Strength of the lithosphere and strain localisation in the Baikal rift

Abstract

We use previously published estimates of Moho and lithosphere–asthenosphere boundary (LAB) depths combined with a 3D thermal model to infer variations of the effective elastic thickness (EET) of the lithosphere in the Baikal rift zone. The predicted continuous EET map is validated using EET values from previous forward flexural models across separate profiles. This map presents a present-day snapshot of the lithosphere mechanical behaviour. It suggests that the cratonic keel bounding the NW part of the rift is characterized by a strong lithosphere where the crust and mantle are mechanically coupled, whereas the more recent (Palaeo-Mesozoic) fold zone located SE of the rift is significantly weaker. Predicted spatial EET variations correlate well with the spatial distribution of earthquakes: where EET abruptly drops from high (~ 50 km) to low (~ 25 km) values, epicentre distribution also changes from very localised to diffuse, suggesting that the degree of crust–mantle coupling responsible for EET drops also controls strain localisation. Finally, we suggest that even though the current thermal state of the lithosphere does not favour partial melting near the LAB depth, the latter could have been promoted in case of moderately hotter lithosphere (+ 120 °C compared to the temperature at the current base). This could be the case during the earlier volcanic period, which spanned from the Oligocene to the Pleistocene.