LATERAL HETEROGENEITY IN THE UPPER-MANTLE AND SS-S TRAVEL-TIME INTERVALS FOR SS-RAYS REFLECTED FROM THE TIBETAN PLATEAU AND ITS SURROUNDINGS

We report large differences between measured and calculated intervals between arrival times of S waves and SS phases whose bounce points lie beneath different parts of the Tibetan Plateau and its surroundings. When compared with traveltimes calculated from the laterally homogeneo us Preliminary Reference Earth Model (PREM), the shortest intervals be tween SS and S correspond to paths with SS reflected from the Karakoru m and westernmost parts of the Tibetan Plateau and Himalaya. Comparabl e, but more scattered, SS - S residuals characterize paths with SS ref lected beneath southern Tibet. The longest such intervals typify paths with SS reflected beneath north central Tibet. If upper mantle struct ure near the bounce points of SS causes these average apparent advance s of 7 s and delays of up to 6 s, then these data suggest that materia l with high S-wave speeds underlie the Karakorum, and low S-wave speed s underlie north central Tibet, a result that concurs with other seism ological observations. If such high- or low-speed regions were confine d to the upper 250 km of the mantle, they would imply a difference on the order of 13% in S-wave speeds between these two regions. Heterogen eity in the lower mantle, inferred in part from these data, can, howev er, account for part of the large variation in residuals that we obser ve. After correcting for large-scale heterogeneity in the upper and lo wer mantle, roughly 9 s of difference persists, which would correspond to average differences of 9% or 0.4 km/s in S-wave speeds in the uppe r 250 km of the mantle beneath these regions. Only at subduction zones can one find such a difference in average S-wave speeds in the mantle beneath regions only 500-1000 km apart, suggesting that dynamic flow in the mantle presently occurs in the upper mantle beneath the Tibetan Plateau. This difference supports the contention that small-scale con vection occurs beneath the Tibetan Plateau.