A High-Resolution Full-Waveform Seismic Model of Asia and Constraints on Seismic Azimuthal Anisotropy in the Northwestern Himalaya

Banerjee, Rupak (2026) A High-Resolution Full-Waveform Seismic Model of Asia and Constraints on Seismic Azimuthal Anisotropy in the Northwestern Himalaya. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Rupak Banerjee (18RS021)) 18RS021.pdf - Submitted Version Restricted to Repository staff only Download (110MB)

Abstract

Asia hosts a complex interplay of varying tectonic settings shaped by continental collision, subduction and intraplate volcanism. This thesis investigates the large-scale structure, evolution, and deformation of the mantle beneath Asia using seismic imaging and azimuthal anisotropy analysis. The study is divided into two components: (i) development of a high-resolution three-dimensional seismic velocity model for Asia using adjoint-based full-waveform inversion, and (ii) characterization of mantle deformation associated with the India–Eurasia collision through analysis of shear-wave splitting from core-refracted (SKS) and direct S waves. Our tomographic model (FWEA23) reveals pronounced slow shear-wave velocity (Vs) anomalies beneath Southeast Asia extending to depths of ∼660 km, consistent with plume-like upwellings beneath Hainan Island. At depths <220 km, the slow anomaly spreads laterally beneath the majority of the southeast Asia, indicating widespread thermal perturbation of the upper mantle. High Vs anomalies, disconnected from the present day subducting slabs, detected in the mantle transition zone (MTZ) beneath the South China Sea, are interpreted as remnants of older subducted lithosphere. These remnants flat slabs, at the base of the MTZ, may act as a heat blanket and contribute to super-adiabatic conditions conducive to plume initiation from the lower mantle. Radial anisotropy further indicates strong horizontal mantle flow away from the plume source, consistent with lateral plume-head spreading and associated lithospheric thinning. These results provide new constraints on the mantle dynamics of Southeast Asia and support a slab-modulated mechanism for plume generation and intraplate volcanism. The Vs model beneath the Indo-Burma and Andaman-Sumatra subduction systems provides 3D disposition of the high Vs subducted Indian slab and its present day deformation from the overlain seismicity distribution. From the Vs models we identify the occurrences of along-strike and along-dip slab segmentation, slab roll-back at greater depths (>400 km) and slab-tear at a shallow depth (100-200 km) in the southern segment. We interpret that the along-strike slab segmentation and the along-dip slab-tear occur due to the change in the convergence obliquity along the subduction zone and the subduction of a buoyant aseismic ridge. To investigate the deformation pattern in the northwest Himalaya, we performed a shear-wave splitting analysis using an unprecedented dataset of core-mantle boundary refracted shear (SKS) and direct shear (S) phases, from 15 stations. The study reveals a vertically coherent deformation pattern in the lithosphere and asthenosphere, beneath the northwest Himalaya, with the anisotropic signals largely dominated by the plate-motion derived mantle flow. Comparative analysis of the splitting signals across the northwest, central, and eastern Himalaya reveals a systematic variation of the fast axes orientation, consistent with the increased contribution of the lithospheric anisotropy in the central and eastern Himalaya. Variations in the splitting characteristics between the TYR and PXR lineaments in the eastern Himalaya is consistent with the increase in the dip of convergence of the Indian lithosphere. Overall, this thesis provides new insights into the mantle structure beneath Asia by computing and interpreting a high-resolution seismic model. We explain the occurrence of the intraplate volcanism in southeast Asia. Additionally, we explore the plausible role of the stagnant lithospheric remnants in creating a thermal boundary layer and facilitating the Hainan Plume formation. We model the 3D disposition and identify the variation in the geometry of the Indian subducting lithosphere beneath Indo-Burma and Andaman-Sumatra subduction systems. We postulate that the subduction obliquity plays an important role in the segmentation of the slab. Furthermore, by analyzing seismic azimuthal anisotropy, we highlight the variation in the mantle deformation pattern along-strike of the Himalayan collision zone, from the northwest to the eastern Himalaya.

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