Following a lengthy hiatus and some apparent soul-searching from bandleader Robin Pecknold, Fleet Foxes aim for dramatic reinvention on their cerebral third LP, Crack-Up.

When they debuted in 2008, they were widely designated as torchbearers of the burgeoning indie folk movement, but there was always an academic element to the Seattle band's work that vaulted them into a class of their own.

Their exultant vocal harmonies rose like a misty hybrid of the Beach Boys and Steeleye Span and their complex chamber pop arrangements recalled the autumnal splendor of the Zombies paired with the melodic complexity of early Yes.

On the band's long-awaited third effort, it's the latter of those two references that jumps to the fore as they deliver what is easily their most progressive album to date.

Named for an essay by F.

Scott Fitzgerald and bearing references to Spanish painter Francisco Goya, the American Civil War, sociopolitical anxiety, and inner-band strife, Crack-Up is dense and difficult, but ultimately rewarding.

At the album's vanguard is "I Am All That I Need/Arroyo Seco/Thumbprint Scar," an ambitious three-part suite in which the familiar strains of Fleet Foxes' trademark wall of harmonies become suddenly hijacked by crudely mumbled interludes and various forms of rhythmic and tonal dissonance.

It's a method employed throughout Crack-Up's 11 tracks, which seem to zig and zag through zones of chaos, fellowship, and transcendence as Pecknold the scholar unveils his strange architecture in layers of detail and nuance.

That the nearly nine-minute centerpiece, "Third of May/Ōdaigahara," was chosen as the album's lead single says something about the availability of easily digestible material on Crack-Up, and yet its aspirations are the glue that holds it all together.

Orchestral, experimental, and more challenging than either of the band's previous releases, it's a natural fit for the Nonesuch label, whose heritage was built on such attributes.

For Fleet Foxes, it represents a shift away from their more idyllic early days into a period of artistic growth and sophistication.