Crustal thicknesses and Poisson's ratios in the eastern Tibetan Plateau and their tectonic implications

Abstract

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 2006–2008. Results 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 (ν ≤ 0.26); (2) the southern Songpan‐Garze terrane, the Sanjiang fold system, and the Sichuan Basin with high‐to‐intermediate ratios (0.27 ≤ ν < 0.30); and (3) the southwestern Yangtze platform with low to intermediate ratios (ν ≤ 0.27). Joint interpretation of the low average Vp values from deep seismic sounding profiles and the low Vp/Vs 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 (ν ≥ 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.