On the segmentation of the slow spreading Mid-Atlantic Ridge between Atlantis and Oceanographer Transform (29.5 N to 35.5 N)

Mid-Ocean Ridges (MOR) are accretionary plate boundaries where new seafloor is created by seafloor spreading. In the early 1980s, these features were mapped for the first time in high detail using multi-beam echosounders and researchers found that the ridge crest of this approximately 70.000 km long rift system has many lateral discontinuities that partition its axis into segments. Discontinuities differ in form and behaviour and are often deeper and less active volcanically than the segments they define. As a result, the crest of the MOR undulates up and down by hundreds of meters over distances of several to hundreds of kilometres. The most prominent ridge offsets are the oceanic transform faults which typically offset the ridge axis by over 20 km. Long transform faults generally form deep valleys, while shorter discontinuities (non-transform offsets) displacing the spreading axis by only a few kilometres to tens of kilometres may show more complex tectonic features.

Even 60 years after the plate tectonic revolution and the introduction of seafloor spreading, much of the classification of ridges crest segmentation is still based on the study of fast-spreading ridges dominated by robust magma supply where discontinuities along the spreading axis are readily identified by offsets of the crest-like ridge axis, including overlapping and often migrating Overlapping Spreading Centres (OSC). It is generally believed that slow spreading ridges show analogue features. Yet observations of prominent median valleys at slow spreading ridges show a much more diverse segmentation. Here, we revisit the segmentation of the slow spreading Mid-Atlantic Ridge (MAR) between 29°30’N (south of Atlantis transform) to 35°30’N (north of Oceanographer transform) using data collected in September and October of 2024 aboard the German RV METEOR during the cruise M204 running a swath-mapping survey along the axis of the MAR. In analogy to fast spreading ridges, we find transform faults and overlapping volcanic centres, but we also map large dome-like features, en-échelon spreading segments, and offsets revealing bookshelf faulting. These structures provide insight into both the various styles of non-transform offsets, and the parameters controlling the different shear accommodation styles.