My opinion is that the Valle Marineris is entirely volcanic in nature. It issues from the vicinity of Olympus Mons, and much if it is filled with blown dust that must have accumulated millimeter by millimeter over tens or hundreds of millions of years, or even billions of years, dating from the era of volcanic activity. I gathered this thought from my examination of the setting of the "monolith", shown below. Looking at the first three images of the series of "zooms", you can see that these massive dune valleys are the merest dimple in the filled floor of the first image of the "headlands" of the Valle Marineris.
