Whats tis bark disease

This is what is happening to my Gleditsia. The split bark has now extended horizontally and vertically. The puzzle is the leaves and tree look healthy.
Do you think I should chop it down in case it spreads.

 

I CANT BELIEVE IT . I HAVE MANAGED TO POST A PHOTO. MANY THANKS DAISY IM OVER THE MOON. YOUR DIRECTIONS ARE IDIOTPROOF

 

It looks to me like whatever the problem was, its sorted itself out.
You can see the new bark forming at the edges of where the dead part is,
Its obviously not looking good but I dont think the tree is dying, although it may never make a good strong trunk.
I'd leave nature to it and just watch what happens.

 

Hi Will the bark grow back then? Do you know what caused it?