Sevier Orogeny!: The Cordilleran Thrust System
The Sevier Orogeny is characterized by thin-skinned deformation where only the upper crust consisting of Proterozoic to Mesozoic sequences and formations accomodates horizontal compression from a convergent plate boundary zone. The Farrallon Plate started subducting under the North American craton west of the Sevier front when Pangea broke apart in the early Mesozoic. Sevier timing is constrained from paleontalogical data, sedimentation, magmatic events, and older strata overlapping younger strata. Generally, the Sevier thrust system began in the Late Jurassic and ended in the Eocene and can be traced from Mexico, through the western U.S., to Canada. Rates of shortening had increased from 1.4 -1.7 mm/yr in the early to mid Cretaceous to 3 mm/yr in the late Cretaceous to early Tertiary (Allmendinger, 1992) for an overall 50% shortening across the region (Erselev, 1993). The Sevier Orogeny is characterized by eastward progression of a wedge of stacked strata.

Critical Taper Wedge Theory

Thin-skinned Sevier deformation is best described so far by the Critical Taper Wedge Theory where the fronts of the Sevier thrusting forms a wedge geometry with a taper at the underdeformed end. The deformation and advancement of the wedge is controlled by the angles of the top and bottom slopes of the taper (Decelles and Mitra, 1995). The angles then depend on the basal shear strength, the internal strength, and upper surface weathering and erosion rates (Decelles and Mitra, 1995).