"The Farallon Plate is also responsible for transporting old island arcs and various fragments of continental crustal material rifted off from other distant plates and accreting them to the North American Plate. These fragments from elsewhere are called terranes (sometimes, "exotic" terranes). Much of western North America is composed of these accreted terranes."At 30 Ma, the spreading ridge (a remnant of which is now known as the Juan de Fuca Ridge) on the western margin of the Farallon had moved under the edge of North America, leading to the birth of the San Andreas Fault.
(This and the next illustrations from here) So one question I want to answer is "Did the subducted spreading ridge continue generating magma and spreading under the continent, or did it become inactive? Was it effectively destroyed, or is it still down there?" The current configuration is below:
There's very little doubt that much of the Farallon is still down there, the ghost of a deceased seafloor haunting the middle mantle. Tough Cookie at Magma Cum Laude has a good write-up that covers much more on this plate. Below is a representation of the current disposition of the plate from Wikipedia.
And below is another from The Art of Science- nice descripition of the photo here.
There are two competing theories (that I know of) to explain Basin and range spreading. First, the spreading ridge is still down there. In that case, the western US is in the process of giving birth to a new ocean. This is the story line so delightfully developed in John McPhee's "Basin and Range." The second theory is the subduction of the spreading ridge, which I can only imagine would be like me trying to swallow a watermelon without chewing, buttressed and compressed western North America, and caused an over thickening of the crust. In this narrative, basin and range spreading is simply a relaxation, a temporary expansion, following the removal of a previously compressive regime. This was the story line preferred by my profs 20+ years ago.
Here's a Google Earth Image of the basin and range as it looks today:
And here's a map and list of major structural features associated with basin and range (hand drawn from memory, so do not cite this):
(note major features in bold yellow; approximate boundary of BR not associated with any features I know of in light yellow) From roughly north to south,
And one final possibility that was the preferred hypothesis of a geologist from southern California whose lecture I attended a few years ago... I really like this one myself, but I don't have the background to judge how plausible it really is.
From Terranes was my home assembled. To terranes will my home return? Note that I have used major structures as the edges, but internal disruption is entirely arbitrary.