 Thinning of the Cordilleran has previously been thought to be evidence and reason for flat subduction in the Sevier and Laramide orogenic events.However, isotopic data suggests that preservation of Cordilleran lithosphere implies Cordilleran thinning is not a sufficient answer for Sevier and Laramide flat subduction.This implies thinning and shearing of the Cordilleran was confined to the fore-arc region.