Numerous models have been presented to explain the late Cretaceous/early Cenozoic Laramide orogeny, which affected the foreland region of the western cordillera within the US1. The most attractive models invoke low-angle subduction2-4, which can develop for various reasons5,6. Evidence from South America indicates that one such reason may be the subduction of buoyant ocean floor. We here extend the low-angle subduction models by attributing the shallowing of the subduction angle during the Laramide orogeny to subduction of a large oceanic plateau of anomalously thick and buoyant oceanic crust. Pacific plate history indicates that a twin to the Hess Rise may represent the oceanic plateau that triggered Laramide events.