It looks to me like a blast area.
Hypothesis: A large moon in the vicinity exploded from massive internal pressures or perhaps fissioning, the shock and debris hit this hemisphere causing the blackened state and also the ridge. This could also be the cause of the rings.
I believe that there were more explosions in the early solar system. Because of the blackened state of large numbers of asteroids and the several different belts, this would fit with observations. There is reason to believe the solar system may have harboured as many as 12 "actual" original planets.