Is it likely that the impact which created the Martian crater Hellas Planitia also caused the Tharsis Bulge?
There is some evidence that your theory could be correct. The largest crater on Mercury, Caloris Basin, is even larger than Hellas, although not as deep. On the opposite side of Mercury from Caloris is the region known as chaotic terrain, which is believed to result from ground shifts occuring as a result of the Caloris impact. Callisto also has an extremely large crater. It mioght be worth checking to see if there is evidence of an effect on the opposite side of Callisto. We would need better information on the ages of the Tharsis ridge and its volcanoes. You should note, though, that Tharsis aligns with Valles Marineris, and could also be a consequence of whatever geological event caused it.
