Christopher McCandless

Since most of the belt is made up of chronological geographical representations of his trip, and this is between Las Vegas, Nevada and Astoria, Oregon, my guess would be, The Continental Divide. There is a nice trail (nickname is the King of trails) that stretches from Mexico into Canada along the Continental Divide it's called Continental Divide Trail and is often abbreviated as CDT. He might have hiked some part of it.

Or perhaps "The Continental Divide"?

I know that this thread is almost a decade old, but for any future person who had the same question I did:

I believe that TCD stands for The Camera Died, as this is the only major event that we know happened between Law Vegas and Astoria. It could stand for something else, but I think that this makes sense.

I found this answer on some Yahoo answers page, and I thought I'd bring it here as I'd hate to see this question go unanswered.
(I made an account just for this post lol)