Causes of oceanic crustal thickness oscillations along a 74-M Mid-Atlantic ridge flow line

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 20, (2019): 6123-6139, doi: 10.1029/2019GC008711.Gravity, magnetic, and bathymetry data collected along a continuous 1,400‐km‐long spreading‐parallel flow line across the Mid‐Atlantic Ridge indicate significant tectonic and magmatic fluctuations in the formation of oceanic crust over a range of time scales. The transect spans from 28 Ma on the African Plate to 74 Ma on the North American plate, crossing the Mid‐Atlantic Ridge at 35.8°N. Gravity‐derived crustal thicknesses vary from 3–9 km with a standard deviation of 1.0 km. Spectral analysis of bathymetry and residual mantle Bouguer anomaly show a diffuse power at >1 Myr and concurrent peaks at 390, 550, and 950 kyr. Large‐scale (>10 km) mantle thermal and compositional heterogeneities, variations in upper mantle flow, and detachment faulting likely generate the >1 Myr diffuse power. The 550‐ and 950‐kyr peaks may reflect the presence of magma solitons and/or regularly spaced ~7.7 and 13.3 km short‐wavelength mantle compositional heterogeneities. The 390‐kyr spectral peak corresponds to the characteristic spacing of faults along the flow line. Fault spacing also varies over longer periods (>10 Myr), which we interpret as reflecting long‐lived changes in the fraction of tectonically versus magmatically accommodated extensional strain. A newly discovered off‐axis oceanic core complex (Kafka Dome) found at 8 Ma on the African plate further suggests extended time periods of tectonically‐dominated plate separation. Fault spacing negatively correlates with gravity‐derived crustal thickness, supporting a strong link between magma input and fault style at mid‐ocean ridges.Data and supplemental materials are available at the Woods Hole Open Access Server (doi.org/10.26025/1912/24796). We would like to thank the Woods Hole Oceanographic Institution, National Science Foundation, Naval Oceanographic Office, and the captain and crew of R/V Neil Armstrong for making the SCARF cruise possible. We would also like to thank Eboné Pierce for her help during the cruise. We thank Meghan Jones for advice using MBSystem. We also thank Maurice Tivey, John Greene, and Masako Tominaga for advice on processing the magnetic data sets. We would like to thank Peter Huybers for sharing his spectral analysis codes. We would like to thank Rob Sohn for his help on interpreting the spectral analysis. We would like to thank Del Bohnenstiel, Milena Marjanović, one anonymous reviewer, and Editor Thorsten Becker for their very helpful comments that improved this manuscript. Funding was provided for this research by NSF OCE‐14‐58201.2020-05-1