Invited ReviewA review of crust and upper mantle structure beneath the Indian subcontinent
Introduction
The Indian subcontinent is formed of a mosaic of various Precambrian tectonic provinces, with stable shields in peninsular India to actively deforming collision belts in the Himalaya, and has experienced extensive volcanism and rifting. India lies on a fast moving plate and has covered a large distance since its separation from the other components of Gondwana (ca 130 Ma). The influence of the fast drift on the stability of cratons, and removal of lithospheric roots are key issues which are much debated (Kumar et al., 2007), but as yet are not fully understood.
In the century since the detection of the Mohorovičić discontinuity (Mohorovičić, 1910) from earthquake observations, both controlled source and passive seismic studies have made impressive advances in understanding the nature of the crust and uppermost mantle (Prodehl et al., 2013). Multiple facets of seismic wave propagation can be brought to bear on the structure of the Earth's interior, and help to resolve the key issues related to evolution and nature of the continental crust and upper mantle. To date there have been only limited attempts to provide a full picture of the Indian crust and upper mantle. There have been reviews of heat flow (Roy and Rao, 2000) and deep seismic sounding studies (Kaila and Krishna, 1992, Reddy and Rao, 2013). However, the full range of available information on the crust and upper mantle available from passive source studies have not previously been exploited.
The foundation stones of seismology in India were laid by the pioneering works of Dr. T. Oldham and Dr. R. D. Oldham, the father–son duo. The great Shillong earthquake of 12th June, 1897 is well documented and reported in the works of Oldham (1899). This deadly Shillong earthquake achieved the maximum intensity XII on MM scale (Richter, 1958), and provided the impetus for a series of initiatives to install seismographs in India to monitor earthquakes. The first few installations were made of Milne's self registering seismographs in Alipore (Calcutta, now Kolkata), Colaba (Bombay, now Mumbai) and Madras (now Chennai) (Tandon, 1992). An Omori-Ewing seismograph was installed in Simla as a response to the great Kangra earthquake of 5th April, 1905. In the years from 1929 to 1930, the country was equipped with a few more Milne-Shaw seismographs, initially installed at Colaba observatory Mumbai, then Bombay and later at few more places in Agra, Calcutta, Hyderabad and Kodaikanal. In the early 1960s five World Wide Standard Seismograph Network (WWSSN) stations were installed at various places across the country following the recommendations of Berkner (1959). After the devastating Latur earthquake of September 30th, 1993 the India Meteorological Department upgraded ten of its observatories to the standard of Global Seismograph Network, and later complemented this network with 14 more broadband stations during 1999–2000. At present the India Meteorological Department runs nearly 80 seismic stations in the national network, supplemented by various temporary networks operated by other organizations. Temporary and permanent networks in different parts of India have been operated by the National Geophysical Research Institute, Indian Institute of Technology Bombay, Wadia Institute of Himalayan Geology, Tezpur University and the Institute of Seismological Research. The National Geophysical Research Institute has established more than 200 broadband seismic stations at various points of time, and so plays a major role in passive source seismology in India.
Deep seismic probing of Indian crust, started in 1972 with refraction/wide-angle reflection work, but subsequently was dominated from the early nineties by deep seismic reflection. A good deal has been achieved (Kaila and Krishna, 1992), with more than 10,000 km of profiles carried out in various experiments using controlled sources. A major supplementary source of information on Indian structure comes from the use of seismic receiver functions exploiting the recordings of distant earthquakes. Receiver functions provide a tool to map the Earths response beneath a single three-component seismic station, and extract information on the seismic discontinuities at depth from the conversions and reverberations associated with the main seismic phases. The first receiver functions for the Indian region used data from the Hyderabad station (HYB) in India, using P-to-s converted waves (Gaur and Priestley, 1997). Since then the role of receiver functions in determining crust and upper mantle discontinuities (Moho, lithosphere–asthenosphere boundary, mantle transition zone discontinuities 410 and 660) has been routine practice. Further information comes from seismic anisotropic studies using SKS/SKKS phases and heat flow that provide links to help understand both geodynamics and structure. The present work presents as a complete picture of the Indian crust and upper mantle as possible, compiled from various sources with emphasis on passive source seismic datasets. We synthesize results from seismic studies, heat flow and seismic anisotropy to develop a comprehensive map of the properties of the crust and upper mantle beneath the Indian subcontinent, with links into the Himalaya and Tibet to provide a wider perspective and understanding of the whole region.
Section snippets
Tectonic setting
The major tectonic units of peninsular India comprise Precambrian terranes (Fig. 1). A vast region in between the peninsula and the actively deforming regions of Himalaya and Tibet is covered by quaternary sediments. These sediments, mainly of Himalayan origin, form the Indo-Gangetic plains with very thick sedimentary deposits (> 8 km).
The western central portion of India is overlain by flood basalts known as the Deccan Traps or the Deccan Volcanic Province (DVP). The Indian plate has crossed
Crust and crust–mantle boundary
The seismological definition of the Moho is linked to the rapid rise in seismic wavespeeds between the crust and the mantle (Prodehl et al., 2013). The transition from crust to mantle is not always sharp, and there can be differences in the interpretation of the seismological and petrological definitions of the base of the crust (O'Reilly and Griffin, 2013). However, seismological results provide the most comprehensive coverage of the nature of the crust and its boundary with the mantle
Lithospheric mantle
There are two major tools to investigate the mantle component of the lithosphere. The exploitation of the fundamental and higher modes of surface waves in tomography can determine the broad scale patterns of seismic wavespeed variation in 3-D (Priestley and Tilmann, 2009). The surface waves provide dominantly horizontal sampling, with depth information through the character of the different modes. Horizontal resolution is not better than 250 km with about 40 km resolution in depth. This is
The 410 and 660 mantle discontinuities
Major discontinuities associated with mineralogical phase transitions are the mantle transition zone at 410 and 660 km. The properties of these discontinuities provide important constraints on the thermal and compositional nature of the mantle. The mineralogical phase transition from olivine α-phase to the modified spinel β-phase at the 410 km discontinuity and the ringwoodite γ-phase to perovskite and magnesiowustite at the 660 km discontinuity match well with the seismic results. Variations in
Summary
With the aid of the growing body of results from passive seismic studies we have been able to provide a summary description of the crustal properties of much of the Indian subcontinent, and also to characterise the upper mantle beneath. Geographic coverage of peninsular India is good, but information is still limited in the more northern parts of the cratons, the region of the Indo-Gangetic plains and parts of the Himalayan foothills.
More detailed crustal information is currently available only
Acknowledgements
We thank M. Ravi Kumar and R. K. Chadha for the useful discussions at different points of time. This study has been supported by a grant from the Ministry of Earth Sciences (MoES), IITKGP/CKH. We thank Stewart Fishwick and an anonymous reviewer for their valuable comments and suggestions which helped us to significantly improve the manuscript.
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