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Figure 1. ODP Leg 205 Costa Rica drilling area including isochrons derived from mapped seafloor magnetic anomalies (from Barckhausen et al., 2001). Numbers indicate crustal ages in million years. Tectonic boundaries, locations of DSDP Leg 84 and ODP Leg 170 drill sites, as well as arc volcanoes (triangles) are shown. FS = Fisher Seamount, QSC = Quesada Sharp Contortion, MAT = Middle America Trench.

Figure 2. Bathymetry of the ODP Leg 205 Costa Rica drilling area (Ranero et al., 2000b), showing the Leg 170 and the proposed Leg 205 drill sites. Line marks two coinciding multichannel seismic lines (line CR-20, Shipley et al., 1992; line BGR-99-44, C. Reichert and C. Ranero, pers. comm., 2001).

Figure 3. Summary plots of recovered lithology during Leg 170 at Sites 1039, 1040, and 1043, with lithology of DSDP Site 495, on the incoming plate outboard of Guatemala, for comparison. Note the similarity of incoming sediment sections at Sites 1039 and 495, as well as the repetition of Site 1039 sequence and lithology below the décollement at Sites 1040 and 1043. Lithologic columns modified from Kimura et al., 1997.

Figure 4. A. Depth-migrated multichannel seismic line CR-20 across proposed Leg 205 drill sites (Stoffa et al., 1991; Shipley et al., 1992). Positions of proposed Leg 205 drill sites are identical to positions of the Leg 170 Sites 1039, 1040, and 1043. B. Post-stack time-migrated multichannel seismic line BGR-99-44 (processing by C. Ranero, GEOMAR, Kiel, Germany) across proposed Leg 205 drill sites (C. Ranero and C. Reichert, pers. comm., 2001). Positions of proposed Leg 205 drill sites are identical to positions of Sites 1039, 1040, and 1043 cored during Leg 170. CDP = common depth point.

Figure 5. Plot of cosmogenic 10Be vs. depth in sediment column at Site 1040. Made by cosmic rays in the atmosphere and decaying with a 1.5-m.y. half-life, measurable 10Be enrichments are seen in sediments younger than 7-10 Ma. The underthrust sediments beneath the décollement have high values typical of the incoming sediment section. Throughout the prism sediments of the upper plate, 10Be concentrations are very low, typical of sediments that are older than 3-5 Ma.

Figure 6. A. Strontium isotope ratios from pore water samples from Site 1039 are indicated by the solid line, with the dashed line showing the Sr seawater curve appropriate to the sediment age. Note the basal pore water trend toward modern seawater composition, with values greater than Miocene seawater, indicating a strong seawater component in basement fluids. B. High lithium concentration in pore water samples from the décollement and fluid conduit at Site 1040; see text for discussion of correlative indicators of higher-temperature (>110°-150°C) deeply sourced fluids advecting from depth along the décollement and upper conduit. C. Barium concentration in pore water samples in the uppermost part of the underthrust section from Site 1040; high pore water Ba concentrations from sulfate reduction have not diffused or advected into the décollement, indicating a separate fluid flow regime in the underthrust sediments.

Figure 7. Schematic illustration of physical and chemical characteristics of the three distinct hydrological systems in the Legs 170 and 205 area. Figure courtesy of D. Saffer and based on Saffer et al., 2000 and Silver et al., 2000.

Figure 8. A. Schematic of the proposed modified CORK for Site 1039R. B. Schematic of the proposed modified CORK for Site 1040R underthrust sequence. C. Schematic of the proposed modified CORK for Site 1040R décollement system. ROV = remotely operated vehicle.

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