Evidence for lowermost mantle anisotropy from discrepant splitting intensity of XKS and SKKS phases recorded in India

Abstract

This study comprehensively examines the shear wave splitting measurements of XKS (SKS and PKS) - SKKS pairs on the same seismograms recorded at 357 broadband stations spanning India, to characterize anisotropy in the lowermost mantle. This resulted in the identification of 104 XKS-SKKS pairs at 62 stations, of which 27 pairs were found to be discrepant, based on the difference in splitting intensity of XKS and the corresponding SKKS phases. These discrepant pairs dominantly sample a portion of the lowermost mantle beneath Southeast Asia and the Indian Ocean. The majority of these pairs represent null-split and split-split cases, with the delay time of SKKS being larger than that of XKS for the latter. This suggests that the XKS phases primarily sample the isotropic (weakly anisotropic) or anisotropic regions with a cancelling effect in the lowermost mantle, while the corresponding SKKS phases sample the anisotropic region of the D${}^{″}$ layer. In addition, there are three discrepant pairs in the split-null category, suggesting anisotropy in the vicinity of southern Tibet, where discrepant pairs from other cases are not observed. This implies an apparent change in the anisotropy of the D${}^{″}$ layer for the regions sampled by XKS and SKKS, although they are associated with high-velocity anomalies. In these regions, the fast polarization azimuths of the discrepant pairs are in the NE-SW and ENE-WSW, and NNE-SSW directions, respectively. These do not coincide with the trend of mantle flow in the lowermost mantle, suggesting an association with paleo-subducted slabs. The observed deformation is probably due to phase transformation of bridgmanite to a more stable post-perovskite, causing Crystallographic Preferred Orientation of the lowermost mantle, which is the candidate mechanism for lowermost mantle anisotropy beneath Southeast Asia and the Indian Ocean.