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.

4. CoAxial Segment and Its Relation to Axial Volcano

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°00–15N) 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 [1986] 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 80–100% 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.

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