Volcanic mound fields on the East Pacific Rise, 16 –19 S: Low effusion rate eruptions at overlapping spreading centers for the past 1 Myr

[1] Volcanic mound fields identified on SeaMARC II and HMR1 12 kHz side-scan data from the southern East Pacific Rise (SEPR) occur near overlapping spreading centers (OSCs) and migration traces of OSCs. The volcanic mound fields appear as a distinctive hummocky seafloor fabric due to side-scan backscatter reflections from clusters of moundshaped reflectors. The lack of growth of the mound fields away from the ridge axis, and their occurrence in association with OSC traces, suggests that mound fields form along the ridge crest near OSCs. Volcanic mound fields are found where 120 kHz side-scan and visual observations find fields of pillow mounds. Since pillow mounds are constructed by low effusion rate eruptions, the volcanic mound fields found near the OSCs and in their migration traces indicate that volcanic effusion rates tend to be lower near ridge discontinuities than midsegment regions. This tendency for low effusion rate eruptions at OSCs is documented for the past 1 Myr. Three independent measurements of ridge segmentation, (1) volcanic segment boundaries marked by the low effusion rate eruptions, (2) tectonic segments defined by OSCs, and (3) magmatic segment boundaries based on continuity of parental magma composition, all coincide in the study area. High backscatter off-axis lava fields not associated with seamounts are found on seafloor younger than 0.2 Ma. The 0.2 Ma corridor corroborates previous results from the distribution of small isolated volcanoes that indicates randomly distributed off-axis eruptions mainly occur on crust younger than 0.2 Ma.