EXTENSIVE DISTRIBUTION OF HYDROTHERMAL PLUMES ALONG THE SUPERFAST SPREADING EAST PACIFIC RISE, 13-DEGREES-30'-18-DEGREES-40'S

A general model relating relative hydrothermal activity to the rate of plate creation requires data from the full spectrum of mid-ocean ridg e spreading rates. To obtain data from a superfast spreading environme nt, the Japanese/U.S. Ridge Flux Project used continuous hydrographic/ optical tow-yes in 1993 to map the distribution of hydrothermal plumes along the southern East Pacific Rise from 13 degrees 30' to 18 degree s 40'S. Plume incidence, the fraction of the spreading axis length ove rlain by a significant plume, was 0.6, including a virtually continuou s vent field stretching 150 km from 17 degrees 20' to 18 degrees 40'S. Hydrothermal venting was most concentrated along portions of the ridg e crest with an inflated cross-sectional area and an observable axial magma chamber reflector. This pattern agrees with previous results fro m the northern East Pacific Rise. Such consistency implies that hydrot hermal circulation on fast spreading ridges is vigorous where the rela tive volume of partial melt is high and meager where melt volume is lo w or undetectable; ''hot rock'' alone is generally insufficient to dri ve significant hydrothermal circulation. Using a mean temperature anom aly of 0.014 +/- 0.010 degrees C, we estimated the hydrothermal heat f lux from the study area as (1.5 +/- 1.1 x 10(7) MW)U-x, where U-x is t he cross-axis flow at the ridge crest. No direct measurements of U-x a re yet available. Combining the plume distribution found here with pri or data from slow, intermediate, and fast spreading ridges yields a si gnificant linear correlation between plume incidence and spreading rat e that extends across the full range of plate motion. This correlation , which mirrors that for near-axis heat flux calculated by recent mode ls, implies that magma supply rate is the principal control on the lar ge-scale distribution of axial hydrothermal venting.