P-to-S conversions from the two primary discontinuities near 410 and 660 km depth (P410s and P660s phases) are evident in particle motions and receiver functions of teleseismic body waves recorded by broadband seismic stations in Iceland. The average arrival times of P410s and P660s are later by 7.0¿0.5 s (standard error) and 4.7¿0.2 s, respectively, than predicted by the iasp91 Earth model. Differential P660s-P410s travel times indicate that the upper mantle transition zone between the 410- and 660-km discontinuities is 23¿9 km thinner than in the iasp91 model. From estimates for the Clapeyron slopes of the phase transitions associated with these discontinuities, this lesser transition zone thickness is consistent with temperatures 180¿70 K hotter than normal. We interpret this result as evidence for upwelling mantle flow at 400--700 km depth beneath Iceland and the adjacent Mid-Atlantic Ridge. ¿ American Geophysical Union 1996