Figure 1. Location map of central Juan de Fuca Ridge with place names used in text. Inset at bottom right corner shows location of area relative to northwest United States and Canada. AV is abbreviation for Axial Valley, V is for Volcano, and FZ is for fracture zone. Location of epicenters from June–July 1993 seismic swarms. Dashed lines show trends of Axial Volcano North Rift Zone (AVNRZ) and CoAxial neovolcanic zone. Diameters of circles are approximate length along axis of venting as observed in 1993. Contour interval is 200 m. Boxes outline areas covered by Figures 2, 5, 8, and 11.
Figure 2. Interpretative map of Axial Volcano and CoAxial segment based on SeaMARC II side-scan sonar and side-lit SeaBeam bathymetry. Dark shaded areas are major off-axis volcanos, medium shaded zones are high backscatter areas marking most recent off-axis lava flows, and light shaded zone marks approximate limit of major hotspot edifices of Axial Volcano and Brown Bear Seamount to the east. Faults are hachured. SeaBeam contours are lightly shaded and their limit marks limit of SeaBeam coverage. Coverage of SeaMARC II side-scan sonar surveys in 1983 and 1984 are shown in inset. SRZ is south rift zone, NRZ is north rift zone, SOBB is Son of Brown Bear Volcano.
Plate 1. Perspective side-lit SeaBeam bathymetry of CoAxial segment of Juan de Fuca Ridge with locations of major features discussed in text. View is looking northwest from eastern side of Axial Volcano. Latitudes and longitudes are only approximate because of foreshortening of image. Dashed lines with arrows are orientations and extent of diking based on geologic evidence for 1993 (red) and 1981–93 dikes.
Figure 3. (a) Sea MARC II swath of Coaxial segment and (b) interpretation. Note distinct graben containing northernmost young lavas. (c) AMS-60 side-scan mosaic of junction of AVNRZ and CoAxial axial valley and (d) interpretation. Note high backscatter lavas erupted along AVNRZ covering older (lower backscatter) fissured lavas.
Figure 4. Sr isotopes versus latitude, central Juan de Fuca Ridge.
Figure 5. Bathymetric map of northernmost portion of CoAxial segment, including the Flow site. Bathymetry is side-lit from west with 50-m contours superimposed. Darker shades are deeper. Box shows area of Figure 6. Location on regional bathymetry shown in Figure 1.
Figure 6. AMS-60 side-scan mosaic (two swaths) of 1993 and 1982–1991 lava flows at the Flow site, CoAxial segment. White lines are boundaries of young lavas determined by SeaBeam differencing anomalies and ROPOS and Alvin dives [Chadwick et al., 1995]. Dotted lines denote depth cross-section locations shown in Figure 7. Location shown in Figure 5.
Figure 7. Depth cross-sections along submersible traverses of young lava flows at the Flow site, CoAxial segment. Young lavas are highlighted by thicker lines. Locations shown in Figure 6.
Plate 2. A series of 35-mm photographs from ROPOS (R) and Alvin (A) of lava flows and vents on CoAxial segment. Origin and year of photograph are given by, e.g., R93: (a) Lobes of pillow lavas on east flank of 1993 flow. Darker bands on pillow (~0.75 m diameter) in upper middle right are very delicate veneer of glass which sloughed off on contact. White particles are fallout in water column (Figure 6, ROPOS Dive 219, R93). (b) Sediment-free glassy tubes and pillows on east flank of 1993 eruption mound (R93). Pillow tube in center is ~0.5 m across (Figure 6, ROPOS Dive 219, R93). (c) Crest of 1993 lava flow showing extensive coating of Fe-Si deposit probably precipitated by Fe-oxidizing bacteria. Temperature probe (~1 m long) at lower right measured exit vent fluid temperatures as high as 51°C in the vicinity of this site (R93). (d) Extensive venting along fissure south of 1993 eruption (R93). (e) Brecciated zone along path of venting south of 1993 eruption site (Figure 7, Alvin Dive 2990, A95). Pieces are ~0.25 m across. (f) Flank of 1982–91 lava site lying 500 m east of the 1993 flow (A93). (g) Crest of the 1982–1991 lava flow; note duller color than Plate 2c (A93). (h) "Floc" drifts amidst older pillow lavas (note thick sediment pockets) along center of axial valley at about 46°19.4N (R93). (i) Turbid vent fluid rising out of fissure near Huge Diffuse Vent (HDV) at Floc site (Figure 9) (A93). Fissure is ~1 m wide.
Figure 8. Bathymetric map of central CoAxial segment, including the Floc site. See Figure 5 caption for details.
Figure 9. AMS-60 side-scan mosaic of fissure swarm (solid white lines) and 1981–1991 lava flows (white outline) at the Floc site. Boundaries of young lavas, extent of venting along fissure, and vent sites (e.g., Mkr 18) determined from camera tows (Figure 8) and Alvin dives. Location of map shown by box in Figure 8. Dotted lines are depth cross-sections shown in Figure 10.
Plate 3. A series of 35-mm photographs from ROPOS, Alvin , and towed camera of Floc and Source sites: (a) Bacterial mats coating wall of fissure at Floc site (A-1993). (b) Nemerteans (each ~3 cm in length) on bacterial mats on wall of fissure at southern Floc site near marker 18 (Figure 9) (A95). (c) Tubeworms (~0.3 m long) at HDV vent, Floc site (A95). (d) Crabs feeding on remnants of tubeworm community at the HDV vent at the Floc site (R96). (e) Young lavas (upper) coating wall of older lavas in fissure at Floc site near marker 18 (A95). (f) The 1981–1991 pillow lavas at the Floc site. Note small but distinct sediment pockets in these lavas (A95). Pillow is ~1 m across. (g) Photo from USGS towed camera of sediment-covered lavas between the AVNRZ and the CoAxial neovolcanic zone. Photo is ~5 m across. (h) Top (~2 m) of 7-m chimney ("Mongo") at Source site. (i) Small chimneys (~0.5–1.0 m high) at Source vent lining up on structure oblique to primary chimneys.
Figure 10. Depth cross sections of Floc site. Locations shown in Figure 9.
Figure 11. Bathymetric map of southern CoAxial segment including the Source site. See Figure 5 caption for details. Location of Figure 12 is shown by box.
Figure 12. Bathymetry of "Source" site with camera, dive tracks, and locations of sites discussed in text. Side-lit from west. Contour interval is 5 m.
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