Formation mechanism of continental mid-lithosphere discontinuity and its effects on craton instability under variable tectonic regimes

Abstract

The continental mid-lithospheric discontinuity (MLD) has been widely detected within most cratons, with the dominant depth of 70-100 km and a significant drop of shear-wave velocity of 2-12%. However, the formation mechanism and corresponding strength of the MLD are widely debated, which may strongly affect the roles of MLD on craton evolution. In this study, we have conducted systematic numerical models with hydrated blocks generation routine to simulate the formation of MLD. Model results indicate that the MLD may be induced by the accumulation of hydrous minerals within cratonic lithosphere, and acts as a water collector during craton evolution. Further on, we focus on the roles of MLD in craton evolution. Based on the comparison among variable mechanisms, the viscosity of MLD may vary from the relatively high viscosity induced by wet olivine to the rather low viscosity induced by antigorite. Thus, systematic numerical modeling has been conducted with the MLD of contrasting strengths, i.e. the wet olivine-induced MLD or antigorite-induced MLD, to investigate the effects of MLD on the craton instability under variable tectonic regimes (stable, extension, compression, mantle flow traction, or mantle plume). Model results indicate that the wet olivine-induced MLD could not lead to lithospheric delamination under all the tested tectonic regime. In contrast, the weak antigorite-induced MLD could localize large strain and decouple the overlying and underlying lithosphere significantly; despite this, the lithospheric delamination requires additional conditions. Craton destruction only occurs with the connection of the weak antigorite-induced MLD and the sub-plate asthenosphere during craton extension or mantle plume activity. The partial melting process during large amount of extension or upwelling of mantle plume with high temperature anomaly and large size is a key condition. In addition, the depleted cratonic lithospheric mantle with low density would increase the intrinsic buoyancy of lithosphere, and inhibit the lithospheric delamination and craton destruction. Therefore, the effect of MLD on the craton destruction is not as significant as previously considered in the models, which requires additional strict conditions that are not widely satisfied on the Earth. This may explain the general stability of most cratons with widespread MLDs.