The role of oceanic plateau subduction in the Laramide orogeny

The cause of the Laramide phase of mountain building remains uncertain. Conceptual models implicate the subduction of either ocean ridges or conjugates of the buoyant Hess or Shatsky oceanic plateaux. Independent verification of these models has remained elusive, because the putative ridges or plateaux are no longer at the Earth's surface. Inverse convection models have identified two prominent seismic anomalies on the recovered Farallon plate. Here we combine inverse convection models with reconstructions of plate motions, to show that these seismic anomalies coincide palaeogeographically with the restored positions of the Shatsky and Hess conjugate plateaux as they subducted beneath North America. Specifically, the distribution of Laramide crustal shortening events tracked the passage of the Shatsky conjugate beneath North America, whereas the effects of the Hess conjugate subduction were restricted to the northern Mexico foreland belt. We propose that continued subduction caused the oceanic crust to undergo the basalt–eclogite phase transformation, during which the Shatsky conjugate lost its extra buoyancy and was effectively removed. Increases in slab density and coupling between the overriding and subducting plates initially dragged the surface downward, followed by regional-scale surface rebound. We conclude that Laramide uplift resulted from the removal, rather than emplacement, of the Shatsky conjugate.