[OokieWonderslug (OP)]

I have read that the New Madrid rift is a failed rift. Then I read that it is still spreading at 1mm per year. I then did a quick calculation and found that at 1mm a year it is exactly as wide as it should be for it's age. So if it is failed, why is it still moving? And why is it as wide as it should be if it never stopped moving at that rate? Sure, it might be slow, but apparently it is still moving.

Granted, it is filling as fast as it is getting wider so it won't ever make an ocean.

But how can a continent that is being pushed at both sides be splitting apart both in New Madrid and in Nevada? How can it rift apart while being crushed together by both the Pacific and the Atlantic? Something smells fishy to me.

[Bass]

Most active rifts are speading at rates of >2 cm/year.  From my dim memory, I think the New Madrid rift was active around 750 million years ago, then failed to tear apart the continent.  I believe it is called "failed" because the rift stopped developing and never truly split the continent.

Remember that tectonic forces are dynamic.  You're right that Nevada (or more accurately, the Basin and Range) is extensional.  This may have to do with rotation, especially with the San Andreas and Walker Lane motions- or may be a mantle phenomena associated with the Yellowstone hotspot.  Look up "back arc basins" for other examples of extensional terrains in compressive regimes.

[OokieWonderslug (OP)]

But the New Madrid is still spreading at 1mm a year. It is spreading as fast as it fills with sediment which I read is more than a couple miles deep now. At that rate it will never form an ocean basin. Even if it spreads forever. So you're saying it is failed because it never spread fast enough to get ahead of sedimentation. That sorta makes sense.