How Cratonic Roots Survive With Weak Mid-Lithosphere Discontinuities

Abstract

The stability of cratons has long been attributed to their neutral buoyancy and strong roots. However, recent seismic studies have revealed widespread mid-lithospheric discontinuities (MLDs) within cratonic roots. Previous studies, based on geophysical and xenolith evidence, mainly suggest that MLDs are formed through metamorphic processes and have low strength. This raises the question of how most cratons remain stable with weak MLDs. Using numerical models and theoretical analysis, we show that the spatial distribution of MLDs can influence cratonic behavior during plate motion. Mantle flow can shear the cratonic root along a weak, laterally continuous MLD, leading to cratonic destruction. However, when the MLDs are laterally intermittent and contain several 10's–1,000's km-wide gaps, especially near the side boundaries, they can generate a viscous coupling that can counterbalance the basal traction force from mantle flow and contribute to stabilizing the cratons.