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.
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