Shear wave splitting analyses in Tian Shan: Geodynamic implications of complex seismic anisotropy

Abstract

AbstractThe Tian Shan is a tectonically complex intracontinental orogenic belt situated between the Tarim Basin and the Kazakh Shield. The vast majority of the previous shear wave splitting (SWS) measurements were presented as station averages, which are only valid when the anisotropy structure can be approximated by a single layer of anisotropy with a horizontal axis of symmetry, i.e., a model of simple anisotropy. A variety of anisotropy‐forming hypotheses have been proposed based on the station‐averaged measurements. In this study, we measure the splitting parameters at 25 stations that recorded high‐quality data from a wide back azimuthal range for the purpose of identifying and characterizing complex anisotropy. Among the 25 stations, 15 of them show systematic azimuthal variations in the observed splitting parameters with a 90° periodicity that is consistent with a model of two‐layered anisotropy. The fast orientations of the upper layer range from 50° to 90° measured clockwise from the north, which are subparallel to the strike of the orogenic belt, and the splitting times are between 0.9 and 1.9 s. The corresponding values for the lower layer are −45° to −85° and 1.2–2.2 s, respectively. The remaining 10 stations demonstrate azimuthally invariant splitting parameters with strike‐parallel fast orientations, and can be represented by a single layer of anisotropy with a horizontal axis of symmetry. We propose that the strike‐parallel anisotropy is caused by lithospheric shortening, and anisotropy in the lower layer is associated with WNW‐ward flow of asthenospheric material sandwiched between the subducting Tarim lithosphere and the thick Kazakh lithospheric root.