Magmatic versus tectonic control on the spreading style of ridge segments inside a slow-slipping multi-transform fault: the St. Paul system, Equatorial Atlantic

The St. Paul multi-transform system is formed by four slow-slipping transform faults, bounding three short intra-transform ridge segments (ITRS), which offset the Equatorial Mid Atlantic Ridge by 630 km. The system formed ca 40 My ago when a change in plate motion induced extension at the large-offset paleo-St Paul transform fault. The opening of the transform domain resulted in the progressive formation of the ITRS. The spreading styles of the three segments are very different and appear to depend on the sub-lithospheric mantle temperature and composition as well as on the structure of the lithosphere. The spreading is more magmatic in the northern ITRS, whilst it is dominated by tectonic processes in the southern ITRS, which displays large Oceanic Core Complexes, and the central ITRS, where mantle is exposed at the seafloor and which displays the strongest obliquity. The significantly lower melt supply at this segment is very likely linked to the marked obliquity, but a small scale compositional change in the underlying mantle may also contribute to decrease the melt supply. Our observations suggest that shallow tectonics and seafloor morphology result from the interplay between thermal structure and local mantle source composition. The coupled change in mantle properties and tectonic style suggest a deep control of the internal segmentation of a multi-transform domain.