Recent high-resolution geological and hydrothermal studies along the
intermediate spreading Juan de Fuca Ridge (full spreading rate of
5-6 cm/yr) in the northeast Pacific have indicated that individual
segments are at different stages of magmatic and tectonic evolution,
resulting in differences in the observed hydrothermal activity.
Studies have focused on three areas: the Endeavour segment, Axial
Seamount, and the northern part of the Cleft Segment. The Endeavour
hydrothermal site is located at the along-axis high within the wide
(0.5-1 km) axial valley of the Endeavour segment, which appears to be
in an extensional rather than volcanic phase of spreading. Recent
submersible studies [ Delaney et al., 1992] have confirmed earlier
suggestions that the boundary fault network at the base of the western
wall of the axial valley is the primary control on the locations of
active and relict vent sites. The main active vent field (
180 m wide
and 350 m long), which is in a region of older basaltic pillow and
lobate flows, includes more than 15 large (up to 30 m in diameter and
>20 m in height), actively venting structures and numerous smaller,
less active or inactive structures. This within-field variation in
size and level of activity of the structures argues for a secondary
control on the distribution of hydrothermal flow channels.
Delaney et al. [1992] suggest that localization of mineral
deposition sufficient to build the large structures involves
intersecting ridge-parallel normal faults and other fractures
and fissures trending oblique or perpendicular to the ridge.
At Axial Seamount, a large, axial volcano located on the central part of the Juan de Fuca Ridge, the distribution and style of venting is controlled by a combination of localized fissuring and faulting and lava morphology. Although there are a number of low temperature vents within the caldera, the major sites of venting are associated with fractures and fissures along the margin of the caldera [ CASM, 1985; Embley et al., 1990; Johnson and Embley, 1990]. The location of the ASHES vent field, which contains the only high temperature vents in the caldera, is controlled by a fault zone that defines the western boundary of the summit caldera [ Hammond, 1990]. Compared to the Endeavour structures, the actively venting chimneys in the ASHES vent field are small (several meters in diameter and 4-5 m in height) and are confined to an 80 m x 80 m area. Diffuse venting is more widespread. Areas of focused venting appear to correlate with flow surface roughness and tend to be concentrated within smooth sheet and lobate flows, suggesting that, on a local scale, the effective permeabilities of lavas of different morphologies determine where conduits can be formed [ Hammond, 1990].
In contrast with the Endeavour segment to the north, recent observations
along the southern Juan de Fuca Ridge indicate that it appears to be in
a stage of renewed active volcanism. The discovery of ``megaplumes''
(radially symmetric hydrothermal plumes >20 km in diameter) in 1986
and 1987 in the water column overlying the northern end of the Cleft
Segment and the southern end of the Vance Segment [ Baker et al.,
1987, 1989] led to a broad research effort in this area. Megaplumes
are thought to result from sudden voluminous discharges of fluids
from pre-existing hydrothermal systems, which would require a
significant increase in the local permeability. This could
be caused by intrusion and tectonic fracturing [ Baker et al.,
1989; Cathles, 1993], or by hydrofracturing of a sealed
cap on the pre-existing hydrothermal system [ Cann and Strens,
1989]. A comparison of two high-resolution sea floor mapping and
acoustic imaging surveys of the area revealed that a fissure
eruption occurred sometime between 1983 and 1987 [ Chadwick
et al., 1991; Embley et al., 1991; Fox et al., 1992].
Two distinct types of very recent lava flows were observed: isolated
mounds of pillow lava (up to 4 km long, 0.5 km wide and tens of meters
high) and a broad sheet flow covering 3.5 km
[ Chadwick and
Embley, 1994], both of which were fed from dikes intruded along
different parts of the same fissure system [ Embley et al.,
1991; Embley and Chadwick, 1994]. The highest density of
diffuse venting and the high temperature black-smokers occur
along this fissure system on one side of the sheet flow, and
geochronological studies of the mineral deposits indicate the
development of at least one new high temperature site as well
as the rejuvenation of older, lower temperature structures
[ Koski et al., 1994]. The lack of hydrothermal activity
on the younger pillow mounds is interpreted to be a reflection
of their being fed by lateral dike injection from a magma
reservoir underlying the sheet flow region [ Embley and
Chadwick, 1994]. The magma reservoir acts as the heat source
for the long-lived, well-organized hydrothermal system, but
only short-lived venting is associated with the lateral dike
intrusion event beneath the pillow mounds.
Time-series submersible observations at the Cleft Segment between
1988 and 1991 have demonstrated that the hydrothermal activity
changed considerably. Some vents have died out, while others have
developed; diffuse hydrothermal fluids have changed from being low
salinity and depleted in metals to fluids enriched in chloride and
metals [ Butterfield and Massoth, 1994]. In 1988, widespread
diffuse flow at temperatures of 60 C was associated with extensive
bacterial mats. By 1991, venting was less extensive and had decreased
in temperature, and the abundance of vent fauna had dramatically
decreased. A significant reduction in venting was also indicated by
time-series measurement on the intensity and distribution of
hydrothermal plumes [ Baker, 1994]. The waning of hydrothermal
activity along the northern Cleft Segment suggests that the impact
of diking events on the overall hydrothermal system may be localized
and short-lived. If the megaplume events in 1986 and 1987 represented
the onset of seawater circulation through newly opened cracks and
fissures, then the waning of activity observed in 1991 suggests time
cycles on the order of 5 years. This contrasts with the vent field
centered at 
44 40
N in the southern Cleft segment, where temperature
and vent fluid chemistry have been stable over a 7-year period
[ Massoth et al., 1994], and supports the suggestion of
Embley and Chadwick [1994] that long-lived hydrothermal
activity reflects the presence of an underlying magma reservoir.