Construction of the neo-volcanic area of a slow-spreading ridge in a cold mantle context: Mid-Atlantic Ridge Eastern Intersection with Romanche Transform Fault

At mid-oceanic ridges, mantle temperature and magma supply influence the structure of the neo-volcanic zone. Due to the large Romanche offset, a strong “cold edge” effect is present at its eastern intersection with the Mid-Atlantic Ridge. This effect decreases with the distance from the transform fault, making this region an ideal area to study the impact of the thermal gradient on the architecture of the neo-volcanic zone. We analyzed seafloor videos and photos from submersible dives, as well as bathymetry and backscatter data collected during the SMARTIES cruise (2019), from the Ridge-Transform Intersection (RTI) to approximately 80 km to the south of it. We produced maps at local and regional scales and quantified the morphology of volcanoes (height, diameter, height/diameter ratio, volume and surface). Visual observations have showed that the seafloor is mainly made up of pillows or elongated pillows and rare massive lava flows. Within 30 km of the RTI, the neo-volcanic area is characterized by clusters of volcanoes, affected by faults trending N120-130° E, oblique to the extension and to the transform fault orientation, and by faults trending E-W. At 30 km to 50 km from the RTI, the Central segment displays a robust volcanic ridge oriented N150°E built by a pilling of volcanoes and narrow ridges. Its eastern and southern parts are old and characterized by oblique faults and narrow ridges (N130-140°E), while the northwest portion is more recent, faults and ridges are almost normal to the extension. The southernmost segment, located at 80 km from the RTI, is orthogonal to the spreading direction and asymmetric, bounded at the west by a detachment fault. Recent volcanic edifices were observed from the center of the segment to the base of the detachment. Our observations suggest that the neo-volcanic area is fed by more and more magma from north to south. This increase in magma supply is marked by a more structured volcanic axis, volcanoes that are more voluminous and a change in the orientation of the segments and faults. Changes in the orientation of faults and off-axis abyssal hills also reveal variations in the magmatic supply over time.