A Cold Mantle Transition Zone Beneath the Baikal Rift

Like most other major continental rifts, the Baikal rift zone (BRZ) in Siberia is presumably underlain by a hot and partially molten mantle, which has a reduced seismic velocity relative to surrounding areas. Several recent seismic tomography studies, however, gave conflicting results about the depth extent and even the existence of the low-velocity anomaly beneath the BRZ, suggesting that additional constraints are needed. Here we present results from stacking of more than 1700 P-to-S receiver functions beneath the rift. A clear uplift of the 410 km discontinuity (d410) with a magnitude of about 40 km relative to the south margin of the Siberian platform is observed beneath the rift. This anomaly corresponds to a 500 degree C reduction in temperature in the vicinity of the d410. In addition, the 660 km discontinuity (d660), although not fully imaged due to uneven distribution of global seismicity, shows a downward trend toward the rift, suggesting that the entire mantle transition zone might have a low temperature. The thickening of the mantle transition zone rules out the possibility that the rifting is caused by a mantle plume originated near or beneath d410. We attribute the cold mantle transition zone to the excessive lose of heat due to the opening of the rift.