U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
The spatial frequency of hydrothermal plumes along second- to fourth-order tectonic segments on the Juan de Fuca Ridge and the East Pacific Rise is positively correlated with ridge axis cross-sectional area and elevation and with the spatial frequency of an axial magma chamber reflector. The correlation holds weakly on the EPR, but not on the JDFR, for the weight concentration of MgO in basalt glasses from the axis. There is no segment-by-segment correlation with spreading rate, an expected consequence of its slight and uniform along-axis variation. Segments with a cross-sectional area >~3.5 km2, a net elevation >~0.35 km, AMC coverage >~60%, and (for the EPR) MgO > ~7 wt% have a much higher mean incidence of hydrothermal plumes than other segments. The best single predictor of hydrothermal activity is the cross-sectional area.
Despite these correlations, there is no simple functional relationship that links plume spatial frequency with any of the geological indexes. Rather, the data suggest two classes of ridge segments. Those segments with index values lower than the mean of the studied population exhibit uniformly low plume incidence. These segments apparently have current magmatic budgets too weak to power widespread hydrothermal activity. Segments with index values above the mean have a plume incidence ranging from low to continuous. Their magma supply rates are sufficient to produce frequent intrusions that create or renew hydrothermal activity. The spatial variation in hydrothermal activity among these segments may be a measure of the relative temporal frequency of hydrothermal activity and thus of magmatic events.
If ridge segments with both Axs > 3.5 km2 and Enet > 0.35 km have the highest probability of present-day hydrothermal activity, then the most active areas on the northern (5°18°N) EPR are expected to be just north of the Orozco transform fault and the area between the Clipperton transform fault and 6°N. On the southern (4°33°S) EPR, active areas should be found around the Gofar transform fault, between the Garrett transform fault and 19°S, and along the segments adjacent to the large overlap zone at 29°S.
On a regional scale, mean values of area, net elevation, and spreading rate for each region are linearly correlated with each other and with plume incidence. We thus cannot conclude from the available data which index is the best predictor of hydrothermal activity on the multisegment scale. A detailed plume survey along the EPR between 19° and 23°S, where the greatest departure is found from the JDFR/EPR trend of spreading rate versus area and depth, will provide the most useful new information for deciphering the relation between ridge characteristics and hydrothermal venting.
The indexes described here for intermediate to superfast ridge segments are not likely to be similarly useful on slow spreading ridges. Alternative indexes that show some degree of correlation with hydrothermal activity are mantle Bouguer (gravity) lows and zones of cross-cutting tectonization.
Acknowledgments. This research was supported by the NOAA VENTS Program. I thank S. Walker for CTD data processing, and colleagues from NOAA, the University of Hawaii, and the Geological Survey of Japan who cooperated in the collection of plume data on the Juan de Fuca Ridge and the East Pacific Rise. D. J. Fornari and R. W. Embley provided helpful comments on the manuscript. D. Scheirer's sharing of detailed cross-section and depth data from the EPR was indispensable, as was similar generosity on the part of R. Detrick, G. Kent, M. Perfit, and J. Sinton. R. Batiza and J. Karsten provided useful and detailed reviews. NOAA Pacific Marine Environmental Laboratory contribution 1692.
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