Axial magnetic anomaly amplitude along the Mid‐Atlantic Ridge between 20°N and 40°N

Abstract

After reduction to correct for the topography, spreading rate, latitude, and lineation azimuth effects, axial magnetic anomalies amplitudes are compared with bathymetric, gravity, seismological, and geochemical data along a 2600‐km‐long section of the Mid‐Atlantic Ridge axis between 20°N and 40°N. Significant results are obtained at two different scales, a regional one (>100 km) related to sublithospheric thermal perturbations such as those associated to hotspots and a local one (<100 km) related to ridge segmentation. At a regional scale, two long‐wavelength highs of the axial magnetic anomaly amplitudes are observed between 37.5°N and 40°N and between 27°N and 30°N. The first high, associated with marked bathymetric, gravity, geoid, seismic velocity, and geochemical anomalies, is related to the Azores hotspot and is interpreted as reflecting the existence of a thick and/or Fe‐Ti enriched magnetic source layer. A second long‐wavelength high centered at 28.5°N corresponds to a weaker bathymetric and gravity signature; it is associated with a large low seismic velocity anomaly and a marked intermediate‐wavelength positive geoid high which suggest higher sublithospheric temperatures, deeper and more extensive partial melting, and a thicker and/or Fe‐Ti enriched magnetic source layer. At a local scale, all but one of the 25 ridge segments investigated show that axial magnetic anomaly amplitude is higher at segment ends by a factor of 2 compared to segment centers. A clear correlation is observed between the range of axial magnetic anomaly amplitude and the range of the mantle Bouguer anomaly (ΔMBA) within each segment. More complex relationships exist between median amplitude, amplitude at segment centers and at segments ends, and ΔMBA. Separation of segments in two groups (“hotter” and “colder”) based on ΔMBA shows different types of variations among segment, with grouped amplitudes at segment centers and scattered amplitudes at segment ends for hotter segments and the opposite for colder segments. These observations support shallow fractionation and the presence of serpentinized bodies in the vicinity of discontinuities as the major processes which control the axial magnetic anomaly amplitude variations.