3D Crustal Density Distribution of Northeast India from Seismically-Constrained Gravity Inversion

The Northeast India is a tectonically active region situated at the complex junction of the Indian, Eurasian, and Burmese plates. This area, encompassing the eastern Himalayas, the Indo-Burmese Ranges, the Shillong Plateau, the Assam Valley and Bengal Basin, exhibits a highly heterogeneous crustal structure and composition resulting from the continental collision and ongoing subduction of the Indian plate. The objective of this study is to present the first 3D crustal density model for northeast India, obtained through a novel tesseroid-based gravity inversion that accounts for the curvature of the Earth and utilises a Gauss-Newton optimization scheme. This framework is initialised and constrained by a local seismic tomography-based 3D reference density model. The inversion employs a tesseroid mesh parameterisation, in which each density contrast of the tesseroid is solved to minimise a composite objective function that balances data misfit, depth-weighted regularization, and 3D smoothness relative to the seismic reference model. The inversion utilizes the filtered residual gravity anomaly, derived by systematically removing the upper-mantle gravitational effects from the observed Bouguer anomaly and isolating the crustal signal using third-order regional-residual separation, enabling stable recovery of short-wavelength density contrasts.
The resulting 3D crustal density structure reveals: (i) High-density material within the upper crust (~10 km) and the lower crust of the Shillong Plateau indicates the presence of basic intrusions, while the uplifted structural configuration suggests a rigid Archean-Proterozoic basement of the Shillong Plateau exhumed through pop-up tectonics. (ii) Thickened, low-density crust beneath the Eastern Himalaya and Indo-Burmese Ranges reflects ongoing Indian-plate underthrusting and subduction, supported by density gradients that dip north to ~25°N and east to ~93°E, imaging the progressive burial of the Indian crust beneath the Eastern Himalayan arc and Indo-Burmese Ranges, respectively. (iii) Pronounced low-density zones beneath the Indo-Burmese Ranges, indicative of crustal weakening and hydrated fabrics. (iv) Adjacent low-density anomalies within the upper crust of the Assam Valley and the Bengal Basin clearly image the sedimentary fill, while high density at ~25 km depth beneath the Bengal Basin is associated with the presence of oceanic crust (or continental to oceanic transition). These contrasting signatures collectively highlight strong vertical and lateral density variations across the region.
These first-order results provide new quantitative constraints on the crustal density characteristics of major tectonic features in Northeast India, significantly contributing to the understanding of the regional stress field and geodynamic setting of this seismically active region.