Roles of Continental Mid-Lithosphere Discontinuity in the Craton Instability Under Variable Tectonic Regimes

The continental mid-lithosphere discontinuity (MLD) is widely detected within cratons, with the dominant depth range of 70-100 km and a significant reduction 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 in craton evolution. The comparisons among variable mechanisms indicate that the strength of the MLD varies from the relatively high viscosity of wet olivine to the rather low viscosity of antigorite. Thus, systematic numerical modeling has been conducted with the MLD of contrasting strengths, that is, the wet olivine-induced MLD or antigorite-induced MLD, to investigate the roles of MLD in the craton instability under variable tectonic regimes (stable, extension, compression, mantle flow traction, or mantle plume). The models show that the cratonic lithosphere with wet olivine-induced MLD maintains its stability under all the tectonic regimes. In contrast, the antigorite-induced MLD with lowest viscosity could significantly promote the decoupling of lithosphere, and facilitate the lithospheric deformation. However, lithospheric delamination only occurs with the rather weak MLD interacting with the sub-plate asthenosphere upwelling during craton extension or mantle plume activity. The sufficient amount of melts is essential for this process, which requires a large amount of extension or a mantle plume with rather high temperature anomaly and large size. Therefore, craton destruction is still difficult, and requires additional strict conditions. This may explain the general stability of most cratons with widespread MLDs. Geophysical observations have revealed a seismic discontinuity within cratons, which is defined as continental mid-lithosphere discontinuity (MLD). The MLD is often considered to be rather weak, which could facilitate the lithospheric delamination, and even the craton destruction. However, most cratons preserve the stability during long-term evolution. Thus, the viscosity of the MLD and its roles in craton evolution require further investigations. In this study, we have conducted systematic numerical modeling to investigate the effects of the MLD on craton instability under variable tectonic regimes. The wet olivine-induced MLD and antigorite-induced MLD are two end-members of MLD with contrasting strengths, which are compared in the present models. The model results indicate that only the connection and interaction between the weak antigorite-induced MLD and the sub-plate asthenosphere could cause craton destruction during tectonic extension or upwelling of mantle plume. The sufficient amount of melts is essential for this process to destroy the lower part of lithosphere. Thus, craton destruction requires additional strict conditions, which may explain the stability of most cratons on the Earth, although with the presence of MLD. The viscosity of mid-lithosphere discontinuity varies from relatively high viscosity of wet olivine to the rather low value of antigorite The wet olivine-induced mid-lithosphere discontinuity could not lead to craton destruction in any tectonic regimes Craton destruction occurs with the presence of antigorite-induced mid-lithosphere discontinuity combing additional strict conditions