Jason London -- My Marriage is Dunzo

Just like his sharting incident in the back of a cop car last year ... Jason London's marriage is in the dumps.

TMZ has learned London's wife Sofia Karstens filed for legal separation last week.

It's unclear why Sofia decided to end things ... but we're guessing his arrest last year factored in -- the one where he was arrested for disorderly conduct/fighting and assault.

Jason and Sofia released a joint statement saying they love and respect each other ... but feel the breakup is necessary to "focus on some growth independently."