My adviser will be out of the town during the semester when I'm planning to defend
1)  agree with others that this is the 21st century.  committee members (chair or not) should be able to skype in to a defense.  this is common practice at all 3 universities i've been associated with.

2)  there should be a graduate advisor type person in your department or college with whom you can consult.  this person is likely someone who helps with recruitment, ensuring students are properly registered for courses and comprehensive/qualifying exams, helping students navigate the "rules" of defending, etc.  talk to this person and explain the situation.  they will likely be able to help you with #1.