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Geological evidence, including the presence of two passive margin platforms, juxtaposed and mismatched deformation between North America and more outboard terranes, as well as the lack of rift deposits, suggest that North America was the lower plate during both the Sevier and Laramide events and that subduction dipped westward beneath the Cordilleran Ribbon Continent (Rubia). Terranes within the composite ribbon continent, now present in the Canadian Cordillera, collided with western North America during the 125–105 Ma Sevier event and were transported northward during the ~80–58 Ma Laramide event, which affected the Cordillera from South America to Alaska. New high-resolution mantle tomography beneath North America reveals a huge slab wall that extends vertically for over 1000 km, marks the site of long-lived subduction, and provides independent verification of the westward-dipping subduction model. Other workers analyzed paleogeographic trajectories and concluded that the initial collision took place in Canada at about 160 Ma – a time and place for which there is no deformational thickening on the North American platform – and later farther west where subduction was not likely westward, but eastward. However, by utilizing a meridionally corrected North American paleogeographic trajectory, coupled with the geologically most reasonable location for the initial deformation, the position of western North America with respect to the relict superslab parsimoniously accounts for the timing and extents of both the Sevier and Laramide events. SOMMAIRELes indications geologiques, en particulier la presence de deux marges de plateforme passives, de deformations adjacentes non-conformes entre l’Amerique du Nord et les terranes exterieurs, ainsi que l’absence de gisements de rift, permet de croire que l’Amerique du Nord etait la plaque sous-jacente durant les evenements de Sevier et de Laramide et que la subduction plongeait vers l’ouest sous le continent rubane de la Cordilleres (Rubia).  Les terranes du continent rubane composite, maintenant au sein de la Cordillere canadienne, sont entres en collision avec l’ouest de l’Amerique du Nord durant l’evenement Sevier (125-105 Ma), et ont ete transportes vers le nord durant l’evenement Laramide (~80–58 Ma), laquelle a affecte la Cordillere, de l’Amerique du Sud a l’Alaska.  Une nouvelle tomographie haute resolution du manteau sous l’Amerique du Nord montre la presence d’un gigantesque mur de plaques vertical qui s’etend sur 1 000 km, marque le site d’une subduction de longue haleine, et offre une validation independante du modele d’une subduction a pendage vers l’ouest.  D’autres chercheurs ont analyse les trajectoires paleogeographiques et conclu que la collision initiale s’est produite au Canada vers 160 Ma – un moment et un endroit sans epaississement par deformation sur la plateforme d’Amerique du Nord – et plus tard plus a l’ouest, la ou la subduction n’etait vraisemblablement pas vers l’ouest, mais vers l’est.  Cela dit, en considerant une trajectoire paleogeographique de l’Amerique du Nord corrigee longitudinalement, avec la position geologique la plus probable de la deformation initiale, la position de la portion ouest de l’Amerique du Nord par rapport aux restes de la super-plaque explique alors facilement la chronologie et l’etendue des episodes Sevier et Laramide.

geoscience canada

Geology, Mantle Tomography, and Inclination Corrected Paleogeographic Trajectories Support Westward Subduction During Cretaceous Orogenesis in the North American Cordillera