the slope--triangle would be inverted---pointing down.
Not enough expansion---'lava' to form mountains!
Matter of 'fact'...the top layers would be the actual layers from the sides of the plate but the lower levels would be fresh layers pushed out from underneath to fill the widening cracks!
The diameter of the earth could be increasing and/or the plates just settling---sinking pushing the material up to form the canyon walls/layering!!
The canyons would be different/same than the plates because as the plates cracked--widening... the slope--triangle would be inverted---pointing down.
Canyons, with the exception of the rift valleys, are exclusively erosional features. If they formed by cracking, they'd all have normal or reverse faults in the bottom and you just don't see that.