In colloquial English, we don't say "Thanks for keep encouraging me". While telling someone to "keep encouraging" another is correct, we don't necessarily thank people for that. Instead, you can say (to the same effect):
Thank you for continuing to encourage me.
This implies that the person you're thanking has been and still is encouraging you.
EDIT: In addition to this, as commentor @Rathony pointed out, the preposition "for" is followed by a verb ending with -ing (when it's followed by a verb, of course). "Keep" is treated similarly.
Keep walking/talking/eating/encouraging, etc.
Neither of these words can be followed by an infinitive verb (such as "to encourage").