Josh Oglesby added 10 for the Hawkeyes (15-9, 4-7 Big Ten), who won for just the second time in six games.
Marble, Iowa's second-leading scorer, put together his best scoring night in nearly two months as the Hawkeyes bounced back from a double-overtime loss at Wisconsin on Wednesday. Marble entered play in a staggering spiral, with just 21 baskets on 83 tries in Big Ten play and just two points in his last two games.
Reggie Hearn had 13 to lead Northwestern, which let the Hawkeyes take control with a 10-0 run midway through the second half.
The Wildcats (13-11, 4-7) lost for the third time in four games.
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