Recent eruptions on the CoAxial segment of the Juan de Fuca Ridge: Implications for mid-ocean ridge accretion processes

R. W. Embley,¹ W. W. Chadwick,² M. R. Perfit,³ M. C. Smith,³ and J. R. Delaney⁴

¹Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Hatfield Marine Science Center, Newport, Oregon 97365
²Cooperative Institute for Marine Resources Studies, Oregon State University, Hatfield Marine Science Center, Newport, Oregon 97365
³Department of Geology, University of Florida, Gainesville, Florida, 32611
⁴School of Oceanography, University of Washington, Seattle, WA 98195

Journal of Geophysical Research, 105(B7), 16,501–16,525 (2000).
Copyright ©2000 by the American Geophysical Union. Further electronic distribution is not allowed.

7. Floc Site

The July/August 1993 CTDT (conductivity, temperature, depth, light transmission) profiles showed high light attenuation values in the plumes located around 46°18N and centered above the CoAxial neovolcanic zone in the middle of the axial valley [Baker et al., 1995; Embley et al., 1995]. This area became known as the "Floc site" (Figures 1, 8, and 9). It should be noted that at the latitude of the Floc site, almost all of the epicenters in the 1993 T wave swarm are west of the CoAxial neovolcanic zone. The Floc site is where a mismatch between the T wave locations and seafloor geology becomes clear. A ROPOS dive at the Floc site in 1993 observed large white particles up to 200 m above the bottom and "snowdrifts" of these flocculent particles on the seafloor (Plate 2h). After localizing the source area with the Scripps Deep-Tow system in August [Spiess and Hildebrand, 1993], Alvin dives in October 1993 documented extensive diffuse venting along a fissure and graben system, including some places where vent fluid was so laden with large white particles that the sites became known as "snow blower" vents (Plate 2i) [Haymon et al., 1993]. Taylor and Wirsen [1997] propose that production of these particles is controlled by direct excretion of filamentous sulfur from sulfur oxidizing bacteria. A high production of sulfur-rich particles was produced at the 9°45–52N East Pacific Rise site [Haymon et al., 1993], and there is photographic evidence (Embley, unpublished data, 1987) for a large particle accumulation on the seafloor following the mid-1980s eruption at the north Cleft site [Embley and Chadwick, 1994]. Hyperthermophilic Archaea (with optimal growth ranges of 90°C) were cultured from vent fluids sampled from the Floc site in October 1993 (but not subsequently), strongly implying the presence of a viable, substantive subsurface microbial biomass at this site in the months following the 1993 dike intrusion [Delaney et al., 1998; Holden et al., 1998].

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.

Observations from the Alvin dives and camera tows presented here and from the plume surveys [Baker et al., 1998] show that diffuse venting was occurring over >5 km along strike from 46°16 to 46°19N (between Mkr 18 and Mkr 5 on Figure 9). Alvin dives in the fall of 1993 observed only bacterial mats at these vent sites (Plates 2i and 3a), but subsequent dives in 1994 and 1995 to the same marked sites documented a rapidly evolving macrofaunal community [Tunnicliffe et al., 1997] (Plates 3b and 3c). By 1995, visible venting had declined to just a few isolated sites, notably at the Huge Diffuse Vent (HDV) and marker 18 sites (Figure 9) and by 1996, observations (using ROPOS) of extensive predation by crabs marked the rapid decline in the macrofaunal communities at the HDV site (V. Tunnicliffe, personal communication, 1996) (Plate 3d). Time series observations of hydrothermal plumes for the same period also show a rapid decline. By 1996, heat and particle content above the Floc and Flow sites had returned nearly to background levels [Baker et al., 1998]. These observations are consistent with the interpretation that the 1993 dike intruded close to the surface beneath the Floc site but by 1996 had cooled to the point that it was no longer producing enough HS flux to support chemosynthesis at the seafloor/rock interface. The rapid contraction of hydrothermal/biological sites above a dike intrusion was also observed at the Cleft segment following its mid-1980s eruption [Embley and Chadwick, 1994; Embley et al., 1994].

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.

Structural control of the venting at the Floc site is revealed by side-scan imagery, which shows a braided and anastomosing fracture system 100–200 m wide cutting through low-relief seafloor from about 46°16N to 46°19.5N (Figures 9 and 10). This zone probably consists largely of preexisting structures that were reactivated during the 1993 intrusion (in contrast to the Flow site where the structures hosting active venting appeared to be new [Chadwick and Embley, 1998]). The southern part of the zone was apparently an eruptive vent in the recent past; submersible observations showed extensive drainback of young sheet flows between 46°17.5N and 46°18.5N. An old sequence of pillow lavas was observed to be partly "coated" with a veneer of younger sheet flows characterized by "bathtub rings" (Plate 3e). Such drainback features are characteristic of eruptive fissures observed (mostly) on intermediate to fast portions of the MOR. An interesting aspect of the venting at the Floc site was that while it was located within a zone that consists of many individual structures, the venting at any particular location was always localized along a single fissure or graben, usually the westernmost structure in the swarm. Hydrothermal fluids vented from the floor, walls, and rims of the structures.

Figure 10. Depth cross sections of Floc site. Locations shown in Figure 9.

Sohn et al. [1998] located nine microearthquakes at the center of the CoAxial axial valley between 46°13N and 46°17N during a 1-month deployment of an ocean bottom seismometer array in 1994, 1 year after the eruption. Sohn et al. [1998] attribute this activity to post diking tectonic adjustments, which is consistent with the geologic evidence presented here.

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