Mantle transition zone discontinuities beneath 3 the Baikal rift and adjacent areas

6 [1] Like most other major continental rifts, the Baikal rift zone (BRZ) in Siberia is 7 presumably underlain by a hot and partially molten mantle, which has a reduced seismic 8 velocity relative to surrounding areas. Recent seismic tomography studies, however, gave 9 conflicting results about the depth extent and even the existence of the low-velocity 10 anomaly beneath the BRZ, suggesting that additional constraints are needed. Here we 11 present results from stacking of about 1700 radial P-to-S receiver functions from a 12 single long-running seismic station, TLY, located at the SW tip of Lake Baikal. A clear 13 uplift of the 410 km discontinuity (d410) with a magnitude of about 47 km relative to 14 the south margin of the Siberian platform is observed beneath the rift. Currently available 15 seismic results suggest that the uplift is unlikely to be caused by addition of water to 16 mantle transition zone (MTZ) silicates but is the result of a 550 C reduction in temperature 17 in the vicinity of the d410. In addition, the 660 km discontinuity (d660) shows a 18 downward trend toward the rift from the south, suggesting that the entire MTZ might have 19 a low temperature beneath the rift. The thickening of the MTZ suggests a high-velocity 20 anomaly of about 2% in the MTZ, and rules out the possibility that the rifting is caused by 21 a mantle plume originated in or beneath the mantle transition zone.