Sevier-Laramide overthrusting was generated by both relative North American (NA)-Farallon and absolute NA plate motions. The magmatic arc thermal axis (MATA), adjacent thermally weakened hinterland metamorphic "core" (HMC), and mechanical anisotropies in the upper crust contributed to variations in thrust belt development. Latest Jurassic to earliest Cretaceous west-northwest-directed absolute motion of NA plate collapsed the Cordillera toward the east-southeast, causing southeast to east vergent thrusting along the hinterland and Sevier thrust belt. Relative NA-Farallon and absolute NA plate motions increased dramatically in the mid-Cretaceous between 105 and 85 m.y.B.P. causing the MATA-HMC to migrate eastward. Rapid west-northwest absolute motion of NA collapsed the Co dillera toward the east-southeast, causing major southeast to east vergent thrusting along the Sevier thrust belt. Absolute NA motion slowed after 85 m.y.B.P., but increasing relative NA-Farallon motion forced shallowing of Farallon subduction angle and further eastward migration of the MATA-HMC. This allowed relative plate convergence stresses to be transferred into the Laramide foreland (LF). Latest Cretaceous-Paleocene (72-56 m.y.B.P.) rapid (13 cm/yr, 5 in./yr), east-northeast-directed NA-Farallon plate convergence created northwest to north-trending, southwest to east or west vergent overthrusting and west-northwest-trending sinistral faulting in the LF from southern Arizona to northern Montana. Extremely rapid (15 cm/yr, 6 in./yr), north-northeast-directed NA-Farallon plate converg nce in the Eocene (56-43 m.y.B.P.) generated intense northwest-trending, southwest vergent overthrusting in southern Arizona, north-trending dextral faulting in the southern LF, and east-west-trending north to south vergent overthrusting in the northern LF.

End_of_Article - Last_Page 501------------