Basaltic lava domes, lava lakes, and volcanic segmentation on the southernEast Pacific Rise

Meter-scale DSL-120 sonar mapping and coregistered Argo II photographic obs ervations reveal changes in eruptive style that closely follow the third-or der structural segmentation of the ridge axis on the southern East Pacific Rise, 17 degrees 11'-18 degrees 37'S. Near segment ends we observe abundant basaltic lava domes which average 20 m in height and 200 m in basal diamet er and have pillow lava as the dominant lava morphology. The ubiquity of pi llow lava suggests low effusion rate eruptions. The abundance of lava domes suggests that the fissure eruptions were of sufficient duration to focus a nd produce a line of volcanic edifices. Near segment centers we observe few er but larger lava domes, voluminous drained and collapsed lava lakes, and smooth lobate and sheet lava flows with very little pillow lava. The abunda nce of sheet flows suggests that high effusion rate eruptions are common. F ewer lava domes and large lava lakes suggest that fissure eruptions do not focus to point sources. This pattern was observed on eight third-order ridg e segments suggesting that a fundamental volcanic segmentation of the ridge occurs on this scale. The third-order segment boundaries also correlate wi th local maxima in the seismic axial magma chamber reflector depth througho ut the study area and decreased across-axis width of the region of seismic layer 2A thickening along the one segment where sufficient cross-axis seism ic lines exist. The geochemically defined magmatic segment boundaries in th e study area match the locations of our volcanic segment boundaries, althou gh rock sampling density is not adequate to constrain the variation across all the third-order volcanic segments chat we identify. These observations suggest that variation in the processes of crustal accretion along axis occ urs at a length scale of tens of kilometers on superfast spreading (>140 km /Myr full rate) mid-ocean ridges.