Introduction

During the Laramide Orogeny (approx 70-40 Ma.) a significant crustal thickening event occurred in the Rocky Mountain Foreland and the Great Plains. Current elevations of up to 2 km above mean sea level and evidence from Phanerozoic shallow sea fossils help constrain the amount and timing of this increase in elevation. Crustal thickness measurements by various methods agree that the thick crust witnessed in the Colorado plateau gradually decreases over hundreds of km into the Great Plains (Sheehan et. al. 1995, Sandvol et. al. 2001). Five mechanisms of crustal thickening; sedimentation, intrusion, crustal shortening, passive flow in the lower crust and simple shear of the lower crust, are generally used to describe this crustal thickening (Bird, 1984). The majority of mass movement is attributed to movement of lower crustal material. This is then broken up into simple shear and passive flow. Simple shear in this case is attributed to the flat subduction of the Farallon plate below the Rocky Mountains. Passive flow would be caused by extrusion of lower crust from below the up to 70 km thick crust to the west of the Great Plains. These models will be explained and analyzed here. There is significant debate about the exact nature of material flow at depths greater than about 5 km and more debate about the whether or not flat subduction of the Farallon plate even occurred. These uncertainties must be considered when examining such a complex region.