The realization in the 1960s that the world-encircling mid-ocean ridge (MOR) generates most of the Earth's volcanic activity has led to an increasing focus on this system with progressively more powerful mapping systems [Detrick, 1986]. Although the scale of the feature (~60,000 km) and its relative remoteness have been impediments to a comprehensive study, the combination of digital swath mapping bathymetric and side-scan sonar systems, detailed MOR basalt (MORB) geochemical analyses, common depth point seismic profiling, and the use of deep-towed cameras and submersibles have resulted in significant progress during the last 2 decades in defining the major volcanic and tectonic "units" of the system [Detrick et al., 1987; Lonsdale, 1977; Macdonald and Fox, 1983; Sempere et al., 1993] and in understanding some of the fundamental accretionary processes which occur at the ridge crest [Ballard et al., 1979; Kappel and Ryan, 1986; Macdonald and Fox, 1983; Tucholke and Lin, 1994]. Although these studies have provided good definition of long-term (~1010 years) patterns of crustal accretion and the tectonic framework of the oceanic crust, a detailed understanding of the temporal and spatial dynamics of MOR magmatism has been lacking. Unlike terrestrial volcanic areas, many of which are closely monitored with local seismic arrays capable of detecting and locating volcanogenic seismic events (M < 4), most of the MOR is monitored solely by the worldwide seismic net, which is limited to a detection threshold of M 4 [Fox, 1993/1994].
The first significant data about processes associated with discrete accretion events along the fast and intermediate spreading rate portions of the MOR became available from studies of sites on the Juan de Fuca Ridge (JdFR) and northern East Pacific Rise through the serendipitous discovery of seafloor eruptions [Chadwick et al., 1991; Embley et al., 1991; Haymon et al., 1993]. Detection of volcanic events on the Juan de Fuca and Gorda Ridges became possible in 1991 when the U.S. Navy allowed the National Oceanic and Atmospheric Administration (NOAA) access to the Sound Surveillance System (SOSUS) submarine hydrophone arrays in the northeast Pacific. The high efficiency of sound propagation in seawater in the Sound Fixing and Ranging (SOFAR) channel of the upper ocean lowers the detection threshold by about 2 orders of magnitude and increases the location precision by about 1 order of magnitude, making possible remote detection of, and subsequent response to, volcanic events in this area [Fox, 1993/1994]. As of late 1999, three eruptions on northeast Pacific spreading centers have been detected with this method and verified in the field [Dziak and Fox, 1999; Dziak et al., 1995; Fox and Dziak, 1998]. The first one was at the CoAxial segment in 1993.
In June 1993, within a week after the real-time acoustic monitoring system became operational, an unusual seismic swarm began on the northern part of JdFR just north of Axial Volcano (Figure 1) [Fox et al., 1995]. The swarm initially migrated north along the ridge axis for at least 25 km [Dziak et al., 1995] to the northern (deeper) end of the segment, where it subsequently became concentrated at approximately the center of the axial valley at 46°31N [Embley et al., 1995] (Figure 1). The only known eruption associated with the 1993 event occurs within this cluster of seismicity [Embley et al., 1995] (Figure 1). Several rapid response cruises took place between July and October 1993 and included water column plume surveys from the research vessels Tully, Discoverer, and Atlantis II, dives with the Remotely Operated Platform for Ocean Science (ROPOS) and manned submersible Alvin, and near-bottom surveys with the Scripps Deep-Tow and a U.S. Geological Survey (USGS) towed camera system. The details of the 1993 seismic swarm and the initial documentation of the water column and seafloor manifestation of the diking/eruption episode have previously been described in a series of papers [e.g., Fox, 1995].
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 JuneJuly 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.
The initial data sets generated several intriguing questions that have major implications not only for this particular event but also for the nature of MOR crustal accretion. An important first-order question was: Where did the dike come from? The tertiary water-borne seismic wave (T wave) epicenters were initially located west of CoAxial's axial valley, and the activity was originally thought to have begun on the north rift zone of Axial Volcano, which overlaps with the southern part of the CoAxial segment (Figure 1). However, the fresh lava and the active hydrothermal sites discovered during the 1993 cruises were aligned along the neovolcanic zone of CoAxial, with no indication that Axial had been involved. Water column and bottom surveys during the 1993 cruises localized three discrete venting sites (Flow site, Floc site, Source site) which were separated by 1520 km from one another (Figure 1). So, a second question was: What was the geologic and hydrothermal manifestation of the event along the path of dike injection? Detailed seafloor mapping of the segment and time series measurements at these vent sites over several years subsequent to the diking event have helped us interpret what parts of the segment were perturbed by this event and which were unaffected. A third question was: Was the dike an isolated event or part of a longer-lasting episode of crustal accretion for this section of ridge? On the central volcanos of Iceland (our closest subaerial analogy to the MOR), stress builds up over long periods of time (hundreds of years) and is released during multiple events that occur on a decadal time scale [Björnsson, 1985], but until now, there has not been an analogous record of crustal accretion observations anywhere on the submerged portion of the MOR.
To answer these questions, the initial event response expeditions were followed by several other surface ship and submersible cruises in 19931996. Submersible dives in 19941995 continued the chemical and biological time series studies at the vent sites discovered in 1993 and conducted detailed mapping of the sites. A segment-scale mapping program in 19941996 used deep-towed side-scan sonar surveys, camera tows, and rock coring along the CoAxial neovolcanic zone and the north rift zone of Axial Volcano in order to place the detailed work at the active vent sites into a regional context.
This paper synthesizes the available SeaBeam, side-scan, towed camera, and submersible data to relate the 1993 CoAxial accretion event to the recent structural and magmatic history of the area and to the broader issue of MOR accretion processes. We believe our data demonstrate that (1) the 1993 dike intruded from a magma source beneath the CoAxial segment (rather than Axial Volcano) with a significant component of lateral propagation along the neovolcanic zone of the segment, and (2) the 1993 eruption was not an isolated event but was one of at least three volcanic events in the 12-year period between 1981 and 1993 which must have relieved a significant level of stress along the segment.
The initial SeaBeam surveys of the axis of the Juan de Fuca Ridge were collected in 19811982 on the NOAA ship Surveyor and were navigated by Loran-C, which provided good relative positioning for swath matching but only approximate geodetic control. These data were subsequently shifted to geodetic "space" using Transit satellite fixes taken during the original surveys. Following the 1990 discovery of the young eruptive mounds on the southern Juan de Fuca Ridge [Chadwick et al., 1991; Fox et al., 1992] another survey of the entire neovolcanic zone of the Juan de Fuca Ridge was made from the NOAA ship Discoverer in 1991. A third survey with GPS navigation over the northern portion of Axial Volcano and the CoAxial segment was made immediately after the 1993 eruption. These three surveys were the basis for the SeaBeam differencing calculations presented by Chadwick et al. . The database used for the SeaBeam base maps in this manuscript is a combination of the older Loran-C/Transit navigated data and the 1993 GPS survey.
Data from two types of side-scan sonar systems, SeaMARC II and AMS-60, are used in this study to provide an excellent regional structural context within which to view higher-resolution data sets. The SeaMARC II system is a surface-towed side-scan sonar system operating at 12 kHz. The SeaMARC II data used in this study were collected in 1983 and 1985 by the Canadian Pacific Geoscience Center, the USGS, the University of Washington, and the University of Hawaii. These surveys were navigated by Loran-C and Transit satellite fixes. In 1996, comprehensive surveys of the neovolcanic zone of CoAxial and the north rift zone of Axial Volcano were made using an AMS-60 (60 kHz) deep-towed side-scan operated by Williamson and Co., Seattle, Washington, towed from the NOAA ship Discoverer. The position of the side-scan sonar was calculated using the setback of the towfish from depth, wire out, and the GPS position of the ship. The AMS-60 side-scan data were processed to a resolution of 1 m using the USGS Mini Image Processing System (MIPS) software. The AMS-60 side-scan data provides information on the morphology, structure, and texture of the seafloor and can be used to distinguish young volcanic terrain from older tectonized seafloor.
These data were collected using the NOAA Pacific Marine Environmental Laboratory (PMEL) and the USGS deep-towed camera systems (19931994), ROPOS (1993), and the manned submersible Alvin (19931995). The ROPOS and Alvin data used in this paper include video and still photographs and microbathymetry (derived from adding the pressure depth and altimeter output). The deep-towed camera data consist of color 35-mm film, color video, and temperature from the USGS system and monochrome video, color 35-mm film, temperature, and microbathymetry from the NOAA camera system. The seafloor images were digitized and characterized according to the scheme of Fox et al. . All of the Alvin dives and most of the deep-towed camera data were collected within long-baseline transponder nets that provide a positional accuracy of ~20 m. These near-bottom observations were used to delineate the location of the neovolcanic zone on both the CoAxial segment and the north rift zone of Axial Volcano.
The Juan de Fuca Ridge extends from the Blanco Fracture Zone at 44°30N northward to the Sovanco Fracture Zone at 48°45N (Figure 1). General descriptions of the ridge and its various segments have been published previously [Crane et al., 1985; Delaney et al., 1981; Embley and Chadwick, 1994; Embley et al., 1990; Johnson and Holmes, 1989; Kappel and Normark, 1987; Kappel and Ryan, 1986]. The dominant morphologic feature on the Juan de Fuca Ridge is the anomalously shallow edifice of Axial Volcano, the youngest in a series of volcanos generated by the Cobb-Eickelberg hotspot [Desonie and Duncan, 1990] (Figure 1). The general characteristics of the Axial Volcano segment have been described by Delaney et al. , Johnson and Holmes , Johnson and Embley , Appelgate , and Embley et al. . The bulk of the volcanic edifice lies between about 45°30N and 46°20N and consists of a summit plateau and caldera centered at 45°55N, 130°00W, two major rift zones around the caldera, and several other smaller plateaus and volcanic rift zones and ridges (Figures 1 and 2 and Plate 1). Axial Volcano and its major rift zones probably overlie the spreading axis between the Vance and CoAxial segments [Delaney et al., 1981].
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 198193 dikes.
To better understand the segmentation of the ridge in this complex area, we constructed a structural map based on the SeaBeam bathymetry and SeaMARC II side-scan data (Figure 2). In older maps based on these data [Davis and Currie, 1993; Johnson and Holmes, 1989] the Axial Volcano North Rift Zone (AVNRZ) overlaps with the Cobb segment beginning at about 46°40N and what is now known as the CoAxial segment was not recognized as an active spreading segment. However, the CoAxial segment is clearly delineated as a separate spreading segment by inward facing faults between 46°00N and 46°40N (Figures 2, 3a, and 3b). These faults structurally define an axial valley that is between 3 and 5 km in width with up to 150 m of relief. The southern portion of the CoAxial segment overlaps with the AVNRZ for at least 25 km (46°0015N), and the northern end of CoAxial segment overlaps with the southern portion of the Cobb segment by 10 km. The southern end of the CoAxial valley was named "Helium Basin" because of the unusually high (He)% found in hydrothermal plumes from this area in the early 1980s [Lupton, 1990].
Most of the faults digitized from the SeaMARC II and SeaBeam data are oriented at or close to the trend of the Juan de Fuca Ridge (~N020°E), but oblique or curvilinear ridges occur at several locations. Curvilinear constructional volcanic ridges occur on the eastern side of the AVNRZ and at the southern and northern ends of the CoAxial segment where it overlaps with the AVNRZ and the Cobb segment, respectively. These curvilinear ridges are interpreted to be formed by crack interaction between overlapping spreading segments such as those described at faster spreading centers [Macdonald and Fox, 1983; Sempere and Macdonald, 1986]. A set of oblique ridges on older crust found northeast of Axial Volcano (Figure 2) could be traces of an older overlapping zone.
A striking feature in Figure 2 is the asymmetric distribution of volcanic cones and lava flows on the west side of the ridge and the concomitant apparent lack of faulting. Sediment cover is thinner on the west side of the ridge because the ridge blocks the sediments from the continent, so this asymmetry in faulting is not caused by sediment cover. It is probably due either to burial of the faults by more active off-axis volcanism and/or because faulting has been less developed on the west side, possibly due to the shoaling of isotherms in the vicinity of the hotspot. Although there are a few large volcanoes on the east flank of the ridge axis, most of the recent off-axis lava flows are confined to the flank of Axial Volcano and Rogue Volcano. An exception is one isolated but well-defined off-axis lava flow at 46°40N, 129°20W (Figure 2).
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.
The neovolcanic zone of a ridge segment is the narrow zone within which recent volcanic eruptions have been focused; it is essentially the present axis of accretion and spreading. Although the exact location of the neovolcanic zone along the CoAxial segment and the AVNRZ is not always clear from bathymetry alone, it is well-defined from a combination of side-scan and camera data. The CoAxial segment extends from the Helium Basin at the base of the steep northeast flank of Axial Volcano (~46°00N; 129°57W) to about 46°40N where it overlaps with the southern end of the Cobb segment (Figures 1 and 2). The shoalest portion of the neovolcanic zone of the CoAxial segment lies between 46°03N and 46°15N along a ridge that bisects the bowl-shaped Helium Basin. The segment deepens to the northeast by 425 m over 60 km, from 2125 m at 46°08N to 2550 m at 46°40N. This relatively steep topographic gradient probably reflects a decrease of the long-term magma supply away from the hotspot.
Morphology of the neovolcanic zone along the CoAxial segment exhibits considerable along-strike variability (Figure 2 and Plate 1). The southern portion of the axial valley (46°0015N) contains a series of prominent ridges and conical volcanoes. From about 46°15N to 46°30N, the center of the axial valley consists of an alternating series of low-relief depressions, small volcanos, and ridges with no well-defined central neovolcanic ridge. North of 46°30N, a series of narrow volcanic ridges are again prominent. These curve eastward in the overlap zone with the Cobb segment north of about 46°33N. Small circular volcanic cones (<1 km in diameter) are found within the axial valley for its entire length, but most of the largest cones lie along the southern portion of the segment.
Between 46°12N and 46°25N the CoAxial segment is bounded on both the west and east sides by large, fault-bounded ridges, which have outlines in map view like two halves of a pear split along the axis of the segment (Figure 2 and Plate 1). We interpret that these "fault-block ridges" initially formed on-axis at CoAxial by magmatic processes and were then split apart by subsequent plate spreading, as in the model proposed by Kappel and Ryan  for the southern JdFR. The western fault-block ridge physically abuts, and is aligned with, the AVNRZ to the south. Some investigators [Sohn et al., 1997] have assumed that the western fault-block ridge is an extension of the AVNRZ and therefore that the T wave epicenters tracked the 1993 dike as it intruded from AVNRZ to the eruption site on the CoAxial segment. However, side-scan, towed camera, and geochemical data show that this would have been unprecedented. Those data clearly show that the lavas erupted from the AVNRZ do not extend farther north than about 46°18N. South of this boundary, the hummocky morphology of constructional volcanism is clearly present in the side-scan imagery (Figures 2, 3a, and 3c), young lavas are photographed in camera tows, and the lavas have a geochemical affinity with basalts recovered from Axial Volcano [Smith, 1999; Smith et al., 1997]. North of this boundary, the recent volcanism abruptly ends and gives way to much older seafloor with numerous fissures and faults and a lower acoustic backscatter value (Figure 3a, 3b, 3c, and 3d) probably caused by increased sediment cover of up to 80100% in this area as seen on bottom photos. The northern termination of young lavas from AVNRZ occurs within a 3-km-wide graben extending from 46°16N to 46°19N that bisects the top of the western fault block ridge (Figure 3). Our interpretation of these relationships is that the AVNRZ and CoAxial segment behave as separate but overlapping ridge segments, with separate shallow magma supplies, and the young volcanic constructions of the AVNRZ have been superimposed upon the older western fault block ridge.
Geochemical data are key to evaluating the level of interaction between the neovolcanic zones of Axial Volcano and the CoAxial segment. Isotropic ratios are one of the most effective tools for discriminating different parental magmas and their mantle sources because of their geochemical transparency to melting and most shallow level crustal processes. Assuming crustal assimilation has been negligible, significant differences in radiogenic isotopic ratios (e.g., 87Sr/86Sr and 143Nd/144Nd) of two basalts indicate that their mantle sources were different [Rogers and Hawkesworth, 1999]. However, covariation between incompatible element abundances and isotopic ratios in MORB from many portions of the MOR, including the JDFR, suggests that some of the isotopic variability is a consequence of mixing of melts from distinct mantle domains (i.e., enriched versus depleted) or that it is a result of variable extents of melting of heterogeneous mantle [White et al., 1987]. Although the Sr and Nd isotopic values of MORB from the JdFR do not vary much in comparison to other MOR segments affected by hot spots, the data allow us to distinguish a few spatial domains along the length of the JdFR. In particular, MORB from the CoAxial segment have distinctly non-radiogenic Sr/Sr and 143Nd/144Nd, suggesting they were derived from long-term depleted mantle that has not been recently "enriched" by another component. A plot of Sr/Sr of basalts against latitude (Figure 4) reveals that the recent source for the CoAxial segment is geochemically distinct from both Axial Volcano and the rest of the JdFR. This distinction is particularly clear when comparing the youngest samples from the AVNRZ and CoAxial segment. The lack of any overlap between the fields that encompass the CoAxial segment and Axial Volcano/AVNRZ provide compelling evidence that the magmatic plumbing systems beneath Axial Volcano and the CoAxial segment are separate and that although these ridge segments overlap, they do not interact magmatically.
Figure 4. Sr isotopes versus latitude, central Juan de Fuca Ridge.
The dives made at the 1993 eruption site ("Flow site") with the ROPOS and the submersible Alvin mapped out the extent of the fresh lava flow and diffuse hydrothermal venting associated with a zone of fissures and narrow grabens extending several kilometers to its north and south [Chadwick et al., 1995; Embley et al., 1995] (Figures 5, 6, and 7). The eruption took place along an older volcanic ridge that extends north of Cage Volcano, a prominent 1-km diameter volcanic cone (Figure 6).
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 19821991 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.
Differencing of SeaBeam bathymetric grids from surveys made in 1982, 1991, and 1993 shows a well-defined positive depth anomaly coinciding with the glassy, unsedimented lava observed (Figure 6 and Plates 2a and 2b) [Chadwick et al., 1995]. One of the thicker (~30 m) portions of the lava flow centered at 46°32.4N (Figure 6) coincides with an area of intense hydrothermal activity observed during ROPOS and Alvin dives in 1993 (Figure 6 and Plate 2c). Seafloor observations made during ROPOS dives (e.g., R219, Figure 6) in August 1993 (probably less than 2 weeks after the eruption) showed that the crestal region of the lava flow was emitting an extensive flow of warm water up to 50°C from an area of drained-out flattened pillow lavas. Fe-Si precipitates coated large areas of the crestal area where active venting was taking place (Plate 2c) [Embley et al., 1995; Juniper et al., 1995].
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 198291 lava site lying 500 m east of the 1993 flow (A93). (g) Crest of the 19821991 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.
High-resolution near-bottom bathymetric surveys using an AMS-60 deep-towed side-scan sonar and a Mesotech scanning sonar mounted on Alvin mapped well-defined narrow (~20 m wide) grabens at both the southern and northern ends of the 1993 lava flow (Figure 6). Chadwick and Embley  propose that these structures formed above the 1993 dike that fed the eruption. In this paper we use the terms "fissure" and "graben" as by Chadwick and Embley . A fissure is a fracture whose walls have moved apart by pure extension. A graben is a linear topographic depression created by subsidence along inward facing normal faults. Small grabens are distinguished from fissures in that they have a well-defined downdropped center block.
Surveys and seafloor observations using the ROPOS and Alvin mapped at least three small patches of lava north of the main lava flow (the northernmost is at 46°32.5N) that erupted out of narrow grabens. Low-level diffuse venting was observed within the northern graben in August 1993, but none was observed in July 1994. An Alvin dive within the graben at the southern end of the main lava flow (Figure 6) in October 1993 found extensive diffuse hydrothermal venting and bacterial mats along its eastern wall, but venting had ceased by September 1995. Extensive venting was also found southwest of the 1993 flow on the plateau south of Cage Volcano in August 1993 at sites extending for 5 km SSW (~N202°E) of the main lava flow (Figure 5 and Plate 2d). All six ROPOS transects, including the southernmost one at 46°28.4N, crossed a narrow zone of venting associated with small grabens and or fissures that cut through pre-1993 lavas (Figure 5). No recently erupted lava was observed on any transect south of the main lava flow.
Camera tows in 1994 and Alvin dives in 1995 (Figure 5) did not detect any active hydrothermal venting south of the lava flow where ROPOS observed it in 1993, but Alvin transects at two sites revealed "fresh" appearing fissures and small grabens characterized by bright white and yellow staining on the broken rock faces (Figure 5). At 46°29N, Alvin (dives 2990 and 2992) encountered a very unusual fractured zone characterized by brecciated, pervasively stained basalt lying within a shallow graben up to 10 m wide (Plate 2e). Careful observation from Alvin's viewports confirmed that this rubble was generated in situ, not tectonic talus generated from a nearby scarp. The zone appeared to be continuous for at least 500 m along strike and located along the line of (apparent) continuous venting observed by ROPOS in 1993.
In summary, the 1993 CoAxial eruption site consists of a 3.8-km-long, (up to) 500-m-wide lava flow, a zone of grabens and fissures extending at least 1 km northward which fed several small eruptions, and a zone extending at least ~4.5 km to the south characterized by grabens, fractured seafloor, and extensive diffuse venting (in 1993), but no fresh lavas. This entire active zone is at least 8.4 km in length. A plot of the T wave epicenters (Figure 1) shows that the events in the second half of the swarm(s) (after JD184) form a "bulls-eye" pattern at the eruption site that has the same diameter (~ 8 km) as the length of known along-axis venting and eruption (southern limit shown on Figure 5). The character and geometry of these structures as well as the short duration of hydrothermal venting along them are all consistent with the interpretation that they formed directly above the 1993 dike and thus reveal the path that the dike took along this section of the CoAxial segment. There is no evidence for any other 1993 eruption sites, but repeat SeaBeam surveys and camera tows have identified two pre-1993 eruption sites along the CoAxial segment, which are described in sections 6 and 8.
The first dive by ROPOS at the Flow site in August 1993 crossed over a largely sediment-free, glassy lava flow lying about 700 m east of the 1993 flow (Figures 5, 6, and 7). Subsequent comparison of SeaBeam surveys clearly showed that this lava was erupted between 1982 and 1991 [Chadwick et al., 1995]. This older flow appears to be morphologically similar to the 1993 flow, consisting of mostly pillow lavas (Plate 2f) but with a distinct zone of flattened and partially drained-out pillow flows along the crest of the mound. The crest of the 19821991 lava flow has a veneer of dull yellow Fe-Si sediment (Plate 2g), which is likely the Fe-Si precipitates observed forming during the cooling phase of the 1993 lava flow (Plate 2c). Similar precipitates were also found on the crest of the lava mounds on the northern Cleft segment that erupted in the mid-1980s [Chadwick and Embley, 1994], so the presence of these deposits is now a well-documented indicator of the cooling phase of recent pillow lavas and their feeder dikes. The northern extent of this flow is currently unknown due to lack of near-bottom photographic coverage. The separation between this eruptive mound and the 1993 mound is ~800 m (centerline to centerline).
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  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°4552N 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 19811991 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 19811991 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 (~0.51.0 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 100200 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.  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.  attribute this activity to post diking tectonic adjustments, which is consistent with the geologic evidence presented here.
The 1994 deep-towed camera survey delineated a series of young, glassy lava flows extending for 7 km along a ridge ~500 m west of the actively venting fissure swarm at the Floc site (Figures 8, 9, and 10). The lava erupted along a preexisting ridge; in some locations it flowed down the east side of the ridge and at other places down the west side (Figure 10). Application of the SeaBeam differencing technique revealed a small but distinct depth difference anomaly (up to 25 m) between the 1981 and 1991 surveys which coincides exactly with the young lava mound delineated by the towed camera at 46°17.5N, 129°43.3W. Note that this SeaBeam anomaly is different than the "southern 19811991 SeaBeam anomaly" tentatively identified at 46°26.2N by Chadwick et al. , which our 1994 camera tows revealed to be false. The Floc site anomaly discussed here was also assumed to be an artifact [Chadwick et al., 1995] because it was relatively small in area and did not have any photographic groundtruthing at the time. The absence of additional SeaBeam difference anomalies coinciding with the other areas of glassy lavas mapped by the towed camera suggests that most of the 19811991 lava flows at the Floc site are thinner than ~515 m, which is the vertical resolution of this technique [Fox et al., 1992]. The SeaBeam differencing results do not allow age discrimination between the 19821991 lava flow at the Flow site and this one at the Floc site. However, close examination of Alvin hand-held photographs of the two sites suggests that the Floc site pillow mounds are slightly older because they have small sediment pockets and do not have as vitreous a luster as the northern lavas (compare Plates 2f and 3f). The towed camera imagery and video traverses made with the submersible Alvin show that neither of the 19811991 flows were fractured, nor were they colonized with sessile organisms.
The southern third of the CoAxial axial valley (south of about 46°14N) is characterized by a distinct median volcanic ridge (Plate 1 and Figure 11). The ridge summit is covered with young pillow lavas with small pockets of sediment and small areas of sheet flows at its southern end (~46°07N) where it broadens. Seafloor between the median ridge and the AVNRZ (dashed line on Figure 11) consists of heavily sedimented sheet flows (Plate 3g). A young pillow mound on the axis of the AVNRZ centered at 46°10N, 129°55W (Figure 11) mapped by the towed camera probably represents a single eruption from a recent diking episode originating at or near the summit of Axial Volcano. No SeaBeam anomaly was found at this site, so it apparently precedes all of the historic CoAxial eruptions.
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.
CTDT tows along this part of the CoAxial segment in October 1993 encountered a hydrothermal plume centered around 46°09N (Figure 11) that was clearly separate from the plumes originating at the known Floc and Flow sites to the north [Baker et al., 1998] . The high-temperature vent site subsequently discovered on Alvin dive 2681 in October 1993 became known as the "Source site" because it was postulated that this site may have represented the magma source region that generated the 1993 dike injection [Butterfield et al., 1997].
The Source vent field is located on the west side of the neovolcanic ridge of the CoAxial segment (Figure 12) along a fissure/fault system. This ridge is the shallowest part of the CoAxial neovolcanic zone, rising to ~2100 m in the vicinity of the vent site and thus is the most likely position of a long-term hydrothermal system on this segment [Ballard and Francheteau, 1982; Crane, 1985]. The vent site consists of four major vents aligned at about N022°E along a west facing scarp (Plate 3h). The chimneys are spaced ~1020 m apart and the entire vent field is <100 m in length. Less vigorous venting also occurs on small chimneys west of and slightly deeper than the large ones. The small chimneys are oriented along trends about N040°W and intersect the primary vents (Plate 3i). This pattern suggests that the locations of the vents may be controlled by intersection of two structural trends such as has been documented at the Endeavour Main Field [Delaney et al., 1992]. The metal-poor vents emit a clear or grayish "smoke" with a maximum temperature of 294°C. Butterfield et al.  concluded that these fluids have a relatively long crustal residence time compared to other hydrothermal systems on the JdFR, but the mechanism for this is unclear.
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.
The only other active hydrothermal activity found nearby was an an area of diffuse venting a few tens of meters on a side about 3.5 km to the south (Figure 12). Small mounds of iron oxides had a fluid temperature only ~4°C above ambient. A line of extinct sulfide chimneys was found several hundred meters northwest of the diffuse vent. These Zn-rich chimneys lie along a small fault aligned along N020°E and extend for at least 100 m along the same trend as the active high-temperature vent field to the north (Figure 12). The presence of these older chimneys indicates that this site has had a long-term history of high-temperature venting. To date, no other high-temperature activity (either past or present) has been found along the length of the CoAxial segment.
Several observations lead one to the conclusion that the Source site predated and was probably not affected by the 1993 seismic swarm. First, the initial seismic activity was to the north and northwest of the site (Figure 1); there were no SOSUS-detected events located along this ridge. Second, the submersible dives and camera tows across and along the ridge yielded no geologic or biologic evidence of very recent tectonic events or young venting such as observed at the Floc site. Last, analyses of a time series of vent fluid samples from this site taken in 1993, 1994, and 1995 [Butterfield et al., 1997] do not show the rapid changes in temperature and chlorinity that characterize other recent eruption sites [Butterfield et al., 1997; Butterfield and Massoth, 1994; Von Damm et al., 1995]. Considering these data, it now seems unlikely that the Source site was significantly perturbed by the 1993 event.
Where did the 1993 dike originate and how does the spatial pattern of the 1993 T wave swarm relate to the physical manifestations of the 1993 dike intrusion on the seafloor? A puzzling problem has been the discrepancy between the trend of the 1993 T wave swarm and the trend of the neovolcanic zone at CoAxial as defined by geologic mapping. We think that the 1993 dike intruded from a magma source beneath the southern CoAxial segment and propagated northward along the CoAxial neovolcanic zone and that Axial Volcano was not involved in the 1993 event, for the following reasons:
Therefore all of the geologic and geochemical evidence leads to the conclusion that the 1993 dike originated within the CoAxial segment and intruded northward along its neovolcanic zone and did not follow the path suggested by the T wave epicenters from AVNRZ to CoAxial. What could explain the oblique trend of the T wave swarm, particularly since the T wave epicenters cluster perfectly above the 1993 eruption site at the northern end of the swarm? There are two hypotheses that could explain this discrepancy. One hypothesis, suggested by Fox et al. , is that the first half of the T wave swarm consisted of deeper earthquakes [Schreiner et al., 1995] and the relatively shallow seafloor to the west of the central CoAxial segment may have radiated more of the acoustic energy and so added a systematic location error to the epicenters in the southern part of the swarm. A second hypothesis is that the early T wave events might have been associated with faulting along the boundary of the western fault block ridge rather than directly associated with the dike itself, located on-axis. Intruding dikes have triggered normal faulting in volcanic rift zones on land [Rubin, 1992; Sigurdsson, 1980], and several normal fault earthquakes located on the bounding faults of the summit caldera of Axial Volcano were triggered during the early stages of the 1998 dike injection there [Dziak and Fox, 1998, 1999].
Three separate eruptions have occurred on the CoAxial segment within the 12-year period from 1981 until 1993 (Plate 1). The two younger eruptions occurred near the northern end of the segment, and the oldest event occurred at the midpoint of the segment at 46°1521N.
How much strain has been released during this time period and what are the implications for the space-time pattern of strain and volcanism for the rest of the ridge system? It seems reasonable to assume that the two 19811991 eruptions represent diking events similar to the 1993 event because since (1) all the lava flows are similar in volume and are elongated along the axis of the neovolcanic zone, and (2) all of the other known historical eruptions documented to date along the northeast Pacific spreading centers [Chadwick et al., 1998; Dziak and Fox, 1998; Dziak et al., 1995; Embley and Chadwick, 1994] have been associated with dikes that have intruded over tens of kilometers along axis. The T wave epicenters suggest that the 1993 dike was injected from a latitude between the Floc and Source sites (~46°12N), so presumably a magma reservoir exists beneath the axis of CoAxial around this location. If we assume that all three recent eruptions were fed by dikes that intruded northward from this area, this may have interesting implications for the pattern of stress accumulation and release from this section of the ridge. At a spreading rate of 5.5 cm/yr, each meter thickness of intruded dike relieves about 18 years of accumulated stress. Although we cannot directly estimate the thickness of any of the dikes intruded during the CoAxial events, estimates of dike widths from sections of oceanic crust exposed in ophiolite sections are typically 0.51.5 m [Gillis, 1995]. The three documented eruptions occurring over a 12-year period represent only a minimum number of diking events because additional events may have taken place without producing eruptions in the 19811991 period prior to SOSUS monitoring and after the initial SeaBeam survey. Also, the lack of teleseismic events (M 4) along this portion of the ridge (R. Dziak, personal commununication, 1999) in the 196693 time period suggests that faulting was not a significant factor in stress release for this period. Nevertheless, the record of recent eruptions at the CoAxial segment represents the first direct evidence of the style of stress release over the timescale of a about a decade along a portion of the MOR. The CoAxial diking events of 19811993 must have relieved a significant amount of stress along the segment. No diking events have been detected by SOSUS in the 19931999 period.
Although the period of observation is very short, this pattern could be analogous to the behavior of Icelandic central volcanos. The long-term record of volcanism in Iceland shows that the central volcanos release accumulated stress in a series of major rifting/volcanic events about every 100150 years [Björnsson et al., 1979]. During the latest of these episodes, the Krafla Volcano produced 20 diking/rifting events between 1975 and 1984, only about half of which erupted lava at the surface [Björnsson, 1985]. Whether the CoAxial "rifting episode" has ended remains to be seen, although at the time of this publication, the segment has been seismically quiet for 6 years. Geodetic monitoring of the Krafla episode suggests that as each diking event relieves some of the regional stress, it becomes more difficult to inject additional dikes into an increasingly compressive stress field [Ewert et al., 1991].
How magma is delivered along segments of the MOR is poorly known because of the lack of direct measurements of 3-D seismicity patterns and deformation during diking events. Models of magma delivery systems have been mostly inferred from surface morphology, lava geochemistry, and geophysical measurements [Batiza, 1996]. Even in Iceland, where a rifting episode was monitored in 19751984 at the Krafla Volcano and where there has been extensive geologic mapping, there remains some controversy as to how magma is delivered to the upper crust. The seismicity patterns during the Krafla rifting events showed that at least some of the dikes were propagated laterally from beneath Krafla caldera [Einarsson and Brandsdottir, 1980] and this led Sigurdsson  to propose that lateral propagation of dikes is an important process on Icelandic central volcanos. On the other hand, Gudmundsson  concludes that many of the long dikes generated during episodes on Icelandic central volcanos are propagated primarily vertically from deeper magma sources. Another source of information about magma delivery systems comes from ophiolites. Interestingly, a study of flow directions in the dikes of the Troodos ophiolite [Varga et al., 1998] showed a significant lateral component of magma transport during diking.
Embley et al.  proposed that the dike(s) that produced the 1980s eruption(s) at the north Cleft segment had a significant lateral component. This was based primarily on the observation that hydrothermal cooling proceeded more rapidly at the end of the dike even though that is where the largest eruption occurred. Their preferred interpretation of this observation was that the distal end of the dike is "rootless"; that is, it did not erupt vertically from a local magma source but was fed laterally from a more robust magma source. The longer-lasting high-temperature venting associated with a sheet flow eruption to the south was assumed to be over the location of the primary magma source.
What was the mode of dike injection during the 1993 event? The northward propagation of the T waves during the 1993 CoAxial event is certainly consistent with a lateral dike injection [Dziak et al., 1995]. Also, Butterfield et al.  concluded that the chemistry of the 1993 pillow mound site vent fluids could be explained by the cooling of the dike and lava flow alone (i.e., "rootless portion"), whereas the chemistry of the fluids at the Floc site constrained them to a larger and hotter source. Although there were no observations of active venting on the older pillow lava eruptive mounds at the CoAxial site, the presence of the yellowish sediments on the 19821991 mound is consistent with a low-temperature reaction zone of a rapidly cooling dike and lava flow. Smith  explained a decrease in MgO content in the lavas at the Floc site 19811991 eruption mounds as crystal fractionation within a northward propagating dike. Finally, two of the CoAxial eruptions and the north Cleft eruption [Chadwick and Embley, 1994] were characterized by 80100% of the eruption by volume being emplaced at the deep end of the segments. This pattern is also consistent with a lateral dike injection in that it is [Fialko and Rubin, 1998] hydraulically easier to erupt lava as the seafloor deepens.
Thus the spatial and temporal patterns of eruption and hydrothermal cooling and the lava geochemistry of the 1993 and older events are consistent with a dike that had a significant lateral component of injection. Admittedly, this conjecture is largely circumstantial, unfortunately; there are very few data (e.g., seismic reflection) on the along-axis melt supply of the Juan de Fuca Ridge. However, some new and more direct evidence for lateral dike injection on a segment of the Juan de Fuca Ridge was the 3-m drop in the caldera of Axial Volcano (Figure 1) measured by a seafloor pressure gauge coupled with eruption of lava on the rift zone during the seismicity episode in 1998 [Fox, 1999; Embley et al., 1999]. Clearly, monitoring of diking events (at a range of spreading rates) with arrays of seismometers and instruments to measure vertical and horizontal strain is needed to provide more definitive data on the geometry of MOR crustal level magma plumbing systems.
Acknowledgments. Without the goodwill of the U.S. Navy in opening up their SOSUS system to the scientific community, the JuneJuly 1993 eruption would almost certainly have gone undetected and the detailed studies reported in this paper would never have been carried out. The NOAA VENTS Program provided support for onshore data analysis and a good portion of the seagoing effort. We particularly thank the ROPOS and Alvin groups and the crew and scientific parties of the research vessels Discoverer and Atlantis II for their excellent support. Williamson and Co. of Seattle, Washington, provided excellent support for the 1996 AMS-60 side-scan survey, and we thank Stuart Sides and Miguel Valasco of the U.S. Geological Survey for their excellent at-sea processing of the data. H.P. Johnson generously included the senior author on the 1995 Alvin expedition and allowed us use of some of the dive data. R. Koski and Hank Chezar generously provided data collected by the U.S. Geological Survey's deep-towed camera system in 19931994. The NOAA Undersea Research Program and the National Science Foundation provided generous support for the Alvin dives in 19931995. We especially thank D. Kelley, J. Getsiv, T. Gregg, S. Hanneman, R. Greene, A. Bobbitt, and D. Wright for their many long hours on watch during the Discoverer and Atlantis II expeditions. S. Merle and Paul D. Johnson provided excellent technical and graphics support. We are also grateful to Ralph Currie, H. P. Johnson, and Mark Holmes for providing tapes of the SeaMARC II data, and to V. Tunnicliffe for Plate 3d. PMEL contribution 2063.
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