The character of the incoming plate subducting at convergent margins and the processes affecting it as it passes below the shallow forearc may play a major role in the nature and extent of hazardous intraplate seismicity as well as the magnitude of volcanism and the chemistry of lavas produced in the overlying volcanic arc. The fate of incoming sediments and ocean crust and of their associated volatiles as they pass through the shallow levels of a subduction zone (0–50 km depth) has profound effects on the behavior of the seismogenic zone, which produces most of the world's destructive earthquakes and tsunamis. Fluid pressure and sediment porosity influence fault localization, deformation style, and strength and may control the updip limit of the seismogenic zone. Fluids within both fault zones and sediments underthrust at the trench affect early structural development and are a key agent in transport of chemical species. The mineralogy and chemistry of any subducted sediments and their dehydration reactions during subduction may control the physical properties of the deeper subduction interface and, hence, the updip and downdip limits of the seismogenic zone wherein interplate earthquakes are generated. The mineralogy, composition, and volatile content of the slab, transformed during its progress through the shallow subduction zone, will govern the flux of fluids or melts from slab to mantle wedge, which is an important control on the extent of mantle melting and formation of arc lavas.

Costa Rica is an important area for studies of the seismogenic zone and subduction factory for several reasons. As one of the few modern arcs subducting a carbonate-rich sediment section, Central America permits study of CO₂ recycling through a subduction zone. Changes along strike in seismicity, plate coupling, and volume and composition of the arc lavas (between Nicaragua and Costa Rica) appear to correlate with changes in sediment dynamics. This balance between sediment accretion, underplating, erosion, and subduction may ultimately result from changing bathymetry, thermal structure, or hydrological behavior along the margin.

Science objectives for Leg 205 have two primary foci, both related to seismogenic zone and subduction factory questions. The first is to determine the igneous and alteration history of the uppermost part of the downgoing plate at reference Site 1253, along with the inferred distribution of fracture permeability in the core and borehole. The second is to characterize and monitor two of the three hydrological systems inferred from Leg 170 results: in basement at Site 1253 and along the décollement (or upper fault zone) at Sites 1254 and 1255. These goals will be accomplished by (1) targeted coring of selected intervals, (2) downhole temperature and pressure measurements, (3) logging at Site 1253, and (4) installation of long-term observatories (modified CORK-IIs) to monitor temperature and pressure and to sample fluids and gases in each of the hydrologic systems. In the décollement zone, instruments will also be deployed to attempt to measure fluid flow rates. Temporal variation of fluid composition in the sealed-off intervals will be obtained by using osmotic fluid samplers. The samplers and temperature loggers will be recovered for analysis 1 to 2 years after installation, pressure data will be downloaded, and new samplers and temperature probes will be installed.

Science objectives specific to the reference Site 1253 center on mass flux to the subduction trench (and ultimately the volcanic arc) as well as the permeability and hydrology of the downgoing igneous section. In conjunction with Leg 170, the coring at Leg 205 provides samples that will allow an estimate of the sedimentary carbonate flux to the trench to be made. Although not a primary focus of the cruise, coring from the two legs also provides an extensive ash stratigraphy as the plate moved from near the Galapagos to outboard of the Middle America Trench. Leg 205 coring into the igneous section will be used to investigate the extent of sill emplacement and their origin and contribution to the bulk composition of the subducting plate along with their tectonic and magmatic implications. If a transition to basement is recognized in the lower igneous section, the extensive recovery of Leg 205 can be used to determine the primary and secondary mineralogy and bulk chemistry of the uppermost part of the downgoing plate. These results can be integrated with those from the deep basement hole planned for Leg 206 (also on the part of the Cocos plate generated at the East Pacific Rise) to constrain the composition of the oceanic crust subducting at this margin. Microbial samples were taken (and contamination tests run) from the sedimentary horizons and from larger veins and fractures in the lower part of the igneous sections for postcruise studies. Logging, coring, and physical properties measured during Leg 205 establish key characteristics relevant to fluid flow and deformation, such as porosity, density, fracture distribution, orientation, and strength, ultimately to be used in conjunction with pressure, temperature, and chemical data from the CORK-II seafloor observatory.

Science objectives for the prism Sites 1254 and 1255 center on the development of the décollement, the use of pore fluid chemistry to infer local diagenetic and deeper dehydration reactions, and the installation of two CORK-IIs, ideally, in the region of maximum fluid flow within the décollement or related fault zones. Leg 205 coring intersected a thrust fault, as well as the décollement zone, in two locations slightly different from the two sites cored during Leg 170. Together, they provide the opportunity to examine variability over short distances in the development of the fault zones, the role of variable sediment lithology in strain partitioning and the style of décollement development, and the interplay between structural features and zones of fluid flow. Postcruise analysis of structural fabrics and experiments on whole-round samples will better constrain hydrological modeling and permit integration of fluid flow and deformation models. Sediments, gases and interstitial waters collected in the thrust fault and décollement during Legs 170 and 205 have some chemical features expected of deeply sourced fluids and provide samples for postcruise chemical and isotopic analyses, as well as for identifying key horizons for the CORK-II long-term seafloor observatory. Whole-round samples from the décollement and contamination testing allow for postcruise microbiological investigations to determine the resident microbial ecology of the zone for comparison to eventual microbial experiments on fluids collected from the décollement.

Installation of long-term seafloor observatories was a major focus of Leg 205. Briefly, the CORK-II design is built around and within a 4¹/₂-in casing string and monitors a single interval below a single packer. Deployments here combine pressure monitoring as used during Leg 196 with retrievable OsmoSamplers at the horizon of interest (containing osmotic fluid and gas samplers and temperature loggers.) At the prism site, OsmoFlow meters are included, which use the dilution of a chemical tracer injected at a constant rate to estimate flow rates.

At Site 1253 on the incoming plate, we cored 230 m, including ~170 m within two igneous units, where the upper unit is a 30-m-thick gabbro sill with sediments above and below. We also logged up to 150 m, primarily in the lower igneous section, and installed a CORK-II observatory. An exceptionally thick ash from Central America was recovered just above the sill, deposited when the plate was >1650 km from the arc; grain size and thickness imply an eruption comparable to the largest ever recorded from Toba Volcano. Interstitial water chemistry from above and below the sill indicates that there is fluid flow, of near-seawater composition, at depth in the igneous section. The lower igneous unit is composed of microgabbro with occasional layers of fine-grained gabbro and rare intervals with basaltic texture; all are of basaltic composition. It may be an exceptionally thick sill composed of multiple intrusions or a series of often thick and slowly cooled lava flows of oceanic crust created at the East Pacific Rise or represent a transition between extrusive and intrusive activity at the ridge. It is more extensively altered and fractured below ~510–513 meters below seafloor (mbsf). A CORK-II was installed, with temperature probes and osmotic fluid and gas samplers at 497–504 mbsf and also at 512–519 mbsf. Pressure monitoring is within the upper OsmoSampler zone and above the packer at ~453 mbsf.

At Site 1254 in the prism, we cored through a thrust fault zone at ~197–219 mbsf and the décollement zone at 338–365 mbsf. Despite drilling disturbance, it is possible to see that deformation, particularly brecciation and brittle shearing, generally increase downward in both zones but with concentration of shear along specific horizons. High concentrations of thermogenic hydrocarbons in the gases and sediments and unique pore water chemistry are seen within both zones, indicating advection of deeply sourced fluids preferentially along sandy horizons showing brittle fracture. The base of the décollement lies within the uppermost part of the underthrust section at Site 1254, as opposed to between the prism and underthrust sediment at Site 1040, which is 50 m away. Three attempts were made to install a CORK-II at Site 1254, twice into the décollement zone and once into the shallower thrust fault. All failed because of a combination of operational difficulties and hole conditions.

At Site 1255, ~0.4 km inboard of the deformation front, we conducted very limited coring and installed a CORK-II into the plate boundary fault. The base of the décollement was placed at 144 mbsf and corresponds to the lithologic boundary between prism and underthrust sediments. The chemical signature of deeply sourced fluids was observed just above the décollement, but is weaker than at Site 1254. The CORK-II was installed successfully, with the packer centered at 129 mbsf and the OsmoSampler centered at 140 mbsf, along with a temperature logger and pressure monitoring screen.