Regarding the Northern basin, another possibility could be that we are seeing the results of glaciation from an extended Northern polar cap.
http://www.agu.org/pubs/pip/2004JE002367.pdf
The above paper on the long term evolution of transient liquid water on Mars, which has been brought up before on this forum, touches on the fact that the current argument of pergigee and eccentric orbit means that aphelion coincides with the Southern winter. However if the argument of perihelion is precessed by 180 degrees, so that the Northern winter coincides with the aphelion, a much more extensive Northern cap would be in evidence.
This precession could have occurred as recently as 25,000 years ago, and as the paper states, would have occurred many tens of times over the last million years.
