DISPLACEMENT TRANSFER BETWEEN THICK- AND THIN-SKINNED DECOLLEMENT SYSTEMS IN THE CENTRAL NORTH AMERICAN CORDILLERA

Late Cretaceous-early Tertiary contractional deformation along the Cordilleran margin of North America is represented by two distinct styles of foreland deformation, thin-skinned and thick-skinned, that primarily differ in depth to their respective basal décollements. Given the coeval nature of contraction in regions experiencing different styles of deformation, displacement on deep level detachments associated with thick-skinned basement-cored uplifts in the southern Rocky Mountains were kinematically linked with displacement along shallow level detachments in the southern Canadian Rockies. In the central North American Cordillera, the north to south transition from thin- to thick-skinned foreland deformation is accommodated by a steep southwest-facing crustal ramp that linked the shallow décollement system of the southern Canadian Rockies and a deep décollement beneath the basement uplifts of the Laramide foreland. The structural ramp was oblique to east-northeast regional shortening and transferred deep crustal displacement of Laramide foreland structures to the Shuswap duplex in the hinterland of the southern Canadian Rockies. The oblique ramp extended from the foreland transition in south-central Montana, northwest to northern Idaho and has a surface manifestation in the belt of basement-cored uplifts in southwestern Montana and exposure of plutonic and high-grade metamorphic rocks of the Idaho batholith in north-central Idaho. Late Cretaceous to Paleocene displacement on the oblique ramp produced uplift and exhumation of these high-grade metamorphic and plutonic rocks of the Idaho batholith between the northwest-striking Lewis and Clark line and Orofino shear zone. The location of the oblique ramp system apparently was nucleated by pre-existing crustal boundaries and the ramp evolved as an expression of displacement transfer required to preserve volumetric and displacement balance within a transpressional orogen. While accommodating transfer between differing structural levels, displacement along the ramp resulted in truncation of the transpressional western Idaho shear zone, and may have localized the site of Eocene extension in this portion of the Cordillera.