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We have reviewed available visual information from the seafloor, and recently acquired microbathymetry for several traverses across the Lucky Strike segment, to evaluate the distribution of hydrothermal activity. We have identified a new on-axis site with diffuse flow, Ewan, and an active vent structure ∼1.2 km from the axis, Capelinhos. These sites are minor relative to the Main field, and our total heatflux estimate for all active sites (200–1200 MW) is only slightly higher than previously published estimates. We also identify fossil sites W of the main Lucky Strike field. A circular feature ∼200 m in diameter located on the flanks of a rifted off-axis central volcano is likely a large and inactive hydrothermal edifice, named Grunnus. We find no indicator of focused hydrothermal activity elsewhere along the segment, suggesting that the enhanced melt supply and the associated melt lenses, required to form central volcanoes, also sustain hydrothermal circulation to form and maintain large and long-lived hydrothermal fields. Hydrothermal discharge to the seafloor occurs along fault traces, suggesting focusing of hydrothermal circulation in the shallow crust along permeable fault zones. [ABSTRACT FROM AUTHOR]

The structure of the oceanic upper volcanic crust is less understood at slow-spreading ridges than at faster ones. Its construction is dominated by pillow lavas, reflecting lower effusion rates than those at fast spreading ridges, where sheet and lobate flows are common and flow off-axis while thickening the extrusive volcanic layer. Based on optical and high-resolution bathymetry data from the Lucky Strike segment (Mid-Atlantic Ridge), we document a mode of volcanic emplacement that likely operates at some magmatically robust slow- and ultra-slow spreading ridge segments, in the presence of strong gradients in magma supply. In these settings, sheet flows may efficiently transport melt away from magmatically robust segment centers in the along-axis direction, steered by normal faults, and exploiting along-axis topographic gradients, with limited across-axis flow. Surface lineations of sheet lava flows tend to be subparallel to fault scarps and the overall segment orientation, and show whorls, well-developed lava channels with associated levees, and surface fold structures at flow fronts. This mode of lava emplacement transitions away from the segment center to pillow-dominated seafloor near the segment ends, associated often with hummocks and axial volcanic ridges. This results in local lava emplacement due to a melt supply that is lower than at the segment center. We propose that fault-steering of lava flows along-axis, limiting off-axis transport as observed at fast-spreading systems, may be common at both slow- and ultra-slow spreading ridges with significant along-axis changes in magma supply linked to topographic gradients. As a result, the nature and properties of the extrusive volcanic layer may vary significantly along axis owing to changes in the modes of volcanic emplacement, as transitions from sheet flows to pillow lavas may impact the porosity structure and hence the seismic properties of the extrusive layer in the oceanic upper crust. • Faults steer sheet flows over a few km along axis, impacting upper crust construction. • Along-axis changes in lava flow morphologies, evidence for melt supply gradients. • Different lava flow morphologies may affect seismic properties of Layer 2A. [ABSTRACT FROM AUTHOR]