Pervasive Double-Layer Anisotropy beneath the Central Tien Shan and Its Geodynamic Implications

The Tien Shan is an intraplate mountain belt situated between two stable Precambrian continental blocks, the Tarim Basin and the Kazakh shield. Contrasting mechanisms have been proposed to explain shear-wave splitting measurements obtained in the central part of Tien Shan, including N-S shortening originated from the India-Eurasia collision, mantle plume, small-scale mantle convection, and relative motion between the lithosphere and the underlain asthenosphere. Virtually all of the previous studies presented and interpreted their results in the form of station-averaged splitting parameters. Such a practice is valid only for the simplest form of anisotropy, i.e., a single layer with a horizontal axis of symmetry. In addition, all the studies only used the SKS phase in the 84-130 degree epicentral distance range. Consequently, only events from a narrow back-azimuthal range were used, making it impossible for identifying the existence of complex anisotropy which is diagnosed by systematic azimuthal variations of the resulting splitting parameters. Here we present a comprehensive analysis of newly-measured individual (rather than station-averaged) splitting parameters at about 40 broadband stations with data available at the IRIS Data Center. A total of about 900 pairs of well-defined SKS, SKKS, and PKS splitting parameters were obtained by using a robust shear-wave splitting measurement procedure that we developed (Liu, 2009, G-cubed). The use of all the three XKS phase and the full epicentral distance range (up to 180 degree) dramatically improved the azimuthal coverage, and provided clear evidence for the existence of pervasive double-layer anisotropy, as demonstrated by systematic azimuthal variations of the splitting parameters with a 90-degree periodicity. Ongoing effort to grid-search the four parameters suggests that beneath most of the stations with sufficient azimuthal coverages, the upper layer is in the lithosphere with an approximately E-W fast direction, which is parallel to the strike of the mountain belt, and the lower layer is in the NE-SW direction which might be related to mantle flow associated with the relative motion between the lithosphere and the asthenosphere