5. Site 1254¹

Shipboard Scientific Party²

SITE SUMMARY

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. F1, F2, F3). 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. F3) 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. At Site 1254, we 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. The high recovery also made it possible to better correlate intervals of maximum inferred fluid flow to specific structural horizons.

The seismic record (Fig. F3) in the vicinity of Site 1254 (common midpoint 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 two-way traveltime, which marks the boundary between margin sediments and the underthrust sequence, cored at 361 meters below seafloor (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 rotary core barrel (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 sampling aboard the ship for physical property and paleomagnetic studies and provided personal samples for postcruise studies.

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. F4). 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 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 Volcano 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.

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. F4). 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. A 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. F4) 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. F4). 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 as marking the top of a relatively less deformed rock volume that may be the footwall of the fault identified between 319.3 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. F4, F5). 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 that 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. F6). 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. F7). 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.

Paleomagnetic measurement on archive-half sections and discrete samples are severely degraded by pervasive drilling disturbance and drill string overprints. Natural remanent magnetization (NRM) 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.

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 SO₄ concentrations (Fig. F4). Samples taken from between 305 and 366 mbsf were analyzed for Li, Ca, K, Mg, and Na in "real time" on the inductively coupled plasma-atomic emission spectrophotometer (ICP-AES) aboard the ship 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 suggest 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 SO₄ 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.

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. F4) 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.

Samples for microbiological investigations were taken and either frozen or fixed for postcruise adenosine triphosphate (ATP) quantification, deoxyribonucleic acid (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 perfluorocarbon tracer 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.

Porosities and bulk densities at Site 1254 (Fig. F4) exhibit trends similar to those seen in 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.

We attempted three downhole measurements of formation temperature and pressure, two with the Davis-Villinger Temperature-Pressure Probe (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.

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.

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²Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.

Ms 205IR-105