Good afternoon.
Which was first, the New Madrid fault, or the Miss. River?
5.56mm
I can recall something about mid continent rifts both in the Great Lakes area / St. Lawrence River and then along the line of the Mississippi/New Madrid. So I’d say the faults and the river co evolved with some rift induced faulting leading the way. Fault zones are the products of plate movement which can paint complicated pictures when you go back a while.
The New Madrid fault has to precede the New Rockies which go back 65 million years or so. The Mississippi couldn’t precede the last ice age which ended 12,000 years more or less, ago. During the last ice age, as well as previous ice ages, the lower Miss. couldn’t have been more than a creek.