Shallow-marine rocks exposed on the 2-km-high, 45-km-thick Colorado Plateau in the western United States indicate that it was near sea level during much of the Phanerozoic. Isostatic calculations, however, illuminate the difficulty in maintaining a 45-km-thick crust at or near sea level. We propose that an isostatically balanced, 30-km-thick, proto–Colorado Plateau crust was thickened during the Late Cretaceous to early Tertiary by intracrustal flow out of an overthickened Sevier orogenic hinterland. This plateau would have been supported by a thick (>70 km) crustal root, which is proposed to have been the source region for hot and weak mid-crustal material that flowed eastward from the plateau toward the low-elevation proto–Colorado Plateau.

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