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. data suggests throughout the sevier-laramide thrusting the crust was also uplifted and extended. the modern chilean subduction is thought to be a parallel model of the sevier and laramide events so there are possibly answers to this question in this modern model. explanations may include a combination of plate motion rates increasing, the underriding oceanic plate becoming younger as the older portion subducts, and thus the underriding plate being hotter and more buoyant.