JOURNAL GEOLOGICAL SOClETY 0' INDIA
VQ1. 29, Mar. 1987,
362 to 363
"p.
POSSIBLE CAUSE OF EASTERLY TILT OF THE SOUTHERN
PART OF INDIA
J(. S. SUBRAMANIAN
Oeological Survey of India
Abstract
The Precambrian terrain In South India is marked by a well defined overall slope
towards east and a westerly slope in Malagasy has been recorded. The definition of
opposing directions of slope in the two lands is considered to be the result of crustal
upwarping along a NNW-SSE 'trending axis prior to (ile separation of Malagasy from
India about lOOm.y.-80m.y. ago. The change in the courses of the Cauvery and a
few other rivers in South India from northeast to east is attributable to the easterly tilt.
Introduction
To the south of the 12°N Latitude in South India, the Western Ghat hill ranges
are marked by a number of peaks ri~ing to altitudes of more than 2000 m above
mean sea level. Farther east, there is perceptible fall in altitude and latitudinal cross~
sections across the Peninsula bear out a well-defined slope element towards east
(Subramanian and Muraleedharan, 1985). King and Bruce Foote (1864) observed
in the central part of Tamil Nadu ' a decided slope to the east' . It is interesting to
note that in Malagasy which is shown to be juxtaposed against the west coast of
India in the Gondwanaland, the overall slope of the terrain is towards west (Nairn,
1978). The opposing slopes in the two lands which constituted a single entity in
the Gondwanaland suggest that the easterly tilt of South India and the westerly tilt
of Malagasy are possibly related to the tectonic processes which led to the separation of the two lands as a part of the rifting and fragmentation of the supercontinent.
Discussion
Magnetic lineament patterns and DSDP site data from the Indian Ocean (Barron
and Harrison, 1981) suggest thCl-t the east coast of India evolved about 140-120 m.y.
ago (Late Jurassic-Early Cretaceous), whereas the west coast came into being later
about 100-80 m. y. ago (Midclle to Late Cretaceous), as a result of the separation of
Malagasy rifting and fragmentation of the western continental edge of India would
have been understandably preceded by crustal up warping and continental distension.
In this context, the definition of an axis of crustal upwarp in NNW-SSE direction
off the west coast of India, suggested by Va.idyanadhan (1976) is interesting. Obviously, the axis lay in the western continental extension of south India, when Malagasy was a part of the Indian plate. Such a situation would suggest that the separation of India and Malagasy was preceded by crustal upwarping along an axis
disposed in a NNW~SSE direction; .the upwarping of the continental crust resulted
jn an easterly tilt of the Indian plate and a westerJy tiJt of MaJagasy ; and tbe rjftjng
and fragmentation of the cOID()osite land along the weak zone of the axis marked the
initial stage in the separation of India and Malagasy (Fig I). Since the separation
of the lands took place 100-80 m.y. ago (Middle to Late Cretaceous), the initiation
of the easterly slope element i.n the Indian plate should have been earlier, probably
in Early Cretaceous.
0016 -7622/87/29-3-362/$1.00
©
Goo!. Soc. India.
RESEARCH NOTES
363
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AXIS OF UPWARP
DIRECTION OF SLOPE
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Figure 1.
Evolution of slope element in India and Malagasy.
In the analysis of the evolution of the Cauvery river and a few other rivers in
South India, Vaidyanadhan (1971) opines that the initial courses of the rivers were
in a northeasterly direction, controlled by NE·SW trending crustal fractures in the
Precambrian terrain and the courses drifted by stages to an easterly direction. It
is but reasonable to suggest that the change in the river courses from northeast to
east was the logical and natural outcome of the easterly tilt of the Indian plate
preceding separation of 1ndia and Malagasy in MiddJe to Late Cretaceous.
References
E. J. and HAlUI.ISON. C. G. N., (1980) An anaiysis of past plate motions: The South
Atlantic and the Indian Oceans. Mechanics of continental drift and plate tectonics.
Edited by P. A. Davies and S. K. Runcorn. Academic press, pp. 89·109.
EARRON,
Kn..:o,
and FOOl E, R. BRUCE, (1965) On the geological structure of portions of the
districts of Trichinopoly, Salem and South Areot, Madras included in sheet No. 79 of the
Indian Atlas. Geol. Surv. India, Mem. v. 4. Art. 2, p. IS.
WILLIAM
NAIRN,
A. E. M., (1978) Northern and Eastern Africa. The Phanerozoic Geology of the world.
Edited by M. Moullade and A. E. M. Nairn. Elsevier, v. 2, pp. 361-365.
K. S. and MURAUfDHARAN, (985) Origin of the Palghat Gap in South IndiaA synthesis. Jour. Geol. Sec. India, v. 26, no. 1, p. 29.
SUBRAMANIAN,
R., (1971) Evolution of the drainage of Cauvery in South India. Jour. Geo!.
Soc. India, v. 12, no. 1, pp. 22·23.
VAIDYAl'ADHAN,
(1976) Evolution of landforms over Precambrian terrains in India. In Studies In: Precambrians. Dept. of Geology, Banga)ore Univirsity, Bangalore, pp. 283-291.