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The Juan de Fuca Ridge in the northeast Pacific extends from the Blanco transform
fault at 44°30N to the Sovanco transform fault
at 48°45
N (Figure
1a). The full spreading rate is 55 mm yr1 [Elvers
et al., 1973; DeMets
et al., 1994]. The ridge consists of six tectonic segments that vary
considerably in morphology and are bounded by first- or second-order discontinuities.
I consider here only Cleft, Vance, Cobb, and Endeavour; the Axial Volcano, West
Valley, and Middle Valley segments are tectonically complex and largely unexplored
for hydrothermal activity. Along-axis seismic data are scarce on the JDFR. Morton
et al. [1987] ran three MCS lines over Cleft and Vance in 1981, and
McDonald
et al. [1994] used seismic refraction to examine the shallow extrusive
layer in the same area. Rohr
et al. [1988] ran a single MCS line across the bathymetric high of
Endeavour. Site-specific seismic refraction experiments have been conducted
by McClain
and Lewis [1982] and Christeson
et al. [1993] on Cobb, and by Cudrak
and Clowes [1993] on Endeavour. Karsten
et al. [1990], Rhodes
et al. [1990], and Smith
et al. [1994] reported chemistry of axial lavas. Baker
and Hammond [1992] summarized the distribution of hydrothermal plumes.
Figure 1. Location maps of tectonic segments studied along the (a) Juan de Fuca Ridge, (b) northern East Pacific Rise (EPR), and (c) southern EPR. Segments ClippertonOrozco2 and CO3 lie immediately north of segment CO1 on the northern EPR; H, G2, and G1 lie immediately south of I on the southern EPR. Segment labeling on the southern EPR from Sinton et al. [1991].
The northern EPR between 9° and 11°50N includes
three segments bounded by first-, second-, or third-order discontinuities [Macdonald
et al., 1992] (Figure 1b). Full-rate
spreading increases from 95 mm yr1 at 11°50
N
to 104 mm yr1 at 9°N [DeMets
et al., 1990, 1994].
Scheirer
and Macdonald [1993] calculated axial cross sections for this area,
and Detrick
et al. [1987] reported an MCS survey of the area. Petrological sampling
has been dense, particularly between 9° and 10°N [Thompson
et al., 1985; Langmuir
et al., 1986; Batiza
and Niu, 1992; Perfit
et al., 1994; Batiza
et al., 1996]. Baker
et al. [1994] mapped the hydrothermal plume distribution.
The southern EPR between 13°50 and 18°40
S
includes seven segments bounded by first-, second-, third-, and fourth-order
discontinuities (Figure 1c). This ridge section
is segmented by overlapping spreading centers (OSCs) noted by Lonsdale
[1989] and Scheirer
et al. [1996], plus large devals at 15°S and 17°05
S.
These OSCs and devals also correspond to petrologic boundaries [Sinton
et al., 1991]. Full-rate spreading is ~145 mm yr1
[DeMets
et al., 1990, 1994].
Scheirer
and Macdonald [1993] calculated axial cross sections for this area,
Detrick
et al. [1993] obtained MCS profiles from along and across axis, and
Sinton
et al. [1991] sampled axial lavas. Urabe
et al. [1995] and Baker
and Urabe [1996] have reported the distribution of along-axis hydrothermal
plumes.
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