The brittle-ductile transition zone (its proper name) is characterized by a certain depth where the temperature is hot enough to change the rocks from a solid state to a more plastic and fluid state. Above the transition zone where the rocks are colder and more rigid, they break more easily when subjected to stress when compared to rocks below the transition zone where rocks are hotter and plastic enough to where the plastic can be bent/folded more easily.
I think it's saying fractures are closed below the transition zone because the rocks are more fluid and the rock is able to bend so that the fractures are closed within the rock, although I'm not 100% sure... that would be my guess.
EDIT: Sorry Unome, I was typing my answer as you submitted your's so I didn't see your answer, and I don't know how to delete my post. I guess there are 2 answers now!