High density crustal intrusive bodies beneath Shillong plateau and Indo Burmese Range of northeast India revealed by gravity modeling and earthquake data
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.
Section snippets
General geology and tectonics
The study area forms a part of the northeastern region of India that lies between the spatial coordinates 23°–27°N Latitude and 90°–95°E Longitude (Fig. 1). Geologically, it consists of distinct tectonic domains of eastern Himalayan arc, Mishmi block, uplifted Shillong Plateau, Brahmaputra valley and northern part of Indo Burmese arc (Fitch, 1970; Molnar and Tapponniar, 1975; Verma et al., 1977; Seeber et al., 1981; Mukhopadhyay, 1984; Kayal, 1996; Nandy, 2001; Bilham and England, 2001; Panda
Data and methodology
The crustal heterogeneity, nature of faulting and deformation causing the seismicity in the northeastern region is analyzed through Bouguer gravity, earthquake and magnetotelluric data. The Bouguer gravity data used for this study are obtained by digitizing the gravity map of India (GSI-NGRI, 2006) and incorporating a new datasets of 4025 gravity observations (Pathak et al., 2014; Ravi Kumar et al., 2017). We have applied different signal processing techniques such as conventional 2D radially
Discussion of results
The gravity anomaly map and its derivatives, data of earthquake epicenters, composite fault plane solutions along with ‘b’ values provide clue to the nature of deformation and seismic characteristics of northeastern region. The outlined gravity discontinuities and body axes represent the tectonic fabric oriented in N-S, NNE-SSW, NE-SW, NW-SE, E-W and NNE-SSW directions. The deciphered subsurface structural trends by gravity discontinuities/structural fabric coincide with weak zones represented
Conclusions
The present study highlights the relation between lateral heterogeneities of gravity interfaces and subsurface structures, nature of deformation and seismic characteristics of northeastern region of India. Scaled spectral analysis of gravity data, ‘b’ value analysis of earthquake data and fault plane solutions provide deep insights into crucial thrust tectonics and seismogenic nature of this region. The 2D subsurface density modeling and 3D Euler deconvolution depth solutions along regional
CRediT authorship contribution statement
Ch. Ravi Kumar: Conceptualization, Data curation, Methodology, Formal analysis, Validation, Visualization, Writing - original draft, Writing - review & editing. A. Selin Raj: Data curation, Formal analysis. B. Pathak: Visualization, Data curation, Formal analysis. Saumen Maiti: Formal analysis. K. Naganjaneyulu: Visualization, Writing - review & editing.
Declaration of competing interest
All authors must disclose any financial and personal relationships with other people or organizations that could inappropriately influence (bias) their work or state if there are no interests to declare.
The authors state there are no interests to declare.
Acknowledgements
CHRK express his gratitude to Deputy Director General & HOD of RSAS, GSI Bangalore for utilizing the facilities of GSI. CHRK express sincere thanks to colleagues who have given valuable suggestions for improving the paper. SM is thankful to Director, IIT(ISM) for giving the permission to publish the work. KN is supported by CSIR project GEOMET (MLP-0002-28-FBR-2). The authors are thankful to Prof. Mark Jellinek, Editor, Prof. M. Santosh and three other anonymous reviewers for providing positive
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