U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
Hydrothermal venting occurs on mid-ocean ridge axes with a diverse array of
morphological, structural, and petrological characteristics. Consequently, it
is not clear if certain geological environments are more conducive than others
to hydrothermal activity. This paper uses complementary data sets recently collected
on multiple-tectonic-segment scales along intermediate to superfast spreading
ridge sections to systematically examine this issue. The spatial density of
venting is gauged by plume incidence, the fraction of linear ridge crest overlain
by hydrothermal plumes. Five geological indexes are compared with plume incidence:
ridge-axis elevation, ridge-axis cross-sectional area, the percentage of ridge
axis underlain by an axial magma chamber (AMC) reflector, MgO wt% in basaltic
glass, and spreading rate. A mean value for plume incidence and each index,
where available, is calculated for each of 14 second- to fourth-order tectonic
segments from the Juan de Fuca Ridge (44°30
to 48°45
N) and the northern
(9° to 11°50
N) and southern
(13°50
to 18°40
S)
East Pacific Rise. Plumes are most common on segments with a cross-sectional
area >3.5 km2, a net elevation >0.35 km, AMC coverage
>60%, and MgO >7 wt%. Spreading rate is not deterministic on the segment
scale. The data suggest two classes of ridge segments. Segments with relatively
low geological index values have a uniformly low plume incidence indicative
of a presently feeble magmatic budget. Those with high index values have a plume
incidence ranging from low to complete coverage, apparently a consequence of
a magma supply rate sufficient to produce frequent dike intrusions and highly
variable hydrothermal activity. The best individual predictors of hydrothermal
activity are cross-sectional area and net elevation. Applying these indexes
to the East Pacific Rise between 18°N and 33°S suggests that the unexplored
segments with the highest probability of activity are those at 15°30
-16°N,
6°-9°N, 5°-6°S, 27°-28°S, and 30°30
-33°S.