Continental lithosphere

Recent contributions of seismology, electrical, geochemical, petrological and geodynamic measurements to the understanding of craton structure and formation.

In this lecture, we look at the structure of the lithosphere in the oldest parts of the continents, the "cratons", which are more than 2 billion years old. These regions have particular characteristics: lower-than-average temperatures down to depths of around 200 kilometers, high seismic velocities, but lower densities as evidenced by their stability over time and the absence of gravity anomalies. This implies a mineralogical composition that is "depleted" in heavy elements, particularly iron. We presented and contrasted the various hypotheses put forward on the formation process of the Archean crust and lithosphere, and in particular the possibility of a major change in tectonic regime at the end of the Archean era.

The lecture ended with a description of our recent work on the stratification of the cratonic lithosphere by anisotropic seismic tomography, which shows the presence of two distinct lithospheric layers, the first (superficial) of which could correspond to the Archean part of depleted composition, and the second (deeper) of which would have formed later by accretion of a cold root within the framework of tectonics similar to those taking place today (oceanic subduction).