Love-wave Group-Velocity Tomography of India–Tibet: Insights into Radially-Anisotropic S-Wave Velocity Structure
Authors: Ghosh Monumoy , Dey Siddharth , Mitra Supriyo 
Published: February 2025

Summary
Love (LQ) and Rayleigh-wave (LR) dispersion data provide essential constraints on radially-anisotropic shear-wave velocity structure. Vertically-polarized S-wave velocity ($\rm S_V$) structure of India–Tibet are available from modeling of LR dispersion data, but reliable LQ dispersion measurements are limited. This is due to poor signal-to-noise (SNR) ratio on horizontal component waveforms, off great-circle-arc propagation and overtone interference. We overcome these limitations by performing systematic SNR tests, polarization analysis and minimization of overtone interference, to compute fundamental-mode LQ group-velocity dispersion and tomography across India, Himalaya and Tibet, for period range of 10–120 s. Fundamental-mode group-velocity dispersion, in this period range, is sensitive to the crust and upper-mantle structure. Lateral resolution of the group-velocity maps vary from 4$\rm ^{\circ }$ (10–40 s) to 9$\rm ^{\circ }$(90–120 s). Group-velocities (absolute and anomaly) and its lateral variations match the regional-scale geologic and tectonic features. Up to 20 s period low-velocity sedimentary layers are observed in the Bengal and Indus delta-fan complexes, Himalaya and Suleiman Mountain foreland basins, Qaidam, Tarim and Eastern Tadjik Basins. The Indian Cratons have higher group-velocities compared to the thickened Tibetan Plateau crust across the entire range of periods. At lower periods the western Tibetan Plateau is underlain by high-velocity anomaly and the central-eastern Plateau has a broad zone of low-velocity anomaly. To test the null-hypothesis of isotropy, we use isotropic $\rm S_V$ models to predict the observed LQ dispersion data. Our test negates the null hypothesis and suggest radially-anisotropic structure. Finally, we invert the LQ dispersion data to obtain horizontally-polarized shear-wave ($\rm S_H$) velocity structure beneath India–Tibet. $\rm S_H$ velocity models have an uncertainty of ∼5%. These models reveal high average $\rm S_H$ velocity (3.7–3.9 $\rm km \,\, s^{-1}$) in the Indian Cratonic crust, which extends beyond its geological outcrops, beneath the Deccan and Marwar Plateau, Southern Indian Granulite Terrane, and the Himalayan foreland basin (HFB). This is underlain by a high $\rm S_H$ velocity mantle-lid with variable thickness beneath cratons. The thickest mantle lids are beneath the Dharwar (∼150 km) and Bastar (∼180 km) Cratons. The high velocity Indian Cratonic crust and upper-mantle underthrust the Tibetan Plateau entirely in the west, up-to the Altyn-Tagh Fault (ATF); and half-way in the centre-east, up-to the Bangong-Nujiang Suture (BNS). This has lead to crustal-doubling beneath Tibet and a large-scale mid-crustal low velocity layer, bound by active faults along its edges. We use the $\rm S_H$ velocity of ≤3.0 $\rm km \,\, s^{-1}$ to delineate the regional-scale sedimentary basins. These include the Bengal and Indus delta-fan complexes with sedimentary layer thickness &gt;15 km; the intra-cratonic HFB, Eastern Tadjik and Qaidam Basins with up-to 10 km sedimentary layer thickness. The Tarim basin has strong E-W variation in sedimentary thickness with the deepest depocentre beneath the eastern basin (&gt;15 km), shallowing significantly west of the Mazhar-Tagh Fault to &lt;10 km. The HFB is also marked by lateral variations in sedimentary thickness, with the thickest layers beneath the Eastern and Western Gangetic Basins (∼6–8 km) and the thinnest beneath the Indus Basin and Brahmaputra Valley (&lt;2 km). These sub-basins are segmented by basement ridges, and the sedimentary thickness variation is controlled by flexural bending of the underthrusting Indian plate beneath the Himalaya.