We have evolved 3‐D seismic velocity structures in northeast India region and its adjoining areas to understand the geodynamic processes of Indian lithosphere that gently underthrusts under the Himalayas and steeply subducts below the Indo‐Burma Ranges. The region is tectonically buttressed between the Himalayan arc to the north and the Indo‐Burmese arc to the east. The tomographic image shows heterogeneous structure of lithosphere depicting different tectonic blocks. Though our results are limited to shallower depth (0–90 km), it matches well with the deeper continuation of lithospheric structure obtained in an earlier study. We observe low‐velocity structure all along the Eastern Himalayas down to ~70 km depth, which may be attributed to deeper roots/thicker crust developed by underthrusting of Indian plate. Parallel to this low‐velocity zone lies a high‐velocity zone in foredeep region, represents the Indian lithosphere. The underthrusting Indian lithosphere under the Himalayas as well as below the Indo‐Burma Ranges is well reflected as a high‐velocity dipping structure. The buckled up part of bending Indian plate in study region, the Shillong Plateau‐Mikir Hills tectonic block, is marked as a high‐velocity structure at shallower depth. The Eastern Himalayan Syntaxis, tectonic block where the two arcs meet, is identified as a high‐velocity structure. The Bengal Basin, tectonic block to the south of Shillong Plateau, shows low velocity due to its thicker sediments. Based on the tomographic image, a schematic model is presented to elucidate the structure and geodynamics of Indian lithosphere in study region.

Abstract. We estimate variations of the crustal thickness beneath the Nepal Himalayas based on tomographic inversion of regional earthquake data. We have obtained a low-velocity anomaly in the upper part of the model down to depths of 40 to 80 km and proposed that the lower limit of this anomaly represents variations of the Moho depth. This statement was supported by results of synthetic modeling. The obtained variations of crustal thickness match fairly well with the free-air gravity anomalies: thinner crust patterns correspond to lower gravity values and vice versa. There is also some correlation with magnetic field: higher magnetic values correspond to the major areas of thicker crust. We propose that elevated magnetic values can be associated with more rigid segments of the incoming Indian crust which cause more compression in the thrust zone and lead to stronger crustal thickening.

Abstract. We estimate variations of the crustal thickness beneath the Nepal Himalayas based on tomographic inversion of regional earthquake data. We have obtained a low-velocity anomaly in the upper part of the model down to depths of 40 to 80 km and proposed that the lower limit of this anomaly represents variations of the Moho depth. This statement was supported by results of synthetic modeling. The obtained variations of crustal thickness match fairly well with the free-air gravity anomalies: thinner crust patterns correspond to lower gravity values and vice versa. There is also some correlation with magnetic field: higher magnetic values correspond to the major areas of thicker crust. We propose that elevated magnetic values can be associated with more rigid segments of the incoming Indian crust which cause more compression in the thrust zone and leads to stronger crustal thickening.

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