High density crustal intrusive bodies beneath Shillong plateau and Indo Burmese Range of northeast India revealed by gravity modeling and earthquake data

Abstract

Modeling and analysis of gravity and earthquake data of a critically dynamic northeastern region of India assumes a special significance for understanding the seismicity and geodynamics. In this paper, the relation between lateral heterogeneity of gravity interfaces with subsurface structures, and seismic characteristics of northeast India are studied in detail to understand the seismicity and dynamics of the region using Bouguer gravity, earthquake and magnetotelluric data. The modern scaled power spectral analysis of gravity data has brought out crustal lateral heterogeneity at a depth of 8.33 km, 19.73 km and 37.14 km that correlate well with 10 km upper crust, 20 km lower crust and a Moho at 28–42 km revealed by 1D seismic crustal velocity and tomography studies. The inferred crustal depths support thinner crust over uplifted Shillong Plateau and gradual increase of depths over subduction interface of Indo Burmese Range. 2D gravity modeling constrained by seismic velocity model and 3D Euler deconvolution technique along two specific E-W and three N-S regional gravity transects delineated three significant high density anomalous heterogeneous intrusive bodies presumably due to underplating beneath Shillong Plateau, Mikir Massif and Indo Burmese Range. Based on the seismicity and stress pattern analysis through seismic ‘b’-value variation, we infer that the study region exhibits compressive stress concentrations in the form of brittle deformation at shallow depths and weakening of lower crust by high density intrusive (mafic/Ultramafic) bodies of ductile nature which may cause intense seismicity besides the ongoing collision of north drifting Indian plate with Eurasia to the north and the eastern subduction interface of Indo Burmese Range. We also infer that the NE to NW trending V-shaped areas outlined north of Shillong plateau, south of Mikir massif and NE-SW trending thrust Belt of Schuppen exhibit aseismic nature and may be the future impounding seismic sources in this region.