Accretionary processes in the axial valley of the Mid-Atlantic Ridge 27 degrees N-30 degrees N from TOBI side-scan sonar images

We analyse TOBI side-scan sonar images collected during Charles Darwin crui se CD76 in the axial valley of the Mid-Atlantic Ridge (MAR) between 27 degr ees N and 30 degrees N (Atlantis Transform Fault). Mosaics of the two side- scan sonar swaths provide a continuous image of the axial valley and the in ner valley walls along more than six second-order segments of the MAR. Tect onic and volcanic analyses reveal a high-degree intra-segment and inter-seg ment variability. We distinguish three types of volcanic morphologies: humm ocky volcanoes or volcanic ridges, smooth, flat-topped volcanoes, and lava flows. We observe that the variations in the tectonics from one segment to another are associated with variations in the distribution of the volcanic morphologies. Some segments have more smooth volcanoes near their ends and in the discontinuities than near their mid-point, and large, hummocky axial volcanic ridges. Their tectonic deformation is usually limited to the edge s of the axial valley near the inner valley walls. Other segments have smoo th volcanoes distributed along their length, small axial volcanic ridges, a nd their axial valley floor is affected by numerous faults and fissures. We propose a model of volcano-tectonic cycles in which smooth volcanoes and l ava flows are built during phases of high magmatic flux. Hummocky volcanic ridges are constructed more progressively, by extraction of magma from pock ets located preferentially beneath the centre of the segments, during phase s of low magma input. These cycles might result from pulses in melt migrati on from the mantle. Melt arrival would lead to the rapid emplacement of smo oth-textured volcanic terrains, and would leave magma pockets, mostly benea th the centre of the segments where most melt is produced. During the end o f the volcanic cycle magma would be extracted from these reservoirs through dikes with a low magma pressure, building hummocky volcanic ridges at low effusion rates. In extreme cases, this volcanic phase would be followed by amagmatic extension until a new magma pulse arrives from the mantle.