Ashton Kutcher [PHOTO] Partying SOLO on Anniversary

TMZ has obtained photos of Ashton Kutcher partying his face off on his 6th wedding anniversary last weekend -- problem is ... Demi Moore was 3,000 miles away.

Sources tell us, Ashton -- who married Demi on September 24th, 2005 -- was burning the midnight oil at FLUXX nightclub in San Diego early Saturday morning ... while Demi promoted her new Lifetime movie in New York.

But they're Ashton and Demi -- plenty of couples spend their anniversaries apart ... partying ... with strangers ... right?