Large-scale crustal heterogeneities and lithospheric strength in cratons

The rheology and thermal structure of the continental lithosphere are intim ately linked. In old cratons, the effective elastic thickness of the lithos phere has been estimated by various spectral (inverse) methods based on the correlation between topography and gravity anomalies. Estimates vary withi n a very large range from approximate to 40 km to 120 km depending on the m ethod used. In this paper, we use forward models to account for lateral var iations 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 hav e calculated the strain/stress distributions and displacement fields. Verti cal integration of the stress permits a local determination of the effectiv e elastic thickness. The computed displacements were used to calculate rela ted Bouguer and free-air gravity anomalies and compare them with the observ ations. The analysis is applied to the 2000-Ma Kapuskasing uplift (in the S uperior Province of the Canadian Shield) where the presence of a high-densi ty block in the upper crust is due to the upthrusting of midcrustal rocks a long a major thrust fault. The study shows that the stability of this struc ture on geologic time scales requires a strong lower crustal rheology, a co ld geotherm, and the fault to be healed. This study also shows that, becaus e of stress dependence of the non-linear rheology, crustal heterogeneities may cause significant (approximate to 40%) local reductions of the lithosph eric strength. Away from the Kapuskasing structure, the average strength of the Lithosphere remains high (EET approximate to 100 km). Conventional met hods for estimating the elastic thickness would not resolve such local stre ngth reductions in cratons, but would predict, depending on the method used , highly overestimated or instead, underestimated EET. (C) 1998 Elsevier Sc ience B.V. All rights reserved.