South Atlantic Transect: Variations in Oceanic Crustal Structure at 31°S

We present an analysis of geophysical data acquired along a transect of 0–62 Ma crust located on the western flank of the Mid‐Atlantic Ridge at 31°S; all crust was formed at the same ridge segment. Crustal thickness, constrained by five wide‐angle profiles, has mean values of 5.6 km at 6.6 and 15.2 Ma, 7.0 km at 30.6 Ma, 5.5 km at 49.2 Ma, and 3.6 km at 61.2 Ma. Crustal thickness is uniform along each ridge‐parallel profile (standard deviations 0.1–0.3 km), indicating uniform along‐axis magmatic accretion over lateral distances of 40–60 km. The crustal structure of 61.2 Ma crust is not only anomalously thin compared to the other profiles but also contains regions with a linear velocity gradient from seafloor to Moho, which suggests that intense fracturing may extend to the base of the thin crust. Abyssal hill root‐mean‐square heights in the study region are 57–142 m and have an inverse correlation with spreading rate. These values are lower than the average root‐mean‐square height of 196 m elsewhere on the southern Mid‐Atlantic Ridge and indicate relatively high mantle temperatures in our study area. Unsedimented or lightly sedimented basement highs are prevalent at all ages; we argue that bottom currents scour the high topography, transporting sediment into adjacent basement lows. All drillsites planned for International Ocean Discovery Program Expeditions 390 and 393 are within 1–10 km of unsedimented or lightly sedimented basement highs, which should facilitate fluid flow and continued geochemical exchange between crust and seafloor. Plain Language Summary We present results from a site survey for scientific ocean drilling. The survey was conducted along a transect from the present day to 62 million years, with all crust formed at the same setting on the ridge axis. Multiple observations (bathymetric features, magnetic lineations, and crustal thickness variations) suggest that this region has relatively high mantle temperatures. Drilling at 30.6 and 61.2 Ma crust should encounter crust formed with a high and low magma supply, respectively. All drill sites are located near features that should facilitate fluid flow and geochemical exchange between crust and seawater. Key Points Geophysical observations indicate a persistent magma supply in the study area for crust formed at the southern Mid‐Atlantic Ridge 31°S The 61.2 Ma crust is thin (3.6 km) and has a crustal structure in some regions that may indicate fracturing to the base of the crust Prevalent uns