[1] We combine results from seismic tomography and plate motion history to investigate slabs of subducted lithosphere in the lower mantle beneath the Americas. Using broadband waveform cross correlation, we measured 37,000 differential P and S traveltimes, 2000 PcP-P and ScS-S times along a wide corridor from Alaska to South America. We invert the data simultaneously to obtain P and S wave velocity models. We interpret slab structures and unravel subduction history by comparing our V S tomographic images with reconstructed plate motion from present-day up to 120 Myr. Convergence of the Pacific with respect to the Americas is computed using either (1) the Pacific and Indo-Atlantic hot spot reference frames or (2) the plate circuit passing through Antarctica. Around 800 km depth, four distinctive fast anomalies can be associated with subduction of the Nazca, Cocos, and Juan de Fuca plates beneath South, Central, and North America, respectively, and of the Pacific plate beneath the Aleutian island arc. The large fast anomalies in the lowermost mantle, which are most pronounced in the S wave models, can be associated with Late Cretaceous subduction of the Farallon plate beneath the Americas. Near 2000 km depth, the images record the post-80 Myr fragmentation of the proto-Farallon plate into the Kula plate in the north and the Farallon plate in the northeast. Near 1000 km depth, we infer separate fast anomalies interpreted as the Kula-Pacific, Juan de Fuca, and Farallon slabs. This interpretation is consistent with the volume and length of slabs estimated from the tomographic images and the plate history reconstruction.Citation: Ren, Y., E. Stutzmann, R. D. van der Hilst, and J. Besse (2007), Understanding seismic heterogeneities in the lower mantle beneath the Americas from seismic tomography and plate tectonic history,

[1] While several mechanisms have been suggested to explain the evolution of the Tibetan Plateau, observational constraints on the deep lithospheric processes have been sparse, and previous seismic studies were mostly along profiles perpendicular to the collision front of the Indian and Eurasian plates. In this study, we show tomographic evidence for the delamination of the mantle lithosphere beneath southeastern Tibet, a process in which the entire mantle lithosphere peels away from the crust along the Moho and thus is a mechanism for rapid thinning of the lithosphere. Our P and S wave velocity models show the presence of a low-velocity anomaly in the crust and upper mantle down to $300 km depth beneath a north-south trending rift zone in southeastern Tibet. This low-velocity anomaly is situated above a tabular, high-dipping-angle, high-velocity anomaly that extends into the upper mantle transition zone. The V P /V S ratio of this high-velocity anomaly suggests that temperature variations are not the only cause of the anomaly and a highly melt-depleted mantle is required. These observations suggest a causal relationship between the delamination of mantle lithosphere and the formation of the north-south trending rift in southeastern Tibet.

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