The Rio Grande rift's tectonic evolution is fairly complex. The fundamental change in the western margin of the North American plate from one of subduction to a transform boundary occurred during Cenozoic time. The Farallon plate continued to be subducted beneath western North America for at least 100 million years during Late Mesozoic and early Cenozoic time. Compressional and transpressional deformation incurred by the Laramide Orogeny lasted until about 40 Ma in New Mexico. This deformation may have been a result of the coupling between the subducting Farallon plate and the overlying North American plate. Crustal thickening occurred due to Laramide compression. After the Laramide Orogeny and until 20 Ma, a major period of volcanic activity occurred throughout the southwestern United States. Injection of hot magmas weakened the lithosphere and allowed for later extension of the region.