Two-stage subduction history under North America inferred from multiple-frequency tomography

Eastward subduction of oceanic tectonic plates has shaped the geologic history of western North America over the past 150 million years(1-4). The mountain-building and volcanism that brought forth the spectacular landscapes of the West are credited to the vast ancient Farallon plate, which interacted mechanically and chemically with the overlying continent as it plunged back into the mantle. Here, we use finite-frequency travel-time and amplitude measurements of teleseismic P-waves in seven frequency bands to obtain a high-resolution tomographic image to similar to 1,800km depth. We discover several large, previously unknown pieces of the plate which show that two distinct stages of whole-mantle subduction are present under North America. The currently active one descends from the Pacific northwest coast to 1,500km depth beneath the Great Plains, whereas its stalled predecessor occupies the transition zone and lower mantle beneath the eastern half of the continent. We argue that the separation between them is linked to the Laramide era 70-50Myr ago, a time of unusual volcanism and mountain-building far inland generally explained by an episode of extremely flat subduction(5).