LAB - Transition between Fossil and Present-Day Flow-Related Seismic Anisotropy

Abstract:

Topography of the lithosphere-asthenosphere boundary (LAB) and structure of the continental lithosphere record geodynamic development of the outer parts of the Earth. Studying the architecture of plates and mapping the LAB discontinuity relief can help to answer questions how and when the plates assembled and to what extent they were later deformed. Differences in changes of physical properties across the “discontinuities” are significant, but sharpness of the transitions depends on optics we are using. Moreover, the nature of the boundary/transition remains uncertain. Fundamental difference in origin and orientation of seismic anisotropy in the mantle lithosphere and in the sub-lithospheric mantle has led us to develop an original approach of LAB modelling. We associate the LAB with a transition of fossil anisotropy in the rigid high-velocity lithosphere to present-day related anisotropy in the more viscous underlying asthenosphere. We present a global LAB model calculated from depth-dependences of azimuthal and radial anisotropy of surface waves (Plomerova et al., 2002) and a uniform model of the European LAB (Plomerova and Babuska, 2010), calculated from P-wave travel times collected during regional passive experiments. In the latter, the LAB is modelled according to an empirical residual-depth relation. The high velocity contrast across the LAB (δv_P ~ 0.6 km/s) required by the empirical gradient cannot be accomplished by isotropic velocities of materials forming the upper mantle, while it can be explained by different orientations of the high-velocities above and below the LAB. Relief of the lower plate boundary can vary considerably on short lateral distances and it is exposed to erosion from asthenosphere. On the other hand, dipping fabrics of often sharply bounded lithospheric domains, created probably during a plate formation, indicate stability of such a fossilized structure, which alters only near the domain boundaries.