Complex evolution of the St. Paul transform system: mantle exhumation and deformation

The COLMEIA cruise, held in the Equatorial Atlantic, in the area of the St. Paul transform system, is part of a joint effort between France and Brazil for the study of the Mid-Atlantic ridge near the St. Peter St. Paul’s Rocks. The scientific objective of the cruise was to study in detail the temporal evolution of the complex Saint Paul transform plate boundary, and the origin of the St. Peter -St. Paul mylonitic massif. This area of the Mid-Atlantic ridge was considered to be a mantle “cold spot” by different authors, thus associated with a low melting regime. Both the bathymetry data and the rocks recovered by dredging suggest that the image of a regional amagmatic Mid-Atlantic ridge is a simplistic view of the processes active in the St. Paul system. The ridge segments are short and narrow, with deep axial valleys. Axial depths are below 4000 m on average, and reach 5400 m in some nodal basins. There is no evidence for a clearly defined neo-volcanic ridge on the axial valley floors, but a few round volcanoes were observed in the axial valley of the central segment. The pattern of off-axis abyssal hills is highly variable from one segment to another. The northern segment displays a long sequence of magmatic abyssal hills. The central segment shows both hummocky ridges probably of magmatic origin, but also ridges where peridotites have been dredged. The southern segment shows few short, symmetric ridges made of peridotite and gabbros. Both the central and the southern segments display asymmetric core complexes nucleating at segments ends. This variety of off-axis morphologies suggest that accretionary processes along the intra-transform segments are unstable and highly variable in space and time. Thus, significant variations in the spreading style were recognized, with a more magmatic northern segment and comparatively less magmatic central and southern segments. However, the existence of long-lived core complexes at the western flanks of these later segments suggests that, instead of an amagmatic regime with mantle exhumation, we are observing a reduced melt extraction regime probably controlled by a cold, thick lithosphere where magma is retained in the crust to create large gabbro bodies. Another striking result is the evidence for compressive stresses across the area, which can be linked to the uplift of the Saint Paul mylonitic massif.