occurs at ~820 mbsf, about 20 m shallower than the prominent reflector.

Stratigraphic control on the location of plate boundary décollements has been proposed in the past on the basis of well log and seismic data from other subduction zones (e.g., Wallace et al., 2003). Here, the décollement occurs within a thick clay-rich layer. If this represents stratigraphic control on the location of the décollement, then the position of the plate boundary interface across the region may be inferred from seismic data.

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