Numerical modelling of lithosphere shortening: Application to the Laramide erogenic province, western USA
A two-dimensional model of lithosphere shortening is presented which quantifies crustal thickening, temperature perturbations and flexural isostatic components. The model assumes that the upper brittle layer of the crust deforms by thrusting, while the lower ductile lithosphere accommodates compressional deformation by a pure shear or "squashing" mechanism. Model results suggest that basement uplift, foreland basin development and underlying crustal structure are controlled by the amount and configuration of the compressional deformation in the upper and lower lithosphere, the perturbation of the temperature field, erosion and the flexural rigidity of the lithosphere. The model is applicable to regions of thick-skinned thrust tectonics and is applied to the Laramide orogenic province in the western USA, Model simulations of structural cross-sections across the Laramide province show basement uplifts and adjacent foreland basins that are comparable in magnitude with those suggested by geological data. The model has also been used to provide insights into the tectonic evolution of the Laramide province and an attempt has been made to determine the flexural rigidity of the lithosphere at the time of Laramide deformation as well as quantify both the effects of the post-shortening re-equilibration of the temperature field and thrust-uplift erosion. The amount of crustal thickening and post-shortening cooling of the geotherm predicted by the two-dimensional model of lithosphere shortening are used in strength calculations to determine the relative strength or weakness of the lithosphere. The results suggest that the lithosphere is relatively strong immediately following shortening, due to the cooling of the geotherm. Following shortening, however, gradual weakening occurs as the temperature field returns back to its unperturbed state in the presence of an enhanced crustal thickness. The results are compatible with the evolution of the Laramide province, which has experienced a change in tectonic regime to one of extension over the last 20-30 Ma.