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Two crustal cross sections through the eastern margin of the Tibetan Plateau are jointly determined from deep seismic sounding. The E–W trending line AA’ passes through the western Sichuan plateau (including the Songpan‐Garze terrane and the Longmenshan fault belt) and ends in the Sichuan basin (a part of the Yangtze craton). Line BB’ has a trend of NNE and crosses the Songpan‐Garze terrane. Two‐dimensional crustal structures along the profiles were jointly determined by the additional use of existing deep seismic sounding data. Our seismic velocity models indicate that the western Sichuan plateau and the Sichuan basin have crustal thicknesses of 62 and 43 km, average crustal P wave velocities of 6.27 and 6.45 km/s and lower crustal (Vp > 6.5 km/s) thicknesses of 27 and 15 km, respectively. Density models constructed from the seismic velocity models are consistent with observed Bouguer gravity anomalies. We infer that collision between the Tibetan Plateau and the Yangtze craton has caused thickening of the lower crust and uplift of the western Sichuan plateau. We detect a low‐velocity layer in the upper crust of the western Sichuan plateau but observe no equivalence in the Sichuan basin; west dipping thrusts may detach into this low‐velocity layer. The seismic phase PmP in the western Sichuan plateau has low amplitude, suggesting high attenuation in the lower crust (Qp of 100–300). We suggest that the high attenuation is a consequence of lower crustal flow caused by the large lower crustal thickness beneath the western Sichuan plateau.

[1] Using P and S arrival times from 4625 local and regional earthquakes recorded at 174 seismic stations and associated geophysical investigations, this paper presents a threedimensional crustal and upper mantle velocity structure of southwestern China (21°-34°N, 97°-105°E). Southwestern China lies in the transition zone between the uplifted Tibetan plateau to the west and the Yangtze continental platform to the east. In the upper crust a positive velocity anomaly exists in the Sichuan Basin, whereas a large-scale negative velocity anomaly exists in the western Sichuan Plateau, consistent with the upper crustal structure under the southern Tibetan plateau. The boundary between these two anomaly zones is the Longmen Shan Fault. The negative velocity anomalies at 50-km depth in the Tengchong volcanic area and the Panxi tectonic zone appear to be associated with temperature and composition variations in the upper mantle. The Red River Fault is the boundary between the positive and negative velocity anomalies at 50-km depth. The overall features of the crustal and the upper mantle structures in southwestern China are a low average velocity, large crustal thickness variations, the existence of a highconductivity layer in the crust or/and upper mantle, and a high heat flow value. All these features are closely related to the collision between the Indian and the Asian plates.

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[1] Crustal thicknesses and Poisson's ratios in the eastern Tibetan Plateau were estimated using the H-k stacking method on teleseismic receiver functions. The data came mainly from the regional seismic networks in 2006 show that the crust thickens from east to west, with a thickness difference of ∼30 km. A sharp change of crustal thickness from 41.5 to 52.5 km is observed across the Longmen Shan. Crustal thickness predictions based on the Airy-type isostasy are consistent with the estimations from H-k stacking, implying that the topography is approximately compensated. The study area can be divided into three parts according to the Poisson's ratios: (1) the northern Songpan-Garze terrane and the western Qinling orogenic belt with low ratios (n ≤ 0.26); (2) the southern Songpan-Garze terrane, the Sanjiang fold system, and the Sichuan Basin with high-to-intermediate ratios (0.27 ≤ n < 0.30); and (3) the southwestern Yangtze platform with low to intermediate ratios (n ≤ 0.27). Joint interpretation of the low average V p values from deep seismic sounding profiles and the low V p /V s ratios from receiver function analysis favors the presence of felsic crust rather than significant volumes of partial melts in the northern Songpan-Garze terrane and the western Qinling orogenic belt. The junction of the Longmen Shan fault zone, the Xianshuihe fault zone, and the Anninghe fault zone has an ultrahigh Poisson's ratio (n ≥ 0.30), which could be considered as an evidence of local partial melt. We propose that the eastward movement of upper and middle crustal material in the southern Songpan-Garze terrane is obstructed by the more rigid Sichuan Basin, resulting in strain accumulation along the Longmen Shan fault zone.

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We present 59 new SKS/SKKS and combine them with 69 previously published data to infer the mantle deformation field in SE Tibet. The dense set of anisotropy measurements in the eastern Himalayan syntaxis (EHS) are oriented along a NE‐SW azimuth and rotate clockwise in the surround regions. We use GPS measurements and geologic data to determine a continuous surface deformation field that is then used to predict shear wave spitting directions at each station. Comparison of splitting observations with predictions yields an average misfit of 11.7° illustrating that deformation is vertically coherent, consistent with previous studies. Within the central EHS in areas directly surrounding the Namche‐Barwa metamorphic massif, the average misfit of 11 stations increases to 60.8°, and vertical coherence is no longer present. The complexity of the mantle anisotropy and surface observations argues for local alteration of the strain fields here associated with recent rapid exhumation of the Indian crust.