I married a person who was not a citizen. She has since obtained her green card. Now she has left me. I suspect it was fraudulent and that she married me only for her green card. What legal recourse do I have?

Just having a Green Card (Lawful Permanent Resident) doesn't mean that they can't be deported. If you were married less than two years, it calls into question the validity of the Green card. If someone with a Green Card commits an aggravated felony or Domestic Violence, they can be stripped of their Green Card and be deported as well.

Contact ICE through the Dept. Homeland Security website and notify them. They will look into it and make a determination.

First I have to say 1/2 of all marriages in the US fail, so just your suspecting is not really proof, you can report the fact that you are now divorced ( if you have divorced her) and see if that will effect her status.