SITE SUMMARIES
Site 1253 is located ~200 m seaward of the deformation front in the deepest part of the Middle America Trench (Figs. F3, F13). Operationally, one primary goal for this site was to recore the sediments immediately above the sill encountered during Leg 170, drill and core for the first time through the sediments below the sill, and core >100 m into the oceanic section. The other major task was to install a CORK-II observatory in the deep igneous section, where coring and logging were used to identify depths at which to set the packer and osmotic fluid and gas samplers.
There were two primary science objectives for this site. The first goal was to determine the bulk composition and distribution within the incoming plate of key element and isotopic tracer; this would provide a baseline in physical, mineralogical, and chemical characteristics against which changes during shallow subduction processes may be measured or inferred. They also provided a starting point from which to examine the recycling from subduction trench to volcanic arc (or deeper mantle) of important components such as CO2. The second major objective at this site was to investigate the hydrology and thermal structure of the igneous section entering the trench. This objective will be addressed using temperature and pressure data and fluid and gas samples recovered from the observatory 1-2 and 5-6 yr postcruise. Interstitial water chemistry analyzed shipboard during Leg 205, together with that from Leg 170, provides evidence for contemporary flow of seawater at depth at both reference sites.
The seismic record (Fig. F13) in the vicinity of Hole 1253A (CMP 3210) images the sedimentary sequence of middle Miocene age with a clear change in seismic reflectivity at ~6.1 s TWT. This represents the top of the calcareous Unit U3 (Kimura, Silver, Blum, et al., 1997) at ~180 mbsf. Beneath the sedimentary sequence, the strong reflector at 6.34 s TWT images the top of a gabbro sill as revealed by drilling results from Leg 170 at Site 1039. The coherent reflection pattern below the top of the sill is difficult, if not impossible, to interpret below the drilled depth of 600 mbsf.
At Site 1253, we drilled one hole that was partially cored and into which we installed a long-term hydrologic borehole observatory. After setting a reentry cone and 16
-in casing into the seafloor, we reentered this hole with the rotary core barrel (RCB) and drilled without coring to ~370 mbsf. RCB coring below 370 mbsf penetrated 30 m of calcareous and locally clay-rich sediments with intermittent ash layers (average recovery = 75%) before encountering a gabbro sill between 400 and 431 mbsf (average recovery = 74%). Below the sill was ~30 m of partially lithified calcareous sediments with intermittent ash layers (average recovery = 20%). This interval was followed by coring ~140 m into a second igneous unit (average recovery = 75%), with local zones of 30%-50% recovery.
After coring, operations focused on preparing the hole for logging and CORK-II installation. The hole was opened to 14
in; 10-in casing was installed to a depth of ~413 mbsf and cemented in place to inhibit communication between the borehole and the formation. After drilling out the cement shoe and drilling a rathole with an RCB bit, we logged the hole. Because we conducted operations at or very near Leg 170 drill sites where LWD was conducted, our logging focused on the igneous section at Site 1253. Here, we ran the triple combination (triple combo) and Formation MicroScanner (FMS)-sonic tool strings to determine physical properties, fracture distribution, and structure of the basement rocks. After an initial logging run encountered an impassable bridge in the shallow sediment section, casing was run into the uppermost part of the sill to stabilize the hole for subsequent CORKing operations. The subsequent logging run encountered a bridge at 530 mbsf that limited the triple combo and first FMS-sonic run to the interval between 530 and 413 mbsf; on a second pass, the FMS-sonic tool string passed below the bridge and the hole was logged upward from 564 mbsf. A miniaturized temperature logger was run along with the Lamont-Doherty Earth Observatory Temperature/Acceleration/Pressure (TAP) tool.
After logging, we assembled the CORK-II components, including a 4-in casing screen, casing packer, and casing made up to the instrument hanger. The entire assembly was then lowered into the hole and latched in to seal the borehole outside of the 4-in casing. The OsmoSampler with integral temperature sensors was lowered through the center of, and latched into the bottom of, the 4-in casing. The final operation was to inflate the packers and shift spool valves that would connect the CORK-II pressure monitoring system to the formation, which would completely seal the zone to be monitored. Problems with the go-devil used for this step made it difficult to determine whether the packer had inflated or the valves had turned for pressure monitoring. Alvin dives since then have confirmed that the installation is fully operational. Three absolute pressure gauges including a data logger are installed within the instrument hanger head. One sensor monitors pressure within the sealed-off fluid sampling zone at the bottom of the hole; one monitors pressure variations present within the borehole above the sealed-off section; and the third sensor provides seafloor reference pressures. One additional sampling line extends from the CORK-II head all the way down to the screened interval below the packer and is available for future pressure/fluid sampling purposes. The specifics of the CORK-II installation, relative to the structure and petrology of the igneous sections, are discussed in more detail below.
Lithostratigraphy and Sediment Geochemistry
Sediment coring began at Site 1253 at 370 mbsf, where nannofossil chalks with minor clay interlayers were recovered, closely similar to those at Leg 170 Site 1039. Other significant grains identified are siliceous sponge spicules, diatoms, and zeolites derived from the degradation of volcanic glass shards. Volcanic detritus (glass, altered glass, and mineral fragments) is ubiquitous, varying between ~3% and 10% of the total. Tephra layers (<1% of total stratigraphic thickness) are typically thin (<5 cm), with mafic layers accounting for >70% of the layers identified. A thick (8 cm) siliceous white tephra was recovered at 398.8 mbsf. Diagenesis has resulted in moderate lithification in the section, except immediately above the gabbro sill, where the sediment is much more clay rich, laminated, and lithified. This section (Core 205-1253-4R) is less calcareous (<2 wt% CaCO3). Clays and zeolites form increasingly large volumes of the sediment in the last 3 m above the gabbro sill, and quartz from diatom and/or spicule opal recrystallization to quartz is seen as thin chert layers at 395.4 mbsf (interval 205-1253A-4R-1, 53-61 cm). Below the gabbro sill, in Core 205-1253A-10R, less lithified nannofossil chalks were recovered. These are identified as the same lithologic unit as above the sill, but they are dominated by a clastic granular limestone, defined as packstone with clay. Minor amounts of baked sediments, usually inferred to be out of place, were recovered within and below the gabbro sill. Bulk sediment chemistry, by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), largely map the minor variations in lithology, with SiO2 increasing and CaO and Sr decreasing in the more clay-rich interval immediately above the sill. The TiO2 and Al2O3 contents in the sediments are largely controlled by the ash contribution; relatively constant Ti/Al ratios through the calcareous and clay sediment sections suggest relatively homogeneous amounts of volcanic detritus throughout the section. Baked sediments have chemistry similar to the dominant lithology. No appreciable increases in Fe, Mn, or transition metal concentrations were noted above the sill, in contrast to the increases in Cu, Ni, Zn, and V observed for ~80 m above the sill at Site 1039.
Biostratigraphy
Because of the small amount of new sediment recovery expected during Leg 205, the shipboard science party did not include a micropaleontologist. Samples were taken for a shore-based participant. Results are expected to help constrain the ages of the gabbro sill and the lower igneous unit.
Igneous and Metamorphic Petrology
Coring at this site penetrated two separate igneous subunits (Fig. F14). The upper subunit is a gabbro sill (Fig. F15) and is similar to that encountered at Leg 170 Holes 1039B, 1039C, and 1040C. The sill (Subunit 4A) has been further subdivided tentatively into two subsections, Subunits 4A-1 and 4A-2, based on the distribution of voids, veins, grain size variation with depth, and the proportions of plagioclase to pyroxene. The lower igneous subunit (Subunit 4B) has been tentatively subdivided into seven subsections using the same criteria. Both the upper and lower igneous sections contain plagioclase and clinopyroxene phenocrysts, with rare olivine, orthopyroxene, ilmenite, and magnetite (Fig. F16). Subunit 4B, particularly below 513 mbsf, is more glass rich and more altered. Phenocrysts are set in a groundmass that typically varies between microcrystalline and fine grained, with occasional medium-grained horizons. A 1.3-m-thick interval of cryptocrystalline material is present at 513 mbsf (Fig. F17), where larger amounts of glass and a greater degree of alteration are observed. The petrologic data suggest that Subunit 4B is either a sill complex with multiple intrusions or a series of thick and slowly cooled lava flows. Its possible that changes in petrology and physical properties at ~513 mbsf mark the change from a sill complex to basement; postcruise dating and detailed analysis will be necessary to evaluate the two possible origins of Subunit 4B. Note that some characteristics of Subunit 4B are similar to those seen in some horizons recovered during Leg 206 coring, which intersected thick ponded lava flows (Wilson, Teagle, Acton, et al., in press).
Discrete alteration is highest at the tops of the subunits and generally higher in the lowermost cores, but is generally low (1%-5%) overall. Veins sampled below 485 mbsf in Subunit 4B contain up to ~11 wt% carbonate, although quantification is difficult because vein material is mixed with various amounts of igneous rock. Diffuse alteration of the bulk rock, in the form of zeolite formation and clay replacement of minerals and glass, ranges from ~10%-50%, with higher levels of alteration seen below 513 mbsf. Chemically, all rocks from both subunits are of basaltic composition (46-49 wt% SiO2 and 6-9 wt% MgO), with compositional variation in part due to olivine, clinopyroxene, and plagioclase fractionation. Variations in elements such as Ti, V, Ba, and Zr indicate that Subunits 4A and 4B are not comagmatic and possibly could have been derived from different mantle sources. Chemical and isotopic analyses beyond those available shipboard will be necessary to determine whether these mantle sources are associated with the Galapagos hotspot, the EPR, or both.
Structural Geology
The most evident feature of the sediments above the magmatic intrusion (in cores firm enough to preserve original structures) is the tilted bedding with 30° average dip; paleomagnetically reoriented dip azimuths cluster at 221°, similar to the geometry observed at Sites 1039 and 1040. Small normal faults perpendicular to bedding, with millimeter-scale offsets are common throughout, as are pressure solution structures 3-10 cm long. The close association of reworked pelagic sediment in the lower part of Subunit U3C, westward-tilted bedding, and the magmatic intrusion suggest that the lowermost part of the sedimentary section was deformed during the gabbro emplacement. At Site 1253, the sediments do not exhibit subhorizontal shortening related to incipient subduction, despite the location of the deformation front only several hundred meters to the east.
The igneous units were carefully analyzed with respect to the CORK-II experiment and to provide data for comparison to the FMS data (Fig. F18). A sediment/gabbro contact was recovered in interval 205-1253A-27R-1, 1-6 cm, and it dips 72°, although this piece sits at the top of the core and may not have been recovered in place. The igneous units are commonly cut by magmatic veins. Dilational joints are also frequent, usually filled with a film of green minerals (clay and zeolite?), and are rarely present as open fractures (Fig. F19). The paleomagnetic reorientation of the fractures to the real geographical coordinates has been done with particular care; some joints share preferred orientations with magmatic veins, but many do not. Overall, fracture density increases with depth (Fig. F18). Brittle shear zones, represented by en echelon Riedel shears, are common in the lower part of the deeper igneous unit (Core 205-1253A-36R and below) and usually show a reverse sense of movement.
Physical Properties
Variations in physical properties correlate with major lithologic changes between sediments and igneous units (Fig. F14). A limited number of measurements indicate decreased porosity and increased grain density and P-wave velocity within sediments immediately above and between the igneous units; these differences may reflect alteration (recrystallization) and porosity reduction caused by emplacement of the igneous units.
Clear trends in the physical property data are (1) the small but systematic increase in velocity, bulk density, and grain density and decrease in porosity within the lower igneous unit and (2) the higher natural gamma radiation (NGR) in the upper igneous unit and the clear shift in NGR emissions at 512 mbsf within the lower igneous unit. The cause of these trends in porosity, density, grain density, and velocity with depth in the lower igneous unit is unclear. The differences in NGR emissions suggest chemical differences between and within the igneous units, which may reflect primary compositional differences or varying degrees of alteration within igneous units that were initially chemically similar. The fact that the trends in porosity, density, and velocity are not correlated with the NGR trend suggests that the processes that control porosity, density, and P-wave velocity are separate from the chemical or lithologic processes that affect the NGR emissions.
Paleomagnetism
Shipboard magnetic studies on the archive-half sections and discrete samples (Fig. F14) established a reliable set of magnetic polarity changes and investigated rock magnetism, especially mineral composition and the domain state of magnetic minerals in the sediments and igneous rocks. The small amount of coring above the sill yielded at least two reversed polarities consistent with those seen in Holes 1039B, 1039C, and 1040C during Leg 170. Sediments below the sill generally showed negative polarity, but low recovery and high drilling disturbance preclude identification of a magnetic chron or subchron. Good recovery in the gabbro sill allows identification of several intervals of normal and reversed magnetic polarity. In the lower igneous unit, the upper part (between 450 and 513 mbsf) is primarily within an interval of reversed polarity. Two brief intervals of possibly normal polarity are identified, but discrepancies between archive-half and discrete sample results preclude firm identification. Below this depth, multiple intervals of normal and reversed polarity are observed. Postcruise age dating will be necessary to provide an absolute framework for this chronostratigraphy. Saturation isothermal remanent magnetization and Lowrie's test of the sediments show three separate unblocking temperatures, interpreted to reflect the presence of goethite, pyrrhotite (or griegite), and magnetite. In the igneous section, magnetization is often unstable and appears to reflect largely multidomain (>100 µm) magnetic minerals, presumed to be magnetite. Intervals of more stable magnetization and high magnetic intensity are observed at 400 mbsf in the upper unit and at 462-474, 513-523, and 572-593 mbsf in the lower unit.
Inorganic Geochemistry
Interstitial water chemistry of sediment pore fluids was used to investigate in situ diagenetic reactions and the possibility of fluid flow in basement (Fig. F20). Several features in the pore water chemistry suggest a role for enhanced ash alteration and associated authigenic mineral formation above and below the sill. Higher Na and much lower K and Si are observed just above the sill, and the Ca and Sr gradients remain constant at this depth. Cl concentrations are very slightly freshened (1.5%) relative to seawater, which may reflect opal-A or clay dehydration reactions immediately above the sill. The implied liberation of Na, Ca, and Sr to the fluids suggests ash alteration and carbonate recrystallization. The sharp decrease in K and Si is consistent with the uptake of these liberated elements via the authigenic formation of clays, zeolites, and quartz, also observed lithologically. Just below the sill, the Mg concentration in the fluid is quite low, consistent with the authigenic formation of more Mg-rich clays associated with ash alteration. Clear overall gradients with depth are noted for Ca, Sr, SO42-, Si, and Li. The gradients parallel those measured during and after Leg 170 (Kimura, Silver, Blum, et al., 1997) but are shifted deeper by ~40 m, thus maintaining the same depth relationship to the top of the sill. The gradients trend toward values typical of modern seawater in the intervals just above and below the sill.
Organic Geochemistry
Organic geochemistry at this site reflects the low heat flow of the incoming plate, with all hydrocarbon concentrations measured for shipboard safety requirements being below the detection limit of the gas chromatographs. Calcium carbonate concentrations in the sediments above and below the sill range from 32 to 65 wt% and overlap those of Site 1039, except in the laminated, clay-rich sediments just above the sill, where values drop to <2 wt%. CaCO3 in the igneous rock is low (<0.4 wt%, except in veins), even in the top and bottom of Subunit 4A and the top of Subunit 4B, immediately adjacent to the sediment section. Veins below 485 mbsf typically contain carbonate (<11 wt%), where some of the differences may be due to variable dilution with matrix igneous material. Total organic carbon is low, and frequently below detection limit, throughout. Sulfur concentrations are low in the sediment sections and near zero in the igneous section, except for one vein sampled at 546.1 mbsf.
Microbiology
Sediment whole rounds (5 cm) were taken for contamination testing (microspheres and PFT) and postcruise microbiological measurements (adenosine-5´-triphosphate [ATP] assay, cell counts, and DNA extractions and analysis). As expected with RCB coring in partially lithified sediments, contamination was significant and variable. In the igneous section, veined intervals were taken as whole rounds (up to 40 cm) and split under sterile conditions. Aliquots will be used for DNA extraction and analysis, culturing experiments and cell counts, fluorescent in situ hybridization studies, and for studies of mineral alteration and chemical change associated with microbial activity. Contamination tests, although difficult to use quantitatively, indicate that the tracers were delivered to all but one cored interval. Interior tracer concentrations are variable, but microsphere concentrations are lower to very low in the interiors.
Downhole Measurements
At Site 1253, the Davis-Villinger Temperature-Pressure Probe (DVTPP) was deployed twice in an attempt to determine the in situ temperature and pressure of the formation. The first measurement was performed directly beneath the casing of the reentry cone at a depth of 60 mbsf; the second was at a depth of 150 mbsf. Prior to these measurements, the bottom water temperature was determined using a high-resolution and calibrated miniaturized temperature data logger (MTL) (Pfender and Villinger, 2002) attached to the video system during reentry, giving a bottom water temperature of 1.989°C at Site 1253. At Site 1039, the bottom water temperature was 1.81°C, as measured by two different tools (water-sampling temperature probe and Adara). The cause of this difference is not clear. The MTL was also affixed to the triple combo logging tool near the TAP tool and run during logging. Unfortunately, thermal changes attributed to the curing of the cement used to tag the casing to the formation created a large signal visible in the temperature record in the upper logged interval, and temperature differences recorded between the two runs indicate that equilibrium formation temperatures had not been attained. The two DVTPP runs encountered difficulties with electronic noise and excessive tool motion, precluding their use to provide high-quality temperature or pressure measurements at this site.
Downhole Logging
The hole was logged upward with one pass of the triple combo and one pass of the FMS tool string from 530 mbsf to the bottom of the casing shoe at 413 mbsf (Figs. F14, F18). On the second pass, the FMS slid past the obstruction at 530 mbsf and the hole was logged upward from 564 mbsf, where a second bridge was encountered. Measured inclination of the hole was very small (0.5°-1.6°). The caliper data indicate that the hole diameter in the logged portions of the upper and lower igneous units (423-431 and 461-561 mbsf) was relatively uniform, ranging mostly between 10 and 12 in. Thin intervals of increased hole diameter are present at 482, 485, 487-489, and 502-504 mbsf. The caliper reached maximum extension between 435 and 461 mbsf, corresponding to the sedimentary section between the igneous units.
The logs can be clearly separated into three intervals on the basis of obvious changes in hole diameter, velocity, resistivity, bulk density, and porosity, corresponding to the upper igneous subunit (Subunit 4A; gabbro sill), the sediments below, and the lower igneous subunit (Subunit 4B) (Fig. F14). In the logged part of sill (413-431 mbsf, with a 14-in hole above 423 mbsf and a 9
-in hole below), porosities are low and densities, resistivities, and P-wave velocities are high. In the sedimentary section of enlarged borehole (431-461 mbsf), high porosities and low bulk densities, resistivities, and P-wave velocities identify sediments. A return to high bulk densities, resistivities, and P-wave velocities at 461 mbsf indicates the top of the lower igneous subunit. The logs better identify the exact depth of the lower igneous subunit than do core depths because of partial recovery and the standard curatorial practices of moving any recovered material to the top of the core. The NGR intensity is distinctly higher in Subunit 4A than in the sediments and Subunit 4B. Natural gamma logs are not available below 513 mbsf, where NGR emissions measured on cores using the multisensor track (MST) suggest a small increase in K, U, or Th concentrations in the lower part of the lower igneous subunit.
The logging data identify a change in the character of the resistivity and P-wave velocity logs in Subunit 4B at ~491-493 mbsf. Above this depth in the lower subunit, values are relatively homogeneous; below, the logs have similar average values but a more spiky character. FMS images indicate a change in character at a depth of ~508 mbsf. Above that depth, conductive features are generally discontinuous. Below, more closely spaced, thin, near-horizontal to slightly dipping conductive features are present in several intervals that are separated by intervals of poor images and irregular borehole size that could be fractured material or sediment interlayers. Intervals of decreased bulk density but no corresponding velocity decrease may indicate a fractured interval rather than sediment interlayers, which should cause a velocity decrease. Based on the bulk density and sonic logs, potential fractured intervals are inferred at 466-468, 484-486, 490-493, and 506-508 mbsf. Sediment interlayers thinner than the vertical resolution of the sonic tool (107 cm), would not be clearly distinguishable in this log, but the general high density and low porosity in areas of smaller borehole diameter preclude the presence of any significant sediment layers.
FMS images can be used to characterize structure and fabric in the igneous units. The gabbro sill (Subunit 4A) between 419 and 426 mbsf exhibits a blocky texture with an ~0.5-m size to the blocks. Between 426 and 432 mbsf, the formation appears more massive with thin conductive features at a 0.5- to 2-m spacing, although it is difficult to trace the conductive features across the four FMS pads. At the very top of Subunit 4B (463-467 mbsf), curved conductive features (fractures or irregularities in the borehole wall) are common. Between 467 and 493 mbsf, the formation appears more massive to blocky, with 0.5- to 1-m spacing between thin conductive features. These conductive features can be clearly traced across the four pads only between ~472 and 478 mbsf. Between 487 and 493 mbsf, irregular to curved vertical conductive features are present, representing possible fractures or irregularities in the borehole wall. From 493 to 498 mbsf, conductive features are rare, becoming more common again between 498 and 508 mbsf. At 508 mbsf, the character of the FMS image changes to more closely spaced conductive features (<0.5-m spacing). In rare cases, such as at 513-514 mbsf, these conductive features can be traced across the four pads and suggest a low dip angle. Image quality between 514.5 and 518 mbsf is poor because of an enlarged borehole. Relatively low (3800-4000 m/s) P-wave velocities and low (5-15 
m) spherically focused resistivities occur at similar depths. Below 518 mbsf, the layered character returns but the absolute value of resistivity increases. Imaging is poor from 525 to 527, from 534 to 539, and from 542 to 555 mbsf. From 539 to 541 and 555 to 563 mbsf, the image is characterized by more closely spaced (<0.5 m), thin, nearly horizontal conductive features. These conductive features appear to dip to the southwest. The static FMS images indicate that both intervals have high resistivity. Therefore, it appears unlikely that these are sediment layers.
Synthesis Topics
Fluid Flow in the Incoming Plate
As at Site 1039, 1.4 km to the west, interstitial water chemistry determined at Site 1253 is also indicative of fluid flow in or below Subunit 4B, where the chemical composition of the fluid is inferred to approach values typical of modern seawater. Figure F20 shows depth profiles for major, minor, and biogeochemical components determined shipboard in the sediment interval above and below the sill. In the limited sediment interval cored, Site 1253 profiles for Ca and Sr mimic those at Site 1039 (Fig. F20). High Ca values (~18.5 mM) seen at ~300 mbsf at Site 1039 likely reflect the effects of mafic ash alteration, which liberates Ca. Mg calcite and dolomite production are also suggested by Mg and Mg/Ca profiles at Site 1039 (Kimura, Silver, Blum, et al., 1997). In the pore waters from the deeper sediments at Site 1253, Ca and Sr decrease by ~20%-30%, toward, but not to, values typical of seawater. A similar magnitude change is seen in the Si content of the pore fluids above and below the sill, excluding the exceptionally low values seen at the immediate boundary with the sill (where quartz precipitation was noted). Li contents increase by ~60% over the lowermost 30 m of the section above the sill. Sulfate concentrations in the pore waters from Site 1253 are relatively uniform (27.2-28.6 mM, with no clear depth variation) and nearly of seawater composition. This contrasts with values of 12-20 mM measured higher in the section at Site 1039. These gradients are in directions opposite to those expected for most biogeochemical and fluid/rock reactions in deep siliceous and calcareous sediments at either low or elevated temperatures, which would be expected to reduce sulfate and to release Si, Ca, and Sr while consuming Mg and Li. The gradients observed at Site 1253, like those at Site 1039, suggest communication with a fluid of nearly seawater composition presumably flowing at depths below those from which interstitial waters have been recovered. At Site 1039, residence time calculations based on Sr and Li isotopes and concentrations (Silver et al., 2000) indicate that the gradients toward seawater are maintained by flow within the last 15-20 k.y. The gradients at Site 1253 are closely similar to those at Site 1039, supporting an argument for recent flow here also, which may have extracted heat from the plate to produce the unusually low heat flow in this region. The nature of this large regional-scale flow system, presumed responsible for the large heat flow anomalies as well as the chemical gradients, remains enigmatic; the CORK-II was installed at this site in hopes of providing necessary new information for better understanding the flow system.
Igneous Stratigraphy
The petrology of the igneous units can be combined with paleomagnetic and rock magnetism studies and logging results to better understand the nature of the two units and their internal structure (Fig. F14). Paleomagnetic results show that the sill (Subunit 4A) spans several polarity reversals, implying multiple pulses of magma intrusion, although the elapsed time cannot be evaluated until age dating is completed postcruise. Magnetic intensity is highest at the top of the sill, indicating more stable magnetization, probably because of the presence there of finer-grained magnetite than at deeper levels in the sill. The petrologic boundary between Subunits 4A-1 and 4A-2 approximately corresponds to a polarity reversal boundary. Subunit 4A-2 is composed entirely of microcrystalline gabbro. Logging results show a large hole diameter at the top of Subunit 4A-2, which corresponds to the 14-in hole drilled to provide a rathole for the casing installation. Seismic velocity and shallow resistivity (considered more reliable in the igneous units; see "Downhole Logging") are relatively high and uniform, and the cores recovered are massive in appearance, breaking into large pieces (see "Site 1253 Visual Core Descriptions"). At the base of the sill, recovery drops, the hole size increases, and velocity and resistivity decrease in general and exhibit a more spiky character, suggesting that fractured rock is present or possibly thin (<1 m) sediment interlayers.
The lower igneous subunit (Subunit 4B) begins at a depth of 450 mbsf in the core reference frame, which was used for petrologic and paleomagnetic work, and at ~460 mbsf in the logging data. A depth of 460 mbsf for the top of Subunit 4B is considered more reliable, given the very low recovery at the top of the subunit and the standard curatorial practice of moving all recovered material to the top of the core. Subunit 4B was subdivided into seven subsections, using the same criteria used for Subunit 4A. Within each subsection, multiple alternations between microcrystalline and fine-grained material may indicate the presence of multiple cooling units. Subunits 4B-1 through 4B-3 all formed during what may be a single reversed polarity interval, although dating is required. Magnetic intensity is again high at the top of Subunit 4B and decreases with depth. The logging data show that Subunits 4B-1 through 4B-3 are characterized by high and relatively uniform resistivity and P-wave velocity. There is a marked increase in resistivity at the top of Subunit 4B-4, which corresponds to a short massive interval that was drilled very slowly (0.75 m/hr) with high recovery. In this interval, conductive features are rare in the FMS data (see "Downhole Logging"). There is a hint of increased P-wave velocity at and below this interval, seen in the logging data and as measured in the cores. From ~490 mbsf to the base of the logged section, the borehole character becomes more heterogeneous, with intermittent highs and lows in resistivity and seismic velocity. Below 508 mbsf, FMS images change to more closely spaced conductive features, which are continuous across all four pads at rare intervals, such as 513-514 mbsf. This is an interesting depth, as it corresponds to a thin layer of rock with true basaltic texture and a return to high magnetic susceptibility, similar to that seen at the top of the sill and the top of Subunit 4B, and interpreted as indicating single-domain (<100 µm) magnetite. Below this depth, several clear sets of polarity reversals are seen, indicating multiple periods of magmatic activity. The MST natural gamma measurements on the core suggest increased K, U, or Th concentrations in this lower part of Subunit 4B. Glass is more abundant below this depth, discrete and diffuse alteration is more extensive, and carbonate-bearing veins are present. Despite these differences, the generally microcrystalline and fine-grained material below this depth shares many textural, mineralogical, and chemical similarities with the overlying sections.
CORK-II Installation
Details of the core and borehole at the levels of the CORK-II installation in Hole 1253A are shown in Figure F21, with the petrological and structural character of key depths as shown in Figures F14 and F18. The center of the packer was set at ~473 mbsf, with the inflatable element being between 471.5 and 475.5 mbsf. The cores indicate that this is an interval of high recovery of massive rock with relatively few fractures. The logging results (see "Downhole Logging") show this to be in an area of relatively uniform physical properties (high resistivity, bulk density, and P-wave velocity). Interpretation of FMS images indicates a massive-blocky formation, with 0.5- to 1-m spacing between thin conductive features, which can be traced across the four pads. The upper OsmoSampler, located inside a 7.35-m-long screen in the 4-in casing, is set between 496.7 and 504 mbsf (Fig. F18). A 2-m pressure screen is located within the casing screen, and a fluid sampling line runs from this screen to the CORK-II wellhead. Figure F18 shows this to be an interval of modest recovery of moderately fractured rock composed of alternating microcrystalline and fine-grained material (see "Igneous and Metamorphic Petrology"). Logging data in Figure F14 show this to be an interval of generally uniform hole diameter, with minor variations in bulk density and P-wave velocity. FMS images show closely spaced conductive features. The lower sampler is dangled in the open hole between 512.1 and 519.5 mbsf. This is again a zone of moderate recovery and fracture density in a cryptocrystalline (basaltic) to microcrystalline part of the section, with relatively high concentration of voids and 10% to locally 50% secondary mineral formation. The logging data (Fig. F14) show this to be an interval of decreased resistivity and sonic velocity and variable hole diameter. In the upper part of this interval, FMS images show closely spaced (<0.5 m) shallowly dipping conductive features that are continuous and can be traced across the four FMS pads. The intervals for the osmotic samplers were chosen using a combination of scientific and operational constraints. Originally, the intervals between 513-521 (now OsmoSampler 2) and 560-568 mbsf were targeted, where the latter is a zone of high fracture density and maximum alteration in largely microcrystalline rock. However, the bridge encountered by the logging tools at 530 mbsf restricted the OsmoSampler deployment to shallower levels. The upper pressure screen, located above the packer, was set into the sediments between the two igneous subunits, where sediments collapsing around the screen are expected to make an effective seal. The final installed configuration for this modified CORK-II geochemical and hydrologic borehole observatory is shown in Figure F21. For details of the postcruise submersible visit to this site, see "Postcruise
Alvin Submersible Visit to Site 1253 and 1255
CORK-IIs".
Site 1254 is located ~1.5 km arcward from the deformation front at a water depth of 4183 m, close to the holes drilled at Site 1040 during Leg 170 (Kimura, Silver, Blum, et al., 1997). Hole 1254A is positioned ~15 m west of Hole 1040C, and Hole 1054B is ~50 m northeast of Hole 1040C (Figs. F3, F22, F23). Therefore, all comparisons to Leg 170 results are to Hole 1040C at Site 1040, as it was the only one that penetrated the décollement and underthrust.
The primary objective of Site 1254 was to investigate a fault zone in the prism, investigate the décollement, and install a long-term observatory for monitoring of fluid flow, pressure, and temperature in the décollement. Results from Site 1040 (Kimura, Silver, Blum, et al., 1997) and seismic data (Fig. F23) provided the framework for drilling the sedimentary sequence and the interpretation of pore fluid geochemistry and structure. Site 1040 geochemical anomalies suggest that deeply sourced fluids, perhaps from seismogenic depths, are migrating along the décollement and prism fault. Site 1254 was intended to investigate in detail the structure and geochemistry of these zones and install an observatory in the décollement. Although perturbed by drilling disturbance, high recovery at Site 1254 enabled detailed structure observations (where they were considered reliable) and higher-resolution chemical sampling than was possible during Leg 170. It is also possible to better correlate intervals of maximum inferred fluid flow to specific structural horizons.
The seismic record (Fig. F23) in the vicinity of Site 1254 (CMP 3130) shows no coherent reflections above the décollement. This reflects the general chaotic sedimentary pattern observed in cores from Hole 1254A. The first prominent reflector relevant for drilling objectives is at 6 s TWT, which marks the boundary between margin sediments and the underthrust sequence, cored at 361 mbsf. The prism fault zone is not imaged in the seismic data.
After setting the reentry cone in Hole 1254A, we cored the prism fault zone (150-230 mbsf) and the décollement (300-367.5 mbsf) with the RCB. Recovery averaged ~88% throughout the cored interval. With generally good hole conditions, we planned to case the hole with 10-in casing. However, after running the casing to 232 mbsf, the casing could no longer advance and had to be pulled up. Soon it became clear that the reentry cone had hung up on the casing; when the sections that were jammed into the cone were pulled up into the moonpool, it became obvious that the casing had collapsed in the throat of the reentry cone for unknown reasons. Hole 1254B, the second attempt for a CORK-II installation, was offset 50 m to the northeast. However, drilling conditions there prevented us from deepening the hole to >278 mbsf, when the drill string got stuck during several attempts to deepen the hole. Therefore, we decided to install the osmotic fluid sampler in the upper fault zone with the screen located at 225 mbsf; this interval, cored and analyzed in Hole 1254A, was not recored because of time constraints. The depth for the screen was determined by inference from the geochemical results of Hole 1254A, which indicate that deeply sourced fluids containing thermogenic hydrocarbons are present in the target zone. After a successful installation of the 10-in casing, the installation of the CORK-II failed as it got stuck ~20 m above the final depth. Attempts to penetrate further probably caused the 4-in casing to break right below the CORK-II head. Thus, we had to abandon Hole 1254B with ~20 m of casing sticking out of the reentry cone.
In total, we drilled 367.5 m at Site 1254, with 140.5 m cored and 227 m drilled and washed. Because of the nature of the tectonic structures encountered, part of the core was heavily disturbed by RCB drilling, which makes structural and paleomagnetic studies difficult. However, the generally good recovery (average = 89%) allowed extensive whole-round sampling of the cored sections for pore water and organic geochemistry in addition to shipboard sampling for physical property and paleomagnetic studies and provided personal samples for postcruise studies.
Lithostratigraphy
The sedimentary sequence recovered at Site 1254, Subunit P1B after Leg 170, is dominated by structureless and typically unsorted dark greenish gray claystones with variable subsidiary quantities of silt and rare interbedded volcanic ashes, sandstone, and redeposited limestone clasts, spanning a sparsely dated sequence of presumed Pliocene-Pleistocene age (Fig. F24). Recovered cores often show moderate to extreme degrees of drilling disturbance; nonetheless, coherent fragments of more lithified sedimentary rocks do indicate that much of the section is either massive or slightly mottled, which is suggestive of moderate bioturbation.
The dominance of clay minerals within the sequence is readily apparent from smear slides, as is the downcore decrease in volcanic ash. Fresh volcanic glass is present at low (<10%) and moderate (<30%) levels above 230 mbsf, becoming heavily altered deeper (>300 mbsf) in the section. The continental provenance of the sediments cored in Hole 1254A is clear from the abundance of quartz and feldspar grains and also from the bright, brownish red biotite mica flakes that are found at all stratigraphic levels. The terrigenous nature of the sediments is confirmed by the very low biogenic component (<5%) of the sediment, restricted to occasional nannofossils above 200 mbsf and below 360 mbsf. Below Section 205-1254A-15R-2 (360.62 mbsf), the proportion of diatoms increases sharply (>10%). The appearance of diatoms is considered important for understanding the structure of the forearc prism because the uppermost sedimentary subunit in the subducting Pacific stratigraphy (Subunit U1A) recorded high percentages of diatom abundance (Kimura, Silver, Blum, et al., 1997).
Redeposited blocks of shallow-water peloid limestones, lithified prior to incorporation within mudstones, are found throughout the section, which is consistent with fluidized gravity and debris flows being the dominant mode of sedimentation. The cobbles show evidence for a shallow-water depositional environment, identified by shallow-water bivalve shell fragments and small gastropods.
Compared to the sequence of well-preserved tephra found at ODP Sites 1039 and 1253 on the subducting Cocos plate, there is little well-preserved tephra stratigraphy found at Site 1254. Although occasional thin altered ash layers are recognized, they are rare, typically <2 cm thick, and often completely altered to claystone. Volumetrically, the tephra represents <1% of the total section. Two thicker coherent ash layers are recorded at Site 1254 (intervals 205-1254A-5R-8, 14-20 cm, at 193.49-193.55 mbsf, and 8R-8, 22-65 cm, at 222.37-222.80 mbsf). Both the thicker ashes preserve relatively fresh glass shards and are interpreted to be the product of primary air fall deposition followed by settling through the water column. The base of the tephra recovered in Section 205-1254A-8R-8 was not recovered, resulting in a minimum thickness estimate of 43 cm. Because Site 1254 is ~150 km from the nearest arc volcano in Central America, this thickness at this range indicates that this must have been a very large eruption, comparable to the Minoan Ash from Santorini as the closest analog (Watkins et al., 1978). Major and trace element analyses of this tephra (interval 205-1254A-8R-8, 22-65 cm) characterize its source as being the volcanic arc of Central America.
Structural Geology
Coring at Site 1254 targeted two different structural domains based on Site 1040 results: (1) a fault zone from 150 to 223 mbsf containing fractured sediment and locally steep bedding dips called the prism fault zone and (2) the décollement zone from 300 to 368 mbsf (Fig. F24). A variety of deformation structures is present at Site 1254, and description of deformation was based on breccia size, foliation, hardness of breccia clasts, and the presence of polished surfaces. Because structural observations in poorly lithified material require good quality cores and the recovered cores are sometimes severely disturbed by drilling, it is difficult to distinguish natural from drilling-induced features.
Cores from 150 to 223 mbsf show various degrees of deformation, with the intensity of deformation, particularly brecciation and brittle shearing, increasing downward, reaching a peak at ~219 mbsf. Deformation is discontinuous, being focused along sheared horizons, 20 cm to 2 m thick. These horizons are characterized by stratal disruption, foliated breccia with fragments as small as a few millimeters in length, brittle shear zones, deformation bands, and distinctly inclined bedding. Concentration of deformation structures at ~210 and 219 mbsf documents that this is indeed a fault zone. Riedel shears within a well-preserved foliated breccia (interval 205-1254A-8R-1, 0-24 cm; 213 mbsf) indicate reverse movement. Paleomagnetic reorientation of this shear zone suggests that the fault is a northeast- or southwest-dipping feature, implying that it is a thrust fault (Fig. F24) that strikes parallel to the deformation front.
The second interval cored started at 300 mbsf, and well-preserved structures are observed starting at Core 205-1254A-11R (319.30 mbsf) (cf. Fig. F24). Cores typically show pervasive drilling disturbance, previously described during Leg 170 as "spiraliferous" (Kimura, Silver, Blum, et al., 1997), consisting of a spiral rotation of clay-rich sections. Despite the drilling disturbance, some bedding plane orientations were observed. Bedding and fissility show various dips, indicating heterogeneity of deformation, but the paleomagnetic reorientation shows that they consistently dip northeast or southwest, with strike parallel to the deformation front. The recovered section from 319.30 to 367.50 mbsf is characterized by intense deformation. The deformation is heterogeneous, and brecciation, usually associated with a strong foliation, is the basis for dividing the deformed interval in two zones.
The upper zone from 319.30 to 328.90 mbsf is characterized by generally increasing brecciation with depth, producing fragments of <0.3 cm in length. Foliation is common throughout Core 205-1254A-11R, resulting in a clear alignment of clasts, which are equidimensional but internally strongly foliated. Below 324.15 mbsf (Core 205-1254A-12R) deformation sharply decreases and consolidated and coarsely brecciated sand layers become common. These sandstone layers have steeply dipping laminations and a few web structures. We interpret this well-defined change in deformation intensity to mark the top of a relatively less deformed rock volume that may be the footwall of the fault identified between 319. and 328.9 mbsf and may be related to the décollement zone. This indicates a more articulate structural geometry for the décollement and associated faults than that observed at Site 1040 (Kimura, Silver, Blum, et al., 1997; Tobin et al., 2001).
The upper boundary of the décollement zone at 338.5 mbsf is defined by increasing deformation intensity in Core 205-1254A-13R. The upper boundary of the décollement is difficult to define precisely because the deformation gradually increases in intensity with depth. A sharp increase in deformation is not observed between Cores 205-1254A-12R and 13R. The décollement zone itself is heterogeneous, with a general downward increase of brecciation intensity, fragment aspect ratio, and hardening of the sediments. Despite the good recovery, "spiraliferous" drilling disturbance affects the cores, although less extensively than at Site 1040. Unlike at Site 1040, "spiraliferous" disturbance is not concentrated in the lowermost part of the décollement zone. Brecciation can be pervasive and severe with fragments characterized by polished surfaces; the development of scaly fabric is precluded by the abundant silt and sand in the sediments. From 354.8 to 355.9 mbsf, sandstone layers are brecciated and foliated. At 360.60 mbsf the appearance of diatoms in the sediments marks the lithologic boundary with the hemipelagic Subunit U1A of the underthrust (Figs. F24, F25). The lithologic boundary is present below 50 cm of finely brecciated sand and 10 cm of highly sheared clay indicating a surface of ductility contrast which appears as a major structural discontinuity. The hemipelagic sediments below the lithologic boundary are still intensely deformed and brecciated with aligned clasts showing a strong internal foliation (Fig. F26). The base of the décollement is placed at 364.2 mbsf and is below the lithologic boundary. Deformation starts to decrease and becomes localized below 364.2 mbsf, where intact sediments are separated by 3- to 8-cm-thick brittle shear zones producing gouge or Riedel shears (Fig. F27). These brittle shear zones show exceptionally consistent normal movement and landward dips when reoriented to the geographical coordinates. The hemipelagic sediments above 364.2 mbsf are also deformed by normal faults; a few of them are present as conjugate features. At Site 1254 the décollement zone has a thickness of 25.7 m. Based on this interpretation, the décollement has cut down into the uppermost underthrust section, incorporating a small amount (4.2 m) of Subunit U1A into its base. The complex geometry of the décollement system at Site 1254 contrasts with that described at Sites 1040 and 1043, where the top of the décollement was identified by an increase in brecciation and the lithologic boundary between the prism and the hemipelagic subunit coincides with the base of the décollement.
Paleomagnetism
Paleomagnetic measurement on archive-half sections and discrete samples are severely degraded by pervasive drilling disturbance and drill string overprints. Natural remanent magnetization inclinations are still variable after alternating-field demagnetization and make the firm identification of magnetic polarity changes and the construction of a magnetostratigraphy difficult. However, the declination data were useful in carefully selected intervals to reorient core segments for structural interpretation. Demagnetization curves of discrete samples from the prism sediments (Subunit P1B) are often poorly behaved, indicating that they have a very unstable magnetization. Two significant high magnetic intensity and susceptibility zones were observed in the intervals from 184 to 202 mbsf and from 310 to ~350 mbsf. The interval of the first anomaly is close to the prism fault zone at ~210 to 220 mbsf, and the second anomaly is within the décollement zone. These variations suggest changes in concentration, grain size, and chemical components of magnetic minerals related to lithology and/or chemical alteration perhaps related to fluid flow.
Inorganic Geochemistry
A total of twenty 35- to 45-cm whole rounds were sampled at Site 1254 for pore fluid geochemistry. Pore waters were analyzed for Ca, Mg, K, Na, B, Ba, Fe, Mn, Sr, H4SiO4, NH4+, and SO42- concentrations (Fig. F24). Samples taken from between 305 and 366 mbsf were analyzed for Li, Ca, K, Mg, and Na in "real time" on the shipboard ICP-AES to identify the horizon of maximum flow of deeply sourced fluid within the décollement zone based on correlation to nearby Site 1040. The "real-time" chemical analyses were available 2 hr after core recovery and, together with careful observations of hydrocarbon gas concentrations and penetration rate, helped to identify the top of the underthrust section.
The pore fluid salinity in the prism sediments (Subunit P1B) is lower than that of seawater by 20%, and thin excursions of higher dilutions up to a maximum of 29% are present at 218 and ~351 mbsf (13 m from base of the décollement zone). The two main salinity minima also show C3H8, Li, and Ca concentration maxima, as well as Mg/Ca, K, and Mg minima. The geochemical excursions between 210 and 218 mbsf are present within a highly fractured interval interpreted as a fault zone, whereas the excursions at ~351 mbsf are located within the décollement zone and appear to coincide with a brecciated, moderately indurated, sandy interval. A small peak in Ca, Li, and C3H8 concentrations is present at 330 mbsf and may also be associated with a similar sandy, brecciated interval within the décollement. These data, together with results from the entire interval cored during Leg 170, suggest that fluid has migrated along conduits and permeated the lower half of the deformed wedge. Assuming that the geothermal gradient is ~20°-30°C/km, the source region must be present at >4 km depth because the minimum temperature required for thermogenic gas formation is 80°-90°C. The minima in K concentrations at 218 and 351 mbsf further suggests that the deformed sediments have been permeated by a fluid from an elevated temperature source of 80° to 120°C, where the illitization reaction, which consumes potassium, is effective. Also, the K depletion signature of this fluid provides an approximate upper limit to the temperature at the source of ~<150°C, although the data do not preclude the possibility of mixing between fluids from greater depths (>150°C) with shallower fluids along the flow path. Above this temperature, fluid-rock reactions leach potassium from the rocks. Lithium, like K, is partitioned into solids at low to moderate temperatures. At higher temperatures, >100°C but <250°C, Li is released into the fluid phase (Chan and Kastner, 2000). The precise threshold temperatures for the partitioning of Li and K into the solid or fluid phases are as yet unknown. Clay and other silicate mineral dissolution or alteration releases B into the fluid phase; however, clay, especially illite, formation consumes B and may be responsible for the low B concentrations within the deformed sediments. The deeply sourced fluid, however, is not enriched in dissolved silica.
Geochemical excursions in Ca, Li, C3H8, K, and Mg are present at ~218 mbsf within the prism fault zone at Site 1254. Similar increases in Ca, Li, and C3H8 concentrations, as well as marked decreases in Mg and K concentrations, were observed at an observed prism fault zone at Site 1040; however, the prism fault zone was present between ~180 and 200 mbsf. Therefore, the upper geochemical excursion in Hole 1254A is ~20 m below the same anomaly observed during Leg 170.
The geochemical change at ~218 mbsf separates intervals with pore fluid chemistry typical of clay-rich sediments from those permeated by a fluid from an elevated temperature source, and it seems to be independent of lithology. Bulk sediment chemistry is also relatively homogeneous throughout the entire prism. Changes in pore water chemistry in a lithologically and chemically homogeneous sediment section likely result from fluid advection into the lower half of the deformed sediment section. The chemical changes observed at the base of the fault zone (conduit) at ~218 mbsf are similar to those observed near the bottom of the décollement zone. Except for the biogeochemical components, the pore fluid concentration depth profiles of the underthrust section are similar to those at the reference Site 1039. The concentrations themselves are slightly different in magnitude than those at Site 1039, presumably reflecting the changes in solubilities and dissolution rates of the major sediment components under the new pressure regime as they are underthrust. In contrast to Site 1039, the higher NH4+ concentrations and the absence of SO42- at the interface between décollement and the underthrust sediments reflect the fact that all the sulfate is reduced at Site 1254 by microbiological activity. Sulfate reduction thus reaches completion in the uppermost few meters of the underthrust hemipelagic section, resulting in somewhat elevated CH4 concentrations within the zero-sulfate depth interval. These geochemical patterns are similar to those observed in Hole 1040C.
Organic Geochemistry
Volatile hydrocarbon gases were sampled by headspace and vacutainer techniques at a higher frequency than pore water samples to assist in determining the exact depths of the inferred fluid conduits associated with fault zones discovered at Site 1040. Analyses of the vacutainer samples (Fig. F24) show that the gas mainly consists of CH4 but also contains considerable amounts of higher alkanes up to C5H12. Methane concentrations were very high (7-9 x 105 ppmv) throughout the cored interval but dropped to ~4 x 104 ppmv directly below the décollement zone at 364 mbsf. Propane, which is a strong indicator of deeply sourced fluids because of its thermogenic origin (>80°-90°C required), shows one peak at 216 mbsf and another in the basal part of the décollement zone at 355 mbsf, with maximum levels of 326 and 370 ppmv, respectively. These high C3H8 concentrations correlate with structurally identified fault zones. Similar patterns, at much lower concentrations, were also observed in the headspace gas samples.
Microbiology
Samples for microbiological investigations were taken and either frozen or fixed for postcruise ATP quantification, DNA assessment, or cell counts. Samples of drilling water were frozen to evaluate contamination of cores. The chemical tracer for quantifying microbiological contamination was not deployed during coring at Site 1254 because of concern that the trace element chemistry of the PFT may affect postcruise pore fluid geochemical analyses. Particulate tracer tests yielded fluorescent microsphere counts suggesting very low to no particulate contamination in the interior of the microbiology whole rounds.
Physical Properties
Porosities and bulk densities at Site 1254 (Fig. F24) exhibit trends similar to those seen at Hole 1040C. Variations in porosity and density within the structurally defined décollement zone correlate with core descriptions: in general, zones of lower porosity (40%-45%) correspond to zones characterized by "spiral" deformation interpreted as drilling disturbance; zones of higher porosity (50%-55%) correspond to zones characterized by brecciation. Porosity is also low (42%-44%) between 358 and 361 mbsf, within and adjacent to a zone of localized shear. Porosity increases and bulk density decreases sharply below 361 mbsf across the lithologic boundary between prism sediments and Pleistocene diatomaceous claystone.
Downhole Measurements
We attempted three downhole measurements of formation temperature and pressure, two with the DVTPP at 50 and 200 mbsf and one at 150 mbsf with the Davis-Villinger Temperature Probe (DVTP). The temperature measurement at 200 mbsf was the only deployment with an interpretable decay curve and indicated a temperature of 3.59°C. This is in good agreement with measurements from Hole 1040C. All pressure measurements were unsuccessful because of tool movement when in formation. However, pressures measured at the mudline and bottom of the hole are in very good agreement with expected hydrostatic pressures expected at that depth, which clearly demonstrates that the pressure sensor is reliable.
Summary
In summary, the analyses of structural fabric and geochemical anomalies allowed us to identify a fault and geochemical boundary at ~218 mbsf. The region above has pore fluids typical of clay-rich sediments; below, the section is lithologically homogeneous but permeated by a fluid from a source at elevated temperature. At ~338.5 mbsf a fault marks the upper boundary of the décollement zone, which extends into the upper meters of the underthrust sequence at 364.2 mbsf. Maximum pore fluid chemical anomalies, indicative of active fluid flow, may preferentially follow zones characterized by brittle fabric. Analysis of cores from the two intervals allowed us to select the optimal depth interval for the long-term borehole fluid sampler experiment. However, because of unstable hole conditions, the attempts to install a CORK-II failed and Site 1254 had to be abandoned because of time constraints.
Site 1255 is located ~0.4 km arcward from the deformation front at a water depth of 4311.6 m and in close vicinity to the holes of Site 1043 drilled during Leg 170 (Kimura, Silver, Blum, et al., 1997). Hole 1255A is ~20 m east of Hole 1043A and ~30 m northwest of Hole 1043B (Figs. F3, F28, F29). In Hole 1043A the complete section was cored to 282 mbsf in the underthrust sequence (Unit U3), whereas Hole 1043B was logged using LWD to 482 mbsf, the top of the igneous basement. As both holes penetrated the décollement, their results were used to plan the drilling strategy and the installation of the CORK-II observatory.
The objective of Site 1255 was to identify the décollement with "real-time" geochemical analyses and penetration rate and to install a long-term observatory for monitoring of fluid flow, pressure, and temperature in the décollement. Because of time constraints, only four cores were taken from 123 to 157 mbsf. We recovered 7.2 m (21%) from the 34-m cored section. Because of the limited recovery and whole-round sections taken for pore water analyses for locating the décollement, other studies on the cores were limited.
The seismic record (Fig. F29) in the vicinity of Hole 1255A (CMP 3174) shows no coherent reflections above the décollement. This reflects the general chaotic sedimentary pattern observed already in cores from Site 1254 and in Leg 170 results. The confused seafloor reflection pattern, masking a clear seafloor identification, is probably due to side echoes, generated by local bathymetric relief (Fig. F3). The first prominent reflector relevant for drilling is at 5.96 s TWT, which marks the boundary between margin sediments and the underthrust sequence, cored at 144 mbsf in Hole 1255A. The current seismic data do not show any evidence for fault zones above the décollement.
After setting the reentry cone in Hole 1255A, we drilled to a depth of 123 mbsf with a 14-in bit. We then installed 10-in casing to a depth of 117 mbsf and cemented it in. Coring started at 123 mbsf, after drilling out the cement shoe, and stopped at 157 mbsf when a sudden increase in penetration rate during cutting of the fourth core indicated that the underthrust sediments were reached. The installation of the CORK-II was successful and completed with the deployment of the remotely operated vehicle (ROV) platform. A postcruise Alvin dive showed the installation to be fully operational, and pressure data showed a return to hydrostatic conditions within the borehole.
Lithostratigraphy
The section recovered from Hole 1255A can be separated into lithostratigraphic Unit T1 (equivalent to Subunit P1B at Site 1254) and Subunit U1A just beneath the level of the décollement at 144.08 mbsf. The division marks a sharp junction between a series of structureless greenish gray claystones with silts and few or no diatoms above an underlying series of diatom-rich claystones with interbedded silts, sands, and occasional fine-grained conglomerates. The clastic sediments in the underthrust section (Subunit U1A), interpreted as near-trench turbidites during Leg 170, differ from the purely hemipelagic diatom ooze recovered at Site 1043 (Kimura, Silver, Blum, et al., 1997). The fact that these turbidites were not recovered at Site 1043 may indicate that this section is very thin and simply not recovered or not present at all because of lateral facies changes over short distances. Blocks of reworked carbonate in Unit T1 indicate that these sediments are mostly debris flow deposits, in part derived originally from shallow nearshore environments. The presence of a pelagic nannofossil chalk interval and the larger proportion of diatoms at Site 1255 suggests that this site experienced a larger amount of pelagic sedimentation than did Site 1254. The underthrust section represents a trench depositional setting, with turbidite silts and sands interbedded with hemipelagic mudstones. Unlike other sites cored during Leg 205, sediments at Site 1255 contain no primary ash layers in the short section recovered; however, fresh and altered volcanic glass shards do compose a significant proportion (10%-15%) of the prism sediments.
Structural Geology
At Site 1255, structural deformation, with brecciation and polished clast surfaces as an indication of incipient scaly fabric, increases within the recovered section from 132.7 mbsf to the base of the décollement at 144.08 mbsf. The top of the décollement zone could not be defined as a result of limited coring and recovery. The base of the décollement is sharp and well defined and coincides with the division between Unit T1 and Subunit U1A. Only one measurement of bedding dip (44°) was possible in the décollement zone. The hemipelagic layers in the underthrust section below 144.08 mbsf show some medium-scale brecciation (1- to 3-cm fragments) with unpolished surfaces, whereas the sandy layers are undeformed.
Organic Geochemistry
Volatile hydrocarbon gases were sampled using the vacutainer and headspace technique. As no gas voids were apparent at Site 1255, in contrast to Site 1254, vacutainer samples show large air contamination. Headspace CH4 concentrations drop rapidly from >3000 ppmv in Core 205-1255A-2R to 5 ppmv below the décollement (145 mbsf). Propane, as an indicator for deeply sourced fluids, is low in the prism section (~1 ppmv) and absent below the décollement.
Inorganic Geochemistry
Only three whole rounds (one per core) could be taken for geochemical analyses of pore waters because of low recovery, and all conclusions are therefore somewhat speculative. As observed at Site 1254, the chemical composition of the pore fluids at Site 1255 also is distinctly different in the wedge and underthrust section, with a less sharp transition at the base of the décollement zone at ~144 mbsf. Fluid flow is indicated by salinity, Na and Ca concentration minima, and a Li concentration maximum within the décollement sample at 134.2 mbsf. Similar concentration variations were observed at Site 1043 along with a magnesium concentration maximum within the same interval. Across the décollement, changes in Ca and Mg concentrations are in the opposite direction to those seen at Site 1254. In general, the pore fluids within the upper fault zone and in the décollement at Sites 1254 and 1040 are characterized by having a significantly stronger signature of a deeply sourced fluid than the pore fluids from Sites 1255 and 1043 (see "Inorganic Geochemistry" in the "Site 1254" chapter). At Sites 1255 and 1043, the pore fluid chemistry in the wedge and the uppermost underthrust sediments appears to reflect some mixing between the lower wedge and uppermost hemipelagic pore fluids, thereby partially obscuring the deeply sourced fluid signature observed at Site 1254. This pore water mixing could be achieved by advection of fluid from the underthrust section across the décollement and into the lower wedge.
Microbiology
Two samples for microbiological investigations were taken and either frozen or fixed for postcruise ATP quantification, DNA assessment, or cell counts. Samples of drilling water were frozen as well to evaluate contamination of cores. The chemical tracer for quantifying microbiological contamination was not deployed during coring in Hole 1255A because of concern that the trace element chemistry of the PFT may affect postcruise pore fluid geochemical analyses. Particulate tracer tests yielded fluorescent microsphere counts suggesting very low to no particulate contamination in the interior of the microbiology whole rounds.
Paleomagnetism
Paleomagnetic declination and inclination, measured on discrete samples and archive halves, disagree only in the upper part (132.76-134.84 mbsf) of the cored interval, making any magnetostratigraphic interpretation questionable within this interval. Across the décollement, a clear polarity change can be seen in both archive halves and discrete samples. However, the polarity changes cannot be assigned to a particular chron and are not usable for dating purposes. Magnetic intensities and susceptibilities are generally low in the upper part of the section but increase substantially in the sandy layers of the underthrust sequence.
Physical Properties
Sample porosities and LWD porosities from Leg 170 show a clear increase at the base of the décollement from values of ~55% in the prism section to values of ~70% in the underthrust. Sample porosities from Hole 1255A clearly confirm the values in the prism, but the few data points below the décollement are not representative of the porosity of the underthrust sequence but, rather, show the influence of sampling clayey or sandy material. Only the magnetic susceptibility shows a marked increase below the décollement, which reflects a presumably higher magnetite content in the turbidites of the underthrust.
CORK-II Installation
A CORK-II observatory was successfully installed as shown in Figure F30. The center of the packer is at 129 mbsf and the center of the screen at 140 mbsf, in the middle of the geochemical anomaly as determined from Site 1255 data and Site 1043 results. The second pressure port inside a small screen was installed just above the upper packer. This CORK-II installation was also visited by Alvin shortly after Leg 205 and was found to be fully operational (see next section).
