The 'bottom' of Valles Marinerus is 1-kilometer 'above' the area surrounding the high-elevation, North American-sized uplift on top of the Tharsis Bulge.
The 'Valles' canyons radiate from the center of the bulge, in perfect accord with the understanding that, this entire feature is an artifact of ancient flowing water erosion.
Tharsis Bulge on Mars