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Abstract

We investigate the lithospheric and sub-lithospheric primary and shear wave velocity structure beneath northwestern India through body(P) wave and surface wave tomography(SWT). The study region witnessed the Neoproterozoic silicic magmatism, Cretaceous Barmer-Sanchor rifting and the pre-Deccan magmatism which resulted in lithospheric modifications that can be imaged through seismic tomography. P-wave tomography was done using ≈8000 P-arrival time data from teleseismic waveforms recorded by 29 broadband stations. Relative residuals estimated through adaptive stacking were inverted in terms of P-velocity perturbations using the fast forward marching technique. About 2100 regional earthquake waveforms were used to construct the Rayleigh wave phase velocity regionalized dispersion maps in the time periods 36s to 200s which were inverted in terms of shear velocity perturbations. The P-tomograms reveal ≈100km wide cylindrical zone of low velocities (-2%) extending from 80km to 140km beneath the rift. SWT reveals a thin lithosphere and ≈200km wide low velocity(-5%) zone in the depth interval 150-250km coinciding with the rift axis. We interpret the shallow anomaly to represent a conduit through which the pre-Deccan magmatism occurred due to lithospheric thinning and partial melting caused by a mantle plume.Geochemical studies on the alkaline complexes associated with pre-Deccan volcanism favour carbonatite metasomatism of the lithosphere. Metasomatism reduces the solidus temperature resulting in melting and formation of low velocity zones(LVZ) due to the thermal anomaly caused by a mantle plume. The LVZ may represent both thermal and compositional imprints and suggests that the signatures of the Neoproterozoic magmatism were overprinted by the later Cretaceous events.