Dear Terri,

Thank you for your question and accompanying pictures. There are a number of potential causes for bark cracking/splitting, and trees that are thin-barked are more susceptible. The following article explains this problem.  Preparations to Prevent Southwest Injury | Purdue Extension Forestry & Natural Resources

Because I am not familiar with the Ivory Silk Lilac tree, I searched and found numerous articles such as this one but could not find any information as to if the Ivory Silk is a thin-barked tree. The bark splitting could also be due to the age and size or your tree. Syringa reticulata subsp. reticulata 'Ivory Silk' (Japanese Tree Lilac, Lilac tree) | North Carolina Extension Gardener Plant Toolbox (ncsu.edu).  

It might be good to have the tree assessed by a certified arborist. He/she could give you suggestions as to how best to care for the future health of your tree. How to Hire an Arborist | Ohioline (osu.edu)

I hope you'll find this information helpful.