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

Using multibeam bathymetry, we identified 86 axial and 1290 off-axis seamou nts on the western flank of the (near-circular volcanoes with heights great er than or equal to 70 m) in an area of 75,000 km(2) Mid-Atlantic Ridge (MA R), 25 degrees 25'N to 27 degrees 10'N, extending: similar to 400 km from t he inner rift valley floor to similar to 29 Ma crust. Our study shows that seamounts are la common morphological feature of the North-Atlantic seafloo r. Seamount-producing volcanism: occurs primarily on the inner rift valley floor, and few, if any, seamounts are formed on the rift valley walls or th e 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 axi al valley to the ridge flank; on crust >4 Ma. Significant changes in seamou nt abundances, sizes, and shapes are attributed to the effects of faulting between similar to 0.6 and 2 m.y. off axis in the lower rift valley walls. Few seamounts are completely destroyed by (inward facing) faults, and popul ation abundances are-similar to those on axis. However, faulting reduces th e characteristic height of the seamount-population significantly. In the up per portions of the rift valley, on 2-4 Ma crust, crustal aging processes ( sedimentation and mass wasting), together with additional outward facing fa ults, destroy and degrade a significant number of seamounts. Beyond the cre st of the rift mountains (>4 Ma crust) faulting is no longer active, and ch anges in the off-axis seamount population reflect crustal aging processes a s well as temporal changes in seamount production that occurred at the ridg e axis. Estimates of population density for off-axis seamounts show a posit ive 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 popu lation density along isochron within individual ridge segments. Possible ex planations are that along-axis production of seamounts is uniform or that s eamount production is enhanced in some regions (e.g., segment centers), but many seamounts do not meet our counting criteria because they are masked b y younger volcanic eruptions and low-relief flows.