Seismic Anisotropy and Lithospheric Deformation of the Eastern (Chinese) Tianshan Orogen

Abstract

Plate tectonics theory predicts strong deformation along the plate boundaries. But intracontinental orogeny may occur far away from the boundaries, for example, the Tianshan orogen within the Eurasian plate, whose mechanism remains debated. In this study, we deployed a dense array of broadband seismic stations (∼15 km spacing) along an ∼600 km profile across the eastern (Chinese) Tianshan orogen. We measured SKS/SKKS splitting parameters to map spatial variations in seismic anisotropy and lithospheric deformation from the orogen to the adjacent blocks. In the Tianshan orogen, fast polarization directions are primarily aligned with the orogen's strike, indicating a dominant contribution from the lithosphere. Delay times of up to 1.2 s may be explained by the presence of a thick lithosphere (∼150 km). In the stable Tarim and Junggar basins/blocks, the orogen oblique or perpendicular fast polarization directions and shorter delay times (0.3–0.8 s) mainly reflect weak fossil anisotropy frozen into the lithosphere. Our results suggest pure-shear deformation of the orogen lithosphere under horizontal contraction between the rigid Tarim and Junggar blocks. Further comparison between the western and eastern Tianshan (separated by ∼80°E) suggests two-stage processes of the intracontinental orogeny. The eastern Tianshan is in an early stage with dominant lithosphere thickening while the western Tianshan represents a later stage with significant lithospheric delamination.