Hmmm... This confort my intuition about what the Mars mantle is largely heterogenous.
But if there is an overal overturning of the mantle, we should see surface features about the marks left by it. Furthermore, even if most of the overturning took place in the very early Mars history, some residual (smaller dimention - slower) movements could have taken place more lately, and even recently.
The Tharsis dome would be a consequence of the overturning, and relatively recent faults around it too.
But we should also see subsidence zones: hollows.
And we can find such hollows around hellas Planitia (which stands out in dark blue in
Google Mars). We find two
"umbilics" hollows at the south west of Hellas, and a large flat volcano. There are two volcanoes at east (or at least features which look like volcanoes) and one more spectacular "umbilic" with a circular network of faults at the north of Hellas. (Sorry, Google Mars don't give latitudes and longitudes, which makes of it rather a toy than a real tool). At a pinch, I even wonder if Hellas itself would be really an impact bassin. Theres is another such large bassin south east of Tharsis dome (in green), and it is much shallower. Large bassins tend to be larger and larger compared to depth, and Hellas is simply much too deep, and in more it is oval, and even a bit tear-drop shaped.
It is difficult to tell what exactly are the "umbilics" as they also ressemble some caldeiras, like the one of t Arsia Mons. But there overal shape strongly suggest some subsidence process.