Mesozoic subducted slabs under Siberia

Recent results from seismic tomography demonstrate that subducted oceanic l ithosphere can be observed globally as slabs of relatively high seismic vel ocity in the upper as well as lower mantle(1,2). The Asian mantle is no exc eption, with high-velocity slabs being observed downwards from the west Pac ific subduction zones under the Kurile Islands, Japan and farther south(3-5 ), as well as under Asia's ancient Tethyan margin. Here we present evidence for the presence of slab remnants of Jurassic age that were subducted when the Mongol-Okhotsk and Kular-Nera oceans closed between Siberia, the combi ned Mongolia-North China blocks and the Omolon block(6-8). We identify thes e proposed slab remnants in the lower mantle west of Lake Baikal down to de pths of at least 2,500 km, where they join what has been interpreted as a ' graveyard'(9) of subducted lithosphere at the bottom of the mantle. Our int erpretation implies that slab remnants in the mantle can still be recognize d some 150 million years or more after they have been subducted and that su ch structures may be useful in associating geodynamic to surface-tectonic p rocesses.