Image by NASA/JPL/MSSS
Following Donald Patten and many others, I think it likely that the incoming asteroid or whatever that created the Hellas crater may have also created the Tharsis Dome and the attendant volcanoes on the opposite side of the planet.
We have no real way of dating these two events so far, so it's as good a guess as any, for now. The seismic shock waves from the impact would travel both around and through the planet, to all collide and release huge amounts of energy for heating, on the opposite side, where the wave forms met and interfered. The exact locale would depend on the angle of impact of Hellas.
The Hellas collision probably also created and is in a sense still creating the Valles Marineris. This great set of cracks is adjacent to the Tharsis Dome, and has formed right about where the doming would have forced maximum adjustment and deformation of the crust.
The likely volume of the impacting body, capable of producing a crater 2,300km across, is of the right order of magnitude to account for the Tharsis Dome, its volcanoes and the Valles Marineris. The crust seems too thick now for plate tectonics to accommodate adjustment, so large additional material inputs would have to cause crustal expansion and cracking.
That scenario explains why there is not nearly enough eroded material anywhere to account for the canyons and with the fact that the system is deepest in the centre.
The missing, supposedly eroded material never existed. The crust is simply being forced apart from below, not being elastic enough to stretch without cracking to cope with the gradual increase in the diameter of the underlying non-crustal material, as the Tharsis Dome subsides.
The topographic map of Tharsis and the neighbouring regions produced by the Mars Orbital Laser Altimeter suggests that what sedimentary material did move, may have flowed from Valles Marineris and the Tharsis Dome first eastwards and then north to the northern hemisphere lowlands.
This does not work if water flow is invoked, as the water has to go uphill several kilometres to exit Valles Marineris, but is no problem if the system is wind-driven.
All the material moved off the Tharsis Dome since its formation would also then be spread by wind over the northern plains, explaining their surface smoothness. They are after all the lowest available topography. Material reaching there would perhaps be trapped in the region by high latitude circum-polar winds, analogous to the Roaring Forties here.
If that view is correct, or partly so, then we also have a huge relatively recent trigger event, releasing enormous amounts of energy, to start every unstable pile of sediment and dry ice on the planet moving.
This seismic energy and also that of other lesser shocks, may have sublimated or partly sublimated the dry ice permafrost to a most unusual depth (after all, it seems to have melted vast amounts of rock, producing spectacular volcanoes), which would destabilise a lot of the unconsolidated material on steep slopes.