Examining the mantle upwelling beneath the northern Mid-Atlantic Ridge

We examine a mantle seismic anomaly beneath the northern Mid-Atlantic Ridge (MAR) by forward numerical simulations using the spectral element method (SEM). The MAR anomaly is imaged in a recent global upper-mantle tomographic model, FFSW1, obtained by finite-frequency tomography using 3-D sensitivity kernels for fundamental-mode Love and Rayleigh waves [Zhou et al.,2005]. In model FFSW1, the MAR anomaly beneath the slow-spreading seafloor extends down to at least the transition zone, and is characterized by (1) strong SH-wave velocity reduction in the upper mantle, and (2) negative radial anisotropy (SH waves travel slower than SV waves) at depths greater than 220 km. This deep-extending anomaly has been proposed to be associated with mantle upwellings that drive plate tectonics at the initial opening of the seafloor. In this study, we show that direct SH waves at an epicentral distance of about 45° are significantly delayed compared to direct SV waves, for earthquakes along the Mid-Atlantic ridge recorded at stations off the east coast of the Atlantic ocean. This observation is consistent with the presence of negative anisotropy in the lower part of the upper mantle in model FFSW1. We show that the depth extent of the radially anisotropic MAR anomaly can be better determined by 3-D SEM forward modeling, taking into account the delay times of direct S waves and higher-mode as well as fundamental-mode surface waves.