It doesn't matter that Mastodon was born of Atlanta's hardcore thrash scene or that for years the band was on the ultimate thrash label, Relapse.

At every turn in doom-rock's scenario, guitarist-singer Bill Kelliher and drummer Bränn Dailor - members of Lethargy who started Mastodon in 1999 - have probably shocked the diehards by growing into smart metal mavens with soft spots and grand arrangements.

They've done this while winning fans with richly diverse major-label records like Blood Mountain.

"We try not to think too much about being too radical," Dailor says about bringing new noise to death-rock's door. "When we first started playing and heard Brent [Hinds, Mastodon guitarist] doing this weird chicken-picking bluegrass stuff in minor keys, you know, to sound evil, we knew it would all wind up sounding different."

Mastodon wanted to be bolder and more innovative still with 2006's Blood Mountain, and so the group expanded on the beautiful melodies of 2004's Leviathan. What it didn't expect was that its progressive roots would show.

"Early Genesis, Zappa, King Crimson, Miles Davis - they suddenly peeked their head out," Dailor says.

Mastodon's dark tales - like those that co-lyricist Dailor wrote for Blood Mountain - indeed have a Tolkien-like pagan death match vibe.

Take "Circle Cysquatch." "Oh yeah, they're a wise group warning you of the dangers ahead in your quest for the crystal skull," Dailor says with a laugh. "I love that stuff."

Mastodon, with Against Me! and Cursive, at 7 tonight at yjr Electric Factory, Seventh and Willow Street. Tickets: $20, $23 Phone: 215-336-2000, 856-338-9000.