Icarus http://dx.doi.org/10.1016/j.icarus.2011.10.023 (2011)

The South Province of Mars is almost devoid of a magnetic signature. Analyses of crustal structure, combined with demagnetization models, suggest that the lack of a magnetic signal here may indicate a weak magnetic field early in Mars's history.

Jafar Arkani-Hamed of the University of Toronto and Daniel Boutin of McGill University, Montreal, tested whether large asteroid impacts on Mars may have re-melted parts of the crust long after the magnetic field had died out, erasing the signature in the South Province. They combined analyses of crustal structure with models of impact-induced demagnetization and found that asteroid impacts in the South Province would have been sufficient to erase any existing magnetic signature around the impact basin. However, they also show that the entire region is devoid of a strong magnetic signal, including areas not affected by impacts.

Rather than undergoing an impact-induced removal of the signature, the entire South Province may have instead formed during a period of weak magnetic field strength on young Mars.