Numerous tectonic processes are responsible for the modification and evolution of continental lithosphere. The continents, however, are generally resilient through geologic time and keep a record of Earth’s tectonic activity, both past and present. The focus of this work is to better understand the modification and evolution of continental lithosphere associated with continent-continent collisions. We do this by studying two orogenic systems: the Alpine Orogeny, associated with the ongoing collision between the African and Eurasian plates, and the Trans-Hudson Orogeny, associated with the initial formation of the North American craton during the Precambrian. This research focuses on the westernmost edge of the Alpine system in the western Mediterranean, where subduction and slab rollback have caused significant extension and Africa-Iberia convergence has caused simultaneous contraction. Here we calculate Pds receiver functions to constrain the discontinuity structure. Additionally, we jointly invert Pds receiver functions and Rayleigh wave phase velocity dispersion data to create a 3-D shear velocity model. These results show a deep Moho around the western portion of the Gibraltar Arc. Below this deep Moho we see the Alboran Slab extending down to ~250 km. In the eastern Gibraltar Arc, there is a very shallow Moho where the slab has detached from the surface and removed continental lithosphere. In the Trans-Hudson Orogen we use receiver functions and gravity data to determine the discontinuity and density structure of the shallow lithosphere. This analysis reveals crustal-scale thrusting associated with the Wyoming-Superior suture zone. We also find a relatively low Moho density contrast throughout the Trans-Hudson and northern Yavapai Province. This low Moho density contrast is associated with a deep Moho (>50 km) and is interpreted to be evidence of a dense lower crustal layer resulting from mafic underplating. Finally, we investigate the contribution that this dense thick crust may have played in the isostatic stabilization of the North American craton as well as other cratons around the world. We find that the lithospheric mantle must provide a negative component to cratonic lithospheric buoyancy in order to account for the low elevations observed along with thick crust in the cratons.