Mike Marshall and Chris Thile, when they come to the Ark on Monday, December 15, will likely deliver the most virtuosically spectacular concert you'll hear this year. If this weren't music for the arcane combination of two mandolins, you'd be reading about it in the paper every week. You might think, hmmm, two mandolins — I can miss that. Big mistake.

Thile ("THEE-lee," with the "th" as in thin) is the mandolinist from the new acoustic band Nickel Creek, the quartet of photogenic and ferociously talented young Californians who, without a shred of permission from the marketing powers that be, landed in the Billboard pop top twenty recently. He's been making recordings since before his teens, and he's now twenty. He's got blazing speed in quiet spaces; he's so fast that the ear can't quite keep up. You're not sure you really heard what you just heard.

In Mike Marshall, a veteran of progressive bluegrass who played for years with David Grisman and has worked at one time or another with a host of musicians in the genre, Thile has found a partner who can channel his prodigal energies. Marshall turns Thile away from the acoustic alternative rock that Nickel Creek sometimes plays and toward the improvisatory spaces of the bluegrass far left field. On their album Into the Cauldron, which Marshall and Thile took their time putting together and honed to perfection, they offer complex duo structures with openings for individual improvisation. Thile bursts out unexpectedly all over, and the shifting point where improvisation meets quite ambitious composition will fascinate a listener.

Their music doesn't demolish genre boundaries. They do a Brazilian piece and Charlie Parker's "Scrapple from the Apple," but there are also several traditional tunes on Into the Cauldron. In general, Marshall and Thile are firmly within the tradition of bluegrass-jazz fusion begun by Grisman and elaborated by a group of marginally well-known but fabulously committed players. Compared with banjoist Bela Fleck, whose trio precedes Marshall and Thile in town by three days, they're less exotic and more classical — in more ways than one, for they play a luminous version of one of J. S. Bach's Goldberg Variations. Their music elaborately fills in possibilities Fleck has bypassed.

So go to hear Mike Marshall and Chris Thile to find out what the post-Bela Fleck generation is up to, if you like. That pair of concerts presents lovers of progressive bluegrass and new acoustic experiment — we're a rare but hardy breed! — with an extremely unusual double feature (or triple feature if you count Special Consensus on December 5). I'll be there to hear a great concert in its own right; reports from other towns describe a pair of master musicians pushing themselves to the limit, exchanging high-fives in enthusiasm, and finishing up in near exhaustion.