Crust and Upper mantle structure of India and surrounding regions from Rayleigh wave Dispersion analysis

Dey, Siddharth (2022) Crust and Upper mantle structure of India and surrounding regions from Rayleigh wave Dispersion analysis. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Siddharth Dey (13IP033)) 13IP033.pdf - Submitted Version Restricted to Repository staff only Download (102MB)

Abstract

We model the 3D isotropic shear-wave velocity structure of crust and upper mantle of India, Himalaya, Tibet and surrounding regions. From a set of 3625 earthquakes, recorded in 572 stations, we obtain 14706 regional-waveforms for the analysis. The waveforms are windowed for the fundamental mode Rayleigh wave, filtered between 10 s and 120 s period, and analyzed using the Multiple Filter technique to compute 1D group-velocity dispersion for 14706 ray-paths. These 1D measurements are combined through ray-based tomography to compute 2D group-velocity maps at 1⁰ nodes between periods of 10 s and 120 s. The 2D maps have 3⁰x3⁰ lateral resolution and reveal strong correlation with the regional geology. 3D shear-wave velocity structure is obtained by isotropic inversion of the tomographic dispersion at each node and interpolation using triangulation. The shear-wave velocity (Vs) structure is presented as depth sections and profiles to study the lateral variations across the Indian subcontinent, Himalaya, Tibetan Plateau and surrounding basins and Subduction zones. Focal mechanisms of moderate-to-large earthquakes Wimpenny and Watson (2021), Moho depth computed from isostatic compensation and free-air gravity anomaly maps (GOCE) are presented and discussed with the Vs structure to unravel the tectonic evolution. Precambrian cratons have high average-crustal Vs (3.5 4.0 km s⁻¹) and thicker crust (45 50 km), except for Eastern-Dharwar and Singhbhum Cratons (30 35 km). Sedimentary basins have slowest velocity, with the base of the sedimentary-layer defined by Vs of 2.9 km s⁻¹. The Bengal and Indus delta-fan complexes, Katawaz Basin and Indus paleodelta have the thickest sedimentary deposits (16 18 km), followed by Tarim and Tadjik Basins (1014 km), Qaidam Basin (~8 km) and Eastern-Ganga Basin (8 10 km). Steep Vs gradient across the Himalaya, double crustal-thickness (~80 km) beneath southern Tibet, paired low-to-high free-air gravity anomaly, and shallow thrust-fault earthquakes, attest to flexural bending and underthrusting of India. Mid-crustal low-velocity layer beneath central Tibet, with 5 10% Vs anomaly, reveals radiogenic heating of the under-thrust Indian upper-crust. From lower-crustal Vs, Moho-depth contours and earthquake focal mechanisms (Mw≥5.5) beneath Tibet, we conjecture that the Indian crust under-thrusts upto the Altyn-Tagh Fault in western Tibet and up to the New Kunlun Fault in the central and eastern Tibet. High Vs of 4.6 4.8 km s⁻¹ in the upper mantle (80 120 km) beneath the Indian subcontinent, Tarim and Tadjik Basins is speculated as an eclogite layer within the upper-mantle peridotite. This layer is absent beneath the Deccan Volcanic Province, possibly due to plume-volcanism related thermal anomaly. Finally, the output model of this study is going to be a crucial starting model for modelling the lower mantle, performing joint anisotropic inversion using Love and Rayleigh waves, executing local earthquake inversion, or calculation of the attenuation characteristics. We put forth several future works that could be potentially achieved with the results of this study.

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