“”You have to ask what kind of blast might peter out by the time it gets to Mexico and not have much effect on South America,”
Well, for one thing, an explosion’s force falls off as the cube of the distance (not the square) so the difference between Michigan and Michoacan could be considerable.
Tunguska flattened square miles when it hit yet Petrograd was unscathed.
I think it is interesting that not one Clovis Point has been found to have been made after this event.