Thermal regime and post-orogenic extension in collision belts

Abstract

In order to assess the conditions leading to post-orogenic extension, the temperature and strength of the lithosphere during continental collision are calculated. The calculations assume that heat is transported by conduction and either that the lithosphere is homogeneously thickened or that only the crust is thickened during the collision. Despite increased crustal thickness and heat production, the temperature does not increase if the rate of shortening is moderate (5 × 10 −16 s −1, leading to 100% increase in crustal thickness in 40 Myears). The temperature distribution is used to determine a rheological profile that is compared with the stress induced by compensated topography. The calculations show that, for homogeneous lithospheric thickening, the strength of the lithosphere increases and is higher than the tensile stress except in the shallow crust. The total strength of stable continental lithosphere at the end of shortening is on the order of 10 13 N m −1; it is larger by one order than the force induced by compensated topography (10 12 N m −1). It appears that homogeneous lithospheric thickening would not lead to post-orogenic extension unless the initial conditions are very special (with the lithosphère hotter than normal). For crustal thickening only, the strength of the lithosphere decreases slightly and is on the same order as the tensile stress. Extension does not necessarily follow from thickening of the crust only but it could take place for a relatively wide range of initial conditions (with initial surface heat-flow 60 mW m −2 or higher). Alternatively, an event such as rapid removal of the mantle lithosphere by delamination or small-scale convection would increase the tensile stress, heat rapidly the lithosphere, reduce its strength, and always trigger extension.