Large-scale crustal heterogeneities and lithospheric strength in cratons

Abstract

The rheology and thermal structure of the continental lithosphere are intimately linked. In old cratons, the effective elastic thickness of the lithosphere has been estimated by various spectral (inverse) methods based on the correlation between topography and gravity anomalies. Estimates vary within a very large range from ≈40 km to 120 km depending on the method used. In this paper, we use forward models to account for lateral variations in mechanical properties and their effect on the equivalent elastic thickness (EET) of the lithosphere. From these models, which allow brittle–elastic–ductile rheologies and mechanical discontinuities (faults), we have calculated the strain/stress distributions and displacement fields. Vertical integration of the stress permits a local determination of the effective elastic thickness. The computed displacements were used to calculate related Bouguer and free-air gravity anomalies and compare them with the observations. The analysis is applied to the 2000-Ma Kapuskasing uplift (in the Superior Province of the Canadian Shield) where the presence of a high-density block in the upper crust is due to the upthrusting of midcrustal rocks along a major thrust fault. The study shows that the stability of this structure on geologic time scales requires a strong lower crustal rheology, a cold geotherm, and the fault to be healed. This study also shows that, because of stress dependence of the non-linear rheology, crustal heterogeneities may cause significant (≈40%) local reductions of the lithospheric strength. Away from the Kapuskasing structure, the average strength of the lithosphere remains high (EET ≈ 100 km). Conventional methods for estimating the elastic thickness would not resolve such local strength reductions in cratons, but would predict, depending on the method used, highly overestimated or instead, underestimated EET.