Chapter 1: Cenozoic geometry and thermal state of the subducting slabs beneath western North America

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Abstract
We have reconstructed the isochron pattern of the Farallon and Vancouver plates in order to predict the thermal state and geometry of subducting slabs beneath western North America during the Cenozoic. Slabs do not last indefinitely; they warm up by conduction when bathed in the asthenosphere. As they warm up, they lose the ability to have earthquakes. Studies of modern subduction zones show that slabs become aseismic after a duration approximately equal to one-tenth their age upon subduction. Combined with a mathematical heat conduction model, these studies give us confidence that the thermal state of a slab can be characterized if we know the time since subduction and the age upon subduction. We reconstruct isochrons on subducted plates using the magnetic anomalies recorded in the Pacific plate, assuming symmetrical spreading and taking into account propagating rifts. Using the improved global plate reconstructions of Stock and Molnar (1988), we position the reconstructed plates with respect to North America to obtain maps of time since subduction and age upon subduction. The result is a series of maps of the slab geometry and approximate thermal condition at six times during the Cenozoic. With these maps we examine postulated relations between the presence and condition of the underlying slab and the occurrence of volcanism and tectonism in the overlying plate. We find that the very long flat slab proposed to have caused the Laramide Orogeny could have easily reached Colorado because of its fast average subduction rate and moderate age upon subduction, and because of the tendency for shallowly dipping slabs to last longer because they heat up more gradually while passing beneath the overriding plate. We find that the eastern edge of the proposed late Cenozoic “slab window” never existed, because of the young age of the slab. Instead, a region of effectively no slab gradually developed as early as 35 Ma, and it was farther inland than the proposed “slab window” Lacking an eastern edge, the “slab window” is better described as a “slab gap.” The southern boundary of the gap is diffuse, and its location is poorly constrained, whereas the northern edge is sharp and has clear, predictable geologic manifestations.

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