[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.

[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,

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The extraction of Green's functions by cross correlation of continuous seismic records at station pairs is best achieved in a diffuse wave field, where energies radiated by random sources have equal power. To partially satisfy the diffuse wave-field condition, original seismic records must be normalized as they are highly nonstationary and may have amplitude variations of orders of magnitude within the cross-correlation time window. We adapt a frequency-time normalization method to obtain seismograms with an even spectrum at all times within the data-processing unit. Compared with the commonly used one-bit normalization, the new method improves the signal-to-noise ratio of empirical Green's functions by a factor of ∼2 and increases the effective data-recording duration by a factor of 4, assuming random local and instrument noise. It yields useful Rayleigh waves at periods up to 300 s for Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL)-type deployments and 600 s for permanent stations with very-broadband sensors. Thus, the new method makes it possible to extend surface-wave signals to frequencies beyond those in traditional earthquake-based surface-wave tomography at both the high-and lowfrequency ends.

During the breakup of continents in magmatic settings, the extension of the rift valley is commonly assumed to initially occur by border faulting and progressively migrate in space and time toward the spreading axis. Magmatic processes near the rift flanks are commonly ignored. We present phase velocity maps of the crust and uppermost mantle of the conjugate margins of the southern Red Sea (Afar and Yemen) using ambient noise tomography to constrain crustal modification during breakup. Our images show that the low seismic velocities characterize not only the upper crust beneath the axial volcanic systems but also both upper and lower crust beneath the rift flanks where ongoing volcanism and hydrothermal activity occur at the surface. Magmatic modification of the crust beneath rift flanks likely occurs for a protracted period of time during the breakup process and may persist through to early seafloor spreading.

[1] We analyze a new set of seismic data from seismograph stations in California. This data set consists of nearly 5000 S receiver functions for 47 seismograph stations. As a rule, the stacked SRFs display a distinct S410p seismic phase (S wave converted to P at the 410 km discontinuity). The wave paths of S410p sample the upper mantle beneath California and the neighboring region of the Pacific. In northernmost California the S410p travel times are close to those of the IASP91 global model. Further south, S410p usually arrives about 2 s earlier than predicted by the IASP91 model. This early arrival can be explained either by an anomalously high Vp/Vs velocity ratio (1.9 in a 125 km thick layer of the upper mantle versus 1.8 in IASP91), by a depression of the 410 km discontinuity of 15 km, or by a combination of both effects with smaller amplitudes. We observe systematically S350p phase which is converted from a negative discontinuity (with a lower S velocity at the lower side) near a depth of 350 km. The observations of S350p are indicative of a low S velocity layer a few tens of kilometers thick atop the 410 km discontinuity beneath southern California and the neighboring oceanic region. Some receiver functions also display S480p phase, which is interpreted as evidence of an intermittent low-velocity layer in the transition zone.

[1] Using traveltimes of teleseismic body waves recorded by several temporary local seismic arrays, we carried out finite-frequency tomographic inversions to image the threedimensional velocity structure beneath southern Tibet to examine the roles of the upper mantle in the formation of the Tibetan Plateau. The results reveal a region of relatively high P and S wave velocity anomalies extending from the uppermost mantle to at least 200 km depth beneath the Higher Himalaya. We interpret this high-velocity anomaly as the underthrusting Indian mantle lithosphere. There is a strong low P and S wave velocity anomaly that extends from the lower crust to at least 200 km depth beneath the Yadong-Gulu rift, suggesting that rifting in southern Tibet is probably a process that involves the entire lithosphere. Intermediate-depth earthquakes in southern Tibet are located at the top of an anomalous feature in the mantle with a low Vp, a high Vs, and a low Vp/Vs ratio. One possible explanation for this unusual velocity anomaly is the ongoing granulite-eclogite transformation. Together with the compressional stress from the collision, eclogitization and the associated negative buoyancy force offer a plausible mechanism that causes the subduction of the Indian mantle lithosphere beneath the Higher Himalaya. Our tomographic model and the observation of north-dipping lineations in the upper mantle suggest that the Indian mantle lithosphere has been broken laterally in the direction perpendicular to the convergence beneath the north-south trending rifts and subducted in a progressive, piecewise and subparallel fashion with the current one beneath the Higher Himalaya.