On Physical Meaning of Global Seismic Models for the Structure of Continental Lithosphere

In spite of the increasing resolution in seismic models, their physical interpretation is still unclear. We performed a regionalized cluster analysis on a family of global isotropic shear-velocity (VS) models to extract robust features on the seismic structure of the continental lithosphere. In agreement with the resolution of surface waves, the long wavelength component of the models maps well large continental provinces. The physical interpretation of the mean 1-D seismic profiles for the largest and seismically most uniform continental regions (the Australian, North-American and East-European cratons), indicate that the continental keels reach, at least, a 300 km depth. An increase in the seismic VS gradient with depth around 250 km is observed beneath continents. This feature, however, does not represent a thermal transition, rather reflects the global compositional nature of the mantle and the presence of mineralogical phase transitions. A similar increase in gradient is also observed beneath oceans and thus is a trait of the average seismic model. The thermal transition from conductive to smaller quasi-adiabatic gradients is instead found around 300 km depth. Additional changes in (VS) gradients with depth beneath continents, in particular a decrease between 150 and 250 km are consistent with a gradual enrichment in re-fertilized material within the continental lithosphere, such as observed globally in cratonic mantle peridotites. At 300 km depth, all cratonic regions look seismically similar and with VS more than 1% faster than global average values. Unless low mantle temperatures associated with deep lithospheric keels extend down to the transition zone, compositional difference between the deep part of the continental (lithospheric) mantle and the surrounding mantle can play an important role in producing high wave-speeds. At temperature of 1700 K, depleted compositions associated with old, continental lithosphere can produce ca. 0.8% faster VS anomalies at this depth. In conclusion, we find that the long-wavelength component of isotropic shear-velocity models (that is their best-constrained feature) provides a profound insight on the thermo-chemical nature of the continental lithosphere and on the thickness of the continental keels.