Upper-mantle anisotropy beneath the Altai–Sayan region of central Asia

Abstract

The Altai–Sayan region of central Asia is actively deforming despite its being located several thousands of kilometers away from the collisional front between India and Eurasia. To add constraints to the role of the upper mantle in this type of tectonic environment, we analyzed intermediate-period, analogue records of SKS and SKKS to determine the state of mantle anisotropy in this region. We find that for all stations located in actively deforming areas, the fast direction of anisotropy correlates well with indicators of Cenozoic to the present-day deformation, and correlates with older structures only when they happen to align with younger ones. The general east-northeast trend of the fast direction of anisotropy in active regions also aligns with the principle extensional strain axis determined from earthquake moment tensors. At one station (NVS), located outside of any actively deforming region, the N–S fast direction may correspond to the long-wavelength strain field of the mantle beneath much of Siberia. These correlations, along with the size of the delay times (1–2 s), suggest that the anisotropy in the upper mantle beneath the Altai–Sayan is responding to recent tectonic activity.