Figure 1. Location map showing tectonic setting of the sediment-covered spreading centers at Middle Valley and Escanaba Trough on the Juan de Fuca-Gorda Ridge spreading system.

Figure 2. Bathymetry of Middle Valley (Davis and Villinger, 1992) shown as contours drawn at 10-m intervals. Dead Dog vent field in the area of active venting (Sites 856 and 1035) and Bent Hill (Sites 858 and 1036) are outlined.

Figure 3. Map of Bent Hill (Middle Valley area) showing the location of Bent Hill and the two mounds (Bent Hill and Ore Drilling Program massive sulfide) to the south. The positions of holes drilled during Leg 139 (black circles) and Leg 169 (white circles) are shown. Modified from Goodfellow and Peter (1994).

Figure 4. Map of Dead Dog vent field (Middle Valley area) showing location of the major hydrothermal mounds, active vents, and the holes drilled on Leg 169 (Holes 1036A, B, and C). The limit of the vent field was determined as the contour of the acoustic side scan sonar reflector. Modified from Butterfield et al. (1994).

Figure 5. Map of Escanaba Trough in the southern portion of the Gorda Ridge spreading center showing the sediment-filled portion of the trough (light shading), intra-trough terraces (intermediate shading), and the volcanic centers (dark shading) that rise through and locally pierce the sediment cover. The location of Sites 1037 (reference hole) and 1038 drilled during Leg 169 are shown. The white box shows the position of Fig. 9.

Figure 6. Detailed map of Central Hill (Escanaba Trough area) and the Northern Escanaba Trough study area (NESCA) showing the position of holes drilled at Site 1038 during Leg 169. Location of active vents, fault scarps, and exposed volcanic rock are based on camera tows and submersible tracks shown as thin lines.

Figure 7. Bent Hill Massive Sulfide in Middle Valley. Schematic graphic Log of Hole 856H showing, from the seafloor down, the massive sulfide mound, the underlying feeder zone, and the sulfide-rich banded sandstone at its base, the turbidite hemipelagic sediment, the basaltic sill complex, and the lower basaltic flows.

Figure 8. Ore Drilling Program Massive Sulfide in Middle Valley. Schematic graphic Log of Hole 1035H showing the complex succession of three massive sulfide zones each of which is underlined by a feeder zone.

Figure 9. Diagram showing the depth to basement in the two Middle Valley holes which were reinstrumented with CORKs. Basement (shaded) at Site 857 is defined as the top of a sill-sediment complex at 470 mbsf. Basement rises to 250 mbsf under Site 858 and is extrusive basalt (from Davis and Becker, 1994b). Lines and circles at the center of the hole represent the temperature string and the position of the thermistors (black circles).

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