HOT MANTLE TRANSITION ZONE BENEATH ICELAND AND THE ADJACENT MID-ATLANTIC RIDGE INFERRED FROM P-TO-S CONVERSIONS AT THE 410 AND 660-KM DISCONTINUITIES

P-to-S conversions from the two primary discontinuities near 410 and 6 60 km depth (P410s and P660s phases) are evident in particle motions a nd receiver functions of teleseismic body waves recorded by broadband seismic stations in Iceland. The average arrival times of P410s and P6 60s are later by 7.0 +/- 0.5 s (standard error) and 4.7 +/- 0.2 s, res pectively, than predicted by the iasp91 Earth model. Differential P660 s - P410s travel times indicate that the upper mantle transition zone between the 410- and 660-km discontinuities is 23 +/- 9 km thinner tha n in the iasp91 model. From estimates for the Clapeyron slopes of the phase transitions associated with these discontinuities, this lesser t ransition 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.