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°57
W) to about 46°40
N
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°03
N
and 46°15
N 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°08
N
to 2550 m at 46°40
N. 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°0015
N)
contains a series of prominent ridges and conical volcanoes. From about 46°15
N
to 46°30
N, 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°30
N,
a series of narrow volcanic ridges are again prominent. These curve eastward
in the overlap zone with the Cobb segment north of about 46°33
N.
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°25
N
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°18
N.
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°16
N
to 46°19
N 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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