Deciphering the Mass Structure of Upper Mantle Flows by Multi-disciplinary Analyses of Satellite Gravity Data

Abstract:

Imaging the mass structure of the mantle at the highest possible resolution is a major undertaking, providing key information in order to decipher the nature of its convective flows. Satellite gravity data record the gravitational signatures of the convecting mantle mass sources, but they are overprinted by large signals from the lithosphere. This constitutes an important limitation in retrieving information on the deeper mantle structure from the gravity data. Here, we present a method based on gravitational gradients analysis in order to enhance the identification and separation of the mantle sources at all depths below the lithosphere. It is based on a detailed description of the spatial geometry of the gravity signals using a multi-directional analysis, which can then be compared to the geometry of seismic velocities at depth. This way, we have been able to extract and reconstruct 1600-2000-km scale gravity and seafloor topography signals, periodically aligned with the present-day absolute plate motions over wide areas of the Pacific and Northern Indian basins. These signals are consistent with 2000-km wavelength slow shear velocity anomalies sharing a similar orientation at upper mantle depths (French et al., 2013), and their amplitudes show that the slow seismic signals must be associated with excess mass sources below seafloor depressions. We discuss the possible implications of these observations.