Axial instability and time evolution at an ultra-cold slow-spreading ridge: the Mid-Atlantic Ridge-Romanche transform intersection. First results from the SMARTIES cruise

Abstract:

Along slow spreading ridges, transform faults are often associated with areas of lower magmatic budget as the contact at ridge-transform intersections of an older and colder oceanic lithosphere with the ridge axis reduces the mantle temperature, yielding lower melting rates.

At the Eastern intersection between the Mid-Atlantic Ridge (MAR) axis and the Romanche transform fault, the 50 My age difference results in very low mantle temperatures and dominance of tectonic over magmatic processes at the ridge axis. The bathymetry reveals complex spatial and temporal patterns of spreading. Several large low-angle surfaces, observed on both flanks of the ridge, were interpreted as fragments of detachment faults and are more similar to the succession of detachments observed at the South-West Indian Ridge than to the typical discrete Oceanic Core complexes (OCC) observed in the MAR. Immediately south of the transform fault, a different pattern is observed. The eastern wall of the nodal basin is dominated by a large detachment forming a heavily faulted OCC. On the western flank, two features can be interpreted as fragments of ancient OCCs, possibly ruptured by ridge jumps. Several Nautile submersible dives along these features retrieved peridotites.

The axial strike changes from near transform-parallel in the north to a direction orthogonal to the spreading in the south, with a succession of basins and relay zones. Volcanism occurs in the axial basins, forming clusters of volcanoes in the north and more structured axial volcanic ridges in the south. All volcanic terrains are heavily faulted. Off axis, on the SW flank, elongated abyssal hills parallel to the ridge axis reflect a period of more abundant melt supply. The oblique pattern of normal faults observed further north, as well as a large detachment surface and a large peridotite hill along the axial valley eastern wall, suggest that phases of lower melt supply are associated with strong axial obliquity.

In the central part of the area, the structural pattern shows strong obliquity and lower melt supply off-axis, and a more recent increase in the melt supply closer to the axial area, accompanied by a change in the direction of the ridge axis. The northern area, closer to the transform, remained under a low magma supply while the south, conversely, displays a more magmatic spreading style.