South Atlantic Transect: Variations in Oceanic Crustal Structure at 31 degrees 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 degrees 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.