The effects of mantle composition on Cenozoic magmatism and tectonism in the western U.S.

Starting with the Laramide orogeny and continuing through the Cenozoic, the U.S. Cordillerian origin is unusual for its width, nature of uplift, and style of tectonic and magmatic activity. We present teleseismic tomography evidence for North America thickness of about 200 km below Colorado and about 150 km below New Mexico. Existing explanations for uplift and magmatism cannot accommodate lithosphere this thick. Imaged mantle structure is low in seismic velocity roughly below the Rocky Mountains and high to the east and west, beneath the tectonically intact Great Plains and Colorado Plateau. Structure internal to the low-velocity volume has a NE grain suggestive of influence by inherited Precambrian sutures. We conclude that the high-velocity upper mantle is Precambrian lithosphere, and the low-velocity mantle is partially molten Precambrian North America mantle. This view of North America is supported by the chemistry of Tertiary basalts. We suggest, as others have, that the Farallon slab made contact with the lithosphere beneath most of western U.S. during the Laramide orogeny. We further suggest that slab de-watering under the increasingly cool conditions of slab contact with North America hydrated the base of the continental lithosphere, causing a steady regional uplift of western U.S. during the Laramide. Imaged low-velocity upper mantle is attributed to hydration-induced melting of lithosphere beneath much of the southern Rocky Mountains. Magmatic ascent led to lithospheric heating and weakening, which in turn allowed horizontal shortening in the mantle to occur beneath the region of Laramide thrusting in the southern Rocky Mountains. Subsequent Farallon slab removal and asthenospheric contact triggered magmatism where asthenosphere made contact with the hydrated and relatively thin and fertile lithosphere of what now is the Basin and Range.