Study of the Crust and Upper Mantle Anisotropy in the Nepal Himalaya

Mandal, Ritwik (2022) Study of the Crust and Upper Mantle Anisotropy in the Nepal Himalaya. Masters thesis, Indian Institute of Science Education and Research, Kolkata.

Text (MS dissertation of Ritwik Mandal (17MS104)) 17MS104_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (2MB)

Abstract

Azimuthal anisotropy present in the crust or upper mantle can be best detected using shear wave splitting . It is critical to research contemporaneous deformational processes occurring in the collisional zone in order to obtain critical information regarding the dynamics of orogeny creation. Seismic anisotropy allows us to explore the structural geology of deep formations remotely. The crust and mantle anisotropy in the central and eastern Nepal Himalaya is studied using the data from YL and the XF network. A total of 188 good and fair splitting result and 179 good and fair null has been observed. A significant variation in the anisotropic signature is indicated by the shear wave splitting measurements. The results obtained from the stations of the YL network shows a NE-SW trend of ϕ varying from 38° to 52° in the south with delay time varying between 0.6s to 0.9s. However as we go towards the north of Nepal the direction of ϕ changes to a more E-W trending pattern with ϕ between 83° to -79° with delay time varying between 0.9s to 1.4s. The results from the XF network in central part of Nepal shows a totally different direction of ϕ as it varies from 74° to -61° with delay times varying hugely within 0.5s to 1.5s. A sudden abrupt fluctuation in ϕ is also noticed in several stations. The eastern part of Nepal shows dominance of asthenospheric flow in terms of contribution to the splitting measurements, whereas the central part of Nepal Himalaya shows lithospheric mantle to be the highest contributor to the splitting parameters. Asthenospheric flow parallel to the slab of the downwelling Indian lithosphere is suggested by measurements from the north eastern part of the Nepal Himalaya. The wide range of splitting parameters implies the presence of more than one layer of anisotropy and a complicated anisotropic pattern under the Nepal Himalaya.

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