SITE 1150

Site 1150 is located in the deep-sea terrace on the landward side of the Japan Trench. The primary objective was to establish a borehole geophysical observatory to monitor seismo-geodetic signals immediately above the active portion of the seismogenic zone where large interplate thrust earthquakes recur. The first successful emplacement of a borehole geophysical observatory (NEREID-1) with a three-component strainmeter, a two-component tiltmeter, and three-component broadband seismometers was completed on 28 July. The sensing sections are <11 m in length bottoming at 1120 mbsf and were cemented in the 105-m-long open hole at Hole 1150D. Because the instrument string and battery frame could not be installed simultaneously, the electrical connection to the downhole instruments was made after Leg 186. The observatory sites were visited by the Dolphin 3K of the Japan Marine Science and Technology Center (JAMSTEC) between 2 and 10 September to start the systems, check the status, and collect initial data. The observatories are designed to be serviced at least once a year.

All of the cores from 0 to 722.6 mbsf from Hole 1150A are dominated by diatomaceous silty clay. The upper 200 m of sediment consists of interbedded diatomaceous ooze and clay, with common siliceous biogenic grains, diatoms, radiolarians, and sponge spicules and rare foraminifers and nannofossils. Volcanic glass and siliciclastic grains are also observed in smear slides, though they compose <10% of the total sediment. From 260 to ~620 mbsf, the sediments gradually become firmer and more hemipelagic with increasing depth. Compared with cores from the upper 200 m, volcanic ash, reworked ash layers, volcanic pebbles, thin turbiditic sand, and silt layers are rare but present in the lower section. Below ~620 mbsf, cores from both Hole 1150A (620-722 mbsf) and Hole 1150B (703-1181.6 mbsf) consist mainly of well-lithified diatomaceous silty claystone. Authigenic glauconitic sand is interbedded with the dominant clayey lithology below 430 mbsf in Hole 1150A and from the start of the cored interval (703 mbsf) at Hole 1150B. Local occurrences of detrital glauconite in minor amounts are rare within both holes. Bioturbation (Chondrites, Zoophycos, and Planolites) is abundant in Hole 1150B. A few carbonaceous layers and nodule-type accumulations are also found interbedded in the largely homogeneous diatomaceous silty claystone in Hole 1150B. Volcanic ash layers are rare in this hole.

Twelve diatom zonations were identified from core-catcher samples from Holes 1150A and 1150B. The age of the lowermost sediment is interpreted to be younger than 9.9 Ma because the first occurrence of Denticulopsis dimorpha was not observed in the studied interval. The average sedimentation rate is 119 m/m.y., with higher sedimentation rates (>200 m/m.y.) occurring between 6.65-3.74 Ma and between 0.3 and 0.0 Ma. The lowest sedimentation rate occurs between 2.0 and ~1.24 Ma (18 m/m.y.). The Pliocene/Pleistocene boundary lies at ~110 mbsf, and the Pliocene/Miocene boundary is at ~500 mbsf. Datums of calcareous nannofossils were difficult to determine accurately because of poor preservation and a low abundance of these fossils. Of the 11 nannofossil datums identified, seven gave ages younger than those indicated by the diatom datums.

Chemical analyses of pore waters from Hole 1150A cores show that chlorinity gradually decreases with depth from ~550 mM at the top of the hole to 500 mM at ~200 mbsf. Chlorinity concentrations remain at about this value for 350 m downhole. From ~550 mbsf, values abruptly decrease with depth to reach a minimum of 350 mM at ~700 mbsf. A similar trend is observed in the magnesium, potassium, and alkalinity profiles. Values gradually decrease in the upper 200 m (from ~50 to 28 mM for magnesium; from ~11 to 8 mM for potassium; and from ~50 to 26 mM for alkalinity). Values remain fairly constant from ~200 to 500 mbsf and drastically decrease with depth to reach minimum values of 11 mM for magnesium, 6 mM for potassium, and 11 mM for alkalinity. Calcium concentrations increase from ~2 to 8 mM in the upper 200 m, remain fairly constant to a depth of 400 mbsf, and then increase to 10 mM at 600 mbsf.

Bulk-elemental analyses for the upper 350 m of samples from Hole 1150A show relatively high concentrations of organic carbon. Values fluctuate with depth between 1.8 and 0.5 wt%. Typical values are ~1 wt%. Sulfur abundances also are relatively high in this interval. Typical values are ~0.9 wt%, fluctuating between 1.3 and 0.5 wt%. Nitrogen abundances are typically ~0.13 wt%, fluctuating between 0.6 and 0.1 wt%.

Gas analyses for Hole 1150B indicate that methane concentrations are between 2% and 8% as measured from headspace gas analysis. Ethane concentrations are typically ~6 ppm from 700 to 900 mbsf. Values then slightly increase with depth to typical values of ~14 ppm. Methane/ethane ratios tend to decrease gradually with depth from ~5000 at 700 mbsf to 2800 at the final depth of ~1200 mbsf. Other hydrocarbon gases are below the detection limits.

Physical properties data show several systematic trends that correlate with downhole chemical and lithologic changes, appearing to indicate variations in hydrological and mechanical conditions. Gas expansion and drilling disturbance (formation of drilling biscuits) affect the physical properties of most cores from Hole 1150A. The interval from the mudline to 222 mbsf consists of hemipelagic diatomaceous ooze and clay. Inverse trends in index properties are observed from ~113 to ~200 mbsf. Bulk density decreases from ~1.7 to 1.3 g/cm3, and porosity increases from ~56% to 79%. Constant to slightly inverse trends with uniform values of index properties are observed from ~200 to 600 mbsf. The top of this interval coincides with a small change in lithologic composition, whereas the bottom corresponds to the change from firm sediments to sedimentary rocks. Bulk density and porosity range from ~1.3 to 1.6 g/cm3 and 63% to 77%, respectively.

Because of scattering and attenuation of the signal in the sediment, P-wave velocity was only measured sporadically above 304 mbsf. The horizontal velocity drops to >1.50 km/s below 750 mbsf, which is followed by a generally increasing trend to >1.90 km/s at the base of Hole 1150B. Vertical P-wave velocity was measured from below 710 mbsf and is generally somewhat lower than horizontal P-wave velocity. Horizontal P-wave velocity gradually increases downhole from 1.45 km/s at 304 mbsf to 1.95 km/s at 710 mbsf. There may be a bias in index properties and P-wave velocity measurements in this interval because only coherent and undisturbed pieces were sampled. Measurements in sedimentary rocks show a wide scatter in porosity, bulk density, and P-wave velocity but suggest generally decreasing porosity and increasing bulk density and P-wave velocity trends. Preliminary analyses of sonic anisotropy combined with paleomagnetic declination data indicate that the sedimentary rock from 730 to 1180 mbsf is anisotropic, with maximum, intermediate, and minimum principal axes along the west-northwest to east-southeast, north-northwest to south-southwest, and vertical orientations, respectively. In the interval where the borehole instruments were installed, the porosity, bulk density, and P-wave velocity are 55%, 1.65 g/cm3, and 2.0 km/s, respectively.

The magnetization of the first eight cores has an unambiguous normal polarity direction, with a steep downward inclination and a northward declination after applying the Tensor-tool orientation correction. The Brunhes/Matuyama boundary is most likely located in Core 186-1150A-10H at ~84-88 mbsf, though coring disturbance makes interpretation tenuous in this interval. Above 850 mbsf, the position of several reversals are evident following alternating-field demagnetization. These should be useful in establishing a few magnetostratigraphic tie points in the Pliocene-Miocene section at this site. The magnetization of the RCB cores from Hole 1150B is fairly stable, and polarity is dominantly normal below 850 mbsf. The declinations from the Hole 1150B cores have proved useful for reconstructing structural orientations of the numerous microfaults and fractures observed in the core. For example, after reorienting fracture and fault planes into geographic coordinates, we find that most in the depth range from 703 to 940 have north-south strikes and dips of 45 to 80, with a clear preference for eastward-dipping planes. Normal offset is observed on most of the fault planes, suggesting that an east-west extensional stress field is responsible for the deformation observed in this interval. The extensional stress direction changes downhole, so that below 1080 mbsf the dominant direction is west-northwest to east-southeast.

Equilibrium temperatures obtained from the APC temperature tool (Adara) and the Davis-Villinger temperature probe in the interval from 0 to 154.8 mbsf give a geothermal gradient of 28.9C/km. The calculated heat flow is 20.1 mW/m2, which, though low relative to global values, is typical for the tectonic environment.

Three logging tool combinations were used in Hole 1150B: the triple combination tool string, the Formation MicroScanner (FMS)/digital sonic tool string, and the borehole televiewer tool string.

Although operational difficulties prevented logging at ~650 mbsf, three strings were deployed down to ~1170 mbsf after a wiper trip operation. Data quality is good throughout the logged intervals. The whole logged section can be divided into six units using the resistivity log in conjunction with other logs. These units are consistent with core descriptions and core measurements. The FMS data show borehole geometries to be oval below ~750 mbsf with east-west elongation (20 cm in north-south and 30-35 cm in east-west). The in situ physical properties values in the lowermost 100 m (i.e., in the interval where the borehole instruments were installed) are ~1.95 km/s for P-wave velocity, ~1.7 g/cm3 for bulk density, 55%-60% for porosity, 1.2 m for resistivity, and ~50 cps for spectral natural gamma ray.