Bulletin Volcanologique

, Volume 35, Issue 3, pp 602–618 | Cite as

The sylhet traps, their tectonic history, and their bearing on problems of indian flood basalt provinces

  • S. C. Talukdar
  • M. V. N. Murthy
Session «Geology And Structure»


Recent studies of the Sylhet Traps (? Jurassic) and the overlying Cretaceous-Tertiary sedimentary cover in the southern part of the Khasi Hills, Shillong Plateau in Assam have led to a reconstruction of the tectonic history of the area since Jurassic times; a clear picture regarding the nature of volcanism has also emerged. The history begins with effusion of tholeiitic basalts, apparently through E-W fissures developed in the peneplaned crystalline basement. One of these fractures became a fault (the Raibah fault) along which the northern non-volcanic block moved up relative to the southern block experiencing volcanism. The fault was active during and after the volcanism till Upper Cretaceous times. The sequence of eruption was as follows: (1) tholeiitic basalts, (2) minor alkali basalts (nepheline tephrite), (3) tholeiitic basalts, (4) localised explosive effusion of minor rhyolites and acid tuffs, and (5) tholeiitic basalts.

Neither feeder dykes nor volcanic vents have been noted in the Sylhet Traps. There are no agglomerates among the basic flows; the fragmental rocks are actually flow breccias. The formation of the various structures such as flow breccias, layering and flow folds in many of the basalt flows are thought to have been controlled by the angle of slope and the rate of flow. Thus, the Sylhet Trap flood basalts are characterised by quiet effusion through linear fissures.

The effusion was followed by a dyke phase, intruding also along E-W fractures, expecially in the monoclinally bent southern portion; the subsequent tectonic history of the area is also characterised by relative uplift and downsinking of different basement blocks.

It is concluded that in the Shillong Plateau uparching of the basement led to fracturing, effusion of basalts apparently along some zones of fissuring along which differential vertical movement of basement blocks was taking place.

In the light of the foregoing conclusions, available data on the tectonics of the Rajmahal and the Deccan Traps are examined; both these flood basalt provinces have suffered broadly similar tectonic histories as the Sylhet Traps.

The various features of flood basalts, viz., large extent, huge thickness, subaerial nature, a post-volcanic dyke phase are interpreted as a consequence of fusion of the Upper Mantle, development of tensional fractures eruptions apparently along fractures between adjoining basement blocks undergoing differential uplift.


Deccan Trap Flood Basalt Alkali Basalt Tholeiitic Basalt Dyke Swarm 
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  1. Adden, J. B., 1949,Dykes in Western India. Trans. Nat. Inst. Sci. India, Vol. 3, p. 123–157.Google Scholar
  2. Beloussov, V., 1962,Basic problems in geotectonics. McGraw Hill Book Co. Inc. Ltd., 816 pp.Google Scholar
  3. Bott, M. H. P., 1964,The upper Mantle in relation to the origin of vertical movements at the Earth’s surface. The U. M. Symp., 1964, p. 20–28.Google Scholar
  4. Chatterjfe, G. C., Biswas, A. B., Bast, S. andNiyogi, B. N., 1964,Geology and groundwater resources of the greater Calcutta industrial area, West Bengal. Bull. Geol. Surv. India, Ser. B. no. 21.Google Scholar
  5. Crookshank, H., 1936,Geology of the Northern slopes of the Stapuras between the Manas and Shar rivers. Mem. Geol. Surv. India, Vol. 65, pt. 2.Google Scholar
  6. De Swardt, A. M. I., 1965,Rift Faulting in Zambia, East Africa Rift System. U. M. Committee UNESCO Seminar, Naitobi, p. 109.Google Scholar
  7. Glennie, E. A., 1932,Gravity anomalies and structure of the Earth’s crust. Prof. Paper Surv. India, no. 27.Google Scholar
  8. Macdonald, G. A., 1953,‘Pahochoc’, ‘an’ and block lava. Am. Jour. Sci., Vol. 251, p. 169–191.CrossRefGoogle Scholar
  9. Mukherjee, P. K., 1955,Geology of the area around Simra, Santhal Paraganas. Quart. J. Geol. Mining and Metallurgical Soc. India, Vol. 27, no. 2, p. 67–84.Google Scholar
  10. Pascoe, E. H., 1964,A manual of the geology of India and Burma. Vol. 3, 3rd Ed.Google Scholar
  11. Palmer, R. W., 1923,Geology of a part of the Khasi and Jaintia Hills, Assam. Rec. Geol. Surv. India, Vol. 55, pt. 2, p. 143–168.Google Scholar
  12. Qureshy, M. N., 1965,A geological analysis of Bouger anomaly map of Peninsular India. Proc. Nat. Inst. Sci. India, Vol. 30, no. 5.Google Scholar
  13. Ray, S., 1963,Fifty years of science in India: Progress in Geology. Ind. Sci. Cong. Assn.Google Scholar
  14. -----, 1967,Geology of Peninsular India and the Upper Mantle. Proc. Symp. on U. M. Project, 1967, p. 249–258.Google Scholar
  15. Rittmann, A.,Volcanoes and their activity. Intersci. Publ., J. Wiley and Sons, N. Y. London, 305 pp.Google Scholar
  16. Sarbadiiikari, T. R., 1958,On orthopyroxene bearing rocks from Tinapahar in the Rajmahal Hills area. Quart. J. Geol., Mining and Metallurgical Soc. India, Vol. 30, no. 4, p. 221–227.Google Scholar
  17. Sengupta, S. N., 1966,Geological and geophysical studies in western part of Bengal Basin, India. Bull. Assn. Pet. Geologists, Vol. 5, p. 1001–1017.Google Scholar
  18. Talukdar, S. C., 1967a, Rhyolite and alkali basalt from the Sylhet Traps, Khasi Hills, Assam. Curr. Sci., Vol. 36, no. 9, p. 238–239.Google Scholar
  19. —————, 1967b, On the carbonaceous material found in the Sylhet Traps, Khasi Hills, Assam. Curr. Sci., Vol. 37, no. 5, p. 139–140.Google Scholar
  20. -----, 1967c, Tectonic significance of Mesozoic volcanicity in Eastern India. (Paper accepted for reading at the 23rd Int. Geol. Cong., Prague).Google Scholar
  21. Walker, G. P. L., 1969,Some observations and interpretations on the Deccan Traps. Unpubl. Report, Centre of Advanced Study in Geology, Univ. of Saugar, Saugar, M. P.Google Scholar
  22. West, W. D., 1959,The source of the Deccan Trap flows. J. Geol. Soc. India, Vol. 1, p. 44–52.Google Scholar
  23. Woollard, G. P., 1958,Area of tectonic activity in the U.S. as indicated by the earthquake epicenters. Trans. Am. Geophys. Union, Vol. 39, p. 1135–1150.Google Scholar

Copyright information

© Stabilimento Tipografico Francesco Giannini & Figli 1970

Authors and Affiliations

  • S. C. Talukdar
    • 1
  • M. V. N. Murthy
    • 1
  1. 1.Geological Survey of IndiaShillong

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