Southeast of the Valles Marineris lies the feature which Schiaparelli named the Noachian terra.
The light colors, yellow and red, are higher elevations, the dark colors, green and blue, are lower elevations. As you can see, the surface of mars was once a shooting gallery. A good many of those impacts may have been ejecta from "the Big Hit," which i will get to a little later. The dark feature to the south east is the Hellas Basin (Hellas is the name the Greeks give to their own country).
The green and blue area is the Hellas basin. The dark feature at the bottom is the Martian south pole. It is an impact crater which makes the Chicxulub crater on Earth look like a puddle. It is about 2500 kilometers in diameter (depending on how one defines the basin), which is over 1200 miles. If it were in the middle of the United States, it would reach from Denver to Louisville.
Some geologists (although i believe they are a minority) think that the Tharsis bulge may have been caused by this massive impact on the opposite side of the planet from Tharsis. They believe that the impact turned a good deal of the basement rock to liquid, and that the Tharsis bulge is a giant magma chamber pushed up on the opposite side of the planet.
It is possible that the Hellas basin is not the largest impact crater on Mars. The Daedalia Planum, south of Tharsis, may well be an impact crater about 4500 kilometers in diameter--wider than the continental United States. It has been obscured by later strikes and whatever event lead to the rise of the Tharsis bulge.