Abstract

The formation of the Rocky Mountains, an event referred to as the Laramide orogeny, has presented a dilemma since the introduction of plate tectonics. In scale and tectonic style, the Late Cretaceous–early Tertiary orogeny resembles collisional orogenies such as the Himalayas. However, no collider remains adjacent to the continental margin at the latitude of Laramide features (i.e., California to Washington). Terranes of western British Columbia and southeast Alaska, known together as “Baja BC,” may constitute the colliding plate. Geologic studies and paleomagnetic data suggest that Baja BC collided with North America at the latitude of modern Baja California (the “hit”) and was subsequently translated northward during oblique convergence (the “run”) to its present latitude between 94 Ma and ca. 40 Ma. Northward translation of Baja BC is spatially and temporally coincident with the ca. 80 to 45 Ma Laramide orogeny, suggesting a causal link between the events.

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