The Costa Rica convergent margin is formed by subduction of the Cocos plate beneath the Carribean plate off Costa Rica (Fig. F1). The subducted oceanic crust, created at the East Pacific Rise (EPR), is anomalously cold with a mean heat flow ~30 mW/m2 in contrast to the expected value of 90–120 mW/m2 for its crustal age of 20–24 Ma (Fisher et al., 2003). This is interpreted to be the result of hydrothermal circulation in exceptionally highly permeable upper oceanic crust, which effectively removes heat from the incoming plate (Fisher et al., 2003). This hypothesis is supported by geochemical pore water investigations on sediments recovered from an interval just above the sediment/basement interface at Site 1039 (ODP Leg 170) that suggests fluid flow of near-seawater composition below the sediments (Kimura, Silver, Blum, et al., 1997). Both the heat flow anomaly and pore fluid profiles have been modeled in terms of active fluid flow at rates of ~1 m/yr (Silver et al., 2000), having residence time of at least ~15,000 yr.
How and where this fluid flow takes place in the downgoing oceanic crust was one focus of Leg 205 (Fig. F1). Building on Leg 170, this leg was designed to investigate the igneous and alteration history of the downgoing plate and the hydrological activity across the Costa Rica margin. All Leg 205 (Sites 1253, 1254, and 1255) and Leg 170 (Sites 1039, 1040, and 1043) drill sites are located on one multichannel seismic profile (Fig. F1) that shows the structure of the subduction zone. At Sites 1039 and 1253, a 400-m-thick sequence of sediments overlays a gabbro sill as documented in cores from Site 1039 (Leg 170; Kimura, Silver, Blum, et al., 1997). It forms the strong reflector in the seismic section (Fig. F1) at 6.34 s two-way traveltime (TWT) and makes the top of oceanic basement below very difficult to identify.
Science objectives specific to reference Site 1253, located ~200 m seaward of the MAT (Fig. F1), center on mass flux to the subduction zone (and ultimately the volcanic arc) as well as the permeability, thermal structure, and hydrology of the downgoing igneous section. These goals were accomplished by coring, downhole temperature and pressure measurements, and logging (Shipboard Scientific Party, 2003a). A long-term pressure and temperature observatory CORK-II (Jannasch et al., 2003) with temperature probes and two osmotic fluid and gas samplers was installed in the basement; results are described in Heesemann et al., this volume, and Davis and Villinger (2006).
At Site 1253 on the incoming plate, we cored 230 m, including ~170 m within two igneous units. The upper unit extends from 400 to 430 mbsf and is a gabbro sill with sediments above and below (Fig. F2). It is comparable to similar units cored at Leg 170 Holes 1039B and 1039C (Kimura, Silver, Blum, et al., 1997), which form the strong reflector in the seismic image in Figure F1. The second, lower igneous unit was drilled from 460 to 600 mbsf and is more extensively altered below 510–513 mbsf. Logging in these formations (Fig. F2) consisted of one run with the triple combination tool string (triple combo; Shipboard Scientific Party, 2003c), comprising density, porosity, and resistivity measurements, and two runs with the FMS-sonic tool string: the FMS for an electrical imaging of the borehole walls and the DSI tool for sonic measurements. DSI records allow for full waveform processing of P-waves, S-waves, and Stoneley waves.