Detailed geological mapping of two contrasting second-order segments of the Mid-Atlantic Ridge between Oceanographer and Hayes fracture zones (33 degrees 30 ' N-35 degrees N)

We present detailed geological mapping of the axial valley floor and ridge flanks of two neighboring but contrasting spreading segments (OH1 and OH3) of the Mid-Atlantic Ridge between Oceanographer and Hayes fracture zones. N ew in situ observations from the submersible Nautile correlated with swath bathymetry and acoustic backscattering data from these segments reveal that the along- and across-axis volcanic-tectonic variability within second-ord er segments is larger than commonly acknowledged. Segment OH1 is a long and robust segment with a narrow and shallow axial valley. The most intense ma gmatic activity is found at the segment center where the thickest crust has been imaged, suggesting focused magma supply. Away from this part of the s egment, faulting and sedimentary cover predominate. In contrast, the center of segment OH3 is dominated by extensive sedimentary cover, fissuring, and faulting. Furthermore, the most recent constructional volcanism is located away from the segment center in a region of greater depth and thinner crus t. This relocation of volcanism suggests either a recent shift in the magma tic source or the last vestige of a centrally located source fed by lateral dike injection. Segment tip magmatic oscillations are suggested by the dis tribution of rock types at both segment ends. Serpentinized peridotites and associated dolerites are exposed at the massifs located at the intersectio n with nontransform offsets (NTOs), whereas only basaltic rocks crop out on the nodal basin floors. We suggest that the combination of low magmatic bu dget and extension taking place at the NTOs during a segment retreat favors the uplift and exposure of ultramafic massifs.