The Rockies are the product of two lithospheres passing each other one travelling west the other travelling east. (Top moving west the bottom lithosphere moving east.) the angle of subduction became shallow, and as a consequence, no magmatism occurred in the central west of the continent, and the underlying oceanic lithosphere actually caused drag on the root of the overlying continental lithosphere. One cause for shallow subduction may have been an increased rate of plate convergence. Another proposed cause was subduction of thickened oceanic crust.