Finite-element models of evolution for the Kapuskasing structural zone

Abstract

Finite-element modeling was used to assess the conditions for the feasibility of a mechanism of evolution for the Kapuskasing structural zone. The proposed mechanism calls for formation of the Kapuskasing structural zone during an intracratonic shortening event which led to overthrusting in the brittle upper crust and formation of a crustal root by ductile deformation in the lower crust. Simple calculations show that, during overthrusting, the temperature in the lower crust is high and the effective viscosity of rocks could be reduced to 1021–1022 Pa∙s for 10−14 s−1 strain rate. The viscosity of the upper mantle below the Moho is higher by 1 to 2 orders of magnitude. The actual shortening could have been completed in 4–20 Ma at a rate on the order of 0.5–2.5 cm∙a−1, which leads to strain rates consistent with the assumed effective viscosity and to a geologically reasonable level of stress [Formula: see text]. The exposure of different crustal levels by erosion was already underway at the end of the shortening event. The calculations show that the formation of a crustal root by ductile flow in the lower crust requires the viscosity to be higher than 1020 Pa∙s; less viscous material will flow and not produce the observed crustal thickening. The preservation of the crustal root is possible provided that the mantle rheology is much stronger than that of the lower crust. The Moho temperature should have been around 800 °C for the formation and preservation of the root to be possible. Such temperature conditions are compatible with the thermal regime of this part of the Canadian Shield between 2.0 and 2.5 Ga, as estimated from present heat-flow data.