In short, ASP.NET Identity is an identity management framework (storing user account data, like usernames and passwords). This article goes into depth on the features and design of ASP.NET Identity: http://brockallen.com/2013/10/20/the-good-the-bad-and-the-ugly-of-asp-net-identity/
Thinktecture.IdentityModel is a security helper library for many missing features in WIF/Claims, ASP.NET, MVC and Web API. An example is security token processing in Web API v1 -- Web API had nothing like this. Another example is CORS support -- again, in Web API v1 there was no help for that.
They serve two different purposes.