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After the discovery of thrust-and-nappe structure near the turn of the twentieth century, mountain belts were viewed as a direct expression of horizontal shortening of the continental crust, and continental rifting was viewed as a phenomenon distinct from it. By mid-century, broad consensus had emerged, mainly on the basis of physical reasoning, that thrust-and-nappe structure instead reflected gravity sliding secondary to vertical motions of the crust, as embodied in the influential stockwerk folding hypothesis. In a noteworthy period from 1977 to 1982, informally referred to here as the ‘detachment era’, not only did the last vestiges of support for the stockwerk hypothesis evaporate, but large-magnitude extension was discovered throughout the Cordillera, manifest primarily by extensional detachments and metamorphic core complexes. Soon afterward extensional detachments were recognized as a global phenomenon, forcing first-order reinterpretation of field relations in most orogens. Although plate tectonics is indisputably the most profound discovery in Earth sciences in the twentieth century, the detachment era arguably had commensurate impact on field-based interpretations of continental tectonics. Three decades later, controversy persists over the origin of metamorphic tectonites in core complexes, and over the existence and mechanics of slip on shallowly dipping extensional detachments.

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