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Robin Thicke, Vampire Weekend top Virgin Mobile FreeFest 2013 lineup

Indie-rock prepsters Vampire Weekend, candy-pop duo Icona Pop and R&B provocateur Robin Thicke -- whose dance track "Blurred Lines" became the earworm of summer -- are among the nearly two-dozen acts that will headline the Virgin Mobile FreeFest when the no-cost music blowout returns Sept. 21 to Merriweather Post Pavilion.

The bill, which organizer Virgin Mobile USA announced Monday morning, will also include such acts as the psychedelic pop group MGMT, neo-country crooners the Avett Brothers and compelling funk-soul revivalist Black Joe Lewis.

Tickets to the festival will be available Aug. 9 at noon via Ticketfly; a "pre-sale," available to Virgin Mobile Facebook fans and previous FreeFest ticketholders (among others), will be Aug. 8.

FreeFest is notable among major music festivals not only because it is free to attend, but also because of its reliably eclectic mashup of mainstream rockers, rappers and acts whose stock is on the rise. The previous FreeFest's main-stage performers included Texas rockers ZZ Top, rapper Nas, Americana group the Alabama Shakes and former White Stripes frontman Jack White.

But this year's headliners signal a shift from last year's rock-heavy bill to one more focused on electronic dance music. Several electronic acts will perform on the two main stages, including sample-happy DJ act Pretty Lights, Scottish synth-poppers CHVRCHES and dance chanteuse Sky Ferreira. And the festival will also reprise the "Dance Forest," a separate, electronic-music-focused stage that will feature Madeon, Washed Out, Ghost Beach and a slew of others.

Surprisingly, no rap act made the bill this year. Thicke, whose only East Coast date so far is FreeFest, represents a nod to R&B fans.

About 50,000 people attend the annual concert, which is in its fifth year at the outdoor Columbia venue. Fans can choose between vying for free tickets or purchasing a "freemium" package, which includes a ticket and such novelty items as a T-shirt.

Festival organizers took an unusual tack in revealing this year's acts, incorporating the names into a wheatpaste mural created by the young tenants of the Southeast Washington homeless youth shelter built with donations raised during previous FreeFest concerts. The mural at the Sasha Bruce Re*Generation House was unveiled Monday morning.

The full lineup:

Vampire Weekend
Robin Thicke
The Avett Brothers
Pretty Lights
City and Colour
Icona Pop
The Knocks
Black Joe Lewis
Sky Ferreira
Little Green Cars
Gareth Emery
Washed Out
Manufactured Superstars Congorock
Ghost Beach