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

Highlights

• Gravity and seismic characteristics of northeastern part of India is discussed.

• Spectral analysis provide information on thrust tectonics and seismogenic nature.

• Reactivation of basement faults causing diffused seismicity in shallow depths.

• Imaged underplating beneath Shillong Plateau, Mikir massif and Indo Burmese Ranges.

• Crustal thickness varies from 30 km to 44 km.

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.

Introduction

The northeastern region of India is tectonically complex with frequent seismic activity where it is trapped between two young mountains, chains of Himalayas to the north and Burmese orogenic belt to the east. The northeast India and its surrounding areas are tectonically complex due to active N-S convergence along Himalayan orogenic belt and E-W subduction under Burmese plate along folding and thrust belts of Indo Burmese Range. The Indian plate is believed to be underthrusting at low angles beneath the Himalaya and subducting at dip angles (>30°) under Burmese arc (Fitch, 1970; Molnar and Tapponniar, 1975; Molnar and Tapponnier, 1977; Ni and Barazangi, 1984; Curray, 1989). The ongoing collision of north drifting Indian plate with Eurasian plate at the north and the north-south trending subduction interface of Indo Burmese Range to the east implicate large-scale active continental deformation and intense seismicity (Verma et al., 1977; Seeber et al., 1981; Mukhopadhyay, 1984; Kayal, 1996; Nandy, 2001; Bilham and England, 2001; Khan, 2005; Seno and Rehman, 2011; Panda et al., 2018 and references therein). This region has experienced three most damaging earthquakes and more than twenty large earthquakes. Besides these, moderate to low earthquakes are quite a common (Kayal, 2008; Khan et al., 2009). The geology and tectonics of the northeastern region is studied well, when compared to geophysical studies. Several tectonic models have been proposed to understand the seismicity and geodynamics of this region (Evans, 1964; Verma et al., 1976; Dasgupta, 1977; Rao, 1983; Mukhopadhyay, 1984; Acharyya et al., 1990; Acharyya, 1991; Kayal and Zhao, 1998; Bilham and England, 2001; Ravi Kumar et al., 2004; Mitra et al., 2005; Kayal et al., 2006; Khan and Chakraborty, 2007; Nayak et al., 2008; Bhattacharya et al., 2010; Sharma and Baruah, 2017; Dipok et al., 2018). Geophysical studies have been conducted in the northeastern region since early 1830's which includes gravity survey for oil exploration in the plains of Assam around Digboi during 1925. Studies were also conducted by measuring the gravity field to understand the subsurface geology, tectonics of the northeastern region (Evans, 1964; Verma and Mukhopadhyay, 1977; Khan and Chakraborty, 2007; Nayak et al., 2008; Saha, 2012). The results of gravity studies reveal that the underlying basement dips gradually towards northern and eastern part of the northeastern region (Verma and Mukhopadhyay, 1977). However, the lateral heterogeneities with crustal thickness variations and nature of seismic sources of this complex tectonic region are not adequately studied. In this study, we have made an attempt to understand the subsurface heterogeneities related to seismicity and tectonics of the region. Particularly, the present study aims to image the crustal heterogeneities, nature of deformation and seismicity of the northeastern region using Bouguer gravity, earthquake and magnetotelluric data. We have examined the relation between the lateral heterogeneities of gravity interfaces along with subsurface structures and its seismic characteristics. The results of 2D subsurface modeling and Euler deconvolution along specific regional gravity transects are constrained by seismic and magnetotelluric data. The computed seismic ‘b’ values along with fault plane solutions provide deep insights into crucial thrust tectonics and seismicity of the northeastern region.