Document Type

Article

Abstract

Meter-scale DSL-120 sonar mapping and coregistered Argo II photographic observations reveal changes in eruptive style that closely follow the third-order structural segmentation of the ridge axis on the southern East Pacific Rise, 17o11'-18o37'S. Near segment ends we observe abundant basaltic lava domes which average 20 rn in height and 200 rn in basal diameter and have pillow lava as the dominant lava morphology. The ubiquity of pillow lava suggests low effusion rate eruptions. The abundance of lava domes suggests that the fissure eruptions were of sufficient duration to focus and produce a line of volcanic edifices. Near segment centers we observe fewer but larger lava domes, voluminous drained and collapsed lava lakes, and smooth lobate and sheet lava flows with very little pillow lava. The abundance of sheet flows suggests that high effusion rate eruptions are common. Fewer lava domes and large lava lakes suggest that fissure eruptions do not focus to point sources. This pattern was observed on eight third-order ridge segments suggesting that a fundamental volcanic segmentation of the ridge occurs on this scale. The third-order segment boundaries also correlate with local maxima in the seismicaxial magmac hamber reflector depth throughout the study area and decreased across-axis width of the region of seismic layer 2A thickening along the one segment where sufficient cross-axis seismic lines exist. The geochemically defined magmatic segment boundaries in the study area match the locations of our volcanic segment boundaries, although rocks ampling density is not adequate to constrain the variation across all the third-order volcanic segments that we identify. These observations suggest that variation in the processes of crustal accretion along axis occurs at a length scale of tens of kilometers on superfast spreading (> 140 km/Myr full rate) mid-ocean ridges.

Share

COinS