WESTWARD SUBDUCTION OF NORTH AMERICA DURING THE SEVIER EVENT

In the Great Basin area, the Sevier fold-thrust belt formed from 125 to ~105 Ma; was the first deformational thickening known on the North American platform terrace since it formed in the Cambrian; and represents the initial collision of North America with the Rubian superterrane. Sevier-age deformation is not known to the west in the Great Basin hinterland. Within Canada, the platform terrace wasn’t deformationally thickened until the Santonian or younger as evidenced by the Borgeau thrust, which as the westernmost thrust to cut the North American platform margin there, has Santonian-Campanian marine shales in its footwall syncline. Rocks immediately to the west on the Windermere High were deformed and thrust prior to 108 Ma. Thus, there is a significant mismatch in time of deformation in both regions.

The problem can be readily resolved because the age of deformation in the Canadian sector matches that of the Sevier thrust belt and so were most likely the rocks that collided with western North America during the Sevier event. The eastwardly migrating 125 to ~100 Ma Ominica batholith of the Canadian sector likely represents arc magmatism in the upper plate prior to slab failure. After thrusting, a 1000 km-long linear array of 100+ small metalliferous plutons were emplaced into Selwyn basin from 99-92 Ma, and likely represent slab break-off magmas. In addition to the matching of deformation and plutonic ages, rocks of the earlier amalgamated Rubian superterrane, now located in the Yukon territory, are unconformably overlain by the 70 Ma Carmacks volcanics, located at the latitude of San Francisco when erupted. Thus, after Laramide terminal collision, rocks of the entire Canadian Cordillera outboard of the platform edge migrated northward into their present position.

Although inexact, geodynamic plate modeling utilizing the non-TPW corrected paleogeographic model of Torsvick et al. (2008) in Gplates, and correcting it for inclination flattening as per Kent and Irving (2010), puts the paleoposition of the Sevier belt at ~125 Ma, directly above the vertical East Coast high-velocity anomaly in the mid-mantle, interpreted by others to represent a relict west-dipping subducted slab. This reconstruction supports a west-dipping subduction model for the Sevier event – the initial deformational phase of the Cordilleran orogeny.