Interplay Between Volcanic and Tectonic Axial Processes in a Cold Slow-spreading Ridge: the Mid-Atlantic Ridge South of the Romanche Transform Tault

Abstract:

Axial volcanic processes at the eastern Mid-Atlantic ridge-Romanche transform fault intersection are investigated through the joint analysis of video recordings acquired with the deep-sea submersible Nautile and of ship bathymetry and backscatter. Data were acquired during the recent SMARTIES cruise.

The axial domain displays a complex morphology. In the southernmost part, ~60 km south of the ridge-transform intersection (RTI), the MAR axis strikes about N165°E, perpendicular to the spreading direction. This segment is borded by a large detachment surface on its western flank and large normal faults on the East. Further north, towards the RTI, the axial direction becomes more oblique, forming a relay zone, with a deep basin enclosed by two topographic highs. North of this relay zone, another segment displays a more oblique direction. Nearing the transform fault, the obliquity increases progressively and the magmatism occurs inside elongated basins with a marked obliquity to the spreading direction, forming a N110°E transfer area between the MAR axis and the Romanche transform.

Two dives, located at the extreme limits of our study area are presented here. The northernmost dive crossed an area where volcanic features form a near-circular cluster at the transfer area. This peculiar volcanic area is located south of a large faulted oceanic core complex, about 20 km south of the Romanche. The lava flows are recent, some covered by a thin veneer of sediment and faulted. There is a close interplay between tectonics and volcanism, as some flows are cut by very recent faults while others cover older faults. The fault direction is mostly N110°E, parallel to the strike of the transfer area and slightly oblique to the strike of the Romanche (N75°E).

The southernmost dive crossed the neovolcanic ridge at the northern end of the ridge south segment, just southeast of a large flat-topped volcano. The seafloor observations show again very recent lava flows. The flows seem to be larger than those observed at the northern dive and also possibly more recent, as the sediment cover appears to be thinner. Faults also appear to be more recent at this location. The faults are mainly parallel to the axial valley (N165°E) but faults oriented N110°E are also observed, suggesting the existence of intra-segment transfer zones between the main volcanic ridges.