You are correct in saying that alkanes are generally unreactive. The do however combust and also react with halogens and undergo a reaction called radical halogenation. That being said, given these reaction conditions, I would guess the reaction mechanism would involve the breaking of C-H bonds and the formation of new H-H bonds. It would also contain the breaking of C-C bonds and the formation of new C-H bonds. But for the reaction to proceed it would require a large amount of energy to break the original bonds. So I would say the reaction would produce H2 (g) and smaller straight chain alkanes such as methane, ethane, propane, and butane. Im not sure whether or not this is the case but its worth a guess. lol