The Cordilleran foreland fold and thrust belt collapsed and spread to the west during a middle Eocene to early Miocene (ca. 49–20 Ma) episode of crustal extension. The sedimentary and structural record of this event is preserved in a network of half grabens that extends from southern Canada to central Utah. Extensional structures superposed on this allochthonous terrain are rooted to the physical stratigraphy, structural relief, and sole faults of preexisting thrust-fold structures. The sole faults dip 3°–6° west above an undeformed Precambrian crystalline basement and accommodated tectonic transport of a thick (up to 20+ km) eastward-tapering hanging wall during regimes of crustal shortening and extension. The chronology of tectonism for the foreland fold and thrust belt is established here by dating latest thrusting and initial normal faulting and is best defined where thrusts and normal faults are linked by common detachment surfaces. Dated movement on two extensionally reactivated thrusts, the Lewis thrust of northwest Montana and southeast British Columbia and the Medicine Butte thrust of southwest Wyoming and northeast Utah, suggests that the hiatus between the end of crustal shortening in the early or early middle Eocene and the start of extension in the early middle Eocene was brief.
Lateral spreading and extensional basin formation in the Cordilleran foreland fold and thrust belt were partly concurrent with formation of metamorphic core complexes and regional magmatism. Conceptually linking extensional processes that were simultaneously deforming both the hinterland and foreland of the late Paleogene Cordilleran orogenic wedge is accomplished by applying the extensional-wedge Coulomb critical-taper model. The rapid drop in North America–Pacific plate convergence rate and/or steepening of the subducted oceanic slab at ca. 50 Ma resulted in a large reduction in east-west horizontal compressive stress in the Cordillera. As a result, the Cordilleran orogenic wedge was left unsupported, and it gravitationally collapsed and horizontally spread west until a new equilibrium was established at ca. 20 Ma. Subsequently, crustal extension and magmatism during the Basin and Range event (ca. 17–0 Ma) overprinted much of this earlier phase of extension.