Mantle upflow under North America and plate dynamics

In the development of plate dynamics there are two fundamental questions. First, do the plates slide over a passive asthenosphere, or are they driven by a convecting upper mantle? Second, are all ocean ridges passive, pull-apart plate margins, or do some lie above upcurrents in the mantle and in that sense become active margins? The combination of stress orientations in the crust of North America with other geophysical and geological information favours an active mantle interpretation, both for the North American plate and for the mantle upflow which lay beneath the East Pacific Rise in Tertiary times and which now lies beneath the western United States.