Finite-Frequency Imaging of the Global Lithosphere-Asthenosphere Boundary (LAB)

Abstract:

The lithosphere-asthenosphere boundary (LAB) divides the cold, rigid shell from the underlying weaker mantle and is of great significance in understanding plate tectonics and mantle convection. However, its depth and defining mechanism are currently poorly understood. We build a global dataset of finite-frequency traveltime measurements using SS precursors and invert for global depth perturbations of the LAB based on finite-Frequency boundary sensitivity calculated in the framework of traveling-wave mode summation. Inversion result shows that the overall oceanic LAB is shallower than in the reference model(<80km) but show large variations that are not correlated with the age of the seafloor. In the central Pacific, the imaged LAB is much deeper (∼100 km) in regions where strong positive gravity anomalies exist. This is counter-intuitive and cannot be explained by simple half space cooling. The seismically imaged first-order discontinuity may represent an equivalent velocity gradient structure produced by mantle differentiation processes which are also responsible for mass distribution (gravity) in this region. It has been suggested that the LAB is probably a sharp discontinuity (over<30 km) which cannot solely be explained by temperature but might be associated with the presence of partial melting, volatiles, or a change in seismic anisotropy. In this study, we compare seismograms generated in 1-D reference earth models with varying discontinuity sharpness to investigate seismic signals that are sensitive to the LAB structure. The result shows that surface wave overtones are more sensitive to the sharpness of the discontinuity than SS precursors, and can be potentially used to better constrain the LAB.