Rooney Mara did her best Jennifer Lopez impression, stepping out in a plunging dress (it dipped down to her navel!) on Tuesday.

The Girl With the Dragon Tattoo star, who favors minimalist styles and neutral colors, donned the plum-colored J. Mendel design at the 2012 Costume Designers Guild Awards in Beverly Hills.

PHOTOS: Rooney Mara's incredible style transformation

Mara, 26, paired the revealing look with nude peep-toe platforms and deep burgundy lips. Her hair, which she chopped off and dyed jet black to play a punk computer hacker in the flick, was slicked back.

PHOTOS: See what Rooney and more stars wore at the Golden Globes

"People always ask, 'Were you crying when they cut your hair?' And it didn't freak me out at all," she told Allure. "I was really happy to do it."

PHOTOS: Biggest celebrity hair makeovers

And her onscreen character Lisbeth Salander left a mark on the actress' personal style, too. "I used to wear a lot of girlie, frilly things, now I buy things that are more comfortable, and more utilitarian, and sort of boyish."

Get more Us! Follow us on Twitter, Friend us on Facebook, Subscribe to Us Weekly