NUMERICAL MODELING OF LITHOSPHERE SHORTENING - APPLICATION TO THE LARAMIDE OROGENIC PROVINCE, WESTERN USA

A two-dimensional model of lithosphere shortening is presented which q uantifies crustal thickening, temperature perturbations and flexural i sostatic components. The model assumes that the upper brittle layer of the crust deforms by thrusting, while the lower ductile lithosphere a ccommodates 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 am ount and configuration of the compressional deformation in the upper a nd lower lithosphere, the perturbation of the temperature field, erosi on and the flexural rigidity of the lithosphere. The model is applicab le to regions of thick-skinned thrust tectonics and is applied to the Laramide orogenic province in the western USA. Model simulations of st ructural cross-sections across the Laramide province show basement upl ifts and adjacent foreland basins that are comparable in magnitude wit h 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 b oth the effects of the post-shortening re-equilibration of the tempera ture field and thrust-uplift erosion. The amount of crustal thickening and post-shortening cooling of the geotherm predicted by the two-dime nsional model of lithosphere shortening are used in strength calculati ons to determine the relative strength or weakness of the lithosphere. The results suggest that the lithosphere is relatively strong immedia tely following shortening, due to the cooling of the geotherm. Followi ng shortening, however, gradual weakening occurs as the temperature fi eld returns back to its unperturbed state in the presence of an enhanc ed crustal thickness. The results are compatible with the evolution of the Laramide province, which has experienced a change in tectonic reg ime to one of extension over the last 20-30 Ma.