The first image here is a sketch of an oceanic plate subducting beneath a continental plate at a collisional plate boundary. The oceanic plate typically sinks at a fairly high angle (somewhat exaggerated here). A volcanic arc grows above the subducting plate. The second shows the plate tectonic setting during the growth of the Laramide orogeny. The angle of the subducting plate is significantly flatter, moving the focus of melting and mountain building muchy farther inland than is normally expected. (Source: USGS)
Click on either image to see an animation of the subducting plate shifting from steep to shallow.
Diagram of the subduction regime (Livaccari 1993).
Source: Bird 1984.
After the formation of the Rocky Mountains, the flattened Farallon slab began to detach from the North American lithosphere, and sink back into the mantle. This detachment began at the edges of the slab and propagated until the whole slab began to fall away. Thus the reverse migration of the magmatic arc formation seen in the northern and southern United States began about 60-50 Ma and spread to the central United States by 40-30 Ma, causing the cessation of volcanic activity in the Rocky Mountains, and reactivation of activity in the west. This also changed the stress regime of the region, ultimately leading to extension in the Basin and Range area, and uplift of the Colorado Plateau and the Rockies.
Source: Bird 1988.
Source: Bird 1998. Click to enlarge.
On to Tectonics: Uplift History