How to create geological fault using Abaqus 6.9?

[Joyce28]

I am trying to create a model with a number of faults using Abaqus 6.9. I am starting with just one horst and graben fault in an extensional environment. I have tried the XFEM and created a crack successfully but the graben won't slip downward in response to the extensional step. Rather, the crack just grow bigger and bigger in the direction of extension. How do I create faults where blocks actually slip as a respond to the extensional step?

I am new to modeling and programming software. A detailed explanation will help a lot. Thanks everyone!