TERRANE TRANSLATION AND COLLISIONAL OROGENY IN THE NORTH AMERICA CORDILLERA

Evaluation of models for far-travelled terranes requires understanding the paleogeography of North America to be used as a comparison for data from Cordilleran terranes. Data for Late Jurassic to Eocene time will be presented, and questions- for example conflicting view of Late Jurassic paleopoles- will be highlighted. Paleomagnetic results from Cretaceous rocks of stable North America (Housen, 2021; Tikoff et al., 2023) indicate that the data is robust and define two age-grouped paleopoles. The NA pole from 145 to 85 Ma is 71.8 N, 192.7 E, A₉₅=2.4, N=27, placing NA at high latitude. The 84-65 Ma pole for North America is 82.6 N, 184.1 E, A₉₅ = 3.5, N=3, which indicates a southward shift of NA after 80 Ma. The N-S orientation and well-defined set of paleopoles provide an ideal reference for definition of displacements of terranes along the Cordilleran margin.

The paleomagnetic data from the Intermontane and Insular Superterranes indicate that the Intermontane Superterrane had post-100 Ma displacements of ~700 to 1400 km; and the Insular Superterrane had post-100 Ma displacements that are larger (2000-3000 km). This forms the basis for an updated collisional model for Late Cretaceous-Eocene orogenic events. The collisional phase (100-85 Ma) resulted in a nearly simultaneous collision from central Mexico to central Idaho, and the coastal magmatic arcs in this segment experience a rapid and intense episode of dextral tranpression/contractional deformation. This event also resulted in major contraction in the Sevier fold-thrust belt, foreland block uplifts in the northern Rocky Mountains, and significant foreland sedimentation in adjacent North America. A 85-55 Ma “run” phase resulted in continued contraction throughout North America in addition to dextral strike-slip faulting of coastal blocks, and is supported by paleomagnetic data from Paleocene rocks of the Chugach terrane. The contractional deformation in Wyoming and Montana is hypothesized to occur because of the clockwise rotation of a large lithospheric block (Blue Mountains terranes and adjacent Laurentia). This block rotation was accommodated by sinistral motion along the Lewis and Clark deformation zone, and resulted in significant shortening in SW Montana.