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

Abstract

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.

Late Cretaceous Ridge Reorganization, Microplate Formation, and the Evolution of the Rio Grande Rise – Walvis Ridge Hot Spot Twins, South Atlantic Ocean

“…Gravity and VGG lineations result from larger AH and only a few of the AH mapped by multibeam correspond to mapped gravity lineations. Nevertheless, the mean of AH azimuths Christeson et al (2020). Uppermost plot shows the bathymetry without interpretation; the plot below it shows interpreted abyssal hill crests (black lines).…”

“…Modern high‐resolution multibeam bathymetry data are rare in the south Atlantic, but we found two useful test areas. One is a narrow zone surveyed during a 2016 multichannel seismic study (Christeson et al., 2020), located slightly east of ERGR (Figures 4 and 5). It shows small, linear ridges, which are the tops of partly buried AH.…”

“…Multibeam bathymetry data at top are from Christeson et al. (2020). Uppermost plot shows the bathymetry without interpretation; the plot below it shows interpreted abyssal hill crests (black lines).…”

Seismic structure of the St. Paul Fracture Zone and Late Cretaceous to Mid Eocene oceanic crust in the equatorial Atlantic Ocean near 18°W

“…Farther south, between 0° and ~3°S around the Chain FZ, crustal thickness is 4.6-5.9 km (Marjanović et al, 2020) and at 2°S of the MAR the crustal thickness ranges from 5.6 to 6.0 km along a 600 km long flow line profile (Vaddineni et al, 2021). However, Christeson et al (2020) reported from five ridge parallel profiles at 31°S a significant crustal thickness variations of 3.6 to 7.0 km for different crustal ages (6-60 Ma), but an almost constant thickness along each profile and thus for crust of the same age, suggesting that the equatorial and south Atlantic shows consistently thinner crust when compared to the average thickness of 7 km reported by White et al (1992) for the Atlantic. However, we have to note that that data compiled by White et al (1992) occurred predominantly in the North Atlantic with a large number of experiments in the north-western Atlantic where crust is in the order of 7-8 km (e.g., Purdy, 1983;Minshull et al, 1991), suggesting that previous estimates might be biased.…”

Evolution and properties of young oceanic crust: constraints from Poisson's ratio

“…Bratt & Solomon 1984;Collier & Singh 1998), two-dimensional profiles (e.g. Van Avendonk et al 1998;Audhkhasi & Singh 2019;Peirce et al 2019a,b;Wilson et al, 2019;Christeson et al 2020;Davy et al 2020), and three-dimensional tomographic grids (e.g. Toomey et al 1990;Gregory 2018;Robinson et al 2020;Simao et al 2020), that have become more detailed through the use of greater numbers of instrumentation deployed at smaller spatial intervals and able to sample at higher temporal rates.…”