Hydrothermal plumes along segments of contrasting magmatic influence, 15°20′-18°30′N, East Pacific Rise:: Influence of axial faulting

[1] Vertical profiles of light backscattering and temperature recorded on 133 rock cores and dredge hauls between the Orozco and Rivera transform faults on the East Pacific Rise (EPR) (15 degrees 20'- 18 degrees 30'N) provide an opportunity to compare the hydrothermal environment of three adjacent but distinctly different segments that span the maximum range of axial cross section at a relatively constant spreading rate. Contrary to predictions based on data from other Pacific ridges, hydrothermal plumes over the inflated 16 degrees N segment were less extensive and weaker than along the narrower, rifted 17 degrees N segment. Remarkably, the 17 degrees N segment has a plume incidence equal to the mean of superfast spreading segments from the southern EPR. The data suggest that the local permeability environment in this region controls the expression of hydrothermal activity in the water column. The 16 degrees N segment, which has little or no indication of faulting, may have its hydrothermal activity presently suppressed by widespread volcanic flows that act as an impermeable cap over much of the segment. Activity on the 17 degrees N segment may be tectonically enhanced, with hydrothermal fluids circulating through deep faults to a cracking front. Within each segment, intense hydrothermal plumes characteristic of focused discharge seem associated with clearly rifted areas, while weaker water column signals characteristic of diffuse discharge are associated with unrifted portions of the ridge axis that appear dominated by magmatism. Previous studies at intermediate- to-superfast spreading ridges have emphasized a positive correlation between local magmatic budget and hydrothermal activity. Our data suggest, however, that even at fast rates local tectonics can control the extent and nature of hydrothermal activity, as documented for several sites on the slow-spreading Mid-Atlantic Ridge. Despite the segment-scale incongruity between hydrothermal activity and magmatic budget, the fraction of total ridge length between 15 degrees 20' and 18 degrees 30'N overlain by plumes (0.39) follows the existing global correlation between plume incidence and spreading rate.