U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
The Cleft Segment (also known as the Southern Symmetrical Segment or rift A)
of the JDFR is a 55-km-long dome bounded on the south by the Blanco Fracture
Zone and on the north by the Vance Segment, which is offset 5 km to the west
at 45°N (Figure 1). Descriptions of the morphology of this segment by Kappel
and Ryan [1986] emphasize the ubiquity of recent volcanic flows and
constructional features. Throughout most of its length the axial valley of the
Cleft Segment is a 2- to 3-km-wide linear feature enclosed by valley walls of
80-120 m relief. South of 44°42N the floor
of the axial valley has a smooth-textured surface of high acoustical reflectivity
that is split by a remarkably continuous central cleft. The cleft is presumably
the source of the voluminous and evidently fluid volcanic flows that presently
cover the unsedimented valley floor [Kappel
and Ryan, 1986]. From 44°42
N to 44°49
N
the valley floor becomes a disrupted surface broken by faults and volcanic constructions.
The largest volcanic cone is 60 m in height, though most have only 20 to 40
m of relief. North of 44°50
N the central volcanic
cones give way to a nearly continuous steep-sided depression. The eastern wall
correspondingly fades into a linear depression. The disappearance of the eastern
wall is accompanied by a distinct eastward bulge in the depth contours on the
flank of the ridge crest.
Figure 1. Bathymetry of the Cleft and Vance segments on the Juan de Fuca
Ridge. Tracks of the CTD tows used to define megaplume 1 (at ~44°50N)
and megaplume 2 (at ~45°15
N) are also shown.
Two primary centers of hydrothermal activity are present on the Cleft Segment.
The most studied area, a chain of vent fields distributed along a 5-km section
centered on the axial high at ~44°40N, was
discovered in 1981 [Normark
et al., 1983] and sampled by a submersible Alvin in 1984 [U.S.
Geological Survey Juan de Fuca Study Group, 1986] and 1987 [Embley
et al., 1988]. Since 1981, annual surface ship sampling and CTD/transmissometer
tows [Massoth
et al., 1982, 1983;
Baker
et al., 1985; Baker
and Massoth, 1986a, 1987]
have demonstrated an apparently continuous emission of hydrothermal fluids from
this vent field. A much more extensive hydrothermal plume was discovered in
1986 [Baker
and Massoth, 1986b; Baker
et al., 1987] at the northern end of the Cleft Segment. The first megaplume
was found above this vent field [Baker
et al., 1987].
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