Marshall's score -- his second of three TD runs in the game -- broke a 14-14 tie and ignited a second-half surge in which the Ducks (8-0, 5-0 Pac-12) reeled off 28 unanswered points for their 10th straight win dating back to last season.
Marshall finished with 133 yards on 19 carries and Heisman Trophy candidate Marcus Mariota went 21-of-28 through the air for 230 yards and a touchdown.
The Bruins (5-2, 2-2) dropped a second straight after falling to Stanford last week.
Brett Hundley managed just 64 passing yards with a touchdown on 13-of-19 efficiency for the Bruins.
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