My final answer: I want to see what's going to happen with Basin and Range in western North America. Will North America remain a single block as Basin and Range spreading subsides? Will a single subcontinent break off and beome a Texas- or Alaska-sized island heading off to the northwest? Or will this region, composed of numerous terranes stitched together over the last 70-80 million years, break up and form a new generation of lithospheric ships, drifting independently on a Mohorovičić sea?
To put the question in context, we need to go back a few tens of millions of years, and watch the Farallon plate get consumed by subduction under the edge of North America. Between about 60 and 80 Ma, numerous small pieces of continental and thickened oceanic crust carried by the Farallon- terranes- were accreted to North America. Much of Nevada and Washington, and most of California and Oregon are formed of these terranes. According to Wikipedia,
"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.




I haven't kept up with the literature. I don't know what directions ideas have gone in the last two decades. I just want to go see.
Here is an outline map of modern seismicity in the central and southern basin and range (from USGS). I have seen what look to me like fairly modern fault scarps on the east side of Steens Mountain in SE Oregon. There are some clear patterns, but I'm not sure what they tell us. Do we have enough of a record to say anything sensible? I doubt it.
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And here's a map and list of major structural features associated with basin and range (hand drawn from memory, so do not cite this):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,



OWL=Olympic Wallowa Lineament, it's not clear what the significance of this feature is, but it may represent a more northerly analogue of the Brothers Fault Zone (BFZ)
YHS=Yellowstone Hotspot- I've always been tempted to link BR spreading to YHS migration, but I have nothing but intuition to back me up.
BFZ= Brothers Fault Zone- The roughly east-west trending northern termination of Basin and Range spreading across central oregon. The fault system allows accomodation between motion in BR and the (more or less) stable Blue Mountains of NE Oregon. More here.
WF= Wasatch Fault- The eastern boundary of basin and range in Utah.
EC-WRL= Eastern California- Walker Rim Lineament- The fault running through Owens Valley, up toward Reno, creating the eastern edge of the Sierra Nevada, and the apparent half-graben bounding the east side of the southern Oregon Cascades. I'm not finding documentation, but I drew this as a continuous feature; it may not be. I'll try to find more info later. But is this the actual western boundary of the North American plate?
SAF= San Andreas Fault- currently considered the western boundary of southern North America... but is it, really?
GF- Garlock Fault- A left-lateral strike slip fault in southern California, that, like the BFZ in Oregon, allows accomodation of basin and range spreading against non-spreading adjacent rocks.
Now the standard geo-future projection with LA near the lattitude of San Francisco, and seaboard southern California has become so iconic, I won't bother to reproduce it. But if, as I suggested above, the "EC-WRL" is actually the tectonic edge of the North America, and if the subducted ridge is still trying to spread, we might see something like this in about 30-40 million years:



OK, enough chitter chatter! Let's Warp!
1 comment:
I'd better go buy a boat! And if you add in the Walker Lane, we might get at least one more island in that last projection (which is my favorite one).
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