The relationship between buoyant mantle flow, melt migration, and gravity bull's eyes at the Mid-Atlantic Ridge between 33°N and 35°N

Abstract

Crustal thickness and mantle Bouguer anomaly (MBA) patterns along the Mid-Atlantic Ridge (MAR) between the Oceanographer and Hayes transforms (33–35°N) are predicted from a three-dimensional mantle flow model driven by plate spreading and mantle density variations, assuming a two- or three-dimensional model of melt extraction. Our two-dimensional melt extraction model focuses melt to the nearest ridge segment by assuming that melt moves only vertically and parallel to the spreading direction. In contrast, the three-dimensional model assumes melt migrates vertically to the top of the melting region and then up the local slope of this surface. The shape of this surface, which is controlled by enhanced conductive cooling at ridge offsets, directs melt toward the center of segments. Our results suggest that buoyant mantle flow driven by temperature variations and a small retained melt fraction (< 2%) enhances overall crustal production and produces long wavelength variations in MBA and crustal thickness relative to purely plate-driven mantle flow. Three-dimensional mantle flow with two-dimensional melt migration predicts little or no crustal thinning associated with small non-transform offsets. However, our simple model of three-dimensional melt migration shows that even a moderate amount of along-axis melt focusing can produce the observed segment-scale variations in MBA and crustal thickness. We therefore suggest that the along-axis crustal thickness and gravity variations observed at the MAR are the combined result of three-dimensional buoyant mantle upwelling (controlling long wavelength features) and three-dimensional melt migration (controlling short wavelength features including the gravity bull's eyes).