TES OBSERVATIONS OF CHRYSE AND ACIDALIA PLANITIAE: MULTIPLE WORKING
HYPOTHESES FOR DISTRIBUTIONS OF SURFACE COMPOSITIONS.
Lunar and Planetary Science XXXIII (2002)
http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1554.pdf

The report authored by members of the TES team discusses the noted distribution of andesitic rock in the north and basaltic rock in the south. Interestingly they note there is also a transitional region showing a mixture of these two lithologies.
They present two hypotheses to explain the distribution of the two kinds of rock. Firstly they suggest the mixing band is due to sedimentary transport of basaltic rock from the south. Secondly they suggest the prevalence of andesitic rock in the north could have been due to a former standing ocean.
Note as well that the sedimentary transport idea might still support the theory of a former northern ocean:

Outflow Channels May Make a Case for a Bygone Ocean on Mars
Written by Linda M.V. Martel
Hawai'i Institute of Geophysics and Planetology
http://www.psrd.hawaii.edu/June01/MarsChryse.html

Bob Clark