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

Abstract

The 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.

Tomographic Pn Velocity and Anisotropy Structure in Mongolia and the Adjacent Regions

“…It is worth noting that the fast directions for 65 km depth in our Pn tomographic model are characterized by a strong NW‐SE fabric beneath the Hentey Mountains and northern Mongolia, which correlates well with the shear wave splitting results (Barruol et al, ; Gao, Davis, Liu, Slack, Zorin, Mordvinova, et al, ; Qiang et al, ). What is more, the ENE‐WSW fast orientation at 50 km depth and the WNW‐ESE fast orientation at 65 km depth are in good concordance with the two‐layered anisotropy in the Tien Shan from shear wave splitting (Cherie et al, ; Li et al, ). We do not show the Pn anisotropy model for the whole data set in this study, because the anisotropy model has a bias for longer paths introduced to the parts of the model with longer paths.…”

“… Pn anisotropy tomographic models from arrivals with epicentral distance variations of (a) 2–7° and (b) 7–12°, sampling depths of about 50 and 65 km, respectively. Red bars show the shear wave splitting results of Barruol et al (), Gao, Davis, Liu, Slack, Zorin, Mordvinova, et al (), Qiang et al (), Cherie et al (), Jiang et al (), and Li et al () and from the Incorporated Research Institutions for Seismology. Yellow arrows are GPS vectors with respect to Eurasia (Calais et al, ).…”

Seismic anisotropy and mantle dynamics beneath the Malawi Rift Zone, East Africa

“…We next attempt to fit the azimuthal variation of the splitting parameters observed at the four northern MRZ stations under a two‐layered model with a horizontal axis of symmetry using the approach of Silver and Savage []. In order to reduce the well‐known nonuniqueness of the resulting two pairs of splitting parameters when measurements from individual stations are used, we take a two‐step approach that is similar to what we have recently utilized to constrain the two‐layered structure in Tian Shan [ Cherie et al , ]. For the first step, we combine measurements from the four stations and perform a grid search of the two pairs of the splitting parameters using a peak frequency of 0.25 Hz and weighting factors of 0.8 and 0.2 for ϕ and δ t , respectively [ Gao and Liu , ].…”

Seismic Anisotropy in the Java‐Banda and Philippine Subduction Zones and its Implications for the Mantle Flow System Beneath the Sunda Plate

“…Information for all events used in this study is given in Table S1 in the supporting information. Anisotropic parameters of 30 stations used for removing the anisotropic contributions beneath the stations are compiled from a range of previous publications (Barruol & Ben Ismail, ; Barruol & Hoffman, ; Chang et al, , ; Cherie et al, ; Fouch & Fischer, ; Hanna & Long, ; Helffrich et al, ; Huang et al, ; Király et al, ; Liu et al, ; Makeyeva et al, ; Vinnik et al, ; Wolfe & Vernon III, ). A full list is provided in Table S2.…”