Effects of a Weak Lower Crust on the Flexure of Continental Lithosphere

Previous studies of flexure in continental settings assert that the elastic thickness of a multilayer lithosphere is controlled by the mechanically competent layer thicknesses only, following a cubic rule. More specifically, the cubic rule states T-e = (T-e1(3)+T-e2(3)+ horizontal ellipsis T-en(3))(1/3), where T-e is the total elastic thickness, and T-ei is the elastic thickness of each competent layer. However, it is not necessarily clear that T-e should be insensitive to the properties of intermediate weak layers (e.g., a weak lower crust) which may act to decouple the surface load from lower competent layers. To test this idea, we formulate 2D viscoelastic loading models with layered viscosity to compute the fully dynamic, time-dependent response of a multilayer lithosphere with a weak lower crust. Results show that the flexural response of a multilayer lithosphere to a surface load is initially consistent with the cubic rule. However, this solution is transient because the stress associated with the load cannot be transmitted through the weak lower crust on long timescales. Stress in the mantle lithosphere relaxes with time and eventually does not support the load at all due to the decoupling effect of flow in the weak lower crust. The steady state flexure of a multilayer lithosphere is controlled solely by the mechanically competent upper crust such that T-e = T-e1, and this new rule is the major finding of this study. Our new findings now explain small estimates of T-e in continental settings with thick mantle lithosphere such as the Northern Tien Shan, which previously, were poorly explained by the cubic rule.

Automated summary

This study demonstrates that the initial flexural response of a multilayer lithosphere to a surface load is consistent with the cubic rule, but the decoupling effect of the weak lower crust eventually leads to the load support being solely controlled by the mechanically competent upper crust.