SCIENTIFIC OBJECTIVES

During ODP Leg 205, Cork-II observatories were installed at two sites across the Middle America Trench off the Nicoya Peninsula, Costa Rica (Fig. F1), drilled previously during Leg 170 (Kimura, Silver, Blum, et al., 1997; Morris, Villinger, Klaus, et al., 2003). The observatories are designed to monitor pressure and temperature changes through time in a horizon of subseafloor fluid flow and to collect a time series of fluid and gas samples for subsequent chemical analysis (Jannasch et al., 2003). One observatory was installed at Site 1253 on the incoming oceanic plate with instruments located within the fractured igneous section at 494–504 and 512–520 meters below seafloor (mbsf). Another CORK-II was installed at Site 1255, 0.4 km inboard of the deformation front, to monitor and sample the region of maximum fluid advection within the décollement at 136–144 mbsf. Downhole instrumentation in the décollement includes a flow meter, such that dilution of tracers injected at a constant rate translates to fluid flux and the four sampling ports may allow identification of anisotropy in fluid flow.

The science goals of the present operation remain those of ODP Leg 205—to investigate active fluid flow across the Costa Rica margin and its implications for the seismogenic zone and subduction factory. One specific opportunity this operation affords is combination of pressure data recovered during the Atlantis cruise with downhole temperature variations through time and the time series chemical data to investigate the transient pressure events recorded at Site 1253 and the overpressured décollement at Site 1255.

There is strong evidence for vigorous shallow flow of cool fluids, which may affect the updip limit of seismicity, in the oceanic section of the subducting plate at Sites 1039 and 1253. East Pacific Rise (EPR)-generated oceanic crust (~24 Ma) at the drill sites is part of a large regional low heat flow anomaly; at the Leg 170 and 205 sites, heat flow is ~15% of that expected for the plate age, implying significant advection of cool fluids (Langseth and Silver, 1996). Heat flow data collected during recent cruises show that seamounts are sites of fluid discharge and recharge (Fisher et al., 2003), and modeling suggests that lateral flow rates of 3–30 m/y in zones within the upper 600 m of high-permeability (10^–10 to 10^–8 m²) basement are required to match the low heat flow on EPR-generated crust (Hutnak et al., submitted, 2003). Chemical data also suggest vigorous and recent/contemporaneous fluid flow. For example, Sr isotopic compositions measured in pore fluids squeezed from sediments show a strong mixing trend toward modern seawater ratios in the basal sediments. These basal sediment values are distinct from those appropriate for seawater contemporaneous with the sediment age or for pore fluid compositions modified by ash weathering, as seen higher in the sediment column (Silver et al., 2000). Simple modeling suggests that unless supported, the gradients, also observed for Li, Ca, and SO₄, would normalize by diffusion in ~15 k.y. Just south of the drill sites, plate reorganizations juxtapose cool EPR crust and ~22 Ma crust generated at the Cocos-Nazca spreading (CNS) center (Barckhausen et al., 2001), which is characterized by heat flow consistent with conductive lithospheric cooling models. This juxtaposition apparently corresponds to a significant change in the updip limit of the seismogenic zone. At 75 km from the trench, where cool EPR crust is subducting, this zone is at ~20 km depth; at ~60 km from the trench, where warmer CNS crust is subducting, this zone is at ~10 km depth (Newman et al., 2002). At Site 1253, the interval below 473 mbsf is packed off and two OsmoSamplers with temperature loggers are centered within fractured intervals at 500 and 516 mbsf, respectively.

Fluids from the décollement zone can be analyzed for a variety of chemical tracers to identify fluid sources, map fluid and element transport, constrain fluid fluxes, and possibly help constrain mineralogy at the updip limit of the seismogenic zone. At the décollement sites (1040, 1043, 1254, and 1255), pore fluid analyses across the plate boundary show strong, narrow (less than the full depth of the décollement zone) anomalous abundances of thermogenic hydrocarbons through C₁₀ and other tracers (e.g., Ca, K, and Li). Taken together, the compositional anomalies indicate vigorous advection within the décollement transporting species generated at temperatures >150°C (i.e., at or near temperatures thought to exist at the updip limit of the seismogenic zone). The persistence of local compositional anomalies suggests transient flow. The OsmoSampler and OsmoFlowmeter are located within the décollement at Site 1255. Tracers such as K/Li ratios and B and Cl isotopes in the fluids may constrain the extent of smectite-illite reaction in the fluid source region; adding O and Sr isotope ratios should further constrain bulk composition and temperature of the fluid source region. Tracers of interest to geochemists investigating element recycling in volcanic arcs via subduction (e.g., U, Pb, Rb, Sr, Ba, Cs, As, B, and Li) will also be analyzed in the sampled fluids. Pumping at a constant rate, the OsmoFlowmeters inject density-compensated iodate-tagged artificial seawater, Cs, and Rb into the borehole below the OsmoSampler. Four sampling ports on a plane with the injection port collect and archive a time series of tagged fluids for subsequent recovery and analyses. Dilution of these tracers will constrain fluxes and, possibly, anisotropy (although not directionality in a geographic sense) of fluid flow. Flux rates of elements from the subducting plate carried in fluids advected from the deeper source will be useful for investigating methane fluxes and the impact of shallow slab dewatering on ocean chemistry and composition of the residual subducted slab at greater depths (ultimately to depths of magma generation).

Dive operations during the Atlantis cruise included downloading data from the multilevel CORKs at Sites 1253 (sampling/monitoring screens at two levels in uppermost igneous basement) and 1255 (screens at the décollement and in the overthrust section). Pressure variations are dominated by tides at the seafloor, response to seafloor tidal loading in the formation, and overpressures at Site 1255. Complete hydrologic sealing took several weeks; the most significant leakage in the first few weeks of monitoring is inferred to have been associated with the high-pressure polypack glands that seal the CORK liner and main casing. Once the seals seated, signals ranging in period from weeks to minutes are observed from barometric, oceanographic, and tectonic sources. Several observations, summarized in the records shown in Figure F2, are of particular interest from a hydrologic and geochemical perspective.

At Site 1253, basement is underpressured relative to the local geotherm hydrostat by ~7 kPa (Fig. F2A), from which it can be inferred that the basement is highly permeable and provides a close-to-hydrostatic drainage path to the ocean for the seaward part of the underthrust sediment section. The degree to which fluids squeezed from the subduction zone sediment complex influence basement fluid composition remains unknown, but it is clear that upper permeable basement provides a link to deep-sourced fluids. Thus, obtaining basement fluid samples is an important priority. Because of the subhydrostatic basement state, this fluid sampling can be done only with an in situ sampler sealed in the hole.

At Site 1255, fluid pressures in the décollement and the overlying overthrust sediments are superhydrostatic, varying with time (Fig. F2B). Maximum pressures are a significant fraction of lithostatic and decline steadily over the first few months of recording. Several events of tectonic (elastic) or hydrologic (diffusional) origin are observed at both screens. One of these (labeled "first event") is seen at the upper screen roughly 2 days before the décollement screen. This precludes the possibility that the event is associated with motion of the packer and indicates a hydrologic source. Observations of fluid-compositional variations will be critical for determining the cause of such events and the slow pressure decline.