Figure 1A. Location map showing the tectonic setting of the sediment-covered spreading centers at Middle Valley and Escanaba Trough on the Juan de Fuca-Gorda spreading system. Modified from Davis, Mottl, Fisher, et al. (1992).
Figure 1B. Bathymetry of the Middle Valley (Davis and Villinger, 1992) shown as contours drawn at 10 m intervals. Areas of proposed drilling sites are the Dead Dog vent field (Site 858, DD1-4) in the area of active venting and Bent Hill (Site 856, BH1-6, BH8). Modifed from Mottl, Davis, Fisher, and Slack, (Eds.), 1994 .
Figure 1C. Bathymetry for the Escanaba Trough shown as 200 m intervals. Black areas are the igneous centers. Proposed drill sites are within the NESCA (northern Escanaba Trough) study area. SESCA is the southern Escanaba Trough study area. Modified from Zierenberg and Shanks (1994).
Figure 2. Triangular diagrams illustrating the (A) Pb- and (B) Au-rich nature of the Escanaba Trough (ET) massive sulfides relative to samples from Middle Valley (MV) or bare-rock hydrothermal systems. Other abbreviations used: TAG-Trans-Atlantic Geotraverse site; GAL- Galapagos spreading center; EPR- East Pacific Rise; END- Endeavour segment, Juan de Fuca Ridge; AX- axial seamount, Juan de Fuca Ridge; SJDF- southern Juan de Fuca Ridge; GB- Guaymas Basin. Modified from Koski et al. (1994).
Figure 3. Pb isotope composition of sulfide, sediment, and basalt samples for Escanaba Trough and Middle Valley. Escanaba Trough sulfide composition indicates a large contribution of sediment-derived lead to the hydrothermal deposit whereas the Middle Valley sulfides are intermediate between basalt - and sediment-derived fields. Isotopic data are from Zierenberg et al. (1993) and Goodfellow and Franklin (1993).
Figure 4. Map of the Dead Dog vent field in Middle Valley showing the location of the major hydrothermal mounds, active vents, holes drilled on Leg 139, and proposed sites for Holes DD1-4. 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. East-West multichannel seismic reflection profiles crossing the Leg 139 and the proposed Leg 169 drilling sites in Middle Valley (Sites DD and BH). Modified from Davis, Mottl, Fisher, et al. (1992).
Figure 6. Diagram showing the depth to the basement in the two Middle Valley holes that were instrumented with CORKs. Basement (shaded) at Site 857 is defined as the top of a sill-sediment complex at 470 mbsf. Basement under Site 858 rises to 250 mbsf and is extrusive basalt (from Davis and Becker, 1994a).
Figure 7. Map of Site 856 in Middle Valley area showing the location of the Bent Hill deposit and the two sulfide mounds to the south. A ridge-parallel normal fault bounds the west side of the sulfide deposits, Bent Hill, and similar uplifted sediment hills that occur south of the map area. Proposed sites BH1, BH2-6, and BH8 are shown. Modified from Goodfellow and Peter (1994).
Figure 8. North-south cross section (5X vertical exaggeration) of Middle Valley Site 856 area showing the exent of penetration of the massive sulfide deposit south of Bent Hill and the location of basaltic sills beneath Bent Hill. Modified from Davis, Mottl, Fisher, et al. (1992).
Figure 9. Map of the southern portion of the Gorda Ridge spreading center showing the sediment-filled portion of the Escanaba Trough (light shading), intratrough terraces (intermediate shading), and the volcanic centers (dark shading) that locally pierce the sediment cover.
Figure 10. Bathymetric map of the NESCA area of Escanaba Trough showing the location of the Southwest Hill, Central Hill, on-axis volcanic rocks, sulfide deposits (black), and active vent sites. Location of the proposed Sites ET1-5 and ET7 are shown. The location of faults is constrained by the seismic profiles, camera and submersibles scarps. Modified from Zeirenberg et al. (1994).
Figure 11. Heat-flow profiles in Escanaba Trough along (A) seismic line 3 (ET1-5) and (B) line 5 (ET7). Modified from Davis and Becker, (1994b).
Figure 12. Escanaba Trough. Detailed map of the Central Hill area of the NESCA site showing proposed drilling transect for Sites ET1-5 across the hydrothermally active massive sulfide deposits (outlined). Location of active vents, fault scarps, and exposed volcanic rock are based on camera tows and submersible tracks shown as thin lines.
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