Record of seamount production and off-axis evolution in the western North Atlantic Ocean, 25°25′–27°10′N

Abstract

Using multibeam bathymetry, we identified 86 axial and 1290 off-axis seamounts (near-circular volcanoes with heights ≥70 m) in an area of 75,000 km2 on the western flank of the Mid-Atlantic Ridge (MAR), 25°25′N to 27°10′N, extending ∼400 km from the inner rift valley floor to ∼29 Ma crust. Our study shows that seamounts are a common morphological feature of the North Atlantic seafloor. Seamount-producing volcanism occurs primarily on the inner rift valley floor, and few, if any, seamounts are formed on the rift valley walls or the ridge flank. The high abundance of off-axis seamounts is consistent with 1–3 km wide sections of oceanic crust being transferred intact from the axial valley to the ridge flank on crust >4 Ma. Significant changes in seamount abundances, sizes, and shapes are attributed to the effects of faulting between ∼0.6 and 2 m.y. off axis in the lower rift valley walls. Few seamounts are completely destroyed by (inward facing) faults, and population abundances are similar to those on axis. However, faulting reduces the characteristic height of the seamount population significantly. In the upper portions of the rift valley, on 2–4 Ma crust, crustal aging processes (sedimentation and mass wasting), together with additional outward facing faults, destroy and degrade a significant number of seamounts. Beyond the crest of the rift mountains (>4 Ma crust) faulting is no longer active, and changes in the off-axis seamount population reflect crustal aging processes as well as temporal changes in seamount production that occurred at the ridge axis. Estimates of population density for off-axis seamounts show a positive correlation to crustal thickness inferred from analysis of gravity data, suggesting that increased seamount production accompanies increased magma input at the ridge axis. We find no systematic variations in seamount population density along isochron within individual ridge segments. Possible explanations are that along-axis production of seamounts is uniform or that seamount production is enhanced in some regions (e.g., segment centers), but many seamounts do not meet our counting criteria because they are masked by younger volcanic eruptions and low-relief flows.