Along-axis magmatic oscillations and exposure of ultramafic rocks in a second-order segment of the Mid-Atlantic Ridge (33 degrees 43'N to 34 degrees 07'N)

New in situ observations on the Mid-Atlantic Ridge demonstrate that the along- and across-axis volcano-tectonic variability within second-order segments is larger than commonly acknowledged. In our study area, the segment center, far from being the place with present-day maximum volcanic activity, is dominated by extensive sedimentary cover, fissuring, and faulting. Furthermore, the area of most recent magmatic activity is located away from the segment center in a region of greater depth and thinner crust. Segment-tip magmatic oscillations are suggested by the distribution of rock types at both segment ends. Serpentinized peridotites and associated dolerites are exposed at the massifs located at the intersection with nontransform discontinuities (NTDs), whereas only basaltic rocks crop out on the nodal basin floors. We suggest that the combination of low magmatic budget and extension taking place at the NTDs during a segment retreat favors the uplift and exposure of ultramafic massifs.