Global mapping of Lithosphere/Asthenosphere Boundary from surface wave tomography

The coupling between rigid lithosphere and the weaker underlying asthenosphere is a key point of Plate Tectonics and Mantle dynamics. The characterization of the properties of the Lithosphere/Asthenosphere Boundary (LAB) is essential for understanding the Upper Mantle. Recent studies, using receiver functions for example, provide local constraints. In this study a global view by surface wave tomography is given. A large amount of data from different groups (Harvard, Boulder, Utrecht, Paris) has been collected. There are more than 100,000 phase and group velocicties measurements on the fundamental mode of Rayleigh and Love waves. This global scale dataset in the period range 15s-200s, enables us to investigate the LAB with an approximative lateral resolution of 500km. The regionalization of the path-averaged velocities is performed to extract isotropic and azimuthally anisotropic terms of local velocities. First we derive our own crustal model (taking account of topography-bathymetry, sediments and crustal thickness) by a MonteCarlo inversion with the shorter periods of the data. Second, to estimate the LAB properties and obtain a global map, we choose a very simple parameter space ajusted with the larger periods of the data. We report here a good correlation between oceanic inverted LAB depth (~45km average) and ocean floor age, in the sense of the classical thickening of oceanic lithosphere. The determination of LAB beneath continents is more difficult, shallower depths (~90km average) are found in comparison with heat flux studies.