Crust and Upper Mantle Structure in Northeast India

Anand, Aakash (2023) Crust and Upper Mantle Structure in Northeast India. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Aakash Anand (15RS003)) 15RS003.pdf - Submitted Version Restricted to Repository staff only Download (21MB)

Abstract

Northeast India is one of the most diverse, tectonically complex, and earthquake-prone regions in the Indian subcontinent, consists of distinct geological blocks: the Eastern Himalaya (EH), Shillong Plateau (SP), Mikir Hills (MH), Assam Valley (AV), Bengal Basin (BB), and Indo Burma Ranges (IBR). In this thesis, crust and upper mantle shear velocity structures in northeast India have been produced by modelling receiver functions along with surface-wave data beneath 24 stations spread across the region. The following key observations can be made from the thesis: a) A high-resolution Rayleigh wave group velocity map is produced for the first time in the region. From these tomography data along with receiver functions, 3-D shear velocity images are produced down to the depth of 80 km, which will be helpful in imaging deeper structures and locating earthquakes more accurately. b) Significant variations of Moho are observed in the region, viz., thicker Moho in IBR (43 - 62 km) and EH (40 - 53 km); thinner Moho in AV (33 - 38 km) and BB (37 km), and the thinnest Moho in SP (30 - 32 km). Average crust shear velocity is lesser in SP and AV (3.14 – 3.37 km/s) compared to that in the BB (3.49 km/s), IBR (3.33 – 3.57 km/s) and EH (3.15 – 3.44 km/s). Almost all stations in the region, apart from IBR stations showed felsic to intermediate average crustal composition (Vp/Vs ~1.73 – 1.8). The lower crust is observed to be thicker in the vicinity of the Kopili fault vis-à-vis the SP and Mikir Hills. c) Most of the earthquakes in IBR are confined to the crust in the west to sub-crustal intra-slab earthquakes in the east, suggesting dipping Indian plate beneath Burmese Plate. The Moho dip angle varies from north (7.8°) to south (9.5°). High shear velocity (3.5 km/s) and the highest Vp/Vs (1.8) were prevalent at all the stations in the IBR, associated with mafic prehnite-pumpellyite facies basalt and hydroxyl zeolite facies basalt, suggesting dehydrated Indian slab beneath the Myanmar microplate, causing intra-slab earthquakes. d) Reported for the first time the presence of the Hales discontinuity at a variable depth of 56 – 72 km beneath the AV, SP, and the Mikir Hills, which can be due to the addition of Cr³⁺ and/or Fe²⁺ that can shift the spinel-garnet stability field in the upper mantle. The uppermost mantle shear-wave velocity (Vsn) is found to be lower beneath the SP, Mikir Hills, and AV (~ 4.1 km/s). The lower Vsn could be attributed to the combined effect of factors, like, rock composition, grain size, increase in temperature, and a small amount of partial melt (<1%) in the upper mantle.

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