Causes of Oceanic Crustal Thickness Oscillations Along a 74-M Mid-Atlantic Ridge Flow Line

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 degrees 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 similar to 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.