Synthesis On The Tectonics And Geochemistry Of The St. Paul Transform Fault, Equatorial Atlantic

The tectonic evolution through time of the<br>Saint Paul’s Fracture Zone System (SPFZ) was<br>determined based on the analyses of free air<br>anomalies, predicted bathymetry, magnetic isochrons,<br>as well as data from 13 deep submersible dives. The<br>SPFZ is located at 1°N on the MAR and represents<br>one of the largest discontinuities, which offsets the<br>MAR axis roughly 600 km. The relative age<br>difference between the opposite sides of the transform<br>fault is about 36 Ma.<br>The evolution through time show that in the<br>inactive segment of SPFZ changes its configuration<br>from a triple FZ in the transform fault to a double FZ<br>and can be traced up to the continental shelf break in<br>Brazil. A fossil spreading segment is observed at<br>33°W. Towards the west (37°50W, 60Ma), the SPFZ<br>is cut by a N25°W structure, and at 40°W (80Ma) by<br>the North Brazilian Ridge (NBR).<br>Evidence from analysis of the ridge axis<br>morphology, petrology of both basalts and uplifted<br>abyssal peridotites, high ultramafic/volcanic ratio,<br>osmium isotope composition, plate kinematics, and<br>mantle tomography suggest that the mantle<br>surrounding the SPFZ is cooler than other areas in the<br>Atlantic Ocean. Re-depleted model ages ranging<br>from 0.56 to 1.1 Ga for peridotites samples indicate<br>that the lithosphere entrained in this part of the<br>oceanic mantle could have formed in the Pan-<br>African-Brazilian orogeny. Chemical and physical<br>tools support the hypothesis that parts of the mantle in<br>the Equatorial Atlantic preserve an older, subducted<br>slab formed in a collision event period prior to 460 Ma.