Continental lithosphere and its interactions with the asthenosphere: New insights from seismic imaging and petrological inversion

Archean cratons have thick, cold lithosphere that is remarkably stable, thanks to its compositional buoyancy and mechanical strength. Despite this stability, cratonic lithosphere can, sometimes, be modified and eroded, following the impact of a mantle plume, episodes of subduction and continental collision, or stretching and rifting. Although the chemical modification and removal of the Archean lithospheric material are permanent, there is intriguing evidence for re-growth in cratonic lithosphere’s thickness in some locations. In order to understand the enigmatic lithospheric evolution of cratons and continental blocks adjacent to them, we need the knowledge of the thermo-chemical structure of the lithosphere and of the dynamics of the lithosphere-asthenosphere interaction.

Seismic surface waves yield abundant evidence on the thermal structure and thickness of the lithosphere and on the temperature of the underlying upper mantle. Tomographic maps resolve in fine regional detail the boundaries between high-velocity (cold) cratons and lower-velocity (warm) neighbouring blocks. The radial structure and thickness of the lithosphere, however, are not resolved by tomographic models quite as well, due to their non-uniqueness. As a result, seismic-velocity profiles from tomographic models are normally incompatible with plausible geotherms. How, then, can we determine the structure and thickness of the lithosphere?

Recently developed methods for computational-petrology-powered inversion (e.g., Fullea et al. 2021) relate seismic, topography, heat-flow and other data directly to temperature and composition of the lithosphere and underlying asthenosphere. The misfit valleys in the surface-wave-dominated parameter space are still broad, and it is essential to have accurate measurements and low data-synthetic misfits. Here, we achieve remarkably low misfits of ~0.1% of the surface-wave phase-velocity values by precise tuning of the petrological inversion, its parameterisation and regularisation. The data are fit closely by models with depleted harzburgite mantle compositions within the lithosphere of cratons. The inversions tightly constrain the thickness of cratonic lithosphere, which we find to vary in the ~150-300 km range over different cratons. The plume-lithosphere interactions and the associated surface uplift and volcanism are controlled, to a large extent, by the lithospheric thickness  (e.g., Civiero et al. 2022), which, in turn, evolves with time, influenced by the processes. High-resolution seismic imaging and the petrological inversion of the resulting data yield exciting new discoveries on the evolution of continental lithosphere and its interactions with the underlying mantle.

References

Civiero, C., Lebedev, S., Celli, N. L., 2022. A complex mantle plume head below East Africa-Arabia shaped by the lithosphere-asthenosphere boundary topography. Geochemistry, Geophysics, Geosystems, 23, e2022GC010610.

Fullea, J., Lebedev, S., Martinec, Z., Celli, N.L., 2021. WINTERC-G: mapping the upper mantle thermochemical heterogeneity from coupled geophysical–petrological inversion of seismic waveforms, heat flow, surface elevation and gravity satellite data. Geophysical Journal International, 226(1), 146-191.