Plate tectonics characterize transform faults as conservative plate boundaries where the lithosphere is neither created nor destroyed. In the Atlantic, both transform faults and their inactive traces, fracture zones, are interpreted to be structurally heterogeneous, representing thin, intensely fractured, and hydrothermally altered basaltic crust overlying serpentinized mantle. This view, however, has recently been challenged. Instead, transform zone crust might be magmatically augmented at ridge-transform intersections before becoming a fracture zone. Here, we present constraints on the structure of oceanic crust from seismic refraction and wide-angle data obtained along and across the St. Paul fracture zone near 18°W in the equatorial Atlantic Ocean. Most notably, both crust along the fracture zone and away from it shows an almost uniform thickness of 5–6 km, closely resembling normal oceanic crust. Further, a well-defined upper mantle refraction branch supports a normal mantle velocity of 8 km/s along the fracture zone valley. Therefore, the St. Paul fracture zone reflects magmatically accreted crust instead of the anomalous hydrated lithosphere. Little variation in crustal thickness and velocity structure along a 200 km long section across the fracture zone suggests that distance to a transform fault had negligible impact on crustal accretion. Alternatively, it could also indicate that a second phase of magmatic accretion at the proximal ridge-transform intersection overprinted features of starved magma supply occurring along the St. Paul transform fault.

Plain Language Summary

Transform faults represent plate boundaries where two plates move past each other without producing new or destroying the existing lithosphere. Most of the Atlantic transform faults and their inactive traces, fracture zones, were characterized by fractured and altered, thin-crust overlying serpentinized mantle rocks. However, recent results reveal that the crust beneath fracture zones may not be as thin and challenge the standard view, introducing a mechanism of secondary magma supply at the intersection between the ridge axis and transform fault. Here, we present results from seismic experiments at the St. Paul fracture zone near 18°W in the equatorial Atlantic. Our results suggest that the subsurface of the St. Paul fracture zone is represented by a nearly uniform crustal thickness of 5–6 km and an upper mantle with a velocity of 8 km/s. Both observations argue for a crust of magmatic origin and the absence of strong alteration of the upper mantle. Collectively, constant crustal thickness and little variation in seismic velocities along the profile crossing the fracture zone suggest that the crustal formation process does not vary as a function of distance from the fracture zone. Alternatively, secondary magma supply at the ridge-transform intersection could overprint any anomalous formation conditions.