While we have no evidence as to when the Grand Canyon came into existence, chances are that it happened after the Flood. That is also where there were huge movements beneath the surface of the earth.
In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened. (Gen 7:11)
JOHN GILL'S COMMENTARY
...the same day were all the fountains of the great deep broken up, and the windows of heaven were opened; and by both these the flood of waters was brought upon the earth, which drowned it, and all the creatures in it: by the former are meant the vast quantities of subterraneous waters, which are more or greater than we know; and might be greater still at the time of the deluge:
"there are large lakes, (as Seneca observes (z),) which we see not, much of the sea that lies hidden, and many rivers that slide in secret:''so that those vast quantities of water in the bowels of the earth being pressed upwards, by the falling down of the earth, or by some other cause unknown to us, as Bishop Patrick observes, gushed out violently in several parts of the earth, where holes and gaps were made, and where they either found or made a vent, which, with the forty days' rain, might well make such a flood as here described: