3-D Anisotropic Structure of the Upper Mantle beneath the Iranian Plateau Using SKS Splitting Intensity Tomography

The Iranian plateau, characterized by the Arabia-Eurasia continental collision in the Zagros and the Makran oceanic subduction system, presents a unique opportunity to investigate the underlying processes of lithospheric deformation and upper-mantle dynamics. Previous studies of upper mantle seismic anisotropy, mostly using SK(K)S splitting and occasionally direct S waves revealed complex patterns for the fast axes. The observed rotations of fast axis obtained from S and SK(K)S waves along different tectonic setting, the difference in fast directions between S and SK(K)S phases in central Iran, evidence for two-layer anisotropy, and ambiguity regarding the depth origin of observed anisotropy emphasize the need for further studies. These challenges, together with the pronounced tectonic heterogeneity of the Iranian plateau, call for tomographic approaches that allows for the localization of anisotropic structure. In this study, we utilize SKS splitting intensity tomography to elucidate the depth distribution of the anisotropic properties of the upper mantle beneath the Iranian plateau. Our dataset includes teleseismic events with magnitude above 5.5 and epicentral distances between 90 and 130 degrees recorded by 151 permanent (2015-2021) and 296 temporary seismic stations (2003-2021). We employ a three-dimensional full-wave anisotropy tomography method using splitting intensity, which provides enhanced depth resolution compared to traditional shear wave splitting methods. This method utilizes perturbation theory to establish the linear relationship between splitting intensity and anisotropic parameters, including the azimuth of fast axis and anisotropy strength, and incorporates Green's function databases to efficiently compute the sensitivity or Fréchet kernels. Splitting intensity measurements are inverted to construct a three-dimensional model of upper-mantle azimuthally anisotropic structure beneath the study area. Results show clear lateral differences in anisotropic strength and fast-axis orientation specifically between the Zagros and Alborz regions, as well as the adjacent domains. Vertical profiles illustrate depth-dependent heterogeneous anisotropic structures across the lithosphere–asthenosphere boundary into the asthenospheric upper mantle. These variations likely reflect the combined influence of regional tectonic processes, continental collision, lithospheric deformation, and present-day mantle flow patterns beneath the Iranian plateau. Our results highlight the potential of SKS splitting intensity tomography to resolve complex mantle anisotropy and shed new light on the three-dimensional deformation structure of the upper mantle. The observed lateral and depth variations in anisotropy provide new insights into how the relationship between surface tectonics and upper-mantle deformation varies spatially across major tectonic domains such as the Zagros, Makran, Central Iran, and the Alborz.