I've been struggling to come up with a way to animate this cracked surface over the passed few days, and after dismissing blend shapes as an option I decided to experiment with an animated texture that could be used for the bump map.
Here's a playblast showing the scene as it is at the moment:
The animated bump-map:
And a rendered still:
With a bit of tweaking and improvement, this scene will be finished tomorrow. And just for encouragement, here's my progress sheet:
Hope everyone's having a good holiday so far.