U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
Cleft Segment: Overview
The morphology of the southern Juan de Fuca Ridge has been generally described by Embley et al. [1983], Kappel and Ryan [1986], and Kappel and Normark [1987]. The Cleft segment (Figure 1 and Plate 1) is the southernmost discrete spreading segment of the Juan de Fuca Ridge, beginning at 44°27'N at its intersection with the Blanco Fracture Zone and ending in an overlapping relationship with the Vance segment at or slightly north of 45°10'N [Kappel and Ryan, 1986; Murphy and Embley, 1988; Johnson and Holmes, 1989; Embley et al., 1991]. The along-axis high is at 44°38'N and from here seafloor depth drops off rapidly to the south into the Blanco Fracture Zone and more gradually to the north, dropping less than 100 m over the first 50 km and then another 100 m in the next 15 km along a pillow mound ridge (Plate 1 and Figures 1 and 2). Although the present along-axis high point occurs at 44°38'N, the smoother and shallower relief of the flanks of the ridge between about 44°50'N and 45°02'N (Figure 1) implies a higher average rate of extrusive volcanism versus tectonism along the northern third of the segment over the past few hundred thousand years.
Plate 1. (Opposite) Sea Beam bathymetric map of southern Juan de Fuca Ridge. Track line spacing approximately 1.8 km to produce nearly 100% coverage. Contour interval is 20 m, color interval is 50 m. Grid spacing is 100 m.
The morphology of the Cleft segment varies along its 80-km length (Plate 1 and Figure 2). The segment's name is derived from the presence of a nearly continuous cleft that is 10 km in length, 30 to 50 m in width, and 20 m in depth, extending along the segment's along-axis high from about 44°35'N to 44°44'N within a broad, 3-km-wide axial valley. This cleft is centered along an untectonized lava plain that extends 15 km along the axis from 44°35'N to 44°43'N. High-temperature vents are found at three locations along the cleft (Figure 1), probably the sites of eruptive centers that fed the lava plain sheet flows [U.S. Geological Survey (USGS) Juan de Fuca Study Group, 1986].
North of the smooth lava plain, beginning at about 44°44'N, the axial valley of the Cleft segment deepens and becomes more fissured, although a zone of relatively untectonized constructional volcanic terrain covers the axial valley from 44°46'N to 44°49'N. Between 44°49'N and 44°55'N, a distinct 500-m-wide inner graben is present (Plates 1 and 2 and Figures 2 and 3). From 44°54'N to 44°59'N, a series of small constructional cones and ridges lies on a line roughly bisecting the neovolcanic zone. North of here, the neovolcanic zone can be traced along the crest of a gentle volcanic ridge to about 45°10'N (Plate 1 and Figure 1). This ridge overlaps the Vance axial valley by about 18 km and curves slightly to the west. The southern portion of the Vance axial valley, which is distinctly wider and deeper than the Cleft axial valley, has a central volcanic constructional ridge (split cone ridge or SCR) that overlaps with and curves toward the Cleft neovolcanic ridge, in the style of the overlapping spreading centers of the East Pacific Rise [Macdonald et al., 1984].
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